Mixed Guest Research Articles

Crystallization experiments of the wheel-and-axle host compound, 1,4-phenylene-bis(di-p-tolylmethanol), from pyridine and methylpyridine mixtures

1,4-Phenylene-bis(di-p-tolylmethanol) (H), a host compound with the wheel-and-axle design, formed complexes with organic guest solvents pyridine (PYR) and 2-, 3- and 4-methylpyridine (2MP, 3MP and 4MP). The host: guest (H: G) ratios were 1:3, 1:2, 1:2 and 1:1, respectively. Host crystallization experiments from mixed guests demonstrated H to prefer both PYR and 4MP; however, it was established that these difficult-to-separate (by fractional distillations) guest mixtures cannot be purified/separated by means of H through supramolecular chemistry strategies owing to these selectivities for PYR and 4MP being less than optimal. Single crystal X-ray diffraction experiments showed that each guest compound was held in its complex by means of a classical hydrogen bond with H, and that one of the preferred guests, PYR, experienced a significantly shorter contact of this type than the other guests. Hirshfeld surface analyses demonstrated that PYR was also involved in a greater percentage of (guest)N···H(host) interactions compared with the other guest molecules. Thermal analyses, on the other hand, revealed that 4MP (also a favoured guest species) formed the most stable complex of the four in this investigation. These results with H were compared to those obtained when employing a closely related host compound from a previous report, 1,4-bis(diphenylhydroxymethyl)benzene: while both host species preferred the same guests (PYR and 4MP), the extent of the selectivity of that host compound compared with H in the present work was significantly more enhanced. Thus, minor modifications may deleteriously affect the selectivity behaviour of closely related host compounds.

Selectivity Considerations of the Roof-Shaped Host Compound trans-α,α,α′,α′-Tetrakis(4-chlorophenyl)-9,10-dihydro-9,10-ethanoanthracene-11,12-dimethanol in Mixed Anisoles

The roof-shaped host compound trans-α,α,α′,α′-tetrakis(4-chlorophenyl)-9,10-dihydro-9,10-ethanoanthracene-11,12-dimethanol H1 was assessed for its selectivity behavior in various anisole (ANI) and ortho-, meta-, and para-methylanisole (oMANI, mMANI, and pMANI) mixtures. Guest/guest competition experiments in equimolar binary mixed guest solvents revealed remarkable host selectivities for ANI or mMANI when oMANI or pMANI was also present (90.7–95.3%). Furthermore, the oMANI/pMANI experiment proved H1 to be nearly completely selective for oMANI (91.8%). In ternary experiments, unexpectedly high selectivities toward ANI (82.6%) and mMANI (89.5%) were noted when the solutions were ANI/oMANI/pMANI and oMANI/mMANI/pMANI. Data obtained from binary competition experiments in which the molar quantity of each guest species was sequentially varied correlated exactly with these equimolar experiments, and high selectivity coefficients were calculated for ANI and mMANI when either oMANI or pMANI was also present. Unfortunately, SCXRD analyses on H1·ANI, 2H1·oMANI, and 2H1·pMANI revealed extreme guest disorder, and these could not be modeled. However, in H1·mMANI, the guest molecules were retained in the crystals by means of a close (host)C–H···π(guest) contact (2.97 Å, 143°). Finally, thermal analyses proved less useful in explaining the observed host selectivity. This work demonstrates that H1 is an extremely selective host compound in very many of the anisole mixtures used in this investigation and, therefore, is a highly suitable candidate to effectively separate and purify such mixtures.

Formulating noncovalent interactions to predict structural transition in mixed guest hydrates

AbstractThis work aims at formulating the noncovalent interactions of the mixed guest hydrate lattices of sI and sII structures. The reduced density plots obtained at the B3LYP level with 6‐31G(d) split valence set unveil the crucial contributions of hydrogen bonding, dispersion, van der Waals interaction, and steric effects toward lattice stability. These contributions are rather unreported in the hydrate phase equilibrium modeling. With this research gap, we attempt to formulate the natural regularity in the nonstoichiometric hydrates by the spherical harmonics shape descriptor. The van der Waals and hydrogen‐bond interactions are described with Kihara spherical cell potential and ab initio based calculations, respectively. Further, the ionization potential and polarizability of the guest molecule describe the dispersion interactions. Combining these contributions, we introduce a theoretical framework for the phase equilibrium modeling of mixed guest hydrates. This novel formulation offers various appealing advantages as: significantly reduced parametric structure, standardization of the parameter value for host–host pair, and generalized model framework. Despite of making the proposed theory so simple, it consistently outperforms the existing latest models, which employ various state equations, ab initio based calculations and CSMGem, for a wide variety of systems (total 26 systems tested) with reference to the experimental data.

Theoretical Investigation of the Fusion Process of Mono-Cages to Tri-Cages with CH4/C2H6 Guest Molecules in sI Hydrates.

Owing to a stable and porous cage structure, natural gas hydrates can store abundant methane and serve as a potentially natural gas resource. However, the microscopic mechanism of how hydrate crystalline grows has not been fully explored, especially for the structure containing different guest molecules. Hence, we adopt density functional theory (DFT) to investigate the fusion process of structure I hydrates with CH4/C2H6 guest molecules from mono-cages to triple-cages. We find that the volume of guest molecules affects the stabilities of large (51262, L) and small (512, s) cages, which are prone to capture C2H6 and CH4, respectively. Mixed double cages (small cage and large cage) with the mixed guest molecules have the highest stability and fusion energy. The triangular triple cages exhibit superior stability because of the three shared faces, and the triangular mixed triple cages (large-small-large) structure with the mixed guest molecules shows the highest stability and fusion energy in the triple-cage fusion process. These results can provide theoretical insights into the growth mechanism of hydrates with other mono/mixed guest molecules for further development and application of these substances.

Guest-Host Supramolecular Assembly of Injectable Hydrogel Nanofibers for Cell Encapsulation.

The fibrous architecture of the extracellular matrix (ECM) is recognized as an integral regulator of cell function. However, there is an unmet need to develop mechanically robust biomaterials mimicking nanofibrous tissue topography that are also injectable to enable minimally invasive delivery. In this study, we have developed a fibrous hydrogel composed of supramolecularly assembled hyaluronic acid (HA) nanofibers that exhibits mechanical integrity, shear-thinning behavior, rapid self-healing, and cytocompatibility. HA was modified with methacrylates to permit fiber photo-cross-linking following electrospinning and either "guest" adamantane or "host" β-cyclodextrin groups to guide supramolecular fibrous hydrogel assembly. Analysis of fibrous hydrogel rheological properties showed that the mixed guest-host fibrous hydrogel was more mechanically robust (6.6 ± 2.0 kPa, storage modulus (G')) than unmixed guest hydrogel fibers (1.0 ± 0.1 kPa) or host hydrogel fibers (1.1 ± 0.1 kPa) separately. The reversible nature of the guest-host supramolecular interactions also allowed for shear-thinning and self-healing behavior as demonstrated by cyclic deformation testing. Human mesenchymal stromal cells (hMSCs) encapsulated in fibrous hydrogels demonstrated satisfactory viability following injection and after 7 days of culture (>85%). Encapsulated hMSCs were more spread and elongated when cultured in viscoelastic guest-host hydrogels compared to nonfibrous elastic controls, with hMSCs also showing significantly decreased circularity in fibrous guest-host hydrogels compared to nonfibrous guest-host hydrogels. Together, these data highlight the potential of this injectable fibrous hydrogel platform for cell and tissue engineering applications requiring minimally invasive delivery.

Naturally Occurring Hydrate Formation and Dissociation in Marine Sediment: Experimental Validation

Fundamental understanding of gas hydrate as a natural storehouse of carbon and a potential energy resource found in permafrost layers and marine sediments is crucial in extracting hydrocarbon fuel from hydrate reservoirs. With this, a theoretical framework is grounded involving the complicated physics of naturally occurring hydrate formation and dissociation with pure and mixed guest gases in the unconsolidated silica sand and seawater. The proposed formulation addresses various practical concerns, including the collective influence of pure water, salt ion, and porous medium; surface renewal at the interface between bulk guest and aqueous phase; pore irregularity in heterogeneous porous materials; hydrate growth and decay in those nanometer-sized pores along with interstitial spaces; and influence of surface tension, among others. To show its versatility, the dynamic model framework is tested under various experimental conditions that mimic the actual field conditions in terms of the type of guest gases (i.e., methane, and pure and mixed carbon dioxide); the concentration of salt ions in the liquid phase; the size, shape, and amount of porous silica sand; and the operating temperature and pressure. Under every aforementioned condition, the proposed model outperforms the existing models, which is quantified in terms of the percentage of average absolute relative deviation (AARD). For gas hydrate formation cases, the proposed model keeps AARD in the range of 1.23–9.57%, which is 5.46–19.58% for the existing models. On the other hand, for gas hydrate decomposition, the AARD of the proposed formulation varies from 2.73 to 9.78%, which is reasonably high (11.89–22.26%) for the existing model.

Modeling of Hydrate Dissociation Curves with a Modified Cubic-Plus-Association Equation of State

A modified cubic-plus-association equation of state (CPA EoS) has been presented in recent works that enforces the representation of the pure component critical point, is based on an extended Mathias–Copeman α function, and uses a volume shift to improve liquid densities. In this work, this same version of the CPA model is applied together with the hydrate van der Waals and Platteeuw model for the description of hydrate dissociation curves. The model is applied to the description of both single and mixed guest hydrates, the results being compared with the available experimental data, and the predictions obtained using the commercial hydrates model from the Multiflash software. This analysis shows that, for most cases, this approach is accurate, with results similar to those of Multiflash. The accuracy of the model for describing the three phase compositions in hydrate + liquid + gas systems is also discussed.

Compounds N,N′-bis(9-cyclohexyl-9-xanthenyl)ethylenediamine and its thio derivative, N,N′-bis(9-cyclohexyl-9-thioxanthenyl)ethylenediamine, as potential hosts in the presence of xylenes and ethylbenzene: Conformational analyses and molecular modelling considerations

Two novel crystalline compounds, N,N′-bis(9-cyclohexyl-9-xanthenyl)ethylenediamine (OED) and its thio derivative, N,N′-bis(9-cyclohexyl-9-thioxanthenyl)ethylenediamine (SED), were designed and synthesized in our laboratories, and assessed for their potential as host compounds for the four C8 aromatic compounds, namely o-, m- and p-xylene (o-Xy, m-Xy, p-Xy), and ethylbenzene (EB). Despite the only difference between the two compounds being the heteroatoms in their B rings, immense behaviour differences were noted: only OED displayed host behaviour in these conditions, clathrating all but m-Xy, while SED failed to form complexes with any of the four organic solvents. These observations prompted an investigation into the conformations of OED and SED through single crystal diffraction (SCXRD) analyses as well as computational studies with surprising results. SCXRD was also employed to analyse the three complexes that successfully formed with OED, and thermal analyses (TA) assisted in understanding the selectivity behaviour of OED when presented with mixed guests.

Complexes of host compound (−)-(2R,3R)-2,3-dimethoxy-1,1,4,4-tetraphenylbutane-1,4-diol (DMT) with guests anisole and the methyl-substituted anisoles: host selectivity, thermal and single crystal diffraction considerations

(−)-(2R,3R)-2,3-Dimethoxy-1,1,4,4-tetraphenylbutane-1,4-diol (DMT) was determined to be a highly efficient host compound for anisole and the methyl-substituted anisoles (MA), clathrating each of these organic solvents with 2:1 host:guest ratios. Furthermore, this host displayed selectivity when presented with mixed guests, and guest–guest competition studies involving the methylanisoles revealed a host selectivity order of p-MA > m-MA > o-MA, whilst the addition of unsubstituted anisole to these experiments showed this guest to now be the preferred one (anisole > p-MA > m-MA > o-MA). Single crystal diffraction analyses indicated the absence of hydrogen bonding between host and guest molecules, and that guests were retained within the host crystal only by means of CH–π, π–π stacking and other short interaction types. The latter involved host aromatic carbon or hydrogen atoms and guest methoxyl or methyl hydrogens and aromatic carbons, as well as host and guest methoxyl hydrogens. Results from thermal analyses provided the reason for the observed selectivity order, with complexes comprising preferred guests anisole and p-MA displaying enhanced thermal stabilities relative to those with o- and m-MA.

Inclusion of chlorinated hydrocarbon guests by 9,9′-bianthracene host and its selectivity

Inclusion compounds consisting of 9,9′-bianthracene host and five chlorinated ethene and ethane guests were prepared. All the guest molecules were included in this host in 1:1 ratio as crystalline inclusion compounds as determined by NMR spectroscopy and thermal analyses. X-ray structures of the inclusion compounds attested that the guest molecules were accommodated in cavities formed by the host molecules and the packing mode was dependent on the size and shape of the guest molecules. Competitive inclusion experiments were performed by using mixed guest systems. The ratios of included molecules indicated that the selectivity decreased in the order of CHCl2CH2Cl, Cl2C=CHCl, cis-ClHC=CHCl, CH2ClCH2Cl, and trans-ClHC=CHCl.

Light‐Harvesting Fluorescent Supramolecular Block Copolymers Based on Cyanostilbene Derivatives and Cucurbit[8]urils in Aqueous Solution

AbstractA novel system of light‐harvesting supramolecular block copolymers (SBCPs) in water is demonstrated. To realize cucurbit[8]uril (CB[8])‐based SBCPs generating artificial light‐harvesting in water, finely color‐tuned supramolecular homopolymers (SHPs) comprising CB[8] host and different cyanostilbene guests (named as B, G, Y, and R) emitting blue, green, yellow, and red fluorescence are first synthesized and characterized, respectively. Light‐harvesting SBCPs with mixed guest emitters are then simply produced by mixing blue and red‐emitting SHPs according to the dynamic host–guest exchange interaction. The light‐harvesting SBCPs show highly efficient energy transfer from B (donor D) to R (acceptor A) attributed to the D/A proximity and parallel orientation of their transition dipoles secured in the block copolymer structure. It is comprehensively shown that cyanostilbene/CB[8]‐based fluorescent SBCPs represent a novel and fascinating class of eco‐friendly artificial light‐harvesting system.

Prediction of vapor-liquid-hydrate equilibrium conditions for single and mixed guest hydrates with the SAFT-VR Mie EOS

Abstract In this work, a thermodynamic modeling approach is utilized to predict the formation conditions of natural gas hydrates. Our approach uses van der Waals-Platteeuw (vdWP) model for the hydrate phase and the SAFT-VR Mie equation of state (EOS) for the vapor and liquid phases. We employ three different association schemes into the SAFT-VR Mie EOS to account for hydrogen bonds among water molecules using square-well (SW), Lennard-Jones (LJ) and Mie potentials. The SAFT-VR Mie EOS is used to determine the solubility of hydrate formers in liquid water. We study single and mixed guest hydrates and compare the results with experimental data. The comparisons with experimental data show that our model gives excellent to satisfactory predictions of the dissociation pressures for single and mixed guest hydrates without re-adjustment of the Langmuir constants.

Structure and Thermochemistry of Perrhenate Sodalite and Mixed Guest Perrhenate/Pertechnetate Sodalite.

Treatment and immobilization of technetium-99 (99Tc) contained in reprocessed nuclear waste and present in contaminated subsurface systems represents a major environmental challenge. One potential approach to managing this highly mobile and long-lived radionuclide is immobilization into micro- and meso-porous crystalline solids, specifically sodalite. We synthesized and characterized the structure of perrhenate sodalite, Na8[AlSiO4]6(ReO4)2, and the structure of a mixed guest perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)2-x(TcO4)x. Perrhenate was used as a chemical analogue for pertechnetate. Bulk analyses of each solid confirm a cubic sodalite-type structure (P4̅3n, No. 218 space group) with rhenium and technetium in the 7+ oxidation state. High-resolution nanometer scale characterization measurements provide first-of-a-kind evidence that the ReO4- anions are distributed in a periodic array in the sample, nanoscale clustering is not observed, and the ReO4- anion occupies the center of the sodalite β-cage in Na8[AlSiO4]6(ReO4)2. We also demonstrate, for the first time, that the TcO4- anion can be incorporated into the sodalite structure. Lastly, thermochemistry measurements for the perrhenate sodalite were used to estimate the thermochemistry of pertechnetate sodalite based on a relationship between ionic potential and the enthalpy and Gibbs free energy of formation for previously measured oxyanion-bearing feldspathoid phases. The results collected in this study suggest that micro- and mesoporous crystalline solids maybe viable candidates for the treatment and immobilization of 99Tc present in reprocessed nuclear waste streams and contaminated subsurface environments.

Molecular dynamics study on the nucleation of methane + tetrahydrofuran mixed guest hydrate

The nucleation of methane (CH4), tetrahydrofuran (THF), and CH4 + THF hydrates are investigated by microsecond MD simulations. These three systems exhibit distinct structural developments in the aqueous phase quantified by the formation of cage structures of hydrogen bonded water molecules. The development of a cluster of cages in the CH4 system is limited by the scarce CH4 molecules in the solution, while in the THF system it is limited by the short lifetime of cages. In the CH4 + THF mixed guest system, a small cluster of caged CH4 molecules can be rapidly stabilized by abundant neighboring cages of THF molecules. Therefore, the induction time of the CH4 + THF mixed guest system is found to be significantly shorter than that of the pure CH4 and pure THF systems. Furthermore, the structure of cages found in the initially formed cage clusters are often different from the typical 5(12)6(n) (n = 0, 2, 3, 4) cages observed in clathrate hydrate systems. The cluster of cages may grow or transform into structure I or II clathrate hydrate in the later stages.

Dynamically controlled one-pot synthesis of heterogeneous core-shell MOF single crystals using guest molecules.

A new mixed guest approach for the synthesis of heterogeneous core-shell MOF crystals was exemplified by one-pot assembly of photoactive guests into an anionic host framework. The formation mechanism, photophysical properties and oxygen gas sensing properties of as-synthesized core-shell MOF crystals were also investigated.

How to monitor guest exchange in host–guest systems

The process of guest-exchange in host–guest compounds is reviewed, with a brief discussion of the mechanism and followed by an outline of various analytical techniques, which are commonly employed to follow the progress of the exchange reaction. These include thermal gravimetry (TG), differential scanning calorimetry (DSC), gas–liquid chromatography, nuclear magnetic resonance (NMR), colour change and other more specialised methods. In-situ methods are emphasized and structural changes as monitored by powder X-ray diffraction or single-crystal to single-crystal transformation are considered. An example is given of a guest exchange in an organic system comprising a xanthenol host that initially contains chlorobenzene as guest, which is exchanged with toluene. The procedure is carried out on a single crystal and the structure is monitored as a function of time. The refinement of the structures, which exhibit disordered mixed guests, poses special problems, and a good starting model for the site occupancies of the guest atoms is important. Many such reactions may be carried out using a simple apparatus.

Synthesis and crystal structures of 2,4,6-pyridine-tricarboxylic anions/guanidinium and tetraalkylammonium inclusion compounds

Two different hydrogen-bonded inclusion compounds, [2,4,6-C5H2N(COO−)3]0.5·[C(NH2)3+]0.5·[(C2H5)4N+]·2H2O (1) and [2,4,6-C5H2N(COO−)3]·[C(NH2)3+]·[(C2H5)4N+]·[(C3H7)4N+]·6H2O (2) are reported in this paper, in which 2,4,6-pyridine-tricarboxylic anions, guanidiniums and water molecules jointly construct host lattices while tetraalkylammonium cations are accommodated as guest species. Both two compounds formed sandwich-like hydrogen-bond inclusion compounds. In compound 1, the dimers composed of 2,4,6-pyridine-tricarboxylic anions and guanidiniums form 2D hydrogen-bonded layers by connecting with water molecules. In compound 2, 2,4,6-pyridine-tricarboxylic anions, guanidiniums and water molecules contribute to generate an undulate rosette hydrogen-bonded architecture. Interestingly, in compound 2, there are two species of guest molecules, tetraethylammonium and tetrapropylammonium, which are alternately arranged between the neighboring layers. Mixed guest cations accommodated in hydrogen-bonded inclusion compounds are seldom seen.

Hydrate-containing phase equilibria for mixed guests of carbon dioxide and nitrogen

Hydrate-containing phase equilibria for mixtures composed of carbon dioxide, nitrogen and water are of potential importance for the flow assurance in the transportation of captured carbon dioxide. Literature data for such mixture tend to be reported on water free basis. In this study three- and four-phase equilibria were experimentally studied at the pressure ranging from 5 to 20 MPa. Isobaric dissolution temperatures of formed hydrates were measured and reported for accurately determined loading compositions. Complex phase behaviors composed of two-, three- and four- phases were observed and they were analyzed by comparing with calculations using GSMGem program developed by Sloan and Koh [1]. Phase equilibria were found to be sensitive to water contents in water dominating mixtures. CSMGem and HYSYS (version 7.1) from AspenTech calculations were found in general agreements with present and literature data.

Selectivity and Enantiomeric Resolution in Inclusion Chemistry: A Systematic Study of Chiral Discrimination through Crystallization

The mechanism of enantiomeric resolution of 2-butylamine has been elucidated by employing a chiral host which enclathrated (R)- and (S)-2-butylamine in different proportions. We first measured the selectivity curve for a mixture of pyridine and morpholine, whose proportions in the crystals were determined by the refinement of single crystal X-ray structure determination and independently by 1H NMR spectroscopy and showed that X-ray diffraction is a robust and accurate method for determining the composition of mixed guests. We then measured the enantiomeric selectivity of this host for 2-butylamine and found correlation between selectivity, the torsional flexibility of the phenyl moieties of the host, and the concomitant remaining volume that accommodates the guest. This mechanism was confirmed by repeating the competition experiments with a related chiral host.

Polymer-induced improvements in ferroelectric liquid crystal

The chevron geometry in the SmC* phase of ferroelectric liquid crystals (FLC) has been a major obstacle in the use of FLC in displays as it results in poor electro-optical performance. The present paper reports a novel method to overcome this problem, by doping a small amount of polymer in the FLC matrix. In addition to the improvement in smectic ordering, polymer doping is also found to be useful in improving the vital electro-optical properties. The various electro-optical parameters like switching time, tilt angle, contrast ratio etc. show improvement in polymer mixed guest host mixture of FLC samples. POLYM. COMPOS., 31:1776–1781, 2010. © 2010 Society of Plastics Engineers.