We report the first synthesis of norbornyl-bridged acene dimers (2 and 3) with well-defined and controlled spatial relationships between the acene chromophore subunits. We employ a modular 2-D strategy wherein the central module, common to all our compounds, is a norbornyl moiety. The acenes are attached to this module using the Diels–Alder reaction, which also forms one of the acene rings. Manipulation of the Diels–Alder adducts provides the desired geometrically defined bis-acenes. The modular nature of this synthesis affords flexibility and allows for the preparation of a variety of acene dimers, including functionalized tetracene dimers.

2,2′:6′,2″-Terpyridyl (tpy) ligands modified by fluorine (dftpy), chlorine (dctpy), or bromine (dbtpy) substitution at the 6- and 6″-positions are used to synthesize a series of bis-homoleptic Fe(II) complexes. Two of these species, [Fe(dctpy)2]2+ and [Fe(dbtpy)2]2+, which incorporate the larger dctpy and dbtpy ligands, assume a high-spin quintet ground state due to substituent-induced intramolecular strain. The smaller fluorine atoms in [Fe(dftpy)2]2+ enable spin crossover with a T1/2 of 220 K and a mixture of low-spin (singlet) and high-spin (quintet) populations at room temperature. Taking advantage of this equilibrium, dynamics originating from either the singlet or quintet manifold can be explored using variable wavelength laser excitation. Pumping at 530 nm leads to ultrafast nonradiative relaxation from the singlet metal-to-ligand charge transfer (1MLCT) excited state into a quintet metal centered state (5MC) as has been observed for prototypical low-spin Fe(II) polypyridine complexes such as [Fe(tpy)2]2+. On the other hand, pumping at 400 nm excites the molecule into the quintet manifold (5MLCT ← 5MC) and leads to the observation of a greatly increased MLCT lifetime of 14.0 ps. Importantly, this measurement enables an exploration of how the lifetime of the 5MLCT (or 7MLCT, in the event of intersystem crossing) responds to the structural modifications of the series as a whole. We find that increasing the amount of steric strain serves to extend the lifetime of the 5,7MLCT from 14.0 ps for [Fe(dftpy)2]2+ to the largest known value at 17.4 ps for [Fe(dbtpy)2]2+. These data support the design hypothesis wherein interligand steric interactions are employed to limit conformational dynamics and/or alter relative state energies, thereby slowing nonradiative loss of charge-transfer energy.

A detailed photophysical picture is elaborated for a structurally well-defined and symmetrical bis-tetracene dimer in solution. The molecule was designed for interrogation of the initial photophysical steps (S1 → 1TT) in intramolecular singlet fission (SF). (Triisopropylsilyl)acetylene substituents on the dimer TIPS-BT1 as well as a monomer model TIPS-Tc enable a comparison of photophysical properties, including transient absorption dynamics, as solvent polarity is varied. In nonpolar toluene solutions, TIPS-BT1 decays via radiative and nonradiative pathways to the ground state with no evidence for dynamics related to the initial stages of SF. This contrasts with the behavior of the previously reported unsubstituted dimer BT1 and is likely a consequence of energetic perturbations to the singlet excited-state manifold of TIPS-BT1 by the (trialkylsilyl)acetylene substituents. In polar benzonitrile, two key findings emerge. First, photoexcited TIPS-BT1 shows a bifurcation into both arm-localized (S1-loc) and dimer-delocalized (S1-dim) singlet exciton states. The S1-loc decays to the ground state, and weak temperature dependence of its emissive signatures suggests that once it is formed, it is isolated from S1-dim. Emissive signatures of the S1-dim state, on the other hand, are strongly temperature-dependent, and transient absorption dynamics show that S1-dim equilibrates with an intramolecular charge-transfer state in 50 ps at room temperature. This equilibrium decays to the ground state with little evidence for formation of long-lived triplets nor 1TT. These detailed studies spectrally characterize many of the key states in intramolecular SF in this class of dimers but highlight the need to tune electronic coupling and energetics for the S1 → 1TT photoreaction.

Halogen substitution at the 6 and 6″ positions of terpyridine (6,6″-Cl2-2,2:6′,2″-terpyridine = dctpy) is used to produce a room-temperature high-spin iron(II) complex [Fe(dctpy)2](BF4)2. Using UV−vis absorption, spectroelectrochemistry, transient absorption, and TD-DFT calculations, we present evidence that the quintet metal-to-ligand charge-transfer excited state (5MLCT) can be accessed via visible light absorption and that the thermalized 5,7MLCT is long-lived at 16 ps, representing a > 100 fold increase compared to the 1,3MLCT within species such as [Fe(bpy)3]2+. This result opens a new strategy for extending iron(II) MLCT lifetimes for potential use in photoredox processes.

We report the effect of crystal structure and crystallite grain size on singlet fission (SF) in polycrystalline tetracene, one of the most widely studied SF and organic semiconductor materials. SF has been comprehensively studied in one polymoprh (Tc I), but not in the other, less stable polymorph (Tc II). Using carefully controlled thermal evaporation deposition conditions and high sensitivity ultrafast transient absorption spectroscopy, we found that for large crystallite size samples, SF in nearly pure Tc II films is significantly faster than SF in Tc I films. We also discovered that crystallite size has a minimal impact on the SF rate in Tc II films, but a significant influence in Tc I films. Large crystallites exhibit SF times of 125 ps and 22 ps in Tc I and Tc II, respectively, whereas small crystallites have SF times of 31 ps and 33 ps. Our results demonstrate first, that attention must be paid to polymorphism in obtaining a self-consistent rate picture for SF in tetracene and second, that control of polymorphism can play a significant role towards achieving a mechanistic understanding of SF in polycrystalline systems. In this latter context we show that conventional theory based on non-covalent tetracene couplings is insufficient, thus highlighting the need for models that capture the delocalized and highly mobile nature of excited states in elucidating the full photophysical picture.

The photophysics of a norbornyl-bridged covalent tetracene (Tc) dimer BT1 and a monomer analogue Tc-e were studied in room-temperature nonpolar solvents. Notably in BT1, a Davydov-split band is observed in UV absorption, heralding interchromophore electronic interactions. Emission spectra indicate an acene-like vibronic progression mirroring the lowest-energy visible absorption. For BT1, this argues against excited-state excimer formation. Evidence of intramolecular singlet fission (SF) comes from a comparison of time-resolved emission decay signals collected for BT1 versus Tc-e in toluene. In BT1, the multiexcitonic 1TT state is produced in 70 ns in 6% yield. A ratio of fission versus fusion rate constants provides an experimental measure of the SF reaction free energy at 52 meV in good agreement with previous calculations. The low SF yield corroborates our expectations that orbital symmetry effects on diabatic coupling for SF are important for dimers that cannot rely on more favorable thermodynamics.

To explore the impact of dye structure on photoinduced interfacial electron-transfer (ET) processes, a series of systematically tuned 4′-aryl-substituted terpyridyl ruthenium(II) complexes have been studied in TiO2 film and dye-sensitized solar cell (DSSC) device settings. Structural tuning is achieved by the introduction of methyl substituents at the ortho positions of a ligand aryl moiety. Solar power conversion efficiencies are measured, and these values are deconstructed to better understand the fundamental processes that control light-to-current conversion. Injection yields are identified as the primary factor limiting efficiencies, due in large part to significant nonradiative decay pathways in these bis-terpyridyl Ru(II) systems. Encouragingly, the addition of methyl steric bulk is found to inhibit charge recombination, with measured recombination lifetimes increasing by over 12-fold across the series of structurally tuned complexes. If injection yields can be improved, the structural tuning of recombination rate constants may be an important design strategy for improving solar conversion efficiency in solar cells and water-splitting devices.

While singlet fission (SF) has developed in recent years within material settings, much less is known about its control in covalent dimers. Such platforms are of fundamental importance and may also find practical use in next-generation dye-sensitized solar cell applications or for seeding SF at interfaces following exciton transport. Here, facile theoretical tools based on Boys localization methods are used to predict diabatic coupling for SF via determination of one-electron orbital coupling matrix elements. The results expose important design rules that are rooted in point group symmetry. For Cs-symmetric dimers, pathways for SF that are mediated by virtual charge transfer excited states destructively interfere with negative impact on the magnitude of diabatic coupling for SF. When dimers have C2 symmetry, constructive interference is enabled for certain readily achievable interchromophore orientations. Three sets of dimers exploiting these ideas are explored: a bis–tetracene pair and two sets of aza-substituted tetracene dimers. Remarkable control is shown. In one aza-substituted set, symmetry has no impact on SF reaction thermodynamics but leads to a 16-fold manipulation in SF diabatic coupling. This translates to a difference of nearly 300 in kSF with the faster of the two dimers (C2) being predicted to undergo the process on a nearly ultrafast 1.5 ps time scale.

Singlet fission (SF) offers opportunities for wavelength-selective processing of solar photons with an end goal of achieving higher efficiency inexpensive photovoltaic or solar-fuels-producing devices. In order to evaluate new molecular design strategies and for theoretical exploration of dynamics, it is important to put in place tools for efficient calculation of the electronic coupling between single-exciton reactant and multiexciton product states. For maximum utility, the couplings should be calculated at multiple nuclear geometries (rather than assumed constant everywhere, i.e., the Condon approximation) and we must be able to evaluate couplings for covalently linked multichromophore systems. With these requirements in mind, here we discuss the simplest methodology possible for rapid calculation of diabatic one-electron coupling matrix elements—based on Boys localization and rediagonalization of molecular orbitals. We focus on a covalent species called BT1 that juxtaposes two tetracene units in a partially cofacial geometry via a norbornyl bridge. In BT1, at the equilibrium C2v structure, the “nonhorizontal” couplings between HOMOs and LUMOs (tHL and tLH) vanish by symmetry. We then explore the impact of molecular vibrations through the calculation of tAB coupling gradients along 183 normal modes of motion. Rules are established for the types of motions (irreducible representations in the C2v point group) that turn on tHL and tLH values as well as for the patterns that emerge in constructive versus destructive interference of pathways to the SF product. For the best modes, calculated electronic coupling magnitudes for SF (at root-mean-squared deviation in position at 298 K), are within a factor of 2 of that seen for noncovalent tetracene dimers relevant to the molecular crystal. An overall “effective” electronic coupling is also given, based on the Stuchebrukhov formalism for non-Condon electron transfer rates.

Synthesis, electrochemical potentials, static emission, and temperature-dependent excited-state lifetimes of several 4′-aryl-substituted terpyridyl complexes of ruthenium(II) are reported. Synthetic tuning is explored within three conceptual series of complexes. The first series explores the impact of introducing a strong σ-donating 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine (tbtpy) opposite to an arylated terpyridine ligand 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (ttpy). It is found that 3MLCT (triplet metal-to-ligand charge-transfer state) stabilization concomitant with 3MC (triplet metal-centered state) destabilization in the heteroleptic parent complex [Ru(ttpy)(tbtpy)]2+ leads to an extended excited-state lifetime relative to the structurally related bis-homoleptic species [Ru(ttpy)2]2+. The second series explores the impact of introducing a carboxylic acid or a methyl ester moiety at the para-position of the arylterpyridyl ligand (R1 = R2 = H) within heteroleptic complexes as a platform for future semiconductor attachment studies. This substitution leads to further lifetime enhancements, understood as arising from 3MLCT stabilization. Such complexes are referred to as [Ru(1)(tbtpy)]2+ (for the acid at R3) and [Ru(1′)(tbtpy)]2+ (for the ester at R3). In the final series, methyl substituents are sequentially added at the R1 and R2 positions for both the acid ([Ru(2)(tbtpy)]2+ and [Ru(3)(tbtpy)]2+) and ester ([Ru(2′)(tbtpy)]2+ and [Ru(3′)(tbtpy)]2+) analogues to eventually explore dynamical electron transfer coupling at dye/semiconductor interfaces. In these complexes, sequential addition of steric bulk decreases excited state lifetimes. This can be understood to arise primarily from the increase of the 3MLCT level, as excited-state electron delocalization is limited by inter-ring twisting in the lower-energy arylated ligand. The introduction of a dimethylated sterically encumbered ligand lead to a notable 14-fold increase in knr from [Ru(1′)(tbtpy)]2+ to [Ru(3′)(tbtpy)]2+ (or [Ru(1)(tbtpy)]2+ to [Ru(3)(tbtpy)]2+).

Computational studies of alicyclic carbodiimides (RN═C═NR) (rings five through twelve) at the MP2/6-31G(d,p)//MP2/6-31G(d,p) level of theory were conducted to locate the transition states between carbodiimides isomers. Transition states for rings six through twelve were found. The RNCNR dihedral angle is ∼0° for even-numbered rings, but deviates from 0° for rings seven, nine, eleven, and twelve. The even- and odd-numbered ring transition states have different symmetry point groups. Cs transition states (even rings) have an imaginary frequency mode that transforms as the asymmetric irreducible representation of the group. C2 transition states (odd rings) have a corresponding mode that transforms as the totally symmetric representation. Intrinsic reaction coordinate analyses followed by energy minimization along the antisymmetric pathways led to enantiomeric pairs. The symmetric pathways give diastereomeric isomers. The five-membered ring carbodiimide is a stable structure, possibly isolable. A twelve-membered ring transition state was found only without applying symmetry constraints (C1). Molecular mechanics and molecular dynamics studies of the seven-, eight-, and nine-membered rings gave additional structures, which were then minimized using ab initio methods. No structures beyond those found from the IRC analyses described were found. The potential for optical resolution of the seven-membered ring is discussed.

Photophysics of the MLCT excited-state of [Ru(bpy)(tpy)(OH2)]2+ (1) and [Ru(bpy)(tpy)(OD2)]2+ (2) (bpy = 2,2′-bipyridine and tpy = 2,2′:6′,2″-terpyridine) have been investigated in room-temperature H2O and D2O using ultrafast transient pump-probe spectroscopy. An inverse isotope effect is observed in the ground-state recovery for the two complexes. These data indicate control of excited-state lifetime via a pre-equilibrium between the 3MLCT state that initiates H-bond dynamics with the solvent and the 3MC state that serves as the principal pathway for nonradiative decay.

We describe the charge transfer interactions between photoexcited CdS nanorods and mononuclear water oxidation catalysts derived from the [Ru(bpy)(tpy)Cl]+ parent structure. Upon excitation, hole transfer from CdS oxidizes the catalyst (Ru2+ → Ru3+) on a 100 ps to 1 ns timescale. This is followed by 10–100 ns electron transfer (ET) that reduces the Ru3+ center. The relatively slow ET dynamics may provide opportunities for the accumulation of multiple holes at the catalyst, which is necessary for water oxidation.

Three new photoinduced electron donor−acceptor (D−A) systems are reported which juxtapose a Ru(II) excited-state donor with a bipyridinium acceptor via a conformationally active asymmetric aryl-substituted bipyridine ligand participating in the bridge between D and A. Across the series of complexes 1−3, steric bulk is sequentially added to tune the inter-ring dihedral angle θ between the bipyridine and the aryl substituent. Driving forces for photoinduced electron transfer (ΔGET) and back electron transfer (ΔGBET) are reported based on electrochemical measurements of 1−3 as well as Franck−Condon analysis of emission spectra collected for three new donor model complexes 1′−3′. These preserve the substitution patterns on the aryl substituent in their respective D–A complexes but remove the bipyridinium acceptor. Both ΔGET and ΔGBET are invariant to within 0.02 eV across the series. Upon visible photoexcitation of each of the D−A systems with ∼100 fs laser pulses at 500 ± 10 nm, an electron-transfer (ET) photoproduct is observed to form with a time constant of τET = 29 ps (1), 37 ps (2), and 57 ps (3). That ET remains relatively rapid throughout this series, even as steric bulk significantly increases the inter-ring dihedral angle θ, is attributed to the effects of ligand-based torsional dynamics driven by intraligand electron delocalization in the D*−A excited state manifold prior to ET. The lifetimes of the charge-separated states (τBET) are also reported with τBET = 98 ps (1), 217 ps (2), and 789 ps (3), representing a more than 8-fold increase across the series. This is attributed to reverse conformational dynamics in D+−A− driven by steric repulsions, which serves to minimize electronic coupling to the ground state. Steric control of ligand geometry and the range over which θ changes during conformational dynamics provides a new strategy to facilitate the formation and storage of charge-separated excited states.

We report the preparation and characterization of Cr(III) coordination complexes featuring the dimethyl 2,2′-bipyridine-4,4′-dicarboxylate (4-dmcbpy) ligand: [(phen)2Cr(4-dmcbpy)](OTf)3 (1), [(Ph2phen)2Cr(4-dmcbpy)](OTf)3 (4), [(Me2bpy)2Cr(4-dmcbpy)](OTf)3 (7), and [Cr(4-dmcbpy)3](BF4)3 (8), where phen is 1,10-phenanthroline, Ph2phen is 4,7-diphenyl-1,10-phenanthroline, and Me2bpy is 4,4′-dimethyl-2,2′-bipyridine. Static and nanosecond time-resolved absorption and emission properties of these complexes dissolved in acidic aqueous (1 M HCl) solutions are reported. Emission spectra collected at 297 K show a narrow spectrum with an emission maximum ranging from 732 nm (1) to 742 nm (4). The emissive state is thermally activated and decays via first order kinetics at all temperatures explored (283 to 353 K). At 297 K the observed lifetime ranges from 7.7 μs (8) to 108 μs (4). The photophysical data suggest that in these acidic aqueous environments these complexes store ∼1.7 eV for multiple microseconds at room temperature. Of the heteroleptic species, complex 4 shows the greatest absorption of visible wavelengths (ε = 1270 M−1 cm−1 at 491 nm), and homoleptic complex 8 has improved absorption at visible wavelengths over [Cr(bpy)3]3+. The electrochemical properties of 1, 4, 7, and 8 were investigated by cyclic voltammetry. It is found that inclusion of 4-dmcbpy shifts the “CrIII/II” E1/2 by +0.22 V compared to those of homoleptic parent complexes, with the first reduction event occurring at −0.26 V versus Fc+/Fc for 8. The electrochemical and photophysical data allow for excited state potentials to be determined: for 8, CrIII*/II lies at +1.44 V versus ferrocenium/ferrocene (∼+2 V vs NHE), placing it among the most powerful photooxidants reported.

Computational studies using density functional theory (DFT) are reported for a series of donor−acceptor (DA) transition metal complexes and related excited-state and electron transfer (ET) photoproduct models. Three hybrid Hartree−Fock/DFT (HF/DFT) functionals, B3LYP, B3PW91, and PBE1PBE, are employed to characterize structural features implicated in the dynamical control of productive forward and energy wasting back ET events. Energies and optimized geometries are reported for the lowest energy singlet state in [Ru(dmb)2(bpy-ϕ-MV)]4+ (DA1), [Ru(dmb)2(bpy-o-tolyl-MV)]4+ (DA2), [Ru(dmb)2(bpy-2,6-Me2-ϕ-MV)]4+ (DA3), and [Ru(tmb)2(bpy-2,6-Me2-ϕ-MV)]4+ (DA3′), where dmb is 4,4′-dimethyl-2,2′-bipyridine, tmb is 4,4′,5,5′-tetramethyl-2,2′-bipyridine, MV is methyl viologen, and ϕ is a phenylene spacer. These indicate that the dihedral angle θ1 between the aryl substituent and the bipyridine fragment to which it is bound, systematically increases with the addition of steric bulk. Energies, optimized geometries, and unpaired electron spin densities are also reported for the lowest energy triplet state of [Ru(dmb)2(4-p-tolyl-2,2′-bipyridine)]2+ (D1*), [Ru(dmb)2(4-(2,6-dimethylphenyl)-2,2′-bipyridine)]2+ (D2*), [Ru(dmb)2(4-mesityl-2,2′-bipyridine)]2+ (D3*), and [Ru(tmb)2(4-mesityl-2,2′-bipyridine)]2+ (D3′*). Each of these serves as a model of a reactant excited state in the forward electron-transfer photochemistry allowing us to qualify and quantify the role of excited-state intraligand electron delocalization in driving substantial geometry changes (especially with respect to θ1) relative to its respective DA counterpart. Next, energies, optimized geometries, and spin densities are reported for the lowest energy triplet of each DA species: 3DA1, 3DA2, 3DA3, and 3DA3′. These are used to model the ET photoproduct and they indicate that θ1 increases following ET, thus, verifying switch-like properties. Finally, we report data for geometry optimized DA1 and 3DA1 in a continuum model of room temperature acetonitrile. This study shows a complete recovery of θ1 to its ground state value which has implications in efforts to trap electrons in charge-separated states.

Synthesis, ground-, and excited-state properties are reported for two new electron donor-bridge-acceptor (D-B-A) molecules and two new photophysical model complexes. The D-B-A molecules are [Ru(bpy)2(bpy-ϕ-MV)](PF6)4 (3) and [Ru(tmb)2(bpy-ϕ-MV)](PF6)4 (4), where bpy is 2,2′-bipyridine, tmb is 4,4′,5,5′-tetramethyl-2,2′-bipyridine, MV is methyl viologen, and ϕ is a phenylene spacer. Their model complexes are [Ru(bpy)2(p-tol-bpy)](PF6)2 (1) and [Ru(tmb)2(p-tol-bpy)](PF6)2 (2), where p-tolyl-bpy is 4-(p-tolyl)-2,2′-bipyridine. Photophysical characterization of 1 and 2 indicates that 2.17 eV and 2.12 eV are stored in their respective 3MLCT (metal-to-ligand charge transfer) excited state. These values along with electrochemical measurements show that photoinduced electron transfer (D*-B-A → D+-B-A−) is favorable in 3 and 4 with ΔG°ET = −0.52 eV and −0.62 eV, respectively. The driving force for the reverse process (D+-B-A− → D-B-A) is also reported: ΔG°BET = −1.7 eV for 3 and −1.5 eV for 4. Transient absorption (TA) spectra for 3 and 4 in 298 K acetonitrile provide evidence that reduced methyl viologen is observable at 50 ps following excitation. Detailed TA kinetics confirm this, and the data are fit to a model to determine both forward (kET) and back (kBET) electron transfer rate constants: kET = 2.6 × 1010 s−1 for 3 and 2.8 × 1010 s−1 for 4; kBET = 0.62 × 1010 s−1 for 3 and 1.37 × 1010 s−1 for 4. The similar rate constants kET for 3 and 4 despite a 100 meV driving force (ΔG°ET) increase suggests that forward electron transfer in these molecules in room temperature acetonitrile is nearly barrierless as predicted by the Marcus theory. The reduction in electron transfer reorganization energy necessary for this barrierless reactivity is attributed to excited-state electron delocalization in the 3MLCT excited states of 3 and 4, an effect that is made possible by excited-state conformational changes in the aryl-substituted ligands of these complexes.

We report the effect of crystal structure and crystallite grain size on singlet fission (SF) in polycrystalline tetracene, one of the most widely studied SF and organic semiconductor materials. SF has been comprehensively studied in one polymoprh (Tc I), but not in the other, less stable polymorph (Tc II). Using carefully controlled thermal evaporation deposition conditions and high sensitivity ultrafast transient absorption spectroscopy, we found that for large crystallite size samples, SF in nearly pure Tc II films is significantly faster than SF in Tc I films. We also discovered that crystallite size has a minimal impact on the SF rate in Tc II films, but a significant influence in Tc I films. Large crystallites exhibit SF times of 125 ps and 22 ps in Tc I and Tc II, respectively, whereas small crystallites have SF times of 31 ps and 33 ps. Our results demonstrate first, that attention must be paid to polymorphism in obtaining a self-consistent rate picture for SF in tetracene and second, that control of polymorphism can play a significant role towards achieving a mechanistic understanding of SF in polycrystalline systems. In this latter context we show that conventional theory based on non-covalent tetracene couplings is insufficient, thus highlighting the need for models that capture the delocalized and highly mobile nature of excited states in elucidating the full photophysical picture.