Abstract
The quinoxalineimide (QI) unit, containing the electron-withdrawing quinoxaline and imide groups, is an electron-deficient building block for organic semiconductor materials. In this study, three fluorinated or chlorinated QIs (QI-1F, QI-2F, and QI-2Cl), have been designed and developed. We report the impact of the fluorination or chlorination of the QI unit on the electronic structures and charge carrier transport properties as compared to unsubstituted QI (QI-2H) bearing the same n-hexyl side chains. The frontier molecular orbital energy levels downshifted with the incorporation of fluorine or chlorine atoms onto the π-framework of QI. Single-crystal structure analyses revealed that all QI-based molecules have an entirely planar backbone and are packed into two-dimensional slipped stacks with diagonal electronic coupling that enables two-dimensional charge carrier transport. Notably, the doubly fluorinated or chlorinated QIs formed compact molecular packing in the single-crystal structures through an infinite intermolecular network relative to unsubstituted QI (QI-2H). The field-effect transistor-based QI molecules exhibited typical n-channel transport properties. As compared to unsubstituted QI (QI-2H), the chlorinated QI exhibited improved electron mobilities up to 7.1 × 10−3 cm2 V−1 s−1. The threshold voltages of the fluorinated or chlorinated QI devices were clearly smaller than that of QI-2H, which reflects the lowest unoccupied molecular orbital levels of the molecules. This study demonstrates that the fluorinated or chlorinated QIs are versatile building blocks in creating n-channel organic semiconductor materials.
Highlights
Introduction nChannel organic semiconductors are highly attractive for a wide variety of potential applications including n-channel organic eld-effect transistors (OFETs), organic photovoltaics (OPVs), and organic thermoelectrics.[1,2,3,4,5] in contrast to the high performance p-channel materials, n-channel mobilities are still lower than those of p-channel organic materials.[6]
To design an n-channel organic semiconductor, controlling the lowest unoccupied molecular orbital (LUMO) level is a key factor, in which low LUMO levels are desirable for facilitating electron injection from electrodes and enhancing the electrochemical stability of organic materials.[8,9]
The detailed synthetic procedures and characterizations are shown in the Electronic supplementary information (ESI).† The decomposition temperatures de ned by a 5% weight-loss temperature (T5% weight loss) in thermal gravimetric analysis (TGA) estimated to be 285 C for 1, C for 2, C for 3, and 295 C for 4 (Fig. S2(a)†)
Summary
Introduction nChannel organic semiconductors are highly attractive for a wide variety of potential applications including n-channel organic eld-effect transistors (OFETs), organic photovoltaics (OPVs), and organic thermoelectrics.[1,2,3,4,5] in contrast to the high performance p-channel materials, n-channel mobilities are still lower than those of p-channel organic materials.[6]. The chemical modi cation with strong electron-withdrawing groups directly contributes to lowering the LUMO levels and effectively stabilizing the reductant species of the p-conjugated molecules.
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