Cationic merocyanine dyes by photomodulation of [4-(naphthopyran-3-yl)phenyl]methines

A series of mono-, bis- and tris-4-(naphthopyran-3-yl)phenyl substituted methanols were efficiently synthesised using a 4-(naphthopyran-3-yl)phenyllithium intermediate. The resulting substituted methanols displayed good photochromism leading to the reversible generation of redeorange photomerocyanines. Acidic dehydration of the methanols gave intensely coloured cations which, upon
subsequent UV irradiation, reversibly generated cationic photomerocyanines with long wavelength
absorption maxima at ca. 640 nm.