RAMAN STUDIES OF PORPHYRIN DIMERS AND BINUCLEAR SYSTEMS

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The elucidation of the mechanism of energy transduction in biological systems requires the characterization of the structural, magnetic and spectroscopic properties of (i) dimeric and larger aggregates of chromophores and (ii) molecules in which multiple metal centers lie in close proximity. The purpose of the proposed research is to probe these properties of a series of representative dimeric and binuclear systems and their constituent monomers using resonance Raman (RR) spectroscopy. These systems include (i) monomeric Fe, Ru Os, Co, Rh, Ir, Cr, Mn and Hg porphyrins, (ii) monomeric Mg, Ni, and Fe chlorins and pheophorbides (iii) single-atom bridged Fe porphyrin and chlorin dimers, (iv) metal-metal bonded (Ru, Os, Rh) and sandwich (Hg, Ce) porphyrin dimers and (v) nonporphyrin binuclear complexes containing exchange-coupled Fe, Cu and Cr ions. The RR spectra of the metalloporphyrins and metallochlorins are being obtained at a variety of temperatures and at excitation wavelengths ranging from the UV to the red region of the spectrum. RR excitation profiles are being constructed and the dispersions of the depolarization ratios of the Raman bands are being measured. The specific aims of these RR studies are threefold: (i) rigorously characterize the vibrational spectra and electronic structure of the systems, (ii) elucidate perturbations of the monomer system induced by dimerization and (iii) search for evidence of significant coupling (exciton interactions) between the constituent monomers of the dimeric unit. Electron paramagnetic resonance and theoretical studies are also being conducted on a number of the systems in order to characterize their magnetic properties. The studies of nonporphyrin binuclear transition-metal complexes involve obtaining RR spectra in the near UV region of the spectrum, where ligand-metal charge-transfer transitions occur. The goal of these studies is to determine whether transitions between the low-lying electronic states which comprise the spin manifold of the exchange-coupled system can be directly observed with RR spectroscopy. Taken together, the studies of the various monomeric, dimeric and binuclear systems should help to provide a more comprehensive picture of the nature of molecular and electronic interactions.