Lanthanide and Actinide Ion Complexes Containing Organic Ligands Investigated by Surface-Enhanced Infrared Absorption Spectroscopy.

A new technique, surface-enhanced infrared absorption (SEIRA) spectroscopy, was used for the structural investigation of lanthanide (Ln) and actinide (An) complexes containing organic ligands. We synthesized thiol derivatives of organic ligands with coordination sites similar to those of 2-[N-methyl-N-hexanethiol-amino]-2-oxoethoxy-[N',N'-diethyl]-acetamide [diglycolamide (DGA)], Cyanex-272, and N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN), which have been used for separating Ln and An through solvent extraction. These ligands were attached on a gold surface deposited on an Si prism through S-Au covalent bonds; the gold surface enhanced the IR absorption intensity of the ligands. Aqueous solutions of Ln (Eu3+, Gd3+, and Tb3+) and An (Am3+) ions were loaded onto the gold surface to form ion complexes. The IR spectra of the ion complexes were obtained using Fourier transform infrared spectroscopy in the attenuated total reflection mode. In this study, we developed a new sample preparation method for SEIRA spectroscopy that enabled us to obtain the IR spectra of the complexes with a small amount of ion solution (5 µL). This is a significant advantage for the IR measurement of radiotoxic Am3+ complexes. In the IR spectra of DGA, the band attributed to C═O stretching vibrations at ∼1630 cm-1 shifted to a lower wavenumber by ∼20 cm-1 upon complexation with Ln and An ions. Moreover, the amount of the red shift was inversely proportional to the extraction equilibrium constant reported in previous studies on solvent extraction. The coordination ability of DGA toward Ln and An ions could be assessed using the band position of the C═O band. The Cyanex-272- and TPEN-like ligands synthesized in this report also showed noticeable SEIRA signals for Ln and An complexes. This study indicates that SEIRA spectroscopy can be used for the structural investigation of ion complexes and provides a microscopic understanding of selective extraction of Ln and An.

Brush Printing Creates Polarized Green Fluorescence: 3D Orientation Mapping and Stochastic Analysis of Conductive Polymer Films.

Brush printing is a unique method used to obtain uniaxially oriented films, whereby a polymer solution is brushed onto a substrate. However, there have been only a few reports on the brush-printing method. Here, we report the preparation of a uniaxially oriented film of a green light-emitting conductive polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). The fluorescence polarization ratio of the oriented F8BT films was as high as 11.3, and the average orientation factor reached 0.74 ± 0.06. The orientation factor and the torsion angle of F8BT were visualized by two mappings of fluorescence and Raman spectral measurements by confocal spectromicroscopy, respectively. These two x-y mapping data with many pixels (∼750 pixels) were evaluated by x-y-z mapping of the film thickness at a single position and were used to reveal the three-dimensional (3D) orientation mechanism from a stochastic approach. Polarized green fluorescence originates from polymer chains uniaxially oriented along the brush direction. The high orientation for a film thickness < 100 nm is established by shear stress, faster capillary flow, and flow-induced chain extension for a thin solution film on a substrate. The high orientation factor was also demonstrated by a high brushing speed, whereas an optimized brushing speed existed. We found that this optimization is attributed to the property of a non-Newtonian fluid. By applying this brush-printing method to the fabrication of an optoelectrical device, polarized green electroluminescence was preliminarily demonstrated by the OLED assembled from an oriented F8BT film.

Si nanocrystal solution with stability for one year.

Colloidal silicon nanocrystals (SiNCs) are a promising material for next-generation nanostructured devices. High-stability SiNC solutions are required for practical use as well as studies on the properties of SiNC. Here, we show a solution of SiNCs that was stable for one year without aggregation. The stable solution was synthesized by a facile process, i.e., pulsed laser ablation of a Si wafer in isopropyl alcohol (IPA). The long-term stability was due to a large ζ-potential of -50 mV from a SiNC passivation layer composed of oxygen, hydrogen, and alkane groups, according to the results of eight experiments and theoretical calculations. This passivation layer also resulted in good performance as an additive for a conductive polymer film. Namely, a 5-fold enhancement in carrier density was established by the addition of SiNCs into an organic conductive polymer, poly(3-dodecylthiophene), which is useful for solar cells. Furthermore, it was found that fresh (<1 day) and aged (4 months) SiNCs give the same enhancement. The long-term stability was attributed to a great repulsive energy in IPA, whose value was quantified as a function the distance between SiNCs.

Uniaxial orientation of P3HT film prepared by soft friction transfer method.

The realization of room-temperature processes is an important factor in the development of flexible electronic devices composed of organic materials. In addition, a simple and cost-effective process is essential to produce stable working devices and to enhance the performance of a smart material for flexible, wearable, or stretchable-skin devices. Here, we present a soft friction transfer method for producing aligned polymer films; a glass substrate was mechanically brushed with a velvet fabric and poly(3-hexylthiophene) (P3HT) solution was then spin-coated on the substrate. A P3HT film with a uniaxial orientation was obtained in air at room temperature. The orientation factor was 17 times higher than that of a film prepared using a conventional friction transfer technique at a high temperature of 120 °C. In addition, an oriented film with a thickness of 40 nm was easily picked up and transferred to another substrate. The mechanism for orientation of the film was investigated using six experimental methods and theoretical calculation, and was thereby attributed to a chemical process, i.e., cellulose molecules attach to the substrate and act as a template for molecular alignment.

Solvation of Esters and Ketones in Supercritical CO2.

Vibrational Raman spectra for the C═O stretching modes of three esters with different functional groups (methyl, a single phenyl, and two phenyl groups) were measured in supercritical carbon dioxide (scCO2). The results were compared with Raman spectra for three ketones involving the same functional groups, measured at the same thermodynamic states in scCO2. The peak frequencies of the Raman spectra of these six solute molecules were analyzed by decomposition into the attractive and repulsive energy components, based on the perturbed hard-sphere theory. For all solute molecules, the attractive energy is greater than the repulsive energy. In particular, a significant difference in the attractive energies of the ester-CO2 and ketone-CO2 systems was observed when the methyl group is attached to the ester or ketone. This difference was significantly reduced in the solute systems with a single phenyl group and was completely absent in those with two phenyl groups. The optimized structures among the solutes and CO2 molecules based on quantum chemical calculations indicate that greater attractive energy is obtained for a system where the oxygen atom of the ester is solvated by CO2 molecules.

Si-nanocrystal/P3HT hybrid films with a 50- and 12-fold enhancement of hole mobility and density: films prepared by successive drop casting.

Hybrid silicon nanocrystal (Si-NC)/poly(3-hexylthiophene) (P3HT) films serve as the active layers of quantum dot/polymer hybrid photovoltaics. To achieve effective photovoltaic properties, it is necessary to enhance the charge carrier mobility and carrier density of the P3HT films. A 50- and 12-fold enhancement of the hole mobility and hole density, respectively, was achieved along the out-of-plane direction of a Si-NC/P3HT hybrid film, which corresponds to the carrier-migration direction between the photovoltaic electrodes. According to time-of-flight, electronic absorption, Raman, atomic force microscopy, photoluminescence lifetime, and X-ray diffraction measurements, the significant enhancement of the mobility and density was attributed to both an increase in the P3HT crystallinity and the dissociation efficiency of P3HT excitons on the addition of Si-NCs to the P3HT films. These enhancements were achieved using a film preparation method developed in the present study, which has been named successive drop casting.

Investigation of attractive and repulsive interactions associated with ketones in supercritical CO2, based on Raman spectroscopy and theoretical calculations.

Carbonyl compounds are solutes that are highly soluble in supercritical CO2 (scCO2). Their solubility governs the efficiency of chemical reactions, and is significantly increased by changing a chromophore. To effectively use scCO2 as solvent, it is crucial to understand the high solubility of carbonyl compounds, the solvation structure, and the solute-solvent intermolecular interactions. We report Raman spectroscopic data, for three prototypical ketones dissolved in scCO2, and four theoretical analyses. The vibrational Raman spectra of the C=O stretching modes of ketones (acetone, acetophenone, and benzophenone) were measured in scCO2 along the reduced temperature Tr = T∕Tc = 1.02 isotherm as a function of the reduced density ρr = ρ∕ρc in the range 0.05-1.5. The peak frequencies of the C=O stretching modes shifted toward lower energies as the fluid density increased. The density dependence was analyzed by using perturbed hard-sphere theory, and the shift was decomposed into attractive and repulsive energy components. The attractive energy between the ketones and CO2 was up to nine times higher than the repulsive energy, and its magnitude increased in the following order: acetone < acetophenone < benzophenone. The Mulliken charges of the three solutes and CO2 molecules obtained by using quantum chemistry calculations described the order of the magnitude of the attractive energy and optimized the relative configuration between each solute and CO2. According to theoretical calculations for the dispersion energy, the dipole-induced-dipole interaction energy, and the frequency shift due to their interactions, the experimentally determined attractive energy differences in the three solutes were attributed to the dispersion energies that depended on a chromophore attached to the carbonyl groups. It was found that the major intermolecular interaction with the attractive shift varied from dipole-induced dipole to dispersion depending on the chromophore in the ketones in scCO2. As the common conclusion for the Raman spectral measurements and the four theoretical calculations, solute polarizability, modified by the chromophore, was at the core of the solute-solvent interactions of the ketones in scCO2.

Significant substitution effects in dipolar and non-dipolar supercritical fluids.

Vibrational Raman spectra of C=C stretching modes of ethylene derivates (cis-C(2)H(2)Cl(2), cis-stilbene, and trans-stilbene) were measured in supercritical fluids along an isotherm as functions of their densities. The substitution effect of the Raman shift is so significant that a difference among three solutes can be 20 times and is observed similarly in dipolar (CHF(3)) and non-dipolar (CO(2)) fluids. In particular, the shifts of trans-stilbene were enormously large among all systems for studies of vibrational spectroscopies of supercritical fluids and were equivalent to those of typical hydrogen-bonded fluids. Such large shifts arising from the significant attractive energy between solute and solvent molecules were attributed to a site-selective solvation around a phenyl group, which was driven by a dispersion force in the absence of steric hindrance. We found that the absence of steric hindrance causes the significant local density augmentation. To the best of our knowledge, Raman experiments and their theoretical analysis are the first ones quantifying how the difference of steric hindrance produces solvation structures in solution as well as supercritical solutions.

Site-selective solvation in supercritical CO2 observed by Raman spectroscopy: phenyl group leads to greater attractive energy than chloro group.

Vibrational Raman spectra of the C=C stretching modes of cis-stilbene and cis-1,2-dichloroethylene (C(2)H(2)Cl(2)) were measured in supercritical CO(2) in a density range of 0.08 < ρ(r) = ρ/ρ(c) < 1.5 at an isotherm of T(r) = T/T(c) = 1.02. As the fluid density increased, the peak frequencies of cis-stilbene and cis-C(2)H(2)Cl(2) shifted toward the low-energy side. The shifted frequencies of cis-stilbene were consistently greater than those of cis-C(2)H(2)Cl(2) in all density regions, by a factor of 4. By analyzing these density dependencies using the perturbed hard-sphere theory, the shifted frequencies were decomposed into attractive and repulsive components. By quantifying these components as a function of fluid density, we investigated how each solute is solvated in supercritical CO(2). The results indicate that the attractive energy between cis-stilbene and CO(2) is twice that between cis-C(2)H(2)Cl(2) and CO(2). A local density augmentation around the solute molecule was not observed in the cis-C(2)H(2)Cl(2)/CO(2) system, but it was observed in the cis-stilbene/CO(2) system because of site-selective solvation around the phenyl group of cis-stilbene. To the best of our knowledge, this is the first time that the site-selective solvation of a solute molecule has been observed using Raman spectral measurements of a solution system. Based on theoretical calculations and Raman spectral measurements of cis-stilbene in the supercritical fluid of dipolar CHF(3), it is concluded that a driving force for site-selective solvation is the dispersion force.

Solute-solvent intermolecular interactions in supercritical Xe, SF6, CO2, and CHF3 investigated by Raman spectroscopy: greatest attractive energy observed in supercritical Xe.

Vibrational Raman spectra of the C=C stretching modes of cis- and trans-1,2-dichloroethylene (C(2)H(2)Cl(2)) were measured in supercritical Xe, SF(6), CO(2), and CHF(3). The spectra were collected over a wide range of densities of supercritical fluids at a fixed solute mole fraction and isotherm of T(r) = T/T(c) = 1.02. In all fluids, as the density increased, the peak frequencies of the C=C stretching modes shifted toward the low-energy side. By analyzing these density dependencies using the perturbed hard-sphere theory, the shifted amounts were characterized into attractive and repulsive components. The attractive shifts of both isomers were almost equivalent in supercritical CHF(3), CO(2), and SF(6), whereas they were significantly larger in supercritical Xe. The attractive shifts obtained experimentally were compared with the ones calculated on the basis of dispersion, dipole-dipole, dipole-induced-dipole, and dipole-quadrupole interactions between solute and solvent molecules. The experimental attractive shifts in supercritical Xe were 2-3 times greater than the calculated shifts. The large attractive shifts were ascribed to both an anisotropic solvation structure and to a strong interaction (charge transfer) between Xe and C(2)H(2)Cl(2) molecules.

Solvation structures of cis- and trans-1,2-dichloroethylene in supercritical CO2 investigated by Raman spectroscopy and attractive energy calculations.

Vibrational Raman spectra of the C horizontal lineC stretching modes of cis- and trans-1,2-dichloroethylene (C(2)H(2)Cl(2)) were measured in supercritical carbon dioxide (CO(2)). The spectra were collected at a fixed solute mole fraction by varying the fluid density by a factor of 20. As the density increased, the peak frequencies of the C horizontal lineC stretching modes shifted toward the low-energy side at isotherms of reduced temperature, T(r) = T/T(c) = 1.02, 1.06, and 1.20. By analyzing these density dependences using the perturbed hard-sphere theory, we decomposed the shifts into attractive and repulsive components. The repulsive shifts of cis-C(2)H(2)Cl(2) were almost equivalent to those of trans-C(2)H(2)Cl(2). However, the attractive shifts of nonpolar trans-C(2)H(2)Cl(2) were significantly greater than those of polar cis-C(2)H(2)Cl(2) at all densities and temperatures. To evaluate the difference in the isomers, we calculated the attractive shifts of the C horizontal lineC stretching modes of each isomer, composing of dispersion, dipole-induced-dipole, and dipole-quadrupole interactions between solute C(2)H(2)Cl(2) and solvent CO(2) molecules. These three interactions were quantified by considering molecular configurations and orientations, and solvation structures around the isomers were elucidated by 3D schematic diagrams. As a result, it was shown that the anisotropic solvation structure around trans-C(2)H(2)Cl(2) was responsible for the larger attractive shifts in the supercritical CO(2). The difference of solvation structures between the isomers was significant at T(r) = 1.02 but became minor as the temperature increased to T(r) = 1.20.

Difference of solute-solvent interactions of cis- and trans-1,2-dichloroethylene in supercritical CO2 investigated by raman spectroscopy.

Vibrational Raman spectra of CC stretching modes of both cis- and trans-1,2-dichloroethylene (C2H2Cl2) were measured as a function of density in supercritical carbon dioxide (CO2). Measurements were performed with solute mole fraction of 0.01 at an isotherm of T r = T/ T c = 1.02. As the density of CO2 increased, peak frequencies of the CC stretching modes shifted toward the low energy side. By analyzing these density dependences using perturbed hard-sphere theory, we decomposed the shifted amounts into attractive and repulsive components. The amounts of repulsive shifts were almost equivalent, whereas those of the attractive shifts of trans-C2H2Cl 2 were larger than those of cis-C2H2Cl2 at all densities. This means that the nonpolar solute, trans-C2H2Cl2, shows stronger solute-solvent interactions than those of the polar solute cis-C2H2Cl2. The difference of attractive interactions between these isomers is the greatest at a density where local density enhancement of supercritical CO2 reaches the maximum.

Time evolution of density fluctuation in the supercritical region. 2. Comparison of hydrogen- and non-hydrogen-bonded fluids.

The time evolution of the density fluctuation of molecules is investigated by dynamic light scattering in six neat fluids in supercritical states. This study is the first to compare the dynamics of density inhomogeneity between hydrogen- and non-hydrogen-bonded fluids. Supercritical methanol and ethanol are used as hydrogen-bonded fluids, whereas four non-hydrogen-bonded fluids were used: CHF(3), C(2)H(4), CO(2), and Xe. We measure the time correlation function of the density fluctuation of each fluid at the same reduced temperatures and densities and investigate the relationship between the dynamic and static density inhomogeneities of those supercritical fluids. In all cases, the profile of the time correlation function of the density fluctuation is characterized by a single-exponential function, whose decay is responsible for the dynamics characterized by hydrodynamic conditions. We obtain correlation times from the time correlation function and discuss dynamic and static inhomogeneity using the Kawasaki theory and the Landau-Placzek theory. While the correlation times in the six fluids show noncoincidence, those values agree well with each other except for the supercritical alcohols when scaled to a dimensionless parameter. Although the principle of corresponding state is observed in the non-hydrogen-bonded fluids, both the supercritical methanol and ethanol deviate from that principle. This deviation is attributed to the presence of hydrogen bonding among alcohol molecules at high temperature and low density. The average cluster size of each fluid is estimated under the same thermodynamic conditions, and it is shown that the clusters of supercritical alcohols are on average 1.5-1.7 times larger than those of the four non-hydrogen-bonded fluids. Moreover, the thermal diffusivity of each neat fluid is obtained over wide ranges of density and temperature.

Time evolution of density fluctuation in supercritical region. I. Non-hydrogen-bonded fluids studied by dynamic light scattering.

The time evolution of the density fluctuation of molecules inhomogeneously dispersing in a mesoscopic volume is investigated by dynamic light scattering in several fluids in supercritical states. This study is the first time-domain investigation to compare the dynamics of density fluctuation among several fluids. The samples used are non-hydrogen-bonded fluids in the supercritical states: CHF(3), C(2)H(4), CO(2), and xenon. These four molecules have different properties but are of similar size. Under these conditions, the relationship between dynamic and static density inhomogeneities is studied by measuring the time correlation function of the density fluctuation. In all cases, this function is characterized by a single exponential function, decaying within a few microseconds. While the correlation times in the four fluids show noncoincidence, those values agree well with each other when scaled to a dimensionless parameter. From the results of this scaling based on the Kawasaki theory and Landau-Placzek theory, the relation between dynamics and static structures is analyzed, and the following four insights are obtained: (i) viscosity is the main contributor to the time evolution of density fluctuation; (ii) the principle of corresponding state is observed by the use of time-domain data; (iii) the Kawasaki theory and the Landau-Placzek theory are confirmed to be applicable to polar, nonpolar, and nondipolar fluids that have no hydrogen bonding, at temperatures relatively far from critical temperature; and (iv) the density fluctuation correlation length and the value of density fluctuation are estimated from the time-domain data and agree with the values from other experiments and calculations.

Dynamics of density fluctuation of supercritical fluid mapped on phase diagram.

A time evolution of polar molecules inhomogeneously dispersing in mesoscale is investigated by dynamic light scattering around the gas-liquid critical point. The dynamics evaluated on phase diagrams produces a contour map of critical slowing down and suggests a ridge to be a trace of intermediate lines between gas and liquid states. A good coincidence between dynamics and static inhomogeneities is confirmed in the wide density, and it is consistent with theoretical expressions.