| abstract |
The invention concerns a water soluble compound of formula (a) wherein: A represents naphthyl or phenyl; and Ar 1 and Ar 2 independently represent a saturated or aromatic carbocyclic group; X a , X b are independently selected among an amino group, an ammonium group and an amino group modified by a linear polyoxyalkylene chain, provided that at least one of X a and X b represents ammonium or modified amino. |