In the last two decades, metal−organic frameworks (MOFs) have appeared as new multifunctional materials that show a variety of framework architectures depending on the choice of metal ions and ligands. Among several features that have been displayed by MOF, one of the important properties is its luminescent behavior that can arise from the ligand unit, metal ions, or the cavity guest molecules. Ratiometric fluorescence (RF) sensors can be made by changing the fluorescence intensity of emission peaks associated with multiemission probes, but the synthesis of such materials is always a challenge, as it requires multifluorescence groups having the same excitation with diverse emission wavelengths. In the case of MOFs, such features can be included by modifying the ligand unit and selecting the suitable metal ions or by using a desired guest molecule. As a result, MOF can act as a promising candidate for the construction of ratiometric fluorescence (RF) sensors. Thus, our aim in this review is to summarize some of the recent development that has been done in terms of synthesis, mechanism, and applications of ratiometric sensors by using MOF materials.

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Syntheses, structures, and magnetic properties of two hexanuclear [5×1 + 1×1] lanthanide cocrystal complexes [Ln5(L)4(LH2)2(tfa)4]·[Ln(tfa)4]·2CH3OH [Ln = Dy(1) and Tb(2)] derived from the polyhydroxy ligand 2‐(hydroxymethyl)‐6‐carbaldehyde‐4‐methylphenol (LH2) are reported. Compounds 1 and 2 crystallize in the triclinic system with space group P1. X‐ray crystallography reveals that 1 and 2 are cocrystals comprising one cationic pentanuclear [Ln5(L)4(LH2)2(tfa)4]+ unit and one anionic mononuclear [Ln(tfa)4]–unit as well as two molecules of methanol as solvent of crystallization. Both complexes form a [2.2] spirocyclic topology fashioned core due to merging of two triangular geometries composed of Ln(III) ions through a common vertex. The central Ln(III) ion in the spirocyclic pentanuclear assembly is eight‐coordinate with distorted square antiprism geometry. On the other hand, the peripheral eight‐coordinate Ln(III) ions are in a distorted trigonal dodecahedron geometry. The comprehensive magnetic study reveals that compound 1 displays slow relaxation of magnetism.