Magnetic circular dichroism study of porphyrin π cation radical species

Abstract

Absorption and magnetic circular dichroism (MCD) spectra of selected metalloporphyrins and their bromine oxidized derivatives with bromide or perchlorate counter ions have been measured. The resultant data clearly discriminate between metal and porphyrin π ring oxidation. Further, in MCD spectrum is sensitive to the orbital ordering in the ground state of the π cation radical species formed from ZnTPP, CuOEP and CooEP. The [ZnTPP] +• Brt-, [ZnTPP] +• ClO − 4. [CuOEP] +• ClO − 4, and [Co(III)OEP] 2+ 2ClO − 4 species each of which has been assined a 2A 2u ground state, exhibit well-defined Faraday A terms under the B (Soret) band in the 400 nm region. In contrast, the data observed for [Co(III)OEP] 2+• 2Br −, a 2A 1u ground state species, does not have the A term character associated with the B band. Both classes of oxidized porphyrins exhibit an apparently charateristic series of weak, overlapping bands between 500 nm and 700 nm in the absorption and MCD spectra. The data show that this region of the MCD spectrum provides an assignment criterion for porphyrin π cation radical species.