INTERPRETATION OF PORPHYRIN AND METALLOPORPHYRIN SPECTRA

Spectral data from metalloporphyrin-ligand complexes show shifts in maxima which are significantly larger for the B (Soret) transitions than for the Q (visible) transitions, in contrast to previous findings not involving ligands. The shifts in the B transitions are ascribed primarily to a stereoelectronic effect. The data suggest weaker configuration interaction than previously assigned. The possibility that visible absorption bands in porphyrins may be due to n → π* and 1 → π* transitions is considered. Evidence in favor of this interpretation is presented, based on the examination of the spectra of monoprotonated porphyrins and on spectra in fluorosulfonic acid-antimony pentafluoride solution. Other possible interpretations of the data are also discussed. © 1968, American Chemical Society. All rights reserved.