Photoswitchable "turn-on" Fluorescence Diarylethenes: Substituent Effects on Photochemical Properties and Electrochromism

A series of "turn-on" fluorescence diarylethenes derived from 2,3-bis(2-methylbenzo[b]thiophen-3-yl)-5,6-dihydro-4H-thieno[2,3-b]thiopyran-4-one (1) with alkyl and acetyl substituents were synthesized. The photochemical and photophysical properties of these derivatives, including the photoreaction of crystalline 1, were thoroughly investigated to reveal substituent effects on their properties. The results indicated that alkyl substituents did not significantly affect the absorption and emission spectra of the diarylethenes. However, large absorption and emission wavelength shifts were observed for the diarylethene with an acetyl substituent due to extension of π-π conjugation. Significantly, all of the fluorescent ring-closed forms of the compounds isomerized to their ring-open forms in the presence of Cu²⁺ in the dark. EPR results provide clear evidence for the formation of the compound 1 radical cation intermediate that might be generated in the reaction between c-1 and Cu²⁺. DFT calculations found that the ground-state activation energy for ring-opening of 1^.+ was approximately 9.2 kcal mol^-1 lower than that of 1 without Cu²⁺, such that a Cu²⁺-catalyzed oxidative cycloreversion reaction at room temperature might be possible.