White Light Emission Properties of Defect Engineered Metal–Organic Frameworks by Encapsulation of Eu³⁺ and Tb³⁺

Defect engineering of metal–organic frameworks (MOFs) has already found applications in gas adsorption, storage, and heterogeneous catalysis. However, the role of defects in determining electrical, optical, and mechanical properties of MOFs is yet to be explored. Herein, we demonstrate the white light emission properties of two defect engineered MOFs (UiO-66 and MIL-53­(Al)) through the encapsulation of lanthanide ions (Eu³⁺ and Tb³⁺). The defect sites are created by the use of a modulator (4-acetyl benzoic acid; AB) during the synthesis of MOFs. The color of the lanthanide functionalized UiO-66 could be tuned from white (UiO-66-AB) to green (Tb@UiO-66-AB), pale pink (Eu@UiO-66-AB), and yellowish-white (Tb/Eu@UiO-66-AB) when placed under a laboratory ultraviolet lamp (365 nm). Under the same conditions, the lanthanide functionalized MIL-53­(Al) showed color changes from blue (MIL-53­(Al)-AB) to cyan (Tb@MIL-53­(Al)-AB), pink (Eu@MIL-53­(Al)-AB), and white (Tb/Eu@MIL-53­(Al)-AB). The luminescence properties of all the lanthanide encapsulated modulated MOFs have been well studied. Notably, by varying the excitation wavelength, a close to white light was obtained for the Tb/Eu@UiO-66-AB material (λ_ex = 340 nm; CIE coordinates (x = 0.3290, y = 0.3934)) and Tb/Eu@MIL-53­(Al)-AB (λ_ex = 320 nm; CIE coordinates (x = 0.3204, y = 0.3873)). These observations indicate that lanthanide encapsulation at defect sites of MOFs allows more room for design and color tuning. The high thermal stabilities, good crystallinities, and tunable luminescence properties of MOFs reveal that these materials may have potential applications in white light emitting devices.