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Here we discuss the use of deuteriated probe molecules in NMR studies of the nematic (N) and twist-bend nematic (NTB) phases formed by the generic liquid crystal dimer CB7CB. The key properties of three probes dissolved in the NTB phase investigated are the orientational order Szz B and the conical angle, θ0, of the director with respect to the spectrometer field. In addition for one probe 5CB-d2 it is possible to obtain a measure of the phase chirality from the difference in the quadrupolar splittings given by the prochiral pair of deuterons. The relative simplicity of the probes facilitates the determination of the key properties which are obtained using two different approaches.

The biological light-harvesting process offers an unlimited source of inspiration. The high level of control, adaptation capability, and efficiency challenge humankind to create artificial biomimicking nanoarchitectures with the same performances to respond to our energy needs. Here, in the extensive search for design principles at the base of efficient artificial light harvesters, an approach based on self-assembly of pigment-peptide conjugates is proposed. The solvent-driven and controlled aggregation of the peptide moieties promotes the formation of a dense network of interacting pigments, giving rise to an excitonic network characterized by intense and spectrally wide absorption bands. The ultrafast dynamics of the nanosystems studied through two-dimensional electronic spectroscopy reveals that the excitation energy is funneled in an ultrafast time range (hundreds of femtoseconds) to a manifold of long-living dark states, thus suggesting the considerable potentiality of the systems as efficient harvesters.

The lipid bilayer is a flexible matrix that is able to adapt in response to the perturbation induced by inclusions, such as peptides and proteins. Here we use molecular dynamics simulations with a coarse-grained model to investigate the effect of a helical inclusion on a lipid bilayer in the liquid disordered phase. We show that the helical inclusion induces a collective tilt of acyl chains, with a small, yet unambiguous difference between a right- and a left-handed inclusion. This behavior is rationalized using the elastic continuum theory: The magnitude of the chiral (twist) deformation of the bilayer is determined by the interaction at the lipid/inclusion interface, and the decay length is controlled by the elastic properties of the bilayer. The lipid reorganization can thus be identified as a generic mechanism that, together with specific interactions, contributes to chiral recognition in phospholipid bilayers. An enhanced response is expected in highly ordered environments, such as rafts in biomembranes, with a potential impact on membrane-mediated interactions between inclusions.

Recent nonequilibrium Molecular Dynamics (NEMD) simulations revealed preferential orientation, induced by a temperature gradient, in fluids of uncharged dumbbell-like particles. The magnitude of this phenomenon, called thermo-orientation, was found to be linear in the applied temperature gradient and to increase with the difference in shape or mass between the two beads of the particles. The underlying mechanism and the microscopic determinants of the phenomenon are not obvious. Here, after examination of the general symmetry requirements for thermo-orientation, we have extended the NEMD simulations to uncharged particles of various shapes and mass distribution, including chiral cases. The numerical results are rationalized by a microscopic model, based on the assumption of local equilibrium. This allows us to correlate the thermo-orientation response of arbitrarily shaped particles to quantities that characterize their shape and mass distribution.

The existing microscopic theories for elasticity of nematics are challenged by recent findings on systems, whether bent molecules or semiflexible polymers, which do not comply with the model of rigid rodlike particles. Here, we propose an extension of Onsager-Straley second-virial theory, based on a model for the orientational distribution function that, through explicit account of the director profile along a particle, changes in the presence of deformations. The elastic constants reveal specific effects of particle morphology, which are not captured by the existing theories. This paves the way to microscopic modeling of the elastic properties of semiflexible liquid crystal polymers, which is a longstanding issue.

Biomembranes, which are made of a lipid bilayer matrix where proteins are embedded or attached, constitute a physical barrier for cell and its internal organelles. With regard to the distribution of their molecular components, biomembranes are both laterally heterogeneous and transversally asymmetric, and because of this they are sites of vital biochemical activities. Lipids may translocate from one leaflet of the bilayer to the opposite either spontaneously or facilitated by proteins, hence they contribute to the regulation of membrane asymmetry on which cell functioning, differentiation, and growth heavily depend. Such transverse motion—commonly called flip-flop—has been studied both experimentally and computationally. Experimental investigations face difficulties related to time-scales and probe-induced membrane perturbation issues. Molecular dynamics simulations play an important role for the molecular-level understanding of flip-flop. In this review we present a summary of the state of the art of computational studies of spontaneous flip-flop of phospholipids, sterols and fatty acids. Also, we highlight critical issues and strategies that have been developed to solve them, and what remains to be solved.

Correction for ‘Thermo-orientation in fluids of arbitrarily shaped particles’ by Andrea Gardin et al., Phys. Chem. Chem. Phys., 2019, 21, 104–113.

The appearance of some special features in 2H-NMR spectra, which could not be explained by the conventional nematic order, was crucial for the discovery of the twist-bend nematic (NTB) phase. Such features reflect the order and symmetry of the local environment of a probe and in principle their analysis can provide quantitative insights into the structural parameters of the phase. We have developed a methodology for the analysis of experimental data, different from the approaches that are normally used for two main reasons: (i) the structural features of the NTB phase, specifically its local polarity, biaxiality and chirality, are consistently taken into account; (ii) the molecular features are introduced in a detailed way, through molecular modelling. We have applied this methodology to analyse 2H-NMR quadrupolar splittings for two representative systems, CB7CB and CB6OCB. The integration of theory, computational modelling and experiment allows us to extract quantitative information from the experimental data and to predict various properties, even not directly probed by measurements. We discuss how the molecular differences between CB7CB and CB6OCB translate into a different phase behaviour and we address issues concerning the distinguishing features of the NTB phase, such as the definition of a local tilted director.

Chromonics are a class of liquid crystals made of aqueous solutions of plank-like molecules, which self-assemble into semiflexible chains. At a given temperature a nematic phase is formed when the system reaches a sufficiently high concentration. Among the unusual properties of chromonic liquid crystals, particularly prominent is the large anisotropy of elastic constants, which leads to new phenomenologies in confined volumes. To gain insights into the microscopic origin of this behavior, we have investigated the elastic properties of a model system that undergoes self-assembly driven nematization by using Monte Carlo simulations and an Onsager-like theory. The relative magnitude of the elastic constants and their dependence on temperature and density show the distinguishing features found in chromonic liquid crystals. We identify the relevant microscopic determinants of this behavior, and we discuss the role played by both the molecular self-assembly and the intrinsic flexibility of aggregates.

Recent numerical simulations of hard helical particle systems unveiled the existence of a novel chiral nematic phase, termed screw-like, characterised by the helical organization of the particle C symmetry axes round the nematic director with periodicity equal to the particle pitch. This phase forms at high density and can follow a less dense uniform nematic phase, with relative occurrence of the two phases depending on the helix morphology. Since these numerical simulations were conducted under three-dimensional periodic boundary conditions, two questions could remain open. First, the real nature of the lower density nematic phase, expected to be cholesteric. Second, the influence that the latter, once allowed to form, may have on the existence and stability of the screw-like nematic phase. To address these questions, we have performed Monte Carlo and molecular dynamics numerical simulations of helical particle systems confined between two parallel repulsive walls. We have found that the removal of the periodicity constraint along one direction allows a relatively-long-pitch cholesteric phase to form, in lieu of the uniform nematic phase, with helical axis perpendicular to the walls while the existence and stability of the screw-like nematic phase are not appreciably affected by this change of boundary conditions., G. C. thanks the Government of Spain for the award of a Ramón y Cajal research fellowship and acknowledges its financial support under Grant Nos. FIS2013-47350-C5-1-R and MDM-2014-0377. This work was also supported by Italy MIUR via No. PRIN-2010-2011 programme under Grant No. 2010LKE4CC.

Biomembrane asymmetry, whose regulation is important for function, is maintained by the movement of lipids from one bilayer leaflet to the other (flip-flop). During the last decades a number of studies have been done to characterize this process, and it was found that it can be spontaneous or assisted by protein transporters. It can be accelerated or inhibited by various factors, e.g., it can be induced by mechanical stresses. It was also found that flip-flop rate and mechanism strongly depend on the molecular structure of the flipping lipid and on the composition and physical state of the membrane. Yet, large discrepancies exist among the data available in the literature, and a quantitative and comprehensive understanding of this process is still missing. This chapter reviews our current knowledge of the molecular aspects of spontaneous (or passive) flip-flop. An overview of experimental studies is presented, together with a summary of the state of the art of computer simulation studies, which enable a direct insight at the molecular level. The achievements and limitations of experimental and computational approaches are pointed out, as well as the challenges that remain to be addressed.

This is the unedited authors' version of Chapter 3 appearing in the following book: Self-Assembly Systems: Theory and Simulations Ed. Li-Tang Yan John Wiley & Sons, Ltd, Chichester, pp. 53-84 (2017)

The shape of the building blocks plays a crucial role in directing self-assembly towards desired architectures. Out of the many different shapes, the helix has a unique position. Helical structures are ubiquitous in nature and a helical shape is exhibited by the most important biopolymers like polynucleotides, polypeptides and polysaccharides as well as by cellular organelles like flagella. Helical particles can self-assemble into chiral superstructures, which may have a variety of applications, e.g. as photonic (meta)materials. However, a clear and definite understanding of these structures has not been entirely achieved yet. We have recently undertaken an extensive investigation on the phase behaviour of hard helical particles, using numerical simulations and classical density functional theory. Here we present a detailed study of the phase diagram of hard helices as a function of their morphology. This includes a variety of liquid-crystal phases, with different degrees of orientational and positional ordering. We show how, by tuning the helix parameters, it is possible to control the organization of the system. Starting from slender helices, whose phase behaviour is similar to that of rodlike particles, an increase in curliness leads to the onset of azimuthal correlations between the particles and the formation of phases specific to helices. These phases feature a new kind of screw order, of which there is experimental evidence in colloidal suspensions of helical flagella., G. C. thanks the Government of Spain for the award of a Ramón y Cajal research fellowship and the financial support under the grant FIS2013-47350-C5-1-R. This work was also supported by MIUR PRIN-COFIN2010-2011 (contract 2010LKE4CC).

The synthesis and characterisation of the nonsymmetric liquid crystal dimer, 1-(4-cyanobiphenyl-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl)hexane (CB6OCB) is reported. An enantiotropic nematic (N)-twist-bend nematic (NTB) phase transition is observed at 109 °C and a nematic-isotropic phase transition at 153 °C. The NTB phase assignment has been confirmed using polarised light microscopy, freeze fracture transmission electron microscopy (FFTEM), (2)H-NMR spectroscopy, and X-ray diffraction. The effective molecular length in both the NTB and N phases indicates a locally intercalated arrangement of the molecules, and the helicoidal pitch length in the NTB phase is estimated to be 8.9 nm. The surface anchoring properties of CB6OCB on a number of aligning layers is reported. A Landau model is applied to describe high-resolution heat capacity measurements in the vicinity of the NTB-N phase transition. Both the theory and heat capacity measurements agree with a very weak first-order phase transition. A complementary extended molecular field theory was found to be in suggestive accord with the (2)H-NMR studies of CB6OCB-d2, and those already known for CB7CB-d4. These include the reduced transition temperature, TNTBN/TNI, the order parameter of the mesogenic arms in the N phase close to the NTB-N transition, and the order parameter with respect to the helix axis which is related to the conical angle for the NTB phase.

Unveiling the subtle rules that control the buildup of macroscopic chirality starting from chiral molecular elements is a challenge for theory and computations. In this context, a remarkable phenomenon is the formation of helically twisted nematic (cholesteric) phases, with pitch in the micrometer range, driven by self-assembly of relatively small chiral species into supramolecular semiflexible polymers. We have developed a theoretical framework to connect the cholesteric organization to the shape and chirality of the constituents, described with molecular detail, in this kind of system. The theory has been tested against new accurate measurements for solutions of short DNA duplexes. We show that the cholesteric organization is determined by steric repulsion between duplexes, and we identify distinctive features of linear self-assembly in the temperature and concentration dependence of the pitch.

Recent experiments have evidenced some unconventional features in the elasticity of nematics, which cannot be explained by standard microscopic theories. Here, in the framework of a second-virial density functional theory, we have developed a general approach, relaxing the usual assumption that the angular distribution of particles with respect to their local director is unaffected by the deformation. We show that, for particles with polar symmetry, a new contribution to the splay and bend deformation free energy arises, associated with the onset of polar order. Calculations for conical and bent-shaped particles reveal dramatic softening of the splay and the bend mode, respectively, which eventually may lead to spontaneous deformation.

The elastic theory of liquid crystals can be traced back to the early 1930s, but the origin of the molecular theory of elastic constants must be postponed to more than 30 years later, when Alfred Saupe wrote his famous papers on this subject. At approximately the same time, the seminal works by Priest and Straley also appeared. Since then, several theories have been developed to connect intermolecular interactions to curvature deformations, on a quite different length-scale, in liquid crystals. This field was particularly alive between the end of the 1970s and the beginning of the 1980s, in parallel with experimental investigations. In more recent times, a renewed interest was aroused by the controversy about the second-order splay-bend contribution, which appears in the Nehring–Saupe expression for the deformation energy density. In the first part of the present contribution the molecular theory of elastic constants is briefly reviewed. This paper focuses on the effects of molecular structure on the elas...

Doping nematic liquid crystals with nonracemic chiral compounds induces a twisted nematic (cholesteric) phase. The ability of solutes to twist the nematic phase may be related to the overall shape of the chiral dopant and consequently to its absolute configuration. The cholesteric induction is therefore a powerful tool complementary to chiroptical techniques to obtain stereochemical information on chiral molecules.

The spontaneous diffusion of solutes through lipid bilayers is still a challenge for theoretical predictions. Since permeation processes remain beyond the capabilities of unbiased molecular dynamics simulations, an alternative strategy is currently adopted to gain insight into their mechanism and time scale. This is based on a monodimensional description of the translocation process only in terms of the position of the solute along the normal to the lipid bilayer, which is formally expressed in the solubility-diffusion model. Actually, a role of orientational and conformational motions has been pointed out, and the use of advanced simulation techniques has been proposed to take into account their effect. Here, we discuss the limitations of the standard solubility-diffusion approach and propose a more general description of membrane translocation as a diffusion process on a free energy surface, which is a function of the translational and rotational degrees of freedom of the molecule. Simple expressions for the permeability coefficient are obtained under suitable conditions. For fast solute reorientation, the classical solubility-diffusion equation is recovered. Under the assumption that well-defined minima can be identified on the free energy landscape, a mechanistic interpretation of the permeability coefficient in terms of all possible permeation paths is given.

Why should achiral particles organize into a helical structure? Here, using theory and molecular dynamics simulations we show that at high concentration crescent-shaped particles interacting through a purely repulsive potential form the twist-bend nematic phase, which features helical order of the twofold symmetry axes of particles, with doubly degenerate handedness. Spontaneous breaking of the chiral symmetry is driven by the entropic gain that derives from the decrease in excluded volume in the helical arrangement. Crucial to this purpose is the concave shape of particles. This study is based on a general formulation of the Onsager theory, which includes biaxiality and polarity of phase and particles, in addition to the space modulation of order. Molecular dynamics simulations corroborate the theoretical predictions and provide further insights into the structure of the helical phase.

Time decay of the fluorescence anisotropy ratio due to diffusion of dyes in micelles is usually interpreted by decoupling the reorientational dynamics of the molecule with respect to the local director from the translational diffusion of the dye. However, while such a kind of assumption is justified in other contexts (like for reorientations of a small mobile fragment in a macromolecule, decoupled from the motion of the macromolecule as a whole, as invoked in the well-known model by Lipari and Szabo), here it is not based on physical grounds. In this work we develop the stochastic model for the full description of the roto-translational dynamics of a dye in the micellar environment, by employing the Fokker–Planck–Smoluchowski equation for the positional and orientational variables. Then we simplify the model to the situation of strong confinement of the molecule at the micelle interface. Finally, by employing a time-scale separation between fast reorientational dynamics and slow lateral diffusion of the dye (which holds if the micelle radius is much larger than the size of the dye), and by resorting to a model like the “wobbling in a cone”, we show that a bi-exponential form can be obtained for the fluorescence anisotropy ratio, but with the remarkable difference that the fast-relaxing component is not affected by the slow motion.

A theoretical model for the dielectric permittivity of nematics has been recently proposed [1], based on the atomistic representation of a probe molecule interacting with a medium which is characterised by its macroscopic properties. Electrostatic interactions are described through the classical model of a charge distribution contained in a molecular- shaped cavity embedded in an anisotropic dielectric continuum. Short-range intermolecular interactions are parameterized in terms of the anisometry of the molecular surface, which is defined according to the "rolling sphere" representation. The results obtained for the isotropic and nematic phases of 4,4'-pentyl-cyanobiphenyl and 4,4'-pentyl-cyanobicyclohexyl are reported; a good agreement with experiment appears, with a significant improvement with respect to the Maier-Meier theory.

A method for the calculation of the dielectric permittivity of isotropic and anisotropic homogeneous fluids is presented which, in the framework of the continuum approximation, adopts a realistic description of the molecular features, so overcoming some of the limits of the Onsager model. The Poisson equation for the molecular charge distribution contained in a cavity in a dielectric continuum in the presence of an external field is solved by a boundary element technique, which allows a detailed description of the cavity shape associated with a given molecular structure. The charge distribution is described in terms of point charges derived from ab initio calculations in vacuum, in addition to a set of interacting atom dipoles induced by all the electric fields experienced by the molecule in the condensed phase. The link between molecular features and bulk properties is established in a general way suitable for isotropic liquids and nematic phases, through the orientational distribution function of the mo...

A nitroxide [60]fullerene adduct containing a pyrrolidine-1-oxyl group has been synthesized. Its orientational order in the nematic phase of the liquid crystal solvent 4,4′-azoxyanisole (PAA) has been measured from the variation of the EPR spectral parameters on passing from the isotropic to the nematic phase. Highly resolved EPR lines allow for precise evaluation of the shifts of the g, a N and a H values. Since the g and the hyperfine tensors are known, the order matrix could be obtained. This is compared with the one calculated with a theoretical model based on short range solute-solvent interactions, which predicts a considerable degree of orientation of the molecular axes, despite the almost spherical shape of the molecule. The agreement with experimental findings is quite good and it is further improved if a bent structure of the pyrrolidine ring is taken into account.

The orientational order parameters of four aromatic solutes in nematic solvents, derived from proton dipolar couplings in NMR spectra, show a correlation with the dielectric anisotropy of the solvents. Explanation of the experimental behaviour is provided by a molecular theory based on the reaction field approximation.

It has been found that bent-shaped achiral molecules can form a liquid crystal phase, called the Twist-Bend Nematic (NTB), which is locally polar and spontaneously twisted having a tilted director, with a conglomerate of degenerate chiral domains with opposite handedness and pitch of a few molecular lengths. Here, using a major extension of the Maier–Saupe molecular field theory, we can describe the transition from the nematic (N) to the NTB phase. We provide a consistent picture of the structural and elastic properties in the two phases, as a function of the molecular bend angle, and show that on approaching the transition there is a gradual softening of the bend mode in the N phase. This points to the crucial role of the molecular shape for the formation of modulated nematic phases and their behaviour.

We have determined experimentally the magnitude of the difference in the splay and bend flexoelectric coefficients, |e 1 − e 3|, of an oxadiazole bent-core liquid crystal by measuring the critical voltage for the formation of flexodomains together with their wave number. The coefficient |e 1 − e 3| is found to be a factor of 2–3 times higher than in most conventional calamitic nematic liquid crystals, varying from 8 pCm−1 to 20 pCm−1 across the ∼60 K—wide nematic regime. We have also calculated the individual flexoelectric coefficients e 1 and e 3, with the dipolar and quadrupolar contributions of the bent-core liquid crystal by combining density functional theory calculations with a molecular field approach and atomistic modelling. Interestingly, the magnitude of the bend flexoelectric coefficient is found to be rather small, in contrast to common expectations for bent-core molecules. The calculations are in excellent agreement with the experimental values, offering an insight into how molecular parameters contribute to the flexoelectric coefficients and illustrating a huge potential for the prediction of flexoelectric behaviour in bent-core liquid crystals.

Hard helices can be regarded as a paradigmatic elementary model for a number of natural and synthetic soft matter systems, all featuring the helix as their basic structural unit: from natural polynucleotides and polypeptides to synthetic helical polymers; from bacterial flagella to colloidal helices. Here we present an extensive investigation of the phase diagram of hard helices using a variety of methods. Isobaric Monte Carlo numerical simulations are used to trace the phase diagram: on going from the low-density isotropic to the high-density compact phases, a rich polymorphism is observed exhibiting a special chiral screw-like nematic phase and a number of chiral and/or polar smectic phases. We present a full characterization of the latter, showing that they have unconventional features, ascribable to the helical shape of the constituent particles. Equal area construction is used to locate the isotropic-to-nematic phase transition, and results are compared with those stemming from an Onsager-like theory. Density functional theory is also used to study the nematic-to-screw-nematic phase transition: within the simplifying assumption of perfectly parallel helices, we compare different levels of approximation, that is second- and third-virial expansions and Parsons-Lee correction., Comment: 19 pages, 20 figures, accepted in Soft Matter

We present a computational investigation of the nematic phase of the bent-core liquid crystal A131. We use an integrated approach that bridges density functional theory calculations of molecular geometry and torsional potentials to elastic properties through the molecular conformational and orientational distribution function. This unique capability to simultaneously access different length scales enables us to consistently describe molecular and material properties. We can reassign (13)C NMR chemical shifts and analyze the dependence of phase properties on molecular shape. Focusing on the elastic constants we can draw some general conclusions on the unconventional behavior of bent-core nematics and highlight the crucial role of a properly-bent shape.

Using an Onsager-like theory, we have investigated the relationship between the morphology of hard helical particles and the features (pitch and handedness) of the cholesteric phase that they form. We show that right-handed helices can assemble into right- (R) and left-handed (L) cholesterics, depending on their curliness, and that the cholesteric pitch is a non-monotonic function of the intrinsic pitch of particles. The theory leads to the definition of a hierarchy of pseudoscalars, which quantify the difference in the average excluded volume between pair configurations of helices having (R) and (L)-skewed axes. The predictions of the Onsager-like theory are supported by Monte Carlo simulations of the isotropic phase of hard helices, showing how the cholesteric organization, which develops on scales longer than hundreds of molecular sizes, is encoded in the short-range chiral correlations between the helical axes.

The helical structure of the chiral nematic phases induced by chiral dopants in nematic solvents provides a macroscopic image of the molecular chirality of the dopant promoted by the orientational order. Chiral biphenyls are challenging systems because their twisting ability shows a strong dependence on the molecular structure, which does not conform to empirical correlation rules. This points out the need for adequate interpretative tools, able to establish a link between molecular properties and macroscopic response. In this paper the twisting ability of chiral biphenyls is reviewed, by reporting examples taken from the literature together with some new experimental results. The microscopic origin of the observed behavior is explained in terms of chirality and anisotropy of short-range and electrostatic-induction interactions. These are described, respectively, by a shape model and a reaction field method, having the common characteristics of a realistic representation of the structure and properties of the chiral dopants in terms of molecular surface, atom charges, and distributed polarizabilities.

A mean-field model for the analysis of electrostatic interactions in liquid crystal phases is proposed. The model is based on a realistic description of the probe molecule and a continuum approximation for its environment. Electrostatic interactions, treated with the reaction field model, are superimposed to short-range interactions, which are accounted for by a shape method. The model is used to predict orientational order parameters of some representative solutes, characterized by different shapes and charge distributions, dissolved in nematics with different dielectric permittivities.

The sign and the magnitude of the helical twisting power β for a series of helicenes have been calculated by the Surface Chirality model. The principal contribution to β derives from the helicity in the direction perpendicular to what can be defined as the main molecular plane; the cholesteric axis is also predicted to be along this direction, in agreement with a view of the cholesteric induction by helical molecules based on empirical observations. In the case of non-rigid biphenanthryl derivatives, the value of β is predicted to vary with the dihedral angle between the phenanthryl moieties, changing sign at about 90° where the conformation passes from s-cis to s-trans, i.e. when the stereochemical descriptor of the biaryl moiety exchanges the M and P indices. Comparison between experimental and calculated values indicates an s-trans conformation for the flexible dopants, in agreement with a previous conclusion drawn from empirical correlations. The Surface Chirality model appears to be a promising technique to assess the absolute configuration of rigid molecules by the comparison of experimental and calculated β values. For flexible molecules, the quality of the information depends critically on the degree of knowledge of their conformational freedom.

NMR T1 relaxation times have been measured for 13C nuclei in the alkyl chains of symmetric tetra-n-alkylstannanes dissolved in CDCl3. The results are interpreted in terms of conformational transitions occurring in the aliphatic chains superimposed to rotational diffusion of the whole molecule. A comparison with analogous tetra-n-alkylammonium salts is performed. Differences are ascribed to changes in the overall rotational diffusion deriving from effects of charge upon formation of ion-pairs and larger aggregates.

The incoherent quasi-elastic neutron scattering from the heptyl and octyl derivatives of the acta-alkylphthalocyanines, CnPcH2, in their crystalline and liquid crystalline (discotic columnar) phases has been measured. Diffusive motion is observed on the 10-10 to 10-12 s timescale. This is interpreted as alkyl chain motion. A detailed comparison of the mesophase data for C7PcH2 has been made with the predictions of the rotational isomeric state model, and an 2 extra motion on this timescale has been invoked, which probably is a localized translational motion. A small discontinuity in the elastic fraction of the quasi-elastic scattering is consistent with a change in the molecular tilt at the transition between the two Dhd phases.

The twisting ability of a novel series of bridged binaphthol derivatives with substituents of various lengths and chemical nature in the 6,6-positions, recently synthesized and used as dopants in nematic solvents, is investigated with the help of the model based upon surface- helicity tensors. Structures of the low energy conformers of these compounds have been generated by molecular mechanics calculations. Their orientational behaviour and the coupling between anisotropy in the alignment and molecular chirality, which are at the origin of the helical twisting power, are analysed on the basis of the anisometry and the chirality of the shape.

A property termed helicity tensor, defined on the basis of the molecular shape, is proposed to quantify the chirality of arbitrary molecules. Numerical implementation of the model can be easily done, once the molecular geometry is known. The principal components of the tensor represent helicities of the molecular surface as viewed along the three perpendicular axes. Results are presented for a number of systems, going from asymmetrically substituted methane derivatives to complex helicoidal structures. Although originally derived to predict the twisting power of chiral probes dissolved in liquid crystal solvents, the concept of helicity tensor provides a useful tool for many applications of molecular engineering.

Nine well studied (polar and non-polar) planar solutes have been used as probes to investigate the mechanism for orientational ordering in the nematic phase. Second-rank orientational order parameters have been derived from proton dipolar coupling constants, obtained from the NMR spectra of these molecules dissolved in a 55 wt% ZLI1132 + EBBA nematic mixture. The experimental data are interpreted in terms of an orienting potential experienced by molecules in a liquid crystal environment. A model for the orienting potential is presented which includes, in addition to short-range repulsion interactions described by the surface tensor model, long-range contributions treated according to a generalized reaction field approach. Least-squares fits of solvent dependent parameters to the experimental data have been performed to estimate the weight of the various contributions to the solute orientational order, and the results obtained in this way are discussed. Finally, the results obtained with the surface tensor...

The following article appeared in Journal of Chemical Physic 138.16 (2013): 164906 and may be found at http://scitation.aip.org/content/aip/journal/jcp/138/16/10.1063/1.4802005, We investigate the isotropic-to-nematic phase transition in systems of hard helical particles, using Onsager theory and Monte Carlo computer simulations. Motivation of this work resides in the ubiquity of the helical shape motif in many natural and synthetic polymers, as well as in the well known importance that the details of size and shape have in determining the phase behaviour and properties of (soft) condensed matter systems. We discuss the differences with the corresponding spherocylinder phase diagram and find that the helix parameters affect the phase behaviour and the existence of the nematic phase. We find that for high helicity Onsager theory significantly departs from numerical simulations even when a modified form of the Parsons-Lee rescaling is included to account for the non-convexity of particles, E.F. acknowledges the University of Padova for a scholarship (Ph.D. School in Materials Science and Engineering). H.B.K. acknowledges MIUR for a Ph.D. scholarship. A.F. and A.G acknowledge financial support from PRIN-COFIN2010- 2011 (Contract No. 2010LKE4CC). G.C. is grateful to the Government of Spain for the award of a Ramón y Cajal research fellowship.

The connection between the molecular structure of liquid crystals and their elastic properties, which control the director deformations relevant for electro-optic applications, remains a challenging objective for theories and computations. Here, we compare two methods that have been proposed to this purpose, both characterized by a detailed molecular level description. One is an integrated molecular dynamics-statistical mechanical approach, where the bulk elastic constants of nematics are calculated from the direct correlation function (DCFs) and the single molecule orientational distribution function [D. A. McQuarrie, Statistical Mechanics (Harper & Row, New York, 1973)]. The latter is obtained from atomistic molecular dynamics trajectories, together with the radial distribution function, from which the DCF is then determined by solving the Ornstein-Zernike equation. The other approach is based on a molecular field theory, where the potential of mean torque experienced by a mesogen in the liquid crystal phase is parameterized according to its molecular surface. In this case, the calculation of elastic constants is combined with the Monte Carlo sampling of single molecule conformations. Using these different approaches, but the same description, at the level of molecular geometry and torsional potentials, we have investigated the elastic properties of the nematic phase of two typical mesogens, 4 '-n-pentyloxy-4-cyanobiphenyl and 4 '-n-heptyloxy-4-cyanobiphenyl. Both methods yield K-3(bend) > K-1 (splay) > K-2 (twist), although there are some discrepancies in the average elastic constants and in their anisotropy. These are interpreted in terms of the different approximations and the different ways of accounting for the structural properties of molecules in the two approaches. In general, the results point to the role of the molecular shape, which is modulated by the conformational freedom and cannot be fully accounted for by a single descriptor such as the aspect ratio. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794920]

Axially chiral alleno-acetylenes, modified with lateral aryl groups via short and convenient synthetic pathways, are presented as efficient chiral dopants for producing cholesteric liquid crystals.

The elastic and optical properties of a bent-core thiadiazole nematic liquid crystal are reported. The system exhibits unusually large values of birefringence (∼0.4), which can be partly attributed to significant conjugation in the molecular structure. It is found that the splay (K11), twist (K22) and bend (K33) elastic constants, determined from studies of the Freedericksz transition, satisfy K33 > K11 > K22 across the wide nematic range studied. Such behaviour is analogous to that commonly observed in calamitic nematic liquid crystals but is in contrast to all other bent-core nematic materials reported to date where it is found that K33 < K11. Such a result questions some of the current explanations that have been put forward to explain the elastic behaviour of bent-core materials. Using a molecular field theory, combined with DFT calculations of the molecular structure, we can ascribe the different elastic behaviour to the value of the bend angle in the molecular core. This angle is shown to play a subtle and substantial role in determining the physical properties of bent-core nematic liquid crystals.

In this work we analyse the consequences of relaxing the rotational isomeric state (RIS) approximation in predicting the transitional properties of nematogenic systems formed by two mesogenic moieties linked by a flexible spacer, as in the homologous series of α,ω-bis(4′-cyanobiphenyl-4-yl)alkanes and α,ω-bis(4′-cyanobiphenyl-4-yloxy)alkanes, generically referred to as CB dimers. Comparison is made therefore between predictions of the nematic-isotropic transition temperatures, entropy changes, and order parameters obtained from the RIS model, and those by taking into account the full distribution of torsional angles. The continuous model predicts a damping of the even-odd effects for long chain systems, with entropy changes and order parameters at the nematic-isotropic transition tending to those of the corresponding monomers.

The conformational dynamics of alkyl chains is usually analysed within the framework of the rotational isomeric state (RIS) approximation, where only the conformers corresponding to the configurational potential minima are considered, and torsional fluctuations about the minima are ignored. In order to test the validity of this procedure, 13 C NMR T 1 relaxation times have been measured in the series of symmetric tetra- n -alkylammonium ions (C N H 2 N +1 ) 4 N + , with N = 8, 10, 12, 16 and 18, and compared with theoretical results predicted by the RIS model. Spin relaxation rates of the terminal carbon atoms show a tendency to level off for increasing chain length, in contrast with the monotonic increase predicted by the RIS model. The experimental behaviour reveals that deviations from RIS results appear for chains with more than ten carbon atoms.