Photoswitchable Fluorescent Diarylethene Derivatives with Thiophene 1,1-Dioxide Groups: Effect of Alkyl Substituents at the Reactive Carbons.

Photoswitching and fluorescent properties of sulfone derivatives of 1,2-bis(2-alkyl-4-methyl-5-phenyl-3-thienyl)perfluorocyclopentene, -, having methyl, ethyl, -propyl, -propyl, and -butyl substituents at the reactive carbons (2- and 2'-positions) of the thiophene 1,1-dioxide rings were studied. Diarylethenes - underwent isomerization reactions between open-ring and closed-ring forms upon alternate irradiation with ultraviolet (UV) and visible light and showed fluorescence in the closed-ring forms. The alkyl substitution at the reactive carbons affects the fluorescent property of the closed-ring isomers. The closed-ring isomers - with ethyl, -propyl, -propyl, and -butyl substituents show higher fluorescence quantum yields than with methyl substituents. In polar solvents, the fluorescence quantum yield of markedly decreases, while - maintain the relatively high fluorescence quantum yields. Although the cycloreversion quantum yields of the derivatives with methyl, ethyl, -propyl, and -propyl substituents are quite low and in the order of 10, introduction of -butyl substituents was found to increase the yield up to the order of 10. These results indicate that appropriate alkyl substitution at the reactive carbons is indispensable for properly controlling the photoswitching and fluorescent properties of the photoswitchable fluorescent diarylethenes, which are potentially applicable to super-resolution fluorescence microscopies.