Surface hopping simulations reveal deactivation pathways of a charge transfer system with planarizing and twisting motion

Nonadiabatic surface hopping simulations are used to investigate the relaxation pathways after photoexcitation of 4-(indol-1-ylamino)benzonitrile, a prototypical example of <i>N</i>-aryl-substituted 1-aminoindoles. This molecule combines functional groups that can undergo in the excited state twisted as well as planarized intramolecular charge transfer and are potential building blocks for molecular motors. The results of the nonadiabatic dynamics simulation show that after excitation with light in the range of 4.2-4.8 eV, the <i>N</i>-aryl-substituted 1-aminoindole decays rapidly from S₃ and S₂ into the S₁ state. On the S₁ potential energy surface, the system relaxes then to one of the charge-transfer minima with twisted amino-phenyl and planarized amino-indole torsional angles. The twist and the planarization occur nearly synchronously, with a slight advance of the twist. The geometric relaxation is accompanied by a change of the electronic structure from a ππ* excitation localized on indole to a charge transfer excitation from the indole and amino groups into the benzonitrile moiety. The simulated time-resolved fluorescence spectrum exhibits a bright band from a locally excited state and a weaker charge-transfer band. The transition energies suggest that the experimentally observed emission happens from the S₁ minima. After transition to the ground state, half of the trajectories relax back to the original minimum and half of the trajectories to its enantiomer. The twist and planarization occur nearly synchronously, with planarization slightly leading. The combination of preference to rotate over twisted intermediate structure in the excited state and over more twisted structures in the ground state indicates that some kinetic work is done during a full cycle. Even though the kinetic work done per cycle is small, this might be increased by <i>e.g.</i> introducing substituents.