Co-reporter:Renato N. Sampaio, Brian N. DiMarco, and Gerald J. Meyer
ACS Energy Letters October 13, 2017 Volume 2(Issue 10) pp:2402-2402
Publication Date(Web):September 13, 2017
DOI:10.1021/acsenergylett.7b00759
Three ruthenium(II) sensitizers, [Ru(L)2(dcb)]2+, were anchored to mesoporous TiO2 thin films where the ligand L = 4,4′-(CH3)2-bpy (dmb), 4,4′-(C(CH3)3)2-bpy (dtb), and 4,4′-(CF3)2-bpy (bpyCF3) controls the thermodynamics and electronic coupling for self-exchange intermolecular RuIII/II “hole hopping”. Apparent electron difussion coefficients, Dapp, were reported to increase in the order bpyCF3 ≪ dtb < dmb. Nanosecond transient absorption measurements made over an 80° temperature range were conducted to abstract average charge recombination rate constants, kcr, under conditions of sub-percolation and saturated sensitizer surface coverages. For sensitizers [Ru(dmb)2(dcb)]2+ and [Ru(dtb)2(dcb)]2+, the kcr values at saturation coverages were significantly larger than those at low coverages, by a degree that followed the trend in Dapp. The inability of [Ru(bpyCF3)2(dcb)]2+ to introduce hole transport was afirmed by recombination kinetic data that were insensitive to the sensitizer surface converage. An Arrhenius analysis indicated that lateral RuIII/II hole hopping decreased the barrier for electron transfer that ultimately led to faster recombination rates.
Co-reporter:Timothy J. Barr and Gerald J. Meyer
ACS Energy Letters October 13, 2017 Volume 2(Issue 10) pp:2335-2335
Publication Date(Web):September 7, 2017
DOI:10.1021/acsenergylett.7b00569
Charge recombination between electrons injected into TiO2 (TiO2(e–)s) and acceptors at the dye-sensitized electrolyte interface have been quantified by measurement of the open-circuit photovoltage, VOC, as a function of the incident photon flux. Literature reports indicate that the order of the reaction with respect to TiO2(e–)s is less than unity, typically 0.5–0.85. Herein, an alternative model is proposed and tested that incorporates a characteristic temperature T0 to model the density of acceptor states and photon flux-dependent TiO2(e–) lifetimes to account for shorter lifetimes at higher concentrations. Tests of this model with standard dye-sensitized solar cells based on the ditetrabutylammonium salt of cis-Ru(dcb)2(NCS)2 (N719, where dcb is 4,4′-(CO2H)2-2,2′-bipyridine) sensitizers in iodide/iodine acetonitrile electrolytes under 0.1–5 sun illumination revealed a reaction that is first-order in TiO2(e–)s with T0 = 1150 K. The first-order reactivity is consistent with an underlying TiIV/III redox reaction, and the kinetic data under 1 sun illumination suggests recombination to molecular iodine, I2. Other implications for solar energy conversion and quantitative analysis of dye-sensitized solar cells are discussed.
Co-reporter:Matthew D. Brady, Renato N. Sampaio, Degao Wang, Thomas J. Meyer, and Gerald J. Meyer
Journal of the American Chemical Society November 8, 2017 Volume 139(Issue 44) pp:15612-15612
Publication Date(Web):October 23, 2017
DOI:10.1021/jacs.7b09367
Hydrobromic acid (HBr) has significant potential as an inexpensive feedstock for hydrogen gas (H2) solar fuel production through HBr splitting. Mesoporous thin films of anatase TiO2 or SnO2/TiO2 core–shell nanoparticles were sensitized to visible light with a new RuII polypyridyl complex that served as a photocatalyst for bromide oxidation. These thin films were tested as photoelectrodes in dye-sensitized photoelectrosynthesis cells. In 1 N HBr (aq), the photocatalyst undergoes excited-state electron injection and light-driven Br– oxidation. The injected electrons induce proton reduction at a Pt electrode. Under 100 mW cm−2 white-light illumination, sustained photocurrents of 1.5 mA cm–2 were measured under an applied bias. Faradaic efficiencies of 71 ± 5% for Br– oxidation and 94 ± 2% for H2 production were measured. A 12 μmol h–1 sustained rate of H2 production was maintained during illumination. The results demonstrate a molecular approach to HBr splitting with a visible light absorbing complex capable of aqueous Br– oxidation and excited-state electron injection.
Co-reporter:Tyler C. Motley, Ludovic Troian-Gautier, M. Kyle Brennaman, and Gerald J. Meyer
Inorganic Chemistry November 6, 2017 Volume 56(Issue 21) pp:13579-13579
Publication Date(Web):October 25, 2017
DOI:10.1021/acs.inorgchem.7b02321
The synthesis, electrochemistry, and photophysical characterization are reported for 11 tris(bidentate) cyclometalated ruthenium(II) compounds, [Ru(N^N)2(C^N)]+. The electrochemical and photophysical properties were varied by the addition of substituents on the 2,2′-bipyridine, N^N, and 2-phenylpyridine, C^N, ligands with different electron-donating and -withdrawing groups. The systematic tuning of these properties offered a tremendous opportunity to investigate the origin of the rapid excited-state decay for these cyclometalated compounds and to probe the accessibility of the dissociative, ligand-field (LF) states from the metal-to-ligand charge-transfer (MLCT) excited state. The photoluminescence quantum yield for [Ru(N^N)2(C^N)]+ increased from 0.0001 to 0.002 as more electron-withdrawing substituents were added to C^N. An analogous substituent dependence was observed for the excited-state lifetimes, τobs, which ranged from 3 to 40 ns in neat acetonitrile, significantly shorter than those for their [Ru(N^N)3]2+ analogues. The excited-state decay for [Ru(N^N)2(C^N)]+ was accelerated because of an increased vibronic overlap between the ground- and excited-state wavefunctions rather than an increased electronic coupling as revealed by a comparison of the Franck–Condon factors. The radiative (kr) and non-radiative (knr) rate constants of excited-state decay were determined to be on the order of 104 and 107–108 s–1, respectively. For sets of [Ru(N^N)2(C^N)]+ compounds functionalized with the same N^N ligand, knr scaled with excited-state energy in accordance with the energy gap law. Furthermore, an Arrhenius analysis of τobs for all of the compounds between 273 and 343 K was consistent with activated crossing into a single, fourth 3MLCT state under the conditions studied with preexponential factors on the order of 108–109 s–1 and activation energies between 300 and 1000 cm–1. This result provides compelling evidence that LF states are not significantly populated near room temperature unlike many ruthenium(II) polypyridyl compounds. On the basis of the underlying photophysics presented here for [Ru(N^N)2(C^N)]+, molecules of this type represent a robust class of compounds with built-in design features that should greatly enhance the molecular photostability necessary for photochemical and photoelectrochemical applications.
Co-reporter:Timothy J. Barr, Renato N. Sampaio, Brian N. DiMarco, Erica M. James, and Gerald J. Meyer
Chemistry of Materials May 9, 2017 Volume 29(Issue 9) pp:3919-3919
Publication Date(Web):April 4, 2017
DOI:10.1021/acs.chemmater.6b05470
Mesoporous thin films comprised of ∼15 nm diameter SnO2 nanocrystallites were synthesized and characterized in acetonitrile electrolytes by electrochemical and spectroscopic techniques. Spectroelectrochemical reduction of the thin films resulted in broad, non-superimposable UV/vis absorption changes. Simultaneous analysis of potential-dependent spectra, by a process termed “potential associated spectra”, resulted in the identification of three unique absorption spectra for reduced SnO2, while only one spectrum was identified for TiO2. Reduction of SnO2 resulted in the appearance of (1) a broad absorption that spans across the visible and near-IR regions, (2) a blue-shifted fundamental absorption, and (3) an absorption band in the blue region. The absorption onsets were dependent on the electrolyte cation, present as the perchlorate salt of Li+, Na+, Mg2+, Ca2+, and TBA+, where TBA+ is tetrabutylammonium. Correlations between the charge within the thin film and the absorbance intensity revealed that significant charge was transferred to SnO2 films before significant visible color changes were observed. This suggested the presence of electrons within the SnO2 thin films that did not absorb visible light and were termed “phantom electrons”.
Co-reporter:Andrew B. Maurer, Ke Hu, and Gerald J. Meyer
Journal of the American Chemical Society June 21, 2017 Volume 139(Issue 24) pp:8066-8066
Publication Date(Web):May 27, 2017
DOI:10.1021/jacs.7b01793
The titration of iodide into acetonitrile solutions of BiI3 resulted in the formation of [BiI6]3–. Ligand-to-metal charge transfer (LMCT) excitation of [BiI6]3– yielded a transient species assigned as the diiodide anion I2•– directly ligated to Bi, [Bi(I2•–)Ix]n. With 20 ns time resolution, transient absorption measurements revealed the appearance of two species assigned on the analysis of the iodine molecular orbitals as an η2 ligated I2•–, [(η2-I2)BiI4]3– (λmax = 640 nm), and an η1 species [(η1-I2)BiI4]3– (λmax = 750 nm). The rapid appearance of this intermediate was attributed to intramolecular I–I bond formation. The [(η2-I2)BiI4]3– subsequently reacted with 1 equiv of iodide to yield [(η1-I2)BiI5]4–. Interestingly, [(η1-I2)BiI5]4– decayed to ground state products with a first-order rate constant of k = 2 × 103 s–1. Under the same experimental conditions, I2•– in CH3CN rapidly disproportionates with a tremendous loss of free energy, ΔGo = −2.6 eV. The finding that metal ligation inhibits this energy wasting reaction is of direct relevance to solar energy conversion. The photochemistry itself provides a rare example of one electron oxidized halide species coordinated to a metal ion of possible relevance to reductive elimination/oxidation addition reaction chemistry of transition metal catalysts.
Co-reporter:Laura Casarin, Wesley B. Swords, Stefano Caramori, Carlo A. Bignozzi, and Gerald J. Meyer
Inorganic Chemistry July 3, 2017 Volume 56(Issue 13) pp:7324-7324
Publication Date(Web):June 12, 2017
DOI:10.1021/acs.inorgchem.7b00819
An anionic CoII complex, [Co(TTT) (NCS)3]− (TTT = 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine and NCS = isothiocyanate), was synthesized for use in dye-sensitized solar cells (DSSCs). The CoII complex was found to ion-pair with the hexacationic sensitizer [Ru(tmam)2(dcb)]6+ (tmam = 4,4′-bis(trimethylaminomethyl)-2,2′-bipyridine and dcb = 4,4′-(CO2H)2-2,2′-bipyridine) anchored to TiO2 thin films immersed in acetonitrile solution. Visible light excitation of the ion pairs resulted in excited-state injection followed by rapid static regeneration of the oxidized sensitizer (<10 ns). The static component to regeneration gave an ion-pair equilibrium constant of 6000 M–1. This value is an order of magnitude smaller than the equilibrium constant determined for [Ru(tmam)2(deeb)]6+ (deeb = 4,4′-(CO2Et)2-2,2′-bipyridine) dissolved in acetonitrile. DSSC studies employing [Co(TTT) (NCS)3]− or the cationic [Co(DTB)3]2+ (DTB = 4,4′-di-tert-butyl-2,2′-bipyridine) as redox mediators revealed a 3 fold photocurrent increase in the presence of the anionic cobalt complex. As the regeneration step was greatly enhanced through the formation of Coulombic ion pairs, both electron injection and regeneration were complete within 10 ns which is unprecedented for dye-sensitization. The results obtained reveal that ground-state ion-pairing can be a powerful strategy for DSSC optimization.
Co-reporter:Ludovic Troian-GautierEvan E. Beauvilliers, Wesley B. Swords, Gerald J. Meyer
Journal of the American Chemical Society 2016 Volume 138(Issue 51) pp:16815-16826
Publication Date(Web):December 2, 2016
DOI:10.1021/jacs.6b11337
Ion-pair interactions between a cationic ruthenium complex, [Ru(dtb)2(dea)][PF6]2, C12+ where dea is 4,4′-diethanolamide-2,2′-bipyridine and dtb is 4,4′-di-tert-butyl-2,2′-bipyridine, and chloride, bromide, and iodide are reported. A remarkable result is that a 1:1 iodide:excited-state ion-pair, [C12+, I–]+*, underwent diffusional electron-transfer oxidation of iodide that did not occur when ion-pairing was absent. The ion-pair equilibrium constants ranged 104–106 M–1 in CH3CN and decreased in the order Cl– > Br– > I–. The ion-pairs had longer-lived excited states, were brighter emitters, and stored more free energy than did the non-ion-paired states. The 1H NMR spectra revealed that the halides formed tight ion-pairs with the amide and alcohol groups of the dea ligand. Electron-transfer reactivity of the ion-paired excited state was not simply due to it being a stronger photooxidant than the non-ion-paired excited state. Instead, work term, ΔGw was the predominant contributor to the driving force for the reaction. Natural bond order calculations provided natural atomic charges that enabled quantification of ΔGw for all the atoms in C12+ and [C12+, I–]+* presented herein as contour diagrams that show the most favorable electrostatic positions for halide interactions. The results were most consistent with a model wherein the non-ion-paired C12+* excited state traps the halide and prevents its oxidation, but allows for dynamic oxidation of a second iodide ion.
Co-reporter:Sarah J. C. Simon, Fraser G. L. Parlane, Wesley B. Swords, Cameron W. Kellett, Chuan Du, Brian Lam, Rebecca K. Dean, Ke Hu, Gerald J. Meyer, and Curtis P. Berlinguette
Journal of the American Chemical Society 2016 Volume 138(Issue 33) pp:10406-10409
Publication Date(Web):August 12, 2016
DOI:10.1021/jacs.6b06288
We report here an enhancement in photovoltage for dye-sensitized solar cells (DSSCs) where halogen-bonding interactions exist between a nucleophilic electrolyte species (I–) and a photo-oxidized dye immobilized on a TiO2 surface. The triarylamine-based dyes under investigation showed larger rate constants for dye regeneration (kreg) by the nucleophilic electrolyte species when heavier halogen substituents were positioned on the dye. The open-circuit voltages (VOC) tracked these kreg values. This analysis of a homologous series of dyes that differ only in the identity of two halogen substituents provides compelling evidence that the DSSC photovoltage is sensitive to kreg. This study also provides the first direct evidence that halogen-bonding interactions between the dye and the electrolyte can bolster DSSC performance.
Co-reporter:Ryan M. O’Donnell; Renato N. Sampaio; Guocan Li; Patrik G. Johansson; Cassandra L. Ward;Gerald J. Meyer
Journal of the American Chemical Society 2016 Volume 138(Issue 11) pp:3891-3903
Publication Date(Web):February 22, 2016
DOI:10.1021/jacs.6b00454
Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [RuII(btfmb)2(LL)]2+, [RuII(dtb)2(LL)]2+, and [RuII(bpy)2(LL)]2+, where bpy is 2,2′-bipyridine, btfmb is 4,4′-(CF3)2-bpy, and dtb is 4,4′-((CH3)3C)2-bpy, and LL is either dcb = 4,4′-(CO2H)2-bpy or mcb = 4-(CO2H),4′-(CO2Et)-2,2′-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long τ > 100 ns excited state lifetimes. The mcb compounds had very similar ground state pKa’s of 2.31 ± 0.07, and their characterization enabled accurate determination of the two pKa values for the commonly utilized dcb ligand, pKa1 = 2.1 ± 0.1 and pKa2 = 3.0 ± 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [RuIII(btfmb–)L2]2+* localized excited state and a [RuIII(dcb–)L2]2+* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent pKa* values. The ability to make a single ionizable group photobasic or photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s).
Co-reporter:Evan E. Beauvilliers and Gerald J. Meyer
Inorganic Chemistry 2016 Volume 55(Issue 15) pp:7517-7526
Publication Date(Web):July 8, 2016
DOI:10.1021/acs.inorgchem.6b00876
The visible absorption and photoluminescence (PL) properties of the four neutral ruthenium diimine compounds [Ru(bpy)2(dcb)] (B2B), [Ru(dtb)2(dcb)] (D2B), [Ru(bpy)2(dcbq)] (B2Q), and [Ru(dtb)2(dcbq)] (D2Q), where bpy is 2,2′-bipyridine, dcb is 4,4′-(CO2–)2-bpy, dtb is 4,4′-(tert-butyl)2-bpy, and dcbq is 4,4′-(CO2–)2-2,2′-biquinoline, are reported in the presence of Lewis acidic cations present in fluid solutions at room temperature. In methanol solutions, the measured spectra were insensitive to the presence of these cations, while in acetonitrile a significant red shift in the PL spectra (≤1400 cm–1) was observed consistent with stabilization of the metal-to-ligand charge transfer (MLCT) excited state through Lewis acid–base adduct formation. No significant spectral changes were observed in control experiments with the tetrabutylammonium cation. Titration data with Li+, Na+, Mg2+, Ca2+, Zn2+, Al3+, Y3+, and La3+ showed that the extent of stabilization saturated at high cation concentration with magnitudes that scaled roughly with the cation charge-to-size ratio. The visible absorption spectra of D2Q was particularly informative due to the presence of two well-resolved MLCT absorption bands: (1) Ru → bpy, λmax ≈ 450 nm; and (2) Ru → dcbq, λmax ≈ 540 nm. The higher-energy band blue-shifted and the lower-energy band red-shifted upon cation addition. The PL intensity and lifetime of the excited state of B2B first increased with cation addition without significant shifts in the measured spectra, behavior attributed to a cation-induced change in the localization of the emissive excited state from bpy to dcb. The importance of excited-state localization and stabilization for solar energy conversion is discussed.
Co-reporter:André Bessette, Mihaela Cibian, Janaina G. Ferreira, Brian N. DiMarco, Francis Bélanger, Denis Désilets, Gerald J. Meyer and Garry S. Hanan
Dalton Transactions 2016 vol. 45(Issue 26) pp:10563-10576
Publication Date(Web):06 Jun 2016
DOI:10.1039/C6DT00961A
In the on-going quest to harvest near-infrared (NIR) photons for energy conversion applications, a novel family of neutral ruthenium(II) sensitizers has been developed by cyclometalation of an azadipyrromethene chromophore. These rare examples of neutral ruthenium complexes based on polypyridine ligands exhibit an impressive panchromaticity achieved by the cyclometalation strategy, with strong light absorption in the 600–800 nm range that tails beyond 1100 nm in the terpyridine-based adducts. Evaluation of the potential for Dye-Sensitized Solar Cells (DSSC) and Organic Photovoltaic (OPV) applications is made through rationalization of the structure–property relationship by spectroscopic, electrochemical, X-ray structural and computational modelization investigations. Spectroscopic evidence for photo-induced charge injection into the conduction band of TiO2 is also provided.
Co-reporter:Wesley B. Swords;Sarah J. C. Simon;Fraser G. L. Parlane;Dr. Rebecca K. Dean;Cameron W. Kellett;Dr. Ke Hu; Gerald J. Meyer; Curtis P. Berlinguette
Angewandte Chemie International Edition 2016 Volume 55( Issue 20) pp:5956-5960
Publication Date(Web):
DOI:10.1002/anie.201510641
Abstract
A homologous series of donor–π–acceptor dyes was synthesized, differing only in the identity of the halogen substituents about the triphenylamine (TPA; donor) portion of each molecule. Each Dye-X (X=F, Cl, Br, and I) was immobilized on a TiO2 surface to investigate how the halogen substituents affect the reaction between the light-induced charge-separated state, TiO2(e−)/Dye-X+, with iodide in solution. Transient absorption spectroscopy showed progressively faster reactivity towards nucleophilic iodide with more polarizable halogen substituents: Dye-F < Dye-Cl < Dye-Br < Dye-I. Given that all other structural and electronic properties for the series are held at parity, with the exception of an increasingly larger electropositive σ-hole on the heavier halogens, the differences in dye regeneration kinetics for Dye-Cl, Dye-Br, and Dye-I are ascribed to the extent of halogen bonding with the nucleophilic solution species.
Co-reporter:Wesley B. Swords;Sarah J. C. Simon;Fraser G. L. Parlane;Dr. Rebecca K. Dean;Cameron W. Kellett;Dr. Ke Hu; Gerald J. Meyer; Curtis P. Berlinguette
Angewandte Chemie 2016 Volume 128( Issue 20) pp:6060-6064
Publication Date(Web):
DOI:10.1002/ange.201510641
Abstract
A homologous series of donor–π–acceptor dyes was synthesized, differing only in the identity of the halogen substituents about the triphenylamine (TPA; donor) portion of each molecule. Each Dye-X (X=F, Cl, Br, and I) was immobilized on a TiO2 surface to investigate how the halogen substituents affect the reaction between the light-induced charge-separated state, TiO2(e−)/Dye-X+, with iodide in solution. Transient absorption spectroscopy showed progressively faster reactivity towards nucleophilic iodide with more polarizable halogen substituents: Dye-F < Dye-Cl < Dye-Br < Dye-I. Given that all other structural and electronic properties for the series are held at parity, with the exception of an increasingly larger electropositive σ-hole on the heavier halogens, the differences in dye regeneration kinetics for Dye-Cl, Dye-Br, and Dye-I are ascribed to the extent of halogen bonding with the nucleophilic solution species.
Co-reporter:Brian N. DiMarco
The Journal of Physical Chemistry C 2016 Volume 120(Issue 26) pp:14226-14235
Publication Date(Web):June 9, 2016
DOI:10.1021/acs.jpcc.6b04438
Self-exchange intermolecular RuIII/II electron transfer, a process commonly referred to as “hole-hopping”, is of great interest as it provides a means of charge transport across the surface of nanocrystalline (anatase) TiO2 mesoporous thin films without the loss of free energy. This process was characterized by cyclic voltammetry and chronoabsorptometry for three homologous Ru diimine compounds of the general form [Ru(LL)2(dcbH2)](PF6)2, where LL is 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (dmb), or 4,4′-di-tert-butyl-2,2′-bipyridine (dtb) and dcbH2 is 2,2′-bipyridyl-4,4′-dicarboxylic acid. Apparent electron diffusion coefficients, D, abstracted from this data increased with dtb < bpy < dmb. Both techniques were consistent with this trend, despite differences in the magnitude of D between the two methods. Temperature dependent measurements revealed an activation barrier for electron self-exchange of 250 ± 50 meV that was within this error the same for all three diimine compounds, suggesting the total reorganization energy, λ, was also the same. Application of Marcus theory, with the assumption that the 900 ± 100 meV total reorganization energy for self-exchange electron transfer was independent of the Ru compound, revealed that the electronic coupling matrix element, HAB, followed the trend dtb (0.02 meV) < bpy (0.07 meV) < dmb (0.10 meV). The results indicate that insulating side groups placed on redox active molecules can be utilized to tune the electronic coupling and hence self-exchange rate constants without significantly altering the reorganization energy for electron transfer on TiO2 surfaces.
Co-reporter:Timothy J. Barr, Amanda J. Morris, Atefeh Taheri, and Gerald J. Meyer
The Journal of Physical Chemistry C 2016 Volume 120(Issue 48) pp:27173-27181
Publication Date(Web):November 9, 2016
DOI:10.1021/acs.jpcc.6b09761
Three pyridinium molecules were synthesized and were anchored to the mesoporous TiO2 thin films that are commonly used in dye-sensitized solar cells. The first reduction potential of the pyridiniums spanned a 660 mV range and occurred commensurate with or after the direct reduction of TiO2. The interfacial redox chemistry was non-Nernstian and was modeled by the inclusion of ideality factors. The kinetics for TiO2 and pyridinium reduction were quantified spectroscopically after a potential step. The reduction rate of TiO2 and of the pyridiniums were within experimental error the same consistent with a band-filling or “cup of wine” model. In contrast, oxidation of the reduced pyridiniums was dependent on the pyridinium formal reduction potential: Those with the most positive formal reduction potential required the most time. The behavior is understood based on the overlap of the pyridinium redox states with the acceptor states in TiO2 and provides a means for optimization of vectorial electron transfer at these interfaces.
Co-reporter:Renato N. Sampaio, Guocan Li, and Gerald J. Meyer
ACS Energy Letters 2016 Volume 1(Issue 4) pp:846
Publication Date(Web):September 27, 2016
DOI:10.1021/acsenergylett.6b00380
The electric field present while electrons injected into TiO2 recombine with oxidized sensitizers has been quantified for the first time. This advance was enabled by transient study of [Ru(NH3)5(ina)]2+, where ina is isonicotinic acid, anchored to the mesoporous TiO2 thin films used in dye-sensitized solar cells. Light excitation of the characteristic metal-to-ligand charge transfer (MLCT) resulted in a significant change in the molecular dipole, Δμ = 9.1 D, that enabled the surface electric field to be transiently quantified after pulsed light excitation. The field present 70 ns after excited-state injection was E = 0.35 MV/cm, and this value decreased continuously with charge recombination. The observed behavior is most consistent with these surface-anchored sensitizers experiencing a continuous contraction of the electric fields due to delocalized electrons, rather than a discrete number of sensitizers experiencing a localized field, as recombination proceeds.
Co-reporter:Guocan Li; William M. Ward;Gerald J. Meyer
Journal of the American Chemical Society 2015 Volume 137(Issue 26) pp:8321-8323
Publication Date(Web):June 17, 2015
DOI:10.1021/jacs.5b04549
Visible light excitation of [Ru(deeb)(bpz)2]2+ (deeb = 4,4′-diethylester-2,2′-bipyridine; bpz = 2,2′-bipyrazine), in Br– acetone solutions, led to the formation of Br–Br bonds in the form of dibromide, Br2•–. This light reactivity stores ∼1.65 eV of free energy for milliseconds. Combined 1H NMR, UV–vis and photoluminescence measurements revealed two distinct mechanisms. The first involves diffusional quenching of the excited state by Br– with a rate constant of (8.1 ± 0.1) × 1010 M–1 s–1. At high Br– concentrations, an inner-sphere pathway is dominant that involves the association of Br–, most likely with the 3,3′-H atoms of a bpz ligand, before electron transfer from Br– to the excited state, ket = (2.5 ± 0.3) × 107 s–1. In both mechanisms, the direct photoproduct Br• subsequently reacts with Br– to yield dibromide, Br• + Br– → Br2•–. Under pseudo-first-order conditions, this occurs with a rate constant of (1.1 ± 0.4) × 1010 M–1 s–1 that was, within experimental error, the same as that measured when Br• were generated with ultraviolet light. Application of Marcus theory to the sensitized reaction provided an estimate of the Br• formal reduction potential E(Br•/Br–) = 1.22 V vs SCE in acetone, which is about 460 mV less positive than the accepted value in H2O. The results demonstrate that Br– oxidation by molecular excited states can be rapid and useful for solar energy conversion.
Co-reporter:Wesley B. Swords, Guocan Li, and Gerald J. Meyer
Inorganic Chemistry 2015 Volume 54(Issue 9) pp:4512-4519
Publication Date(Web):April 14, 2015
DOI:10.1021/acs.inorgchem.5b00344
A series of three highly charged cationic ruthenium(II) polypyridyl complexes of the general formula [Ru(deeb)3–x(tmam)x](PF6)2x+2, where deeb is 4,4′-diethyl ester-2,2′-bipyridine and tmam is 4,4′-bis[(trimethylamino)methyl]-2,2′-bipyridine, were synthesized and characterized and are referred to as 1, 2, or 3 based on the number of tmam ligands. Crystals suitable for X-ray crystallography were obtained for the homoleptic complex 3, which was found to possess D3 symmetry over the entire ruthenium complex. The complexes displayed visible absorption spectra typical of metal-to-ligand charge-transfer (MLCT) transitions. In acetonitrile, quasi-reversible waves were assigned to RuIII/II electron transfer, with formal reduction potentials that shifted negative as the number of tmam ligands was increased. Room temperature photoluminescence was observed in acetonitrile with quantum yields of ϕ ∼ 0.1 and lifetimes of τ ∼ 2 μs. The spectroscopic and electrochemical data were most consistent with excited-state localization on the deeb ligand for 1 and 2 and on the tmam ligand for 3. The addition of tetrabutylammonium iodide to the complexes dissolved in a CH3CN solution led to changes in the UV–vis absorption spectra consistent with ion pairing. A Benesi–Hildebrand-type analysis of these data revealed equilibrium constants that increased with the cationic charge 1 < 2 < 3 with K = 4000, 4400, and 7000 M–1. 1H NMR studies in CD3CN also revealed evidence for iodide ion pairs and indicated that they occur predominantly with iodide localization near the tmam ligand(s). The diastereotopic H atoms on the methylene carbon that link the amine to the bipyridine ring were uniquely sensitive to the presence of iodide; analysis revealed that an iodide “binding pocket” exists wherein iodide forms an adduct with the 3 and 3′ bipyridyl H atoms and the quaternized amine. The MLCT excited states were efficiently quenched by iodide. Time-resolved photoluminescence measurements of 1 revealed a static component consistent with rapid electron transfer from iodide in the “binding pocket” to the Ru metal center in the excited state, ket > 108 s–1. The possible relevance of this work to solar energy conversion and dye-sensitized solar cells is discussed.
Co-reporter:Guocan Li;Ke Hu;Kiyoshi C. D. Robson; Serge I. Gorelsky; Gerald J. Meyer; Curtis P. Berlinguette; Michael Shatruk
Chemistry - A European Journal 2015 Volume 21( Issue 5) pp:2173-2181
Publication Date(Web):
DOI:10.1002/chem.201405261
Abstract
Two novel tris-heteroleptic Ru–dipyrrinates were prepared and tested as sensitizers in the dye-sensitized solar cell (DSSC). Under AM 1.5 sunlight, DSSCs employing these dyes achieved power conversion efficiencies (PCEs) of 3.4 and 2.2 %, substantially exceeding the value achieved previously with a bis-heteroleptic dye (0.75 %). As shown by electrochemical measurements and DFT calculations, the improved PCEs stem from the synthetically tuned electronic structure, which affords more negative excited state redox potentials and favorable electron injection into the TiO2 conduction band. Electron injection was quantified by nanosecond transient absorption spectroscopy, which revealed that the highest injection yield is achieved with the dye that acts as the strongest photoreductant.
Co-reporter:Cassandra L. Ward
The Journal of Physical Chemistry C 2015 Volume 119(Issue 45) pp:25273-25281
Publication Date(Web):October 16, 2015
DOI:10.1021/acs.jpcc.5b08617
The sensitizer [Ru(dtb)2(dcb)]2+, where dtb is 4,4′-di-tert-butyl-2,2′-bipyridine and dcb is 4,4′-dicarboxylic acid-2,2′-bipyridine, was anchored to mesoporous TiO2 thin films and characterized by visible spectroscopy in 0.1 M Na+, Li+, Mg2+, and Ca2+ perchlorate acetonitrile solutions on nanosecond and longer time scales. Relative to neat acetonitrile, the presence of these electrolyte cations induced a red shift in the metal-to-ligand charge transfer (MLCT) absorption of Ru(dtb)2(dcb)/TiO2. The magnitude of the shift increased with increasing valence of the metal cation. Pulsed 532 nm light excitation of Ru(dtb)2(dcb)/TiO2 resulted in the appearance of a long-lived bleach that returned to pre-excitation values on an approximately millisecond time scale under all conditions studied. Global analysis, spectral modeling, and single wavelength kinetic analysis revealed that two dynamic processes were operative: (1) charge recombination, RuIII(dtb)2(dcb)/TiO2(e–) → RuII(dtb)2(dcb)/TiO2, and (2) an electric field created by the injected electron. These two distinct nonexponential processes were observed in the same spectral region and on similar time scales. The ability of global analysis, specifically the decay-associated spectra, to kinetically and spectrally resolve these two processes was assessed. Single wavelength kinetic measurements and spectral modeling provided quantitatively different rate constants, but both led to the surprising conclusion that there was no evidence for charge screening of the electric field by cations present in the electrolyte. The decay of the electric field was cation independent, behavior very different from that previously reported in the presence of redox mediators. The charge recombination kinetics revealed a small yet measurable dependence on the nature of the cation present in the electrolyte with the divalent cations inducing the fastest recombination.
Co-reporter:Gerald J. Meyer
PNAS 2015 Volume 112 (Issue 30 ) pp:9146-9147
Publication Date(Web):2015-07-28
DOI:10.1073/pnas.1511569112
Co-reporter:Atefeh Taheri and Gerald J. Meyer
Dalton Transactions 2014 vol. 43(Issue 47) pp:17856-17863
Publication Date(Web):23 Sep 2014
DOI:10.1039/C4DT01683A
The metal-to-ligand charge transfer (MLCT) excited states of two related heteroleptic Ru(II) compounds [Ru(bpy)2(deeb)]2+ and [Ru(bpy)2(deebq)]2+, where bpy is 2,2′-bipyridine, deeb is 4,4′-(CO2CH2CH3)2-2,2′-bipyridine and deebq is 4,4′-(CO2CH2CH3)2-2,2′-biquinoline, were characterized in fluid acetonitrile by temperature dependent photoluminescence spectroscopies as well as quenching by iodide ions. Photoluminescence emanates from a manifold of thermally equilibrated excited states referred to as the thexi states. Evidence for activated internal conversion to a 4th MLCT excited state was garnered from an Arrhenius analysis of temperature dependent lifetime data. The activation energy was found to be 550 cm−1 for [Ru(bpy)2(deeb)]2+* and 1200 cm−1 for [Ru(bpy)2(deebq)]2+*. The pre-exponential factor abstracted from the Arrhenius analysis of the [Ru(bpy)2(deebq)]2+* data suggested that ligand field excited states might be populated, however there was no evidence for ligand loss photochemistry under the conditions studied. The excited states were found to quench iodide by a dynamic process in good agreement with the Stern–Volmer model. Transient absorption data showed that the quenching mechanism was electron transfer to generate an iodine atom and a reduced ruthenium compound as products. The quenching rate constants abstracted from temperature dependent Stern–Volmer quenching data were corrected for diffusion and activated complex formation to yield electron transfer rate constants that were found to increase markedly with temperature. An Arrhenius analysis of the electron transfer data revealed that electron transfer from iodide to the d-orbitals of the excited state was an activated process with an Ea of 2400 cm−1 for [Ru(bpy)2(deeb)]2+ and 3300 cm−1 for [Ru(bpy)2(deebq)]2+.
Co-reporter:Erinn C. Brigham, Darren Achey, Gerald J. Meyer
Polyhedron 2014 82() pp: 181-190
Publication Date(Web):
DOI:10.1016/j.poly.2014.07.023
Co-reporter:Ryan M. O’Donnell ; Renato N. Sampaio ; Timothy J. Barr ;Gerald J. Meyer
The Journal of Physical Chemistry C 2014 Volume 118(Issue 30) pp:16976-16986
Publication Date(Web):April 21, 2014
DOI:10.1021/jp500493t
The photophysical and electron transfer properties of mesoporous nanocrystalline (anatase) TiO2 thin films sensitized to visible light with [Ru(dtb)2(dcb)](PF6)2, where dtb is 4,4′-(tert-butyl)2-2,2′-bipyridine and dcb is 4,4′-(CO2H)2-2,2′-bipyridine, were quantified in acetonitrile solutions that contained 100 mM concentrations of Li+, Na+, Mg2+, or Ca2+ perchlorate salts. The presence of these salts resulted in a dramatic and cation dependent bathochromic (red) shift of the metal-to-ligand charge transfer (MLCT) absorption and photoluminescence (PL) spectra of Ru(dtb)2(dcb)/TiO2 relative to the value measured in neat or 100 mM TBAClO4, where TBA is tetrabutyl ammonium cation, acetonitrile solutions. The magnitude of the shifts followed the trend: Na+ < Li+ < Ca2+ < Mg2+. The PL intensity was also found to decrease in this same order and comparative actinometry studies showed that this was due to MLCT excited state electron transfer quenching by the TiO2 acceptor states. The RuIII/II redox chemistry was found to be non-Nernstian; the ideality factors were cation-dependent, suggestive of an underlying electric field effect. Electrochemical reduction of the TiO2 resulted in a black coloration and a blue shift of the fundamental (VB → CB) absorption, the normalized spectra were cation independent. Reduction of sensitized TiO2 also resulted in a blue shift of the MLCT absorption, the magnitude of which was used to determine the surface electric fields. Under conditions where about 20 electrons were present in each anatase nanocrystallite, the electric field strength reported by the Ru compound followed the trend Na+ < Li+ < Mg2+ < Ca2+, with Na+ being 1.1 MV/cm and Ca2+ 2.3 MV/cm. In pulsed laser experiments, the first-derivative absorption signature was observed transiently after excited state injection and iodide oxidation. These absorption amplitudes were time-dependent and decayed over time periods where the number of injected electrons was constant, with behavior attributed to screening of the surface electric field by cations present in the electrolyte. The monovalent cations screened charge much more rapidly than did the dications, kLi+,Na+ = 5.0 × 104 s–1 and kMg2+,Ca2+ = 5.0 × 102 s–1, presumably because the small number of injected electrons resulted in spatially isolated singly reduced Ti(III) sites that were more easily screened by the monocations.
Co-reporter:Renato N. Sampaio, Ryan M. O’Donnell, Timothy J. Barr, and Gerald J. Meyer
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 18) pp:3265-3268
Publication Date(Web):September 8, 2014
DOI:10.1021/jz5016444
The electric fields generated by excited-state electron injection into anatase TiO2 nanocrystallites are screened by cations present in the external electrolyte. With some assumptions, a newly discovered electroabsorption signature enables quantification of the electric field strength experienced by surface-anchored dye molecules. Here, it was found that the fields increased in the order Na+ < Li+ < Mg2+ < Ca2+, with magnitudes of 1.1 MV/cm for Na+ and 2.2 MV/cm for Ca2+, values that were insensitive to whether the anion was iodide or perchlorate. The magnitude of the field was directly related to average TiO2(e–) + I3– → charge recombination rate constants abstracted from time-resolved kinetic data. Extrapolation to zero field provided an estimate of recombination dynamics when diffusion alone controlled I3– mass transport, k = 300 s–1. The decreased rate constants measured after excited-state injection were attributed to migration of I3– away from the TiO2. Cation transference coefficients were tabulated that ranged from t = 0.97 for Ca2+ to 0.40 for Na+ and represented the ability of the unscreened electric field to block the TiO2(e–) + I3– → charge recombination reaction. This data provides the first compelling evidence that the anionic nature of I3– inhibits unwanted charge recombination in dye-sensitized solar cells.Keywords: charge screening; dye-sensitized solar cell (DSSC); ruthenium; titanium dioxide;
Co-reporter:André Bessette, Mihaela Cibian, Janaina G. Ferreira, Brian N. DiMarco, Francis Bélanger, Denis Désilets, Gerald J. Meyer and Garry S. Hanan
Dalton Transactions 2016 - vol. 45(Issue 26) pp:NaN10576-10576
Publication Date(Web):2016/06/06
DOI:10.1039/C6DT00961A
In the on-going quest to harvest near-infrared (NIR) photons for energy conversion applications, a novel family of neutral ruthenium(II) sensitizers has been developed by cyclometalation of an azadipyrromethene chromophore. These rare examples of neutral ruthenium complexes based on polypyridine ligands exhibit an impressive panchromaticity achieved by the cyclometalation strategy, with strong light absorption in the 600–800 nm range that tails beyond 1100 nm in the terpyridine-based adducts. Evaluation of the potential for Dye-Sensitized Solar Cells (DSSC) and Organic Photovoltaic (OPV) applications is made through rationalization of the structure–property relationship by spectroscopic, electrochemical, X-ray structural and computational modelization investigations. Spectroscopic evidence for photo-induced charge injection into the conduction band of TiO2 is also provided.
Co-reporter:Atefeh Taheri and Gerald J. Meyer
Dalton Transactions 2014 - vol. 43(Issue 47) pp:NaN17863-17863
Publication Date(Web):2014/09/23
DOI:10.1039/C4DT01683A
The metal-to-ligand charge transfer (MLCT) excited states of two related heteroleptic Ru(II) compounds [Ru(bpy)2(deeb)]2+ and [Ru(bpy)2(deebq)]2+, where bpy is 2,2′-bipyridine, deeb is 4,4′-(CO2CH2CH3)2-2,2′-bipyridine and deebq is 4,4′-(CO2CH2CH3)2-2,2′-biquinoline, were characterized in fluid acetonitrile by temperature dependent photoluminescence spectroscopies as well as quenching by iodide ions. Photoluminescence emanates from a manifold of thermally equilibrated excited states referred to as the thexi states. Evidence for activated internal conversion to a 4th MLCT excited state was garnered from an Arrhenius analysis of temperature dependent lifetime data. The activation energy was found to be 550 cm−1 for [Ru(bpy)2(deeb)]2+* and 1200 cm−1 for [Ru(bpy)2(deebq)]2+*. The pre-exponential factor abstracted from the Arrhenius analysis of the [Ru(bpy)2(deebq)]2+* data suggested that ligand field excited states might be populated, however there was no evidence for ligand loss photochemistry under the conditions studied. The excited states were found to quench iodide by a dynamic process in good agreement with the Stern–Volmer model. Transient absorption data showed that the quenching mechanism was electron transfer to generate an iodine atom and a reduced ruthenium compound as products. The quenching rate constants abstracted from temperature dependent Stern–Volmer quenching data were corrected for diffusion and activated complex formation to yield electron transfer rate constants that were found to increase markedly with temperature. An Arrhenius analysis of the electron transfer data revealed that electron transfer from iodide to the d-orbitals of the excited state was an activated process with an Ea of 2400 cm−1 for [Ru(bpy)2(deeb)]2+ and 3300 cm−1 for [Ru(bpy)2(deebq)]2+.
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