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)-containing solvent molecules by copper (Cu)-porphyrins has been observed and studied in detail by the methods of resonance Raman (RR), resonance coherent anti-Stokes Raman scattering (RCARS), and picosecond absorption spectroscopy. It was found that the formation of the excited complex (exciplex) [(CuP)*-L] occurs when an O-containing molecule L is attached as an axial Ligand to a Cu-porphyrin in the excited tripdoublet-quartet 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-lying excited (d,d) state, which involves the promotion of an electron from 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 and in transient difference absorption spectra by characteristic derivative-like absorption changes. The decay of the excited (d,d) state having a lifetime of hundreds of picoseconds is accompanied by the exciplex 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) state 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 = dimethyl sulfoxide (DMSO), dimethyl formamide (DMF)) this relaxation channel via the CT state dominates. The vibrational analysis of transient Raman spectra is done to elucidate the structural changes of five-coordinate Cu-porphyrins occurring both in the excited (d,d) state at ambient temperature and in the ground electronic state, being stable at liquid nitrogen temperature (77 K).