Naphthopyran photochromic dyes

R. Pardo, M. Zayat and D. Levy, The influence of sol-gel processing parameters on the photochromic spectral and dynamic behaviour of a naphthopyran dye in an ormosil coating, J. Mater. Chem., 2005, 15, 703. 

Naphthopyran dyes show an interesting photochromic effect. Exposure to UV light converts these colorless compounds reversibly to colored compounds, as shown below. These compounds can be used in self darkening eyeglasses. Identify the pericyclic reaction and describe the allowed stereochemistry [57]. 

Gabbutt, C.D., Gelbrich, T., Hepworth, J.D., Heron, B.M., Hursthouse, M.B., and Partington, S.M. (2002) Synthesis and photochromic properties of some fluorine-containing naphthopyrans. Dyes Pigm., 54, 79-93. 

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