Signaling Recognition Events with Fluorescent Sensors and Switches.

Design principles of fluorescent molecular sensors for cation recognition

In the 21st century, soft materials will become more important as functional materials because of their dynamic nature. Although soft materials are not as highly durable as hard materials, such as metals, ceramics, and engineering plastics, they can respond well to stimuli and the environment. The introduction of order into soft materials induces new dynamic functions. Liquid crystals are ordered soft materials consisting of self-organized molecules and can potentially be used as new functional materials for electron, ion, or molecular transporting, sensory, catalytic, optical, and bio-active materials. For this functionalization, unconventional materials design is required. Herein, we describe new approaches to the functionalization of liquid crystals and show how the design of liquid crystals formed by supramolecular assembly and nano-segregation leads to the formation of a variety of new self-organized functional materials.

Functional liquid-crystalline assemblies: self-organized soft materials.

Crown ethers: sensors for ions and molecular scaffolds for materials and biological models.

Calixarenes are macrocyclic molecules, like crown ethers and cyclodextrins.1-7 Calixarenes made up of phenol and methylene units have many conformational isomers because of two possible rotational modes of the phenol unit: the oxygen-through-theannulus rotation and the para-substituent-throughthe-annulus rotation (Figure 1). The conformational isomers thus yielded afford a great number of unique cavities with the different size and the different shape. Recently, a number of strategies have been exploited by which not only the conformation of calix[4]arenes, but also those of calix[6]arenes and calix[8]arenes, can be immobilized. This means that our group can now design various calixarene-based receptors that show high selectivity for guest molecules and metal cations. In this review article, our group describe novel strategies for cavity design using calix[n]arene skeletons, strategies that are intended to allow complexation of specific molecular targets or metal ions.

Novel Cavity Design Using Calix[n]arene Skeletons: Toward Molecular Recognition and Metal Binding.

Conformations and Structures of Tetra- O -alkyl-p-tert -butylcalix[4]arenes. How Is the Conformation of Calix[4]arenes Immobilized?

The {sup 1}H and {sup 13}C NMR spectra of a complex formed from tetrasodium p-sulfonatocalix(4)arene (1{sub 4}) and trimethylanilinium chloride (2) in D{sub 2}O showed that 2 is bound to the cavity of cone-shaped 1{sub 4}: in the acidic pH region (pD 0.4) the phenyl moiety resides in the cavity, whereas in the neutral pH region (pD 7.3) both the trimethylammonium and the phenyl moiety are included nonspecifically in the cavity. The difference was accounted for by the change in the electron density in the benzene {pi}-systems induced by the dissociation of OH groups. The complex was crystallized from acidic aqueous solution. The solid-state {sup 13}C NMR (CP-MAS) supports the inclusion of the phenyl moiety in the cavity. The X-ray crystallographic studies established that (1) 1{sub 4} adopts a cone conformation, (2) 1{sub 4} and 2 form a host-guest-type complex with the phenyl moiety inserted into the cavity, (3) not only the hydrophobic force but also the electrostatic force operates for guest inclusion, and (4) the overall structure is similar to that of clays, 1{sub 4} molecules being arranged into bilayers. These findings serve as a useful cross-link between solution complexes and solid-state complexes.

NMR and crystallographic studies of a p-sulfonatocalix(4)arene-guest complex

Syntheses of all possible conformational isomers of O-alkyl-p-t-butylcalix[4]arenes

It was shown that an artificial acetylcholine (neurotransmitter) detection system using resorcin[4]arene (1:  Inouye et al. J. Am. Chem. Soc. 1994, 116, 5517) has substantial problems:  both acetylcholine and a fluorescent receptor molecule decompose in 0.01 M KOH/MeOH solution. In their system the strong alkaline medium was necessitated to dissociate OH groups in 1 and to enhance the fluorescence quenching ability. To solve this problem we exploited a new artificial acetylcholine detection system using water-soluble calix[n]arene-p-sulfonates which are useful even in aqueous (water/methanol) neutral solution. This achievement is due to several outstanding properties of these calix[n]arenes such as low pKa values, cation−π interactions, high water-solubility, etc.

Reinvestigation of Calixarene-Based Artificial-Signaling Acetylcholine Receptors Useful in Neutral Aqueous (Water/Methanol) Solution

Chiral Recognition of α‐Amino Acid Derivatives with a Homooxacalix[3]arene: Construction of a Pseudo‐C2‐Symmetrical Compound from a C3‐Symmetrical Macrocycle

Koji Araki

Papers

Conformations and Structures of Tetra- O -alkyl-p-tert -butylcalix[4]arenes. How Is the Conformation of Calix[4]arenes Immobilized?

NMR and crystallographic studies of a p-sulfonatocalix(4)arene-guest complex

Syntheses of all possible conformational isomers of O-alkyl-p-t-butylcalix[4]arenes

Reinvestigation of Calixarene-Based Artificial-Signaling Acetylcholine Receptors Useful in Neutral Aqueous (Water/Methanol) Solution

Chiral Recognition of α‐Amino Acid Derivatives with a Homooxacalix[3]arene: Construction of a Pseudo‐C2‐Symmetrical Compound from a C3‐Symmetrical Macrocycle