Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 324-353).

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

The central focus of this dissertation pertains to the synthesis and study of novel N-Heterocyclic carbene architectures. This pursuit has led to advances in carbine structure and bonding, and application of NHCs in materials chemistry in the role of monomer and catalyst / text

published_or_final_version / Chemistry / Master / Master of Science

Discovery of cisplatin as an effective anticancer agent has stimulated the development of metal based medicine. The recent advances in research on platinum, ruthenium and gold complexes have received much attention in medicinal chemistry, and studies of other less explored metal complexes may reveal alternative mode of mechanism as novel therapeutic agents. A series of dirhodium(II,II) complexes with carboxylate and carboxamidate ligands and thiourea complexes of coinage metals have been prepared in this study. Their biological activities and mechanisms of action have been studied. Dirhodium(II,II) carboxylate complexes with variations of alkyl and benzoyl side chains were synthesized and displayed remarkable cytotoxicities to cancer cells with potency down to submicromolar level. The cytotoxicities of rhodium complexes were found to significantly correlate with the cellular uptake of the rhodium complexes. As revealed by oligonucleotide microarray and bioinformatic analysis, the mode of action of the rhodium carboxylate complexes are highly similar to that of a proteasome inhibitor. Further cellular and biochemical studies showed that rhodium carboxylate complexes induced an accumulation of ubiquitinated proteins, inhibited the proteolytic activities of purified 20S proteasome and proteasomal deubiquitinating enzyme. These results corroborate that the impairment of the ubiquitin-proteasome system is linked to the cytotoxic action of rhodium carboxylate complexes. Silver is known to be an anti-inflammatory agent for topical treatment. A silver complex of N, N’-disubstituted cyclic thiourea that is reasonably stable towards reduced glutathionewas found to potently inhibit the NF-B transcriptional activity. Treatment of cells with silver thiourea inhibited TNF-α-stimulated IκB kinase activity, IκBα phosphorylation and degradation, nuclear translocation of NF-κB p65 and eventually the stimulated gene expression of inflammatory cytokines. Suppression of IκB kinase activity was associated with modification of sensitive cysteine residues and disruption of IκB kinase assembly. These data demonstrated that the inhibitory properties of Ag+ ions on an anti-inflammatory and anti-cancer drug target could be effectively delivered via the thiourea ligand. Silver is also an antimicrobial metal, and this study was also extended to understand the silver-bacteria interaction using a silver resistant bacteria as a model. Many silver resistant bacteria often produce considerable amount of silver particles when exposed to high concentrations of silver salts but the mechanism of biosynthesis is not well understood. A silver resistant E. coli that displays active silver efflux was shown to synthesize zero-valent silver nanoparticles in the periplasmic space through reduction of silver ions under anaerobic conditions. As the microbial c-type cytochromes are known to mediate respiratory metal reduction, their role in the biosynthesis of silver nanoparticles was examined. A deletion mutant of the cytoplasmic membrane-anchored tetra-heme c-type cytochrome subunit of periplasmic nitrate reductase (NapC) showed marked reduction of accumulation of silver nanoparticles. This study identified a molecular mechanism of biosynthesis of silver nanoparticles that may have implication in bioenvironmental processes and synthetic biology of metal nanomaterials. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

A series of photochromic dithienylethene-containing triarylboranes has been synthesized and characterized. The X-ray crystal structure of (4,5-bis(2,5-dimethylthiophen-3-yl)-thiophen-2-yl)dimesitylborane has been determined. An intense electronic absorption band was observed in all the triarylborane-containing compounds, corresponding to the intramolecular charge-transfer transition from the π-conjugated thiophene / fused thiophene / bithiophene to the vacant pπorbital of boron. This class of compounds was also found to bind fluoride ion such that the trigonal planar geometry of the boron center would be changed to tetrahedral, leading to a significant perturbation in the photophysical properties. Upon photo-excitation into the transition band, (4,5-bis(2,5-dimethylthiophen-3-yl)-thiophen-2-yl)dimesityl- borane and (4,5-bis-(2,5-dimethylthiophen-3-yl)-thieno[3,2-b]thiophen-2-yl)- dimesitylborane displayed multi-addressable photochromic reactivities upon addition of fluoride, in which a green solution was found in the closed form of the unbound compounds, while they changed to a purple color upon fluoride binding. Interestingly, both (4',5'-bis(2,5-dimethyl-thiophen-3-yl)-2,2'- bithiophen-5-yl)dimesitylborane and (4',5'-bis(2,5-dimethyl-thiophen-3-yl)-2,2'- bithiophen-3-yl)dimesitylborane displayed gated photochromic reactivities upon addition of fluoride, while only typical photochromic reactivity was observed in (4',5'-bis(2,5-dimethyl-thiophen-3-yl)-2,2'-bithiophen-4-yl)- dimesitylborane, indicative of a strong influence of the substituent effect on the photochromic reactivity upon incorporation of the -BMes2 group. A series of photochromic dithienylethene-containing β-diketones and the corresponding boron(III) compounds, appended with -BF2, -B(C6F5)2 and -BPh2, have been designed and synthesized. Upon photo-excitation into the transition band, typical photochromism was observed in all the dithienylethene- containing β-diketone ligands with the color changed from pale yellow to green in degassed benzene solution. Upon incorporation of the -BF2 and -B(C6F5)2 groups, near-infrared responsive photochromic behavior was observed, in which the absorption of the closed form was found to be at ca. 758-863 nm, suggesting that a pronounced perturbation of the dithienylethene was achieved by coordination of the boron(III) center. Moreover, a significant reduction in luminescence intensity of over 90 % was observed upon photocyclization, indicative of their potential application as photoswitchable luminescence materials. The thermal stability of the closed form and the activation energy for the thermal cycloreversion have also been studied on the representative compounds, 1-(4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2- yl)-butane-1,3-dione and difluoroboryl(1-(4,5-bis(2,5-dimethylthiophen-3-yl)- thiophen-2-yl)butane-1,3-dionate). A series of photochromic dithienylethene-containing BODIPY has been successfully prepared and characterized. The electronic absorption properties have been studied. These compounds showed an intense visible absorption band, corresponding to the 0-0 band of a strong S0→S1 transition of the BODIPY core, with a weak and broad absorption band at higher energy, corresponding to the S0→ S2 transition of the BODIPY core. An absorption band in the UV region was also observed in 2,3-bis(2,5-dimethyl- thiophen-3-yl)thiophene-containing derivatives, corresponding to the π→π* transitions of the dithienylethene moiety. The photochromic reactivities have also been studied such that the photostationary state was found to be reached upon irradiation in the UV region for over eight hours, while no observable changes would be obtained upon photo-excitation in the visible region. The slow conversion rate has been suggested to be attributed to the fact that a rapid excitation energy transfer could occur from the excited Th-DTE to the BODIPY moiety, rendering the suppression of the photocyclization. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

A series of alkynylplatinum(II) polypyridine complexes with 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine and 4,4′-dicarboxy-2,2′-bipyridine as TiO2 anchoring functionalities, has been successfully synthesized. Their photophysical, electrochemical and luminescence properties have been extensively studied. The excited state properties were probed using nanosecond transient absorption spectroscopy. [Pt(tctpy)(C≡C-Th-BTD-Th)][NnBu4]2 displayed a long-lived transient signal which was tentatively assigned to result from the formation of a charge-separated state, which could be alternatively described as a [Pt(tctpy)???(C≡C-Th-BTD-Th)+?] state, with the charge recombination rate constant determined to be 2.9 × 105 s?1. The excited state redox potentials for the oxidation process were determined and the data confirmed the ability of the complexes to inject an electron into the conduction band of TiO2. The majority of the complexes were found to sensitize the nanocrystalline TiO2 and exhibit photovoltaic properties, which have been characterized by current-voltage measurements under illumination of air mass (AM) 1.5G sunlight (100 mW cm–2). A new class of molecular dyads comprising metalloporphyrin-linked alkynylplatinum(II) polypyridine complexes was synthesized and characterized. Their photophysical, electrochemical and luminescence properties have been studied in detail. The excited state properties were probed using nanosecond transient absorption spectroscopy which indicated the formation of a charge-separated state involving the porphyrin radical anion, [Por??－(C≡C)Pt+?]. The excited state redox potentials for the oxidation process were also determined with the data supporting the capability of the complexes to inject an electron into the conduction band of TiO2. The majority of the complexes were found to sensitize the nanocrystalline TiO2 and exhibit photovoltaic properties, as characterized by current-voltage measurements under illumination of air mass (AM) 1.5G sunlight (100 mW cm–2). A series of organic materials consisting of a triphenylamine-based donor with oligothiophene or oligothienylenevinylene based-conjugated linker and dicyanovinyl, tricyanovinyl or cyanacrylic acid groups as acceptor, was synthesized and characterized. Their photophysical, electrochemical, thermal and luminescence properties were studied. Transient absorption spectra of TPA-OTV-DCN in dichloromethane solution on the pico- to nanosecond timescale were recorded after femtosecond laser excitation at 400 nm. A transient signal at ca. 700 nm was tentatively assigned to result from the formation of a charge-separated [(TPA-OTV)+??DCN??] state with the charge recombination rate constant determined to be 5.3 × 109 s?1. The energy levels of the LUMOs of TPA-OTV1-DCN, TPA-OTV2-DCN, TPA-OTV3-DCN TPA-TAZ1-DCN, TPA-TAZ2-DCN and TPA-o-4Th-DCN were calculated to be of ca. ?3.9 eV, establishing the formation of a downhill driving force for the energetically favorable electron transfer process involving the injection of an electron into the LUMO of the C60 acceptor. The majority of the compounds were found to exhibit photovoltaic properties. The photovoltaic responses were characterized by current-voltage measurements under illumination of air mass (AM) 1.5G sunlight (100 mW cm–2). / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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The first example of an N,N'-chelated [beta]-diketiminato phosphenium salt has been isolated by careful tuning of both the ligand architecture and the electronic characteristics of the phosphorus containing moiety. Using a two-electron reduction approach, the first example of a phosphinidene valence isomer has been isolated and structurally characterized. The mechanism of formation of this valence isomer was elucidated via Density Functional Theory (DFT) calculations. These calculations revealed that the formation of this valence isomer involves the intermediacy of a triplet diradical. In turn, the triplet diradical decays by two further steps to generate the observed product. A cationic phosphinous acid has been prepared via the hydrolysis of a cationic phosphenium complex in basic solution. DFT calculations reveals that the introduction of a positive charge stabilizes the phosphinous acid form. Two N,C-bonded phosphenium cation salts have also been obtained, the crystal structures of which reveal that the formation of these compounds involves C-H activation of a methyl group on the [beta]-diketiminate ligand. Three synthetic methods, namely halide abstraction, aluminum/boron halide exchange process and salt metathesis, have been explored for the preparation of boron cations supported by [beta]-diketiminate ligands. The first structurally characterized boron cation supported by a [beta]-diketiminate ligand has been isolated as its [Al₂Cl₇]⁻ salt. A Ga-Fe organometallic complex supported by a [beta]-diketiminate ligand has also been synthesized via salt metathesis reaction and fully characterized. Finally, several bisamidinate supported boron complexes have been synthesized and structurally characterized. Such complexes may have applications as bifunctional catalysts or as building blocks for novel polymers.