Ineffective OH Pinning of the Flipping Dynamics of a Spherical Guest within a Tight‐Fitting Tube

A supramolecular/synthetic method has been devised to affix a sterically hindered substituent onto a fullerene guest encapsulated in a tubular host. A two‐wheeled complex of (C₅₉N)‐(C₅₉N) with a tubular host was oxidatively bisected to afford a C₅₉N⁺ cation captured in the tube. The C₅₉N⁺ cation in the tube was then trapped by ethanol or water, which led to an oxy substituent pinned on the guest. The guest motions within the tube were modulated by the pinned substituent, and up‐and‐down flipping motions were halted by an ethoxy substituent. A hydroxy substituent, however, was ineffective in halting the flipping motions, despite the tight‐fitting relationship between the tubular host and the spherical guest. Theoretical calculations of the dynamics revealed that the flipping motions were assisted by OH‐π hydrogen bonds between the guest and the carbon‐rich wall and that sliding motions of the OH group were also facilitated by deformations of the tube.     

Restricted Conformational Flexibility of a Triphenylamine Derivative on the Formation of Host–Guest Complexes with Various Macrocyclic Hosts

Herein, we report the host–guest‐type complex formation between the host molecules cucurbit[7]uril (CB[7]), β‐cyclodextrin (β‐CD), and dibenzo[24]crown‐8 ether (DB24C8) and a newly synthesized triphenylamine (TPA) derivative 1 X₃ as the guest component. The host–guest complex formation was studied in detail by using ¹H NMR, 2D NOESY, UV/Vis fluorescence, and time‐resolved emission spectroscopy. The chloride salt of the TPA derivative was used for recognition studies with CB[7] and β‐CD in an aqueous medium. The restricted internal rotation of the guest molecule on complex formation with either of these two host molecules was reflected in the enhancement of the emission quantum yield and the average excited‐state lifetime for the triphenylamine‐based excited states. Studies with DB24C8 as the host molecule were performed in dichloromethane, a medium that maximizes the noncovalent interaction between the host and guest fragments. The Förster resonance energy transfer (FRET) process involving DB24C8 and 1 (PF₆)₃, as the donor and acceptor fragments, respectively, was established by electrochemical, steady‐state emission, and time‐correlated single‐photon counting studies.     

Solid‐State Molecular Nanomagnet Inclusion into a Magnetic Metal–Organic Framework: Interplay of the Magnetic Properties

Single‐ion magnets (SIMs) are the smallest possible magnetic devices and are a controllable, bottom‐up approach to nanoscale magnetism with potential applications in quantum computing and high‐density information storage. In this work, we take advantage of the promising, but yet insufficiently explored, solid‐state chemistry of metal–organic frameworks (MOFs) to report the single‐crystal to single‐crystal inclusion of such molecular nanomagnets within the pores of a magnetic MOF. The resulting host–guest supramolecular aggregate is used as a playground in the first in‐depth study on the interplay between the internal magnetic field created by the long‐range magnetic ordering of the structured MOF and the slow magnetic relaxation of the SIM.     

The Host/Guest Type of Excited‐State Proton Transfer; a General Review

Contemporary progress regarding guest/host types of excited‐state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H‐bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited‐state double proton transfer (ESDPT) in 7‐azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non‐catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen‐bonded complexes both hydrogen‐bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited‐state thermodynamics are of importance to fine‐tune the proton transfer reaction in the non‐catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton‐transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited‐state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in de tail.     

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.     

Dynamic Gas‐Inclusion in a Single Crystal

In solid‐state science, most changing phenomena have been mysterious. Furthermore, the changes in chemical composition should be added to mere physical changes to also cover the chemical changes. Here, the first success in characterizing the nature of gas inclusion in a single crystal is reported. The gas inclusion process has been thoroughly investigated by in situ optical microscopy, single‐crystal X‐ray diffraction analyses, and gas adsorption measurements. The results demonstrated an inclusion action of a first‐order transition behavior induced by a critical concentration on the phase boundary. The transfer of phase boundary and included gas are strongly related. This relationship can generate the dynamic features hidden in the inclusion phenomena, which can lead to the guest capturing and transfer mechanism that can apply to spatiotemporal inclusion applications by using host solids.     

High Degree of Polymerization in a Fullerene‐Containing Supramolecular Polymer

Supramolecular polymers based on dispersion forces typically show lower molecular weights (MW) than those based on hydrogen bonding or metal–ligand coordination. We present the synthesis and self‐assembling properties of a monomer featuring two complementary units, a C₆₀ derivative and an exTTF‐based macrocycle, that interact mainly through π–π, charge‐transfer, and van der Waals interactions. Thanks to the preorganization in the host part, a remarkable log K_a=5.1±0.5 in CHCl₃ at room temperature is determined for the host–guest couple. In accordance with the large binding constant, the monomer self‐assembles in the gas phase, in solution, and in the solid state to form linear supramolecular polymers with a very high degree of polymerization. A MW above 150 kDa has been found experimentally in solution, while in the solid state the monomer forms extraordinarily long, straight, and uniform fibers with lengths reaching several microns.     

Retarded Solid‐State Rotations of an Oval‐Shaped Guest in a Deformed Cylinder with CH–π Arrays

Upon encapsulating an oval‐shaped hydrocarbon guest, a cylindrical host deforms its shape to maximize intermolecular contacts. Structure–assembly relationship studies with a series of hydrocarbon guests disclosed the importance of molecular shapes and CH–π contacts. Multiple contacts and weak CH–π hydrogen bonds resulted in an optimal assembly; however, the shape deformation resulted in severe retardation of rotational motions in the crystal. Thus, unlike a circular guest, the oval‐shaped guest did not change its orientation in the host. Unexpectedly, the planar guest did not affect the packing structure to form a double helix in intertwined host arrays.     

Encapsulated guest-host dynamics: guest rotational barriers and tumbling as a probe of host interior cavity space

The supramolecular host assembly [Ga(4)L(6)](12-) (1; L = 1,5-bis[2,3-dihydroxybenzamido]naphthalene) encapsulates cationic guest molecules within its hydrophobic cavity and catalyzes a variety of chemical transformations within its confined interior space. Despite the well-defined structure, the host ligand framework and interior cavity are very flexible and 1 can accommodate a wide range of guest shapes and sizes. These observations raise questions about the steric effects of confinement within 1 and how encapsulation fundamentally changes the motions of guest molecules. Here we examine the motional dynamics (guest bond rotation and tumbling) of encapsulated guest molecules to probe the steric consequences of encapsulation within host 1. Encapsulation is found to increase the Ph-CH(2) bond rotational barrier for ortho-substituted benzyl phosphonium guest molecules by 3 to 6 kcal/mol, and the barrier is found to depend on both guest size and shape. The tumbling dynamics of guests encapsulated in 1 were also investigated, and here it was found that longer, more prolate-shaped guest molecules tumble more slowly in the host cavity than larger but more spherical guest molecules. The prolate guests reduce the host symmetry from T to C(1) in solution at low temperatures, and the distortion of the host framework that is in part responsible for this symmetry reduction is observed directly in the solid state. Analysis of guest motional dynamics is a powerful method for interrogating host structure and fundamental host-guest interactions.     

Solid‐State Conformational Flexibility at Work: Zipping and Unzipping within a Cyclic Peptoid Single Crystal

A peptidomimetic compound undergoes a reversible single‐crystal‐to‐single‐crystal transformation upon guest release/uptake with the transformation involving a drastic conformational change. The extensive and reversible alteration in the solid state is connected to the formation of an unprecedented “CH–π zipper” which can reversibly open and close (through the formation of CH–π interactions), thus allowing for guest sensing.     

Recognition of N‐Alkyl and N‐Aryl Acetamides by N‐Alkyl Ammonium Resorcinarene Chlorides

N‐Alkyl ammonium resorcinarene chlorides are stabilized by an intricate array of intra‐ and intermolecular hydrogen bonds that leads to cavitand‐like structures. Depending on the upper‐rim substituents, self‐inclusion was observed in solution and in the solid state. The self‐inclusion can be disrupted at higher temperatures, whereas in the presence of small guests the self‐included dimers spontaneously reorganize to 1:1 host–guest complexes. These host compounds show an interesting ability to bind a series of N‐alkyl acetamide guests through intermolecular hydrogen bonds involving the carbonyl oxygen (C?O) atoms and the amide (NH) groups of the guests, the chloride anions (Cl^?) and ammonium (NH₂⁺) cations of the hosts, and also through CH ??? π interactions between the hosts and guests. The self‐included and host–guest complexes were studied by single‐crystal X‐ray diffraction, NMR titration, and mass spectrometry.     

Investigation of chloromethane complexes of cryptophane‐A analogue with butoxy groups using 13C NMR in the solid state and solution along with single crystal X‐ray diffraction

Host‐guest complexes between cryptophane‐A analogue with butoxy groups (cryptophane‐But) and chloromethanes (chloroform, dichloromethane) were investigated in the solid state by means of magic‐angle spinning ¹³C NMR spectroscopy. The separated local fields method with ¹³C‐¹H dipolar recoupling was used to determine the residual dipolar coupling for the guest molecules encaged in the host cavity. In the case of chloroform guest, the residual dipolar interaction was estimated to be about 19 kHz, consistent with a strongly restricted mobility of the guest in the cavity, while no residual interaction was observed for encaged dichloromethane. In order to rationalize this unexpected result, we performed single crystal X‐ray diffraction studies, which confirmed that both guest molecules indeed were present inside the cryptophane cavity, with a certain level of disorder. To improve the insight in the dynamics, we performed a ¹³C NMR spin‐lattice relaxation study for the dichloromethane guest in solution. The system was characterized by chemical exchange, which was slow on the chemical shift time scale but fast with respect to the relaxation rates. Despite these disadvantageous conditions, we demonstrated that the data could be analyzed and that the results were consistent with an isotropic reorientation of dichloromethane within the cryptophane cavity. Copyright © 2015 The Authors. Magnetic Resonance in Chemistry published by John Wiley & Sons Ltd.     

Acyclic Janus‐AT Nucleoside Host Channels Precisely Lock Water into Single‐File Wires with Local Rotational Flexibility

Confining polar water molecules to particular geometries demands sophisticated intermolecular interactions, and not many small synthetic molecules have accomplished such a task. Herein, regioisomeric acyclic Janus‐AT nucleosides ( 1 and 2 ), with a self‐complementary fused genetic alphabet and conformationally flexible side chains, have been selectively synthesized. 1 and 2 adopt disparate base‐pair motifs from the π–π stacked hydrophobic base moieties and distinct hydrogen bond (HB) interconnections from the hydrophilic sugar residues, which in turn lead to divergent, intricate intermolecular interaction networks with different capacities to confine water molecules. Under the precise control of the host framework of the N⁸‐regioisomer, separate ordered single‐file water wires can be locked through special three‐HB clamps into unique inter‐ and intra‐wire geometrical alignments. Localized dynamic synchronized rotations within the fixed framework coordinated by both the host hydroxy groups and guest water molecules were observed in a temperature‐induced reversible single‐crystal‐to‐single‐crystal transition (SCSCT).     

Dyes in Liquid Crystals: Experimental and Computational Studies of a Guest–Host System Based on a Combined DFT and MD Approach

Practical applications of guest–host liquid crystal systems are critically dependent on the alignment of the guest species within the liquid crystal host. UV/Vis absorption spectroscopy shows that the 1,5‐dihydroxy‐2,6‐bis‐(4‐propylphenyl)‐9,10‐anthraquinone dye aligns within the E7 nematic host, giving an experimental dichroic ratio of 9.40 and dye order parameter of 0.74. This alignment was modelled by using a combination of density functional theory (DFT) and molecular dynamics (MD) computational approaches that do not require the input of experimental data. Time‐dependent DFT calculations show that the electronic transition dipole moment is highly aligned with the long molecular axis of the dye. Fully atomistic MD simulations show that the long axis of the dye is less highly aligned within the E7 host, indicating that this contribution limits the overall dye alignment and, thereby, the potential practical applications of this particular system. Importantly, this study demonstrates an experimental and combined DFT and MD computational approach that may be applied generally to guest–host systems, providing a potential route to their rational design.     

Resorcinarene Bis‐Thiacrowns: Prospective Host Molecules for Silver Encapsulation

Mixed‐donor atom tetramethoxy resorcinarene bis‐thiacrown hosts, in which the crown unit contains both hard oxygen and soft sulfur donor atoms, were synthesized for soft metal cation binding. The binding properties were investigated both in solution and in the solid state by NMR spectroscopy and X‐ray crystallography. It was found that the resorcinarene bis‐thiacrowns were able to complex silver cations with remarkable affinity forming readily 1:2 host–guest complexes in solution. The solid state structures also revealed that the bis‐thiacrowns form silver complexes in an unanticipated endo‐ and exo‐cavity fashion within the same host molecule. Both the solution and solid state studies indicated the sulfur atoms to be the major contributing donor atoms in forming the binding interactions with silver cations.     

Electrospray‐Ionization Mass Spectrometry for Screening the Specificity and Stability of Single‐Stranded‐DNA Templated Self‐Assemblies

Supramolecular complexes consisting of a single‐stranded oligothymine ( dTn ) as the host template and an array of guest molecules equipped with a complementary diaminotriazine hydrogen‐bonding unit have been studied with electrospray‐ionization mass spectrometry (ESI‐MS). In this hybrid construct, a supramolecular stack of guest molecules is hydrogen bonded to dTn . By changing the hydrogen‐bonding motif of the DNA host template or the guest molecules, selective hydrogen bonding was proven. We were able to detect single‐stranded‐DNA (ssDNA)–guest complexes for strands with lengths of up to 20 bases, in which the highest complex mass detected was 15 kDa; these complexes constitute 20‐component self‐assembled objects. Gas‐phase breakdown experiments on single‐ and multiple‐guest–DNA assemblies gave qualitative information on the fragmentation pathways and the relative complex stabilities. We found that the guest molecules are removed from the template one by one in a highly controlled way. The stabilities of the complexes depend mainly on the molecular weight of the guest molecules, a fact suggesting that the complexes collapse in the gas phase. By mixing two different guests with the ssDNA template, a multicomponent dynamic library can be created. Our results demonstrate that ESI‐MS is a powerful tool to analyze supramolecular ssDNA complexes in great detail.     

Co‐Conformational Isomerism in a Neutral Ion‐Paired Supramolecular System

Two different counter‐ion‐free host–guest complexes have been prepared and isolated. These compounds were formed from two equally and opposite doubly‐charged species, the viologen guests 1 a ²⁺ and 1 b ²⁺ and the anti‐disulfodibenzo[24]crown‐8 [ DSDB24C8] ^2? host, which gave rise to the 1:1 neutral complexes [ 1 a?DSDB24C8 ] and [ 1 b?DSDB24C8 ]. These species are held together by hydrogen bonding and π stacking, as well as strong electrostatic interactions. The investigation of these neutral ion‐paired supramolecular systems in solution and in the solid state allowed us to establish their co‐conformational preferences. Compound [ 1 a?DSDB24C8 ], with small methyl groups as substituents on the viologen unit, may adopt three different geometries, 1) an exo nonthreaded, 2) a partially threaded, and 3) a threaded arrangement, depending on the relative spatial orientation between the host and guest: The partially‐threaded structure is preferred in solution and in the solid state. The presence of bulky tert‐butylbenzyl groups in the viologen moiety in compound [ 1 b?DSDB24C8 ] restricts the possible geometrical arrangements to one: The exo nonthreaded arrangement. This structure was confirmed in the solid state by X‐ray crystallography. The stability of the neutral complexes in solution was determined by UV/Vis spectrophotometry. The stoichiometry of the complexes was established by continuous variation experiments, and overall equilibrium constants and ΔG° values were determined on the basis of dilution experiments. The results observed are a consequence of only the intrinsic stability of the complexes as there are no additional contributions from counter ions.     

Janusarene: A Homoditopic Molecular Host

A homoditopic molecular host, janusarene, is presented that has two back‐to‐back compactly arranged nanocavities for guest complexation. The unique two‐face structural feature of janusarene allows it to bind and align various guest compounds concurrently, which include spherical pristine fullerene C₆₀ and planar polycyclic aromatic hydrocarbons (PAHs), such as pyrene, perylene, and 9,10‐dimethylanthracene. The host–guest interactions were characterized by single‐crystal X‐ray diffraction. A pairwise encapsulation of the PAH guests by janusarene enables PAH dimers to be obtained that deliver spectroscopic properties distinct from those of PAHs dissolved in solution, or in the bulk state. A monotopic control host was also synthesized and used to characterize the host–guest complexing behavior in solution.     

Halogen‐Bond‐Templated [2+2] Photodimerization in the Solid State: Directed Synthesis and Rare Self‐Inclusion of a Halogenated Product

A ditopic halogen‐bond acceptor organizes a diiodooctafluorostilbene for a [2+2] photodimerization reaction to take place between two stilbene molecules in the solid state. The resultant cyclobutane product is functionalized with halogen atoms and undergoes self‐assembly to form a channel‐type host–guest compound that exhibits a very rare form of self‐inclusion.