Abstract
In this work, we report the one-pot synthesis and characterization of four water-soluble 2-hydroxybenzo[e][1,2]oxaphosphinine 2-oxides. The compounds were obtained by cascade reactions of (2-ethoxyvinyl)phosphonic dichloride with phenol or naphthol derivatives, and their acid–base, structural, and photophysical properties were investigated using a combination of experimental and computational methods. These compounds exhibit UV–vis absorption maxima at 209–341 nm and fluorescence maxima at 300–394 nm. Notably, these cyclic phosphonic acids exhibit unusually strong acidity with pKa values from −1.3 to 0, comparable to mineral acids; complete protonation is not achieved even in concentrated HCl. The acidity trends and spectra were further analyzed by DFT using both explicit and implicit solvation models.