Liquid-crystalline medium and liquid-crystal display having high twist

A liquid-crystalline medium having a helically twisted structure comprising:

• a nematic component and an optically active component;
• wherein said optically active component comprises one or more chiral compounds whose helical twisting power and concentration are selected in such a way that the helix pitch of the medium is ≦1 μm;
• and
• wherein said nematic component comprises one or more compounds selected from the group consisting of compound of formula I and II
• in which
• a is 1 or 2,
• b is 0 or 1,
• R1 and R2 are each, independently of each other, alkyl having from 1 to 12 C-atoms, wherein one or two non-adjacent CH2-groups are each optionally replaced by —O—, —CH═CH—, —C≡C—, —CO—, —OCO— or —COO— in such a way that O atoms are not linked directly to one another,
• Z1 and Z2 are each, independently of each other, —CH2CH2—, —CH═CH—, —CF2O—, —OCF2—, —CH2O—, —OCH2—, —COO—, —OCO—, —C2F4—, —CF═CF—, —CH═CHCH2O—, or a single bond, and Z1 in formula II may also denote —C≡C—, and
• L1-4 are each, independently of each other, F, Cl, OCF3, CF3, CH3, CH2F, or CHF2.