Compound of the general formula (Ia), (Ib) and (Ic) R50 and R60 are different form each other and are selected from the group consisting of -R70, -ZR70, -Z-OH, -Z-NH2, -Z-SH, -Z-OC(O)R70, -OC(O)R70, - COOH and its corresponding salts, -C(O)NH2, -C(O)NH-R70, -C(O)N-(R70)2, - COOR70, -Z-COOH and its corresponding salts, -Z-C(O)NH-R70, -Z-C(O)NH2, -Z- C(O)N-(R70)2, -Z-COOR70, -CH(COOH)2 and its corresponding salts, -CH(COOR70)2, and -Z-SO3- wherein R70 is selected from the group consisting of a linear or branched C1 to C20 alkyl, (C1 to C10)-alkyloxy-(C1 to C10)-alkyl, C2 to C10 alkenyl, C6 to C12 aryl, C3 to C10 cycloalkyl, cycloalkylalkyl and cycloalkylalkenyl, wherein Z is a linear or branched C1 to C10 alkyl, linear or branched C3 to C10 cycloalkyl, a linear or branched C6 to C10 aryl or a (C1 to C10)-alkyloxy- (C1 to C10)-alkyl, cycloalkylalkyl and cycloalkylalkenyl.