Tomoki Ogoshi

para-Bridged Symmetrical Pillar[5]arenes: Their Lewis Acid Catalyzed Synthesis and Host–Guest Property

Condensation of 1,4-dimethoxybenzene (DMB) with paraformaldehyde in the presence of BF3.O(C2H5)2 gave novel para-bridged pentacyclic pillar DMB (DMpillar[5]arene). Moreover, para-bridged pentacyclic hydroquinone (pillar[5]arene) was prepared. Pillar[5]arene formed 1:1 host-guest complexes with dialkyl viologen and alkyl pyridinium derivatives. However, pillar[5]arene did not form complexes with the diadamantyl viologen derivative since a bulky adamantyl group was unable to thread the cavity of pillar[5]arene.

Chemical Sensors Based on Cyclodextrin Derivatives

This review focuses on chemical sensors based on cyclodextrin (CD) derivatives. This has been a field of classical interest, and is now of current interest for numerous scientists. First, typical chemical sensors using chromophore appended CDs are mentioned. Various “turn-off” and “turn-on” fluorescent chemical sensors, in which fluorescence intensity was decreased or increased by complexation with guest molecules, respectively, were synthesized. Dye modified CDs and photoactive metal ion-ligand complex appended CDs, metallocyclodextrins, were also applied for chemical sensors. Furthermore, recent novel approaches to chemical sensing systems using supramolecular structures such as CD dimers, trimers and cooperative binding systems of CDs with the other macrocycle [2]rotaxane and supramolecular polymers consisting of CD units are mentioned. New chemical sensors using hybrids of CDs with π-conjugated polymers, peptides, DNA, nanocarbons and nanoparticles are also described in this review.

Synthesis of novel pillar-shaped cavitands “Pillar[5]arenes” and their application for supramolecular materials

In 2008, we reported a new class of macrocyclic hosts and named “Pillar[5]arenes”. They combine the advantages and aspects of traditional hosts and have a composition similar to those of typical calix[n]arenes. Pillar[5]arenes have repeating units connected by methylene bridges at the para-position, and thus they have a unique symmetrical pillar architecture differing from the basket-shaped structure of meta-bridged calix[n]arenes. Pillar[5]arenes show high functionality similar to cyclodextrins, and can capture electron accepting guest molecules within their cavity similarly to cucurbit[n]urils. In this review, the synthesis, structure, rotation, host–guest properties, planar chirality and functionality of pillar[5]arenes are discussed, along with pillar[5]arene-based supramolecular architectures and the challenges in synthesizing pillar[6]arenes.

Photoreversible switching of the lower critical solution temperature in a photoresponsive host-guest system of pillar[6]arene with triethylene oxide substituents and an azobenzene derivative.

A new water-soluble thermoresponsive pillar[6]arene with triethylene oxide groups was synthesized. The pillar[6]arene showed lower critical solution temperature behavior in aqueous solution. Its clouding point was photoreversibly switched based on a photoresponsive host-guest system. The trans form of an azobenzene guest formed a stable 1:1 complex with the pillar[6]arene. Complexation increased the clouding point. Irradiation with UV light induced a conformation change for the azobenzene guest from the trans to cis form, and dethreading occurred because of a size mismatch between the cis form and the pillar[6]arene cavity. This dethreading decreased the clouding point. The photoresponsive host-guest system was reversible, and the clouding point could be switched by alternating irradiation with UV or visible light. We demonstrated photoresponsive reversible clear-to-turbid and turbid-to-clear transitions for the solution based on the reversible switching of the clouding point using the photosensitive host-guest system.

Supramolecular polymers with alternating pillar[5]arene and pillar[6]arene units from a highly selective multiple host–guest complexation system and monofunctionalized pillar[6]arene

A highly selective multiple host–guest complexation system based on pillar[5]arene and pillar[6]arene is reported. A pyridinium cation moiety is included in the pillar[5]arene cavity, but hardly forms a stable host–guest complex with pillar[6]arene. A 1,4-diazabicyclo[2.2.2]octane cation moiety forms a host–guest complex with pillar[6]arene, but not with pillar[5]arene. A synthetic route to mono-reactive pillar[6]arene is also developed, enabling the synthesis of pillar[6]arene with a pyridinium cation group at a single position. Based on the highly selective multiple host–guest complexation system and mono-reactive pillar[6]arene, a supramolecular polymer with alternating pillar[5]arene and pillar[6]arene units is constructed.

Reversibly tunable lower critical solution temperature utilizing host-guest complexation of pillar[5]arene with triethylene oxide substituents.

A thermoresponsive macromolecule consisting of 10 outer triethylene oxide groups and a pillar[5]arene core was prepared. The macromolecule showed lower critical solution temperature behavior. Moreover, its clouding point can be reversibly tuned based on the addition of guest and host compounds; the clouding point increased upon addition of a guest didecylviologen salt and decreased when the competitive host cucurbit[7]uril was added.

Facile, rapid, and high-yield synthesis of pillar[5]arene from commercially available reagents and its X-ray crystal structure.

We monitored the progress of formation of dimethoxypillar[5]arene by size-exclusion chromatography. Surprisingly, the cyclization reaction completely finished in just 3 min. By improving the reaction conditions and purification process, we successfully obtained dimethoxypillar[5]arene in a short time and in high yield (71%) from commercially available reagents. By improving the deprotection reaction of the methoxy moieties, pillar[5]arene was isolated quantitatively. Single crystal X-ray analysis confirmed the structure of pillar[5]arene in the solid state.

Organic–inorganic polymer hybrids prepared by the sol-gel method

This review focuses on some aspects of organic-inorganic hybrid materials prepared by the sol-gel method. This field has been studied worldwide as one of the nanotechnologies, and is now of current interest for both organic and inorganic scientists. The elaboration of organic–inorganic polymer hybrid materials using the sol-gel process can be accomplished by various approaches. The simplest method is increasing the compatibility by using physical interactions, covalent bonding and compatibilizer between organic polymer and silica gel. Other novel approaches, such as an in-situ method, NHSG (Non- Hydrolysis Sol-Gel) process, and use of reactive polymer hybrids resulted in the preparation of novel transparent organic–inorganic polymer hybrid materials. Stimulus responsive polymer hybrids are also mentioned. Furthermore, nano-structured organic–inorganic polymer hybrids are created by using supermolecular and self-assembly of organic molecules or polymers recently. The obtained nano-structured hybrid materials showed unique properties that could not be found in amorphous hybrid materials. The possibilities and applications of organic–inorganic polymer hybrid materials are also described in this review.

Synthesis and Conformational Characteristics of Alkyl-Substituted Pillar[5]arenes

A series of pillar[5]arene derivatives with alkyl groups of different length were synthesized. The new alkyl-substituted pillar[5]arene derivatives 1,4-bis(ethoxy)pillar[5]arene (C2), 1,4-bis(propoxy)pillar[5]arene (C3), 1,4-bis(butoxy)pillar[5]arene (C4), 1,4-bis(pentyloxy)pillar[5]arene (C5), 1,4-bis(hexyloxy)pillar[5]arene (C6), and 1,4-bis(dodecanoxy)pillar[5]arene (C12) were obtained by Lewis acid-catalyzed condensation of dialkoxybenzene monomers with paraformaldehyde. The conformational characteristics of the pillar[5]arene derivatives were investigated by dynamic (1)H NMR measurements. When the alkyl substituents were bulkier than methyl groups, the rotation of phenolic units in the pillar[5]arenes was suppressed and their conformation was immobilized. As their length increased, the alkyl substituents packed at the upper and lower rims and thus lowered the conformational freedom of the pillar[5]arenes.

Planar-Chiral Macrocyclic Host Pillar[5]arene: No Rotation of Units and Isolation of Enantiomers by Introducing Bulky Substituents

Enantiomers of bulky percyclohexylmethyl-substituted pillar[5]arene (Cy-C1-Pillar) were able to be separated by chiral column chromatography, and the separated enantiomers did not racemize. Even though modified with the bulky cyclohexylmethyl-substituents at both rims, Cy-C1-Pillar was able to capture a guest molecule.