Radical cations of a series of a-methyl-substituted bi-, ter-and quaterthiophenes were investigated by EPR and, in part, ENDOR spectroscopy. In the case of terthiophenes, the electronic and steric effects of methyl substituents were studied. The experimentally determined spin density distributions were rationalized by means of semiempirical quantum-mechanical calculations (MNDO, PM3, RHF-INDO/SP). Sterically noo-hindered oligothiop hene radical cations are planar and form mixtures of cis and trans conformers, whereas uniform species were observed in the case of sterically hindered, non-planar radical cations.

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CW ESR studies of the photoexcited triplet states of a series of porphyrins and covalently linked porphyrinquinones have been performed in isotropic and anisotropic (liquid-crystalline) frozen solutions. In frozen nematic solutions, the appearance of the spectra is strongly dependent on the orientation of the sample relative to the external magnetic field. This fact allows a differentiation between the z and x,y axes and hence a more accurate determination of the zero-field splitting parameters D and E. Spectrum simulation and fitting aided in the interpretation of the experimental results and the extraction of the relevant parameters. In the case of some porphyrin-quinones, steady-state triplet ESR signals cannot be observed, which is ascribed to singlet-electron transfer enhanced by folding of the quinone acceptor over the porphyrin donor. The doublet signal of porphyrin or chlorin cation radicals, which is also qbserved, appears in emission under certain conditions, indicative of the radical triplet pair mechanism.

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Various substimted p-benzosemiquinone radical anions, inter alia ubisemiquinone and de¡ have been investigated in 2-propanol and in reversed micelles by EPR and ENDOR spectroscopy. Unsymmetrical semiquinones, with respect to the oxygen atoms, experience remarkable hyperfine shifts depending on the medium. This effect even allows differentiafion between stereoisomers. Immobilization of the semiquinone molecules at the watersurfactant interface in reversed micelles gives ¡ to pronounced asymmetric linewidth effects. In the case of 2-cyclohexyl-3-methyl-l,4-benzosemiquinones, mixtures of two species (conformers) have been observed.