MCD of transitions to Jahn-Teller unstable states in the metalloporphins of Mg, Zn, Cu, Pd and Pt

Publisher Website
Google Scholar
Abstract
MCD spectra of solutions of magnesium, zinc, copper, palladium and platinum porphin in EPA measured at 77 K are reported and analysed. It is concluded from the experimental data that the Jahn-Teller (JT) coupling in the first excited singlet states S 1 of palladium porphin (PdP) and platinum porphin (PtP) is very weak but that it is sizable in zinc porphin (ZnP). The JT-active mode of ZnP is identified with the 170–180 cm-1 vibration of this molecule. For the JT-distortion parameter α we find for PdP and PtP: αS 1 state we find for PdP and PtP: Λ=(4·0±0·4) and for ZnP: Λ = (3·7±0·4). From the value of the A-type MCD integrated over the whole S 1-S 0 absorption band it is deduced that in PdP 40 per cent of the intensity in this band is induced by b-type vibrations and 20 per cent by a-type vibrations. For PtP these figures amount to 30 per cent and 20 per cent. Analysis of the MCD spectra with the help of the dispersion method reveals that in solution the degeneracy of the S 1 state of ZnP, PdP and PtP is lifted. The concomitant zero-field splitting amounts to about 60 cm-1 for all three compounds.