tes at least one of which is transparent, formula (1) being wherein X??? and Y??? may be the same or different and are each independently a counter anion selected from the group consisting of an halogen anion, ClO4???, BF4???, PF6???, CH3COO??? and CH3(C6H4)SO3???, R1 is hydrogen or a C1-C5 alkyl group, R2 is a C1-C30 divalent hydrocarbon residue, R3 is a C1-C20 hydrocarbon residue and a is an int