Nonadiabatic Dynamics Simulation of the Wavelength-Dependent Photochemistry of Azobenzene Excited to the nπ* and ππ* Excited States

Azobenzene is one of the most ubiquitous photoswitches in photochemistry and a prototypical model for photoisomerizing systems. Despite this, its wavelength-dependent photochemistry has puzzled researchers for decades. Upon excitation to the higher energy ππ* excited state instead of the dipole-forbidden nπ* state, the quantum yield of isomerization from trans- to cis-azobenzene is halved. The difficulties associated with unambiguously resolving this effect both experimentally and theoretically have contributed to lasting controversies regarding the photochemistry of azobenzene. Here, we systematically characterize the dynamic photoreaction pathways of azobenzene by performing first-principles simulations of the nonadiabatic dynamics following excitation to both the ππ* and the nπ* states. We demonstrate that ground-state recovery is mediated by two distinct S1 decay pathways: a reactive twisting pathway and an unreactive planar pathway. Increased preference for the unreactive pathway upon ππ* excitation largely accounts for the wavelength-dependent behavior observed in azobenzene.

偶氮苯（Azobenzene）是光化学中最常见的光开关之一，同时也是光异构化体系的典型模型。尽管如此，其波长依赖的光化学行为数十年来一直困扰着研究者。当分子被激发至高能量的ππ*激发态而非偶极禁阻的nπ*态时，反式偶氮苯向顺式偶氮苯异构化的量子产率会降低一半。无论是实验还是理论层面，明确解析这一效应均存在极大困难，这也使得围绕偶氮苯光化学的争议长期存在。本研究通过对激发至ππ*和nπ*态后的非绝热动力学开展第一性原理模拟，系统表征了偶氮苯的动态光反应路径。研究证实，基态恢复由两条截然不同的S1衰变路径介导：一条为反应性扭曲路径，另一条为非反应性平面路径。当分子被ππ*态激发时，对非反应性路径的偏好性增强，这在很大程度上解释了偶氮苯中观测到的波长依赖行为。