Yoshiro Yonezawa

Layer-by-layer self-assembly of polyelectrolyte and water soluble cyanine dye

Abstract We have fabricated multilayer assemblies consisting of cyanine dye and polyelectrolyte by means of alternate adsorption method. In this study we used two kinds of water soluble thiacyanine dyes and poly(diallyldimethylammonium chloride) (PDDA) as polycation. Anionic cyanine dye molecules were adsorbed onto the positively charged surface of PDDA layer, giving rise to the J-aggregate. The sequential alternate adsorption yielded the PDDA-(dye/PDDA)n assemblies (n, repeating number). UV-vis spectroscopy measurements indicated that peak absorbance at the J-band increased linearly with the increasing adsorption cycles. These results demonstrate the advantage of the alternate adsorption method for investigating J-aggregate-containing multilayer assemblies.

Photochemical formation of colloidal metals

Colloidal silver and gold have been formed by irradiation of AgClO4 and HAuCl4 solutions with 253.7 nm light in the presence of protective agents such as sodium dodecylsulphate, sodium alginate and colloidal silica. Absorption maxima of colloidal silver and gold have been observed at 390–420 and 540–560 nm, respectively. Colloidal metals have been purified by dialysis and their amounts analysed quantitatively. The quantum yields of metal deposition are 0.053–0.15 for colloidal silver and 0.030–0.039 for colloidal gold. The quantum yield is increased several times by the use of protective agents, with the exception of a AgClO4–sodium alginate solution.

Photochemical formation of colloidal silver: peptizing action of acetone ketyl radical

When an aqueous AgClO4–sodium dodecylsulphate (SDS)–isopropyl alcohol (Pri OH) solution was photolysed with 253.7 nm light, a sharp and intense absorption band of colloidal silver (λ= 390–400 nm) appeared after a relatively long induction period. In contrast, the sharp colloidal absorption rapidly developed when an AgClO4–SDS–acetone [(CH3)2CO] solution was irradiated under the same conditions. It has been shown by chemical analysis and TEM characterization of the reduced silver that some of the colloidal silver agglomerates formed as a result of photolysis were peptized to fine silver particles during irradiation. Acetone ketyl radical formed by excitation of (CH3)2CO with 253.7 nm light seems to have an ability to peptize colloidal agglomerate in the solution phase (peptizing action). A novel ‘photo-acetone method’ available for preparation of fine metal particles has been proposed and applied to obtain colloidal gold, copper and platinum.

Tuning of the spectroscopic properties of composite nanoparticles by the insertion of a spacer layer: effect of exciton-plasmon coupling.

Composite nanoparticles (NPs) having a double-shell structure, Au core, spacer layer (inner shell), and J-aggregate (JA) layer (outer shell) (Au/spacer/JA) have been synthesized. The spacer layer composed of N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride played an important role in promoting the J-aggregation of anionic cyanine dyes on the surface, as evidenced by the successful formation of the JA layers with four kinds of anionic cyanine dyes. It was found that the presence of a spacer layer causes a significant change in the line shape of the absorption spectrum, particularly near the J-band; there is the appearance of a peak type absorption for the composite NPs with the double-shell structure, while there is a dip type absorption for the ones without the spacer layer. The change from the peak type absorption to the dip type absorption in the Au/spacer/JA NPs occurs when the size of the Au core is varied from 5 to 15 nm. These observations would indicate that the strength of exciton-plasmon coupling between the Au core and the JA layer is enhanced with the increase in the core size or the decrease in the separation between the Au core and the JA shell. The photoluminescence arising from the JA can be detected for the composite NPs with the double-shell structure, showing that the quenching by the Au core is effectively suppressed by the spacer layer.

Laser flash photolysis study of the photochemical formation of colloidal Ag nanoparticles in the presence of benzophenone

Sensitized photoreduction of Ag+ ions and subsequent formation of colloidal Ag nanoparticles take place when an aqueous solution of a mixture of AgClO4, sodium dodecylsulfate (SDS) and benzophenone (BP) is irradiated with near-UV light. In this study, the mechanism and kinetics of the reaction have been studied by means of nanosecond laser flash photolysis, time-resolved emission spectroscopy and also steady-state photolysis. Benzophenone ketyl radial (BPK) is formed by the hydrogen abstraction reaction of the benzophenone triplet (BPT) from SDS, which reduces Ag+ ions. It is found that the yield of colloidal Ag nanoparticle formation decreases remarkably with increasing concentration of Ag+ ions, indicating the efficient quenching process of BPT by Ag+ ions. Rate constants for the oxidation of BPK as well as the quenching of BPT by Ag+ ions in SDS micellar solution are considerably larger than those expected in the bulk solution, reflecting that Ag+ ions and BP molecules are concentrated in the SDS micelles. The rate of colloidal Ag nanoparticle formation, VAg, and BP photobleach, VBP, under steady-state irradiation with near-UV light (λ = 365 nm) were evaluated and the dependence of VAg and VBP on the concentration of Ag+ ions is interpreted quantitatively in terms of rate constants estimated by laser flash photolysis.

Photochemical formation of silver metal films from silver salt of natural high molecular carboxylic acid

Thin films of silver salt of alginic acid, a typical high molecular carboxylic acid in nature, were photolyzed by 253.7 nm light. On irradiating with a 15‐W sterilization lamp in air at relative humidity of more than 70%, the silver alginate films first became yellow‐brown colored due to formation of photolytic colloidal silver particles. When irradiation was continued, the irradiated surface of the films finally changed into clear silver mirror. The morphology of these films was observed by means of a high‐resolution scanning electron microscope. Colloidal silver particles (10–50 nm diam) formed by a short‐time irradiation were sparsely distributed at the film surface. As a result of prolonged irradiation for ∼180 min, film surface was covered with aggregated colloidal silver. The x‐ray diffraction study of the irradiated films revealed sharp diffraction lines, indicating that the colloidal silver was in a highly crystalline state. A preliminary observation of a microtomed cross section of the film showe...

Luminescence and excitation energy transfer in monolayer assemblies incorporating organic dye molecules. II

Excitation energy transfer between organic dye molecules in monolayer assemblies is studied. Typical cyanine dyes: N, N’‐dioctadecyl‐oxacyanine perchlorate and N,N’‐dioctadecyl‐thiacyanine perchlorate are employed as a donor and acceptor, respectively. Inclusion of the acceptor into the monolayer composed of isolated donor and matrix molecules induces quenching of monomer fluorescence of the donor and consequent sensitization of acceptor fluorescence. Quenching of donor fluorescence by the acceptor obeys the Stern–Volmer relation with a weakly temperature‐dependent Stern–Volmer constant. Lowering the temperature from T=293 to 77 K brings about the increase of both the Stern–Volmer constant and the intensity ratio of the sensitized fluorescence to the donor fluorescence. These observations indicate that the rate of thermal deactivation of the excited donor molecule is proportional to temperature in spite of temperature‐insensitive energy‐transfer rate. Energy transfer between donor and acceptor molecules o...

Nonlinear optical properties of silver sols prepared by photoreduction method

Abstract The third-order nonlinear susceptibility ( χ (3) of silver sol prepared by a photochemical method was measured by the degenerate four-wave mixing method. The χ (3) of the silver sol of brown-green color was larger than that of yellow-brown color. The photo-induced aggregation of silver particles affects the χ (3) of the sol. It has been found that the χ (3) of the silver sol is proportional to the intensity of surface-enhanced raman scattering from carboxylate ions adsorbed on the silver particle. The increase of χ (3) by the enhanced local electric field due to surface plasmon resonance is proposed.

Spin polarization and spin delocalization in unrestricted hartree-fock method

Abstract A theory for the separation of the spin density calculated with the unrestricted Hartree-Fock method into the two components due to the spin polarization and spin delocalization mechanisms is given and applied to methyl, ethyl and vinyl radicals.

Excited-state dynamics of the mixed J-aggregate of two kinds of cyanine dyes in layer-by-layer alternate assemblies

Abstract Layer-by-layer alternate assemblies, which incorporate two kinds of cyanine dyes in the form of mixed J-aggregates, were fabricated by means of the alternate adsorption method. The excited-state dynamics of the mixed J-aggregates were examined by steady-state fluorescence spectroscopy and picosecond lifetime measurements. In these assemblies, the behavior of excitation energy transfer between the donor and acceptor J-aggregates obeyed Stern–Volmer kinetics in the low concentration region of the acceptor molecules. It was found that both the fluorescence lifetime and quantum yield of the acceptor aggregates decrease with increasing concentration of the acceptor molecules, which indicates that fluorescence self-quenching occurred in the acceptor aggregates.