Structure Control of Synthetic Bilayer Membranes from Single-Chain Amphiphiles Containing the Schiff Base Segment: I. Conformation Control and Spectral Characterization

Abstract

Fifteen single-chain ammonium amphiphiles with a rigid Schiff base segment, CnSBCmN+(m= 4, 6, 10;n= 4, 7, 10, 12, 16), were prepared, and their aggregational behaviors in aqueous dispersions were investigated by electron microscopy, differential scanning calorimetry and electronic absorption spectra. The agreement of absorption spectra between aqueous dispersions and cast films, and the temperature-dependent spectral features, have been employed as criteria for bilayer formation. The results showed that except for the five short-chain-lengthed amphiphiles with the sum of tail and spacer not larger than C14, stable bilayers were obtainable for the other amphiphiles. Through the spectral assignments of the planar and nonplanar conformations of the Schiff base segment in the monomer spectra, the electronic absorption spectra of the stable bilayer assemblies were studied. It was observed systematically for the first time that with increasing the alkyl chain length, the spectra change from a single peak at ca. 286 nm characteristic of the loose packing of the nonplanar Schiff base segments in the bilayers, cascading through a doublet band with a 286-nm absorption overlapped by a 330-nm band, which is indicative of the coexistence of the two types of conformations in the bilayer matrix, finally to the fine structured band along with the 286-nm absorption representing the strong aggregation of long-chain-lengthed amphiphiles with the sum of tail and spacer larger than C20. Therefore, the conformation control and spectral characterization of the supramolecular systems containing a nonplanar chromophore have been established.