Syntheses and photochromic properties of diaryl acenaphthylene derivatives

A series of diaryl acenaphthylenes has been synthesized, and their photochromic properties are studied both in solution and in the crystalline state. 2,4-Dimethyl-5-phenylthiophene derivative 1a showed no photochromic reaction, while 2-methyl-5-phenylthiophene derivative 2a and 5-methyl-2-phenylthiazol derivative 3a showed reversible photochromism in solution. Fluorescence spectrum of compound 3a changed upon photoirradiation. In the single-crystal state, compound 3a showed no photochromic reaction possibly because of π–π and CH/S inter-molecular interactions. The photocyclization quantum yield of 3a was determined to be as low as 0.7%. The closed-isomer 3b showed relatively high thermal stability by the rigidity of acenaphtylene.