. . "Preferred chemical compounds of the formula A conform to the general formulae I, II and III: in which R1 are identical or different and are a hydrogen atom, a halogen atom, a boron-free C1-C40-hydrocarbon-containing group, such as C1-C20-alkyl, C1-C20-haloalkyl, C1-C10-alkoxy, C6-C20-aryl, C6-C20-haloaryl, C6-C20-aryloxy, C7-C40-arylalkyl, C7-C40-haloarylalkyl, C7-C40-alkylaryl or C7-C40-haloalkyl" . . .