We synthesized a series of new low-band gap donor–acceptor copolymers containing 4,4′-bis(alkyl)-[6,6′-bithieno[3,2-b]pyrrolylidene]-5,5′(4H,4′H)-dione. This acceptor unit, so-called dithienoketopyrrole (DTKP), is an analogue of isoindigo, the phenyl rings of which are replaced by thiophenes. Donor moieties such as benzodithiophene, cyclopentadithiophene, fluorene, and dithienothiophene are polymerized with DTKP in an alternating fashion by Stille or Suzuki–Miyaura coupling methods. Exceedingly low-band gaps (E_g = 1.0–1.6 eV) were achieved in these copolymers through internal charge transfer interactions between the donor and acceptor moieties. The structural, photophysical, and electrochemical properties of the resultant copolymers were characterized, and field-effect transistor (FET) mobilities were measured. The copolymers showed electronic absorption spectra extending to the near infrared region (600–1400 nm) with absorption maxima at 745–971 nm, along with a low-lying LUMO of −3.8 eV. Density functional theory (DFT) calculation indicated high planarity for the copolymer backbone when compared to that of its phenyl-isoindigo counterparts. FET hole mobilities on the order of 10⁻⁴ to 10⁻³ cm² V⁻¹ s⁻¹ were obtained, demonstrating a feasibility to use them in organic photovoltaic cells.