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 theA _zz component of the hyperfine tensors of all α-protons in the reference frame of the zero field splitting tensor. Dipolar and isotropic contributions toA _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 · 10⁵ s^?1.