Alignment by Langmuir/Schaefer monolayers of bent-core liquid crystals Wilder Iglesias, a Timothy J. Smith, b Prem B. Basnet, c Sharon R. Stefanovic, c Carsten Tschierske, d Daniel J. Lacks, b Antal J akli a and Elizabeth K. Mann * c Received 29th March 2011, Accepted 5th July 2011 DOI: 10.1039/c1sm05546a Langmuir films of bent-core molecules at the air/water interface are transferred onto a solid surface by the inverse-Langmuir-Schaefer (ILS) technique. Previous work by the authors demonstrated that ILS films of a symmetric bent-core molecule can serve as effective planar alignment layers for a nematic bent-core liquid crystal cell, but the Langmuir films were unstable and formed multilayers at very low pressures. Here, films of bent-core molecules with one hydrophilic end connected to the bent core by a short aliphatic chain are tested as alignment layers. The hydrophilic group led to much more stable Langmuir films, and also to a molecular tilt at the surface which could be controlled by molecular area. This interpretation of the molecular behavior was supported by a combination of atomically accurate molecular dynamics simulations of up to 36 bent core molecules at a water surface and by the continuous variation of tilt induced by ILS films in a rod-like liquid crystal cell. The ILS films were then tested as alignment layers for a bent-core nematic: highly-compressed films induced perpendicular alignment, which provides a significant step towards their practical application. I. Introduction The discovery of the mesogenic properties of bent-core (BC) molecules 1 has opened up a major new and exciting dimension in the science of thermotropic liquid crystals (LCS). These meso- gens have a rich phase diagram including a vast number of smectic and columnar phases denoted by B1–B7. 2–5 Seminal findings, having broad implications for the general field of soft condensed matter, include the observation of ferroelectricity and of spontaneous breaking of chiral symmetry in smectic phases composed of molecules without chiral stereo-centers. 6 These phases are candidates for devices with fast switching times. Nematic phases of BC molecules are much less common and their study lags behind that of the smectic and columnar phases, but they show a number of interesting rheological 7 and electro- mechanical properties. 8 Effective alignment layers, which yield controlled molecular configurations at the surface that propagate into the bulk liquid crystal, are crucial both to improve control of experiments designed to explore the behavior of bent-core molecules and to effectively exploit this behavior. Good alignment has been hard to achieve with the common methods used for rod-like (cala- mitic) molecules because of the symmetry breaking of the bent- core mesogens (Fig. 1). An alignment layer made up of molecules of similar shape has a major advantage: the shape and symmetry of the molecules in the alignment film and in the bulk liquid crystal are compatible. Langmuir films, molecularly thin films self-organized on the smooth water surface, allow external control of the phase and orientation within the film through the average molecular area, as dictated by the interaction of the molecules with each other and the substrate. These films, transferred to a solid substrate by Fig. 1 Cartoon models comparing the alignment of rod-shaped (a,b) and bent core (c,d) molecules according to two common models: (a,c): the groove model; (b,d): a hydrocarbon layer model. The symmetry argu- ments are similar for more accurate models. a Liquid Crystal Institute, Kent State University, Kent, OH, 44242, USA b Chemical Engineering, Case Western Reserve University, Cleveland, OH, 44106, USA c Physics Department, Kent State University, Kent, OH, 44242, USA d Institute of Organic Chemistry, Martin-Luther-University Halle- Wittenberg, Halle, D06120, Germany This journal is ª The Royal Society of Chemistry 2011 Soft Matter , 2011, 7, 9043–9050 | 9043 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2011, 7, 9043 www.rsc.org/softmatter PAPER Downloaded by University of Manitoba on 30 November 2012 Published on 11 August 2011 on http://pubs.rsc.org | doi:10.1039/C1SM05546A View Article Online / Journal Homepage / Table of Contents for this issue