Novel Hybrid Nanomaterials Based on Dendritic Azobenzen

This thesis describes the synthesis, characterization and applications of new hybrid nanomaterials based on dendritic azobenzene. Dendritic azobenzenes were prepared via the copper-catalyzed Huisgen 1,3-dipolar cycloaddition between azides and alkynes, also known as "click" reaction. The new dendritic azobenzene compounds were characterized by nuclear magnetic resonance (NMR) spectroscopy, matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS) and electrospray ionization mass spectrometry (ESI-MS). The dendritic azobenzene thus obtained were used to prepare novel hybrid nanomaterials.;Part I) A series of melanin derivatives has been prepared by functionalizing melanin with dendritic azobenzene compounds in basic condition with potassium carbonate (K2CO3) and without potassium ion (such as triethylamine Et3N). The dendritic melanins were characterized by time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy and UV-Visible absorption spectroscopy. Interestingly, the dendritic melanins treated with K2CO3 show excellent solubility in water and tetrahydrofuran (THF). This may due to the multivalent chelating properties between the heteroatoms of melanin and potassium ions. These highly soluble dendritic melanins were coupled with titanium dioxide (TiO2) nanotube arrays facilely for photocurrent generation with significant signal enhancement. This result suggests that the dendritically functionalized melanins can be potentially coupled with different nanomaterials or biomaterials for further applications. In addition, a thin film of dendritically functionalized melanin was fabricated by spin casting on a SiO2/Si surface, to give a thickness of ∼300 nm and reasonable conductivity. Unfortunately, the thin films do not show any field-effect transistor (FET) signal.;Part II) Dendritic azobenzene encapsulated gold nanoparticles (Au NPs) were prepared. The size and morphology of NPs were determined by transmission electron microscopy (TEM). The size of as-prepared G2- N-hydroxylsuccimide (NHS) dendron encapsulated Au NPs is around 3 to 4 nm, while the size of as-prepared G3-NHS dendron encapsulated Au NPs is around 2 to 3 nm. However, the dendritic Au NPs are not stable when they are stored in particular solvents, such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). This may due to weak interactions between the substituted 1,2,3-triazoles and Au atoms, leading to a change of morphology. The Au NPs that were prepared by the G3-azobenzene dendron with acetylene core, show excellent stability in DMSO. The size of the resulting dendritic Au NPs is around 1.5 +/- 0.3 nm. The di-acetylene functionality of the Au NPs was characterized by the formation of Au trimers after a palladium-catalyzed oxidative homocoupling reaction. TEM images show that there were significant amounts of Au NP dimer and trimers. Furthermore, the weight percentages of Au and dendritic azobenzene ligands were measured by thermogravimetric analysis (TGA), and agreed well with the results obtained from TEM images. In addition, the effect of photoisomerization and the core group of dendritic azobenzene is investigated to understand more about the interaction of the dendritic encapsulated Au NPs.