pubs.acs.org/cm Published on Web 11/12/2009 r 2009 American Chemical Society Chem. Mater. 2009, 21, 5867–5874 5867 DOI:10.1021/cm902634r Homeotropic and Planar Alignment of Discotic Liquid Crystals: The Role of the Columnar Mesophase Guillaume Schweicher, Gabin Gbabode, Florence Quist, Olivier Debever, Nicolas Dumont, Sergey Sergeyev, and Yves H. Geerts* Laboratoire de Chimie des Polym eres, Facult e des Sciences, Universit e Libre de Bruxelles (ULB), CP206/1, Boulevard du Triomphe, 1050 Brussels, Belgium Received August 26, 2009. Revised Manuscript Received October 26, 2009 Blends of two metal-free phthalocyanine mesogens exhibiting different mesophases (Col r phase from room temperature to isotropization for the first one, and Col r at room temperature then Col h from around 60 °C to isotropization for the second one) have been studied in order to determine the relationship between the type of mesophase and the alignment behavior. The phase diagram of this system has been built and evidence of full solid-state miscibility of the two pure constituents in all proportions and temperatures is presented. Investigation of phase alternation at room temperature as a function of composition revealed that border compositions exhibit Col r phases similar to the pure constituents, whereas Col h mesophase was stabilized for intermediate compositions. Combined polarized optical microscopy observations and X-ray diffraction measurements showed that home- otropic alignment is adopted only for mixed samples exhibiting Col h mesophase, thus demonstrating that the presence of a Col h mesophase is a necessary condition for homeotropic alignment. Introduction Organic semiconductors have aroused much interest in the past few years 1 and among the different materials under study, π-conjugated liquid crystals have attracted particular attention due notably to their self-assembling ability, their malleability and their ease of processing. Generally speaking, there exist two types of liquid crystal- line mesogens, which differ not only in their shape, but also in the type of self-assembled structure they yield and in the dimensionality of charge transport and exciton migration within such structures. 1 Rod-shaped calamitics can form nematic (orientational order) or smectic (orientational as well as positional order) phases and present a two-dimensional charge transport in directions perpendicular to the long axis of the molecules. In addi- tion to forming nematic mesophases, disk-shaped disco- tics can self-assemble into columns, which can then arrange into a hexagonal, a rectangular, a tetragonal, an oblique, or a cubic lattice. 2 Charge transport in such columnar mesophases is one-dimensional and its direc- tion is parallel to the column axis. To construct efficient devices, it is therefore necessary to control the self- assembled structures formed by such materials and, in particular, to control the charge transport direction rela- ting to the alignment of the mesophase. Although the alignment of low-molecular-weight calamitic nematic mesophases onto surfaces, through rubbing or photo- alignment techniques in particular, has been extensively studied (notably because of their use in liquid crystal displays, LCD 2-4 ), the alignment of discotic columnar mesophases has not received as much attention. The latter can present two types of alignment with respect to a substrate: planar alignment, in which the column axes lie parallel to the surface and homeotropic alignment where the column axes are perpendicular to the surface. These two orientations can be used in different applica- tions. For example, uniaxial planar orientation is needed for field-effect transistors applications, whereas home- otropic alignment can be used in solar cells or light- emitting diodes. 1 A few examples of induced alignment of discotic co- lumnar mesophases, through construction of Langmuir- Blodgett thin films, 5,6 zone-casting techniques, 7-9 photo- patterning 10 or use of magnetic fields 11 have been reported *Corresponding author. E-mail: ygeerts@ulb.ac.be. (1) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36, 1902–1929. (2) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; H€ agele, C.; Giusy Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni, M. Angew. 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