Magnetic circular dichroism and absorption spectra of the porphyrins. Part I

Abstract

We have measured the magnetic circular dichroism and absorption spectra of free base porphyrin, mono- and di-acid forms and a number of metalloporphyrins. Three of these, free base octaethylporphyrin, zinc and copper octaethylporphyrins have been studied in rigid media at liquid helium temperatures, the remainder as solutions at room temperature. From studying the protonated forms we find no support for the n→π* interpretation of Corwin. The remaining spectra fit reasonably well with established theory in that the metal is only weakly coupled to the ring π system and takes little part in the optical transitions in the metalloporphyrins studied in this work. Close examination of the bandwidths of the free base spectra at low temperatures suggests an underlying weak transition which may be n→π* in character. Much vibronic structure is observed at low temperature in the Q₁ band and it is clear that several vibronic states are excited which accounts for the observation by Anex of mixed polarisation in second band. Using the functions of Gouterman we obtain some justification for the experimental observation that the Q bands are split much more than is the Soret band on going from metalloporphyrin to free base.