A time-resolved resonance Raman study of the T[sub 1] excited state of zinc(II) octaalkylporphyrins

The T[sub 1] 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[sub [beta]]-C[sub [beta]] stretching modes [nu][sub 2] and [nu][sub 11] are seen from the ground-state (S[sub 0]) values, determined by an analysis of [beta]-[sup 13]C-labeled zinc(II) etioporphyrin I. The magnitude of these S[sub 0]/T[sub 1] shifts are smaller than those observed for the same modes in the T[sub 1] state of zinc(II) tetraphenylporphyrin (ZnTPP), consistent with expected differences in electronic configuration between the T[sub 1] states of meso-tetraphenyl and [beta]-octaalkyl substituents [[sup 3](a[sub 2u]e[sub g]) vs [sup 3](a[sub 1u]e[sub g]), respectively]. The direction of frequency shifts seen for these modes is opposite to that observed for the T[sub 1] state of free-base octaethylporphyrin (H[sub 2]OEP); we speculate that these differences result from a Jahn-Teller effect occurring in the T[sub 1] states of ZnOEP and ZnEtio. None of the modes observed showed sensitivity to meso-d[sub 4] substitution, implying that they do not contain significant C[sub m]-H stretching or bending character. 22 refs., 2 figs.