Luminescence and Energy Transfer Phenomena in Tb³⁺/Eu³⁺-Mixed Polyoxometallolanthanoates K₁₅H₃[Tb_1.4Eu_1.6(H₂O)₃(SbW₉O₃₃)(W₅O₁₈)₃]·25.5H₂O and Na₇H₁₉[Tb_4.3Eu_1.7O₂(OH)₆(H₂O)₆Al₂(Nb₆O₁₉)₅]·47H₂O

The energy dissipation of Tb³⁺/Eu³⁺ cations in both heterolanthanide multinuclear polyoxometalates, K₁₅H₃[Tb_1.4Eu_1.6(H₂O)₃(SbW₉O₃₃)(W₅O₁₈)₃]·25.5H₂O and Na₇H₁₉[Tb_4.3Eu_1.7O₂(OH)₆(H₂O)₆Al₂(Nb₆O₁₉)₅]·47H₂O is studied by crystal structures, emission and excitation spectra, and emission decay dynamics. The excitation of the Tb^{3+ 7}F₆ → 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[Tb_1.4Eu_1.6(H₂O)₃(SbW₉O₃₃)(W₅O₁₈)₃]·25.5H₂O (with Tb−Eu separation of 5.05 Å) and Na₇H₁₉[Tb_4.3Eu_1.7O₂(OH)₆(H₂O)₆Al₂(Nb₆O₁₉)₅]·47H₂O (with 3.76 Å separation), respectively. The low symmetry (C_s 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 ⁷F_J ↔ ⁵D₀ (J = 0,1) transitions which leads to a faster transter rate at high temperatures.