Transient resonance Raman spectroscopic studies of some paramagnetic metalloporphyrins: Effects of axial ligand on charge-transfer and photoreduction processes

Time-resolved resonance Raman spectroscopic measurements were undertaken to probe the axial ligand and solvent effects on the photophysical and photochemical processes of some paramagnetic metalloporphyrins (Cu(II), Cr(III), Mn(III) and Fe(III) porphyrins). The presence of odd electrons in d-orbitals of the central metal ions of these metalloporphyrins gives rise to a strong interaction with porphyrin π-electronic system, which affects the energy relaxation pathways of their photoexcited states. The formation and energy of porphyrin mutually implies metal (π,d) or (d,π*) charge-transfer (CT) and metal (d,d) states are sensitive to the solvents employed, especially to the axial ligation ability of the solvents. Based on the change in Raman wavenumbers induced by the structural change in the exciplex formation of photoexcited Cu(II)TPP with the solvents, two intermediate (π,d) CT states below the ²T/⁴T (π,π*) states are proposed, one being mess affected by the ligating solvent (CT₁; Cu(II) porphyrins*) and the other strongly influenced by a σ-donor axial ligand [CT₂; Cu(II) porphyrins* (ligand)]. As for XCr(III)TPP (X = Cl or Br), the photodissociation of axial ligand halogen atoms was observed in benzene. On the other hand, in THF upon photoexcitation, the axial ligand halogen atoms were replaced by the solvent molecules to form the five-coordinate Cr(III)TPP(THF). A significant reduction in the lifetimes of photoexcited ClCr(III)TPP in THF was observed, which is probably due to the participation of the low-lying (π,d(π)) CT state in the energy relaxation of the five-coordinate excited Cr(III)TPP(THF)*. The transient Raman spectra of ClMn(III)TPP in THF under intense laser pulse illumination at 416 nm exhibit the spectral features characteristic of Mn(II)TPP. The photoinduced absorption spectra of ClMn(III)TPP in THF are also similar to the steady-state absorption spectra of Mn(II)TPP, indicating the photoreduction of Mn(III) to Mn(II) porphyrins. Upon photoexcitation of ClFe(III)OEP in THF, the photoreduction to yield Fe(II)OEP(THF) occurred. The photoreduced Fe(II)OEP(THF) species exist as the high-spin complex, which was confirmed by a comparative analysis of the Raman spectrum of chemically prepared low-spin ClFe(II)OEP(pip)₂. Copyright (C) 2000 John Wiley and Sons, Ltd.