Acridone-Benzimidazole Ring-Fused Ligands: A New Class of Sensitizers of Lanthanide Luminescence via Low-Energy Excitation

Two monotopic tridentate ligands, namely HLAB1 and HLAB2 have been synthesized which feature an acridone chromophore fused on a benzimidazolepyridine framework. They differ from each other by their connection points between a N-methylacridone and a N-methylbenzimidazole chromophoric moieties. Both ligands self-assemble with Ln(III) ions in neat acetonitrile to form thermodynamically stable neutral complexes of general formula [Ln(L-ABX)(3)] (log beta(13) in the range 22-25). Subsequent photophysical investigations conducted in the same solvent demonstrate that ligand HLAB1 sensitizes europium luminescence (Q(Eu)(L) = 10% and tau(Eu) = 0.93 ms) whereas ligand HLAB2 sensitizes the luminescence of NIR-emitting Ln(III) ions, in particular Yb-III (Q(Yb)(L) = 0.86 % and tau(Yb) = 29.3 mu s). The sensitization efficiencies eta(sens) of both ligands have been determined for these two complexes and found to be around 50-60%. The main advantage of these ligands is their excitation wavelength which lies in the visible range (410-430 nm).