pubs.acs.org/cm Published on Web 02/15/2010 r 2010 American Chemical Society Chem. Mater. 2010, 22, 2079–2092 2079 DOI:10.1021/cm903444n Fine Tuning of Solid-State Properties of Septithiophenes by Tailoring the Substituents Andreas Kreyes, † Stefan Ellinger, † Katharina Landfester, †,‡ Matthieu Defaux, §,^ Dimitri A. Ivanov, § Andreas Elschner, ) Timo Meyer-Friedrichsen, ) and Ulrich Ziener* ,† † University of Ulm, Institute of Organic Chemistry III, Albert-Einstein-Allee 11, D-89081 Ulm, Germany, ‡ Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany, § Institut de Sciences des Mat eriaux de Mulhouse (CNRS LRC 7228), 15 rue Jean Starcky, B.P. 2488, 68057 Mulhouse Cedex, France, ) H.C. Starck Clevios GmbH, Chemiepark, Geb.B202, D-51368 Leverkusen, Germany. ^ Present address: DWI an der RWTH Aachen, Pauwelsstrasse 8, D-52056 Aachen, Germany. Received November 11, 2009 A series of five R,ω-substituted septithiophenes, which differ in the geometry of the peripheral branched alkyl substitutents, is presented. The position of the branching point has a substantial effect on the solubility and melting points of the oligomers. In contrast, organic field-effect transistors (OFETs) that have been prepared from those materials via vapor deposition show all mobilities in the same range (0.18-0.018 cm 2 V -1 s -1 ). The relative unsusceptibility of the mobilities on the molecular structure is attributed to the fact that all septithiophenes form smectic-like structures at room temperature under solid-state conditions, as revealed by temperature-dependent X-ray diffraction. Furthermore, four of the five oligomers exhibit thermotropic liquid crystalline smectic C phases. Strong interactions between the thiophene cores are assumed as driving forces for those structural features. Thus, tailoring the peripheral substituents makes it possible to fine-tune the thermal and solubility properties and, to a certain extent, the ordering under solid-state conditions. The ordering and the electrical properties are mainly dominated by the length of the core of the oligothiophenes. Introduction Organic semiconductors that contain conjugated π-elec- tron systems have been intensively investigated as active materials in electronic applications such as organic field- effect transistors (OFETs), organic light-emitting diodes (OLEDs), and photovoltaic cells (PVCs). 1-5 The possibi- lity of large area processing from solution on flexible substrates such as polymer foils makes these materials particularly suitable for new low-cost, lightweight applica- tions as electronic papers, radio-frequency identification (RFID) tags, or flexible displays. 6,7 One major advantage of organic semiconductors is the possibility to design the materials on the level of molecular structure to optimize their properties for the desired application. One very promising class of materials for use in OFETs is based on R,ω-substituted oligothiophenes. 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