The invention relates to compounds, to the complexes they form with a lanthanide, and to the use of the complexes for fluorescence marking or NMR imaging.The complex consists of an Ln ion and a ligand R2—C(X—R1)(R3)—NR4R5. R1 is a functional group, X is a single bond or a hydrocarbon-based chain consisting of at least one alkylene or alkenylene group optionally comprising at least one hetero atom or an arylene. R2 is an anionic group A2 or a C1-C4 alkylene or alkenylene group bearing at least one such group A2 and optionally comprising at least one hetero atom. R3 is H or a C1-C5 alkylene or alkenylene group optionally containing at least one hetero atom, and optionally bearing at least one anionic group A3. R4 is a substituent with light-absorbing properties that forms chelate rings with Ln. R5 is a substituent that forms chelate rings with Ln.