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. (C) 2012 Elsevier B.V. All rights reserved.