Elucidating white light emissions in Tm/Dy codoped polyoxometalates: a color tuning and energy transfer mechanism study.

The double-tartaric bridging Tm-substituted POM derivative [N(CH3)4]6K3H7[Tm(C4H2O6)(α-PW11O39)]2·27H2O (1) was successfully synthesized and well characterized by various physico-chemical analyses. Furthermore, the mixed Dy3+/Tm3+ ion-based POM derivatives [N(CH3)4]6K3H7[DyxTm1-x(C4H2O6)(α-PW11O39)]2·27H2O (3-8) were first synthesized and confirmed by PXRD and IR spectra, indicating compounds 3-8 are isomorphic with 1. The detailed analyses of Ln-O-W bond angle and coordinated aqua ligands around emitting Ln3+ ions have revealed that the mentioned negative factors do not effectively affect the luminescence of emitting Ln3+ ions in 1-8. Investigations of PL emissions reveal that 3-8 can display color-tunable PL properties, emitting color from blue to white to yellow. The study of time-resolved emission spectroscopy of 6 indicates the energy can transfer from the LMCT excited state of POM fragments to Tm3+ and/or Dy3+ ions. Furthermore, the decreased luminescent lifetime of Tm3+ ions in 3-8 reasonably verifies the energy transfer from Tm3+ to Dy3+ ions to efficiently facilitate emissions of the Dy3+ centre.