Synthesis, characterization and spectroscopic investigation of new tetrakis(acetylacetonato)thulate(III) complexes containing alkaline metals as countercations

In this work a series of tetrakis complexes C[Tm(acac)4], where C+=Li+, Na+ and K+ countercations and acac=acetylacetonate ligand, were synthesized and characterized for photoluminescence investigation. The relevant aspect is that these complexes are water-free in the first coordination sphere. The emission spectra of the tetrakis Tm3+-complexes present narrow bands characteristic of the 1G4→3H6 (479 nm), 1G4→3F4 (650 nm) and 1G4→3H5 (779 nm) transitions of the Tm3+ ion, with the blue emission color at 479 nm as the most prominent one. The lifetime values (τ) of the emitting 1G4 level of the C[Tm(acac)4] complexes were 344, 360 and 400 ns for the Li+, Na+ and K+ countercations, respectively, showing an increasing linear behavior versus the ionic radius of the alkaline ion. An efficient intramolecular energy transfer process from the triplet state (T) of the ligands to the emitting 1G4 state of the Tm3+ ion is observed. This fact, together with the absence of water molecules in first coordination sphere, allows these tetrakis Tm3+-complexes to act as efficient blue light conversion molecular devices.