Tozer Oliver Robert, Barford William
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, United Kingdom.
J Chem Phys. 2015 Aug 28;143(8):084102. doi: 10.1063/1.4929378.
We investigate exciton dynamics on a polymer chain in solution induced by the Brownian rotational motion of the monomers. Poly(para-phenylene) is chosen as the model system and excitons are modeled via the Frenkel exciton Hamiltonian. The Brownian fluctuations of the torsional modes were modeled via the Langevin equation. The rotation of monomers in polymer chains in solution has a number of important consequences for the excited state properties. First, the dihedral angles assume a thermal equilibrium which causes off-diagonal disorder in the Frenkel Hamiltonian. This disorder Anderson localizes the Frenkel exciton center-of-mass wavefunctions into super-localized local exciton ground states (LEGSs) and higher-energy more delocalized quasi-extended exciton states (QEESs). LEGSs correspond to chromophores on polymer chains. The second consequence of rotations-that are low-frequency-is that their coupling to the exciton wavefunction causes local planarization and the formation of an exciton-polaron. This torsional relaxation causes additional self-localization. Finally, and crucially, the torsional dynamics cause the Frenkel Hamiltonian to be time-dependent, leading to exciton dynamics. We identify two distinct types of dynamics. At low temperatures, the torsional fluctuations act as a perturbation on the polaronic nature of the exciton state. Thus, the exciton dynamics at low temperatures is a small-displacement diffusive adiabatic motion of the exciton-polaron as a whole. The temperature dependence of the diffusion constant has a linear dependence, indicating an activationless process. As the temperature increases, however, the diffusion constant increases at a faster than linear rate, indicating a second non-adiabatic dynamics mechanism begins to dominate. Excitons are thermally activated into higher energy more delocalized exciton states (i.e., LEGSs and QEESs). These states are not self-localized by local torsional planarization. During the exciton's temporary occupation of a LEGS-and particularly a quasi-band QEES-its motion is semi-ballistic with a large group velocity. After a short period of rapid transport, the exciton wavefunction collapses again into an exciton-polaron state. We present a simple model for the activated dynamics which is in agreement with the data.
我们研究了溶液中聚合物链上由单体的布朗旋转运动引起的激子动力学。选择聚对苯撑作为模型系统，并通过弗伦克尔激子哈密顿量对激子进行建模。扭转模式的布朗涨落通过朗之万方程进行建模。溶液中聚合物链中单体的旋转对激发态性质有许多重要影响。首先，二面角呈现热平衡，这导致弗伦克尔哈密顿量中的非对角无序。这种无序使弗伦克尔激子质心波函数安德森定域到超定域的局域激子基态（LEGSs）和能量更高、更离域的准扩展激子态（QEESs）。LEGSs对应于聚合物链上的发色团。旋转的第二个后果——即低频旋转——是它们与激子波函数的耦合导致局部平面化并形成激子极化子。这种扭转弛豫导致额外的自定域。最后，至关重要的是，扭转动力学导致弗伦克尔哈密顿量随时间变化，从而导致激子动力学。我们确定了两种不同类型的动力学。在低温下，扭转涨落对激子态的极化子性质起微扰作用。因此，低温下的激子动力学是激子极化子整体的小位移扩散绝热运动。扩散常数的温度依赖性呈线性关系，表明是一个无活化过程。然而，随着温度升高，扩散常数以快于线性的速率增加，表明第二种非绝热动力学机制开始占主导。激子被热激活到能量更高、更离域的激子态（即LEGSs和QEESs）。这些态不会因局部扭转平面化而自定域。在激子暂时占据一个LEGS——特别是一个准带QEES——期间，其运动是具有大群速度的半弹道运动。经过短时间的快速传输后，激子波函数再次坍缩成激子极化子态。我们提出了一个与数据相符的活化动力学简单模型。
