Chiral nematic, also known as cholesteric, liquid crystalline materials are useful in cholesteric liquid crystal displays a variety of applications including various liquid crystal (e.g.: LC) display components, reflective films, optical filters, polarizers, paints, and inks, among others. Methods for preparing such materials are well established. See for example: G. Gottarelli and G. Spada, Mol. Cryst. Liq. Crys., 123, 377 (1985); G. Spada and G. Proni, Enantiomer, 3, 301 (1998). However, improvement is still needed. While early uses of chiral nematic compositions relied upon mixtures composed mostly of chiral components, more recently such materials are composed of nematic liquid crystal (LC) mixtures combined with small amounts of chiral dopants. In such new compositions the properties of the nematic host material, for example: viscosity, birefringence, electrical anisotropy, and magnetic anisotropy among others, are tailored to the desired usage by altering the chemical composition of the nematic mixture and then a chiral dopant is incorporated to induce helical twisting so as to provide the desired chiral nematic pitch. It is apparent that the properties of this chiral nematic composition are therefore a combination of the properties of the nematic host plus those of the dopant.
Chiral nematic liquid crystals can be formulated to reflect various wavelength of incident electromagnetic radiation, and it is well understood that the reflected light is circularly polarized, depending upon the sense of chirality of the helical pitch. Thus a chiral nematic displaying a right-handed helical meso-structure will reflect right-handed incident light. For many applications it is useful to be able to reflect both right-handed and left-handed sense of circularly polarized light, for example, in a vertically layered structure. It is further well known that enantiomers of a chiral dopant structure induce the opposite polarity of helical rotation and, therefore, afford oppositely polarized light reflections. For this reason the preparation of enantiomeric dopants for use in separate light modulating layers can be particularly useful.
For some applications it is desirable to have liquid crystal mixtures that exhibit a strong helical twist and thus a short pitch length. A short pitch can be achieved by using high amounts of dopant or by using a dopant with a high helical twisting power. However, using chiral dopants in high amounts can negatively affect the properties of the liquid crystalline host mixture, for example; the dielectric anisotropy, the viscosity, and the driving voltage or the switching times among others. In liquid crystalline mixtures that are used in selectively reflecting cholesteric displays, the pitch has to be selected such that the maximum of the wavelength reflected by the cholesteric helix is in the range of visible light. Another possible application is polymer films with a chiral liquid crystalline phase for optical elements, such as cholesteric broadband polarizers or chiral liquid crystalline retardation films.
Such liquid crystalline materials can be used for the preparation of polymer films with a chiral liquid crystalline phase, for active and passive optical elements or color filters and for liquid crystal displays, for example STN, TN, AMD-TN, temperature compensation, guest-host or phase change displays, or polymer free or polymer stabilized cholesteric texture (PFCT, PSCT) displays. Such liquid crystal displays can include a chiral dopant in a liquid crystalline medium and a polymer film with a chiral liquid crystalline phase obtainable by (co)polymerizing a liquid crystalline material containing a chiral dopant and a polymerizable mesogenic compound.
Biphenyl chiral dopants with 4,4′ substitution have been long known and well studied, and are reported to display generally small helical twisting power when they are used as dopants in nematic liquid crystals. See for example the review: R. Eelkema and B. L. Feringa, Org. Biomol. Chem., 2006, 4, 3729-3745. However, biphenyl chiral dopants with 2,2′ substitution have received much less study. The 2,2′-dimethylbiphenyl derivatives described by R. Holzwarth, R. Bartsch, Z. Cherkaoui, and G. Solladie in Eur. J. Org. Chem. 2005, 3536-3541, and Chem. Eur. J. 2004, 10, 3931-3935, and by M. R. Wilson and D. J. Earl in J. Mater. Chem., 2001, 11, 2672-2677 are reported to show small to moderate helical twisting powers. Biphenyl liquid crystalline materials with multiple oxygen linked substituents have been reported in studies of liquid crystalline polymers. See polymers described by T.-H.-Tong, et. al., in Polymer (2000), 41(11), 4127-4135, Macromolecules (1998), 31(11), 3537-3541, and Polymer Preprints (1998), 39(1), 252-253. Also, binaphthyl materials with multiple oxygen linked substituents have been reported for polymer preparations by G. Bernatz and A. Taugerbeck in European Patent Application EP1911828 A1 20080416.
Thus there is a considerable demand for new chiral dopants with a high helical twisting power which can be easily synthesized in individual enantiomers, which can be used in low amounts, show improved temperature stability of the cholesteric pitch for utilizing a temperature invariant reflection wavelength and do not affect the properties of the liquid crystal host mixture.
We have found new inventive chiral dopants of the tetraoxybiphenyl esters which provide these properties, can be prepared easily, have high helical twisting power, and do not have the disadvantages of the dopants of the state of the art as discussed above.