# Fluorescence Properties and Mg2+ Selectivity of Aryl‐Alkynyl Derivatives of the o‐Aminophenoltriacetate (APTRA) Ligand

## TL;DR

This paper shows how modifying APTRA ligands can improve their selectivity for magnesium over calcium by using electron-withdrawing groups.

## Contribution

The study demonstrates a strategy to enhance Mg2+ selectivity over Ca2+ in APTRA derivatives using aryl-alkynyl fluorophores.

## Key findings

- Electron-withdrawing substituents reduce Ca2+ affinity more than Mg2+ affinity, increasing Mg2+ selectivity.
- The effect is stronger when the APTRA nitrogen is para to the alkyne, enhancing conjugation.
- Fluorescence changes are minimal in water, but absorption shifts allow for measuring binding affinities.

## Abstract

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.

It is a challenge to improve the selectivity of high‐denticity ligands for Mg2+ over Ca2+. We show that in aryl‐alkynyl derivatives of APTRA, electron‐withdrawing substituents lower the binding affinity for Ca2+ more than Mg2+, making the ligand more selective for Mg2+. The effect is larger when the APTRA nitrogen is para to alkyne, but the associated charge‐transfer fluorescence is weak in water.