Metal–Organic Framework Luminescence in the Yellow Gap by Codoping of the Homoleptic Imidazolate _∞³[Ba(Im)₂] with Divalent Europium

The rare case of a metal-triggered broad-band yellow emitter among inorganic–organic hybrid materials was achieved by in situ codoping of the novel imidazolate metal–organic framework _∞³[Ba­(Im)₂] with divalent europium. The emission maximum of this dense framework is in the center of the yellow gap of primary light-emitting diode phosphors. Up to 20% Eu²⁺ can be added to replace Ba²⁺ as connectivity centers without causing observable phase segregation. High-resolution energy-dispersive X-ray spectroscopy showed that incorporation of even 30% Eu²⁺ is possible on an atomic level, with 2–10% Eu²⁺ giving the peak quantum efficiency (QE = 0.32). The yellow emission can be triggered by two processes: direct excitation of Eu²⁺ and an antenna effect of the imidazolate linkers. The emission is fully europium-centered, involving 5d → 4f transitions, and depends on the imidazolate surroundings of the metal ions. The framework can be obtained by a solvent-free in situ approach starting from barium metal, europium metal, and a melt of imidazole in a redox reaction. Better homogeneity for the distribution of the luminescence centers was achieved by utilizing the hydrides BaH₂ and EuH₂ instead of the metals.