Supramolecular Chirogenesis in Porphyrin-Based Systems: Chirality Transfer from Anionic Chiral Surfactants to Cationic, Achiral Porphyrins

The chirality transfer from chiral domains to achiral molecules is an important theoretical and applicative issue. In this work, we have investigated the interaction between two anionic chiral surfactants bearing a proline residue as hydrophilic head and the cationic, achiral porphyrin Zn(II) [5-{4-(3-trimethylammonium)propyloxyphenyl}-10,15,20-triphenylporphyrinyl]chloride to assess the effects of the structural variations in both units on the chirality transfer efficiency and amplification. We showed that the efficiency of transferring molecular information depends on the surfactant's features, namely the chiral configuration of the polar head, the length of the aliphatic chain, and the aggregation state. At the same time, the presence of a coordinated metal and the peripheral charged group on the porphyrin macrocycle are key factors. In detail, the study of the hetero-aggregates formed at a surfactant concentration below the critical micellar concentration (cmc) indicates that the chirality depends on the synergy of hydrophobic effect, coordination interaction, and electrostatic forces. If the surfactant concentration is higher than the cmc, at a low concentration, porphyrins are included in micelles as monomers. Under these conditions, no chirality transfer is evident. When the porphyrin is in excess with respect to the micelles, an efficient asymmetry induction is again observed, transmitted from the chiral polar head to the porphyrin oligomers included in the micelle, through the polar heads and the hydrocarbon chains of the surfactants.