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

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+.