Structure and Phase Behavior of a Discotic Columnar Liquid Crystal Confined in Nanochannels Carole V. Cerclier, † Makha Ndao, † Re ́ mi Busselez, † Ronan Lefort, † Eric Grelet, ‡ Patrick Huber, §,∥ Andriy V. Kityk, ⊥ Laurence Noirez, # Andreas Schö nhals, ∇ and Denis Morineau* ,† † Institut de Physique de Rennes, CNRS UMR 6251, Universite ́ de Rennes 1, 35042 Rennes, France ‡ Centre de Recherche Paul-Pascal, CNRS UPR 8641, Universite ́ de Bordeaux 1, 33600 Pessac, France § Experimental Physics, Saarland University, 66041 Saarbrü cken, Germany ∥ Materials Physics and Technology, Hamburg University of Technology, 21073 Hamburg, Germany ⊥ Faculty of Electrical Engineering, Czestochowa University of Technology, 42-200 Czestochowa, Poland # Laboratoire Le ́ on Brillouin (CEA-CNRS), CEA Saclay, 91191 Gif sur Yvette, France ∇ BAM Bundesanstalt fü r Materialforschung und −prü fung, 12205 Berlin, Germany ABSTRACT: The confinement of discotic columnar liquid crystal in nanoporous templates is a promising strategy to design nanofibers with potential applications in organic electronics. However, for many materials, geometric nanoconfinement has been shown to induce significant modifications of the physical properties, such as structure or phase behavior. We address the case of a discotic columnar liquid crystal confined in various templates. The influence of the size, the roughness, and the chemical nature of pores was investigated for a pyrene derivative by small-angle neutron scattering, X-ray diffraction, and calorimetry on a wide range of temperatures. A homeotropic anchoring (face-on orientation of the disk-shape molecules at the interface) is favored in all smooth cylindrical nanochannels of porous alumina while surface roughness of porous silicon promotes more disordered structures. The hexagonal columnar− isotropic phase transition is modified as a result of geometrical constraints and interfacial interactions. ■ INTRODUCTION Organic electronics is a field of intense scientific activity because of promising applications toward the fabrication of effective low-cost, portable and disposable devices such as organic light emitting diodes, photovoltaic devices, field effect transistors, memory elements or sensors. 1 Among organic semiconductors, discotic columnar liquid crystals (DCLC) are a promising class of materials, 2,3 which consist of disklike molecules composed of a rigid aromatic core surrounded by flexible aliphatic chains. These molecules self-assemble by stacking on top of one another and form columns, which arrange in a regular 2D lattice. 4 DCLC combine unique material properties, such as fluidity and the self-healing of structural defects with anisotropic mechanical and optical properties. Thanks to their self-organization, a strong orbital overlap occurs in one dimension and allows charge carriers to move easily along the columns. Thus, depending on potential applications, a large effort is required to control the parameters that influence the alignment mechanism in the suitable geometry for devices. Two different possible organizations of columns at the solid surface are illustrated in Figure 1 and correspond respectively to the homeotropic (face-on orienta- tion of the molecules) and the planar (edge-on orientation of the molecules) anchoring conditions. To obtain efficient photovoltaic cells, columns must form a homeotropically (face-on) aligned open film on the surface of one electrode prior to deposit the second electrode. 5 Usually, columns exhibit planar (edge-on) alignment in open thin films, 6,7 while they align homeotropically when sandwiched between two solid substrates. 7 The organization results from the competition between homeotropic and planar alignment due to the difference in interfacial tensions between air/liquid crystal and liquid crystal/substrate. 8 Several works have reported on homeotropic alignment of hexagonal DCLC in thin films 9−13 and the strategies for homeotropic alignment were the controlled thermal annealing of DCLC in open film or in confined geometry between two interfaces, 8,11,12,14,15 or the Received: April 17, 2012 Revised: August 21, 2012 Published: August 22, 2012 Figure 1. Schematic representation of the different orientations of a discotic columnar liquid crystal with regard to a surface. Article pubs.acs.org/JPCC © 2012 American Chemical Society 18990 dx.doi.org/10.1021/jp303690q | J. Phys. Chem. C 2012, 116, 18990−18998