A theoretical investigation about the excited state behavior for 2-(6'-hydroxy-2'-pyridyl)benzimidazole: The water-assisted excited state proton transfer process

In this work, based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, we theoretically investigate the excited-state process of the 2-(6'-hydroxy-2'-pyridyl)benzimidazole (2HPB) system in acetonitrile and water solvents. Since acetonitrile is an aprotic solvent, it has no effect on the solvent-assisted excited-state proton transfer (ESPT) process. Therefore, the 2HPB molecule cannot transfer the proton in acetonitrile, which is consistent with previous experimental observation. On the other hand, 2HPB can combine one water molecule (which is a protic solvent), forming the 2HPB-H2O complex in the S-0 state. After photoexcitation, the intermolecular hydrogen bonds O1H2O3 and O3H4N5 both get strengthened in the S-1 state, which leads to the possibility of a water-assisted ESPT process. Further, the charge redistribution reveals the tendency of ESPT. By exploring the potential energy curves for the 2HPB-H2O complex in water, we confirm that a stepwise double proton transfer process occurs in the S-1 state. Water-assisted ESIPT can occur along O1H2O3 or O3H4N5 because of their similar potential barriers. Based on the stepwise ESPT mechanism, we reinterpret the absorption and fluorescence spectra mentioned in the experiments and confirm the rationality of the water-assisted ESPT process.