Construction And Photo Regulation Of Azobenzene Containing Supramolecular Assemblies

In this thesis work we have designed and synthesized a series of azobenzene containing molecules to construct either organogels or metallo-supramolecular polymers. Photoresponsive behaviors of these supramolecular assemblies have been carefully studied through a variety of methods.In the first part of the work we have synthesized an organogelator bearing a cholesterol and a10,12-pentacosadiynoic acid group tethered at each side of the azobenzene through amide linkers. It can gelate organo solvents like ethyl acetate and dioxane successfully. Introduction of azo species leads to a photo induced gel-sol transition. Through UV-vis、CD spectra, TEM, SEM, XRD analysis we have carefully studied the structural change in the isomerization process. The gel shows helical fibers and spheres twisted from fibers in TEM and SEM. Wide angle and small angel X-ray diffraction reveals ordered columnar structures and CD spectra shows supramolecular chirality. Blue shifts in UV-vis spectra further confirmed the H aggregates. However, after photo induced gel-sol transition, the system shows a totally different structure. The distorted molecules lower the order to make it CD silent and the periodicities in XRD increase. Interestingly, the UV-vis and CD spectra taken during the transition process draw a map of the gel-sol dynamics. Upon irradiation with365nm light, the CD signal disappears but a new CD band increases and reaches to a maximum, it gradually becomes CD silent with further irradiation. Moreover, the diacetylene parts can bring another kind of photo response, that is the topological polymerization. The gel here can be successfully polymerized to stabilize the assembly structures.The other part is azo containing metallo-supramolecular polymers. We have designed a linear ditopic ligand that features an azobenzene group and two2,6-bis triazopyridine moieties. Ru2+, Cd2+, Pd2+was selected to bind to the ligand to construct supramolecular polymers. Ru2+, Cd2+are six coordinated and Pd2+is square-planar four coordinated and the first metal ion forms dynamic inert complexes while the other two are labile. All the molecular structures have been confirmed by H-NMR and ESI-MS. Due to the inertness of the Ru2+complexes, its yields strong polymers with the ditopic ligand. Yet the labile Cd2+forms supramolecular polymers successfully. UV-vis titration studies show that the supramolecular polymers reach a maximum degree in the molar ratio of1:1. Isomerization of these azo systems has also been studied by UV-vis spectra. Comparing to pure ligand, the Ru2+-polymers have a much lower ratio of isomerization due to the inert and strong coordination bonds. The labile Cd2+supramolecular polymers have a much higher ratio than Ru2+-polymers. The Pd2+complex hardly shows any response to365nm light but has some little extent of isomerization under435nm irradiation, which may be caused by red shifts of the π-π*band and overlaps of other bands. Further work is ongoing.