Effects of Substituted Alkyl Chain Length on ... - ACS Publications

May 20, 2015 - Reiji Kumai,. ‡. Makoto Yoneya,. † and Tatsuo Hasegawa*. ,†,§. †. National Institute of Advanced Industrial Science and Techno...
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Effects of Substituted Alkyl Chain Length on SolutionProcessable Layered Organic Semiconductor Crystals Satoru Inoue, Hiromi Minemawari, Jun'ya Tsutsumi, Masayuki Chikamatsu, Toshikazu Yamada, Sachio Horiuchi, Mutsuo Tanaka, Reiji Kumai, Makoto Yoneya, and Tatsuo Hasegawa Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00810 • Publication Date (Web): 20 May 2015 Downloaded from http://pubs.acs.org on May 25, 2015

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Chemistry of Materials

Effects of Substituted Alkyl Chain Length on Solution-Processable Layered Organic Semiconductor Crystals Satoru Inoue, *,† Hiromi Minemawari,† Jun’ya Tsutsumi,† Masayuki Chikamatsu,† Toshikazu Yamada,† Sachio Horiuchi,† Mutsuo Tanaka,† Reiji Kumai,‡ Makoto Yoneya,† and Tatsuo Hasegawa*,†,§ †

National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8562, Japan

Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan §

Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan

ABSTRACT: We systematically investigated the effects of the substituted alkyl chain length on solution-processable layered crystalline organic semiconductors, namely 7-alkyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophenes (Ph-BTBT-Cn; 0 ≤ n ≤ 14). The solubilities of the compounds in organic solvents sharply increased with increasing alkyl chain length at n ≤ 3, but decreased gradually at n ≥ 5; these latter compounds exhibit liquid-crystal phases at high temperature. The properties are strongly correlated with the alkyl length dependence of the layered molecular packing, as revealed by singlecrystal structural analysis: compounds with n ≤ 4 do not form independent alkyl chain layers, whereas those with n ≥ 5 form isomorphous bilayer-type crystal structures in which independent alkyl chain layers are formed and enhance the cohesive energy of the crystals.

Organic semiconductors have unique characteristic among semiconducting materials, in that they can be processed in solution under ambient conditions. 1-4 Recently, this solution processability has attracted considerable attention worldwide, because it may enable low-cost production of flexible and large-area electronic products using print production (or “printed electronics”) techniques.5-7 There is therefore a need to design and develop organic molecular materials with suitable solubilities and thermal stabilities, in addition to semiconducting properties. 6,13-Bis(triisopropylsilylethynyl)pentacene8,9 or benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives10-14 are well-known examples of solution-processable smallmolecule organic semiconductors, although the solubility in organic solvents decreases considerably for materials with extended π-electron skeletons by increasing cohesion.15 Alkyl chain substitution is an effective chemical modification for improving the solubilities of organic semiconductors.1,10,16 Alkyl substitution of some prototypical organic semiconductors is also known to play a crucial role in the formation of layered crystal structures, a property that is essential for providing high-performance organic-thin-film transistors (TFTs).17-22 Among them, it was reported that decyl-substituted Ph-BTBT afforded high performance organic TFTs with mobility as high as 14.7 cm2/Vs by using the spin-coated films under heat treatment,23 with showing good thermal stability and high degree of layered crystallinity.24 Such asymmetric substitution with various substituents is expected to provide higher designability to the materials. However, a systematic understanding of the effects of substituted alkyl

chains on solution-processable organic semiconductors has not yet been obtained. In this communication, we present an investigation of the alkyl chain length dependence on the solubilities, and thermal and structural properties of layered crystalline organic semiconductors, Ph-BTBT-Cn (0 ≤ n ≤ 14). We found that the solubility depends on the alkyl chain length and shows a maximum at around n = 3. This characteristic is closely correlated with changes in the crystal structure with changes in n. On the basis of these findings, we suggest that the cohesive energy between the alkyl chains plays an important role in the high degree of layered crystallinity in Ph-BTBT-Cn. Ph-BTBT-Cn (n = 0–10, 12, and 14) was obtained by fourstep synthesis from BTBT according to the literature procedure [see the Supporting Information (SI)].25 Final purification of the products was achieved by repeated recrystallization from a mixed solution of chloroform and ethanol. In evaluating the solubility, the material was first ground to a powder and then dissolved in an organic solvent to form a saturated solution at 20 °C. The concentration was frequently checked, based on UV absorption at 328 nm using standard curve analysis. The Ph-BTBT-Cn compounds dissolve in chloroform, ethyl acetate, and some aromatic solvents, but are poorly soluble in hexane and non-aromatic polar solvents such as dimethyl sulfoxide and acetonitrile (