Crystallographic characterization of the polyoxotungstate [Eu3(H2O)3(SbW9O33)(W5O18)3]18– and energy transfer in its crystalline lattices

Abstract

The potassium salt of the new mixed-polyoxotungstate anion [Eu₃(H₂O)₃(SbW₉O₃₃)(W₅O₁₈)₃]^18–(1) has been prepared from WO₃, Sb₂O₃, Eu(NO₃)₃·6H₂O, and KOH in water and isolated in crystalline form as K₁₅H₃[Eu₃(H₂O)₃(SbW₉O₃₃)(W₅O₁₈)₃]·25.5 H₂O. It crystallizes in the monoclinic space group C2/m, with a= 30.250(7), b= 18.568(5), c= 22.101 (6)Å, β= 109.19(8)°, and Z= 4. A central Eu₃(H₂O)₃ core is co-ordinated by a B-type α-SbW₉O₃₃ unit and three W₅O₁₈ units with tetrahedral conformation. Each Eu³⁺ in the core exhibits eight-co-ordination by oxygen atoms belonging to H₂O, SbW₉O₃₃, and W₅O₁₈ units. The intramolecular energy transfer from the O→W charge-transfer levels for the polyoxotungstate crystalline lattices to the emitting ⁵D₀ level of Eu³⁺ takes place efficiently at least over 6.9 Å which is the largest distance between W and Eu atoms in the anion. A comparison of the lifetime and quantum yield of the emission among (1), [Eu(W₅O₁₈)₂]^9–, and [Eu(SiW₁₁O₃₉)₂]^13– indicates that the hopping of a d¹ electron between WO₆ octahedra is the predominant deactivation channel of the O→W charge-transfer levels, which reduces drastically the communication with the excited levels of Eu³⁺.