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

Ortho-aminophenol-N,N,O-triacetate (APTRA) has been widely adopted for binding and sensing Mg²⁺, although it binds Ca²⁺ more strongly. This contribution investigates APTRA derivatives that incorporate an aryl-alkynyl fluorophore, addressing how the binding affinities for Mg²⁺ and Ca²⁺ 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, CF₃, 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 CF₃, 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 Ca²⁺ more than Mg²⁺, leading to a net improvement in selectivity for Mg²⁺, 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.