TRANSIENT RAMAN-SPECTROSCOPIC INVESTIGATIONS ON RUIITPP(L)(2) (L EQUALS PYRIDINE AND PIPERIDINE) AND RUIITPP(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 (RuTPP)-T-II(L)(2) (L = pyridine and piper idine) and RuIITPP(CO)(py) in various solvents such as benzene, tetrah ydrofuran, pyridine, and piperidine. This work is intended to investig ate the nature of the charge transfer states accompanied by photoexcit ation. It was found that the electronic structure of the lowest energy charge transfer (CT) state is largely dependent on not only the natur e of axial ligands attached to the central metal but also the solvents employed. As for (RuTPP)-T-II(L)(2) (L = pyridine and piperidine) in neat pyridine and piperidine solvents, the transient species were obse rved to be different in structure depending on the axial ligand. Sever al reasons for this observation were considered, including the discuss ion on the nature of the CT states such as (pi,d), (d,d), (d(pi),pi(L )) (L = axial ligand), and (d pi,pi(ring)) states, and the configurat ion interaction between the two e(g)(pi) orbitals. The present transi ent Raman spectroscopic studies on photoexcited (RuTTPP)-T-II(pip)(2) has manifested the Jahn-Teller effects, which may result in a diamond distortion of the porphyrin ring superimposed by a minor rectangular d istortion. This major B-2g distortion accounts for the observation of a variety of unusual Raman spectral features in photoexcited (RuTPP)-T -II(pip)(2). Meanwhile, the introduction of a CO ligand to Ru-II porph yrin also altered the nature of the CT state in nonligating and weakly ligating solvents. It was observed that the CO ligand of the carbonyl ated Ru-II porphyrin was substantially replaced by the solvent molecul es such as pyridine and piperidine in the excited state. These experim ental results were interpreted in terms of the change in the charge di stribution among the porphyrin ring, the central metal and the axial l igand in the metal-to-ring (d(pi)e(g)(pi)) CT state. The relevance of our findings to the photodynamics observed by the previous picosecond transient absorption measurements of photoexcited Ru(II) porphyrins i s discussed in terms of the conformational changes of photoexcited spe cies.