Tuning the Band Gap of Low-Band-Gap Polyselenophenes and Polythiophenes: The Effect of the Heteroatom

A series of new low-band-gap thieno- or selenolo-fused polyselenophenes (P5 and P6) and selenolo-fused polythiophene (P4) (as well as previously reported thieno-fused polythiophene, P3) was prepared systematically by electropolymerization (P4-P6) and by solid-state polymerization (P3, P5 and P6). The 2,5-dibrominated monomers (3Br(2), 5Br(2), and 6Br(2)) undergo solid-state polymerization under slight heating and produce insoluble P3, P5, and P6 as black conducting powders. The spectroelectrochemically measured optical band gaps of P4-P6 films are 0.96, 0.72, and 0.76 eV, respectively. DFT calculations performed on P3-P6 provide excellent estimations of the experimental band gaps of these polymers. The band gap of the polyselenophenes (P5 and P6) is 0.2 eV lower than that of the corresponding polythiophenes (P3 and P4). We introduced a new scheme for band gap control in conjugated polymers by replacing the sulfur atom with a selenium atom in the main and/or peripheral ring, which leads to significant and predictable changes in the band gap of the polymers. This is due to the lower aromaticity of a selenophene ring compared to a thiophene ring. Thus, we have achieved band gap control in very low band gap (similar to 0.7-1.0 eV) polymers through the use of different combinations of selenium and sulfur atoms in the main and peripheral rings.