Molecular Relaxation Dynamics Research Articles

Molecular relaxation dynamics in organic monolayer junctions

The complex admittance of various molecular junctions based on long alkyl chains (C18) has been measured and resolved into different components. The dipolar contribution of the response function has been analyzed in terms of the generalized Langevin equation. This formalism allows us to extract from experimental data the spectral density of polarization noise and to characterize the fluctuation dynamics of the molecules around their equilibrium positions. This spectral density is of $1/f$ type, characteristic of fractional Gaussian noise. It is suggested that the structural disorder of the junctions is at the origin of these polarization fluctuations. Possible relation between the $1/f$ tunnel current and polarization noises is also discussed.

Dynamic Crossover of α′ Relaxation in Poly(vinyl acetate) above Glass Transition via Mechanical Spectroscopy

The molecular relaxation dynamics of poly(vinyl acetate) (PVAc) has been studied by mechanical spectroscopy above the glass transition temperature (T(g)) within a frequency range from 1 mHz to 100 Hz. The temperature-dependent mechanical spectra reveal the existence of two relaxation modes: alpha, ascribed to the glass transition, and alpha', which may be related to the softening dispersion, composed of the sub-Rouse modes and the Rouse modes. The alpha' mode has a weaker temperature dependence than the alpha mode. Furthermore, the alpha' mode from the frequency-domain spectra exhibits a similar dynamic crossover at temperature T(B) approximately 387 K as the alpha mode through the temperature dependence of relaxation time, relaxation strength, and shape parameter. However, the crossover of the alpha' mode occurs at a time of about 0.08 s, longer than 10(-6)-10(-7) s for the alpha mode by dielectric spectroscopy. According to the coupling model, the crossover is suggested to be caused by the strong increase of intermolecular cooperativity below T(B).

Composition-dependent damping and relaxation dynamics in miscible polymer blends above glass transition temperature by anelastic spectroscopy

Anelastic spectroscopy is used to study the composition dependence of the damping and molecular relaxation dynamics in miscible poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) blends above the glass transition temperature. The ultrahigh damping peak of the relaxation type is shown to be associated with the liquid-liquid transition of PMMA. A higher PEO concentration leads to a higher damping performance and a lower transition temperature. The decreasing activation energy with increasing PEO concentration indicates a drastic increase in molecular mobility. Moreover, the relaxation time reveals a transition from the Vogel–Fulcher–Tamman behavior to the Arrhenius behavior due to the intermolecular guest-host interactions.

Effects of blending with fluorescing molecules on the dynamics of the β, α, and α′ relaxations observed in poly(methyl methacrylate)

Perturbations in typical dielectric or calorimetric features of the molecular relaxation dynamics in amorphous poly(methyl methacrylate) and two blends with a neutral/apolar brominated perylene dye were analyzed to explore guest-host nanoscale interactions and their role on lasing properties. Aside from a gradual increase of the relaxation strength Δεβ (plasticization), the local β relaxation is nearly independent of the presence of dye. By contrast, segmental dynamics are moderately hampered: both the dielectric (Tα) and calorimetric (Tg) estimates of the glass transition temperature rise with increasing dye content, probably as a result of modifications in the free volume (antiplasticization). Analogous shifts are observed in the peak temperature of the space-charge relaxation (Tρ) and the α′ (liquid-liquid) transition (TLL), with Tρ∕Tα=1.04±0.01 and TLL∕Tα=1.10±0.01.

Influence of molecular relaxation dynamics on quartz-enhanced photoacoustic detection of CO2 at λ =2 μm

Carbon dioxide (CO2) trace gas detection based on quartz enhanced photoacoustic spectroscopy (QEPAS) using a distributed feedback diode laser operating at λ=2 μm is performed, with a primary purpose of studying vibrational relaxation processes in the CO2-N2-H2O system. A simple model is developed and used to explain the experimentally observed dependence of amplitude and phase of the photoacoustic signal on pressure and gas humidity. A (1σ) sensitivity of 110 parts-per-million (with a 1 s lock-in time constant) was obtained for CO2 concentrations measured in humid gas samples.

Molecular Relaxation Dynamics of Self-Assembled Monolayers

Dielectric relaxation spectroscopy is used to quantify molecular motion in alkylsilane SAMs coated on porous glass over a broad temperature range, -30 to -150 degrees C. Systematic measurements using SAMs with variable coating densities allow us to determine the effect of monolayer disorder on molecular mobility in thin molecular films. A relaxation process with an activation energy of approximately 25 kJ/mol is found to dominate dynamics of SAM-chain segments near the substrate. By introducing polar CN groups at the ends of the chain, we show that the relaxation process in the monolayer canopy can be isolated and studied. This approach can be generalized to other substituent polar groups to probe localized relaxation dynamics in surface-grafted monolayer films.

Interplay of surface and confinement effects on the molecular relaxation dynamics of nanoconfined poly(methyl methacrylate) chains.

The thermally stimulated current (TSC) signatures of the primary (alpha) transition and its precursor, the Johary-Goldstein (beta) relaxation, are used to probe effects of nanoconfinement on the dielectric relaxation dynamics of poly(methyl methacrylate) (PMMA) radically polymerised in situ 50 angstroms mean pore size silica-gel. Nanoconfinement leads to a broadened and low-temperature-shifted beta band (peaking at Tbeta, with deltaTbeta = T(conf.)beta - T(bulk)beta = -15 degrees C for a heating rate of 5 deg/min), signifying the occurrence of faster relaxing moieties compared to the bulk-like PMMA film. Furthermore, both TSCs and differential scanning calorimetry (DSC) estimate a rise of the glass transition temperature for the confined phase ([Formula: see text]= +13 degrees C) and an increased width for the corresponding transition signals, relative to the signals in the bulk. Simple free-volume and entropy models seem inadequate to provide a collective description of the above perturbations. The observation of a spatial heterogeneity regarding the relaxation dynamics is discussed in terms of the presence of a motional gradient, with less mobile segments near the interface and more mobile segments in the core, and the interplay of adsorption ( e.g., strong physical interactions that slow down molecular mobilities) and confinement effects ( e.g., lower entanglements concentration and local density fluctuations that provide regions of increased free space). The results suggest that in the case of high-molecular-weight polymers confined in small-pore systems, adsorption effects have considerable bearing on the glass transition phenomenon whereas confinement primarily influences side-chains' rotational mobilities. The confinement effect is expected to dominate over adsorption for PMMA phases occluded in higher pore sizes and silanised walls.

Local Dynamics and Molecular Origin of Polymer Network−Water Interactions as Studied by Broadband Dielectric Relaxation Spectroscopy, FTIR, and Molecular Simulations

An investigation was carried out of the molecular interactions and local relaxation dynamics in glassy polymer networks exposed to moisture. Stoichiometric and off-stoichiometric mixtures of the diglycidyl ether of bisphenol A (DGEBA) and diethylene triamine (DETA) were prepared, cured, and investigated. The physical and the chemical nature of the interactions between the network and the absorbed moisture were studied by broadband dielectric relaxation spectroscopy (DRS), Fourier transform near-infrared (FT-NIR) spectroscopy, and molecular simulations. Dry networks are characterized by two Arrhenius-like local relaxations: the β process, associated with hydroxyl groups, and the γ process, associated with primary and secondary amine, and glycidyl ether groups. Absorbed water interacts with the network and affects the dynamics of β and γ processes. FT-NIR spectra reveal the presence of three forms of water molecules, differing in the number of hydrogen atoms (0, 1, or 2) that participate in hydrogen bonds....

Substituent Effects in Molecular Electronic Relaxation Dynamics via Time-Resolved Photoelectron Spectroscopy: ππ* States in Benzenes

We study the applicability of femtosecond time-resolved photoelectron spectroscopy to the study of substituent effects in molecular electronic relaxation dynamics using a series of monosubstituted benzenes as model compounds. Three basic types of electronic substituents were used: CC (styrene), CO (benzaldehyde), and C⋮C (phenylacetylene). In addition, the effects of the rigidity and vibrational density of states of the substituent were investigated via both methyl (α-methylstyrene, acetophenone) and alkyl ring (indene) substitution. Femtosecond excitation to the second ππ* state leads, upon time-delayed ionization, to two distinct photoelectron bands having different decay constants. Variation of the ionization laser frequency had no effect on the photoelectron band shapes or lifetimes, indicating that autoionization from super-excited states played no discernible role. From assignment of the energy-resolved photoelectron spectra, a fast decaying component was attributed to electronic relaxation of the ...

Symmetry of the all-optical orientation dynamics of an octupolar azo-dye salt

Optical patterning of the molecular order in dye doped polymers offers interesting and quite rich prospects. The generalization of the principles of holography to the recording of an interference pattern between mutually coherent fields of different frequencies, such as fundamental and second harmonic has been demonstrated to enable a full 3D polar control of the molecules. Together with exhibiting fairly large second order nonlinearities, donor–acceptor substituted azobenzene type materials were demonstrated to be very efficiently ordered materials under all-optical poling conditions. Photoinduced cis/ trans isomerization results indeed in a free rotation of the molecule leading to an efficient molecular redistribution following the molecules selective polar excitation. In order to investigate the possible influence of the poling process on the resulting symmetry of the optically induced macroscopic susceptibility, a tetrahedral organotin azobenzene derivative was synthetized. All-optical orientation dynamics and symmetry of the second-order susceptibility χ (2) induced in PMMA rods doped with such a T d space-group molecule are reported and discussed. The molecular relaxation dynamics is shown to have a determining influence on the symmetry resulting from the all-optical poling process.

Experimental determination of the cooperative length scale of a glass-forming liquid near the glass transition temperature

Photon correlation spectroscopy and dielectric relaxation are used to examine the molecular reorientation relaxation dynamics of a fragile glass-forming liquid Aroclor (a mixture of polychlorinated biphenyls), modified by the addition of low- and high-molecular-weight polyisoprene and polybutadiene as a function of temperature and polymer solute concentration. Concentration fluctuation contributes a temperature-dependent broadening of the relaxation spectrum of Aroclor. The rate of change of the Aroclor relaxation spectrum with temperature is more pronounced when the polymers added are of low molecular weight and exhibits a steplike decrease in the neighborhood of some characteristic molecular weight. The radius of gyration of the polymer with this characteristic molecular weight is about 15 \AA{}, which determines the cooperative length scale L(T) of Aroclor to be approximately 30 \AA{} near and above the glass transition temperature.

Microsecond to Subnanosecond Molecular Relaxation Dynamics of the Interaction of Ca2+ with Some Carbohydrates in Aqueous Solution

Ultrasonic absorption spectra in the frequency range 1−500 MHz are reported for solutions of methyl-β-d-arabinopyranoside and of 1,6-anhydro-β-d-glucopyranoside, both with and without added Ca2+ and Ba2+ ions. These carbohydrates have been chosen because neither monosaccharide shows any relaxation process, over the same frequency range, in salt-free aqueous solutions. Spectra for both carbohydrates in aqueous solution with added Ca(ClO4)2, CaCl2, or Ba(ClO4)2 can be represented by an asymptotic high-frequency contribution and a Debye relaxation term centered between 50 and 200 MHz, corresponding to a process with a relaxation time of approximately 3−4 ns. Spectra for solutions of Ca(ClO4)2 + 1,6-anhydro-β-d-glucopyranoside also exhibit a second relaxation process at lower frequencies, with a relaxation time of approximately 30 ns. These results clearly indicate cation−carbohydrate complex formation. The observed relaxation terms are interpreted as a modified Eigen−Winkler-type multistep process, associated with formation of monodentate cation−carbohydrate complexes and, where possible, subsequent rearrangement to tridentate complexes.

The effects of pressure and temperature on molecular dynamics during linear-chain polymerization by dielectric measurements

Molecular relaxation of the various states formed during the course of growth of a linear-chain polymer by addition reactions of the amine group of cyclohexylamine with the epoxy groups of a diepoxide has been studied at isobaric conditions of hydrostatic pressures up to 206 bar and at several temperatures from 300 K to 314 K, by using dielectric measurements for a fixed frequency of 1 kHz, which are adequate for obtaining information on the relaxation time during the course of polymerization. The reaction occurs faster at high pressures and the curves of permittivity and loss against the polymerization time, which resemble the corresponding spectra, bodily shift to a shorter time. At 206 bar and 307.5 K, the ε″ plot shows contributions from a second, high frequency relaxation. The increase in relaxation time, when the reaction occurred at high pressures, has been discussed in terms of both (a) an increase due to the increase in the rate of chemical reaction and (b) the usual physical effect of pressure on molecular kinetics, and an attempt made to resolve the two effects. The effect of hydrostatic pressure predominates the molecular relaxation dynamics through an increase in the polymerization rate. Formalisms relating the chemical and physical processes are given, but not examined by experiments. The decrease in the configurational entropy is formulated in terms of the polymerization rate and pressure. The increase of the static permittivity of the mixture on compression is marginal. It decreases more rapidly with the progress of polymerization at high pressures. Two issues on obtaining information on molecular dynamics of a time-variant system from single-frequency measurements, raised by others since our earliest studies, have been elaborated and analytically clarified. By using simulated dielectric data it has been shown that the dc conductivity and interfacial polarization alter the shape of the dielectric permittivity and loss plots to make misleadingly alternative parameter fits possible.

Dynamics and photoexcitation in MX and MXX′ chain solids

Nonlinear adiabatic dynamics associated with nonlinear excitations in MX and MXX′ chain materials are numerically studied within a discrete, 3/4-filled, two-band, tight-binding extended Peierls-Hubbard model. Both Hartree-Fock (HF) adiabatic molecular relaxation and molecular dynamics techniques are employed to investigate the time evolution of solitons, polarons, bipolarons in charge-density-wave (CDW), bond-order-wave (BOW) and spin-density-wave (SDW) ground state materials. Photoexcitations are performed between ( i) continuum levels, ( ii) localized levels, ( iii) continuum and localized levels. The subsequent formation and time evolution of excitons, defect pairs, breathers, and, for the MXX' solids, charge separation are studied in terms of energy levels and distortion patterns. These results are compared with the dynamics of previously studied 1/2-filled, one-band, SSH-type electron-phonon models, used for studying polyenes and polyynes, and experimental data.

Molecular relaxation dynamics of lithium perchlorate in acyclic polyether solvents

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMolecular relaxation dynamics of lithium perchlorate in acyclic polyether solventsPaul Firman, Meizhen Xu, Edward M. Eyring, and Sergio PetrucciCite this: J. Phys. Chem. 1993, 97, 14, 3606–3613Publication Date (Print):April 1, 1993Publication History Published online1 May 2002Published inissue 1 April 1993https://doi.org/10.1021/j100116a027RIGHTS & PERMISSIONSArticle Views131Altmetric-Citations26LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (819 KB) Supporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Femtosecond dynamics, photoexcitation and ESR spectra of MX chain solids

Non-linear adiabatic dynamics and ESR spectra associated with non-linear excitations in MX chain (M = Pt; X = Cl, Br, I) materials are numerically studied within a discrete, 3/4-filled, two-band, tight-binding extended Peierls-Hubbard model. Both Hartree-Fock (HF) adiabatic molecular relaxation and molecular dynamics techniques are employed to investigate the time evolution of solitons, polarons, bipolarons in charge-density-wave (CDW) ground state materials. The time evolution of excitons, defect pairs and/or breathers is studied subsequent to photoexcitation. The ESR spectra associated with the resulting spin carrying non-linear excitations such as neutral solitons, triplet excitons as well as electron and hole polarons are then calculated within the above model. The superhyperfine structure in the ESR spectra is attributed to a small spin density present on the halogen sublattice.

Activation energy barriers in the molecular relaxation dynamics of Li+ interacting with acyclic and cyclic polyethers in acetonitrile

AbstractUltrasonic relaxation spectra of LiClO4 plus the open chain polyether triglyme (TG) at CLiClO4 ≃ 0.8 M, molar ratio R = [TG]/[LiClO4] = 1, and at temperatures of 15, 25, 35, and 45°C in dry acetonitrile are reported. Corresponding spectra for LiClO4 plus the macrocycle 12‐crown‐4 (12C4) at R = [12C4]/[LiClO4] = 1, at 10, 15, 20, 25, 30, and 40°C, and CLiClO4 = 0.32 M in acetonitrile are also reported.Both relaxation envelopes, for the two systems, have been fitted by two Debye relaxation processes and interpreted by the Eigen‐Winkler reaction scheme: where “PE” symbolizes the polyether, Li+ … PE is a solvent separated species and LiPE+ and (LiPE)+ are two complex species. The temperature dependence of the relaxation spectra permits the calculation of the activation parameters for the forward and reverse processes for the system LiClO4 + TG and for one of the two processes for the LiClO4 + 12C4 system.Comparisons between the two sets of data lead to the conclusion that the much slower second relaxation process, in the case of the macrocycle, is attributable to a combination of an enthalpic and an entropic effect on the energy barrier for the present systems.

Deconvolution problems in a pulsed photoacoustic measurement of methane vibrational lifetimes at 77 K, 0.01 Torr

A conventional gas phase photoacoustic measurement has proved to be less than straightforward due to difficulties associated with deconvolution of the microphone (Bruel and Kjær) response from the vibrational quenching lifetimes measured for methane in various binary buffer gases. The application of the lifetime data is to the aeronomy of Jupiter and preliminary results have been reported [Halthore et al., “Implications of the methane relaxation time measurements for the atmospheres of outer planets and Titan,” 21st Ann. Mtg. Div. Planetary Sci., AAS, 1989]. However, when extensive, accurate recordings were obtained this summer, a systematic error was noted that was attributed to convolution of the microphone's impulse response into the molecular relaxation dynamics. For this reason, it is felt that cw chopped laser excitation would be superior to the Nd‐YAG pulsed laser excitation presently used to pump the asymmetric stretch mode of methane at 3.3 μm. [Work supported by the Institute for Space and Terrestrial Physics, Space Astrophysics Laboratory, Toronto.]

Structure and molecular relaxation dynamics of lithium perchlorate + macrocycle solutions in 1,3-dioxolane at 25.degree.C

Structure and molecular relaxation dynamics of lithium perchlorate + macrocycle solutions in 1,3-dioxolane at 25.degree.C

Temperature dependence of ionic association and of molecular relaxation dynamics of lithium hexafluoroarsenate in 2-methyltetrahydrofuran

Abstract : Electrical conductance data in the temperature range 25 to -35 C, analyzed by the Fuoss-Kraus triple-ions theory of conductance, reveal the presence of both ion pairs and triple ions. Evidence of the presence of the latter ones persists (with the formation constant K sub T somewhat increased) even by allowing for measured change of solvent permittivity with concentration of electrolyte. This dispenses any remaining doubt on the existence of triple ions. Formation constants K sub P and K sub T at various temperature for ion-pairs and triple ions respectively have been determined. These figures are compared with values calculated by the Bjerrum theory of ion-pairs formation and by the Delsignore-Bjerrum theory for formation of triple ions, respectively. Ultrasonic relaxation spectra, in the concentration range 0.1M to 0.4M of LiAsF6 and is the frequency range 0.5 to 400 MHz at 15 and 5 C in 2MeTHF, are reported. Infrared spectra in the Nu sub 3 band envelope of the AsF6ion, in the concentration range 0.05 to 0.8M, reveal the presence of three separated bands.