Orwik

Columnar phases and field induced biaxiality of a Gay–Berne discotic liquid crystal

We have studied a system of discotic particles with a central transverse dipole using Monte Carlo (MC) computer simulations at constant pressure. We have investigated several temperatures corresponding to nematic and columnar liquid crystal phases an... expand abstractd determined the molecular and dipolar organizations. Low temperature columnar phases are characterized by local biaxial ordering of dipoles, even if the system is on the whole uniaxial. However, simulations in the presence of a transverse field for different field and dipole strengths show that the system has a large susceptibility and easily becomes biaxial, making it potentially interesting for switching applications. An explicit formulation for the susceptibility in terms of biaxial invariants is derived. collapse abstract

Monte Carlo simulations of zero electric field gradient liquid crystal mixtures

How Does the Trans–Cis Photoisomerization of Azobenzene Take Place in Organic Solvents?

The trans–cis photoisomerization of azobenzene-containing materials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely unknown. Herein, we study the isomerization in a va... expand abstractcuum and in various solvents via a modified molecular dynamics simulation adopting an ab initio torsion–inversion force field in the ground and excited states, while allowing for electronic transitions and a stochastic decay to the fundamental state. We determine the trans–cis photoisomerization quantum yield and decay times in various solvents (n-hexane, anisole, toluene, ethanol, and ethylene glycol), and obtain results comparable with experimental ones where available. A profound difference between the isomerization mechanism in vacuum and in solution is found, with the often neglected mixed torsional–inversion pathway being the most important in solvents. collapse abstract

How does the trans-cis photoisomerization of azobenzene take place in organic solvents?

The trans-cis photoisomerization of azobenzene-containing materials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely unknown. Herein, we study the n, pi* isomerization in a v... expand abstractacuum and in various solvents via a modified molecular dynamics simulation adopting an ab initio torsion-inversion force field in the ground and excited states, while allowing for electronic transitions and a stochastic decay to the fundamental state. We determine the trans-cis photoisomerization quantum yield and decay times in various solvents (n-hexane, anisole, toluene, ethanol, and ethylene glycol), and obtain results comparable with experimental ones where available. A profound difference between the isomerization mechanism in vacuum and in solution is found, with the often neglected mixed torsional-inversion pathway being the most important in solvents. collapse abstract

EFFECT OF NANO–CONFINEMENT ON LIQUID CRYSTAL POLYMER CHAINS.

We apply a Monte Carlo polymerization model for Gay-Berne monomers that we have recently introduced [J. Chem. Phys. 121, 9123 (2004)] to investigate with computer simulations the effects of nanoconfinement and anchoring type on the structure of the m... expand abstractain chain liquid-crystal polymers formed in thin films, in the presence of several types of surface alignment: parallel to the interface (random and uniform) or perpendicular to it (homeotropic). We perform first a study of the confined monomers and then we examine the features of the polymer chains obtained from an isotropic or nematic sample. We find a significant effect of the anchoring conditions on the characteristics of the chains and particularly striking differences between planar and homeotropic boundaries. Furthermore, our results indicate that the choice of different anchorings could be used to tune the linearity and degree of polymerization of the chains. 1 1 collapse abstract

REVIEW ARTICLE Computer simulations of biaxial nematics

Abstract. Biaxial nematic (Nb) liquid crystals are a fascinating condensed matter phase that has baffled for more than thirty years scientists engaged in the challenge of demonstrating its actual existence, and which has only recently been experiment... expand abstractally found. During this period computer simulations of model Nb have played an important role, both in providing the basic physical properties to be expected from these systems, and in giving clues about the molecular features essential for the thermodynamic stability of Nb phases. However, simulations studies are expected to be even more crucial in the future for unravelling the structural features of biaxial mesogens at molecular level, and for helping in the design and optimisation of devices towards the technological deployment of Nb materials. This review article gives an overview of the simulation work performed so far, and relying on the recent experimental findings, focusses on the still unanswered questions which will determine the future challenges in the field. collapse abstract

Computer simulation and molecular design of model

liquid crystals

Theor Chem Acc DOI 10.1007s00214-007-0271-0 REGULAR ARTICLE A computer simulation of model discotic dimers

Abstract We set up a model for discotic liquid crystal dimers and study, by means of Monte Carlo simulations, their phase behaviour and self–assembling properties, in comparison with the simpler monomeric case. Each discotic dimer is described by two... expand abstract oblate Gay–Berne ellipsoids connected by a flexible spacer, modelled by a harmonic “spring ” of three different lengths. We find that dimerization in general yields produces a significant change on the phase behaviour, with an increase of the columnar–nematic transition temperature, a widening of the nematic region and the apparent suppression of the crystalline phase in favour of the columnar phase up to very low temperatures. Longer spacers prove to ease the formation of columns and to increase the orientational order. Keywords Monte Carlo · Triphenylenes · Gay–Berne · Columnar phase · Liquid crystals collapse abstract

c○World Scientific Publishing Company MONTE CARLO SIMULATIONS OF ROD-LIKE GAY BERNE MESOGENS WITH TRANSVERSE DIPOLES

We report the results of a Monte Carlo investigation of a system of rod-like particles interacting via the Gay–Berne potential with an embedded transverse dipole. We describe the effect that the dipole has on the molecular organization. Keywords: Com... expand abstractputer Simulations; Liquid Crystals; Dipolar Systems. 1. collapse abstract

LIQUID CRYSTAL OBSERVABLES: STATIC AND DYNAMIC PROPERTIES

Abstract. In this Chapter we introduce the description of single and pair particle static properties of liquid crystals, and discuss their calculation from computer simulations. We also briefly describe the calculation of dynamic properties from mole... expand abstractcular dynamics simulations using Linear Response theory. 1. Single particle properties We consider a system of N molecules at certain specified thermodynamic conditions. Typically we shall consider that volume V and temperature T are fixed together with N (canonical conditions), but we shall also refer to the case where pressure P is fixed together with T (isobaric conditions). We assume the molecules to be classical, rigid particles with centre of mass at position r and orientation ω given for instance by a set of three Euler angles (α, β, γ), or only two angles (α, β) as illustrated in fig. 1 if we can assume that the molecules have cylindrical symmetry [1]. We shall discuss the calculation of observables in liquid crystals [2] in fairly general terms, but adopting a rather special point of view, that of computer simulations. As will be clear from the contributions in this book, computer simulations techniques [3] actually generate configurations of the system, i.e. sets of positions ri =(xi,yi,zi) and orientations ωi of all the particles. In particular the Monte Carlo (MC) method generates equilibrium configurations, albeit non necessarily in the proper time order, while Molecular Dynamics (MD) actually generates configurations time step after time step in their natural time sequence. In this last case configurations consists not only of positions and orientations, but also of the full set of linear and angular velocities. 18 Figure 1. The two angles α, β defining the orientation of a cylindrically symmetric molecule (a) and the three Euler angles α, β, γ required for a generic rigid particle (b). A complete static information about the system is represented by a sufficiently large set of M of its configurations. Indeed if we can calculate the value of a property dependent on molecular positions and orientations A(r1,ω1,..., rN,ωN) in each of these equilibrium configurations (J), then the average value of A is collapse abstract

Distribution of Hydrophobic Probe Molecules in Lipid Bilayers. 2. Time-Resolved Fluorescence Anisotropy Study of Perylene in Vesicles

vesicles of palmitoyl-δ-9-oleoyl(16:0,18:1)phophatidylcholine (POPC) was studied using time-resolved fluorescence anisotropy. The experimental anisotropy decay curves were analyzed using a global target approach. Here, anisotropy curves obtained at s... expand abstractix temperatures above the gel-lamellar phase transition of the vesicles as well as the two combinations of excitation and emission wavelengths, (256, 470 nm) and (410, 470 nm), were fitted simultaneously. We have utilized two sets of orientational distributions of perylene in the bilayer. One set contains various one-population orientational distributions, while the other consists of models in which the probes are distributed over two distinct orientational populations. We find that in general the models yield statistically equivalant solutions, though the two-population models need a substantially smaller number of fit parameters. The existence of two distinct orientational populations of perylene in the lipid vesicle bilayer is in agreement with the results of Monte Carlo dynamics simulations in the preceding paper, but we argue that additional independent information is needed in order to remove the remaining ambiguities. We conclude that time-resolved anisotropy experiments on macroscopically unoriented samples do not provide sufficient information in order to fully characterize the orientational distribution of probe molecules in the bilayers. collapse abstract

World Scientific Publishing Company PHASE DIAGRAM AND ORIENTATEONAL ORDER OF A SYSTEM WITH SECOND AND FOURTH RANK INTERACTIONS

A simple generalized Lebwohl-Lasher model for liquid crystals, where a fourth rank interact~on is added to the usual second rank one, is investigated in detail. We have obtained the phase diagram of the system performing extensive Monte Carlo compute... expand abstractr simulations for a range of the fourth to the second rank relative strengths and we compare it with the prediction of Mean Field and Two Site Cluster theories. We show that the addition of a non-negligible fourth rank term significantly changes the temperature dependence of the order parameter. Fourth rank contributions larger than 20 % worsen the agreement of the model with the typical temperature behavior of the order in nematics. 1. collapse abstract

Prion Proteins Leading to Neurodegeneration

Prion diseases are fatal neurodegenerative disorders related to the conformational alteration of the prion protein (PrP C) into a pathogenic and protease-resistant isoform PrP Sc. PrP C is a cell surface glycoprotein expressed mainly in the central n... expand abstractervous system and despite numerous efforts to elucidate its physiological role, the exact biological function remains unknown. Many lines of evidences indicate that prion is a copper binding protein and thus involved in the copper metabolism. Prion protein is not expressed only in mammals but also in other species such as birds, reptiles and fishes. However, it is noteworthy to point out that prion diseases are only observed in mammals while they seem to be spared to other species. The chicken prion protein (chPrP C) shares about 30 % of identity in its primary sequence with mammal PrP C. Both types of proteins have an N-terminal domain endowed with tandem amino acid repeats (PHNPGY in the avian protein, PHGGGWQ in mammals), followed by a highly conserved hydrophobic core. Furthermore, NMR studies have highlighted a similar globular domain containing three α-helices, one short 310-helix and a short antiparallel β-sheet. Despite this structural similarity, it should be noted that the normal isoform of mammalian PrP C is totally degraded by proteinase K, while avian PrP C is not, thereby collapse abstract

to be published in Faraday Discussions (2009) A molecular level simulation of a twisted nematic cell

We have performed a Monte Carlo simulation of a sub–micrometric twisted nematic cell with nearly 10 6 particles using an off–lattice molecular model of a liquid crystal. This computer experiment is a proof of principle that molecular models can be pu... expand abstractshed to the limit of the system sizes addressable with finite elements models thus bridging the mesoscopic gap for multiscale modelling while providing a direct molecular level view of the working of the display. This approach, that allows a direct prediction of molecular organisations, properties, and responses of device systems without the requirement of prior estimate or knowledge of material properties (e.g. elastic constants), is particularly important in view of simulating materials and devices for which these quantities are not known. Results for the molecular organisation are discussed, with particular regard to its helical nature in the field–off state. 1 collapse abstract

Ž. Chemical Physics Letters 297 1998 8–14 A Gay–Berne potential for dissimilar biaxial particles

We present a generalized biaxial Gay–Berne potential describing the interaction between two arbitrary, not necessarily identical, Ž AB. ellipsoidal particles. The model does not rely on approximate combination rules like that of Berthelot, but the pa... expand abstractir energy is nevertheless computed efficiently using molecular parameters, optimized for the homogeneous cases Ž AA. and Ž BB., such as their respective shape and interaction biaxialities. q 1998 Elsevier Science B.V. All rights reserved. The Gay–Berne Ž GB. pair potential wx 1 is a simple one-site attractive–repulsive interaction that represents an anisotropic generalization of a shifted 6–12 Lennard–Jones potential. The GB potential between two identical particles 1, 2 of a certain type A can be written as U u ˆ,u ˆ,r collapse abstract

hexarepeat

the role of histidine and tyrosine residues on the conformation of the avian prion

A molecular level simulation of a twisted nematic cell.

We have performed a Monte Carlo simulation of a sub-micrometric twisted nematic cell with nearly 106 particles using an off-lattice molecular model of a liquid crystal. This computer experiment is a proof of principle that molecular models can be pus... expand abstracthed to the limit of the system sizes addressable with finite element models thus bridging the mesoscopic gap for multiscale modelling while providing a direct molecular level view of the working of the display. This approach, that allows a direct prediction of molecular organisations, properties, and responses of device systems without the requirement of prior estimate or knowledge of material properties (e.g., elastic constants), is particularly important in view of simulating materials and devices for which these quantities are not known. Results for the molecular organisation are discussed, with particular regard to its helical nature in the field-off state. collapse abstract

accepted for publication in Journal of Physical Chemistry B (2009). Order and dynamics inside H-PDLC nanodroplets: an ESR spin probe study

We have performed a detailed study of the order and dynamics of the commercially available BL038 liquid crystal (LC) inside nanosized (50–300 nm) droplets of a reflection-mode Holographic Polymer Dispersed Liquid Crystal (H-PDLC) device where LC nano... expand abstractdroplets layers and polymer layers are alternately arranged, forming a diffraction grating. Here, we have determined the macroscopic configuration of the LC local nematic domain director and derived a model of the nanodroplet organization inside the layers. To achieve this, we have taken advantage of the high sensitivity of the electron spin resonance (ESR) spin probe technique, not previously used to study these materials. The spectroscopic analysis was conducted at a series of temperatures ranging from the nematic to the isotropic phase of the LC. In conjunction with SEM images of the H-PDLC cross-section, which provide additional information on the nanodroplet size and shape distribution, the observed director configuration has been modeled as a bidimensional distribution of elongated nanodroplets (prolate ellipsoid) whose long axis is, on the average, parallel to the layers and whose internal director configuration is a quasi-monodomain (“stretched ” bipolar) aligned along the nanodroplet long axis. Interestingly, at room temperature the molecules tend to keep an average orientation parallel to the nanodroplet layers even when these are Università di Bologna collapse abstract

Biaxial liquid crystal elastomers: a lattice model

Abstract. We present a lattice model for biaxial liquid crystal elastomers and perform large-scale Monte Carlo simulations in this model system. Uniaxial and biaxial orientational ordering clearly reflects in macroscopic spontaneous sample deformatio... expand abstractns. The simulation output is used to predict calorimetry data and deuterium magnetic resonance spectra. PACS. 61.30.Vx Polymer liquid crystals – 61.30.Cz Molecular and microscopic models and theories of liquid crystal structure – 61.41.+e Polymers, elastomers, and plastics 1 collapse abstract

Adv. Functional Materials (2009) Modeling Polymer DielectricsPentacene Interfaces: On the Role of Electrostatic Energy Disorder on Charge Carrier Mobility

Force-field and quantum-chemical calculations have been combined to model at the atomistic level the packing of pentacene molecules on two polymer dielectric layers (PMMA versus polystyrene – PS) widely used in field-effect transistors and to assess ... expand abstractthe impact of electrostatic interactions at the interface on the charge mobility values in the pentacene layers, respectively. The results show unambiguously that the electrostatic interactions introduce a significant energetic disorder in the pentacene layer in contact with the polymer chains; a drop in the hole mobility by a factor of 5 is predicted with PS chains while a factor of 60 is obtained for PMMA due to the presence of polar carbonyl groups. 2 The field of organic electronics has developed tremendously over the last twenty years. [1] The key advantages of organic semiconductors compared to their inorganic equivalents are the versatility of chemical synthesis, the ease of processing (in particular over large areas), and low production costs. Among the numerous applications, organic field-effect transistors (OFETs) are attractive devices planned to collapse abstract

Journal of Physical Chemistry, accepted (2009) Theoretical Characterization of the Structural and Hole Transport Dynamics in

We present a joint Molecular Dynamics (MD) / Kinetic Monte-Carlo (KMC) study aiming at the atomistic description of charge transport in stacks of liquid-crystalline tetra alkoxy-substituted metal-free phthalocyanines. The molecular dynamics simulati... expand abstractons reproduce the major structural features of the mesophases, in particular a phase transition around 340 K between the rectangular and hexagonal phases. Charge transport simulations based on a Monte-Carlo algorithm show an increase by two orders of magnitude in the hole mobility when accounting for the rotational and translational dynamics. The results point to the formation of dynamical structural defects along the columns. 2 collapse abstract

A soft-core Gay-Berne model for the simulation of liquid crystals by Hamiltonian replica exchange.

The Gay-Berne (GB) potential has proved highly successful in the simulation of liquid crystal phases, although it is fairly demanding in terms of resources for simulations of large (e.g., N>10(5)) systems, as increasingly required in applications. He... expand abstractre, we introduce a soft-core GB model, which exhibits both liquid crystal phase behavior and rapid equilibration. We show that the Hamiltonian replica exchange method, coupled with the newly introduced soft-core GB model, can effectively speed up the equilibration of a GB liquid crystal phase by frequent exchange of configurations between replicas, while still recovering the mesogenic properties of the standard GB potential. collapse abstract

Theoretical characterization of the structural and hole transport dynamics in liquid-crystalline phthalocyanine stacks.

We present a joint molecular dynamics (MD)/kinetic Monte Carlo (KMC) study aimed at the atomistic description of charge transport in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines. The molecular dynamics simulations ... expand abstractreproduce the major structural features of the mesophases, in particular, a phase transition around 340 K between the rectangular and hexagonal phases. Charge transport simulations based on a Monte Carlo algorithm show an increase by 2 orders of magnitude in the hole mobility when accounting for the rotational and translational dynamics. The results point to the formation of dynamical structural defects along the columns. collapse abstract

Modeling Polymer DielectricPentacene Interfaces: On the Role of Electrostatic Energy Disorder on Charge Carrier Mobility

Force-field and quantum-chemical calculations are combined to model the packing of pentacene molecules at the atomic level on two polymer dielectric layers (poly(methyl methacrylate) (PMMA) versus polystyrene (PS)) widely used in field-effect transis... expand abstracttors and to assess the impact of electrostatic interactions at the interface on the charge mobility values in the pentacene layers. The results show unambiguously that the electrostatic interactions introduce a significant energetic disorder in the pentacene layer in contact with the polymer chains; a drop in the hole mobility by a factor of 5 is predicted with PS chains while a factor of 60 is obtained for PMMA due to the presence of polar carbonyl groups. collapse abstract

Ferroelectric Response and Induced Biaxiality in the Nematic Phase of BentCore Mesogens

The still undiscovered fluid ferroelectric nematic phase is expected to exhibit a much faster and easier response to an external electric field compared to conventional ferroelectric smectic liquid crystals; therefore, the discovery of such a phase c... expand abstractould open new avenues in electro-optic device technology. Here, experimental evidence of a ferroelectric response to a switching electric field in a low molar mass nematic liquid crystal is reported and connected with field-induced biaxiality. The fluid is made of bent-core polar molecules and is nematic over a range of 120 °C. Combining repolarization current measurements, electro-optical characterizations, X-ray diffraction and computer simulations, ferroelectric switching is demonstrated and it is concluded that the response is due to field-induced reorganization of polar cybotactic groups within the nematic phase. This work represents significant progress toward the realization of ferroelectric fluids that can be aligned at command with a simple electric field. collapse abstract