the ?? symbols represent, independently of each other and of ??, (i) a hydrogen atom, (ii) a halogen atom other than an iodine atom, or (iii) a linear or branched C1-C20 alkyl group; ?? resents (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 and C1-C8 alkyl groups, (iii) a group ???