Revisiting the excited state proton transfer dynamics in N-oxide-based fluorophores: a keto–enol/enolate interplay to detect trace water in organic solvents

Abstract

This work highlights the unique proton transfer ability observed in cyanoquinoxaline N-oxide-based fluorophores. A fluorophore HCQ was synthesized for this purpose, and a detailed study of its photophysical characteristics was undertaken. Preliminary structural characterization by NMR and single crystal XRD techniques indicated the possibility of a ground-state proton transfer (GSPT) reaction in the molecule. The absorbance and fluorescence spectroscopic studies further confirmed its sensitivity to the solvent environment and the possibility of the existence of three species enol (E)/enolate (RO⁻)/keto (K), with the predominance for a particular form based on the polarity of the solvent. Measurement of the fluorescence lifetime of the molecule allowed us to establish the role of protic solvents in the deprotonation of HCQ. In addition, the involvement of water in crystal packing and the significantly reduced lifetime of the molecule in water indicated the involvement of GSIPT and ESIPT processes. Based on the unique response of the molecule in the aqueous medium, its application for water detection in organic solvents was explored. HCQ demonstrated a water-induced fluorescence switch from its enol (E) form to the deprotonated (RO⁻)/keto (K) form, showcasing distinct spectral responses across various solvents with LOQ values in the range of 0.09–0.9%. The results were finally validated through Karl Fischer titration, showing similar outcomes.