Merocyanine Dyes and Photochromic Naphthopyrans

Diarylnaphthopyrans 1 and 2 have been widely used as the photochromic component in commercial ophthalmic sun lenses. Through the careful choice of dye structure and polymer matrix the color and persistence of the photomerocyanine dyes 3 and 4 can be controlled (Scheme 1).
The use of photochromic dyes to modulate various phenomena, including fluorescence, has developed into an attractive research area. We have previously reported on the influence of the UV-promoted electrocyclic pyran ring-opening and – ring-closing on the color of traditional cationic dye-naphthopyran conjugates 5 and 6 and also on the fluorescence of BODIPY-naphthopyran conjugates 7 (Scheme 2). Of significant importance to such ‘switching’ studies is a detailed understanding of the influence of the geminal (hetero)aryl substituents upon the photophysical properties of the naphthopyrans and their photomerocyanines. We now describe our recent observations on the position of substituents on the (hetero)aryl substituents and their impact on the structure, persistence, absorbance and emission properties of the resulting photomerocyanines.