Donor–acceptor alternating copolymers with chlorine atoms on the backbone are synthesized via a straightforward Stille polycondensation. Thiophene and chlorine-substituted 2,3-diphenylquinoxaline are chosen as the donor and acceptor moieties, respectively. Compared with the corresponding non-substituted and fluorine-substituted analogues, which are also synthesized for systematical studies, the chlorine-bearing copolymer exhibits the deepest lowest unoccupied molecular orbital (LUMO). The highest occupied molecular orbital (HOMO) level of the chlorine-bearing copolymer can be modified by applying different donor moieties. Additionally, these chlorine-bearing copolymers show low self-absorption and large Stokes Shift.