Disclosed herein are compounds of formula (I): wherein: the groups R1, R2, R3, R4, R5, R6, R7, R8, R1′, R2′, R3′, R4′, R5′, R6′, R7′, R8′, R1″, R2″, R3″, R4″, R5″, R6″, R7″, R8″ are defined herein; and at least one of the following conditions is satisfied: two radicals from at least one of the four following pairs of radicals are not the same radical: R1′ and R8′, R2′ and R7′, R3′ and R6′, R4′ and R5′, and two radicals from at least one of the four following pairs of radicals are not the same radical: R1″ and R8″, R2″ and R7″, R3″ and R6″, R4″ and R5′.