Solvent effects on the photochromic reactions of diarylethene derivatives

Photochromic reactions of diarylethene derivatives, 1,2-bis(2,4,5-trimethylthiophene-3-yl)maleic anhydride (1a) and 1,2-bis(2-methylbenzothiophene-3-yl)maleic anhydride (2a) were found to depend on solvent polarity. The ring-closure quantum yield of Is decreased with the increasing solvent polarity from 0.13 in n-hexane to 0.003 in acetonitrile. The yield of 2a decreased from 0.12 in n-hexane to 0.03 in acetonitrile. Fluorescence measurement (fluorescence maxima, quantum yields, and decay times) revealed that there existed two kinds of conformations in the excited states, reactive planar and inactive twisted ones, and the state energies for the conformations were dependent on the solvent polarity. The inactive twisted conformation became more stable and dominant in polar solvents, and this explained the low quantum yields. The ring-opening quantum yields were scarcely affected by the solvent polarity.