Force-induced cleavage of C–O bond in photochromic naphthopyrans

Force-induced cleavage of C–O bond in photochromic naphthopyrans in the pure microcrystalline solid state is described. The ring-closed isomers (colorless form) of naphthopyrans undergo cleavage of C–O bond under stress and are converted to ring-open isomers (colored form) in solid state. It is found that the threshold stress required to produce cleavage of C–O bond in naphthopyran derivatives in pure microcrystalline solid state is more than 2 MPa. Besides, the cleavage of C–O bond in photochromic naphthopyrans in the pure microcrystalline solid state can also be achieved by grinding.