Metal Ion Mediation of Interfacial Chiral Supramolecular Formation of Amphiphilic Schiff Bases Studied by In Situ Second Harmonic Generation
Man Xue, Li Zhang, Minghua Liu, Yu Bai, Yuan Guo, Zhen Zhang
Abstract
The coordination effects between chiral Schiff bases and metal ions can provide an effective strategy for fabricating chiral supramolecular self-assemblies. We studied the supramolecular self-assembly of the amphiphilic Schiff base enantiomer, 2-hydroxy-1-naphthylmethylamino-N,N′-bis(octadecyl)-l/d-glutamic diamide (l/dGJ), at the air/water interfaces by using in situ second harmonic generation linear dichroism (SHG-LD) technique combined with UV–vis spectroscopy and atom force microscopy. lGJ and dGJ monolayers can form mirror-image structures of each other at different interfaces, which can be mediated by metal ions. When Mg2+ and Zn2+ ions were added into the subphases, the l/dGJ monolayers self-assembled into a nanofiber-like structure and showed significant chirality similar to that at the pure water interface. However, when Cu2+ ions were added into the subphase, the chirality of the l/dGJ monolayer was destroyed because of the Cu2+ coordination effect. The degree of the chirality excess (DCE) value decreased with the increment of the concentration of Cu2+ in the subphase. Furthermore, when the surface pressure increased, the DCE value of Cu2+–l/dGJ complexes increased gradually, which indicated that the Cu2+–l/dGJ complex aggregated into a chiral supramolecular structure through lateral molecular packing.