published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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A variety of functionalised 1,3-dithiole derived donor-π-acceptor systems linked by an alkenic bridge have been synthesised, the acceptor units being dicyanomethylene and cyanoimine. They exhibit intramolecular electron transfer from the electron donor moiety to the electron acceptor moiety. The presence of electron donating substituents on the 1,3-dithiole ring increases the degree of overall charge transfer; this process has been studied by UV-VIS spectroscopy and X-ray crystallography. The latter technique indicates a significant amount of intramolecular electron transfer in the ground state. A selection of functionalised donor-π-acceptor systems linked by an anthracene spacer unit have been prepared which possess greater donor-Ti-acceptor characteristics; however, the degree of electron transfer measured by X-ray crystallography of one derivative appears to be less. The 1,3-diselenole donor unit has also been utilised as an electron donor unit in donor-π-acceptor systems however, for all derivatives the diselenole unit is inferior to the 1,3- dithiole unit. The linking of a 1,3-dithiole donor unit to two acceptor units produces charge transfer bands that are red shifted compared to the parent donor-π-acceptor systems. Exchange of one acceptor unit for a donor unit leads to a similar red shift. Solution electrochemical studies, non-linear optical properties, ultra-fast spectroscopy and solvatochromic analysis have been performed on a range of systems.

The stratosphere of Saturn contains a photochemical haze that appears thicker at the poles and may originate from chemistry driven by the aurora. Models suggest that the formation of hydrocarbon haze is initiated at high altitudes by the production of benzene, which is followed by the formation of heavier ring polycyclic aromatic hydrocarbons. Until now there have been no observations of hydrocarbons or photochemical haze in the production region to constrain these models. We report the first vertical profiles of benzene and constraints on haze opacity in the upper atmosphere of Saturn retrieved from Cassini Ultraviolet Imaging Spectrograph stellar occultations. We detect benzene at several different latitudes and find that the observed abundances of benzene can be produced by solar-driven ion chemistry that is enhanced at high latitudes in the northern hemisphere during spring. We also detect evidence for condensation and haze at high southern latitudes in the polar night.

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This dissertation will focus on NMR studies and photochemical experiments performed in a supramolecular host - guest assembly in aqueous media. Cavity of the synthetic resorcinarene based host molecule used in this study acts as a hydrophobic pocket for guests like dibenzyl ketones, N-alkyl-2-pyridone and tropolone ethers. Two molecules of the host self-assemble to form a capsular complex in the presence of a guest. It was possible to identify the binding modes of the guest molecules inside the cavity of the host using 1D and 2D NMR spectroscopy techniques like NOESY, TOCSY and DOSY. Photochemical reactivity of the bound guest molecules were found to be different from their reaction in isotropic solutions illustrating the ability of the host to confine the guests, the intermediate radicals and the photoproducts. Additionally, unlike many other supramolecular hosts, generally less significant substituent effects were found to play a major role in the binding motif and consequently, reactivity of the guest substrates. Dibenzyl ketones were used to probe the stability of the complex. By introducing different alkyl substituents in the para position and in the alpha-position, binding modes of the guests were altered. Their photochemical behavior correlated well with the structure established by NMR studies. Dynamic nature of the intermediate radicals bound to the guest was confirmed by formation of rearrangement products. Diastereoselective cyclization reactions of N-alkyl-2-pyridones, tropolone ethers and cyclohexadienone carboxamide appended to a chiral auxiliary were also studied. Observed selectivity in the reaction of methyl-substituted pyridones depended on position of the methyl substituent on the pyridyl ring.