Compounds of the formula ##STR1## wherein R.sup.1 denotes a group R.sup.3 or R.sup.3 --A.sup.4 --Z.sup.3 -- and R.sup.2 denotes a group R.sup.4 or --Z.sup.4 --A.sup.5 --R.sup.4 ; m and n each individually stand for the number 0 or 1; A.sup.1, A.sup.2, A.sup.3, A.sup.4 and A.sup.5 each independently represent unsubstituted or halogen-, cyano- and/or methyl-substituted 1,4-phenylene in which optionally 1 CH group or 2 CH groups is/are replaced by nitrogen, trans-1,4-cyclohexylene in which optionally 2 non-adjacent methylene groups are replaced by oxygen and/or sulphur, 1-cyano-trans-1,4-cyclohexylene, bicyclo[2.2.2]octane-1,4-diyl, naphthalene-2,6-diyl, tetralin-2,6-diyl or trans-decalin-2,6-diyl; Z.sup.1, Z.sup.2, Z.sup.3 and Z.sup.4 each independently signify a single covalent bond, --COO--, --OOC--, --CH.sub.2 O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2, --C.tbd.C--, --(CH.sub.2).sub.3 O--, --O(CH.sub.2).sub.3 -- or the trans form of --CH.dbd.CH--CH.sub.2 O-- or --OCH.sub.2 --CH.dbd.CH--; R.sup.3 and R.sup.4 each independently denote halogen, cyano, --NCS, --CF.sub.3, --OCF.sub.3 or alkyl in which optionally one is replaced by and/or optionally one methylene group or two non-adjacent methylene groups is/are replaced by --O--, --COO-- and/or --OOC-- and/or optionally one methylene group is replaced by --CHX--; and X signifies halogen, cyano or methyl,as well as liquid crystalline mixtures and their use for electro-optical purposes.