pH Switched sensitisation of europium(III) by a dansyl group

The Eu(III), Tb(III) and Y(III) complexes of an eight-coordinate ligand containing a pendant dansylamidoethyl group were synthesised. Reversible intramolecular binding of the dansylamide nitrogen was demonstrated by UV absorption and luminescence pH titration, although no sensitisation of the Eu(III) or Tb(III) luminescent emission was seen due to lack of population of the chromophore triplet state. However, sensitisation of Eu(III) was achieved by protonation of the dimethylamino group on the naphthyl ring. Deprotonation constants of the dimethylamino (pK_a1) and dansylamido (pK_a2) group were determined in 0.1 M NaCl. Values of pK_a1 3.9 (ligand), 3.6 (Eu), 3.7 (Tb), 3.8 (Y) and pK_a2 (log K_MLH) 10.8 (ligand), 5.8 (Eu), 6.0 (Tb), 6.2 (Y) were obtained.