Photobehaviour of diarylethenes with thiophenes as aryl groups and dithiole-2-thione and dithiole-2-one at the ethenic bond

Four diarylethenes, which were expected to be thermally irreversible photochromic compounds, containing thiophene groups as aryls and dithiole-2-thione or dithiole-2-one at the ethenic bond, were investigated by steady state and time-resolved spectroscopy to obtain information about their photochemistry and the competitive relaxation paths of their electronically excited states. Only for the 4,5-bis(2-methyl-1-benzothien-3-yl)-1,3-dithiole-2-thione (BTDT), a good photochromic response was observed. The photocolouration reaction was undetectable for the 4,5-bis(2,5-dimethylthien-3-yl)-1,3-dithiole-2-thione (TDT), whereas for the two analogous compounds with oxygen (-ones) the occurrence of side reactions reduced their photochromic properties. For the photoreactive compounds, the quantum yields of the UV-photoinduced ring-closing reaction and the visible-stimulated cycloreversion reaction were determined. Nanosecond time-resolved experiments showed the formation of transients which were assigned to n, π* triplets for thiones and π, π* triplets for ones, respectively. 1H NMR measurements allowed the equilibrium ratio between parallel and antiparallel conformations for the benzo-annellated compounds to be determined.