Intramolecular sensitisation of lanthanide(iii) luminescence by acetophenone-containing ligands: the critical effect of para-substituents and solvent

Tetraazamacrocyclic ligands have been prepared in which three of the four nitrogen atoms are functionalised with carboxylate donors and the fourth is alkylated with a para-substituted acetophenone group {–CH2C(O)C6H4–X, where X = H, OMe, NMe2}. The europium(III), gadolinium(III) and (for X = H) terbium(III) complexes of these potentially octadentate ligands have been prepared. The unsubstituted and methoxy-substituted ligands sensitise the europium(III) emissive state with high efficiency in aqueous solution. Calculation of the pure radiative lifetime, based on the contribution of the 5D0 → 7F1 transition to the total integrated emission intensity, has allowed an estimate of the efficiency of energy transfer to be made. Sensitisation of europium luminescence also occurs in the complex of the dimethylamino-substituted ligand but the process is highly solvent-dependent: very efficient sensitisation is observed in dichloromethane and DMSO, but the complex is non-emissive in water and only slightly emissive in acetonitrile. UV-Visible absorption spectra indicate that in all the solvents investigated, the carbonyl oxygen of the ligand is directly coordinated to the metal ion. The intensity of the hypersensitive 5D0 → 7F2 transition in the three complexes mirrors the degree of polarisation of the ketone CO bond.