Double-Oxalate-Bridging Tetralanthanide Containing Divacant Lindqvist Isopolytungstates with an Energy Transfer Mechanism and Luminous Color Adjustablility Through Eu3+/Tb3+ Codoping

A double-oxalate-bridging tetra-Gd³⁺ containing divacant Lindqvist dimeric isopolytungtate Na₁₀[Gd₂(C₂O₄)(H₂O)₄(OH)W₄O₁₆]₂·30H₂O (Gd₄W₈) was obtained based on the reaction of Na₂WO₄·2H₂O, H₂C₂O₄, and GdCl₃ in aqueous solution. Its dimeric polyoxoanion is established by two divacant Lindqvist [W₄O₁₆]^8- segments connected by a rectangular tetra-nuclearity [Gd₄(C₂O₄)₂(H₂O)₈(OH)₂]⁶⁺ cluster. Notably, neighboring trinuclear [Na₃O₄(H₂O)₁₁]^5- clusters are interconnected to construct a picturesque 1-D sinusoidal Na-O cluster chain. The most outstanding characteristic is that 1-D sinusoidal Na-O cluster chains combine [Gd₂(C₂O₄)(H₂O)₄(OH)W₄O₁₆]₂^10- polyoxoanions together, giving rise to an intriguing 3-D extended porous framework. The red emitter Eu³⁺ ions and green emitter Tb³⁺ ions are first codoped into Gd₄W₈ to substitute Gd³⁺ ions for the exploration of the energy transfer (ET) mechanism between Eu³⁺ and Tb³⁺ ions and the color-tunable PL property in the isopolytungtate system. The PL emission spectra and decay lifetime measurements of the Eu³⁺/Tb³⁺ codoped Gd₄W₈ system illustrate that under excitation at 370 nm, Tb³⁺ ions can transfer energy to Eu³⁺ ions. When the molar concentration of Tb³⁺ ions is fixed at 0.9 and that of the Eu³⁺ ions gradually increases from 0.01 to 0.08, the calculated ET efficiency (η_ET) from Tb³⁺ to Eu³⁺ ions increases from 7.9% for Gd_0.36Tb_3.6Eu_0.04W₈ to 67.3% for Gd_0.08Tb_3.6Eu_0.32W₈. The energy transfer mechanism (Tb³⁺ → Eu³⁺) is a nonradiative dipole-dipole interaction. Furthermore, upon excitation at 370 nm, Eu₄W₈ and Tb₄W₈ show visible red- and green-emitting lights, respectively. When codoping trace amounts of Eu³⁺ ions in Tb₄W₈, under excitation at 370 nm, Tb_3.92Eu_0.08W₈ displays near white-light emission.