Synthesis Of New Photochromic Diarylethenes Bearing Both Five-membered And Six-membered Moieties And Investigation Of Their Optoelectronic Properties

Organic photochromic materials have received considerable attention because of their potential applications in optical memories, optical switches, and full-displays. Among many known photochromic systems, diarylethenes are the most promising candidates for optoelectronic devices because of their remarkable thermal stability of isomers and excellent fatigue resistance. Currently, design and synthesis of novel diarylethenes as well as investigations of their properties is a hotspot in the researching field.In this dissertation, we have synthesized three kinds of new diarylethenes with different aryl moieties and investigated systematically their optoelectronic properties, especially in exploring the effects of substitution. The main research results are shown as follows:1. Twenty-one new unsymmetrical diarylethenes were synthesized and their structures were identified by NMR, IR and elemental analysis. Ten diarylethene crystals were prepared, and their structures were determined by single-crystal X-ray diffraction analysis.2. These diarylethene derivatives, with the exception of DT-15~19, had good photochromism both in hexane and in PMMA film. In the crystalline phase, diarylethenes bearing a benzene ring, such as compounds DT-6, DT-15~17, showed no photochromism although they packed with an anti-parallel conformation and the distances between the two reactive carbon atoms were less than 4.2 ?. On the contrary, the diarylethenes with a pyridine moiety showed good photochromism in their respective crystalline phase, which were, to the best of our knowledge, the first examples of photochromic crystals bearing six-membered aryl moieties. The effects of substitution on the photochromic properties of the diarylethenes were also investigated. Electron-donating substituents can shift the absorption maxima of the closed-ring isomers to a longer wavelength and decrease their cyclization quantum yields. However, the electron-withdrawing substituents can significantly increase cyclization and cycloreversion quantum yields. For the diarylethenes with a pyridine moiety, either electron-donating or withdrawing group can increase the molar absorption coefficients of the both isomers.3. All diarylethenes were thermal stability in hexane both at room temperature and at 80 oC. The fatigue resistance of diarylethenes bearing a pridine unit was much stronger than that bearing a biphenyl moiety, especially in PMMA amorphous film. After 200 repeated cycles, compounds DT-8~10 still showed good photochromism with the absorbance maintained above 80% basically.4. The fluorescence spectra of diarylehtenes both in hexane and in PMMA amorphous film have been investigated. The results illustrated that all the compounds exhibited fluorescence and functioned as a fluorescent photoswitch both in solution and in PMMA film. Furthermore, the concentration effect on the fluorescent property was also remarkable. In the diarylethene system bearing a biphenyl moiety, the electron-donating substituent could be effective to increase the efficiency of the fluorescence and decrease the emission peak. When going from electron-donating to electron-withdrawing substituents, the fluorescence quantum yields decreased from 0.0221 to 0.0201. However, when the biphenyl unit was replaced with pyridine moiety, the electron-withdrawing cyano group improved the fluorescence quantum yields from 0.0083 to 0.015 (compared with DT-9), and shifted the emission peak to a shorter wavelength.5. Cyclic voltammetry tests indicated that the different substituents had a significant effect on the electrochemical behaviour of these diarylethenes. Under the same experimental condition, the oxidation potential onsets of the open-ring isomers were clearly higher than those of the closed-ring isomers for most diarylethenes. This is because the longer conjugation length of the ring-closed isomers generally leads to a less positive potential.