Abstract: The present invention relates to a compound of formula (1) (I), wherein Y3 is O or S, wherein s, t, u and w are 0 or 1, wherein i is an integer of from 1 to 3, wherein j is an integer of from 3 to 5, and wherein Z1, Z2 and Z3 are selected from the group consisting of CO2H, —SO2H, —SO3H, —OSO3H, and -0P03H2, R1 is —CH3 or H, X is selected from the group consisting of alkylaryl, aryl, alkylheteroaryl and heteroaryl, Y1 and Y2 are selected from the group consisting of aryl, alkylaryl, cycloalkyl, heterocycloalkyl, heteroaryl and alkylheteroaryl, and wherein A is a chelator residue having a structure selected from the group consisting of (la), (lb) and (lc), wherein R2, R3, R4 and R5 are selected from the group consisting of H, —CH2—COOH and —CH2—C(=0)-NH2 or wherein R2 and R4 form a —(CH2)n— bridge with n being an integer of from 1 to 3, wherein n is preferably 2, and wherein r, v and q are 0 or 1, with the proviso that in case u and w are 0, q and v are 0, and (A) wherein u and w are 1, or (B) wherein u is 0 an