　　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 non-luminous 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*10^4 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 (phi-f = 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 hours storage in the dark at 80 degC. The closed-ring isomers slowly returned to the initial open-ring isomers upon irradiation with visible (lamda > 480 nm) light. The ring-opening quantum yields (phi-co) 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 (phi-f = 0.61-0.78) under irradiation with 532 nm light. The ring-opening reactions are very slow (phi-co < 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 (tau-f = 4 and 22 ps). The closed-ring isomer 8b in 1,4-dioxane exhibits excellent fatigue resistant property under irradiation with visible light (lamda > 440 nm) superior to the stability of Rhodamine 101 in ethanol.