Storage of methane and freon by interstitial van der Waals confinement

Pillar[5]arene-based supramolecular organic frameworks for highly selective CO2-capture at ambient conditions

Low-density, solid-state, porous supramolecular organic frameworks are constructed using pillarenes. The frameworks have a honeycomb-like structure, permanent porosity, high thermal stability, and selective and reversible sorption properties toward CO2. The exceptionally selective CO2-sorption properties (375/1, 339/1) of one framework over N2 and CH4 indicate potential applications in CO2-capture for post-combustion power plants and natural gas sweetening.

Amphiphilic inclusion spaces for various guests and regulation of fluorescence intensity of 1,8-bis(4-aminophenyl)anthracene crystals

A host framework for inclusion of various guest molecules was investigated by preparation of inclusion crystals of 1,8-bis(4-aminophenyl)anthracene (1,8-BAPA) with organic solvents. X-ray crystallographic analysis revealed construction of the same inclusion space incorporating 1,8-BAPA and eight guest molecules including both non-polar (benzene) and polar guests (N,N-dimethylformamide, DMF). Fluorescence efficiencies varied depending on guest molecule polarity; DMF inclusion crystals exhibited the highest fluorescence intensity (ΦF=0.40), four times as high as that of a benzene inclusion crystal (ΦF=0.10). According to systematic investigations of inclusion phenomena, strong host–guest interactions and filling of the inclusion space led to a high fluorescence intensity. Temperature-dependent fluorescence spectral measurements revealed these factors effectively immobilised the host framework. Although hydrogen bonding commonly decreases fluorescence intensity, the present study demonstrated that such strong interactions provide excellent conditions for fluorescence enhancement. Thus, this remarkable behaviour has potential application toward sensing of highly polar molecules, such as biogenic compounds.

Fluorocarbon adsorption in hierarchical porous frameworks

Metal-organic frameworks comprise an important class of solid-state materials and have potential for many emerging applications such as energy storage, separation, catalysis and bio-medical. Here we report the adsorption behaviour of a series of fluorocarbon derivatives on a set of microporous and hierarchical mesoporous frameworks. The microporous frameworks show a saturation uptake capacity for dichlorodifluoromethane of >4 mmol g(-1) at a very low relative saturation pressure (P/Po) of 0.02. In contrast, the mesoporous framework shows an exceptionally high uptake capacity reaching >14 mmol g(-1) at P/Po of 0.4. Adsorption affinity in terms of mass loading and isosteric heats of adsorption is found to generally correlate with the polarizability and boiling point of the refrigerant, with dichlorodifluoromethane > chlorodifluoromethane > chlorotrifluoromethane > tetrafluoromethane > methane. These results suggest the possibility of exploiting these sorbents for separation of azeotropic mixtures of fluorocarbons and use in eco-friendly fluorocarbon-based adsorption cooling.

Anti-sieve effect in guest inclusion by thiacalix[4]arene giving a surge in thermal stability of its clathrates prepared by solid-phase guest exchange

The unusual exclusion of smaller guests by host may be avoided by solid-phase exchange in preparation of clathrates, which thus become extremely thermally stable.

Reversible and irreversible chemisorption in nonporous-crystalline hybrids

The tools of synthetic chemistry allow us to fine-tune the reactivity of molecules at a level of precision not yet accessible with inorganic solids. We have investigated hybrids that couple molecules to the superior thermal and mechanical properties of solids. Herein we present, to the best of our knowledge, the first demonstration of reactivity between hybrid perovskites and substrates. Reaction with iodine vapor results in a remarkable expansion of these materials (up to 36 % in volume) where new covalent CI bonds are formed with retention of crystallinity. These hybrids also show unusual examples of reversible chemisorption. Here, solid-state interactions extend the lifetime of molecules that cannot be isolated in solution. We have tuned the half-lives of the iodinated structures from 3 h to 3 days. These nonporous hybrids drive substrate capture and controlled release through chemical reactivity. We illustrate the strengths of the hybrid by considering radioactive iodine capture.

How much do van der Waals dispersion forces contribute to molecular recognition in solution?

The emergent properties that arise from self-assembly and molecular recognition phenomena are a direct consequence of non-covalent interactions. Gas-phase measurements and computational methods point to the dominance of dispersion forces in molecular association, but solvent effects complicate the unambiguous quantification of these forces in solution. Here, we have used synthetic molecular balances to measure interactions between apolar alkyl chains in 31 organic, fluorous and aqueous solvent environments. The experimental interaction energies are an order of magnitude smaller than estimates of dispersion forces between alkyl chains that have been derived from vaporization enthalpies and dispersion-corrected calculations. Instead, it was found that cohesive solvent-solvent interactions are the major driving force behind apolar association in solution. The results suggest that theoretical models that implicate important roles for dispersion forces in molecular recognition events should be interpreted with caution in solvent-accessible systems.

Phase and glass transitions observed by adiabatic calorimetry of host p-tert-butylcalix[4]arene and guest toluene inclusion crystal, suggesting the progress of the combined order-disorder process of the host-guest molecules

Heat capacities of crystalline p-tert-butylcalix[4]arene·toluene host-guest compound were measured in a temperature range between 3 K and 345 K by adiabatic calorimetry. The crystal showed one second-order phase transition at 256.4 K accompanied by wide heat-capacity tails on both the high and low temperature sides and two first-order phase transitions with heat-capacity peaks at 127 K and 139 K. The total entropy of a sequence of the phase transitions was assessed experimentally to be 47 J K(-1) mol(-1), indicating that the number of allowed distinguishable molecular configurations is over 100 in the high-temperature disordered state. Many such distinguishable configurations are understood to be produced by a special intermolecular interaction of C-H···(π-electron) bonds formed between p-tert-butylcalix[4]arene and toluene molecules, and the transitions were interpreted as originating from orientational order-disorder of both the guest toluene molecule and the tert-butyl groups of host arene in their combination.

Through-the-annulus threading of the larger calix[8]arene macrocycle

A complete study of the through-the-annulus threading of the larger calix[8]arene macrocycle with di-n-alkylammonium cations has been performed in the presence of the "superweak" TFPB counterion. Thus, it was found that such threading occurs only upon partial preorganization of the calix[8]arene macroring by intramolecular bridging. In particular, 1,5-bridged calix[8]arenes with a meta- or para-xylylene bridge (2 and 3) gave pseudo[2]rotaxanes in which one dialkylammonium axle (4a-4e(+)) was threaded into one of the two subcavities of the calix[8]-wheel. Conformational studies by using chemical shift surface maps and DFT calculations evidenced a 3/4-cone geometry for these subcavities. Higher pseudorotaxane K(ass) values were obtained for calix[8]-wheels 2 and 3, with respect to calix[6]-host 1a, due to the cooperative effect of their two subcavities. Dynamic NMR studies on calix[8]-pseudorotaxanes evidenced a direct correlation between K(ass) (and ΔG(ass)) values and energy barriers for calix inversion due to the effectiveness of thread templation. In accordance with DFT calculations, an endo-alkyl preference, over the endo-benzyl one, was observed by threading calix[8]-wheel 3 with the directional n-butylbenzylammonium axle 4d(+).

Concomitant pseudopolymorphs of 10-deacetyl baccatin III

Three new solvates [mono-dimethyl sulfoxide (mono-DMSO), mono-dimethyl acetamide (mono-DMA) and mono-dimethyl formamide (mono-DMF)] of 10-Deacetyl baccatin III, were generated by slow evaporation in DMSO, DMF, and DMSO/DMA (1:1) solvent systems respectively. Two concomitant forms mono-DMSO(a new form) and di-DMSO (a known form) were obtained in the DMSO solvent system. Yet two other concomitant forms mono-DMA (a new form) and di-DMSO (a known form) were obtained in DMSO/DMA (1:1) solvent system. A fourth solvate mono-DMF (a new form) was crystallized in unimolar ratio using DMF as a solvent. These solvates were characterized using powder X-ray diffraction, differential scanning calorimeter, thermogravimetric analysis (TGA), and spectroscopic [(13)C solid-state nuclear magnetic spectroscopy, solution (1)H NMR, and Fourier transform infrared] techniques. The interactions between host and guest molecules were elucitated by single-crystal X-ray diffraction data. In all the cases, guest molecules are connected to the host molecules by O-H∙∙∙O hydrogen bonds. A remarkable difference in the desolvation onset temperatures of di- and mono-DMSO solvates was observed which was also featured by a corresponding weight loss during TGA analysis.

Simulations of guest transport in clathrates of Dianin's compound and hydroquinone

Clathrates have been proposed for use in a variety of applications including gas storage, mixture separation and catalysis due to the potential for controlled guest diffusion through their porous lattices. Here molecular dynamics simulations are employed to study guest transport in clathrates of hydroquinone (HQ) and Dianin's compound (DC). Systems investigated were HQ with methanol and acetonitrile, and DC with methanol and ethanol. Simulations were set up with one guest in the pore, two guests in the pore and one vacancy in the pore and a filled pore, and free-energy barriers for movement between cavities of the pore were estimated for all cases. Comparison between these simulations indicates that guest transport most likely proceeds by molecules moving from full to empty cavities consecutively, one by one, rather than in a concerted manner. Thus, the presence of empty cavities is very important for guest transport, which becomes more energetically demanding in fully loaded systems. Flexibility of the host can assist guest transport. In the studied DC clathrates transport occurs via an intermediate conformation in which the hydroxyl group of the alcohol guest molecule participates in the hydrogen-bonded ring of the host. We also address the issue of the number of methanol guest molecules that DC accommodates, for which conflicting information exists. We found that this is likely to be temperature dependent and suggest that under some conditions the system is most likely non-stoichiometric.

Solution-processable molecular cage micropores for hierarchically porous materials

Macroscopic inorganic porous beads are imbibed with a "porous molecular additive" by simple solution processing techniques, resulting in controllable loading and increased surface area. The porous additive consists of soluble organic cage molecules that precipitate as microporous crystals when solutions of opposite chirality are mixed.

Modular assembly of metal-organic supercontainers incorporating sulfonylcalixarenes

A new strategy for the design of container molecules is presented. Sulfonylcalix[4]arenes, which are synthetic macrocyclic containers, are used as building blocks that are combined with various metal ions and tricarboxylate ligands to construct metal-organic "supercontainers" (MOSCs). These MOSCs possess both endo and exo cavities and thus mimic the structure of viruses. The synthesis of MOSCs is highly modular, robust, and predictable. The unique features of MOSCs are expected to provide exciting new opportunities for the exploration of their functional applications.

Molecular recognition of hydrocarbon guests by a supramolecular capsule formed by the 4:4 self-assembly of tris(Zn(2+)-cyclen) and trithiocyanurate in aqueous solution

We have previously reported that the trimeric Zn(2+)-cyclen complex (tris(Zn(2+)-cyclen), [Zn(3)L(1)](6+)) and the trianion of trithiocyanuric acid (TCA(3-)) assembled in a 4:4 ratio to form a cuboctahedral supramolecular cage, [(Zn(3)L(1))(4)(TCA(3-))(4)](12+) (hereafter referred to as a Zn-cage), in neutral aqueous solution (cyclen=1,4,7,10-tetraazacyclododecane). Herein, we examined the molecular recognition of C(1)-C(12) hydrocarbons (C(n)H((2n+2)) (n≈1-12)), cyclopentane, cyclododecane, cis-decalin, and trans-decalin by the Zn-cage under normal atmospheric pressure. This cage complex was also able to encapsulate guest molecules that had larger volumes than that of the inner cavity of the Zn-cage, thereby suggesting that the inner shape of the Zn-cage was flexible. Computational simulations of Zn-cage-guest complexes provided support for this conclusion. Moreover, the solvent-accessible surface areas (SASA) of the Zn-cage host, guest molecules, and the Zn-cage-guest complexes were calculated and the data were used to explain the order of stability determined by the guest-replacement experiments. The storage of volatile molecules in aqueous solution by the Zn-cage is also discussed.