Liquid-crystalline medium

A liquid-crystalline medium based on a mixture of polar compounds, comprising at least one compound of formula I

• in which
• R11 and R12 each, independently of one another, denote an alkyl or alkenyl radical having up to 15 C atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, in which optionally one or more CH2 groups are replaced by —O—, —S—,
• —C≡C—, —CF2O—, —OC—O— or —O—CO— in such a way that O atoms are not linked directly to one another,
• each, independently of one another, denote
• a and b each, independently of one another, denote 0 or 1, where a+b≧1.