New ion-doped fluorinated smectics for smart windows and memory displays

Abstract

The dynamic scattering effect based on smectics (DSS) is an important candidate for many applications, wherein information either is not changed quickly or the same information has been presented for a long time – for example information notices, electronic books or smart windows. Using a mesogen at the smectic A phase (SmA), long time live recording of information is feasible to obtain without refreshing the voltage needed. The DSS Ad (of a positive dielectric anisotropy) uses the movement of ions. This movement is restricted since it is more free along interlayer space. The molecular director of the smectic medium is oriented in parallel to the electric field (perpendicular to electrodes) and ionic dopants having a negative anisotropy of conductivity migrate along the smectic layers under the applied electric field. Charges immerse the smectic layer in defected places only. Such a charge movement induces forces leading to the local disorientation of the molecular director. Under a movement of ionic dopants, the uniform layers of the SmA structure are disrupted and blocks of it start to rotate. As a result, a strongly scattering state is observed. The scattering state becomes ‘frozen’ due to the high viscosity of the SmA phase. This state is called the ‘write state’. Scattering state at the SmA phase does not need any driving electric field, as is in the case of the scattering effect at the nematic phase. As to induce a clear transparent state of SmA structure, called here ‘erase state’, it is necessary to apply the electric field of a rather high frequency, usually above 0.5 kHz. If the frequency is high enough, ions are oscillating without pronounced translational movement so the dielectrically positive smectic medium keeps the molecular director collinear with an electric field (homeotropic arrangement induced and preserved). Simultaneously, ionic dopants exhibit the property pretending of a surface active compound what progresses additionally the homeotropic orientation of the director. The clear homeotropic state of the SmA structure is called ‘erase state’. DSS effect does not need any polarisers and orienting layers. This profoundly decreases the device fabrication cost from one side but from the other side, a direct penetration of broad spectrum light allows for a destructive action on liquid crystalline materials and reduces the durability of devices. This is especially important in the case of a smart windows typically being under influence of a strong sun ultraviolet radiation for a long time. A special kind of a glass may partially limit this destructive action. Compounds such as fluoro-substituted biphenyls, cyclohexylbiphenyls, bicyclohexylbenzenes and terphenyls exhibit high chemical and photochemical stability and, therefore, they are commonly used in devices as nematic liquid crystallinemedia. Liquid crystals with fluorine atoms in the terminal position exhibit the positive dielectric anisotropy and they may have nematogenic as well as smectogenic features. They may exhibit the single nematic phase only or subsequently nematic and smectic phases. Fluorinated liquid crystals form a monolayer smectic structure (type SmA1) wherein the layer spacing d is near equal the molecular length l (d ~ 1). SmA layers have diffusion character. The neighbouring smectic layers intercalate so molecules can migrate slowly from one layer to the other. The SmAd phase consists of monomeric (single molecules) species and dimeric species. The SmAd phase has stronger diffusion character than the others. This feature favours a lower threshold voltage for the write state as well as for the erase state. The rigidity of SmA1 layer can be decreased by composing SmA1 type mixtures with molecules of different lengths and polarity. The presence of several fluorine atoms inducing in a molecule local dipole pointing at the same direction enables to increase the polarity of compounds up to the level characteristic for cyano compounds or even higher. Therefore, the compounds with two, three, four and six fluorine atoms as well as comprising a pyrimidine ring were chosen to compose desired mixtures with as high as possible positive dielectric anisotropy. Presence of fluorine atoms in the phenyl ring prefers the orthogonal smectic phases, especially SmA1 phase. Fluorinated liquid crystals show low conductivity because they have a small solubility of ionic substances and small ability to the solvation of ions and their extraction from the electrodes. In the case of smectic materials used for DSS mode, a desire level of conductivity is hardly achievable. Complexes of crown ethers with alkaline metal salts are well soluble ionic agents in different organic solvents. They are used to increase solubility of inorganic salts in organic solvents and activity of anions in different chemical reactions of nucleophilic substitution or increase of the transport of ions through membranes. Crown ethers LIQUID CRYSTALS TODAY 2018, VOL. 27, NO. 4, 113–114 https://doi.org/10.1080/1358314X.2018.1570596