Thermally Irreversible Photochromic Systems. Reversible Photocyclization of Non-Symmetric Diarylethene Derivatives

Non-symmetric diarylethenes with an indole ring on one end and a thiophene, a benzo[b]thiophene, or a pyrrole ring on the other end of the double bond were synthesized in an attempt to get thermally irreversible photochromic compounds having absorption bands at longer wavelengths. 2-(1,2-Dimethyl-3-indolyl)-3-(2,4,5-trimethyl-3-thienyl)maleic anhydride(8a) underwent photoinduced cyclization/ring-opening reactions with relatively high quantum yields (cyclization quantum yield: 0.15; ring-opening quantum yield: 0.40), and the both isomers were thermally stable. The closed-ring form had the absorption maximum at 595 nm with the edge extending to 760 nm. Cyclization/ring-opening reactions of 8a and 2-(1,2-dimethyl-3-indolyl)-3-(2-methyl-3-benzo[b]thienyl)maleic anhydride (9a) were induced by Ar ion (488 nm) and He–Ne (633 nm) lasers.