Time-resolved Resonance Raman spectroscopic study on copper(II) porphyrins in various solvents: Solvent effects on the charge transfer states

Time-resolved resonance Raman (TR³) spectroscopic investigations were done on copper(II) tetraphenylporphyrin (Cu^IITPP) and copper(II) octaethylporphyrin (Cu^IIOEP) in various solvents to elucidate the electronic nature of the excited states involved in the deactivation processes. In noncoordinating solvents, the frequency shift and enhancement pattern in the Raman spectrum of excited state Cu^IITPP were identified as the simultaneous contribution from the ²T/⁴T (π,π*) manifold and the (π,d) charge transfer state (CT₁). These observations were also consistent with the small energy gap between the ²T/⁴T (π,π*) manifold and the (π,d) charge transfer state of Cu^IITPP, as suggested by the previous picosecond transient absorption results. On the contrary, our TR³ spectrum of Cu^IIOEP represented the characteristic features mainly induced by the ²T/⁴T (π,π*) manifold with no indication of the contribution from the (π,d) charge transfer state, which was suggested to be located in an energy well above the ²T/⁴T (π,π*) manifold. In coordinating solvents, however, the TR³ spectra of Cu^IITPP and Cu^IIOEP, as indicated by the significant shifts of the v₂ and v₄ bands relative to those in noncoordinating solvents, exhibited the involvement of a new (π,d) charge transfer state (CT₂) associated with the formation of the five-coordinated species with solvent as an axial ligand in the excited state. The identification of the electronic nature and the axial ligand dynamics in the excited states of Cu^II-TPP and Cu^IIOEP was demonstrated by examining the relationship between the core size expansion upon axial ligation and the shift and enhancement pattern of characteristic Raman bands.