International School of Liquid Crystals

15th Workshop

Liquid Crystal Phases and Nano-Structures

A Workshop to celebrate ten years of the International Liquid Crystal School and Claudio Zannoni 60th birthday

E. Majorana Centre for Scientific Culture, Erice

27 October – 1 November 2008

TALKS

Directors of the Workshop: Director of the School: P. Pasini and S. Žumer C.Zannoni

Sponsors: Italian Liquid Crystal Society, INSTM-CRIMSON, E4 Computer Engineering

Thermotropic Biaxial Nematics: An Enduring Challenge?

Geoffrey Luckhurst

School of Chemistry, University of Southampton, Southampton SO17 1BJ, United Kingdom

The discovery of a new liquid crystal phase is an exciting event especially if the phase has been predicted theoretically. In principle, one example of such a phase is the biaxial nematic first predicted by Marvin Freiser1 in 1970. Following his prediction there was much theoretical work but apparently no experimental attempts to find the phase until 1980. Then, in a seminal study, Yu and Saupe discovered the biaxial nematic but for a lyotropic liquid crystal2. Since then although simulations3 of model systems have certainly revealed the existence of the thermotropic biaxial nematic the discovery of a real system has remained elusive. However, many compounds have been claimed to form a biaxial nematic but the evidence was not always strong and when studied with deuterium NMR it was invariably found that the nematic is uniaxial4. In recent years more detailed evidence in support of claims has been presented and the existence of the biaxial nematic seems more convincing. One example of such a claim is for a tetrapode in which four mesogenic groups are tethered by flexible spacers to a central silicon atom5. In addition, it seems that as well as finding the elusive biaxial phase the FTIR studies also determined the four orientational order parameters characterising both the uniaxial and the biaxial nematics5. The identification of the phases was then supported by NMR using a deuteriated calamitic nematogen as a spin probe6.

Given the reassuring nature of these investigations we had decided to undertake our own studies. In the first of these the four order parameters were used to test a rather general molecular field theory of the biaxial nematic in which the molecular biaxiality is characterised by two parameters. Attempts to fit the theory to the experimental results proved to be interesting which prompted us to compare the order parameters found for the uniaxial and biaxial nematic phases of a biaxial Gay-Berne mesogen3(b) again with interesting results7. Independently of these theoretical studies we had also determined the phase symmetry with the aid of NMR and a selection of deuteriated spin probes over the entire nematic range of the tetrapode. To help appreciate these NMR results the orientational ordering of the probes in a phase of known biaxiality was determined. Further studies of the tetrapode at low temperatures were inadvertently undertaken and these suggest how some of our puzzling results, both experimental and theoretical, can be understood.

The thermotropic biaxial nematic phase would appear to present an enduring challenge at least as far this tetrapode is concerned.

1 M J Freiser, Phys Rev Lett 24, 1041 (1970). 2 L L Yu and A Saupe, Phys Rev Lett 45, 1000 (1980). 3 (a) G R Luckhurst and S Romano, Mol Phys 40, 129 (1980); (b) R Berardi and C Zannoni, J Chem Phys 113, 5971 (2000) 4 G R Luckhurst, Thin Solid Films 393, 40 (2001) 5 K Merkel, A Kocot, J K Vij, R Korlacki, G H Mehl and T Meyer, Phys Rev Lett 93, 237801 (2004). 6 J L Figueirinhas, C Cruz, D Filip, G Feio, A C Ribeiro, Y Frere, T Meyer and G Mehl, Phys Rev Lett 94, 107802 (2005). 7 F Bisi, G R Luckhurst and E G Virga, Phys Rev E 78, 021710 (2008).

Superstructures in nematic colloids

Slobodan Žumer1,2, Miha Ravnik1, and Brina Črnko1

1 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia;

2 Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia

Our recent theoretical predictions are contrasted with the latest experimental studies of colloidal structures in spatially confined nematic liquid crystals. Modeling is based on phenomenological Landau- de Gennes type free energy where also effects of confinement and external fields are taken into account. The complexity of effective inter-colloidal couplings in a nematic solvent leads to numerous colloidal structures not present in simple liquids. Here we focus on thin nematic films where colloidal particles can share nematic defects. In such a case the coupling is string-like and is therefore much more robust as compared to an interaction based on an array of localized disclinations. These structures can be realized either via -1/2 disclination lines that entangle two or more colloidal particles [1] or via -1 disclination loops with nonsingular cores where topological constrains result in sharing of the corresponding areas of deformed nematic by neighboring particles. The resulting colloidal dimmers, chains, lattices, and braids are the first steps toward assembling of robust nematic colloidal crystals and hierarchical structures [2] that are of potential interest for photonic applications.

[1] M. Ravnik, M. Škarabot, S. Žumer, U. Tkalec, I. Poberaj, D. Babič, N. Osterman, and I. Muševič, Entangled Nematic Colloidal Dimers and Wires, Phys. Rev. Lett. 99, 247801 (2007) [2] M. Škarabot, M. Ravnik, S. Žumer, U. Tkalec, I. Poberaj, D. Babič, I. Muševič, Hierarchical self-assembly of nematic colloidal superstructures, Phys. rev. E 77, 061706 (2008),

Evolutionary strategies for solving extremely complicated NMR spectra of solutes in nematic solvents

C.A. de Lange

Laser Centre, Vrije Universiteit De Boelelaan 1081, 1081 HV Amsterdam

The Netherlands

E.E. Burnell Chemistry Department, University of Britisch Columbia

2036 Main Mall, Vancouver (B.C.) V6T 1Z1 Canada

W.L. Meerts

Molecular- and Biophysics Group, Institute for Molecules and Materials Radboud Universiteit Nijmegen, P.O. Box 9010, 6500 GL Nijmegen

The Netherlands The complexity of NMR spectra of solutes in partially ordered solvents such as liquid crystals increases rapidly with the number of spins. Spectra of simple solutes with sufficient symmetry and containing not too many spins (typically ~ 6) are readily analysed. The analysis of larger spin systems is more difficult, and often impossible. In this contribution the application of a general automated Evolutionary Algorithm to solving the highly complex proton NMR spectrum of the twelve-spin system pentane will be presented. This molecule interconverts rapidly among several symmetry-unrelated conformations on the NMR time scale. The interpretation of the spectral parameters that are obtained from the analysis requires the use of a model to connect relative orientational orders in symmetry-unrelated conformers.

International workshop, “Liquid Crystal Phases and Nanostructures”, Erice, Italy, 27 Oct.-1 Nov. 2008

RECENT ADVANCES IN THE DYNAMICS OF CONFINED NEMATIC LIQUID

CRYSTALS WITH DEFECTS AND BOUNDARY NON-HOMOGENEITIES

A. F. Martins* and A. Véron

Depart. Materials Science, The New University of Lisbon, 2829-516, Caparica, Portugal. (* e-mail: asfm@fct.unl.pt)

Abstract In spite of intense research and significant progress in the last thirty years, some aspects of the dynamics of real nematic liquid crystals confined between parallel plates (LCD geometry) are still not clear. In particular, local non-homogeneities in the boundary alignment of the molecules may have a significant effect on the electric field driven orientational dynamics that has not been recognized until recently [1]. This effect will be discussed in the present communication. We present 2-D computer simulations of the electric field driven director dynamics in thin nematic samples confined between parallel plates, in the presence of topological defects or boundary or volume non-homogeneities in the sample. Deuterium NMR spectra directly computed from the numerical results are good agreement with the experimental data [2]. We propose that in addition to the well known collective modes of reorientation (observed simultaneously over the whole sample), a qualitatively different mode of reorientation, that we may call progressive mode, exists and may dominate the director reorientation. This mode has now been observed in thin samples of low-molecular mass liquid crystals [2] as well as in polymeric systems [3]. The progressive mode originates in the local non-homogeneities and then propagates through the whole sample building up a periodic pattern. Its deuterium NMR signature – a doublet with time-dependent intensity and constant splitting [1,2] - is clearly distinct from that of collective modes - a doublet with time-dependent splitting and constant intensity [4-6].

[1] A.F. Martins, A. Véron, Thin Solid Films (2008), doi: 10.1016/j.tsf.2008.09.052. [2] G.R. Luckhurst, A. Sugimura, B.A. Timimi and H. Zimmermann, Liquid Crystals 32,

1389 (2005). [3] A.F. Martins et al., to be published. [4] A.F. Martins, P.Esnault and F. Volino, Phys. Rev. Lett. 57, 1745 (1986). [5] A. Véron, A.E. Gomes, C.R. Leal, J. vd Klink, A.F. Martins, Mol. Cryst. Liquid Cryst.

331, 499 (1999). [6] A.F. Martins, A.E. Gomes, A. Polimeno, L. Orian, Phys. Rev. E 62, 2301 (2000).

Nematic fluctuations and semi-soft elasticity in liquid crystal elastomers

Martin Čopič1,2 and Andrej Petelin2

1Faculty of Mathematics and Physics, University of Ljubljana

Jadranska 19, SI-1000 Ljubljana, Slovenia

2J. Stefan Institute Jamova 39, SI-1000, Ljubljana, Slovenia

In liquid crystal elastomers the nematic director is coupled to the strain of the polymer network. This leads to some interesting properites of the LC elastomers like very large length change with temperature, potentially useful as artificial muscles, and soft elasticity. With light scattering we measured the relaxation rate of the orientational fluctuations which is finite at zero wave vector due to the internal stress, frozen in the sample at the time of preparation. There is a weak fast signal also above the nematic-isotropic transition showing that there is a residual nematic order also in the high temperature phase, in aggreement with other experiments showing that the system is above critical point. When the strain is applied perpendicular to the director, so that the elastic response is semisoft, the relaxation rate of the director flucutations decreases to a very low value. The data are in good agreement with the theory of semisoft elasticity.

An atomistic model for the elastic constants of nematics

Mirko Cestari, Alessandro Bosco¶ and Alberta Ferrarini

Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy

¶ present address: SISSA, via Beirut 2-4, Trieste, Italy e-mail: alberta.ferrarini@unipd.it

Several molecular models for the elastic constants of nematics have been presented, since

the early work of Nehring and Saupe,1 and have contributed to shed light on the connection between the intermolecular interactions and the elastic response of liquid crystals to director distortions [for a review see ref. 2]. However, the prediction of elastic coefficients on the basis of the molecular structure remains a challenge. This is a difficult task even for Molecular Dynamics techniques, since atomistic simulations of liquid crystals are very demanding and suitable theoretical models are needed to link long-scale deformations to observables on the length-scale of the simulation box.3 Modeling approaches, allowing for a realistic account of the molecular structure, can be a useful complement to simulations.

We have derived molecular expressions for the elastic constants of nematics on the basis of the so called 'Surface Interaction' (SI) model, which was developed some years ago and since then successfully used to predict order and thermodynamic properties of nematics,4 as well as cholesteric induction5 and flexoelectric coefficients.6 Within this framework, the anisotropy of the molecular field experienced by molecules in the nematic environment is described in terms of the coupling between the molecular surface and the field of the nematic director. The chemical structure of mesogens can be introduced through a suitable representation of the molecular surface. In this way, molecular-scale features and long-scale elastic deformations are simultaneously taken into account. The elastic free energy is derived from the orientational distribution function in the presence of director distortions. The molecular flexibility can be easily handled, which is important for mesogenic systems. The method has been applied to typical mesogens; elastic constants have been calculated by averaging over conformers with geometries calculated at the quantum mechanical level7 and the Sanner representation of the molecular surface.8 In this communication, the derivation of molecular expressions for the elastic constants will be presented and the results obtained for PAA and 5CB will be discussed, highlighting the role of features like molecular shape and rigidity.

1. J. Nehring, A. Saupe, J. Chem. Phys. 54, 337 (1971); 56, 5527 (1972). 2. M.P. Allen, A.J. Masters, J. Mat. Chem. 11, 2678 (2001); M.R. Wilson, Int. Rev. Phys. Chem.

24, 421 (2005). 3. S. Singh, Phys. Rep. 277, 283 (1996). 4. A. Ferrarini, G.J. Moro, P.L. Nordio, G.R. Luckhurst, Mol. Phys. 77, 1 (1992). 5. A. Ferrarini, G.J. Moro, P.L. Nordio, Phys. Rev. E 53, 681 (1996). 6. A. Ferrarini, Phys. Rev. E 64, 021710 (2000). 7. M.J. Frisch et al , Gaussian 03, Revision C.02 (Gaussian Inc., Wallingford CT, 2004). 8. M.F. Sanner, J.C. Spehner, A.J. Olson, Biopolymers 38, 305 (1996).

Simulation studies of structure and organisation in chromonic phases and macromolecular liquid crystals.

M. R. Wilson, J. S. Lintuvuori and F. Chami

Department of Chemistry, Durham University,

Science Laboratories, South Road, Durham, DH1 3LE, UK

1) Molecular order within a chromonic phase is not well understood. From system to system, little is known about how molecules order. Moreover, we know little about the strength of interactions between molecules in stacks or the ordering of water molecules around stacks. We presents new results from an atomistic simulation study designed to answer some of these fundamental questions. Results are presented from atomistic simulations of a chromonic stack for the molecule edicol (sunset yellow) in water, looking at the ordering of edicol molecules in the columns, the structure of the bulk phases and the free energy change on binding to a chromonic stack. Results are also presented for quantum predictions of NMR shielding. Simulation results are linked to recent experimental measurements in solution, which aim to providing a fundamental understanding of how self-assembly occurs in chromonics.

2) Macromolecular liquid crystals composed of polyphilic segments are of considerable importance in soft matter chemistry. In such systems, unfavourable molecular interactions between different parts of a molecule can lead to nanophase/microphase separation and the formation of a range of fascinating structures. Careful, tuning molecular interactions provides the possibility to produce self-assembled structures for future molecular electronic devices or for biomimetic applications. We present new coarse-grained potentials, which allow us to see molecular self-assembly in complex architectures. We show results for single site systems as well as for a multipedal liquid crystal. For the latter, we see spontaneous self assembly into a lamellar phase, with a coupled change in molecular structure. We show also how the same methodology can be applied to a range of other macromolecular systems.

3) We show also how our new coarse-grained potentials can be combined with the recently developed technique of Statistical Temperature Molecular Dynamics (STMD). The latter uses the relation between statistical temperature and density of states; effectively combining multicanonical molecular dynamics with Wang-Landau sampling by dynamic update of the statistical temperature estimate. This can be used as a way of sampling a temperature window efficiently to identify different mesophases; and also of providing free energies and entropies, which are difficult to obtain by conventional simulation methodologies.

----------------------------------- 1 Hughes, Z. E., Stimson, L. M., Slim H., Lintuvuori, J, S., Ilnytskyi, J. M., Wilson, M. R., Comput. Phys. Comm., 2008, 178, 724. 2 Lintuvuori, J. S., Wilson, M. R., J. Chem. Phys., 2008, 128, 044906. 3 Kim, J., Straub, J. E., Keyes, T., Phys. Rev. Lett., 2006, 97, 05061, Kim, J., Straub, J. E., Keyes T., J. Chem. Phys., 2007, 126, 135101, Kim, J., Straub, J. E., Keyes, T., Phys. Rev. E., 2007, 76, 011913.

Computer simulation of columnar phases in T and X shaped bolaamphiphiles

Martin A. Bates and MartinWalker Department of Chemistry, University of York, York YO10 4HU, UK.

Dissipative particle dynamics simulations are used to investigate the structure of the columnar phases in T and X-shaped bolaamphiphiles. These amphiphilic molecules consist of a rod-like aromatic core, with a polar group at each end. A single lateral chain is present in the T-shaped molecules and two lateral chains are present for X-shaped molecules. The phase behaviour is examined as a function of the length of the lateral chain(s). The simulations indicate that both types of molecules exhibit columnar phases, in which the walls of the columns are formed by the aromatic core, the polar groups are located at the edges and the lateral chains fill the interiors of the columns. Square columnar phases are observed for short chains and hexagonal lamellar phases are observed as the length of the chain is increased, due to the larger volume necessary for the chains. Lamellar phases are observed on increasing the length further. For the T-shaped molecules, the columns are always double walled (Figure 1). In contrast, for the X-shaped molecules, the columnar phases are single walled, in agreement with X-ray diffraction studies. For the X-shaped bolaamphiphiles, we also examine a model that corresponds to fluorinating one of the chains, leading to a larger repulsion between the chains. For the square phase, local demixing does not disrupt the structure of the columns. However, the hexagonal phase cannot be ‘coloured’ with two different types and so is frustrated. We will discuss various outcomes of this frustration, and how these relate to the molecular structure.

Figure 1: Double and single walled hexagonal columnar phases for T and X-shaped bolaamphiphiles.

Computer simulation of ordered fluids and soft solids Doug Cleaver

Head of Materials Modelling Group and Head of Postgraduate Research Materials and Engineering Research Institute

Sheffield Hallam University Howard Street

Sheffield, S1 1WB, UK

Soft solid cubic phases have been observed experimentally in lyotropic and thermotropic liquid crystals and in block copolymers. We have now succeeded in observing free formation of this phase using MD and MC simulations of remarkably simple molecular systems [1]. From these, we can see that, on a molecular scale, there is very little to distinguish soft solids from ordered fluid phases. Here we will first discuss the types of model with which we have observed soft solid behaviour (so far), and probe which observables offer the best indications of their formation. We will then discuss which routes are most likely to yield such phases for more complex simulation models. ----------------------------------- [1] Ellison, LJ, Michel, DJ, Barmes, F; Cleaver DJ. Phys. Rev. Letts, 2006, 97, 237801.

Computer simulations of biaxial nematics

Silvia Orlandi, Luca Muccioli, Matteo Ricci, Roberto Berardi, and Claudio Zannoni

Dipartimento di Chimica Fisica e Inorganica, and INSTM–CRIMSON,

Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy

The recently discovered biaxial nematic phase is an example of the new mesogenic materials which are reinvigorating research, both at fundamental and industrial levels, in view of designing faster and less energy consuming electro-optical devices and displays. In this talk we report results from molecular dynamics computer simulations of static and dynamic properties of model biaxial nematic mesogens in the bulk and confined between planar surfaces [1].

[1] R. Berardi, L. Muccioli, S. Orlandi, M. Ricci and C. Zannoni, “Computer simulations of biaxial nematics”, J. Phys. Condens. Matter, accepted (2008).

Dynamics of liquid crystal colloids studied by 3D microscopy Oleg D. Lavrentovich Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, OH 44242 Liquid crystals (LCs) produce a rich variety of complex 3D structures that are easily changed by external fields, boundary conditions, embedded particles, etc. In the latter case of the liquid crystal with colloidal inclusions (liquid crystal colloid), the equilibrium is often achieved by introducing topological defects in the director field, stabilized by surface anchoring at interfaces. This report presents recent experiments on the dynamics of micron-size colloids in the nematic and smectic liquid crystals driven by AC and DC electric fields. The 3D director structure around the particles is determined by fluorescence confocal polarizing microscopy (FCPM). Elastic distortions around the colloid cause repulsion between the particle and bounding walls; these forces are typically strong enough to overcome gravity and to keep the particle levitating in the liquid crystal bulk. The electric field causes translational and rotational motion of the colloids; the underlying mechanisms include specific liquid crystal effects such as backflow (coupling of director reorientation and mass flow) and mechanisms of general nature, valid in both anisotropic and isotropic fluids, such as electrorotation and hydrodynamic interaction between the particle and a confining wall. The direction of particle motion can be controlled by the symmetry of director distortions, opening the door for a variety of colloidal motors. The work has been supported by NSF DMR 0504516, DOE, W.M. Keck Foundation, CMPND.

Poly-domain to mono-domain phase transitions and field-induced macroscopic biaxial order in nematics

S.D. Peroukidis, P.K.Karahaliou, A.G. Vanakaras and D.J. Photinos

Department of Materials Science, University of Patras, 26504 Patras, Greece The recent experimental observation of phase biaxiality in thermotropic nematic liquid crystals, first in bent-core nematogens [1] and shortly afterwards in radial tetramers of laterally tethered rod-like nematogens [2], has motivated intensive research in the directions of (i) studying key physical properties of these systems for electro-optic applications [3] (ii) synthesis of other thermotropic biaxial nematic compounds [4] and (iii) molecular theory and computer simulation studies for exploring the ordering [5] and the dynamic response [6] of these systems in relation to their molecular structure. The major objective regarding the technological potential of biaxial nematics is to produce room temperature biaxial nematics that would be suitable for use in novel electro-optic devices exploiting the speed advantage of the “short” director reorientations over the presently used operating modes of conventional (uniaxial) nematic LCs which are based on the reorientations of the primary director n. Besides the pursuit of these technological goals, the investigations so far have given rise to certain issues of fundamental significance to LC science, such as the identification of experimentally measurable signatures of nematic phase biaxiality, the analysis of conditions for the achievement of monodomain biaxial samples and the related question of whether the measured macroscopic biaxial order is not induced by these conditions. In addition, the relation between the observed biaxiality in bent-core nematics and the possible existence of transverse polar ordering is still unresolved. On the other hand, the findings of the fist electro-optic biaxial response experiments [3] are strongly suggestive of a picture according to which the observed macroscopic biaxial ordering is a result of the externally induced (field or surface alignment) mutual alignment of existing biaxial microscopic domains which, in the absence of an external aligning mechanism, are uniformly distributed about the primary director n. This has led us [7] to the formulation of a phenomenological theory for the field-induced phase transitions from a phase of uniaxially distributed biaxial domains to a macroscopically biaxial nematic phase. In this work we show that all the available experimental results on the observation of biaxiality in bent-core nematics can be consistently interpreted with this picture. We extend the theory to treat the more general case of coexisting biaxial and polar order, which is argued to be of direct relevance to the bent-core systems. Further, we consider the different symmetries that are compatible with nematic biaxiality and their implications on the measurements of biaxial order parameters by the various experimental techniques that require aligned samples. We show that these implications can lead directly to the experimental discrimination between biaxial nematic phases in which n is a twofold symmetry axis from those in which it is not. Acknowledgement: This work is supported by the project BIND-216025 (Biaxial Nematic Devices), FP7 / ITC-1-3.2 / STREP-CP-FP-INFSO. References [1] L.A.Madsen et al, Phys. Rev. Lett, 92, 145506 (2004). [2] K.Merkel et al, Phys. Rev. Lett., 93, 237801 (2004); J.L.Figueirinhas et al, Phys. Rev. Lett., 94, 107802 (2005). [3] Lee et al., J. Appl. Phys., 101, 034105 (2007). [4] M.Lehmann et al, Chem. Com. (15), 1768 (2008). [5] J. Pelaez, M. Wilson, Phys. Rev. Lett., 97, 267801 (2006). [6] R. Berardi, L. Muccioli, and C. Zannoni, J. Chem. Phys., 128, 024905 (2008). [7] A.G. Vanakaras and D.J. Photinos, J. Chem. Phys., 128, 154512 (2008).

Biaxiality in the Bent-Core ODBP Mesogens

Edward T. Samulski1, T. J. Dingemans

2, and L.A. Madsen

3

1

Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599 USA, e-mail: et@unc.edu

2 Delft University of Technology, Aerospace Engineering, Delft, The Netherlands,

e-mail: T.J.Dingemans@LR.TUDelft.NL 3 Department of Chemistry, Virginia Tech University, Blacksburg, VA 24061 USA,

e-mail: lmadsen@vt.edu

Abstract Forty years ago Marvin Freiser

1 suggested that a biaxial

nematic phase could in principle exist for monomeric calamitic liquid crystals—molecules with a single, biaxially-shaped mesogenic core. Today, this deceptively simple suggestion continues to challenge us. Bent-core calamitics based on the oxadiazole heterocycle

2 appear to exhibit Nb phases as

evidenced by deuterium NMR3. However, to date reports of

Nb phases for ODBP mesogens employed a labeled probe (hexamethylbenzene-d18) dissolved (~1-2 wt%) in the LC phases. We will present new data on deuterium-labeled ODBP mesogens, including some with labels on the core like ODBP-Ph-d4-OC12

as well as mesogens with terminally labeled

tails such as ODBP-Ph-C7-d3 (see phase map below). Marvin Freiser ~ 1969

[1] M. J. Freiser, Phys. Rev. Lett., 1970, 24, 1041. [2] (a) K. Semmler, T. J. Dingemans, and E. T. Samulski, Liq. Cryst., 1998, 24, 799. (b) T.J.Dingemans and E. T. Samulski, Liq. Cryst., 2000, 27, 131. (c) T.J. Dingemans, N. S. Murthy, and E. T. Samulski, J. Phys. Chem. B, 2001, 105, 8845. [3] (a) L. A. Madsen, T. J. Dingemans, M. Nakata, and E. T. Samulski, Phys. Rev. Lett., 2004,

92, 14505. (b) B. R. Acharya, A. Primak, T. J. Dingemans, et al., Pramana J. Phys. 2003, 61,

231, and Phys. Rev. Lett. 2004, 92, 145506.

From Models to Molecules – in Search of the Biaxial Nematic Phase

Duncan W. Bruce

Department of Chemistry, University of York, Heslington, YORK YO10 5DD, UK

db519@york.ac.uk

One of the joys of working as a synthetic chemist within networks and collaborations of various types has been, and continues to be, interactions with those concerned with theory, modelling and simulation. Their models provide thought-provoking challenges which, as synthetic chemists, we do our best to try to make reality.

Perhaps one of the greatest challenges laid down both from theory and in silico has related to the biaxial nematic phase. Predicted more than 30 years ago, its existence is known beyond doubt, yet it has been, and probably remains, an area of controversy and many good and respected authors have found grief of one sort or another in trying to show the existence of the phase.

We have been interested in the biaxial nematic phase for some time and have often taken our inspiration and design from the work of those who would seek to predict how the phase might be found. This talk will give a brief overview of some of that work and will then concentrate on two systems, namely non-covalent bent-core materials and covalently linked rod-disc dimers.

3-D ordering of mesogen covered gold naoparticles.

L.. Cseh1, X. Zeng2, F. Liu2, A. G. Fowler2, G. Ungar2, J. E.Macdonald3, G. H. Mehl1

1Dept. of Chemistry, University of Hull, Hull HU6 7RX; 2Dept. of Engineering Materials, University of Sheffield, Sheffield UK; 3School of Physics and Astronomy, Cardiff University, Cardiff .UK A considerable research effort has been directed towards the investigation of spherical colloidal particles dispersed in low molecular liquid crystals. The spatial arrangement of the particles has often been manipulated by external fields or laser tweezers resulting in chains or raft like assemblies. The interest in smaller nanometric sized particles in LC matrices has been driven, in the main, by their ability to alter the dielectric behaviour of LCs, potentially resulting in faster switching devices. The investigation of small LC functionalized nanoparticles leading to systems where the self assembly behaviour is determined by an interplay of the particles’ structure and the self-assembly behaviour of mesogens is surprisingly limited. Either the LC behaviour of mesogen functionalized nanoparticles was not fully determined, or occurred only in mixtures or cubic lattices expected for the packing of spherical particles were detected. ref We reported recently on a number of mesogen-covered gold nanoparticle systems were combinations of mesogen size, spacer length and mole fraction of unsubstituted alkylthiol were varied. A calamitic mesogen motif with a lateral connection to the nanoparticles was selected as such systems promote either nematic phase behaviour at low temperatures or columnar assemblies. Based on DSC and OPM experiments nematic phase behaviour of the mesogens functionalized nanoparticle structure was detected However detailed X-ray diffraction experiments carried out on powder and oriented samples permitted to detect the formation of a more complex self-assembly behaviour. The calculation of electron density maps allowed the identification of superlattices, where the nanoparticles are organised in 3D strings embedded in a nematic like matrix, indicating a delicate balance in the interplay of the self assembly behaviour of the nanoparticles and the nematic mesogens.

Nanostructured surfaces for tailored anchoring of liquid crystals

Hiroshi YOKOYAMA

Nanotechnology Research Institute

and Liquid Crystal NanoSystem

Tsukuba, Japan Surface alignment of liquid crystal orientation at the cell wall is critical for functionality of liquid crystal devices. In contrast to the conventional approach pursuing uniform and robust alignment, the advent of nanotechnology has opened up a novel area of nano-engineering of surface alignment. Nanoscopic design and fabrication of patterned alignment surfaces (sometimes including the tuning of the anchoring strength as well as orientation) allow the surface alignment to be actively functional realizing, for example, multistability that leads to a substantial reduction of power consumption. This talk will give the historical and technological background, theoretical and design principles, and device implementation by emerging nanotechnologies.

Disorder effects at the HexB-SmA transition in 65OBC Liquid Crystal

F. Mercuri, S. Paoloni, U. Zammit and M.Marinelli

Dipartimento di Ingegneria Meccanica – Università di Roma “Tor Vergata”

The universality class of the HexB-SmA is a long standing puzzle. If one consider the Hexatic order parameter, it should belong to the 3D XY class, but experimental results do not confirm this hypothesis. Specific heat measurements, in particular, gives a critical exponent

, that is very far from the expected value of An attempt to explain this discrepancy has been the one in which an additional order parameter, due to the in-plane herringbone ordering of the molecules, couples to the Hexatic one. The main results of this approach is that the transition should be weakly first order, but cp measurements in compounds in which the herringbone ordering is not present, give similar discrepancy. On the other hand, recent results demonstrate that the transition is indeed a weakly first one and a quasitriciritcal behavior has been suggested, though the value for the critical exponent is close to a tetracritical value.

We have performed high resolution calorimetric measurements on samples whose thicknesses were in the micrometer range. It turns out that there is a subtle influence of the amount of disorder on the critical exponent. Annealing effects do take place heating and cooling a bulk

sample around the transition temperature but, again, . Measurements have been then performed in not aligned and monodomain sample and the thickness of the cell has been varied from 10 to 100 µm. It has been found that in same particular experimental conditions (thin and

aligned sample), , a value that is between the one of a bulk sample and free standing

films ( ). We speculate that this is due to a coupling between the hexatic order parameter and the in-plane strain and that this coupling can be modulated by a thermal annealing and by an aligning field. According to this hypothesis it turns out that appropriate experimental conditions are crucial for any attempt to establish the universality class of the HexB-SmA transition.

Complexity in Liquid Crystals

Carsten Tschierske 1, Robert Kieffer 1, Benjamin Glettner 1, Marko Prehm 1, Karsten Pelz 1, Ute Baumeister 1, Goran Ungar 2, Feng Liu 2, Xiangbing Zeng 2

1 Institute of Chemistry, Martin-Luther University Halle-Wittenberg

Kurt-Mothes Str.2, 06120 Halle/Saale, Germany, e-mail: carsten.tschierske@chemie.uni-halle.de 2 Department of Engineering Materials and Centre for Molecular Materials, University of Sheffield,

Mappin Street, Sheffield S1 3JD, UK, e-mail: g.ungar@sheffield.ac.uk

Abstract We have recently introduced the concept of polyphilic molecular tectons to design new and complex types of organization in LC systems. Among them, there are liquid crystalline honeycomb-like arrays of triangular, square, pentagonal and hexagonal cylinders, which were formed by self-assembly of ternary block molecules. [1] More complex cylinder nets composed of cylinders with very different shape and having different materials in the interior of the cylinders were now achieved by using molecular tectons composed of four instead of only three incompatible segments (quaternary block molecules). These mesophases can be described by a multi-color tiling of space. Among these new structures there is the liquid crystalline Kagome (a periodic packing combining hexagonal and triangular cylinders), a multi-color hexagonal super lattice and several other tiling patterns composed of up to five different types of columns forming one uniform superlattice. Acknowledgements This work was supported by the ESF in the framework of the EUROCORES project SONS. References [1] (a) X.-H. Cheng, M. Prehm, M. K. Das, J. Kain, U. Baumeister, S. Diele, D. Leine, A. Blume, C.

Tschierske, J.Am. Chem. Soc., 2003, 125, 10977; (b) B. Chen, X. Zeng, U. Baumeister, G. Ungar, C. Tschierske, Science, 2005, 307, 96; (c) M. Prehm, F. Liu, U. Baumeister, X. Zeng, G. Ungar, C. Tschierske, Angew. Chem. Int. Ed. 2007, 46, 7972; (d) C. Tschierske, Chem. Soc. Rev. 2007, 36, 1930.

Molecular Dynamics simulations of liquid-crystalline dendrimers phases

Daniel Guillon, Bertrand Donnio and Cyril Bourgogne Institut de physique et Chimie des Matériaux de Strasbourg

23, rue du Loess, BP 43 67034 Strasbourg Cedex 2, France

We report here a few examples of the self-organisation behaviour of some liquid crystalline dendrimers. The original design of the molecules imposes the use of all-atomic molecular dynamics methods to model correctly every intra- and intermolecular effects. The selected materials are octopus dendrimers with block anisotropic side-arms, segmented amphiphilic block codendrimers, multicore and star-shaped oligomers, fullero-dendrimers and multi-functionalised manganese clusters. The molecular organisation in lamellar or columnar phases occurs due to soft / rigid parts self-recognition, hydrogen-bonding networks or from the molecular shape intrinsically. All of these materials have been studied using the Discovery molecular mechanics software from Accelrys , the models were built as Periodic Boundary Condition (PBC) cells in pseudo-2D or 3D symmetries, according to X-rays experimental information. The energy relaxation and the MD thermodynamic equilibration of the systems were performed using some general purpose force fields available in the software (cvff, pcff or esff). References : L. Gehringer, C. Bourgogne, D. Guillon & B. Donnio, J. Am. Chem. Soc. 2004, 126, 3856 I. Bury, B. Heinrich, C. Bourgogne, D. Guillon & B. Donnio, Chem. Eur. J. 2006, 12, 8396 A. Zelcer, B. Donnio, C. Bourgogne, F. Cukiernik & D. Guillon, Chem. Mater. 2007, 19, 1992

Deuterium NMR study of the director dynamics for a low molar mass nematic

1Geoffrey R. Luckhurst, 2Akihiko Sugimura*, 1Bakir A. Timimi, 3Herbert Zimmermann

1School of Chemistry, University of Southampton,

Highfield, Southampton SO17 1BJ, United Kingdom 2Department of Information Systems Engineering, Osaka Sangyo University,

3-1-1 Nakagaito, Daito-Shi, Osaka 574-8530, Japan 3Max-Planck-Institut für Medizinische Forschung, Department of Biophysics,

Jahnstrasse 29, D-69120 Heidelberg, Germany

There have been many investigations of the alignment of nematic liquid crystals by either a magnetic and/or an electric field. There have also been studies of the effect of applying two constraints to a nematic sample, such as an electric field and a surface torque. As a result, the basic features of these hydrodynamic processes have been characterized for the systems in their equilibrium and non-equilibrium states. To complement the experiments theoretical models, based on continuum theory, have been developed which successfully describe these phenomena. Such macroscopic behaviour has been investigated with deuterium nuclear magnetic resonance (NMR) spectroscopy. This technique has proved to be especially important for the investigation of liquid crystals [1]. Since the quadrupolar splitting for deuterons observed in the liquid crystal phase is determined by the angle between the director and the magnetic field, time-resolved deuterium NMR spectroscopy has been employed to investigate the dynamic director alignment process in a thin nematic film following the application or removal of an electric field [2,3]. One of the prime advantages in the use of deuterium NMR spectroscopy to determine the director orientation is that the form of the spectrum is influenced by the distribution of the director with respect to the magnetic field. This situation obtains because when the director is not uniformly aligned the observed spectrum is a weighted sum of the spectra from all director orientations. For a number of years we have been studying such director dynamics for low molar mass nematics using deuterium NMR but with an electric as well as a magnetic field to rotate the director. At the school we shall describe recent progress in our experimental studies and discuss the unanswered questions created by the measurements of the director dynamics. [1] G. R. Luckhurst, T. Miyamoto, A. Sugimura, T. Takashiro, B. A. Timimi, J. Chem. Phys.,

114, 10493 (2001). [2] G. R. Luckhurst, T. Miyamoto, A. Sugimura, B. A. Timimi, J. Chem. Phys., 116, 5099

(2002). [3] G. R. Luckhurst, A. Sugimura, B. A. Timimi, H. Zimmermann, Liq. Cryst., 32, 1389 (2005).

Vibrational Circular Dichroism in Spontaneously Chiral Segregated Domains

of Ester Molecules

Hideo TAKEZOE Department of Organic and Polymeric Materials

Tokyo Institute of Technology Mail Box: S8-42

2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan We found spontaneous deracemization in liquid crystalline molecules containing ester moiety. In addition to CD measurements in thin cells and freely suspended films, VCD measurements were also conducted. We found several VCD peaks at ester-ralated vibrational wavenumbers. I will show these experimental data with calculated spectra and discuss whether or not we can infer the conformation of molecules in chiral domains.

What NMR of solutes in liquid-crystalline solvents can tell about the ordering potential

Elliott Burnell

Chemistry Department, University of British Columbia, Vancouver V6T 1Z1, Canada The NMR of orientationally ordered solutes in liquid crystalline solvents gives accurate values of order parameters, and these in turn can be interpreted in terms of the anisotropic intermolecular potential that causes the orientational ordering [1,2]. A wealth of experimental evidence leads to the conclusion that a single mechanism is incapable of describing solute orientational order in different liquid crystals. We shall demonstrate that experimental results can be rationalized in terms of two independent second-rank mechanisms whose relative importance varies with liquid-crystal solvent [3]. There is strong indication that the dominant mechanism involves short-range interactions that depend on solute size and shape. The second mechanism is less certain, and various possibilities will be discussed with the favourite involving the molecular quadrupole. Nematic liquid crystals exhibit orientational order while smectic liquid crystals exhibit, in addition, positional order. Obtaining information from NMR experiments about the positional order is challenging. We have applied the ideas of Kobayashi / McMillan theory for smectic ordering to the results obtained from a collection of solutes in several liquid-crystal solvents that exhibit both a smectic A phase and a higher-temperature nematic phase [4]. The main problem in the analysis involves extrapolating the nematic potential into the smectic A phase. We use the idea (above) that the nematic potential can be described in terms of a maximum of two second-rank interactions in order to obtain estimates for the positional order parameter and for the strength of the coupling between positional and orientational ordering. References

1] "Prediction from Molecular Shape of Solute Orientational Order in Liquid Crystals", E.E.

Burnell and C.A. de Lange, Chemical Reviews 98, 2359-2387 (1998). 2] NMR of Ordered Liquids, edited by E. Elliott Burnell and Cornelis A. de Lange, (Kluwer

Academic, Dordrecht, 2003). 3] “Mechanisms of solute orientational order in nematic liquid crystals”, E. Elliott Burnell,

Leon C. ter Beek and Zhengmin Sun, J. Chem. Phys. 128, 164901 (2008). 4] "The smectic potential in a liquid crystal with a reentrant nematic phase: NMR of solutes",

Anand Yethiraj, E. Elliott Burnell and Ronald Y. Dong , Chemical Physics Letters 441 , 245-249 (2007), and references therein.

E4 Computer Engineering S.p.A.

Technical Presentation

1

Molecular and Nano- Modified Liquid Crystalline Materials

Iam-Choon Khoo, J. Liou, J. Park, J. Huang, Y. Ma, M. V. Stinger and A. Diaz. Department of Electrical Engineering

The Pennsylvania State University University Park, PA 16802 USA

Abstract

In their many mesophases, liquid crystals possess extraordinarily large optical nonlinearities originating from individual or collective molecular processes [1]. In conjunction with nano-structures or nano-particulates, these materials exhibit not only greatly enhanced nonlinear optical responses, but also many emergent electronics and optical properties and functions not possible with other materials [2-7]. In this presentation, we will discuss examples of such liquid crystalline nano-metamaterials including: (a) bulk aligned nematic liquid crystals with randomly distributed nano-particulates; (b) periodic nano-structures with aligned nematic layer. These metamaterials exhibit emergent optical properties such as sub-unity and negative refractive indices, enhanced (electrically or optically) tunable birefringence, and ultrafast al-optical switching capabilities. In particular, we will discuss detailed theories and experimental observations of a nano-modified nematic liquid crystalline system capable of very broadband (from visible to infrared) all-optical switching with microseconds [8] and possibly sub-microseconds speed [9]. Recent experimental observations of all-optical switching via laser induced birefringence changes in ultra-thin (250 nm) nematic layer and their implications in tunable nano-structured metamaterials will also be discussed. References: 1. I. C. Khoo, Liquid Crystals, 2nd Edition (Wiley Inter-Science, NJ 2007). 2. I. C. Khoo, Jae-Hong Park and Justin Liou, Appl. Phys. Lett. 91, 143122 (2007). See also, I. C. Khoo, M. V. Wood, M. Y. Shih and P. H. Chen, Optics Express, Vol. 4, no. 11, pp 431-442 (1999). 3. E. Graugnard, J. S. King, S. Jain, C. J. Summers, Y. Zhang-Williams and I. C. Khoo, Phys. Rev. B72, 233105 (2005) 4. I. C. Khoo, D. H. Werner, X. Liang, A. Diaz and B. Weiner, Optics Letts. 31, 2592 (2006); 5. I. C. Khoo, A. Diaz, S. Kubo, J. Liou, Mike Stinger, T. Mallouk and J. H. Park, Molecular Crystals Liquid Crystals 485:1, pp. 934-944 (2008). 6. A. Diaz, J. H. Park and I. C. Khoo, JNOPM Vol. 16, pp 533 – 549 (2007). 7. Bossard, J. A., Liang, X., Li, L., Werner, D. H., Weiner, B., Cristman, P. F., Diaz, A., & Khoo, I. C. (2008), IEEE Transactions on Antennas and Propagation, 56, pp. 1308 - 1320 (2008). 8. Iam-Choom Khoo, Jae-Hong Park and Justin D. Liou, J. Opt. Soc. Am. B [Accepted for publication, 2008]. 9. see for example, I. C. Khoo and R. Normandin, Opt. Letts., 9, 285-287 (1984); I. C. Khoo, R. G. Lindquist, R. R. Michael, R. J. Mansfield and P. Lopresti, J. Appl. Phys., 69, 3853 (1991); T. Ikeda and A. Shishido et al, J. Am. Chem. Soc. 119 pp. 7791-7796 (1997).

Complex photonic structures in soft materials.

Giancarlo Abbate Dipartimento di Scienze Fisiche Università di Napoli Federico II

Via Cintia Monte S.Angelo I-80126 Napoli, Italy

Multiple-beam holography has been widely used for the realization of photonic quasicrystals with high rotational symmetries not achievable by the conventional periodic crystals. Accurate control of the properties of the interfering beams is necessary to provide photonic band gap structures. Here we show, by FDTD simulation of the transmission spectra of 8-fold quasiperiodic structures, how the geometric tiling of the structure affects the presence and properties of the photonic band-gap for low refractive index contrast. Hence, we show a novel approach to the fabrication of photonic quasicrystals based on the use of a programmable Spatial Light Modulator encoding Computer-Generated Holograms, that permits an accurate control of the writing pattern with almost no limitations in the pattern design. Using this single beam technique we fabricated quasiperiodic structures with high rotational symmetries and different geometries of the tiling, demonstrating the great versatility of our technique.

Electro-optical properties of liquid crystal dispersions

Fiore Pasquale Nicoletta Dipartimento di Scienze Farmaceutiche

Università della Calabria Liquid crystal dispersions show interesting electro-optical properties. They have attracted the interest of researchers in recent years from both a basic and an applicative point of view. The operation principle of a liquid crystal dispersion film is the electrically controlled re-orientation of liquid crystal directors. Upon application of a suitable electric field, liquid crystal directors will change their original alignment and the device will be characterized by a different electro-optical state. It is known that the electro-optical performance of liquid crystal dispersions is dependent on several parameters including domain size and shape, chemical-physical properties of components, and liquid crystal boundary conditions. In this talk some ways to control the electro-optical response of liquid crystal dispersions will be reviewed.

POLICRYPS structure and applications

C. Umeton1,

R. Caputo1, A. De Luca1, L. De Sio1, L. Pezzi1, A. Veltri1, R. Asquini2, A. d’Alessandro2, D. Donisi2, R. Beccherelli3,and A.V. Sukhov4

1Laboratorio Italiano di Cristalli Liquidi LICRYL, CNR-INFM,

Centro di Eccellenza Materiali Innovativi Funzionali CEMIF.CAL and Dipartimento di Fisica -Università della Calabria

87036 Arcavacata di Rende (CS), Italy

2Dipartimento di Ingegneria Elettronica, Università di Roma La Sapienza 00184 Roma, Italy

3Istituto per la Microelettronica e Microsistemi, CNR-IMM

00133 Roma, Italy

4Institute for Problems in Mechanics, Russian Academy of Science Moscow 119526, Russia

Abstract POLICRYPS is a nano/micro composite structure made of slices of almost pure polymer alternated to films of well aligned liquid crystal (LC). The structure is obtained by irradiating a homogeneous syrup of LC, monomer and curing agent molecules with an interference pattern of UV/visible light under suitable experimental and geometrical conditions; the spatial periodicity can be easily varied from almost nanometric (200 nm) to micrometric (15 µm) scale. Where the effect on an impinging light beam is concerned, the POLICRYPS can be utilized both in transmission and reflection (depending on geometrical configuration and used substrate) with negligible scattering losses, while the effect of a spatially modulation of the refractive index (from polymer to LC) can be switched ON and OFF by application of an external electric field of the order of few V/µm. We review: - The general features of the POLICRYPS structure, that is the “recipe” to fabricate them, along with a chemical – diffusive model that indicates the best conditions to make samples with good morphological, optical and electrooptical properties; - Utilisation of POLICRYPS with a light beam impinging almost perpendicularly to the structure: switchable diffraction grating, switchable optical phase modulator; - Utilisation of POLICRYPS with a light beam impinging parallel to the structure and to the channels: switchable array of waveguides for discrete diffraction and discrete solitons, tunable array of microlasers; - Utilisation of POLICRYPS with a light beam impinging parallel to the structure and perpendicular to the channels: tunable Bragg filter, sensor.

LIQUID CRYSTALLIZATION AND PHASE SEPARATION IN CONCENTRATED SOLUTIONS

OF ULTRASHORT DNA AND RNA OLIGONUCLEOTIDES

Tommaso Bellini1, Marco Buscaglia1, Giuliano Zanchetta1, Michi Nakata2, Noel A. Clark2

1 - Dipartimento di Chimica, Biochimica e Biotecnologie per la Medicina, Università di Milano,

Milano, Italy.

2 - Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, CO 80309-0390, USA. Ultrashort complementary DNA and RNA oligomers, down to 6 base pairs in length, are found to exhibit nematic and columnar liquid crystal phases, even though such duplexes lack the shape anisotropy required for liquid crystal orientational ordering. These phases are produced by the end-to-end adhesion and consequent living polymerization of the duplex oligomers into polydisperse anisotropic rod-shaped aggregates, which can order. Upon cooling mixed solutions of ultrashort oligonucleotides, in which only a fraction of the sample is composed of complementary sequences, the duplex-forming oligomers phase separate into liquid crystal droplets, leaving the unpaired single strands in isotropic solution. We interpret this behavior, that we find also in mixtures of DNA sequences and PEG, as the combined result of the energy gain from the end-to-end stacking of duplexes and of depletion-type interactions favoring the segregation of the more rigid duplexes from the flexible single strands or PEG chains. This new form of spontaneous partitioning of complementary nDNA can be extended to solutions of ultrashort oligonucleotides with random sequences, resulting in an intriguing randomness-length phase diagram. The condensation of liquid crystallites of complementary sequences from random environment appears a new route to purification of short duplex oligomers and, if in the presence of ligation, could provide a mode of positive feedback for the preferential synthesis of longer complementary oligomers, a mechanism of possible relevance in prebiotic environments.

Robust spatial optical solitons in nematic liquid crystals: light trapping, routing and steering

Gaetano Assanto

Department of Electronic Engineering

University of Rome "Roma Tre" Via della Vasca Navale, 84 - 00146 - Roma – Italy

Self-confined light beams can effectively form and propagate in nematic liquid crystals. Such solitary waves or nematicons are robust against perturbations; hence, they can readily be deviated, refracted or reflected. Nematicons can even get trapped and/or released from a potential.

G.J. Vroege Van ’t Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, The Netherlands Goethite nanorods and their modifications: extraordinary mineral liquid crystals Goethite (α-FeOOH) can be synthesized as boardlike crystalline nanorods (of typical dimensions 200 x 40 x 20 nm3 which can be varied over a wide range). These particles can be stabilized electrostatically in water of pH=3. Dispersions of these particles are nice examples of mineral liquid crystals, forming nematic, columnar [1] and smectic [2] liquid crystal (LC) phases. We employed (microradian) small-angle x-ray diffraction at 2 different beamlines of the European Synchrotron Radiation Facility to investigate these different structures. The original particles are very polydisperse (above 50%), but alternative synthesis methods and repeated centrifugation steps can be used [3] to reduce the polydispersity to about 15%. We will discuss the important role sedimentation and fractionation play on the occurrence of the different LC phases for systems of different polydispersities. Since all 3 dimensions of the particles are considerably different, in some systems biaxial LC phases could be expected of which we will present a preliminary measurement. Finally, Goethite particles also show interesting magnetic properties. They possess a permanent magnetic moment along their long axis combined with an induced moment with an easy axis along the shortest particle dimension. This combination leads to an extreme sensitivity to magnetic fields and peculiar re-orientation phenomena when the strength of the external magnetic field is varied. We show this can even lead to magnetic-field induced phase transitions. [1] B. J. Lemaire et al, Phys. Rev. Lett. 88, 125507 (2002). [2] G. J. Vroege et al, Adv. Mater. 18, 2565 (2006). [3] D. M. E. Thies-Weesie et al, Chem. Mater. 19, 5538 (2007).

About the Nature and the Role of Long-range Orientational Interactions in Nematic Mesophases Giorgio Celebre, Andreea Ionescu LXNMR_S.C.An. Laboratory - Dipartimento di Chimica, Università della Calabria, 87036, Rende (CS), Italy. The property of orientational ordering represents the peculiar feature of liquid crystalline systems, and the full understanding of the underlying mechanisms is very important both for theoretical advancements and technological applications in a host of fields. An effective way to study the ordering is by LXNMR spectroscopy of small probe-molecules dissolved in nematic solvents. Starting from a mean-field interpretation of the order parameters of several small solutes in a certain number of different nematics, it has been recently formulated the ansatz that just two independent mechanisms are sufficient to rationalize the ordering phenomena [1]. As a matter of fact, it is quite usual (since a long time) to adopt the approximation of describing the anisotropic orientational potential U(Ω) as a sum of two contributions [2]: a short-range term Usr(Ω) (effective at distances comparable to the molecular dimensions, accounting for size-and-shape interactions and commonly recognized as dominant [3a], at least for solutes larger than H2) and a second, longer-range potential Ulr(Ω), whose nature and role is not completely understood until now. Several mechanisms (based on electrostatic, induction and dispersion effects; see, for example, [2-7] and references therein) have been suggested (and often contradicted [7,8]) as responsible of this second contribution to the orientational ordering. One of the most known hypotheses (based on the Buckingham multipole expansion approach [3]) involves the interaction between the electric quadrupole moment (Q) of the solute and the (only nominal ?) Average Electric Field Gradient (AEFG) of the solvent [9,3]. This approach has been quite successful (for its simplicity and practical implications) but also debated and criticized (for its naïveté and conceptual weakness [6c,7,8]). In spite of its clear limitations, the crude model of “Q-AEFG“ coupling as describing the main long-range orientational interaction allowed for the very useful creation of the so-called “Zero-AEFG” nematic mixtures (nicknamed Magic Mixtures [3a]) where the practical experience shows that the ordering of the solutes (also larger than H2) can be predicted (within a 10% of error) by taking into account only short-range orientational interactions [3,10-12] (recently new possible Magic Mixtures have been conjectured [13] and an effective relationship between the so-called AEFG and the dielectric properties of the solvent has been obtained in an approximated way [14]). On the other hand, as said above, the simple “Q-AEFG“ mechanism suffers of manifest flaws and different criticisms have been raised to it in the past: 1) there are incontrovertible theoretical [15], experimental [5b,11,12] and computer simulation evidences [16,17] that the so-called AEFG is not an exclusive property of the solvent, but it depends also on the solute (in ref. [14] this dependence is obtained explicitly); 2) the value (and sometimes the sign) of the AEFG obtained from LXNMR data of small solutes dissolved in a given nematic solvent often disagrees with results from NMR of noble gas isotopes dissolved in the same solvent [8]; c) in dense systems, the convergence of the multipole expansion for the shortest solute-solvent distances represents a serious problem and it should be considered [3a,18] (a study about polar and apolar solutes in nematic solvents indicates that electrostatic contributions are operative over very short intermolecular distances [6c]: under this point of view, also the “long-range” definition conventionally given to these orientational interactions should be questioned). This implies the need of using distributed quadrupoles for the particles involved in MC simulations [18] (moreover, MC simulations by using particles with central point quadrupoles cannot predict negative order parameters of H2 and acetylene dissolved in EBBA [18]). All the points listed above are not necessarily independent; besides, some of them clearly depend on the strategic choice of adopting the simple and attractive (but sometimes, probably, too crude) mean-field approach to describe the orientational phenomena. Intrigued by all these contrasting evidences and looking for a “steady” point of reference to shed more light on the open questions, we decided to try to solve in a general way (starting from the first principles of classical electrostatics [19]) the fundamental problem of calculating the electric field gradient experienced by a highly idealized solute (represented by the equivalent series of point multipoles defined at a centre [20]) immersed in a

anisotropic uniaxial medium characterized by its dielectric tensor. In the case of a non-ionic and apolar solute, we deal (in a first approximation) with a point quadrupole (fully characterized by its tensor Q) positioned at the centre of a spherical (for simplicity) cavity of radius R (strictly speaking, in this description the quadrupolar solute is just a point singularity of the electric field [21]). We tackled the problem by using the generalized Reaction Field Model [2,14,22] (based upon the original ideas of Kirkwood [23] and Onsager [21]) in the linear response approximation [24]. There exist in literature studies about dipole-induced reaction fields in anisotropic media, also for non-spherical cavities (see, for example, ref. 19-23 reported in [14]); anyway, to the best of our knowledge, the general problem of quadrupole-induced reaction field in anisotropic media has never been solved before (in ref. [14] is reported an approximated solution of the problem for a particular case: an axial quadrupole representing the H2 molecule). By exploiting the theoretical apparatus developed in [24] to study the solvation in nematic phases comprised of polar mesogenic molecules and by resorting to the mathematical technique of working in the reciprocal (Fourier-tranformed) space, we were successful in solving this fundamental problem in a close, quite elegant form. The obtained formulas were used to calculate the Ulr(Ω) contribution for real cases and the theoretical predictions were compared with experimental results from literature about biaxial and uniaxial solutes in different nematic solvents [9,11,12]. Successes and failures of the approach in the predictions will be discussed at length in order to gather (if possible) useful hints about the general problem of orientational interactions. Acknowledgment: this work has been supported by MIUR (PRIN #2006034121) References: [1] E. E. Burnell, L. C. ter Beek and Z. Sun J. Chem. Phys., 128, 164901 (2008); [2] G. Celebre, G. De Luca and A. Ferrarini Mol. Phys., 92, 1039 (1997) and references therein; [3] a) E. E. Burnell, C. A. de Lange Chem. Rev. 98, 2359 (1998); b) E. E. Burnell, C. A. de Lange, Eds., NMR of Ordered Liquid, Kluwer: Dordrecht, The Nederlands (2003) and references therein; [4] a) A. di Matteo, A. Ferrarini, G. J. Moro J. Phys. Chem. B 104, 7764 (2000); b) A. di Matteo, A. Ferrarini J. Phys. Chem. B 105, 2837 (2001); [5] a) R. T. Syvitski, E. E. Burnell Chem. Phys. Lett. 281, 199 (1997); b) R. T. Syvitski, E. E. Burnell J. Chem. Phys. 113, 3452 (2000); [6] a) D. J. Photinos, C. D. Poon, E. T. Samulski and H. Toriumi J. Phys. Chem. 96, 8176 (1992); b) D. J. Photinos and E. T. Samulski J. Chem. Phys. 98, 10009 (1993); c) T. Dingemans, D. J. Photinos, E. T. Samulski, A. F. Terzis and C. Wutz J. Chem. Phys. 118, 7046 (2003); [7] A. F. Terzis and D. J. Photinos Mol. Phys. 83, 847 (1994); [8] J. Jokisaari, P. Ingman, J. Lounila, O. Pulkinnen, P. Diehl and O. Muenster Mol. Phys. 78, 41 (1993) and references therein; [9] G. N. Patey, E. E. Burnell, J. G. Snijders and C. A. de Lange Chem. Phys. Lett. 99, 271 (1983); [10] G. Celebre and G. De Luca J. Phys. Chem. B 107, 3243 (2003); [11] G. Celebre and G. De Luca Chem. Phys. Lett. 368, 359 (2003); [12] C. Aroulanda, G. Celebre, G. De Luca and M. Longeri J. Phys. Chem. B 110, 10485 (2006); [13] L. C. ter Beek and E. E. Burnell Chem. Phys. Lett. 426, 96 (2006); [14] G. Celebre J. Phys. Chem. B 111, 2565 (2007) + ADDITIONS AND CORRECTIONS J. Phys. Chem. B 111, 5773 (2007); [15] J. W. Emsley, W. E. Palke and G. N. Shilstone Liq. Cryst. 9, 643 (1991); [16] J. M. Polson and E. E. Burnell Phys. Rev. E 55, 4321 (1997); [17] E. E. Burnell, R. Berardi, R. T. Syvitski and C. Zannoni Chem. Phys. Lett. 331, 455 (2000); [18] J.S.J. Lee, R. O. Sokolovskii, R. Berardi, C. Zannoni and E.E. Burnell Chem. Phys. Lett. 454, 56 (2008) and references therein; [19] J. D. Jackson Classical Electrodynamics (2nd Edition) J. Wiley & Sons: New York (1975); [20] J. L. Rivail and D. Rinaldi Chem. Phys. 18, 233 (1976); [21] L. Onsager J. Am. Chem. Soc. 58, 1486 (1936); [22] V. Dillet, D. Rinaldi, J. G. Angyan and J. L. Rivail Chem. Phys. Lett. 202, 18 (1993); [23] J. G. Kirkwood J. Chem. Phys. 2, 351 (1934); [24] V. Kapko and D. V. Matyushov J. Chem. Phys. 124, 114904 (2006) and references therein.

Physical properties of n-cyanobiphenyls from Molecular Dynamics simulations

Luca Muccioli, Giustiniano Tiberio, Roberto Berardi, and Claudio Zannoni

Dipartimento di Chimica Fisica e Inorganica and INSTM Università di Bologna, viale Risorgimento 4, 40136 Bologna, Italy

emails: Luca.Muccioli@unibo.it, Claudio.Zannoni@unibo.it

The n-cyano biphenyl mesogens (nCB) are probably the most studied and characterized liquid crystalline compounds ever, as well as the paradigmatic case of the nematic LC employed in displays. The first homologues of the nCB series (n=5-8) possess a rather limited nematic temperature range (4CB is not nematic) and their nematic-isotropic transition temperatures show a small but noticeable odd-even effect, which is somehow challenging to reproduce with molecular modeling. For this reason, and for the availability of rich experimental data, the nCB have also been widely studied theoretically with quantum chemistry [1] and simulation [2] methods. Despite this, a satisfactory computer simulation of the phase transition of these compounds was still lacking, probably due to limitations of trajectory length, but also to insufficient accuracy of the available force fields, which need to be at least partly reparametrized, as demonstrated by the comprehensive work of Tani and coworkers. Recently a paper from the same group on cyanobiphenyls has appeared, where with a fully atomistic resolution for the cores obtaining static and dynamic nCB properties with good success [3]. Here we choose instead a

united-atom resolution force field, which has the advantage of reducing the number of centers and of allowing larger integration time steps and accelerated dynamics, increasing the affordable system size and the time window that the computer simulation can span. In particular, differently from all previous simulation studies of nCB to date, we prepare the samples in a cooling down sequence from the isotropic phase, observing the spontaneous onset of order upon cooling isotropic samples of 250 molecules [4]. In this presentation the derivation of the force field will be described, as well as the calculation of physical properties from the simulation, focusing on the comparison with the experimental data. It will be shown that the experimental isotropic-nematic transition

temperatures are reproduced within 4 K, allowing for a molecular level interpretation of the odd-even effect along the series. Other properties, like densities, orientational order parameters, NMR residual dipolar couplings, static dielectric constants are also well reproduced, demonstrating the feasibility of predictive in silico modelling of nematics from the molecular structure. [1] Adam CJ, Ferrarini A, Wilson MR, Ackland GJ, Crain J. Mol. Phys. 97, 541-550, 1999 [2] see e.g. Komolkin AV, Laaksonen A, Maliniak A. J. Chem. Phys. 101, 4103-4116, 1994; McDonald AJ, Hanna S. J. Chem. Phys. 124, 164906, 2006; Lansac Y, Glaser MA, Clark NA. Phys. Rev. E 64, 051703, 2001 [3] Cacelli I, De Gaetani L, Prampolini G, Tani A. J. Phys. Chem. B 111, 2130-2137, 2007; Cifelli M, De Gaetani L, Prampolini G, Tani A. J. Phys. Chem. B 112, 9777-9786, 2008 [4] Tiberio G, Muccioli L, Berardi R, Zannoni C. ChemPhysChem, 9,11-13, 2008

Average simulated order parameters P2 and P4 (filled symbols) for 7CB as a function of T-TNI, compared with different sets of experimental values (empty symbols). The Haller fitting function is shown as a dotted line.

Understanding and Controlling Anisotropy in Ionic Polymer Membranes and Micellar Solutions

Jing Li, Kyle G. Wilmsmeyer, Jianbo Hou, and Louis A. Madsen Department of Chemistry and Macromolecules and Interfaces Institute

Virginia Tech, Blacksburg, VA 24061 USA

When tailored ionic polymers such as Nafion© are used in fuel cells, protons conduct through a nm-scale hydrophilic phase. Previous proton conductivity measurements both in the membrane plane and across the plane have demonstrated substantial conductivity anisotropy, but no quantitative understanding exists for these observations. We use 2H NMR to quantify channel alignment in swollen ionomer membranes by using D2O as a probe molecule. The observed 2H quadrupole splittings, demonstrating uniform channel alignment. Different ionic polymer membranes show a diversity of alignment modes, including a case where channels are biaxially oriented (h = 0.12 +/-0.02) in the membrane plane, and cases where channels are oriented perpendicular to the membrane. Additionally, we quantify the anisotropy of diffusion in these membranes, which correlates well with our aligned morphology measurements. Other materials exhibit lamellar morphologies, with corresponding diffusion anisotropy. We are building models to quantitatively relate anisotropy with bulk polymer properties. Control over the direction and extent of orientational order of the hydrophilic channels will allow increased material design freedom and improvements in ionomer device performance.

Extruded membrane

Cast membrane

blue: water channels yellow: polymer

We will also present studies on director dynamics in cylindrical (wormlike) micelles ear and NMR, or “rheo-NMR,” we ccontrol and track the degree, direcand time evolution of molecular alignment.

aligned by shear and/or a magnetic field. Using sh an tion,

.

2H NMR again provides aconvenient probe of order. We will notably discuss a novel field alignment that depends on shear history. Here we focus on understanding model systems designed for broad relevance tounderstanding behaviors of macromolecular fluids and gels

0

15

30

45

60

75

90

0 30 60 90 120 150 180Time (min.)

An

gle

(d

eg.)

Micelle alignment with shear and B field

Microscopic Mechanisms of Tilt, Biaxiality and FerroelectricOrdering in Smectic Liquid Crystals.

M.A.Osipov and M.V.GorkunovDepartment of Mathematics, University of Strathclyde, Glasgow G1 1XH, UK

In spite of apparent simplicity the nature of the SmA-SmC transition has been de-bated for three decades [1]. Elementary picture of the tilt transition assumes a collectivetilt of strongly ordered molecules which resembles a structural transformation with no en-tropy change. In contract, in the de Vries scenario the system undergoes an order-disordertransition which corresponds to a different microscopic mechanism. The situation is evenmore complicated due to the effects of molecular biaxiality which also leads to ferroelectricordering.

We present a molecular theory of the SmA-SmC transition which is based o a rathergeneral model interaction potential and employs a complete set of order parameters. Thecoefficients of the model potential are calculate numerically for a number of particularintermolecular interactions. The theory enables one to describe smectics with conventionaland anomalously weak layer contraction without using the cone model of de Vries [2,3].The influence of both weak and strong molecular biaxiality on the SmA-SmC transitionis also considered [4] including the description of the tilting transition induced by biaxialordering.

The theory also enables one to describe ferroelectric ordering is chiral smectics C* andto consider the corresponding molecular mechanisms [5]. The relation between spontaneouspolarization, tilt and biaxial order parameters is considered in detail, and the general theoryis illustrated by a [articular model of a chiral molecule [6]. The results are compared withexperimental data.

[1]. J.P.F. Lagerwall and F. Giesselmann, ChemPhysChem 7, 20 (2006)[2]. M.V.Gorkunov, M.A.Osipov, J.P.F.Lagerwall and F.Giesselmann. Phys.Rev. E, 76,051706 (2007)[3]. M.V. Gorkunov and M.A.Osipov, J. Phys.: Condens. Matter, 20, 465101 (2008).[4]. M.V. Gorkunov and M.A.Osipov, J.Phys. A: Math. Theor., 41, 295001 (2008).[5]. M.A.Osipov and M.V.Gorkunov, Phys. Rev. E, 77, 031701 (2008).[6]. M.A.Osipov, M.V.Gorkunov, H.F.Gleeson and S.Jaradat, Eur. Phys. J. E,26, 395 (2008).

Director fluctuations in ordered phases and transverse magnetization relaxation

Giorgio J. Moro

Dipartimento di Scienze Chimiche, Università di Padova, Italy

In the past, NMR measurements of the longitudinal relaxation have been often employed to characterize the molecular effects of director fluctuations. [1]. However, such a type of observations does not allow a clear separation between the molecular tumbling with respect to the director and the effects of director fluctuations. On the contrary, because of the different time scales of these two types of motions, the time delay dependence of the transverse magnetization in pulsed experiments leads directly to the identification of the slow director fluctuations and to their dispersion in frequency as well. However, the analysis of such a type of experiments is not obvious at all, since the standard Redfield theory cannot be employed because of the slow time scale of director fluctuations, and one has to solve the full Stochastic Liouville Equation for the coupled evolution of the spin degrees of freedom and of the director field. In recent years, a joint research effort of the Padova Theory group with the research teams of G. Kothe (Freiburg University, Germany) and of J.H. Freed (Cornell University, USA) has produced the theoretical tools for the interpretation of the effects of director fluctuations in pulsed experiments of NMR and ESR type [2-5], which have been applied to measurements in polymeric liquid crystals and membranes leading to a characterization of the viscoelastic properties of these ordered fluids [6-9]. [1] R.Y. Dong: Nuclear Magnetic Resonance of Liquid Crystals, Springer, Berlin, 1994. [2] D. Frezzato, G. Kothe, G.J. Moro: Transverse Nuclear Spin Relaxation Due to Director Fluctuations in Liquid Crystals – A Slow-Motional Theory, J. Phys. Chem. B 105 (2001) 1281-1292. [3] D. Frezzato, G.J. Moro, G. Kothe: Transverse nuclear spin relaxation due to director fluctuations in liquid crystals. II. Second-order contributions of the fluctuating director, J. Chem. Phys. 119 (2003) 6031-6045. [4] D. Frezzato, G. Kothe, G.J. Moro: Transverse nuclear spin relaxation due to director fluctuations in liquid crystals. II. Second-order contributions of the fluctuating director, J. Chem. Phys. 119 (2003) 6046-6058. [5] D. Frezzato, G. Kothe, G.J. Moro: Director Fluctuations and ESR Spectra: A Slow-Motional Treatment, J. Phys. Chem. B 108 (2004) 9505-9515. [6] G. Althof, D. Frezzato, M. Vilfan, O. Stauch, R. Schubert, I. Vilfan, G.J. Moro, G. Kothe: Transverse Nuclear Spin Relaxation Studies of Viscoelastic Properties of Membrane Vesicles. I. Theory, J. Phys. Chem. B 106 (2002) 5506-5516. [7] G. Althof, O. Stauch, M. Vilfan, D. Frezzato, G.J. Moro, P. Hauser, R. Schubert, G. Kothe: Transverse Nuclear Spin Relaxation Studies of Viscoelastic Properties of Membrane Vesicles. II. Experimental Results, J. Phys. Chem. B 106 (2002) 5517-5526. [8] D. Frezzato, G.J. Moro, M. Titelbach, G. Kothe: Transverse nuclear spin relaxation induced by director fluctuations in a nematic liquid crystal polymer. Evaluation of the anisotropic elastic constants: J. Chem. Phys. 119 (2003) 4060-4069. [9] B. Fresch, D. Frezzato, G.J. Moro, G. Kothe, J.H. Freed , Collective Fluctuations in ordered Fluids Investigated by Two-Dimensional Electron-Electron Resonance Spectroscopy, J. Phys. Chem. B 110 (2006) 24238-24254.

Discotic Liquid Crystals : a New Generation of Organic Semiconductors

Yves Henri Geerts, ygeerts@ulb.ac.be

Université Libre de Bruxelles, Laboratoire de chimie des polymères, CP 206/1, Bd du Triomphe, B-1050

Bruxelles, Belgique, http://www.ulb.ac.be/sciences/chimpoly

The discovery that 2,3,6,7,10,11-hexahexylthiotriphenylene 1 exhibits a charge carrier mobility (µ) on the order of µ = 0.1 cm2V-1s-1[1], has created wide scientific and technological interests in discotic liquid crystal as potential candidates for optoelectronic applications [2]. Since then, µ of several mesogenes based on hexabenzocoronene, triphenylene, and phthalocyanine aromatic cores have been studied [3]. Values of µ as high as 0.5 cm2V-1s-1 have been reported for the columnar mesophase of hexabenzocoronene derivatives [4]. Such mobility approaches the corresponding value for the intersheet µ in graphite (3 cm2V-1s-1) and matches those of single crystals of aromatic compounds [4]. However, most of the discotic mesogenes reported so far have in common to be better hole carriers than electron carriers [3]. Only a few examples of electron carrier discotic mesogenes exist to date [5,6] creating, therefore, the need for new materials. We will report the synthesis, mesophase characterisation and property investigation of new type of discotic molecules such as 2,3. The presence of six nitrogen atoms in the aromatic core is anticipated to significantly increase the first reduction potential facilitating electron injection and high electron mobility [7]. Surprisingly, it was discovered that the six nitrogen atoms caused a dramatic change in the mesomorphic behaviour [8]. Whatever its side chains, compound 2 is not liquid crystalline whereas structurally related 1 and 3 exhibit one to several liquid crystalline phases. The occurrence of liquid crystalline mesophases appears for mesogen 4 when thioalkyl side chains are replaced by side chains connected by amide functions. An extremely short intracolumnar distance of 3.18 Å is observed for this compound that exhibit an unusually high charge carrier mobility [9]. This short distance is imposed by intermolecular Hydrogen bonding. Compound 4 can be viewed as a supramolecular polymer where repeating units are connected by non-covalent bonds. We will also report on highly fluorescent columnar mesophases and crystals. The concept of non-parallel transition dipoles has then be successfully extended to mesogen 6 that forms columnar hexagonal mesophases. A fluorescence quantum yield as high as φ = 0.6, i.e. superior to the values encountered for poly(fluorene)s and poly(p-phenylenevinylene)s commonly used in LED! [10]

NN

NN

N

N

O

HN

R

OHNR

O

NHR O NH

R

O

NH

R

O

HNR

SRRS

SRRS

SR

SR

NN

NN

N

N

SRRS

RSSR

SR

SR

NN

NN

N

N

SRRS

SRRS

SR

SR

NN

NN

N

NS

S

SSR

RS

SR

RSSR

SR

1 2 3 4 5

RO OR

OR

RO OR

RO

RO OR

RO

RO

OR

OR

6

1a) D. Adam et al. Nature, 1994, 371, 141. 2) A.N. Cammidge and R.J. Bushby “Synthesis and structural features” in “Handbook of Liquid Crystals”, Vol. 2B, D. Demus, J.W. Goodby, G.W. Gray, H.-W. Spiess & V. Vill editors, Wiley VCH, Weinheim, 1998. 3) A. van de Craats et al. Adv. Mater., 2001, 13, 130. 4) A. van de Craats et al. Adv. Mater., 1998, 10, 36. A. van de Craats et al. Adv. Mater., 1999, 11, 1469. 5) C W. Struijk et al. J. Am. Chem. Soc., 2000; 122; 11057. 6) A. van de Craats, Ph.D. Dissertation, Delft University of Technology, June 2000. 7) F. Uckert et al. Adv. Mater., 2000, 12, 905. 8) O. Roussel et al. Molec. Cryst. Liq. Cryst., 2003, 396, 35-39. 9) R.I. Gearba et al. Adv. Mater. 2003, 15, 1614. 10) The value of φ = 0.6, measured by Dr. Anna Köhler with an integrating sphere

Organic field effect transistors revisited:

from charge tunneling devices to label-free transducers

Fabio Biscarini CNR-Istituto per lo Studio dei Materiali Nanostrutturati, Bologna Italy

e-mail: f.biscarini@bo.ismn.cnr.it Organic field effect transistors (OFET), where ordered conjugated molecules act as charge transport material, are low-dimensional devices, as charge carriers are transported within the first few mono-layers in contact with the gate dielectric. [1] This is largely the outcome of a growth transition occurring at about two monolayers, and not limited by the Debye length of the electric field in the organic semiconductor. Therefore, the structural/morphological and electronic control of the interfaces is crucial to the physics and the performance of the device.

I will discuss the physics of the charge injection interface, specifically a pentacene FET, that can be changed by functionalizing the charge injection interface with Self-Assembly Monolayers (SAM) of increasingly longer chain length. We observe a non-monotonic crossover from charge transport to charge injection-controlled device from short to long chain lengths. The charge mobility exhibits large fluctuations correlated to the odd/even number of the methylene units in the alkyl chains. This shows that injection occurs via non-resonant tunneling through the alkyl chain. We are able to measure the charge tunneling average inverse decay length, viz. 0.58 Å-1. Thus, OFETs can be operated as gauges for molecular electronics. [2]

In the final part of the talk I will show how this interfacial sensitivity of OFETs can be exploited for label-free biological transducers. I will show operations an ultra-thin film transistor under water, then its response to the presence of increasingly larger concentrations of biomolecules (DNA, peptide 1-40 of beta-amyloid). These results show a promising route for the detection and monitoring of biological molecules in vitro and during their evolution. Finally, I will show our first attempts to integrate living cells on organic semiconductors [3] and devices. Support from EU Projects IP-NAIMO, BIODOT and Emilia Romagna Region NetLAB NANOFABER is acknowledged. [1] F. Dinelli, M. Murgia, J. F. Moulin, M. Cavallini, P. Levy, F. Biscarini, D. De Leeuw, Phys. Rev. Lett. 92, 116802-116805 (2004). [2] P. Stoliar, R. Kshirsagar, M. Massi, P. Annibale, C. Albonetti, D. M. de Leeuw, Fabio Biscarini J. Am. Chem. Soc. (2007). [3] E. Bystrenova, M. Jelitai, I. Tonazzini, A. Lazar, M. Huth, P. Stoliar, C. Dionigi, M. G. Cacace, B. Nickel, E. Madarasz, and F. Biscarini, in press Adv. Funct. Materials (2008).

Models of ferroelectric nematic colloidal behaviour

T.J. Sluckin School of Mathematics, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom

In the last decade there has been much interest, both experimental and theoretical, in ferroelectric liquid crystal nanocolloids [1–4]. These are highly dispersed suspensions of nanoscopic particles in a nematic liquid crystal (LC) host. The dopants possess a permanent electric polarization, and provide strong LC director anchoring at the particle surface. At low dopant concentrations, these LC nanocolloids resemble pure LCs, without any apparent evidence, at least to the senses, of dissolved particles [1]. They are stable against colloidal aggregation and possess enhanced dielectric anisotropy. A particularly anomalous feature is that, curiously for nematic colloids, the nematic-isotropic transition temperature increases rather than decreases. However, the Frederiks transition threshold voltage can under some circumstances nevertheless exhibit an apparently anomalous decrease [1]. I present theoretical models which discuss properties of ferroelectric LC nanocolloids, concentrating on: (a) the statistical mechanical behaviour, and (b) the dielectric properties of the effective medium. In the former case, we examine some scenarios of the internal nanocolloidal structure, which can affect the manner in which the interparticle orientational interaction acts. In the latter case, we consider a heterogeneous system within mean field theory, starting with the theory of Fu and Resca [5]. In the work presented here, the electric response is calculated for impurities which possess a permanent electric dipole moment. We find that doping the LC host with ferroelectric colloidal nanoparticles causes dielectric feedback effects which can indeed significantly reduce the electric Frederiks transition threshold voltage, even at low dopant concentrations. [1] Yu. Reznikov, O. Buchnev, O. Tereshchenko, V. Reshetnyak, A. Glushchenko

and J.L. West, Appl. Phys. Lett. 82, 1917 (2003). [2] F. Li, O. Buchnev, C.I. Cheon, A. Glushchenko, V. Reshetnyak, Yu. Reznikov,

T.J. Sluckin, and J.L. West, Phys. Rev. Lett. 97, 147801 (2006). [3] V.Yu. Reshetnyak, S.M. Shelestiuk, and T.J. Sluckin, Mol. Cryst. Liq. Cryst. 454,

201/[603] (2006). [4] M. Čopič, A. Mertelj, O. Buchnev, and Yu. Reznikov, Phys. Rev. E 76, 011702

(2007). [5] L. Fu and L. Resca, Phys. Rev. B 50 (21), 15719 (1994) The author acknowledges numerous conversations with Yuri Reznikov, Victor Reshetnyak, Sergei Shelestiuk, all from Kiev. He also acknowledges financial support from the Royal Society (London) and NATO, which have funded exchange projects between Southampton and Kiev.

CONTROLLING SURFACE DEFECT VALENCEIN COLLOIDS

G. Skacej1 and C. Zannoni2

1 Fakulteta za matematiko in fiziko, Univerza v Ljubljani,Jadranska 19, SI-1000 Ljubljana, Slovenia

2 Dipartimento di Chimica Fisica ed Inorganica, Universita di Bologna, and INSTM,Viale Risorgimento 4, I-40136 Bologna, Italy

A few years ago Nelson [1] suggested the fascinating possibility of doing chemistry usingcolloidal particles coated with a layer of liquid crystals, rather than atoms. In this scenario,topological defects in the liquid-crystalline ordering play an important role since theydetermine the colloidal valence. Therefore, there has been growing experimental andtheoretical interest in nematic shell systems over the last years [2-4]. — In our study,we perform large-scale Monte Carlo simulations of orientational ordering in sphericalnematic shells and study the type and position of topological defects when an externalelectric field (homogeneous or quadrupolar) is applied. The field-induced variation of thedefect number (and strength) can be used to change the colloidal valence [5].

[1] D. R. Nelson, Nanoletters 2, 1125 (2002).[2] A. Fernandez-Nieves et al, Phys. Rev. Lett. 99, 157801 (2007).[3] M. A. Bates, J. Chem. Phys. 128, 104707 (2008).[4] H. Shin et al, Phys. Rev. Lett. 101, 037802 (2008).[5] G. Skacej and C. Zannoni, Phys. Rev. Lett. 100, 197802 (2008).