Quinoline-Substituted Excited-State Intramolecular Proton Transfer Fluorophores as Stimuli-Sensitive Dual-State Fluorophores

Abstract

This article describes the synthesis along with full photophysical and ab initio properties of a series of 2-(2’-hydroxyphenyl)benzoxazole (HBO) fluorophores, a family of compounds prone to undergo 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 photoluminescent quantum yield in solution. We also demonstrate the proton-sensitive nature of these dyes in solution, where protonation does not only trigger fluorescence enhancement, but acts as a transition switch between two excited states, with different emission profiles. HBO-isoquinoline displays very intense fluorescence intensity in neutral and protonated dichloromethane solutions in the green-yellow region, but also in the solid-state. Moreover, a record Stokes shift of 11,000 cm-1 was recorded for this dye.