Alberta Ferrarini

A shape model for molecular ordering in nematics

An explicit form for the orientating potential acting in uniaxial liquid crystal phases is derived, by analogy with the surface anchoring potential which determines the orientation of macroscopic anisometric particles. The surface of a molecule is determined by describing the molecule as an assembly of van der Waals spheres. The model is successfully applied to predict the ordering tensors for a variety of systems, namely, solutions in nematic solvents of small and rigid probes, or relatively long n-alkanes with many degrees of internal freedom, and pure nematogens formed by aromatic cores attached to flexible chains.

Understanding the unusual transitional behaviour of liquid crystal dimers

Abstract The transitional properties of liquid crystal dimers within a homologous series differ significantly depending on whether the spacer joining the two mesogenic groups contains an even or an odd number of atoms. In order to obtain a physical understanding of this unusual behaviour we have developed a simple model in which the component molecules can adopt just two conformations, one linear and the other bent. An analysis of the model within the molecular field approximation reveals that it is able to provide us with a good understanding of the behaviour of liquid crystal dimers. In addition it predicts the existence of a nematicnematic transition.

Broken Helix in Vesicle and Micelle-Bound α-Synuclein: Insights from Site-Directed Spin Labeling-EPR Experiments and MD Simulations

The region 35-43 of human alpha-Synuclein bound to small unilamellar lipid vesicles and to sodium dodecyl sulfate micelles has been investigated by site-directed spin labeling and electron paramagnetic resonance spectroscopy. The distance distributions obtained from spectral fitting have been analyzed on the basis of the allowed rotamers of the spin-label side-chain. Very similar results have been obtained in the two environments: an unbroken helical structure of the investigated region can be ruled out. The distance distributions are rather compatible with the presence of conformational disorder, in agreement with previous findings for micelle-bound alpha-Synuclein. The propensity for helix breaking is confirmed by molecular dynamics simulations.

Crucial role of molecular curvature for the bend elastic and flexoelectric properties of liquid crystals: mesogenic dimers as a case study

Using a molecular field theory with atomistic modelling, we provide a complete description of the elastic and flexoelectric properties of the nematic phase formed by liquid crystal dimers which, depending on the parity of the number of atoms in the spacer, have either a bent (odd) or a straight (even) average shape. We can then estimate the flexoelastic ratio and make a direct comparison with the outcome of flexoelectro-optic measurements. Our results demonstrate the extreme sensitivity of the bend elasticity and flexoelectricity to the molecular structure, with dramatic differences between even and odd dimers. An unusually low bend elastic constant is predicted for the latter; we discuss the implications of this result for the high flexoelastic response and the existence of Blue Phases stable over a wide temperature range, which were both recently claimed for odd liquid crystal dimers.

SHAPE MODEL FOR ORDERING PROPERTIES OF MOLECULAR DOPANTS INDUCING CHIRAL MESOPHASES

An extension to chiral phases of a model derived previously to interpret orientational properties in liquid crystal solvents of molecules with arbitrary shape leads, quite naturally, to the definition of a traceless chirality tensor, useful for predicting the structures of twisted nematics induced by chiral solutes. The magnitude and handedness of the helical pitch in the induced chiral phase depend on the interplay of the chirality tensor components and the ordering of the different axes of the chiral probe in a given nematic solvent. The effect of changing the molecular structure of the solute and the temperature is discussed for a number of typical dopants, including biphenyl, binaphthyl and heptalene derivatives. Although the model has been derived in a form suitable for application to induced cholesteric structures, it contains the basic ingredients to account for the physical behaviour of chiral smectic-C* mesophases.

Prediction of the transitional properties of liquid crystal dimers. A molecular field calculation based on the surface tensor parametrization

The transitional properties of the liquid crystal dimers formed by two mesogenic groups linked by a flexible chain exhibit a pronounced dependence on the number of atoms in the spacer. Here we present the results of a theoretical calculation of the nematic–isotropic transition temperature, the entropy of transition, and the second rank orientational order parameters at the transition for two homologous series of cyanobiphenyl dimers. In one the alkyl spacer is attached directly to the mesogenic groups while in the other the alkyl chain is linked via ether units to the two cyanobiphenyl groups. At the heart of the molecular field theory is its parametrization based on the surface tensor approach. In this theory the potential of mean torque for each conformer is related to its topology by the surface tensor, which ensures that molecules are aligned such that their surfaces overlap to a maximal extent. The transitional properties predicted by the theory are found to depend in a sensitive manner on the geomet...

Photoinduced Reorganization of Motor-Doped Chiral Liquid Crystals : Bridging Molecular Isomerization and Texture Rotation

We recently reported that the photoisomerization of molecular motors used as chiral dopants in a cholesteric liquid crystal film induces a rotational reorganization which can be observed by optical microscopy and produces the motion of microscopic objects placed on top of the film (Feringa, B. L.; et al. Nature 2006, 440, 163; J. Am. Chem. Soc. 2006, 128, 14397). The mechanism underlying the mesoscopic manifestation of the molecular process was not fully understood, and here we present a joint theoretical and experimental investigation, which provides a detailed insight into the mechanism of texture rotation. This description allows us to identify the interplay between the chemical structure of the chiral dopant and the material properties of the liquid crystal host, and to quantify their role in the observed dynamic phenomenon. We have found that a crucial role is played by the hybrid anchoring of the liquid crystal, with the director parallel to the substrate and perpendicular to the interface with air; in this configuration an almost unperturbed cholesteric helix, with its axis normal to the substrate, is present in most of the film, with strong deformations only close to the free interface. The texture rotation observed in the experiment reflects the rotation of the director during the unwinding of the cholesteric helix, produced by the change in shape of the chiral dopant under photoisomerization. The rotational reorganization is controlled by the photochemical process, via the coupling between the chirality of the dopant and the elastic properties of the liquid crystal host.

Controlling chirality with helix inversion in cholesteric liquid crystals

The helical organization of cholesteric liquid crystals is omnipresent in living matter. Achieving control over the structure of the cholesteric helix consequently holds great potential for developing stimuli-responsive materials matching the level of sophistication of biological systems. In particular, inversion of a cholesteric helix is associated with inversion of the circular polarization of the light it reflects. While control over the cholesteric period has been thoroughly investigated, reports on helix inversion are scarcer. Predicting which systems display helix inversion also remains a challenge because of the subtle balance of contributions underlying this phenomenon. Here we provide an overview on recent advances in controlling and understanding helix inversion in cholesteric liquid crystals.

Understanding the dependence of the transitional properties of liquid crystal dimers on their molecular geometry

Abstract The characteristic feature of liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, is often thought to be the strong odd-even effect exhibited by their transitional properties. That is, the nematic-isotropic transition temperature and the entropy of transition are large for dimers with an even number of groups in the spacer in comparison with those for neighbouring dimers with an odd number of groups. However, the magnitude of the odd-even effect along a homologous series of dimers is found to depend strongly on the nature of the link between the mesogenic group and the spacer. This dependence is thought to originate in the variation of the molecular geometry with the linking group, a view which is supported by detailed molecular field theory calculations involving all of the conformational states. Here we are concerned with developing a more transparent understanding of this geometrical effect using a simple model of the dimers in which all of the conformational ...

From the double-stranded helix to the chiral nematic phase of B-DNA: a molecular model.

B-DNA solutions of suitable concentration form left-handed chiral nematic phases (cholesterics). Such phases have also been observed in solutions of other stiff or semiflexible chiral polymers; magnitude and handedness of the cholesteric pitch are uniquely related to the molecular features. In this work we present a theoretical method and a numerical procedure which, starting from the structure of polyelectrolytes, lead to the prediction of the cholesteric pitch. Molecular expressions for the free energy of the system are obtained on the basis of steric and electrostatic interactions between polymers; the former are described in terms of excluded volume, while a mean field approximation is used for the latter. Calculations have been performed for 130 base pair fragments of B-DNA. The theoretical predictions provide an explanation for the experimental behavior, by showing the counteracting role played by shape and charge chirality of the molecule.