Excited-state Relaxation Mechanisms of 2,2’-(1-Phenyl-1H-1,2,4-triazole-3,5-diyl)diphenol: Single or Double Proton Transfer

Triazole compounds are important organic systems with excellent electronic properties, which has diagnostic potential in the fields of organic electronics and organic photovoltaics. The important photophysical nature of these systems is the transformation between the enol and keto forms after excited-state proton transfer. In this work, the IR vibrational spectrum, ESIPT mechanism and excited-state decay dynamics of 2,2'-(1-Phenyl-1H-1,2,4-triazole-3,5-diyl)diphenol (ExPh) are explored by using electronic structure calculations and non-adiabatic dynamics simulations. Two S1/S0 conical intersections with distinct proton transfer (ESIPT-I and ESIPT-II) involved were obtained. The associated two-dimensional S1 minimum-energy potential energy surface indicates that the dynamical roles of these two S1/S0 conical intersections in the S1 excited-state decay are quite different. The ESIPT-I reaction is more favorable to occur than the ESIPT-II process. Our dynamics simulations support this suspect with the whole trajectories decay to the ground state via the S1S0-1 conical intersection which involves ESIPT-I process. The ESIPT-involved efficient deactivation pathway can be partially responsible for the decrease of fluorescence emission. These results and ESIPT mechanisms are helpful for understanding the decay pathways of similar systems.