STUDIES OF THE VIBRATIONAL AND ELECTRONIC-STRUCTURE OF THE S-1 EXCITED-STATES OF BETA-SUBSTITUTED PORPHYRINS BY PICOSECOND TIME-RESOLVED RESONANCE RAMAN-SPECTROSCOPY

Resonance Raman (RR); spectra are reported for the S-1 excited states of zinc(II) complexes of octaethylporphyrin (OEP) and protoporphyrin-IX-dimethyl ester (PPDME). Skeletal stretching modes exhibit large downshifts in the S-1 state of ZnOEP relative to the ground state, indicating an overall expansion of the macrocycle upon photoexcitation. Vibrational modes are assigned using N-15 and meso-d(4) isotopomers of OEP: the isotopic sensitivity is observed to be similar in the S-0 and S-1 states, implying that the ground-state normal mode structure is preserved in the singlet excited state. Excited-state bond orders are calculated using INDO1/S methods, and the large observed downshifts of skeletal stretching modes v(2), v(3), and v(4) in the S-1 state are found to be consistent with the description of this states as an admixture of (a(1u) --> e(g)) and (a(2u) --> e(g)) excitations. Substituent vibrational frequencies in the S-1 state exhibit minor (<5 cm(-1)) downshifts from their ground-state values, indicating that substituent bonds are minimally affected by (pi,pi*) excitation and that the strong S-1-resonant enhancement of these modes in the ground state does not take place by an A-term mechanism. The frequency shift patterns for skeletal vibrational modes of ZnPPDME in the S-1 excited state closely resemble those of ZnOEP, implying a similar electronic configuration; the vinyl C=C stretching mode appears to be only slightly perturbed in the excited states, although the exact position cannot be accurately determined. The results of these studies support the description of metalloporphyrin singlet excited states provided by Gouterman's four-orbital model.