Visible-Light-Induced Exciton Dynamics and Trans-to-Cis Isomerization in Azobenzene Aggregates: Insights from Surface Hopping / Semiempirical Configuration Interaction Molecular Dynamics Simulations

Assemblies of photochromic molecules feature exciton states, which govern photochemical and photophysical processes in multichromophoric systems. Understanding photoinduced dynamics of the assemblies requires nonadiabatic treatment involving multiple exciton states and numerous nuclear degrees of freedom, thus posing a challenge for simulations. In this work, we address this challenge for aggregates of azobenzene, a prototypical molecular switch, performing on-the-fly surface hopping calculations combined with semiempirical configuration interaction electronic structure and augmented with transition density matrix analysis to characterize exciton evolution. Specifically, we consider excitation of azobenzene tetramers in the nπ* absorption band (located in the visible (blue) part of the electromagnetic spectrum) thus extending our recent work on dynamics after ππ* excitation (corresponding to the ultraviolet region) [Titov, J. Phys. Chem. C 2023, 127, 13678–13688]. We find that the nπ* excitons, which are initially strongly localized by ground state conformational disorder, undergo further (very strong) localization during short-time photodynamics. This excited-state localization process is extremely ultrafast, occuring within first 10 fs of photodynamics. We observe virtually no exciton transfer of the localized excitons in the nπ* manifold. However, the transfer may occur via secondary pathways involving ππ* states or the ground state. Moreover, we find that nπ* quantum yields of the trans-to-cis isomerization are reduced in the aggregated state.