Using Stille and Suzuki polymerization reactions we incorporate thienoisoindigo (TII) as an acceptor co-monomer into a series of alternating π-conjugated copolymers with combinations of benzene, thiophene and carbazole as donor co-monomers. By changing the nature and length of the donor segments, the optical band gap of these soluble TII copolymers can be varied over a large range from 1.52 eV down to 0.87 eV. The semiconducting properties of the TII copolymers were established in bottom-gate bottom-contact field-effect transistors that provide hole mobilities for these materials in the range of 10⁻³ to 10⁻² cm² V⁻¹ s⁻¹.