HOLE LOCALIZATION AND SPIN COUPLING IN PI-MONOCATIONS AND PI-DICATIONS OF MU-OXOPORPHYRIN DIMERS - RELEVANCE TO STRUCTURE OF OXIDIZED SPECIAL PAIR IN PHOTOSYNTHETIC REACTION CENTERS

The antiferromagnetically coupled cofacial mu-oxoiron(III) dimers of octaethylporphyrin (OEP) and tetraphenylporphyrin (TPP) can be oxidized in two one-electron steps to yield pi-monocations and pi-dications. Hitherto unreported absorption bands are observed in the mid-infrared region in the spectra of the monocations. These are identified as intervalence transitions of dynamically trapped holes. The electron-phonon coupling energies (CH2Cl2 solution, 295 K) are 0.31 +/- 0.04 eV, and the inter-ring, predominantly through-space, electronic coupling constants J are 0.07 and 0.03 eV, respectively. The small values of J are a consequence of the large mean plane separation of the porphyrin rings (4.53 angstrom). Since it is improbable that the electron-phonon coupling energies will vary greatly between cofacial porphyrin dimers, it is suggested that absorption bands around 1 eV in the pi-monocations of 'sandwich'' compounds such as M(III)P2 or M(IV)P2+, where P is a porphyrin and M are lanthanide, actinide, or transition metals (3-12) observed under similar conditions will have electronic ground states that are delocalized on at least the vibrational time scale, with frequency maxima equal to 2J in the tight-binding approximation. The thermal inter-ring hole-transfer times in the mu-oxo dimer monocations are estimated to be approximately 1 ps. The relevance of these results to the problem of the structure of the oxidized 'special pair'' in the photosynthetic reaction center is briefly discussed. The pi-dications exhibit a band in the near-infrared region which is attributed to a transition between antiferromagnetically coupled pi-states, from which, using the Hubbard hamiltonian, inter-ring exchange integrals K are calculated to be 0.39 +/- 0.02 eV, and the singlet-triplet separations (S)DELTA(T) calculated to be 0.025 and 0.0024 eV, respectively. The monocations are stable to disproportionation in CH2Cl2 solution (K(com) (295 K) = 3 X 10(5) and 2 x 10(4), respectively).