Molecular Orientation Distribution Research Articles

Visualizing 3D Anisotropic Molecular Orientation in Polarization Holographic Optical Elements via Dielectric Tensor Tomography

AbstractThe ability to unveil the spatial distribution of refractive index (RI) within volumetric holographic optical elements (HOEs) is critical for quantitating their diffractive behaviors. Angle‐resolved far‐field measurements of diffractive intensity have been prevalent toward this end. However, this century‐old approach is unable to directly visualize the spatial distribution of RI at mesoscopic scale. More significantly, visualization of molecular orientation within photoaddressable polymers (PAPs), which serve as standard recording media for polarization HOEs (pHOEs), remains uncharted territory. The recent advent of dielectric tensor tomography (DTT) has paved the way for full characterization of 3D anisotropic dielectric tensors, encompassing principal RIs and their optic axes. This study embarks on direct visualization of the 3D spatial distribution of anisotropic molecular orientations within holographically recorded PAPs. Illuminating these PAPs with polarized light at varying angles, the diffracted vector fields essential for reconstructing the dielectric tensor tomogram are captured. After diagonalizing the dielectric tensors, periodic rotations of the anisotropic molecule orientations can be visualized in the PAPs, which have never been achieved so far. The homogeneity of grating patterns produced under diverse manufacturing conditions is also examined and juxtaposed.

Investigating the impact of pore structure and surface chemistry on CO2 adsorption in graphitic slit-pores using GCMC simulation

Carbon capture, storage, and utilization from post-combustion processes using adsorption phenomena has emerged as a promising solution to the greenhouse gas emission crisis. The capacity of solid porous adsorbents dictates the performance of such carbon capture processes. Developments of porous adsorbents with favourable structural and chemical characteristics of the pore have been a major research area. Computational tools, notably the Grand Canonical Monte Carlo (GCMC) simulation, are widely employed to characterize the adsorption process in crystalline adsorbents like metal-organic frameworks and zeolites. Activated carbons are becoming promising alternatives to conventionally-used zeolite adsorbents for CO2 adsorption, because of their abundant microporosity and cost performance. However, the amorphous nature of the activated carbon materials poses a challenge in accurately modelling their pore characteristics and their adsorption process. Therefore, this study systematically explores the effect of pore size distribution and type of functional groups on the adsorption of CO2 on activated carbons using a simplified slit-pore graphite structure representing the activated carbon adsorbent. Four different pore sizes (7 Å, 8.9 Å, 18.5 Å, and 27.9 Å) and three oxygen-containing functional groups (Carbonyl, Hydroxyl, and Carboxyl) were selected to model the graphite structures. Results from the GCMC simulation reveal a significant rise in the CO2 adsorption capacity (from 4 mmol/g to 21 mmol/g) as the pore size was increased from 7 Å to 27.9 Å. Likewise, the functional groups enhance the low-pressure adsorption process by reducing the onset pressure of the pore filling by a factor of 100, especially in ultra-micropores. Likewise, we demonstrate the increase in the isosteric heat of adsorption due to the reduction in the pore size and the presence of functional groups. Additionally, the study illustrates the adsorbed phase behaviour of CO2 concerning the pore characteristics, a facet often overlooked in the existing literature. The adsorbed phase local density and molecular orientation distribution are analysed to understand the variation in the adsorption uptake and isosteric heat of adsorption properties. The study further identifies the adsorbed phase monolayer to multilayer transition and the ‘T′-shaped orientation of the adsorbed CO2 molecules as the key contributors to the high isosteric heat of adsorption in 8.9 Å pore size. It is envisaged that this study will navigate the precision adsorbent development for an efficient carbon capture process.

Electric field as a crystallization switch of heterogeneous ice formation

Using molecular dynamics simulations, we investigated the effect of external electric field on ice formation with the present of a substrate surface. It turns out that the electric field can affect the ice formation on substrate surface by altering the dipole orientation of interfacial water molecules (IWs): a crossover from inhibiting to promoting ice formation with the increase of electric field strength. According to the influence of the electric field on ice formation, the electric field strength range of 0.0 V nm−1–7.0 V nm−1 can be divided into three regions. In the region I and region III, there are both ice formation on the substrate surface. While, the behavior of IWs in the region I and region III are distinguished, including the arrangements of oxygen atoms and the dipole orientation distribution. In region II, ice formation does not occur in the system within 5 × 200 ns simulations. The IWs show a disorder structure, preventing the ice formation process on substrate. The interfacial water molecular orientation distribution and two-dimensional free energy landscape reveals that the electric field can alter the dipole orientation of the interfacial water and lead a free energy barrier, making the ice formation process harder. Our result demonstrates the external electric field can regulate the behavior of IWs, and further affect the ice formation process. The external electric field act as a crystallization switch of ice formation on substrate, shedding light into the studies on the control of ice crystallization.

Vertical Distribution of Molecular Orientation and Its Implication on Charge Transport in Floating Films of Conjugated Polymers

Fabrication of uniform and oriented thin films of conjugated polymers (CPs) with controlled thickness is essential for the implementation of organic devices. Over the years, unidirectional floating film transfer method (UFTM) has emerged as a facile technique for the fabrication of oriented thin films but its distribution of orientation in vertical (thickness) direction has not been explored yet. To investigate this, thin films of nonregioregular poly(3‐hexylthiophene) are fabricated using UFTM with different concentrations of CPs to obtain thickness variation from 11 to 47 nm. Thin films of 4% and 9% polymer solution have similar dichroic ratio of 5.5 but have an appreciable difference in film thickness of 12 and 47 nm where thicker films are disordered as probed by polarized absorption spectra. Organic field‐effect transistors fabricated using thinner films (12 nm) exhibit a saturated charge carrier mobility of 2.3 × 10−3cm2 V−1 s−1, an order of magnitude higher than its thicker (47 nm) counterpart. This difference in device performance can be attributed to the fact that charge transport is principally governed by the top polymer surface at air–polymer interface, which is found to be more disordered owing to the vertical distribution of orientation.

Molecular simulation of adsorption and diffusion behavior of CO2 in pyrophyllite

Understanding the microscopic process of adsorption and diffusion of CO2 in clay minerals is important for geological storage of CO2. In this study, Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations are used to obtain the adsorption and diffusion behavior of CO2 in pyrophyllite slit pores. The influence of split pore size, temperature, pressure, and water content were analyzed in detail. The adsorption characteristics of CO2 in pyrophyllite were revealed by discussing the distribution of CO2 density profile, adsorption capacity, isosteric heat of adsorption, interaction energy, mean square displacement, self-diffusion coefficient, and molecular orientation distribution. It shows that the excess adsorption isotherm of CO2 in pyrophyllite slit pores demonstrate significant supercritical adsorption characteristics. Surface attractions originating from both pore walls overlap in the middle of pore, leading to an enhanced adsorption capacity of CO2 in pyrophyllite slit pores when pore aperture is lower than 3 nm. In pyrophyllite pores, the presence of water has an enhancing effect on CO2 adsorption when the water content is below 30 wt%. A maximum value of CO2 adsorption amount is reached at the water content of 10 wt%, while negative growth occurs at 35 wt%. At low water content, water molecules form a weak adsorption layer due to the weak attraction of pyrophyllite to water. As water content increases, the self-diffusion coefficient of the CO2 decreases, and the diffusion capacity decreases. The adsorption state of the CO2 molecule on the mineral surface is primarily a parallel configuration, while the water molecule has no clear orientation. Our calculated results can provide guidance for further study of clay-water-adsorbate interface reaction of hydrophobic minerals.

Physicochemical properties of high-content rubber modified bio-asphalt using molecular simulation

High-content rubber modified asphalt would let out lots of volatile organic compounds (VOCs), bio-oil was used to reduce the VOCs emission and mixing temperature of high-content rubber modified asphalt. The molecular model of high-content rubber modified bio-asphalt (HRMBA) was built to calculate the solubility parameter, free volume, molecular orientation, and radial distribution function. The effects of bio-oil on the physicochemical properties of HRMBA during VOCs emission process were studied deeply using molecular dynamic simulations and first principles. The differential scanning calorimeter (DSC), Fourier infrared spectrometer (FT-IR), and gel chromatography (GPC) were used to test the glass transition temperature, functional groups, and molecular aggregate and verify the simulated results. The results show that the optimum contents of waste tire in HRMBA is 35%, and bio-oil can promote the compatibility between rubber and asphalt. Free volume of HRMBA shows a strong correlation with the probe Connolly radius, and Connolly radius increases the molecular movements and dissolution of HRMBA. Moreover, the compatibility between bio-oil and rubber are good. Bio-oil can improve the dissolution of rubber into asphalt and inhibit the VOCs emissions of HRMBA. The compatibility between rubber and asphalt are damaged after VOCs emission. A new calculation method was used to evaluate the physicochemical properties of HRMBA, and experiment methods were also used to verify these.

Effects of primary, secondary and tertiary structures on functional properties of thermoplastic starch biopolymer blend films

To better understand the correlation between structure and properties in thermoplastic starch biopolymer blend films, the effects of amylose content, chain length distribution of amylopectin and molecular orientation of thermoplastic sweet potato starch (TSPS) and thermoplastic pea starch (TPES) on microstructure and functional properties of thermoplastic starch biopolymer blend films were studied. After thermoplastic extrusion, the amylose contents of TSPS and TPES decreased by 16.10 % and 13.13 %, respectively. The proportion of the chains with the degree of polymerization between 9 and 24 of amylopectin in TSPS and TPES increased from 67.61 % to 69.50 %, and from 69.51 % to 71.06 %, respectively. As a result, the degree of crystallinity and molecular orientation of TSPS and TPES films increased as compared to sweet potato starch and pea starch films. The thermoplastic starch biopolymer blend films possessed a more homogeneous and compacter network. The tensile strength and water resistance of thermoplastic starch biopolymer blend films increased significantly, whereas thickness and elongation at break of thermoplastic starch biopolymer blend films decreased significantly.

Modulating the Mixing Gibbs Free Energy to Enhance Solid–Liquid Phase Separation for High‐Performance Organic Solar Cells

AbstractOrganic solar cells (OSC) feature a hierarchical structure with the electron donor/acceptor layer sandwiched by anode and cathode, which raises the importance of controlling the molecular crystal orientation, domain size, and vertical distribution to facilitate the charge collection at electrodes. However, the similar conjugated backbone of donor/acceptor material and fast film‐formation kinetics have led to spinodal‐decomposition‐orientated phase separation that result in the film presenting an intimately mixed morphology and random molecular orientation. To solve the issue, the mixing Gibbs free energy‐triggered solid–liquid phase separation during the film formation process is enhanced by solidifying one component and solvating the other based on a liquid additive. Following the liquid evaporation process, a favorable vertical distribution is obtained. Meanwhile, the prolonged solvation process enlarges the domain size and assists the molecules to diffuse and orient properly, enabling better exciton/charge dynamics during the power conversion processes. As a result, the fabricated devices exhibit a fill factor over 80% and an efficiency of 18.72%, which is one of the top efficiencies for binary OSCs. Insights and a methodology is provided here to manipulate the organic donor/acceptor phase separation in terms of mixing Gibbs free energy.

Retrieval of the molecular orientation distribution from atom-pair angular distributions

Imaging laser-induced rotational dynamics is an important and active field due to its applications in capturing reactions in the molecular frame and in molecular imaging. Experimental measurement of the molecular orientation distribution, as a function of the Euler angles, has been demonstrated for special cases when the detectable signal is generated along the molecular symmetry axis. Here we developed the general theory that maps the probability density distribution of the molecular orientation to the atom-pair angular distributions for nonlinear molecules. With the theory, the molecular orientation distribution can be retrieved from the measured atom-pair angular distribution, which we demonstrate experimentally using ultrafast electron diffractive imaging of impulsively aligned trifluoro-iodomethane molecules. The retrieved molecular orientation distribution is in good agreement with direct numerical simulations of the time-dependent Schr\"odinger equation using the experimental conditions. Unlike the existing retrieval methods, the retrieval method does not require solving the Schr\"odinger equation, works for any alignment method, and is in principle applicable to asymmetric top molecules.

Effect of temperature on heavy hydrocarbon crystallization in natural gas

Understanding the mechanisms of nucleation and crystallization of heavy hydrocarbons in natural gas is significant for controlling alkane crystals, improving nucleation theory, and developing new processing technologies. However, the mechanism of heavy hydrocarbon crystallization at different temperatures is unknown. In this study, the effects of temperature on heavy hydrocarbon crystallization were investigated thermodynamically and kinetically using molecular dynamics simulations. Furthermore, the temperature dependence of the nucleation pathway was elucidated. The results suggest that a crystalline monolayer will first form on the supercooled heavy hydrocarbon droplet surface, followed by layer-by-layer bulk crystallization. As subcooling increases, the droplets and potential energy indicate four states: frustrated crystallization, rapid crystallization, surface crystallization, and vitrification. A temperature range of 130–110 K is ideal for heavy hydrocarbon droplet nucleation. Furthermore, bulk crystallization is determined by the competition between induced nucleation by the initial surface crystalline layer and homogeneous nucleation based on internal nucleation sites. And this depends on the degree of supercooling and determines the final morphology and molecular orientation distribution of crystals. The pathway of heavy hydrocarbon nucleation shifts from a non-classical two-step to a classical process as supercooling increases.

Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry.

Quantitative determination of the molecular orientation distribution function in samples of liquid crystals with a complex director geometry was performed using the numerical simulation of electron paramagnetic resonance (EPR) spectra of the spin probes in a liquid-crystalline medium. To achieve the quantitative agreement of experimental and simulated EPR spectra, the hierarchy of the orientation order was explicitly taken into account, namely, the local ordering of liquid crystal molecules by the mean-field potential of surrounding molecules, and the partial disordering of local directors within the sample. The samples under study are planar and twist LC cells with liquid crystal 8CB cooled from the nematic into the smectic A phase in the magnetic field. The presence of the magnetic field perpendicular to the cell director leads to distortion of the orientation of the liquid crystal in the cell. The spin probe technique was successfully employed for the reliable measurement of orientation distribution functions of the low nonorthorhombic symmetry. Orientation order parameters up to 12th rank were measured, including nonaxial and nonorthorhombic order parameters. It is shown that the presence of several contradicting aligning forces leads to the tilt of the preferential director toward the direction, which is a compromise between the orienting forces.

Characterization of the Interfacial Orientation and Molecular Conformation in a Glass-Forming Organic Semiconductor.

The ability to control structure in molecular glasses has enabled them to play a key role in modern technology; in particular, they are ubiquitous in organic light-emitting diodes. While the interplay between bulk structure and optoelectronic properties has been extensively investigated, few studies have examined molecular orientation near buried interfaces despite its critical role in emergent functionality. Direct, quantitative measurements of buried molecular orientation are inherently challenging, and many methods are insensitive to orientation in amorphous soft matter or lack the necessary spatial resolution. To overcome these challenges, we use polarized resonant soft X-ray reflectivity (p-RSoXR) to measure nanometer-resolved, molecular orientation depth profiles of vapor-deposited thin films of an organic semiconductor Tris(4-carbazoyl-9-ylphenyl)amine (TCTA). Our depth profiling approach characterizes the vertical distribution of molecular orientation and reveals that molecules near the inorganic substrate and free surface have a different, nearly isotropic orientation compared to those of the anisotropic bulk. Comparison of p-RSoXR results with near-edge X-ray absorption fine structure spectroscopy and optical spectroscopies reveals that TCTA molecules away from the interfaces are predominantly planar, which may contribute to their attractive charge transport qualities. Buried interfaces are further investigated in a TCTA bilayer (each layer deposited under separate conditions resulting in different orientations) in which we find a narrow interface between orientationally distinct layers extending across ≈1 nm. Coupling this result with molecular dynamics simulations provides additional insight into the formation of interfacial structure. This study characterizes the local molecular orientation at various types of buried interfaces in vapor-deposited glasses and provides a foundation for future studies to develop critical structure-function relationships.

Fringe fields are important when examining molecular orientation in a cold ammonia beam

External fields have been widely adopted to control and manipulate the properties of gas-phase molecular species. In particular, electric fields have been shown to focus, filter and decelerate beams of polar molecules. While there are several well-established approaches for controlling the velocity and quantum-state distribution of reactant molecules, very few of these methods have examined the orientation of molecules in the resulting beam. Here we show that a buffer gas cell and three-bend electrostatic guide (coupled to a time-of-flight set-up) can be configured such that 70% of ammonia molecules in the cold molecular beam are oriented to an external electric field at the point of detection. With a minor alteration to the set-up, an approximately statistical distribution of molecular orientation is seen. These observations are explained by simulations of the electric field in the vicinity of the mesh separating the quadrupole guide and the repeller plate. The combined experimental apparatus therefore offers control over three key properties of a molecular beam: the rotational state distribution, the beam velocity, and the molecular orientation. Exerting this level of control over the properties of a molecular beam opens up exciting prospects for our ability to understand what role each parameter plays in reaction studies.

Influence of Temperature on Molecular Adsorption and Transport at Liposome Surfaces Studied by Molecular Dynamics Simulations and Second Harmonic Generation Spectroscopy.

A fundamental understanding of the kinetics and thermodynamics of chemical interactions at the phospholipid bilayer interface is crucial for developing potential drug-delivery applications. Here we use molecular dynamics (MD) simulations and surface-sensitive second harmonic generation (SHG) spectroscopy to study the molecular adsorption and transport of a small organic cation, malachite green (MG), at the surface of 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (DOPG) liposomes in water at different temperatures. The temperature-dependent adsorption isotherms, obtained by SHG measurements, provide information on adsorbate concentration, free energy of adsorption, and associated changes in enthalpy and entropy, showing that the adsorption process is exothermic, resulting in increased overall entropy. Additionally, the molecular transport kinetics are found to be more rapid under higher temperatures. Corresponding MD simulations are used to calculate the free energy profiles of the adsorption and the molecular orientation distributions of MG at different temperatures, showing excellent agreement with the experimental results.

Molecular-Scale Chemical Imaging of the Orientation of an On-Surface Coordination Complex by Tip-Enhanced Raman Spectroscopy.

Metal-organic coordination structures at interfaces play an essential role in many biological and chemical systems. Understanding the molecular specificity, orientation, and spatial distribution of the coordination complexes at the nanometer scale is of great importance for effective molecular engineering of nanostructures and fabrication of functional devices with controllable properties. However, fundamental properties of such coordination systems are still rarely studied directly. In this work, we present a spectroscopic approach on the basis of tip-enhanced Raman spectroscopy (TERS) to investigate cobalt(II) tetraphenyl-porphyrine coordination species on the scale of a single molecule under ambient conditions. Coordination species anchored on gold surfaces modified with pyridine thiol self-assembled monolayers can be spectroscopically distinguished and mapped with ca. 2 nm resolution. In addition, in combination with density functional theory simulations, the adsorption configuration and molecular orientation of the coordination complexes are also revealed using TERS imaging.

Label-free multimodal nonlinear optical microscopy for biomedical applications

This paper addresses the application of multimodal nonlinear optical (MNLO) microscopy to clinical research within the context of label-free non-invasive molecular imaging. Here, a compact MNLO microscope based on a laser scanning microscope, a femtosecond laser, a time-correlated single-photon counting system, and a photonic crystal fiber are introduced for biomedical applications. By integrating two-photon fluorescence, two-photon fluorescence lifetime imaging, second-harmonic generation, and coherent anti-Stokes Raman scattering microscopy, the proposed scheme provides profound insights into the physicochemical properties related to 3D molecular orientation distribution, inter- and intra-molecular interactions, and disease progression in biological systems and organs. The high peak power and the low average intensity of near-infrared laser pulses allow for deep-penetration imaging without compromising sample vitality. Linking nonlinear optical phenomena with time/spectral/polarization-resolved imaging also makes it possible to obtain multidimensional information to address complex biomedical questions.

Disassociated molecular orientation distributions of a composite cellulose–lignin carbon fiber precursor: A study by rotor synchronized NMR spectroscopy and X-ray scattering

Cellulose–lignin composite carbon fibers have shown to be a potential environmentally benign alternative to the traditional polyacrylonitrile precursor. With the associated cost reduction, cellulose–lignin carbon fibers are an attractive light-weight material for, e.g. wind power and automobile manufacturing. The carbon fiber tenacity, tensile modulus and creep resistance is in part determined by the carbon content and the molecular orientation distribution of the precursor. This work disassociates the molecular orientation of different components in cellulose–lignin composite fibers using rotor-synchronized solid-state nuclear magnetic resonance spectroscopy and X-ray scattering. Our results show that lignin is completely disordered, in a mechanically stretched cellulose–lignin composite fiber, while the cellulose is ordered. In contrast, the native spruce wood raw material displays both oriented lignin and cellulose. The current processes for fabricating a cellulose–lignin composite fiber cannot regain the oriented lignin as observed from the native wood.

Orientation Distribution of Molecules: Characterization and Experimental Determination by Means of Magnetic Resonance

The article presents a comparative review of experimental methods used for characterization of orientational ordering of the molecules in a partially aligned media. The optical, X-ray and magnetic resonance techniques are considered. The potential and limitations of different approaches are discussed. The magnetic resonance is concluded to be the most informative technique for detailed characterization of the molecular orientation distribution.

Polarized Raman Spectroscopy Strategy for Molecular Orientation of Polymeric Fibers with Raman Tensors Deviating from the Molecular Frame

Polarized light is frequently used to identify molecular anisotropy in polymers, biological systems, and other materials. The influence of the Raman tensor by polarized light reveals not only the chemical structure but also information on the morphology of polymers. The orientation distribution of molecules in polymers has previously been determined for systems with the principal axis components of the Raman tensor parallel to the molecular frame. In many cases, the Raman tensor principal axis is not parallel to the molecular chain axis. Therefore, the orientation of the Raman tensor, relative to the molecular chain axis, is crucial if accurate information about the molecular orientation distribution is sought for. This work presents a strategy for separating the Raman tensor orientation angles from the molecular orientation angles for polymeric samples with fiber symmetry. Composite polymeric materials often experience signal overlap in the X-ray scattering wide-angle region, where the anisotropy is often resolved. While X-ray scattering investigates intermolecular distances, Raman spectroscopy resolves chemical information, and anisotropy, by the influence of Raman scattering. The quantitative principles presented here may aid in the evaluation of anisotropy in such composite materials.

Correction to Molecular Orientation Distribution of Regenerated Cellulose Fibers Investigated with Polarized Raman Spectroscopy

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to Molecular Orientation Distribution of Regenerated Cellulose Fibers Investigated with Polarized Raman SpectroscopyLeo SvenningssonLeo SvenningssonMore by Leo Svenningsson, Yuan-Chih LinYuan-Chih LinMore by Yuan-Chih Linhttp://orcid.org/0000-0002-0028-7481, Maths KarlssonMaths KarlssonMore by Maths Karlssonhttp://orcid.org/0000-0002-2914-6332, Anna MartinelliAnna MartinelliMore by Anna Martinellihttp://orcid.org/0000-0001-9885-5901, and Lars Nordstierna*Lars NordstiernaMore by Lars Nordstiernahttp://orcid.org/0000-0002-6580-0610Cite this: Macromolecules 2020, 53, 7, 2764Publication Date (Web):March 18, 2020Publication History Published online18 March 2020Published inissue 14 April 2020https://doi.org/10.1021/acs.macromol.0c00569Copyright © 2020 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views515Altmetric-Citations-LEARN 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 (514 KB) Get e-Alerts Get e-Alerts