Molecular Tilt Research Articles

Spatial variation in the molecular tilt orientational order within the solid domains of phase-separated, mixed dialkylphosphatidylcholine monolayers

The miscibility of the solid-phase-forming distearoylphosphatidylcholine (DSPC) and the fluid-phase-forming dilauroylphosphatidylcholine (DLPC) at the air/water interface was investigated by the Langmuir film balance. Surface pressure–area isotherms suggest that mixtures containing 25.0–62.5-mol% DLPC (range of composition investigated) are phase-separated. The lateral structure of the DSPC/DLPC monolayers was imaged by Brewster angle microscopy (BAM) as a function of the surface pressure. Quasi-circular condensed domains appeared at pressures between 0 and 0.5 mN m −1, and these structures were already fully developed at ∼1 mN m −1. Further compression of the monolayers above 1 mN m −1 merely brought the domains closer together. The mixed monolayers consisted of solid domains of DSPC, ∼3–20 μm in size, in a fluid matrix of DLPC. BAM and the phase contrast mode of intermittent-contact atomic force microscopy (AFM) revealed that the quasi-circular DSPC domains are divided into segments of different reflectivities (BAM) or phase shift (AFM) that arise from abrupt changes in the long-range orientational order of the tilted hydrocarbon chains. The DSPC domains in DSPC/DLPC internally exhibited star and cardioid textures that were heretofore only reported for single-component lipid monolayers in the phase coexistence region.

Superlattice Epitaxy of Vinylidene Fluoride/Trifluoroethylene Copolymer Crystals on Highly Oriented Pyrolytic Graphite

Vinylidene fluoride/trifluoroethylene (VDF/TrFE) copolymer crystals were grown from the dilute solution on cleaved (0001) surface of highly oriented pyrolytic graphite (HOPG). Using transmission electron microscopy, the orientational relation between the copolymer crystals and cleaved (0001) surface of HOPG is clarified, as well as the supperlattice matching. Electron diffraction (ED) patterns with 6-fold symmetry were observed for the copolymer crystals with VDF molar contents of 59, 65, and 71%. It suggests that a rectangular array of (20\bar1) plane for the high-temperature phase (HP) of the copolymer crystal faces HOPG(0001) plane, from which the molecular chains tilt by 46.7°. The ED pattern with such 6-fold symmetry is attributed to superposition of diffractions from the HP crystallites with three different orientations on HOPG. Such results suggest that epitaxial growth of HP occurred and it was frozen or stabilized even at room temperature due to epitaxial effect or interaction between copolymer m...

Molecular orientational properties of a high-tilt chiral smectic liquid crystal determined from its infrared dichroism

The orientational characterisitics and the temperature dependencies of the molecular apparent tilt angle of a partly fluorinated chiral smectic liquid crystal (/S/)-4-(1-methylheptyloxycarbonyl)phenyl 4'-[6-(3,4,4,4-tetrafluoro-3-trifluoromethylbutylcarbonyloxy)hexyloxy] biphenyl-4-carboxylate (acronym MHPHFHHOBC) are studied using the polarized Fourier transform infrared (FTIR) spectroscopy. The molecular orientational distributions and the orientational order parameters for a homogeneously aligned liquid crystalline sample at various temperatures and external electric fields are examined. The analysis uses the dichroic parameters of the phenyl and the carbonyl bands. For a temperature range of 65-80 degrees C corresponding to the antiferroelectric SmCA* phase, the molecular apparent tilt angle lies within the range 43 degrees -44 degrees; antiferroelectric smectic structure being rather close to the orthoconic SmCA* phase. An application of sufficiently high dc field across the cell in its SmA* phase surprisingly shows that the dichroism first increases slowly and then rapidly in two stages and finally a saturated apparent tilt angle of approximately 30 degrees is reached. The IR dichroic data is used to estimate the polar angles and the degree of rotational biasing of the carbonyl groups with respect to the molecular long axis. In the SmA* phase, the sample appears to demonstrate some of the typical properties of a de Vries material.

Direct Probing Molecular Twist and Tilt in Aromatic Self-Assembled Monolayers

Using a nitrile tailgroup as a spectroscopic marker, both twist and tilt of the aromatic backbones in several typical aromatic SAMs on Au(111) have been directly determined in a single experiment. Whereas the exact value of the twist angle depends on the molecular architecture, it was found to be quite noticeable in all SAMs (40−50°) and close to the respective value for aromatic bulk systems (32°).

Deciphering Structural Domains of Alkanethiol Self-Assembled Configurations by Friction Force Microscopy

High resolution and high sensitivity friction force microscopy (FFM) is used to distinguish between different crystallographic domains of standing up molecular configurations of self-assembled alkanethiols partially covering Au(111) surfaces. We propose two suitable methods to decipher structural domains of the same configuration depending on the two-dimensional (2D) symmetry of the organic adlayer. For the hexagonal (radical3xradical3)R30 degrees where no differences among equivalent domains are expected in lattice-resolved scanning force imaging, different molecular domains however can be observed in lateral force images because of the friction asymmetry caused by domains presenting different relative orientations between the molecular tilt direction and the tip scanning direction. Since no lateral packing anisotropy is expected in this close-packed configuration, no friction anisotropy however is observed. Conversely, because of its rectangular space group symmetry, lattice resolved stick-slip imaging is enough to solve between the existing domains for the (2xradical3) rectangular configuration.

Analysis of mechanically induced processes in the Langmuir film

The understanding of processes in the monolayer at the air–water interface induced by mechanical compression is important as a part of basic research of the system with reduced dimensionality as well as for the investigation of processes during the Langmuir–Blodgett deposition. The Maxwell displacement current technique provides a substantial contribution for the study of structural and electrical properties. Analysis based on imperfect gas approximation with semi-empirical intermolecular potentials is used. Detail theoretical study of molecular tilt in a continual lateral compression and dielectric relaxation phenomena (step-compression) is presented. Obtained results are confronted with standard surface pressure analysis and surface potential measurement.

Tilted and Non-tilted Liquid Crystalline Langmuir Monolayers: Analogy to Bulk Smectic Phases

Hydrophobic compounds of smectic liquid crystals are spontaneously spread on a liquid surface to form softly-condensed monomolecular films. The constituent molecules are coherently tilted from the surface normal when they form smectic-C phase in the bulk state, while the molecules are aligned perpendicular to the surface when they form smectic-A or -B phase in the bulk. The result proves that the tilting property in the smectic liquid crystals should be determined only by the intralayer molecular interaction. The molecular dynamics simulations can reproduce the experimental result satisfactorily and also suggest that the molecular dipole moment should play an essential role to cause the coherent molecular tilt in smectic liquid crystals.

Vesicle shape, molecular tilt, and the suppression of necks

Can the presence of molecular-tilt order significantly affect the shapes of lipid bilayer membranes, particularly membrane shapes with narrow necks? Motivated by the propensity for tilt order and the common occurrence of narrow necks in the intermediate stages of biological processes such as endocytosis and vesicle trafficking, we examine how tilt order inhibits the formation of necks in the equilibrium shapes of vesicles. For vesicles with a spherical topology, point defects in the molecular order with a total strength of +2 are required. We study axisymmetric shapes and suppose that there is a unit-strength defect at each pole of the vesicle. The model is further simplified by the assumption of tilt isotropy: invariance of the energy with respect to rotations of the molecules about the local membrane normal. This isotropy condition leads to a minimal coupling of tilt order and curvature, giving a high energetic cost to regions with Gaussian curvature and tilt order. Minimizing the elastic free energy with constraints of fixed area and fixed enclosed volume determines the allowed shapes. Using numerical calculations, we find several branches of solutions and identify them with the branches previously known for fluid membranes. We find that tilt order changes the relative energy of the branches, suppressing thin necks by making them costly, leading to elongated prolate vesicles as a generic family of tilt-ordered membrane shapes.

Role of molecular tilt in thermal fluctuations of lipid membranes

Long-wavelength thermal fluctuations of lipid membranes are adequately described by the Helfrich elastic model. On the other hand, fluctuations of wavelengths comparable with bilayer thickness exhibit significant deviations from the prediction of the elastic model and are typically assumed to be dominated by microscopic surface tension due to protrusion of lipid molecules into the solvent. We present evidence that the short-wavelength modes of a lipid membrane are dominated by fluctuations of the tilt of lipid molecules with respect to the membrane normal rather than the microscopic surface tension. We obtain an expression for the spectral intensity of the thermal membrane fluctuations by appealing to the Hamm-Kozlov model, which accounts for both membrane bending and lipid tilt contributions to the total membrane energy but neglects the contributions of the microscopic surface tension. The tilt and the bending fluctuations obtained from our coarse-grained molecular dynamics simulations of a dipalmitoylphosphatidylcholine lipid bilayer show good agreement with the theory. Furthermore, the obtained tilt and bending moduli are in close agreement with experimentally determined values. The magnitude of the microscopic protrusion tension estimated from our simulations is significantly smaller than that of the tilt modulus. These results indicate that the membrane fluctuations can be adequately described by a macroscopic elastic model down to scales of interlipid distance provided one accounts for the tilt fluctuations.

Molecular modeling of key elastic properties for inhomogeneous lipid bilayers

Fusion and fission of biological membranes play a crucial role in intracellular transport. Until recently, it was believed that membrane shape transformations involved in these processes are driven by proteins. However, recent evidence shows that lipids, by themselves, can drive membrane deformations. It has been hypothesized that the localized formation of certain lipids changes elastic properties of a membrane in such a way that the membrane deforms spontaneously. This study represents a step towards a systematic investigation of the role of various lipids in local changes of membrane elastic properties. We use coarse-grained molecular dynamics (CGMD) simulations to determine possible effects of addition of phosphatidylinositol-4-phosphate (PI4P) lipids on elastic properties of dipalmitoyl phosphatidyl choline (DPPC) lipid bilayers. We investigate the splay (bending) and the molecular tilt moduli of mixed DPPC/PI4P bilayers, as well as the line tension between domains of pure DPPC and mixed DPPC/PI4P bilayers. Although our results indicate negligible effects of PI4P on elastic properties of DPPC bilayers, the developed methodology can be applied to a wide range of lipid systems.

Possible model of an antiferroelectric twist grain boundary phase

Using x-ray and optical methods we have probed the structural organization of an antiferroelectric twist grain boundary phase (TGBC(a)) lying between the regular antiferroelectric smectic-C (SmC(a)* and the smectic-Q (SmQ) or isotropic phase. We find that the twist axis is everywhere perpendicular to the local smectic layer normal and that the helical superstructure is incommensurate with the smectic layer structure. The twist grain boundaries consist of a periodic lattice of alternating +1/2 and -1/2 dispirations, i.e., unit screw dislocations in combination with half unit disclinations. The molecular tilt plane is alternatingly parallel and perpendicular to the twist axis. We find that the optically measured tilt angle in the SmC(a)* phase is smaller than that measured by x rays, which is the opposite to what is found in the SmC* phase. This means that the core part tilts less than the end chains in the SmC(a)* phase, while it tilts more in the SmC* phase. On entering the TGB phase a clear decrease is measured in the tilt angle. This is explained by the elastic influence from the disclinations, which appear in this phase.

Effect of Molecule−Substrate Interaction on Thin-Film Structures and Molecular Orientation of Pentacene on Silver and Gold

Evaporated pentacene thin films with thicknesses from several nm to 150 nm on gold and silver substrates have been studied by ultraviolet photoelectron spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). It was found that pentacene thin-film structures, particularly their molecular orientations, are strongly influenced by the metal substrates. UPS measurements revealed a distinct change in the valence band structures of pentacene on Au compared to those on Ag, which is attributed to the different packing between adjacent molecules. Using NEXAFS, we observed 74+/-5 degrees and 46+/-5 degrees molecular tilt angles on Ag and Au, respectively, for all measured thicknesses. We propose that pentacene molecules stand up on the surface and form the "thin-film phase" structure on Ag. On Au, pentacene films grow in domains with molecules either lying flat or standing up on the substrate. Such a mixture of two crystalline phases leads to an average tilt angle of 46 degrees for the whole film and the change in valence band structures. STM and distance-voltage (z-V) spectroscopy studies confirm the existence of two crystalline phases on Au with different conducting properties. z-V spectra on the low conducting phase clearly indicate its nature as "thin-film phase".

Structure and stability of bent core liquid crystal fibers

Recently it was found that fluid smectic phases of bent core liquid crystals formed freestanding fibers of extremely high slenderness ratios. Studies of these fibers showed that their structure was composed of concentric cylindrical smectic layers. For this configuration to be stable there must be an energy term that desires bending of the smectic layers. We show that an energy term that deals with the divergence of the dipolar direction can encourage layer bending if the layer chirality value is allowed to vary. The energy term associated with holding the layer chirality is closely related to layer compressions and electrical self-interactions. For our model, we assumed a simple smectic-C geometry with constant molecular tilt and cone angle defined by the director with respect to the layer normal, but allowed a constant variation of the polar direction about the director. Applying this simplified model to a free energy which accounts for director distortions, divergence of the polar direction, biaxial layer strain, surface tension, and electrical self-interactions, we were able to show consistency between the stable fiber radius and other properties predicted in our model to results from experimental studies.

Molecular Orientation and Ordering during Initial Growth of Copper Phthalocyanine on Si(111)

The molecular orientation and order in the initial growth of copper phthalocyanine on Si(111) was investigated in situ by angular-dependent X-ray absorption spectroscopy. A transition from lying-down to standing-up configuration occurs in less than one monolayer. After the Si surface is passivated by a monolayer of the molecules, only the standing-up structure persists in subsequent growth. The molecular film also exhibits an order−disorder−order transition in the distribution of molecular tilt angle during growth. These transitions are related to the changes of interactions in the system at different stages.

Supramolecular Tilt Chirality Derived from Symmetrical Benzene Molecules: Handedness of the 21 Helical Assembly

Achiral molecules can form aggregates with chirality. This depends on the relative position of the molecules, in other words, the tilt of the molecules (so-called supramolecular tilt chirality). In this paper, we describe supramolecular chirality appearing in a 2(1) column composed of symmetrical benzene molecules, which is formed in the host cavity of inclusion crystals of cholic acid. Moreover, we determined the handedness, that is, right or left, of the 2(1) helical column of benzene on the basis of the molecular tilt. Determination of the 2(1) helical handedness was performed on assemblies of other benzene derivatives in cholic acid crystals and benzene assemblies in other host frameworks selected from the Cambridge Structural Database. Finally, we demonstrated complementarity of the handedness between the 2(1) symmetrical host framework of cholic acid and the benzene column.

Structural Characterization of 4-Bromostyrene Self-Assembled Monolayers on Si(111)

Organic functionalization of silicon holds promise for a variety of applications ranging from molecular electronics to biosensing. Because the performance and reliability of organosilicon devices will be intimately tied to the detailed structure of the organic adlayers, it is imperative to develop systematic strategies for forming and characterizing self-assembled monolayers (SAMs) on silicon with submolecular spatial resolution. In this study, we use 4-bromostyrene for the photochemical growth of Br-terminated SAMs on Si(111). A variety of experimental and theoretical techniques including atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), and density functional theory (DFT) have been employed to determine the coverage and conformation of the 4-bromostryene molecules within the SAM. In particular, AFM verifies a continuous and atomically flat SAM, and the XRR data indicate a SAM thickness of 8.50 A and a molecular coverage of 46% of the surface silicon atoms. Because the DFT calculations indicate a molecular length of 8.89 A, the measured XRR thickness implies a molecular tilt angle of approximately 17 degrees. The XRR analysis also suggests that the Br atoms are preserved on top of the SAM in agreement with XPS measurements that show bromine bound solely to carbon and not to silicon. XRF reveals a Br atomic coverage of 50%, again in close agreement to that found by XRR. Single-crystal Bragg diffraction XSW is used to generate a three-dimensional map of the Br distribution within the SAM, which in conjunction with the XRR result suggests that the 4-bromostyrene molecules are tilted such that the Br atoms are located over the T4 sites at a height of 8.50 A above the top bulklike Si(111) layer. The direction of molecular tilt toward the T4 sites is consistent with that predicted by the DFT calculation. Overall, through this unique suite of complementary structural characterization techniques, it is concluded that the Br functional handle is preserved at the top of the SAM and is available for further substitutional chemistry.

The dipole interaction model of the molecular aggregation in Y-type Langmuir-Blodgett film

Based on the classical electrostatic theory, the model of dipole-dipole interaction of rod-like amphiphilic molecules in Y-type Langmuir-Blodgett (LB) film is presented in the paper. The relations between the structure and spectral absorption peak of LB films were obtained by using this model. For Y-type films, the relationship of molecular aggregation state and the structure parameters, i.e. molecular separation distance a, separation d between two adjacent layers, molecular tilt angle θ, and layer number g, were studied using our model. Then we compared the similarities and differences of aggregation behavior between Y-type and Z-type films. The theoretical results agree well with experimental data.

Study of dependence of molecular orientation and optical properties of zinc phthalocyanine grown under two different pressure conditions

Zinc phthalocyanine (ZnPc) thin films were prepared by organic molecular beam deposition on Si (111) covered with a thin native oxide layer under two different pressure conditions. The samples are characterized by infrared spectroscopy and ellipsometry measurements aiming at the determination of dielectric functions and average molecular orientation in the ZnPc films. A two order of magnitude increase in the pressure during growth increases the average molecular tilt angle with respect to the substrate surface from ∼19° to ∼61° implying a change of crystal phase as well. Likewise sizeable differences are observed in the anisotropic dielectric functions of the ZnPc thin films in the infrared as well as in the visible and ultraviolet spectral ranges.

Four Kinds of 21 Helical Assemblies with the Molecular Tilt as Well as Three-directional and Facial Chirality

Abstract Four kinds of 21 helical assemblies with a right- or left-handed screw with belly- or back-inside were confirmed in cocrystals of brucine. Discrimination of the different helicities owes to the molecular tilt as well as three-directional and facial chirality of the brucine molecule.

Epitaxial structures of self-organized, standing-up pentacene thin films studied by LEEM and STM

Growth of pentacene (Pn) thin films has been studied in situ by means of low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). A very low nucleation density of Pn grains has been observed on Bi(0 0 0 1)/Si(1 1 1) template, resulting in formation of large, monolayer-high Pn grains with diameter exceeding several hundreds of micrometers. We determined that formation of self-organized, standing-up Pn epitaxial layers was stabilized by a weak interaction between the substrate and Pn molecules and by the presence of the commensurate structure between the oblique Pn lattice and trigonal substrate surface lattice. The ‘point-on-line’ commensurability has been found along a -axis of Pn and one of the primitive vectors of substrate surface lattice. Strong ‘point-on-line’ commensurability in Pn/Bi(0 0 0 1)/Si(1 1 1) system resulted in a bulk-like epitaxial thin film growth, starting from the first layer. The presence of twins, often having a mirror line parallel to the direction of the ‘point-on-line’ matching, has been also detected using an asymmetric dark-field imaging mode in LEEM experiments, which, we believe, is the first LEEM demonstration of molecular tilt imaging.