Au, Ag, and Au/Ag colloidal nanoparticles coated with the J-aggregate of an anionic cyanine dye, 3,3′disulfopropyl-5,5′-dichlorothiacyanine sodium salt (TC), have been prepared for the first time. The absorption spectrum of TC-coated Au colloidal nanoparticles is not a simple sum of the contributions of colloidal gold and TC but is characterized by an evident absorption dip at the position corresponding to the J-band of TC. These spectral features are reproduced by the simulation based on the Maxwell-Garnett-type treatment of Gao et al. The alternate adsorption technique allowed us to deposit dye-coated Au/Ag composite nanoparticles at the surface of a cationic polymer, poly(diallyldimethylammonium chroride).
The layer-by-layer alternate assemblies incorporating two kinds of cyanine dyes have been fabricated by alternately adsorbing a cationic polyelectrolyte and anionic cyanine dyes on the quartz plate. A thiacyanine dye (dye I) was employed as the donor and two kinds of thiacarbocyanine dye having a meso-alkyl groups m-ethyl (dye II), m-methyl (dye III)sas the acceptor. The mole fraction of the acceptor in the mixed J-aggregate, χ, was varied from 0 to 1. It is confirmed that these dye combinations form the mixed J-aggregate in the alternate assemblies. From steady-state fluorescence spectra of the molecular assemblies, excitation energy transfer from the donor J-aggregate to the acceptor J-aggregate is observed, whose kinetics obeys the Stern-Volmer relationship. The experimentally determined rate constant of energy transfer, k ET , is fairly large, indicating efficient energy transfer due to exciton migration through the donor J-aggregate. The relative fluorescence quantum yield and the fluorescence lifetime of the acceptor aggregate decrease with increasing χ, implying the considerable self-quenching of acceptor fluorescence. The relative change of the coherent size of the dye II aggregate has been estimated from the J-band line width and the radiative decay rate constant. It is found that the coherent size of the dye II aggregate is increased by a factor of 4-5 with increasing χ from 0.008 to 1.
The composite gold nanorods (Au NRs) having a double-shell structure composed of Au NR (core), spacer layer (inner shell), and J-aggregate (JA) layer (outer shell) have been synthesized to examine the spectroscopic properties of the hybrid system in which the localized surface plasmon is coupled with the molecular exciton of JA. The spacer layer consisting of N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride plays a significant role in the formation of JA shell for several kinds of cyanine dyes. The absorption spectra of composite NRs are characterized by a distinct dip near the J-band when the plasmon energy of Au core is close to the exciton energy of JA shell, whereas a normal J-band peak appears when two energies are widely different from each other. The gradual change from the dip type to peak type absorption was observed when the plasmon energy was modulated by varying the aspect ratio of Au NR. Furthermore, composite NRs with thicker spacer layers have been fabricated by inserting the multilayer shell of polyelectrolytes between TMA and JA layers. They exhibited an alteration of the spectral line shape from the dip type to peak type with increase in the thickness of spacer layer. These observations have been interpreted in terms of the strength of the exciton-plasmon coupling, which is sensitive to the configuration of composite NRs as well as the relative difference between plasmon and exciton energies.
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.
Silver nanoparticles (Ag NPs) covered with a double-shell consisting of a spacer layer and cyanine dye J-aggregate (JA) layer were synthesized to examine the hybrid effects of the molecular exciton and the surface plasmon on the spectroscopic properties. The absorption spectral features of the composite NPs are classified into two types: a peak type or dip type in which the absorption of JA appears as a normal peak or an anomalous dip, respectively. The peak type is observed when the plasmon energy of the Ag NP core is far from the exciton energy of the JA shell, while the dip type is observed when both energies are close to each other, indicating that exciton−plasmon interaction is weak for the peak type and strong for the dip type, respectively. Both types of absorption spectra could be reproduced by the theoretical calculation based on the quasistatic approximation, which provides a qualitative explanation for the absorption spectral features of composite NPs. From the observations of photoluminescence lifetimes and relative quantum yields of composite NPs, it was suggested that the radiative rate of composite NPs was also influenced by the exciton−plasmon coupling.
The influence of solvent viscosity on the intramolecular charge-transfer (CT)-state formation in the excited S 1 state for 4-(N,N-dimethylamino) triphenylphosphine (DMATP) in alcohol solvents has been investigated by measuring the steady-state and time-resolved fluorescence spectra at high pressures. The kinetic mechanism of the intramolecular CT reaction has been examined as a function of solvent shear viscosity. In the lower viscosity region the reaction is controlled by the solvent relaxation. With increasing pressure, the reaction path shifts toward the "high-viscosity regime" in which the molecule moves along the nonrelaxed path on the free energy surface. The viscosity dependence of R = 0.33, where R is the power law parameter, can be interpreted as the extreme value in which the reaction is controlled by the dynamic solvent effect due to intrinsic collisional interaction of barrier crossing. The coupling between the intramolecular CT-state formation dynamics of DMATP and the solvent relaxation dynamics is discussed.
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