108 research outputs found
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
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 (CC 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
Vibronic coupling models for donor-acceptor aggregates using an effective-mode scheme: Application to mixed Frenkel and charge-transfer excitons in oligothiophene aggregates
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