Understanding the pressure-induced emission enhancement for triple fluorescent compound with excited-state intramolecular proton transfer

An organic compound, N-salicylidene-3-hydroxy-4-(benzo[d]thiazol-2-yl)phenylamine, with two intramolecular hydrogen bonding and triple fluorescence bands has been synthesized. With increasing pressure, enhanced emission of the triple fluorescence has been observed in succession. Within the pressure region up to 54.2 kbar, the luminescence of the SalHBP solution changed gradually from blue to cyan, until it became almost white. The three bands were assigned to the emission from the enol-enol, keto-enol, and enol-keto excited states of the molecules. The mechanism of the pressure-induced emission enhancement (PIEE) has been investigated with the aid of photophysical measurements and theoretical calculations. The results indicated that the PIEE phenomenon was significantly dependent on factors of hydrogen bond, viscosity, and molecular free volume.