Zeeman Effect in Porphyrins: Zero-Field Splitting of the Excited Electronic States

Recent experimental studies have reported anomalous and contradictory results for the absorption of left and right circularly polarized light (LCPL and RCPL) by zinc and magnesium coproporphyrins in the presence of a magnetic field: for both compounds, the absorption bands for right and left circularly polarized light have distinctly different shapes at room temperature; at 77°K unexplained shoulders appear in the absorption spectra for both polarizations for the magnesium but not for the zinc compound; the value of the angular momentum of the lowest energy excited state computed from the separation of the peaks of the LCPL and RCPL absorption bands differs from the value obtained from magnetic circular dichroism (MCD) experiments by nearly 50%; no shape anomalies were observed in the MCD spectra. We derive a general solution for the mixing of two states by a magnetic field and show that all of the anomalous experimental data are explained if (a) the presumably degenerate pair of excited states are split in zero field by an energy comparable to the Zeeman energy but less than the spectral bandwidths, and also (b) the overlapping transitions have unequal intensities. Our results show that MCD should give better estimates of excited state angular momenta while the direct measurement of LCPL and RCPL is superior in detecting nondegeneracy.