Abstract
Benzo [1, 2, 3-kl : 4, 5, 6-k'l']dixanthene (1a) and its endoperoxide (2a) are well known as thermally stable photochromic compounds. Authors have already reported the synthesis and photochromic properties of sulfur analogue of (1a), benzo [1, 2, 3-kl : 4, 5, 6-k'l'] bisthioxanthene (1b), and its endoperoxide (2b). Relative rate for thermolysis of (2b) to give (1b) in refluxing xylcne compared with that of (2a) to give (1a) was 0.03. This result will be explained on the heats of formation or the molecular geometries of (1a) and (1b) in terms of dihedral angles between planes of xanthene (or thioxanthene) rings as revealed by PM3 full geometry optimization method.S
