Outer-sphere Reorganization Energy Research Articles

Unraveling the mechanisms of triplet state formation in a heavy-atom free photosensitizer.

Triplet excited state generation plays a pivotal role in photosensitizers, however the reliance on transition metals and heavy atoms can limit the utility of these systems. In this study, we demonstrate that an interplay of competing quantum effects controls the high triplet quantum yield in a prototypical boron dipyrromethene-anthracene (BD-An) donor-acceptor dyad photosensitizer, which is only captured by an accurate treatment of both inner and outer sphere reorganization energies. Our ab initio-derived model provides excellent agreement with experimentally measured spectra, triplet yields and excited state kinetic data, including the triplet lifetime. We find that rapid triplet state formation occurs primarily via high-energy triplet states through both spin-orbit coupled charge transfer and El-Sayed's rule breaking intersystem crossing, rather than direct spin-orbit coupled charge transfer to the lowest lying triplet state. Our calculations also reveal that competing effects of nuclear tunneling, electronic state recrossing, and electronic polarizability dictate the rate of non-productive ground state recombination. This study sheds light on the quantum effects driving efficient triplet formation in the BD-An system, and offers a promising simulation methodology for diverse photochemical systems.

Non-adiabatic molecular dynamics simulations provide new insights into the exciton transfer in the Fenna-Matthews-Olson complex.

A trajectory surface hopping approach, which uses machine learning to speed up the most time-consuming steps, has been adopted to investigate the exciton transfer in light-harvesting systems. The present neural networks achieve high accuracy in predicting both Coulomb couplings and excitation energies. The latter are predicted taking into account the environment of the pigments. Direct simulation of exciton dynamics through light-harvesting complexes on significant time scales is usually challenging due to the coupled motion of nuclear and electronic degrees of freedom in these rather large systems containing several relatively large pigments. In the present approach, however, we are able to evaluate a statistically significant number of non-adiabatic molecular dynamics trajectories with respect to exciton delocalization and exciton paths. The formalism is applied to the Fenna-Matthews-Olson complex of green sulfur bacteria, which transfers energy from the light-harvesting chlorosome to the reaction center with astonishing efficiency. The system has been studied experimentally and theoretically for decades. In total, we were able to simulate non-adiabatically more than 30 ns, sampling also the relevant space of parameters within their uncertainty. Our simulations show that the driving force supplied by the energy landscape resulting from electrostatic tuning is sufficient to funnel the energy towards site 3, from where it can be transferred to the reaction center. However, the high efficiency of transfer within a picosecond timescale can be attributed to the rather unusual properties of the BChl a molecules, resulting in very low inner and outer-sphere reorganization energies, not matched by any other organic molecule, e.g., used in organic electronics. A comparison with electron and exciton transfer in organic materials is particularly illuminating, suggesting a mechanism to explain the comparably high transfer efficiency.

Borylation in the Second Coordination Sphere of FeII Cyanido Complexes and Its Impact on Their Electronic Structures and Excited-State Dynamics

Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B(C6F5)3 resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two FeII cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the FeII-based t2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV–vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV–vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B(C6F5)3 attachment at the cyanido N lone pair.

Bioinorganic Chemistry on Electrodes: Methods to Functional Modeling

One of the major goals of bioinorganic chemistry has been to mimic the function of elegant metalloenzymes. Such functional modeling has been difficult to attain in solution, in particular, for reactions that require multiple protons and multiple electrons (nH+/ne-). Using a combination of heterogeneous electrochemistry, electrode and molecule design one may control both electron transfer (ET) and proton transfer (PT) of these nH+/ne- reactions. Such control can allow functional modeling of hydrogenases (H+ + e- → 1/2 H2), cytochrome c oxidase (O2 + 4 e- + 4 H+ → 2 H2O), monooxygenases (RR'CH2 + O2 + 2 e- + 2 H+ → RR'CHOH + H2O) and dioxygenases (S + O2 → SO2; S = organic substrate) in aqueous medium and at room temperatures. In addition, these heterogeneous constructs allow probing unnatural bioinspired reactions and estimation of the inner- and outer-sphere reorganization energy of small molecules and proteins.

AutoSolvate: A toolkit for automating quantum chemistry design and discovery of solvated molecules.

The availability of large, high-quality datasets is crucial for artificial intelligence design and discovery in chemistry. Despite the essential roles of solvents in chemistry, the rapid computational dataset generation of solution-phase molecular properties at the quantum mechanical level of theory was previously hampered by the complicated simulation procedure. Software toolkits that can automate the procedure to set up high-throughput explicit-solvent quantum chemistry (QC) calculations for arbitrary solutes and solvents in an open-source framework are still lacking. We developed AutoSolvate, an open-source toolkit, to streamline the workflow for QC calculation of explicitly solvated molecules. It automates the solvated-structure generation, force field fitting, configuration sampling, and the final extraction of microsolvated cluster structures that QC packages can readily use to predict molecular properties of interest. AutoSolvate is available through both a command line interface and a graphical user interface, making it accessible to the broader scientific community. To improve the quality of the initial structures generated by AutoSolvate, we investigated the dependence of solute-solvent closeness on solute/solvent identities and trained a machine learning model to predict the closeness and guide initial structure generation. Finally, we tested the capability of AutoSolvate for rapid dataset curation by calculating the outer-sphere reorganization energy of a large dataset of 166 redox couples, which demonstrated the promise of the AutoSolvate package for chemical discovery efforts.

Probing the thermodynamics and kinetics of ethylene carbonate reduction at the electrode-electrolyte interface with molecular simulations.

Understanding the formation of the solid-electrolyte interphase (SEI) in lithium-ion batteries is an ongoing area of research due to its high degree of complexity and the difficulties encountered by experimental studies. Herein, we investigate the initial stage of SEI growth, the reduction reaction of ethylene carbonate (EC), from both a thermodynamic and a kinetic approach with theory and molecular simulations. We employed both the potential distribution theorem and the Solvation Method based on Density (SMD) to EC solvation for the estimation of reduction potentials of Li+, EC, and Li+-solvating EC (s-EC) as well as reduction rate constants of EC and s-EC. We find that solvation effects greatly influence these quantities of interest, particularly the Li+/Li reference electrode potential in EC solvent. Furthermore, we also compute the inner- and outer-sphere reorganization energies for both EC and s-EC at the interface of liquid EC and a hydroxyl-terminated graphite surface, where total reorganization energies are predicted to be 76.6 and 88.9 kcal/mol, respectively. With the computed reorganization energies, we estimate reduction rate constants across a range of overpotentials and show that EC has a larger electron transfer rate constant than s-EC at equilibrium, despite s-EC being more thermodynamically favorable. Overall, this manuscript demonstrates how ion solvation effects largely govern the prediction of reduction potentials and electron transfer rate constants at the electrode-electrolyte interface.

Contributions to cytochrome c inner- and outer-sphere reorganization energy.

Cytochromes c are small water-soluble proteins that catalyze electron transfer in metabolism and energy conversion processes. Hydrogenobacter thermophilus cytochrome c552 presents a curious case in displaying fluxionality of its heme axial methionine ligand; this behavior is altered by single point mutation of the Q64 residue to N64 or V64, which fixes the ligand in a single configuration. The reorganization energy (λ) of these cytochrome c552 variants is experimentally determined using a combination of rotating disc electrochemistry, chronoamperometry and cyclic voltammetry. The differences between the λ determined from these complementary techniques helps to deconvolute the contribution of the active site and its immediate environment to the overall λ (λTotal). The experimentally determined λ values in conjunction with DFT calculations indicate that the differences in λ among the protein variants are mainly due to the differences in contributions from the protein environment and not just inner-sphere λ. DFT calculations indicate that the position of residue 64, responsible for the orientation of the axial methionine, determines the geometric relaxation of the redox active molecular orbital (RAMO). The orientation of the RAMO with respect to the heme is key to determining electron transfer coupling (HAB) which results in higher ET rates in the wild-type protein relative to the Q64V mutant despite a 150 mV higher λTotal in the former.

The effect of inner-sphere reorganization on charge separated state lifetimes at sensitized TiO2 interfaces.

Improving the efficiency of photo-electrocatalytic cells depends on controlling the rates of interfacial electron transfer to promote the formation of long-lived charge separated states. Ultimately, for efficient catalytic assemblies to see widespread implementation, repeated electron transfer in the absence of charge recombination needs to be realized. In this study, a series of manganese-based transition metal complexes known to undergo charge transfer-induced spin crossover are employed to study how significant increases in inner-sphere reorganization energy affect the rates of interfacial electron transfer. Each complex is characterized by transient spectroscopic and electrochemical methods to calculate the rate of electron transfer to a model chromophore anchored to the surface of a TiO2 film. Likewise, open-circuit voltage decay measurements were used to determine the voltage-dependent lifetime of injected electrons in TiO2 in the presence of each complex. To further characterize the rates of electronic recombination, density functional theory was used to calculate the inner-sphere and outer-sphere reorganization energy for each complex. These calculations were then combined with classical Marcus theory to determine the theoretical rate of back-electron transfer from the TiO2 conduction band. These results show that, in model complexes, a significant reduction in the recombination rate constant is achieved for complexes possessing a significant inner-sphere reorganization energy.

Electron Transfer Reorganization Energies in the Electrode-Electrolyte Double Layer.

The total reorganization energy, λ, for interfacial electron transfer, ET, from a conductive electrode to redox-active molecules at fixed positions within the electric double layer, EDL, has been determined experimentally. Conductive indium-tin-oxide (ITO, In2O3:Sn) mesoporous films were functionalized with 4-[N,N-di(p-tolyl)-amino]benzylphosphonic acid (TPA) and/or [RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+ (RuP), where bpy is 2,2'-bipyridine. The small inner-sphere reorganizations, λi, for RuIII/IIP and TPA+/0 make them excellent probes of outer-sphere reorganization energy, λo, as λi ≪ λo such that λ = λi + λo ≈ λo. Consecutive layer-by-layer addition of ZrIV-bridged methylenediphosphonic acid enabled positioning at distances from 4 to 27 Å from the ITO. Excited-state injection into the ITO by RuP* generated ITO(e-)|RuIIIP. For ITO cofunctionalized with TPA and RuP, subnanosecond lateral ET yielded ITO(e-)|TPA+. The kinetics for ET from ITO to RuIIIP or TPA+ were quantified spectroscopically as a function of applied potential (Eapp) and hence driving force, -ΔG°. Marcus-Gerischer analysis of this data provided λ. Significantly, λo was near zero at close electrode proximity, λ = 0.11 eV at a distance of ∼4 Å, as manifest by kinetics largely insensitive to Eapp. In agreement with dielectric continuum theory, λ increased to values expected in CH3CN solution when the molecule was positioned at a distance of ∼27 Å (λ = 0.94 eV). The data reveal small intrinsic barriers for electron transfer proximate to conductive interfaces, which is an exploitable behavior in solar energy conversion and other applications that utilize transparent conductive oxides to accept or deliver electrons.

An Insulating Al2O3 Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized TiO2 Surfaces.

Three chromophores of the general form [Ru(bpy')2(4,4'-(PO3H2)2-2,2'-bipyridine)]2+, where bpy' is 4,4'-(C(CH3)3)2-2,2'-bipyridine (Ru(dtb)2P); 4,4'-(CH3O)2-2,2'-bipyridine (Ru(OMe)2P), and 2,2'-bipyridine (RuP) were anchored to mesoporous thin films of TiO2 nanocrystallites at saturation surface coverages to investigate lateral self-exchange RuIII/II intermolecular hole hopping in 0.1 M LiClO4/CH3CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E1/2 (RuIII/II) potential or by pulsed laser (532 nm, 8 ns fwhm) excitation and monitored by visible absorption chronoabsorptometry and time-resolved absorption anisotropy measurements, respectively. The hole hopping rate constant kR extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO2|Ru(OMe)2P (ket = 1.4 × 106 s-1) > TiO2|RuP (7.1 × 105 s-1) > TiO2|Ru(dtb)2P (6.5 × 104 s-1). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO2|RuP and TiO2|Ru(dtb)2P that were within experimental error the same as that measured with the potential step. The hole hopping rate constants were found to trend with the TiO2(e-)|RuIII → TiO2|RuII charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of Al2O3 on top of the dye-sensitized films was found to prevent hole hopping by both initiation methods even though the chromophore surface coverage exceeded the percolation threshold and excited-state injection was efficient. The dramatic hole hopping turnoff was attributed to a larger outer-sphere reorganization energy for self-exchange due to the restricted access of electrolyte to the redox active chromophores. The implications of these findings for solar energy conversion applications are discussed.

What Are We Missing by Not Measuring the Net Charge of Proteins?

The net electrostatic charge (Z) of a folded protein in solution represents a bird's eye view of its surface potentials-including contributions from tightly bound metal, solvent, buffer, and cosolvent ions-and remains one of its most enigmatic properties. Few tools are available to the average biochemist to rapidly and accurately measure Z at pH≠pI. Tools that have been developed more recently seem to go unnoticed. Most scientists are content with this void and estimate the net charge of a protein from its amino acid sequence, using textbook values of pKa . Thus, Z remains unmeasured for nearly all folded proteins at pH≠pI. When marveling at all that has been learned from accurately measuring the other fundamental property of a protein-its mass-one wonders: what are we missing by not measuring the net charge of folded, solvated proteins? A few big questions immediately emerge in bioinorganic chemistry. When a single electron is transferred to a metalloprotein, does the net charge of the protein change by approximately one elementary unit of charge or does charge regulation dominate, that is, do the pKa values of most ionizable residues (or just a few residues) adjust in response to (or in concert with) electron transfer? Would the free energy of charge regulation (ΔΔGz ) account for most of the outer sphere reorganization energy associated with electron transfer? Or would ΔΔGz contribute more to the redox potential? And what about metal binding itself? When an apo-metalloprotein, bearing minimal net negative charge (e.g., Z=-2.0) binds one or more metal cations, is the net charge abolished or inverted to positive? Or do metalloproteins regulate net charge when coordinating metal ions? The author's group has recently dusted off a relatively obscure tool-the "protein charge ladder"-and used it to begin to answer these basic questions.

Heterogeneous Rate Constants of the Electron-Transfer of Iron- and Ruthenium-bipyridine Complexes in Imidazolium-Based Ionic Liquids

The heterogeneous rate constants (ks) for the oxidation of iron- and ruthenium-bipyridine complexes, [Fe(bpy)3]²⁺/³⁺ and [Ru(bpy)3]²⁺/³⁺ have been determined using Nicholson's method in imidazolium-based ionic liquids with five different types of cation/anion over a range of temperature from 298–318 K. The heterogeneous rate constants of these redox reactions range from 10⁻⁴ to 10⁻³ cm s⁻¹, depending on the dynamic viscosity and type of cation/anion of the ILs. Marcus-Hush theory is used to explain the activation energies, Ea. Similar activation energies are found for the Fe-bipyridine and Ru-bipyridine complexes indicating that inner-sphere reorganization energy has only a small influence. For the calculation of the solvent-dependent outer sphere reorganization energy a dipole-free expression is used, adopted to the different dielectric properties of ionic liquids compared to organic solvents. i-Ru drops have been compensated using the corresponding experimental uncompensated Ru-values obtained from temperature dependent Bode-plots.

(Invited) Observation of the Marcus Inverted Region of Electron Transfer from Asymmetric Chemical Doping of Pristine (n,m) Single-Walled Carbon Nanotubes

The concept of electrical energy generation based on asymmetric chemical doping of single-walled carbon nanotube (SWNT) papers is presented. We explore 27 small, organic, electron-acceptor molecules that are shown to tune the output open-circuit voltage (VOC) across three types of pristine SWNT papers with varying (n,m) chirality distributions. A considerable enhancement in the observed VOC, from 80 to 440 mV, is observed for SWNT/molecule acceptor pairs that have molecular volume below 120 Å3 and lowest unoccupied molecular orbital (LUMO) energies centered around −0.8 eV. The electron transfer (ET) rate constants driving the VOC generation are shown to vary with the chirality-associated Marcus theory, suggesting that the energy gaps between SWNT and the LUMO of acceptor molecules dictate the ET process. When the ET rate constants and the maximum VOC are plotted versus the LUMO energy of the acceptor organic molecule, volcano-shaped dependencies, characteristic of the Marcus inverted region, are apparent for three distinct sources of SWNT papers with modes in diameter distributions of 0.95, 0.83, and 0.75 nm. This observation, where the ET driving force exceeds reorganization energies, allows for an estimation of the outer-sphere reorganization energies with values as low as 100 meV for the (8,7) SWNT, consistent with a proposed image-charge modified Born energy model. These results expand the fundamental understanding of ET transfer processes in SWNT and allow for an accurate calculation of energy generation through asymmetric doping for device applications. Reference: J. Am. Chem. Soc., 2017, 139 (43), 15328–15336

Observation of the Marcus Inverted Region of Electron Transfer from Asymmetric Chemical Doping of Pristine (n,m) Single-Walled Carbon Nanotubes.

The concept of electrical energy generation based on asymmetric chemical doping of single-walled carbon nanotube (SWNT) papers is presented. We explore 27 small, organic, electron-acceptor molecules that are shown to tune the output open-circuit voltage (VOC) across three types of pristine SWNT papers with varying (n,m) chirality distributions. A considerable enhancement in the observed VOC, from 80 to 440 mV, is observed for SWNT/molecule acceptor pairs that have molecular volume below 120 Å3 and lowest unoccupied molecular orbital (LUMO) energies centered around -0.8 eV. The electron transfer (ET) rate constants driving the VOC generation are shown to vary with the chirality-associated Marcus theory, suggesting that the energy gaps between SWNT and the LUMO of acceptor molecules dictate the ET process. When the ET rate constants and the maximum VOC are plotted versus the LUMO energy of the acceptor organic molecule, volcano-shaped dependencies, characteristic of the Marcus inverted region, are apparent for three distinct sources of SWNT papers with modes in diameter distributions of 0.95, 0.83, and 0.75 nm. This observation, where the ET driving force exceeds reorganization energies, allows for an estimation of the outer-sphere reorganization energies with values as low as 100 meV for the (8,7) SWNT, consistent with a proposed image-charge modified Born energy model. These results expand the fundamental understanding of ET transfer processes in SWNT and allow for an accurate calculation of energy generation through asymmetric doping for device applications.

On the Control of Electron Transfer Rates through the Morphology of Nanoparticles

It will be shown how the morphology and dimension of inorganic nanostructures in a solution should affect the rates of outer sphere electron transfer for nearby redox ions much differently than is predicted for a macroscopic planar interface. Simple Marcus theory and electrostatic evaluations of the self-energy of the ions near these nanostructures are used to calculate this effect, which is controlled by the degree to which the environment about the ion is the high dielectric permittivity of the solid rather than solution. A higher degree results in a lower reorganization energy for the ion and a smaller activation energy for electron transfer. The extreme of interest to this discussion is that of a redox ion within a spherical or cylindrical nanocavity of an inorganic solid.It has long been known how simple Marcus theory predicts electron transfer rates for donor and acceptor couples as a function of the difference in the free energy of the reaction and experimentation1 has confirmed the predicted parabolic form of transfer rates and the “inverted region,” where rates slow down. Variations in the outer sphere reorganization energies of donor-acceptor couples also result in changes of charge transfer rates predicted by this theory and also indicate this decrease in transfer rates.2 Recently, it has been demonstrated how the temperature dependence of electron transfer rates for a specific donor-acceptor couples also describes this parabolic behavior according to theory.3 This presentation on nanostructures and outer sphere electron transfer explores how the dielectric permittivity within Marcus theory may be an additional variable to control rates of electron transfer reactions.Geometric configurations will be examined in this work in the limits where the radius of the nanocavity of the solid varies from large compared to that of the redox ion within it to where it approaches the size of the redox ion. The electron transfer will be calculated as a self-exchange rate for reduced and oxidized forms of a redox couple to avoid considerations of electron exchange with the solid. To illustrate the principles involved, the solution will be taken as acetonitrile and the solid as ZnO. Spherical nanoshells will also be considered to illustrate how the same redox ion can have an accelerated electron transfer rate within the cavity while its rate for ions outside the shell slows down.4

Very Long-Lived Photoinduced Charge-Separated States of Triphenylamine-Naphthalenediimide Dyads in Polymer Matrices.

Photoinduced electron transfer was studied in dyads (dyad1 and dyad2) containing triphenylamine (MTA) and naphthalenediimide (MNDI) linked with oligo(phenyleneethynylene) dispersed in rigid polymer matrices of polystyrene (PS), poly(vinyl chloride), and poly(methyl methacrylate). Photoexcitation of these dyads yielded long-lived charge-separated (CS) states involving MTA+ and MNDI-. The quantum yields of charge separation in dyad1 and dyad2 were approximately 0.4 and 0.3, respectively, in the polymer matrices. The CS lifetime for dyad2 in PS was longer (400 ms) than those in poly(vinyl chloride) (120 ms) and poly(methyl methacrylate) (65 ms) at 298 K. In addition, CS state had a very long lifetime of 5.4 s in glassy toluene at 100 K. Below glass transition temperatures, polymer side chain motions with various relaxation rates should affect the charge recombination processes. The energy gap (ΔG) and outer-sphere reorganization energy (λ) in the charge recombination process were estimated using a slow-frequency component for dielectric constants. By use of ΔG and λ values, the matrix dependence of the CS lifetimes was successfully rationalized based on Marcus theory, and the charge recombination process in PS with low polarity and high polarizability should be in a deeper inverted region than the other polymer matrices. It also suggested that the rigidity of the polymer effectively suppressed intramolecular motions promoting the charge recombination process.

Single-Molecule Conductance of Viologen-Cucurbit[8]uril Host-Guest Complexes.

The local molecular environment is a critical factor which should be taken into account when measuring single-molecule electrical properties in condensed media or in the design of future molecular electronic or single molecule sensing devices. Supramolecular interactions can be used to control the local environment in molecular assemblies and have been used to create microenvironments, for instance, for chemical reactions. Here, we use supramolecular interactions to create microenvironments which influence the electrical conductance of single molecule wires. Cucurbit[8]uril (CB[8]) with a large hydrophobic cavity was used to host the viologen (bipyridinium) molecular wires forming a 1:1 supramolecular complex. Significant increases in the viologen wire single molecule conductances are observed when it is threaded into CB[8] due to large changes of the molecular microenvironment. The results were interpreted within the framework of a Marcus-type model for electron transfer as arising from a reduction in outer-sphere reorganization energy when the viologen is confined within the hydrophobic CB[8] cavity.

Electron Transfer Rate Maxima at Large Donor-Acceptor Distances.

Because of their low mass, electrons can transfer rapidly over long (>15 Å) distances, but usually reaction rates decrease with increasing donor-acceptor distance. We report here on electron transfer rate maxima at donor-acceptor separations of 30.6 Å, observed for thermal electron transfer between an anthraquinone radical anion and a triarylamine radical cation in three homologous series of rigid-rod-like donor-photosensitizer-acceptor triads with p-xylene bridges. Our experimental observations can be explained by a weak distance dependence of electronic donor-acceptor coupling combined with a strong increase of the (outer-sphere) reorganization energy with increasing distance, as predicted by electron transfer theory more than 30 years ago. The observed effect has important consequences for light-to-chemical energy conversion.

Structure–Property Relationships for Exciton and Charge Reorganization Energies of Dipolar Organic Semiconductors: A Combined Valence Bond Self-Consistent Field and Time-Dependent Hartree-Fock and DFT Study of Merocyanine Dyes

We present an analysis of the optoelectronic properties of merocyanine dyes by means of valence bond self-consistent field (VBSCF), time-dependent Hartree-Fock (TD-HF), density functional theory (DFT), and high-level ab initio calculations. The electronic structure of merocyanines can be described as a superposition of two resonance structures, a neutral one and a zwitterionic one. Calculated valence bond (VB) weights for these resonance structures demonstrate the importance of strong accepting groups when increasing the weight of the zwitterionic structures of different merocyanines. The dependence of exciton and charge reorganization energies on VB weights’ composition is analyzed, demonstrating that the special case of equal contributions of both structures, the so-called cyanine limit, goes along with minimal exciton and charge reorganization energies. For the latter, it is shown that the external (outer-sphere) reorganization energy plays a crucial role. Furthermore, a careful investigation of the ex...

Influence of a nearby substrate on the reorganization energy of hole exchange between dye molecules.

A numerical method is presented to estimate the influence of a nearby substrate on the polarization energy and outer sphere reorganization energy (λo) for intermolecular hole transfer for a series of dye molecules. The calculation considers the net charge distribution of the oxidised molecule (determined from quantum chemical calculation of the highest occupied molecular orbital of the neutral molecule within the frozen orbital approximation) encapsulated within a conformal cavity, by the molecules total electron density. An analytical point charge approximation was used at longer range. The molecular cavity was either surrounded by a single polarizable continuum, or, to simulate a nearby substrate, embedded at different positions relative to the interface between two semi-infinite slabs with different dielectric constants. The calculated λo values in the single dielectric medium were linearly related to the outer-sphere reorganisation energy calculated from DFT with a polarizable continuum model, validating the approach. In the two phase system, variations in λo was sensitive to the position of the substrate relative to the molecule and differences in the Pekar factor (1/εo - 1/εr) for the media. For dye molecules in ACN positioned touching a TiO2 substrate λo was typically about 20% lower than in pure ACN depending on the molecular configuration. Our approach can be adapted to systems of more than two media.