Quinoline-substituted excited-state intramolecular proton transfer fluorophores as stimuli-sensitive dual-state fluorophores

Abstract

This article describes the synthesis, along with comprehensive photophysical and ab initio characterization, of a series of 2-(2′-hydroxyphenyl)benzoxazole (HBO) fluorophores, a family of compounds prone to undergoing an excited-state intramolecular proton transfer (ESIPT) process, functionalized with different positional isomers of quinoline or isoquinoline. We notably show that the position of the nitrogen atom at the azaheterocycle site has a key influence on both the emission profile and the photoluminescence quantum yield in solution. We also demonstrate the proton-sensitive nature of these dyes in solution, where not only does protonation trigger fluorescence enhancement, but it also acts as a transition switch between two excited states, with different emission profiles. HBO-isoquinoline displays very intense fluorescence not only in neutral and protonated dichloromethane solutions in the green-yellow region, but also in the solid state. Moreover, this dye exhibited a record Stokes shift of 11 000 cm⁻¹.