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New p -fluoro-substituted indeno-fused 3,3-diphenyl-3 H -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 3 H -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(1 H )-one core, which renders the thermal bleaching very difficult and thus increases the life-time and concentration of these coloured species. |
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