A turn-on mode fluorescent diarylethene: Solvatochromism of fluorescence

Environment-sensitive fluorophores have been used as molecular probes to acquire physical and chemical information in microstructures. Here, we report on solvatochromism of a fluorescent diarylethene, which is switched by photos radiation. A turn-on mode photoswitchable fluorescent diarylethene derivative, 1,2-bis(2-ethyl-6-(5-methylthiophen-2-yl)-1-benzothiophen-1,1-dioxide-3-yl)perfluorocyclopentene (1a), underwent a cyclization reaction upon irradiation with ultraviolet (UV) light to form fluorescent closed-ring isomer 1b. 1b dramatically changed the fluorescent color in response to solvent polarity, such as green in n-hexane, yellow in tetrachloromethane (CCl4), orange in dichloromethane (CH2Cl2), and red in dimethyl sulfoxide (DMSO), and showed high fluorescence quantum yields of 0.6-0.8 in these solvents. The fluorescent solvatochromism is ascribed to the intramolecular charge-transfer (ICT) character of 1b, which consists of electron-donating thiophene rings at both ends and central electron-accepting benzothiophene 1,1-dioxide groups. The analysis based on the Mataga-Lippert plot revealed that the difference in the dipole moments between the excited and ground states of 1b is as large as 12 D. Such a solvatochromic fluorophore with photoswitching ability can find potential applications in super-resolution fluorescence imaging of microscopic polarity in biological cells and materials.