Conjugated Donor−Acceptor Copolymer Semiconductors with Large Intramolecular Charge Transfer:  Synthesis, Optical Properties, Electrochemistry, and Field Effect Carrier Mobility of Thienopyrazine-Based Copolymers

Several conjugated thieno[3,4-b]pyrazine-based donor−acceptor copolymers were synthesized by Stille and Suzuki copolymerizations, and their optical, electrochemical, and field-effect charge transport properties were characterized. The new copolymers, poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine) (BTTP), poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine-alt-2,5-thiophene) (BTTP-T), poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine-alt-9,9-dioctyl-2,7-fluorene) (BTTP-F), and poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine-alt-1,4-bis(decyloxy)phenylene) (BTTP-P), had moderate to high molecular weights, broad optical absorption bands that extend into the near-infrared region with absorption maxima at 667−810 nm, and small optical band gaps (1.1−1.6 eV). They showed ambipolar redox properties with low ionization potentials (HOMO levels) of 4.6−5.04 eV. The field-effect mobility of holes varied from 4.2 × 10-4 cm2/(V s) in BTTP-T to 1.6 × 10-3 cm2/(V s) in BTTP-F. These results show that thieno[3,4-b]pyrazine-based donor−acceptor copolymers combine small optical band gaps with fairly high carrier mobilities and thus are promising for organic electronic devices.