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J. Phys. Chem. 1996, 100, 4180-4187
Charge Transfer Switching in Photoexcited Ru(II) Porphyrins: A Time-Resolved Resonance Raman and Spectroelectrochemical Study S. E. Vitols, Ranjit Kumble, Milton E. Blackwood, Jr., J. Scott Roman, and Thomas G. Spiro* Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544 ReceiVed: September 7, 1995X
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 (TR3) spectroscopy. The spectra reveal unexpected differences between the two species. The TR3 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 eg* 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 TR3 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 TR3 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.
Introduction The past 15 years have witnessed renewed interest in ruthenium porphyrin spectroscopy.1-10 Detailed knowledge of the excited state dynamics of Ru(II) porphyrins is important for a number of reasons. Some types of ruthenium porphyrins undergo photoinduced intramolecular electron transfer on a nanosecond time scale.2-5 Characterization of these chargeseparated states could lead to the design of molecular systems that convert solar energy into a chemical potential. Ru(II) porphyrin oligomers have recently shown considerable promise as molecular wires.11-13 Other types of ruthenium porphyrins exhibit synthetically useful photochemistry via axial ligand photolysis.14,15 Ruthenium porphyrins have also attracted attention because of their analogy to the biologically important Fe(II) porphyrins found in heme proteins.16,17 In particular, Ru(II) porphyrins are excellent systems for studying the effects of π acceptor ligands, such as CO, on the photophysics of biologically relevant iron porphyrin complexes. The effects of axial ligation on the photodynamics of Fe(II) porphyrins cannot be studied directly because these complexes undergo ultrafast (
