Fluorescent Cross‐Linked Supramolecular Polymer Constructed by Orthogonal Self‐Assembly of Metal–Ligand Coordination and Host–Guest Interaction

A new host molecule consists of four terpyridine groups as the binding sites with zinc(II) ion and a copillar[5]arene incorporated in the center as a spacer to interact with guest molecule was designed and synthesized. Due to the 120 ° angle of the rigid aromatic segment, a cross‐linked dimeric hexagonal supramolecular polymer was therefore generated as the result of the orthogonal self‐assembly of metal–ligand coordination and host–guest interaction. UV/Vis spectroscopy, ¹H NMR spectroscopy, viscosity and dynamic light‐scattering techniques were employed to characterize and understand the cross‐linking process with the introduction of zinc(II) ion and guest molecule. More importantly, well‐defined morphology of the self‐assembled supramolecular structure can be tuned by altering the adding sequence of the two components, that is, the zinc(II) ion and the guest molecule. In addition, introduction of a competitive ligand suggested the dynamic nature of the supramolecular structure.     

Advances in the study of the host-guest interaction by using coronene as the guest molecule

This review supplied direct insight of host-guest molecule system by using COR as the guest molecule.     

The self-assembly and structural regulation of a hydrogen-bonded dimeric building block formed by two NH···O hydrogen bonds on HOPG

The self-assembled behavior of an unsymmetric molecule (BCDTDA) with one imidazole group as center and benzoic acid group as functional group is studied, and the regulatory behaviors of coronene (COR) and three bipyridine derivatives (named BP, PEBP-C4 and PEBP-C8) on BCDTDA self-assembly structures are also investigated. Based on highly oriented pyrolytic graphite (HOPG) substrate, scanning tunneling microscopy (STM) is used to observe the variation of assembled behaviors at the solid-liquid interface. Because of the concentration effect, BCDTDA molecules can assemble into grids and Kagomés structures in the form of NH···O hydrogen bonded dimers. BCDTDA molecules still maintain dimeric structures in the regulation of COR and BP molecules to BCDTDA self-assembly. However, PEBP-C4 and PEBP-C8 destroy the structure of the dimers, and form a variety of co-assembled structures with BCDTDA. Different guest molecules coordinate the host molecules differently, which makes the experiment more meaningful. Combined with density functional theory (DFT) calculation, the discovery of molecular interactions provides a promising strategy for the construction of functional nanostructures and devices.     

Host Perturbation in a β‐Hydroquinone Clathrate Studied by Combined X‐ray/Neutron Charge‐Density Analysis: Implications for Molecular Inclusion in Supramolecular Entities

X‐ray/neutron (X/N) diffraction data measured at very low temperature (15 K) in conjunction with ab initio theoretical calculations were used to model the crystal charge density (CD) of the host–guest complex of hydroquinone (HQ) and acetonitrile. Due to pseudosymmetry, information about the ordering of the acetonitrile molecules within the HQ cavities is present only in almost extinct, very weak diffraction data, which cannot be measured with sufficient accuracy even by using the brightest X‐ray and neutron sources available, and the CD model of the guest molecule was ultimately based on theoretical calculations. On the other hand, the CD of the HQ host structure is well determined by the experimental data. The neutron diffraction data provide hydrogen anisotropic thermal parameters and positions, which are important to obtain a reliable CD for this light‐atom‐only crystal. Atomic displacement parameters obtained independently from the X‐ray and neutron diffraction data show excellent agreement with a |ΔU| value of 0.00058 Ų indicating outstanding data quality. The CD and especially the derived electrostatic properties clearly reveal increased polarization of the HQ molecules in the host–guest complex compared with the HQ molecules in the empty HQ apohost crystal structure. It was found that the origin of the increased polarization is inclusion of the acetonitrile molecule, whereas the change in geometry of the HQ host structure following inclusion of the guest has very little effect on the electrostatic potential. The fact that guest inclusion has a profound effect on the electrostatic potential suggests that nonpolarizable force fields may be unsuitable for molecular dynamics simulations of host–guest interaction (e.g., in protein–drug complexes), at least for polar molecules.     

Host–Guest Complexation Using Pillar[5]arene Crystals: Crystal-Structure Dependent Uptake,Release, and Molecular Dynamics of an Alkane Guest

Host–guest complexation has been mainly investigated in solution, and it is unclear how guest molecules access the assembled structures of host and dynamics of guest molecules in the crystal state. In this study, we studied the uptake, release, and molecular dynamics of n-hexane vapor in the crystal state of pillar[5]arenes bearing different substituents. Pillar[5]arene bearing 10 ethyl groups yielded a crystal structure of herringbone-type 1:1 complexes with n-hexane, whereas pillar[5]arene with 10 allyl groups formed 1:1 complexes featuring a one-dimensional (1D) channel structure. For pillar[5]arene bearing 10 benzyl groups, one molecule of n-hexane was located in the cavity of pillar[5]arene, and another n-hexane molecule was located outside of the cavity between two pillar[5]arenes. The substituent-dependent differences in molecular arrangement influenced the uptake, release, and molecular dynamics of the n-hexane guest. The substituent effects were not observed in host–guest chemistry in solution, and these features are unique for the crystal state host–guest chemistry of pillar[5]arenes.     

Adsorption dynamics of gases and vapors on the nanoporous metal organic framework material Ni2(4,4'-bipyridine)3(NO3)4: guest modification of host sorption behavior

This study combines measurements of the thermodynamics and kinetics of guest sorption with powder X-ray diffraction measurements of the nanoporous metal organic framework adsorbent (host) at different adsorptive (guest) loadings. The adsorption characteristics of nitrogen, argon, carbon dioxide, nitrous oxide and ethanol and methanol vapors on Ni2(4,4'-bipyridine)3(NO3)4 were studied over a range of temperatures as a function of pressure. Isotherm steps were observed for both carbon dioxide and nitrous oxide adsorption at approximately 10-20% of the total pore volume and at approximately 70% of total pore volume for methanol adsorption. The adsorption kinetics obey a linear driving force (LDF) mass transfer model for adsorption at low surface coverage. At high surface coverage, both methanol and ethanol adsorption follow a combined barrier resistance/diffusion model. The rates of adsorption in the region of both the carbon dioxide and methanol isotherm steps were significantly slower than those observed either before or after the step. X-ray diffraction studies at various methanol loadings showed that the host structure disordered initially but underwent a structural change in the region of the isotherm step. These isotherm steps are ascribed to discrete structural changes in the host adsorbent that are induced by adsorption on different sites. Isotherm steps were not observed for ethanol adsorption, which followed a Langmuir isotherm. Previous X-ray crystallography studies have shown that all the sites are equivalent for ethanol adsorption on Ni2(4,4'-bipyridine)3(NO3)4, with the host structure undergoing a scissoring motion and the space group remaining unchanged during adsorption. The activation energies and preexponential factors for methanol and ethanol adsorption were calculated for each pressure increment at which the linear driving force model was obeyed. There was a good correlation between activation energy and ln(preexponential factor), indicating a compensation effect. The results are discussed in terms of reversible adsorbate/adsorbent (guest/host) structural changes and interactions and the adsorption mechanism. The paper contains the first evidence of specific interactions between guests and functional groups leading to structural change in flexible porous coordination polymer frameworks.     

Theoretical Study on the Conformational Conversion of 1,3-Dioxane Inside a Capsular Host

The mechanism for the conformational conversion of 1,3-dioxane guest encapsulated inside a capsular host was theoretically investigated using semiempirical PM3 method and DFT methods. The free-state process of the conformational conversion of 1,3-dioxane was also investigated to make a comparison between the two different states using the same theory. The influences of the inner phase of the capsule on the conformational conversion of guest molecule were discussed via analyzing the comparative results. It was found that the capsular host could accommodate 1,3-dioxane within its cavity by the weak attractive interactions between host and guest, and it responds to the conformational conversion of guest by the deformation of hydrogen-bonding seam at the middle of the capsule. When entrapped in the capsule, the guest molecule undergoes the conformational conversion from chair form to twist-boat form slower than that under the free condition. The deformation of the capsule is favorable to maximize the attractive interactions between host and guest.     

Complexation between Methyl Viologen (Paraquat) Bis(Hexafluorophosphate) and Dibenzo[24]Crown‐8 Revisited

Paraquat bis(hexafluorophosphate) undergoes stepwise dissociation in acetone. All three species—the neutral molecule, and the mono‐ and dications—are represented significantly under the experimental conditions typically used in host–guest binding studies. Paraquat forms at least four host–guest complexes with dibenzo[24]crown‐8. They are characterized by both 1:1 and 1:2 stoichiometries, and an overall charge of either zero (neutral molecule) or one (monocation). The monocationic 1:1 host–guest complex is the most abundant species under typical (0.5–20 mM ) experimental conditions. The presence of the dicationic 1:1 host–guest complex cannot be excluded on the basis of our experimental data, but neither is it unambiguously confirmed to be present. The two confirmed forms of paraquat that do undergo complexation—the neutral molecule and the monocation—exhibit approximately identical binding affinities toward dibenzo[24]crown‐8. Thus, the relative abundance of neutral, singly, and doubly charged pseudorotaxanes is identical to the relative abundance of neutral, singly, and doubly charged paraquat unbound with respect to the crown ether in acetone. In the specific case of paraquat/dibenzo[24]crown‐8, ion‐pairing does not contribute to host–guest complex formation, as has been suggested previously in the literature.     

Elucidating the mechanism of binding of chromate to cucurbit[6]uril: a DFT study

We report the binding of chromate, a toxic heavy metal ion to the macrocyclic host molecule, cucurbituril using density functional theory. Due to the anionic nature of the guest molecule and the portals of the host molecule, we propose that the binding mechanism should be assisted by cations. The calculated barrier for chromate binding to cucurbituril is found to be?~17?kcal?mol^?1. The large barrier can be attributed to portal opening of the host molecule, electrostatic repulsion between the guest molecule and the portals of the host molecule and the solvent re-organization around guest molecule.     

Selective effect of guest molecule length and hydrogen bonding on the supramolecular host structure

A host supramolecular structure consisting of bis-(2,2':6',2' '-terpyridine)-4'-oxyhexadecane (BT-O-C16) is shown to respond to guest molecules in dramatic ways, as observed by using scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite surface under ambient conditions. It is observed that small linear molecules can be encapsulated within the host supramolecular lattice. The characteristics of the host structure were nearly unaffected by the encapsulated guest molecules of terphthalic acid (TPA) dimers, whereas appreciable changes in cavity dimension can be observed with azobenzene-4,4'-dicarboxylic acid. The STM study and density functional theory (DFT) analysis reveal that intermolecular hydrogen bonding interaction plays an essential role in forming the assembling structures. The difference in guest molecule length is considered the important cause for the different guest-host complexes.     

Tunable Spin‐Crossover Behavior of the Hofmann‐like Network {Fe(bpac)[Pt(CN)4]} through Host–Guest Chemistry

A study of the spin‐crossover (SCO) behavior of the tridimensional porous coordination polymer {Fe(bpac)[Pt(CN)₄]} (bpac=bis(4‐pyridyl)acetylene) on adsorption of different mono‐ and polyhalobenzene guest molecules is presented. The resolution of the crystal structure of {Fe(bpac)[Pt(CN)₄]} ? G (G=1,2,4‐trichlorobenzene) shows preferential guest sites establishing π???π stacking interactions with the host framework. These host–guest interactions may explain the relationship between the modification of the SCO behavior and both the chemical nature of the guest molecule (electronic factors) and the number of adsorbed molecules (steric factors).     

Guest-to-host transmission of structural changes for stimuli-responsive adsorption property

We show that structural changes of a guest molecule can trigger structural transformations of a crystalline host framework. Azobenzene was introduced into a flexible porous coordination polymer (PCP), and cis/trans isomerizations of the guest azobenzene by light or heat successfully induced structural transformations of the host PCP in a reversible fashion. This guest-to-host structural transmission resulted in drastic changes in the gas adsorption property of the host-guest composite, displaying a new strategy for creating stimuli-responsive porous materials.     

β-环糊精和部分有机物分子识别作用的焓熵判断

应用气相色谱实验技术,测定了一些有机溶剂（或溶质）在β-环糊精（β-CD）固体表面上的吸附热力学函数.实验结果表明,气相中的有机物质在β-CD表面上的吸附过程之焓、熵互补现象与它们之间的分子识别作用密切相关,它们的绝对值之和可以作为分子识别作用强弱的判据.     

Computer modelling studies of 18-crown-6 with urea and thiourea

Abstract

Computer modelling studies have been carried out on the interaction of 18-crown-6 with a variety of guest molecules, including urea, thiourea and substituted ureas. The five known crystal structures of these host/guest systems were used as models. We were interested to establish whether the arrangement of guest molecules around a host molecule in the crystal was indicative of the lowest energy configuration for a host/guest fragment or was a consequence of packing effects. Two models were therefore considered for each structure and the structures minimised via molecular mechanics. In the first mode, the structure consisted of one unit cell and periodic boundary conditions were used in the calculation. Coulombic effects were calculated using the Ewald summation. In the second model, the structure consisted of an 18-crown-6 molecule surrounded by two hydrogen-bonded guest molecules. Both models were minimised using the CERIUS package using the DreidingII forcefield.

The crystal structure minimisations reproduced the structures very well with an average change in cell volume of 3.6% and a mean r.m.s. positional deviation of 0.20 Å. The fits for the fragment models were significantly larger for all structures (mean 0.30 Å) but even so it can be concluded that the arrangement in the crystal gave a good indication of the lowest energy configuration of the host/guest in vacuo.     

含偶氮苯主-客体复合物的光致异构化反应对结合能与几何构象的影响

在分子尺度上构建光驱动的人工分子机器是超分子化学研究的一个热点。偶氮苯是一类具有双稳态的光致开关分子,能够完成高效、可逆的反式(E)$\to $顺式(Z)的光致异构化过程,因而可以作为人工分子机器的功能单元。本文采用密度泛函理论(DFT)和反应分子动力学(RMD)模拟,研究了含偶氮苯封端基团的互锁型超分子体系中冠醚主体与二烷基铵客体间结合强度,模拟了偶氮苯Z$\to $E异构化反应的动态过程,讨论了异构化反应对主客体分子构象的影响。在偶氮苯封端基团通过发生Z$\to $E异构化实现体系单向可控运动时,较强的主-客体间结合能力是保证互锁型超分子体系稳定的必要前提。顺式客体与主体大环氢键相互作用比反式客体更强,因此顺式复合异构体具有比反式异构体更大的结合强度。偶氮苯基团发生E$\to $Z光致异构化引入位阻效应,使得顺式复合物只能从环戊基准封端处进行脱环。主客体复合过程对偶氮苯基团的几何结构没有明显影响。偶氮苯光致异构化发生的速度快于客体脱环的速度是实现单向运动的动力学上的必要条件。在异构化反应后的500 ps内,大环会经历一个明显的结构驰豫过程。冠醚大环主体的柔性构象有助于实现在偶氮苯光致异构化发生过程中主客体间持续稳定的结合。各种超分子体系中,尽管客体组成各不相同,但是包含相似的主客体识别位点的超分子体系具有相似的结合能,显示了机械互锁型复合体系中各种功能性构建单元间主客体相互作用具有正交性。引入双稳态的偶氮苯功能基团对客体其他部分的几何结构影响很小。理论计算结果有助于理性设计更复杂的刺激响应性人工分子机器。     

Computational NMR Spectra of o-Benzyne and Stable Guests and Their Hemicarceplexes

The incarceration of o-benzyne and 27 other guest molecules within hemicarcerand 1 , as reported experimentally by Warmuth, and Cram and co-workers, has been studied by density functional theory (DFT). The ¹H NMR chemical shifts, rotational mobility, and conformational preference of the guests within the supramolecular cage were determined, which showed intriguing correlations of the chemical shifts with structural parameters of the host–guest system. Furthermore, based on the computed chemical shifts reassignments of some NMR signals are proposed. This affects, in particular, the putative characterization of the volatile benzyne molecule inside a hemicarcerand, for which our CCSD(T) and KT2 results indicate that the experimentally observed signals are most likely not resulting from an isolated o-benzyne within the supramolecular host. Instead, it is shown that the guest reacted with an aromatic ring of the host, and this adduct is responsible for the experimentally observed signals.     

Synthesis of α‐Cyclodextrin‐Conjugated Poly(ε‐lysine)s and Their Inclusion Complexation Behavior

Novel functional polymers utilizing specific host/guest interactions were designed by introducing α‐CD host molecules into poly(ε‐lysine) chains as side groups. An interesting phase separation was observed as a result of the inclusion complexation between the polymeric host and 3‐(trimethylsilyl)propionic acid as a model guest in aqueous media. This water‐soluble polymeric host would be useful for various applications, particularly drug delivery, due to its biodegradability, low toxicity, and unique functionality represented as a complexation‐induced phase separation.     

Influence of Equilibration Time in Solution on the Inclusion/Exclusion Topology Ratio of Host–Guest Complexes Probed by Ion Mobility and Collision‐Induced Dissociation

Host–guest complexes are formed by the creation of multiple noncovalent bonds between a large molecule (the host) and smaller molecule(s) or ion(s) (the guest(s)). Ion‐mobility separation coupled with mass spectrometry nowadays represents an ideal tool to assess whether the host–guest complexes, when transferred to the gas phase upon electrospray ionization, possess an exclusion or inclusion nature. Nevertheless, the influence of the solution conditions on the nature of the observed gas‐phase ions is often not considered. In the specific case of inclusion complexes, kinetic considerations must be taken into account beside thermodynamics; the guest ingression within the host cavity can be characterized by slow kinetics, which makes the complexation reaction kinetically driven on the timescale of the experiment. This is particularly the case for the cucurbituril family of macrocyclic host molecules. Herein, we selected para‐phenylenediamine and cucurbit[6]uril as a model system to demonstrate, by means of ion mobility and collision‐induced dissociation measurements, that the inclusion/exclusion topology ratio varies as a function of the equilibration time in solution prior to the electrospray process.     

Nano‐Saturn: Experimental Evidence of Complex Formation of an Anthracene Cyclic Ring with C60

An anthracene cyclic hexamer was synthesized by the coupling reaction as a macrocyclic hydrocarbon host. This disk‐shaped host included a C₆₀ guest in 1:1 ratio to form a Saturn‐type supramolecular complex in solution and in crystals. X‐ray analysis unambiguously revealed that the guest molecule was accommodated in the middle of the host cavity with several CH???π contacts. The association constant K_a determined by NMR titration measurements was 2.3×10³ L mol^?1 at 298 K in toluene. The structural features and the role of CH???π interactions are discussed with the aid of DFT calculations.     

How important is the dispersion interaction for cyclobis(paraquat‐p‐phenylene)‐based molecular “shuttles”? A theoretical study

Inclusion complexes of cyclobis(paraquat‐p‐phenylene) and various aromatic molecules in their neutral and oxidized form were studied at the LMP2/6‐311+G**//BHandHLYP/6‐31G* level of theory, which represents the highest level theoretical study to date for these complexes. The results show that it is dispersion interaction that contributes most to the binding energy. One electron oxidation of a guest molecule leads to complete dissociation of inclusion complex generating strong repulsion potential between guest and host molecules. Electrostatic interactions also can play an important role, provided the guest molecule has a dipole moment; however, dispersion interactions always dominate in binding energy. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005