In Situ Preparation of Highly Fluorescent Dyes upon Photoirradiation

Photoswitchable or photoactivatable fluorescent dyes are potentially applicable to ultrahigh density optical memory media as well as super-resolution fluorescence imaging when the dyes are highly fluorescent and have large absorption coefficients. Here, we report on highly fluorescent photochromic dyes, which are initially nonluminous in solution under irradiation with visible light but activated to emit green or red fluorescence upon irradiation with ultraviolet (UV) light. The dyes 5a–9a are sulfone derivatives of 1,2-bis(2-ethyl-6-phenyl(or thienyl)-1-benzothiophen-3-yl)perfluorocyclopentene. It was found that substitution of phenyl or thiophene rings at 6 and 6′ positions of the benzothiophene-1,1-dioxide groups is effective to increase the fluorescence quantum yields of the closed-ring isomers over 0.7 and absorption coefficients over 4 × 104 M–1 cm–1. The phenyl-substituted derivatives 5a–7a undergo photocyclization reactions to produce yellow closed-ring isomers 5b–7b, which emit brilliant green fluorescence at around 550 nm (ΦF = 0.87–0.88) under irradiation with 488 nm light. Any absorption intensity change of the closed-ring isomers was not observed even after 100 h storage in the dark at 80 °C. The closed-ring isomers slowly returned to the initial open-ring isomers upon irradiation with visible (λ > 480 nm) light. The ring-opening quantum yields (ΦC→O) were measured to be (1.6–4.0) × 10–4. When the phenyl substituents are replaced with thiophene rings, such as compounds 8a and 9a, the absorption bands of the closed-ring isomers shift to longer than 500 nm. The closed-ring isomers exhibit brilliant red fluorescences at around 620 nm (ΦF = 0.61–0.78) under irradiation with 532 nm light. The ring-opening reactions are very slow (ΦC→O < 1 × 10–5). The fluorescence lifetimes of these sulfone derivatives were measured to be around 2–3 ns, which is much longer than the value of the closed-ring isomer of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene (τF = 4 and 22 ps). The closed-ring isomer 8b in 1,4-dioxane exhibits excellent fatigue resistant property under irradiation with visible light (λ > 440 nm) superior to the stability of Rhodamine 101 in ethanol.