Theoretical study on the ESIPT of fluorescent probe molecules N-(2-(4-(dimethylamino)phenyl)-3-hydroxy-4-oxo-4h -chromen-6-yl) butyramide in different solvents

Abstract

The solvatochromic effect and excited-state intramolecular proton transfer (ESIPT) of N-(2-(4-(dimethylamino)phenyl)-3-hydroxy-4-oxo-4H-chromen-6-yl)butyramide (NDHB) molecules in different kinds of solvents were studied using density functional theory and time-dependent density functional theory. Hydrogen bonding is the driving force in proton transfer (PT). Functional analysis of reduced density gradient confirms that the intramolecular hydrogen bond is strongly reinforced and thus has facilitated the PT in the first excited state (S1). In addition, the dielectric constant of the solvent can influence this process. These results are also confirmed by the comparison of relevant structural parameters and analysis of infrared vibration spectrum. Frontier molecular orbitals are analyzed in different kinds of solvents, illuminating the solvatochromic effect on spectral properties. Quantitative analyses of NBO and Mulliken's charge reveal that charge redistribution upon photoexcitation enhances the hydrogen bond. Potential energy curves are also scanned to clarify the effect of solvents on ESIPT. Compared with the energy barriers of PT in different kinds of solvents, ESIPT becomes highly favorable when the solvent dielectric constant decreases.