Photochromic Properties of a New Spirobenzothiopyran Compound

Photochromic processes of 1′-hexyl-5′,8-dimethoxy-3′,3′-dimethyl-6-nitrospiro[2H-1-benzothiopyran-2,2′-indoline] were investigated with laser flash techniques. Photocoloration was found to proceed via the excited singlet state, and the quantum yield of coloration was an order of magnitude smaller than those of ordinary spirobenzopyrans. The main route of the decoloration was found to be the thermal reaction. Remarkably different solvent effects for the thermal fading rate of the colored form were observed, which indicates that the zwitterionic structure is more significant for the new spirobenzothiopyran than ordinary spirobenzopyrans. Repetitive cycles of photocoloration and thermal fading were achieved.