Coherent Charge Transfer Exciton Formation in Regioregular P3HT: A Quantum Dynamical Study

Coherent Vibrations Promote Charge-Transfer across a Graphene-Based Interface

Discerning the impact of the coherent motion of the nuclei on the timing and efficiency of charge transfer at the donor-acceptor interface is essential for designing performance-enhanced optoelectronic devices. Here, we employ an experimental approach using photocurrent detection in coherent multidimensional spectroscopy to excite a donor aromatic macrocycle and collect the charge transferred to a 2D acceptor layer. For this purpose, we prepared a cobalt phthalocyanine-graphene (CoPc-Gr) interface. Unlike blends, the well-ordered architecture achieved through the physical separation of the two layers allows us to unambiguously collect the electrical signal from graphene alone and associate it with a microscopic understanding of the whole process. The CoPc-Gr interface exhibits an ultrafast electron-transfer signal, stemming from an interlayer mechanism. Remarkably, the signal presents an oscillating time evolution modulated by coherent vibrations originating from the laser-excited CoPc states. By performing Fourier analysis on the beatings and correlating it with the Raman features, along with a comprehensive first-principles characterization of the vibrational coupling in the CoPc excited states, we successfully identify both the orbitals and molecular vibrations that promote the charge transfer at the interface.

Bexcitonics: Quasiparticle approach to open quantum dynamics

We develop a quasiparticle approach to capture the dynamics of open quantum systems coupled to bosonic thermal baths of arbitrary complexity based on the Hierarchical Equations of Motion (HEOM). This is done by generalizing the HEOM dynamics and mapping it into that of the system in interaction with a few bosonic fictitious quasiparticles that we call bexcitons. Bexcitons arise from a decomposition of the bath correlation function into discrete features. Specifically, bexciton creation and annihilation couple the auxiliary density matrices in the HEOM. The approach provides a systematic strategy to construct exact quantum master equations that include the system-bath coupling to all orders even for non-Markovian environments. Specifically, by introducing different metrics and representations for the bexcitons it is possible to straightforwardly generate different variants of the HEOM, demonstrating that all these variants share a common underlying quasiparticle picture. Bexcitonic properties, while unphysical, offer a coarse-grained view of the correlated system-bath dynamics and its numerical convergence. For instance, we use it to analyze the instability of the HEOM when the bath is composed of underdamped oscillators and show that it leads to the creation of highly excited bexcitons. The bexcitonic picture can also be used to develop more efficient approaches to propagate the HEOM. As an example, we use the particle-like nature of the bexcitons to introduce mode-combination of bexcitons in both number and coordinate representation that uses the multi-configuration time-dependent Hartree to efficiently propagate the HEOM dynamics.

Coherent Transient Localization Mechanism of Interchain Exciton Transport in Regioregular P3HT: A Quantum-Dynamical Study

Transient localization has been proposed as a transport mechanism in organic materials, for both charge carriers and excitons. Here, we characterize a quantum coherent transient localization mechanism using full quantum simulations of an H-aggregated model system representative of regioregular poly(3-hexylthiophene) (rrP3HT). A Frenkel-Holstein Hamiltonian parametrized from first principles is considered, including local high-frequency modes and anharmonic, site-correlated interchain modes. Quantum-dynamical calculations are carried out using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for a 13-site system with 195 vibrational modes, under periodic boundary conditions. It is shown that temporary localization of exciton polarons alternates with resonant transfer driven by interchain modes. While the transport process is mainly determined by exciton-polarons at the low-energy band edge, persistent coupling with the excitonic manifold is observed, giving rise to a nonadiabatic excitonic flux. This elementary transport mechanism remains preserved for limited static disorder and gives way to Anderson localization when the static disorder becomes dominant.

Mapping electronic decoherence pathways in molecules

Establishing the fundamental chemical principles that govern molecular electronic quantum decoherence has remained an outstanding challenge. Fundamental questions such as how solvent and intramolecular vibrations or chemical functionalization contribute to the decoherence remain unanswered and are beyond the reach of state-of-the-art theoretical and experimental approaches. Here we address this challenge by developing a strategy to isolate electronic decoherence pathways for molecular chromophores immersed in condensed phase environments that enables elucidating how electronic quantum coherence is lost. For this, we first identify resonance Raman spectroscopy as a general experimental method to reconstruct molecular spectral densities with full chemical complexity at room temperature, in solvent, and for fluorescent and non-fluorescent molecules. We then show how to quantitatively capture the decoherence dynamics from the spectral density and identify decoherence pathways by decomposing the overall coherence loss into contributions due to individual molecular vibrations and solvent modes. We illustrate the utility of the strategy by analyzing the electronic decoherence pathways of the DNA base thymine in water. Its electronic coherences decay in [Formula: see text]30 fs. The early-time decoherence is determined by intramolecular vibrations while the overall decay by solvent. Chemical substitution of thymine modulates the decoherence with hydrogen-bond interactions of the thymine ring with water leading to the fastest decoherence. Increasing temperature leads to faster decoherence as it enhances the importance of solvent contributions but leaves the early-time decoherence dynamics intact. The developed strategy opens key opportunities to establish the connection between molecular structure and quantum decoherence as needed to develop chemical strategies to rationally modulate it.

Pulse-Induced Dynamics of a Charge-Transfer Complex from First Principles

The ultrafast dynamics of charge carriers in organic donor-acceptor interfaces are of primary importance to understanding the fundamental properties of these systems. In this work, we focus on a charge-transfer complex formed by quaterthiophene p-doped by tetrafluoro-tetracyanoquinodimethane and investigate electron dynamics and vibronic interactions also at finite temperatures by applying a femtosecond pulse in resonance with the two lowest energy excitations of the system with perpendicular and parallel polarization with respect to the interface. The adopted ab initio formalism based on real-time time-dependent density-functional theory coupled to Ehrenfest dynamics enables monitoring the dynamical charge transfer across the interface and assessing the role played by the nuclear motion. Our results show that the strong intermolecular interactions binding the complex already in the ground state influence the dynamics, too. The analysis of the nuclear motion involved in these processes reveals the participation of different vibrational modes depending on the electronic states stimulated by the resonant pulse. Coupled donor-acceptor modes mostly influence the excited state polarized across the interface, while intramolecular vibrations in the donor molecule dominate the excitation in the orthogonal direction. The results obtained at finite temperatures are overall consistent with this picture, although thermal disorder contributes to slightly decreasing interfacial charge transfer.

Multistate Reaction Coordinate Model for Charge and Energy Transfer Dynamics in the Condensed Phase

Constructing multistate model Hamiltonians from all-atom electronic structure calculations and molecular dynamics simulations is crucial for understanding charge and energy transfer dynamics in complex condensed phases. The most popular two-level system model is the spin-boson Hamiltonian, where the nuclear degrees of freedom are represented as shifted normal modes. Recently, we proposed the general multistate nontrivial extension of the spin-boson model, i.e., the multistate harmonic (MSH) model, which is constructed by extending the spatial dimensions of each nuclear mode so as to satisfy the all-atom reorganization energy restrictions for all pairs of electronic states. In this work, we propose the multistate reaction coordinate (MRC) model with a primary reaction coordinate and secondary bath modes as in the Caldeira-Leggett form but in extended spatial dimensions. The MRC model is proven to be equivalent to the MSH model and offers an intuitive physical picture of the nuclear-electronic feedback in nonadiabatic processes such as the inherent trajectory of the reaction coordinate. The reaction coordinate is represented in extended dimensions, carrying the entire reorganization energies and bilinearly coupled to the secondary bath modes. We demonstrate the MRC model construction for photoinduced charge transfer in an organic photovoltaic caroteniod-porphyrin-C60 molecular triad dissolved in tetrahydrofuran as well as excitation energy transfer in a photosynthetic light-harvesting Fenna-Matthews-Olson complex. The MRC model provides an effective and robust platform for investigating quantum dissipative dynamics in complex condensed-phase systems since it allows a consistent description of realistic spectral density, state-dependent system-bath couplings, and heterogeneous environments due to static disorder in reorganization energies.

Nanoscale and Real-Time Nuclear-Electronic Dynamics Simulation Study of Charge Transfer at the Donor-Acceptor Interface in Organic Photovoltaics

Charge-transfer (CT) processes in donor-acceptor interfaces of organic photovoltaics have been challenging targets for computational chemistry owing to their nanoscale and ultrafast nature. Herein, we report real-time nuclear-electronic dynamics simulations of CT processes in a nanometer-scale donor-acceptor interface model composed of a donor poly(3-hexylthiophene-2,5-diyl) crystal and an acceptor [6,6]-phenyl-C₆₁-butyric acid methyl ester aggregate. The simulations were realized using our original reduced-scaling computational technique, namely, patchwork-approximation-based Ehrenfest dynamics. The results illustrated the CT pathway with atomic resolution, thereby rationalizing the observed excitation-energy dependence of the quantity of CT. Further, nuclear motion, which is affected by the electronic dynamics, was observed to play a significant role in the CT process by modulating molecular orbital energies. The present study suggests that microscopic CT processes strongly depend on local structures of disordered donor-acceptor interfaces as well as coupling between nuclear and electronic dynamics.

Quantum dynamical study of inter-chain exciton transport in a regioregular P3HT model system at finite temperature: HJ vs. H-aggregate models

We report on quantum dynamical simulations of inter-chain exciton transport in a model of regioregular poly(3-hexylthiophene), rr-P3HT, at finite temperature, using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for a system of up to 63 electronic states and 180 vibrational modes. A Frenkel Hamiltonian of HJ aggregate type is used, along with a reduced H-aggregate representation; electron-phonon coupling includes local high-frequency modes as well as anharmonic intermolecular modes. The latter are operative in mediating inter-chain transport, by a mechanism of transient localization type. Strikingly, this mechanism is found to be of quantum coherent character and involves non-adiabatic effects. Using periodic boundary conditions, a normal diffusion regime is identified from the exciton mean-squared displacement, apart from early-time transients. Diffusion coefficients are found to be of the order of 3 x 10-3 cm2/s, showing a non-monotonous increase with temperature.

Effect of mixed Frenkel and charge transfer states in time-gated fluorescence spectra of perylene bisimides H-aggregates: Hierarchical equations of motion approach

We theoretically investigated the effect of mixed Frenkel (F) and charge transfer (CT) states on the spectral properties of perylene bisimide (PBI) derivatives, focusing on the role of strong electron-phonon interactions. The model consists of a four-level system described by the Holstein Hamiltonian coupled to independent local heat-baths on each site, described by Brownian spectral distribution functions. We employ the reduced hierarchical equations of motion (HEOM) approach to calculate the time evolution of the system and compare it to the pure F exciton cases. We compute the absorption and time-gated fluorescence (TGF) spectra for different exciton transfer integrals and F-CT band gap conditions. The coherence length of excitons ($N_{coh}$) is evaluated employing two different definitions. We observe the presence of an excited hot state peak whose intensity is associated with the delocalization of the excited species and ultrafast dynamics that are solely dependent on the frequency of the local bath. The results indicate that the inclusion of CT states promotes localization of the excitons which is manifested in a decrease in the intensity of the hot state peak and the 0--1 peak, and an increase in the intensity of the 0--0 emission peak in TGF spectrum, leading to a decrease of $N_{coh}$.

Representation of Diabatic Potential Energy Matrices for Multiconfiguration Time-Dependent Hartree Treatments of High-Dimensional Nonadiabatic Photodissociation Dynamics

Conventional quantum mechanical characterization of photodissociation dynamics is restricted by steep scaling laws with respect to the dimensionality of the system. In this work, we examine the applicability of the multi-configurational time-dependent Hartree (MCTDH) method in treating nonadiabatic photodissociation dynamics in two prototypical systems, taking advantage of its favorable scaling laws. To conform to the sum-of-product form, elements of the ab initio diabatic potential energy matrix (DPEM) are re-expressed using the recently proposed Monte Carlo canonical polyadic decomposition method, with enforcement of proper symmetry. The MCTDH absorption spectra and product branching ratios are shown to compare well with those calculated using conventional grid-based methods, demonstrating its promise for treating high-dimensional nonadiabatic photodissociation problems.

Direct observation of ultrafast exciton localization in an organic semiconductor with soft X-ray transient absorption spectroscopy

The localization dynamics of excitons in organic semiconductors influence the efficiency of charge transfer and separation in these materials. Here we apply time-resolved X-ray absorption spectroscopy to track photoinduced dynamics of a paradigmatic crystalline conjugated polymer: poly(3-hexylthiophene) (P3HT) commonly used in solar cell devices. The π→π^* transition, the first step of solar energy conversion, is pumped with a 15 fs optical pulse and the dynamics are probed by an attosecond soft X-ray pulse at the carbon K-edge. We observe X-ray spectroscopic signatures of the initially hot excitonic state, indicating that it is delocalized over multiple polymer chains. This undergoes a rapid evolution on a sub 50 fs timescale which can be directly associated with cooling and localization to form either a localized exciton or polaron pair.

A detailed understanding of ultrafast exciton dynamics is crucial for improving the efficiency of organic light-harvesting-devices. Here, the authors track exciton localization on a sub-50 fs timescale in an organic semiconductor using time resolved soft x-ray absorption spectroscopy.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Signatures of coherent vibronic exciton dynamics and conformational control in two-dimensional electronic spectroscopy of conjugated polymers

Two-dimensional electronic spectroscopy (2DES) signals for homo-oligomer J-aggregates are computed, with a focus on the role of structural change induced by low-frequency torsional modes along with quasi-stationary trapping effects induced...

(De)localization dynamics of molecular excitons: comparison of mixed quantum–classical and fully quantum treatments

Molecular excitons play a central role in processes of solar energy conversion, both natural and artificial. It is therefore no wonder that numerous experimental and theoretical investigations in the last...

Computational and data driven molecular material design assisted by low scaling quantum mechanics calculations and machine learning

Electronic structure methods based on quantum mechanics (QM) are widely employed in the computational predictions of the molecular properties and optoelectronic properties of molecular materials. The computational costs of these QM methods, ranging from density functional theory (DFT) or time-dependent DFT (TDDFT) to wave-function theory (WFT), usually increase sharply with the system size, causing the curse of dimensionality and hindering the QM calculations for large sized systems such as long polymer oligomers and complex molecular aggregates. In such cases, in recent years low scaling QM methods and machine learning (ML) techniques have been adopted to reduce the computational costs and thus assist computational and data driven molecular material design. In this review, we illustrated low scaling ground-state and excited-state QM approaches and their applications to long oligomers, self-assembled supramolecular complexes, stimuli-responsive materials, mechanically interlocked molecules, and excited state processes in molecular aggregates. Variable electrostatic parameters were also introduced in the modified force fields with the polarization model. On the basis of QM computational or experimental datasets, several ML algorithms, including explainable models, deep learning, and on-line learning methods, have been employed to predict the molecular energies, forces, electronic structure properties, and optical or electrical properties of materials. It can be conceived that low scaling algorithms with periodic boundary conditions are expected to be further applicable to functional materials, perhaps in combination with machine learning to fast predict the lattice energy, crystal structures, and spectroscopic properties of periodic functional materials.

The Ultrafast Quantum Dynamics of Photoexcited Adenine-Thymine Basepair Investigated with a Fragment-based Diabatization and a Linear Vibronic Coupling Model

In this contribution we present a quantum dynamical study of the photoexcited hydrogen bonded base pair adenine–thymine (AT) in a Watson–Crick arrangement. To that end, we parametrize Linear Vibronic Coupling (LVC) models with Time-Dependent Density Functional Theory (TD-DFT) calculations, exploiting a fragment diabatization scheme (FrD) we have developed to define diabatic states on the basis of individual chromophores in a multichromophoric system. Wavepacket propagations were run with the multilayer extension of the Multiconfiguration Time-Dependent Hartree method. We considered excitations to the three lowest bright states, a ππ* state of thymine and two ππ* states (L_a and L_b) of adenine, and we found that on the 100 fs time scale the main decay pathways involve intramonomer population transfers toward nπ* states of the same nucleobase. In AT this transfer is less effective than in the isolated nucleobases, because hydrogen bonding destabilizes the nπ* states. The population transfer to the A → T charge transfer state is negligible, making the ultrafast (femtosecond) decay through the proton coupled electron transfer mechanism unlikely, in line with experimental results in apolar solvents. The excitation energy transfer is also very small. We carefully compare the predictions of LVC Hamiltonians obtained with different sets of diabatic states, defined so to match either local states of the two separated monomers or the base pair adiabatic states in the Franck–Condon region. To that end we also extend the flexibility of the FrD-LVC approach, introducing a new strategy to define fragments diabatic states that account for the effect of the rest of the multichromohoric system through a Molecular Mechanics potential.

Fragment Diabatization Linear Vibronic Coupling Model for Quantum Dynamics of Multichromophoric Systems: Population of the Charge-Transfer State in the Photoexcited Guanine-Cytosine Pair

We introduce a method (FrD-LVC) based on a fragment diabatization (FrD) for the parametrization of a linear vibronic coupling (LVC) model suitable for studying the photophysics of multichromophore systems. In combination with effective quantum dynamics (QD) propagations with multilayer multiconfigurational time-dependent Hartree (ML-MCTDH), the FrD-LVC approach gives access to the study of the competition between intrachromophore decays, like those at conical intersections, and interchromophore processes, like exciton localization/delocalization and the involvement of charge-transfer (CT) states. We used FrD-LVC parametrized with time-dependent density functional theory (TD-DFT) calculations, adopting either CAM-B3LYP or ωB97X-D functionals, to study the ultrafast photoexcited QD of a guanine-cytosine (GC) hydrogen-bonded pair, within a Watson-Crick arrangement, considering up to 12 coupled diabatic electronic states and the effect of all of the 99 vibrational coordinates. The bright excited states localized on C and, especially, on G are predicted to be strongly coupled to the G → C CT state, which is efficiently and quickly populated after an excitation to any of the four lowest energy bright local excited states. Our QD simulations show that more than 80% of the excited population on G and ∼50% of that on C decay to this CT state in less than 50 fs. We investigate the role of vibronic effects in the population of the CT state and show that it depends mainly on its large reorganization energy so that it can occur even when it is significantly less stable than the bright states in the Franck-Condon region. At the same time, we document that the formation of the GC pair almost suppresses the involvement of dark nπ* excited states in the photoactivated dynamics.

Electronic Circular Dichroism Spectra of DNA Quadruple Helices Studied by Molecular Dynamics Simulations and Excitonic Calculations including Charge Transfer States

We here investigate the Electronic Circular Dichroism (ECD) Spectra of two representative Guanine-rich sequences folded in a Quadruple helix (GQ), by using a recently developed fragment diabatisation based excitonic model (FrDEx). FrDEx can include charge transfer (CT) excited states and consider the effect of the surrounding monomers on the local excitations (LEs). When applied to different structures generated by molecular dynamics simulations on a fragment of the human telomeric sequence (Tel21/22), FrDEx provides spectra fully consistent with the experimental one and in good agreement with that provided by quantum mechanical (QM) method used for its parametrization, i.e., TD-M05-2X. We show that the ECD spectrum is moderately sensitive to the conformation adopted by the bases of the loops and more significantly to the thermal fluctuations of the Guanine tetrads. In particular, we show how changes in the overlap of the tetrads modulate the intensity of the ECD signal. We illustrate how this correlates with changes in the character of the excitonic states at the bottom of the La and Lb bands, with larger LE and CT involvement of bases that are more closely stacked. As an additional test, we utilised FrDEx to compute the ECD spectrum of the monomeric and dimeric forms of a GQ forming sequence T30695 (5′TGGGTGGGTGGGTGGG3′), i.e., a system containing up to 24 Guanine bases, and demonstrated the satisfactory reproduction of the experimental and QM reference results. This study provides new insights on the effects modulating the ECD spectra of GQs and, more generally, further validates FrDEx as an effective tool to predict and assign the spectra of closely stacked multichromophore systems.

Polythiophene-Doped Resorcinol-Formaldehyde Resin Photocatalysts for Solar-to-Hydrogen Peroxide Energy Conversion

The generation of hydrogen peroxide (H₂O₂) from water and dioxygen by sunlight-driven heterogeneous photocatalysis is a promising method for the artificial photosynthesis of a liquid solar fuel. We previously found that resorcinol-formaldehyde (RF) resin powders prepared by high-temperature hydrothermal synthesis act as highly active semiconductor photocatalysts for H₂O₂ generation. Herein, we report that RF resin powders doped with poly(3-hexylthiophene-2,5-diyl) (RF/P3HT) exhibit enhanced photocatalytic activities. The highly dispersed P3HT within the resin particles created charge transfer complexes with the conduction band of the resin via electron donation, facilitating efficient transfer of the photogenerated conduction band electrons through P3HT. This enhanced charge separation promoted efficient water oxidation and O₂ reduction. The solar-to-chemical conversion efficiency for H₂O₂ generation on the RF/P3HT resin in water under simulated sunlight irradiation with atmospheric pressure of O₂ was ∼1.0%, which is the highest efficiency reported for powder catalysts in artificial photosynthesis.

Revealing generation, migration, and dissociation of electron-hole pairs and current emergence in an organic photovoltaic cell

Using an innovative quantum mechanical method for an open quantum system, we observe in real time and space the generation, migration, and dissociation of electron-hole pairs, transport of electrons and holes, and current emergence in an organic photovoltaic cell. Ehrenfest dynamics is used to study photoexcitation of thiophene:fullerene stacks coupled with a time-dependent density functional tight-binding method. Our results display the generation of an electron-hole pair in the donor and its subsequent migration to the donor-acceptor interface. At the interface, electrons transfer from the lowest unoccupied molecular orbitals (LUMOs) of thiophenes to the second LUMOs of fullerene. Further migration of electrons and holes leads to the emergence of current. These findings support previous experimental evidence of coherent couplings between electronic and vibrational degrees of freedom and are expected to stimulate further work toward exploring the interplay between electron-hole pair (exciton) binding and vibronic coupling for charge separation and transport.

Solar to hydrocarbon production using metal-free water-soluble bulk heterojunction of conducting polymer nanoparticle and graphene oxide

This work demonstrates the first example of interfacial manipulation in a hybrid photocatalyst based on poly(3-hexylthiophene-2,5-diyl) (P3HT) nanoparticle and graphene oxide (GO) bulk heterojunctions to efficiently reduce CO₂ into selective industrial hydrocarbons under gas-phase reaction and visible-light illumination. High selectivity of chemical products (methanol and acetaldehyde) was observed. Moreover, the hybrid photocatalyst's solar-to-fuel conversion efficiency was 13.5 times higher than that of pure GO. The increased production yield stems from the co-catalytic and sensitizing role of P3HT in the hybrid system due to its ability to extend light absorption to the visible range and improve interfacial charge transfer to GO. The hybrid P3HT-GO formed a type II heterojunction, and its static and dynamic exciton behaviors were examined using fluorescence spectroscopy and exciton lifetime mapping. A reduced fluorescence decay time was observed by interfacial manipulation for improved dispersion, indicating a more efficient charge transfer from the excited P3HT to GO. Thus, the conducting polymer nanoparticles, 2D nanocarbon, have demonstrated superior performance as a metal-free, non-toxic, low-cost, and scalable heterogeneous photocatalyst for CO₂ reduction to solar fuel, a solid-gas system.

Vibration-mediated resonant charge separation across the donor-acceptor interface in an organic photovoltaic device

Examination of a recent open-system Ehrenfest dynamics simulation suggests that a vibration-mediate resonance may play a pivotal role in the charge transfer across a donor-acceptor interface in an organic solar cell. Based on this, a concise dissipative two-level electronic system coupled to a molecular vibrational mode is proposed and solved quantum mechanically. It is found that the charge transfer is enhanced substantially when the vibrational energy quanta is equal to the electronic energy loss across the interface. This vibration-mediate resonant charge transfer process is ultrafast, occurring within 100 fs, comparable to experimental findings. The open-system Ehrenfest dynamics simulation of the two-level model is carried out, and similar results are obtained, which confirms further that the earlier open-system Ehrenfest dynamics simulation indeed correctly predicted the occurrence of the resonant charge transfer across the donor-acceptor interface.

Quantum Dynamics of Exciton Transport and Dissociation in Multichromophoric Systems

Due to the subtle interplay of site-to-site electronic couplings, exciton delocalization, nonadiabatic effects, and vibronic couplings, quantum dynamical studies are needed to elucidate the details of ultrafast photoinduced energy and charge transfer events in organic multichromophoric systems. In this vein, we review an approach that combines first-principles parameterized lattice Hamiltonians with accurate quantum dynamical simulations using advanced multiconfigurational methods. Focusing on the elementary transfer steps in organic functional materials, we address coherent exciton migration and creation of charge transfer excitons in homopolymers, notably representative of the poly(3-hexylthiophene) material, as well as exciton dissociation at polymer:fullerene heterojunctions. We emphasize the role of coherent transfer, trapping effects due to high-frequency phonon modes, and thermal activation due to low-frequency soft modes that drive a diffusive dynamics.

Excitonic Model for Strongly Coupled Multichromophoric Systems: The Electronic Circular Dichroism Spectra of Guanine Quadruplexes as Test Cases

We here propose a general and flexible approach, based on fragment diabatization, which incorporates charge transfer states and significantly increases the reliability of excitonic Hamiltonians for systems where the chromophores are very close. This model (FrDEx) is used to compute the electronic circular dichroism and absorption spectra of two prototype guanine-rich DNA sequences folded in quadruple helices (GQs), i.e., a fragment of the human telomeric sequence (Tel21, antiparallel), and (TGGGGT)₄ (TG4T, parallel). Calculations on different subsets of Tel21 and TG4T, from dimers to tetramers, show that FrDEx provides spectra close to the reference full quantum mechanical (QM) ones (obtained with time-dependent density functional theory), with significant improvements with respect to "standard" excitonic Hamiltonians. Furthermore, these tests enable the most cost-effective procedure for the whole GQ to be determined. FrDEx spectra of Tel21 and TG4T are also in good agreement with the QM and experimental ones and give access to interesting insights into the chemical-physical effects modulating the spectral signals. FrDEx could be profitably used to investigate many other biological and nanotechnological materials, from DNA to (opto)electronic polymers.

Intermolecular conical intersections in molecular aggregates

Conical intersections (CoIns) of multidimensional potential energy surfaces are ubiquitous in nature and control pathways and yields of many photo-initiated intramolecular processes. Such topologies can be potentially involved in the energy transport in aggregated molecules or polymers but are yet to be uncovered. Here, using ultrafast two-dimensional electronic spectroscopy (2DES), we reveal the existence of intermolecular CoIns in molecular aggregates relevant for photovoltaics. Ultrafast, sub-10-fs 2DES tracks the coherent motion of a vibrational wave packet on an optically bright state and its abrupt transition into a dark state via a CoIn after only 40 fs. Non-adiabatic dynamics simulations identify an intermolecular CoIn as the source of these unusual dynamics. Our results indicate that intermolecular CoIns may effectively steer energy pathways in functional nanostructures for optoelectronics.

First-Principles Quantum and Quantum-Classical Simulations of Exciton Diffusion in Semiconducting Polymer Chains at Finite Temperature

We report on first-principles quantum-dynamical and quantum-classical simulations of photoinduced exciton dynamics in oligothiophene chain segments, representative of intrachain exciton migration in the poly(3-hexylthiophene) (P3HT) polymer. Following up on our recent study (Binder R.; Burghardt, I. Faraday Discuss. 2020, 221, 406), multilayer multiconfiguration time-dependent Hartree calculations for a short oligothiophene segment comprising 20 monomer units (OT-20) are carried out to obtain full quantum-dynamical simulations at finite temperature. These are employed to benchmark mean-field Ehrenfest calculations, which are shown to give qualitatively correct results for the present system. Periodic boundary conditions turn out to significantly improve earlier estimates of diffusion coefficients. Using the Ehrenfest approach, a series of calculations are subsequently carried out for larger lattices (OT-40 to OT-80), leading to estimates for temperature-dependent mean-squared displacements, which are found to exhibit a near-linear dependence as a function of time. The resulting diffusion coefficient estimates are an increasing function of temperature, whose detailed functional form depends on the degree of static disorder. With a realistic static disorder parameter (σ_s ≃ 0.06 eV), the diffusion coefficients decrease from D ∼ 1 × 10^-2 cm² s^-1 to D ∼ 1 × 10^-3 cm² s^-1, in qualitative agreement with experimental data for P3HT. The dynamical scenario obtained from our simulations shows that exciton migration in P3HT-type chains is a largely adiabatic process throughout the temperature regime we investigated (i.e., T = 50-300 K). The resulting picture of exciton migration is a coherent, but not bandlike, motion of an exciton-polaron driven by fluctuations induced by low-frequency modes. This process acquires partial hopping character if static disorder becomes prominent and Anderson localization sets in.

A hierarchical effective mode approach to phonon-driven multilevel vibrational relaxation dynamics at surfaces

We discuss an efficient Hierarchical Effective Mode (HEM) representation of a high-dimensional harmonic oscillator bath, which describes phonon-driven vibrational relaxation of an adsorbate-surface system, namely, deuterium adsorbed on Si(100). Starting from the original Hamiltonian of the adsorbate-surface system, the HEM representation is constructed via iterative orthogonal transformations, which are efficiently implemented with Householder matrices. The detailed description of the HEM representation and its construction are given in the second quantization representation. The hierarchical nature of this representation allows access to the exact quantum dynamics of the adsorbate-surface system over finite time intervals, controllable via the truncation order of the hierarchy. To study the convergence properties of the effective mode representation, we solve the time-dependent Schrödinger equation of the truncated system-bath HEM Hamiltonian, with the help of the multilayer extension of the Multiconfigurational Time-Dependent Hartree (ML-MCTDH) method. The results of the HEM representation are compared with those obtained with a quantum-mechanical tier-model. The convergence of the HEM representation with respect to the truncation order of the hierarchy is discussed for different initial conditions of the adsorbate-surface system. The combination of the HEM representation with the ML-MCTDH method provides information on the time evolution of the system (adsorbate) and multiple effective modes of the bath (surface). This permits insight into mechanisms of vibration-phonon coupling of the adsorbate-surface system, as well as inter-mode couplings of the effective bath.

First-principles quantum simulations of exciton diffusion on a minimal oligothiophene chain at finite temperature

High-dimensional multiconfigurational quantum dynamics simulations are carried out at finite temperature to simulate exciton diffusion on an oligothiophene chain, representative of a segment of the poly(3-hexylthiophene) (P3HT) polymer. The ab initio parametrized site-based Hamiltonian of Binder et al. [Phys. Rev. Lett., 2018, 120, 227401] is employed to model a 20-site system, including intra-ring and inter-ring high-frequency modes as well as torsional modes which undergo thermal fluctuations induced by an explicit harmonic oscillator bath. The system-bath dynamics is treated within the setting of a stochastic mean-field Schrödinger equation. For the 20-site excitonic system, a total of 20 Frenkel states and 248 modes are propagated using the multi-layer multi-configuration time-dependent Hartree (ML-MCTDH) method. The resulting dynamics can be interpreted in terms of the coherent motion of an exciton-polaron quasi-particle stochastically driven by torsional fluctuations. This dynamics yields a near-linear mean squared displacement (MSD) as a function of time, from which a diffusion coefficient can be deduced which increases with temperature, up to 5.7 × 10-3 cm2 s-1 at T = 300 K.

Coherent Real-Space Charge Transport Across a Donor-Acceptor Interface Mediated by Vibronic Couplings

There is growing experimental and theoretical evidence that vibronic couplings, couplings between electronic and nuclear degrees of freedom, play a fundamental role in ultrafast excited-state dynamics in organic donor-acceptor hybrids. Whereas vibronic coupling has been shown to support charge separation at donor-acceptor interfaces, so far, little is known about its role in the real-space transport of charges in such systems. Here we theoretically study charge transport in thiophene:fullerene stacks using time-dependent density functional tight-binding theory combined with Ehrenfest molecular dynamics for open systems. Our results reveal coherent oscillations of the charge density between neighboring donor sites, persisting for ∼200 fs and promoting charge transport within the polymer stacks. At the donor-acceptor interface, vibronic wave packets are launched, propagating coherently over distances of more than 3 nm into the acceptor region. This supports previous experimental observations of long-range ballistic charge-carrier motion in organic photovoltaic systems and highlights the importance of vibronic coupling engineering as a concept for tailoring the functionality of hybrid organic devices.