Thermally stable nonlinear optical activity in a smectic-A liquid crystal

Order and frustration in liquid-crystalline dendrimers

X-ray diffraction has been used to elucidate the structure and phase behavior of several liquid-crystalline dendrimers with a different surface topology of the terminal chains. This includes second-generation liquid-crystalline block and statistical dendrimers with mixed aliphatic and mesogenic terminal groups as well as homo-dendrimers of several generations containing only mesogenic end groups. The homo-dendrimers of generation one to four display a monolayer smectic phase, while the fifth generation shows a more ordered columnar phase. The block-dendrimer of the second generation has a bilayer smectic phase. The precise structure of the lamellar ordering has been determined by X-ray reflectivity from thin films on a substrate. The second-generation statistical dendrimer does not show any mesogenic phase. The observed phase behavior is discussed in terms of the frustration due to competition between the stiff geometry of the dendritic matrix and the close-packing conditions of the terminal chains.

Organic materials for second-harmonic generation: advances in relating structure to function

The relationships between molecular structure and the nonlinear optical phenomenon second-harmonic generation (SHG) are discussed. New-found relationships built up from basic structural axioms that were deduced in the 1970s and 1980s are the particular focus of this article, using structural results from X-ray and neutron-diffraction studies. The molecular and supramolecular manifestations of the SHG effect are borne out, although ways to optimize the effect on the molecular scale feature predominantly, since control of SHG on the supramolecular scale remains difficult given present limitations. The use of a variety of templates to generate head-to-tail oriented host-guest species thereby bypassing such limitations is described. The paper concludes with a look ahead at next generation 'octupolar' SHG-active compounds, the prediction of new series of SHG-active compounds via data-mining computational procedures, and developments in diffraction technology that may enable structural movies of a molecule to be captured during the SHG process. A practical assessment of the viability of organic SHG materials for industrial application is reviewed with a positive outcome, thus indicating a promising future for organic SHG materials.

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 composed of two-dimensional ion-pair-based domains separated by the insulating hydrocarbon layers, was probed by second harmonic generation (SHG) analysis. Despite the ordinary amphiphilic self-assemblies containing no pi-electron moieties, the solid-state thin films retaining a smectic layer structure showed 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 multilayer structure plays a crucial role for the SHG from the phosphonium thin films and the ionic layers act as an SHG-active site. The most significant characteristic of this system is to possess an ability to control SHG intensity electrically. The efficiency of the SHG process in the thin-film assemblies was enhanced by applying an external electric field parallel to the layer plane. Furthermore, through evaluation of thermal stability of the sample films, it was revealed that the SHG signals were detected only in the solid-state temperature range and the disappearance of the SHG occurs earlier than the solid-to-liquid crystalline phase transition. These results demonstrated 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.