hydroxy-3,5,6-trichlorophenyl)-methane).Preferred compounds having the general formula II are those in which R.sup.7 denotes an alkyl, alkoxy or aralkyl residue, in particular an alkoxy residue and R.sup.5, R.sup.6, R.sup.8 and R.sup.9 are the same or different and each denotes a nitrogroup, halogen, a cyano group, an alkylsulfonyl group or an alkyl group substituted with halogen.