Disclosed are compounds of formula I ##STR1## wherein R is C.sub.1-12 -alkyl or halogenoalkyl wherein one or two non-adjacent CH.sub.2 groups are optionally replaced by --O--,A is ##STR2## Q is a bivalent radical of formula (2), and L.sup.0, L.sup.1 and L.sup.2 are each independently H or F, with the proviso that at least one of L.sup.0, L.sup.1 and L.sup.2 present in the molecule denotes F.