The effect of a sulphur bridge on the photochromic properties of indeno-fused naphthopyrans

The synthesis of four new 2,2-diphenyl-2H-naphtho[1,2-b]pyrans with a fused indeno group at the f face and a sulphur junction between the 2,2-phenyl groups is described. The photochromic properties in solution of these novel compounds were investigated under continuous irradiation. Compared to known indeno-fused naphthopyrans, these new compounds showed a significant bathochromic shift in the spectra of the open forms, faster ring closure kinetics and an expected decrease in the colourabilities.