Takahiro Kakuta

Separation of Linear and Branched Alkanes Using Host–Guest Complexation of Cyclic and Branched Alkane Vapors by Crystal State Pillar[6]arene

Activated crystals of pillar[6]arene produced by removing the solvent upon heating were able to take up branched and cyclic alkane vapors as a consequence of their gate-opening behavior. The uptake of branched and cyclic alkane vapors by the activated crystals of pillar[6]arene induced a crystal transformation to form one-dimensional channel structures. However, the activated crystals of pillar[6]arene hardly took up linear alkane vapors because the cavity size of pillar[6]arene is too large to form stable complexes with linear alkanes. This shape-selective uptake behavior of pillar[6]arene was further utilized for improving the research octane number of an alkane mixture of isooctane and n-heptane: interestingly, the research octane number was dramatically improved from a low research octane number (17 %) to a high research octane number (>99 %) using the activated crystals of pillar[6]arene.

Stimuli-Responsive Supramolecular Assemblies Constructed from Pillar[n]arenes

Supramolecular assemblies are constructed from at least two molecules through various noncovalent bonding modes such as hydrogen bonding, cationic-anionic electrostatic interactions, aromatic interactions, metal-ligand bonding, hydrophobic-hydrophilic interactions, and charge-transfer interactions. Owing to the dynamic and reversible nature of these noncovalent bonds, the assembly and disassembly of these molecules are dynamic and reversible. Molecules self-assemble to form the most conformationally and thermally stable structures through these noncovalent interactions. The formation of these noncovalent interactions is affected by the properties of the environment such as its polarity, temperature, and pressure; thus, the structure of the assembled compounds is determined by the environment. The sizes and shapes of the supramolecular assemblies play an important role in determining their functions. Therefore, controlling their size and shape is important. Introducing stimuli-responsive groups into supramolecular assemblies is a useful way to control their size and shape. Controlling supramolecular structures and motions with external stimuli, i.e., periodic and rotational motions on the molecular scale, structures, and molecular weights at the nano- and micrometer scales, visible shrinking/expansion, and adhesive behavior at a macroscopic scale, is very useful. Macrocyclic host molecules are useful building blocks for the construction of stimuli-responsive supramolecular assemblies because their host ability can be tuned by changing the shape and electron density of the cavity. The size-dependent hosting ability of the cavity is similar to the lock-and-key model in biological systems. Stimuli-responsive supramolecular assemblies have been developed by using macrocyclic compounds such as cyclodextrins, cucurbit[ n]urils, calix[ n]arenes, crown ethers, and related macrocycles. We successfully developed new pillar-shaped macrocyclic hosts in 2008, which were coined pillar[ n]arenes. The unique structural features of pillar[ n]arenes allowed new properties. This year, 2018, marks one decade of research into pillar[ n]arene chemistry, and in that time the properties of pillar[ n]arenes have been widely investigated by various scientists. Thanks to their efforts, the characteristic properties of pillar[ n]arenes that result from their pillar-shaped structures have been elucidated. Their host ability, the chirality of their pillar-shaped structure, and their versatile functionality are unique features of pillar[ n]arenes not seen in other well-known hosts, and these properties are very useful for the creation of new stimuli-responsive supramolecular assemblies. In this Account, we describe photo-, pH- and redox-responsive supramolecular assemblies based on pillar[ n]arenes. First, we discuss molecular-scale stimuli-responsive supramolecular assemblies, i.e., pseudorotaxanes, pseudocatenanes, and supramolecular polymers. We also highlight subnanometer- and micrometer-scale stimuli-responsive supramolecular assembles such as particles and vesicles. Finally, we discuss the macroscopic stimuli-responsive structural changes of surfaces and gels. This Account will provide useful information for researchers working on not only pillar[ n]arene chemistry but also the chemistry of other macrocyclic hosts, and it will inspire new discoveries in the field of supramolecular assemblies and systems containing macrocyclic hosts.

Solid-state self-inclusion complexation behaviour of a pillar[5]arene-based host–guest conjugate

A host–guest conjugate consisting of a pillar[5]arene and an ethylene moiety containing a triazole group at one end and a perfluorooctyl group at the other end was synthesized. The host–guest conjugate displayed unusual real-time scale solid-state self-inclusion complexation behaviour. The host–guest conjugate formed a de-threaded supramolecular structure only in solution, even at high concentrations. However, heating the solid-state host–guest conjugate led to the formation of a self-inclusion complex structure at a real-time scale; at 100 °C, it took ca. 20 h to form the self-inclusion complex at the equilibrium state at a conversion of 85%. The self-inclusion complex converted to the de-threaded form at a real-time scale when it was stored in solvents with small molecules, which worked as competitive guests.

Molecular Space Chemistry Based on Pillar[n]arenes

In 2008, our group reported on pillar[n]arenes as novel-shaped macrocyclic compounds. Owing to the para-bridge connection between 1,4-dialkoxybenzene units, pillar[n]arenes adopt a highly symmetrical pillar and polygonal-shaped structure, which is different from typical host molecules. Because pillar[n]arenes exhibit these highly symmetrical structures with high functionality, many chemists have used pillar[n]arenes as building blocks to construct supramolecular assemblies. Therefore, pillar[n]arenes have applications in various fields such as material science and biochemistry. We discuss the host–guest ability of pillar[n]arenes, application of pillar[5]arenes as catalysts and reductants, and assembly of pillar[n]arenes on surfaces based on their pillar-shaped structures. The highly ordered assembled structures, based on their highly symmetrical polygonal prism shape, are also discussed.

Synthesis of optically active π-stacked compounds based on planar chiral tetrasubstituted [2.2]paracyclophane

Optically active X-shaped compounds based on planar chiral [2.2]paracyclophane were synthesized. These compounds composed of two different π-electron systems stacked at central aromatic rings through chemoselective Sonogashira–Hagihara coupling. The optical and chiroptical properties of the obtained compounds were investigated, which exhibited large molar absorption coefficients, good photoluminescence quantum efficiency, and intense circularly polarized luminescence (CPL) with large dissymmetry factors on the order of 10−3, indicating that the compounds are excellent CPL emitters.

Spacer Length-Independent Shuttling of the Pillar[5]arene Ring in Neutral [2]Rotaxanes

Abstract For a series of neutral [2]rotaxanes consisting of a pillar[5]arene ring and axles possessing two stations separated by flexible spacers of different lengths, the free energies of activation for the ring shuttling between the stations were found to be independent of the spacer length. The constitution of the spacer affects the activation energies: replacement of CH2 groups by repulsive oxygen atoms in the axle increases the barrier. The explanation for the observed length‐independence lies in the presence of a barrier for re‐forming the stable co‐conformation, which makes the ring travel back and forth along the thread in an intermediate state.

Pillar[ n ]arenes: Versatile Macrocyclic Receptors for Supramolecular Chemistry

In 2008, we reported new pillar-shaped macrocyclic hosts, known as “pillar[ n ]arenes.” Today, 8 years after our first report, pillar[ n ]arenes are recognized as important key players in supramolecular chemistry because of their many unique advantages. Pillar[ n ]arenes can be produced in high yields in a one-step reaction and have a unique pillar-shaped structure, versatile functionality, and interesting host–guest properties. In this article, first, we provide a brief description of the synthesis and structure of pillar[ n ]arenes. Secondly, we discuss typical host–guest properties of simple peralkylated pillar[ n ]arenes. Finally, we describe the synthesis of mono-, di-, penta-, and perfunctionalized pillar[ n ]arenes and provide several examples of the application of these functionalized pillar[ n ]arenes.