N‐Aryl or N‐Alkyl Pyridinium‐Substituted Excited‐State Intramolecular Proton Transfer Fluorophores

In this article, it describes the synthesis of a series of fluorophores consisting of N-alkyl or N-aryl pyridinium groups connected at different positions of a 2-(2'-hydroxyphenyl)benzoxazole scaffold and the exploration of the photophysical properties in solution (dichloromethane) and in the solid state, as amorphous powders. All dyes display a bathochromically shifted fluorescent transition from an excited keto state, formed after excited-state intramolecular proton transfer process. A full chemical engineering study was performed by changing the nature of the substitution at the pyridinium site (alkyl or aryl), the position of the pyridinium substitution and the nature of the counterion (six examples). The nature of the radiative transitions observed in these fluorescent dyes was confirmed by Time-dependent density functional theory (TD-DFT) calculations.