Kazuki Sada

“Clickable” Metal−Organic Framework

We demonstrated the metal-organic framework bearing the azide group in the organic linkers and in situ click reactions with some small alkynes. The XRPD patterns indicated that the click reaction proceeded without any decomposition of the original MOF network. Controlling the organic linkers and incorporation of the azide groups should provide the designer-made MOFs that have controlled molecular cavities with the desired steric dimensions and functionality.

Post-polymerization of preorganized assemblies for creating shape-controlled functional materials.

Combination of supramolecular chemistry with molecular recognition has been successfully applied to creating large superstructures with a wide variety of morphologies. Control of shapes and patterns of ordered molecular assemblies in nano and micro scales has attracted considerable interest as promising bottom-up technology. It is known, however, that these molecular assembling superstructures are fragile, reflecting the characteristic of the non-covalent interaction, a driving force operating in these molecular systems. In fact, they easily collapse or change by small perturbation in the environmental conditions. Thus, over the last decade, researchers have been seeking possible methods for the immobilization these superstructures. This critical review focuses on recent advances in in situ post-modification under the influence of the molecular assemblies as templates and polymerization of ordered molecular assemblies such as organogel fibers and crystals to preserve their original superstructures and intensify their mechanical strength.

Binary organogelators which show light and temperature responsiveness.

The gelation ability of 10 alkylammonium (CnH(2n+1)NH3+ where n=4-11, 12 and 16) anthracene-9-carboxylates (1n) has been evaluated. In cyclohexane, 1(4), 1(5), 1(6) and 1(7) only provided precipitates whereas 1(11), 1(12) and 1(16) provided very viscous solutions. In contrast, 1(8) 1(9) and 1(10) resulted in gels. The critical gelation concentration of 1(10) was very low (5.0 x 10(-4) mol dm(-3)). SEM observations showed that in the gel phase the morphology changes from straight fibrils to frizzy fibrils with the increase in n, whereas in the sol phase the formation of the sheet-like, two-dimensional aggregate is recognized. When the cyclohexane 1(10) gel was photoirradiated (lambda > 300 nm), the UV-VIS absorption bands assignable to monomeric anthracene were decreased and the gel was changed into the sol. It was confirmed by dark-field optical microscopy that the fibrillar bundles supporting the gel formation gradually disappear with photoirradiation time. When this sol was warmed at 30 degrees C in the dark, the gel was not regenerated but the precipitation of 1(10) resulted. When this sol was heated once at the bp of cyclohexane and cooled to 15 degrees C, the solution was changed into the gel again. This finding indicates that the fibrillar structure required for the gel formation is not reconstructed at 30 degrees C but obtained only when the hot cyclohexane solution is cooled.

Intercalation of alkylamines into an organic polymer crystal

Organic solid-state synthesis allows formation of products that are difficult or impossible to produce by conventional methods. This feature, and the high degree of reaction selectivity that can be achieved, is a direct result of the control over the relative orientation of the reactants afforded by the solid state. But as the successful development of ‘topochemical reactions’ requires the careful design of suitable reactant crystals, the range of both reactions and products amenable to this approach has been limited. However, recent advances in organic crystal engineering, particularly the rational design of complex solid architectures through supramolecular preorganization, have renewed interest in topochemical reactions. Previously, we have orientated muconate monomers—diene moieties with a carboxylate group on each end—using long-chain n-alkylammonium ions, such that the topochemical photopolymerization of the solid-state reactants produces layered crystals of stereoregular and high-molecular-mass polymers. Here we show that these polymer crystals are capable of repeated, reversible intercalation by conversion to the analogous poly(carboxylic acid), followed by transformation into a number of poly(alkylammonium muconate)s upon addition of the appropriate amine. Introduction of functional groups into these crystals may allow the design of organic solids for applications such as molecular recognition, separation and catalysis, thereby extending the range and practical utility of current intercalation compounds.

Importance of Packing Coefficients of Host Cavities in the Isomerization of Open Host Frameworks: Guest‐Size‐Dependent Isomerization in Cholic Acid Inclusion Crystals with Monosubstituted Benzenes

The crystal structures of inclusion compounds of cholic acid (CA) with 28 monosubstituted benzenes have been systematically investigated. All of the crystals belong to the monoclinic space group P2(1) and have bilayer structures with one-dimensional molecular channels that can include guest compounds. They are classified into four types of host frameworks that depend on the conformations and stacking modes of the host compound. The host frameworks and the host-guest ratios depend primarily on the molecular volumes of the guest compounds. The packing coefficient of the host cavity (PCcavity), which is the volume ratio of the guest compound to the host cavity, is used to clarify the relationship between the guest volume and isomerization of the host frameworks. The value of PCcavity, for stable inclusion compounds lies in the range of 55-70%. Compounds out of this range induce isomerization of the host frameworks. The packing coefficients of other host-guest compounds, in which the guest components are included in the host cavities through steric dimensions and van der Waals forces, are also in this range. These results indicate that PCcavity is a useful parameter correlation for guest recognition and isomerization of the host frameworks.

Transcription of chirality in the organogel systems dictates the enantiodifferentiating photodimerization of substituted anthracene.

An organogelator (G) that contains 2-anthracenecarboxylic acid (2Ac) attached covalently to a gelator counterpart that consists of 3,4,5-tris(n-dodecan-1-yloxy)benzoic acid by means of a chiral amino alcohol linkage has been synthesized. G acts as an efficient gelator of organic solvents, including mixed solvents and chiral solvents. Photodimers isolated after the photoreaction of the gel samples display different degrees of stereoselectivity. In the gel state, the formation of head-to-head (h-h) photodimers is always favored over head-to-tail (h-t) photodimers. Enantiomeric excess (ee) values of the major h-h photodimers reached as high as -56% in the case of the gels with enantiomeric glycidyl methyl ethers. Here, the solvent chirality is outweighed by the intrinsic chirality of the gelator molecule. The packing of the chromophore in the gel state has been characterized by the absorption and the emission behaviors and their variations during the course of gel-to-sol phase transition. Whereas for the hexane gel, emission intensity increases with an increase in temperature, other systems show a decrease in emission intensity. Redshift of the lambda(max) in the gel spectra indicates the J-aggregate arrangement of the chromophores. Chiral transcription in the gel state has been investigated by CD spectroscopy, which shows a decrease in CD intensity during the gel-to-sol phase transition. The X-ray diffraction study clearly differentiates among the gels in terms of the order of molecular arrangements. The gel systems are categorized as strong, moderately strong, and weak, that originate from the cooperative or individual participations of intermolecular hydrogen-bonding and pi-pi interactions, fine-tuned by the solvent polarity and the gelation temperature. A simple model based on the experimental findings and the molecular preorientation as evidenced by the stereochemistry of the photodimers has been proposed.

Polymer Phase‐Transition Behavior Driven by a Charge‐Transfer Interaction

Charge-transfer (CT) interactions are intermolecular interactions between p-electron-rich (donor molecules) and pelectron-deficient species (acceptor molecules). The high specificity of CT interactions enables the arrangement between a donor and an acceptor to be controlled, and the resulting CT complex shows a characteristic absorption band in the visible region; this absorption band provides information on the association of supramolecular complexes. Therefore, CT interactions are among the most powerful tools for the design of supramolecular complexes, such as organic crystalline materials with superconductivity or conductivity, low-molecular-weight organic gelators, preorganized building blocks for rotaxane, and supramolecular polymers. In many cases, the supramolecular CT complexes can be collapsed readily by heating, since a CT interaction provides a bond of modest strength unless a cavity, such as that of cucurbit[8]uril, is used to stabilize the CT complex. Herein, we demonstrate the precise and facile control of the lower critical solution temperature (LCST) of a polymer solution, that is, its phase-transition behavior, by the use of a CT interaction. The well-known phase-transition behavior associated with the LCST of a polymer solution occurs as a result of primitive molecular recognition between the polymer chains and solvent molecules (e.g., H2O). It has attracted broad interest with respect to the development of stimuli-sensitive materials because the solubility of the polymer chains changes drastically upon heating. However, one of the major drawbacks of this phenomenon is the restriction imposed by the required conditions (temperature, solvent, or pressure); as result, stimuli-sensitive materials are limited to those based on intrinsic LCST polymers, such as poly(N-isopropylamide) (PNIPAM) and poly(ethylene glycol) (PEG). Sophisticated supramolecular control of the LCST has been paid much attention, because such studies can uncover not only a fundamental perspective of the phenomenon but also potential applications in smart materials. As an outstanding example, Ritter and co-workers reported that a polymer bearing adamantyl moieties showed LCST behavior in the presence of methylated b-cyclodextrins (b-CD) in water owing to the dissociation of b-CD from the adamantyl group in the polymer chain upon heating and the subsequent association of adamantyl groups through a hydrophobic interaction. We also demonstrated readily adjustable LCST behavior based on hydrogen bonding between a polymer bearing urea units and an alcohol as an “effector” for polymer solubility. In both cases, the dissociation of supramolecular complexes at an increased temperature triggered a drastic change in solubility of the polymers. These findings inspired us to explore the applicability of other weak intermolecular interactions between polymers and effectors to clarify the molecular design of LCST polymer chains and external effectors. Herein, we demonstrate the first example of LCST behavior based on a CT interaction as the driving force (Figure 1). As the result of the appearance of a CT band in the UV/Vis absorption spectrum, a quantitative evaluation of the relationship between LCST behavior and the formation of supramolecular complexes was possible. Previously, the distinction between polymer–polymer interactions and polymer–effector (solvent) interactions was considered problematic in the common case of LCST behavior driven by hydrogen bonding.

Molecular Rotation in Self-Assembled Multidecker Porphyrin Complexes

An alkyl chain-substituted multidecker porphyrin (a cerium double-decker porphyrin (CeDDP) and a lanthanum triple-decker porphyrin (LaTDP)) complexes were arranged in a monolayer array on Au(111) substrate. By using a pulse injection deposition method, both multidecker complexes were deposited on the surface intact to form a well-defined two-dimensional array. Low-temperature scanning tunneling microscopy (STM) allowed the measurement of the topographic heights of the multidecker porphyrin complexes and visualization of their internal structures clearly. The STM images suggest that the top porphyrin ligand in CeDDP rarely rotates under nondestructive imaging condition, while the top porphyrin ligand in LaTDP exhibits flip-flop rotation even under the nondestructive imaging condition at sub-pA tunneling currents. These results provide the future applications of molecular-scale mechanical machines and single molecule storage memory.

Construction of superhydrophobic surfaces by fibrous aggregation of perfluoroalkyl chain-containing organogelators

Superhydrophobic surfaces, characterized by water contact angles greater than 150 degrees, can be produced by means of intermediate organogels, which were formed by perfluoroalkyl chain-containing organogelators with volatile organic solvents.

Alternate Layer-by-Layer Adsorption of Single- and Double-Walled Carbon Nanotubes Wrapped by Functionalized β-1,3-Glucan Polysaccharides

A great deal of attention has been focused on exploiting novel methods to fabricate thin carbonaceous capsules from multiple components for advanced materials. A layer-by-layer (LbL) method is therefore being introduced to synthesize thin and multi-carbon nanotube (CNT)-based hollow capsules from CNT complexes with cationic or anionic complementarily functionalized beta-1,3-glucans as building-blocks. These ionic beta-1,3-glucans wrap around single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) to form water-soluble complexes with ionic groups on their exterior surface. Alternate self-assembly of these CNT complexes on the silica particles is demonstrated in solution by electrostatic interactions. The LbL adsorption processes were carefully monitored by zeta-potential measurements, frequency shifts of a quartz crystal microbalance (QCM), and electron micrographs. Silica particles were then dissolved away by HF acid to obtain CNT-based hollow capsules composed of SWNTs and DWNTs. We believe that these novel surface adsorption methods are useful for potential design of CNT-based advanced functional materials.