Co-reporter:Eric A. Margulies, Nicolas Kerisit, Przemyslaw Gawel, Catherine M. Mauck, Lin Ma, Claire E. Miller, Ryan M. Young, Nils Trapp, Yi-Lin Wu, François Diederich, and Michael R. Wasielewski
The Journal of Physical Chemistry C October 5, 2017 Volume 121(Issue 39) pp:21262-21262
Publication Date(Web):August 10, 2017
DOI:10.1021/acs.jpcc.7b07870
Cyano-substituted tetracenes (5,11-dicyano-6,12-diphenyltetracene, Tet) undergo exoergic singlet fission (SF), a spin-allowed photophysical process that generates a pair of triplet excitons from one singlet exciton. To elucidate substituent effects on SF, we have measured the SF dynamics and triplet yields of thin films, formed by Tet bearing hydrogen (H), methyl (Me), fluoro (F), and trimethylsilyl (TMS) substituents on the p-phenyl positions and on the 3 and 9 positions of the tetracene core, by time-resolved spectroscopy in the vis-NIR and IR regions. The H-, Me-, and F-Tet display strong intramolecular electronic coupling (π–π distances <4 Å), and SF gives high triplet exciton yields up to 200% (quantitative). In addition, a charge-transfer state mediates SF in F-Tet films, while H-Tet and Me-Tet show no evidence for such a state. Correlations between the SF yields and the crystal structure show that chromophore slippage along both their short and long axes allows efficient SF and that a large degree of π contact between the chromophores is not necessary for rapid and efficient SF in the solid state. As expected, the large interchromophore distance in TMS-Tet (>4 Å) reduces its SF triplet yield to about 60%.
Co-reporter:Jenna L. Logsdon, Patrick E. Hartnett, Jordan N. Nelson, Michelle A. Harris, Tobin J. Marks, and Michael R. Wasielewski
ACS Applied Materials & Interfaces October 4, 2017 Volume 9(Issue 39) pp:33493-33493
Publication Date(Web):April 21, 2017
DOI:10.1021/acsami.7b02585
Orthogonal attachment of polar and nonpolar side-chains to a zinc porphyrin-perylenediimide dyad (ZnP-PDI, 1a) is shown to result in self-assembly of ordered supramolecular ribbons in which the ZnP and PDI molecules form segregated π-stacked columns. Following photoexcitation of the ordered ribbons, ZnP+•-PDI–• radical ion pairs form in <200 fs and subsequently produce a 30 ± 3% yield of free charge carriers that live for about 100 μs. Elongating the side chains on ZnP and PDI in 1b enhances the order of the films, but does not result in an increase in free charge carrier yield. In addition, this yield is independent of temperature, free energy of reaction, and the ZnP-PDI distance in the covalent dyad. These results suggest that the free charge carrier yield in this system is not limited by a bound charge transfer (CT) state or promoted by a vibronically hot CT state. Instead, it is likely that π-stacking of the segregated donors and acceptors within the ribbons results in delocalization of the charges following photoexcitation, allowing them to overcome Coulombic attraction and generate free charge carriers.Keywords: charge transport; EPR; femtosecond transient absorption spectroscopy; GIWAXS; organic photovoltaics; self-assembly; thin films;
Co-reporter:Yilei Wu, Jiawang Zhou, Brian T. Phelan, Catherine M. Mauck, J. Fraser Stoddart, Ryan M. Young, and Michael R. Wasielewski
Journal of the American Chemical Society October 11, 2017 Volume 139(Issue 40) pp:14265-14265
Publication Date(Web):September 7, 2017
DOI:10.1021/jacs.7b08275
Facile exciton transport within ordered assemblies of π-stacked chromophores is essential for developing molecular photonic and electronic materials. Excimer states having variable charge transfer (CT) character are frequently implicated as promoting or inhibiting exciton mobility in such systems. However, determining the degree of CT character in excimers as a function of their structure has proven challenging. Herein, we report on a series of cyclophanes in which the interplanar distance between two phenyl-extended viologen (ExV2+) chromophores is varied systematically using a pair of o-, m-, or p-xylylene (o-, m-, or p-Xy) covalent linkers to produce o-ExBox4+ (3.5 Å), m-ExBox4+ (5.6 Å), and p-ExBox4+ (7.0 Å), respectively. The cyclophane structures are characterized using NMR spectroscopy in solution and single-crystal X-ray diffraction in the solid state. Femtosecond transient mid-IR and stimulated Raman spectroscopies show that the CT contribution to the excimer states formed in o-ExBox4+ and m-ExBox4+ depends on the distance between the chromophores within the cyclophanes, while in the weak interaction limit, as represented by p-ExBox4+ (7.0 Å), the lowest excited singlet state of ExV2+ exclusively photo-oxidizes the p-Xy spacer to give the p-Xy+•-ExV+• ion pair. Moreover, the vibrational spectra of the excimer state show that it assumes a geometry that is intermediate between that of the locally excited and CT states, approximately reflecting the degree of CT character.
Co-reporter:Brandon K. Rugg, Brian T. Phelan, Noah E. Horwitz, Ryan M. Young, Matthew D. Krzyaniak, Mark A. Ratner, and Michael R. Wasielewski
Journal of the American Chemical Society November 8, 2017 Volume 139(Issue 44) pp:15660-15660
Publication Date(Web):October 26, 2017
DOI:10.1021/jacs.7b10458
Controlling spin–spin interactions in multispin molecular assemblies is important for developing new approaches to quantum information processing. In this work, a covalent electron donor–acceptor–radical triad is used to probe spin-selective reduction of the stable radical to its diamagnetic anion. The molecule consists of a perylene electron donor chromophore (D) bound to a pyromellitimide acceptor (A), which is, in turn, linked to a stable α,γ-bisdiphenylene-β-phenylallyl radical (R•) to produce D-A-R•. Selective photoexcitation of D within D-A-R• results in ultrafast electron transfer to form the D+•-A–•-R• triradical, where D+•-A–• is a singlet spin-correlated radical pair (SCRP), in which both SCRP spins are uncorrelated relative to the R• spin. Subsequent ultrafast electron transfer within the triradical forms D+•-A-R–, but its yield is controlled by spin statistics of the uncorrelated A–•-R• radical pair, where the initial charge separation yields a 3:1 statistical mixture of D+•-3(A–•-R•) and D+•-1(A–•-R•), and subsequent reduction of R• only occurs in D+•-1(A–•-R•). These findings inform the design of multispin systems to transfer spin coherence between molecules targeting quantum information processing using the agency of SCRPs.
Co-reporter:Nicholas D. Eastham, Alexander S. Dudnik, Thomas J. Aldrich, Eric F. Manley, Thomas J. Fauvell, Patrick E. Hartnett, Michael R. Wasielewski, Lin X. Chen, Ferdinand S. Melkonyan, Antonio Facchetti, Robert P. H. Chang, and Tobin J. Marks
Chemistry of Materials May 23, 2017 Volume 29(Issue 10) pp:4432-4432
Publication Date(Web):May 3, 2017
DOI:10.1021/acs.chemmater.7b00964
Perylenediimide (PDI) small molecule acceptor (SMA) crystallinity and donor polymer aggregation and crystallinity effects on bulk-heterojunction microstructure and polymer solar cell (PSC) performance are systematically investigated. Two high-performance polymers, semicrystalline poly[5-(2-hexyldodecyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-yl-alt-4,4″dodecyl-2,2′:5′,2″-terthiophene-5,5″-diyl] (PTPD3T or D1) and amorphous poly{4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-2-carboxylate-2,6-diyl) (PBDTT-FTTE or D2), are paired with three PDI-based SMAs (A1–A3) of differing crystallinity (A1 is the most, A3 is the least crystalline). The resulting PSC performance trends are strikingly different from those of typical fullerene-based PSCs and are highly material-dependent. The present trends reflect synergistic aggregation propensities between the SMA and polymer components. Importantly, the active layer morphology is templated by the PDI in some blends and by the polymer in others, with the latter largely governed by the polymer aggregation. Thus, PTPD3T templating capacity increases as self-aggregation increases (greater Mn), optimizing PSC performance with A2, while A3-based cells exhibit an inverse relationship between polymer aggregation and performance, which is dramatically different from fullerene-based PSCs. For PBDTT-FTTE, A2-based cells again deliver the highest PCEs of ∼5%, but here both A2 and PBDTT-FTTE (medium Mn) template the morphology. Overall, the present results underscore the importance of nonfullerene acceptor aggregation for optimizing PSC performance and offer guidelines for pairing SMAs with acceptable donor polymers.
Co-reporter:Claire E. Miller, Michael R. Wasielewski, and George C. Schatz
The Journal of Physical Chemistry C May 18, 2017 Volume 121(Issue 19) pp:10345-10345
Publication Date(Web):April 26, 2017
DOI:10.1021/acs.jpcc.7b02697
Singlet fission (SF) is being explored as a way to improve the efficiency of organic photovoltaics beyond the Shockley-Queisser limit; however, many aspects of the SF mechanism remain unresolved. The generally accepted mechanisms provide simplified models of SF that equivocate over whether a charge transfer (CT) state is involved in SF. A one-step superexchange model allows the CT state to act as a virtual state, reducing the effect of large Gibbs free energy values from SF rate calculations. Also, extending superexchange to an excimer-mediated process allows for further refinement of the triplet formation model. Application of the superexchange and excimer-mediated models to a variety of rylene and diketopyrrolopyrrole derivatives provides new insights into the role of the CT and excimer states, providing a semiquantitative description of SF that is dictated by the CT state energy.
Co-reporter:Tomáš Šolomek, Natalia E. Powers-Riggs, Yi-Lin Wu, Ryan M. Young, Matthew D. Krzyaniak, Noah E. Horwitz, and Michael R. Wasielewski
Journal of the American Chemical Society March 8, 2017 Volume 139(Issue 9) pp:3348-3348
Publication Date(Web):February 22, 2017
DOI:10.1021/jacs.7b00233
We present the stereoselective synthesis of a chiral covalent organic cage consisting of three redox-active naphthalene-1,4:5,8-bis(dicarboximide) (NDI) units by dynamic imine chemistry. Single crystal X-ray diffraction analysis shows that host–guest interactions and racemic cocrystallization allow for controlling the solid state structure. Electronic interactions between the NDI units probed by absorption and circular dichroism spectroscopies, electrochemistry and theoretical calculations are shown to be weak. Photoexcitation of NDI leads to intracage charge separation with a longer lifetime than observed in the corresponding monomeric NDI and dimeric NDI cyclophane imines. The EPR spectrum of the singly reduced cage shows that the electron is localized on a single NDI unit at ambient temperatures and transitions to rapid hopping among all three NDI units upon heating to 350 K. Dynamic covalent chemistry thus promises rapid access to covalent organic cages with well-defined architectures to study charge accumulation and electron transport phenomena.
Co-reporter:Jordan N. Nelson, Matthew D. Krzyaniak, Noah E. Horwitz, Brandon K. Rugg, Brian T. Phelan, and Michael R. Wasielewski
The Journal of Physical Chemistry A March 23, 2017 Volume 121(Issue 11) pp:2241-2241
Publication Date(Web):March 3, 2017
DOI:10.1021/acs.jpca.7b00587
Photoinitiated subnanosecond electron transfer within covalently linked electron donor–acceptor molecules can result in the formation of a spin-correlated radical pair (SCRP) with a well-defined initial singlet spin configuration. Subsequent coherent mixing between the SCRP singlet and triplet ms = 0 spin states, the so-called zero quantum coherence (ZQC), is of potential interest in quantum information processing applications because the ZQC can be probed using pulse electron paramagnetic resonance (pulse-EPR) techniques. Here, pulse-EPR spectroscopy is utilized to examine the ZQC oscillation frequencies and ZQC dephasing in three structurally well-defined D–A systems. While transitions between the singlet and triplet ms = 0 spin states are formally forbidden (Δms = 0), they can be addressed using specific microwave pulse turning angles to map information from the ZQC onto observable single quantum coherences. In addition, by using structural variations to tune the singlet–triplet energy gap, the ZQC frequencies determined for this series of molecules indicate a stronger dependence on the electronic g-factor than on electron–nuclear hyperfine interactions.
Co-reporter:Xirui Gong, Ryan M. Young, Karel J. Hartlieb, Claire Miller, Yilei Wu, Hai Xiao, Peng Li, Nema Hafezi, Jiawang Zhou, Lin Ma, Tao Cheng, William A. Goddard III, Omar K. Farha, Joseph T. Hupp, Michael R. Wasielewski, and J. Fraser Stoddart
Journal of the American Chemical Society March 22, 2017 Volume 139(Issue 11) pp:4107-4107
Publication Date(Web):February 22, 2017
DOI:10.1021/jacs.6b13223
Molecules capable of performing highly efficient energy transfer and ultrafast photoinduced electron transfer in well-defined multichromophoric structures are indispensable to the development of artificial photofunctional systems. Herein, we report on the synthesis, characterization, and photophysical properties of a rationally designed multichromophoric tetracationic cyclophane, DAPPBox4+, containing a diazaperopyrenium (DAPP2+) unit and an extended viologen (ExBIPY2+) unit, which are linked together by two p-xylylene bridges. Both 1H NMR spectroscopy and single-crystal X-ray diffraction analysis confirm the formation of an asymmetric, rigid, box-like cyclophane, DAPPBox4+. The solid-state superstructure of this cyclophane reveals a herringbone-type packing motif, leading to two types of π···π interactions: (i) between the ExBIPY2+ unit and the DAPP2+ unit (π···π distance of 3.7 Å) in the adjacent parallel cyclophane, as well as (ii) between the ExBIPY2+ unit (π···π distance of 3.2 Å) and phenylene ring in the closest orthogonal cyclophane. Moreover, the solution-phase photophysical properties of this cyclophane have been investigated by both steady-state and time-resolved absorption and emission spectroscopies. Upon photoexcitation of DAPPBox4+ at 330 nm, rapid and quantitative intramolecular energy transfer occurs from the 1*ExBIPY2+ unit to the DAPP2+ unit in 0.5 ps to yield 1*DAPP2+. The same excitation wavelength simultaneously populates a higher excited state of 1*DAPP2+ which then undergoes ultrafast intramolecular electron transfer from 1*DAPP2+ to ExBIPY2+ to yield the DAPP3+•–ExBIPY+• radical ion pair in τ = 1.5 ps. Selective excitation of DAPP2+ at 505 nm populates a lower excited state where electron transfer is kinetically unfavorable.
Co-reporter:Yilei Wu, Matthew D. Krzyaniak, J. Fraser Stoddart, and Michael R. Wasielewski
Journal of the American Chemical Society March 1, 2017 Volume 139(Issue 8) pp:2948-2948
Publication Date(Web):February 14, 2017
DOI:10.1021/jacs.7b00515
Crystalline supramolecular frameworks consisting of charged molecules, held together by hydrogen bonds and Coulomb interactions, have attracted great interest because of their unusual structural, chemical, electronic, and magnetic properties. Herein, we report the preparation, structure, and magnetic properties of the triradical trianion of a shape-persistent chiral equilateral molecular triangle having three naphthalene-1,4:5,8-bis(dicarboximide)s ((+)-NDI-Δ3(−•)). Single-crystal X-ray diffraction of its tris(cobaltocenium) salt ([(+)-NDI-Δ3(−•)(CoCp2+)3]) reveals accessible one-dimensional tubular cavities, and variable-temperature electron paramagnetic resonance spectroscopy shows that a dilute solution of [(+)-NDI-Δ3(−•)(CoCp2+)3] in an organic glass has a spin-frustrated doublet ground state and a thermally accessible quartet state. Furthermore, SQUID magnetometry from 5 to 300 K of solid [(+)-NDI-Δ3(−•)(CoCp2+)3] shows ferromagnetic ordering with a Curie temperature TC = 20 K. The successful preparation of hybrid ionic materials comprising macrocyclic triradical trianions with spin-frustrated ground states and accessible 1D pores offers routes to new organic spintronic materials.
Co-reporter:Noah E. Horwitz, Brian T. Phelan, Jordan N. Nelson, Catherine M. Mauck, Matthew D. Krzyaniak, and Michael R. Wasielewski
The Journal of Physical Chemistry A June 15, 2017 Volume 121(Issue 23) pp:4455-4455
Publication Date(Web):May 22, 2017
DOI:10.1021/acs.jpca.7b03468
Photoexcitation of electron donor–acceptor molecules frequently produces radical ion pairs with well-defined initial spin-polarized states that have attracted significant interest for spintronics. Transfer of this initial spin polarization to a stable radical is predicted to depend on the rates of the radical ion pair recombination reactions, but this prediction has not been tested experimentally. In this study, a stable radical/electron donor/chromophore/electron acceptor molecule, BDPA•–mPD–ANI–NDI, where BDPA• is α,γ-bisdiphenylene-β-phenylallyl, mPD is m-phenylenediamine, ANI is 4-aminonaphthalene-1,8-dicarboximide, and NDI is naphthalene-1,4:5,8-bis(dicarboximide), was synthesized. Photoexcitation of ANI produces the triradical BDPA•–mPD+•–ANI–NDI–• in which the mPD+•–ANI–NDI–• radical ion pair is spin coupled to the BDPA• stable radical. BDPA•–mPD+•–ANI–NDI–• and its counterpart lacking the stable radical are found to exhibit spin-selective charge recombination in which the triplet radical ion pair 3(mPD+•–ANI–NDI–•) is in equilibrium with the 3*NDI charge recombination product. Time-resolved EPR measurements show that this process is associated with an inversion of the sign of the polarization transferred to BDPA• over time. The polarization transfer rates are found to be strongly solvent dependent, as shifts in this equilibrium affect the spin dynamics. These results demonstrate that even small changes in electron transfer dynamics can have a large effect on the spin dynamics of photogenerated multispin systems.
Co-reporter:Daniel D. Powell, Michael R. Wasielewski, and Mark A. Ratner
The Journal of Physical Chemistry B July 27, 2017 Volume 121(Issue 29) pp:7190-7190
Publication Date(Web):June 29, 2017
DOI:10.1021/acs.jpcb.7b02748
Coherence effects on electron transfer in a series of symmetric and asymmetric two-, three-, four-, and five-site molecular model systems for photosystem I in cyanobacteria and green plants were studied. The total site energies of the electronic Hamiltonian were calculated using the density functional theory (DFT) formalism and included the zero point vibrational energies of the electron donors and acceptors. Site energies and couplings were calculated using a polarizable continuum model to represent various solvent environments, and the site-to-site couplings were calculated using fragment charge difference methods at the DFT level of theory. The Redfield formalism was used to propagate the electron density from the donors to the acceptors, incorporating relaxation and dephasing effects to describe the electron transfer processes. Changing the relative energies of the donor, intermediate acceptor, and final acceptor molecules in these assemblies has profound effects on the electron transfer rates as well as on the amplitude of the quantum oscillations observed. Increasing the ratio of a particular energy gap to the electronic coupling for a given pair of states leads to weaker quantum oscillations between sites. Biasing the intermediate acceptor energies to slightly favor one pathway leads to a general decrease in electron transfer yield.
Co-reporter:Taiga Fujii, Arun K. Thazhathveetil, Ilyas Yildirim, Ryan M. Young, Michael R. Wasielewski, George C. Schatz, and Frederick D. Lewis
The Journal of Physical Chemistry B August 31, 2017 Volume 121(Issue 34) pp:8058-8058
Publication Date(Web):July 17, 2017
DOI:10.1021/acs.jpcb.7b04996
The dynamics of electron injection have been investigated in intramolecular i-motif conjugates possessing stilbenediether (Sd) and perylenediimide (PDI) chromophores separated by either four or six hemiprotonated cytosine C–C base pairs assembled with synthetic loops. Circular dichroism spectra are consistent with the formation of i-motif structures in the absence or presence of Sd and PDI chromophores. The fluorescence of the Sd chromophore is essentially completely quenched by neighboring C–C base pairs, consistent with their function as an electron donor and electron acceptor, respectively. However, the fluorescence of the PDI chromophore is only partially quenched. The dynamics of electron injection from singlet Sd into the i-motif and subsequent charge recombination has been determined by femtosecond transient absorption (fsTA) spectroscopy and compared with the results for electron injection and charge recombination in Sd-linked hairpins possessing cytosine–guanine (C–G) or 5-fluorouracil–adenine (F–A) base pairs. While charge injection is ultrafast (<0.8 ps) for the i-motifs, charge transport across the i-motif C–C base pairs to the PDI electron trap is not observed. The absence of electron transport is related to the structure of the stacked C–C base pairs in the i-motif.
Co-reporter:Kristen E. Brown, Arunoday P. N. Singh, Yi-Lin Wu, Ashutosh Kumar Mishra, Jiawang Zhou, Frederick D. Lewis, Ryan M. Young, and Michael R. Wasielewski
Journal of the American Chemical Society August 30, 2017 Volume 139(Issue 34) pp:12084-12084
Publication Date(Web):August 2, 2017
DOI:10.1021/jacs.7b06998
The hole transport dynamics of DNA hairpins possessing a stilbene electron acceptor and donor along with a modified guanine (G) nucleobase, specifically 8-(4′-phenylethynyl)deoxyguanosine, or EG, have been investigated. The nearly indistinguishable oxidation potentials of EG and G and unique spectroscopic characteristics of EG+• make it well-suited for directly observing transient hole occupation during charge transport between a stilbene electron donor and acceptor. In contrast to the cation radical G+•, EG+• possesses a strong absorption near 460 nm and has a distinct Raman-active ethynyl stretch. Both spectroscopic characteristics are easily distinguished from those of the stilbene donor/acceptor radical ion chromophores. Employing EG, we observe its role as a shallow hole trap, or as an intermediate hole transport site when a deeper trap state is present. Using a combination of ultrafast absorption and stimulated Raman spectroscopies, the hole-transport dynamics are observed to be similar in systems having EG vs G bases, with small perturbations to the charge transport rates and yields. These results show EG can be deployed at specified locations throughout the sequence to report on hole occupancy, thereby enabling detailed monitoring of the hole transport dynamics with base-site specificity.
Co-reporter:Catherine M. Mauck, Patrick E. Hartnett, Yi-Lin Wu, Claire E. Miller, Tobin J. Marks, and Michael R. Wasielewski
Chemistry of Materials August 22, 2017 Volume 29(Issue 16) pp:6810-6810
Publication Date(Web):July 23, 2017
DOI:10.1021/acs.chemmater.7b01845
Nanoparticles (NPs) of the singlet fission chromophore 3,6-bis(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (PhTDPP) having average hydrodynamic diameters of 63–193 nm were prepared by rapidly injecting variable concentrations of PhTDPP solutions in tetrahydrofuran into water. These PhTDPP NPs are stable over months in water and exhibit fluorescence quantum yields ≪1%. Femtosecond transient absorption spectroscopy shows that singlet fission is more rapid in smaller NPs, likely reflecting their greater surface area-to-volume ratio and consequent exposure of more molecules to the high dielectric aqueous environment. These observations suggest that charge transfer states, whose energy is sensitive to the dielectric constant of the surrounding medium, serve as virtual intermediates in PhTDPP NP singlet fission. However, the lifetime of the triplet excitons produced by singlet fission is longest in the larger NPs having greater long-range order, which allows the triplet excitons to diffuse further from one another thus slowing triplet–triplet annihilation.
Co-reporter:Arun K. Thazhathveetil, Michelle A. Harris, Ryan M. YoungMichael R. Wasielewski, Frederick D. Lewis
Journal of the American Chemical Society 2017 Volume 139(Issue 5) pp:1730-1733
Publication Date(Web):January 17, 2017
DOI:10.1021/jacs.6b10265
Co-reporter:Peter Spenst, Ryan M. YoungBrian T. Phelan, Michel KellerJakub Dostál, Tobias Brixner, Michael R. Wasielewski, Frank Würthner
Journal of the American Chemical Society 2017 Volume 139(Issue 5) pp:2014-2021
Publication Date(Web):January 12, 2017
DOI:10.1021/jacs.6b11973
A series of semirigid perylene bisimide (PBI) macrocycles with varied ring size containing two to nine PBI chromophores were synthesized in a one-pot reaction and their photophysical properties characterized by fluorescence, steady-state, and transient absorption spectroscopy as well as femtosecond stimulated Raman spectroscopy. These macrocycles show solvent-dependent conformational equilibria and excited-state properties. In dichloromethane, the macrocycles prevail in wide-stretched conformations and upon photoexcitation exhibit symmetry-breaking charge separation followed by charge recombination to triplet states, which photosensitize singlet oxygen formation. In contrast, in aromatic solvents folding of the macrocycles with a distinct odd–even effect regarding the number of PBI chromophore units was observed in steady-state and time-resolved absorption and fluorescence spectroscopy as well as femtosecond stimulated Raman spectroscopy. These distinctive optical properties are attributable to the folding of the even-membered macrocycles into exciton-vibrational coupled dimer pairs in aromatic solvents. Studies in a variety of aromatic solvents indicate that these solvents embed between PBI dimer pairs and accordingly template the folding of even-membered PBI macrocycles into ropelike folded conformations that give rise to solvent-specific exciton-vibrational couplings in UV–vis absorption spectra. As a consequence of the embedding of solvent molecules in the coiled double-string rope architecture, highly solvent specific intensity ratios are observed for the two lowest-energy exciton-vibrational bands, enabling assignment of the respective solvent simply based on the absorption spectra measured for the tetramer macrocycle.
Co-reporter:Rita E. Cook, Brian T. PhelanRebecca J. Kamire, Marek B. Majewski, Ryan M. Young, Michael R. Wasielewski
The Journal of Physical Chemistry A 2017 Volume 121(Issue 8) pp:
Publication Date(Web):February 13, 2017
DOI:10.1021/acs.jpca.6b12644
The use of multiple chromophores as photosensitizers for catalysts involved in energy-demanding redox reactions is often complicated by electronic interactions between the chromophores. These interchromophore interactions can lead to processes, such as excimer formation and symmetry-breaking charge separation (SB-CS), that compete with efficient electron transfer to or from the catalyst. Here, two dimers of perylene bound either directly or through a xylyl spacer to a xanthene backbone were synthesized to probe the effects of interchromophore electronic coupling on excimer formation and SB-CS using ultrafast transient absorption spectroscopy. Two time constants for excimer formation in the 1–25 ps range were observed in each dimer due to the presence of rotational isomers having different degrees of interchromophore coupling. In highly polar acetonitrile, SB-CS competes with excimer formation in the more weakly coupled isomers followed by charge recombination with τCR = 72–85 ps to yield the excimer. The results of this study of perylene molecular dimers can inform the design of chromophore–catalyst systems for solar fuel production that utilize multiple perylene chromophores.
Co-reporter:Catherine M. Mauck, Ryan M. YoungMichael R. Wasielewski
The Journal of Physical Chemistry A 2017 Volume 121(Issue 4) pp:
Publication Date(Web):January 3, 2017
DOI:10.1021/acs.jpca.6b11388
Excimer formation plays a significant role in trapping excitons within organic molecular solids. Covalent dimers of perylene-3,4:9,10-bis(dicarboximide) (PDI) are useful model systems for studying these processes as their intermolecular geometries can be precisely tuned. Using femtosecond visible-pump infrared-probe (fsIR) spectroscopy in the shortwave- and mid-infrared regions, we characterize two PDI dimers with a cofacial and a slip-stacked geometry that are coupled through a triptycene bridge. In the mid-infrared region, fsIR spectra for the strongly coupled dimers are highly blue-shifted compared to spectra for monomeric 1*PDI. The perylene core stretching modes provide a directly observable probe of excimer relaxation, as they are particularly sensitive to this process, which is associated with a small blue shift of these modes in both dimers. The broad Frenkel-to-CT state electronic transition of the excimer, the edge of which has previously been detected in the NIR region, is now fully resolved to be much broader and to extend well into the shortwave infrared region for both dimers and is likely a generic feature of π-extended aromatic excimers.
Co-reporter:Rebecca J. Kamire, Kelly L. Materna, William L. HoffeditzBrian T. Phelan, Julianne M. ThomsenOmar K. Farha, Joseph T. Hupp, Gary W. BrudvigMichael R. Wasielewski
The Journal of Physical Chemistry C 2017 Volume 121(Issue 7) pp:
Publication Date(Web):January 27, 2017
DOI:10.1021/acs.jpcc.6b11672
Improving stability and slowing charge recombination are some of the greatest challenges in the development of dye-sensitized photoelectrochemical cells (DSPECs) for solar fuels production. We have investigated the effect of encasing dye molecules in varying thicknesses of Al2O3 deposited by atomic layer deposition (ALD) before catalyst loading on both the stability and the charge transfer dynamics in organic dye-sensitized TiO2 photoanodes containing iridium-based molecular water-oxidation catalysts. In the TiO2|dye|Al2O3|catalyst electrodes, a sufficiently thick ALD layer protects the perylene-3,4-dicarboximide (PMI) chromophores from degradation over several weeks of exposure to light. The insulating capacity of the layer allows a higher photocurrent in the presence of ALD while initial charge injection is slowed by only 1.6 times, as observed by femtosecond transient absorption spectroscopy. Rapid picosecond-scale catalyst oxidation is observed in the presence of a dinuclear catalyst, IrIr, but is slowed to tens of picoseconds for a mononuclear catalyst, IrSil, that incorporates a long linker. Photoelectrochemical experiments demonstrate higher photocurrents with IrSil compared to IrIr, which show that recombination is slower for IrSil, while higher photocurrents with IrIr upon addition of ALD layers confirm that ALD successfully slows charge recombination. These findings demonstrate that, beyond stability improvements, ALD can contribute to tuning charge transfer dynamics in photoanodes for solar fuels production and may be particularly useful for slowing charge recombination and accounting for varying charge transfer rates based on the molecular structures of incorporated catalysts.
Co-reporter:Won-Sik Han, Brad S. Veldkamp, Scott M. Dyar, Samuel W. Eaton, Michael R. Wasielewski
Tetrahedron 2017 Volume 73, Issue 33(Issue 33) pp:
Publication Date(Web):17 August 2017
DOI:10.1016/j.tet.2017.06.021
We report the synthesis and photophysical characterization of isomeric donor-acceptor1-acceptor2 (D-A1-A2) triads each comprising a perylene donor (D) connected by a xylene bridge to a naphthalene-1,8-dicarboximide primary acceptor (NMI, A1) that is, in turn, covalently linked to a naphthalene-1,4:5,8-bis(dicarboximide) secondary acceptor (NDI, A2) through a benzene ring at its ortho, meta or para positions (Per-Xy-NMI-x-NDI). Selective photoexcitation of Per produces both Per+• and NDI‒• simultaneously with time constants of ∼50 ps in benzonitrile and ∼80 ps in toluene with a >98% yield. This is much faster than expected for single-step electron transfer from 1*Per to NDI and is consistent with a two-step mechanism, 1*Per-Xy-NMI-x-NDI → Per+•-Xy-NMI‒•-x-NDI → Per+•-Xy-NMI-x-NDI‒•, in which the second step is much faster than the first step. In addition, preserving long-lived charge separation in these systems is found to depend on inhibiting charge recombination by a triplet radical ion pair recombination pathway.Download high-res image (157KB)Download full-size image
Co-reporter:Amod Timalsina;Patrick E. Hartnett;Ferdinand S. Melkonyan;Joseph Strzalka;Vari S. Reddy;Antonio Facchetti;Tobin J. Marks
Journal of Materials Chemistry A 2017 vol. 5(Issue 11) pp:5351-5361
Publication Date(Web):2017/03/14
DOI:10.1039/C7TA00063D
The synthesis of a new tetrafluorinated semiconducting donor polymer, poly[(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene)-alt-(5,6-difluoro-4,7-(4-(2-ethylhexyl)-dithien-2-yl-2,1,3-benzothiadiazole)] (PBTZF4), and its photovoltaic performance in bulk heterojunction (BHJ) blends with the non-fullerene molecular acceptor [1,2:3,4]-bis-[N,N′-bis-1-pentylhexyl-perylenediimide-1,12-yl]-benzene (bPDI2P), are reported. PBTZF4:bPDI2P solar cells exhibit a high open circuit voltage (Voc) of 1.118 V, a short circuit current density (Jsc) of 10.02 mA cm−2, and a fill factor (FF) of 49.5%, affording a power conversion efficiency (PCE) of 5.55%. Interestingly, a lower PCE of 3.68% is obtained with the difluorinated analogue, poly[(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene)-alt-(5,6-difluoro-4,7-(4-(2-ethyl-hexyl)-dithien-2-yl-2,1,3-benzothiadiazole)] (PBTZF2). Both PBTZF4:bPDI2P and PBTZF2:bPDI2P cells benefit from complementary (donor/acceptor) light absorption and very low geminate recombination, with bimolecular recombination being the dominant loss mechanism, as established by femtosecond transient absorption spectroscopy. DFT computation and physicochemical characterization data argue that the “additional” tetrafluorination planarizes the PBTZF4 backbone and enhances aggregation versusPBTZF2, affording superior charge carrier transport as assayed by field-effect mobility. In addition, fluorine-originated HOMO stabilization, −5.41 eV for PBTZF4versus −5.33 eV for PBTZF2, and a superior blend microstructure afford a higher PBTZF4:bPDI2P solar cell PCE versusPBTZF2:bPDI2P.
Co-reporter:Amod Timalsina;Patrick E. Hartnett;Ferdinand S. Melkonyan;Joseph Strzalka;Vari S. Reddy;Antonio Facchetti;Tobin J. Marks
Journal of Materials Chemistry A 2017 vol. 5(Issue 11) pp:5351-5361
Publication Date(Web):2017/03/14
DOI:10.1039/C7TA00063D
The synthesis of a new tetrafluorinated semiconducting donor polymer, poly[(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene)-alt-(5,6-difluoro-4,7-(4-(2-ethylhexyl)-dithien-2-yl-2,1,3-benzothiadiazole)] (PBTZF4), and its photovoltaic performance in bulk heterojunction (BHJ) blends with the non-fullerene molecular acceptor [1,2:3,4]-bis-[N,N′-bis-1-pentylhexyl-perylenediimide-1,12-yl]-benzene (bPDI2P), are reported. PBTZF4:bPDI2P solar cells exhibit a high open circuit voltage (Voc) of 1.118 V, a short circuit current density (Jsc) of 10.02 mA cm−2, and a fill factor (FF) of 49.5%, affording a power conversion efficiency (PCE) of 5.55%. Interestingly, a lower PCE of 3.68% is obtained with the difluorinated analogue, poly[(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene)-alt-(5,6-difluoro-4,7-(4-(2-ethyl-hexyl)-dithien-2-yl-2,1,3-benzothiadiazole)] (PBTZF2). Both PBTZF4:bPDI2P and PBTZF2:bPDI2P cells benefit from complementary (donor/acceptor) light absorption and very low geminate recombination, with bimolecular recombination being the dominant loss mechanism, as established by femtosecond transient absorption spectroscopy. DFT computation and physicochemical characterization data argue that the “additional” tetrafluorination planarizes the PBTZF4 backbone and enhances aggregation versusPBTZF2, affording superior charge carrier transport as assayed by field-effect mobility. In addition, fluorine-originated HOMO stabilization, −5.41 eV for PBTZF4versus −5.33 eV for PBTZF2, and a superior blend microstructure afford a higher PBTZF4:bPDI2P solar cell PCE versusPBTZF2:bPDI2P.
Co-reporter:Jose F. Martinez;Nathan T. La Porte;Catherine M. Mauck
Faraday Discussions 2017 (Volume 198) pp:235-249
Publication Date(Web):2017/06/02
DOI:10.1039/C6FD00219F
The naphthalene-1,4:5,8-bis(dicarboximide) radical anion (NDI−˙), which is easily produced by mild chemical or electrochemical reduction (−0.5 V vs. SCE), can be photoexcited at wavelengths as long as 785 nm, and has an excited state (NDI−˙*) oxidation potential of −2.1 V vs. SCE, making it a very attractive choice for artificial photosynthetic systems that require powerful photoreductants, such as CO2 reduction catalysts. However, once an electron is transferred from NDI−˙* to an acceptor directly bound to it, a combination of strong electronic coupling and favorable free energy change frequently make the back electron transfer rapid. To mitigate this effect, we have designed a molecular triad system comprising an NDI−˙ chromophoric donor, a 9,10-diphenylanthracene (DPA) intermediate acceptor, and a Re(dmb)(CO)3 carbon dioxide reduction catalyst, where dmb is 4,4′-dimethyl-2,2′-bipyridine, as the terminal acceptor. Photoexcitation of NDI−˙ to NDI−˙* is followed by ultrafast reduction of DPA to DPA−˙, which then rapidly reduces the metal complex. The overall time constant for the forward electron transfer to reduce the metal complex is τ = 20.8 ps, while the time constant for back-electron transfer is six orders of magnitude longer, τ = 43.4 μs. Achieving long-lived, highly reduced states of these metal complexes is a necessary condition for their use as catalysts. The extremely long lifetime of the reduced metal complex is attributed to careful tuning of the redox potentials of the chromophore and intermediate acceptor. The NDI−˙–DPA fragment presents many attractive features for incorporation into other photoinduced electron transfer assemblies directed at the long-lived photosensitization of difficult-to-reduce catalytic centers.
Co-reporter:Rebecca J. Kamire;Marek B. Majewski;William L. Hoffeditz;Brian T. Phelan;Omar K. Farha;Joseph T. Hupp
Chemical Science (2010-Present) 2017 vol. 8(Issue 1) pp:541-549
Publication Date(Web):2016/12/19
DOI:10.1039/C6SC02477G
The design of efficient hydrogen-evolving photocathodes for dye-sensitized photoelectrochemical cells (DSPECs) requires the incorporation of molecular light absorbing chromophores that are capable of delivering reducing equivalents to molecular proton reduction catalysts at rates exceeding those of charge recombination events. Here, we report the functionalization and kinetic analysis of a nanostructured NiO electrode with a modified perylene-3,4-dicarboximide chromophore (PMI) that is stabilized against degradation by atomic layer deposition (ALD) of thick insulating Al2O3 layers. Following photoinduced charge injection into NiO in high yield, films with Al2O3 layers demonstrate longer charge separated lifetimes as characterized via femtosecond transient absorption spectroscopy and photoelectrochemical techniques. The photoelectrochemical behavior of the electrodes in the presence of Co(II) and Ni(II) molecular proton reduction catalysts is examined, revealing reduction of both catalysts. Under prolonged irradiation, evolved H2 is directly observed by gas chromatography supporting the applicability of PMI embedded in Al2O3 as a photocathode architecture in DSPECs.
Co-reporter:Nathan T. La Porte;Jose F. Martinez;Svante Hedström;Benjamin Rudshteyn;Brian T. Phelan;Catherine M. Mauck;Ryan M. Young;Victor S. Batista
Chemical Science (2010-Present) 2017 vol. 8(Issue 5) pp:3821-3831
Publication Date(Web):2017/05/03
DOI:10.1039/C6SC05103K
A major goal of artificial photosynthesis research is photosensitizing highly reducing metal centers using as much as possible of the solar spectrum reaching Earth's surface. The radical anions and dianions of rylenediimide (RDI) dyes, which absorb at wavelengths as long as 950 nm, are powerful photoreductants with excited state oxidation potentials that rival or exceed those of organometallic chromophores. These dyes have been previously incorporated into all-organic donor–acceptor systems, but have not yet been shown to reduce organometallic centers. This study describes a set of dyads in which perylenediimide (PDI) or naphthalenediimide (NDI) chromophores are attached to Re(bpy)(CO)3 through either the bipyridine ligand or more directly to the Re center via a pyridine ligand. The chromophores are reduced with a mild reducing agent, after which excitation with long-wavelength red or near-infrared light leads to reduction of the Re complex. The kinetics of electron transfer from the photoexcited anions to the Re complex are monitored using transient visible/near-IR and mid-IR spectroscopy, complemented by theoretical spectroscopic assignments. The photo-driven charge shift from the reduced PDI or NDI to the complex occurs in picoseconds regardless of whether PDI or NDI is attached to the bipyridine or to the Re center, but back electron transfer is found to be three orders of magnitude slower with the chromophore attached to the Re center. These results will inform the design of future catalytic systems that incorporate RDI anions as chromophores.
Co-reporter:Edward J. Dale, Nicolaas A. Vermeulen, Michal Juríček, Jonathan C. Barnes, Ryan M. Young, Michael R. Wasielewski, and J. Fraser Stoddart
Accounts of Chemical Research 2016 Volume 49(Issue 2) pp:262
Publication Date(Web):February 2, 2016
DOI:10.1021/acs.accounts.5b00495
Acting as hosts, cationic cyclophanes, consisting of π-electron-poor bipyridinium units, are capable of entering into strong donor–acceptor interactions to form host–guest complexes with various guests when the size and electronic constitution are appropriately matched. A synthetic protocol has been developed that utilizes catalytic quantities of tetrabutylammonium iodide to make a wide variety of cationic pyridinium-based cyclophanes in a quick and easy manner. Members of this class of cationic cyclophanes with boxlike geometries, dubbed ExnBoxm4+ for short, have been prepared by altering a number of variables: (i) n, the number of “horizontal” p-phenylene spacers between adjoining pyridinium units, to modulate the “length” of the cavity; (ii) m, the number of “vertical” p-phenylene spacers, to modulate the “width” of the cavity; and (iii) the aromatic linkers, namely, 1,4-di- and 1,3,5-trisubstituted units for the construction of macrocycles (ExBoxes) and macrobicycles (ExCages), respectively.This Account serves as an exploration of the properties that emerge from these structural modifications of the pyridinium-based hosts, coupled with a call for further investigation into the wealth of properties inherent in this class of compounds. By variation of only the aforementioned components, the role of these cationic receptors covers ground that spans (i) synthetic methodology, (ii) extraction and sequestration, (iii) catalysis, (iv) molecular electronics, (v) physical organic chemistry, and (vi) supramolecular chemistry. Ex1Box4+ (or simply ExBox4+) has been shown to be a multipurpose receptor capable of binding a wide range of polycyclic aromatic hydrocarbons (PAHs), while also being a suitable component in switchable mechanically interlocked molecules. Additionally, the electronic properties of some host–guest complexes allow the development of artificial photosystems. Ex2Box4+ boasts the ability to bind both π-electron-rich and -poor aromatic guests in different binding sites located within the same cavity. ExBox24+ forms complexes with C60 in which discrete arrays of aligned fullerenes result in single cocrystals, leading to improved material conductivities. When the substitution pattern of the ExnBox4+ series is changed to 1,3,5-trisubstituted benzenoid cores, the hexacationic cagelike compound, termed ExCage6+, exhibits different kinetics of complexation with guests of varying sizes—a veritable playground for physical organic chemists.The organization of functionality with respect to structure becomes valuable as the number of analogues continues to grow. With each of these minor structural modifications, a wealth of properties emerge, begging the question as to what discoveries await and what properties will be realized with the continued exploration of this area of supramolecular chemistry based on a unique class of receptor molecules.
Co-reporter:Catherine M. Mauck, Patrick E. Hartnett, Eric A. Margulies, Lin Ma, Claire E. Miller, George C. Schatz, Tobin J. Marks, and Michael R. Wasielewski
Journal of the American Chemical Society 2016 Volume 138(Issue 36) pp:11749-11761
Publication Date(Web):August 22, 2016
DOI:10.1021/jacs.6b05627
Singlet fission (SF) in polycrystalline thin films of four 3,6-bis(thiophen-2-yl)diketopyrrolopyrrole (TDPP) chromophores with methyl (Me), n-hexyl (C6), triethylene glycol (TEG), and 2-ethylhexyl (EH) substituents at the 2,5-positions is found to involve an intermediate excimer-like state. The four different substituents yield four distinct intermolecular packing geometries, resulting in variable intermolecular charge transfer (CT) interactions in the solid. SF from the excimer state of Me, C6, TEG, and EH takes place in τSF = 22, 336, 195, and 1200 ps, respectively, to give triplet yields of 200%, 110%, 110%, and 70%, respectively. The transient spectra of the excimer-like state and its energetic proximity to the lowest excited singlet state in these derivatives suggests that this state may be the multiexciton 1(T1T1) state that precedes formation of the uncorrelated triplet excitons. The excimer decay rates correlate well with the SF efficiencies and the degree of intermolecular donor–acceptor interactions resulting from π-stacking of the thiophene donor of one molecule with the DPP core acceptor in another molecule as observed in the crystal structures. Such interactions are found to also increase with the SF coupling energies, as calculated for each derivative. These structural and spectroscopic studies afford a better understanding of the electronic interactions that enhance SF in chromophores having strong intra- and intermolecular CT character.
Co-reporter:Michelle A. Harris; Ashutosh Kumar Mishra; Ryan M. Young; Kristen E. Brown; Michael R. Wasielewski;Frederick D. Lewis
Journal of the American Chemical Society 2016 Volume 138(Issue 17) pp:5491-5494
Publication Date(Web):April 15, 2016
DOI:10.1021/jacs.6b00702
The excited state behavior of DNA hairpins possessing a diphenylacetylenedicarboxamide (DPA) linker separated from a single guanine–cytosine (G–C) base pair by zero-to-six adenine–thymine (A–T) base pairs has been investigated. In the case of hairpins with zero or one A–T separating DPA and G, formation of both DPA anion radical (DPA–•) and G cation radical (G+•) are directly observed and characterized by their transient absorption and stimulated Raman spectra. For hairpins with two or more intervening A–T, the transient absorption spectra of DPA–• and the adenine polaron (An+•) are observed. In addition to characterization of the hole carriers, the dynamics of each step in the charge separation and charge recombination process as well as the overall efficiency of charge separation have been determined, thus providing a complete account of the mechanism and dynamics of photoinduced charge transport in these DNA hairpins.
Co-reporter:Eric A. Margulies, Jenna L. Logsdon, Claire E. Miller, Lin Ma, Ethan Simonoff, Ryan M. YoungGeorge C. Schatz, Michael R. Wasielewski
Journal of the American Chemical Society 2016 Volume 139(Issue 2) pp:663-671
Publication Date(Web):December 15, 2016
DOI:10.1021/jacs.6b07721
Co-reporter:Patrick E. Hartnett, Catherine M. Mauck, Michelle A. Harris, Ryan M. YoungYi-Lin Wu, Tobin J. Marks, Michael R. Wasielewski
Journal of the American Chemical Society 2016 Volume 139(Issue 2) pp:749-756
Publication Date(Web):December 27, 2016
DOI:10.1021/jacs.6b10140
Photodriven electron transfer from a donor excited state to an assembly of electronically coupled acceptors has been proposed to enhance charge transfer efficiency in functional organic electronic materials. However, the circumstances under which this may occur are difficult to investigate in a controlled manner in disordered donor–acceptor materials. Here we investigate the effects of anion delocalization on electron transfer using zinc meso-tetraphenylporphyrin (ZnTPP) as a donor and a perylene-3,4:9,10-bis(dicarboximide) dimer as the acceptor (PDI2). The PDI units of the dimer are positioned in a cofacial orientation relative to one another by attachment of the imide group of each PDI to the 4- and 5-positions of a xanthene spacer. Furthermore, the distal imide group of one PDI is linked to the para-position of one ZnTPP phenyl group to yield ZnTPP-PDI2. The data for the dimer are compared to two different ZnTPP-PDI monomer reference systems designed to probe electron transfer to each of the individual PDI molecules comprising PDI2. The electron transfer rate from the ZnTPP lowest excited singlet state to PDI2 is increased by 50% relative to that in ZnTPP-PDI, when the data are corrected for the statistics of having two electron acceptors. Femtosecond transient IR absorption spectroscopy provides evidence that the observed enhancement in charge separation results from electron transfer producing a delocalized PDI2 anion.
Co-reporter:Lin Ma, Feng Hao, Constantinos C. Stoumpos, Brian T. Phelan, Michael R. Wasielewski, and Mercouri G. Kanatzidis
Journal of the American Chemical Society 2016 Volume 138(Issue 44) pp:14750-14755
Publication Date(Web):October 17, 2016
DOI:10.1021/jacs.6b09257
The dynamics of photoexcited lead-free perovskite films, CH3NH3SnI3, were studied using broadband transient absorption and time-resolved fluorescence spectroscopy. Similar to its lead analogue CH3NH3PbI3, we show that free carrier (electrons and holes) recombination is also the dominant relaxation pathway in CH3NH3SnI3 films. The slow hot carrier relaxation time is 0.5 ps. Long carrier diffusion lengths for electrons (279 ± 88 nm) and holes (193 ± 46 nm) were obtained from fluorescence quenching measurements. We also show that SnF2 doping in the CH3NH3SnI3 film effectively increases the fluorescence lifetime up to 10 times and gives diffusion lengths exceeding 500 nm. These results suggest that the photophysics of CH3NH3SnI3 perovskite are as favorable as those of CH3NH3PbI3, demonstrating that it is a promising nontoxic lead-free replacement for lead iodide perovskite-based solar cells.
Co-reporter:Ryan M. Young; Stephen C. Jensen; Kedy Edme; Yilei Wu; Matthew D. Krzyaniak; Nicolaas A. Vermeulen; Edward J. Dale; J. Fraser Stoddart; Emily A. Weiss; Michael R. Wasielewski;Dick T. Co
Journal of the American Chemical Society 2016 Volume 138(Issue 19) pp:6163-6170
Publication Date(Web):April 25, 2016
DOI:10.1021/jacs.5b13386
Time-resolved optical spectroscopies reveal multielectron transfer from the biexcitonic state of a CdS quantum dot to an adsorbed tetracationic compound cyclobis(4,4′-(1,4-phenylene) bipyridin-1-ium-1,4-phenylene-bis(methylene)) (ExBox4+) to form both the ExBox3+• and the doubly reduced ExBox2(+•) states from a single laser pulse. Electron transfer in the single-exciton regime occurs in 1 ps. At higher excitation powers the second electron transfer takes ∼5 ps, which leads to a mixture of redox states of the acceptor ligand. The doubly reduced ExBox2(+•) state has a lifetime of ∼10 ns, while CdS+•:ExBox3+• recombines with multiple time constants, the longest of which is ∼300 μs. The long-lived charge separation and ability to accumulate multiple charges on ExBox4+ demonstrate the potential of the CdS:ExBox4+ complex to serve as a platform for two-electron photocatalysis.
Co-reporter:Patrick E. Hartnett, Eric A. Margulies, H. S. S. Ramakrishna Matte, Mark C. Hersam, Tobin J. Marks, and Michael R. Wasielewski
Chemistry of Materials 2016 Volume 28(Issue 11) pp:3928
Publication Date(Web):May 11, 2016
DOI:10.1021/acs.chemmater.6b01230
The perylenediimide (PDI)-based molecules N,N-bis(1-ethylpropyl)-2,5,8,11-tetraphenyl-PDI (3-pentyl), N,N-bis(3,7-dimethyloctyl)-2,5,8,11-tetraphenyl-PDI (3,7-DMO), N,N-bis(2-ethylhexyl)-2,5,8,11-tetraphenyl-PDI (2-EH), and N,N-bis(n-octyl)-2,5,8,11-tetraphenyl-PDI (n-octyl) are synthesized and investigated for photovoltaic response. Single-crystal X-ray structures reveal that these molecules crystallize in either herringbone or slip-stacked geometries and that the crystal packing morphology can be manipulated by changing the solubilizing alkyl substituents or the crystallization conditions. The herringbone structure is shown to result in limited electronic coupling between adjacent chromophores, while the slip-stacked geometry promotes strong coupling. In bulk-heterojunction blend films with the donor polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PBDTT-FTTE), the herringbone acceptors undergo more rapid charge separation than the slip-stacked acceptors but also suffer from increased geminate recombination, as measured by femtosecond transient absorption. This tendency toward recombination decreases short-circuit currents and therefore decreases power conversion efficiency from 3.9% in the purely slip-stacked system to 2.5% in the purely herringboned system. The ratio between the slip-stacked geometry and the herringbone geometry can be reliably controlled in PBDTT-FTTE:3,7-DMO blends using the solvent additive diiodooctane (DIO),and is monitored using grazing incidence wide-angle X-ray scattering (GIWAXS). At low DIO concentrations, diffraction peaks corresponding to the slip-stacked geometry predominate while at high concentrations those corresponding to the herringbone geometry predominate. This microstructural change correlates with changes in charge carrier generation efficiency and thus device power conversion efficiency. This work also provides insights on crystalline acceptor materials which are rare in comparison to amorphous materials, and these results argue that strong coupling between neighboring acceptor molecules is important for efficient charge separation in such systems.
Co-reporter:Peter Spenst, Ryan M. Young, Michael R. Wasielewski and Frank Würthner
Chemical Science 2016 vol. 7(Issue 8) pp:5428-5434
Publication Date(Web):18 May 2016
DOI:10.1039/C6SC01574C
Cofacial positioning of two perylene bisimide (PBI) chromophores at a distance of 6.5 Å in a cyclophane structure prohibits the otherwise common excimer formation and directs photoexcited singlet state relaxation towards intramolecular symmetry-breaking charge separation (τCS = 161 ± 4 ps) in polar CH2Cl2, which is thermodynamically favored with a Gibbs free energy of ΔGCS = −0.32 eV. The charges then recombine slowly in τCR = 8.90 ± 0.06 ns to form the PBI triplet excited state, which can be used subsequently to generate singlet oxygen in 27% quantum yield. This sequence of events is eliminated by dissolving the PBI cyclophane in non-polar toluene, where only excited singlet state decay occurs. In contrast, complexation of electron-rich aromatic hydrocarbons by the host PBI cyclophane followed by photoexcitation of PBI results in ultrafast electron transfer (<10 ps) from the guest to the PBI in CH2Cl2. The rate constants for charge separation and recombination increase as the guest molecules become easier to oxidize, demonstrating that charge separation occurs close to the peak of the Marcus curve and the recombination lies far into the Marcus inverted region.
Co-reporter:Patrick E. Hartnett, H. S. S. Ramakrishna Matte, Nicholas D. Eastham, Nicholas E. Jackson, Yilei Wu, Lin X. Chen, Mark A. Ratner, Robert P. H. Chang, Mark C. Hersam, Michael R. Wasielewski and Tobin J. Marks
Chemical Science 2016 vol. 7(Issue 6) pp:3543-3555
Publication Date(Web):09 Feb 2016
DOI:10.1039/C5SC04956C
A series of perylenediimide (PDI) dimers are evaluated as acceptors for organic photovoltaic (OPV) cells. The materials are characterized using a wide variety of physical and computational techniques. These dimers are first linked at the bay position of each PDI molecule via an aromatic spacer; subsequent photocyclization affords ring-fused dimers. Thus, photocyclization of the thiophene-linked dimer 2,5-bis-[N,N′-bis-perylenediimide-1-yl]-thiophene (T1) affords the twisted acceptor [2,3-b:2′,3′-d]-bis-[N,N′-bis-perylenediimide-1,12-yl]-thiophene (T2), while photocyclization of the thienothiophene-linked dimer, 2,5-bis-[N,N′-bis-perylenediimide-1-yl]-thienothiophene (TT1) affords the planar acceptor [2,3-b:2′,3′-d]-bis-[N,N′-bis-perylenediimide-1,12-yl]-thienothiophene (TT2). Furthermore, a dimer linked by a phenylene group, 1,4-bis-[N,N′-bis-perylenediimide-1-yl]-benzene (Ph1), can be selectively photocyclized to form either the twisted dimer, [1,2:3,4]-bis-[N,N′-bis-perylenediimide-1,12-yl]-benzene (Ph1a) or the planar dimer [1,2:4,5]-bis-[N,N′-bis-perylenediimide-1,12-yl]-benzene (Ph2b). Ring-fusion results in increased electronic coupling between the PDI units, and increased space-charge limited thin film electron mobility. While charge transport is efficient in bulk-heterojunction blends of each dimer with the polymeric donor PBDTT-FTTE, in the case of the twisted dimers ring fusion leads to a significant decrease in geminate recombination, hence increased OPV photocurrent density and power conversion efficiency. This effect is not observed in planar dimers where ring fusion leads to increased crystallinity and excimer formation, decreased photocurrent density, and decreased power conversion efficiency. These results argue that ring fusion is an effective approach to increasing OPV bulk-heterojunction charge carrier generation efficiency in PDI dimers as long as they remain relatively amorphous, thereby suppressing excimer formation and coulombically trapped charge transfer states.
Co-reporter:Rebecca J. Lindquist, Brian T. Phelan, Anna Reynal, Eric A. Margulies, Leah E. Shoer, James R. Durrant and Michael R. Wasielewski
Journal of Materials Chemistry A 2016 vol. 4(Issue 8) pp:2880-2893
Publication Date(Web):25 Aug 2015
DOI:10.1039/C5TA05790F
Perylene-3,4-dicarboximide (PMI) based chromophores have demonstrated the ability to inject electrons into TiO2 for dye-sensitized solar cell applications and to accept electrons from metal complexes relevant to water oxidation, but they are nearly unexplored for use in photoelectrochemical cells (PECs) for solar fuels generation. A series of related PMIs with high oxidation potentials and carboxylate binding groups was synthesized and investigated for this purpose. Charge injection and recombination dynamics were measured using transient absorption (TA) spectroscopy on the picosecond to second timescales. The dynamics and electron injection yields were correlated with the PMI energetics and structures. Injection began in less than 1 ps for the dye with the best performance and a significant charge-separated state yield remained at long times. Finally, this chromophore was used to oxidize a covalently bound water oxidation precatalyst following electron injection into TiO2 to demonstrate the utility of the dyes for use in PECs.
Co-reporter:Alejo M. Lifschitz, Ryan M. Young, Jose Mendez-Arroyo, C. Michael McGuirk, Michael R. Wasielewski, and Chad A. Mirkin
Inorganic Chemistry 2016 Volume 55(Issue 17) pp:8301-8308
Publication Date(Web):May 10, 2016
DOI:10.1021/acs.inorgchem.6b00095
Herein, we report the first allosteric photoredox catalyst regulated via constructively coupled structural and electronic control. While often synergistically exploited in nature, these two types of control mechanisms have only been applied independently in the vast majority of allosteric enzyme mimics and receptors in the literature. By embedding a model of photosystem II in a supramolecular coordination complex that responds to chloride as an allosteric effector, we show that distance and electronic control of light harvesting can be married to maximize allosteric regulation of catalytic activity. This biomimetic system is composed of a Bodipy photoantenna, which is capable of transferring excited-state energy to a photoredox pair, wherein the excitation energy is used to generate a catalytically active charge-separated state. The structural aspect of allosteric regulation is achieved by toggling the coordination chemistry of an antenna-functionalized hemilabile ligand via partial displacement from a RhI structual node using chloride. In doing so, the distance between the antenna and the central photoredox catalyst is increased, lowering the inherent efficiency of through-space energy transfer. At the same time, coordination of chloride lowers both the charge of the RhI node and the reduction potential of the RhII/I couple, to the extent that electronic quenching of the antenna excited state is possible via photoinduced electron transfer from the metal center. Compared to a previously developed system that operates solely via electronic regulation, the present system demonstrates that coupling electronic and structural approaches to allosteric regulation gives rise to improved switching ratios between catalytically active and inactive states. Contributions from both structural and electronic control mechanisms are probed via nuclear magnetic resonance, X-ray diffraction, electrochemical, spectroelectrochemical, and transient absorption studies. Overall, this work establishes that intertwined electronic and structural regulatory mechanisms can be borrowed from nature to build stimuli-responsive inorganic materials with potential applications in sensing, catalysis, and photonic devices.
Co-reporter:Rita E. Cook, Brian T. Phelan, Leah E. Shoer, Marek B. Majewski, and Michael R. Wasielewski
Inorganic Chemistry 2016 Volume 55(Issue 23) pp:12281-12289
Publication Date(Web):November 18, 2016
DOI:10.1021/acs.inorgchem.6b02013
Two new covalently linked chromophore–CO2 reduction catalyst systems were prepared using a perylene chromophore and a bis[(dicyclohexylphosphino)ethyl]phenylphosphinopalladium(II) catalyst. The primary goal of this study is to probe the influence of photosensitizer attachment on the electrocatalytic performance. The position either para or meta to the phosphorus on the phenyl group of the palladium complex was linked via a 2,5-xylyl group to the 3 position of perylene. The electrocatalytic CO2 reduction activity of the palladium complex is maintained in the meta-linked system, but is lost in the para-linked system, possibly because of unfavorable interactions of the perylene chromophore with the glassy carbon electrode used. Following selective photoexcitation of the perylene, an enhanced perylene excited-state decay rate was observed in the palladium complexes compared to perylene attached to the free ligands. This decrease is accompanied by formation of the perylene cation radical, showing that electron transfer from perylene to the palladium catalyst occurs. Electron transfer and charge recombination were both found to be faster in the para-linked system than in the meta-linked one, which is attributed to stronger electronic coupling in the former. These results illustrate the need to carefully tune the electronic coupling between a photosensitizer chromophore and the catalyst to promote photodriven electron transfer yet inhibit adverse electronic effects of the chromophore on electrocatalysis.
Co-reporter:Guan-Jhih Huang, Michelle A. Harris, Matthew D. Krzyaniak, Eric A. Margulies, Scott M. Dyar, Rebecca J. Lindquist, Yilei Wu, Vladimir V. Roznyatovskiy, Yi-Lin Wu, Ryan M. Young, and Michael R. Wasielewski
The Journal of Physical Chemistry B 2016 Volume 120(Issue 4) pp:756-765
Publication Date(Web):January 5, 2016
DOI:10.1021/acs.jpcb.5b10806
Connecting electron donors and acceptors to a benzene ring in a meta or para relationship results in quantum interference effects that can strongly influence charge separation (CS) and charge recombination (CR) processes in these systems. We report on the energy and electron transfer behavior of chlorophyll-based para- and meta-linked donor–bridge–acceptor (D-B-A) dyads, where the semisynthetic chlorophyll a derivative, zinc methyl 3-ethyl-pyrochlorophyllide a (D), is covalently attached at its 20-position to the para position of one phenyl of diphenylacetylene (B). The meta or para position of the phenyl in B distal to the donor is in turn attached to perylene-3,4:9,10-bis(dicarboximide) (PDI) (A). Photoexcitation of the D-B-A dyads produces long-lived radical ion pairs D•+-B-A•–, which recombine to the ground state and to both 3*D-B-A and D-B-3*A. Time-resolved optical and electron paramagnetic resonance spectroscopies were used to monitor the charge transfer and triplet energy transfer (TEnT) processes. At longer times, TEnT occurs from 3*D-B-A to D-B-3*A. Surprisingly, the D-B-A molecules linked via the meta linkage exhibit faster CS, CR, and TEnT rates than do those with the para linkage in contrast to most other meta/para-linked D-B-A molecules previously examined.
Co-reporter:Arun K. Thazhathveetil, Michelle A. Harris, Ryan M. Young, Michael R. Wasielewski, Frederick D. Lewis
Journal of Photochemistry and Photobiology A: Chemistry 2016 Volume 331() pp:160-164
Publication Date(Web):1 December 2016
DOI:10.1016/j.jphotochem.2016.03.002
•Capped hairpins with seven base pairs and one mismatch are stable at 25 °C.•T-T and C-C mismatches effectively block hole transport.•Hole transport via an A-A mismatch is very efficient, G-G is less efficient.•A-to-G strand crossing is more efficient than is A-to-A strand crossing.The efficiency and dynamics of hole transport via base mismatches and strand crossings have been determined for 10 DNA hairpins by means of femtosecond transient absorption spectroscopy. The hairpins possess a stilbenedicarboxamide hole donor and stilbenediether hole acceptor separated by seven base pairs. Hole transport occurs via an intra- or interstrand A3G4 diblock purine sequence that contains either a single base pair mismatch or strand crossing. Pyrimidine-pyrimidine mismatches (TT or CC) effectively block intrastrand hole transport; whereas purine-purine mismatches (AA or GG) permit efficient interstrand hole transport. Purine-to-purine intrastrand hole transport between adjacent matched base pairs is also moderately efficient (AG > AA > GG). Based upon these results AA mismatches are being employed in the design of three-way junctions for incorporation in prototype charge splitting and charge combining devices.
Co-reporter:Patrick E. Hartnett, Eric A. Margulies, Catherine M. Mauck, Stephen A. Miller, Yilei Wu, Yi-Lin Wu, Tobin J. Marks, and Michael R. Wasielewski
The Journal of Physical Chemistry B 2016 Volume 120(Issue 7) pp:1357-1366
Publication Date(Web):January 28, 2016
DOI:10.1021/acs.jpcb.5b10565
Singlet exciton fission (SF) is a promising strategy for increasing photovoltaic efficiency, but in order for SF to be useful in solar cells, it should take place in a chromophore that is air-stable, highly absorptive, solution processable, and inexpensive. Unlike many SF chromophores, diketopyrrolopyrrole (DPP) conforms to these criteria, and here we investigate SF in DPP for the first time. SF yields in thin films of DPP derivatives, which are widely used in organic electronics and photovoltaics, are shown to depend critically on crystal morphology. Time-resolved spectroscopy of three DPP derivatives with phenyl (3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhDPP), thienyl (3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, TDPP), and phenylthienyl (3,6-di(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhTDPP) aromatic substituents in 100–200 nm thin films reveals that efficient SF occurs only in TDPP and PhTDPP (τSF = 220 ± 20 ps), despite the fact that SF is most exoergic in PhDPP. This result correlates well with the greater degree of π-overlap and closer π-stacking in TDPP (3.50 Å) and PhTDPP (3.59 Å) relative to PhDPP (3.90 Å) and demonstrates that SF in DPP is highly sensitive to the electronic coupling between adjacent chromophores. The triplet yield in PhTDPP films is determined to be 210 ± 35% by the singlet depletion method and 165 ± 30% by the energy transfer method, showing that SF is nearly quantitative in these films and that DPP derivatives are a promising class of SF chromophores for enhancing photovoltaic performance.
Co-reporter:Noah E. Horwitz, Brian T. Phelan, Jordan N. Nelson, Matthew D. Krzyaniak, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2016 Volume 120(Issue 18) pp:2841-2853
Publication Date(Web):April 23, 2016
DOI:10.1021/acs.jpca.6b02621
Photoinduced electron transfer reactions in organic donor–acceptor systems leading to long-lived radical ion pairs (RPs) have attracted broad interest for their potential applications in fields as diverse as solar energy conversion and spintronics. We present the photophysics and spin dynamics of an electron donor − electron acceptor − stable radical system consisting of a meta-phenylenediamine (mPD) donor covalently linked to a 4-aminonaphthalene-1,8-dicarboximide (ANI) electron-accepting chromophore as well as an α,γ-bisdiphenylene-β-phenylallyl (BDPA) stable radical. Selective photoexcitation of ANI produces the BDPA–mPD+•–ANI–• triradical in which the mPD+•–ANI–• RP spins are strongly exchange coupled. The presence of BDPA is found to greatly increase the RP intersystem crossing rate from the initially photogenerated BDPA–1(mPD+•–ANI–•) to BDPA–3(mPD+•–ANI–•), resulting in accelerated RP recombination via the triplet channel to produce BDPA–mPD–3*ANI as compared to a reference molecule lacking the BDPA radical. The RP recombination rates observed are much faster than those previously reported for weakly coupled triradical systems. Time-resolved EPR spectroscopy shows that this process is also associated with strong spin polarization of the stable radical. Overall, these results show that RP intersystem crossing rates can be strongly influenced by stable radicals nearby strongly coupled RP systems, making it possible to use a third spin to control RP lifetimes down to a picosecond time scale.
Co-reporter:Junling Sun; Yilei Wu; Yuping Wang; Zhichang Liu; Chuyang Cheng; Karel J. Hartlieb; Michael R. Wasielewski;J. Fraser Stoddart
Journal of the American Chemical Society 2015 Volume 137(Issue 42) pp:13484-13487
Publication Date(Web):October 6, 2015
DOI:10.1021/jacs.5b09274
A tristable [2]catenane, composed of a macrocyclic polyether incorporating 1,5-dioxynaphthalene (DNP) and tetrathiafulvalene (TTF) units along with a 4,4′-bipyridinium (BIPY•+) radical cation as three very different potential recognition sites, interlocked mechanically with the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT4+), was synthesized by donor–acceptor templation, employing a “threading-followed-by-cyclization” approach. In this catenane, movement of the CBPQT4+ ring in its different redox states among these three potential recognition sites, with corresponding color changes, is achieved by tuning external redox potentials. In the starting state, where no external potential is applied, the ring encircles the TTF unit and displays a green color. Upon oxidation of the TTF unit, the CBPQT4+ ring moves to the DNP unit, producing a red color. Finally, if all the BIPY2+ units are reduced to BIPY•+ radical cations, the resulting CBPQT2(•+) diradical dication will migrate to the BIPY•+ unit, resulting in a purple color. These readily switchable electrochromic properties render the [2]catenane attractive for use in electro-optical devices.
Co-reporter:Yilei Wu; Ryan M. Young; Marco Frasconi; Severin T. Schneebeli; Peter Spenst; Daniel M. Gardner; Kristen E. Brown; Frank Würthner; J. Fraser Stoddart
Journal of the American Chemical Society 2015 Volume 137(Issue 41) pp:13236-13239
Publication Date(Web):September 29, 2015
DOI:10.1021/jacs.5b08386
We report on a visible-light-absorbing chiral molecular triangle composed of three covalently linked 1,6,7,12-tetra(phenoxy)perylene-3,4:9,10-bis(dicarboximide) (PDI) units. The rigid triangular architecture reduces the electronic coupling between the PDIs, so ultrafast symmetry-breaking charge separation is kinetically favored over intramolecular excimer formation, as revealed by femtosecond transient absorption spectroscopy. Photoexcitation of the PDI triangle dissolved in CH2Cl2 gives PDI+•–PDI–• in τCS = 12.0 ± 0.2 ps. Fast subsequent intramolecular electron/hole hopping can equilibrate the six possible energetically degenerate ion-pair states, as suggested by electron paramagnetic resonance/electron–nuclear double resonance spectroscopy, which shows that one-electron reduction of the PDI triangle results in complete electron sharing among the three PDIs. Charge recombination of PDI+•–PDI–• to the ground state occurs in τCR = 1.12 ± 0.01 ns with no evidence of triplet excited state formation.
Co-reporter:Ryan M. Young; Arunoday P. N. Singh; Arun K. Thazhathveetil; Vincent Y. Cho; Yuqi Zhang; Nicolas Renaud; Ferdinand C. Grozema; David N. Beratan; Mark A. Ratner; George C. Schatz; Yuri A. Berlin; Frederick D. Lewis
Journal of the American Chemical Society 2015 Volume 137(Issue 15) pp:5113-5122
Publication Date(Web):March 30, 2015
DOI:10.1021/jacs.5b00931
DNA-based molecular electronics will require charges to be transported from one site within a 2D or 3D architecture to another. While this has been shown previously in linear, π-stacked DNA sequences, the dynamics and efficiency of charge transport across DNA three-way junction (3WJ) have yet to be determined. Here, we present an investigation of hole transport and trapping across a DNA-based three-way junction systems by a combination of femtosecond transient absorption spectroscopy and molecular dynamics simulations. Hole transport across the junction is proposed to be gated by conformational fluctuations in the ground state which bring the transiently populated hole carrier nucleobases into better aligned geometries on the nanosecond time scale, thus modulating the π–π electronic coupling along the base pair sequence.
Co-reporter:Nanjia Zhou; Xugang Guo; Rocio Ponce Ortiz; Tobias Harschneck; Eric F. Manley; Sylvia J. Lou; Patrick E. Hartnett; Xinge Yu; Noah E. Horwitz; Paula Mayorga Burrezo; Thomas J. Aldrich; Juan T. López Navarrete; Michael R. Wasielewski; Lin X. Chen; Robert. P. H. Chang; Antonio Facchetti;Tobin J. Marks
Journal of the American Chemical Society 2015 Volume 137(Issue 39) pp:12565-12579
Publication Date(Web):September 8, 2015
DOI:10.1021/jacs.5b06462
As effective building blocks for high-mobility transistor polymers, oligothiophenes are receiving attention for polymer solar cells (PSCs) because the resulting polymers can effectively suppress charge recombination. Here we investigate two series of in-chain donor–acceptor copolymers, PTPDnT and PBTInT, based on thieno[3,4-c]pyrrole-4,6-dione (TPD) or bithiopheneimide (BTI) as electron acceptor units, respectively, and oligothiophenes (nTs) as donor counits, for high-performance PSCs. Intramolecular S···O interaction leads to more planar TPD polymer backbones, however backbone torsion yields greater open-circuit voltages for BTI polymers. Thiophene addition progressively raises polymer HOMOs but marginally affects their band gaps. FT-Raman spectra indicate that PTPDnT and PBTInT conjugation lengths scale with nT catenation up to n = 3 and then saturate for longer oligomer. Furthermore, the effects of oligothiophene alkylation position are explored, revealing that the alkylation pattern greatly affects film morphology and PSC performance. The 3T with “outward” alkylation in PTPD3T and PBTI3T affords optimal π-conjugation, close stacking, long-range order, and high hole mobilities (0.1 cm2/(V s)). These characteristics contribute to the exceptional ∼80% fill factors for PTPD3T-based PSCs with PCE = 7.7%. The results demonstrate that 3T is the optimal donor unit among nTs (n = 1–4) for photovoltaic polymers. Grazing incidence wide-angle X-ray scattering, transmission electron microscopy, and time-resolved microwave conductivity measurements reveal that the terthiophene-based PTPD3T blend maintains high crystallinity with appreciable local mobility and long charge carrier lifetime. These results provide fundamental materials structure-device performance correlations and suggest guidelines for designing oligothiophene-based polymers with optimal thiophene catenation and appropriate alkylation pattern to maximize PSC performance.
Co-reporter:Patrick E. Hartnett, Scott M. Dyar, Eric A. Margulies, Leah E. Shoer, Andrew W. Cook, Samuel W. Eaton, Tobin J. Marks and Michael R. Wasielewski
Chemical Science 2015 vol. 6(Issue 1) pp:402-411
Publication Date(Web):16 Sep 2014
DOI:10.1039/C4SC02551B
The photophysics of a covalently linked perylenediimide–diketopyrrolopyrrole–perylenediimide acceptor–donor–acceptor molecule (PDI–DPP–PDI, 1) were investigated and found to be markedly different in solution versus in unannealed and solvent annealed films. Photoexcitation of 1 in toluene results in quantitative charge separation in τ = 3.1 ± 0.2 ps, with charge recombination in τ = 340 ± 10 ps, while in unannealed/disordered films of 1, charge separation occurs in τ < 250 fs, while charge recombination displays a multiexponential decay in ∼6 ns. The absence of long-lived, charge separation in the disordered film suggests that few free charge carriers are generated. In contrast, upon CH2Cl2 vapor annealing films of 1, grazing-incidence X-ray scattering shows that the molecules form a more ordered structure. Photoexcitation of the ordered films results in initial formation of a spin-correlated radical ion pair (electron–hole pair) as indicated by magnetic field effects on the formation of free charge carriers which live for ∼4 μs. This result has significant implications for the design of organic solar cells based on covalent donor–acceptor systems and shows that long-lived, charge-separated states can be achieved by controlling intramolecular charge separation dynamics in well-ordered systems.
Co-reporter:Matthew D. Krzyaniak, Lukáš Kobr, Brandon K. Rugg, Brian T. Phelan, Eric A. Margulies, Jordan N. Nelson, Ryan M. Young and Michael R. Wasielewski
Journal of Materials Chemistry A 2015 vol. 3(Issue 30) pp:7962-7967
Publication Date(Web):30 Jun 2015
DOI:10.1039/C5TC01446H
Selective photoexcitation of the donor in an electron donor–acceptor1–acceptor2 (D–A1–A2) molecule, in which D = perylene and both A1 and A2 = naphthalene-1,8:4,5-bis(dicarboximide), results in sub-nanosecond formation of a spin-correlated singlet radical pair 1(D+˙–A1−˙–A2) having a large electron spin–spin exchange interaction, 2J, which precludes its observation by transient EPR spectroscopy. Subsequent selective photoexcitation of A1−˙ rapidly produces 1(D+˙–A1–A2−˙), resulting in a dramatic decrease in 2J, which allows coherent spin evolution to mix the singlet (S) radical pair state 1(D+˙–A1–A2−˙) with the T0 triplet sublevel of 3(D+˙–A1–A2−˙) in an applied magnetic field, where B ≫ 2J. A spin-polarized transient EPR spectrum characteristic of the spin-correlated radical pair D+˙–A1–A2−˙ is then observed. The time delay between the two laser pulses was incremented to measure the rate of decoherence in 1(D+˙–A1−˙–A2) in toluene at 295 K, which was found to be 8.1 × 107 s−1. Deuteration of the perylene donor or the toluene solvent decreases the decoherence rate constant of 1(D+˙–A1−˙–A2) to 4.3 × 107 s−1 and 4.6 × 107 s−1, respectively, while deuteration of both the perylene donor and the toluene solvent reduced the decoherence rate constant by more than half to 3.4 × 107 s−1. The data show that decreasing electron-nuclear hyperfine interactions significantly increases the zero quantum coherence lifetime of the spin-correlated radical pair.
Co-reporter:Scott M. Dyar, Amanda L. Smeigh, Steven D. Karlen, Ryan M. Young, Michael R. Wasielewski
Chemical Physics Letters 2015 Volume 629() pp:23-28
Publication Date(Web):1 June 2015
DOI:10.1016/j.cplett.2015.04.020
•The synthesis of DABP, a bi-functional perylene redox chromophore is reported.•DABP retains the electronic properties of perylene, yet eliminates regioisomers.•Ultrafast electron transfer reactions in DABP donor–acceptor systems are described.The excited state and redox properties of a new bi-functional perylene redox chromophore, 2,3-dihydro-1-azabenzo[cd]perylene (DABP), are described. Perylene has been widely used in electron donor–acceptor molecules in fields ranging from artificial photosynthesis to molecular spintronics. However, attaching multiple redox components to perylene to carry out multi-step electron transfer reactions often produces hard to separate regioisomers, which complicate data analysis. The use of DABP provides a strategy to retain the electronic properties of perylene, yet eliminate regioisomers. Ultrafast photo-initiated single- and two-step electron transfer reactions in three linear electron donor–acceptor systems incorporating DABP are described to illustrate its utility.
Co-reporter:Almaz S. Jalilov;Ryan M. Young;Samuel W. Eaton;Frederick D. Lewis
Photochemistry and Photobiology 2015 Volume 91( Issue 3) pp:739-747
Publication Date(Web):
DOI:10.1111/php.12360
Abstract
The mechanism and dynamics of photoinduced electron transfer in two families of DNA hairpins possessing Michler's ketone linkers have been investigated by means of steady state and time-resolved transient absorption and emission spectroscopies. The excited state behavior of the diol linker employed in hairpin synthesis is similar to that of Michler's ketone in methanol solution. Hairpins possessing only a Michler's ketone linker undergo fast singlet state charge separation and charge recombination with an adjacent purine base, attributed to well-stacked ground state conformations, and intersystem crossing to the triplet state, attributed to poorly stacked ground state conformations. The failure of the triplet to undergo electron transfer reactions on the 7 ns time scale of our measurements is attributed to the low triplet energy and reduction potential of the twisted triplet state. Hairpins possessing both a Michler's ketone linker and a perylenediimide base surrogate separated by four base pairs undergo photoinduced hole transport from the diimide to Michler's ketone upon excitation of the diimide. The efficiency of hole transport is dependent upon the sequence of the intervening purine bases.
Co-reporter:Daniel M. Gardner, Hsiao-Fan Chen, Matthew D. Krzyaniak, Mark A. Ratner, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2015 Volume 119(Issue 29) pp:8040-8048
Publication Date(Web):June 22, 2015
DOI:10.1021/acs.jpca.5b03048
Transient electron paramagnetic resonance (TREPR) spectroscopy has been used to study the spin–spin interactions in a novel U-shaped electron donor–chromophore–acceptor–radical (D–C–A–R•) system in which a xanthene bridge holds a tert-butylphenyl nitroxide (BPNO•) radical in close proximity to a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) acceptor. Photoexcitation of the 4-aminonaphthalene-1,8-dicarboximide (ANI) chromophore results in rapid, two-step electron transfer to generate the triradical (D+•–C–A–•–R•). The large through-bond distance between A–• and R• makes their spin–spin exchange interaction (2JAR) negligibly small, whereas their short through-space distance results in a strong dipolar interaction (DAR), which is observed as a set of broad lines in the TREPR spectra of D+•–C–A–•–R• in solid toluene solution at 85 K. Transient nutation experiments show that these transitions belong to a species with spin S = 1, whereas experiments on D+•–C–A–•–R• in the oriented nematic liquid crystal 4-cyano-4′-n-pentylbiphenyl at 85 K demonstrate the anisotropy of DAR.
Co-reporter:Scott M. Dyar, Eric A. Margulies, Noah E. Horwitz, Kristen E. Brown, Matthew D. Krzyaniak, and Michael R. Wasielewski
The Journal of Physical Chemistry B 2015 Volume 119(Issue 43) pp:13560-13569
Publication Date(Web):May 26, 2015
DOI:10.1021/acs.jpcb.5b02378
We report on a novel small organic molecule comprising a perylene chromophore fused to a six-membered ring containing a persistent nitroxide radical to give a perylene-nitroxide, or PerNO•. This molecule is a robust, compact molecule in which the radical is closely bound to the chromophore but separated by saturated carbon atoms, thus limiting the electronic coupling between the chromophore and radical. We present both ultrafast transient absorption experiments and time-resolved EPR (TREPR) studies to probe the spin dynamics of photoexcited PerNO• and utilize X-ray crystallography to probe the molecular structure and stacking motifs of PerNO• in the solid state. The ability to control both the structure and electronic properties of molecules having multiple spins as well as the possibility of assembling ordered solid state materials from them is important for implementing effective molecule-based spintronics.
Co-reporter:Samuel W. Eaton, Stephen A. Miller, Eric A. Margulies, Leah E. Shoer, Richard D. Schaller, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2015 Volume 119(Issue 18) pp:4151-4161
Publication Date(Web):April 9, 2015
DOI:10.1021/acs.jpca.5b02719
Two terrylene chromophores, 2,5,10,13-tetra(tert-butyl)terrylene (1) and 2,5-di(tert-butyl)terrylene (2), were synthesized and studied to determine their singlet exciton fission (SF) efficiencies. Compound 1 crystallizes in one-dimensional stacks, whereas 2 packs in a slip-stacked, herringbone pattern of dimers motif. Strongly quenched fluorescence and rapid singlet exciton decay dynamics are observed in vapor-deposited thin films of 1 and 2. Phosphorescence measurements on thin films of 1 and 2 show that SF is only 70 meV endoergic for these chromophores. Femtosecond transient absorption experiments using low laser fluences on these films reveal rapid triplet exciton formation for both 1 (τ = 120 ± 10 ps) and 2 (τ = 320 ± 20 ps) that depends strongly on film crystallinity. The transient absorption data are consistent with formation of an excimer state prior to SF. Triplet exciton yield measurements indicate nearly quantitative SF in thin films of both chromophores in highly crystalline solvent-vapor-annealed films: 170 ± 20% for 1 and 200 ± 30% for 2. These results show that significantly different crystal morphologies of the same chromophore can both result in high-efficiency SF provided that the energetics are favorable.
Co-reporter:Catherine M. Mauck, Kristen E. Brown, Noah E. Horwitz, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2015 Volume 119(Issue 22) pp:5587-5596
Publication Date(Web):May 11, 2015
DOI:10.1021/acs.jpca.5b01048
A covalent dyad was synthesized in which perylene-3,4,:9:10-bis(dicarboximide) (PDI) is linked to β-apocarotene (Car) using a biphenyl spacer. The dyad is monomeric in toluene and forms a solution aggregate in methylcyclohexane (MCH). Using femtosecond transient absorption (fsTA) spectroscopy, the monomeric dyad and its aggregates were studied both in solution and in thin films. In toluene, photoexcitation at 530 nm preferentially excites PDI, and the dyad undergoes charge separation in τ = 1.7 ps and recombination in τ = 1.6 ns. In MCH and in thin solid films, 530 nm excitation of the PDI-Car aggregate also results in charge transfer that competes with energy transfer from 1*PDI to Car and with an additional process, rapid Car triplet formation in <50 ps. Car triplet formation is only observed in the aggregated PDI-Car dyad and is attributed to singlet exciton fission (SF) within the aggregated PDI, followed by rapid triplet energy transfer from 3*PDI to the carotenoid. SF from β-apocarotene aggregation is ruled out by direct excitation of Car films at 414 nm, where no triplet formation is observed. Time-resolved electron paramagnetic resonance measurements on aggregated PDI-Car show the formation of 3*Car with a spin-polarization pattern that rules out radical-pair intersystem crossing as the mechanism of triplet formation as well.
Co-reporter:Junling Sun, Yilei Wu, Zhichang Liu, Dennis Cao, Yuping Wang, Chuyang Cheng, Dongyang Chen, Michael R. Wasielewski, and J. Fraser Stoddart
The Journal of Physical Chemistry A 2015 Volume 119(Issue 24) pp:6317-6325
Publication Date(Web):May 18, 2015
DOI:10.1021/acs.jpca.5b04570
We describe a visible light-driven switchable [2]catenane, composed of a Ru(bpy)32+ tethered cyclobis(paraquat-p-phenylene) (CBPQT4+) ring that is interlocked mechanically with a macrocyclic polyether consisting of electron-rich 1,5-dioxynaphthalene (DNP) and electron-deficient 4,4′-bipyridinium (BIPY2+) units. In the oxidized state, the CBPQT4+ ring encircles the DNP recognition site as a consequence of favorable donor–acceptor interactions. In the presence of an excess of triethanolamine (TEOA), visible light irradiation reduces the BIPY2+ units to BIPY(•+) radical cations under the influence of the photosensitizer Ru(bpy)32+, resulting in the movement of the CBPQT2(•+) ring from the DNP to the BIPY(•+) recognition site as a consequence of the formation of the more energetically favorable trisradical complex, BIPY(•+) ⊂ CBPQT2(•+). Upon introducing O2 in the dark, the BIPY(•+) radical cations are oxidized back to BIPY2+ dications, leading to the reinstatement of the CBPQT4+ ring encircled around the DNP recognition site. Employing this strategy of redox control, we have demonstrated a prototypical molecular switch that can be manipulated photochemically and chemically by sequential reduction and oxidation.
Co-reporter:Leah E. Shoer, Samuel W. Eaton, Eric A. Margulies, and Michael R. Wasielewski
The Journal of Physical Chemistry B 2015 Volume 119(Issue 24) pp:7635-7643
Publication Date(Web):December 2, 2014
DOI:10.1021/jp511624s
A series of electron donor–acceptor compounds based on substitution of perylene-3,4:9,10-bis(dicarboximide) (PDI) with four electron donors at the 2,5,8,11-positions were synthesized and characterized using femtosecond transient absorption spectroscopy. The distance between the PDI and the N,N-dimethylaniline or phenothiazine donors was varied using one or two phenyl groups. Photoexcitation of PDI results in rapid charge separation followed by charge recombination with time constants ranging from tens of picoseconds to nanoseconds. The electron transfer time constants are compared with those of the corresponding molecules in which the donor is attached to the PDI through its imide nitrogen atom. The electron transfer reactions through the 2,5,8,11-positions of PDI are generally much faster than those through the imide nitrogen positions, in concert with stronger donor electronic coupling to the PDI acceptor core and in contrast to substituents at the imide positions through which the HOMO and LUMO nodal planes pass.
Co-reporter:Nanjia Zhou;Hui Lin;Sylvia J. Lou;Xinge Yu;Peijun Guo;Eric F. Manley;Stephen Loser;Patrick Hartnett;Hui Huang;Lin X. Chen;Robert P. H. Chang;Antonio Facchetti;Tobin J. Marks
Advanced Energy Materials 2014 Volume 4( Issue 3) pp:
Publication Date(Web):
DOI:10.1002/aenm.201300785
Co-reporter:Erin T. Chernick; Rubén Casillas; Johannes Zirzlmeier; Daniel M. Gardner; Marco Gruber; Henning Kropp; Karsten Meyer; Michael R. Wasielewski; Dirk M. Guldi;Rik R. Tykwinski
Journal of the American Chemical Society 2014 Volume 137(Issue 2) pp:857-863
Publication Date(Web):December 5, 2014
DOI:10.1021/ja510958k
Understanding the fundamental spin dynamics of photoexcited pentacene derivatives is important in order to maximize their potential for optoelectronic applications. Herein, we report on the synthesis of two pentacene derivatives that are functionalized with the [(2,2,6,6-tetramethylpiperidin-1-yl)oxy] (TEMPO) stable free radical. The presence of TEMPO does not quench the pentacene singlet excited state, but does quench the photoexcited triplet excited state as a function of TEMPO-to-pentacene distance. Time-resolved electron paramagnetic resonance experiments confirm that triplet quenching is accompanied by electron spin polarization transfer from the pentacene excited state to the TEMPO doublet state in the weak coupling regime.
Co-reporter:Patrick E. Hartnett ; Amod Timalsina ; H. S. S. Ramakrishna Matte ; Nanjia Zhou ; Xugang Guo ; Wei Zhao ; Antonio Facchetti ; Robert P. H. Chang ; Mark C. Hersam ; Michael R. Wasielewski ;Tobin J. Marks
Journal of the American Chemical Society 2014 Volume 136(Issue 46) pp:16345-16356
Publication Date(Web):October 28, 2014
DOI:10.1021/ja508814z
Perylenediimide (PDI)-based acceptors offer a potential replacement for fullerenes in bulk-heterojunction (BHJ) organic photovoltaic cells (OPVs). The most promising efforts have focused on creating twisted PDI dimers to disrupt aggregation and thereby suppress excimer formation. Here, we present an alternative strategy for developing high-performance OPVs based on PDI acceptors that promote slip-stacking in the solid state, thus preventing the coupling necessary for rapid excimer formation. This packing structure is accomplished by substitution at the PDI 2,5,8,11-positions (“headland positions”). Using this design principle, three PDI acceptors, N,N-bis(n-octyl)-2,5,8,11-tetra(n-hexyl)-PDI (Hexyl-PDI), N,N-bis(n-octyl)-2,5,8,11-tetraphenethyl-PDI (Phenethyl-PDI), and N,N-bis(n-octyl)-2,5,8,11-tetraphenyl-PDI (Phenyl-PDI), were synthesized, and their molecular and electronic structures were characterized. They were then blended with the donor polymer PBTI3T, and inverted OPVs of the structure ITO/ZnO/Active Layer/MoO3/Ag were fabricated and characterized. Of these, 1:1 PBTI3T:Phenyl-PDI proved to have the best performance with Jsc = 6.56 mA/cm2, Voc = 1.024 V, FF = 54.59%, and power conversion efficiency (PCE) = 3.67%. Devices fabricated with Phenethyl-PDI and Hexyl-PDI have significantly lower performance. The thin film morphology and the electronic and photophysical properties of the three materials are examined, and although all three materials undergo efficient charge separation, PBTI3T:Phenyl-PDI is found to have the deepest LUMO, intermediate crystallinity, and the most well-mixed domains. This minimizes geminate recombination in Phenyl-PDI OPVs and affords the highest PCE. Thus, slip-stacked PDI strategies represent a promising approach to fullerene replacements in BHJ OPVs.
Co-reporter:Ashutosh Kumar Mishra ; Ryan M. Young ; Michael R. Wasielewski ;Frederick D. Lewis
Journal of the American Chemical Society 2014 Volume 136(Issue 44) pp:15792-15797
Publication Date(Web):October 9, 2014
DOI:10.1021/ja509456q
The dynamics of charge separation and charge recombination have been determined for lipid complexes of DNA capped hairpins possessing stilbene electron-acceptor and -donor chromophores separated by base-pair domains that vary in length and base sequence in chloroform solution by means of femtosecond time-resolved transient absorption spectroscopy. The results obtained for the DNA–lipid complexes are compared with those previously obtained in our laboratories for the same hairpins in aqueous buffer. The charge separation and charge recombination times for the lipid complexes are consistently much shorter than those determined in aqueous solution and are only weakly dependent on the number of base pairs separating the acceptor and donor. The enhanced rate constants for forward and return charge transport in DNA–lipid complexes support proposals that solvent gating is responsible, to a significant extent, for the relatively low rates of charge transport for DNA in water. Moreover, they suggest that DNA–lipid complexes may prove useful in the development of DNA-based molecular electronic devices.
Co-reporter:Rebecca J. Lindquist ; Kelly M. Lefler ; Kristen E. Brown ; Scott M. Dyar ; Eric A. Margulies ; Ryan M. Young
Journal of the American Chemical Society 2014 Volume 136(Issue 42) pp:14912-14923
Publication Date(Web):September 23, 2014
DOI:10.1021/ja507653p
Robust perylene-3,4-dicarboximide (PMI) π-aggregates provide important light-harvesting and electron–hole pair generation advantages in organic photovoltaics and related applications, but relatively few studies have focused on the electronic interactions between PMI chromophores. In contrast, structure–function relationships based on π–π stacking in the related perylene-3,4:9,10-bis(dicarboximides) (PDIs) have been widely investigated. The performance of both PMI and PDI derivatives in organic devices may be limited by the formation of low-energy excimer trap states in morphologies where interchromophore coupling is strong. Here, five covalently bound PMI dimers with varying degrees of electronic interaction were studied to probe the relative chromophore orientations that lead to excimer energy trap states. Femtosecond near-infrared transient absorption spectroscopy was used to observe the growth of a low-energy transition at ∼1450–1520 nm characteristic of the excimer state in these covalent dimers. The excimer-state absorption appears in ∼1 ps, followed by conformational relaxation over 8–17 ps. The excimer state then decays in 6.9–12.8 ns, as measured by time-resolved fluorescence spectroscopy. The excimer lifetimes reach a maximum for a slip-stacked geometry in which the two PMI molecules are displaced along their long axes by one phenyl group (∼4.3 Å). Additional displacement of the PMIs by a biphenyl spacer along the long axis prevents excimer formation. Symmetry-breaking charge transfer is not observed in any of the PMI dimers, and only a small triplet yield (<5%) is observed for the cofacial PMI dimers. These data provide structural insights for minimizing excimer trap states in organic devices based on PMI derivatives.
Co-reporter:Vladimir V. Roznyatovskiy, Daniel M. Gardner, Samuel W. Eaton, and Michael R. Wasielewski
Organic Letters 2014 Volume 16(Issue 3) pp:696-699
Publication Date(Web):January 13, 2014
DOI:10.1021/ol403736m
A series of electron-deficient perylene and naphthalene imides and diimides (1–4) with varying degrees of trifluoromethylation were synthesized. Single crystal X-ray analysis afforded detailed structural information, while spectroelectrochemical and EPR spectroscopy provided characterization of the radical anions of 1–4. This study reveals that trifluoromethylation of the imides and diimides makes their one-electron reduction potentials substantially more positive relative to the unsubstituted counterparts, while their other properties remain largely unchanged.
Co-reporter:Eric A. Margulies, Leah E. Shoer, Samuel W. Eaton and Michael R. Wasielewski
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 43) pp:23735-23742
Publication Date(Web):17 Sep 2014
DOI:10.1039/C4CP03107E
Excitation energy transfer in perylene-3,4:9,10-bis(dicarboximide) (PDI) aggregates is of interest for light-harvesting applications of this strongly absorbing and π–π stacking chromophore. Here we report the synthesis and characterization of two PDI dimers in which the chromophores are covalently linked by a redox-inactive triptycene bridge in orientations that are cofacial (1) and slip-stacked along their N–N axes (2). Femtosecond transient absorption experiments on 1 and 2 reveal rapid exciton delocalization resulting excimer formation. Cofacial π–π stacked dimer 1 forms a low-energy excimer state absorption (λmax = 1666 nm) in τ = ∼2 ps after photoexcitation. Inserting a phenyl spacer on the bridge to generate a slip-stacked PDI–PDI geometry in 2 results in a less stable excimer state (λmax = 1430 nm), which forms in τ = ∼12 ps due to decreased electronic coupling. The near-infrared (NIR) excimer absorption of cofacial dimer 1 is ∼120 meV lower in energy than that of slip-stacked dimer 2, further highlighting electronic differences between these states.
Co-reporter:Bradley D. Rose, Leah E. Shoer, Michael R. Wasielewski, Michael M. Haley
Chemical Physics Letters 2014 Volumes 616–617() pp:137-141
Publication Date(Web):25 November 2014
DOI:10.1016/j.cplett.2014.10.031
•Indenofluorene and fluorenofluorene are found to be non-emissive.•Femtosecond spectroscopy measured excited state lifetimes of approximately 9–12 ps.•Computations indicate a conical intersection for efficient relaxation to the electronic ground state.The ultrafast excited state decay rates for indenofluorene and fluorenofluorene derivatives as measured by transient absorption spectroscopy are reported. The excited state lifetimes of the molecules were extremely short (ca. 9–12 ps) and are a result of the p-xylylene motif in indenofluorenes and the corresponding expanded 2,6-naphthoquinonedimethide in fluorenofluorene. Quantum chemical calculations indicate that the fast relaxation to the ground state results from a potential energy surface crossing between the S0 and S1 states. This process in turn provides an efficient route for excited state deactivation and explains why this class of quinoidal molecules is non-emissive.
Co-reporter:Charusheela Ramanan ; Chul Hoon Kim ; Tobin J. Marks
The Journal of Physical Chemistry C 2014 Volume 118(Issue 30) pp:16941-16950
Publication Date(Web):March 11, 2014
DOI:10.1021/jp500475c
Perylenediimides (PDIs) offer a number of attractive characteristics as alternatives to fullerenes in organic photovoltaics (OPVs), including favorable orbital energetics, high extinction coefficients in the visible spectral region, photostability, and the capacity to self-assemble into ordered nanostructures. However, energy transfer followed by charge separation in PDI assemblies must kinetically out-compete excimer formation that limits OPV performance. We report on the excitation energy transfer (EET) rate in a covalently linked PDI tetramer in which the PDI chromophores are arranged in a tetrahedral geometry about a tetraphenyladamantane core. Transient absorption spectroscopy of the tetramer in CH2Cl2 reveals a laser intensity-dependent fast absorption decay component indicative of singlet–singlet annihilation resulting from intramolecular EET. Femtosecond fluorescence anisotropy measurements show that the EET time constant τ = 6 ps, which is similar to that predicted for a through-space Förster EET mechanism. Concentration-dependent steady-state spectroscopic studies reveal the formation of intermolecular aggregates of the tetramers in toluene. The aggregates are formed by cofacial π-stacking interactions between PDIs of neighboring tetramers. Transient absorption spectra of the aggregated tetramers in toluene solution demonstrate long-lived excited-state decay dynamics (τ ∼ 30 ns) in agreement with previous observations of PDI excimers.
Co-reporter:Dr. Maher Fathalla;Dr. Jonathan C. Barnes; Ryan M. Young;Dr. Karel J. Hartlieb;Scott M. Dyar;Samuel W. Eaton;Dr. Amy A. Sarjeant; Dick T. Co; Michael R. Wasielewski; J. Fraser Stoddart
Chemistry - A European Journal 2014 Volume 20( Issue 45) pp:14690-14697
Publication Date(Web):
DOI:10.1002/chem.201403744
Abstract
Understanding the mechanism of efficient photoinduced electron-transfer processes is essential for developing molecular systems for artificial photosynthesis. Towards this goal, we describe the synthesis of a donor–acceptor dyad comprising a zinc porphyrin donor and a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) acceptor. The X-ray crystal structure of the dyad reveals the formation of a dimeric motif through the intermolecular coordination between the triazole nitrogen and the central Zn metal of two adjacent units of the dyad. Photoinduced electron transfer within the dyad in MeCN was investigated by femtosecond and nanosecond transient absorption spectroscopy, as well as by transient EPR spectroscopy. Photoexcitation of the dyad produced a weakly coupled ZnP+.–CBPQT3+. spin-correlated radical-ion pair having a τ=146 ns lifetime and a spin–spin exchange interaction of only 0.23 mT. The long radical-ion-pair lifetime results from weak donor–acceptor electronic coupling as a consequence of having nine bonds between the donor and the acceptor, and the reduction in reorganization energy for electron transfer caused by charge dispersal over both paraquat units within CBPQT3+..
Co-reporter:Kristen E. Brown, Walter A. Salamant, Leah E. Shoer, Ryan M. Young, and Michael R. Wasielewski
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 15) pp:2588-2593
Publication Date(Web):July 14, 2014
DOI:10.1021/jz5011797
Energy transfer in perylene-3,4:9,10-bis(dicarboximide) (PDI) aggregates is often limited by formation of a low-energy excimer state. Formation dynamics of excimer states are often characterized by line shape changes and peak shift dynamics in femtosecond visible transient absorption spectra. Femtosecond near-infrared transient absorption experiments reveal a unique low-energy transition that can be used to identify and characterize this state without overlapping excited singlet-state absorption. Three covalently bound PDI dimers with differing PDI–PDI distances were studied to probe the influence of interchromophore electronic coupling on the PDI excimer transient spectra and dynamics.Keywords: charge transfer; electronic coupling; femtosecond spectroscopy; molecular aggregates;
Co-reporter:Kelly M. Lefler, Chul Hoon Kim, Yi-Lin Wu, and Michael R. Wasielewski
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 9) pp:1608-1615
Publication Date(Web):April 21, 2014
DOI:10.1021/jz500626g
Unlike the widely studied perylene-3,4:9,10-bis(dicarboximide) (PDI) dyes, self-assembly of the corresponding perylene-3,4-dicarboximide (PMI) dyes into large arrays and studies of their excited state properties have received far less attention. Two symmetric PMI trefoils were synthesized by connecting the 9-position of the perylene core either directly (1) or through a phenylene linker (2) to the 1,3,5-positions of a central benzene ring. Synchrotron-based small- and wide-angle X-ray scattering measurements in methylcyclohexane show that trefoil 1 self-assembles into cofacial trimers (13) on average, while trefoil 2 forms much larger assemblies that are tridecamers (213) on average. Their photophysics were characterized using steady-state as well as transient absorption and emission spectroscopy. Time-resolved spectroscopy reveals that both 13 and 213 initially form excitonically coupled excited states that subsequently relax to excimer states having 20 and 8.4 ns lifetimes, respectively, which decay to ground-state primarily nonradiatively. The data are consistent with stronger electronic coupling between the PMI molecules in 213 relative to 13.Keywords: energy transfer; excimer; exciton coupling; ultrafast spectroscopy; X-ray scattering;
Co-reporter:Dr. Vladimir V. Roznyatovskiy;Dr. Raanan Carmieli;Scott M. Dyar;Kristen E. Brown ; Michael R. Wasielewski
Angewandte Chemie International Edition 2014 Volume 53( Issue 13) pp:3457-3461
Publication Date(Web):
DOI:10.1002/anie.201309335
Abstract
Zinc tetrabenzotetraphenyl porphyrin (ZnTBTPP) covalently attached to four perylenediimide (PDI) acceptors self-assembles into a π-stacked, segregated columnar structure, as indicated by small- and wide-angle X-ray scattering. Photoexcitation of ZnTBTPP rapidly produces a long-lived electron–hole pair having a 26 Å average separation distance, which is much longer than if the pair is confined within the covalent monomer. This implies that the charges are mobile within their respective segregated ZnTBTPP and PDI charge conduits.
Co-reporter:Alyssa-Jennifer Avestro;Daniel M. Gardner;Dr. Nicolaas A. Vermeulen;Eleanor A. Wilson;Dr. Severin T. Schneebeli;Dr. Adam C. Whalley;Dr. Matthew E. Belowich; Raanan Carmieli; Michael R. Wasielewski; J. Fraser Stoddart
Angewandte Chemie International Edition 2014 Volume 53( Issue 17) pp:4442-4449
Publication Date(Web):
DOI:10.1002/anie.201309680
Abstract
The controlled self-assembly of well-defined and spatially ordered π-systems has attracted considerable interest because of their potential applications in organic electronics. An important contemporary pursuit relates to the investigation of charge transport across noncovalently coupled components in a stepwise fashion. Dynamic oligorotaxanes, prepared by template-directed methods, provide a scaffold for directing the construction of monodisperse one-dimensional assemblies in which the functional units communicate electronically through-space by way of π-orbital interactions. Reported herein is a series of oligorotaxanes containing one, two, three and four naphthalene diimide (NDI) redox-active units, which have been shown by cyclic voltammetry, and by EPR and ENDOR spectroscopies, to share electrons across the NDI stacks. Thermally driven motions between the neighboring NDI units in the oligorotaxanes influence the passage of electrons through the NDI stacks in a manner reminiscent of the conformationally gated charge transfer observed in DNA.
Co-reporter:Scott M. Dyar;Jonathan C. Barnes;Dr. Michal Jurí&x10d;ek; J. Fraser Stoddart; Dick T. Co; Ryan M. Young; Michael R. Wasielewski
Angewandte Chemie International Edition 2014 Volume 53( Issue 21) pp:5371-5375
Publication Date(Web):
DOI:10.1002/anie.201402444
Abstract
Molecules capable of accepting and storing multiple electrons are crucial components of artificial photosynthetic systems designed to drive catalysts, such as those used to reduce protons to hydrogen. ExBox4+, a boxlike cyclophane comprising two π-electron-poor extended viologen units tethered at both ends by two p-xylylene linkers, has been shown previously to accept an electron through space from a photoexcited guest. Herein is an investigation of an alternate, through-bond intramolecular electron-transfer pathway involving ExBox4+ using a combination of transient absorption and femtosecond stimulated Raman spectroscopy (FSRS). Upon photoexcitation of ExBox4+, an electron is transferred from one of the p-xylylene linkers to one of the extended viologen units in ca. 240 ps and recombines in ca. 4 ns. A crystal structure of the doubly reduced species ExBox2+ was obtained.
Co-reporter:Yilei Wu;Dr. Marco Frasconi;Daniel M. Gardner;Dr. Paul R. McGonigal;Dr. Severin T. Schneebeli; Michael R. Wasielewski; J. Fraser Stoddart
Angewandte Chemie International Edition 2014 Volume 53( Issue 36) pp:9476-9481
Publication Date(Web):
DOI:10.1002/anie.201403816
Abstract
Investigating through-space electronic communication between discrete cofacially oriented aromatic π-systems is fundamental to understanding assemblies as diverse as double-stranded DNA, organic photovoltaics and thin-film transistors. A detailed understanding of the electronic interactions involved rests on making the appropriate molecular compounds with rigid covalent scaffolds and π–π distances in the range of ca. 3.5 Å. Reported herein is an enantiomeric pair of doubly-bridged naphthalene-1,8:4,5-bis(dicarboximide) (NDI) cyclophanes and the characterization of four of their electronic states, namely 1) the ground state, 2) the exciton coupled singlet excited state, 3) the radical anion with strong through-space interactions between the redox-active NDI molecules, and 4) the diamagnetic diradical dianion using UV/Vis/NIR, EPR and ENDOR spectroscopies in addition to X-ray crystallography. Despite the unfavorable Coulombic repulsion, the singlet diradical dianion dimer of NDI shows a more pronounced intramolecular π–π stacking interaction when compared with its neutral analog.
Co-reporter:Mehdi Zarea, Raanan Carmieli, Mark A. Ratner, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2014 Volume 118(Issue 24) pp:4249-4255
Publication Date(Web):May 28, 2014
DOI:10.1021/jp5039283
Subnanosecond radical pair (RP) formation by electron transfer from an excited singlet state or by bond breaking produces two correlated spins coupled by their spin–spin exchange (J) and magnetic dipole (D) interactions. In the high magnetic field limit, the two-spin system can be described by a singlet state (S) and three triplet states (T0, T+1, T–1). When J is small relative to the electron Zeeman interaction, |T0⟩ is the only triplet state that is populated by coherent spin mixing with the |S⟩ state because the |T+1⟩ and |T–1⟩ states are well-separated from |S⟩ by a large energy gap. Herein, we describe the spin dynamics for RPs having restricted geometries in which J is similar in magnitude to the electron Zeeman interaction and does not fluctuate significantly. Under these circumstances, depending on the sign of J, the energies of |T+1⟩ or |T–1⟩ are close to that of |S⟩ so that weak isotropic electron–nuclear hyperfine coupling leads to population of |T+1⟩ or |T–1⟩. An approximate relationship for the triplet quantum yield is developed for a RP in the large J regime, where one or both electrons interact with nearby spin-1/2 nuclei. This relationship also yields the net spin polarization transfer to the nuclear spins.
Co-reporter:Arun Kalliat Thazhathveetil, Anton Trifonov, Michael R. Wasielewski, and Frederick D. Lewis
The Journal of Physical Chemistry A 2014 Volume 118(Issue 45) pp:10359-10363
Publication Date(Web):June 9, 2014
DOI:10.1021/jp502974s
The effect of Mg2+ cations on the electronic spectra and dynamics and efficiency of hole transport has been determined by means of femtosecond time-resolved transient absorption spectroscopy for DNA hairpins possessing stilbene electron acceptor and donor chromophores. The results are compared with those obtained previously for the same hairpins in the presence of Na+ cations and for one hairpin with no added salt. Quantum yields and rate constants for charge separation are smaller in the presence of Mg2+ than Na+, the largest differences being observed for the hairpins with the largest number of base pairs. Slower charge separation is attributed to minor groove binding by Mg2+, which results in a stiffer duplex structure rather than a change in ground state geometry. Reduction in the Na+ concentration has little effect on either the dynamics or efficiency of hole transport.
Co-reporter:Dr. Vladimir V. Roznyatovskiy;Dr. Raanan Carmieli;Scott M. Dyar;Kristen E. Brown ; Michael R. Wasielewski
Angewandte Chemie 2014 Volume 126( Issue 13) pp:3525-3529
Publication Date(Web):
DOI:10.1002/ange.201309335
Abstract
Zinc tetrabenzotetraphenyl porphyrin (ZnTBTPP) covalently attached to four perylenediimide (PDI) acceptors self-assembles into a π-stacked, segregated columnar structure, as indicated by small- and wide-angle X-ray scattering. Photoexcitation of ZnTBTPP rapidly produces a long-lived electron–hole pair having a 26 Å average separation distance, which is much longer than if the pair is confined within the covalent monomer. This implies that the charges are mobile within their respective segregated ZnTBTPP and PDI charge conduits.
Co-reporter:Alyssa-Jennifer Avestro;Daniel M. Gardner;Dr. Nicolaas A. Vermeulen;Eleanor A. Wilson;Dr. Severin T. Schneebeli;Dr. Adam C. Whalley;Dr. Matthew E. Belowich; Raanan Carmieli; Michael R. Wasielewski; J. Fraser Stoddart
Angewandte Chemie 2014 Volume 126( Issue 17) pp:4531-4538
Publication Date(Web):
DOI:10.1002/ange.201309680
Abstract
The controlled self-assembly of well-defined and spatially ordered π-systems has attracted considerable interest because of their potential applications in organic electronics. An important contemporary pursuit relates to the investigation of charge transport across noncovalently coupled components in a stepwise fashion. Dynamic oligorotaxanes, prepared by template-directed methods, provide a scaffold for directing the construction of monodisperse one-dimensional assemblies in which the functional units communicate electronically through-space by way of π-orbital interactions. Reported herein is a series of oligorotaxanes containing one, two, three and four naphthalene diimide (NDI) redox-active units, which have been shown by cyclic voltammetry, and by EPR and ENDOR spectroscopies, to share electrons across the NDI stacks. Thermally driven motions between the neighboring NDI units in the oligorotaxanes influence the passage of electrons through the NDI stacks in a manner reminiscent of the conformationally gated charge transfer observed in DNA.
Co-reporter:Yilei Wu;Dr. Marco Frasconi;Daniel M. Gardner;Dr. Paul R. McGonigal;Dr. Severin T. Schneebeli; Michael R. Wasielewski; J. Fraser Stoddart
Angewandte Chemie 2014 Volume 126( Issue 36) pp:9630-9635
Publication Date(Web):
DOI:10.1002/ange.201403816
Abstract
Investigating through-space electronic communication between discrete cofacially oriented aromatic π-systems is fundamental to understanding assemblies as diverse as double-stranded DNA, organic photovoltaics and thin-film transistors. A detailed understanding of the electronic interactions involved rests on making the appropriate molecular compounds with rigid covalent scaffolds and π–π distances in the range of ca. 3.5 Å. Reported herein is an enantiomeric pair of doubly-bridged naphthalene-1,8:4,5-bis(dicarboximide) (NDI) cyclophanes and the characterization of four of their electronic states, namely 1) the ground state, 2) the exciton coupled singlet excited state, 3) the radical anion with strong through-space interactions between the redox-active NDI molecules, and 4) the diamagnetic diradical dianion using UV/Vis/NIR, EPR and ENDOR spectroscopies in addition to X-ray crystallography. Despite the unfavorable Coulombic repulsion, the singlet diradical dianion dimer of NDI shows a more pronounced intramolecular π–π stacking interaction when compared with its neutral analog.
Co-reporter:Brad S. Veldkamp, Won-Sik Han, Scott M. Dyar, Samuel W. Eaton, Mark A. Ratner and Michael R. Wasielewski
Energy & Environmental Science 2013 vol. 6(Issue 6) pp:1917-1928
Publication Date(Web):19 Apr 2013
DOI:10.1039/C3EE40378E
Using visible and near-infrared transient absorption spectroscopy to track distinct excited state, cation, and anion signals, we report a detailed kinetic analysis of photoinitiated multi-step charge separation and ultrafast charge transfer induced dissociation in a self-assembled donor–bridge–acceptor–cobaloxime triad. The donor–bridge–acceptor ligand consists of a perylene chromophore linked via a xylene bridge to a pyridyl-substituted 1,8-naphthalimide electron acceptor. Coordination of the ligand to the catalyst [Co(dmgBF2)2(L)2], where dmgBF2 = (difluoroboryl)dimethylglyoximato and L = water or a solvent molecule, yields a donor–bridge–acceptor–catalyst triad assembly. Photoexcitation with 416 nm laser pulses generates the perylene S1 excited state. Subsequent electron transfer from perylene to the acceptor occurs in τ = 9.0 ± 0.1 ps followed by electron transfer to the catalyst in τ = 6 ± 1 ps. Of the charge-separated state population formed, 79 ± 1% undergoes charge recombination to either the singlet ground state (τ = 0.8 ± 0.1 ns) or the perylene triplet state (τ = 4.3 ± 0.1 ns). Co(I)-pyridyl bond dissociation with τ = 2.4 ± 0.2 ns competes with intramolecular charge recombination resulting in a 21 ± 1% yield of dissociated oxidized photosensitizer and reduced catalyst. Subsequent diffusional charge recombination occurs with k = (1.8 ± 0.2) × 1010 M−1 s−1. This detailed analysis of the electron transfer and dissociation dynamics of an integrated photosensitizer–catalyst system will inform the rational design of novel molecular assemblies that efficiently absorb photons, transfer electrons, and catalyze fuel-forming reactions.
Co-reporter:Samuel W. Eaton ; Leah E. Shoer ; Steven D. Karlen ; Scott M. Dyar ; Eric A. Margulies ; Brad S. Veldkamp ; Charusheela Ramanan ; Daniel A. Hartzler ; Sergei Savikhin ; Tobin J. Marks
Journal of the American Chemical Society 2013 Volume 135(Issue 39) pp:14701-14712
Publication Date(Web):September 6, 2013
DOI:10.1021/ja4053174
The crystal structure of N,N-bis(n-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide), 1, obtained by X-ray diffraction reveals that 1 has a nearly planar perylene core and π–π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π–π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of 1 results in a 140 ± 20% yield of triplet excitons (3*1) in τSF = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis.
Co-reporter:Raanan Carmieli ; Arun K. Thazhathveetil ; Frederick D. Lewis
Journal of the American Chemical Society 2013 Volume 135(Issue 30) pp:10970-10973
Publication Date(Web):July 15, 2013
DOI:10.1021/ja4055405
DNA hairpins having both a tethered anthraquinone (Aq) end-capping group and a perylenediimide (PDI) base surrogate were synthesized, wherein Aq and PDI are each separated from a G-C base pair hole trap by A-T and I-C base pairs (G = guanine, A = adenine, T= thymine, C = cytosine, I = inosine). Selective photoexcitation of PDI at 532 nm generates a singlet radical ion pair (RP), 1(G+•-PDI–•), while selective photoexcitation of Aq at 355 nm generates the corresponding triplet RP, 3(G+•-Aq–•). Subsequent radical pair intersystem crossing within these spin-correlated RPs leads to mixed spin states that exhibit spin-polarized, time-resolved EPR spectra in which the singlet- and triplet-initiated RPs have opposite phases. These results demonstrate that a carefully designed DNA hairpin can serve as a photodriven molecular spin switch based on wavelength-selective formation of the singlet or triplet RP without significant competition from undesired energy transfer processes.
Co-reporter:Yi-Lin Wu ; Kristen E. Brown
Journal of the American Chemical Society 2013 Volume 135(Issue 36) pp:13322-13325
Publication Date(Web):August 27, 2013
DOI:10.1021/ja407648d
We report here a potassium-induced guanine quadruplex as a supramolecular platform for controlled assembly of electron donor–acceptor systems. A monodisperse, C4-symmetric octamer of a guanine–perylene-3,4,9,10-bis(dicarboximide) conjugate (GPDI) was prepared in tetrahydrofuran. The two layers of cyclic guanine tetramers have the same direction of rotation, and the PDI moiety between the layers adopts a nearly eclipsed relationship (H-aggregation), as revealed by small- and wide-angle X-ray scattering, NMR spectroscopy, and steady-state UV/vis absorption. Following photoexcitation of the PDI moiety in the quadruplex, charge separation occurs in τCS = 98 ± 12 ps to give G+•-PDI–• that recombines in τCR = 1.2 ± 0.2 ns, which is >100 times longer than that in the monomeric GPDI dyad. The transient absorption spectrum of G+•-PDI–• within the GPDI quadruplex suggests the formation of a radical anion delocalized over the neighboring PDI units, and this result is consistent with the more favorable electrochemical reduction potential for PDIs in the quadruplex relative to the monomer.
Co-reporter:Michael T. Vagnini, Michael W. Mara, Michael R. Harpham, Jier Huang, Megan L. Shelby, Lin X. Chen and Michael R. Wasielewski
Chemical Science 2013 vol. 4(Issue 10) pp:3863-3873
Publication Date(Web):29 Jul 2013
DOI:10.1039/C3SC51511G
Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(IV) state of the Ir(III)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor–chromophore–Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI–1*PMI–Ir(III) → NDI–PMI˙−–Ir(IV) and NDI–1*PMI–Ir(III) → NDI˙−–PMI˙+–Ir(III) that each proceed with τ < 5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI˙−–PMI–Ir(IV). The fully charge-separated ion pair has a lifetime of 17.2 ± 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the LIII-edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(III) to Ir(IV), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donor–acceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.
Co-reporter:Hsiao-Fan Chen, Daniel M. Gardner, Raanan Carmieli and Michael R. Wasielewski
Chemical Communications 2013 vol. 49(Issue 77) pp:8614-8616
Publication Date(Web):05 Aug 2013
DOI:10.1039/C3CC45129A
Ordered multi-spin assemblies are required for developing solid-state molecule-based spintronics. A linear donor–chromophore–acceptor (D–C–A) molecule was covalently attached inside the 150 nm diam. nanopores of an anodic aluminum oxide (AAO) membrane. Photoexcitation of D–C–A in a 343 mT magnetic field results in sub-nanosecond, two-step electron transfer to yield the spin-correlated radical ion pair (SCRP) 1(D+˙–C–A−˙), which then undergoes radical pair intersystem crossing (RP-ISC) to yield 3(D+˙–C–A−˙). RP-ISC results in S–T0 mixing to selectively populate the coherent superposition states |S′〉 and |T′〉. Microwave-induced transitions between these states and the unpopulated |T+1〉 and |T−1〉 states result in spin-polarized time-resolved EPR (TREPR) spectra. The dependence of the electron spin polarization (ESP) phase of the TREPR spectra on the orientation of the AAO membrane pores relative to the externally applied magnetic field is used to determine the overall orientation of the SCRPs within the pores at room temperature.
Co-reporter:Victoria L. Gunderson, Michael R. Wasielewski
Chemical Physics Letters 2013 Volume 556() pp:303-307
Publication Date(Web):29 January 2013
DOI:10.1016/j.cplett.2012.12.025
Small- and wide-angle X-ray scattering (SAXS/WAXS) and DOSY 1H NMR reveal that a zinc methyl pyrochlorophyllide a (Chl) derivative with a 3-pyridyl ligand at the Chl 20-position self-assembles in toluene solution into a cyclic trimer having a radius of 8.2 ± 0.1 Å. Femtosecond transient absorption spectroscopy shows that singlet–singlet annihilation occurs within the cyclic trimer as a result of energy transfer between the Chls that occurs with an (11 ± 1 ps)−1 rate constant, which agrees with the predictions of Förster theory, even though the Chl–Chl distances are comparable to the length of the Chl transition dipole.Graphical abstractHighlights► A zinc pyrochlorophyllide a with a 3-pyridyl ligand self-assembles into a cyclic trimer. ► Energy transfer between the Chls occurs with an (11 ± 1 ps)−1 rate constant. ► Surprisingly, Förster theory predicts the rate despite the short Chl–Chl distance.
Co-reporter:Dr. Severin T. Schneebeli;Dr. Marco Frasconi;Dr. Zhichang Liu;Yilei Wu;Daniel M. Gardner;Nathan L. Strutt;Chuyang Cheng; Raanan Carmieli; Michael R. Wasielewski; J. Fraser Stoddart
Angewandte Chemie International Edition 2013 Volume 52( Issue 49) pp:13100-13104
Publication Date(Web):
DOI:10.1002/anie.201307984
Co-reporter:Dr. Severin T. Schneebeli;Dr. Marco Frasconi;Dr. Zhichang Liu;Yilei Wu;Daniel M. Gardner;Nathan L. Strutt;Chuyang Cheng; Raanan Carmieli; Michael R. Wasielewski; J. Fraser Stoddart
Angewandte Chemie International Edition 2013 Volume 52( Issue 49) pp:
Publication Date(Web):
DOI:10.1002/anie.201309249
Co-reporter:Dr. Severin T. Schneebeli;Dr. Marco Frasconi;Dr. Zhichang Liu;Yilei Wu;Daniel M. Gardner;Nathan L. Strutt;Chuyang Cheng; Raanan Carmieli; Michael R. Wasielewski; J. Fraser Stoddart
Angewandte Chemie 2013 Volume 125( Issue 49) pp:13338-13342
Publication Date(Web):
DOI:10.1002/ange.201307984
Co-reporter:Dr. Severin T. Schneebeli;Dr. Marco Frasconi;Dr. Zhichang Liu;Yilei Wu;Daniel M. Gardner;Nathan L. Strutt;Chuyang Cheng; Raanan Carmieli; Michael R. Wasielewski; J. Fraser Stoddart
Angewandte Chemie 2013 Volume 125( Issue 49) pp:
Publication Date(Web):
DOI:10.1002/ange.201309249
Co-reporter:Sarah M. Mickley Conron, Leah E. Shoer, Amanda L. Smeigh, Annie Butler Ricks, and Michael R. Wasielewski
The Journal of Physical Chemistry B 2013 Volume 117(Issue 7) pp:2195-2204
Publication Date(Web):January 29, 2013
DOI:10.1021/jp311067q
Two X-shaped, cruciform electron donor2–acceptor–acceptor′2 (D2-A-A′2) molecules, 1 and 2, in which D = zinc 5-phenyl-10,15,20-tripentylporphyrin (ZnTPnP) or zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), respectively, A = pyromellitimide (PI), and A′ = perylene-3,4:9,10-bis(dicarboximide) (PDI), were prepared to study self-assembly motifs that promote photoinitiated charge separation followed by electron and hole transport through π-stacked donors and acceptors. PDI secondary electron acceptors were chosen because of their propensity to form self-ordered, π-stacked assemblies in solution, while the ZnTPnP and ZnTPP donors were selected to test the effect of peripheral substituent steric interactions on the π-stacking characteristics of the cruciforms. Small- and wide-angle X-ray scattering measurements in toluene solution reveal that 1 assembles into a π-stacked structure having an average of 5 ± 1 molecules, when [1] ≅ 10–5 M, while 2 remains monomeric. Photoexcitation of the π-stacked structure of 1 results in formation of ZnTPnP•+-PI-PDI•– in τCS1 = 0.3 ps, which is nearly 100-fold faster than the formation of ZnTPnP•+-PI•– in a model system lacking the PDI acceptor. The data are consistent with a self-assembled structure for 1 in which the majority of the intermolecular interactions have the ZnTPnP donor of one monomer cofacially π-stacked with the PDI acceptor of a neighboring monomer in a crisscrossed fashion. In contrast, 2 remains monomeric in toluene, so that photoexcitation of ZnTPP results in the charge separation reaction sequence: 1*ZnTPP-PI-PDI → ZnTPP•+-PI•–-PDI → ZnTPP•+-PI-PDI•–, where τCS1 = 33 ps and τCS2 = 239 ps. The perpendicular orientation of ZnTPnP and ZnTPP relative to PDI in 1 and 2 is designed to decrease the porphyrin–PDI distance without greatly decreasing the overall number of bonds linking them. This serves to decrease the Coulomb energy penalty required to produce D•+-PI-PDI•– relative to the corresponding linear D-PI-PDI array, while retaining the weak electronic coupling necessary to achieve long-lived charge separation, as evidenced by τCR = 24 ns for ZnTPP•+-PI-PDI•–.
Co-reporter:Michael T. Colvin, Raanan Carmieli, Tomoaki Miura, Sabine Richert, Daniel M. Gardner, Amanda L. Smeigh, Scott M. Dyar, Sarah M. Conron, Mark A. Ratner, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2013 Volume 117(Issue 25) pp:5314-5325
Publication Date(Web):May 30, 2013
DOI:10.1021/jp4045012
A series of donor–chromophore–acceptor–stable radical (D–C–A–R•) molecules having well-defined molecular structures were synthesized to study the factors affecting electron spin polarization transfer from the photogenerated D+•–C–A–• spin-correlated radical pair (RP) to the stable radical R•. Theory suggests that the magnitude of this transfer depends on the spin–spin exchange interaction (2JDA) of D+•–C–A–•. Yet, the generality of this prediction has never been demonstrated. In the D–C–A–R• molecules described herein, D is 4-methoxyaniline (MeOAn), 2,3-dihydro-1,4-benzodioxin-6-amine (DioxAn), or benzobisdioxole aniline (BDXAn), C is 4-aminonaphthalene-1,8-dicarboximide, and A is naphthalene-1,8:4,5-bis(dicarboximide) (1A,B–3A,B) or pyromellitimide (4A,B–6A,B). The terminal imide of the acceptors is functionalized with either a hydrocarbon (1A–6A) or a 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (R•) (1B–6B). Photoexcitation of C with 416-nm laser pulses results in two-step charge separation to yield D+•–C–A–•–(R•). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy using continuous-wave (CW) microwaves at both 295 and 85 K and pulsed microwaves at 85 K (electron spin–echo detection) was used to probe the initial formation of the spin-polarized RP and the subsequent polarization of the attached R• radical. The TREPR spectra show that |2JDA| for D+•–C–A–• decreases in the order MeOAn+• > DioxAn+• > BDXAn+• as a result of their spin density distributions, whereas the spin–spin dipolar interaction (dDA) remains nearly constant. Given this systematic variation in |2JDA|, electron spin–echo-detected EPR spectra of 1B–6B at 85 K show that the magnitude of the spin polarization transferred from the RP to R• depends on |2JDA|.
Co-reporter:Ryan M. Young, Scott M. Dyar, Jonathan C. Barnes, Michal Juríček, J. Fraser Stoddart, Dick T. Co, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2013 Volume 117(Issue 47) pp:12438-12448
Publication Date(Web):October 22, 2013
DOI:10.1021/jp409883a
Multielectron acceptors are essential components for artificial photosynthetic systems that must deliver multiple electrons to catalysts for solar fuels applications. The recently developed boxlike cyclophane incorporating two extended viologen units joined end-to-end by two p-phenylene linkers—namely, ExBox4+—has a potential to be integrated into light-driven systems on account of its ability to complex with π-electron-rich guests such as perylene, which has been utilized to great extent in many light-harvesting applications. Photodriven electron transfer to ExBox4+ has not previously been investigated, however, and so its properties, following photoreduction, are largely unknown. Here, we investigate the structure and energetics of the various accessible oxidation states of ExBox4+ using a combination of spectroscopy and computation. In particular, we examine photoinitiated electron transfer from perylene bound within ExBox4+ (ExBox4+⊂perylene) using visible and near-infrared femtosecond transient absorption (fsTA) spectroscopy. The structure and conformational relaxation dynamics of ExBox3+⊂perylene+ are observed with femtosecond stimulated Raman spectroscopy (FSRS). From the fsTA and FSRS spectra, we observe that the central p-phenylene spacer in one of the extended viologen units on one side of the cyclophane becomes more coplanar with its neighboring pyridinium units over the first ∼5 ps after photoreduction. When the steady-state structure of chemically generated ExBox2+ is investigated using Raman spectroscopy, it is found to have the central p-phenylene rings in both of its extended viologen units rotated to be more coplanar with their neighboring pyridinium units, further underscoring the importance of this subunit in the stabilization of the reduced states of ExBox4+.
Co-reporter:Kelly M. Lefler, Kristen E. Brown, Walter A. Salamant, Scott M. Dyar, Kathryn E. Knowles, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2013 Volume 117(Issue 40) pp:10333-10345
Publication Date(Web):September 13, 2013
DOI:10.1021/jp4083008
Two covalent perylene-3,4:9,10-bis(dicarboximide) (PDI) dimers in which the PDI molecules are attached to a xanthene (Xan) scaffold in which the long axes of the two π–π stacked PDI molecules are slipped by 4.3 and 7.9 Å were prepared. These dimers are designed to mimic J-aggregates and provide insights into the photophysics of triplet state formation in PDI aggregates that target organic electronics. Using ultrafast transient absorption and stimulated Raman spectroscopy, the mechanism of 3*PDI formation was found to depend strongly on a competition between the rate of Xan•+–PDI•– formation involving the spacer group and the rate of excimer-like state formation. Which mechanism is favored depends on the degree of electronic coupling between the two PDI molecules and/or solvent polarity. Singlet exciton fission to produce 3*PDI does not compete kinetically with these processes. The excimer-like state decays relatively slowly with τ = 28 ns to produce 3*PDI, while charge recombination of Xan•+–PDI•– yields 3*PDI more than an order of magnitude faster. The perpendicular orientation between the π orbitals of PDI and the Xan bridge provides a large enough orbital angular momentum change to greatly increase the intersystem crossing rate via Xan•+–PDI•– → 3*PDI charge recombination. These results highlight the importance of understanding inter-chromophore electronic coupling in a wide range of geometries as well as the active role that molecular spacers can play in the photophysics of covalent models for self-assembled chromophore aggregates.
Co-reporter:Annie Butler Ricks ; Kristen E. Brown ; Matthias Wenninger ; Steven D. Karlen ; Yuri A. Berlin ; Dick T. Co
Journal of the American Chemical Society 2012 Volume 134(Issue 10) pp:4581-4588
Publication Date(Web):February 15, 2012
DOI:10.1021/ja205913q
Donor–bridge–acceptor (D–B–A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FNn) bridges (n = 1–3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ+•-An–•, and An–• acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ+•-An–•–FNn–NI → DMJ+•-An–FNn–•–NI → DMJ+•-An–FNn–NI–•. The charge-shift reaction from An–• to NI–• exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å–1, which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FNn–• radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D+•–B–•–A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D–B–A molecule.
Co-reporter:Cheng Wang ; Scott M. Dyar ; Dennis Cao ; Albert C. Fahrenbach ; Noah Horwitz ; Michael T. Colvin ; Raanan Carmieli ; Charlotte L. Stern ; Sanjeev K. Dey ; Michael R. Wasielewski ;J. Fraser Stoddart
Journal of the American Chemical Society 2012 Volume 134(Issue 46) pp:19136-19145
Publication Date(Web):November 9, 2012
DOI:10.1021/ja307577t
The electronic properties of tetrathiafulvalene (TTF) can be tuned by attaching electron-donating or electron-withdrawing substituents. An electron-rich macrocyclic polyether containing two TTF units of different constitutions, namely 4,4′-bis(hydroxymethyl)tetrathiafulvalene (OTTFO) and 4,4′-bisthiotetrathiafulvalene (STTFS), has been synthesized. On two-electron oxidation, a hetero radical dimer is formed between OTTFO•+ and STTFS•+. The redox behavior of the macrocyclic polyether has been investigated by electrochemical techniques and UV–vis and electron paramagnetic resonance (EPR) spectroscopies. The [2]catenane in which the macrocyclic polyether is mechanically interlocked with the cyclobis(paraquat-p-phenylene) (CBPQT4+) ring has also been prepared using template-directed protocols. In the case of the [2]catenane, the formation of the TTF hetero radical dimer is prevented sterically by the CBPQT4+ ring. After a one-electron oxidation, a 70:30 ratio of OTTFO•+ to STTFS•+ is present at equilibrium, and, as a result, two translational isomers of the [2]catenane associated with these electronically different isomeric states transpire. EPR titration spectroscopy and simulations reveal that the radical states of the two constitutionally different TTF units in the [2]catenane still experience long-range electronic intramolecular coupling interactions, despite the presence of the CBPQT4+ ring, when one or both of them are oxidized to the radical cationic state. These findings in the case of both the free macrocyclic polyether and the [2]catenane have led to a deeper fundamental understanding of the mechanism of radical cation dimer formation between constitutionally different TTF units.
Co-reporter:Victoria L. Gunderson ; Amanda L. Smeigh ; Chul Hoon Kim ; Dick T. Co
Journal of the American Chemical Society 2012 Volume 134(Issue 9) pp:4363-4372
Publication Date(Web):February 13, 2012
DOI:10.1021/ja211329k
The synthesis and photoinduced charge transfer properties of a series of Chl-based donor–acceptor triad building blocks that self-assemble into cyclic tetramers are reported. Chlorophyll a was converted into zinc methyl 3-ethylpyrochlorophyllide a (Chl) and then further modified at its 20-position to covalently attach a pyromellitimide (PI) acceptor bearing a pyridine ligand and one or two naphthalene-1,8:4,5-bis(dicarboximide) (NDI) secondary electron acceptors to give Chl–PI–NDI and Chl–PI–NDI2. The pyridine ligand within each ambident triad enables intermolecular Chl metal–ligand coordination in dry toluene, which results in the formation of cyclic tetramers in solution, as determined using small- and wide-angle X-ray scattering at a synchrotron source. Femtosecond and nanosecond transient absorption spectroscopy of the monomers in toluene–1% pyridine and the cyclic tetramers in toluene shows that the selective photoexcitation of Chl results in intramolecular electron transfer from 1*Chl to PI to form Chl+•–PI–•–NDI and Chl+•–PI–•–NDI2. This initial charge separation is followed by a rapid charge shift from PI–• to NDI and subsequent charge recombination of Chl+•–PI–NDI–• and Chl+•–PI–(NDI)NDI–• on a 5–30 ns time scale. Charge recombination in the Chl–PI–NDI2 cyclic tetramer (τCR = 30 ± 1 ns in toluene) is slower by a factor of 3 relative to the monomeric building blocks (τCR = 10 ± 1 ns in toluene–1% pyridine). This indicates that the self-assembly of these building blocks into the cyclic tetramers alters their structures in a way that lengthens their charge separation lifetimes, which is an advantageous strategy for artificial photosynthetic systems.
Co-reporter:Lukáš Kobr ; Daniel M. Gardner ; Amanda L. Smeigh ; Scott M. Dyar ; Steven D. Karlen ; Raanan Carmieli
Journal of the American Chemical Society 2012 Volume 134(Issue 30) pp:12430-12433
Publication Date(Web):July 16, 2012
DOI:10.1021/ja305650x
Photoexcitation of the electron donor (D) within a linear, covalent donor–acceptor–acceptor molecule (D-A1-A2) in which A1 = A2 results in sub-nanosecond formation of a spin-coherent singlet radical ion pair state, 1(D+•-A1–•-A2), for which the spin–spin exchange interaction is large: 2J = 79 ± 1 mT. Subsequent laser excitation of A1–• during the lifetime of 1(D+•-A1–•-A2) rapidly produces 1(D+•-A1-A2–•), which abruptly decreases 2J 3600-fold. Subsequent coherent spin evolution mixes 1(D+•-A1-A2–•) with 3(D+•-A1-A2–•), resulting in mixed states which display transient spin-polarized EPR transitions characteristic of a spin-correlated radical ion pair. These photodriven J-jump experiments show that it is possible to use fast laser pulses to transfer electron spin coherence between organic radical ion pairs and observe the results using an essentially background-free time-resolved EPR experiment.
Co-reporter:Raanan Carmieli ; Amanda L. Smeigh ; Sarah M. Mickley Conron ; Arun K. Thazhathveetil ; Masaaki Fuki ; Yasuhiro Kobori ; Frederick D. Lewis
Journal of the American Chemical Society 2012 Volume 134(Issue 27) pp:11251-11260
Publication Date(Web):June 7, 2012
DOI:10.1021/ja303721j
A series of DNA hairpins (AqGn) possessing a tethered anthraquinone (Aq) end-capping group were synthesized in which the distance between the Aq and a guanine-cytosine (G-C) base pair was systematically varied by changing the number (n – 1) of adenine-thymine (A-T) base pairs between them. The photophysics and photochemistry of these hairpins were investigated using nanosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Upon photoexcitation, 1*Aq undergoes rapid intersystem crossing to yield 3*Aq, which is capable of oxidizing purine nucleobases resulting in the formation of 3(Aq–•Gn+•). All 3(Aq–•Gn+•) radical ion pairs exhibit asymmetric TREPR spectra with an electron spin polarization phase pattern of absorption and enhanced emission (A/E) due to their different triplet spin sublevel populations, which are derived from the corresponding non-Boltzmann spin sublevel populations of the 3*Aq precursor. The TREPR spectra of the 3(Aq–•Gn+•) radical ion pairs depend strongly on their spin–spin dipolar interaction and weakly on their spin–spin exchange coupling. The anisotropy of 3(Aq–•Gn+•) makes it possible to determine that the π systems of Aq–• and G+• within the radical ion pair are parallel to one another. Charge recombination of the long-lived 3(Aq–•Gn+•) radical ion pair displays an unusual bimodal distance dependence that results from a change in the rate-determining step for charge recombination from radical pair intersystem crossing for n < 4 to coherent superexchange for n > 4.
Co-reporter:Hyejin Yoo, Hee Won Bahng, Michael R. Wasielewski and Dongho Kim
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 6) pp:2001-2007
Publication Date(Web):06 Jan 2012
DOI:10.1039/C2CP22377E
The conformation of embedded molecule in a polymer matrix is sensitive to the local nano-environment that the molecule experiences. Particularly, single molecule spectroscopic methods have been utilized to visualize each molecular conformation in local sites of the polymer matrix by monitoring rotational diffusion and fluctuating fluorescence of the molecule. Here, we have performed single molecule spectroscopic experiments on a π-stacked perylenediimide (PDI) dimer and trimer, in which enhanced π–π interaction in π-stacked PDIs makes the fluorescence lifetime longer, embedded in two different polymers, namely poly(methyl methacrylate) (PMMA) and poly(butyl methacrylate) (PBMA), to reveal the conformational change depending on the polymer matrix. The fluorescence lifetimes of π-stacked PDIs are influenced by polymer surroundings because their molecular conformations are dependent on their interactions with the local environment in the polymer matrix. Furthermore, from an in-depth analysis of autocorrelation functions of fluorescence intensity trajectories, we could assign that the first autocorrelation value (lag 1) is larger as the intensity trace becomes more fluctuating. Thus, we expect that π-stacked PDIs, a model system for the formation of PDI excimer-like states, can be utilized to probe the surrounding nano-environment by monitoring the conformational change in real time.
Co-reporter:Hyejin Yoo, Shu Furumaki, Jaesung Yang, Ji-Eun Lee, Heejae Chung, Tatsuya Oba, Hiroyuki Kobayashi, Boris Rybtchinski, Thea M. Wilson, Michael R. Wasielewski, Martin Vacha, and Dongho Kim
The Journal of Physical Chemistry B 2012 Volume 116(Issue 42) pp:12878-12886
Publication Date(Web):September 24, 2012
DOI:10.1021/jp307394x
Perylenediimide (PDI) molecules are promising building blocks for photophysical studies of electronic interactions within multichromophore arrays. Such PDI arrays are important materials for fabrication of molecular nanodevices such as organic light-emitting diodes, organic semiconductors, and biosensors because of their high photostability, chemical and physical inertness, electron affinity, and high tinctorial strength over the entire visible spectrum. In this work, PDIs have been organized into linear (L3) and trefoil (T3) trimer molecules and investigated by single-molecule fluorescence microscopy to probe the relationship between molecular structures and interchromophoric electronic interactions. We found a broad distribution of coupling strengths in both L3 and T3 and hence strong/weak coupling between PDI units by monitoring spectral peak shifts in single-molecule fluorescence spectra upon sequential photobleaching of each constituent chromophore. In addition, we used a wide-field defocused imaging technique to resolve heterogeneities in molecular structures of L3 and T3 embedded in a PMMA polymer matrix. A systematic comparison between the two sets of experimental results allowed us to infer the correlation between intermolecular interactions and molecular structures. Our results show control of the PDI intermolecular interactions using suitable multichromophoric structures.
Co-reporter:James D. Blakemore;Samuel W. Eaton;Robert H. Crabtree;Francis D’Souza;Gary W. Brudvig;Michael T. Vagnini;Nathan D. Schley;Amanda L. Smeigh;Dick T. Co
PNAS 2012 Volume 109 (Issue 39 ) pp:
Publication Date(Web):2012-09-25
DOI:10.1073/pnas.1202075109
Photodriving the activity of water-oxidation catalysts is a critical step toward generating fuel from sunlight. The design
of a system with optimal energetics and kinetics requires a mechanistic understanding of the single-electron transfer events
in catalyst activation. To this end, we report here the synthesis and photophysical characterization of two covalently bound
chromophore-catalyst electron transfer dyads, in which the dyes are derivatives of the strong photooxidant perylene-3,4:9,10-bis(dicarboximide)
(PDI) and the molecular catalyst is the Cp∗Ir(ppy)Cl metal complex, where ppy = 2-phenylpyridine. Photoexcitation of the PDI in each dyad results in reduction of the
chromophore to PDI•- in less than 10 ps, a process that outcompetes any generation of 3∗PDI by spin-orbit-induced intersystem crossing. Biexponential charge recombination largely to the PDI-Ir(III) ground state
is suggestive of multiple populations of the PDI•--Ir(IV) ion-pair, whose relative abundance varies with solvent polarity. Electrochemical studies of the dyads show strong
irreversible oxidation current similar to that seen for model catalysts, indicating that the catalytic integrity of the metal
complex is maintained upon attachment to the high molecular weight photosensitizer.
Co-reporter:Kristen E. Brown, Brad S. Veldkamp, Dick T. Co, and Michael R. Wasielewski
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 17) pp:2362-2366
Publication Date(Web):August 8, 2012
DOI:10.1021/jz301107c
The ultrafast vibrational dynamics of the photoinduced charge-transfer reaction between perylene (Per) and perylene-3,4:9,10-bis(dicarboximide) (PDI) were investigated using femtosecond stimulated Raman spectroscopy (FSRS). Specifically probing the structural dynamics of PDI following its selective photoexcitation in a covalently linked dyad reveals vibrational modes uniquely characteristic to the PDI lowest excited singlet state and radical anion between 1000 and 1700 cm–1. A comparison of these vibrations to those of the ground state reveals the appearance of new 1*PDI and PDI–• stretching modes in the dyad at 1593 and 1588 cm–1, respectively. DFT calculations reveal that these vibrations are parallel to the long axis of PDI and thus then may be integral to the charge separation reaction. The ability to differentiate excited state from radical anion vibrational modes allows the evaluation of the influence of specific modes on the charge transfer dynamics in donor–bridge–acceptor systems based on PDI molecular constructs.Keywords: charge transfer; electron transfer; vibrational spectroscopy;
Co-reporter:Michael T. Colvin, Annie Butler Ricks, Amy M. Scott, Dick T. Co, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2012 Volume 116(Issue 8) pp:1923-1930
Publication Date(Web):February 1, 2012
DOI:10.1021/jp212546w
Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor–bridge–acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ–An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ+•–PI–• or DMJ+•–NI–• quantitatively in 1–4 (τCS ≤ 10 ps), and that charge recombination occurs with τCR = 268 and 158 ps for 1 and 3, respectively, and with τCR = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ+• and PI–• or NI–•, giving 2J > 600 mT for 1–3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of 3*An upon charge recombination occurs by spin–orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1–3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1–4 depend on the donor–acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.
Co-reporter:Michael T. Colvin, Annie Butler Ricks, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2012 Volume 116(Issue 9) pp:2184-2191
Publication Date(Web):February 15, 2012
DOI:10.1021/jp2125735
The impact of donor–acceptor electronic coupling and bridge energetics on the preference for hole or electron transfer leading to charge recombination in a series of donor-bridge-acceptor (D-B-A) molecules was examined. In these systems, the donor is 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) and acceptor is naphthalene-1,8:4,5-bis(dicarboximide) (NI), while the bridges are either oligo(p-phenyleneethynylene) (PEnP, where n = 1–3) 1–3 or oligo(2,7-fluorenone) (FNn, where n = 1–3) 4–6. Photoexcitation of 1–3 and 4–6 produces DMJ+•-An-PEnP-NI–• and DMJ+•-An-FNn-NI–•, respectively, which undergo radical pair intersystem crossing followed by charge recombination to yield both 3*An and 3*NI, which are observed by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. 3*NI is produced by hole transfer from DMJ+• to NI–•, while 3*An is produced by electron transfer from NI–• to DMJ+•, using the agency of the bridge HOMOs and LUMOs, respectively. By monitoring the initial population of 3*NI and 3*An in 1–6, the data show that charge recombination occurs preferentially by selective hole transfer when the bridge is PEnP, while it occurs by preferential electron transfer when the bridge is FNn. Over time, the initial population of 3*NI decreases, while that of 3*An increases, indicating that triplet–triplet energy transfer (TEnT) occurs. The observed distance dependence of TEnT from 3*NI to An is weakly exponential with a decay parameter β = 0.08 Å–1 for the PEnP series and β = 0.03 Å–1 for the FNn series. In the PEnP series, this weak distance dependence is attributed to a transition from the superexchange regime to hopping transport as the energy gap for triplet energy injection onto the bridge becomes significantly smaller as n increases, while in the FNn series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.
Co-reporter:Kelly M. Lefler, Dick T. Co, and Michael R. Wasielewski
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 24) pp:3798-3805
Publication Date(Web):December 6, 2012
DOI:10.1021/jz3018946
We report the synthesis, self-assembly characteristics, and ultrafast electron transfer dynamics of a perylene-3,4-dicarboximide (PMI) covalently linked to an N,N′-bis(3,4,5-tridodecyloxyphenyl)melamine electron donor (D) via a biphenyl spacer (PMI-Ph2-D). Synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) measurements in methylcyclohexane solution show that PMI-Ph2-D self-assembles into π–π stacked, hydrogen-bonded foldamers consisting of two or three hexameric rings or helices. Ultrafast transient absorption spectroscopy reveals that photoinduced charge separation within these nanostructures occurs by a unique pathway that is emergent in the assembly, whereas electron transfer does not occur in the PMI-Ph2-D monomers in tetrahydrofuran.Keywords: electron transfer; emergent properties; femtosecond spectroscopy; supramolecular; X-ray scattering;
Co-reporter:Premaladha Poddutoori, Dick T. Co, Amanda P. S. Samuel, Chul Hoon Kim, Michael T. Vagnini and Michael R. Wasielewski
Energy & Environmental Science 2011 vol. 4(Issue 7) pp:2441-2450
Publication Date(Web):15 Jun 2011
DOI:10.1039/C1EE01334C
Two covalently linked linear electron donor–acceptor triads Fc-ZnTPP-[NMI-FeI-FeI-S2(CO)6] (1) and Fc-Ph-ZnTPP-[NMI-FeI-FeI-S2(CO)6] (2) consisting of a zinc meso-tetraphenylporphyrin (ZnTPP) chromophore, a naphthalene monoimide diiron hydrogenase active site model [NMI-FeI-FeI-S2(CO)6], and a ferrocene (Fc) secondary electron donor have been synthesized along with their corresponding dyad reference molecules ZnTPP-[NMI-FeI-FeI-S2(CO)6] (3), Fc-ZnTPP (4), and Fc-Ph-ZnTPP (5). Time-resolved transient absorption and emission studies in CH2Cl2 show that selective photoexcitation of ZnTPP in triads 1 and 2 results in two competing quenching pathways for 1*ZnTPP: electron transfer from 1*ZnTPP to [NMI-FeI-FeI-S2(CO)6] and energy transfer from 1*ZnTPP to low-lying Fc excited states. Our studies on reference dyads 4 and 5 show that the majority of 1*ZnTPP produced by the laser pulse decays rapidly by energy transfer to Fc in triad 1 (τ < 10 ps), while electron transfer to [NMI-FeI-FeI-S2(CO)6] dominates in triad 2, allowing the second rapid electron transfer step from Fc to ZnTPP+˙ to proceed. Quantum yields of the fully charge separated states Fc+-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] and Fc+-Ph-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] are 0.13 and 0.71, respectively. Charge recombination in Fc+-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] occurs with τCR = 9 ± 1 ns and τCR = 67 ± 2 ns for Fc+-Ph-ZnTPP-[NMI-Fe0-FeI-S2(CO)6]. By incorporating a secondary electron donor, the lifetime of the reduced diironhydrogenase mimic was extended by a factor of >450. Studies of photochemical hydrogen evolution using 1 and 2 reveal that the hydrogen generation efficiency depends on the lifetime of the final charge separated state. The ability to execute a multi-electron proton-coupled electron transfer mechanism in a stepwise manner will allow us to investigate the structural and electronic requirements for each step aiding in overall system optimization. Thus, it is possible to use the same multi-step electron transfer strategy that has been employed to extend the lifetime of charge-separated states in photodriven donor–acceptor systems to extend the lifetime of the reduced states of metal complexes of potential use in catalytic proton reduction.
Co-reporter:Arun K. Thazhathveetil ; Anton Trifonov ; Michael R. Wasielewski ;Frederick D. Lewis
Journal of the American Chemical Society 2011 Volume 133(Issue 30) pp:11485-11487
Publication Date(Web):July 5, 2011
DOI:10.1021/ja204815d
Transport of positive charge or holes in DNA occurs via a thermally activated multi-step hopping mechanism. The fastest hopping rates reported to date are those for repeating poly(purine) sequences in which hopping occurs via a random walk mechanism with rate constants of khop = 4.3 × 109 s–1 for poly(dG) and 1.2 × 109 s–1 for poly(dA). We report here the dynamics of charge separation in DNA conjugates possessing repeating 7-deazaadenine (dzA) sequences. These data provide an estimated value of khop = 4.2 × 1010 s–1 for poly(dzA), an order of magnitude faster than for poly(dG).
Co-reporter:Amy M. Scott
Journal of the American Chemical Society 2011 Volume 133(Issue 9) pp:3005-3013
Publication Date(Web):February 14, 2011
DOI:10.1021/ja1095649
The temperature dependence of spin-selective intramolecular charge recombination (CR) in a series of 2,7-fluorenone (FN1−2) and p-phenylethynylene (PE1−2P) linked donor−bridge−acceptor molecules with a 3,5-dimethyl-4-(9-anthracenyl) julolidine (DMJ−An) electron donor and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor was studied using nanosecond transient absorption spectroscopy in the presence of a static magnetic field. Photoexcitation of DMJ−An into its charge transfer band and subsequent electron transfer to NI results in a nearly quantitative yield of 1(DMJ+•−An−FNn−NI−•) and 1(DMJ+•−An−PEnP−NI−•), which undergo rapid radical pair intersystem crossing (RP-ISC) to produce the triplet RPs, 3(DMJ+•−An−FNn−NI−•) and 3(DMJ+•−An−PEnP−NI−•), respectively. The CR rate constants, kCR, in toluene were measured over a temperature range from 270 to 350 K, and a kinetic analysis of kCR in the presence of an applied static magnetic field was used to extract the singlet and triplet charge recombination rate constants, kCRS and kCRT, respectively, as well as the intersystem crossing rate constant, kST. Plots of ln (kT1/2) versus 1/T for PE1P show a distinct crossover at 300 K from a temperature-independent singlet CR pathway to a triplet CR pathway that is positively activated with a barrier of 1047 ± 170 cm−1. The singlet CR pathway via the FN1 bridge displays a negative activation energy that results from donor−bridge and bridge−acceptor torsional motions about the single bonds joining them. In contrast, the triplet CR pathway via the FN1−2 and PE1−2P bridges exhibits positive activation energies. The activation barriers to these torsional motions range from 1100 to 4500 cm−1 and can be modeled by semiclassical electron transfer theory.
Co-reporter:Tomoaki Miura
Journal of the American Chemical Society 2011 Volume 133(Issue 9) pp:2844-2847
Publication Date(Web):February 14, 2011
DOI:10.1021/ja110789q
We have studied spin-dependent charge transfer dynamics in wirelike donor−bridge−acceptor (D−B−A) molecules comprising a phenothiazine (PTZ) donor, an oligo(2,7-fluorene) (FLn) bridge, and a perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptor, PTZ−FL3−PDI (1) and PTZ−FL4−PDI (2), dissolved in the magnetic field-aligned nematic phase of 4-cyano-4′-n-pentylbiphenyl (5CB) at 295 K. Time-resolved EPR spectroscopy using both continuous wave and pulsed microwaves shows that the photogenerated radical pairs (RPs), PTZ+•−FL3−PDI−• and PTZ+•−FL4−PDI−•, recombine much faster from the singlet RP manifold than the triplet RP manifold. When a strong resonant microwave π pulse is applied following RP photogeneration in 1 and 2, the RP lifetimes increase about 50-fold as indicated by electron spin−echo detection. This result shows that the RP lifetime can be greatly extended by rapidly switching off fast triplet RP recombination.
Co-reporter:Charusheela Ramanan ; Amanda L. Smeigh ; John E. Anthony ; Tobin J. Marks
Journal of the American Chemical Society 2011 Volume 134(Issue 1) pp:386-397
Publication Date(Web):November 23, 2011
DOI:10.1021/ja2080482
The photophysics and morphology of thin films of N,N-bis(2,6-diisopropylphenyl)perylene-3,4:9,10-bis(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10-bis(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ∼100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast singlet fission (1*TIPS-Pn + TIPS-Pn → 2 3*TIPS-Pn) and charge transfer from 1*TIPS-Pn to PDIs 1–3. As the blend films become more homogeneous across the series TIPS-Pn:PDI 1 → 2 → 3, charge separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state 1(TIPS-Pn+•–PDI–•) that undergoes radical pair intersystem crossing to form 3(TIPS-Pn+•–PDI–•), which then undergoes charge recombination to yield either 3*PDI or 3*TIPS-Pn. Energy transfer from 3*PDI to TIPS-Pn also yields 3*TIPS-Pn. These results show that multiple pathways produce the 3*TIPS-Pn state, so that OPV design strategies based on this system must utilize this triplet state for charge separation.
Co-reporter:Shinji Ando, Charusheela Ramanan, Antonio Facchetti, Michael R. Wasielewski and Tobin J. Marks
Journal of Materials Chemistry A 2011 vol. 21(Issue 47) pp:19049-19057
Publication Date(Web):31 Oct 2011
DOI:10.1039/C1JM13397G
A series of donor–acceptor molecules consisting of core-brominated and -cyanated perylene-3,4:9,10-bis(dicarboximide) (PDI) structures covalently linked to two terminal pendant alkylanthracenes (A) is described. These hybrid molecules, having varying alkyl tether lengths as well as PDI electron affinities, were synthesized by condensation of a 1,7-dibromoperylene tetracarboxylic acid anhydride with the appropriate aminoalkylanthracene, followed by cyanation with CuCN. Thermal, optical, and electrochemical properties were characterized. PDI moiety photoexcitation results in pendant anthracene oxidation, generating 1(A+˙-PDI−˙-A) species. The solution dynamics of this one-electron charge separation were characterized by ultrafast transient absorption spectroscopy, and charge separation rates are found to vary with alkyl tether length. Trends in these rates are attributed to solution phase geometric variations of the PDI-A structure, reflecting the flexibility of the spacer.
Co-reporter:Victoria L. Gunderson, Elisha Krieg, Michael T. Vagnini, Mark A. Iron, Boris Rybtchinski, and Michael R. Wasielewski
The Journal of Physical Chemistry B 2011 Volume 115(Issue 23) pp:7533-7540
Publication Date(Web):May 20, 2011
DOI:10.1021/jp2016374
Elucidation of photoinduced charge transfer behavior in organic dye/metal hybrids is important for developing photocatalytic systems for solar energy conversion. We report the synthesis and photophysical characterization of a perylene-3,4:9,10-bis(dicarboximide) (PDI)–ruthenium(II) complex, bis-PDI-2,2′-bipyridineRu(II)Cl2(CNtbutyl)2, which has favorable energetics, ΔGCS ≈ −1.0 eV, for singlet electron transfer from the Ru complex to PDI. Time-resolved optical spectroscopy reveals that upon selective photoexcitation of PDI, ultrafast charge transfer (<150 fs) from the Ru complex to 1*PDI generates the Ru(III)–PDI–• ion pair. The resulting vibrationally hot Ru(III)–PDI–• ion pair exhibits fast relaxation (τ = 3.9 ps) and charge recombination (τCR = 63 ps). Our experimental and computational (DFT and TDDFT) studies show that energy-preserving photodriven singlet electron transfer can dominate in properly designed organic dye/metal complexes, making them of particular interest for use in artificial photosynthetic systems for solar fuels formation.
Co-reporter:Angel J. Jiménez;Dr. Bruno Grimm;Victoria L. Gunderson;Michael T. Vagnini;Sra KrickCalderon;Dr. M. Salomé Rodríguez-Morgade;Dr. Michael R. Wasielewski;Dr. Dirk M. Guldi;Dr. Tomás Torres
Chemistry - A European Journal 2011 Volume 17( Issue 18) pp:5024-5032
Publication Date(Web):
DOI:10.1002/chem.201002963
Abstract
Metal coordination was probed as a versatile approach for designing a novel electron donor/acceptor hybrid [PDIpy4{Ru(CO)Pc}4] (1), in which four pyridines placed at the bay region of a perylenediimides (PDIpy4) coordinate with four ruthenium phthalocyanine units [Ru(CO)Pc]. This structural motif was expected to promote strong electronic coupling between the electron donors and the electron acceptor, a hypothesis that was confirmed in a full-fledged physicochemical investigation focusing on the ground and excited state reactivities. As far as the ground state is concerned, absorption and electrochemical assays indeed reveal a notable redistribution of electron density, that is, from the electron-donating [Ru(CO)Pc] to the electron-accepting PDIpy4. The most important thing to note in this context is that both the [Ru(CO)Pc] oxidation and the PDIpy4 reduction are rendered more difficult in 1 than in the individual building blocks. Likewise, in the excited state, strong electronic communication is the inception for a rapid charge-transfer process in photoexcited 1. Regardless of exciting [Ru(CO)Pc] or PDIpy4, spectral characteristics of the [RuPc] radical cation (broad absorptive features from 425 to 600 nm with a maximum at 575 nm, as well as a band centered at 725 nm) and of the PDI radical anion (780 nm maximum) emerge. The correspondingly formed radical ion pair state lasts for up to several hundred picoseconds in toluene, for example. On the other hand, employing more polar solvents, such as dichloromethane, destabilizes the radical ion pair state.
Co-reporter:Michael T. Colvin, Amanda L. Smeigh, Emilie M. Giacobbe, Sarah M. Mickley Conron, Annie Butler Ricks, and Michael R. Wasielewski
The Journal of Physical Chemistry A 2011 Volume 115(Issue 26) pp:7538-7549
Publication Date(Web):June 1, 2011
DOI:10.1021/jp2021006
tert-Butylphenylnitroxide (BPNO•) and α,γ-bisdiphenylene-β-phenylallyl (BDPA•) stable radicals are each attached to zinc meso-tetraphenylporphyrin (ZnTPP) at a fixed distance using one of the ZnTPP phenyl groups. BPNO• and BDPA• are oriented para (1 and 3, respectively) or meta (2 and 4, respectively) relative to the porphyrin macrocycle. Following photoexcitation of 1–4, transient optical absorption spectroscopy is used to observe excited state quenching of 1*ZnTPP by the radicals and time-resolved electron paramagnetic resonance (TREPR) spectroscopy is used to monitor the spin dynamics of the paramagnetic product states. The presence of BPNO• or BDPA• accelerates the intersystem crossing rate of 1*ZnTPP about 10- to 500-fold in 1–4 depending on the structure compared to that of 1*ZnTPP itself. In addition, the lifetime of 3*ZnTPP in 1 is shorter than that of 3*ZnTPP itself as a result of enhanced intersystem crossing (EISC) from 3*ZnTPP to the ground state. The TREPR spectra of the three unpaired spins produced within 1 and 2 show spin-polarized excited doublet (D1) and quartet (Q) states and subsequent formation of a spin-polarized ground state radical (D0). All three signals are absorptive for 1 and emissive for 2. Polarization inversion of the Q state is observed on a tens of nanoseconds time scale in 2, while no polarization inversion is observed for 1. The lack of polarization inversion in 1 is attributed to the short lifetime of the doublet–quartet manifold as a result of the very large exchange interaction. The TREPR spectra of 3 and 4 show ground state radical polarization at X-band (9.5 GHz) at room temperature, but not at 85 K, and similarly no polarization is observed at W-band (94 GHz). No evidence of excited doublet or quartet states is observed, indicating that the exchange interaction is both weak and temperature dependent. These results show that although ultrafast EISC produces 3*ZnTPP within 1–4, the magnitude of the exchange interactions between the three relevant spins in the resulting 3*ZnTPP–BPNO• and 3*ZnTPP–BDPA• systems dramatically alters their spin dynamics.
Co-reporter:Michael T. Colvin ; Annie Butler Ricks ; Amy M. Scott ; Amanda L. Smeigh ; Raanan Carmieli ; Tomoaki Miura
Journal of the American Chemical Society 2010 Volume 133(Issue 5) pp:1240-1243
Publication Date(Web):December 30, 2010
DOI:10.1021/ja1094815
A covalent, fixed-distance donor−bridge−acceptor (D−B−A) molecule was synthesized that upon photoexcitation undergoes ultrafast charge separation to yield a radical ion pair (RP) in which the spin−spin exchange interaction (2J) between the two radicals is sufficiently large to result in preferential RP intersystem crossing to the highest-energy RP eigenstate (T+1) at the 350 mT magnetic field characteristic of X-band (9.5 GHz) EPR spectroscopy. This behavior is unprecedented in covalent D−B−A molecules, and is evidenced by the time-resolved EPR (TREPR) spectrum at X-band of 3*D−B−A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e,e,e,e,e,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the TREPR triplet spectrum at W-band (94 GHz) of 3*D−B−A displays the (a,e,e,a,a,e) polarization pattern characteristic of a weakly coupled RP precursor, similar to that observed in photosynthetic reaction center proteins, and indicates a switch to selective population of the lower-energy T0 eigenstate.
Co-reporter:Amanda P. S. Samuel ; Dick T. Co ; Charlotte L. Stern
Journal of the American Chemical Society 2010 Volume 132(Issue 26) pp:8813-8815
Publication Date(Web):June 10, 2010
DOI:10.1021/ja100016v
Diiron complexes modeled on the active site of the [FeFe] hydrogenases having the general formula [Fe2(μ-R)(CO)6−n(L)n], where commonly R = alkyl or aryl dithiolate and L = CO, CN−, or PR3, are a promising class of catalysts for use in photodriven H2 production. However, many of these catalysts are difficult to photoreduce using chromophores that absorb visible light. Here we report the synthesis and spectroscopic characterization of a naphthalene-4,5-dicarboximide-1,8-dithiolate diiron complex [NMI-Fe2S2(CO)6, 1] and a covalently linked, fixed-distance zinc 5,10,15-tri-n-pentyl-20-phenylporphyrin-NMI-Fe2S2(CO)6 donor−acceptor dyad (2). The electron-withdrawing nature of the NMI group makes the diiron complex among the most easily reduced hydrogenase mimics reported to date (−0.74 V vs SCE). In the presence of triflic acid, the cyclic voltammogram of 1 showed an increase in current at the first reduction wave at −0.78 V and a new reduction wave at −1.4 V. As the acid concentration was increased, the current at −0.78 V remained constant while the current at −1.4 V increased significantly, which is consistent with a catalytic proton reduction process. Selective photoexcitation of the Zn porphyrin in 2 with 553 nm, 110 fs laser pulses in both toluene and CH2Cl2 yielded transient absorption spectra showing a distinct peak at 616 nm, which has been assigned to [NMI-Fe2S2(CO)6]−• on the basis of spectroelectrochemical measurements on 1. The 616 nm peak was used to monitor the charge separation (CS) and charge recombination (CR) dynamics of 2, which yielded τCS = 12 ± 1 ps and τCR = 3.0 ± 0.2 ns in toluene and τCS = 24 ± 1 ps and τCR = 57 ± 1 ps in CH2Cl2. Photoexcitation of the disulfide precursor to 2 in both toluene and CH2Cl2 produced only the singlet and triplet excited states of the Zn porphyrin, showing that electron transfer is favorable only when the diiron complex is present. Photoexcitation of 2 in the presence of trifluoroacetic acid was shown to generate H2.
Co-reporter:Annie Butler Ricks ; Gemma C. Solomon ; Michael T. Colvin ; Amy M. Scott ; Kun Chen ; Mark A. Ratner
Journal of the American Chemical Society 2010 Volume 132(Issue 43) pp:15427-15434
Publication Date(Web):October 13, 2010
DOI:10.1021/ja107420a
Photoinitiated charge separation (CS) and recombination (CR) in a series of donor−bridge−acceptor (D−B−A) molecules with cross-conjugated, linearly conjugated, and saturated bridges have been compared and contrasted using time-resolved spectroscopy. The photoexcited charge transfer state of 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ−An) is the donor, and naphthalene-1,8:4,5-bis(dicarboximide) (NI) is the acceptor in all cases, along with 1,1-diphenylethene, trans-stilbene, diphenylmethane, and xanthone bridges. Photoinitiated CS through the cross-conjugated 1,1-diphenylethene bridge is about 30 times slower than through its linearly conjugated trans-stilbene counterpart and is comparable to that observed through the diphenylmethane bridge. This result implies that cross-conjugation strongly decreases the π orbital contribution to the donor−acceptor electronic coupling so that electron transfer most likely uses the bridge σ system as its primary CS pathway. In contrast, the CS rate through the cross-conjugated xanthone bridge is comparable to that observed through the linearly conjugated trans-stilbene bridge. Molecular conductance calculations on these bridges show that cross-conjugation results in quantum interference effects that greatly alter the through-bridge donor−acceptor electronic coupling as a function of charge injection energy. These calculations display trends that agree well with the observed trends in the electron transfer rates.
Co-reporter:Sarah M. Mickley Conron ; Arun K. Thazhathveetil ; Michael R. Wasielewski ; Alexander L. Burin ;Frederick D. Lewis
Journal of the American Chemical Society 2010 Volume 132(Issue 41) pp:14388-14390
Publication Date(Web):September 23, 2010
DOI:10.1021/ja106991f
We report the measurement of distance- and temperature-dependent rate constants for charge separation in capped hairpins in which a stilbene hole acceptor and hole donor are separated by A3Gn diblock polypurine sequences consisting of 3 adenines and 1−19 guanines. The longer diblock systems obey the simplest model for an unbiased random walk, providing a direct measurement of khop = 4.3 × 109 s−1 for a single reversible G-to-G hole hopping step, somewhat faster than the value of 1.2 × 109 s−1 calculated for A-tract hole hopping. The temperature dependence for hopping in A3G13 provides values of Eact = 2.8 kcal/mol and A = 7 × 109 s−1, consistent with a weakly activated, conformationally gated process.
Co-reporter:Hyejin Yoo ; Jaesung Yang ; Andrew Yousef ; Michael R. Wasielewski ;Dongho Kim
Journal of the American Chemical Society 2010 Volume 132(Issue 11) pp:3939-3944
Publication Date(Web):February 25, 2010
DOI:10.1021/ja910724x
π-Stacked perylenediimides (PDIs) have strong electronic communication between the individual molecules and show great promise as organic electronic materials for applications in field effect transistors, photovoltaics, and liquid crystal displays. To gain further insight into the relationship between conformational behaviors and electronic structures of π-stacked PDIs, we have investigated changes in the excimer-like state of cofacial PDI oligomers that result from π-stacking in real time by monitoring the single-molecule fluorescence intensity and lifetime trajectories in a PMMA polymer matrix. The fluorescence intensity and lifetime of π-stacked perylenediimides are sensitive to the degree of π-orbital interactions among PDI units, which is strongly associated with molecular conformations in the polymer matrix. Furthermore, our results can be applied to probe the conformational motions of biomolecules such as proteins.
Co-reporter:Thea M. Wilson ; Takaaki Hori ; Min-Chul Yoon ; Naoki Aratani ; Atsuhiro Osuka ; Dongho Kim
Journal of the American Chemical Society 2010 Volume 132(Issue 4) pp:1383-1388
Publication Date(Web):January 5, 2010
DOI:10.1021/ja908605s
A series of zinc porphyrin arrays comprised of a meso−meso linked porphyrin dimer, a meta-phenylene linked dimer, gable-like tetramers consisting of the meso−meso linked dimers bridged via a meta-phenylene linker, and a dodecameric ring composed of this alternating dimeric pattern were singly oxidized, and intramolecular hole hopping between the porphyrin moieties was probed using electron paramagnetic resonance (EPR) spectroscopy. Electron nuclear double resonance (ENDOR) spectroscopy was also used to probe hole hopping within the dimers. Rapid hole hopping occurs between both porphyrins within both dimers and among three porphyrins of the tetramers with rates >107 s−1 at 290 K. Additionally, the hole hops among 8−12 porphyrins in the dodecameric ring with a rate that is >107 s−1 at 290 K, but hopping is slow at 180 K. These results show that hole hopping is rapid even though the meta-phenyl bridges and direct meso−meso linkages do not provide optimal electronic coupling between the porphyrins within these multiporphyrin arrays. This greatly expands the scope of possible structures that can be employed to tailor the design of long distance charge transport systems.
Co-reporter:Josh Vura-Weis ; Mark A. Ratner
Journal of the American Chemical Society 2010 Volume 132(Issue 6) pp:1738-1739
Publication Date(Web):January 21, 2010
DOI:10.1021/ja907761e
The binding energy and electronic coupling of perylenediimide (PDI) π-stacked dimers were calculated using M06-2X/6-31++G** as a function of stacking geometry. Due to shallow minima in the potential energy surface, electronic coupling can vary by over an order of magnitude among energetically accessible geometries. The coupling was then determined for 20 PDI derivatives with various substitutions at the imide region, and several were identified as the most promising candidates for organic thin film transistors (OTFTs). This strategy of side-by-side comparison of binding energy and electronic coupling may prove useful for other π-stacked OTFTs such as pentacene and poly(thiophene) derivatives.
Co-reporter:Victoria L. Gunderson, Sarah M. Mickley Conron and Michael R. Wasielewski
Chemical Communications 2010 vol. 46(Issue 3) pp:401-403
Publication Date(Web):01 Dec 2009
DOI:10.1039/B921084A
Small- and wide-angle X-ray scattering in solution reveals that chlorophyll (Chl) trefoils self-assemble in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) to yield supramolecular cyclic trimers. Two Chls of each trefoil coordinate to DABCO to form the vertices of the hexagonal structure, while the remaining Chl acts like a substituent on the benzene-like assembly.
Co-reporter:Alexander W. Hains, Charusheela Ramanan, Michael D. Irwin, Jun Liu, Michael R. Wasielewski and Tobin J. Marks
ACS Applied Materials & Interfaces 2010 Volume 2(Issue 1) pp:175
Publication Date(Web):December 7, 2009
DOI:10.1021/am900634a
This contribution describes the design, synthesis, characterization, and organic photovoltaic (OPV) device implementation of a novel interfacial layer (IFL) for insertion between the anode and active layer of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. The IFL precursor, 5,5′-bis[(p-trichlorosilylpropylphenyl)phenylamino]-2,2′-bithiophene (PABTSi2), covalently anchors to the Sn-doped In2O3 (ITO) surface via the −SiCl3 groups and incorporates a bithiophene unit to align the highest occupied molecular orbital (HOMO) energy with that of P3HT (5.0 eV). The synthesis and subsequent electrochemical analysis of PABTSi2 indicates a HOMO energy of 4.9 eV, while the lowest uoccupied molecular orbital level remains sufficiently high, at 2.2 eV, to effectively block electron leakage to the OPV ITO anode. For the P3HT:PCBM OPV fabrication, PABTSi2 is used as a spin-coated cross-linked (via −SiCl3 hydrolysis and condensation) 1:2 blend with poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB). Such devices exhibit an average power conversion efficiency of 3.14%, a fill factor of 62.7%, an open-circuit voltage of 0.54 V, and a short-circuit current of 9.31 mA/cm2, parameters rivaling those of optimized PEDOT:PSS-based devices.Keywords: electron blocking; interface; organic photovoltaics
Co-reporter:Tomoaki Miura ; Amy M. Scott
The Journal of Physical Chemistry C 2010 Volume 114(Issue 48) pp:20370-20379
Publication Date(Web):July 23, 2010
DOI:10.1021/jp103441n
Photoinitiated charge separation and thermal charge recombination (CR) in a covalent donor−bridge−acceptor (D-B-A) system consisting of a perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptor, 2,7-oligofluorene bridge (FLn), and phenothiazine donor (PTZ) (PTZ−FLn−PDI) have been shown to transition from superexchange to charge hopping mechanisms as the D−A distance increases. In work presented here, the spin-selective multiple CR pathways in PTZ−FLn−PDI are studied by a detailed analysis of the magnetic field effect (MFE) on the radical ion pair (RP) lifetime and triplet yield. A kinetic analysis of the MFE gives the spin-selective CR rates and the RP singlet−triplet (S-T) relaxation rates for n = 2−4. When n = 2 and 3, where the S-T splitting (2J) of the RP is large, slow S-T relaxation results in a kinetic bottleneck slowing the observed total CR rate at zero magnetic field. These results show that spin state mixing is an important controlling factor for CR reaction rates in these systems. The CR rate constant for the triplet RP (kCRT) obtained by MFE analysis is about 10 times faster than the corresponding rate for the singlet RP (kCRS) when n = 2−4, indicating that kCRT occurs near the maximum of the Marcus rate vs free energy dependence, whereas kCRS is deep in the inverted region. The distance dependence of both kCRS and kCRT is explained by the crossover from superexchange (n = 1 and 2) to distant independent thermal hopping (n = 3 and 4). A possible mechanism of the S-T relaxation is proposed based on S-T dephasing, which may be induced by fluctuations of 2J resulting from bridge torsional dynamics.
Co-reporter:Joseph E. Bullock, Michael T. Vagnini, Charusheela Ramanan, Dick T. Co, Thea M. Wilson, Jay W. Dicke, Tobin J. Marks and Michael R. Wasielewski
The Journal of Physical Chemistry B 2010 Volume 114(Issue 5) pp:1794-1802
Publication Date(Web):January 14, 2010
DOI:10.1021/jp908679c
Ruthenium-catalyzed C−H bond activation was used to directly attach phenethyl groups derived from styrene to positions ortho to the imide groups in a variety of rylene imides and diimides including naphthalene-1,8-dicarboximide (NMI), naphthalene-1,4:5,8-bis(dicarboximide) (NI), perylene-3,4-dicarboximide (PMI), perylene-3,4:9,10-bis(dicarboximide) (PDI), and terrylene-3,4:11,12-bis(dicarboximide) (TDI). The monoimides were dialkylated, while the diimides were tetraalkylated, with the exception of NI, which could only be dialkylated due to steric hindrance. The absorption, fluorescence, transient absorption spectra, and lowest excited singlet state lifetimes of these chromophores, with the exception of NI, are nearly identical to those of their unsubstituted parent chromophores. The reduction potentials of the dialkylated chromophores are ∼100 mV more negative and oxidation potentials are ∼40 mV less positive than those of the parent compounds, while the corresponding potentials of the tetraalkylated compounds are ∼200 mV more negative and ∼100 mV less positive than those of their parent compounds, respectively. Continuous wave electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) data on the radical anion of PDI reveals spin density on the perylene-core protons as well as on the β-protons of the phenethyl groups. The phenethyl groups enhance the otherwise poor solubility of the bis(dicarboximide) chromophores and only weakly perturb the photophysical and redox properties of the parent molecules, rendering these derivatives and related molecules of significant interest to solar energy conversion.
Co-reporter:Chihiro Maeda;Pyosang Kim;Sung Cho;Jong Kang Park;Jong Min Lim;Dongho Kim Dr.;Josh Vura-Weis;MichaelR. Wasielewski Dr.;Hiroshi Shinokubo Dr.;Atsuhiro Osuka Dr.
Chemistry - A European Journal 2010 Volume 16( Issue 17) pp:
Publication Date(Web):
DOI:10.1002/chem.200903195
Abstract
meso-Triazolyl-appended ZnII–porphyrins were readily prepared by CuI-catalyzed 1,3-dipolar cycloaddition of benzyl azide to meso-ethynylated ZnII–porphyrin (click chemistry). In noncoordinating CHCl3 solvent, spontaneous assembly occurred to form tetrameric array (3)2 from meso–meso-linked diporphyrins 3, and dodecameric porphyrin squares (4)4 and (5)4 from the L-shaped meso–meso-linked triporphyrins 4 and 5. The structures of these assemblies were examined by 1H NMR spectra, absorption spectra, and their gel permeation chromatography (GPC) retention time. Furthermore, the structures of the dodecameric porphyrin squares (4)4 and (5)4 were probed by small- and wide-angle X-ray scattering (SAXS/WAXS) measurements in solution using a synchrotron source. Excitation-energy migration processes in these assemblies were also investigated in detail by using both steady-state and time-resolved spectroscopic methods, which revealed efficient excited-energy transfer (EET) between the meso–meso-linked ZnII–porphyrin units that occurred with time constants of 1.5 ps−1 for (3)2 and 8.8 ps−1 for (5)4.
Co-reporter:Michael T. Colvin, Emilie M. Giacobbe, Boiko Cohen, Tomoaki Miura, Amy M. Scott and Michael R. Wasielewski
The Journal of Physical Chemistry A 2010 Volume 114(Issue 4) pp:1741-1748
Publication Date(Web):January 7, 2010
DOI:10.1021/jp909212c
A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO−PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO−PDI show that enhanced intersystem crossing occurs with τ = 45 ± 1 ps, resulting in formation of TEMPO−3*PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO−1*PDI → TEMPO+•−PDI−• occurs with τ = 1.2 ± 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO−3*PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized 3*PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO−3*PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to 3*PDI having a spin polarization pattern characteristic of overpopulation of its T0 sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of 3*PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.
Co-reporter:Jenny V. Lockard, Annie Butler Ricks, Dick T. Co and Michael R. Wasielewski
The Journal of Physical Chemistry Letters 2010 Volume 1(Issue 1) pp:215-218
Publication Date(Web):November 17, 2009
DOI:10.1021/jz900136a
The nature of the lowest-energy charge-transfer (CT) excited state of the donor−acceptor molecule, 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ−An) is investigated using femtosecond stimulated Raman spectroscopy. Transient Raman spectra are presented with subpicosecond time resolution, and peaks are assigned based on the published Raman modes of the reference molecule phenyl anthracene. The results indicate that the CT excited state is dominated by a fully charge separated radical ion pair state with minimal contribution from the local anthracene π−π* state. Keywords (keywords): electron transfer; femtosecond; radical ions; transient; vibrational spectroscopy;
Co-reporter:Ji-Eun Lee, Jaesung Yang, Victoria L. Gunderson, Michael R. Wasielewski and Dongho Kim
The Journal of Physical Chemistry Letters 2010 Volume 1(Issue 1) pp:284-289
Publication Date(Web):December 1, 2009
DOI:10.1021/jz900193c
We have comparatively investigated the single-molecule photophysical properties of two chlorophyll trefoils that feature distinctive electronic couplings induced by differences in linkage: one is an ethynyl-linked chlorophyll trefoil (1) in which relatively short and rigid linkage between the chromophores promotes effective electronic coupling, and the other is a phenyl−ethynyl-linked chlorophyll trefoil (2) in which the phenyl addition induces an orthogonal geometry impeding π-conjugation and provides a longer interchlorophyll distance reducing through-space interaction. By recording single-molecule fluorescence intensity trajectories and their corresponding lifetimes, we observed one-step photobleaching behaviors, less frequent on−off behaviors, a narrower fluorescence lifetime distribution, and higher photostability in 1 as compared with 2. These results indicate that the performance of molecular photosynthetic systems in the solid state is strongly associated with electronic couplings and, thus, give insight into the construction of well-functioning artificial photosynthetic systems.Keywords (keywords): artificial photosynthetic system; chlorophyll; electronic coupling; fluorescence spectroscopy; single-molecule study;
Co-reporter:Qixi Mi, Mark A. Ratner and Michael R. Wasielewski
The Journal of Physical Chemistry C 2010 Volume 114(Issue 32) pp:13853-13860
Publication Date(Web):July 27, 2010
DOI:10.1021/jp103678m
Thanks to the squared Cartesian coordinates and its corresponding ternary diagram, several types of anisotropic physical quantities are expressed by a weighted mean of their principal values. Specifically, in terms of the electron paramagnetic resonance (EPR) spectra of triplet states under various conditions, the anisotropies in the resonance field (due to spin dipole−dipole interaction), in the spin polarization (due to spin−orbit intersystem crossing, SO-ISC), and in the distribution of molecular orientations (due to liquid crystal alignment) are all linearized. The spectral intensity becomes a path integral on the ternary diagram along the field isolines. These major simplifications afford, for the first time, analytical line shape formulas of arbitrarily polarized triplets as sums of elliptical integrals. Even with approximations applied, the analytical results agree almost perfectly with both simulated and experimental spectra and accurately capture the higher-order spectral effects such as peak shifts and net spin polarization. This universal scheme is also promising for other spectroscopic techniques in which anisotropy plays a significant role.
Co-reporter:AmyM. Scott Dr.;Annie ButlerRicks;MichaelT. Colvin ;MichaelR. Wasielewski
Angewandte Chemie 2010 Volume 122( Issue 16) pp:2966-2970
Publication Date(Web):
DOI:10.1002/ange.201000171
Co-reporter:TheaM. Wilson Dr.;TarekA. Zeidan Dr.;Mahesh Hariharan ;FrederickD. Lewis ;MichaelR. Wasielewski
Angewandte Chemie International Edition 2010 Volume 49( Issue 13) pp:2385-2388
Publication Date(Web):
DOI:10.1002/anie.200907339
Co-reporter:AmyM. Scott Dr.;Annie ButlerRicks;MichaelT. Colvin ;MichaelR. Wasielewski
Angewandte Chemie International Edition 2010 Volume 49( Issue 16) pp:2904-2908
Publication Date(Web):
DOI:10.1002/anie.201000171
Co-reporter:Qixi Mi, Mark A. Ratner and Michael R. Wasielewski
The Journal of Physical Chemistry A 2010 Volume 114(Issue 1) pp:162-171
Publication Date(Web):December 8, 2009
DOI:10.1021/jp907476q
This paper expands the established four-state model of spin-correlated radical pairs (SCRPs) to include local nuclear spins which are ubiquitous in real-world systems and essential for the radical pair intersystem crossing (RP-ISC) mechanism. These nuclei are coupled to the unpaired electron spins by hyperfine interaction and split their electron paramagnetic resonance (EPR) lines. Rather than enumerating all possible nuclear states, an algorithm is devised to sort out the net hyperfine offset 2Q, which, along with the electron spin−spin coupling 2J, characterizes the behavior of SCRPs. Using this algorithm, the EPR spectra of SCRPs coupled to arbitrary nuclear spins can be efficiently simulated with only 2J and the EPR spectra of individual radicals as the inputs. Particularly illustrative is the case of a SCRP resulting from photoinduced electron transfer comprised of a spectrally narrow anion radical signal having small hyperfine splittings and a broad cation radical signal having many large hyperfine splittings and a Gaussian width σ, where the EPR peak of the anion radical exhibits an effective splitting of 21/2J2/σ. For SCRPs having singlet and triplet pathways for charge recombination, their kinetic behavior is obtained concisely by considering the decay rate constants kS and kT as imaginary energies, while adhering to the existing derivation of the four-state model. These models are employed to interpret the diverse array of spectral and kinetic modulation patterns observed in the experimental EPR spectra of photogenerated SCRPs and to extract the 2J value, which reflects the donor−acceptor electronic coupling. During the first several hundred nanoseconds following photoexcitation, the spectral and time domain characteristics of the measured time-resolved EPR spectra manifest the consequences of the Uncertainty Principle, and the modulation patterns in either domain result from hyperfine splittings between the unpaired electron and the nuclear spins.
Co-reporter:Tomoaki Miura, Raanan Carmieli and Michael R. Wasielewski
The Journal of Physical Chemistry A 2010 Volume 114(Issue 18) pp:5769-5778
Publication Date(Web):April 14, 2010
DOI:10.1021/jp101523n
Spin-selective charge recombination of photogenerated radical ion pairs within a series of donor−bridge−acceptor (D−B−A) molecules, where D = phenothiazine (PTZ), B = oligo(2,7-fluorenyl), and A = perylene-3,4:9,10-bis(dicarboximide) (PDI), PTZ−FLn−PDI, where n = 1−4 (compounds 1−4), is studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy in which the microwave source is either continuous-wave or pulsed. Radical ion pair TREPR spectra are observed for 3 and 4 at 90−294 K, while the neutral triplet state of PDI (3*PDI) is observed at 90−294 K for 2−4 and at 90 K for 1. 3*PDI is produced by three mechanisms, as elucidated by its zero-field splitting parameters and spin polarization pattern. The mechanisms are spin−orbit-induced intersystem crossing (SO-ISC) in PDI aggregates, direct spin−orbit charge-transfer intersystem crossing (SOCT) from the singlet radical pair within 1, and radical pair intersystem crossing (RP-ISC) as a result of S−T0 mixing of the radical ion pair states in 2−4. The temperature dependence of the spin−spin exchange interaction (2J) shows a dramatic decrease at low temperatures, indicating that the electronic coupling between the radical ions decreases due to an increase in the average fluorene−fluorene dihedral angle at low temperatures. The charge recombination rates for 3 and 4 decrease at low temperature, but that for 2 is almost temperature-independent. These results strongly suggest that the dominant mechanism of charge recombination for n ≥ 3 is incoherent thermal hopping, which results in wire-like charge transfer.
Co-reporter:Michael R. Wasielewski
Accounts of Chemical Research 2009 Volume 42(Issue 12) pp:1910
Publication Date(Web):October 5, 2009
DOI:10.1021/ar9001735
In natural photosynthesis, organisms optimize solar energy conversion through organized assemblies of photofunctional chromophores and catalysts within proteins that provide specifically tailored environments for chemical reactions. As with their natural counterparts, artificial photosynthetic systems for practical solar fuels production must collect light energy, separate charge, and transport charge to catalytic sites where multielectron redox processes will occur. While encouraging progress has been made on each aspect of this complex problem, researchers have not yet developed self-ordering and self-assembling components and the tailored environments necessary to realize a fully-functional artificial system. Previously researchers have used complex, covalent molecular systems comprised of chromophores, electron donors, and electron acceptors to mimic both the light-harvesting and the charge separation functions of photosynthetic proteins. These systems allow for study of the dependencies of electron transfer rate constants on donor−acceptor distance and orientation, electronic interaction, and the free energy of the reaction. The most useful and informative systems are those in which structural constraints control both the distance and the orientation between the electron donors and acceptors. Self-assembly provides a facile means for organizing large numbers of molecules into supramolecular structures that can bridge length scales from nanometers to macroscopic dimensions. The resulting structures must provide pathways for migration of light excitation energy among antenna chromophores, and from antennas to reaction centers. They also must incorporate charge conduits, that is, molecular “wires” that can efficiently move electrons and holes between reaction centers and catalytic sites. The central scientific challenge is to develop small, functional building blocks with a minimum number of covalent linkages, which also have the appropriate molecular recognition properties to facilitate self-assembly of complete, functional artificial photosynthetic systems. In this Account, we explore how self-assembly strategies involving π-stacking can be used to integrate light harvesting with charge separation and transport. Our current strategy uses covalent building blocks based on chemically robust arylene imide and diimide dyes, biomimetic porphyrins, and chlorophylls. We take advantage of the shapes, sizes, and intermolecular interactions—such as π−π and/or metal−ligand interactions—of these molecules to direct the formation of supramolecular structures having enhanced energy capture and charge-transport properties. We use small- and wide-angle X-ray scattering (SAXS/WAXS) from a synchrotron source to elucidate the solution phase structures of these monodisperse noncovalent aggregates. We expect that a greater understanding of self-assembly using π-stacking and molecular designs that combine those features with hydrogen bonding and metal-ligand bonding could simplify the structure of the building blocks for artificial photosynthetic complexes, while retaining their ability to assemble complex, photofunctional structures.
Co-reporter:Raanan Carmieli ; Qixi Mi ; Annie Butler Ricks ; Emilie M. Giacobbe ; Sarah M. Mickley
Journal of the American Chemical Society 2009 Volume 131(Issue 24) pp:8372-8373
Publication Date(Web):May 28, 2009
DOI:10.1021/ja902864h
The distance over which two photogenerated charges are separated in electron donor−acceptor systems for artificial photosynthesis depends on the structure of the system, while the lifetime of the charge separation and, ultimately, its ability to carry out useful redox chemistry depend on the electronic coupling between the oxidized donor and reduced acceptor. The radical ions produced by charge separation are frequently delocalized over the π systems of the final oxidized donor and reduced acceptor, so that there is often significant uncertainty as to the average distance between the separated charges, especially in low dielectric constant media, where the Coulomb attraction of the ions may be significant and the charge distribution of the ions may be distorted, so that the average distance between them may be shorter than that implied by their chemical structures. The charge separation distances between photogenerated radical ions in three donor−acceptor molecules having different donor−acceptor distances were measured directly from their dipolar spin−spin interactions using out-of-phase electron spin echo envelope modulation (OOP-ESEEM). The measured distances in toluene at 85 K compare favorably to the calculated distances between the centroids of the spin distributions of the radical ions within the radical ion pairs. These results show that despite the intrinsically nonpolar nature of medium, the spin (and charge) distributions of the RPs are not significantly distorted by Coulomb attraction over these long distances. This study shows that OOP-ESEEM is well-suited for probing the detailed structural features of charge-separated intermediates that are essential to understanding how to design molecular structures that prolong and control charge separation for artificial photosynthesis.
Co-reporter:Thea M. Wilson ; Michael J. Tauber
Journal of the American Chemical Society 2009 Volume 131(Issue 25) pp:8952-8957
Publication Date(Web):June 9, 2009
DOI:10.1021/ja902258g
A series of linearly linked perylenediimide (PDI) dimers and trimers were synthesized in which the PDI π systems are nearly orthogonal. These oligomers and several model compounds were singly reduced, and intramolecular electron hopping between the PDI molecules was probed by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy. When the functional groups attached to the ends of the oligomers were chosen to make each PDI molecule electronically equivalent, the single electron hops between the PDI molecules with rates that significantly exceed 107 s−1. Rapid electron hopping between pairs of PDI molecules having orthogonal π systems is unexpected and may expand the possible design motifs for organic electronic materials based on PDI.
Co-reporter:Josh Vura-Weis ; Michael R. Wasielewski ; Arun K. Thazhathveetil ;Frederick D. Lewis
Journal of the American Chemical Society 2009 Volume 131(Issue 28) pp:9722-9727
Publication Date(Web):June 26, 2009
DOI:10.1021/ja9015217
The realization of highly efficient photoinduced charge separation across the π-stacked base pairs in duplex DNA remains elusive. The low efficiencies (<5%) typically observed for charge separation over a dozen or more base pairs are a consequence of slow charge transport and rapid charge recombination. We report here a significant (5-fold or greater) enhancement in the efficiency of charge separation in diblock purine oligomers consisting of two or three adenines followed by several guanines, when compared to oligomers consisting of a single purine or alternating base sequences. This approach to wire-like behavior is attributed to both slower charge recombination and faster charge transport once the charge reaches the G-block in these diblock systems.
Co-reporter:Frederick D. Lewis ; Arun K. Thazhathveetil ; Tarek A. Zeidan ; Josh Vura-Weis
Journal of the American Chemical Society 2009 Volume 132(Issue 2) pp:444-445
Publication Date(Web):December 22, 2009
DOI:10.1021/ja908470d
The efficiency of singlet and triplet charge radical ion-pair formation and the dynamics of radical-pair charge recombination in DNA−anthraquinone conjugates have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. Singlet charge separation is more efficient than intersystem crossing, resulting in inefficient formation of the long-lived triplet radical ion pair. Both singlet charge separation and charge recombination are faster when guanine rather than adenine is the neighboring purine base.
Co-reporter:Raanan Carmieli, Tarek A. Zeidan, Richard F. Kelley, Qixi Mi, Frederick D. Lewis and Michael R. Wasielewski
The Journal of Physical Chemistry A 2009 Volume 113(Issue 16) pp:4691-4700
Publication Date(Web):February 24, 2009
DOI:10.1021/jp900230q
A series of short DNA hairpins (nG) using perylene-3,4:9,10-bis(dicarboximide) (PDI) as the hairpin linker was synthesized in which the distance between the PDI and a guanine−cytosine (G-C) base pair is systematically varied by changing the number (n − 1) of adenine−thymine (A-T) base pairs between them. Due to the relatively large hydrophobic surface of PDI, the nG hairpins dimerize in buffer solutions. The photophysics and photochemistry of these hairpins were investigated using femtosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Photoexcitation of the self-assembled PDI dimer within each nG hairpin results in subpicosecond formation of its lower exciton state (1*PDI2) followed by formation of an excimer-like state (1*XPDI2) with τ = 10−28 ps. Both of these states are lower in energy than 1*PDI, so that neither can oxidize A, C, and T. Electron transfer from G to 1*PDI2 is faster than formation of 1*XPDI2 only for 1G. Electron transfer from G to 1*XPDI2 for 2G−8G, occurs by the superexchange mechanism and, thus, becomes exponentially less efficient as the G-PDI2 distance increases. Nevertheless, TREPR studies show that photoexcitation of 2G and 4G produce spin-correlated radical ion pairs having electron spin polarization patterns indicating that a low yield of charge separation proceeds from 1*XPDI2 by the radical pair intersystem crossing (RP-ISC) mechanism to initially yield a singlet radical ion pair. The strong spin-polarization of the radical ion pairs makes it possible to observe them, even though their concentration is low. As expected, the hairpin lacking G (0G) and that having the longest G-PDI2 distance (8G) display no TREPR radical ion pair signals. Hairpins 0G, 2G, 4G, and 8G all exhibit triplet EPR spectra at 85 K. Simulations of the spectra show that 3*PDI is produced mainly by a spin−orbit-induced intersystem crossing mechanism, while the spectra of 2G and 4G have 5% and 21% contributions, respectively, from 3*PDI produced by charge recombination of radical ion pairs that originate from RP-ISC. These low percentages of RP-ISC derived 3*PDI result mainly from the low yield of radical ion pairs in 2G and 4G.
Co-reporter:Kil Suk Kim, Zin Seok Yoon, Annie Butler Ricks, Jae-Yoon Shin, Shigeki Mori, Jeyaraman Sankar, Shohei Saito, Young Mee Jung, Michael R. Wasielewski, Atsuhiro Osuka and Dongho Kim
The Journal of Physical Chemistry A 2009 Volume 113(Issue 16) pp:4498-4506
Publication Date(Web):March 5, 2009
DOI:10.1021/jp8111205
At room temperature, meso-hexaaryl-substituted [28]hexaphyrins(1.1.1.1.1.1) in solution exist largely as an equilibrium between planar antiaromatic and distorted Möbius aromatic conformers. As the temperature decreases, the molecular structure changes into the distorted Möbius topology that commonly occurs in [28]hexaphyrins, which gives rise to longer excited singlet and triplet state lifetimes than planar antiaromatic [28]hexaphyrins. Temperature-dependent two-photon absorption measurements of [28]hexaphyrin indicate that the degree of aromaticity of Möbius [28]hexaphyrin is large, comparable to that of Hückel aromatic planar [26]hexaphyrin. Through our spectroscopic investigations, we have demonstrated that a subtle balance between the strains induced by the size of the [28]hexaphyrin macrocyclic ring and the energy stabilization contributed by π-electron delocalization in the formation of distorted Möbius [28]hexaphyrin leads to the molecular structure change into the Möbius topology as the temperature decreases.
Co-reporter:Victoria L. Gunderson, Thea M. Wilson and Michael R. Wasielewski
The Journal of Physical Chemistry C 2009 Volume 113(Issue 27) pp:11936-11942
Publication Date(Web):May 19, 2009
DOI:10.1021/jp902515p
We report the observation of multiple excitation energy transfer (EET) pathways in a covalently linked, chlorophyll (Chl) tetramer having three different fixed Chl−Chl distances. The tetramer was synthesized by covalently attaching 20-(4-ethynylphenyl)Chl to the 1,3,6,8-positions of pyrene to give Chl4-py. Reference Chl oligomers were prepared by attaching 20-(4-ethynylphenyl)Chl to the 1-position of pyrene (Chl-py), the 1,8 and 3,6 positions of pyrene (para-Chl2-py and ortho-Chl2-py, respectively), and the 1,3,5-positions of benzene (meta-Chl3). The Chlx-py derivatives were studied using femtosecond transient absorption (TA) and transient absorption anisotropy (TAA) spectroscopy, and compared with data obtained earlier on meta-Chl3. Using femtosecond TA, the decay of 1*Chl4-py was monitored after photoexcitation of its Qy band with a 655 nm, 130 fs laser pulse. A triexponential decay of 1*Chl4-py (τ = 7 ps, 152 ps, and 4.2 ns) was observed, with the two shorter time constants being laser intensity dependent. This dependency is indicative of singlet−singlet annihilation and is attributed to EET via two pathways, those involving the two closest Chl’s (ortho/meta), which are kinetically indistinguishable, and the two most distant (para) Chl’s. Further confirmation of multiple EET pathways was made by femtosecond TAA measurements. Förster energy transfer lifetimes were calculated for the three possible energy transfer pathways (ortho, 20 ps; meta, 34 ps; and para, 150 ps) and compared to the experimental results. Our results indicate that EET between nonadjacent chromophores having high oscillator strengths, such as the para Chl’s in Chl4-py, is significant, and the additional utilization of the minor EET contributing pathways may provide greater avenues for designing efficient light harvesting in future artificial photosynthetic systems.
Co-reporter:Zesheng An, Susan A. Odom, Richard F. Kelley, Chun Huang, Xuan Zhang, Stephen Barlow, Lazaro A. Padilha, Jie Fu, Scott Webster, David J. Hagan, Eric W. Van Stryland, Michael R. Wasielewski and Seth R. Marder
The Journal of Physical Chemistry A 2009 Volume 113(Issue 19) pp:5585-5593
Publication Date(Web):April 21, 2009
DOI:10.1021/jp900152r
A series of 1,7-bis(arylethynyl)-N,N′-bis[2,6-diisopropylphenyl]perylene-3,4:9,10-bis(dicarboximide)s has been obtained from Sonogashira coupling of the 1,7-dibromoperylene-3,4:9,10-bis(dicarboximide) with p-substituted phenylacetylenes in which the p-substituents include π-donors (dialkylamino, diarylamino, p-(diarylamino)phenyl, alkoxy) and π-acceptors (diarylboryl, p-(diarylboryl)phenyl). The bis(arylethynyl)-substituted chromophores all show two reversible molecular reductions and are all slightly more readily reduced than unsubstituted perylene-3,4:9,10-bis(dicarboximide)s with the electrochemical potentials being rather insensitive to the π-donor or acceptor nature of the aryl group. The amine derivatives also show reversible molecular oxidations. The UV−vis spectra of the chromophores with alkoxy and boryl substituents show red-shifted absorptions relative to unsubstituted perylene diimides with discernible vibronic structure. In contrast, the lowest energy absorptions of the amino derivatives are broad and structureless, suggesting donor-to-acceptor charge-transfer character. Transient absorption spectra for the amine derivatives were interpreted in terms of ultrafast charge separation, followed by charge recombination on a time scale of ca. 80−2000 ps. Two compounds were also synthesized in which an additional stronger, but more weakly coupled, donor group is linked by a nonconjugated bridge to the p-amine donor, to investigate the effect on the charge recombination lifetimes; however, the lifetimes of the charge-separated states, ca. 150 and 1000 ps, were within the range observed for the simple amine systems. Finally, the two-photon absorption properties of three bis(arylethynyl)-substituted derivatives were investigated, along with those of 1,7-di(pyrrolidin-1-yl)-N,N′-bis[2,6-diisopropylphenyl]perylene-3,4:9,10-bis(dicarboximide). As with other perylene-3,4:9,10-bis(dicarboximide)s and related species, strong two-photon absorption (>1000 GM) was observed for three of these species close to the one-photon absorption edge; however, an additional feature (100−500 GM) was also observed at longer wavelength. An example with (p-aminophenyl)ethylnyl substituents showed a qualitatively different two-photon spectrum with a cross-section >500 GM being observed over a broad wavelength range.
Co-reporter:Brooks A. Jones;Antonio Facchetti;Tobin J. Marks
Advanced Functional Materials 2008 Volume 18( Issue 8) pp:1329-1339
Publication Date(Web):
DOI:10.1002/adfm.200701045
Abstract
A series of eight perylene diimide (PDI)- and naphthalene diimide (NDI)-based organic semiconductors was used to fabricate organic field-effect transistors (OFETs) on bare SiO2 substrates, with the substrate temperature during film deposition (Td) varied from 70–130 °C. For the N,N′-n-octyl materials that form highly ordered films, the mobility (µ) and current on-off ratio (Ion/Ioff) increase slightly from 70 to 90 °C, and remain relatively constant between 90 and 130 °C. Ion/Ioff and µ of dibromo-PDI-based OFETs decrease with increasing Td, while films of N,N′-1H,1H-perfluorobutyl dicyanoperylenediimide (PDI-FCN2) exhibit dramatic Ion/Ioff and µ enhancements with increasing Td. Increased OFET mobility can be correlated with higher levels of molecular ordering and minimization of film morphology surface irregularities. Additionally, the effects of SiO2 surface modification with trimethylsilyl and octadecyltrichlorosilyl monolayers, as well as with polystyrene, are investigated for N,N′-n-octyl dicyanoperylenediimide (PDI-8CN2) and PDI-FCN2 films deposited at Td = 130 °C. The SiO2 surface treatments have modest effects on PDI-8CN2 OFET mobilities, but modulate the mobility and morphology of PDI-FCN2 films substantially. Most importantly, the surface treatments result in substantially increased Vth and decreased Ioff values for the dicyanoperylenediimide films relative to those grown on SiO2, resulting in Vth > 0.0 V and Ion/Ioff ratios as high as 108. Enhancements in current modulation for these high-mobility, air-stable, and solution-processable n-type semiconductors, should prove useful in noise-margin enhancement and further improvements in organic electronics.
Co-reporter:Randall H. Goldsmith, Orlando DeLeon, Thea M. Wilson, Daniel Finkelstein-Shapiro, Mark A. Ratner and Michael R. Wasielewski
The Journal of Physical Chemistry A 2008 Volume 112(Issue 19) pp:4410-4414
Publication Date(Web):April 18, 2008
DOI:10.1021/jp801084v
The temperature dependence of intramolecular charge separation in a series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene FLn (n = 1−4) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FLn bridge. In toluene, the temperature dependence of the charge separation rate constants for PTZ-FLn-PDI, (n = 1−4) is relatively weak and is successfully described by the semiclassical Marcus equation. The activation energies for charge separation suggest that bridge charge carrier injection is not the rate limiting step. The difficulty of using temperature and length dependence to differentiate hopping and superexchange is discussed, with difficulties in the latter topic explored via an extension of a kinetic model proposed by Bixon and Jortner.
Co-reporter:FrederickD. Lewis ;Pierre Daublain;Boiko Cohen;Josh Vura-Weis ;MichaelR. Wasielewski
Angewandte Chemie International Edition 2008 Volume 47( Issue 20) pp:3798-3800
Publication Date(Web):
DOI:10.1002/anie.200705903
Co-reporter:Richard F. Kelley, Michael J. Tauber, Thea M. Wilson and Michael R. Wasielewski
Chemical Communications 2007 (Issue 42) pp:4407-4409
Publication Date(Web):14 Aug 2007
DOI:10.1039/B708318A
A series of linkers constructed from combinations of phenyl and ethynyl groups is shown to permit ultrafast energy transfer between two chlorophylls, while allowing control over radical cation migration between them.
Co-reporter:Michael J. Ahrens, Richard F. Kelley, Zachary E. X. Dance and Michael R. Wasielewski
Physical Chemistry Chemical Physics 2007 vol. 9(Issue 12) pp:1469-1478
Publication Date(Web):22 Feb 2007
DOI:10.1039/B617876F
A bichromophoric electron donor–acceptor molecule composed of a zinc tetraphenylporphyrin (ZnTPP) surrounded by four perylene-3,4:9,10-bis(dicarboximide)
(PDI) chromophores (ZnTPP-PDI4) was synthesized. The properties of this molecule were compared to a reference molecule having ZnTPP covalently bound to a single PDI (ZnTPP-PDI). In toluene, ZnTPP-PDI4 self-assembles into monodisperse aggregates of five molecules arranged in a columnar stack, (ZnTPP-PDI4)5. The monodisperse nature of this assembly contrasts sharply with previously reported ZnTPP-PDI4 derivatives having 1,7-bis(3,5-di-t-butylphenoxy) groups (ZnTPP-PPDI4). The size and structure of this assembly in solution was determined by small angle X-ray scattering (SAXS) using a high flux synchrotron X-ray source. The ZnTPP-PDI reference molecule does not aggregate. Femtosecond transient absorption spectroscopy shows that laser excitation of both ZnTPP-PDI and (ZnTPP-PDI4)5 results in quantitative formation of ZnTPP+˙-PDI−˙ radical ion pairs in a few picoseconds. The transient absorption spectra of (ZnTPP-PDI4)5 suggest that the PDI−˙ radicals interact strongly with adjacent PDI molecules within the columnar stack. Charge recombination occurs more slowly within (ZnTPP-PDI4)5
(τ
= 4.8 ns) than it does in ZnTPP-PDI (τ
= 3.0 ns) producing mostly ground state as well as a modest yield of the lowest triplet state of PDI (3*PDI). Formation of 3*PDI occurs by rapid spin–orbit induced intersystem crossing (SO-ISC) directly from the singlet radical ion pair as evidenced by the electron spin polarization pattern exhibited by its time-resolved electron paramagnetic resonance spectrum.
Co-reporter:Richard F. Kelley;Michael J. Tauber Dr.
Angewandte Chemie 2006 Volume 118(Issue 47) pp:
Publication Date(Web):3 NOV 2006
DOI:10.1002/ange.200603046
Gleichmäßig verteilt: Chlorophyll-Einheiten wurden an starre Linker mit dreifacher Symmetrie angebracht, deren Struktur die Effizienz von Energie- und Ladungstransfer zwischen den Chlorophyll-Einheiten bestimmt. Direkte Ethinyl-Verknüpfungen erleichtern die Verteilung von Ladung über alle drei Chlorophyll-Einheiten auf der Zeitskala von EPR- und ENDOR-Spektroskopie (107 Hz).
Co-reporter:Frederick D. Lewis ;Huihe Zhu;Pierre Daublain;Boiko Cohen
Angewandte Chemie 2006 Volume 118(Issue 47) pp:
Publication Date(Web):30 OCT 2006
DOI:10.1002/ange.200603455
Gute Übereinstimmung: Die Kinetik des photoinduzierten Lochtransports über DNA-A-Bereiche mit 1–7 Basenpaaren wurde durch transiente Femtosekundenabsorptionsspektroskopie bestimmt. Die Werte sind bei bis zu vier Basenpaaren stark entfernungsabhängig, bei größeren Abständen dann aber kaum noch. Diese Kinetik-Ergebnisse (□, ▪) stimmen sehr gut mit den DNA-Strangbruchergebnissen von Giese et al. (○, •) überein.
Co-reporter:Richard F. Kelley;Michael J. Tauber Dr.
Angewandte Chemie International Edition 2006 Volume 45(Issue 47) pp:
Publication Date(Web):3 NOV 2006
DOI:10.1002/anie.200603046
Share and share alike: Chlorophylls are appended to rigid linkers with threefold symmetry. The efficiency of both energy and charge transfer between the chlorophylls is controlled by the structure of the central linker. Direct chlorophyll–ethynyl linkage facilitates charge sharing between all three chlorophylls on the EPR/ENDOR timescale (107 Hz). ENDOR=electron nuclear double resonance.
Co-reporter:Frederick D. Lewis ;Huihe Zhu;Pierre Daublain;Boiko Cohen
Angewandte Chemie International Edition 2006 Volume 45(Issue 47) pp:
Publication Date(Web):30 OCT 2006
DOI:10.1002/anie.200603455
Whole scheme of things: The kinetics and efficiency of photoinduced hole transport across DNA A tracts with 1–7 base pairs have been determined from femtosecond transient absorption spectroscopic data. These values are strongly distance dependent over the first four base pairs, but are relatively insensitive to distance at longer distances. These kinetic results (□, ▪) parallel the DNA-strand-cleavage results of Giese et al. (○, •).
Co-reporter:Yi-Lin Wu; Kristen E. Brown; Daniel M. Gardner; Scott M. Dyar
Journal of the American Chemical Society () pp:
Publication Date(Web):March 4, 2015
DOI:10.1021/jacs.5b00977
We have prepared a G-quadruplex (GQ-1) that incorporates an 8-(4′-aminophenylethynyl)guanine (GEAn) electron donor covalently attached to a 4-aminonaphthalene-1,8-imide (ANI) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) electron acceptor (GEAn-ANI-NDI, 1). In the presence of KPF6 in tetrahydrofuran (THF), 1 self-assembles into a monodisperse, C4-symmetric GQ-1 with small spatial intraquadruplex overlap between the ANI-NDI units. Photoexcitation of monomeric 1 induces the two-step charge transfer GEAn-1*ANI-NDI → GEAn+•-ANI–•-NDI → GEAn+•-ANI-NDI–• that occurs in τCS1 = 5 ps and τCS2 = 330 ps, respectively, while charge recombination in ca. 300 ns. Sharpening of the GEAn+• transient absorption and a shift of the ethynyl vibrational frequency in 1 were observed, concomitant with the stepwise electron transfer from ANI–• to NDI. Formation of GQ-1 from 1 in THF increases the secondary charge-shifting rate (τCS2 = 110 ps) and results in no change in ethynyl vibrational frequency. Charge recombination in GQ-1 is slowed by enhanced radical-pair intersystem crossing driven by the greater number of hyperfine couplings in the assembly. Moreover, time-resolved EPR spectroscopy shows that the spin–spin-exchange interaction (J) between the radicals of GEAn+•-ANI-NDI–• within GQ-1 is smaller than that of 1, suggesting that the spin (charge) density in GEAn+• is more dispersed in GQ-1. The spectroscopic results are consistent with hole sharing among the guanines within the G-quadruplex that is kinetically competitive with the formation of GEAn+•. This suggests that G-quadruplexes can serve as effective hole conduits in ordered donor–acceptor assemblies.
Co-reporter:Eric A. Margulies, Leah E. Shoer, Samuel W. Eaton and Michael R. Wasielewski
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 43) pp:NaN23742-23742
Publication Date(Web):2014/09/17
DOI:10.1039/C4CP03107E
Excitation energy transfer in perylene-3,4:9,10-bis(dicarboximide) (PDI) aggregates is of interest for light-harvesting applications of this strongly absorbing and π–π stacking chromophore. Here we report the synthesis and characterization of two PDI dimers in which the chromophores are covalently linked by a redox-inactive triptycene bridge in orientations that are cofacial (1) and slip-stacked along their N–N axes (2). Femtosecond transient absorption experiments on 1 and 2 reveal rapid exciton delocalization resulting excimer formation. Cofacial π–π stacked dimer 1 forms a low-energy excimer state absorption (λmax = 1666 nm) in τ = ∼2 ps after photoexcitation. Inserting a phenyl spacer on the bridge to generate a slip-stacked PDI–PDI geometry in 2 results in a less stable excimer state (λmax = 1430 nm), which forms in τ = ∼12 ps due to decreased electronic coupling. The near-infrared (NIR) excimer absorption of cofacial dimer 1 is ∼120 meV lower in energy than that of slip-stacked dimer 2, further highlighting electronic differences between these states.
Co-reporter:Hyejin Yoo, Hee Won Bahng, Michael R. Wasielewski and Dongho Kim
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 6) pp:NaN2007-2007
Publication Date(Web):2012/01/06
DOI:10.1039/C2CP22377E
The conformation of embedded molecule in a polymer matrix is sensitive to the local nano-environment that the molecule experiences. Particularly, single molecule spectroscopic methods have been utilized to visualize each molecular conformation in local sites of the polymer matrix by monitoring rotational diffusion and fluctuating fluorescence of the molecule. Here, we have performed single molecule spectroscopic experiments on a π-stacked perylenediimide (PDI) dimer and trimer, in which enhanced π–π interaction in π-stacked PDIs makes the fluorescence lifetime longer, embedded in two different polymers, namely poly(methyl methacrylate) (PMMA) and poly(butyl methacrylate) (PBMA), to reveal the conformational change depending on the polymer matrix. The fluorescence lifetimes of π-stacked PDIs are influenced by polymer surroundings because their molecular conformations are dependent on their interactions with the local environment in the polymer matrix. Furthermore, from an in-depth analysis of autocorrelation functions of fluorescence intensity trajectories, we could assign that the first autocorrelation value (lag 1) is larger as the intensity trace becomes more fluctuating. Thus, we expect that π-stacked PDIs, a model system for the formation of PDI excimer-like states, can be utilized to probe the surrounding nano-environment by monitoring the conformational change in real time.
Co-reporter:Shinji Ando, Charusheela Ramanan, Antonio Facchetti, Michael R. Wasielewski and Tobin J. Marks
Journal of Materials Chemistry A 2011 - vol. 21(Issue 47) pp:NaN19057-19057
Publication Date(Web):2011/10/31
DOI:10.1039/C1JM13397G
A series of donor–acceptor molecules consisting of core-brominated and -cyanated perylene-3,4:9,10-bis(dicarboximide) (PDI) structures covalently linked to two terminal pendant alkylanthracenes (A) is described. These hybrid molecules, having varying alkyl tether lengths as well as PDI electron affinities, were synthesized by condensation of a 1,7-dibromoperylene tetracarboxylic acid anhydride with the appropriate aminoalkylanthracene, followed by cyanation with CuCN. Thermal, optical, and electrochemical properties were characterized. PDI moiety photoexcitation results in pendant anthracene oxidation, generating 1(A+˙-PDI−˙-A) species. The solution dynamics of this one-electron charge separation were characterized by ultrafast transient absorption spectroscopy, and charge separation rates are found to vary with alkyl tether length. Trends in these rates are attributed to solution phase geometric variations of the PDI-A structure, reflecting the flexibility of the spacer.
Co-reporter:Matthew D. Krzyaniak, Lukáš Kobr, Brandon K. Rugg, Brian T. Phelan, Eric A. Margulies, Jordan N. Nelson, Ryan M. Young and Michael R. Wasielewski
Journal of Materials Chemistry A 2015 - vol. 3(Issue 30) pp:NaN7967-7967
Publication Date(Web):2015/06/30
DOI:10.1039/C5TC01446H
Selective photoexcitation of the donor in an electron donor–acceptor1–acceptor2 (D–A1–A2) molecule, in which D = perylene and both A1 and A2 = naphthalene-1,8:4,5-bis(dicarboximide), results in sub-nanosecond formation of a spin-correlated singlet radical pair 1(D+˙–A1−˙–A2) having a large electron spin–spin exchange interaction, 2J, which precludes its observation by transient EPR spectroscopy. Subsequent selective photoexcitation of A1−˙ rapidly produces 1(D+˙–A1–A2−˙), resulting in a dramatic decrease in 2J, which allows coherent spin evolution to mix the singlet (S) radical pair state 1(D+˙–A1–A2−˙) with the T0 triplet sublevel of 3(D+˙–A1–A2−˙) in an applied magnetic field, where B ≫ 2J. A spin-polarized transient EPR spectrum characteristic of the spin-correlated radical pair D+˙–A1–A2−˙ is then observed. The time delay between the two laser pulses was incremented to measure the rate of decoherence in 1(D+˙–A1−˙–A2) in toluene at 295 K, which was found to be 8.1 × 107 s−1. Deuteration of the perylene donor or the toluene solvent decreases the decoherence rate constant of 1(D+˙–A1−˙–A2) to 4.3 × 107 s−1 and 4.6 × 107 s−1, respectively, while deuteration of both the perylene donor and the toluene solvent reduced the decoherence rate constant by more than half to 3.4 × 107 s−1. The data show that decreasing electron-nuclear hyperfine interactions significantly increases the zero quantum coherence lifetime of the spin-correlated radical pair.
Co-reporter:Rebecca J. Lindquist, Brian T. Phelan, Anna Reynal, Eric A. Margulies, Leah E. Shoer, James R. Durrant and Michael R. Wasielewski
Journal of Materials Chemistry A 2016 - vol. 4(Issue 8) pp:NaN2893-2893
Publication Date(Web):2015/08/25
DOI:10.1039/C5TA05790F
Perylene-3,4-dicarboximide (PMI) based chromophores have demonstrated the ability to inject electrons into TiO2 for dye-sensitized solar cell applications and to accept electrons from metal complexes relevant to water oxidation, but they are nearly unexplored for use in photoelectrochemical cells (PECs) for solar fuels generation. A series of related PMIs with high oxidation potentials and carboxylate binding groups was synthesized and investigated for this purpose. Charge injection and recombination dynamics were measured using transient absorption (TA) spectroscopy on the picosecond to second timescales. The dynamics and electron injection yields were correlated with the PMI energetics and structures. Injection began in less than 1 ps for the dye with the best performance and a significant charge-separated state yield remained at long times. Finally, this chromophore was used to oxidize a covalently bound water oxidation precatalyst following electron injection into TiO2 to demonstrate the utility of the dyes for use in PECs.
Co-reporter:Michael T. Vagnini, Michael W. Mara, Michael R. Harpham, Jier Huang, Megan L. Shelby, Lin X. Chen and Michael R. Wasielewski
Chemical Science (2010-Present) 2013 - vol. 4(Issue 10) pp:NaN3873-3873
Publication Date(Web):2013/07/29
DOI:10.1039/C3SC51511G
Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(IV) state of the Ir(III)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor–chromophore–Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI–1*PMI–Ir(III) → NDI–PMI˙−–Ir(IV) and NDI–1*PMI–Ir(III) → NDI˙−–PMI˙+–Ir(III) that each proceed with τ < 5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI˙−–PMI–Ir(IV). The fully charge-separated ion pair has a lifetime of 17.2 ± 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the LIII-edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(III) to Ir(IV), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donor–acceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.
Co-reporter:Peter Spenst, Ryan M. Young, Michael R. Wasielewski and Frank Würthner
Chemical Science (2010-Present) 2016 - vol. 7(Issue 8) pp:NaN5434-5434
Publication Date(Web):2016/05/18
DOI:10.1039/C6SC01574C
Cofacial positioning of two perylene bisimide (PBI) chromophores at a distance of 6.5 Å in a cyclophane structure prohibits the otherwise common excimer formation and directs photoexcited singlet state relaxation towards intramolecular symmetry-breaking charge separation (τCS = 161 ± 4 ps) in polar CH2Cl2, which is thermodynamically favored with a Gibbs free energy of ΔGCS = −0.32 eV. The charges then recombine slowly in τCR = 8.90 ± 0.06 ns to form the PBI triplet excited state, which can be used subsequently to generate singlet oxygen in 27% quantum yield. This sequence of events is eliminated by dissolving the PBI cyclophane in non-polar toluene, where only excited singlet state decay occurs. In contrast, complexation of electron-rich aromatic hydrocarbons by the host PBI cyclophane followed by photoexcitation of PBI results in ultrafast electron transfer (<10 ps) from the guest to the PBI in CH2Cl2. The rate constants for charge separation and recombination increase as the guest molecules become easier to oxidize, demonstrating that charge separation occurs close to the peak of the Marcus curve and the recombination lies far into the Marcus inverted region.
Co-reporter:Richard F. Kelley, Michael J. Tauber, Thea M. Wilson and Michael R. Wasielewski
Chemical Communications 2007(Issue 42) pp:NaN4409-4409
Publication Date(Web):2007/08/14
DOI:10.1039/B708318A
A series of linkers constructed from combinations of phenyl and ethynyl groups is shown to permit ultrafast energy transfer between two chlorophylls, while allowing control over radical cation migration between them.
Co-reporter:Patrick E. Hartnett, Scott M. Dyar, Eric A. Margulies, Leah E. Shoer, Andrew W. Cook, Samuel W. Eaton, Tobin J. Marks and Michael R. Wasielewski
Chemical Science (2010-Present) 2015 - vol. 6(Issue 1) pp:NaN411-411
Publication Date(Web):2014/09/16
DOI:10.1039/C4SC02551B
The photophysics of a covalently linked perylenediimide–diketopyrrolopyrrole–perylenediimide acceptor–donor–acceptor molecule (PDI–DPP–PDI, 1) were investigated and found to be markedly different in solution versus in unannealed and solvent annealed films. Photoexcitation of 1 in toluene results in quantitative charge separation in τ = 3.1 ± 0.2 ps, with charge recombination in τ = 340 ± 10 ps, while in unannealed/disordered films of 1, charge separation occurs in τ < 250 fs, while charge recombination displays a multiexponential decay in ∼6 ns. The absence of long-lived, charge separation in the disordered film suggests that few free charge carriers are generated. In contrast, upon CH2Cl2 vapor annealing films of 1, grazing-incidence X-ray scattering shows that the molecules form a more ordered structure. Photoexcitation of the ordered films results in initial formation of a spin-correlated radical ion pair (electron–hole pair) as indicated by magnetic field effects on the formation of free charge carriers which live for ∼4 μs. This result has significant implications for the design of organic solar cells based on covalent donor–acceptor systems and shows that long-lived, charge-separated states can be achieved by controlling intramolecular charge separation dynamics in well-ordered systems.
Co-reporter:Patrick E. Hartnett, H. S. S. Ramakrishna Matte, Nicholas D. Eastham, Nicholas E. Jackson, Yilei Wu, Lin X. Chen, Mark A. Ratner, Robert P. H. Chang, Mark C. Hersam, Michael R. Wasielewski and Tobin J. Marks
Chemical Science (2010-Present) 2016 - vol. 7(Issue 6) pp:NaN3555-3555
Publication Date(Web):2016/02/09
DOI:10.1039/C5SC04956C
A series of perylenediimide (PDI) dimers are evaluated as acceptors for organic photovoltaic (OPV) cells. The materials are characterized using a wide variety of physical and computational techniques. These dimers are first linked at the bay position of each PDI molecule via an aromatic spacer; subsequent photocyclization affords ring-fused dimers. Thus, photocyclization of the thiophene-linked dimer 2,5-bis-[N,N′-bis-perylenediimide-1-yl]-thiophene (T1) affords the twisted acceptor [2,3-b:2′,3′-d]-bis-[N,N′-bis-perylenediimide-1,12-yl]-thiophene (T2), while photocyclization of the thienothiophene-linked dimer, 2,5-bis-[N,N′-bis-perylenediimide-1-yl]-thienothiophene (TT1) affords the planar acceptor [2,3-b:2′,3′-d]-bis-[N,N′-bis-perylenediimide-1,12-yl]-thienothiophene (TT2). Furthermore, a dimer linked by a phenylene group, 1,4-bis-[N,N′-bis-perylenediimide-1-yl]-benzene (Ph1), can be selectively photocyclized to form either the twisted dimer, [1,2:3,4]-bis-[N,N′-bis-perylenediimide-1,12-yl]-benzene (Ph1a) or the planar dimer [1,2:4,5]-bis-[N,N′-bis-perylenediimide-1,12-yl]-benzene (Ph2b). Ring-fusion results in increased electronic coupling between the PDI units, and increased space-charge limited thin film electron mobility. While charge transport is efficient in bulk-heterojunction blends of each dimer with the polymeric donor PBDTT-FTTE, in the case of the twisted dimers ring fusion leads to a significant decrease in geminate recombination, hence increased OPV photocurrent density and power conversion efficiency. This effect is not observed in planar dimers where ring fusion leads to increased crystallinity and excimer formation, decreased photocurrent density, and decreased power conversion efficiency. These results argue that ring fusion is an effective approach to increasing OPV bulk-heterojunction charge carrier generation efficiency in PDI dimers as long as they remain relatively amorphous, thereby suppressing excimer formation and coulombically trapped charge transfer states.
Co-reporter:Hsiao-Fan Chen, Daniel M. Gardner, Raanan Carmieli and Michael R. Wasielewski
Chemical Communications 2013 - vol. 49(Issue 77) pp:NaN8616-8616
Publication Date(Web):2013/08/05
DOI:10.1039/C3CC45129A
Ordered multi-spin assemblies are required for developing solid-state molecule-based spintronics. A linear donor–chromophore–acceptor (D–C–A) molecule was covalently attached inside the 150 nm diam. nanopores of an anodic aluminum oxide (AAO) membrane. Photoexcitation of D–C–A in a 343 mT magnetic field results in sub-nanosecond, two-step electron transfer to yield the spin-correlated radical ion pair (SCRP) 1(D+˙–C–A−˙), which then undergoes radical pair intersystem crossing (RP-ISC) to yield 3(D+˙–C–A−˙). RP-ISC results in S–T0 mixing to selectively populate the coherent superposition states |S′〉 and |T′〉. Microwave-induced transitions between these states and the unpopulated |T+1〉 and |T−1〉 states result in spin-polarized time-resolved EPR (TREPR) spectra. The dependence of the electron spin polarization (ESP) phase of the TREPR spectra on the orientation of the AAO membrane pores relative to the externally applied magnetic field is used to determine the overall orientation of the SCRPs within the pores at room temperature.
Co-reporter:Victoria L. Gunderson, Sarah M. Mickley Conron and Michael R. Wasielewski
Chemical Communications 2010 - vol. 46(Issue 3) pp:NaN403-403
Publication Date(Web):2009/12/01
DOI:10.1039/B921084A
Small- and wide-angle X-ray scattering in solution reveals that chlorophyll (Chl) trefoils self-assemble in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) to yield supramolecular cyclic trimers. Two Chls of each trefoil coordinate to DABCO to form the vertices of the hexagonal structure, while the remaining Chl acts like a substituent on the benzene-like assembly.
Co-reporter:Nathan T. La Porte, Jose F. Martinez, Svante Hedström, Benjamin Rudshteyn, Brian T. Phelan, Catherine M. Mauck, Ryan M. Young, Victor S. Batista and Michael R. Wasielewski
Chemical Science (2010-Present) 2017 - vol. 8(Issue 5) pp:NaN3831-3831
Publication Date(Web):2017/02/28
DOI:10.1039/C6SC05103K
A major goal of artificial photosynthesis research is photosensitizing highly reducing metal centers using as much as possible of the solar spectrum reaching Earth's surface. The radical anions and dianions of rylenediimide (RDI) dyes, which absorb at wavelengths as long as 950 nm, are powerful photoreductants with excited state oxidation potentials that rival or exceed those of organometallic chromophores. These dyes have been previously incorporated into all-organic donor–acceptor systems, but have not yet been shown to reduce organometallic centers. This study describes a set of dyads in which perylenediimide (PDI) or naphthalenediimide (NDI) chromophores are attached to Re(bpy)(CO)3 through either the bipyridine ligand or more directly to the Re center via a pyridine ligand. The chromophores are reduced with a mild reducing agent, after which excitation with long-wavelength red or near-infrared light leads to reduction of the Re complex. The kinetics of electron transfer from the photoexcited anions to the Re complex are monitored using transient visible/near-IR and mid-IR spectroscopy, complemented by theoretical spectroscopic assignments. The photo-driven charge shift from the reduced PDI or NDI to the complex occurs in picoseconds regardless of whether PDI or NDI is attached to the bipyridine or to the Re center, but back electron transfer is found to be three orders of magnitude slower with the chromophore attached to the Re center. These results will inform the design of future catalytic systems that incorporate RDI anions as chromophores.
Co-reporter:Rebecca J. Kamire, Marek B. Majewski, William L. Hoffeditz, Brian T. Phelan, Omar K. Farha, Joseph T. Hupp and Michael R. Wasielewski
Chemical Science (2010-Present) 2017 - vol. 8(Issue 1) pp:NaN549-549
Publication Date(Web):2016/08/30
DOI:10.1039/C6SC02477G
The design of efficient hydrogen-evolving photocathodes for dye-sensitized photoelectrochemical cells (DSPECs) requires the incorporation of molecular light absorbing chromophores that are capable of delivering reducing equivalents to molecular proton reduction catalysts at rates exceeding those of charge recombination events. Here, we report the functionalization and kinetic analysis of a nanostructured NiO electrode with a modified perylene-3,4-dicarboximide chromophore (PMI) that is stabilized against degradation by atomic layer deposition (ALD) of thick insulating Al2O3 layers. Following photoinduced charge injection into NiO in high yield, films with Al2O3 layers demonstrate longer charge separated lifetimes as characterized via femtosecond transient absorption spectroscopy and photoelectrochemical techniques. The photoelectrochemical behavior of the electrodes in the presence of Co(II) and Ni(II) molecular proton reduction catalysts is examined, revealing reduction of both catalysts. Under prolonged irradiation, evolved H2 is directly observed by gas chromatography supporting the applicability of PMI embedded in Al2O3 as a photocathode architecture in DSPECs.
Co-reporter:Michael J. Ahrens, Richard F. Kelley, Zachary E. X. Dance and Michael R. Wasielewski
Physical Chemistry Chemical Physics 2007 - vol. 9(Issue 12) pp:NaN1478-1478
Publication Date(Web):2007/02/22
DOI:10.1039/B617876F
A bichromophoric electron donor–acceptor molecule composed of a zinc tetraphenylporphyrin (ZnTPP) surrounded by four perylene-3,4:9,10-bis(dicarboximide)
(PDI) chromophores (ZnTPP-PDI4) was synthesized. The properties of this molecule were compared to a reference molecule having ZnTPP covalently bound to a single PDI (ZnTPP-PDI). In toluene, ZnTPP-PDI4 self-assembles into monodisperse aggregates of five molecules arranged in a columnar stack, (ZnTPP-PDI4)5. The monodisperse nature of this assembly contrasts sharply with previously reported ZnTPP-PDI4 derivatives having 1,7-bis(3,5-di-t-butylphenoxy) groups (ZnTPP-PPDI4). The size and structure of this assembly in solution was determined by small angle X-ray scattering (SAXS) using a high flux synchrotron X-ray source. The ZnTPP-PDI reference molecule does not aggregate. Femtosecond transient absorption spectroscopy shows that laser excitation of both ZnTPP-PDI and (ZnTPP-PDI4)5 results in quantitative formation of ZnTPP+˙-PDI−˙ radical ion pairs in a few picoseconds. The transient absorption spectra of (ZnTPP-PDI4)5 suggest that the PDI−˙ radicals interact strongly with adjacent PDI molecules within the columnar stack. Charge recombination occurs more slowly within (ZnTPP-PDI4)5
(τ
= 4.8 ns) than it does in ZnTPP-PDI (τ
= 3.0 ns) producing mostly ground state as well as a modest yield of the lowest triplet state of PDI (3*PDI). Formation of 3*PDI occurs by rapid spin–orbit induced intersystem crossing (SO-ISC) directly from the singlet radical ion pair as evidenced by the electron spin polarization pattern exhibited by its time-resolved electron paramagnetic resonance spectrum.
Co-reporter:Amod Timalsina, Patrick E. Hartnett, Ferdinand S. Melkonyan, Joseph Strzalka, Vari S. Reddy, Antonio Facchetti, Michael R. Wasielewski and Tobin J. Marks
Journal of Materials Chemistry A 2017 - vol. 5(Issue 11) pp:NaN5361-5361
Publication Date(Web):2017/02/09
DOI:10.1039/C7TA00063D
The synthesis of a new tetrafluorinated semiconducting donor polymer, poly[(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene)-alt-(5,6-difluoro-4,7-(4-(2-ethylhexyl)-dithien-2-yl-2,1,3-benzothiadiazole)] (PBTZF4), and its photovoltaic performance in bulk heterojunction (BHJ) blends with the non-fullerene molecular acceptor [1,2:3,4]-bis-[N,N′-bis-1-pentylhexyl-perylenediimide-1,12-yl]-benzene (bPDI2P), are reported. PBTZF4:bPDI2P solar cells exhibit a high open circuit voltage (Voc) of 1.118 V, a short circuit current density (Jsc) of 10.02 mA cm−2, and a fill factor (FF) of 49.5%, affording a power conversion efficiency (PCE) of 5.55%. Interestingly, a lower PCE of 3.68% is obtained with the difluorinated analogue, poly[(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene)-alt-(5,6-difluoro-4,7-(4-(2-ethyl-hexyl)-dithien-2-yl-2,1,3-benzothiadiazole)] (PBTZF2). Both PBTZF4:bPDI2P and PBTZF2:bPDI2P cells benefit from complementary (donor/acceptor) light absorption and very low geminate recombination, with bimolecular recombination being the dominant loss mechanism, as established by femtosecond transient absorption spectroscopy. DFT computation and physicochemical characterization data argue that the “additional” tetrafluorination planarizes the PBTZF4 backbone and enhances aggregation versusPBTZF2, affording superior charge carrier transport as assayed by field-effect mobility. In addition, fluorine-originated HOMO stabilization, −5.41 eV for PBTZF4versus −5.33 eV for PBTZF2, and a superior blend microstructure afford a higher PBTZF4:bPDI2P solar cell PCE versusPBTZF2:bPDI2P.
Co-reporter:Amod Timalsina, Patrick E. Hartnett, Ferdinand S. Melkonyan, Joseph Strzalka, Vari S. Reddy, Antonio Facchetti, Michael R. Wasielewski and Tobin J. Marks
Journal of Materials Chemistry A 2017 - vol. 5(Issue 11) pp:NaN5361-5361
Publication Date(Web):2017/02/09
DOI:10.1039/C7TA00063D
The synthesis of a new tetrafluorinated semiconducting donor polymer, poly[(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene)-alt-(5,6-difluoro-4,7-(4-(2-ethylhexyl)-dithien-2-yl-2,1,3-benzothiadiazole)] (PBTZF4), and its photovoltaic performance in bulk heterojunction (BHJ) blends with the non-fullerene molecular acceptor [1,2:3,4]-bis-[N,N′-bis-1-pentylhexyl-perylenediimide-1,12-yl]-benzene (bPDI2P), are reported. PBTZF4:bPDI2P solar cells exhibit a high open circuit voltage (Voc) of 1.118 V, a short circuit current density (Jsc) of 10.02 mA cm−2, and a fill factor (FF) of 49.5%, affording a power conversion efficiency (PCE) of 5.55%. Interestingly, a lower PCE of 3.68% is obtained with the difluorinated analogue, poly[(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene)-alt-(5,6-difluoro-4,7-(4-(2-ethyl-hexyl)-dithien-2-yl-2,1,3-benzothiadiazole)] (PBTZF2). Both PBTZF4:bPDI2P and PBTZF2:bPDI2P cells benefit from complementary (donor/acceptor) light absorption and very low geminate recombination, with bimolecular recombination being the dominant loss mechanism, as established by femtosecond transient absorption spectroscopy. DFT computation and physicochemical characterization data argue that the “additional” tetrafluorination planarizes the PBTZF4 backbone and enhances aggregation versusPBTZF2, affording superior charge carrier transport as assayed by field-effect mobility. In addition, fluorine-originated HOMO stabilization, −5.41 eV for PBTZF4versus −5.33 eV for PBTZF2, and a superior blend microstructure afford a higher PBTZF4:bPDI2P solar cell PCE versusPBTZF2:bPDI2P.
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