R4 is chosen from the groups corresponding to the formula ???(C)n???C???Z1???C???C???Z3???C-A4 in which n is equal to 1 or 2, Z1 and Z3 represent, independently of each other, a hetero atom chosen from O and N, at least one being a nitrogen atom forming part of an aromatic heterocycle with the two carbon atoms surrounding it, and A4 is a group that is anionic at neutral pH, in which the atom beari