Thermally reversible photochromic behaviour of new naphthopyrans involving an intramolecular [2+2] cyclization reaction

New 4-(2,2-diphenylethenyl)naphthopyrans were synthesized and their photochromic behaviour in solution were studied under continuous UV light irradiation conditions. Although only one coloured photoproduct was expected to be formed, due to the naphthopyran substitution pattern, NMR analysis on degassed UV irradiated solutions, performed at low temperature, showed the formation of different compounds. Among them, the main product is formed through an intramolecular [2+2] cyclization reaction and show thermally reversible photochromic properties.