Charge transfer switching in photoexcited Ru(II) porphyrins: A time-resolved resonance Raman and spectroelectrochemical study

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)(d pi,pi*) 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 pi donation by pyridine and acceptance by OEP in Ru(II)OEP(Py)(2).