Highly luminescent charge-neutral europium(iii) and terbium(iii) complexes with tridentate nitrogen ligands

Abstract

We report on the synthesis of tridentate-nitrogen pyrazole–pyridine–tetrazole (L¹H) and pyrazole–pyridine–triazole (L²H) ligands and their complexation with lanthanides (Ln = Gd(III), Eu(III) and Tb(III)) resulting in stable, charge-neutral complexes Ln(L¹)₃ and Ln(L²)₃, respectively. X-ray crystallographic analysis of the complexes with L¹ ligands revealed tricapped trigonal coordination geometry around the lanthanide ions. All complexes show bright photoluminescence (PL) in the solid state, indicating efficient sensitization of the lanthanide emission via the triplet states of the ligands. In particular, the terbium complexes show high PL quantum yields of 65 and 59% for L¹ and L², respectively. Lower PL efficiencies of the europium complexes (7.5 and 9%, respectively) are attributed to large energy gaps between the triplet states of the ligands and accepting levels of Eu(III). The triplet state energy can be reduced by introducing an electron withdrawing (EW) group at the 4 position of the pyridine ring. Such substitution of L¹H with a carboxylic ester (COOMe) EW group leads to a europium complex with increased PL quantum yield of 31%. A comparatively efficient PL of the complexes dissolved in ethanol indicates that the lanthanide ions are shielded against nonradiative deactivation via solvent molecules.