Liquid crystal materials are between crystals and liquids, and have the characteristics of liquids and crystals. On the one hand, liquid crystals have fluid flow characteristics. On the other hand, liquid crystals exhibit spatial anisotropy of crystals, including dielectric properties, magnetic polarization, and optical refractive index. From the perspective of the composition and the physical conditions for the appearance of the mesophase, liquid crystals can be roughly divided into two categories: thermotropic liquid crystals and lyotropic liquid crystals. A lyotropic liquid crystal is a liquid crystal formed of two or more compounds including a solvent compound, most of which are synthesized from two components, an amphiphilic compound or a surfactant and a polar solvent. Thermotropic liquid crystal is a liquid crystal phase that appears due to temperature changes. It has a crystalline structure at low temperatures and becomes liquid at high temperatures. The temperature is indicated by the melting point and the clearing point. Liquid crystal monomolecules have their own melting points and clearing points, and exist in the form of liquid crystals at intermediate temperatures. Thermotropic liquid crystals are divided into three categories according to the arrangement of liquid crystal molecules: smectic phase, nematic phase and cholesteric phase. The liquid crystal (LC) building block is shown in Figure 1, the square is a mesogenic group, Z is a side group, A is a linking group, and X and Y are terminal groups. The LC building block is similar to biphenyl, diphenylacetylene, diazonium, olefin, ester, bicycloalkane, etc.
Figure 1. Liquid crystal (LC) building blocks
- Liquid crystal display: Under the action of different electric currents and electric fields, the liquid crystal molecules will be arranged in a regular rotation, resulting in a difference in light transmittance. In the process of switching the power on and off, the difference between light and dark can be produced, and the light and dark changes of each pixel can be controlled to form the desired display pattern. Simple liquid crystal structural units often only show black and white changes, and doping with cholesteric materials can achieve color display. In addition, doped dyes and high-scattering materials can enhance diffraction intensity and color contrast, and expand the application of liquid crystal displays, such as 3D display technology, touch technology, etc.
- Organic photovoltaic device (OPV): The order and orientation characteristics of liquid crystal materials can improve the efficiency of OPV from different aspects. As the additive of OPV, LC building blocks can form an ordered liquid crystal phase microstructure under certain conditions.. Using the superior morphology control ability of crystalline materials to control the heterojunction interface and microstructure of OPV can realize the phase separation of donor material and acceptor material on the nanoscale, and can also improve the orderliness of the microstructure of the entire active layer composite material. Thereby facilitating exciton separation, charge transfer and transport, thereby improving the photoelectric conversion efficiency of the device.
- Anti-counterfeiting packaging: Thermotropic liquid crystals are easily affected by temperature and produce color changes. The LC building block and the cholesteric liquid crystal are mixed, and the authenticity can be identified by making liquid crystal anti-counterfeiting ink and liquid crystal anti-counterfeiting paper. Among them, the liquid crystal ink displays light and dark pictures and colors when the temperature changes, and the pictures and texts can present different colors with the change of temperature. It has the advantages of simple implementation, low cost, convenient inspection, good concealment, bright colors, and strong reproducibility. It is the preferred anti-counterfeiting technology for banknotes, tickets and product trademarks in various countries.
- Denis Andrienko. Introduction to liquid crystals [J]. Journal of Molecular Liquids 2018, 267, 520–541.