Europium(III) Luminescence and Intramolecular Energy Transfer Studies of Polyoxometalloeuropates

Abstract

The photoexcitation in the oxygen-to-metal (M = Nb or W) charge transfer (O→M lmct) bands of two X-ray crystallographically characterized polyoxometalloeuropates, Na7H19{[Eu3O(OH)3(H2O)3]2Al2(Nb6O19)5}·47H2O and [Eu(H2O)8]3K2H3[(GeTi3W9O37)2O3]·13H2O, induced Eu3+ emission due to the 5D0→7FJ (J = 0−4) transition with a single-exponential decay. In the latter compound an additional 5D1→7FJ transition with a weak intensity (with the relative intensity of about 9% of the 5D0→7FJ emission at 4.2 K) was observed. No observation of the 5D1→7FJ emission in the former compound was ascribed to the 5D1−5D0 cross-relaxation: Eu3+(5D1) + Eu3+(7F0) → Eu3+(5D0) + Eu3+(7F3). Both nonradiative deactivation of the 5D0 state and intramolecular energy transfer from the O→M lmct states to Eu3+ are discussed together with the luminescence properties of four other structurally characterized polyoxotungsto(or molybdo)europates, Na9[Eu(W5O18)2]·32H2O, K15H3[Eu3(H2O)3(SbW9O33)(W5O18)3]·25.5H2O, [NH4]12H2[Eu4(H2O)16(MoO4)(Mo7O24)4]·13H2O, and [Eu2(H2O)12][Mo8O27]·6H2O, on the basis of differences in the number of aqua and hydroxo ligands at the first coordination sphere of the Eu3+ site, the Eu···Eu distance in the molecule, and the structure of the polyoxometalate ligands among six compounds. A plot of the deviation of the reciprocal 5D0 lifetime from that of Na9[Eu(W5O18)2]·32H2O containing an anhydrous Eu3+ site versus total number of aqua and hydroxo ligands coordinating Eu3+ indicates a good linearity irrespective of the coordination geometry, if the mean distance between Eu and aqua or hydroxo oxygen atoms is less than 2.5 Å. The kinetic analysis of the luminescence reveals that the highly symmetrical polyoxometalate ligand favors the effective nonradiative deactivation of the O→M lmct excitation energy due to a small disparity between the O→M lmct excited and ground states and that the energy transfer into the 5D0 and 5D1 states in the polyoxometaloeuropates occurs via the O→M lmct triplet states.