Deviate from the norm: ambipolar-acceptor frameworks as a new design paradigm for low bandgap conjugated polymers

A new design paradigm is presented for the production of low bandgap (E-g < 1.5 eV) polymers which takes advantage of the dual nature of building blocks that can simultaneously exhibit both strong donor and strong acceptor properties. By replacing the donor unit of commonly applied donor-acceptor (D-A) frameworks with such ambipolar units, the resulting ambipolar-acceptor framework maintains the classical D-A backbone, while effectively adding a second acceptor. As illustrative examples of this approach, the synthesis of a series of alternating copolymers are reported that pair the ambipolar unit thieno[3,4-b]pyrazine (TP) with various traditional acceptors, including 2,1,3-benzothiadiazole, quinoxaline, and 2H-benzotriazole, successfully producing soluble, processible materials with band gaps of 1.02-1.12 eV. Analysis of these materials reveals the tunability of this approach and provides convincing evidence that the LUMOs of the TP and traditional acceptor hybridize to give a delocalized and suitably stabilized polymer LUMO, thus contributing to the low bandgaps of the resulting polymers. To demonstrate the potential of these materials, initial application to bulk heterojunction photonic devices are also reported, revealing photoresponse out to beyond 1100 nm, with initial NIR photodetectors providing detectivity (D*) values as high as 8.9 x 10(11) Jones at 815 nm.