Transient raman spectroscopic investigations on ruⁱⁱtpp(L)₂ (L = Pyridine and Piperidine) and Ru^IITPP(CO)(py) in various solvents: alternation of excited charge transfer depending on axial ligation

Transient Raman spectra with an excitation of nanosecond pulses at 416 nm were first recorded for Ru^IITPP(L)2 (L = pyridine and piperidine) and Ru^IITPP(CO)(py) in various solvents such as benzene, tetrahydrofuran, pyridine, and piperidine. This work is intended to investigate the nature of the charge transfer states accompanied by photoexcitation. It was found that the electronic structure of the lowest energy charge transfer (CT) state is largely dependent on not only the nature of axial ligands attached to the central metal but also the solvents employed. As for Ru^IITPP(L)2 (L = pyridine and piperidine) in neat pyridine and piperidine solvents, the transient species were observed to be different in structure depending on the axial ligand. Several reasons for this observation were considered, including the discussion on the nature of the CT states such as (π,d), (d,d), (d.π(L)) (L = axial ligand), and (d.π;r*(ring)) states, and the configuration interaction between the two eg(π*) orbitals. The present transient Raman spectroscopic studies on photoexcited RuTPP(pip)2 has manifested the Jahn-Teller effects, which may result in a diamond distortion of the porphyrin ring superimposed by a minor rectangular distortion. This major B2g distortion accounts for the observation of a variety of unusual Raman spectral features in photoexcited Ru^IITPP(pip)2. Meanwhile, the introduction of a CO ligand to Ru^II porphyrin also altered the nature of the CT state in nonligating and weakly ligating solvents. It was observed that the CO ligand of the carbonylated Ru^II porphyrin was substantially replaced by the solvent molecules such as pyridine and piperidine in the excited state. These experimental results were interpreted in terms of the change in the charge distribution among the porphyrin ring, the central metal and the axial ligand in the metal-to-ring (dT,eg(π*)) CT state. The relevance of our findings to the photodynamics observed by the previous picosecond transient absorption measurements of photoexcited Ru(II) porphyrins is discussed in terms of the conformational changes of photoexcited species.