A new class of dual fluorescent chemosensors for nitroaromatic compounds (NACs) based on phosphonated pyrene derivatives is reported, showing high selectivity towards trinitrotoluene (TNT). The strong intermolecular interactions (π-π stacking and hydrogen bonding) allow high fluorescence quenching with visual detection in short response times.

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Photoluminescent isotypical phosphonate-based metal–organic frameworks, [Ln­(H5btp)]·2H2O [where Ln3+ = Tb3+ (1) or Gd3+ (2) and H8btp = [1,1′-biphenyl]-3,3′,5,5′-tetrayltetrakis­(phosphonic acid)] prepared under solvo­(hydro)­thermal conditions exhibit a three-dimensional supramolecular structure built up from phosphonate linkers bridging Ln3+ ions. Single-crystal X-ray diffraction of 1 revealed hydrogen bonds along the a-axis between the crystallization water molecules and the phosphonate groups. The network features lozenge-shaped tubular channels with intermolecular interactions between adjacent organic ligand molecules. Crystal packing forces bring together two ligand biphenyl groups in a parallel arrangement, forming an excimer. A ratiometric luminescent thermometer operative close to room temperature was built based on the excimer and the 5D4 → 7F5 Tb3+ emissions, exhibiting a maximum relative thermal sensitivity of 1.26%·K–1 and a minimum temperature uncertainty of 0.75 K at 319 K. This performance is similar to that of a thermometer based on the ligand singlet S 2,1 → S 0 and metal 5D4 → 7F5 transitions. Thus, the same MOF material provides two separate temperature data sets. This work shows the possibility of using excimer emissions in ratiometric luminescence thermometry.

Phosphonate- and yttrium-based metal-organic frameworks (MOFs), formulated as [Y(Hbtp)]·5.5HO (1), [Y(Hbtp)]·2.5HO (2), (HO)[Y(Hbtp)(Hbtp)]·HO (3), and [Y(Hbtp)]·HO·0.5(MeOH) (4), were prepared using a "green" microwave-assisted synthesis methodology which promoted the self-assembly of the tetraphosphonic organic linker [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonic acid) (Hbtp) with Y cations. This new family of functional materials, isolated in bulk quantities, exhibits a remarkable breathing effect. Structural flexibility was thoroughly studied by means of X-ray crystallography, thermogravimetry, variable-temperature X-ray diffraction, and dehydration and rehydration processes, ultimately evidencing a remarkable reversible single-crystal to single-crystal (SC-SC) transformation solely through the loss and gain of crystallization solvent molecules. Topologically, frameworks remained unaltered throughout this interconversion mechanism, with all compounds being binodal 6,6-connected network with a Schäfli symbol of {4.6}{4.6.8}. Results show that this is one of the most stable and thermally robust families of tetraphosphonate-based MOFs synthesized reported to date. Porous materials 2 and 3 were further studied to ascertain their performance as heterogeneous catalysts and proton conductors, respectively, with outstanding results being registered for both materials. Compound 2 showed a 94% conversion of benzaldehyde into (dimethoxymethyl)benzene after just 1 h of reaction, among the best results registered to date for MOF materials. On the other hand, the protonic conductivity of compound 3 at 98% of relative humidity (2.58 × 10 S cm) was among the highest registered among MOFs, with the great advantage of the material to be prepared using a simpler and sustainable synthesis methodology, as well as exhibiting a good stability at ambient conditions (temperature and humidity) over time when compared to others.

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