Nematic liquid crystals are employed in electro-optical devices for the modulation of the transmittant or incident light and for the black and white or colored rendition of numerals, symbols and pictures. The object of the invention is the use of nematic liquid crystals having a high positive dielectric anisotropy, a low threshold and operating voltage and high clear points and a process for making these compounds. It was found that as liquid crystals for electro-optical devices mixtures are useful which, as an essential component, contain one or several compounds from the class of 5-cyano-2-[4-acyloxyphenyl]-pyrimidines of the formula ##STR1## wherein R.sup.1 is ##STR2## C.sub.5 H.sub.11 -- or C.sub.n H.sub.2n+1 O-- R.sup.2 is C.sub.n H.sub.2n+1 ; C.sub.n H.sub.2n+1 O; C.sub.n H.sub.2n+1 S; F; Cl; Br; CF.sub.3 ; C.sub.n H.sub.2n+1 COO; C.sub.n H.sub.2n+1 OCOO; CN; NO.sub.2 ; C.sub.n H.sub.2n+1 NH; or C.sub.n H.sub.2n+1 (CH.sub.3)N, n is 1 to 10, and R.sup.3 is C.sub.n H.sub.2n+1, n is 1 to 10.
向列相液晶被用于电光器件,用于调制透射或入射光,并用于数字,符号和图片的黑白或彩色再现。本发明的目的是使用具有高正电介质各向异性,低阈值和操作电压以及高清晰点的向列相液晶,并提供制造这些化合物的过程。发现作为电光器件液晶的混合物是有用的,其作为基本成分包含来自5-
氰基-2-[4-酰氧基苯基]-
嘧啶类化合物的一个或多个化合物,其
化学式为##STR1##其中R1为##STR2##
C5H11--或CnH2n+1O--,R2为CnH2n+1;CnH2n+1O;CnH2n+1S;F;Cl;Br;
CF3;CnH2n+1COO;CnH2n+1OCOO;CN;
NO2;CnH2n+1NH;或CnH2n+1(
CH3)N,n为1至10,R3为CnH2n+1,n为1至10。