Photochromic systems. Part 2. Studies on the photoreactions and the photopolychromic properties of (E)-(2,5-dimethoxyphenyl-substituted)methylene isopropylidenesuccinic anhydrides

Abstract

Each of the pale yellow title compounds (1), in toluene and other appropriate organic solvents, cyclises on irradiation at 366 nm to give purple solutions containing a mixture of a thermally unstable, blue 4-substituted 1,8a-dihydro-5,8-dimethoxy-1,1-dimethylnaphthalene-2,3-dicarboxylic anhydride (5) and a thermally stable, red 4-substituted 1,8a-dihydro-6,8a-dimethoxy-1,1-dimethylnaphthalene-2,3-dicarboxylic anhydride (7)(1,8a-DHNs). The red and the blue 1,8a-DHNs (7) and (5) undergo the reverse reactions on exposure to white light. The purple solutions turn red at ambient temperature in the dark or on irradiation at wavelengths at which the red 1,8a-DHNs do not absorb, due to thermal or photochemical reversal of the blue 1,8a-DHNs. When a trace amount of an acid is added to the purple solutions, they turn blue due to the rapid acid-catalysed reversal of the red-1,8a-DHNs. On prolonged irradiation at 366 nm, the red 1,8a-DHNs (7) undergo a 1,7-methoxy shift to yield 4,6-dimethoxy-1,4-DHNs (8), and the blue 1,8a-DHNs (5) undergo an oxidative rearrangement to 4-hydroxy-5,8-dimethoxy-1,4-DHNs (9). Photochemical studies on related compounds are described.