Liquid crystal medium and liquid crystal display with a highly twisted structure

#The present invention relates to a liquid-crystalline medium having high twist, to the use thereof for electro-optical applications, in nonlinear optics (NLO) and laser optics, and to electro-optical devices, displays and lasers containing this medium.

#Liquid-crystal displays (LC displays) are known from the prior art. The commonest display devices are based on the Schadt-Helfrich effect and contain an LC medium having a twisted nematic structure, such as, for example, TN (“twisted nematic”) cells having twist angles of typically 90° and STN (“super-twisted nematic”) cells having twist angles of typically 180 to 270°. Also known are ferroelectric LC displays which contain an LC medium having a twisted smectic structure. The twisted structure in these displays is usually achieved by addition of one or more chiral dopants to a nematic or smectic LC medium.

#Also known are LC displays which contain LC media having a chiral nematic or cholesteric structure (CLC displays). CLC media have significantly higher twist compared with the LC media from TN and STN cells, with twist angles which are usually a multiple of 360°.

#The mixture CC1 has a significantly lower VHR compared with mixture C2 from Example 2.

#FIG. 1 shows the absorption curves of nematic mixture N2 from Example 2 (a) and nematic mixture NC1 from the comparative example (b). Both mixtures exhibit absorption in the UV region.

#FIG. 2 shows the absorption curves of cholesteric mixture C2 from Example 2 (a) and cholesteric mixture CC1 from the comparative example (b). In addition to absorption in the UV region, both mixtures exhibit low absorption in the region of the cholesteric reflection band.

#As can be clearly seen from FIGS. 1 and 2, mixtures N2 and C2 according to the invention exhibit lower UV absorption than comparative mixtures NC1 and CC1 respectively. In particular, however, the absorption band of mixtures N2 and C2 according to the invention is shifted 20 nm to shorter wavelengths compared with the comparative mixtures and is thus below the typical laser excitation wavelength of 355 nm. The mixtures according to the invention are therefore particularly suitable for use as host material in an active laser medium.