Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, United Kingdom.
Department of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom.
J Chem Phys. 2018 Jan 21;148(3):034901. doi: 10.1063/1.5009393.
The exciton relaxation dynamics of photoexcited electronic states in poly(p-phenylenevinylene) are theoretically investigated within a coarse-grained model, in which both the exciton and nuclear degrees of freedom are treated quantum mechanically. The Frenkel-Holstein Hamiltonian is used to describe the strong exciton-phonon coupling present in the system, while external damping of the internal nuclear degrees of freedom is accounted for by a Lindblad master equation. Numerically, the dynamics are computed using the time evolving block decimation and quantum jump trajectory techniques. The values of the model parameters physically relevant to polymer systems naturally lead to a separation of time scales, with the ultra-fast dynamics corresponding to energy transfer from the exciton to the internal phonon modes (i.e., the C-C bond oscillations), while the longer time dynamics correspond to damping of these phonon modes by the external dissipation. Associated with these time scales, we investigate the following processes that are indicative of the system relaxing onto the emissive chromophores of the polymer: (1) Exciton-polaron formation occurs on an ultra-fast time scale, with the associated exciton-phonon correlations present within half a vibrational time period of the C-C bond oscillations. (2) Exciton decoherence is driven by the decay in the vibrational overlaps associated with exciton-polaron formation, occurring on the same time scale. (3) Exciton density localization is driven by the external dissipation, arising from "wavefunction collapse" occurring as a result of the system-environment interactions. Finally, we show how fluorescence anisotropy measurements can be used to investigate the exciton decoherence process during the relaxation dynamics.
聚对苯乙炔中光激发电子态的激子弛豫动力学在一个粗粒化模型中进行了理论研究，其中激子和核自由度都被量子力学处理。弗伦克尔-霍尔斯坦哈密顿量用于描述系统中存在的强激子-声子耦合，而内部核自由度的外部阻尼通过林德布拉德主方程来描述。数值上，使用时间演化块约化和量子跳跃轨迹技术计算动力学。与聚合物系统物理相关的模型参数的值自然导致时间尺度的分离，超快动力学对应于激子向内部声子模式（即 C-C 键振动）的能量转移，而较长时间的动力学对应于这些声子模式通过外部耗散的阻尼。与这些时间尺度相关，我们研究了以下过程，这些过程表明系统在聚合物的发射色团上弛豫：（1）激子极化子的形成发生在超快时间尺度上，与 C-C 键振动的半振动时间周期内存在的激子-声子相关。（2）激子退相干由与激子极化子形成相关的振动重叠的衰减驱动，发生在相同的时间尺度上。（3）激子密度局域化由外部耗散引起，这是由于系统-环境相互作用导致的“波函数坍塌”而产生的。最后，我们展示了如何使用荧光各向异性测量来研究弛豫动力学过程中的激子退相干过程。
