No abstract

No abstract

No abstract

The charge transfer (CT) excited states of Ru(II)OEP(Py) 2 and Ru(II)TPP(Py) 2 (TPP ) tetraphenylporphyrin, OEP ) octaethylporphyrin, and Py ) pyridine) have been investigated by nanosecond time-resolved resonance Raman (TR 3 ) spectroscopy. The spectra reveal unexpected differences between the two species. The TR 3 spectrum of [Ru(II)TPP(Py) 2 ]* resembles the resonance Raman (RR) spectrum of the RuTPP(Py) 2 radical anion. Both spectra show strong enhancement of nontotally symmetric modes, resulting from a Jahn-Teller distortion due to the e g * orbital degeneracy. The band frequencies are slightly higher in the CT state than in the radical anion, reflecting the effect of the Ru(II) oxidation. Thus, the TR 3 spectral features support a 3 (dπ,π*) excited state assignment, with an electron transferred from Ru(II) to the porphyrin. In contrast, the TR3 spectrum [Ru(II)OEP(Py) 2 ]* does not resemble the RR spectra of the Ru(II)OEP radical anion. Rather, it contains totally symmetric modes, at frequencies close to those of Ru(III)OEP(Py) 2 . In addition, bound pyridine modes appear at 1208 and 1603 cm -1 in the TR 3 spectrum and they shift upon pyridine perdeuteration. These characteristics imply electron transfer from Ru(II) to bound pyridine, instead of porphyrin, for the CT state of Ru(II)OEP(Py) 2 . The switch in the pathway for photoinduced electron transfer between TPP and OEP is consistent with a much more negative reduction potential for Ru(II)OEP(Py) 2 than for Ru(II)TPP(Py) 2 , > -2.2 versus -1.52 V versus SSCE. This anomalous energetics suggest synergetic π donation by pyridine and acceptance by OEP in Ru(II)OEP(Py) 2 .

The excited states of [tetrakis(2,4,6-trimethylphenyl)porphyrinato]rhodium(II), Rh(II)TMP, have been studied using nanosecond transient absorption spectroscopy, emission spectroscopy, and electrochemistry. Rh(II)TMP has a long-lived excited state, with a 460 nm absorption band, that decays with a time constant of 180 ± 22 ns, in benzene and 205 ± 28 ns in 1,3-difluorobenzene. No transient absorption features are seen between 600 and 700 nm, but weak phosphorescence is detected at 743 nm at 77 K. Thus, the excited state has the characteristics of a triplet (π,π*) state, although a low lying, charge transfer state had been expected from iterative extended Huckel calculations. The phosphorescence is biphasic, and highly temperature dependent, consistent with a trip-quartet [4T] lowest excited state, lying not far below a trip-doublet [2T] state. Electrochemistry establishes that the Rh(II)/(I) reduction potential is more negative than −1.5 V versus SSCE while the porphyrin cation reduction potential is +0.55 V. These potentials place the lowest LMCT state at >2.05 V, well above the triplet states. In addition to elucidating the photophysics of a novel type of metalloporphyrin, these results have significant bearing on the feasibility of using Rh(II) porphyrins in photoinduced catalytic systems.