Furan functionalized naphthalenediimide semiconductors with different N-alkyl chains for n-type organic thin-film transistor applications

Abstract

Development of n-type organic semiconductors (OSCs) is necessary in order to develop complementary electronics based on organic thin-film transistors (OTFTs). In this investigation, we designed and synthesized a series of four novel imide- and core-substituted naphthalene diimide (NDI) molecules. The molecular architecture was altered via introduction of different alkyl side chains (octyl and 2-ethylhexyl) to the imide-N positions and 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) to the NDI core. The NDI-TCF derivatives are abbreviated as NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B, using a “1” or “2” to indicate the number of TCF subunits at the NDI core, and an “A” or a “B” to indicate an octyl or a 2-ethylhexyl alkyl chain at the imide-N position, respectively. All four compounds displayed similar photophysical properties but exhibited different electrochemical properties with lower LUMO levels of −4.25 eV, −4.33 eV, −4.27 eV and −4.35 eV for NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B, respectively. These LUMO levels suggest a capacity for electron transport. Furthermore, the NDI-TCF chromophores were incorporated into OTFTs and systematically studied on the relationship between the NDI-TCF molecular architecture and electron transport properties. NDI-TCF1A and NDI-TCF1B displayed more consistent OTFT performance than NDI-TCF2A and NDI-TCF2B. NDI-TCF1A and NDI-TCF1B exhibited low threshold voltages (VT) of −0.43 V ± 4.78 V and 6.12 V ± 6.63 V, respectively, electron mobilities (μe) of ca. 3 × 10−5 cm2V−1s−1, and on/off (ION/OFF) ratios of ca. 103. Insofar that TCF facilitates electron transport, the OTFT data suggest that the number of TCF subunits affect OTFT performance via thin-film morphology. The thin-film morphology of NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B was studied using atomic force microscopy (AFM) technique and supported by X-ray diffraction measurements.