Radosz W, Pawlik G, Mitus A C
Department of Theoretical Physics, Wroclaw University of Science and Technology , Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland.
J Phys Chem B. 2018 Feb 8;122(5):1756-1765. doi: 10.1021/acs.jpcb.7b11949. Epub 2018 Jan 26.
We study theoretically the kinetics of noninteracting photoswitchable guest molecules (model azo-dye) dispersed at low concentration in host (model polymer matrix) in the all-optical poling process close to the glass transition temperature T. We modify kinetic Monte Carlo model used in our previous studies of nonlinear optical processes in host-guest systems. The polymer matrix is simulated using the bond-fluctuation model. The kinetics of multiple trans-cis-trans cycles is formulated in terms of transition probabilities which depend on local free volume in the matrix and its dynamics. Close to T, the buildup of polar order, monitored in terms of angular probability density functions, follows a power-law in time while the evolution of the nonlinear susceptibilities related to second harmonic generation effect follows the stretched-exponential law. This complex dynamics of guest molecules implies the presence of dynamic heterogeneities of the matrix in space and time which spread the complexity from the matrix to the otherwise simple dynamics of noninteracting guest molecules. A qualitative physical picture of mosaic-like states-intertwined areas of free- and hindered angular motion of guest molecules-is proposed and the role of related short and longer scales in space for the promotion of complex dynamics of guest molecules is discussed. A brief comparison of the theory to available experimental data is given.
我们从理论上研究了在接近玻璃化转变温度(T)的全光极化过程中，低浓度分散在主体（模型聚合物基体）中的非相互作用光开关客体分子（模型偶氮染料）的动力学。我们修改了在之前关于主客体系统非线性光学过程研究中使用的动力学蒙特卡罗模型。聚合物基体采用键涨落模型进行模拟。多个反式 - 顺式 - 反式循环的动力学根据依赖于基体中局部自由体积及其动力学的跃迁概率来表述。接近(T)时，根据角概率密度函数监测的极性有序的建立在时间上遵循幂律，而与二次谐波产生效应相关的非线性极化率的演化遵循拉伸指数律。客体分子这种复杂的动力学意味着基体在空间和时间上存在动态不均匀性，这种不均匀性将复杂性从基体扩展到了原本简单的非相互作用客体分子的动力学。我们提出了一个类似镶嵌态的定性物理图像——客体分子自由和受限角运动相互交织的区域，并讨论了空间中相关短尺度和长尺度对促进客体分子复杂动力学的作用。最后给出了该理论与现有实验数据的简要比较。
