Supramolecular polar thin films built by surfactant liquid crystals: Polarization-tunable multilayer self-assemblies with in-plane ferroelectric ordering of ion-based dipoles

Polar order in the phosphonium liquid crystal thin films, which are compose d of two-dimensional ion-pair-based domains separated by the insulating hyd rocarbon layers, was probed by second harmonic generation (SHG) analysis. D espite the ordinary amphiphilic self-assemblies containing no pi -electron moieties, the solid-state thin films retaining a smectic layer structure sh owed clearly an SHG activity, while the disordered films without the layer structure were not active at all for the SHG. It was found that the multila yer structure plays a crucial role for the SHG from the phosphonium thin fi lms and the ionic layers act as an SHG-active site. The most significant ch aracteristic of this system is to possess an ability to control SHG intensi ty electrically. The efficiency of the SHG process in the thin-film assembl ies was enhanced by applying an external electric field parallel to the lay er plane. Furthermore, through evaluation of thermal stability of the sampl e films, it was revealed that the SHG signals were detected only in the sol id-state temperature range and the disappearance of the SHG occurs earlier than the solid-to-liquid crystalline phase transition. These results demons trated that the origin of polar order in the phosphonium thin films is due to in-plane noncentrosymmetric arrangement (ferroelectric ordering) of ion pairs as an electric dipole, that is, dipole symmetry in an ionic layer.