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Enhanced Layered-Herringbone Packing due to Long Alkyl Chain Substitution in Solution-Processable Organic Semiconductors

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posted on 20.01.2017, 14:49 by Hiromi Minemawari, Mutsuo Tanaka, Seiji Tsuzuki, Satoru Inoue, Toshikazu Yamada, Reiji Kumai, Yukihiro Shimoi, Tatsuo Hasegawa
Herein, we report the stabilization and modulation of layered-herringbone (LHB) packing, which is known to afford high-performance organic thin-film transistors, based on crystal structure analyses and calculations of intermolecular interaction energies for alkyl-substituted organic semiconductor (OSC) crystals. We systematically investigated the alkyl chain-length dependence of the crystal structures, solvent solubilities, and thermal characteristics for three series of symmetrically and asymmetrically alkyl-substituted benzothieno­[3,2-b]­[1]­benzothiophenes (BTBTs). All the series exhibit LHB packing when the BTBTs are substituted with relatively long alkyl chains (−CnH2n+1), i.e., n ≥ 4 for monoalkylated, n ≥ 6 for dialkylated, and n ≥ 5 for phenyl-alkylated BTBTs. LHB packing is also evident in the nonsubstituted and diethyl-substituted BTBTs, although those substituted with short alkyl chains generally did not feature LHB packing because of their lack of interchain ordering. The density functional theory calculations of the intermolecular interactions revealed that the BTBT cores inherently generate LHB packing, and the stability is increasingly enhanced by the alignment of longer alkyl chains. It was also found that the LHB packing is stabilized by keeping the size ratios of the total intermolecular attractive forces between the T-shaped and slipped parallel contacts at about 3:2 for all the LHB compounds, despite the slight structural modifications generated by the substituents. We discuss the effects of alkyl substitutions to modulate the LHB packing of the BTBT cores and thus the two-dimensional carrier transport in layered OSC crystals.