Synthesis and excited state Raman spectroscopy of sterically crowded ruthenium(II) octaethyltetraphenylporphyrin

Pyridine and CO adducts of Ru(II)OETPP (OETPP = 2,3,7,8,12,13, 17,18-octaethyl-5, 10, 15,20-tetraphenylporphinato) have been prepared and characterized by absorption and resonance Raman (RR) spectroscopy. Ru --> porphyrin back-bonding is evident from the ground state RR spectra. Skeletal mode frequency downshifts in Ru(II)OETPP(Py)(2) relative to Ru(II)OETPP(CO)(Py) are comparable to those seen for the TPP (tetraphenylporphyrin) and OEP (octaethylporphyrin) analogs, indicating that distortion of the porphyrin ring from substituent crowding has little effect on the extent of back-bonding. Photoexcited Ru(II)OETPP(Py)(2) has absorption and RR spectra which are similar to those of (RuTPP)-T-II(Py)(2) and are characteristic of a (d pi,pi*) charge transfer state. As for (RuTPP)-T-II(Py)(2), the photoexcited RR spectrum has features indicating porphyrin anion formation and does not have pyridine anion features, as have been observed for photoexcited Ru(II)OEP(Py)(2). The ethyl substituents raise the porphyrin e(g)* orbital above the lowest pyridine pi* orbital in Ru(II)OEP(Py)(2), but not in Ru(II)OETPP(Py)(2), for which the effect of the phenyl substituents is dominant.