Selective Sensitization of Eu(III) and Tb(III) Emission with Triarylboron-Functionalized Dipicolinic Acids

Three triarylboron- (Mes2BAr-) functionalized dipicolinic acids, namely, 4-(4-(dimesitylboranyl)-2,3,5,6-tetramethylphenyl)pyridine-2,6-dicarboxylic acid (H2 L1), 4-(4-(4-dimesitylboranyl)-2,3,5,6-tetramethylphenyl)-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic acid (H2 L2), and 4-(4-(4-dimesitylboranyl) phenyl)-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic acid (H2 L3), have been designed and synthesized. Lanthanide(III) complexes with the general formula of [NBu4]3[LnL3] (Ln = Eu or Tb; L = L1, L2, or L3) were obtained. The new triarylboron-functionalized ligands were found to be effective in the selective sensitization of the emissions of Eu(III) and Tb(III) ions with a high quantum efficiency (e.g., 0.54 for [NBu4]3[TbL1 3] in the solid state) upon excitation at ∼330 nm. An intraligand charge-transfer (ILCT) transition from the mesityl or aryl group to the boron or boron-aryl unit was found to play a key role in the activation of the Eu(III) and Tb(III) emissions, based on TD-DFT computational data and luminescence titration experiments performed using fluoride and cyanide ions.