Multifunctional luminescence sensing and white light adjustment of lanthanide metal–organic frameworks constructed from the flexible cyclotriphosphazene-derived hexacarboxylic acid ligand

Abstract

Considering that cyclotriphosphazene polycarboxylic acid is a kind of organic ligand with fantastic structures and performances and the unique luminescence characteristics of rare earth ions, a series of porous lanthanide metal–organic frameworks (Ln-MOFs) (CH₃)₂NH₂[Ln₃(HCPCP)_1.5(CH₃COO)]·6DMA (Ln = Ce (1), Sm (2), Eu (3), Tb (4), HCPCP = hexa(4-carboxyphenoxy)cyclotriphosphazene, and DMA = N,N-dimethylacetamide) were synthesized with novel topological network structures. Compound 4 exhibited a sensitive recognition of –NO₂, and had a fluorescence quenching phenomenon for seven kinds of nitro aromatic compounds (NACs). In particular, it showed the best fluorescence response to 2,4,6-trinitrophenol (TNP) and 2,4-dinitrophenol (DNP), and the K_SV values were 2.86 × 10⁵ M⁻¹ and 8.21 × 10⁴ M⁻¹, and the limit of detection (LOD) values were 0.20 μM and 0.71 μM, respectively. At the same time, we successfully doped different concentrations of Eu³⁺ into compound 4 to obtain a series of doped Ln-MOF materials x%Eu³⁺@4 (x = 0.5, 2.5, 5, 7.5, 10, 15 and 20). With the increase of Eu³⁺ doping ratios, the characteristic peaks of Tb³⁺ and Eu³⁺ changed regularly, and the energy transfer from Tb³⁺ to Eu³⁺ ions occurred. By changing the excitation wavelength of the samples with different Eu³⁺ doping concentrations, a higher quality white light emitting material 7.5%Eu³⁺@4 (λ_ex = 340 nm) was finally obtained, with a CIE coordinate of (0.3268, 0.3212).