Porphyrin studies. XXXVII. The interpretation of porphyrin and metalloporphyrin spectra

Abstract

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 -* * and 1 -* 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.It is generally agreed that the absorption bands of porphyrins and metalloporphyrins observed in the region 350-700 µ are due to it -* * transitions. Simple Hiickel calculations by Longuet-Higgins, Rector, and Platt4 assign the degenerate pair of eg orbitals as lowest empty orbitals and a2u and alu as highest and next highest filled orbitals, respectively. The near-ultraviolet absorption (Soret band) is then ascribed to a transition of the type alu -eg, while the visible bands are ascribed to a2u -* eg transitions. This theory regards both transitions as allowed and does not account for the large difference in intensity observed between the visible and ultraviolet transitions. Platt5 has modified this one-electron approach by assuming strong configuration interaction between alu and a2u components. These are supposed to be nearly degenerate and give a combined "f" orbital, in Platt's notation, which has angular momentum of 4 au. The(1) (a) Part XXXVII of Porphyrin Studies; for part XXXVI, see

The synthesis, X-ray structure, and substitution lability of chloro(2,3,7,8,12,13,17,18-octaethylporphinato)(triphenylphosphine)rhodium(III)

“…1). These findings are qualitatively consistent with coordination of an additional strong ligand (26). A corresponding reverse blue shift of the spectrum of Ru(OEP)(PnBu3)(S) (S = DMSO or MeCN) compared with that of Ru(OEP)(P"BU~)~ has been noted previously (17).…”

Protonation of porphyrin in iron-free cytochromec: Spectral properties of monocation free base porphyrin, a charge analogue of ferric heme

“…The monocation is known to exist only for some porphyrins in some solvents. 16,34,35 Polarization studies of the monocation porphyrins are consistent with the view that removal of one proton from PH 2ϩ lowers the symmetry group D 4h to D 2h . The splitting is not as large as when two protons are added, however, and consequently bands II and III coincide, and there are apparently only three peaks.…”

Complexes d'hematoporphyrine IX avec le platine(II): synthese, intermediaires, spectres, interet biologique

“…De ces rtsultats, il ressort que si l'espece diprotonte existe en milieu acttique glacial, elle se trouve en tr6s faible quantite. Corwin et al (11) ont risolu le probleme en dissolvant l'ester dans un mtlange acide acttique 60%, acCtone 40%. L'isolation du monocation est a l'ttude.…”