Thermotropic Behavior, Packing, and Thin Film Structure of an Electron Accepting Side-Chain Polymer Peter Kohn, † Lilit Ghazaryan, † Gaurav Gupta, † Michael Sommer, ‡,§ Andr ́ e Wicklein, ‡ Mukundan Thelakkat,* ,‡ and Thomas Thurn-Albrecht* ,† † Institut fü r Physik, Martin-Luther-Universitä t Halle-Wittenberg, 06099 Halle, Germany ‡ Angewandte Funktionspolymere, Makromolekulare Chemie I, Universitä t Bayreuth, 95440 Bayreuth, Germany * S Supporting Information ABSTRACT: We report on the phase behavior and the structure of poly(perylene bisimide acrylate), an electron accepting semiconductor polymer with disclike side-chain units, in comparison to the corresponding low molecular weight perylene bisimide. By combination of DSC, optical microscopy, and temperature-dependent small-angle and wide- angle X-ray scattering, we show that both compounds display a lamello-columnar packing. While the perylene bisimide model compound crystallizes, the polymeric architecture of poly(perylene bisimide acrylate) suppresses order, leading to a 2D lamello-columnar liquid crystalline phase. The structure of the side-chain polymer in thin films with different thermal treatments as observed by GIWAXS correlates well with previously observed largely different electron mobilities. Such a polymeric, liquid crystalline compound combines the advantages of molecular order and easy processability, together with the film forming properties of polymeric materials. ■ INTRODUCTION Perylene bisimide (PBI) derivatives with their high electron mobilities 1 and strong absorption in the visible region 2 are a promising class of electron accepting organic semiconductor materials 3-5 with possible applications in electronic devices, as e.g. field effect transistors or solar cells. 6-9 Generally, transport parameters in organic semiconductors are not a purely molecular property but depend strongly on packing. Structure is therefore important on a local scale as well as on a more global scale. 10 While local order determines the hopping process between molecules, defects like grain boundaries between crystals limit charge transport over larger distances. For this reason, liquid crystalline materials are regarded as especially interesting as they combine local order and the possibility to induce macroscopic alignment by simple means. Depending on the architecture as well as the kind and length of the solubilizing groups which are attached to the aromatic core, low molecular weight PBIs show different crystalline and liquid- crystalline phases of different symmetry. 5,11-14 In most cases columnar structures with a 1D stacking of the perylene bisimide (PBI) cores were reported; in some cases an additional lamellar arrangement of the PBIs in the plane perpendicular to the π- stacking direction was found, leading to so-called lamello- columnar phase. 5,15,16 While in most cases low molecular weight PBIs have been used for electronic applications, polymers are interesting for their film forming properties and mechanical stability as compared to small molecule discotics. Thus, the search for electron accepting polymeric materials with high performance is an intensive field of research, 17 and polymers containing PBI, either in the main chain 18,19 or as side groups attached to a polymeric backbone via flexible alkyl spacers 20,21 (cf. Figure 1), are among the most promosing electron accepting polymers. Furthermore, side-chain polymers containing perylene bisimide were successfully incorporated as one block in donor-acceptor diblock copolymers with possible applications, e.g., in bulk heterojunction organic solar cells. 22-26 In such a system the required donor-acceptor nanostructure could potentially be Received: May 19, 2012 Revised: June 22, 2012 Published: July 10, 2012 Figure 1. Sketch of a discotic side-chain polymer: polymer backbone (solid black line), spacer (dotted black line), and swallow-tail substituents (dashed line). Article pubs.acs.org/Macromolecules © 2012 American Chemical Society 5676 dx.doi.org/10.1021/ma3010197 | Macromolecules 2012, 45, 5676-5683