RESONANCE RAMAN, CARS, AND PICOSECOND ABSORPTION-SPECTROSCOPY OF COPPER PORPHYRINS - THE EVIDENCE FOR THE EXCIPLEX FORMATION WITH OXYGEN-CONTAINING SOLVENT MOLECULES

The reversible process of photoinduced binding of oxygen (O)-containin g solvent molecules by copper (Cu)-porphyrins has been observed and st udied in detail by the methods of resonance Raman (RR), resonance cohe rent anti-Stokes Raman scattering (RCARS), and picosecond absorption s pectroscopy. It was found that the formation of the excited complex (e xciplex) [(CuP)-L] occurs when an O-containing molecule L is attached as an axial Ligand to a Cu-porphyrin in the excited tripdoublet-quart et state manifold T-2,4(1). If L is a molecule of tetrahydrofuran (THF ), dioxane, or cyclohexanone, then the deactivation of the excitation energy inside the five-coordinate Cu-porphyrin proceeds via the low-ly ing excited (d,d) state, which involves the promotion of an electron f rom the highest filled da orbital to the half-filled d(x2-y2) orbital. This state is displayed prominently in transient RR and RCARS spectra by large frequency shifts in selected metalloporphyrin marker Lines a nd in transient difference absorption spectra by characteristic deriva tive-like absorption changes. The decay of the excited (d,d) state hav ing a lifetime of hundreds of picoseconds is accompanied by the excipl ex disruption into initial components. Saturation RCARS studies reveal the existence of the second exciplex deactivation channel, presumably involving the low-lying excited intramolecular charge-transfer (CT) s tate of the Cu-porphyrin, which is competitive with the decay pathway via the (d,d) state. It was found that for some axial Ligands (L = dim ethyl sulfoxide (DMSO), dimethyl formamide (DMF)) this relaxation chan nel via the CT state dominates. The vibrational analysis of transient Raman spectra is done to elucidate the structural changes of five-coor dinate Cu-porphyrins occurring both in the excited (d,d) state at ambi ent temperature and in the ground electronic state, being stable at li quid nitrogen temperature (77 K).