A TIME-RESOLVED RESONANCE RAMAN-STUDY OF THE T(1) EXCITED-STATE OF ZINC(II) OCTAALKYLPORPHYRINS

The T1 excited states of zinc(II) octaethylporphyrin (ZnOEP) and zinc(II) etioporphyrin I (ZnEtio) have been studied by nanosecond time-resolved resonance Raman spectroscopy. The spectra reveal identical features for both compounds, which we attribute to the lowest-lying triplet state. Slight downshifts of C(beta)-C(beta) stretching modes nu2 and nu11 are seen from the ground-state (S0) values, determined by an analysis of beta-C-13-labeled zinc(II) etioporphyrin I. The magnitude of these S0/T1 shifts are smaller than those observed for the same modes in the T1 state of zinc(II) tetraphenylporphyrin (ZnTPP), consistent with expected differences in electronic configuration between the T1 states of meso-tetraphenyl and beta-octaalkyl substituents [3(a(2u)e(g)) vs 3(a(1u)e(g)), respectively]. The direction of frequency shifts seen for these modes is opposite to that observed for the T1 state of free-base octaethylporphyrin (H2OEP); we speculate that these differences result from a Jahn-Teller effect occurring in the T1 states of ZnOEP and ZnEtio. None of the modes observed showed sensitivity to meso-d4 substitution, implying that they do not contain significant C(m)-H stretching or bending character.