Bi-diketopyrrolopyrrole (Bi-DPP) as a novel electron accepting compound in low band gap π-conjugated donor–acceptor copolymers/oligomers

The synthesis and characterization of a novel 2,5-diketopyrrolo[3,4-<i>c</i>]pyrrole(DPP)-based accepting building block with the scheme DPP-neutral small linker-DPP (<b>Bi-DPP</b>) is presented, which was utilized as electron accepting moiety for low band gap π-conjugated donor-acceptor copolymers as well as for a donor-acceptor small molecule. The electron accepting moiety <b>Bi-DPP</b> was prepared <i>via</i> a novel synthetic pathway by building up two DPP moieties step by step simultaneously starting from a neutral phenyl core unit. Characterization of the synthesized oligomeric and polymeric materials via cyclic voltammetry afford LUMO energy levels from -3.49 to -3.59 eV as well as HOMO energy levels from -5.07 to -5.34 eV resulting in low energy band gaps from 1.52 to 1.81 eV. Spin coating of the prepared donor-acceptor oligomers/polymers resulted in well-defined films. Moreover, UV-vis measurements of the investigated donor-acceptor systems showed a broad absorption over the whole visible region. It is demonstrated that <b>Bi-DPP</b> as an electron accepting moiety in donor-acceptor systems offer potential properties for organic solar cell devices.