Thiadiazoloquinoxaline-Fused Naphthalenediimides for n‑Type Organic Field-Effect Transistors (OFETs) Ben-Lin Hu, † Ke Zhang, † Cunbin An, † Wojciech Pisula, †,‡ and Martin Baumgarten* ,† † Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany ‡ Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland *S Supporting Information ABSTRACT: Thiadiazoloquinoxaline-fused naphthalenediimides (TQ-f-NDIs) are designed and synthesized. They show high electron affinities (EAs) of ∼4.5 eV. Organic field-effect transistor (OFET) devices, fabricated by dip-coating, provided maximum high electron mobilities of 0.03 cm 2 /(V·s) with an on/off ratio of 2 × 10 5 . T he development of electron-deficient π-building blocks (electron acceptors) has been a hot topic in the past decade, since they are necessary and indispensable elements to develop n-type and ambipolar organic field-effect transistors (OFETs). 1,2 Numerous donor units have been reported with low ionization potential (IP) and good device performance. 3 However, the development of acceptors, especially the electron- deficient ones with high electron affinities (EAs), e.g. strong electron acceptors, still lags compared to their donor counterparts. Therefore, the development and design of high performance n-type organic semiconductors based on strong acceptor units remain a focal issue in the field of functional π- materials. 4−8 Naphthalenediimide (NDI) is no-doubt the most important kind of electron-deficient π-building block and serves as a promising candidate for organic electronics applications, such as photovoltaic devices and flexible displays, due to their high EAs, high charge carrier mobility, and excellent thermal stability. 9−13 Even though NDIs possess high EA values, some of their derivatives are still unstable under air. When they were applied in optoelectronic devices, for example, a high electron mobility of 6.2 cm 2 /(V·s) has been reported from the crystals of a core-unsubstituted NDI. However, significant degradation was observed in the OFET devices. 14 To improve the air stability of the NDIs, higher electron-deficient units have been generated by grafting N-heteroacene, cyano, perfluoroalkyl, and chloro groups onto the NDI cores. 10,15−26 Furthermore, no report has shown the possibility of developing another electron- deficient unit annulated to the NDI core with high EA. Herein, thiadiazoloquinoxaline, as a strong electron-deficient unit, is annulated on the core position of naphthalenediimide to decrease the LUMO levels. As shown in Figure 1, thiadiazolo- quinoxaline-fused naphthalenediimides (TQ-f-NDI), combin- ing naphthalenediimide (NDI), thiadiazoloquinoxaline (TQ), and N-heteroacene (NHA) in one molecule, are first reported. As expected, the DFT calculated LUMO energy level of TQ-f- NDI is as low as −3.96 eV, which is much lower than that of unsubstituted NDI (−3.41 eV). A series of TQ-f-NDIs are synthesized by the nucleophilic substitution of tetrabromo-NDI with benzothiadiazole diamine, followed by oxidation with PbO 2 . Thereby strong acceptors with high EA of ∼4.5 eV are obtained. OFETs based on the TQ-f-NDI were fabricated by solution process, revealing a good n-channel field-effect response and good stability under ambient conditions. The synthesis of diamine 1a is shown in Scheme S1, starting from 4,7-dibromobenzothiadiazole by a four-step procedure, and benzothiadiazole diamine 1b was synthesized according to a reported procedure. 27 The tetrabromo NDIs 2 were synthesized according to reported procedures with little modification. 28,29 To balance the crystallinity and solubility, Received: September 28, 2017 Figure 1. Chemical structures of [1,2,5]thiadiazolo[3,4-f ]quinoxaline, N-heteroacene, NDI, TQ-f-NDI, and the LUMO and HOMO distribution of NDI and TQ-f-NDI (DFT calculations by B3LYP/6- 31G(d)). Letter pubs.acs.org/OrgLett © XXXX American Chemical Society A DOI: 10.1021/acs.orglett.7b03041 Org. Lett. XXXX, XXX, XXX−XXX Cite This: Org. Lett. XXXX, XXX, XXX-XXX