NMR investigation of the dyes formed under UV irradiation of some photochromic indeno-fused naphthopyrans

New p-fluoro-substituted indeno-fused 3,3-diphenyl-3H-naphtho[2,1-b]pyrans were prepared and their photochromic properties studied by UV–vis and NMR spectroscopy. At room temperature UV irradiation of compound 6, presenting the indene ring fused to the k-face of the 3H-naphtho[2,1-b]pyran gave rise to two diastereoisomeric TT open forms that possess an helical structure with the conjugated 3,3-diaryl-1,3-propdienyl chain out of the plane of the indeno-naphthalen-2(1H)-one core, which renders the thermal bleaching very difficult and thus increases the life-time and concentration of these coloured species.