Molecular Reorientational Dynamics Research Articles

Synthetically tunable polymers, free volume element size distributions, and dielectric breakdown field strengths

Four polyetherimide (PEI) polymers were synthesized with different end groups, while leaving the molecular weights, glass transition temperature, thermal properties, and dielectric constant essentially unchanged. The Restricted Orientation Anisotropy Method (ROAM), an ultrafast infrared laser technique, was used to measure the films’ free volume elements (FVEs) radius probability distribution (RPD) curves. This technique exploits the observation that a vibrational probe’s molecular reorientation dynamics inside a polymer film’s FVEs are sterically restricted by the surfaces of the FVEs. The measured RPD curves displayed significant changes in shape and center positions for the PEI polymers with different end cap. The results demonstrate that structural changes to the end groups of a polymer can significantly modify its microscopic morphology, which is consistent with other experiments that observed changes in macroscopic observables. The thin film samples’ breakdown fields (EBD) were measured, and a correlation between polymer films having a higher probability of large FVEs and lower EBDs was observed. This work demonstrates that the nanoscopic chain packing structure that controls the nature of the FVEs and breakdown field properties of PEI are synthetically tunable, without the need to change the main chain chemical structure. The results provide some experimental evidence for the theoretical model of electron-acceleration inside FVEs as the microscopic origin of polymer film dielectric breakdown. The results also provide a framework for the study of other polymer dielectrics, and suggest that design of high breakdown field polymer films may involve minimizing FVE sizes, particularly the large FVE tails of the size distribution.

Paddle-wheel mechanism in doped succinonitrile-glutaronitrile plastic electrolyte: a joint magnetic resonance, dielectric, and viscosimetry study of Li ion translational and molecular reorientational dynamics.

Mixtures of 60% SN (succinonitrile) and 40% GN (glutaronitrile) doped with LiTFSI or LiPF6 at different concentrations are investigated using dielectric spectroscopy. Room temperature conductivities up to 10-3 S cm-1 are measured along with an overall conductivity enhancement of almost five decades compared to pure SN. Additionally, the dynamics of the methylene (CD2) groups of SN and that of the Li+ ions within the mixture are studied in a wide temperature range using 2H and 7Li NMR relaxometry, respectively. Static-field-gradient proton NMR combined with viscosity measurements probe the molecular diffusion. GN addition and Li doping both enhance the electrical conductivity significantly, while leaving the reorientational motion within the matrix essentially unchanged. The times scales and thus the effective energy barriers characterizing the Li ion motion as well as the molecular reorientations are very similar in the liquid and in the plastic phases, findings that argue in favor of the presence of a paddle-wheel mechanism.

Optical Supercontinuum Generation via Rotational and Vibrational Molecular Cavity Optomechanics

AbstractThe solid high quality (Q) factor whispering gallery mode microcavities are demonstrated as an excellent experimental platform for enhanced light–matter interactions, and are applied in the studies of nonlinear optics, quantum optics, and sensing. However, the interaction between the microcavity and liquid anisotropic molecule, which is unique for abundant vibrational and rotational degrees of freedom, is rarely studied. Here, the gap is bridged by filling an optofluidic microbubble resonator with carbon disulfide, and the related anisotropic molecular cavity optomechanics effects are investigated. Due to the enhanced coupling between the optical mode and the rotation/vibration of molecules, stimulated Rayleigh–Kerr/Raman–Kerr scattering effects are observed. Molecular cavity optomechanics provides a broadband gain for generating Stokes photons in high‐Q optical modes, and eventually allows quasi‐supercontinuum generation with a bandwidth of 436 nm under a low continuous laser pump power of about 10 mW. This work opens a new avenue for strong light–matter interactions by exploring rotational and vibrational molecular cavity optomechanics, which provides a promising platform for nonlinear optics, cavity‐enhanced molecular reorientation dynamics, and novel optical sources.

Water in Mesoporous Confinement: Glass-To-Liquid Transition or Freezing of Molecular Reorientation Dynamics?

The first mechanical relaxation measurements (f = 400 Hz) of water confined in micro-porous silica were performed more than 40 years ago. The authors reported a so called “capillary transition” (here denoted as P3) of water in the core of the pores and a second one at a lower temperature, which they called the “adsorbate transition” (P1 in present work) related to water near the surface of the pores. The capillary transition was identified with the freezing of water in the centre of the pores. However, even 40 years later, the origin of the adsorbate transition is not yet clear. One study relates it to the liquid-to-glass transition of the supercooled water in the pores, and another study to the freezing of the proton reorientations at the lattice defects. The present work shows the data from extensive dynamic mechanical analysis (DMA) measurements (f = 0.1 Hz–70 Hz) of water confined in mesoporous silica (d = 2.5, 5 and 10 nm), which are in favour of a liquid-to-glass scenario.

Gauging Molecular Orientation through Time Domain Simulations of Surface-Enhanced Raman Scattering.

Molecular reorientation dynamics modulate the physical and chemical properties of molecules at interfaces. This is particularly the case for organic molecules interacting with metallic surfaces-structural motifs of current interest to ultrasensitive chemical/biological detection/imaging. In this context, surface-enhanced Raman scattering (SERS) can be used to gauge the orientation of molecules in the immediate vicinity of plasmonic metals. Herein, we analyze SERS spectra of two aromatic thiols (4-mercaptobenzonitrile and thiophenol) chemisorbed onto silver substrates using ab initio molecular dynamics-based Raman spectral simulations that account for the optical response of molecules in the bulk and oriented molecules at the surface. Through a comparison with prior works aimed at gauging the orientation of our two model systems on metallic substrates, we describe the advantages and discuss the limitations of our described models and approach to gauging the orientation of molecules on metals through time domain simulations of their SERS signatures.

Enhancement of the local asymmetry in the hydrogen bond network of liquid water by an ultrafast electric field pulse

Condensed phase electron decomposition analysis based on density functional theory has recently revealed an asymmetry in the hydrogen-bond network in liquid water, in the sense that a significant population of water molecules are simultaneously donating and accepting one strong hydrogen-bond and another substantially weaker one. Here we investigate this asymmetry, as well as broader structural and energetic features of water’s hydrogen-bond network, following the application of an intense electric field square pulse that invokes the ultrafast reorientation of water molecules. We find that the necessary field-strength required to invoke an ultrafast alignment in a picosecond time window is on the order of 108 Vm−1. The resulting orientational anisotropy imposes an experimentally measurable signature on the structure and dynamics of the hydrogen-bond network, including its asymmetry, which is strongly enhanced. The dependence of the molecular reorientation dynamics on the field-strength can be understood by relating the magnitude of the water dipole–field interaction to the rotational kinetic energy, as well as the hydrogen-bond energy.

Plastic-crystalline solid-state electrolytes: Ionic conductivity and orientational dynamics in nitrile mixtures.

Many plastic crystals, molecular solids with long-range, center-of-mass crystalline order but dynamic disorder of the molecular orientations, are known to exhibit exceptionally high ionic conductivity. This makes them promising candidates for applications as solid-state electrolytes, e.g., in batteries. Interestingly, it was found that the mixing of two different plastic-crystalline materials can considerably enhance the ionic dc conductivity, an important benchmark quantity for electrochemical applications. An example is the admixture of different nitriles to succinonitrile, the latter being one of the most prominent plastic-crystalline ionic conductors. However, until now, only few such mixtures were studied. In the present work, we investigate succinonitrile mixed with malononitrile, adiponitrile, and pimelonitrile to which 1 mol. % of Li ions was added. Using differential scanning calorimetry and dielectric spectroscopy, we examine the phase behavior and the dipolar and ionic dynamics of these systems. We especially address the mixing-induced enhancement of the ionic conductivity and the coupling of the translational ionic mobility to the molecular reorientational dynamics, probably arising via a "revolving-door" mechanism.

The activation energy for water reorientation differs between IR pump-probe and NMR measurements.

Molecular reorientation dynamics in liquid water are typically probed using either infrared (IR) pump-probe anisotropy experiments or the NMR spin-echo technique. While it is widely appreciated that the two yield different reorientation times based on the nature of the measurements, little attention has been paid to the implications for the corresponding activation energies. Here, the activation energies associated with reorientation of the OH bond vector in liquid water are calculated to high accuracy directly from simulations at a single temperature using a recently developed method [Z. A. Piskulich et al., J. Chem. Phys. 147, 134103 (2017)]. The results indicate that the reorientation times obtained from IR anisotropy and NMR measurements have different activation energies that, with improved accuracy, should be experimentally distinguishable. The origins of the differences in the two activation energies are examined in detail, including by a decomposition into the contributions to the activation energies due to the kinetic energy and the intermolecular interactions.

Variation of ionic conductivity in a plastic-crystalline mixture.

Ionically conducting plastic crystals (PCs) are possible candidates for solid-state electrolytes in energy-storage devices. Interestingly, the admixture of larger molecules to the most prominent molecular PC electrolyte, succinonitrile, was shown to drastically enhance its ionic conductivity. Therefore, binary mixtures seem to be a promising way to tune the conductivity of such solid-state electrolytes. However, to elucidate the general mechanisms of ionic charge transport in plastic crystals and the influence of mixing, a much broader database is needed. In the present work, we investigate mixtures of two well-known plastic-crystalline systems, cyclohexanol and cyclooctanol, to which 1 mol. % of Li ions were added. Applying differential scanning calorimetry and dielectric spectroscopy, we present a thorough investigation of the phase behavior and the ionic and dipolar dynamics of this system. All mixtures reveal plastic-crystalline phases with corresponding orientational glass-transitions. Moreover, their conductivity seems to be dominated by the "revolving-door" mechanism, implying a close coupling between the ionic translational and the molecular reorientational dynamics of the surrounding plastic-crystalline matrix. In contrast to succinonitrile-based mixtures, there is no strong variation of this coupling with the mixing ratio.

C60 Solvate with (1,1,2)-Trichloroethane: Dynamic Statistical Disorder and Mixed Conformation

We present a full characterization of the orientationally disordered cocrystal of C60 with (1,1,2)-trichloroethane (C2H3Cl3), by means of X-ray diffraction, Raman spectroscopy, and broadband dielectric spectroscopy. Our results include the determination of molecular conformations, lattice structure, positional disorder, and molecular reorientational dynamics down to the microsecond time scale. We find that, while in the disordered solid phase of pure C2H3Cl3 the molecules exist only in the gauche conformation, both gauche and transoid conformers are present in the solvate, where they occupy the largest interstitial cavities between the fullerene species. The two C2H3Cl3 conformers exhibit separate, independent relaxations, exhibiting simply activated behavior in the measured temperature range. The relaxation of the transoid conformer, which has twice the dipole moment of the gauche isomer, is significantly slower than that of the latter, due to the high polarizability of C60 resulting in an electrostatic ...

Molecular Reorientation Dynamics Govern the Glass Transitions of the Amorphous Ices.

The glass transitions of low-density amorphous ice (LDA) and high-density amorphous ice (HDA) are the topic of controversial discussions. Understanding their exact nature may be the key to explaining the anomalies of liquid water but has also got implications in the general context of polyamorphism, the occurrence of multiple amorphous forms of the same material. We first show that the glass transition of hydrogen-disordered ice VI is associated with the kinetic unfreezing of molecular reorientation dynamics by measuring the calorimetric responses of the corresponding H2O, H2(18)O, and D2O materials in combination with X-ray diffraction. Well-relaxed LDA and HDA show identical isotopic-response patterns in calorimetry as ice VI, and we conclude that the glass transitions of the amorphous ices are also governed by molecular reorientation processes. This "reorientation scenario" seems to resolve the previously conflicting viewpoints and is consistent with the fragile-to-strong transition from water to the amorphous ices.

ピコ秒時間分解赤外分光とMDシミュレーションの融合による溶媒再配向ダイナミクスの研究

ピコ秒時間分解赤外分光とMDシミュレーションの融合による溶媒再配向ダイナミクスの研究

Gd-DTPA-Dopamine-Bisphytanyl Amphiphile: Synthesis, Characterisation and Relaxation Parameters of the Nanoassemblies and Their Potential as MRI Contrast Agents.

Here, a new amphiphilic magnetic resonance imaging (MRI) contrast agent, a Gd(III)-chelated diethylenetriaminepentaacetic acid conjugated to two branched alkyl chains via a dopamine spacer, Gd-DTPA-dopamine-bisphytanyl (Gd-DTPA-Dop-Phy), which is readily capable of self-assembling into liposomal nanoassemblies upon dispersion in an aqueous solution, is reported. In vitro relaxivities of the dispersions were found to be much higher than Magnevist, a commercially available contrast agent, at 0.47 T but comparable at 9.40 T. Analysis of variable temperature (17)O NMR transverse relaxation measurements revealed the water exchange of the nanoassemblies to be faster than that previously reported for paramagnetic liposomes. Molecular reorientation dynamics were probed by (1)H NMRD profiles using a classical inner and outer sphere relaxation model and a Lipari-Szabo "model-free" approach. High payloads of Gd(III) ions in the liposomal nanoassemblies made solely from the Gd-DTPA-Dop-Phy amphiphiles, in combination with slow molecular reorientation and fast water exchange makes this novel amphiphile a suitable candidate to be investigated as an advanced MRI contrast agent.

A multi-property fluorescent probe for the investigation of polymer dynamics near the glass transition

AbstractIn addition to the commonly observed single molecule fluorescence intensity fluctuations due to molecular reorientation dynamics, a perylene bisimide-calixarene compound (1) shows additional on-off fluctuations due to its ability to undergo intramolecular excited state electron transfer (PET). This quenching process is turned on rather sharply when a film of poly(vinylacetate) containing 1 is heated above its glass transition temperature (T g), which indicates that the electron transfer process depends on the availability of sufficient free volume. Spatial heterogeneities cause different individual molecules to reach the electron transfer regime at different temperatures, but these heterogeneities also fluctuate in time: in the matrix above T g molecules that are mostly nonfluorescent due to PET can become fluorescent again on timescales of seconds to minutes.The two different mechanisms for intensity fluctuation, rotation and PET, thus far only observed in compound 1, make it a unique probe for the dynamics of supercooled liquids.

Comparison between Cycloalkyl- and n-Alkyl-Substituted Imidazolium-Based Ionic Liquids in Physicochemical Properties and Reorientational Dynamics

We synthesized three series of imidazolium-based ionic liquids (ILs) containing cycloalkyl groups such as cyclopentyl, cyclohexyl, or cycloheptyl groups incorporating bis(trifluoromethanesulfonyl)amide anions and characterized them with respect to physicochemical properties and molecular reorientational dynamics. A comparison of the physicochemical properties revealed that cycloalkyl-substituted imidazolium ILs have higher densities, viscosities, and glass transition temperatures than the respective n-alkyl-substituted imidazolium ILs. Among three series, the cyclopentyl-substituted IL exhibits exceptionally lower viscosity. Observation of correlation times by (13)C NMR spectroscopy revealed that a remarkably lower viscosity for the cyclopentyl-substituted IL and a considerably higher viscosity for the cyclohexyl- and cycloheptyl-substituted ones are closely related to the respective reorientational motion of the cations. The cause of these distinctions is suggested to be attributed to the difference of activation energy for the conformational interconversion of their substituents.

Temperature Effect on the Reorientational Correlation Time of Water in Formamide– and N,N-Dimethylformamide–Water Mixtures

Abstract NMR spin–lattice relaxation times (T1) for nucleus D of D2O in FA-d3–D2O and DMF–D2O mixtures were measured up to 80 mol % of amide at 5, 25, and 45 °C under atmospheric pressure. To discuss the dynamic behavior of water molecules in the mixtures in detail, the reorientational correlation times (τc) of D2O molecules were estimated from the quadrupole coupling equation. In FA-d3–D2O mixtures, τc increased monotonously with increasing FA content at 25 and 45 °C, while we can recognize a shallow minimum at 5 °C in the D2O-rich region. In DMF–D2O mixtures, τc showed a remarkable maximum in the composition dependence at each temperature. In order to investigate the effect of the microscopic environments on the reorientational dynamics of water molecules, we calculated τc for individual environments through the MD simulations. From these results, we discussed the effects of hydrophilic and hydrophobic interactions on the molecular reorientational dynamics.

Consequences of Transient Heating on the Motional Dynamics of Cholesterol-Containing Phospholipid Vesicles

We have investigated the effects of transient heating on the molecular reorientation dynamics of perylene incorporated into the nonpolar acyl chain region of 100 nm diameter unilamellar vesicles comprised of DMPC and cholesterol as a function of system temperature. We observe distinctly different reorientation behavior for perylene, depending upon whether it is excited to its S(1) or S(2) electronic state. This difference is due to the dissipation of excess excitation energy through nonradiative relaxation from the S(2) to the S(1) manifold. The thermal energy deposited into the lipid bath following S(2) ← S(0) excitation results in faster rotational diffusion than that observed for excitation to the S(1) state. These findings are consistent with an average temperature increase of ca. 7 K for system temperatures ≤ 278 K and ca. 13 K for system temperatures >278 K. We believe the variation in average temperature change is a result of a lipid phase transition occurring in the vicinity of 278 K.

Effects of Energy Dissipation on Motional Dynamics in Unilamellar Vesicles

We have investigated the molecular reorientation dynamics of perylene incorporated into the nonpolar acyl chain region of 100 nm diameter phosphocholine unilamellar vesicles. When perylene is excited to the S(1) electronic manifold, it exhibits a different fluorescence anisotropy decay than when it is excited to its S(2) electronic manifold. In addition to differences in the polarization of the two transitions, there is a difference in the amount of excitation energy dissipated nonradiatively, because for both excitation conditions radiative emission is from the S(1) state. The data reveal a faster rotational diffusion constant for excitation to the S(2) manifold, and we interpret this finding in the context of local heating of the chromophore immediate environment by the nonradiative dissipation of energy associated with rapid relaxation from the S(2) to S(1) manifolds. Our results are consistent with transient heating on the order of 4 K, with the actual temperature increase depending subtly on whether the phospholipid acyl chains are in the gel phase or the fluid phase.

On the calculation of rotational anisotropy decay, as measured by ultrafast polarization-resolved vibrational pump-probe experiments

Polarization-resolved vibrational pump-probe experiments are useful for measuring the dynamics of molecular reorientation. The rotational anisotropy observable is usually modeled by the second-Legendre-polynomial time-correlation function of the appropriate molecule-fixed unit vector. On the other hand, more elaborate calculations that include non-Condon effects, excited-state absorption, and spectral diffusion, can be performed using the infrastructure of the nonlinear response formalism. In this paper we present "exact" (within the impulsive limit) results from the nonlinear response formalism, and also a series of approximations that ultimately recover the traditional result mentioned above. To ascertain the importance of these effects not included in the traditional approach, we consider the specific case of dilute HOD in H(2)O. We find that for the frequency-integrated anisotropy decay, it is important to include non-Condon effects. For the frequency-resolved anisotropy decay, non-Condon effects, excited-state absorption, and spectral diffusion are all important. We compare our results with recent experiments.

Influence of confinement on molecular and director reorientational dynamics of liquid crystal

We present the qualitative results of investigations of influence of confinement of liquid crystal in random porous medium on relaxation due to molecular reorientations investigated by dielectric spectroscopy and fluctuations of director orientations (collective relaxation) investigated by photon correlation spectroscopy. The relaxation times of the process due to the molecular rotations in deeply supercooled state of confined liquid crystal were slower than at the temperatures corresponding to nematic phase by a factor of 10 6. This slowing down was accompanied by anomalous broadening of the dielectric spectra. We attribute this qualitative change in molecular relaxation to coupling of molecules to the pore walls and among themselves via molecule-wall interactions. In the nematic phase of bulk liquid crystals, the relaxation process due to director orientation fluctuations (collective mode) was single exponential. The dynamics of this process was also drastically modified by random confinement. The slow relaxation process, which does not exist in the bulk liquid crystal, and a broad spectrum of relaxation times, appear for confined liquid crystal. These results on the influence of confinement on relaxation properties of liquid crystal were complimented by differential scanning calorimetry experiments.