Towards Building Blocks for Supramolecular Architectures Based on Azacryptates

Recent applications of organic cages in sensing and separation processes in solution

Organic cages are three-dimensional polycyclic compounds of great interest in the scientific community due to their unique features, which generally include simple synthesis based on the dynamic covalent chemistry strategies, structural tunability and high selectivity. In this feature article, we present the advances over the last ten years in the application of organic cages as chemosensors or components in chemosensing devices for the determination of analytes (pollutants, analytes of biological interest) in complex aqueous media including wine, fruit juice, urine. Details on the recent applications of organic cages as selective (back-)extractants or masking agents for potential applications in relevant separation processes, such as the plutonium and uranium recovery by extraction, are also provided. Over the last ten years, organic cages with permanent porosity in the liquid and solid states have been highly appreciated as porous materials able to discriminate molecules of different sizes. These features, combined with good solvent processability and film-forming tendency, have proved useful in the fabrication of membranes for gas separation, solvent nanofiltration and water remediation processes. An overview of the recent applications of organic cages in membrane separation technologies is given.

Synthesis and VCD Spectroscopic Characterization of a Series of Azacryptands from a Chiral Valine-Based Derivative of Tris(2-aminoethyl)amine (TREN)

Utilizing experimental and computational vibrational circular dichroism (VCD) spectroscopy, we explored the conformational preferences of a series of chiral C3 -symmetric octaazacryptands with tris(2-aminoethyl)-amine head groups derived from valine. While the spectra of the smallest azacryptand with p-phenyl linkers and its elongated derivative with p-biphenyls linker were found to match well with the computed spectra, the computed conformational preferences of the m-biphenyl-based azacryptand did not seem to reflect the conformations dominating in chloroform solution. A detailed analysis revealed that structural changes resulting in a collapsed cage structure gave a notably better match with the experiment. It could subsequently be concluded from the VCD analysis, that the octaazacryptands prefer a collapsed structure, which is not predicted by density functional theory (DFT) calculations as the global minimum structures. These findings are expected to have consequences also for future studies on inclusion complexes of such azacryptands.

PEEK-WC-Based Mixed Matrix Membranes Containing Polyimine Cages for Gas Separation

Membrane-based processes are taking a more and more prominent position in the search for sustainable and energy-efficient gas separation applications. It is known that the separation performance of pure polymers may significantly be improved by the dispersion of suitable filler materials in the polymer matrix, to produce so-called mixed matrix membranes. In the present work, four different organic cages were dispersed in the poly(ether ether ketone) with cardo group, PEEK-WC. The m-xylyl imine and furanyl imine-based fillers yielded mechanically robust and selective films after silicone coating. Instead, poor dispersion of p-xylyl imine and diphenyl imine cages did not allow the formation of selective films. The H2, He, O2, N2, CH4, and CO2 pure gas permeability of the neat polymer and the MMMs were measured, and the effect of filler was compared with the maximum limits expected for infinitely permeable and impermeable fillers, according to the Maxwell model. Time lag measurements allowed the calculation of the diffusion coefficient and demonstrated that 20 wt % of furanyl imine cage strongly increased the diffusion coefficient of the bulkier gases and decreased the diffusion selectivity, whereas the m-xylyl imine cage slightly increased the diffusion coefficient and improved the size-selectivity. The performance and properties of the membranes were discussed in relation to their composition and morphology.

Sensing and Liquid-Liquid Extraction of Dicarboxylates Using Dicopper Cryptates

We report the investigation of dicopper(II) bistren cryptate, containing naphthyl spacers between the tren subunits, as a receptor for polycarboxylates in neutral aqueous solution. An indicator displacement assay for dicarboxylates was also developed by mixing the azacryptate with the fluorescent indicator 5-carboxyfluorescein in a 50:1 molar ratio. Fluorimetric studies showed a significant restoration of fluorophore emission upon addition of fumarate anions followed by succinate and isophthalate. The introduction of hexyl chains on the naphthalene groups created a novel hydrophobic cage; the corresponding dicopper complex was investigated as an extractant for dicarboxylates from neutral water into dichloromethane. The liquid-liquid extraction of succinate-as a model anion-was successfully achieved by exploiting the high affinity of this anionic guest for the azacryptate cavity. Extraction was monitored through the changes in the UV-visible spectrum of the dicopper complex in dichloromethane and by measuring the residual concentration of succinate in the aqueous phase by HPLC-UV. The successful extraction was also confirmed by 1H-NMR spectroscopy. Considering the relevance of polycarboxylates in biochemistry and in the environmental field, e.g., as waste products of industrial processes, our results open new perspectives for research in all contexts where recognition, sensing, or extraction of polycarboxylates is required.

Organic Cages as Building Blocks for Mechanically Interlocked Molecules: Towards Molecular Machines

The research on systems able to perform controllable motions under external stimuli arises great interest in the scientific community. Over the years, a library of innovative devices has been produced, classified in different categories according to the molecular or supramolecular level of motion. This minireview aims to highlight some representative studies, in which organic cages are used as building blocks for mechanically interlocked molecules, and in which intramolecular motions are triggered by external input. However, the application of organic cages in the construction of molecular machines is hardly achieved. A good compromise must actually be reached, between flexibility and rigidity of the cage's framework for an effective control of the intra- and/or intermolecular motion in the final mechanical device. Our final goal is to stimulate researchers' curiosity towards cage-like molecules, so that they take on the challenge of converting a cage into a molecular machine.