Photochemical and thermal behaviour of new photochromic indeno-fused naphthopyrans

Three new indeno-fused naphthopyrans were prepared and their photochromic behaviour was investigated. UV-vis continuous irradiation of indeno-fused 3H-naphtho[2,1-b]pyrans generates two photoisomers in variable relative amounts with very different thermal stabilities. While one isomer exhibits a fast thermal bleaching the other is thermally very stable and returns to the uncoloured form mainly through a photochemical process promoted by visible light irradiation.