Discrete Heteropolynuclear Yb/Er Assemblies: Switching on Molecular Upconversion Under Mild Conditions

摘要

The salts {Ln(2)Ln*(Hhmq)(3)(OAc)(3)(hfac)(2)(+) Ln*(hfac)(3)(OAc)(MeOH)(-)} (Hhmq=2-methanolquinolin-8-oxide, hfac=hexafluoroacetylacetonate; Ln, Ln*=Er, Gd, Yb) feature a discrete heteronuclear cation consisting of two types of lanthanide atoms. The quinolinoxy O-atom serves as a mu(2)-bridge to two Ln atoms and as a mu(3)-bridge to all three atoms, with metal...metal distances being around 3.7 angstrom. For 1 (Yb2Er(+)), near-infrared downshifted luminescence is switched to competitive upconversion luminescence upon irradiation by a 980 nm laser under an extremely low excitation power (0.288 W cm(-2)) through introduction of fluoride ions. The stability of 1 after addition of fluoride was confirmed by powder X-ray diffraction and multistage mass spectrometry, associated with the H-1 NMR of 6 (La2Eu(+)). More importantly, the at least 20-fold enhancement of the quantum yield in non-deuterated solvents at room temperature under low power densities (2 W cm(-2)) is the highest among the few molecular examples reported.