Effectively controlling the ESIPT behavior and fluorescence feature of 2-(2′-hydroxyphenyl)-4-chloromethylthiazole by changing its π-conjugation: A theoretical exploration

Abstract

Based on density functional theory (DFT) and time-dependent DFT (TD-DFT), the effects of π-conjugation on the excited-state intramolecular proton transfer (ESIPT) process and photophysical properties of 2-(2′-hydroxyphenyl)-4-chloromethylthiazole (HCT) have been studied. The strong electron acceptor with π-conjugated structure (MN1, MN2) have been chosen to analyze the changes of intramolecular H-bond, the frontier molecular orbital, the absorption/fluorescence spectrum and the energy barrier of ESIPT process. Our results show that the strong electron-withdrawing groups (MN1, MN2) weaken the intramolecular H-bond in the S1 state. The substitution effect on the H-bond strength in the S0 state is opposite. For MN1-HCT and MN2-HCT, both the forward and backward ESIPT reactions can occur. The normal and tautomer form can exist simultaneously in the S1 state. The absorption and fluorescent peaks of MN1-HCT and MN2-HCT all red-shift more than 150 nm due to the extended π-conjugated chain.