Fluorescence Properties and Mg 2+ Selectivity of Aryl‐Alkynyl Derivatives of the o ‐Aminophenoltriacetate (APTRA) Ligand

ABSTRACT Ortho ‐aminophenol‐ N,N,O ‐triacetate (APTRA) has been widely adopted for binding and sensing Mg 2+ , although it binds Ca 2+ more strongly. This contribution investigates APTRA derivatives that incorporate an aryl‐alkynyl fluorophore, addressing how the binding affinities for Mg 2+ and Ca 2+ are modulated by a substituent R in the aryl ring of the fluorophore. Six such derivatives have been synthesized via their tris ‐ethyl esters. They feature X ═ CN, CF 3 , or OMe, as a mesomerically electron‐withdrawing, inductively electron‐withdrawing, or electron‐donating substituent, respectively, with the alkyne either para or meta to the APTRA nitrogen. Study of the absorption and fluorescence properties of the esters reveals the importance of intramolecular charge transfer (ICT) states for X ═ CN and CF 3 , but not OMe. The corresponding carboxylate ligands are less emissive in water, and the fluorescence is not strongly modulated by metal ions. However, the absorption spectra change markedly, allowing dissociation constants K d to be evaluated. The key conclusions are that (i) electron‐withdrawing substituents attenuate the affinity for Ca 2+ more than Mg 2+ , leading to a net improvement in selectivity for Mg 2+ , and (ii) the effect is larger when the APTRA nitrogen (as opposed to phenolic oxygen) is para to—and hence directly conjugated with—the alkyne.