Intensive Upconversion Luminescence of Na-Codoped Rare-Earth Oxides with a Novel RE–Na Heterometallic Complex as Precursor

Four
novel heterometallic RE–Na–organic frameworks, [(RE)­Na3(PZTC)2(H2O)4]·2H2O (RE = Yb (1), Ho (2), Er (3), Y (4); PZTC = pyrazine-2,3,5-tricarboxylate),
were synthesized via solvothermal reactions and characterized by IR,
elemental analysis, and single-crystal X-ray diffraction. The results
show that the four complexes are isostructural. In the frameworks,
the trinuclear Na cluster and RE ion acting as nodes are bridged by
the multifunctional PZTC ligand to give a 3-D framework. Codoping
in the frameworks was realized due to their isostructural characteristics.
The codoped complexes were calcinated at 800 °C to give rise
to the corresponding oxides. Investigation of their photophysical
properties shows that the upconversion luminescence (UCL) of the Ho
system is green while that of the Er system is red upon excitation
at 980 nm. With regard to the luminescence color and intensity, the
Er system is preferable to the Ho system for application in bioimaging.
Both the red and the green UCL of the Ho3+ and Er3+ systems involve a two-photon process. In addition, the UCL mechanism
is given. The UCL comparison of Na-doped oxides with non-Na-doped
oxides indicates that doping Na can greatly enhance the UCL of the
Er system.