3,7-Bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b ']difuran-2,6-dione Dicyanides with Engineered Side Chains for Unipolar n-Type Transistors

Air-stable solution-processable unipolar n-type organic semiconductors are highly desired in the field of organic thin film transistors as n-type transistors are indispensable components in low-power-consumption complementary integrated circuits. The present work seeks to address this issue and describes the synthesis of two dicyano-substituted bis(2-oxoindolin-3-ylidene)benzodifurandione (BOIBDD) derivatives with engineered alkyl side chains for n-type transistors. Two kinds of side chains are used in this study: 2-hexyldecyl (C16H33) and 3-hexylundecyl (C17H35). Surprisingly, the small structural difference between side chains induced remarkable differences in the physical properties and electronic performance of the BOIBDD derivatives. In addition, cyano substitution into the backbone of BOIBDD derivatives was shown to be effective at lowering the HOMO and LUMO energy levels of BOIBDD derivatives to obtain unipolar n-type materials. Compared with fluorine substitution, cyano substitution was readily realized by a simple and straightforward reaction using cheap CuCN as a cyanation reagent. Both atomic force microscopy (AFM) and X-ray diffraction (XRD) studies have confirmed that the BOIBDD derivative with 3-hexylundecyl side chains tended to form highly crystalline structures in the solid state. More interestingly, thermal annealing of this compound at a temperature as low as 75 degrees C, well below its melting point of 305 degrees C, can still further improve its crystallinity and thus significantly increase its electron mobility by 43 times to 0.1 cm(2) V-1 s(-1) in air. The insight gained from this study will shed some light on the design of new air-stable high-performance n-type organic semiconductors.