Luminescent sensors for pH, pO2, halide and hydroxide ions using phenanthridine as a photosensitiser in macrocyclic europium and terbium complexes

Abstract

The metal-based luminescence of the Eu and Tb complexes of the octadentate ligands L1 and L2a/2b is a function of pH in aqueous solution: in [EuL1], the red-emission is switched on following N-protonation of the sensitising phenanthridine group (pKa = 4.2, pKS1 = 4.4) with a luminescence enhancement of over 500 for λexc 370 nm. With the Tb complex, the green luminescence switches off following protonation (pKT1 = 5.7) because back-energy transfer occurs rapidly to the low-lying triplet state only in the protonated complex (ET = 21 300 cm–1) and not in the unprotonated (ET = 22 000 cm–1) form. In the corresponding N-methylated complexes, the metal-based emission is quenched by halide ions (e.g.Ksv –1 = 40 mmol dm–3 for Cl– in [MeEuL1]+) and, for the terbium complexes only, by molecular oxygen (Ksv–1 = 45 Torr for [MeTbL1]+).