Tuning Mg(II) Selectivity: Comparative Analysis of the Photophysical Properties of Four Fluorescent Probes with an Alkynyl-Naphthalene Fluorophore

Four alkynyl-1-naphthyl fluorophores have been synthesised with tri- or pentadentate ligating groups suited to the binding of magnesium. Their photophysical and binding properties for magnesium, calcium and zinc ions have been assessed using absorption, emission and excitation spectroscopy. Each compound has a pK_a value between 6.2 and 5.1 and exhibits no significant pH response around pH 7.2. Enhanced selectivity for Mg²⁺ over Ca²⁺ was observed with a pentadentate phosphinate substituted system, compared to its carboxylate analogue, due to a 10-fold lowering of affinity for the Ca²⁺ ion. In each case, the overall dissociation constants for Mg²⁺ fall in the low mm range.