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One approach towards the collection and storage of solar energy as chemical potential involves the photosensitised dissociation of water into hydrogen and oxygen which is the subject of this thesis. Many of the photochemical systems developed so far for the oxidation and/or reduction of water are outlined in Chapter 1, with particular emphasis on the more recently developed photosystems which employ a dye as sensitiser. Most of the experimental techniques used are described in the second chapter. Chapter 3 focuses upon a single dye, Rose Bengal, and its role in a sacrificial system containing EDTA, methyl viologen and a H<SUB>2</SUB>-catalyst for the photoproduction of hydrogen from water. The performance of other Xanthene dyes in an identical photosystem is described in Chapter 4. The design of a novel light-harvesting system coupling Rose Bengal as sensitiser to a 'reaction centre' consisting of anthracene-9-carboxylate, EDTA and methyl viologen for the photoreduction of water is described in the following chapter. Chapter 6 outlines the development of a flow-system containing an oxygen Membrane Polarographic Detector (O<SUB>2</SUB>-MPD) for studying oxygen evolving reactions. In Chapter 7, the flow-system developed in the previous chapter is subsequently used to study the catalytic properties of the various forms of ruthenium dioxide hydrate towards the oxidation of water, leading to the development of a stable reproducible catalyst. The significance of these results to solar energy conversion is discussed.

Chemiluminescence is the emission of safe "cold" light as a result of a chemical reaction. This phenomenon has found diverse applications in environmental and analytical chemistry, and more importantly in diagnostic medicine. Acridinium esters constitute an important class of man-made chemiluminescent materials whose potential still remains to be exploited. Interestingly, while these compounds have been the object of extensive structural modifications, the carbonyl group attached to the crucial position 9 of the acridine ring has never been replaced. The research presented herein is concerned with computational study and design, as well as attempted synthesis, of new acridinium derivatives devoid of a carbonyl group attached to position 9. Part I reviews the most important features of chemiluminescence and its applications in diagnostic medicine (Chapter 1). Part II presents the computational investigation of new acridinium derivatives bearing sulfur-containing groups attached to position 9. While Chapter 2 covers the relevant background aspects of computational chemistry. Chapter 3 presents a study using the two recent semi-empirical molecular orbital methods, AM1 and PM3. This study showed that acridinium-9-thionocarboxylate esters and acridinium-9-sulfonate esters were potentially chemiluminescent and phenyl 10-methylacridinium-9-thionocarboxylate trifluoromethanesulfonate was selected as a synthetic target. Part III details the various approaches to the synthesis of the target thionoester. Chapter 4 gives a general introduction to thionoesters. Chapter 5 describes attempts at direct thionation of phenyl acridine-9-carboxylate and other relevant carboxylic ester derivatives. Chapter 6 presents synthesis of 9-cyanoacridine and attempts at the Pinner reaction followed by sulfhydrolysis as an alternative synthetic route. In Chapter 7m yet another approach is considered, based on the direct esterification of acridine-9-thiocarboxylic acid. In Chapter 8, the reaction of 9-lithioacridine with phenyl chlorothionoformate is investigated.

This thesis consists of five parts. In chapter <B>1</B>, the background to chemiluminescence, chemiluminescent compounds and their applications is given. The synthesis and chemiluminescent behaviour of substituted phenyl acridinium esters (compounds <B>1</B> and <B>2</B>), heterocyclyl acridinium esters and their analogues (compounds <B>3-7</B>) and phenyl 2-halo-6-methoxy acridinium esters (compounds <B>8-11</B>) are detailed in chapters <B>2, 3 </B>and <B>4</B> respectively. The chemiluminescence studies include chemiluminescent kinetics, chemiluminescent efficiencies, chemiluminescent wavelengths and hydrolytic stabilities of the acridinium esters. Modifications on leaving groups show obvious effects on chemiluminescent kinetics and stabilities, but no wavelength changes were observed with such modifications. However, modifications on the acridine ring do not significantly affect chemiluminescent kinetics and stabilities. Chemiluminescent efficiency was decreased by the introduction of halogen atoms into the acridine ring. Surprisingly, chemiluminescent wavelength was found to be influenced by the introduction of halo and methoxy groups to positions 2 and 6, respectively of the acridine ring. Among the newly prepared acridinium esters, compounds <B>2 </B>and <B>3</B> possess better stability and good quantum efficiency, and could be developed as better chemiluminescent labels in chemiluminescent immunoassays over the model compound. Apart from the acridinium esters, three cysteine derivatives (compounds <B>12-14</B>) were also prepared for chemiluminescent immunoassay. The enantiomeric purity of the product was identified by NMR. The details are presented in chapter 5.

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Two novel quinolinyl/pyridinyl bis-urea ligands were coordinated to Au(I) and Re(I). Emission was ligand-centred (Au(I)) or <super>3</super>MLCT (Re(I)).&nbsp; Two <italic>fac</italic>{[Re(CO)₃(di-imine)]₂L}<super> 2+</super> complexes demonstrated luminescent responses to anions in two organic solvents.&nbsp; Emission quenching and modulated <super>3</super>MLCT lifetimes, characteristic of each anion, were observed.

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