Excited-state relaxation mechanisms of 2,2'-(1-phenyl-1 H -1,2,4-triazole-3,5-diyl)diphenol: single- or double-proton transfer?

Triazole compounds are important organic systems with excellent electronic properties, which have 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 study, the IR vibrational spectrum, ESIPT mechanism, and excited-state decay dynamics of 2,2'-(1-phenyl-1 H -1,2,4-triazole-3,5-diyl)diphenol (ExPh) were explored using electronic structure calculations and non-adiabatic dynamics simulations. Two S 1 /S 0 conical intersections with distinct proton transfer (ESIPT-I and ESIPT-II) involved were obtained. The associated two-dimensional S 1 minimum-energy potential energy surface indicated that the dynamical roles of these two S 1 /S 0 conical intersections in the S 1 excited-state decay were quite different. The ESIPT-I reaction was more favorable to occur than the ESIPT-II process. Our dynamics simulations supported this hypothesis with the whole trajectories decaying to the ground state via the S1S0-1 conical intersection, which involved the ESIPT-I process. The ESIPT-Involved efficient deactivation pathway could be partially responsible for the decrease in fluorescence emission. These results and ESIPT mechanisms are helpful for understanding the decay pathways of similar systems.