Nucleophilic addition of phosphorus(III) derivatives to squaraines: colorimetric detection of transition metal-mediated or thermal reversion

Nucleophilic addition of phosphorus(III) agents to the electrophilic core of intensely colored squaraine dyes gives a bleached zwitterionic adduct in good to excellent yields (up to 99%) at room temperature. The process can be reversed by adding specific transition metal complexes with high phosphorous(III) affinity.