Transient Raman Spectroscopic Investigations on Ru^IITPP(L)₂ (L = Pyridine and Piperidine) and Ru^IITPP(CO)(py) in Various Solvents:  Alternation of Excited Charge Transfer Depending on Axial Ligation

Abstract

Transient Raman spectra with an excitation of nanosecond pulses at 416 nm were first recorded for Ru II TPP(L) 2 (L ) pyridine and piperidine) and Ru II TPP(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 II TPP(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 ,π*(L)) (L ) axial ligand), and (d π ,π*(ring)) states, and the configuration interaction between the two e g (π*) orbitals. The present transient Raman spectroscopic studies on photoexcited Ru II TPP(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 B 2g distortion accounts for the observation of a variety of unusual Raman spectral features in photoexcited Ru II TPP(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 (d π ,e g (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.

Transient Absorption and Resonance Raman Investigations on the Axial Ligand Photodissociation of Halochromium(III) Tetraphenylporphyrin

“…The transient RR spectra were obtained by using 416 nm pulses, generated by the hydrogen Raman shifting of the third (355 nm) harmonics from a nanosecond Q-switched Nd:YAG laser. The Raman spectra were collected with an HR 640 spectrograph (Jobin-Yvon), a gated intensified photodiode array detector (Princeton Instruments IRY700), and a pulse generator (Princeton Instruments FG100) …”

Lowest Excited State of Oxovanadyl(IV) Tetraphenylporphyrin

“…The probe pulse at 457 nm was generated by the D 2 Raman shifting of the second harmonic from a Q-switched Nd:YAG laser. The Raman spectra were collected with a HR 640 spectrograph (Jobin-Yvon), a gated intensified photodiode array detector (Princeton Instruments IRY700), and a pulse generator (Princeton Instruments FG100) …”

Comparative Study of the Catalytic Amination of Benzylic C–H Bonds Promoted by Ru(TPP)(py)2 and Ru(TPP)(CO)

“…To analyse the catalytic influence of the electronic features of the axial ligand, we here report the combined experimental and computational study of the catalytic behaviour of Ru(TPP)(py) 2 (2). Although the physicochemical properties of 2 were already elucidated by a variety of experimental methods, such as electrochemistry, [35,36] UV/Vis absorption, emission, resonance Raman [37][38][39][40][41] and picosecond transient absorption, [42,43] the use of this complex to catalyse C-H amination by organic azides has not yet been reported.…”