Suitable substituents R18 to R27 are the same radicals as described for R1-R16 and halogens, for example fluorine, chlorine or bromine, where, if desired, it is also possible for two vicinal radicals R18 to R27 to be linked to form a 5- or 6-membered ring and also to be substituted by halogens, such as fluorine, chlorine or bromine.