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-reso lved resonance Raman spectroscopy. The spectra reveal identical featur es for both compounds, which we attribute to the lowest-lying triplet state. Slight downshifts of C(beta)-C(beta) stretching modes nu2 and n u11 are seen from the ground-state (S0) values, determined by an analy sis of beta-C-13-labeled zinc(II) etioporphyrin I. The magnitude of th ese S0/T1 shifts are smaller than those observed for the same modes in the T1 state of zinc(II) tetraphenylporphyrin (ZnTPP), consistent wit h expected differences in electronic configuration between the T1 stat es of meso-tetraphenyl and beta-octaalkyl substituents [3(a(2u)e(g)) v s 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 r esult from a Jahn-Teller effect occurring in the T1 states of ZnOEP an d ZnEtio. None of the modes observed showed sensitivity to meso-d4 sub stitution, implying that they do not contain significant C(m)-H stretc hing or bending character.