Synthesis and photophysical properties of crystalline [EuW 10 O 36 ] 9- -based polyoxometalates with lanthanide ions as counter cations

To synthesize crystalline [EuW10O36]9--based polyoxometalates with lanthanide ion as counter cation is still quite challenging due to the rapid reaction of polyanion and lanthanide cation, and the rigorous pH range needed in the reaction process. Here the stability of reaction precursor, Na9EuW10O36·32H2O (EuW10), in various pH aqueous solution is first explored. Then the crystalline [EuW10O36]9--based polyoxometalates with Ln(Ⅲ) ion (Eu3+, Sm3+ and Tb3+) as counter cation, namely Eu3EuW10O36·24H2O (S1), Sm3EuW10O36·23H2O (S2) and Tb3EuW10O36·25H2O (S3), are synthesized by facile precipitation method at the pH value of 6.5, and characterized by powder XRD, FTIR spectra, elemental analysis and thermogravimetric analysis, confirming the crystal structures of the as-synthesized compounds are isostructural to EuW10, all containing polyanion of [EuW10O36]9-. Photophysical properties such as fluorescent spectra and decay lifetime are also investigated. Besides the characteristic emission of Eu3+ (5D0→7F0,1,2,3,4) for S1 and S2, S3 also exhibits characteristic emission of Tb3+ (5D4→7F6,4). We attribute it to the different energy transfer mechanisms, that is, S2 obeys resonance mechanism from Sm3+ to Eu3+ while S3 the cross-relaxation mechanism from Tb3+ to Eu3+.