Host–Guest Chemistry Within Cellulose Nanocrystal Gel Receptors

Cellulose nanocrystals (CNCs) spontaneously assemble into gels when mixed with a polyionic organic or inorganic salt. Here, we have used this ion‐induced gelation strategy to create functional CNC gels with a rigid tetracationic macrocycle, cyclobis(paraquat‐p‐phenylene) ( CBPQT ⁴⁺). Addition of [ CBPQT ]Cl₄ to CNCs causes gelation and embeds an active host inside the material. The fabricated CNC gels can reversibly absorb guest molecules from solution then undergo molecular recognition processes that create colorful host–guest complexes. These materials have been implemented in gel chromatography (for guest exchange and separation), and as elements to encode 2‐ and 3‐dimensional patterns. We anticipate that this concept might be extended to design a set of responsive and selective gel‐like materials functioning as, for instance, water‐pollutant scavengers, substrates for chiral separations, or molecular flasks.     

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.     

Size‐Tunable Micron‐Bubbles Based on Fluorous–Fluorous Interactions of Perfluorinated Dendritic Polyglycerols

This paper describes the behavior of various generations of polyglycerol dendrimers that contain a perfluorinated shell. The aggregation in organic solvents is based on supramolecular fluorous–fluorous interactions, which can be described by means of ¹⁹F NMR spectroscopy. In order to study the interaction and aggregation phenomena of dendrimers with perfluorinated shell and perfluoro‐tagged guest molecules we investigated [G3.5]‐dendrimer with a perfluorinated shell in the presence of perfluoro‐tagged disperse red. Noteworthy, the interaction intensities varied in an unexpected manner depending on the equivalents of perfluoro‐tagged guest molecules added to the dendrimers in solution which then formed supramolecular complexes based on fluorous–fluorous interactions. We found that these complexes aggregated around residual air in the solvent to form stable micron‐sized bubbles. Their sizes correlated with the interaction intensities measured for certain dendrimer–guest molecule ratios. Degassing of the solutions led to a quasi phase separation between organic and fluorous phase, whereby the dendrimers formed the fluorous phases. Regassing the sample with air afforded bubbles of the initial size again.     

Host–Guest Interactions of Phosphorescent Molecular Tweezers Based on an Alkynylplatinum(II) Terpyridine System with Polyaromatic Hydrocarbons

Host–guest interactions of a molecular tweezer complex 1 with various planar organic molecules including polyaromatic hydrocarbons (PAHs) were investigated by 1D and 2D ¹H NMR spectroscopy, UV/Vis absorption and emission titration studies. 2D and DOSY NMR spectroscopies support the sandwiched binding mode based on 1:1 host–guest interactions. The binding constants (K_S) of complex 1 for various PAHs were determined by NMR titration studies and the values were found to span up to an order of 10⁴ M ^?1 for coronene to no observable interaction for benzene, indicating that the π‐surface area is important for such host–guest interactions. The substituent effect on the host–guest interaction based on the guest series of 9‐substituted anthracenes was also studied. In general, a stronger interaction was observed for the anthracene guest with electron‐donating groups, although steric and π‐conjugation factors cannot be completely excluded. The photophysical responses of complex 1 upon addition of various PAHs were measured by UV/Vis and emission titration studies. The UV/Vis absorption spectra were found to show a drop in absorbance of the metal‐to‐ligand charge‐transfer (MLCT) and ligand‐to‐ligand charge‐transfer (LLCT) admixture band upon addition of various guest molecules to 1 , whereas the emission behavior was found to change differently depending on the guest molecules, showing emission enhancement and/or quenching. It was found that emission quenching occurred either via energy transfer or electron transfer pathway or both, while emission enhancement was caused by the increase in rigidity of complex 1 as a result of host–guest interaction.     

Effect of Bulkiness on Reversible Substitution Reaction at MnII Center with Concomitant Movement of the Lattice DMF: Observation through Single‐Crystal to Single‐Crystal Fashion

The porous coordination polymer ({[Mn(L)H₂O](H₂O)_1.5(dmf)}_n, 1 ) (DMF=N,N‐dimethylformamide) exhibits variety of substitution reactions along with movement of lattice DMF molecule depending upon bulkiness of the external guest molecules. If pyridine or 4‐picoline is used as a guest, both lattice and coordinated solvent molecules are simultaneously substituted (complexes 6 and 7 , respectively). If a bulky guest like aniline is used, a partial substitution at the metal centers and full substitution at the channels takes place (complex 8 ). If the guest is 2‐picoline (by varying the position of bulky methyl group with respect to donor N atom), one Mn^II center is substituted by 2‐picoline, whereas the remaining center is substituted by a DMF molecule that migrates from the channel to the metal center (complex 9 ). Here, the lattice solvent molecules are substituted by 2‐picoline molecules. For the case of other bulky guests like benzonitrile or 2,6‐lutidine, both the metal centers are substituted by two DMF molecules, again migrating from the channel, and the lattice solvent molecules are substituted by these guest molecules (complex 10 and 11 , respectively). A preferential substitution of pyridine over benzonitrile (complex 12 ) at the metal centers is observed only when the molar ratio of PhCN:Py is 95:5 or less. For the case of an aliphatic dimethylaminoacetonitrile guest, the metal centers remain unsubstituted (complex 13 ); rather substitutions of the lattice solvents by the guest molecules take place. All these phenomena are observed through single crystal to single crystal (SC–SC) phenomena.     

A Solid‐State Fluorescent Host System with a 21‐Helical Column Consisting of Chiral (1R,2S)‐2‐Amino‐1,2‐diphenylethanol and Fluorescent 1‐Pyrenecarboxylic Acid

A solid‐state fluorescent host system was created by self‐assembly of a 2₁‐helical columnar organic fluorophore composed of (1R,2S)‐2‐amino‐1,2‐diphenylethanol and fluorescent 1‐pyrenecarboxylic acid. This host system has a characteristic 2₁‐helical columnar hydrogen‐ and ionic‐bonded network. Channel‐like cavities are formed by self‐assembly of this column, and various guest molecules can be included by tuning the packing of this column. Moreover, the solid‐state fluorescence of this host system can change according to the included guest molecules. This occurs because of the change in the relative arrangement of the pyrene rings as they adjust to the tuning of the packing of the shared 2₁‐helical column, according to the size of the included guest molecules. Therefore, this host system can recognize slight differences in molecular size and shape.     

Tuning Proton Conductivity by Interstitial Guest Change in Size‐Adjustable Nanopores of a CuI‐MOF: A Potential Platform for Versatile Proton Carriers

By exploiting the breathing behavior of nanopores, we have studied for the first time the dependency of the guest‐induced proton conductivity of an interpenetrated Cu^I metal–organic framework (Cu^I‐MOF, [ 1 ]) on various guest molecules. Proton conductivities of over 10^?3 S cm^?1 under humid conditions were induced by a series of guest molecules, namely N,N‐dimethylformamide, dimethyl sulfoxide, diethylamine, 1,4‐dinitrobenzene, nitrobenzene, pyridine, and 1H‐1,2,4‐triazole. A detailed investigation of the guest‐incorporated complexes revealed that low‐energy proton conduction occurs under humid conditions through the Grotthuss mechanism in [ 1 ?NB] and through the vehicle mechanism in the rest of the complexes. Single‐point energy computations revealed considerable stabilization upon guest encapsulation. To the best of our knowledge, [ 1 ] represents the first example in which considerably high protonic conductivity is triggered upon the facile incorporation of small molecules of such a variety. The investigation portrayed herein may be a stepping stone towards the rational design of proton‐conducting materials for practical applications.     

Role‐Allocated Combination of Two Types of Hydrogen Bonds towards Constructing a Breathing Diamondoid Porous Organic Salt

A diamondoid porous organic salt (d‐POS) composed of 8‐hydroxyquinoline‐5‐sulfonic acid (HQS) and triphenylmethylamine (TPMA) shows reversible structure contraction and expansion (“breathing”) in response to guest desorption and adsorption. This flexible structure is designed hierarchically by utilizing two different types of hydrogen bonds. X‐ray crystallographic analysis reveals that the two types of hydrogen bonds are formed separately to play respective roles for constructing the d‐POS. The strong charge‐assisted hydrogen bond between the sulfonate anion of HQS and the ammonium cation of TPMA serves as a static node to provide a supramolecular cluster for a building block. In contrast, the complementary neutral hydrogen bond between the hydroxyl and quinolyl groups of HQS acts as a dynamic linker to connect the clusters. Consequently, these two types of hydrogen bonds yield the d‐POS with one‐dimensional channels through the formation of diamondoid networks. We clarify that the d‐POS undergoes dynamic structure transformation that originates in the cleavage and reformation of the complementary neutral hydrogen bond during guest desorption and adsorption. From the comparative studies, it is also demonstrated that applying the complementary neutral hydrogen bond in the d‐POS provides significant advantages in terms of the responsivity of the structure over applying other weak noncovalent interactions for the connection of the clusters. Furthermore, the resultant d‐POS also modulates fluorescent profiles dynamically responsive to guest adsorption and desorption.     

Designed Enclosure Enables Guest Binding Within the 4200 Å3 Cavity of a Self‐Assembled Cube

Metal–organic self‐assembly has proven to be of great use in constructing structures of increasing size and intricacy, but the largest assemblies lack the functions associated with the ability to bind guests. Here we demonstrate the self‐assembly of two simple organic molecules with Cd^II and Pt^II into a giant heterometallic supramolecular cube which is capable of binding a variety of mono‐ and dianionic guests within an enclosed cavity greater than 4200 ų. Its structure was established by X‐ray crystallography and cryogenic transmission electron microscopy. This cube is the largest discrete abiological assembly that has been observed to bind guests in solution; cavity enclosure and coulombic effects appear to be crucial drivers of host–guest chemistry at this scale. The degree of cavity occupancy, however, appears less important: the largest guest studied, bound the most weakly, occupying only 11 % of the host cavity.     

Achieving Strong Positive Cooperativity through Activating Weak Non‐Covalent Interactions

Positive cooperativity achieved through activating weak non‐covalent interactions is common in biological assemblies but is rarely observed in synthetic complexes. Two new molecular tubes have been synthesized and the syn isomer binds DABCO‐based organic cations with high orientational selectivity. Surprisingly, the ternary complex with two hosts and one guest shows a high cooperativity factor (α=580), which is the highest reported for synthetic systems without involving ion‐pairing interactions. The X‐ray single‐crystal structure revealed that the strong positive cooperativity likely originates from eight C?H???O hydrogen bonds between the two head‐to‐head‐arranged syn tube molecules. These relatively weak hydrogen bonds were not observed in the free hosts and only emerged in the complex. Furthermore, this complex was used as a basic motif to construct a robust [2+2] cyclic assembly, thus demonstrating its potential in molecular self‐assembly.     

Redox‐Active Metal–Organic Frameworks: Highly Stable Charge‐Separated States through Strut/Guest‐to‐Strut Electron Transfer

Molecular organization of donor and acceptor chromophores in self‐assembled materials is of paramount interest in the field of photovoltaics or mimicry of natural light‐harvesting systems. With this in mind, a redox‐active porous interpenetrated metal–organic framework (MOF), {[Cd(bpdc)(bpNDI)] ? 4.5 H₂O ? DMF}_n ( 1 ) has been constructed from a mixed chromophoric system. The μ‐oxo‐bridged secondary building unit, {Cd₂(μ‐OCO)₂}, guides the parallel alignment of bpNDI (N,N′‐di(4‐pyridyl)‐1,4,5,8‐naphthalenediimide) acceptor linkers, which are tethered with bpdc (bpdcH₂=4,4′‐biphenyldicarboxylic acid) linkers of another entangled net in the framework, resulting in photochromic behaviour through inter‐net electron transfer. Encapsulation of electron‐donating aromatic molecules in the electron‐deficient channels of 1 leads to a perfect donor–acceptor co‐facial organization, resulting in long‐lived charge‐separated states of bpNDI. Furthermore, 1 and guest encapsulated species are characterised through electrochemical studies for understanding of their redox properties.     

Load–Collapse–Release Cascades of Amphiphilic Guest Molecules in Charged Dendronized Polymers through Spatial Separation of Noncovalent Forces

The ability to pack guest molecules into charged dendronized polymers (denpols) and the possibility to release these guest molecules from subsequently densely aggregated denpols in a load–collapse–release cascade is described. Charged denpols, which constitute molecular objects with a persistent, well‐defined envelope and interior, are capable of incorporating large amounts of amphiphilic guest molecules. Simultaneously, multivalent ions can coordinate to the surfaces of charged denpols, leading to counterion‐induced aggregation of the already guest‐loaded host structures. Thus, although the local guest concentration in denpol‐based molecular transport might already be initially high due to the dense guest packing inside the dendritic denpol scaffolding, the “local” guest concentration can nonetheless be further increased by packing (through aggregation) of the host–guest complexes themselves. Subsequent release of guest compounds from densely aggregated dendronized polymers is then possible (e.g., through increasing the solution concentration of imidazolium‐based ions). Augmented with this release possibility, the concept of twofold packing of guests, firstly through hosting itself and secondly through aggregation of the hosts, gives rise to a load–collapse–release cascade that strikingly displays the high potential of dendronized macromolecules for future molecular transport applications.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.     

A Guest‐Dependent Approach to Retain Permanent Pores in Flexible Metal–Organic Frameworks by Cation Exchange

Two anionic metal–organic frameworks were successfully prepared based on pre‐designed flexible multicarboxylate ligands and indium cations. Owing to the flexibility of the bridging organic linkers, which could not themselves sustain the frameworks, both of the frameworks showed thermal instability and shrinkage after removal of guest solvent molecules. Inspired by bamboo, we used a guest‐dependent approach to tune the permanent porosity of the MOFs. In this approach, several tetraalkyammonium cations of different sizes were introduced into the channels by cation exchange to act as partitions and to support the main frameworks. This approach significantly enhanced the stability of the framework and its permanent porosity. Moreover, the gas‐adsorption properties (such as gate sorption, hysteresis, and selectivity) of the MOFs were also modulated by the judicious choice of guest cations.     

Clicked Isoreticular Metal–Organic Frameworks and Their High Performance in the Selective Capture and Separation of Large Organic Molecules

Three highly porous metal–organic frameworks (MOFs) with a uniform rht‐type topological network but hierarchical pores were successfully constructed by the assembly of triazole‐containing dendritic hexacarboxylate ligands with Zn^II ions. These transparent MOF crystals present gradually increasing pore sizes upon extension of the length of the organic backbone, as clearly identified by structural analysis and gas‐adsorption experiments. The inherent accessibility of the pores to large molecules endows these materials with unique properties for the uptake of large guest molecules. The visible selective adsorption of dye molecules makes these MOFs highly promising porous materials for pore‐size‐dependent large‐molecule capture and separation.     

Metal–Organic Cyclohelicates as Optical Receptors for Glutathione: Syntheses,Structures, and Host–Guest Behaviors

Two trinuclear zinc‐based cyclohelicates, Zn–PDB (PDB=[5‐(dibenzylamino)‐N1′,N3′‐bis(pyridin‐2‐ylmethylene)isophthalohydrazide]) and Zn–PMB (PMB=[5‐(bodipy‐oxy)‐N1′,N3′‐bis(pyridin‐2‐ylmethylene)isophthalohydrazide]) containing dibenzylamino and BODIPY groups, respectively, were generated by incorporating two amide‐containing tridentate chelators into meta‐positions of a substituted phenyl ring. Single‐crystal structure analysis and related spectroscopic characterizations demonstrated the formation of macrocyclic helicals both in the solid state and in solution. The host–guest behavior of the cyclohelical hosts towards γ‐glutamyl‐cysteinyl‐glycine (GSH) and its component amino acids was investigated by spectroscopic titrations. UV/Vis absorption titration and NMR titrations of Zn–PDB and Zn–PMB upon addition of the above‐mentioned guests suggested that the Glu residue of GSH was positioned within the cavity. The COO⁻ groups interacted with metal ions through static interactions. The Cys moiety of GSH interacted with the amide groups sited in host molecules through hydrogen‐bonding interactions to produce measurable spectral changes. Fluorescent titrations of Zn–PMB upon the addition of GSH and ESI‐MS investigations of the titration solutions confirmed the host–guest interaction modes and revealed the possible 1:1 complexation stoichiometry. These results showed that the recognition of a substrate within the cavity of functionalized metal–organic cage‐like receptors could be a useful method to produce supramolecular sensors for biomolecules.     

A Structurally Variable Porous Organic Salt Based on a Multidirectional Supramolecular Cluster

A porous organic salt (POS) composed of 2‐sulfophenyl anthracene (2‐SPA) and triphenylmetylamine (TPMA) forms five types of porous crystals, POS‐a–e, by recognizing subtle differences in the molecular structure of incorporated guest molecules. This structurally variable POS was hierarchically designed on the basis of a supramolecular cluster with a directionally flexible linker formed by the organic salt. X‐ray crystallographic analysis reveals that the salt forms six conformers attributable to rocking and rotational motions of the phenylene group in 2‐SPA. The clusters form POS crystals through different porous networks according to the conformers. The POS crystals show a wide range of fluorescence spectra that are responsive to differences in the molecular and electronic structure of the guest molecule. This remarkable behavior has potential application in sensitive chemical sensors that are responsive to slight differences in molecular structures.     

Host–Guest [2+2] Cycloaddition Reaction: Postsynthetic Modulation of CO2 Selectivity and Magnetic Properties in a Bimodal Metal–Organic Framework

Simultaneous tuning of permanent porosity and modulation of magnetic properties by postsynthetic modification (PSM) with light in a metal–organic framework is unprecedented. With the aim of achieving such a photoresponsive porous magnetic material, a 3D photoresponsive biporous framework, MOF1, which has 2D channels occupied by the guest 1,2‐bis(4‐pyridyl)ethylene (bpee), H₂O, and EtOH molecules, has been synthesized. The guest bpee in 1 is aligned parallel to pillared bpee with a distance of 3.9 Å between the ethylenic groups; this allows photoinduced PSM of the pore surface through a [2+2] cycloaddition reaction to yield MOF2. Such photoinduced PSM of the framework structure introduces enhanced CO₂ selectivity over that of N₂. The higher selectivity in MOF2 than that of MOF1 is studied through theoretical calculations. Moreover, MOF2 unveils reversible changes in T_c with response to dehydration–rehydration. This result demonstrates that photoinduced PSM is a powerful tool for fabricating novel functional materials.     

Insights into Structure Direction of Microporous Aluminophosphates: Competition between Organic Molecules and Water

A combination of experimental characterisation techniques and computational modelling has allowed us to gain insight into the molecular features governing structure direction in the synthesis of microporous aluminophosphates. The occlusion of three different structure‐directing agents (SDAs), triethylamine (TEA), benzylpyrrolidine (BP) and (S )‐(?)‐N‐benzylpyrrolidine‐2‐methanol (BPM), within the AFI structure during its crystallisation, together with the simultaneous incorporation of water, has been experimentally measured. We found a higher incorporation of organic molecules in the structure obtained with BPM, while a higher water (and lower organic) content is found for the ones obtained with TEA and BP as SDAs. The computational study provides a thermodynamic explanation for the observed behaviour in terms of the relative stabilisation energy of the SDAs and water molecules within the AFI framework compared with when they are in aqueous solution, and demonstrates that a competition for preferential occupation exists between water and organic SDAs, which is a function of the interaction with the inorganic framework. The lower interaction of TEA and BP molecules with the AFI structure promotes the simultaneous incorporation of water molecules in the 12‐membered‐ring (MR) channel, to increase the host–guest interaction energy and thus the thermodynamic stability. The presence of strongly interacting methanol groups in the BPM molecules leads to the incorporation of only organic molecules within the 12‐MR channels. Our results demonstrate the essential role that water molecules play in the stabilisation of hydrophilic microporous aluminophosphates; a minimum amount of organic SDA is, however, essential for a templating role of the microporous architecture.     

Recognition‐Mediated Hydrogel Swelling Controlled by Interaction with a Negative Thermoresponsive LCST Polymer

Most polymeric thermoresponsive hydrogels contract upon heating beyond the lower critical solution temperature (LCST) of the polymers used. Herein, we report a supramolecular hydrogel system that shows the opposite temperature dependence. When the non‐thermosesponsive hydrogel NaphtGel, containing dialkoxynaphthalene guest molecules, becomes complexed with the tetra cationic macrocyclic host CBPQT⁴⁺, swelling occurred as a result of host–guest complex formation leading to charge repulsion between the host units, as well as an osmotic contribution of chloride counter‐ions embedded in the network. The immersion of NaphtGel in a solution of poly(N‐isopropylacrylamide) with tetrathiafulvalene (TTF) end groups complexed with CBPQT⁴⁺ induced positive thermoresponsive behaviour. The LCST‐induced dethreading of the polymer‐based pseudorotaxane upon heating led to transfer of the CBPQT⁴⁺ host and a concomitant swelling of NaphtGel. Subsequent cooling led to reformation of the TTF‐based host–guest complexes in solution and contraction of the hydrogel.