Mechanistic Insights into the Excited-State Intramolecular Proton Transfer (ESIPT) Process of...

The 2-(2-aminophenyl)naphthalene molecule attracted much attention due to excited-state intramolecular proton transfer (ESIPT) from an amino NH₂ group to a carbon atom of an adjacent aromatic ring. The ESIPT mechanisms of 2-(2-aminophenyl)naphthalene are still unclear. Herein, the decay pathways of this molecule in vacuum were investigated by combining static electronic structure calculations and nonadiabatic dynamics simulations. The calculations indicated the existence of two stable structures (S0-1 and S0-2) in the S₀ and S₁ states. For the S0-1 isomer, upon excitation to the Franck-Condon point, the system relaxed to the S₁ minimum quickly, and then there exist four decay pathways (two ESIPT ones and two decay channels with C atom pyramidalization). In the ESIPT decay pathways, the system encounters the S1S0-PT-1 or S1S0-PT-2 conical intersection, which funnels the system rapidly to the S₀ state. In the other two pathways, the system de-excited from the S₁ to the S₀ state via the S1S0-1 or S1S0-2 conical intersection. For the S0-2 structure, the decay pathways were similar to those of S0-1. The dynamics simulations showed that 75 and 69% of trajectories experienced the two ESIPT conical intersections for the S0-1 and S0-2 structures, respectively. Our simulations showed that the lifetime of the S₁ state of S0-1 (S0-2) is estimated to be 358 (400) fs. Notably, we not only found the detailed reaction mechanism of the system but also found that the different ground-state configurations of this system have little effect on the reaction mechanism in vacuum.