Luminescence Screening Assays for the Identification of Sensitizers for Lanthanides Based on the Controlled Formation of Ternary Lanthanide Complexes with DTPA-Bisamide Ligands

The formation of ternary complexes of lanthanide–diethylenetriamine pentaacetic acid (DTPA)–bisamide complexes with different aromatic acids and their application in luminescent screening assays are presented. The europium complexes of DTPA–bisethylamide (BEA), DTPA–bisbutylamide (BUA), DTPA–bis(2‐norbornyl) (NBA), and DTPA–bis(1‐adamantyl) (ADA) have been isolated and the sensitization of emission upon 1:1 formation with aromatic acids has been studied by luminescence spectroscopy. The ternary complexes show stronger luminescence with picolinate (PCA) rather than phthalate (PTA) or benzoate (BZA), with the latter forming 1:2 complexes. Isophthalate and dipicolinate sensitizers do not show a 1:1 formation of the ternary complexes. Electrospray mass spectra show characteristic peaks that confirm the formation of the ternary complexes, and NMR spectroscopic studies demonstrated a conformational locking effect upon formation of the ternary complex. It is shown that europium complexes with bulkier amide arms provide complexes with stronger luminescence enhancement that is clearly attributable to the bulkiness of the amide arms. In a luminescence screening experiment, different substitutions on BZA and PCA were examined. Sensitizers with a long alkyl chain show greater luminescent enhancements than the nonsubstituted acids, which can be attributed to an “umbrella” effect of the alkyl chain that protects the europium luminescent center from the quenching of secondary high‐energy vibrations, in particular OH from water molecules. The same effect is presented for the quinoline derivatives as sensitizers. Quinoline derivatives with salicylate binding units were identified as the best sensitizers when combined with EuADA, which is even bulkier than EuNBA, with HQ‐3COO showing a 50‐fold enhancement of emission upon formation of 1:1 complexes. Screening umbrellas: Sensitizers of europium emission have been screened in luminescence assay experiments to reveal recognition features in the formation of ternary complexes that lead to signal enhancement; bulky aliphatic substituents on a non‐absorbing bisamide complex (NAL) or the sensitizer (S) play the role of an “umbrella” and protect the Eu emission signal from water molecules.