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, 1–5, having methyl, ethyl, n-propyl, i-propyl, and i-butyl substituents at the reactive carbons (2- and 2′-positions) of the thiophene 1,1-dioxide rings were studied. Diarylethenes 1–5 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 2b–5b with ethyl, n-propyl, i-propyl, and i-butyl substituents show higher fluorescence quantum yields than 1b with methyl substituents. In polar solvents, the fluorescence quantum yield of 1b markedly decreases, while 2b–5b maintain the relatively high fluorescence quantum yields. Although the cycloreversion quantum yields of the derivatives with methyl, ethyl, n-propyl, and i-propyl substituents are quite low and in the order of 10−5, introduction of i-butyl substituents was found to increase the yield up to the order of 10−3. 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.

To a dry THF solution (110 mL) containing 2e [S1] (3.0 g, 11 mmol) was slowly added 1.6 M n-BuLi hexane solution (7.6 mL, 12 mmol) at −78°C under a nitrogen atmosphere and the mixture was stirred for 15 min at that temperature. A dry THF solution (10 mL) containing octafluorocyclopentene (0.83 mL, 6.2 mmol) was slowly added at -78°C and the mixture was stirred overnight at that temperature.
The reaction was stopped by adding water. The resulting mixture was extracted with ethyl acetate and the organic layer was washed with brine, dried over MgSO 4 , filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane) to give 2f (1.1 g, 35%

4-Methy-2-propylthiophene (3c)
To a dry THF solution (180 mL) containing di-sec-butylamine (9.8 mL, 57 mmol) and N,N,N′,N′-tetramethylethylenediamine (8.5 mL, 57 mmol) was slowly added 1.6 M n-BuLi hexane solution (36 mL, 58 mmol) at −78°C under a nitrogen atmosphere and the mixture was stirred for 30 min at that temperature. After warmed up to 0°C, the mixture was stirred for 30 min. After cooled down to −78°C again, 3-methylthiophene (5.0 mL, 52 mmol) was slowly added and the mixture was stirred for 1 hour. 1-Iodopropane (5.6 mL, 57 mmol) was slowly added and the mixture was stirred for 30 min. After warmed up to room temperature, the mixture was stirred for 16 hours and then dilute HCl was added. The resulting mixture was extracted with diethyl ether and the organic layer was washed with brine, dried over MgSO 4 , filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane) to give 3c (
The reaction was stopped by adding water. The resulting mixture was extracted with ethyl acetate and the organic layer was washed with brine, dried over MgSO 4 , filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane) to give 3f (1.0 g, 49%

2-Isobutyl-4-methylthiophene (5c)
To a dry THF solution (180 mL) containing di-sec-butylamine (9.8 mL, 57 mmol) and N,N,N′,N′-tetramethylethylenediamine (8.5 mL, 57 mmol) was slowly added 1.6 M n-BuLi hexane solution (36 mL, 58 mmol) at -78°C under a nitrogen atmosphere and the mixture was stirred for 1 hour at that temperature. 3-Methylthiophene (5.0 mL, 52 mmol) was slowly added and the mixture was stirred for 1 hour. 1-Iodo-2-methylpropane (7.4 mL, 64 mmol) was slowly added and the mixture was stirred for 1 hour. After warmed up to room temperature, the mixture was stirred overnight and then dilute HCl was added. The resulting mixture was extracted with diethyl ether and the organic layer was washed with brine, dried over MgSO 4 , filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane) to give 5c (

3-Bromo-2-isobutyl-4-methyl-5-phenylthiophene (5e)
To a dry THF solution (45 mL) containing 5d (1.4 g, 4.5 mmol) and tributyl borate (1.2 mL, 4.5 mmol) was slowly added 1.6 M n-BuLi hexane solution (3.1 mL, 5.0 mmol) at -78°C under a nitrogen atmosphere and the mixture was stirred for 1 hour at that temperature. After warmed up to room temperature, dilute HCl was added. Pd(PPh 3 ) 4 (0.26 g, 0.22 mmol), iodobenzene (0.50 mL, 4.5 mmol), and saturated aqueous K 2 CO 3 (20 mL) was added and the mixture was refluxed for 7 hours. The resulting mixture was extracted with ethyl acetate and the organic layer was washed with brine, dried over MgSO 4 , filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane) to give 5e (0.73 g, 53% 5f To a dry THF solution (20 mL) containing 5e (0.60 g, 1.9 mmol) was slowly added 1.6 M n-BuLi hexane solution (1.3 mL, 2.1 mmol) at -78°C under a nitrogen atmosphere and the mixture was stirred for 15 min at that temperature. A dry THF solution (5 mL) containing octafluorocyclopentene (0.14 mL, 1.0 mmol) was slowly added at -78°C and the mixture was stirred for 1 hour at that temperature.
After warmed up to room temperature, the mixture was stirred for 5 hours. The reaction was stopped by adding water. The resulting mixture was extracted with diethyl ether and the organic layer was washed with brine, dried over MgSO 4 , filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane) to give 5f (0.20 g, 34%