Designing Simple Tridentate Ligands for Highly Luminescent Europium Complexes

Graphical Abstract

Europium Union: Benzimidazole-substituted pyridine-2-carboxylates efficiently sensitise europium luminescence in neutral homoleptic nine-coordinate complexes with quantum yields reaching 71 % in the solid state (see graphic) and 52 % in CH₂Cl₂; the complexes display high absorption, with molar absorption coefficients in the range (5–6)×10⁴ M⁻¹ cm⁻¹.

Abstract

A series of tridentate benzimidazole-substituted pyridine-2-carboxylic acids have been prepared with a halogen, methyl or alkoxy group in the 6-position of the benzimidazole ring, which additionally contains a solubilising N-alkyl chain. The ligands form neutral homoleptic nine-coordinate lanthanum, europium and terbium complexes as established from X-ray crystallographic analysis of eight structures. The coordination polyhedron around the lanthanide ion is close to a tricapped trigonal prism with ligands arranged in an up–up–down fashion. The coordinated ligands serve as light-harvesting chromophores in the complexes with absorption maxima in the range 321–341 nm (ε=(4.9–6.0)×10⁴ M⁻¹ cm⁻¹) and triplet-state energies between 21 300 and 18 800 cm⁻¹; the largest redshifts occur for bromine and electron-donor alkoxy substituents. The ligands efficiently sensitise europium luminescence with overall quantum yields () and observed lifetimes (τ_obs) reaching 71 % and 3.00 ms, respectively, in the solid state and 52 % and 2.81 ms, respectively, in CH₂Cl₂ at room temperature. The radiative lifetimes of the Eu(⁵D₀) level amount to τ_rad=3.6–4.6 ms and the sensitisation efficiency η_sens=(τ_rad/τ_obs) is close to unity for most of the complexes in the solid state and equal to approximately 80 % in solution. The photophysical parameters of the complexes correlate with the triplet energy of the ligands, which in turn is determined by the nature of the benzimidazole substituent. Facile modification of the ligands makes them promising for the development of brightly emissive europium-containing materials.