Central metal ion determined self-assembly of intrinsically chiral porphyrins

Abstract

The aggregation of a tetraaryl-porphyrin with chiral amide-containing side groups depends critically on the central metal ion in the tetrapyrrolic core, an effect shown dramatically in solution as well as in the gel formation by the compounds. In solution, the circular dichroism (CD) spectra of the metalloporphyrins show that they all aggregate to some degree, and in most cases the aggregates of the metal-containing species is more favored than the parent free-base porphyrin. The compound which shows the greatest optical activity is the zinc(II) porphyrin which forms a J-aggregate with large Cotton effects in the CD spectrum. Infrared spectroscopy revealed that this aggregate is favored by interaction of the amide oxygen atom with the zinc(II) ion at the core of the porphyrin. The other metalloporphyrins, containing divalent copper, cobalt, and palladium or manganese(III) acetate all show CD activity, and all but the cobalt compound form gels in hexane or cyclohexane. The morphology of the xerogels formed after evaporation of the solvent from these gels depend greatly on the metal ion, with only the copper porphyrin — which shows a clear H-aggregate in solution — having a fibrous morphology