The invalidity of intermolecular proton transfer triggered twisted intramolecular charge transfer in excited state for 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine

Abstract

AbstractThe excited‐state dynamics of the excited‐state proton transfer and intramolecular twisted charge transfer (TICT) reactions of a molecular photoswitch 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine (DHP) in aprotic and alcoholic solvents have been theoretically investigated by using time‐dependent density functional theory. The excited‐state intramolecular proton transfer (ESIPT) reaction of DHP proceeding upon excitation in all the solvents has been confirmed, and the dual emission has been assigned to the enol and keto forms of DHP. However, for methanol and ethanol solvents within strong hydrogen‐bonded capacity, the intermolecular hydrogen bonds between DHP and methanol/ethanol would promote an excited‐state double proton transfer (ESDPT) along the hydrogen‐bonded bridge. Importantly, the previous proposed ESDPT‐triggered TICT mechanism of DHP in methanol and ethanol was not supported by our calculations. The twist motion would increase the total energy of the system for both the products of ESIPT and ESDPT. According to the calculations of the transition states, the ESDPT reaction occurs much easier in keto form generated by ESIPT. Therefore, a sequential ESIPT and ESDPT mechanism of DHP in methanol and ethanol has been reasonably proposed.