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Double charge transfer absorption in solution. Voigt, Eva-Maria

Abstract

The theory of charge-transfer spectra has been described briefly with respect to the energy, symmetry and intensity of charge-transfer transitions. The concepts derived were then applied to the special case of double charge-transfer absorption in donor/acceptor complexes between substituted benzenes and tetracyanoethylene. A first-order perturbation method treating specifically the perturbation of the substituted donor cation was discussed in detail since it is of particular importance to the interpretation of multiple charge-transfer spectra as investigated here. A detailed experimental study of the various aspects of double charge-transfer absorption has been presented in the second chapter. The equilibrium constants and extinction coefficients for both charge-transfer transitions of a number of increasingly methoxy-substituted benzene/TCNE complexes have been reported. TCNE was found to form stable complexes with these donors. For some complexes the variation of the relative molar extinction coefficients with temperature of the two c.-t. transitions of a complex was measured. A small but definite difference in heat of formation was obtained for transitions I and II. This was taken as evidence for the existence of two configurational isomers in the complexes measured. Extensive data on the effect of polar and nonpolar solvents on the positions, relative intensities, band widths and band separations of the two transitions, respectively, for the p-dimethoxy/TCNE and o-dimethoxy/TCNE complexes were reported. In nonpolar solvents the effect of solvent on the charge-transfer energy could be described as a function of the refractive index of the solvent. In polar solvents, red as well as blue shifts of the charge-transfer bands relative to their position in cyclohexane were observed. The TCNE/ substituted benzene complexes showed high solvent specificity in many polar solvents. The data obtained are summarized in terms of potential energy curves of the complex in dependence on refractive index and polarity of solvent. Such representation could account for most of the effects observed. The dependence of charge-transfer energy on ionization potential for the TCNE/substituted benzene complexes was found to be decidedly anomalous when compared to the behavior of other π-acceptors. The problem was investigated in detail. It could be shown that the abnormal behavior of the TCNE complexes is related to the structure of the acceptor. In the last section the observed double charge-transfer spectra of substituted benzene/TCNE complexes have been compared with theory. However, the perturbation parameters as derived from experiment were found to vary, contrary to theory which defines them as constants. The discrepancy could be resolved when the coulomb interactions of the charge-transfer processes were taken into account.

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