. . "; n represents an integer equal to 1 to 8; the groups X represent, independently of each other and of Y, (i) a hydrogen atom, (ii) a halogen atom other than an iodine atom, or (iii) a linear or branched C1-C20 alkyl group; Y represents (i) a halogen atom other than an iodine atom, (ii) a phenyl group optionally substituted with one or more atoms chosen from halogen atoms other than the iodine atom" . .