Abstract: Compounds of formula (I) RA—CO—N(R2)—(CR12)n—N(R5)—RB and salts or optical active isomers thereof, wherein each R1 independently are the same or different and denotes a hydrogen atom, a hydroxyl group, a C1 to C4 straight of branched alkyl group or a C1 to C4 straight of branched oxyalkyl group; R2 denotes a hydrogen atom or a C1 to C4 straight of branched alkyl group; R5 independently are the same or different and denotes a acyl moiety; RA and RB independently are the same or different and denote a triiodinated phenyl group, preferably a 2,4,6-triiodinated phenyl group further substituted by two groups R3 in the 3 and 5 positions wherein each R3 are the same or different and denote a hydrogen atom or a non-ionic hydrophilic moiety, provided that at least one R3 group in the compound of formula (I) is a hydrophilic moiety; and n denotes a positive integer of 1 to 6.