Saitama University, Faculty of Engineering, Department of Applied Chemistry Shimo-Ohkubo, Urawa, 338 Japan
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 the 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 xylene compared with that of (2a) to give (1a) was 0.03. This result will be explained using the heats of formation or the molecular geometries of (1a) and (1b) in terms of dihedral angl es between planes of xanthene ( or thioxanthene ) rings as revealed by PM3 full geometry optimization method.
Keyword : Photochromism, Benzodixanthene, Geometry optimization, MOPAC$B!!(B
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