Photochromism and fluorescence properties of 1,2-bis(2-alkyl-1-benzothiophene-3-yl)perhydrocyclopentenes

Three 1,2-bis(2-alkyl-1-benzothiophene-3-yl)perhydrocyclopentenes with different alkyl chain lengths (methyl, ethyl, and propyl) at the reactive carbons were synthesized, and their photophysical properties were compared with those of 1,2-bis(2-alkyl-1-benzothiophene-3-yl)perfluorocyclopentenes. The structures of the perhydrocyclopentenes were analyzed by X-ray crystallography, and their cyclization/cycloreversion reaction and fluorescence quantum yields, and fluorescence lifetimes were measured by steady-state spectroscopy and the time-correlated single photon counting method, respectively. From these results, we found the following three differences between perhydrocyclopentenes and perfluorocyclopentenes. (1) For the closed-ring isomers, the absorption bands of the perhydrocyclopentenes (around 450 nm) shifted to a shorter wavelength than those of the perfluorocyclopentenes (around 530 nm) because of the acceptor nature of the perfluorocyclopentene moiety. (2) The cyclization/cycloreversion quantum yields of the perhydrocyclopentenes were slightly larger than those of the perfluorocyclopentenes in hexane. (3) Interestingly, the fluorescence of both the open- and closed-ring isomers was observed only for the perhydrocyclopentenes, and the lifetimes of the closed-ring isomers were estimated to be <30 ps. The fluorescence of the closed-ring isomers of the perhydrocyclopentenes was mainly due to the suppression of nonradiative deactivation from the excited state directly to the ground state compared with the perfluorocyclopentenes.

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