Synthesis and charge-transporting properties of electron-deficient CN2–fluorene based D–A copolymers

In this paper, we report four D–A copolymers on the basis of a novel acceptor (A) unit of 9-fluorenylidene malononitrile (CN2–Fluorene) and four common donor (D) units of 9-alkylfluorene (P1), benzodithiophene (P2), bithiophene (P3), and dithienopyrrole (P4), respectively. These four copolymers were synthesized using Stille or Suzuki coupling and physicochemically characterized. Each of them exhibits a weak near IR absorption band extending down to 800 nm, due to weak intramolecular charge transfer (ICT) between the donor and CN2–Fluorene units. The LUMO energy levels of the copolymers (∼−3.70 eV) are determined mainly by the CN2–Fluorene unit, while the HOMO energy levels are finely tuned by the incorporation of donors, with the electrochemical bandgap decreasing from 2.04 eV for P1 to 1.56 eV for P2, 1.28 eV for P3, and 1.18 eV for P4. TD-DFT calculations on one repeating unit of the copolymer confirm the strong LUMO-determining ability of the CN2–Fluorene unit. The charge transport properties of the copolymers are investigated by fabricating OTFT devices with a bottom-contact configuration. The hole mobility of P3 is 1.43 × 10−3 cm2 V−1 s−1 under ambient conditions and that of P2 and P4 is 1.31 × 10−4 and 1.81 × 10−4 cm2 V−1 s−1, respectively.