We present four new macrocyclic Eu(III) complexes and demonstrate how ligand structure and positional tuning of π-conjugated antennae modulate steric demand and anion binding site geometry. This enables size and shape complementarity for selective ADP recognition in water over competing phosphoanions.

Abstract

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.