Electronic effect on the photophysical properties of 2-(2-hydroxyphenyl)benzothiazole-based excited state intramolecular proton transfer fluorophores synthesized by sonogashira-coupling reaction

Abstract

For excited-state intramolecular proton transfer (ESIPT) fluorophores, the intramolecular hydrogen bond strength has decisive influence on their photophysical properties. Herein, a series of 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives as new ESIPT fluorophores are synthesized through Sonogashira-coupling reaction, and their electronic structures are systematically tuned through various substitutions with different Hammett substituent constants (σ) in the 5-position of the phenol ring. Theoretical calculations and crystal analysis show that the HBT derivative with larger σ contains the stronger intramolecular hydrogen bond, which is less likely to be damaged by external factors, and that it has larger energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. The experimentally observed trends are well in line with the theoretical calculations. More importantly, unlike other HBT derivatives and parent HBT displaying anion emission in PBS medium, the two HBT derivatives bearing methyl ester and cyano substitutions, respectively, emit as keto tautomer, which are expected to be applied in biological aspects.