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Transient resonance Raman spectroscopic studies of some paramagnetic metalloporphyrins: effects of axial ligand on charge‐transfer and photoreduction processes
Author(s) -
Jeoung Sae Chae,
Kim Dongho,
Cho Dae Won
Publication year - 2000
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/(sici)1097-4555(200004)31:4<319::aid-jrs545>3.0.co;2-i
Subject(s) - chemistry , photochemistry , raman spectroscopy , resonance raman spectroscopy , ligand (biochemistry) , excited state , photoexcitation , porphyrin , resonance (particle physics) , photodissociation , atomic physics , biochemistry , physics , receptor , optics
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 ⇋ 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 2 T/ 4 T (π,π * ) states are proposed, one being less affected by the ligating solvent (CT 1 ; Cu II porphyrins * ) and the other strongly influenced by a σ‐donor axial ligand [CT 2 ; 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 © 2000 John Wiley & Sons, Ltd.