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

The metal-based luminescence of the Eu and Tb complexes of the octadentate ligands L-1 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 lambda(exc) greater than or equal to 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 = 21300 cm(-1)) and not in the unprotonated (E-T = 22000 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](+)).