Solvent effect on excited state intramolecular proton transfer and charge transfer in 3-((2E, 4E)-5-(4-(dimethylamino) phenyl) penta-2, 4-dienoyl)-4-hydroxy-2H-chromen-2-one

Abstract

Recently, a new fluorescent probe 3-((2E, 4E)-5-(4-dimethylamino)phenyl) penta-2, 4-dienoyl)-4-hydroxy-2H-chromen-2-one (DPPHC) was designed and synthesized experimentally (Bhattacharyya et al., 2021), which is based on the mechanism of intramolecular charge transfer (ICT) inhibition of excited state intramolecular proton transfer (ESIPT). However, this experimental work failed to qualitatively unveil the solvent effect on photophysical and photochemical properties. Accordingly, we applied density functional theory and time-dependent density functional theory to systematically study the mechanism of solvent effect of DPPHC molecule. The calculated absorption spectra and fluorescence emission spectra are in good agreement with experimental results. It is found that DPPHC molecule takes on significant ICT characteristics instead of ESIPT reactions during photoexcitation. By analyzing the geometric parameters, the infrared vibrational spectra, the Hirshfeld charges and the potential energy curves, it is indicated that the hydrogen bond strength of the DPPHC molecule in S1 state was weakened in the same solvent, which was not conducive to ESIPT. The energy barrier in the first excited state (S1) is much larger than that in the ground state (S0), which directly leads to the failure of ESIPT reaction. With the increase of solvent polarity (from heptane, methylcyclohexane, chloroform, dichloromethane to acetonitrile solvent), the strength of hydrogen bond in both S0 and S1 states gradually decreases. The energy barriers are increasing in both S0 and S1 states. In addition, the electron density of the chalcone part of the DPPHC molecule decreases and that of the dimethylamino part increases gradually. Obviously, the electron redistribution leads to an increase in the electron density of O1 and O2 atoms, which causes a significant decrease in hydrogen bond strength and greatly reduces the likelihood of ESIPT reactions occurring. The detailed investigation on the microscopic mechanism of ICT inhibiting the reaction of ESIPT will provide valuable reference for the subsequent application of the DPPHC molecule as fluorescent probe to detect chloroform.