Luminescence and Energy Transfer Phenomena in Tb3+/Eu3+-Mixed Polyoxometallolanthanoates K15H3[Tb1.4Eu1.6(H2O)3(SbW9O33)(W5O18)3]·25.5H2O and Na7H19[Tb4.3Eu1.7O2(OH)6(H2O)6Al2(Nb6O19)5]·47H2O
1999-09-30T00:00:00Z (GMT) by
The energy dissipation of Tb3+/Eu3+ cations in both heterolanthanide multinuclear polyoxometalates, K15H3[Tb1.4Eu1.6(H2O)3(SbW9O33)(W5O18)3]·25.5H2O and Na7H19[Tb4.3Eu1.7O2(OH)6(H2O)6Al2(Nb6O19)5]·47H2O is studied by crystal structures, emission and excitation spectra, and emission decay dynamics. The excitation of the Tb3+ 7F6 → 5D4 transitions produces not only the emission lines of Tb3+ but also those of Eu3+, accompanied by nonexponential rise and decay curves of the emission from Tb3+ and Eu3+. 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 Tb3+ → Eu3+ energy transfer is identified as a Förster−Dexter-type energy transfer from Tb3+ (donor) to Eu3+ (acceptor). At low temperatures 5D4(Tb) + 7F0(Eu) → 7F4(Tb) + 5D0(Eu) governs the transfer process, and at high temperatures it is governed by 5D4(Tb) + 7F1(Eu) → 7F5(Tb) + 5D1(Eu), 5D4(Tb) + 7F1(Eu) → 7F4(Tb) + 5D0(Eu), and 5D4(Tb) + 7F2(Eu) → 7F5(Tb) + 5D1(Eu) interactions which involve the thermally populated 7F1 and 7F2 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 × 104 and 4.7 × 105 s-1, and the critical radii at 4.2 K are 10.3 and 10.0 Å for K15H3[Tb1.4Eu1.6(H2O)3(SbW9O33)(W5O18)3]·25.5H2O (with Tb−Eu separation of 5.05 Å) and Na7H19[Tb4.3Eu1.7O2(OH)6(H2O)6Al2(Nb6O19)5]·47H2O (with 3.76 Å separation), respectively. The low symmetry (Cs for the former and C1 for the latter) of the LnO8 (Ln = Tb and Eu) coordination polyhedra allows the nonvanishing electric dipole transition probability for the 7FJ ↔ 5D0 (J = 0,1) transitions which leads to a faster transter rate at high temperatures.