Contribution Of Reorientation Research Articles

Photothermal and Reorientational Contributions to the Photomechanical Response of DR1 Azo Dye-Doped PMMA Fibers

This work is a comprehensive experimental and theoretical study aimed at understanding the photothermal and molecular shape-change contributions to the photomechanical effect of polymers doped with azo dyes. Our prototypical system is the azobenzene dye Disperse Red 1 (DR1) doped into poly (methyl methacrylate) (PMMA) polymer formed into optical fibers. We start by determining the thermo-mechanical properties of the materials with a temperature-dependent stress measurement. The material parameters, so determined, are used in a photothermal heating model—with no adjustable parameters—to predict its contribution. The photothermal heating model predicts the observations, ruling out mechanisms originating in light-induced shape changes of the dopant molecules. The photomechanical tensor response along the two principle axes in the uniaxial approximation is measured and compared with another independent theory of photothermal heating and angular hole burning/reorientation. Again, the results are consistent only with a purely thermal response, showing that effects due to light-induced shape changes of the azo dyes are negligible. The measurements are repeated as a function of polymer chain length and the photomechanical efficiencies determined. We find the results to be mostly chain-length independent.

Ionic conductivity and relaxation dynamics in plastic crystals with nearly globular molecules.

We have performed a dielectric investigation of the ionic charge transport and the relaxation dynamics in plastic crystalline 1-cyano-adamantane (CNA) and in two mixtures of CNA with the related plastic crystals adamantane or 2-adamantanon. Ionic charge carriers were provided by adding 1% of Li salt. The molecules of these compounds have nearly globular shape and, thus, the so-called revolving-door mechanism assumed to promote ionic charge transport via molecular reorientations in other PC electrolytes should not be active here. Indeed, a comparison of the dc resistivity and the reorientational α-relaxation times in the investigated PCs reveals complete decoupling of both dynamics. Similar to other PCs, we find a significant mixing-induced enhancement of the ionic conductivity. Finally, these solid-state electrolytes reveal a second relaxation process, slower than the α-relaxation, which is related to ionic hopping. Due to the mentioned decoupling, it can be unequivocally detected and is not superimposed by the reorientational contributions as found for most other ionic conductors.

Empirical Analysis of Optical Kerr Effect Spectra: A Case for Constraint.

Ultrafast optical Kerr effect (OKE) spectroscopy is a widely used method for studying the depolarized, Raman-active intermolecular dynamics of liquids. Through appropriate manipulation of OKE data, it is possible to determine the reduced spectral density (RSD), which is the Bose-Einstein-corrected, low-frequency Raman spectrum with the contribution of diffusive reorientation removed. OKE RSDs for van der Waals liquids can often be fit well to an empirical function that is the sum of a Bucaro-Litovitz function and an antisymmetrized Gaussian (AG). Although these functions are not directly representative of specific intermolecular dynamics, the AG fit parameters can provide useful insights into the microscopic properties of liquids. Here we show that fits using the AG function are typically not well-determined, and that equally good results can be obtained with a wide range of fitting parameters. We propose the use of a physically motivated constraint on the amplitude of the AG function, and demonstrate that this constraint leads to more intuitive trends in the fit parameters for temperature-dependent RSDs in 1,3,5-trifluorobenzene and hexafluorobenzene.

Orientational Mobility in Dendrimer Melts: Molecular Dynamics Simulations

We have simulated the melt of poly(carbosilane) dendrimers using atomistic models and have reproduced the effect predicted by the analytical theory; i.e., orientational autocorrelation functions of a segment from the same layer (numbered from periphery) are practically identical and do not depend on dendrimer size. The frequency dependences of the dielectric and NMR relaxation were obtained and studied in detail. The main contribution to the maxima of these dependences is given by the pulsation process. It leads to a shift of the maxima to low frequencies for the core segment in comparison with the maxima for peripheral segments. The contribution of local reorientation can also be significant, and in some cases this contribution manifests as an additional maximum. The nontrivial scaling laws in the frequency dependences of dielectric permittivity and NMR relaxation rate averaged over all layers of a dendrimer macromolecule are found. A similar scaling law is observed in the experiments on NMR relaxation b...

Translational and rotational mobility of methanol-d4 molecules in NaX and NaY zeolite cages: A deuteron NMR investigation

Nuclear magnetic resonance (NMR) provides means to investigate molecular dynamics at every state of matter. Features characteristic for the gas phase, liquid-like layers and immobilized methanol-d4 molecules in NaX and NaY zeolites were observed in the temperature range from 300K down to 20K. The NMR spectra at low temperature are consistent with the model in which molecules are bonded at two positions: horizontal (methanol oxygen bonded to sodium cation) and vertical (hydrogen bonding of hydroxyl deuteron to zeolite framework oxygen). Narrow lines were observed at high temperature indicating an isotropic reorientation of a fraction of molecules. Deuteron spin–lattice relaxation gives evidence for the formation of trimers, based on observation of different relaxation rates for methyl and hydroxyl deuterons undergoing isotropic reorientation. Internal rotation of methyl groups and fixed positions of hydrogen bonded hydroxyl deuterons in methyl trimers provide relaxation rates observed experimentally. A change in the slope of the temperature dependence of both relaxation rates indicates a transition from the relaxation dominated by translational motion to prevailing contribution of reorientation. Trimers undergoing isotropic reorientation disintegrate and separate molecules become localized on adsorption centers at 166.7K and 153.8K for NaX and NaY, respectively, as indicated by extreme broadening of deuteron NMR spectra. Molecules at vertical position remain localized up to high temperatures. That indicates the dominating role of the hydrogen bonding. Mobility of single molecules was observed for lower loading (86molecules/uc) in NaX. A direct transition from translation to localization was observed at 190K.

ChemInform Abstract: Importance of Orientational Rearrangement During Vitrification of Hydrocarbons: Dependence on Molecular Shape.

On the basis of Monte Carlo calculations of 2,2-dimethylpropane (neopentane), n-pentane, and 2,2-dimethylbutane (neohexane) at several temperatures, thermodynamic properties and radial distribution functions as well as dimerization and bonding energy distribution functions are reported for both liquid and glassy states. Changes in the radial distribution functions on cooling depend on whether the groups are accessible (peripheral) or inaccessible. Peaks in the radial distribution functions corresponding to peripheral groups do not shift to lower distances on cooling and at times display a large increase in the intensity of the first peak. The peaks due to inaccessible groups, on the other hand, shift to lower distances on cooling. The magnitude of the reorientational contribution in determining the resulting structure of the glass is estimated for the different hydrocarbon molecules investigated. The reorientational contribution is highest for neopentane (26%) followed by isopentane (24%), neohexane (22%), and n-pentane (0%). It appears that molecular geometry has an important role in determining the magnitude of the reorientational contribution to the structure of the glass.

Supercontinuum generation in liquid-filled photonic crystal fiber with slow nonlinear response

We theoretically investigate the nonlinear propagation of femtosecond pulses in liquid-core photonic crystal fibers filled with CS2. The effect of slow nonlinearity due to reorientational contribution of liquid molecules on broadband supercontinuum generation in the femtosecond regime is studied using an appropriately modified nonlinear Schrodinger equation. To analyze the quality of the pulse, we perform the stability analysis and study coherence of supercontinuum pulse numerically. We show that the response of the slow nonlinearity not only enhances broadening of the pulse and changes the dynamics of the generated solitons, but also increases coherence of the pulse.

Biosorption of copper(II) and lead(II) onto potassium hydroxide treated pine cone powder

Pine cone powder surface was treated with potassium hydroxide and applied for copper(II) and lead(II) removal from solution. Isotherm experiments and desorption tests were conducted and kinetic analysis was performed with increasing temperatures. As solution pH increased, the biosorption capacity and the change in hydrogen ion concentration in solution increased. The change in hydrogen ion concentration for lead(II) biosorption was slightly higher than for copper(II) biosorption. The results revealed that ion-exchange is the main mechanism for biosorption for both metal ions. The pseudo-first order kinetic model was unable to describe the biosorption process throughout the effective biosorption period while the modified pseudo-first order kinetics gave a better fit but could not predict the experimentally observed equilibrium capacities. The pseudo-second order kinetics gave a better fit to the experimental data over the temperature range from 291 to 347 K and the equilibrium capacity increased from 15.73 to 19.22 mg g −1 for copper(II) and from 23.74 to 26.27 for lead(II). Activation energy was higher for lead(II) (22.40 kJ mol −1) than for copper(II) (20.36 kJ mol −1). The free energy of activation was higher for lead(II) than for copper(II) and the values of ΔH* and ΔS* indicate that the contribution of reorientation to the activation stage is higher for lead(II) than copper(II). This implies that lead(II) biosorption is more spontaneous than copper(II) biosorption. Equilibrium studies showed that the Langmuir isotherm gave a better fit for the equilibrium data indicating monolayer coverage of the biosorbent surface. There was only a small interaction between metal ions when simultaneously biosorbed and cation competition was higher for the Cu-Pb system than for the Pb-Cu system. Desorption studies and the Dubinin–Radushkevich isotherm and energy parameter, E, also support the ion-exchange mechanism.

Mode-Selective Optical Kerr Effect Spectroscopy

We demonstrate that by using two pump pulses with independently controllable polarizations, intensity, and timing, different contributions to the optical Kerr effect (OKE) signal in liquids can be enhanced and suppressed. When both pump pulses have the same polarization, intramolecular vibrations can be enhanced or suppressed without affecting the reorientational diffusion contribution to the signal significantly. Similar control can be exerted over intramolecular vibrations when the pump pulses are perpendicularly polarized, and under these conditions it is also possible to suppress the reorientational diffusion component of the signal completely. When two intramolecular vibrational modes are present in the signal, it is possible to enhance one while completely suppressing the other if the pump polarizations and timing are chosen appropriately. This technique should be a useful means for enhancing contrast in OKE microscopy.

Ion conducting rotor phases—new insights from quasielastic neutron scattering

Ion conducting rotor phases are high-temperature modifications of simple inorganic salts with typically cubic structure. Here the polyatomic tetrahedral anions like SO 4 2− or PO 4 3− show considerable rotational disorder, while the cations exhibit fast ion conductivity. A long lasting dispute concentrates on the question if and how the anion reorientational motion influences or enhances the cation transport in these materials. We report new results from quasielastic neutron scattering experiments on several ion conducting rotor phases, viz., Na 3PO 4, Li 2SO 4, and LiNaSO 4. Sodium ion diffusion in Na 3PO 4 has been examined on the backscattering spectrometer IN16. Here the data can easily be interpreted on the basis of the Chudley–Elliott model, yielding reasonable values for jump distance, mean residence times, and diffusion coefficients. While in Na 3PO 4 the linewidths due to anion reorientation (studied in an earlier time-of-flight measurement) and cation hopping differ by roughly three orders of magnitude, the situation in Li 2SO 4 and LiNaSO 4 is completely different: here both types of motion have a similar time scale, and the time-of-flight experiment reveals both diffusional and reorientational contributions to the quasielastic scattering.

Partitioning of the total reorganization energy into its vibrational and solvent (inertial and reorientational) contributions using resonance Raman intensities

The concept and determination of inertial solvent reorganization energy for an intramolecular charge-transfer process are presented in this Letter. Resonance Raman (RR) spectra have been recorded for 4-nitro,4′-dimethylamino-azobenzene (DA) within the charge-transfer transition, in acetonitrile and benzonitrile. Mode-specific and solvent reorganization energies are determined by simulating the absorption and RR spectra. The results indicate that the vibrational reorganization energy for DA is comparable in the two solvents and the difference is observed mainly in the solvent reorganization energy that has been partitioned into inertial and reorientational components. The inertial response has greater contribution than its reorientational counterpart in acetonitrile, as expected, whereas the reverse trend is observed in benzonitrile.

Dynamical properties in glass forming polymers

Raman low-frequency depolarized light scattering measurements were performed on polymers, namely isotactic polypropylene (iPP), polyethylene (PE) and their blends with hydrogenated oligo cyclo pentadiene (HOCP) at the melting point. In the solid state these blends have a lamellar morphology, crystalline iPP layers alternating to amorphous iPP + HOCP layers. Detailed study of the experimental data showed the main role played by the effective vibrational density of states in comparison with the reorientational diffusion contribution. On the other hand the existence of a boson peak, characteristic of glass forming systems, whose centre-frequency shifts towards higher values, increasing the percentage of HOCP, denotes the disorder effect connected with the presence of this component in the polymeric blends, the occurrence of which is also shown by the evolution of the dynamical correlation length, R c. Furthermore, in the very low-frequency range a crossover ( ω co∼0.1 THz) from a spectral phonon-like contribution to a fracton-like contribution is detected.

Reorientational motion of the NO 3− ion through the liquid-glass transition in Ca 0.4K 0.6(NO 3) 1.4 and Ca(NO 3) 2 + 8H 2O

The reorientational correlation function of the NO 3 − ion in Ca 0.4K 0.6(NO 3) 1.4 and in Ca(NO 3) 2 + 8H 2O has been investigated by Raman spectroscopy from the low viscosity liquid at high temperatures through the liquid-glass transformation region and into the frozen glassy state. It was obtained from the reorientational contribution to the 1050 cm −1 symmetric stretch of the NO 3 − ion, determined from a comparison of the anisotropic and isotropic parts of the Raman band. It was found that the NO 3 − ion displays rapid reorientational motion over the entire temperature range studied, i.e., also well below the glass transition temperature, T g. Two types of rotational motion were revealed from the time evolution of the reorientational correlation function which shows a change of behaviour around 0.2 ps. The fast process was attributed to a free-rotor-type motion and the slower process to reorientational diffusion. The relaxation time of the latter, τ or, was observed to follow an Arrhenius temperature dependence over the range from above the melting temperature to well below T g. It was found that τ or decouples from the structural relaxation time at temperatures below ∼ 1.7 T g in the two systems studied.

Quadratic electroabsorption studies of third-order susceptibility mechanisms in dye-doped polymers

We apply quadratic electroabsorption to thin-film solid solutions of squarylium dye molecules in poly(methyl methacrylate) polymer to study the electronic and reorientational mechanisms in the third-order nonlinear-optical susceptibility. To interpret the data, we have generalized quadratic electro-optic response theory to include both electronic and hindered-molecular-reorientation mechanisms. Such a theory predicts that the ratio of two independent third-order susceptibility tensor components, namely, χ3333(3)/χ1133(3), characterizes the relative contribution of each mechanism. We have designed and built an experiment that measures this ratio as a function of temperature and wavelength. Results show that at room temperature and near the first electronic transition wavelength the response is dominated by the electronic mechanism and that the reorientational contribution dominates when the sample is heated above its glass-transition temperature. Furthermore, far off resonance, the sign of the imaginary part of the third-order susceptibility is positive and is consistent with the two-level model. We also discuss the possible role of electrostriction in our analysis. Quadratic electroabsorption is thus shown to be a versatile tool for measuring the imaginary part of the third-order nonlinear-optical susceptibility, which yields information about the role of the interaction of the polymer host with the dopant molecule

Magic Angle Lifetime Measurements in Evanescent Wave Fluorometry

For fluorescence lifetime measurements in isotropic solutions, it is well known that detection using an emission polarization of 54.7°, the so-called magic angle, eliminates contribution of molecular reorientation to the fluorescence decay function. For macroscopically oriented samples and evanescent wave excitation, the magic angle may not necessarily be the 54.7°. With the use of a typical optical scheme of evanescent wave excitation and detection along the surface normal, a mathematical expression is derived for the magic angle condition. The magic excitation angle is shown to be a function of the angle of incidence of the excitation beam and the refractive indices of the two media involved in total internal reflection. The magic angle relations are valid regardless of the orientational distribution of molecules along the surface normal. The magic angle relations apply to cases where the in-plane orientational distribution is isotropic.

Importance of orientational rearrangement during vitrification of hydrocarbons: dependence on molecular shape

On the basis of Monte Carlo calculations of 2,2-dimethylpropane (neopentane), n-pentane, and 2,2-dimethylbutane (neohexane) at several temperatures, thermodynamic properties and radial distribution functions as well as dimerization and bonding energy distribution functions are reported for both liquid and glassy states. Changes in the radial distribution functions on cooling depend on whether the groups are accessible (peripheral) or inaccessible. Peaks in the radial distribution functions corresponding to peripheral groups do not shift to lower distances on cooling and at times display a large increase in the intensity of the first peak. The peaks due to inaccessible groups, on the other hand, shift to lower distances on cooling. The magnitude of the reorientational contribution in determining the resulting structure of the glass is estimated for the different hydrocarbon molecules investigated. The reorientational contribution is highest for neopentane (26%) followed by isopentane (24%), neohexane (22%), and n-pentane (0%). It appears that molecular geometry has an important role in determining the magnitude of the reorientational contribution to the structure of the glass.

Synthesis and characterization of carbazole polymers exhibiting large nonlinear absorption and refractive index

Poly-(N-[1-butane 4-sulfonate benzyl triethylammonium],-3,6-carbozolyl), PNBSC, the corresponding propane sulfonate derivative, PNPSC, and poly-(N-[2-pyrimido] 3,6 carbazolyl), PCZP were synthetized as candidates for non-linear optical materials. The radical cations of these polymers were distinguished by absorbances at 850–900nm and 1300 nm which were assigned to intramer and intermer charge transfer in the carbazole radical cations. Solutions of PNBCZ and PNPCZ in 0.1 and 1 M aluminum chloride - nitrobenzene or nitromethane or 2:1 aluminum chloride-n-butyl pyridinium chloride solvents, exhibited a complex with a sharp absorption peak at 700 nm and a very broad absorption centered at 1100 nm. Since these absorption characteristics are very similar to those appearing for oxidation with one electron oxidants such as NO + in protonic acids, a similar complexation must be taking place in both cases. The nonlinear optical properties of the aluminum chloride based solutions of PNBCZ and PNPCZ and nitrobenzene solutions of the radical cation of PCZP were measured with a degenerate four wave mixing experiment at 568 nm at the minimum of the linear absorption spectrum using 10 nsec wide pulses from a dye laser. The monomer unit based, nonlinear susceptibilities γ xxxx (all beams copolarized) and γ xyyx (probe and reflected phase conjugate polarized perpendicular to the pump beams) were 3.5→2.8×10 −29 esu and 9.0→5.7 × 10 −30 esu, respectively. We thus estimate that the electronic and reorientational contributions to the x 3 is on the order of 10 −7 − 10 −8 esu. Significant nonlinear absorption was also observed at 1060 nm using 5 nsec pulses. The data could be matched to a simple linear intensity dependence of the absorption coefficient only at lower intensities, whereupon values between 10 −6→10 −5 cm/W/M (monomer unit based) were obtained. We suspect that such large nonlinearities are obtained through the introduction of delocalized charge transfer states into the polymer chain.

Reorientational diffusion of tungsten hexacarbonyl in solution

The oxygen-17 spin-lattice relaxation time in tungsten hexacarbonyl has been measured in the solvent, chloroform-d, and used to determine the reorientational diffusion coefficient, D, as a function of temperature. The experimental activation energy for D was significantly below that predicted by the Gierer-Wirtz ‘microviscosity’ model, but in almost perfect agreement with the value calculated from the more recent Hynes-Kapral-Weinberg (HKW) theory of reorientation in liquids. The HKW model was used to predict the diffusion coefficient and, hence, the reorientational contribution (Δ Reor) to the bandwidths of the E g (Raman) and T lu(IR) vibrational modes of W(CO) 6 in various solvents. These, together with the reported widths (Δ Obs), permitted determination of the contribution of vibrational relaxation (Δ Vib). Both Δ Reor and Δ Vib are sensitively dependent both upon mode and solvent. In general, the latter contribution dominates the observed bandwidth, particularly in the solvents, chloroform and benzene.

Separation of vibrational dephasing and reorientational contributions to the infrared and Raman lineshapes in a simulation of MeCN

The complete lineshapes of the predicted Raman and infrared spectra of the four A 1 modes in simulations of CH 3CN are compared with those obtained by taking the product of vibrational and reorientational time correlation functions. The usual assumption that these two contributions are separable is found to be valid to a good approximation, although small deviations are found in one mode.

Sulfate ion time correlation functions in cubic lithium sulfate

Raman spectra of the sulfate ion intramolecular modes were recorded in the high-temperature plastic phase of lithium sulfate from 621 to 796°C. The spectral data from the ν 1 and ν 2 modes were Fourier transformed to give time correlation functions which describe vibrational dephasing and reorientational motion of the sulfate ion. Analysis of the ν 1 band yields a pure vibrational dephasing correlation function, while analysis of ν 2 yields a complex correlation function containing both reorientational and vibrational dephasing contributions.