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 red–orange 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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Three [(naphthopyran-3-yl)phenyl]methanols were synthesised by a metalation protocol. ► The naphthopyran unit in the methanols displays photochromism upon UV irradiation. ► Treatment of the methanols with strong acid affords intensely coloured cations. ► UV irradiation of the cations reversibly generates cationic merocyanine dyes with λmax at ca. 640 nm.