Polarity change of OFETs based on Dithienocoronene Diimide (DTCDI)-Derived isomeric triads end-capped with Azulene

Abstract

Exploring the polarity change of organic field-effect transistors (OFETs) based on organic semiconductors through the molecular orbital distribution is a challenging work. In this study, we present two dithienocoronene diimide (DTCDI)-derived isomeric triads end-capped with azulene (DTCDI-B2Az and DTCDI-B6Az), which were designed and synthesized by connecting two azulene units at 2-position of five-membered ring (DTCDI-B2Az) or 6-position of seven-membered ring (DTCDI-B6Az) to the π-conjugated electron-deficient DTCDI-core, respectively. The two isomeric triads show different molecular structure, electronic structure, optical and electrochemical properties. DTCDI-B2Az features the lowest unoccupied molecular orbital (LUMO) energy levels (−3.81 eV) and the highest occupied molecular orbitals (HOMO) energy levels (−5.80 eV), and meanwhile the LUMO/HOMO orbital distributing over the whole molecule, which facilitates electron/hole injection and transport under ambient conditions. However, DTCDI-B6Az possesses a LUMO/HOMO energy levels (−4.20/−6.02 eV) and its HOMO orbital is localized on two end-capping azulene units that can decrease the electron transfer barrier for more effective electron injection, and simultaneously suppress the hole injection and transfer. The OFET devices based on the two triads were fabricated by solution-processed method using bottom-gate top-contact (BGTC) device structure. For DTCDI-B2Az, air-stable ambipolar transport characteristics were achieved with the highest hole/electron mobilities of 5.40 × 10−4 and 1.83 × 10−2 cm2 V−1 s−1, respectively. However, for DTCDI-B6Az, air-stable unipolar n-type performances were obtained with the highest electron mobility of 1.56 × 10−2 cm2 V−1 s−1. Overall, this study demonstrates the polarity of OFETs was changed with the DTCDI-derived triads constructed by changing the connection position of the end-capped azulene group to the π-conjugated electron-deficient DTCDI-core.