THE MAGNETIC CIRCULAR DICHROISM OF PORPHYRIN DICATIONS1 *

The utility of magnetic circular dichroism (MCD) spectroscopy in porphyrin chemistry, which is evident from the number of papers cited in several reviews2-* of recent progress in Faraday effect spectroscopy, can be attributed to the high information content of porphyrin MCD spectra. On the one hand, experimental chemists have exploited the analytical application^^-'^ made possible by the richly detailed MCD curves that are characteristic of porphyrins. Whereas, for the theoretical chemist,l3-I6 it is the facile determination of excited state magnetic moments that is of interest since this quantity provides a stringent test of the quality of his wave functions. For example, all theoretical models of porphyrin electronic structure predict the existence of degenerate excited states for square symmetric systems (e.g., metal complexes and dications). The observation of A terms in the MCD spectrum is sufficient to confirm excited state degenera~ies.~ In the MCD spectrum of octaethylporphine dication shown in FIGURE l , note that the S-shaped MCD bands which are characteristic of A terms correlate with the Qo-o and B absorption bands. Quantitative evaluation of A values is only slightly more troublesome than is qualitative evaluation. The values obtained by Dratz'* agree remarkably well with the value calculated by GoutermanI6 using the four-orbital model. Thus, an understanding of the molecular basis of porphyrin MCD is at hand. There are, however, several questions which remain to be resolved. Two of these are particularly relevant : the discrepancy between the magnetic moments determined by direct Zeeman effect mea~urement '~ and by MCD," and the possibility that positive A terms occur within the vibrational components of the Qo-' band.'6-19 We report here on the MCD spectra of a number of porphyrin dications, a series in which vibrational effects are enhanced. We also report the results of using the techniques of deuterium exchange, low temperature, and curve resolution to emphasize and interpret the main features of the vibrational spectrum. To our knowledge, this work represents the first attempt at vibrational analysis in alkylsubstituted porphyrin dications.

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