Liquid crystalline mixtures having a chiral tilted smectic phase

Optically active compounds of the formula I wherein rings A, B and C each independently are unsubstituted or each are 1,4-phenylene substituted with at least one of halogen-, cyano-, methyl- or methoxy in which optionally 1 CH group or 2 CH groups is/are replaced by nitrogen; R1 is a radical of an optically active terpene alcohol after cleavage of the hydroxy group or a group -(CH2)m-C*HX1-R3; R2 is a radical of an optically active terpene alcohol after cleavage of the hydroxy group or a group -(CH2)n-C*HX2-R4; m and n each independently stand for the number O or 1: C* denotes a chiral carbon atom; X1 and X2 each independently are fluorine, chlorine, cyano, methyl or methoxy; R3 and R4 each independently denote a group R, phenyl or phenyl substituted with R, with the proviso that R3 is different from X1 and R4 is different from X2; R is an alkyl group in which optionally one methylene group is replaced by oxygen and/or optionally one group CH-CH is replaced by a group C=C; with the proviso that R1 and R2 are not simultaneously 2-alkyl when rings A, B and C together are p-terphenyl, as well as liquid crystalline mixtures and their use for electro-optical purposes.