Hirosuke Kawabata

Cholesterol as a versatile platform for chiral recognition

The cholesterol moiety is the perfect host. The environment it creates changes to accomodate each new guest. Cholesteric liquid crystals change colour. Cholesteric gels have different phase transition temperatures. Cholesteric LB films change the area they cover.

Molecular design of calixarene-based ion-selective electrodes

In order to obtain insights into relationships between the calix[4]arene structure and the ion selectivity in the electrode system, 20 ionophoric calix[4]arenes were synthesized and their ion selectivity (with Na+ as a standard) estimated. Among these ionophoric calix[4]arenes, 25,26,27,28-tetrakis[(ethoxycarbonyl)methoxy]-p-t-octylcalix[4]arene afforded the highest logKNA,Mpot value (−3.1) in the presence of 2-fluorophenyl-2′-nitrophenylether (10) as the best of 13 plasticizers. This is the first example in which the Na+/K+ selectivity exceeds a factor of 103 in the electrode system based on the neutral carrier. The high Na+ selectivity is attributed to modification of the upper rim which ostensibly has no relation with the component of the cavity. This paper demonstrates the potential relationships between the unique structure of the calix[4]arene-based ligands and selectivity performance for the design of ion-selective electrodes.

Syntheses and aggregation properties of new water-soluble calixarenes

Eighteen water-soluble calixarenes have been synthesized and their aggregation properties in water studied. They are six anionic calixarenes (1n) having sulphonate groups on the upper rim, nine anionic calixarenes (2n) having sulphonate groups on the lower rim, and three cationic calixarenes (3n) having ammonium groups on the upper rim. On the basis of measurements of critical micelle concentrations (c.m.c.s.) by surface tension, conductance, and spectroscopic methods, it was concluded that the aggregation behaviour of these water-soluble calixarenes can be classified into three categories: (i) non-micellar calixarenes, (ii) micelle-forming calixarenes, and (iii) unimolecular micellar calixarenes. The possible structure-aggregation relationships are discussed.

Fluorescent calix[4]arene which responds to solvent polarity and metal ions

A calix[4]arene having two pyrene moieties on the lower rim has been synthesized; the ratio of the monomer vs. excimer emission was sensitively affected by solvent polarity and added metal ions.

New water-soluble calixarenes bearing sulphonate groups on the ‘lower rim’: the relation between calixarene shape and binding ability

Association constants have been determined for pyrene as a guest molecule for 9 water-soluble calix[n]arenes (n= 4, 6, and 8) bearing sulphonate groups on the ‘upper rim’(1) or on the ‘lower rim’(2). The results have established that calixarene (1) has a strong but non-selective binding site whereas calixarene (2) has a relatively weak but selective binding site.

Molecular design and characterizations of new calixarene-based gelators of organic fluids

We have found that certain calix[n]arenes having long acyl groups at the p-positions (e.g., 5,11,17,23,29,35,41,47-octadodecanoylcalix[8]arene-49,50,51,52,53,54,55,56-octol: 28C12) act as excellent and unique gelators of various organic solvents (e.g., toluene, carbon tetrachloride, carbon disulfide, hexane, isopropanol, etc.). The sol–gel phase transition temperatures (Tgel) could be determined by the inverted test-tube method or by the DSC method. The change in the aggregation mode at the phase transition temperature was directly observable by an optical microscope: below Tgel., the fibrillar network (diameter ca. 1 µm) appeared whereas above Tgel, it ‘melted’ down. It was shown on the basis of these measurements that the sol-gel phase transition occurs reversibly. The spectroscopic studies using 1H NMR and IR spectroscopy and the comparative experiments using the analogues of 28C12(nine cyclic and six non-cyclic) established that the prerequisites for the formation of the stable organic gels are (i) the intermolecular CO ⋯ HO hygrogen-bonding interaction to form the three-dimensional network and (ii) the moderate affinity of gelators with solvent molecules. The results offer important strategies useful for the molecular design of new gelators of organic fluids.

Synthesis and optical resolution of an asymmetrically substituted calix[4]arene

A cone-shaped, asymmetrically substituted calix[4]arene, 1, has been synthesized and successfully optically resolved for the first time.

Assembly of functionalized calixarenes

Abstract Calixarene-based functional assemblies having potential for metal ion and/or molecular recognition were obtained: they are (1) gelators and (2) polymers. Calix [n] arenes bearing aliphatic chains at the p-position (2 n C m , n = 4, 6, and 8; m = 6, 12, and 18) have been synthesized by direct acylation from calix [n] arenes (1 m ) and serve as novel gelators of organic fluids. It has been established that the gelation process is thermally reversible, and the formation mechanism of the gels is ascribed to C─O…HO hydrogen bonding. On the other hand, insoluble polymer gels could also be obtained from a mixture of monovinyl- and divinyl-calix [4] arenes and they showed ion-selective swelling.