Theoretical study on ESIPT mechanism and negative solvatochromism effect of 3-(4,5-Diphenyl-1H-imidazol-2-yl)-9-phenyl-9H-carbazol-4-ol compound in different solvents

It is necessary to study the excited-state intramolecular proton transfer of complex molecular systems in solvents, which play important roles in photochemistry, photobiology, and optoelectronic materials. In this work, we have worked on the effect of solvent polarity on the ESIPT behaviour of 3-(4,5-Diphenyl-1H-imidazol-2-yl)-9-phenyl-9,9a-dihydro-4aH-carbazol-4-ol (CHPI). Specifically, an in-depth study of hydrogen bonding behavior and charge complexation in different polar solvents has confirmed the excited state hydrogen bonding enhancement mechanism, which is the primary prerequisite to facilitate the ESIPT process. It is noteworthy that the intra-molecular hydrogen bonding of CHPI compound is enhanced more significantly in nonpolar solvents. To further elaborate the ESIPT process, we analyzed the potential energy curves of the S0 and S1 states, and the comparison shows that the potential barrier decreases as the solvent polarity decreases. Therefore, the nonpolar solvent environment is more favorable for CHPI molecule to undergo the ESIPT process.