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An external reflection (ER) spectrometric device was developed to directly measure adsorbates at the supercritical carbon dioxide (SC-CO2)-water interface. The aggregation of diprotonated species of 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (H4tpps(2-)) at the positively charged SC-CO2-water interface, prepared by adsorption by the cetyltrimethylammonium ion (CTA(+)), was studied using this device. Orientations of the H4tpps(2-) monomers and J-aggregates at the SC-CO2-water interface were assessed using s- and p-polarized external reflection (ER) spectra. It appeared that the porphyrin plane of the H4tpps(2-) monomer was nearly parallel to the SC-CO2-water interface, and that the long axis of the rod-like H4tpps(2-) J-aggregate was also nearly parallel to the interface. Dependence of the ER spectra on CTA(+) concentration and CO2 pressure were investigated, and the interfacial CTA(+) concentration was found to cause changes in the interfacial H4tpps(2-) species present. Increasing the CO2 pressure changed the interfacial species from the H4tpps(2-) monomer to the H4tpps(2-) J-aggregate because the interfacial CTA(+) concentration increased as the pressure increased. This suggests that the interfacial chemical species can be changed by controlling the pressure and temperature of the SC-CO2. This is the first report of direct measurements of the chemical species at the SC-CO2-water interface, as far as we know.

Plastic microchannel-microheater chips were fabricated on the basis of imprinting and photolithography/etching techniques. Knowing the temperature-dependent fluorescence lifetime of Rhodamine B in water, the temperature profile of the solution phase in the microchannel in the vicinity of the heater was evaluated by space- and time-resolved fluorescence microspectroscopies. The fabricated chip was then applied to control the thermal phase transition of an aqueous poly(N-isopropylacrylamide) (PNIPAM) solution. PNIPAM particle formation by the phase transition was controlled by both the applied voltage to the heater and the solution-flow rate. The temperature profile showing below or above the phase-transition temperature (i.e., 32 °C) and solution-flow profile in the microchannel were visualized successfully on the basis of the PNIPAM particle formation. The continuous and automated concentration of a hydrophobic solute by the PNIPAM particles produced by the phase transition was also explored under solutio...

We have fabricated nanometer-sized channels, demonstrated a technique for the introduction of liquid into the channels, and carried out time-resolved fluorescence measurements of aqueous solutions. In this study, 330-nm- and 850-nm-sized channels were fabricated on fused-silica substrates by fast atom beam etching and hydrofluoric acid bonding methods. A liquid introduction method utilizing capillary action was demonstrated. The liquid introduction was observed under an optical microscope, and the liquid velocity during the introduction was analyzed by surface energy and macroscale hydrodynamics. The liquid velocity due to capillary action in the nanometer-sized channel seemed more than four times slower than the estimation. Then, aqueous solutions of rhodamine 6G (R6G), sulforhodamine 101 (SR101), and rhodamine B (RB) in the channels were measured by time-resolved fluorescence spectroscopy; spectra of the same solution in a 250-microm-sized channel were also measured as a reference for the macrospace. Although the fluorescence spectra in the 330-nm-, 850-nm- and 250-microm-sized channels agreed with one another, the fluorescent decays in the nanometer-sized channels were faster for R6G and SR101 and slower for RB than the respective decays in the 250-microm-sized channels. The results suggested the solutions had lower dielectric constants and higher viscosities in the nanometer-sized channels.

The polarity of a water/oil (oil: cyclohexane, carbon tetrachloride, toluene, chlorobenzene, o-dichlorobenzene, or 1,2-dichloroethane) interface was investigated by means of time-resolved total-internal-reflection (TIR) fluorescence spectroscopy of a polarity-sensitive probe: sulforhodamine B (SRB). In bulk solutions, the nonradiative decay rate constant of SRB increased with an increase in a solvent polarity parameter [ET(30)], and this relationship was used to estimate the polarities of water/oil interfaces. For the oil having a relatively low solvent polarity [ET(30)35 kcal/mol], the polarity of the water/ oil interface agreed with that of the arithmetic average of the polarities of the two phases [ET(30)calc]. For water/odichlorobenzene and water/1,2-dichloroethane interfaces [ET(30) of the oil35 kcal/mol], on the other hand, the interfacial polarity determined by TIR spectroscopy was lower than the ET(30)calc. The results are discussed in terms of thickness/roughness of the water/oil interface.

The structures of water, partitioned in cation-exchange resin particles, were studied on the basis of fluorescence dynamics of Nile Blue A (NB). The fluorescence lifetime of NB in the resin was longer than that in water and increased with increasing cross-linking density of the resin (rho). The results demonstrated that the water structures in the resin were significantly different from those in water and dependent on rho. A study on solvation dynamics of NB in the resin, reflecting structured water around the ion-exchange group, revealed the roles of "bound water" molecules in the water structures, since the solvent relaxation time (tauS) in the resin was much longer than that in water and depended on p; tauS increased from 34 to 55 ps with increasing rho from 2 to 8%. The origin of the rho dependence of tauS was discussed in terms of the separation distance between the ion-exchange groups, and the effects of the counterion of the ion-exchange group on the solvation processes were also discussed.

Preparations of a series of face-capped octahedral hexarhenium(III) clusters having two N-heterocyclic ligands, [Bu4N]2[trans-[Re6(mu 3-S)8Cl4(L)2]] (Bu4N+ = tetra-n-butylammonium cation; L = pyrazine (1a), 4,4'-bipyridine (3a), 4-methylpyridine (5a), 4-(dimethylamino)pyridine (6a)) and their cis analogues (1b, 3b, 5b, and 6b, respectively), and their electrochemical and photophysical properties have been reported. An X-ray crystal structure determination has been carried out for 1a to confirm the trans configuration (C40H80N6S8Cl4Re6, orthorhombic, space group Cmca (No. 64), a = 19.560(5) A, b = 19.494(4) A, c = 18.592(4) A, beta = 115.76(2) degrees, Z = 4). The redox potential of the reversible ReIII6/ReIII5ReIV process of these complexes and previously reported [Bu4N]2[trans- and cis-[Re6(mu 3-S)8Cl4(4-cyanopyridine)2]] (2a and 2b, respectively) and [Bu4N]2[trans- and cis-[Re6(mu 3-S)8Cl4(pyridine)2]] (4a and 4b, respectively) in acetonitrile depends linearly on the pKa of the N-heterocyclic ligands, with the potentials being more negative with basic ligands. The ligand-centered-redox waves for 1a, 1b, 2a, and 2b were observed as split waves (delta E1/2 = 90-140 mV), the extent of the splitting being larger for the cis isomer and largest for the pyrazine complexes. Electronic interaction between the two ligands through the [Re6(mu 3-S)8]2+ core has been suggested. The second ligand-reduction wave was also observed for 3a and 3b, the potential being shifted positively to coalesce with the first reduction wave on addition of the weak proton donor imidazole. This is accounted for by the proton-coupled redox reaction at the free pyridyl site of the 4,4'-bipyridine ligands. All of the complexes show luminescence in acetonitrile at room temperature. While the complexes of pyridine and 4-methylpyridine show photophysical characteristics (lambda em 740-750 nm, phi em 0.031-0.057, tau em 4.2-6.2 microseconds) similar to those (770 nm, 0.039, and 6.3 microseconds, respectively) of [Re6(mu 3-S)8Cl6]4-, emissions of other complexes are significantly weak with lambda em, phi em, and tau em values in the ranges 763-785 nm, 0.0010-0.0017, and 0.013-0.029 microsecond, respectively. Suggestions are given for the excited states localized on the cluster core and the ligand pi* orbitals.

A laser trapping-microspectroscopy system combined with a fluid manifold was developed to manipulate and analyze "single" microparticles. A sample solution containing microparticles was introduced to a flow cell set on a microscope stage, and a single particle was trapped by a 1064-nm laser beam. With the particle being trapped, the other particles were pumped out by flowing water to hold the unique microparticle in the flow cell. Under solution-flow conditions, a single microparticle was laser trapped in balance with the gradient (F(g)) and Stokes forces (F(s)) experienced by the particle, and thus, the trapped position was shifted to the downstream side of the 1064-nm laser beam focus. Flow rate and particle size dependencies of this particular positional displacement of the particle were discussed in terms of F(g) and F(s). On the basis of these studies, optical requirements to conduct absorption microspectroscopy of a laser-trapped particle were optimized, and the technique was applied to study a time course of dye adsorption processes in single microparticles. The adsorption rate of Rhodamine B was determined for individual microparticles for the first time.

The fluorescence dynamics of Rhodamine B (RhB) and excitation energy transfer from RhB to Malachite Green (MG) in cation-exchange resin particles were studied with special reference to elucidating effects of the cross-linking density of the resin on the fluorescence dynamics. In the cation-exchange resin, the fluorescence lifetime and rotational relaxation time of RhB were very slow compared to those determined in an aqueous solution, suggesting strong binding of the dye to the ion-exchange group of the resin. Therefore, the spatial distribution characteristics of the dye in the resin particle was used as a measure for that of the ion-exchange group. The apparent structural dimension determined by the fractal dimension analysis of energy-transfer quenching of RhB by MG, representing the distribution characteristics of the dye in the resin, was smaller than Euclidean dimension (3) and decreased from 2.8 to 2.6 with decrease in the cross-linking density of the resin from 8 to 2%. The origin of the fractal-l...

Ion-exchange processes of a cationic dye (Malachite Green, MG) are studied for individual polymer particles (diameter of 20 or 13 μm) by laser trapping microspectroscopy. When a cation-exchange resin, preadsorbed homogeneously with Rhodamine B (RhB), is soaked in an aqueous MG solution, MG is adsorbed in the surface layer of the particle in the initial stage and then diffuses into the inner volume with time. In the MG-diffused layer, RhB fluorescence is quenched by excitation energy transfer from the excited state of RhB to the ground-state MG, as revealed by fluorescence microspectroscopy of individual resin particles in the dye solution. On the basis of the time dependence of the quenching efficiency, the thickness of the MG-diffused layer at a given soaking time is estimated, and the diffusion coefficient of the dye in the particle is determined to be 8 × 10(-)(11) cm(2) s(-)(1). This method is shown to be very useful for determining the diffusion coefficient of a nonfluorescent ion in individual ion-exchange resin microparticles.

Molecular association of malachite green (MG) in single micrometer-sized water droplets dispersed in di-n-butyl phthalate was studied by means of scanning laser manipulation and absorption spectroscopy to elucidate characteristic features of chemical processes at the droplet/oil interface. At a high MG concentration, the droplet showed a new absorption band around 580 nm in addition to the monomer band at 615 nm. Analyses of the concentration-dependent absorption spectrum of MG in the droplet indicated that the band at 580 nm could be assigned to that of the MG dimer. The dimer formation of MG in the water droplet was shown to be facilitated with decreasing the droplet diameter, demonstrating the micrometer size effect on dye association. A role of the water/oil interface for the MG dimer formation is discussed.

We report measurements of the kinetics of polymer diffusion in latex films prepared from pairs of poly(butyl methacrylate) [PBMA] latex particles. One set contains only a small amount of sulfate groups; the others contain significant amounts of carboxylic acid groups at the surface. Neutralization of these acid groups with sodium and barium hydroxide retards the interdiffusion rate but does not suppress it. In these experiments, we employ pairs of particles identical in size, with very similar molecular weights, molecular weight distributions, and surface charge densities, that differ only in the fluorescent chromophore (phenanthrene [Phe] or anthracene [An]) used to label the polymer. Each pair of latexes allows us to use direct nonradiative transfer [DET] experiments to follow polymer interdiffusion in films prepared from their dispersions. Acid group neutralization in the latex converts an acid-rich phase at the interparticle boundary in the film into an ionomer phase. Ba 2+ salts are more effective than Na + at slowing down the polymer diffusion rate, and their influence increases in proportion to the amount of polar groups present. We follow an approach developed by Eisenberg (Macromolecules 1871, 4, 125) to estimate the T g values of the ionomer phase in each film and show that there is a linear relationship between the mean apparent diffusion coefficient of the polymer determined from the DET experiment and T-T g , where T is the annealing temperature of the film

Polymer diffusion in latex films prepared from two pairs of poly(butyl methacrylate) (PBMA) latex particles is investigated. One set contains only a small amount of sulfate groups at the surface; the other contains a significant amount of carboxylic acid groups at the surface. Neutralization of these acid groups with ammonium, sodium, and barium hydroxide retards the interdiffusion rate at 100°C but does not suppress it. In these experiments, we employ pairs of particles identical in size, with very similar molecular weights, molecular weight distributions, and surface charge densities, that differ only in the fluorescent chromophore [phenanthrene (Phe) or anthracene (An)] used to label the polymer

Single micrometer-sized oil droplets formed at the end of a capillary tube in an aqueous surfactant solution were shown to exhibit vigorous auto-oscillated vibrations. The frequency and amplitude of the vibration were dependent on the droplet size, the concentration of the surfactant, and the nature of the oil used. The possible mechanism was discussed.

Spectral fitting was done on time- and temperature-dependent emission spectra of Ru(bpy){sub 3}{sup 2+} (bpy = 2,2{prime}-bipyridine) around the glass-to-fluid transition temperature of ethanol-methanol (4/1 v/v). With increasing time after excitation or with increasing temperature, the emission spectrum showed (i) a low-energy shift of the maximum, (ii) an increase in the intensity of the 0-1 vibrational band relative to that of the 0-0 band, and (iii) broadening the vibrational bands. These changes in the spectrum were ascribed to dynamic solvation change, leading to stabilization of the metal-to-ligand charge-transfer (MLCT) excited state and an increase in distortion between the ground and excited states. The time-dependent emission shift is accompanied by time-dependent nonradiative decay from the MLCT excited state.

Time-resolved measurements were performed on the photochemically induced isothermal phase transition of polymer liquid crystals (PLC) with mesogenic side chains of phenyl benzoate (PAPB3) and cyanobiphenyl (PACB3) under conditions wherein the photochemical reaction of the doped photoresponsive molecule (4-butyl-4-{prime}-methoxyazobenzene, BMAB) was completed within {approximately} 10 ns, and the subsequent phase transition of the matrix PLC from nematic (N) to isotropic (I) state was followed by time-resolved measurements of the birefringence of the system. Formation of a sufficient amount of the cis isomer of BMAB with a single pulse of a laser lowered the N-I phase transition temperature of the mixture, inducing the N-I phase transition of PLCs isothermally in a time range of {approximately} 200 ms. This time range is comparable to that of low molecular weight liquid crystals, indicating that suppression in mobility of mesogens in PLCs does not affect significantly the thermodynamically controlled process.

The pressure (P = 0.06-1.1 GPa) and temperature (T = 100-200 K) dependence of the emission spectrum and lifetime of Ru(bpy){sub 3}{sup 2+} (bpy = 2,2{prime}-bipyridine) was studied in ethanol-methanol (4/1, v/v) by means of nanosecond time-resolved emission spectroscopy. In contrast to a pressure-induced lower energy shift of the emission spectrum at room temperature, application of pressure brings about a distinct sharpening and a higher energy shift of the spectrum at low temperature (100-200 K). The phenomena observed with increasing P were quite analogous to the spectral changes upon cooling at a fixed pressure. Both P and T effects on the emission characteristics are explained by the variation of solvent viscosity. A time-dependent lower energy shift of the emission was observed in the T regions between 100 and 200 K depending on P while it was observed in a narrow T region between 115 and 150 K at atmospheric pressure. Although the emission lifetime of Ru(bpy){sub 3}{sup 2+} increases sharply upon cooling, the lifetime was found to be almost independent of P up to {approx} 1 GPa (100-160 K). The energy and band shapes of the emission spectrum of Ru(bpy){sub 3}{sup 2+} are exclusively determined by viscosity-dependent solvent relaxation while more » the lifetime is governed by deactivation processes involving the upper lying metal-to-ligand charge-transfer (MLCT) excited state(s) and therefore directly by T. The important role of viscosity-dependent solvent relaxation in the MLCT excited state is also discussed. « less

Oxidative quenching of *Ru(bpy){sub 3}{sup 2+} (bpy = 2,2{prime}-bipyridine) by neutral organic electron acceptors such as quinones, nitroaromatics, and aromatic esters in acetonitrile was studied in detail. The quenching exhibited an anomalous negative temperature dependence at {Delta}G{sub 23} (free energy change of an electron-transfer step, k{sub 23}) > {minus}3 kcal/mol, which accompanied large and negative activation entropies ({minus}18 to {minus}44 eu). The abnormal behavior was explained satisfactorily by the large contribution of back electron transfer regenerating the excited-state reactants (k{sub 32}). In a narrow {Delta}G{sub 23} region between {minus}2 and 0 kcal/mol, a bell-shaped Eyring plot was observed for several quenchers. The activation parameters for the forward (k{sub 23}) and backward (k{sub 32}) electron-transfer processes were determined on the basis of the bell-shaped Eyring plot and the standard enthalpy and entropy changes of the process. Although the temperature dependence of the quenching is anomalous, the activation parameters for the elementary process, k{sub 23} or k{sub 32}, are quite normal. The mechanism of the quenching was discussed on the basis of the rate constant as well as of the activation parameters relevant to k{sub 23}, k{sub 32}, and k{sub 30} (deactivation from the product ion pairs).

Time-resolved emission spectroscopy of Ru(bpy)/sub 3/Cl/sub 2/ and cis-Ru(bpy)/sub 2/(CN)/sub 2/, where bpy is 2,2'-bipyridine, was conducted in an ethanol-methanol mixture above 80 K. The emission spectra of both complexes shifted to the red with increasing temperatures (>80 K) as well as with the delay time after excitation (110-150 K). The apparent activation energies for the time-dependent red shift of the emission (110-130 K) were calculated to be 570 and 1360 cm/sup -1/ for Ru(bpy)/sub 3/Cl/sub 2/ and cis-Ru(bpy)/sub 2/(CN)/sub 2/, respectively. It was concluded that charge localization in the excited state was not an intrinsic property of the complex but was induced by solvent relaxation processes. 17 references, 4 figures, 1 table.

An anomalous temperature dependence of the rates was observed for both reductive and oxidative quenching of excited cis-dicyanobis(1,10-phenanthroline)ruthenium(II), *RuCN, by aromatic amines and nitroaromatics, respectively, in acetonitrile. The observed activation enthalpies and entropies were more positive than those observed for oxidative quenching of the excited Ru(bpy){sub 3}{sup 2+} (bpy = 2,2{prime}-bipyridine). The results were satisfactorily explained by the standard and/or electrostatic enthalpy and entropy changes of the electron-transfer step. Efficient back electron transfer from product ion pairs to the excited-state reactants in the redox quenching of *RuCN is ascribable to a favorable entropy change of the process as well as to electrostatic attraction within the ion pairs. The electrostatic interactions within the product ion pairs (repulsive or attractive) determine the overall quenching paths and, thus, the temperature dependence of the quenching.

The rate constants (k{sub q}) and activation parameters for reductive quenching of *Ru(bpy){sub 3}{sup 2+} (bpy = 2,2{prime}-bipyridine) by 19 aromatic amines were determined in acetonitrile. The activation enthalpies ({Delta}H{sub 23}{double dagger}) increased from +1 to +6 kcal/mol with decreasing k{sub q} while the activation entropies were almost constant around {minus}15 eu for all quenchers except for those of highly exoergic reactions. The mechanism of the reductive quenching is discussed on the basis of k{sub q} as well as of the activation parameters. The quenching was shown to depend on the nature of the aromatic amine; the quenching by tertiary amines was always faster than that by primary and secondary amines when k{sub q} are compared at the same free energy change. The results are explained by the variation of outer-sphere reorganization energy with the nature of a quencher. For the quenching by tertiary and primary/secondary amines, the radii of the quenchers (r{sub Q}{sup ex}) were estimated to be 3 and 2.4 {angstrom}, respectively, by analyzing free energy relationships of the activation free energy of the quenching. The difference in r{sub Q}{sup ex} between tertiary and primary or secondary amines (0.5-0.6 {angstrom}) was in good agreement with that between N,N-dimethylaminomore » and anilino groups estimated by the space-filling model (0.7 {angstrom}). The solvent reorganization around the amino group is more relevant than that around an aromatic ring or a whole molecule to the actual outer-sphere reorganization energy.« less

The pressure effects on the relaxation process of excited K{sub 4}(Pt{sub 2}(H{sub 2}P{sub 2}O{sub 5}){sub 4}){center dot}2H{sub 2}O (Pt{sub 2}) crystal have been studied at room temperature. The emission properties such as maximum energy and peak intensity are discussed on the basis of X-ray diffraction study and infrared spectroscopy. Lower energy shifts of the emission energy were observed ({minus}170 and {minus}190 cm{sup {minus}1}/GPa for fluorescence and phosphorescence, respectively), which was interpreted as due to reduction of the intramolecular Pt-Pt distance. The pressure-induced excimer fluorescence appearing at the lower energy side of the normal fluorescence also exhibited a lower energy shift with increasing pressure ({minus}1,000 cm{sup {minus}1}/GPa). This phenomenon was observed only for well-grown Pt{sub 2} crystals.

Electron transfer quenching reactions of excited tris(2,2'-bi-pyridine)ruthenium(II), *Ru(bpy)/sub 3//sup 2 +/, have received special attention as tests of Marcus theory as well as in their application to redox photosensitization. Nevertheless, the mechanism of oxidative quenching of *Ru(bpy)/sub 3//sup 2 +/ in acetonitrile, exhibiting negative temperature dependence in endoergic and moderately exoergic regions, is not explicitly understood. They now report the origin of the negative temperature dependence and the reaction mechanism of oxidative quenching of *Ru(bpy)/sub 3//sup 2 +/. The quenching rate constant (k/sub q/) at a given temperature was obtained by conventional Stern-Volmer plots based on both the emission intensity and the lifetime of *Ru(bpy)/sub 3//sup 2 +/. The effect of diffusion on the observed quenching rate constant was corrected.

The rates of both reductive and oxidative quenching of excited cis-dicyanobis(1,10-phenanthroline)ruthenium(II) by aromatic amines and nitroaromatics in acetonitrile (ionic strength = 0), respectively, exhibited a negative temperature dependence. Comparing the present results with those for Ru(bpy)/sub 3//sup 2 +/ (bpy = 2,2'-bipyridine), the authors concluded that the electrostatic interaction (repulsive or attractive) within product ion pairs decided the quenching mechanism and, consequently, the temperature dependence of the quenching.

Energy migration among chromophores of identical nature is one of the simplest but most fundamental photophysical processes in particular in molecular aggregate systems. Nevertheless, its quantitative rate analysis has not been reported until very recently. The energy transfer process involving either the singlet or triplet state is highly relevant to material science, photoreactions, and expected photonic devices. In the last case, the controlled one-dimensional flow of energy and/or electrons is a key process, which has hardly been surveyed. To test the possibility, we chose X{sub 4}(Pt{sub 2}(P{sub 2}O{sub 5}H{sub 2}){sub 4}) (X = Na{sup +}, K{sup +}, Et{sub 4}N{sup +}, and (n-Bu){sub 4}N{sup +}) crystals and measured the laser-induced phosphorescence.