Preparation of siloxy focal dendron-protected TiO2 nanoparticles and their photocatalysis

TiO2 nanoparticles were synthesized at approximately 0 degrees C by hydrolyzing [(CH3)2CHO]4Ti in 1-propanol solutions of poly(amido amine) dendrons with a siloxy focal point and long alkyl chain spacers. Transmission electron microscopic photographs showed that TiO2 nanoparticle was 1-5 nm in size and protected by dendrons, when prepared at a mixing ratio 1:10 of Ti ion and dendron. At higher contents of Ti ion, TiO2 nanoparticles aggregated up to a maximum size of 90 nm, depending on the dendron generation (first to third). It was confirmed from X-ray photoelectron spectroscopy that Si-O-Ti covalent bond was formed in dendron-protected TiO2 nanoparticles. The ability of dendron-protected TiO2 nanoparticles as a photocatalyst for the photodegradation of 2,4-dichlorophenoxyacetic acid was higher than that of nonprotected nanoparticle and superior at higher generation. It was suggested that the dendrons protecting TiO2 nanoparticle have enough void volume to conserve guest molecules and behave effectively as a reservoir of guest molecules.     

Synthesis and characterization of poly(ethyleneimine) dendrimers

Poly(ethyleneimine) (PEI) dendrimers up to the third generation (G3) were prepared by a divergent synthesis method from an ethylenediamine (EDA) core. The amine terminals were bonded with vinylbromide by a Michael addition reaction. Then, the bromide terminals were converted to amine groups using a Gabriel amine synthesis method. PEI dendrimers displayed pH-dependent luminescence, and their emission intensities at pH 6 increased over time. Fluorescence intensities also increased with increasing dendrimer generation from G1 to G3. Air-bubbling in aqueous solutions of dendrimers made to incorporate detectable amount of oxygen in dendrimers. EDA also behaved similarly in luminescence and oxygen incorporation.

Fluorescence emission from dendrimers and its pH dependence

A strong fluorescence emission was observed from different kinds of dendrimers under acidic condition. There was a remarkable difference in fluorescence properties between second and fourth generation NH2-terminated poly(amido amine) dendrimers. It can be assumed that the backbone of the dendrimer played the key role in forming the novel fluorescent center.     

Preparation of divinylbenzene copolymer particles with encapsulated hexadecane for heat storage application

From the viewpoint of heat storage application, encapsulation of n-hexadecane (HD) was carried out by microsuspension copolymerizations of divinylbenzene (DVB) and acrylic monomers (butyl acrylate, BA; ethyl acrylate, EA) utilizing the self-assembling of phase-separated polymer (SaPSeP) method proposed by the authors. The heat of solidification (H _s) of encapsulated HD in the micron-sized, cross-linked particles was determined by the differential scanning calorimeter (DSC). H _s of the encapsulated HD in poly(DVB) particles was much lower than that of pure HD, but it was increased with BA or EA content copolymerized up to that of pure HD. Such an influence of encapsulation on the H _s was discussed. Part CCIIIC of the series “Studies on suspension and emulsion”.     

Fabrication of metal nanoparticle monolayers on amphiphilic poly(amido amine) dendrimer Langmuir films

A newly designed 1.5th generation poly(amido amine) dendrimer with an azacrown core, hexylene spacers, and octyl terminals was spread on gold nanoparticle (Au-NP) suspension. The surface pressure-area isothermal curves indicated that the molecular area of dendrimer on Au-NP suspension was significantly smaller than that on water, indicating the formation of dendrimer/Au-NP composites. The dendrimer Langmuir films on the Au-NP suspension were transferred to copper grids at various surface pressures and observed by transmission electron microscopy. The transferred films consisted of a fractal-like network of nanoparticles at low surface pressure and of a defect-rich monolayer of nanoparticles at high surface pressure. From these results, it was suggested that the dendrimers bind Au-NPs, and dendrimer/Au-NP composites formed networks or monolayers at the interface. From the intensity decrease of the Au plasmon band of Au-NP suspension after the formation of composite, it was estimated that some (approximately 14) dendrimer molecules bind to one Au-NP. Furthermore, neutron reflectivity at the air/suspension interface and X-ray reflectivity of the film transferred on a silicon substrate revealed that the dendrimer molecules are localized on the upper-half surface of Au-NP. Metal affinity of azacrown, flexibility of hexylene spacer, and amphiphilicity of dendrimer with octyl terminals played important roles for the formation of dendrimer/Au-NP hybrid films. The present investigation proposed a new method to fabricate the self-assembled functional polymer/nanoparticle hybrid film.     

Photovoltaic performance of dye-sensitized solar cells based on donor-acceptor pi-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes with a carboxyl group at different positions of the chromophore skeleton

Donor-acceptor pi-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes 3a, 3b, 8a, and 8b with a carboxyl group at different positions of the chromophore skeleton have been designed and synthesized. The absorption and fluorescence spectra and cyclic voltammograms of the fluorescent dyes agree very well, showing that the position of the carboxyl group has a negligible influence on the photophysical and electrochemical properties of these dyes. When these dyes are used in dye-sensitized solar cells, however, their photovolatic performances are considerably different. The short-circuit photocurrents and energy conversion efficiencies under a simulated solar light increase in the order: 3a (2.12 mA cm(-2), 1.00%) approximately 3b (2.10 mA cm(-2), 1.06%) > 8b (1.50 mA cm(-2), 0.67%) > 8a (0.84 mA cm(-2), 0.34%). Based on semi-empirical molecular orbital calculations (AM1 and INDO/S) together with spectral analyses and their photovolatic performance, the relationships between the observed photovolatic properties and the chemical structures of the benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes are discussed. It is found that strong interaction between a TiO(2) surface and the electron accepting moiety of the dye leads to a high photovoltaic performance.     

Estimation of heterogeneous surface structure of submicron-sized,composite polymer particles consisting of hydrophobic and hydrophilic components by atomic force microscopy*

Surface structure of submicron-sized poly(styrene/2-hydroxyethyl methacrylate) [P(S/HEMA)] composite particles produced by emulsifier-free emulsion polymerization was estimated with atomic force microscopy (AFM). AFM force curves were measured in water at different points of the particle surface; it was clarified that the particle surface had a heterogeneous structure consisting of hard and soft parts, which must be, respectively, based on styrene-rich and 2-hydroxyethyl methacrylate-rich parts.     

Resolution of non-protein amino acids via the microbial protease-catalyzed enantioselective hydrolysis of their N-unprotected esters

In the Aspergillus oryzae protease-catalyzed ester hydrolysis, substitution of N-unprotected amino acid esters for the corresponding N-protected amino acid esters resulted in a large enhancement of the hydrolysis rate, while the enantioselectivity was deteriorated strikingly when the substrates employed were the conventional methyl esters. This difficulty was overcome by employing esters bearing a longer alkyl chain such as the isobutyl ester. Utilizing this ester, amino acids carrying an aromatic side chain were resolved with excellent enantioselectivities (E=50 to >200). With amino acids bearing an aliphatic side chain also, good results in terms of the hydrolysis rate and enantioselectivity were obtained by employing such an ester as the isobutyl ester. Moreover, the enantioselectivity proved to be enhanced further by conducting the reaction at low temperature. This procedure was applicable to the case where the enantioselectivity was not high enough even by the use of the isobutyl ester.