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

Abstract

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 value between 6.2 and 5.1 and exhibits no significant pH response around pH 7.2. Enhanced selectivity for Mg over Ca was observed with a pentadentate phosphinate substituted system, compared to its carboxylate analogue, due to a 10-fold lowering of affinity for the Ca ion. In each case, the overall dissociation constants for Mg fall in the low mm range.

APTRA‐Based Luminescent Lanthanide Complexes Displaying Enhanced Selectivity for Mg2+

“…EuL 2 : In contrast to the emission spectral response observed for EuL 1 , the addition of Mg 2+ , Ca 2+ (Figure ) and Zn 2+ (Figure S7 in the Supporting Information) resulted in a 1.4‐, 2.5‐ and 6‐fold enhancement in the luminescence emission intensity. A non‐ratiometric excitation response was observed for EuL 2 , in accordance with the structurally comparable naphthalene systems containing an aniline nitrogen atom meta to the chromophore …”

“…The complexes EuL 1 and EuL 2 are based upon a pyridylalkynyl chromophore and TbL 3 contains a bi‐aryl chromophore to sensitise lanthanide emission. Comparisons of the cation affinity and selectivity profiles for EuL 1 and EuL 2 are made here with the behaviour of the APTRA analogues containing an alkynyl‐naphthalene fluorophore, recently reported in parallel work, and with the older TbL 4 system . The selection of these three new complexes allows the behaviour of systems with a pyridylalkynylaryl and a bi‐aryl chromophore to be compared to assess the influence of the interannular linking unit on the affinity, selectivity and luminescence response to Mg 2+ .…”

“…Ligands that are able to bind magnesium in water with high selectivity over calcium are unknown. Some promising progress has been reported recently with the development of o ‐aminophenol‐ N , N ‐diacetate‐ O ‐methylene‐methylphosphinate (APDAP), a new pentadentate phosphinate‐based analogue of APTRA . By using UV/Visible absorption spectroscopy, APDAP has been shown to display a significantly enhanced selectivity for Mg 2+ , as a consequence of a 110‐fold reduction in affinity for Ca 2+ .…”

“…By using UV/Visible absorption spectroscopy, APDAP has been shown to display a significantly enhanced selectivity for Mg 2+ , as a consequence of a 110‐fold reduction in affinity for Ca 2+ . The incorporation of the APDAP chelate into an alkynyl‐naphthalene fluorophore has also enabled Mg 2+ concentrations to be studied in a competitive binding environment . Such behaviour suggests that the APDAP chelate has the potential to be used to monitor Mg 2+ fluxes in regions of the body not previously explored, because of their elevated Ca 2+ concentrations …”

“…The incorporation of the APDAP chelate into an alkynyl‐naphthalene fluorophore has also enabled Mg 2+ concentrations to be studied in a competitive binding environment . Such behaviour suggests that the APDAP chelate has the potential to be used to monitor Mg 2+ fluxes in regions of the body not previously explored, because of their elevated Ca 2+ concentrations …”