Electrostatically Driven Guest Binding in Self-Assembled Molecular Network of Hexagonal Pyridine Macrocycle at the Liquid/Solid Interface: Symmetry Breaking Induced by Coadsorbed Solvent Molecules.

We present here the construction of a self-assembled two-dimensional network at the liquid/solid interface using a hexagonal pyridine macrocycle which binds an organic cation in its intrinsic porous space by electrostatic interactions. For this purpose, a hexagonal pyridinylene-butadiynylene macrocycle (PyBM) having six octyloxymethyl groups, PyBM-C8, was synthesized. As guests, tropylium (Tr) tetrafluoroborate and trioxatriangulenium (TOTA) hexafluorophosphate were used. In this study, we focused on (i) the network patterns of PyBM-C8 which change in response to its concentration and (ii) the position of the guest immobilized in the porous space of the macrocycle. Scanning tunneling microscopy (STM) observations at the interface of 1,2,4-trichlorobenzene (TCB) and highly oriented pyrolytic graphite (HOPG) revealed that PyBM-C8 formed four different polymorphs, oblique, loose hexagonal, linear, and rectangular, depending on the solute concentration and annealing treatment. Solvent TCB molecules are likely coadsorbed to not only the intrinsically porous space of PyBM-C8 (internal TCB) but also the space outside of the macrocycle between its alkyl chains (external TCB) in most of the cases. Upon adding the guest cation, whereas small Tr was not visualized in the pore due to size mismatching, larger TOTA was clearly observed in each pore. In addition, based on high-resolution STM images of the rhombus packing pattern of PyBM-C8, we revealed experimentally that TOTA was placed at an off-center position of the deformed hexagonal macrocyclic core in the rhombus pattern. On the basis of the molecular mechanics calculations, we hypothesize that the off-center location of TOTA is due to deformation of the hexagonal macrocycle through interaction with two external TCB molecules located at opposite edges of the macrocyclic core. Symmetry breaking of the macrocyclic host framework induced by coadsorbed surrounding solvent molecules thus plays a significant role in host-guest complexation at the liquid/solid interface.

Electrostatically Driven Guest Binding in a Self-Assembled Porous Network at the Liquid/Solid Interface.

We present here the construction of a self-assembled two-dimensional (2D) porous monolayer bearing a highly polar 2D space to study guest co-adsorption through electrostatic interactions at the liquid/solid interface. For this purpose, a dehydrobenzo[12]annulene (DBA) derivative, DBA-TeEG, having tetraethylene glycol (TeEG) groups at the end of the three alternating alkoxy chains connected by p-phenylene linkers was synthesized. As a reference host molecule, DBA-C10, having nonpolar C10 alkyl chains at three alternating terminals, was employed. As guest molecules, hexagonal phenylene-ethynylene macrocycles (PEMs) attached by triethylene glycol (TEG) ester and hexyl ester groups, PEM-TEG and PEM-C6, respectively, at each vertex of the macrocyclic periphery were used. Scanning tunneling microscopy observations at the 1,2,4-trichlorobenzene/highly oriented pyrolytic graphite interface revealed that PEM-TEG was immobilized in the pores formed by DBA-TeEG at higher probability because of electrostatic interactions such as dipole-dipole and hydrogen bonding interactions between oligoether units of the host and guest, in comparison to PEM-C6 with nonpolar groups. These observations are discussed based on molecular mechanics simulations to investigate the role of the polar functional groups. When a nonpolar host matrix formed by DBA-C10 was used, however, only phase separation and preferential adsorption were observed; virtually no host-guest complexation was discernible. This is ascribed to the strong affinity between the guest molecules which form by themselves densely packed van der Waals networks on the surface.

Hexagonal Molecular Tiling by Hexagonal Macrocycles at the Liquid/Solid Interface: Structural Effects on Packing Geometry.

We present here hexagonal tiling using hexagonal phenylene-ethynylene and phenylene-butadiynylene macrocycles attached by alkyl ester groups, PEM-C6 and PBM-C8, respectively, or triethylene glycol ester groups, PEM-TEG and PBM-TEG, respectively, at each vertex of the macrocyclic periphery at the liquid/solid interface. In this study, we focused on the effects of macrocyclic core size and the chemical properties of side chains attached to macrocyclic cores as well as solute concentrations on the hexagonal geometry of self-assembled monolayers. STM observations at the 1,2,4-trichrolobenzene/graphite interface revealed that PEM-C6 formed a honeycomb structure by van der Waals interactions between the interdigitated alkyl chains. However, upon increasing solute concentration, it changed to more dense hexagonal structure (tentatively called loose hexagonal structure I). In contrast, PBM-C8 formed loose hexagonal structure II of a slightly different packing mode at low concentration, while at high concentration it formed a high-density hexagonal structure in which alkyl chains are not adsorbed on the surface (dense hexagonal structure). In the dense hexagonal structure, macrocyclic cores are linked by hydrogen bonds between the ester carbonyl oxygen and the aromatic hydrogen atoms of the neighboring macrocycles. The packing geometries of loose hexagonal structures of PEM-C6 and PBM-C8 are different due to the different distance between the attachment of the alkyl ester groups which are located in confined space. On the other hand, PEM-TEG and PBM-TEG formed dense hexagonal structures, similar to PBM-C8 at high concentration, with their TEG units not adsorbed on the surface.

Square tiling by square macrocycles at the liquid/solid interface: co-crystallisation with one- or two-dimensional order.

We have systematically investigated the self-assembled monolayers of seven bimolecular mixtures of square-shaped pyridinophanes and cyclophanes bearing alkoxy or alkoxycarbonyl substituents in the presence of the tropylium ion as a marker of pyridinophanes at liquid/graphite interfaces by means of scanning tunnelling microscopy (STM). The purpose of this work was to elucidate the mixing behaviour of these macrocycles highlighting the formation of one- or two-dimensionally ordered square tilings consisting of alternating alignments of different macrocycles as a result of attractive dipole-dipole or hydrogen-bonding interactions; four co-crystals differing in the dimensionality of the ordering of pyridinophane and cyclophane were observed. The different modes of interaction between the functional groups (ether or carbonyl group) in the side-chains of the pyridinophanes and cyclophanes lead to the formation of co-crystals with dimensionally different orderings of the two macrocycles. These observations revealed that a slight modification of the molecular structure may dramatically change the mixing behaviour and structures of the co-crystals.

Axle length does not affect switching dynamics in degenerate molecular shuttles with rigid spacers.

For a series of [2]rotaxane molecular shuttles possessing linear rigid rod-like axles of varying lengths between degenerate recognition sites, the activation barrier for shuttling motion was clearly shown to be constant. Moreover, dynamic NMR studies have revealed that both the entropic and enthalpic contributions to the shuttling motion remain constant regardless of the actual length of the rigid rod-like axles employed herein.

Site-selective guest inclusion in molecular networks of butadiyne-bridged pyridino and benzeno square macrocycles on a surface.

We present here the formation of a modular 2D molecular network composed of two different types of square-shaped butadiyne-bridged macrocycles, having intrinsic molecular voids, aligned alternately at the solid-liquid interface. Site-selective inclusion of a guest cation took place at every other molecular void in the molecular network with two different recognition sites.

Highly selective and high-yielding rotaxane synthesis via aminolysis of prerotaxanes consisting of a ring component and a stopper unit.

A highly rotaxane-selective synthesis via aminolysis of prerotaxanes, which were composed of a phenolic pseudo-crown ether as a ring component and a bulky stopper unit, was developed. The best result was obtained in the case of aminolysis of 3b with 3,5-dimethylbenzylamine which proceeded quantitatively with ca. 100% rotaxane selectivity forming the corresponding rotaxane 5b. The rotaxanes were formed by kinetically controlled attack of the amine from the backside of the ring component of the prerotaxanes.

Preparation and evaluation of a chiral stationary phase covalently bound with a chiral pseudo-18-crown-6 ether having a phenolic hydroxy group for enantiomer separation of amino compounds.

In order to develop a chiral stationary phase (CSP), which has even higher separation ability than the corresponding commercially available crown ether based CSP (OA-8000 having a pseudo-18-crown-6 ether with an OMe group as a selector), chemically bonded type CSP having a phenolic OH group on a crown ring was developed. Normal mobile phases with or without acid additive can be used with this OH type CSP in contrast to the conventional OMe type CSP which has a neutral chiral selector. Enantiomers of 25 out of 27 amino compounds, including 20 amino acids, 5 amino alcohols, and 2 lipophilic amines, were efficiently separated on a column with this CSP. Nine amino compounds out of 27 were separated with better separation factors than the corresponding OMe type CSP. It is noteworthy that the chromatography on this CSP exhibited excellent enantiomer-separations for amines and amino alcohols when triethyl amine was used as an additive in the mobile phase. Comparison of enantiomer separation ability on this OH type of CSP and on the OMe type of CSP and correlation between the enantioselectivity in chiral chromatography and that of the corresponding model compounds in solution imply that the chiral separation arose from chiral recognition in host guest interactions.

Construction of cyclic arrays of Zn-porphyrin units and their guest binding at the solid-liquid interface.

Cyclic arrays consisting of six zinc-porphyrin units are constructed by the supramolecular self-assembly of a dehydrobenzo[12]-annulene derivative having three zinc porphyrin units at the liquid/graphite interface. Binding with C60 furnishes cyclic hexameric arrays of the complexes on the surface.

Preparation and evaluation of a chiral stationary phase covalently bound with chiral pseudo-1 8-crown-6 ether having 1-phenyl-1,2-cyclohexanediol as a chiral unit.

A chiral stationary phase (CSP) has been prepared by chemically bonding a chiral pseudo-18-crown-6 type host having a 1-phenyl-1,2-cyclohexanediol unit to 3-aminopropyl silica gel. The chiral column was prepared by the slurry-packing method in a stainless steel HPLC column. Normal mobile phases can be used with this CSP in contrast to conventional dynamic coating type CSPs. Enantiomers of 20 out of 30 amino compounds, including 20 amino acids, 2 amino acid methyl esters, 6 amino alcohols, and 2 lipophilic amines, were efficiently separated on columns with this CSP. It is noteworthy that 15 amino compounds out of 30 were separated with better separation factors and shorter retention times compared to the corresponding CSP having pseudo-18-crown-6 with 1-phenyl-1,2-ethanediol as a chiral unit. In view of the correlation between the enantiomer selectivities observed in chromatography and those obtained in gas phase FABMS-EL methods and solution phase titrations, chiral recognition in the host-guest interaction likely contributes to enantiomer separation.

Novel Self-Assembly of m-Xylylene Type Dithioureas by Head-to-Tail Hydrogen Bonding.

Dithiourea 1a self-assembles to form an orthogonal dimer structure both in solution and in the solid state, wherein the four thiourea groups establish a closed network of hydrogen bonds through a head-to-tail binding mode. This novel dimer structure was elucidated on the basis of (1)H NMR spectra, vapor pressure osmometry, and X-ray crystal structure analysis. Furthermore, a series of m-xylylene type dithioureas were synthesized and their dimerization constants (K(a)) in CDCl(3) were determined by dilution experiments using (1)H NMR spectroscopy. The magnitude of the K(a) values are dependent on the steric bulk of the side chains, the acidity of the thiourea groups, and the weak intermolecular interaction between the benzene rings of the side chains and the m-xylylene spacer.

Depression of the apparent chiral recognition ability obtained in the host-guest complexation systems by electrospray and nano-electrospray ionization mass spectrometry.

Chiral recognition in the host-guest complexation systems of chiral crown ether hosts and amino ester guests was thoroughly examined using the electrospray ionization (ESI) mass spectrometry/enantiomer labeled (EL)-guest method. In this method, the mass spectra of a mixture of three components in a solution, a chiral host (H), an equal amount of an (S)-enantiomer guest labeled with deuterium atoms (G(S-dn)(+)) and an unlabeled (R)-enantiomer guest (G(R)+), were measured and the relative peak intensity value [I(H + G(R))(+) / I(H + G(S-dn))(+) = IRIS] of the host-guest complex ions, observed with an excess guest concentration, was taken to provide the chiral recognition ability of the host. In our earlier report (1996), we demonstrated that the apparent chiral recognition abilities using a mass spectrometer with a homemade ESI interface were depressed by about one tenth compared with the corresponding abilities obtained by fast-atom bombardment (FAB) MS. In the present study, the enantioselective complexation behaviors of various combinations of chiral crown hosts with chiral guests were further investigated in detail mainly using a modern commercial ESI/ion trap (IT) mass spectrometer. Consequently, it was found that the apparent IRIS values from the ESI-MS/EL-guest method changed significantly, depending upon the instrument used, and in particular, upon the ESI interfaces. Moreover, under the specific measuring conditions in ESI-IT-MS, the degrees of depression of the apparent chiral recognition abilities are roughly grouped into three classes, depending upon the number (or probably the type) of the hydrophobic substituents of the hosts. Representing the degrees by the slopes when plotting the apparent IRIS values in ESI-MS versus those in FAB-MS, the slopes for the three classes are (1) 1.0, (2) 0.7 and (3) 0.3; the higher the hydrophobicity of the hosts (and then, the host-guest complex ions), the lower the slope (the apparent enantioselectivity). Strengthening the degree of depression may be caused by an increase in the local concentration of the host close to the surface of the droplets produced during the electrospary ionization process. The chiral recognition ability (K(R )/ K(S)) in an equilibrated solution agrees quite well with the IRIS value in FAB-MS rather than that in ESI-MS.

Amplification of enantioselectivity and sensitivity based on non-linear response of molecular wire bearing pseudo-18-crown-6 to chiral amines.

Exploiting a non-linear response for chiral discrimination, an intelligent system capable of chirality amplification was designed and constructed as poly(phenyleneethynylene) having chiral pseudo-18-crown-6. Both sensitivity and selectivity were amplified from those limited by the law of mass action.

Novel chiral recognition beyond the limitation due to the law of mass action: highly enantioselective chiral sensing based on non-linear response in phase transition events.

Based on non-linear response in phase transition events exhibiting a sharp transmittance change, an intelligent chiral sensor capable of chirality amplification in aqueous media was designed and constructed as poly(N-isopropylacrylamide) having chiral pseudo-18-crown-6. Both sensitivity and selectivity of the sensor surmount the limitation due to the law of mass action.

Remarkable effects of chirality on deslipping reactions of diastereomeric rotaxanes and relevant mechanism involving pre-equilibrium.

The first clear differentiation on deslipping rates affected by the difference in the diastereomeric stereocenters of rotaxanes up to 8.4 times and unexpectedly large steric kinetic isotope effect on the deslipping reaction (ca. 20%) were observed. On the basis of the kinetic parameters, steric kinetic isotope effect, and (1)H NMR spectra of the nondeuteriated and deuteriated rotaxanes, we propose a deslipping mechanism involving pre-equilibrium.

An anthracene-based photochromic macrocycle as a key ring component to switch a frequency of threading motion.

A concept and demonstration of a switching in frequencies of molecular motions are described using a pseudorotaxane system. The setup consists of dibenzylammonium hexafluorophosphate and a photochromic dianthrylethane-based [24]crown-8-type macrocycle, which we designed as a key ring component for the pseudorotaxane system having photocontrollable threading functionality by changing the size of ring component due to the action of light.

Highly effective and reversible control of the rocking rates of rotaxanes by changes to the size of stimulus-responsive ring components.

We have designed and synthesized rotaxanes whose rates of rocking motion (pendular motion) were switched reversibly through changes to the size of the ring component in response to external stimuli. The ring molecules of the rotaxanes incorporate a metaphenylene unit, which swings like a pendulum, and a dianthrylethane unit, which undergoes reversible isomerization in response to photo- and thermal stimuli and changes the size of the ring component. The rocking rates were estimated quantitatively by variable-temperature (VT) NMR spectroscopy and saturation transfer experiments, which revealed substantial changes in the rates between the open and closed forms, particularly in the case of rotaxanes with an isopropoxy group attached to a phenylene unit.

A shuttling molecular machine with reversible brake function.

Design, synthesis, and demonstration of a prototype of a shuttling molecular machine with a reversible brake function are reported. It is a photochemically and thermally reactive rotaxane composed of a dianthrylethane-based macrocycle as the ring component and a dumbbell shaped molecular unit with two, secondary ammonium stations separated by a phenylene spacer as the axle component. The rate of shuttling motion was shown to be reduced to less than 1 % (from 340 to <2.5 s(-1)) by reducing the size of the ring component from 30-crown-8 to 24-crown-8 macrocycles upon photoirradiation. The ring component was turned back to 30-crown-8 by thermal ring opening, thus establishing a reversible brake function that works in response to photochemical and thermal stimuli.