Organised structures of flexible bolaamphiphiles with trisiloxane spacers: three- and two-dimensional molecular assemblies with different molecular conformation

Abstract

We designed and synthesised new bolaamphiphiles consisting of hydrophobic rigid cores, flexible trisiloxane spacers, and terminal ammonium groups. Organised structures of the bolaamphiphiles with biphenyl and azobenzene cores were examined in the bulk and in the monolayer states. In the bulk states, these molecules exhibited layered liquid crystal phases with an extended molecular conformation. The biphenyl derivative showed only lyotropic liquid crystallinity in the presence of water. In contrast, the azobenzene derivative exhibited both thermotropic and lyotropic liquid crystal behaviour. UV–vis spectroscopy for the materials indicated that the association of the azobenzene cores contributed to the formation of the stable mesomorphic structures. On the other hand, when these molecules were spread on the water surface, monolayer films were formed with both hydrophilic ends on the water surface. The compression of the monolayer films induced the conformational changes in the bolaamphiphiles from a flat-lying extended state to a reverse U-shaped folded state and the core parts were lifted up to the uppermost film surface, maintaining their flat orientation. The cores integrated to the film surface were shown to be monomeric species by UV–vis spectroscopy, whereas the azobenzene cores tend to associate in the bulk and on the water surface at low surface pressures. The trans–cis photoisomerisation and cis–trans thermal isomerisation of the azobenzene cores occurred efficiently in the compressed monolayer films, which is available to tune the surface properties of the monolayer films. For these monolayer films with two-dimensional core arrays, we can expect novel surface-mediated functionalities as photo- and electro-active surfaces, alignment layers for materials synthesis, and molecular sensors due to their tunabilities in core density and polarity.