Layered crystalline organic semiconductors have recently attracted considerable attention in materials science, as they can afford very high performance organic thin-film transistors (OTFTs). It has been demonstrated that a high degree of layered crystallinity is essential for the production of single-crystalline or uniaxially oriented polycrystalline thin films in which high-mobility carrier transport occurs along the film planes. Among various layered crystalline molecular materials, a series of 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophenes (diCn-BTBTs) have been shown to afford high-performance OTFTs with field-effect mobilities in excess of 10 cm2/Vs.[ref.1-5] The coupled functional pi-conjugated skeleton and the alkyl chain moieties are effective in making the molecules highly soluble in organic solvents, raising expectations that these materials might be useful in the emerging field of printed electronics. However, despite these superior features, applications of diCn-BTBTs have been restricted to their use in single-component molecular semiconductors. In this study, we report newly developed two-component molecular semiconductors based on diCn-BTBTs. The compounds are molecular charge-transfer (CT) compounds, (diCn-BTBT)(FmTCNQ), composed of 2,7-diCn-BTBT (n = 4, 8, 12) as electron donors and optionally fluorinated derivatives of tetracyanoquinodimethane (FmTCNQ; m = 0, 2, 4) as electron acceptors.[ref.6] The compounds represent a novel type of pi-conjugated layered crystalline structure consisting of donors and acceptors, which is quite stable even at high temperature in sharp contrast to that of the single component diCn-BTBT. By using the lamellar crystals of the compounds, we successfully fabricated single crystalline field-effect transistors with a maximum electron mobility of 0.4 cm2/Vs. Intriguingly, the operation mode (p- or n-type) of the transistors could be systematically controlled by changing the fluorine substitution of the TCNQ. The feature opens an opportunity for using the CT materials in electronics devices by taking advantage of the nature of the inherent narrow band gap and high mobility arising from the strong donor-acceptor CT interactions.
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