Conformational Dynamics Guides Coherent Exciton Migration in Conjugated Polymer Materials: A First-Principles Quantum Dynamical Study

We report on high-dimensional quantum dynamical simulations of torsion-induced exciton migration in a single-chain oligothiophene segment comprising twenty repeat units, using a first-principles parametrized Frenkel J-aggregate Hamiltonian. Starting from an initial inter-ring torsional defect, these simulations provide evidence of an ultrafast two-time scale process at low temperatures, involving exciton-polaron formation within tens of femtoseconds, followed by torsional relaxation on a ~300 femtosecond time scale. The second step is the driving force for exciton migration, as initial conjugation breaks are removed by dynamical planarization. The quantum coherent nature of the elementary exciton migration step is consistent with experimental observations highlighting the correlated and vibrationally coherent nature of the dynamics on ultrafast time scales.Comment: 4 pages, 4 figure

Dynamics of the molecular geometric phase

The fate of the molecular geometric phase in an exact dynamical framework is investigated with the help of the exact factorization of the wavefunction and a recently proposed quantum hydrodynamical description of its dynamics. An instantaneous, gauge invariant phase is introduced for arbitrary paths in nuclear configuration space in terms of hydrodynamical variables, and shown to reduce to the adiabatic geometric phase when the state is adiabatic and the path is closed. The evolution of the closed-path phase over time is shown to adhere to a Maxwell-Faraday induction law, with non-conservative forces arising from the electron dynamics that play the role of electromotive forces. We identify the pivotal forces that are able to change the value of the phase, thereby challenging any topological argument. Nonetheless, negligible changes in the phase occur when the local dynamics along the probe loop is approximately adiabatic. In other words, the adiabatic idealization of geometric phase effects may remain suitable for effectively describing certain dynamic observables

Exciton dissociation at donor-acceptor polymer heterojunctions: quantum nonadiabatic dynamics and effective-mode analysis

The quantum-dynamical mechanism of photoinduced subpicosecond exciton dissociation and the concomitant formation of a charge-separated state at a TFB:F8BT polymer heterojunction is elucidated. The analysis is based upon a two-state vibronic coupling Hamiltonian including an explicit 24-mode representation of a phonon bath comprising high-frequency (C$=$C stretch) and low-frequency (torsional) modes. The initial relaxation behavior is characterized by coherent oscillations, along with the decay through an extended nonadiabatic coupling region. This region is located in the vicinity of a conical intersection hypersurface. A central ingredient of the analysis is a novel effective mode representation, which highlights the role of the low-frequency modes in the nonadiabatic dynamics. Quantum dynamical simulations were carried out using the multiconfiguration time-dependent Hartree (MCTDH) method

Quantum hydrodynamics of coupled electron-nuclear systems

The quantum dynamics of electron-nuclear systems is analyzed from the perspective of the exact factorization of the wavefunction, with the aim of defining gauge invariant equations of motion for both the nuclei and the electrons. For pure states this is accomplished with a quantum hydrodynamical description of the nuclear dynamics and electronic density operators tied to the fluid elements. For statistical mixtures of states the exact factorization approach is extended to two limiting situations that we call "type-n" and "type-e" mixtures, depending on whether the nuclei or the electrons are, respectively, in an intrinsically mixed state. In both cases a fully gauge invariant formulation of the dynamics is obtained again in hydrodynamic form with the help of mechanical momentum moments (MMMs). Nuclear MMMs extend in a gauge invariant way the ordinary momentum moments of the Wigner distribution associated with a density matrix of positional variables, electron MMMs are operator-valued and represent a generalization of the (conditional) density operators used for pure states. The theory presented here bridges exact quantum dynamics with several mixed quantum-classical approaches currently in use to tackle non-adiabatic molecular problems, offering a foundation for systematic improvements. It further connects to non-adiabatic theories in condensed-phase systems. As an example, we re-derive the finite-temperature theory of electronic friction of Dou, Miao \& Subotnik (Phys. Rev. Lett. 119, 046001 (2017)) from the dynamics of "type-e" mixtures and discuss possible improvements

ULTRAFAST VIBRONIC DYNAMICS OF FUNCTIONAL ORGANIC POLYMER MATERIALS: COHERENCE, CONFINEMENT, AND DISORDER

This talk addresses quantum dynamical studies of ultrafast photo-induced energy and charge transfer in functional organic materials, complementing time-resolved spectroscopic observations [1] which underscore that the elementary transfer events in these molecular aggregate systems can be guided by quantum coherence, despite the presence of static and dynamic disorder. The intricate interplay of electronic delocalization, coherent vibronic dynamics, and trapping phenomena requires a quantum dynamical treatment that goes beyond conventional mixed quantum-classical simulations. Our approach combines first-principles parametrized Hamiltonians, based on TDDFT and/or high-level electronic structure calculations, with accurate quantum dynamics simulations using the Multi-Configuration Time-Dependent Hartree (MCTDH) method [2]. The talk will specifically focus on (i) exciton dissociation and free carrier generation in regioregular donor-acceptor assemblies [3-5], (ii) exciton multiplication in acene materials [6] and (iii) the elementary mechanism of exciton migration and creation of charge-transfer excitons in polythiophene and poly-(p-phenylene vinylene) type materials [7]. Special emphasis is placed on the influence of structural (dis)order and molecular packing, which can act as a determining factor in transfer efficiencies. Against this background, we will comment on the role of temporal and spatial coherence along with a consistent description of the transition to a classical-statistical regime. \noindent[1] A. De Sio and C. Lienau, Phys. Chem. Chem. Phys. 19, 18813 (2017). \noindent[2] G. A. Worth, H.-D. Meyer, H. Köppel, L. S. Cederbaum, and I. Burghardt, Int. Rev. Phys. Chem. 27, 569 (2008). \noindent[3] M. Polkehn, H. Tamura, P. Eisenbrandt, S. Haacke, S. Méry, and I. Burghardt, J. Phys. Chem. Lett. 7, 1327 (2016). \noindent[4] M. Polkehn, P. Eisenbrandt, H. Tamura, and I. Burghardt, Int. J. Quantum Chem. 118:e25502. (2018). \noindent[5] M. Polkehn, H. Tamura, and I. Burghardt, J. Phys. B: At. Mol. Opt. Phys. 51, 014003 (2018). \noindent[6] H. Tamura, M. Huix-Rotllant, I. Burghardt, Y. Olivier, and D. Beljonne, Phys. Rev. Lett. 115, 107401 (2015). \noindent[7] R. Binder, M. Polkehn, T. Ma, and I. Burghardt, Chem. Phys. 482, 16 (2017)

Phonon-driven ultrafast exciton dissociation at donor-acceptor polymer heterojunctions

A quantum-dynamical analysis of phonon-driven exciton dissociation at polymer heterojunctions is presented, using a hierarchical electron-phonon model parameterized for three electronic states and 24 vibrational modes. Two interfering decay pathways are identified: a direct charge separation, and an indirect pathway via an intermediate bridge state. Both pathways depend critically on the dynamical interplay of high-frequency C=C stretch modes and low-frequency ring-torsional modes. The ultrafast, highly non-equilibrium dynamics is consistent with time-resolved spectroscopic observations