Facile preparative redox chemistry of bis(4-dialkylaminophenyl)squaraine dyes

Bis(4-dialkylaminophenyl)squaraine dyes are readily reduced in solution by borohydride to give alkali soluble leuco compounds, which exist in the 3-hydroxy-2,4-(4-dialkylaminophenyl)cyclobutenone tautomeric form. The leuco dyes can be oxidised rapidly back to the squaraine dye, thus providing a means of temporarily solubilising the squaraine dye in water. The leuco compounds undergo substitution reactions typical of vinylogous carboxylic acids.