Conical Intersection-Mediated Nonradiative Decay in an ESIPT-Active Imidazole Derivative

Excited-state intramolecular proton transfer (ESIPT) is a photoinduced process that involves proton migration within an intramolecular hydrogen-bonded framework following electronic excitation and is often associated with large Stokes-shifted emission. However, the excited-state fate after proton transfer is not exclusively governed by radiative decay from a stabilized keto tautomer, and nonradiative relaxation may also play a decisive role. In this work, the excited-state relaxation mechanism of 2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (DIP) was investigated using multireference electronic structure calculations combined with nonadiabatic dynamics simulations. SA-CASSCF/MS-CASPT2 calculations reveal the presence of a low-energy S₁S₀ conical intersection in the keto configuration that provides an efficient nonradiative decay channel. The ESIPT process proceeds through a modest proton-transfer barrier of 7.47 kcal·mol⁻¹, after which the system is directed toward the conical intersection region on the excited-state potential energy surface. Surface-hopping dynamics simulations at the OM2/MRCI level further demonstrate that the relaxation process bifurcates into pathways yielding either enol or keto ground-state products. These results indicate that the excited-state dynamics are governed not only by proton transfer but also by strong nonadiabatic coupling associated with the conical intersection. This work reveals that the excited-state fate of ESIPT systems is strongly governed by conical intersection-mediated nonradiative decay following proton transfer.