D3h‐Symmetrical Shape‐Persistent Macrocycles Consisting of Pyridine–Acetylene–Phenol Conjugates as an Efficient Host Architecture for Saccharide Recognition

Hexagonal shape‐persistent macrocycles (SPMs) consisting of three pyridine and three phenol rings linked with acetylene bonds were developed as a preorganized host for saccharide recognition by push–pull‐type hydrogen bonding. Three tert‐butyl or 2,4,6‐triisopropylphenyl substituents were introduced on the host to suppress self‐aggregation by steric hindrance. In spite of the simple architecture, association constants K_a of the host with alkyl glycoside guests reached the order of 10⁶ m ^?1 on the basis of UV/Vis titration experiments. This glycoside recognition was much stronger than that in the cases of acyclic equivalent hosts because of the entropic advantage brought by preorganization of the hydrogen‐bonding sites. Solid–liquid extraction and liquid–liquid transport through a liquid membrane were demonstrated by using native saccharides, and much preference to mannose was observed.     

Encapsulation‐Induced Remarkable Stability of a Hydrogen‐Bonded Heterocapsule

Remarkably enhanced stability of the self‐assembled hydrogen‐bonded heterocapsule 1?2 by the encapsulation of 1,4‐bis(1‐propynyl)benzene 3 a was found with K_a=1.14×10⁹ M ^?1 in CDCl₃ and K_a2=1.59×10⁸ M ^?2 in CD₃OD/CDCl₃ (10 % v/v) at 298 K. The formation of 3 a @( 1?2 ) was enthalpically driven (ΔH°<0 and ΔS°<0) and there was a unique inflection point in the correlation between ΔH° versus ΔS° as a function of polar solvent content. The ab initio calculations revealed that favorable guest–capsule dispersion and electrostatic interactions between the acetylenic parts (triple bonds) of 3 a and the aromatic inner space of 1?2 , as well as less structural deformation of 1?2 upon encapsulation of 3 a , play important roles in the remarkable stability of 3 a @( 1?2 ).     

Induction of Optical Activity in an Oligothiophene Synchronized with pH‐Sensitive Folding of Amylose

Control of the helical sense in α‐sexithiophene (6T) through pH‐responsive wrapping with left‐handed‐helical amylose is demonstrated. A change in pH of the medium caused a significant conformational change in amylose as the host polymer, which resulted in either supramolecular complexation with 6T as the guest molecule to induce optical activity or decomplexation leading to loss of optical activity. Furthermore, we observed that chirality reversal in 6T does not require hosts of opposite helical chirality, but can be made possible simply by taking advantage of the pH sensitivity of the amylose folding, which is dependent on the pH history of the aqueous medium. In helical amylose, 6T assumes a clockwise‐twisted conformation when the pH is changed from acidic to neutral, but assumes an anticlockwise‐twisted conformation when the aqueous solution is acidified from very basic conditions.     

Construction of different supramolecular polymer systems by combining the host–guest and hydrogen‐bonding interactions

Poly(ethylene glycol) (PEG) can form either the inclusion complex with α‐cyclodextrins (α‐CDs) through host–guest interactions or the interpolymer complex with poly(acrylic acid) (PAA) through hydrogen‐bonding interaction. Mixing α‐CD, PEG, and PAA ternary components in an aqueous solution, the competition between host–guest and hydrogen‐bonding interactions occurs. Increasing feed ratio of α‐CD:EG:AA from 0:1:1 to 0.2:1:1 (molar ratio), various interesting supramolecular polymer systems, such as hydrogen‐bonding complex, dynamic polyrotaxane, crystalline inclusion complex, and thermoresponsive hydrogel, are successively obtained. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1114–1120, 2008     

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.     

Unidirectional Threading into a Bowl‐Shaped Macrocyclic Trimer of Boron–Dipyrrin Complexes through Multipoint Recognition

Bowl‐shaped macrocycles have the distinctive feature that their two sides are differentiated, and thus can be developed into elaborate hosts that fix a target molecule in a controlled geometry through multipoint interactions. We now report the synthesis of a bowl‐shaped macrocyclic trimer of the boron–dipyrrin (BODIPY) complex and its unidirectional threading of guest molecules. Six polarized B^δ+?F^δ‐ bonds are directed towards the center of the macrocycle, which enables strong recognition of cationic guests. Specifically, the benzylbutylammonium ion is bound in a manner in which the benzyl group is located at the convex face of the bowl and the butyl group at its concave face. Furthermore, adrenaline was strongly captured on the convex side of the bowl by hydrogen bonding, Coulomb forces, and C?H???π interactions.     

A Halogen‐Bond‐Induced Triple Helicate Encapsulates Iodide

The self‐assembly of higher‐order anion helicates in solution remains an elusive goal. Herein, we present the first triple helicate to encapsulate iodide in organic and aqueous media as well as the solid state. The triple helicate self‐assembles from three tricationic arylethynyl strands and resembles a tubular anion channel lined with nine halogen bond donors. Eight strong iodine???iodide halogen bonds and numerous buried π‐surfaces endow the triplex with remarkable stability, even at elevated temperatures. We suggest that the natural rise of a single‐strand helix renders its linear halogen‐bond donors non‐convergent. Thus, the stringent linearity of halogen bonding is a powerful tool for the synthesis of multi‐strand anion helicates.     

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.     

Dramatic Enhancement of Binding Affinities Between Foldamer‐Based Receptors and Anions by Intra‐Receptor π‐Stacking

As a synthetic model for intra‐protein interactions that reinforce binding affinities between proteins and ligands, the energetic interplay of binding and folding was investigated using foldamer‐based receptors capable of adopting helical structures. The receptors were designed to have identical hydrogen‐bonding sites for anion binding but different aryl appendages that simply provide additional π‐stacking within the helical backbones without direct interactions with the bound anions. In particular, the presence of electron‐deficient aryl appendages led to dramatic enhancements in the association constant between the receptor and chloride or nitrate ions, by up to three orders of magnitude. Extended stacking within the receptor contributes to the stabilization of the entire folding structure of complexes, thereby enhancing binding affinities.     

Dynamic Properties of Molecular Tweezers with a Bis(2‐hydroxyphenyl)pyrimidine Backbone

4,6‐Bis(2‐hydroxyphenyl)‐2‐alkylpyrimidines with two anthryl or 9‐ethylnylanthryl substituents at the positions para to the OH groups prefer a U‐shaped conformation supported by two intramolecular OH ??? N hydrogen bonds in the solid state and in CDCl₃ solution. The compound with a hexyl substituent on the pyrimidine group and two 9‐ethynylanthryl arms at the hydroxyphenyl groups forms a 1:1 complex with 2,4,7‐trinitrofluorenone. Its association constant K_a was estimated to be 2100 M ^?1 at 298 K, which is larger than those of other molecular tweezers (K_a<1000 M ^?1). DFT calculations suggested that the complex adopts a stable conformation supported by intramolecular hydrogen bonds among the OH groups and the pyrimidine ring as well as by intermolecular π–π interaction between the anthryl groups and 2,4,7‐trinitrofluorenone. Addition of nBu₄NF to a solution of the molecular tweezers or their complexes causes the cleavage of one or two OH ??? N hydrogen bonds, formation of new O ??? HF hydrogen bonds, and changes in the molecular conformation. The resulting structure of the molecular tweezers contains nonparallel anthryl groups, which do not bind the guest molecule. Photochemical measurements on 4,6‐bis(2‐hydroxyphenyl)‐2‐methylpyrimidine with two anthryl substituents showed negligible luminescence (quantum yield ?<0.01), owing to photoinduced electron transfer of the molecule with a U‐shaped structure. However, the O‐hexylated compound exhibits emission from the anthryl groups with ?=0.39.     

A Dynamic and Responsive Host in Action: Light‐Controlled Molecular Encapsulation

The rational design of a flexible molecular box, oAzoBox ⁴⁺, incoporating both photochromic and supramolecular recognition motifs is described. We exploit the E?Z photoisomerization properties of azobenzenes to alter the shape of the cavity of the macrocycle upon absorption of light. Imidazolium motifs are used as hydrogen‐bonding donor components, allowing for sequestration of small molecule guests in acetonitrile. Upon E→Z photoisomerization of oAzoBox ⁴⁺ the guest is expelled from the macrocyclic cavity.     

Janus‐Like Squaramide‐Based Hosts: Dual Mode of Binding and Conformational Transitions Driven by Ion‐Pair Recognition

New tripodal squaramide‐based hosts have been synthesised and structurally characterised by spectroscopic methods. In 2.5 % (v/v) [D₆]DMSO in CDCl₃, compound 4 formed dimeric assemblies [log K_dim=3.68(8)] as demonstrated by ¹H NMR spectroscopy and UV dilution experiments. AFM and SEM analyses revealed the formation of a network of bundled fibres, which indicates a preferential mechanism for aggregation. These C₃‐symmetric tripodal hosts exhibited two different and mutually exclusive modes of binding, each one easily accessible by simultaneous reorientation of the squaramide groups. In the first, a convergent disposition of the NH squaramide protons allowed the formation of an array of N? H???X^? hydrogen bonds with anions. In the second mode, reorientation of carbonyl squaramide groups allowed multiple C?O???H interactions with ammonium cations. The titration of 4 with different tetraalkylammonium iodides persistently showed the formation of 1:1 complexes, as well as 1:2 and 1:3 complexes. The corresponding stoichiometries and binding affinities of the complexes were evaluated by multi‐regression analysis. The formation of high‐order complexes, supported by ROESY, NOESY and mass spectrometry experiments, has been attributed to the insertion of NR₄I ion pairs between the carbonyl and NH protons of the squaramide groups located in adjacent arms of 4 . The observed effects reflect the induction of significant conformational changes in the hosts, mainly in relation to the relative orientation of the squaramide groups adapting their geometries to incoming ion‐pair complementary substrates. The results presented herein identify and fully describe two different modes of ion‐pair recognition aimed at directing conformational transitions in the host, therefore establishing a base for controlling more elaborate movements of molecular devices through ion‐pair recognition.     

Controlled Dynamic Helicity of a Folded Macrocycle Based on a Bisterephthalamide with a Twofold Z‐Shaped Structure

Dynamic helicity in a folded macrocycle and control of the helical preference are described. We designed macrocycle 1 with a dual mode of folding through the integration of two flexible units that are arranged twice to form a cyclic structure. As a folding unit, we used a terephthalamide skeleton and a Z‐shaped hydrocarbon: the former acted as a control unit to induce a preference of a particular sense of dynamic helicity and the latter was just a spacer. A terephthalamide unit provided a binding site for capturing a ditopic hydrogen‐bonding guest when it adopted helically folded syn forms (M/P). Thus, only the terephthalamide unit controlled the helical sense of dynamic helicity in a folded macrocycle through the supramolecular transmission of chirality upon complexation with a chiral ditopic guest. In addition, chirality on a host could also contribute to the control of the helical preference in a folded macrocycle, which led to exceptionally enhanced chiroptical signals.     

Host–Guest Chemistry of a Water‐Soluble Pillar[5]arene: Evidence for an Ionic‐Exchange Recognition Process and Different Complexation Modes

The complexation of an anionic guest by a cationic water‐soluble pillararene is reported. Isothermal titration calorimetry (ITC), ¹H NMR, ¹H and ¹⁹F DOSY, and STD NMR experiments were performed to characterize the complex formed under aqueous neutral conditions. The results of ITC and ¹H NMR analyses showed the inclusion of the guest inside the cavity of the pillar[5]arene, with the binding constant and thermodynamic parameters influenced by the counter ion of the macrocycle. NMR diffusion experiments showed that although a fraction of the counter ions are expelled from the host cavity by exchange with the guest, a complex with both counter ions and the guest inside the pillararene is formed. The results also showed that at higher concentrations of guest in solution, in addition to the inclusion of one guest molecule in the cavity, the pillararene can also form an external complex with a second guest molecule.