Polyoxometalate-based metal–organic framework NENU-5 hybrid materials for photoluminescence tuning by introducing lanthanide ions and their functionalized soft ionogel/thin film

Abstract

Polyoxometalate-based metal–organic framework (POM-MOF) hybrid materials have received increasing attention, and have the advantages of both POMs and MOFs. Herein, we encapsulate lanthanide(III) (Ln³⁺) cations (Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺) in the channels of [Cu₂(BTC)_4/3(H₂O)₂]₆[HPMo₁₂O₄₀]·(C₄H₁₂N)₂·xH₂O (x = 25–30, NENU-5) nanocrystals by exchanging the (C₄H₁₂N)⁺ with Ln³⁺. The structure, composition, thermal stability, morphology, and absorption of ultraviolet and visible light of the obtained hybrids are characterized by PXRD, ICP, FTIR, SEM and UV-vis DRS. The photophysical properties of these materials are investigated in detail under ultraviolet radiation, displaying the characteristic emission line of the corresponding Ln³⁺ ions. These physical characterizations prove that these hybrid materials exhibit a uniform mesoporous structure, good crystallization and multi-color luminescence. Furthermore, photofunctional Ln³⁺ functionalized POM-MOF hybrids are dispersed in an ionic liquid compound (1-(2-hydroxyethyl)-3-methylimidazolium bromide) which has little effect on luminescence to produce a soft ion gel. Finally, we manufacture their thin films by assembling the hybrids on a quartz plate using a polymer linker to achieve further practical application. Both the fluorescence of lanthanide hybrid materials and that of thin films can be adjusted by different Ln³⁺ compositions or excitation wavelengths and finally reach white light output.