All chemical reagents were purchased commercially and used without further purification. X-ray powder diffraction (XRD) was performed on ZTU-6 using a Bruker D8 Advance diffractometer (Cu-Kα radiation, λ = 0.154 nm). The thermal stability of ZTU-6 was tested using a thermogravimetric analyser (Mettler Toledo, Switzerland). Luminescence tests were performed using an Edinburgh FLS980. The LED fluorescence performance of ZTU-6 was measured using a Hangzhou HAAS-2000 photoelectric colour-integrated tester.

Based on previous literature (Zhang et al., 2011), m-H₂BDC (167 mg, 1 mmol), 1,10-Phen (180 mg, 1 mmol), a certain amount of Eu(NO₃)₃·6H₂O, and Tb(NO₃)₃·6H₂O were dissolved in 3 mL N, N′-dimethylformamide (DMF) and 3 mL water in a 25 mL polytetrafluoroethylene reactor and then heated in an oven (120°C) for 72 h and subsequently cooled to room temperature. The prepared samples were washed three times with fresh DMF and acetone and dried at room temperature to obtain the crystal material ZTU-6 (yield 41% based on the m-H₂BDC ligand). Elemental analysis results (%) of C₅₀H₃₅Eu_0.095Tb_1.905N₅O₁₂ (1215.04): theoretical values C, 49.42; H, 2.90; N, 5.76; experimental values C, 49.15; H, 2.599; N, 5.64. The ratio of terbium to europium was 24.07:1.20 in ZTU-6, which was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES).

The ZTU-6 crystal was fully ground, mixed with AB silica gel in a mass ratio of 1:1, and encapsulated on a commercial 365 nm UV LED chip to form the LED device. The LED device was heated at 150°C for 2 h, and its luminescence properties were tested using a HAAS-2000 instrument at room temperature. In addition, ZTU-6 was combined with BaMgAl₁₀O₁₇:Eu²⁺ blue powder (BAM) and silicate yellow–green powder [(Sr,Ba)₂SiO₄:Eu²⁺] at a mass ratio of 3:1:6 and was encapsulated on a commercial 395 nm LED chip to form a warm WLED device (Li et al., 2020).

m-H₂BDC, 1,10-Phen, and Ln³⁺ ions were used as examples of mixed lanthanide organic frameworks (ZTU-6) under hydrothermal conditions. The purity of ZTU-6 was assessed by analysing its XRD pattern obtained using a Bruker D8 Advance instrument. The results shown in Figure 1C demonstrate that the experimental XRD peaks of ZTU-6 corresponded to the simulated XRD peaks, confirming the successful preparation of ZTU-6 in its pure phase (Zhang et al., 2011).

The asymmetric unit of ZTU-6 has one Ln³⁺ ion, an m-BDC²⁻ ligand, one phenanthroline, and half a dimethylamine molecule. The central Ln³⁺ ions in ZTU-6 are nine-coordinated with the seven oxygen atoms from five m-BDC²⁻ ligands and the two nitrogen atoms from 1,10-Phen (Figure 1A). Each Ln³⁺ ion connects to oxygen atoms, forming four bridging structures and two chelate structures with binuclear [Ln₂(COO)₆] secondary structure units (SBUs) (Zhang et al., 2011). In the structure, the m-BDC²⁻ ligand adopts two coordination modes. The first m-BDC²⁻ ligand adopts the (k ²-μ ₂)-(k ¹-k ¹)-μ ₄ mode, which connects the [Ln₂(COO)₆] SBU, forming a two-dimensional structure along the A-axis. The second m-BDC²⁻ ligand adopts the k ²-(k ¹-k ¹)-μ ₃ mode, which connects the adjacent two-dimensional layers (Zhang et al., 2011), forming a three-dimensional frame structure with the typical pcu topology (Figure 1B; Supplementary Figure S1).

The thermal stability of ZTU-6 was tested under nitrogen at a heating rate of 10°C/min in the range of 20°C–800°C; weight loss of guest dimethylamine molecules occurred between 20°C and 300°C. The frameworks began to decompose after 550°C (Figure 1D; Supplementary Figure S2), indicating the high thermal stability of ZTU-6.

Under excitation at λ_ex = 365 nm, ZTU-6 emits a bright orange–yellow light with the characteristic emission (⁵ D ₄→⁷ F ₅) of Tb³⁺ ions at 544 nm and the characteristic emission (⁵ D ₀→⁷ F ₂) of Eu³⁺ ions at 613 nm (Figure 2), which indicates that the Tb³⁺ ions and Eu³⁺ ions simultaneously enter the framework at a ratio of 24.07:1.20 (Zhang et al., 2011; Li et al., 2020). Although the content of Eu³⁺ ions in ZTU-6 is very low, the emission intensity is still dominant, indicating that Eu³⁺ and Tb³⁺ ions are in the same framework and that Eu³⁺ ions can be sensitised by Tb³⁺ ions (Zhang et al., 2011; Li et al., 2020). Furthermore, the quantum yield and fluorescence lifetime of ZTU-6 were 79.15% and 1.09 m, respectively. The ZTU-6 crystal material was fully mixed with AB silica gel in a 1:1 mass ratio and then encapsulated on a commercial 365 nm UV LED chip to obtain orange-light LED devices. At 20 mA, its CIE coordinates are (0.55, 0.37) (Figure 2), and its CCT is 1572 K.

A warm WLED device was created by mixing ZTU-6 with BAM blue powder and silicate yellow–green powder at a mass ratio of 3:1:6 and then encapsulating it on a commercial 395 nm LED chip. The warm WLED device emitted blue light at 460 nm from the BAM blue powder, the characteristic emission of Tb³⁺ ions at 544 nm, the characteristic emission of Eu³⁺ ions at 613 nm, and a yellow region from the commercial silicate yellow–green powder (Figure 3). At 20 mA, the device emitted warm white light with CIE chromaticity coordinates of (0.38,0.36), a CRI of 93.4, and a CCT of 3908 K, demonstrating excellent CRI and CCT and providing design ideas and theoretical references for developing new WLED devices (Wang et al., 2019; Gutiérrez et al., 2020; Karmakar and Li, 2022; Zhang et al., 2022).