EPR AND ENDOR STUDY OF PORPHYRINS AND THEIR COVALENTLY-LINKED DIMERS IN THE PHOTOEXCITED TRIPLET-STATE

The triplet states of several substituted porphyrins (Tetraphenylporphyrin (H2TPP), Zinc-Tetramethylporphyrin (ZnTMP), Octaethylporphyrin (H2OEP) and the Dication of H2TPP (H4TPP2+)) and two covalently linked dimers with H2TPP-subunits in disordered solid solution were studied by EPR and ENDOR at liquid helium temperature. The measurement yields the A(zz) component of the hyperfine tensors of all alpha-protons in the reference frame of the zero field splitting tensor. Dipolar and isotropic contributions to A(zz) are discussed and spin densities derived. The spin densities are compared with results of all-valence-electrons self-consistent field molecular orbital calculations (RHF-INDO/S). One of the dimers shows indications of triplet energy transfer between the porphyrin subunits. The order of magnitude of the transfer rate is estimated to be 5 . 10(5) s-1.