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 L^2a/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 (pK_a = 4.2, pK S₁ = 4.4) with a luminescence enhancement of over 500 for λ_exc 370 nm. With the Tb complex, the green luminescence switches off following protonation (pK T₁ = 5.7) because back-energy transfer occurs rapidly to the low-lying triplet state only in the protonated complex (E_T = 21 300 cm^–1) and not in the unprotonated (E_T = 22 000 cm^–1) form. In the corresponding N-methylated complexes, the metal-based emission is quenched by halide ions (e.g.K_sv ^–1 = 40 mmol dm^–3 for Cl^– in [MeEuL¹]⁺) and, for the terbium complexes only, by molecular oxygen (K_sv ^–1 = 45 Torr for [MeTbL¹]⁺).