In Situ Measurement of the Supramolecular Chirality in the Langmuir Monolayers of Achiral Porphyrins at the Air/Aqueous Interface by Second Harmonic Generation Linear Dichroism

Abstract

Chiral porphyrin assemblies are promising molecular materials because they possess unique biological compatibility and excellent electronic properties. Metal ions can strongly affect the formation of supramolecular chirality. In this paper, we investigated the effect of metal ions in the subphase on the supramolecular chirality of a porphyrin derivative with two long hydrophobic chains (TPPA2a) at the air/aqueous interfaces by means of second harmonic generation linear dichroism (SHG-LD). It was found that TPPA2a can form chiral assemblies at the air/aqueous interface even though the molecule itself is achiral. Furthermore, metal ions added into the subphase have a considerable effect on the interfacial supramolecular chirality: Zn²⁺ inhibits the formation of supramolecular chirality, while Cu²⁺ promotes the formation. We suggest that the effect of metal ions on the supramolecular chirality is due to the coordination between the metal ions and TPPA2a molecules. To clarify the coordination mechanism, we also performed UV–vis measurements of TPPA2a Langmuir–Blodgett (LB) films and SHG-LD experiments on TPPA4, which is similar to TPPA2a but without ester groups. These results revealed that the metal ions did not interact with the central nitrogen of porphyrin rings, while the coordination between metal ions and the ester groups possibly affects the supramolecular chirality. This is a novel mechanism involving coordination between metal cations and side chains of porphyrin derivatives, and it may provide a deeper understanding of the supramolecular chirality of porphyrin assemblies