CONTINUOUS-WAVE ELECTRON-SPIN-RESONANCE STUDIES OF PORPHYRIN AND PORPHYRIN-QUINONE TRIPLET-STATES

CW ESR studies of the photoexcited triplet states of a series of porph yrins and covalently linked porphyrin-quinones have been performed in isotropic and anisotropic (liquid-crystalline) frozen solutions. In fr ozen nematic solutions, the appearance of the spectra is strongly depe ndent on the orientation of the sample relative to the external magnet ic field. This fact allows a differentiation between the z and x,y axe s and hence a more accurate determination of the zero-field splitting parameters D and E. Spectrum simulation and fitting aided in the inter pretation of the experimental results and the extraction of the releva nt parameters. In the case of some porphyrin-quinones, steady-state tr iplet ESR signals cannot be observed, which is ascribed to singlet-ele ctron transfer enhanced by folding of the quinone acceptor over the po rphyrin donor. The doublet signal of porphyrin or chlorin cation radic als, which is also observed.