| Summary: | This thesis reports the results of a study of some donor-acceptor complexes in solution by a variety of physical techniques. The introduction outlines the early work in this field, followed by a discussion of the generally accepted theoretical treatment of these systems, including the constructive critisms of this treatment. The methods available to study molecular complexes are then reviewed. There has been ample thermodynamic evidence reported for complex formation between hydrocarbons as electron donors and hexafluorobenzene as an electron acceptor. These interactions have been attributed to the formation of charge-transfer complexes, although no observation of a characteristic charge-transfer band in the u.v.-visible spectrum has been made. An investigation into the interaction in solution between hexafluorobenzene and aromatic amines revealed new absorption bands which could be ascribed to a charge-transfer transition. The nature of these interactions is discussed, and it is demonstrated that a molecular complex may exist without charge-transfer forces contributing significantly to its stability. The large solvent sensitivity of the pyridinium iodide charge-transfer transition has lead to the energy of this transition being used as an empirical solvent polarity scale. Similar ionic-ionic and ionic-neutral complexes were examined, and it is shown that the solvent sensitivity of molecular complexes is a function of the individual moieties, as distinct from solvation of the unit complex as was previously supposed. It is also shown that these solvent polarity scales are not always directly applicable to solvation studies of similar systems in other regions of the electromagnetic spectrum. The spectra of Nurster's cations show a marked temperature dependence, the observed changes being explicable in terms of a dimerisation. A solvent study of these ions reveals that the proposed dimerisation takes the form of a disproportionation. The interaction of organic cations with amines is reported. Complex formation is generally only observed transiently, being followed by a chemical reaction. The role of a charge-transfer complex as a precursor in the chemical reaction is discussed. The possibility of a charge-transfer mechanism being operative in the fluorescence quenching of organic acceptors was investigated. The mechanism is found to consist of charge-transfer complex formation in the ground state (static quenching), further complex formation in the excited state and collisional deactivation.
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