Influence of ethynyl extension on the dual-state emission properties of pyridinium-substituted ESIPT fluorophores

Abstract

We describe the synthesis and photophysical properties in both solution and in the solid-state of dual-state emitters undergoing an Excited-State Intramolecular Proton Transfer (ESIPT) process. These fluorophores, built around the 2-(2′-hydroxyphenyl)benzoxazole (HBO) scaffold incorporate a N-methylpyridinium unit, at the 3 or 5 position of the phenol ring, prone to reduce the vibrational relaxation in the excited-state thanks to resonance effects and consequently allowing increasing the solution-state quantum yield (QY). In order to further optimize their optical properties, an additional ethynyl-extended triisopropylsilyl (TIPS) spacer was introduced with the aim to combine molecular rigidification and π-delocalization, as cooperative effects to enhance dual-state emission (DSE) properties. Molecular engineering studies and comparison with unsubstituted compounds reveal the significant impact of the position of the ethynyl-TIPS moiety on the photophysical properties, especially the fluorescence color. As several phenomena can compete with the ESIPT process, all dyes were modelled using ab initio calculations to rationalize the nature of the excited-states.