Continuous-wave electron spin resonance studies of porphyrin and porphyrin-quinone triplet states

Abstract

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 observed, appears in emission under certain conditions, indicative of the radical triplet pair mechanism.