Bis(3‐thienyl)perfluorocyclopentenes containing partially fluorinated thiophene rings were synthesized and their behaviors examined. Dithienylethenes containing chlorinated thiophene rings were also prepared as reference compounds. The absorption maxima of the closed‐ring isomers, their thermal stabilities, and their quantum yields varied depending on the substitution position. The absorption maxima showed a red shift upon fluorine substitution at the 2‐ or 4‐positions of the thiophene ring, whereas a blue shift was observed upon substitution at the 5‐position. This shift was explained by the shape of the HOMO and LUMO and was well reproduced by theoretical calculations using time‐dependent density‐functional theory (TD‐DFT). The cycloreversion quantum yield of the closed‐ring isomer of 2‐fluorine‐substituted 1,2‐bis(2‐fluoro‐3‐thienyl)perfluorocyclopentene was about 130 times smaller than that of the closed‐ring isomer of 5‐fluorine‐substituted 1,2‐bis(5‐fluoro‐2‐methyl‐3‐thienyl)perfluorocyclopentene. This low quantum yield was attributed to the change of the π‐conjugation mode in the closed‐ring isomer. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
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Higashiguchi et al. (2004) studied this question.