. . . . ", R5 and R6 are identical or different and have the meaning stated for R3 and R4, with the proviso that R5 and R6 are not hydrogen, R7 is ##STR15## wherein R11, and 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" . . . . . . . .