Development of N-Alkyl-Substituted Bis(pyrrolo[3,4-d])tetrathiafulvalenes as Organic Semiconductors for Solution-Processible Field-Effect Transistors

A series of N-alkyl-substituted bis(pyrrolo[3,4-d])tetrathiafulvalenes (PyTTFs, alkyl = n-butyl, n-octyl, n-dodecyl, n-cetyl, and n-icosyl, 3−7) were synthesized as highly soluble tetrathiafulvalene (TTF) derivatives. Their solid-state structures were characterized by X-ray diffraction, and their suitability for use as semiconductors for solution-processible organic field-effect transistor (OFET) devices was evaluated. Whereas the solubility of the TTF derivatives was enhanced with the introduction of alkyl groups, very long alkyl groups, such as the n-icosyl group, reduced the solubility probably due to intermolecular hydrophobic interactions between the very long alkyl groups. The solid-state structure was also influenced by the length of the alkyl groups; molecules 5−7 having n-dodecyl or longer alkyl groups tended to assume two-dimensional (2-D) molecular ordering both in the bulk single crystals and in the spin-coated thin films. In contrast, 3 and 4, having short n-butyl and n-octyl groups, did not take on a 2-D interactive structure in the solid state. Consistent with the solid-state structure of the PyTTF derivatives, field-effect transistor (FET) characteristics of the solution-processed OFETs markedly depended on the length of the alkyl groups. In contrast to spin-coated thin films of 3 and 4, which were relatively inferior semiconducting layers (μFET = ∼10-5 cm2 V-1 s-1 or no reproducible field effect), OFET devices consisting of spin-coated thin films of 5−7 showed typical p-channel FET characteristics, namely, hole mobilities of ∼10-2 cm2 V-1 s-1 and current on/off ratios of ∼104. The results indicate that an appropriate combination of a π-conjugated core with long alkyl groups could provide soluble organic semiconductors that are applicable to solution-processible OFETs.

本研究合成了一系列N-烷基取代双(吡咯并[3,4-d])四硫富瓦烯（bis(pyrrolo[3,4-d])tetrathiafulvalenes, PyTTFs，烷基分别为正丁基、正辛基、正十二烷基、正十六烷基及正二十烷基，对应化合物3~7），作为高可溶性四硫富瓦烯（tetrathiafulvalene, TTF）衍生物。通过X射线衍射(X-ray diffraction)表征了其固态结构，并评估了其作为可溶液加工有机场效应晶体管(organic field-effect transistor, OFET)半导体材料的适用性。尽管引入烷基可提升该TTF衍生物的溶解性，但过长的烷基链（如正二十烷基）反而会因烷基链间的分子间疏水相互作用降低其溶解度。固态结构同样受烷基链长度影响：带有正十二烷基或更长烷基链的化合物5~7，无论是在本体单晶还是旋涂薄膜中，均倾向于形成二维(2-D)分子有序结构。与之相反，带有短链正丁基与正辛基的化合物3与4，在固态中并未形成二维相互作用结构。与PyTTF衍生物的固态结构结果一致，经溶液加工制备的OFET的场效应晶体管(field-effect transistor, FET)性能也显著依赖于烷基链长度。相较于性能相对较差的化合物3与4的旋涂薄膜（场效应迁移率μFET≈10^-5 cm² V⁻¹ s⁻¹，部分甚至无可重复的场效应），由化合物5~7的旋涂薄膜制备的OFET器件展现出典型的p型沟道FET特性，即空穴迁移率约为10^-2 cm² V⁻¹ s⁻¹，电流开关比约为10⁴。本研究结果表明，将π共轭骨架与合适长度的烷基链相结合，可获得可溶且适用于可溶液加工OFET的有机半导体材料。