Compounds of the formula (I) R.sup.1 --A--Z.sup.1 (I) in which: R.sup.1 is an alkyl radical having 1 to 8 carbon atoms or an alkenyl radical having 2 to 8 carbon atoms, where, in each case, one (nonterminal) --CH.sub.2 -- group may also be replaced by --O-- or --C(.dbd.O)O-- and/or one or more H may be replaced by F A is phenanthrene-2,7-diyl or 9,10-dihydrophenanthrene-2,7-diyl Z.sup.1 is F, OCH.sub.x F.sub.3-x or CH.sub.x F.sub.3-x, where, in each case, x is 0, 1 or 2, OC.sub.2 H.sub.z F.sub.5-z or C.sub.2 H.sub.z F.sub.5-z, where, in each case, z is 0, 1, 2, 3 or 4, CH.dbd.CF.sub.2 or Cl, with the proviso that 2-fluoro-7-methoxy-phenanthrene is excluded.