Modulating the Near-Infrared Luminescence of Neodymium and Ytterbium Complexes with Tridentate Ligands Based on Benzoxazole-Substituted 8-Hydroxyquinolines
An improved synthesis of 2-(2′-benzothiazole)- and 2-(2′-benzoxazole)-8-hydroxyquinoline ligands that combine a tridentate N,N,O-chelating unit for metal binding and extended chromophore for light harvesting is developed. The 2-(2′-benzoxazole)-8-hydroxyquinoline ligands form mononuclear nine-coordinate complexes with neodymium, [Nd- (κ3-ligand)3], and an eight-coordinate complex with ytterbium, [Yb(κ3-ligand)2 · (κ1-ligand) · H2O], as verified by crystallographic characterization of five complexes with four different ligands. The chemical stability of the complexes increases when the ligand contains 5,7-dihalo-8-hydroxyquinoline versus an 8-hydroxyquinoline group. The complexes feature a ligand-centered visible absorption band with a maximum at 508-527 nm and an intensity of (7.5-9.6) × 103 M-1 · cm-1. Upon excitation with UV and visible light within ligand absorption transitions, the complexes display characteristic lanthanide luminescence in the near-infrared at 850-1450 nm with quantum yields and lifetimes in the solid state at room temperature as high as 0.33% and 1.88 μs, respectively. The lanthanide luminescence in the complexes is enhanced upon halogenation of the 5,7- positions in the 8-hydroxyquinoline group and upon the addition of electron-donating substituents to the benzoxazole ring. Facile modification of chromophore units in 2-(2′-benzoxazole)-8-hydroxyquinoline ligands provides means for controlling the luminescence properties of their lanthanide complexes.