Steric and Geometric Tuning of π-Conjugated Antennae in Europium(III) Complexes for Selective ADP Recognition

The selective recognition of adenosine diphosphate (ADP) in water presents a significant challenge for synthetic supramolecular chemistry, driven by its biological importance in cellular energy transfer and enzymatic signaling pathways. Discriminating ADP from structurally similar anions such as ATP requires a high degree of host-guest complementarity. We recently developed [Eu.ADPGlow]^-, a luminescent Eu(III) complex bearing two 6-substituted quinolyl-phenoxyacetate arms, which create a binding site at the central Eu(III) ion that accommodates ADP. Binding induces a tight interaction in water, involving both metal coordination and π-π stacking, switching the emission on with a 33-fold enhancement. Here, we examine how systematic changes in ligand geometry influence anion selectivity, by synthesizing four new Eu(III) complexes with pendant arms at the 4- or 7-positions of the quinoline scaffold. The 4-substituted systems provide a more accessible binding site and bind ADP, ATP, and AMP with limited selectivity between them, while the 7-substituted analogues impose steric hindrance at the Eu(III) center, resulting in minimal response to all tested anions. Only the 6-substituted complex [Eu.ADPGlow]^- achieves optimal geometrical complementarity for ADP binding. These findings reinforce the importance of steric and geometric control in the design of selective lanthanide probes for biological anions in water.