Kinetics of electron-transfer quenching of excited aromatic molecules in solution

The work described in this thesis is concerned with laser kinetic and spectroscopic investigations of two distinct types of charge-separation process following electronic excitation, namely, (i) the kinetics of quenching by intermolecular electron-transfer and (ii) the development of (intramolecular) excited state dipoles as evinced by solvatochromic effects on both fluorescence and triplet-triplet absorption spectra. The kinetic studies refer to several systems, i.e. the fluorescence quenching of 9,10-dicyanoanthracene and 2-aminoanthracene in both polar and non-polar solvents and the triplet quenching of xanthone and thioxanthone, and also singlet thioxanthone, in MeCH/H20 mixtures. Correlation of the logarithm of the quenching rate constant (k23) with the free energy of the electron-transfer (ΔGo23) was attempted in terms of current models for electron transfer due to Marcus, Weller, Polanyi and others, sufficiently wider ranges in ΔGo23 being utilised to effect reasonable discrimination. In the case of 9,10-dicyanoanthracene, fluorescence quenching was effected both by donors (inorganic anions in Me0H/H20 and alkylmetals in cyclohexane) and organic acceptors in MeOH (Chapter 3), while quenching of 2-aminoanthracene was by organic acceptors in MeOH (Chapter 4). In general the Polanyi treatment gave better agreement, although this could not always be distinguished from the Rehm-Weller model. The quenching rates both of the singlet and triplet states of thioxanthone by inorganic anions were determined, enabling a comparison of their reactivity, as were the quenching rates of triplet xanthone, all in MeCN-H20 mixture (3:2 v/v), (Chapter 5). Triplet thioxanthone is quenched 102-fold more slowly by SCN- and Br- ions than singlet excited thioxanthone, which may be understood in terms of a higher value for ΔGo23 for the same acceptor in its singlet as cotrpared with its triplet state. The triplet quenching rates were correlated more satisfactorily with ΔGo23 by the Polanyi treatment. Solvatochromism was examined in two systems (Chapter 6) namely, the fluorescence of 2-aminoanthracene and N,N-dimethyl-2-aminoanthracene, comparison of which confirms (a) that the former is affected by solvent-donor hydrogen-bonding in hydroxylic solvents and (b) that Bilot-Kawski and related expressions provide a more specific analysis of solvatochromic affects than the Dimroth and related solvent parameters. The red-shift in the fluorescence maxima of these fluorophores with increasing solvent polarity was interpreted on the basis of increases in the dipole moment of the state over that of the Sostate. While the fluorescence maxima of xanthone and thioxanthone show substantial red-shifts with increasing polarity, as measured by the polarisation function of Bilot and Kawski, indicating an increase in dipole moment on excitation, the triplet-triplet absorption transient spectra of xanthone, thioxanthone and li-methylacridone are strongly blue-shifted, indicating a decrease in dipole moment as estimated by the Suppan expression