Synthesis And Properties Of Fluorescent Molecular Switches Based On Photochromic Diarylethene And Rhodamine

Diarylethene derivatives undergo a molecular transformation from open-ring isomer to close-ring isomer upon irradiation with UV light. The photoinduced interconversion of one state of a photochromic compound into the other is generally accompanied by color, absorption and fluorescence change. The inverse process from closed-ring isomer to open-ring isomer can be accomplished by irradiation of visible light of appropriate wavelength, and the photochromic compound recovers to the initial state. The photoinduced fluorescence modulation of photochromic diarylethene is one of the most attractive properties for photo-switching devices, optical storage, and anti-counterfeiting medium, because of its excellent properties such as high sensitivity, excellent fatigue resistance. So far, construction of reversible fluorescent switches based on photoisomerization of diarylethenes to modulate fluorescence dye was become a new research hotspot.In this thesis, four new molecular fluorescent switches based on photochromic diarylethenes bearing a rhodamine unit have been synthesized, and their structures are charactered on the NMR, IR, and elemental analysis. Furthermore, five novel diarylethenes were designed and synthesized to blend with rhodamine B to construct different dopant of fluorescent switches.Five novel diarylethenes bearing different heterocycle have been synthesized to investigate the in effects are on reversible fluorescent switches properties. The main results are described briefly as follows.Photochromic properties of diarylethenes DR-1-4and DT-5-14in different medium were investigated. Firstly, upon irradiation with297nm light, DR-1-4in methanol solution showed new visible absorption bands at500-600nm; compounds DT-5-9in solution and film exhibited new visible absorption bands at500-650nm, and DT-10～14also displayed various the absorption maxima in visible area. These diarylethenes could return to the initial states upon irradiation with visible light.Four diarylethenes DR-1-4bearing rhodamine unit have designed and synthesized and Cr(Ⅲ), Al(Ⅲ), and Fe(Ⅲ) ions can be selectively recognized by changes in fluorescence and color. After addition of Cr(III), the compounds in methanol solution changed from colorless to pink accompanied with a remarkable fluorescence enhancement, and the fluorescence emission intensity can be easily quenched by photoisomerization upon irradiation with the UV light. Furthermore, the color and fluorescence of the compounds could return its original state upon irradiation with visible light. In the presence of Cr(Ⅲ), FRET process from the rhodamine moiety (FRET donor) to the closed-ring diarylethene moiety (FRET acceptor) occurs upon UV light stimuli, and emission intensity can be recovered upon irradiation with visible light. An efficient FRET relies on the overlap between the emission band of rhodamine and the absorption band of photochromic component with the interconversion of the latter. DR-1-4bearing different heterocyclic aryl rings lead to fluorescence quenching are variable level. The results indicated that the absorption bands of diarylethene in the closed-ring isomer bearing high molar absorption coefficient in visible range well match the emission band of rhodamine, which is favorable for fluorescence modulation between diarylethenes and dyes. Furthermore, the fluorescence switching on-off of photochromic DR-1-4proceeded very well in methanol solution after10cycles, which resulted from the degradation of photoisomerization.Photochromic compounds DT-5～9and rhodamine B dye are respectively dispersed at molecular level in the polymer film or solution to investigated the fluorescence intensity of dopants. The results showed that the emission intensity of rhodamine B can be reversibly controlled by photoisomerization of DT-5～9, which could result from overlap of the absorption bands (500-650nm) of photochromic DT-5～9in the closed-rings and the emission band of rhodamine B (540-630nm). Therefore, an efficient excitation energy transfer occurred between rhodamine B and the closed form of DT-5～9. The equiv ration and concentration of diarylethenes blend with rhodamine B effect on the fluorescence quenching in dichloromethane solution and in a PMMA film was investigated. Results indicated that the optimal ration for the fluorescence modulation of diarylethene blend with rhodamine B was2.5:1both in dichloromethane solution and in PMMA film, and diarylethenes DT-5-9=(1.0×10-5M) blend with rhodamine B in solution showed the best fluorescent switch behavior. Dopants DR-5～9displayed excellent fluorescence fatigue resistance properties in solution and film after20cycles. In addition, heterocyclic aryl rings (benzofuran, benzothiophene, isoxazole, pyrrole, and thiophene) have obviously effects on the properties of diarylethenes DT-10～14, such as absorption maxima, molar absorption coefficient, the fluorescence spectra. Firstly, upon irradiation with297nm light, DT-10in hexane solution and PMMA film showed the visible absorption peak at the largest wavelength and the highest molar absorption coefficient, but no fluorescence quenching. The fluorescence intensity of DT-10～13exhibited relatively clear fluorescent switches both in hexane solution and PMMA film upon altering irradiation with UV and visible light, and displayed excellent fluorescence fatigue resistance after20cycles.