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

Abstract 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 μ 2 ‐bridge to two Ln atoms and as a μ 3 ‐bridge to all three atoms, with metal⋅⋅⋅metal distances being around 3.7 Å. For 1 ( [Yb 2 Er] + ), 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 1 H NMR of 6 ( [La 2 Eu] + ). 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.