Two Stage Colour Modulation of Triarylmethine Dyes Derived from a Photochromic Naphthopyran

The synthesis of the unsymmetrical triarylmethine dyes 4derived from a lithiated naphthopyran and diaryl ketones is reported. In toluene solution 4 display a typical reversible photochromic response becoming coloured upon irradiation with UV light and fading upon cessation of irradiation. At low pH an intensely coloured dye cation is generated and UV irradiation of this solution results in a further colour change as a consequence of photochromic cycling of the naphthopyran unit