Di Maiolo Francesco, Brey Dominik, Binder Robert, Burghardt Irene
Institute for Physical and Theoretical Chemistry, Goethe University, Max-von-Laue-Str. 7, 60438 Frankfurt/Main, Germany.
J Chem Phys. 2020 Nov 14;153(18):184107. doi: 10.1063/5.0027588.
We report on quantum dynamical simulations of exciton diffusion in an oligo(para-phenylene vinylene) chain segment with 20 repeat units (OPV-20) at finite temperature, complementary to our recent study of the same system at T = 0 K [R. Binder and I. Burghardt, J. Chem. Phys. 152, 204120 (2020)]. Accurate quantum dynamical simulations are performed using the multi-layer multi-configuration time-dependent Hartree method as applied to a site-based Hamiltonian comprising 20 electronic states of Frenkel type and 460 vibrational modes, including site-local quinoid-distortion modes along with site-correlated bond-length alternation (BLA) modes, ring torsional modes, and an explicit harmonic-oscillator bath. A first-principles parameterized Frenkel-Holstein type Hamiltonian is employed, which accounts for correlations between the ring torsional modes and the anharmonically coupled BLA coordinates located at the same junction. Thermally induced fluctuations of the torsional modes are described by a stochastic mean-field approach, and their impact on the excitonic motion is characterized in terms of the exciton mean-squared displacement. A normal diffusion regime is observed under periodic boundary conditions, apart from transient localization features. Even though the polaronic exciton species are comparatively weakly bound, exciton diffusion is found to be a coherent-rather than hopping type-process, driven by the fluctuations of the soft torsional modes. Similar to the previous observations for oligothiophenes, the evolution for the most part exhibits a near-adiabatic dynamics of local exciton ground states (LEGSs) that adjust to the local conformational dynamics. However, a second mechanism, involving resonant transitions between neighboring LEGSs, gains importance at higher temperatures.
我们报告了在有限温度下对具有20个重复单元的聚（对亚苯基乙烯撑）链段（OPV - 20）中激子扩散的量子动力学模拟，这是我们近期对该系统在T = 0 K时研究的补充[R. Binder和I. Burghardt，J. Chem. Phys. 152, 204120 (2020)]。使用多层多组态含时Hartree方法进行精确的量子动力学模拟，该方法应用于基于位点的哈密顿量，该哈密顿量包含20个Frenkel型电子态和460个振动模式，包括位点局部醌式畸变模式以及位点相关的键长交替（BLA）模式、环扭转模式和一个显式的简谐振子浴。采用第一性原理参数化的Frenkel - Holstein型哈密顿量，该哈密顿量考虑了位于同一连接处的环扭转模式与非谐耦合BLA坐标之间的相关性。扭转模式的热致涨落通过随机平均场方法描述，其对激子运动的影响通过激子均方位移来表征。除了瞬态局域化特征外，在周期性边界条件下观察到正常扩散区域。尽管极化子激子种类的束缚相对较弱，但发现激子扩散是一个由软扭转模式的涨落驱动的相干过程而非跳跃型过程。与之前对低聚噻吩的观察结果类似，大部分演化表现出局部激子基态（LEGSs）的近绝热动力学，其适应局部构象动力学。然而，在较高温度下，涉及相邻LEGSs之间共振跃迁的第二种机制变得重要。
