Luminescence and Energy Transfer Phenomena in Tb<sup>3+</sup>/Eu<sup>3+</sup>-Mixed Polyoxometallolanthanoates K<sub>15</sub>H<sub>3</sub>[Tb<sub>1.4</sub>Eu<sub>1.6</sub>(H<sub>2</sub>O)<sub>3</sub>(SbW<sub>9</sub>O<sub>33</sub>)(W<sub>5</sub>O<sub>18</sub>)<sub>3</sub>]·25.5H<sub>2</sub>O and Na<sub>7</sub>H<sub>19</sub>[Tb<sub>4.3</sub>Eu<sub>1.7</sub>O<sub>2</sub>(OH)<sub>6</sub>(H<sub>2</sub>O)<sub>6</sub>Al<sub>2</sub>(Nb<sub>6</sub>O<sub>19</sub>)<sub>5</sub>]·47H<sub>2</sub>O

1999-09-30T00:00:00Z (GMT) by Toshihiro Yamase Haruo Naruke
The energy dissipation of Tb<sup>3+</sup>/Eu<sup>3+</sup> cations in both heterolanthanide multinuclear polyoxometalates, K<sub>15</sub>H<sub>3</sub>[Tb<sub>1.4</sub>Eu<sub>1.6</sub>(H<sub>2</sub>O)<sub>3</sub>(SbW<sub>9</sub>O<sub>33</sub>)(W<sub>5</sub>O<sub>18</sub>)<sub>3</sub>]·25.5H<sub>2</sub>O and Na<sub>7</sub>H<sub>19</sub>[Tb<sub>4.3</sub>Eu<sub>1.7</sub>O<sub>2</sub>(OH)<sub>6</sub>(H<sub>2</sub>O)<sub>6</sub>Al<sub>2</sub>(Nb<sub>6</sub>O<sub>19</sub>)<sub>5</sub>]·47H<sub>2</sub>O is studied by crystal structures, emission and excitation spectra, and emission decay dynamics. The excitation of the Tb<sup>3+</sup> <sup>7</sup>F<sub>6</sub> → <sup>5</sup>D<sub>4</sub> transitions produces not only the emission lines of Tb<sup>3+</sup> but also those of Eu<sup>3+</sup>, accompanied by nonexponential rise and decay curves of the emission from Tb<sup>3+</sup> and Eu<sup>3+</sup>. There is no significant exchange interaction between the lanthanide ions, as a result of the coordination of aqua and/or hydroxo ligands to the lantahanide ions. The mechanism of the Tb<sup>3+</sup> → Eu<sup>3+</sup> energy transfer is identified as a Förster−Dexter-type energy transfer from Tb<sup>3+</sup> (donor) to Eu<sup>3+</sup> (acceptor). At low temperatures <sup>5</sup>D<sub>4</sub>(Tb) + <sup>7</sup>F<sub>0</sub>(Eu) →<sup> 7</sup>F<sub>4</sub>(Tb) + <sup>5</sup>D<sub>0</sub>(Eu) governs the transfer process, and at high temperatures it is governed by <sup>5</sup>D<sub>4</sub>(Tb) + <sup>7</sup>F<sub>1</sub>(Eu) →<sup> 7</sup>F<sub>5</sub>(Tb) + <sup>5</sup>D<sub>1</sub>(Eu), <sup>5</sup>D<sub>4</sub>(Tb) + <sup>7</sup>F<sub>1</sub>(Eu) →<sup> 7</sup>F<sub>4</sub>(Tb) + <sup>5</sup>D<sub>0</sub>(Eu), and <sup>5</sup>D<sub>4</sub>(Tb) + <sup>7</sup>F<sub>2</sub>(Eu) → <sup>7</sup>F<sub>5</sub>(Tb) + <sup>5</sup>D<sub>1</sub>(Eu) interactions which involve the thermally populated <sup>7</sup>F<sub>1</sub> and <sup>7</sup>F<sub>2</sub> levels. The nearest-neighbor energy-transfer rates by electric dipole−dipole interactions between a Tb−Eu pair at 4.2 K are estimated to be 4.5 × 10<sup>4</sup> and 4.7 × 10<sup>5</sup> s<sup>-1</sup>, and the critical radii at 4.2 K are 10.3 and 10.0 Å for K<sub>15</sub>H<sub>3</sub>[Tb<sub>1.4</sub>Eu<sub>1.6</sub>(H<sub>2</sub>O)<sub>3</sub>(SbW<sub>9</sub>O<sub>33</sub>)(W<sub>5</sub>O<sub>18</sub>)<sub>3</sub>]·25.5H<sub>2</sub>O (with Tb−Eu separation of 5.05 Å) and Na<sub>7</sub>H<sub>19</sub>[Tb<sub>4.3</sub>Eu<sub>1.7</sub>O<sub>2</sub>(OH)<sub>6</sub>(H<sub>2</sub>O)<sub>6</sub>Al<sub>2</sub>(Nb<sub>6</sub>O<sub>19</sub>)<sub>5</sub>]·47H<sub>2</sub>O (with 3.76 Å separation), respectively. The low symmetry (<i>C</i><i><sub>s</sub></i> for the former and <i>C</i><sub>1</sub> for the latter) of the LnO<sub>8</sub> (Ln = Tb and Eu) coordination polyhedra allows the nonvanishing electric dipole transition probability for the <sup>7</sup>F<i><sub>J</sub></i> ↔ <sup>5</sup>D<sub>0</sub> (<i>J</i> = 0,1) transitions which leads to a faster transter rate at high temperatures.