Exciton/Charge-Transfer Electronic Couplings in Organic Semiconductors

From Many-Body Ab Initio to Effective Excitonic Models: A Versatile Mapping Approach Including Environmental Embedding Effects

We present an original multistate projective diabatization scheme based on Green's function formalisms that allows the systematic mapping of many-body ab initio calculations onto effective excitonic models. This method inherits the ability of the Bethe-Salpeter equation to describe Frenkel molecular excitons and intermolecular charge-transfer states equally well, as well as the possibility for an effective description of environmental effects in a QM/MM framework. The latter is found to be a crucial element in order to obtain accurate model parameters for condensed phases and to ensure their transferability to excitonic models for extended systems. The method is presented through a series of examples illustrating its quality, robustness, and internal consistency.

Using Adiabatic Energy Splitting To Compute Dexter Energy Transfer Couplings

Dexter energy transfer and transport (DET) are of broad interest in energy science, and DET rates depend on electronic couplings between donor and acceptor species. DET couplings are challenging to compute since they originate from both one- and two-particle interactions, and the strength of this interaction drops approximately exponentially with donor-acceptor distances. Using adiabatic energy splitting to compute DET couplings has advantages because adiabatic states can be calculated directly using conventional quantum chemical methods. We describe a minimum energy splitting method to compute the DET coupling by altering molecular geometries to drive the systems into a T1/T2 energy quasi-degenerate-activated DA complex. We explore the accuracy of various quantum chemical approaches to calculate the Dexter couplings.

Simulating Electron Transfer Reactions in Solution: Radical-Polar Crossover

Single-electron transfer (SET) promotes a wide variety of interesting chemical transformations, but modeling of SET requires a careful treatment of electronic and solvent effects to give meaningful insight. Therefore, a combined constrained density functional theory and molecular mechanics (CDFT/MM) tool is introduced specifically for SET-initiated reactions. Mechanisms for two radical-polar crossover reactions involving the organic electron donors tetrakis(dimethylamino)ethylene (TDAE) and tetrathiafulvalene (TTF) were studied with the new tool. An unexpected tertiary radical intermediate within the TDAE system was identified, relationships between kinetics and substitution in the TTF system were explained, and the impact of the solvent environments on the TDAE and TTF reactions were examined. The results highlight the need for including solvent dynamics when quantifying SET kinetics and thermodynamics, as a free energy difference of >20 kcal/mol was observed. Overall, the new method informs mechanistic analysis of SET-initiated reactions and therefore is envisioned to be useful for studying reactions in the condensed phase.

The role of in Silico/DFT investigations in analyzing dye molecules for enhanced solar cell efficiency and reduced toxicity

Toxicity has been a significant concern for many materials used in the production of solar cells and generally conflicts with its efficacy. Therefore, it is crucial to develop alternative, non-toxic materials to improve the sustainability and safety of solar cell technology. In recent years, computational methods such as Conceptual density functional theory (CDFT) have been increasingly used to study the electronic structure and optical properties of toxic molecules such as dyes, with the goal of designing and modifying these molecules to enhance solar cell efficiency and reduce toxicity. By applying CDFT-based chemical reactivity parameters and electronic structure rules, researchers can gain valuable insights into the performance of solar cells and optimize their design accordingly. In silico studies have been used to screen and design non-toxic dye molecules, which can improve the sustainability and safety of solar cell technology. This review article discusses the applications of CDFT in the analysis of toxic dye molecules for use in solar cells. This review also highlights the importance of using alternative, non-toxic materials in the production of solar cells. The review also discusses the limitations of CDFT and in silico studies and their potential for future research. Finally, the article concludes by emphasizing the potential of in silico/DFT investigations for accelerating the discovery of new and efficient dye molecules for enhancing solar cells' efficiency.

Towards the description of charge transfer states in solubilised LHCII using subsystem DFT

Light harvesting complex II (LHCII) in plants and green algae have been shown to adapt their absorption properties, depending on the concentration of sunlight, switching between a light harvesting and a non-harvesting or quenched state. In a recent work, combining classical molecular dynamics (MD) simulations with quantum chemical calculations (Liguori et al. in Sci Rep 5:15661, 2015) on LHCII, it was shown that the Chl611-Chl612 cluster of the terminal emitter domain can play an important role in modifying the spectral properties of the complex. In that work the importance of charge transfer (CT) effects was highlighted, in re-shaping the absorption intensity of the chlorophyll dimer. Here in this work, we investigate the combined effect of the local excited (LE) and CT states in shaping the energy landscape of the chlorophyll dimer. Using subsystem Density Functional Theory over the classical [Formula: see text]s MD trajectory we look explicitly into the excitation energies of the LE and the CT states of the dimer and their corresponding couplings. Upon doing so, we observe a drop in the excitation energies of the CT states, accompanied by an increase in the couplings between the LE/LE and the LE/CT states facilitated by a shorter interchromophoric distance upon equilibration. Both these changes in conjunction, effectively produces a red-shift of the low-lying mixed exciton/CT states of the supramolecular chromophore pair.

Subsystem density-functional theory: A reliable tool for spin-density based properties

Subsystem density-functional theory compiles a set of features that allow for efficiently calculating properties of very large open-shell radical systems such as organic radical crystals, proteins, or deoxyribonucleic acid stacks. It is computationally less costly than correlated ab initio wave function approaches and can pragmatically avoid the overdelocalization problem of Kohn-Sham density-functional theory without employing hard constraints on the electron-density. Additionally, subsystem density-functional theory calculations commonly start from isolated fragment electron densities, pragmatically preserving a priori specified subsystem spin-patterns throughout the calculation. Methods based on subsystem density-functional theory have seen a rapid development over the past years and have become important tools for describing open-shell properties. In this Perspective, we address open questions and possible developments toward challenging future applications in connection with subsystem density-functional theory for spin-dependent properties.

Diabatic Decomposition Perspective on the Role of Charge Transfer and Local Excitations in Thermally Activated Delayed Fluorescence

Thermally activated delayed fluorescence (TADF) is a phenomenon that relies on the upconversion of triplet excitons to singlet excitons by means of reverse intersystem crossing (rISC). It has been shown both experimentally and theoretically that the TADF mechanism depends on the interplay between charge transfer and local excitations. However, the difference between the diabatic and adiabatic character of the involved excited states is rarely discussed in the literature. Here we develop a diabatization procedure to implement a four-state model Hamiltonian to a set of TADF molecules. We provide physical interpretations of the Hamiltonian elements and show their dependence on the electronic state of the equilibrium geometry. We also demonstrate how vibrations affect the TADF efficiency by modifying the diabatic decomposition of the molecule. Finally, we provide a simple model that connects the diabatic Hamiltonian to the electronic properties relevant to TADF and show how this relationship translates into different optimization strategies for rISC, fluorescence, and overall TADF performance.

Identifying structure-absorption relationships and predicting absorption strength of non-fullerene acceptors for organic photovoltaics

Non-fullerene acceptors (NFAs) are excellent light harvesters, yet the origin of their high optical extinction is not well understood. In this work, we investigate the absorption strength of NFAs by building a database of time-dependent density functional theory (TDDFT) calculations of ∼500 π-conjugated molecules. The calculations are first validated by comparison with experimental measurements in solution and solid state using common fullerene and non-fullerene acceptors. We find that the molar extinction coefficient (ε _d,max) shows reasonable agreement between calculation in vacuum and experiment for molecules in solution, highlighting the effectiveness of TDDFT for predicting optical properties of organic π-conjugated molecules. We then perform a statistical analysis based on molecular descriptors to identify which features are important in defining the absorption strength. This allows us to identify structural features that are correlated with high absorption strength in NFAs and could be used to guide molecular design: highly absorbing NFAs should possess a planar, linear, and fully conjugated molecular backbone with highly polarisable heteroatoms. We then exploit a random decision forest algorithm to draw predictions for ε _d,max using a computational framework based on extended tight-binding Hamiltonians, which shows reasonable predicting accuracy with lower computational cost than TDDFT. This work provides a general understanding of the relationship between molecular structure and absorption strength in π-conjugated organic molecules, including NFAs, while introducing predictive machine-learning models of low computational cost.

Evaluation of molecular photophysical and photochemical properties using linear response time-dependent density functional theory with classical embedding: Successes and challenges

Time-dependent density functional theory (TDDFT) based approaches have been developed in recent years to model the excited-state properties and transition processes of the molecules in the gas-phase and in a condensed medium, such as in a solution and protein microenvironment or near semiconductor and metal surfaces. In the latter case, usually, classical embedding models have been adopted to account for the molecular environmental effects, leading to the multi-scale approaches of TDDFT/polarizable continuum model (PCM) and TDDFT/molecular mechanics (MM), where a molecular system of interest is designated as the quantum mechanical region and treated with TDDFT, while the environment is usually described using either a PCM or (non-polarizable or polarizable) MM force fields. In this Perspective, we briefly review these TDDFT-related multi-scale models with a specific emphasis on the implementation of analytical energy derivatives, such as the energy gradient and Hessian, the nonadiabatic coupling, the spin-orbit coupling, and the transition dipole moment as well as their nuclear derivatives for various radiative and radiativeless transition processes among electronic states. Three variations of the TDDFT method, the Tamm-Dancoff approximation to TDDFT, spin-flip DFT, and spin-adiabatic TDDFT, are discussed. Moreover, using a model system (pyridine-Ag20 complex), we emphasize that caution is needed to properly account for system-environment interactions within the TDDFT/MM models. Specifically, one should appropriately damp the electrostatic embedding potential from MM atoms and carefully tune the van der Waals interaction potential between the system and the environment. We also highlight the lack of proper treatment of charge transfer between the quantum mechanics and MM regions as well as the need for accelerated TDDFT modelings and interpretability, which calls for new method developments.

Transition-Based Constrained DFT for the Robust and Reliable Treatment of Excitations in Supramolecular Systems

Despite the variety of available computational approaches, state-of-the-art methods for calculating excitation energies, such as time-dependent density functional theory (TDDFT), are computationally demanding and thus limited to moderate system sizes. Here, we introduce a new variation of constrained DFT (CDFT), wherein the constraint corresponds to a particular transition (T), or a combination of transitions, between occupied and virtual orbitals, rather than a region of the simulation space as in traditional CDFT. We compare T-CDFT with TDDFT and ΔSCF results for the low-lying excited states (S1 and T1) of a set of gas-phase acene molecules and OLED emitters and with reference results from the literature. At the PBE level of theory, T-CDFT outperforms ΔSCF for both classes of molecules, while also proving to be more robust. For the local excitations seen in the acenes, T-CDFT and TDDFT perform equally well. For the charge transfer (CT)-like excitations seen in the OLED molecules, T-CDFT also performs well, in contrast to the severe energy underestimation seen with TDDFT. In other words, T-CDFT is equally applicable to both local excitations and CT states, providing more reliable excitation energies at a much lower computational cost than TDDFT cost. T-CDFT is designed for large systems and has been implemented in the linear-scaling BigDFT code. It is therefore ideally suited for exploring the effects of explicit environments on excitation energies, paving the way for future simulations of excited states in complex realistic morphologies, such as those which occur in OLED materials.

The Seamless Connection of Local and Collective Excited States in Subsystem Time-Dependent Density Functional Theory

The theoretical understanding of photoinduced processes in multichromophoric systems requires, as an essential ingredient, the possibility of accurately describing their electronically excited states. However, the size of these systems often prohibits the usage of conventional electronic-structure methods, so that often multiscale approaches based on phenomenologically motivated models are employed. In contrast, subsystem time-dependent density functional theory (sTDDFT) allows for a subsystem-based ab initio description of multichromophoric systems and therefore allows for, in principle, an exact description of photoinduced processes. This Perspective aims to outline the theoretical foundations and commonly used practical realizations as well as to illustrate benefits of recent developments and open issues in the field of sTDDFT. Prospective, potential future applications and possible methodological developments are discussed.

Excited state diabatization on the cheap using DFT: Photoinduced electron and hole transfer

Excited state electron and hole transfer underpin fundamental steps in processes such as exciton dissociation at photovoltaic heterojunctions, photoinduced charge transfer at electrodes, and electron transfer in photosynthetic reaction centers. Diabatic states corresponding to charge or excitation localized species, such as locally excited and charge transfer states, provide a physically intuitive framework to simulate and understand these processes. However, obtaining accurate diabatic states and their couplings from adiabatic electronic states generally leads to inaccurate results when combined with low-tier electronic structure methods, such as time-dependent density functional theory, and exorbitant computational cost when combined with high-level wavefunction-based methods. Here, we introduce a density functional theory (DFT)-based diabatization scheme that directly constructs the diabatic states using absolutely localized molecular orbitals (ALMOs), which we denote as Δ-ALMO(MSDFT2). We demonstrate that our method, which combines ALMO calculations with the ΔSCF technique to construct electronically excited diabatic states and obtains their couplings with charge-transfer states using our MSDFT2 scheme, gives accurate results for excited state electron and hole transfer in both charged and uncharged systems that underlie DNA repair, charge separation in donor-acceptor dyads, chromophore-to-solvent electron transfer, and singlet fission. This framework for the accurate and efficient construction of excited state diabats and evaluation of their couplings directly from DFT thus offers a route to simulate and elucidate photoinduced electron and hole transfer in large disordered systems, such as those encountered in the condensed phase.

Electronic couplings for photo-induced processes from subsystem time-dependent density-functional theory: The role of the diabatization

Subsystem time-dependent density-functional theory (sTDDFT) making use of approximate non-additive kinetic energy (NAKE) functionals is known to be capable of describing excitation energy transfer processes in a variety of applications. Here, we show that sTDDFT, especially when combined with projection-based embedding (PbE), can be employed for the entire range of photo-induced electronic couplings essential for modeling photophysical properties of complex chemical and biological systems and therefore represents a complete toolbox for this class of problems. This means that it is capable of capturing the interaction/coupling associated with local- and charge-transfer (CT) excitons. However, this requires the choice of a reasonable diabatic basis. We therefore propose different diabatization strategies of the virtual orbital space in PbE-sTDDFT and show how CT excitations can be included in sTDDFT using NAKE functionals via a phenomenological approach. Finally, these electronic couplings are compared to couplings from a multistate fragment excitation difference (FED)-fragment charge difference (FCD) diabatization procedure. We show that both procedures, multistate FED-FCD and sTDDFT (with the right diabatization procedure chosen), lead to an overall good agreement for the electronic couplings, despite differences in their general diabatization strategy. We conclude that the entire range of photo-induced electronic couplings can be obtained using sTDDFT (with the right diabatization procedure chosen) in a black-box manner.

Electron transfer in extended systems: characterization by periodic density functional theory including the electronic coupling

We describe a new computer implementation of electron transfer (ET) theory in extended systems treated by periodic density functional theory (DFT), including the calculation of the electronic coupling transition element V_AB. In particular, the development opens up the full characterization of electron transfer in the solid state. The approach is valid for any single-determinant wavefunction with localized character representing the electronic structure of the system, from Hartree-Fock (HF) theory, to density functional theory (DFT), hybrid DFT theory, DFT+U theory, and constrained DFT (cDFT) theory. The implementation in CP2K reuses the high-performance functions of the code. The computational cost is equivalent to only one iteration of an HF calculation. We present test calculations for electron transfer in a number of systems, including a 1D-model of ferric oxide, hematite Fe₂O₃, rutile TiO₂, and finally bismuth vanadate BiVO₄.

Coupling to Charge Transfer States is the Key to Modulate the Optical Bands for Efficient Light Harvesting in Purple Bacteria

The photosynthetic apparatus of purple bacteria uses exciton delocalization and static disorder to modulate the position and broadening of its absorption bands, leading to efficient light harvesting. Its main antenna complex, LH2, contains two rings of identical bacteriochlorophyll pigments, B800 and B850, absorbing at 800 and 850 nm, respectively. It has been an unsolved problem why static disorder of the strongly coupled B850 ring is several times larger than that of the B800 ring. Here we show that mixing between excitons and charge transfer states in the B850 ring is responsible for the effect. The linear absorption spectrum of the LH2 system is simulated by using a multiscale approach with an exciton Hamiltonian generalized to include the charge transfer states that involve adjacent pigment pairs, with static disorder modeled microscopically by molecular dynamics simulations. Our results show that sufficient inhomogeneous broadening of the B850 band, needed for efficient light harvesting, is only obtained by utilizing static disorder in the coupling between local excited and interpigment charge transfer states.

Intermolecular Singlet and Triplet Exciton Transfer Integrals from Many-Body Green's Functions Theory

A general approach to determine orientation and distance-dependent effective intermolecular exciton transfer integrals from many-body Green's functions theory is presented. On the basis of the GW approximation and the Bethe-Salpeter equation (BSE), a projection technique is employed to obtain the excitonic coupling by forming the expectation value of a supramolecular BSE Hamiltonian with electron-hole wave functions for excitations localized on two separated chromophores. Within this approach, accounting for the effects of coupling mediated by intermolecular charge transfer (CT) excitations is possible via perturbation theory or a reduction technique. Application to model configurations of pyrene dimers shows an accurate description of short-range exchange and long-range Coulomb interactions for the coupling of singlet and triplet excitons. Computational parameters, such as the choice of the exchange-correlation functional in the density-functional theory (DFT) calculations that underly the GW-BSE steps and the convergence with the number of included CT excitations, are scrutinized. Finally, an optimal strategy is derived for simulations of full large-scale morphologies by benchmarking various approximations using pairs of dicyanovinyl end-capped oligothiophenes (DCV5T), which are used as donor material in state-of-the-art organic solar cells.

Molecular dynamics and charge transport in organic semiconductors: a classical approach to modeling electron transfer

Organic photovoltaics (OPVs) are a promising carbon-neutral energy conversion technology, with recent improvements pushing power conversion efficiencies over 10%. A major factor limiting OPV performance is inefficiency of charge transport in organic semiconducting materials (OSCs). Due to strong coupling with lattice degrees of freedom, the charges form polarons, localized quasi-particles comprised of charges dressed with phonons. These polarons can be conceptualized as pseudo-atoms with a greater effective mass than a bare charge. We propose that due to this increased mass, polarons can be modeled with Langevin molecular dynamics (LMD), a classical approach with a computational cost much lower than most quantum mechanical methods. Here we present LMD simulations of charge transfer between a pair of fullerene molecules, which commonly serve as electron acceptors in OSCs. We find transfer rates consistent with experimental measurements of charge mobility, suggesting that this method may provide quantitative predictions of efficiency when used to simulate materials on the device scale. Our approach also offers information that is not captured in the overall transfer rate or mobility: in the simulation data, we observe exactly when and why intermolecular transfer events occur. In addition, we demonstrate that these simulations can shed light on the properties of polarons in OSCs. Much remains to be learned about these quasi-particles, and there are no widely accepted methods for calculating properties such as effective mass and friction. Our model offers a promising approach to exploring mass and friction as well as providing insight into the details of polaron transport in OSCs.

Planarity and multiple components promote organic photovoltaic efficiency by improving electronic transport

Establishing how the conformation of organic photovoltaic (OPV) polymers affects their electronic and transport properties is critical in order to determine design rules for new OPV materials and in particular to understand the performance enhancements recently reported for ternary blends. We report coupled classical and ab initio molecular dynamics simulations showing that polymer linkage twisting significantly reduces optical absorption efficiency, as well as hole transport rates in donor polymers. We predict that blends with components favoring planar geometries contribute to the enhancement of the overall efficiency of ternary OPVs. Furthermore, our electronic structure calculations for the PTB7-PID2-PC71BM system show that hole transfer rates are enhanced in ternary blends with respect to their binary counterpart. Finally, our results point at thermal disorder in the blend as a key reason responsible for device voltage losses and at the need to carry out electronic structure calculations at finite temperature to reliably compare with experiments.

Electrostatic phenomena in organic semiconductors: fundamentals and implications for photovoltaics

This review summarizes the current understanding of electrostatic phenomena in ordered and disordered organic semiconductors, outlines numerical schemes developed for quantitative evaluation of electrostatic and induction contributions to ionization potentials and electron affinities of organic molecules in a solid state, and illustrates two applications of these techniques: interpretation of photoelectron spectroscopy of thin films and energetics of heterointerfaces in organic solar cells.

A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers

Interest in materials that undergo singlet fission (SF) has been catalyzed by the potential to exceed the Shockley-Queisser limit of solar power conversion efficiency. In conventional materials, the mechanism of SF is an intermolecular process (xSF), which is mediated by charge transfer (CT) states and depends sensitively on crystal packing or molecular collisions. In contrast, recently reported covalently coupled pentacenes yield ∼2 triplets per photon absorbed in individual molecules: the hallmark of intramolecular singlet fission (iSF). However, the mechanism of iSF is unclear. Here, using multireference electronic structure calculations and transient absorption spectroscopy, we establish that iSF can occur via a direct coupling mechanism that is independent of CT states. We show that a near-degeneracy in electronic state energies induced by vibronic coupling to intramolecular modes of the covalent dimer allows for strong mixing between the correlated triplet pair state and the local excitonic state, despite weak direct coupling.

Models of charge pair generation in organic solar cells

A critical perspective on modelling of charge generation in organic photovoltaics, focussing on interfacial electronic states, electrostatics, and dynamic processes.

Structure-property relationships of curved aromatic materials from first principles

CONSPECTUS: Considerable effort in the past decade has been extended toward achieving computationally affordable theoretical methods for accurate prediction of the structure and properties of materials. Theoretical predictions of solids began decades ago, but only recently have solid-state quantum techniques become sufficiently reliable to be routinely chosen for investigation of solids as quantum chemistry techniques are for isolated molecules. Of great interest are ab initio predictive theories for solids that can provide atomic scale insights into properties of bulk materials, interfaces, and nanostructures. Adaption of the quantum chemical framework is challenging in that no single theory exists that provides prediction of all observables for every material type. However, through a combination of interdisciplinary efforts, a richly textured and substantive portfolio of methods is developing, which promise quantitative predictions of materials and device properties as well as associated performance analysis. Particularly relevant for device applications are organic semiconductors (OSC), with electrical conductivity between that of insulators and that of metals. Semiconducting small molecules, such as aromatic hydrocarbons, tend to have high polarizabilities, small band-gaps, and delocalized π electrons that support mobile charge carriers. Most importantly, the special nature of optical excitations in the form of a bound electron-hole pairs (excitons) holds significant promise for use in devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), and molecular nanojunctions. Added morphological features, such as curvature in aromatic hydrocarbon structure, can further confine the electronic states in one or more directions leading to additional physical phenomena in materials. Such structures offer exploration of a wealth of phenomenology as a function of their environment, particularly due to the ability to tune their electronic character through functionalization. This Account offers discussion of current state-of-the-art electronic structure approaches for prediction of structural, electronic, optical, and transport properties of materials, with illustration of these capabilities from a series of investigations involving curved aromatic materials. The class of curved aromatic materials offers the ability to investigate methodology across a wide range of materials complexity, including (a) molecules, (b) molecular crystals, (c) molecular adsorbates on metal surfaces, and (d) molecular nanojunctions. A reliable pallet of theoretical tools for such a wide array relies on expertise spanning multiple fields. Working together with experimental experts, advancements in the fundamental understanding of structural and dynamical properties are enabling focused design of functional materials. Most importantly, these studies provide an opportunity to compare experimental and theoretical capabilities and open the way for continual improvement of these capabilities.

The low-lying electronic states of pentacene and their roles in singlet fission

We present a detailed study of pentacene monomer and dimer that serves to reconcile extant views of its singlet fission. We obtain the correct ordering of singlet excited-state energy levels in a pentacene molecule (E (S1) < E (D)) from multireference calculations with an appropriate active orbital space and dynamical correlation being incorporated. In order to understand the mechanism of singlet fission in pentacene, we use a well-developed diabatization scheme to characterize the six low-lying singlet states of a pentacene dimer that approximates the unit cell structure of crystalline pentacene. The local, single-excitonic diabats are not directly coupled with the important multiexcitonic state but rather mix through their mutual couplings with one of the charge-transfer configurations. We analyze the mixing of diabats as a function of monomer separation and pentacene rotation. By defining an oscillator strength measure of the coherent population of the multiexcitonic diabat, essential to singlet fission, we find this population can, in principle, be increased by small compression along a specific crystal direction.

The Third Dimension of a More O'Ferrall-Jencks Diagram for Hydrogen Atom Transfer in the Isoelectronic Hydrogen Exchange Reactions of (PhX)(2)H(•) with X = O, NH, and CH(2)

A critical element in theoretical characterization of the mechanism of proton-coupled electron transfer (PCET) reactions, including hydrogen atom transfer (HAT), is the formulation of the electron and proton localized diabatic states, based on which a More O'Ferrall-Jencks diagram can be represented to determine the step-wise and concerted nature of the reaction. Although the More O'Ferrall-Jencks diabatic states have often been used empirically to develop theoretical models for PCET reactions, the potential energy surfaces for these states have never been determined directly based on first principles calculations using electronic structure theory. The difficulty is due to a lack of practical method to constrain electron and proton localized diabatic states in wave function or density functional theory calculations. Employing a multistate density functional theory (MSDFT), in which the electron and proton localized diabatic configurations are constructed through block-localization of Kohn-Sham orbitals, we show that distinction between concerted proton-electron transfer (CPET) and HAT, which are not distinguishable experimentally from phenomenological kinetic data, can be made by examining the third dimension of a More O'Ferrall-Jencks diagram that includes both the ground and excited state potential surfaces. In addition, we formulate a pair of effective two-state valence bond models to represent the CPET and HAT mechanisms. We found that the lower energy of the CPET and HAT effective diabatic states at the intersection point can be used as an energetic criterion to distinguish the two mechanisms. In the isoelectronic series of hydrogen exchange reaction in (PhX)(2)H(•), where X = O, NH, and CH(2), there is a continuous transition from a CPET mechanism for the phenoxy radical-phenol pair to a HAT process for benzyl radical and toluene, while the reaction between PhNH(2) and PhNH(•) has a mechanism intermediate of CPET and HAT. The electronically nonadiabatic nature of the CPET mechanism in the phenol system can be attributed to the overlap interactions between the ground and excited state surfaces, resulting in roughly orthogonal minimum energy paths on the adiabatic ground and excited state potential energy surfaces. On the other hand, the minimum energy path on the adiabatic ground state for the HAT mechanism coincides with that on the excited state, producing a large electronic coupling that separates the two surfaces by more than 120 kcal/mol.

Simulation of solution phase electron transfer in a compact donor-acceptor dyad

Charge separation (CS) and charge recombination (CR) rates in photosynthetic architectures are difficult to control, yet their ratio can make or break photon-to-current conversion efficiencies. A rational design approach to the enhancement of CS over CR requires a mechanistic understanding of the underlying electron-transfer (ET) process, including the role of the environment. Toward this goal, we introduce a QM/MM protocol for ET simulations and use it to characterize CR in the formanilide-anthraquinone dyad (FAAQ). Our simulations predict fast recombination of the charge-transfer excited state, in agreement with recent experiments. The computed electronic couplings show an electronic state dependence and are weaker in solution than in the gas phase. We explore the role of cis-trans isomerization on the CR kinetics, and we find strong correlation between the vertical energy gaps of the full simulations and a collective solvent polarization coordinate. Our approach relies on constrained density functional theory to obtain accurate diabatic electronic states on the fly for molecular dynamics simulations, while orientational and electronic polarization of the solvent is captured by a polarizable force field based on a Drude oscillator model. The method offers a unified approach to the characterization of driving forces, reorganization energies, electronic couplings, and nonlinear solvent effects in light-harvesting systems.