Photochromism of thiazole-containing triangle terarylenes

Triangle terarylene derivatives based on the 4,5-diarylthiazole structure have been synthesized and their photochromic properties have been studied both in solution and in the crystalline state. The thiazolyl-substituted terarylenes displayed reversible photochromism in solution, with photocyclization quantum yields as high as 60%, while a 4,5-dithienylthiazole derivative underwent photochromic reaction even in the single-crystal state, although a 4,5-dithiazolylthiazole (terthiazole) showed no photochromic reaction in the crystalline phase. The difference in photochromic reactivity between the 4,5-dithienylthiazole and the 4,5-dithiazolylthiazole was found by X-ray crystallographic analyses to originate from conformational differences in the crystalline state. The thermal cycloreversion activation energies in the solution phase were measured for thiazolyl-substituted terarylenes and for a corresponding terthiophene derivative. The activation energy for the transition from photogenerated closed-ring isomer to open-ring isomer increased with the number of substitutions of thiazole unit for thiophene. The lower aromatic stabilization energy of thiazole in relation to that of thiophene was considered to be responsible for the thermal stability of the closed-ring isomers of thiazolyl-substituted terarylenes.