Structure–property relationship of donor–acceptor type conjugated copolymers with thienoisoquinoline and benzothiadiazole units

Abstract

We synthesized a new series of donor–acceptor (D-A) type conjugated copolymers containing two electron-accepting units, namely thienoisoquinoline (TIQO) and benzothiadiazole (BT) (PTIQO-HBT) or fluorinated BT analogs (PTIQO-FBT and PTIQO-FFBT) connected to thiophene-based electron-donating units. The optical and electrochemical properties, planarity of conjugated backbone, molecular crystalline structure, and charge transport characteristics were investigated to comprehensively study the structure–property relationship of these semiconducting copolymers. When the BT unit carries 0, 1, and 2 fluorine, both the highest occupied and the lowest unoccupied molecular orbital energy levels of the conjugated copolymers gradually decrease. DFT calculations showed no significant difference in the energy gap or the copolymer backbone planarity. However, the grazing incidence X-ray diffraction results confirm that the PTIQO-FBT copolymer film has a longer coherence length of π–π stacking between the molecular chains than PTIQO-HBT and PTIQO-FFBT, which may affect the intermolecular charge transport. Organic field-effect transistors (OFETs) employing the three TIQO-based D-A type semiconductor copolymer films as active layer all exhibited ambipolar characteristics that enable both hole and electron transport. Particularly, the PTIQO-FBT based OFETs (hole mobility: 0.030 cm2/V·s, electron mobility: 0.023 cm2/V·s) demonstrate higher performance with well-balanced hole and electron transport, as compared to the other two copolymers in terms of field-effect mobility. Therefore, introducing a single fluorine into the BT unit in the TIQO-based D-A type conjugated copolymers results in the most appropriately controlled ground-state charge transfer for efficient intramolecular charge transport along the polymer backbone, as well as higher molecular film crystallinity.