Excited-State Deactivation Mechanism of 3,5-bis(2-hydroxyphenyl)-1 H -1,2,4-triazole: Electronic Structure Calculations and Nonadiabatic Dynamics Simulations.

3,5-bis(2-Hydroxyphenyl)-1 H -1,2,4-triazole (bis-HPTA) has attracted wide attention due to the important application in the detection of microorganisms and insecticidal activity. However, the mechanisms of excited-state intramolecular proton transfer (ESIPT) process and decay pathways are still a matter of debate. In this work, we have comprehensively investigated the photodynamics of bis-HPTA by executing combined electronic structure calculations and nonadiabatic surface-hopping dynamics simulations. Based on the computed electronic structure and dynamics information, we propose two nonadiabatic deactivation channels that efficiently populate the ground state from the Franck-Condon region. In the first one, after being excited to the bright S 1 state, bis-HPTA molecule undergoes an ultrafast and barrierless ESIPT-1 process. Then, the system encounters with an energetically accessible S 1 /S 0 conical intersection (CI), which funnels the system to the ground state speedily. Afterward, the keto species either arrives at the keto product or return to its enol species via a ground-state proton transfer in the S 0 state. In the other excited-state decay channel, the S 1 system hops to the ground state through a different CI, which involves the ESIPT-2 process. In our dynamics simulations, about 79.6% of the trajectories decay to the S 0 state via the first CI, while the remaining ones employ the second conical intersection. The results of dynamics simulations also demonstrated that the lifetime of the S 1 state is estimated as 315 fs. The present work will give elaborating mechanistic information of similar compounds in various environments.