Supramolecular Science最新文献

Organic microcrystals which are expected to have interesting and fascinating physical properties were fabricated by a reprecipitation method as aqueous dispersions. Many kinds of organic compounds have been microcrystallized by this convenient method. The size control has been extensively investigated for a polydiacetylene and succeeded in the range from several tens of nanometers to several micrometers by adjusting the temperature and concentration. Linear optical properties of these well-defined polydiacetylene microcrystals have been investigated and interesting size and temperature dependences of excitonic absorption are demonstrated. Nonlinear optical (NLO) properties of polydiacetylene microcrystals have also been evaluated by means of a Z-scan technique, and an extremely high nonlinear refractive index at the resonant wavelength was shown.

This paper reviews our recent progress in synthesis, optical properties and reassembling behaviour of nanosheet-crystallites of quasi-TiO₂. Processing to thin anatase flakes from them is also presented. The colloidal nanosheets were synthesized by soft-chemically exfoliating a layered protonic titanate of $\text{H}{}_{\mathrm{x}}\text{Ti}{}_{\mathrm{2-x/4}}\text{□}{}_{\mathrm{<mtext>x</mtext><mtext>/4</mtext>}}\text{O}{}_4\text{·}\text{H}{}_2\text{O}$ (x∼0.7, □: vacancy). The action of bulky quarternary alkylammonium ion in large excess induced osmotic swelling involving Δd>3 nm. Lowering the dose of the electrolyte promoted further interlayer expansion leading to delamination into elementary host layers. The resulting nanosheets exhibited a sharp UV-visible absorption band which is considerably blue-shifted compared to bulk TiO₂. These distinct features and a well-structured photoluminescence may be interpreted as a reflection of their molecular nature. Slow drying of the colloids brought about spontaneous reassembling of the nanosheets, which was initiated by association of a limited number of the nanosheets with a very large intersheet spacing of >10 nm and finally yielded a well-ordered lamellar structure. Freeze-drying of the suspension produced the lamellar phase in an open microstructure which can be converted into thin flaky particulates of anatase tens nanometers thick. The material, an assembly of the flakes, possessed a high specific surface area (40–110 m² g^-1) associated with mesopores.

Organic photochromic units and molecules can be regarded as light-driven nano molecular machines. Once such molecules are aligned at a surface, the supramolecular organization provides an efficient macroscopic mechanical response in a collective way. Amphiphilic polymers having an azobenzene (Az) side chain are the favorable materials for observation of such effects since they show marked photomechanical response with essentially full reversibility. An in situ Brewster angle microscopic observation showed marked morphological and rheological photoinduced changes in the molecular films. Moreover, we have newly found that the identical photosensitive molecular film transferred on to a solid mica surface shows large morphological changes under highly humid conditions as proven by atomic force microscopy (AFM). It is supposed that the molecular film is driven in the same mechanism both on water and water-adsorbed mica surfaces. These microscopic observations provide new insights of the photomechanical response in photochromic monolayers.

Magnetotactic bacteria orient and migrate along magnetic field lines. This ability is based on a submicron assembly of single-magnetic domain iron mineral particles that elegantly solves the problem of how to construct a magnetic dipole that is large enough to be oriented in the geomagnetic field at ambient temperature, yet fit inside a micron-sized cell. The solution is based on the ability of the bacteria to accumulate high concentrations of iron, and control the deposition, size and orientation of a specific iron mineral at specific locations in the cell.

A novel procedure for SANS data analysis is described which enables one to use the fitted parameters to compute the average mean, Gaussian and square mean curvatures of the interface in systems which show a micro-phase-separated bicontinuous structure. This procedure also leads to a 3-D reconstruction of the connected internal interface which allows one to visualize the mesoscopic scale morphology of the material. The method has been successfully applied to various bicontinuous structures such as microemulsions made of surfactant, water and oil, porous silica glasses and phase-separated homopolymer blends. In this lecture, we show examples of analyses of SANS data taken from one-phase bicontinuous microemulsions at the hydrophile-lipophile-balance temperature and a light scattering intensity taken from a symmetric micro-phase-separated polymer blend at the late stage of spinodal decomposition.

A thin film made of an ion-exchangeable layered compound, K₄Nb₆O₁₇, was prepared by a novel method. Fine particles of K₄Nb₆O₁₇ obtained by wet-grinding of the powder were coated on a substrate and calcined. Recrystallization of the fine particles at 1073 K was confirmed by XRD and SEM, and very flat and large crystals with the b-axis perpendicular to the substrate surface were obtained. The obtained thin film had a layered structure and an ion-exchange property similar to that of K₄Nb₆O₁₇ powder. The adsorption of CO was investigated for the thin film calcined at 1073 K by IR spectroscopy. The initially IR-inactive H⁺ species were suggested to be transformed into OH groups as a result of CO adsorption at the interlayer space.

The Si–C–(Ti)–O inorganic fibers prepared by pyrolyzing poly(titano) carbosilane organic polymer precursors are well known to keep a very high tensile strength of about 250 kg mm^-2 at high temperatures above 1000°C in air for several hours. The structural evolution during the organic-to-inorganic conversion was measured by small angle X-ray scattering (SAXS) using a point-collimated Cu Kα X-ray beam and a two-dimensional imaging plate detector. The SAXS profile for the fibers generally comprises two components; an anisotropic scattering observed in the lower q region (<0.07 Å^-1) and an isotropic one in the higher q region (>0.1 Å^-1), where $\text{q=4π}\text{sin}\text{θ/λ}$ is the scattering vector. The isotropic SAXS profiles, which sensitively depend on the pyrolyzing temperature, are contributed from the β-SiC nanoparticles embedded in the amorphous matrix of the fibers. The very high mechanical strength of the Si–C–(Ti)–O inorganic fibers originates from the formation of a carbon-rich shell-like interface boundary surrounding the β-SiC nanoparticles which is sharply separated from the amorphous matrix.

We studied the structure of poly(vinyl alcohol) (PVA) gels formed in mixtures of dimethyl sulfoxide (DMSO) and water using several scattering techniques such as wide-angle neutron scattering (WANS), small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (U-SANS) and light scattering (LS) to cover a very wide Q range from 10^-4 to 10 Å^-1. The WANS measurements have revealed that the cross-linking points of the gels are crystallites, and the size and its distribution have been evaluated by the SANS measurements. The SANS results have also shown that the structure observed in the low Q range below 10^-2 Å^-1 is dominated by a liquid–liquid-phase separation. The early stage of the phase separation has been studied in detail using the time-resolved LS technique, while the late stage has been investigated by the U-SANS technique because the LS measurements cannot access the opaque samples. On the basis of the results, we present a quantitative sketch of the structure of the PVA gel.

In water, poly(2-vinyl-4,6-diamino-1,3,5-triazine)(PVDAT) selectively binds the derivatives of thymine and uracil through the formation of three hydrogen bonds with the diaminotriazine (DAT) residues. The nucleotides and dinucleotides are bound much more strongly than are nucleic acid bases, due to the additional interactions of their phosphates with the DAT residues. The binding constant of the thymidine 5′-monophosphate-PVDAT adduct (5400 M^-1) is one of the largest values ever reported for the artificial receptors in protic solvents. In contrast, cytosine and its monophosphate are hardly bound to PVDAT. A water-soluble vinyldiaminotriazine–acrylamide copolymer also forms hydrogen bonds with thymine in water, whereas the corresponding monomers do not. A polymer effect is predominantly important for the molecular recognition through hydrogen bonding in water.

A new type of optically active organic-inorganic composite was prepared by a sol–gel method in which tetraethoxysilane (TEOS) is hydrolyzed in the presence of an optically active organic compound (d-lactose, d-glucose, d-sorbitol, d-fructose, l-tartaric acid, l-malic acid, or l-mandelic acid). Optical resolution of tris(pentane-2,4-dionato)metal complexes was performed by using the new sol–gel derived composites, composites prepared by conventional techniques (kneading with l-lactose, l-fructose or l-tartaric acid, and impregnation with an l-lactose, l-fructose or l-tartaric acid solution) and the optically active organic compounds themselves. The sol–gel derived composites showed much higher optical resolution abilities than the composites prepared by conventional techniques. In addition, the optically active organic compounds could not resolve the racemate into the enantiomers under similar conditions. X-ray diffraction and NMR results disclosed that an optically active organic compound in the sol–gel derived composite is highly dispersed, most likely, because it bonds to silicon atoms. Thus, it was deduced that optically active molecules dispersed at a molecular level recognize the chirality of the metal chelate compound. The high-resolution ability of the sol–gel derived composites arises from the combined effect of the silica support (adsorbing power) and the highly dispersed molecules (chiral recognition power).