. . . . . . . "gen, ##STR13## in which R11, R12 and R13 are identical or different and are a hydrogen atom, a halogen atom, a C1 -C10 -alkyl group, a C1 -C10 -fluoroalkyl group, a C6 -C10 -aryl group, a C6 -C10 -fluoroaryl group, a C1 -C10 -alkoxy group, a C2 -C10 -alkenyl group, a C7 -C40 -arylalkyl group, a C8 -C40 -arylalkenyl group or a C7 -C40 -alkylaryl group, or R11 and R12 or R11 and R13 in each case for" . . . . .