A luminescent terbium phosphonate metal–organic framework (MOF) exhibits a remarkable capability to rapidly detect traces of nitroaromatic explosives (as low as 66 ppb) through luminescence quenching. Its sensitivity, fast response, facile synthesis, low usage (less than 0.1 mg), cheapness, and good stability make it one of the most powerful nitroaromatic explosive sensors known.

Three new luminescent zinc phosphonates, [Zn(H2L)(H2O)2]·H2O (1), [Zn(H2L)] (2), and [Zn(H3L)2(H2O)2]·3H2O (3), have been hydrothermally synthesized based on 1-hydroxy-2-(imidazo(1,2-a)pyridin-3-yl)ethylidene-1,1-diphosphonic acid (H4L). Single-crystal X-ray diffraction reveals that compounds 1–3 are a binuclear cluster, two-dimensional polar sandwich-like framework, and mononuclear unit, respectively. Compounds 1–2 exhibit remarkable capabilities to detect the trace of 2,4,6-trinitrophenol rapidly and quantitatively through luminescent quenching. It is worth to note that compounds 1–2 can be readily recovered and reused. On the other hand, compound 1 features dual emissions which facilitates it to serve as a useful self-calibrated ratiometric luminescent thermometer in the temperature range of 77–243 K, as well as a more reliable and sensitive intensity-based luminescent thermometer in the temperature range of 10–60 °C. Furthermore, compound 1 has good stability under simulated physiological conditions, indicating that it can be potentially utilized as a luminescent thermometer for biological applications.

Hydrothermal reactions of Zn(CH3COO)2·2H2O with two new triazine-based phosphonic acids, 2,4,6-triglyphosate-1,3,5-triazine (H9L1) and 4,6-biglyphosate-2-hydroxyl-1,3,5-triazine (H6L2), afforded two new zinc phosphonates, namely, [(C5H14N2)Zn3(HL1)(H2O)]·3.5H2O (1) and [(C5H14N2)Zn2L2]·3H2O (2), respectively. The two zinc phosphonates were structurally characterized through single-crystal X-ray diffraction. Compound 1 is a three-dimensional (3D) framework in that Zn–O–P chains are connected by HL18– anions with hendecadentate modes. In compound 2, each L26– anion links four Zn2 binuclear units to form a 3D framework. In both compounds 1 and 2, the protonated 2-methylpiperazines are encapsulated into the channels of 3D frameworks. Thermogravimetric analysis and powder X-ray diffraction reveal that compounds 1 and 2 are thermally stable up to 200 and 160 °C under an air atmosphere, respectively. Compounds 1 and 2 display near-UV and purple luminescence with maximal bands at 377 and 402 nm, respectively. It worth noting that the luminescence of solids 1 and 2 can be preserved after compounds 1 and 2 are heated at 200 and 160 °C for 2 h under an air atmosphere, respectively. In addition, the ion-exchange property of compound 1 was also studied.

Hydrothermal reactions of ZnII or MnII ion with 1-C10H7-CH2N(CH2COOH)(CH2PO3H2)(H3L1), 3-HOOC-C6H4-CH2N(CH2COOH)(CH2PO3H2)(H4L2), and 4-HOOC-C6H4-CH2N(CH2COOH)(CH2PO3H2)(H4L3) afforded six new layered metal phosphonates, namely, [Zn3(HL1)3]·3H2O (1), [Zn(HL1)]·CH3COOH (2), [Zn(H2L2)] (3), [Mn(H3L2)2] (4), [Mn(H3L3)2] (5), and [Zn(H2L3)]·H2O (6). Compounds 1–6 are characterized by single-crystal X-ray diffraction (XRD), powder XRD, IR spectroscopy, elemental analysis, and thermogravimetric analysis (TGA). In compounds 1–3 and 6, each [ZnO4] tetrahedron shares three corners with three neighboring [PCO3] tetrahedra to generate a Zn–O–P layer, which consists of eight and four member rings (MRs). While in compounds 4 and 5, each [MnO6] octahedron shares four corners with neighboring four [PCO3] tetrahedra into a Mn–O–P layer containing eight MRs. The organic groups hang on two sides of a Zn–O–P or Mn–O–P layer to form two-dimensional (2D) sandwich-like frameworks of compounds 1–6. TGA and powder XRD reveal that 2D frameworks of compounds 2, 3, 4, and 6 are thermally stable up to 180, 250, 230, and 250 °C under an air atmosphere, respectively. It is interesting that compounds 1–2 display bright UV luminescence, which can be irreversibly quenched by UV irradiation. In addition, the blue luminescence of solid 6 can be transformed into blue-green emission by simply a heating treatment.

Hydrothermal reactions of lanthanide oxide or lanthanide chloride with 3-ammonium-1-hydroxypropylidene-1,1-diphosphonic acid (+H3NCH2CH2C(OH)(PO3H2)(PO3H–), H4L) afforded seven new lanthanide phosphonates, namely, [Ln(HL)] (Ln = Eu (1), Tb (2), Sm (3), Gd (4), Er (5), Nd (6), and Pr (7)). Compounds 1–7 are characterized by single-crystal X-ray diffraction, powder XRD, elemental analysis, IR spectroscopy, and thermogravimetric analysis (TGA). Compounds 1–5 crystallize in the orthorhombic Pnma space group, whereas compounds 6 and 7 crystallize in the monoclinic P2(1)/c space group. Compounds 1–7 exhibit a similar 3D open framework with eight-membered ring channels, which are occupied by organic pendants of CH2CH2NH3. TGA and PXRD reveal that compound 1 is thermally stable up to 350 °C under an air atmosphere. It is interesting that compounds 1 and 2 display bright red and green luminescence, respectively, which can be further enhanced obviously upon cooling to 10 K. It is worth noting that the bright red emission of compound 1 can be preserved after heat-treatment at 350 °C under an air atmosphere. Compounds 6 and 7 can give off NIR luminescence and near UV emission, respectively. Furthermore, the magnetic properties of solids 1 and 3–6 have also been studied.

Two new zinc phosphonates with formulas [Zn18(L)12(H2N(CH2)4NH2)2]·25.5H2O (1) and [Zn9(L)6(HN(C2H4NH2)2)2]·10H2O (2) have been synthesized starting from zinc acetate, 1-C10H7-CH2N(CH2COOH)(CH2PO3H2) (H3L) and 1,4-butanediamine (for 1)/diethylenetriamine (for 2). Compounds 1 and 2 are characterized by single-crystal X-ray crystallography, powder XRD, elemental analysis, IR spectroscopy, and thermogravimetric (TG) analysis. In the crystal structures of compounds 1 and 2, Zn7 heptanuclear clusters are bridged by [Zn2(H2N(CH2)4NH2)] and [Zn(HN(C2H4NH2)2)] fragments, respectively, to form two-dimensional (2D) frameworks. Compound 1 is a dynamic framework upon the dehydration–hydration process and displays bright purple emission, while the framework of compound 2 is stable up to 250 °C under an air atmosphere. And the luminescence of compound 2 can be tuned by heating treatment.

Hydrothermal reactions of Cd2+, Mn2+, Co2+ and Ni2+ ions with 4-pyridyl-CH2N(CH2COOH)(CH2PO3H2) (H3L) afforded four new metal phosphonates, namely, (H3O)4[Cd7(L)6]·16H2O (1), (H3O)4[Mn7(L)6]·17H2O (2), [Co(HL)(H2O)]·2H2O (3) and [Ni(HL)(H2O)]·2H2O (4). Compounds 1 and 3 are isomorphous with compounds 2 and 4, respectively. Solid 1 is a 3D open-framework consisting of heptanuclear cadmium clusters. The framework of solid 1 is thermally stable up to 250 °C under an air atmosphere. In 3, HL2− anions exhibit pentadentate modes to combine Co2+ ions into a 2D hybrid layer. It is interesting that solid 1 displays bright blue luminescence, which can be enhanced upon cooling to 10 K. Furthermore, after heat-treatment at 250 °C under an air atmosphere, solid 1-250 also gives off blue emission. And the magnetic properties of 2–4 have also been studied.

Two new molecular zinc phosphonates, (HN(C2H5)3)4[Zn7(L)6]·2H2O (1) and [Zn9(L)6(H2N(CH2)2NH(CH2)2NH(CH2)2NH2)2]·18H2O (2) (H3L = 1-C10H7-CH2N(CH2COOH)(CH2PO3H2)), which consist of heptanuclear and nonanuclear zinc clusters, respectively, display bright luminescence for sensing UV radiation.

Two isomorphous 3D metal phosphonates, [M2(H2L)] (M = Zn (1), Co (2); H6L = 1,4-((H2O3PCH2)(HOOCCH2)NCH2)2 C6H4, are reported; Solid 1 is stable up to 300 °C under an air atmosphere, displays bright tunable luminescence and offers reversible sensing of nitrobenzene.

A zinc diphosphonate, (H2G)0.5[Zn3(L)(HL)]·3H2O (G = 4-picolylamine, H4L = 1-hydroxyl-2-(3-pyridyl)ethylidene-1,1-diphosphonic acid) displays a 3D pillared open-framework with ellipsoid-like 12MR channels, bright tunable luminescence and thermally stable up to 250 °C under an air atmosphere.

A luminescent terbium phosphonate metal–organic framework (MOF) exhibits a remarkable capability to rapidly detect traces of nitroaromatic explosives (as low as 66 ppb) through luminescence quenching. Its sensitivity, fast response, facile synthesis, low usage (less than 0.1 mg), cheapness, and good stability make it one of the most powerful nitroaromatic explosive sensors known.