EUROPIUM(III) LUMINESCENCE AND INTRAMOLECULAR ENERGY-TRANSFER STUDIESOF POLYOXOMETALLOEUROPATES

The photoexcitation in the oxygen-to-metal (M = Nb or W) charge transf er (O-->M lmct) bands of two X-ray crystallographically characterized polyoxometalloeuropates, 9{[Eu3O(OH)(3)(H2O)(3)](2)Al-2(Nb6O19)(5)}.47 H(2)O and [Eu(H2O)(8)](3)K2H3[(GeTi3W9O37)(2)O-3].13H(2)O, induced Eu3 + emission due to the D-5(0)-->F-7(J) (J = 0-4) transition with a sing le-exponential decay. In the latter compound an additional D-5(1)-->F- 7(J) transition with a weak intensity (with the relative intensity of about 9% of the D-5(0)-->F-7(J) emission at 4.2 K) was observed. No ob servation of the D-5(1)-->F-7(J) emission in the former compound was a scribed to the D-5(1)-D-5(0) cross-relaxation: Eu3+(D-5(1)) + Eu3+(F-7 (0)) --> Eu3+(D-5(0)) + Eu3+(F-7(3)). Both nonradiative deactivation o f the D-5(0) state and intramolecular energy transfer from the O-->M l mct states to Eu3+ are discussed together with the luminescence proper ties of four other structurally characterized polyoxotungsto(or molybd o)europates, Na-9[Eu(W5O18)(2)].32H(2)O, K15H3[Eu-3(H2O)(3)(SbW9O33)(W 5O18)(3)].25.5H(2)O, 4](12)H2H[Eu-4(H2O)(16)(MoO4)(Mo7O24)(4)].13H(2)O , and [Eu-2(H2O)(12)][Mo8O27].6H(2)O, on the basis of differences in t he number of aqua and hydroxo ligands at the first coordination sphere of the Eu3+ site, the Eu ... Eu distance in the molecule, and the str ucture of the polyoxometalate ligands among six compounds. A plot of t he deviation of the reciprocal D-5(0) lifetime from that of Na-9[Eu(W5 O18)(2)].32H(2)O containing an anhydrous Eu3+ Site versus total number of aqua and hydroxo ligands coordinating Eu3+ indicates a good linear ity irrespective of the coordination geometry, if the mean distance be tween Eu and aqua or hydroxo oxygen atoms is less than 2.5 Angstrom. T he kinetic analysis of the luminescence reveals that the highly symmet rical polyoxometalate ligand favors the effective nonradiative deactiv ation of the O-->M lmct excitation energy due to a small disparity bet ween the O-->M lmct excited and ground states and that the energy tran sfer into the D-5(0) and D-5(1) states in the polyoxometaloeuropates o ccurs via the O-->M lmct triplet states.