Changes In Molecular Orientation Research Articles

Investigation of Molecular Chain Orientation Change of Polymer Crystals in Phase Transitions by Friction Anisotropy Measurement

Direct observation of the molecular orientation change in polymer crystals provides us visible information for understanding their structural phase-transition mechanisms. In this letter, we successfully identified the main-chain orientation of poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) crystals over all directions using friction anisotropy measured by lateral-modulation friction force microscopy (LM-FFM). This technique made possible our investigation of molecular orientation changes caused by a ferroelectric phase transition and also a fabrication process for artificial nanometer-scale structures. These results give us visual information that is directly connected to the transition mechanisms.

Monolayer to Multilayer Nanostructural Growth Transition in N-Type Oligothiophenes on Au(111) and Implications for Organic Field-Effect Transistor Performance

The evolution in growth morphology and molecular orientation of n-type semiconducting alpha,omega-diperfluorohexyl-quaterthiophene (DFH-4T) on Au(111) is investigated by scanning tunneling microscopy and scanning tunneling spectroscopy as the film thickness is increased from one monolayer to multilayers. Monolayer-thick DFH-4T films are amorphous and morphologically featureless with a large pit density, whereas multilayer films exhibit drastically different terraced structures consisting of overlapping platelets. Large changes in DFH-4T molecular orientation are observed on transitioning from two to four monolayers. Parallel electrical characterization of top-versus-bottom contact configuration DFH-4T FETs with Au source/drain electrodes reveals greatly different mobilities (mu(TOP) = 1.1 +/- 0.2 10(-2) cm(2)V(-1)s(-1) versus mu(BOTTOM) = 2.3 +/- 0.5 10(-5) cm(2)V(-1)s(-1)) and contact resistances (R(C-TOP) = 4-12 MOmegacm vs R(C-BOTTOM) > 1 GOmegacm). This study provides important information on the organic semiconductor-source\drain electrode interfaces and explains why top-contact OFET devices typically have superior performance. By direct visualization, it demonstrates that the DFH-4T film growth transition from monolayer to multilayer on Au is accompanied by dramatic morphology and molecular orientation changes, starting from an amorphous, pitted, and disordered monolayer, to crystalline and smooth bi/tetralayers but with the molecules reoriented by 90 degrees . These chemisorption-derived inhomogenities at the contact-molecule interface and the large monolayer --> multilayer --> bulk microstructural changes are in accord with the large bottom-contact device resistance and poor OFET performance.

Ellipsometric study of an organic template effect: H2Pc/PTCDA

Abstract A key prerequisite for fabricating opto-electronic devices with organic molecules is to control the molecular growth mode. In this work we present an ellipsometric study of the templating effect of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on the growth mode of metal free phthalocyanine (H 2 Pc). The difference in the anisotropic dielectric function between the H 2 Pc films grown on bare glass or silicon substrates compared to the anisotropic dielectric function of H 2 Pc films grown on PTCDA indicates a drastic change in molecular orientation. From the strong in-plane/out-of-plane anisotropy the average molecular tilt angle of H 2 Pc was found to be around 52° for the films grown on bare substrates and around 25° for H 2 Pc films grown on PTCDA. A splitting in the H 2 Pc Q-band was observed for the H 2 Pc films grown on PTCDA which indicates a deviation from the perfect H 2 Pc α-phase crystal.

Influence of alkyl chain substitution on sexithienyl-metal interface morphology and energetics

The interface between Ag(111) and vacuum sublimated α,ω-dihexylsexithienyl (DH6T) was investigated using ultraviolet photoelectron spectroscopy and atomic force microscopy. While the monolayer of DH6T is lying flat on the metal surface, we found that already in the second molecular layer the molecules are almost standing upright. This abrupt change in molecular orientation lowered the hole injection barrier (Δh) of DH6T/Ag by 0.5eV between monolayer and multilayer. Δh for DH6T multilayers was even lowered by 0.8eV compared to unsubstituted sexithienyl multilayers. The reduction of Δh is attributed to the electronic decoupling of molecules in the first from those in the second layer via the hexyl chains.

Synchrotron photoemission studies of pentacene films on Cu(1 1 0)

We have investigated the nature of the interaction between pentacene and Cu(1 1 0) for films grown by two different methods, and the energy level alignment at the resulting pentacene/Cu(1 1 0) interface. The first film was grown in a stepwise fashion at room temperature while the second film was grown using an annealed monolayer to template the growth of the subsequent layers. Synchrotron based photoemission techniques have been used to compare the initial stages of the growth of the two films. A thorough examination of the growth of both films indicates a strong bond between the initial monolayer of pentacene and the Cu(1 1 0) substrate, in addition to a change in molecular orientation for subsequent layers after the completion of the initial monolayer. A comparison of the two films indicates identical film growth for both films with the film templated by the annealed monolayer exhibiting a more uniform growth mode for the film thicknesses investigated.

Molecular Recognition of Cytosine- and Guanine-Functionalized Nucleolipids in the Mixed Monolayers at the Air−Water Interface and Langmuir−Blodgett Films

Molecular recognition of mixed nucleolipids of 1-(2-octadecyloxycarbonylethyl)cytosine and 7-(2-octadecyloxycarbonylethyl)guanine in the monolayers at the air-water interface and Langmuir-Blodgett (LB) films has been investigated in detail using surface pressure/potential-area isotherms, infrared reflection-absorption spectroscopy (IRRAS), and Fourier transform infrared (FTIR) transmission spectroscopy, respectively. Prior to molecular recognition, the cytosine moieties in the monolayer were hydrogen bonded with an almost flat-on orientation, the alkyl chains were uniaxially oriented with respect to the film normal, the guanine moieties in the monolayer were stacked probably through pi-pi interaction with an end-on orientation, and the C-C-C planes of the alkyl chains were preferentially oriented parallel to the water surface. In the monolayer of equimolar mixture, molecular recognition between the cytosine and guanine moieties occurred together with the ring planes of base pairing and the C-C-C planes of the alkyl chains favorably oriented parallel to the water surface. The guanine moieties underwent an orientation change from an end-on mode before molecular recognition to a flat-on one after molecular recognition. The base pairing between the cytosine and guanine moieties in the monolayers was achieved since the N7-substituted guanine derivatives suppressed the formation of guanine tetramers. Both the IRRAS spectra of the monolayers and the FTIR spectra of the LB films presented the exact sites in the cytosine and guanine moieties for the formation of triple hydrogen bonds. The base pairing resulted in a change in molecular orientation and interaction, and the corresponding LB film exhibited a different phase transition behavior from a typical crystal transition for the cytosine-functionalized nucleolipids and an analogous glass transition for the guanine-functionalized nucleolipids. The thermal stability of the mixed LB film was improved in comparison to the LB films of pure components.

Surface Structure and Interface Dynamics of Alkanethiol Self-Assembled Monolayers on Au(111)

Scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS) were used to examine the structural transitions and interface dynamics of octanethiol (OT) self-assembled monolayers (SAMs) caused by long-term storage or annealing at an elevated temperature. We found that the structural transitions of OT SAMs from the c(4 x 2) superlattice to the (6 x square root 3) superlattice resulting from long-term storage were caused by both the dynamic movement of the adsorbed sulfur atoms on several adsorption sites of the Au(111) surface and the change of molecular orientation in the ordered layer. Moreover, it was found that the chemical structure of the sulfur headgroups does not change from monomer to dimer by the temporal change of SAMs at room temperature. Contrary to the results of the long-term-stored SAMs, it was found that the annealing process did not modify either the interfacial or chemical structures of the sulfur headgroups or the two-dimensional c(4 x 2) domain structure. Our results will be very useful for a better understanding of the interface dynamics and stability of sulfur atoms in alkanethiol SAMs on Au(111) surfaces.

Abnormal Temperature Dependence of Photoemission Intensity Mediated by Thermally Driven Reorientation of a Monomolecular Film

The photoemission intensities of organic molecular layers generally obey the Debye-Waller temperature dependence but not always. With the example of a monomolecular film formed from [1,1';4',1''-terphenyl]-4,4''-dimethanethiol, we show that pronounced deviations from Debye-Waller temperature behavior are possible and are likely caused by temperature-dependent changes in molecular orientation.

Conformation study of the membrane models by the Maxwell displacement current technique and oxidative stress

The role of biological membranes as a target in biological radiation damage is still unclear. Recently much attention has been paid to the dynamic behaviour of the cell membrane. Maxwell displacement current technique (MDC) provides new possibility of conformation study of the membrane models. Oxidative stress can impair macromolecules in the cell on a molecular level. MDC technique enables to study the changes in molecular orientations and/or conformations of cell membranes. The combination of different methods in structural biology can clarify membrane chemical and physical properties.

A novel method for determination of collagen orientation in cartilage by Fourier transform infrared imaging spectroscopy (FT-IRIS)

The orientation of collagen molecules is an important determinant of their functionality in connective tissues. The objective of the current study is to establish a method to determine the alignment of collagen molecules in histological sections of cartilage by polarized Fourier transform infrared imaging spectroscopy (FT-IRIS), a method based on molecular vibrations. Polarized FT-IRIS data obtained from highly oriented tendon collagen were utilized to calibrate the derived spectral parameters. The ratio of the integrated areas of the collagen amide I/II absorbances was used as an indicator of collagen orientation. These data were then applied to FT-IRIS analysis of the orientation of collagen molecules in equine articular cartilage, in equine repair cartilage after microfracture treatment, and in human osteoarthritic cartilage. Polarized light microscopy (PLM), the most frequently utilized technique to evaluate collagen fibril orientation in histological sections, was performed on picrosirius red-stained sections for comparison. Thicknesses of each zone of normal equine cartilage (calculated based on differences in collagen orientation) were equivalent as determined by PLM and FT-IRIS. Comparable outcomes were obtained from the PLM and FT-IRIS analyses of repair and osteoarthritis tissues, whereby similar zonal variations in collagen orientation were apparent for the two methods. However, the PLM images of human osteoarthritic cartilage showed less obvious zonal discrimination and orientation compared to the FT-IRIS images, possibly attributable to the FT-IRIS method detecting molecular orientation changes prior to their manifestation at the microscopic level.

Coherent Vibrational Quantum Beats as a Probe of Langmuir−Blodgett Monolayers

We combine frequency- and femtosecond time-domain measurements of vibrational coherences for spectroscopic characterization of surface monolayer films, utilizing 3-wave mixing as the surface-selective technique. Frequency-domain spectra in the CH-stretch region are obtained by infrared + visible sum frequency generation (SFG). Time-domain coherences are measured using SFG free induction decay (SFG-FID), where a 75 fs IR pulse excites several vibrational modes and a delayed 40 fs visible pulse probes the oscillating surface polarization. A unified framework based on optical Bloch equations is used to simultaneously analyze the time- and frequency-domain data. We compare molecular organization of monolayers in different two-dimensional phases. Highly ordered films transferred at high surface pressure are dominated by two transitions in the frequency domain, CH3 symmetric stretch (2875 cm(-1)) and CH3-Fermi resonance with bend overtone (2935 cm(-1)), and a coherent quantum beat in the time-domain at the difference frequency (approximately 540 fs period). At lower surface pressure, relative amplitudes change and additional transitions emerge (CH3 asymmetric stretch and CH2 modes), indicating changes in molecular orientation and onset of disorder. Information redundancy in the combined frequency- and time-domain data allows more accurate determination of the spectral parameters than purely frequency- or time-domain techniques.

Optimizing the Impact Resistance of Matrix-free Spectra®Fiber-reinforced Composites

It has been discovered that Spectra® fiber can shift its peak melting temperature upward by almost 10 if it is maintained at constant length. Based on this, a novel processing method called high-temperature, high-pressure sintering has been developed in which Spectra® cloth is consolidated using heat and pressure to make a matrix-free fiber-reinforced composite. Because of the outstanding properties of the Spectra® fiber, the resulting product is a high modulus, high strength, and high-impact resistant ‘one-polymer composite.’ The three key processing parameters are temperature, time, and pressure. The high temperature is in the vicinity of the fiber’s normal melting point so as to induce adequate, but not excessive melting, which allows for the fusion of the cloths on recrystallization. Time is important in the sense of facilitating heat transfer and promoting interfiber and interlayer adhesion while conserving the fiber’s high crystallinity and orientation. The high pressure exerted laterally on the cloths is required to constrain them and prevent shrinkage, as well as eliminate the voids and consolidate the materials. The very long relaxation time and the rubbery nature of ‘melted ultrahigh molecular weight polyethylene’ retard the loss of orientation, and thus make this process possible. The influence of these parameters on the structure and properties of the consolidated structures are studied. Specifically, the crystallinity changes for specimens processed under different conditions are measured by DSC; the molecular orientation changes for specimens processed under different conditions are studied by WAXD; and the impact properties for specimens processed under different conditions are measured by instrumented puncture impact tests. The established process-structure-property relationship is used as a guideline to fine-tune and optimize the processing conditions in order to achieve the best possible impact properties.

Leucine Fastener Formation Mechanism between Peptide β-Sheets in a Monolayer Studied by Infrared Multiple-Angle Incidence Resolution Spectroscopy

A leucine zipper and a leucine fastener formed between peptide molecules have been hypothetical models of a molecular association via interdigitation. As a molecular interaction mechanism, a "leucine zipper" with the aid of an alpha-helix backbone in biological peptides is believed to play an important role in the molecular association, but no experimental evidence to prove the zipping has been reported thus far. In the same fashion, a "leucine fastener" built on the structure of peptide beta-sheets has never been experimentally captured either. In the present study, very fine changes of molecular stress and orientation in monolayers of a synthesized tetraleucine-containing amphiphile before and after the molecular interdigitation have readily been detected by infrared multiple-angle incidence resolution spectroscopy, which was recently developed for the analysis of structural anisotropy in thin materials. It has been suggested that the conventional molecular orientation model of the leucine fastener should be modified, and the backbone structure (parallel beta-sheet in the present study) plays a necessary role for the interlock of the leucine fastener.

Structure and phase transition in self-assembled films of an anti-ferroelectric liquid crystal studied by two-dimensional correlation FTIR spectroscopy

The FTIR spectra were measured for a self-assembled film of the anti-ferroelectric liquid crystal (AFLC) molecules over a temperature range of 40–150 °C. The frequency and intensity variations of the infrared spectra were discussed in terms of changes in molecular orientation, conformation and intra- and/or inter-molecular interaction during phase transitions. The two-dimensional (2d) correlation analysis revealed that two hydrocarbon chains have different orientation behaviors, the shorter hydrocarbon chain is parallel to molecular long axis and the longer one is perpendicular to it. The splitting of the C O stretching band indicated that the s- cis and the s- trans conformers of the ester group coexist in the self-assembly film. During the phase transitions, in mesogen part the C O group adjacent to the chiral part is first response with temperature changing, following the C O group between two phenyl rings, and eventually the phenyl rings; and the respondence with temperature of methyl is slower than that of methylene in alkyl chains.

Quantification of Redox-Induced Thickness Changes of 11-Ferrocenylundecanethiol Self-Assembled Monolayers by Electrochemical Surface Plasmon Resonance

Redox-induced orientation changes (or monolayer thickness changes) of self-assembled monolayers (SAMs) of 11-ferrocenylundecanethiol (FcC11SH) were quantified by the electrochemical surface plasmon resonance (EC-SPR). EC-SPR enables one to determine the collective effect of the monolayer thickness and refractive index changes resulted from the oxidation of ferrocene (Fc) to ferrocenium. To measure the monolayer volume variation associated with the molecular orientation change, an electrochemical quartz crystal microbalance (EQCM) was used to determine the total number of water molecules accompanying with the ion-pairing between the ferrocenium cation and the counteranion in the solution. With the maximum void space within the SAM for water incorporation known, the Lorentz-Lorenz equation was used to correlate the SPR dip shift to the maximum monolayer thickness variation. In the presence of 0.1 M HClO4 and 0.1 M HNO3, the monolayer thickness changes were deduced to be 0.09 and 0.08 nm, respectively. Thus, upon electrochemical oxidation of the FcC11SH SAM, the swinging of the alkyl chain farther away from the electrode (Ye et al., Langmuir, 1997, 13, 3157) or the rotation or flipping of the Fc cyclopentyldiene ring around the bond between the Fc group and the alkyl chain (Viana et al., J. Electroanal. Chem. 2001, 500, 290) can both lead to the observed film thickness changes, with the former probably being the more important process.

Change of molecular packing and orientation from monolayer to multilayers of hydrogenated and fluorinated carboxylates studied by in-plane X-ray diffraction together with NEXAFS spectroscopy at C K-edge

By applying the in-plane X-ray diffraction together with the NEXAFS spectroscopy to organized molecular films, the molecular orientation and packing from the monomolecular layers to multilayers of hydrogenated and fluorinated long-chain carboxylic acids and carboxylates were characterized. The two-dimensional molecular packing in the monolayers for cadmium salts of fatty acids was clearly different from those for fatty acids as well as those in the multilayers, whereas the fluorinated long-chain carboxylates had hexagonal packing of the rod like molecules common in both mono- and multilayers. This seems to be probably due to difference of molecular cohesive energy between hydrocarbons and fluorocarbons themselves.

Plasmon-waveguide resonance studies of ligand binding to the human beta 2-adrenergic receptor.

Plasmon-waveguide resonance (PWR) spectroscopy is an optical technique that can be used to probe the molecular interactions occurring within anisotropic proteolipid membranes in real time without requiring molecular labeling. This method directly monitors mass density, conformation, and molecular orientation changes occurring in such systems and allows determination of protein-ligand binding constants and binding kinetics. In the present study, PWR has been used to monitor the incorporation of the human beta(2)-adrenergic receptor into a solid-supported egg phosphatidylcholine lipid bilayer and to follow the binding of full agonists (isoproterenol, epinephrine), a partial agonist (dobutamine), an antagonist (alprenolol), and an inverse agonist (ICI-118,551) to the receptor. The combination of differences in binding kinetics and the PWR spectral changes point to the occurrence of multiple conformations that are characteristic of the type of ligand, reflecting differences in the receptor structural states produced by the binding process. These results provide new evidence for the conformational heterogeneity of the liganded states formed by the beta(2)-adrenergic receptor.

Structure and Properties of Biodegradable Polymers: Rolling Effect in Poly (butylene succinate) Sheets

Sheets made from poly (butylene succinate) (PBS) were prepared by extrusion. They have been studied under uniaxial drawing as a function of the draw condition. Changes in molecular orientation and crystallinity were investigated using the criteria of differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile testing, sonic modulus, X-ray diffraction, density, birefringence and measurements of % Haze. Rolling improved the thermo-mechanical and dynamic properties of extruded sheets as a result of changes in molecular orientation and crystallinity.

Improved thermal stability of Langmuir-Blodgett films through an intermolecular hydrogen bond and metal complex.

Langmuir-Blodgett (LB) films of N-octadecanoyl-L-alanine and its silver and zinc complexes have been investigated by variable-temperature Fourier transform infrared transmission spectroscopy. The thermal stability of LB films is improved through an intermolecular hydrogen bond and metal complex. The intermolecular hydrogen-bonding interaction between hydrophilic head groups in the same monolayers and the metal complex between one head group and another in the neighboring monolayers considerably increase the interaction between the corresponding hydrophobic alkyl chains. It is shown that the transformation of the triclinic subcell packing of the molecules in the LB films prior to and after the silver complex into hexagonal packing occurs before the phase transition accompanied with a change in molecular orientation. The phase transition behavior of the LB films is varied from a small temperature interval to large one depending on the hydrogen bond and metal complex.

Probing Adsorption, Orientation and Conformational Changes of Cytochrome c on Fused Silica Surfaces with the Soret Band

The Soret absorption band has been utilized as a probe for the adsorption of cytochrome c to the surface of a fused silica prism in direct contact with bulk protein solutions of various concentrations and pH. Employing linear polarized light and a single-pass total internal reflection absorption technique, we examined in detail the adsorption isotherm, molecular orientation, packing density, and conformational change of the protein bound to the bare (hydrophilic) and silanized (hydrophobic) glass surfaces. An adsorbate density of Γ = 1.4 × 1013 molecules/cm2 was determined for the hydrophilic substrate at pH 7.2 and Cb = 110 μM, indicating that the protein molecules are essentially closely packed on the surface at saturation. The packing density is sensitive to the solution pH as well as the surface hydrophobicity, a result that the protein−surface interaction is governed by both electrostatic and hydrophobic forces. The same forces also govern the molecular orientation, yielding an angle of θμ = 41° between the heme plane and the surface normal at neutral pH. The angle is retained over a wide pH range (4−9) and is fairly independent of the surface coverage on both the hydrophilic and hydrophobic substrates. Reorientation of the protein occurs (41° → 20°) at pH ≈ 3, when the cyt c unfolds and the hydrophobic force becomes dominant in the adsorption process.