Molecular Reorientation Processes Research Articles

Nuclear magnetic relaxation and diffusion study of the ionic liquids 1-ethyl- and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide confined in porous glass.

The molecular dynamics of the room-temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Bmim Tf2N) confined in porous glass is studied by nuclear magnetic resonance (NMR) relaxometry and diffusometry and is compared with the bulk dynamics over a wide temperature range. The molecular reorientation processes for anions and cations alike are found to be significantly affected by the presence of the glass interface at high temperatures. In this respect, the ionic liquid behaves similarly to polar liquids where proton NMR relaxation is governed by reorientations mediated by translational displacements (RMTDs). This process becomes less significant towards lower temperatures when the characteristic translational correlation times of the ions approach a timescale comparable with those of the RMTD process, and the relaxation dispersions in bulk and in confinement become similar below a temperature corresponding to about 1.2Tg , a value where the onset of dynamic heterogeneity has been observed before. The self-diffusion coefficient, on the other hand, is found to be strongly reduced than the bulk within the accessible temperature range of 248K and above and is significantly slower than expected from the tortuosity effect, suggesting that ion-surface interactions affect the macroscopic properties.

Intensity Fluctuations in Single-Molecule Surface-Enhanced Raman Scattering.

Around 20 years ago, the first reports of single-molecule surface-enhanced Raman scattering (SM-SERS) caused a revolution in nanotechnology. Several researchers were quick to recognize the importance of a technique that can provide molecular vibrational fingerprinting at the SM level. Since then, a large amount of work has been devoted to the development of nanostructures capable of SM-SERS detection. A great effort has also been geared toward elucidating the different mechanisms that contribute to the effect. The understanding of the concept of plasmonic SERS hotspots, the role of chemical effects, and the dynamics of atomic and cluster rearrangements in nanometric domains has significantly advanced, driven by new computational and experimental methods used to study SM-SERS. In particular, SERS intensity fluctuations (SIFs) are now recognized as a hallmark of SM-SERS. Interpretation of SM-SERS data must take into consideration temporal and spatial variations as a natural consequence of the extreme localization inherent to surface plasmon resonances. Further analysis of variations in spectral signature, due to either molecular reorientation or photo (or thermal) processes, pointed to a new area that combines the power of SERS fingerprinting at the SM level to modern concepts of catalysis, such as hot-electrons-driven chemistry. This large body of work on the fundamental characteristics of the SM-SERS effect paved the way to the interpretation of other related phenomena, such as tip-enhanced Raman scattering (TERS). Despite all the fundamental progress, there are still very few examples of real applications of SM-SERS. In recent years, our research group has been studying SIFs, focused on different ways to use SM-SERS. The obvious application of SM-SERS is in analytical chemistry, particularly for quantification at ultralow concentrations (below 1 nM). However, quantification using SM-SERS faces a fundamental sampling problem: the analytes (adsorbed in very small amounts, i.e., low surface coverage) must find rare SERS hotspots (areas with intense electric field localization that yields SERS). This limitation leads to strong temporal and spatial variations in SERS intensities, which translates into very large error bars in an experimental calibration curve. We tackled this problem by introducing the concept of "digital SERS". This approach provided a roadmap for SERS quantification at ultralow concentrations and a potential pathway for a better understanding of the "reproducibility problem" associated with SERS. In this Account, we discuss not only the analytical applications but also other implementations of SM-SERS demonstrated by our group. These include the use of SM-SERS as a tool to probe colloidal aggregation, to evaluate the efficiency of SERS substrates, and to characterize the energy of localized resonances. SERS involves a series of random processes: hotspots are rare; surfaces/clusters constantly reconstruct; and molecules diffuse, adsorb, and desorb. All these pathways contribute to strong fluctuations in SERS intensities. Our work indicates that a statistical view of the effect can lead to interesting insights and the potential to fulfill the promise of this SM technique for real-world applications.

Accurate and precise lattice parameters of H2O and D2O ice Ih between 1.6 and 270 K from high-resolution time-of-flight neutron powder diffraction data.

Accurate and precise lattice parameters for D2O and H2O varieties of hexagonal ice (ice Ih, space group P63/mmc) have been obtained in the range 1.6 to 270 K. Precision of the lattice parameters (∼0.0002% in a and 0.0004% in c for D2O, 0.0008% in a and 0.0015% in c for H2O) is ensured by use of the time-of-flight method on one of the longest primary neutron flight-path instruments in the world, the High-Resolution Powder Diffractometer at the ISIS neutron source. These data provide a more precise description of the negative thermal expansion of the material at low temperatures than the previous synchrotron `gold standard' [Röttger et al. (1994). Acta Cryst. B50, 644-648], including the region below 10 K where the lattice parameters saturate. The volume expansivity of both isotopologues turns negative below 59-60 K, in excellent agreement with a recent dilatometry study. The axial expansivities are highly isotropic (differing by < 1% in D2O ice Ih). Furthermore, the c/a ratio of different D2O ice samples exhibit a statistically significant dispersion of ∼0.015% below 150 K that appears to depend on the thermal history of the sample, which disappears on warming above 150 K. Similarly, H2O ice exhibits a `kink' in the c/a ratio at ∼115 K. The most plausible explanation is a freezing-in of the molecular reorientation process on cooling and subsequent relaxation on warming.

Discrimination of photo-induced isomerization and molecular reorientation processes in azobenzene derivative doped in a polymer

In order to investigate the mechanism of photo-induced processes of azo molecules doped in polymer, temporal evolution of the absorption spectrum and transmittance for two polarization components were measured under and after optical pumping. A simple model was applied to decompose the contributions from angular hole burning (AHB) due to photo-isomerization and following molecular reorientation (MRO). The result for disperse red 1 (DR1) in poly-methyl methacrylate (PMMA) indicated the dominance of AHB in the order of seconds while slow accumulation of the MRO effect became significant during a longer time range. Several extraordinary behaviors observed in preceding studies will be explained in context given in this study. Dependence on dye concentration showed that MRO was more significant in highly loaded samples. Further, quantum yield of photo-isomerization process in this system was estimated to be 8%.

Correlations of low-field NMR and variable-field NMR parameters with osteoarthritis in human articular cartilage under load.

NMR experiments carried out at magnetic fields below 1T provide new relaxation parameters unavailable with conventional clinical scanners. Contrast of T1 generally becomes larger towards low fields, as slow molecular reorientation processes dominate relaxation at the corresponding Larmor frequencies. This advantage has to be considered in the context of lower sensitivity and frequently reduced spatial resolution. The layered structure of cartilage is one example where a particularly strong variation of T1 across the tissue occurs, being affected by degenerative diseases such as osteoarthritis (OA). Furthermore, the presence of 1 H-14 N cross-relaxation, leading to so-called quadrupolar dips in the 1 H relaxation time dispersion, provide insight into the concentration and mobility of proteoglycans and collagen in cartilage, both being affected by OA. In this study, low-field imaging and variable-field NMR relaxometry were combined for the first time for tissue samples, employing unidirectional load to probe the mechanical properties. 20 human knee cartilage samples were placed in a compression cell, and studied by determining relaxation profiles without and with applied pressure (0.6MPa) at 50μm in-plane resolution, and comparing with volume-averaged T1 dispersion. Samples were subsequently stored in formalin, prepared for histology and graded according to the Mankin score system. Quadrupolar dips and thickness change under load showed the strongest correlation with Mankin grade. Average T1 and change of maximum T1 under load, as well as its position, correlate with thickness and thickness change. Furthermore, T1 (ω) above 25 mT was found to correlate with thickness change. While volume-averaged T1 is not a suitable indicator for OA, its change due to mechanical load and its extreme values are suggested as biomarkers available in low-field MRI systems. The shape of the dispersion T1 (ω) represents a promising access to understanding and quantifying molecular dynamics in tissue, pointing toward future in vivo tissue studies.

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.

Synthesis and Surface Properties of Fluorinated Hyperbranched Polyglycidol

Using 2,2-difluoro-1,3-dimethylimidazolidine as fluorinating agent, the fluorinated modification of hyperbranched polyglycidol is described. The aggregation behavior and the surface tension of resultant product in aqueous solutions are studied. The adsorption kinetics parameters for dynamic surface tension are determined from diffusion model and diffusion-reorientation model. In the investigated concentration range, the Z-average sizes of the fluorinated polyglycidol almost do not change with increasing concentration, and remain in the range of ∼500 nm to ∼700 nm. The time dependence of the change in surface tension was obviously exhibited, and the surface tension can be reduced to ∼34 mN/m after 1000s at 0.012 mol/L. According to the analysis of adsorption kinetics, the adsorption process of the fluorinated product is diffusion controlled for short time and dilute concentration, and involves molecular reorientation process for long time and high bulk concentration.

Real‐Time Microscopy of Reorientation Driven Nucleation and Growth in Pentacene Thin Films on Silicon Dioxide

AbstractThe role of molecular reorientation processes in the self‐assembly of anisotropic molecules, such as pentacene (Pn) is studied utilizing a unique capability of low‐energy electron microscopy (LEEM) for the real‐time investigation of the film growth. In Pn film on SiO2, a layer‐by‐layer growth is observed, albeit different from the expected Volmer–Weber growth mode typical for the systems with lower adhesion (weak interfacial interaction). The observed growth mechanism is also different than conventional concept of layer‐by‐layer, or Frank van der Merwe growth. In the Pn/SiO2 system the nucleation density decreases in each consecutive layer, at least up to four monolayers. This growth mechanism is hereafter named inverse Stranski‐Krastanov growth. Furthermore, in this growth system the second layer islands nucleate preferentially at the domain boundaries formed by the interconnections of the bottom (first layer) domains. The top layer overgrows bottom layer with its own, initial in‐plane crystal orientation, regardless of the in‐plane orientations in underlying Pn domains. The dark‐field LEEM imaging allows us to distinguish between Pn domains having different azimuthal direction of molecular tilt. LEEM intensity versus start voltage (LEEM I–V) curves taken in the vicinity of mirror potential from the first and second layer Pn islands show that the surface potential of the second layer is higher by about 0.05 eV than that of the first layer, while the surface potentials for the epitaxial and non‐epitaxial parts of the second layer island are identical.

Strongly nonlinear dynamics of ferroelectric liquid crystals

Molecular reorientation processes induced in thin ferroelectric liquid crystal systems by strong alternating external electric fields are studied both by solving numerically the equation of reorientation motion of molecules and by measuring the electro-optic response of thin samples. It is shown that the occurrence of a wide band in nonlinear response spectra above the Goldstone-mode frequency is a consequence of complex partially uncorrelated molecular reorientations enforced within smectic layers by sufficiently high fields of high enough frequencies. Such nonlinear reorientational motions of molecules are argued to have a character of weakly chaotic long-lasting transients, related to almost periodic modulations of the amplitude of rotational oscillations performed by molecules with the field frequency. These modulations have been numerically proved to proceed with lower frequencies than the field frequency and with space-dependent depths of temporal changes. The occurrence of the modulations has experimentally been confirmed by registering distinct contributions to electro-optic response spectra at frequencies less than the running frequency of the applied electric field.

Optical phase conjugation in azo-dye doped chiral liquid crystal

We report on optical phase conjugation phenomenon observed in chiral nematic liquid crystal showing band gap type Bragg reflection. The phase conjugate to the signal beam is observable only in the small temperature interval when the Bragg condition is fulfilled and only for circularly polarized light. The optical phase conjugation signals were observed at low cw laser light intensities (&amp;lt;100 mW/cm2, λ = 532 nm). Estimated value of third order optical susceptibility χ(3) = 2.8 × 10−17 m2/V2 is attributed to enhancement due to photoisomerisation of azo-dye (disperse red 1) inducing molecular reorientation process of liquid crystal molecules.

Photo-orientation of axial molecules

We examine the photo-orientation of molecules in a linearly polarized field and the ensuing optical anisotropy of a sample. We propose a theoretical model that considers both photoinduced reorientation and rotational diffusion, for the case of linear or axial molecules not interacting among them, as in dilute solutions in viscous media. We perform numerical simulations to highlight the dependence on the parameters of the molecular reorientation processes, on the intensity of the exciting light, and on the use of cross polarized pulses. As a realistic example we simulate the photo-orientation of azobenzene in ethylene glycol.

Optically induced static magnetization due to molecular reorientational processes in dc electric field

The molecular theory of optically induced static magnetization is extended to systems acted on with a dc electric field. The two effects, the invers Faraday effect and invers magnetochiral birefringence, are analysed for their dependence on an electric field perpendicular or parallel to the propaga tion direction of the incident light beam. The mechanisms responsible for the effects are described in terms of linear and nonlinear multipole polarizability tensors as well as molecular reorientation functions defined by the generalized Langevin-Kielich functions. The effects at electric saturation are a source of information concerning the tensor components of electric dipole, magnetic dipole and electric quadrupole polarizability.

Study of vibrational relaxation and molecular reorientation in liquids: a Raman spectroscopic study

AbstractThe vibrational relaxation and molecular reorientational processes in liquids have been studied in some carbonyl‐containing molecules using the acetonitrile (ACN) solvent. The vibrational and reorientational processes have been studied in correspondence with correlation times. The screening effect due to dielectric has been studied and the Onsager reaction field model has been tested. The study shows that repulsive types of intermolecular forces play an important role in complex systems. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Criticality of bistability phenomenon in deformed helix ferroelectric liquid crystal

The dynamics of bistability in deformed helix ferroelectric liquid crystal have been investigated by electro-optical method. The threshold value of the voltage required to switch the molecules of deformed helix ferroelectric liquid crystals (DHFLC) material has been studied by applying triangular wave pulse and texture observation under polarizing microscope. Two peaks have been observed in low and high frequency regimes on the application of triangular wave pulse to the sample. It has been proposed that the first peak is due to helix unwinding-winding process and the second is due to molecular reorientation process. The occurrence of double peak has been predicted to have close correlation with the observance of memory effect in DHFLC material, which is critically dependent on frequency and applied voltage, on the application of time delayed square wave pulse to the sample cell. The conditions for dynamic and static memory effects in DHFLC material have also been discussed in detail.

Photoinduced cooperative molecular reorientation on azobenzene side-chain-type copolymers

In order to improve the value of photoinduced birefringence and the long-term stability of azobenzene copolymer, two series of azobenzene side-chain-type copolymers consisting of two repeating units, one bearing cyanoazobenzene (or cyanoazopyridine) and the other bearing a long π-conjugated chromophore (bisazobenzene), have been synthesized and compared with respect to their photoinduced birefringence characteristics. Consequently, a fairly large value (observed highest value was 0.244) and superior stability (initial relaxation value was less than 0.2%) of photoinduced birefringence have been achieved. Furthermore, unusual behavior of the photoinduced birefringence depending on the copolymerization ratio of the two chromophores has been found, i.e., a sharp peak of the photoinduced birefringence has been observed at a copolymerization ratio of around 50:50 mol%. This phenomenon could be attributed to mutual promotion of the photoinduced molecular reorientation process, that is cooperative molecular motion, taking place at a specific stoichiometric composition ratio of the two chromophores.

Vibrational and rotational relaxation of hexafluorobenzene studied by Raman scattering in the supercritical domain

The vibrational and rotational relaxation of hexafluorobenzene (C 6F 6) has been studied by Raman spectroscopy in the supercritical domain. The polarized I VV( ν) and depolarized I HV( ν) profiles associated with the vibrational ν 1( A 1g) ‘breathing’ mode of the molecule have been analysed for the fluid under isothermal ( T ⁎ = T/ T C ∼ 1.23 and 1.02) and isobaric conditions ( P ⁎ = P/ P C ∼ 3.34 and P ⁎ ∼ 4.89) in a wide density domain (0.02 ≤ ρ ⁎ = ρ/ ρ C ≤ 3). The band shapes were found to be close to Lorentzian profiles in the density and temperature range investigated. The band centre position shifts towards higher frequency values with density. These shifts have been discussed in terms of the competition between repulsive and attractive forces using the Buckingham-Ben-Amotz approach. It was found that repulsive interactions are dominant and that the attractive interactions are mostly effective at low densities. As for the vibrational relaxation, the analysis has been done in the framework of the dephasing mechanism. In the domain of liquid-like densities the experimental vibrational times follow the Isolated Binary Collision (IBC) model (Fischer-Laubereau), derived for a repulsive interaction. In contrast, along the isotherms in the gas-like density values, such a model does not apply. As a consequence, it appears that attractive interactions play a significant role, in the gas-like density domain. As the rotational dynamics of the main molecular axis (tumbling motion) obeys the fast modulation condition, the molecular reorientation process is governed by a diffusional process. Along the near critical isotherm and in the high compressible domain, local density inhomogeneities are found to have strong influence on both measured shifts and rotational correlation times. A local density excess δ ρ ⁎ = ρ loc ⁎− ρ ⁎ has been featured from these two observables. Furthermore, a very large activation volume was found (800 cm 3 mol − 1 ) which is due to the clustering phenomenon taking place in the hyper-compressible domain.

FTIR‐Spectroscopy on Segmental Reorientation of a Nematic Elastomer under External Mechanical Fields

AbstractSummary:Fourier transform infrared (FTIR) spectroscopy with polarized light is employed to study the segmental orientation and the order of a nematic liquid crystalline elastomers (NLCEs) with a monodomain structure in response to an external mechanical field. The reorientation and the order parameter of the different molecular moieties are analyzed in detail, revealing information about angular excursion in the rearrangement of the mesogens, the spacer molecules, and the main‐chain. In case of an elongation of the NLCE films perpendicular to the initial mesogen orientation, no reorientation or change of order is observed for an elongation ratio less than 1.3. At higher strain, a molecular reorientation process is induced on all molecular segments and the order of alignment is decreased. When the NLCE‐films are stretched parallel to the mesogens, no molecular reorientation takes place and the order parameters show no significant change.Angular reorientation of different molecular moieties at mechanical strain perpendicular to the initial mesogen orientation.magnified imageAngular reorientation of different molecular moieties at mechanical strain perpendicular to the initial mesogen orientation.

Dynamics in supercooled polyalcohols: Primary and secondary relaxation

We have studied details of the molecular dynamics in a series of pure polyalcohols by means of dielectric spectroscopy and H2 nuclear magnetic resonance (NMR). From glycerol to threitol, xylitol and sorbitol a systematic change in the dynamics of the primary and secondary relaxation is found. With increasing molecular weight and fragility an increase in the width of the α-peak is observed. Details of the molecular reorientation process responsible for the α-relaxation were exploited by two-dimensional NMR experiments. It is found that in the same sequence of polyalcohols the appearance of the secondary relaxation changes gradually from a wing type scenario to a pronounced β-peak. From NMR experiments using selectively deuterated samples the molecular origin of the secondary relaxation could be elucidated in more detail.

Molecular reorientation in intense femtosecond laser fields

A femtosecond double-pulse experiment shows that molecular reorientation during multiple ionization is a very fast process for light molecules such as ${\mathrm{O}}_{2}$ or ${\mathrm{N}}_{2}\mathrm{O}$ using 60-fs laser pulses and variable time delays between \ifmmode\pm\else\textpm\fi{}1 ps in the ${10}^{15}$ $\mathrm{W}/{\mathrm{cm}}^{2}$ intensity range. A unified two-dimensional model of molecular multiple ionization allows one to get the multicharged fragment trajectories and consequently discuss the speed of alignment of a molecule. The numerical results confirm that the molecular reorientation process is faster when the laser intensity increases, in good agreement with the experimental data.

ELLIPSOMETRY OF MOLECULAR REORIENTATION IN A FREELY SUSPENDED FERROELECTRIC LIQUID CRYSTAL FILM

Optical measurements of a field induced molecular reorientation process in a freely suspended ferroelectric liquid crystal film (FSFLC) have been studied. In order to measure a direction and a magnitude of a birefringence of the film at the reorientation, a photoelastic modulator has been used. A complicated response that has broad spectrum of the response speed is observed in the thick FSFLC film that has helical structure.