The effect of ring aromaticity on ESIPT behavior and photophysical properties of 2-(2′-hydroxyphenyl)- 4-chloromethylthiazole derivatives: A TD-DFT study

Abstract

By using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, the effect of ring aromaticity on the excited state intramolecular proton transfer (ESIPT) process and photophysical properties of HCO, HCSe, HPDCT and HPZCT compounds were explored in detail. The strengthened excited state intramolecular hydrogen bonds (Hbonds) of the studied compounds were confirmed by analysis of the structure, infrared vibration frequency, electron density and reduced density gradient. The stronger Hbond is favorable to ESIPT process. Frontier molecular orbitals evidently showed that the proton transfer was induced by the rearrangement of electronic density. The HOMO-LUMO energy gap (Egap) correlated with the Stokes shift in the enol form. A smaller Egap would produce a larger Stokes shift. The heteroatom substitution in thiazole ring and nitrogen substitution in benzene ring of HCT molecule affected the ring aromaticity, and then had an obvious effect on the intramolecular Hbond. The stronger the ring aromaticity, the stronger the intramolecular Hbond, and the easier the ESIPT process to occur.