Donor-acceptor (DA) frameworks have become the most common approach for the generation of reduced and low band gap materials. In such DA frameworks, it is generally believed that the small ionization potential of the donor (D) unit results in an energetically destabilized highest-occupied molecular orbital (HOMO) energy level and the high electron affinity of the acceptor (A) unit results in an energetically stabilized lowest-unoccupied molecular orbital (LUMO) energy level. With the right combinations, the coupling of the donor and acceptor units could then potentially result in a HOMO-LUMO gap smaller than either of the units individually would be able to obtain. Current models of these DA frameworks consist of a donor unit with a relatively high HOMO level and an even higher LUMO level, coupled with an acceptor unit with a relatively low LUMO level, but quite deep HOMO level. More recently, however, thieno[3,4-b]pyrazines (TPs) have been shown to act as a strong donor and strong acceptor unit simultaneously due to both its high HOMO and low LUMO level, resulting in something that can be best described as an ambipolar unit. However, there is has been little investigation as to how these ambipolar units behave in DA frameworks and how this affects the band gap of the resulting polymer.
In order to further probe the parameters of DA effects in conjugated materials, a series of dimers have been prepared in the current study. Homo- and hetero- dimers of compounds considered to be classic acceptor units, 2,1,3-benzothiadiazole (BTD), classic donor units, 3,4-ethylenedioxytiophene (EDOT), and ambipolar TP units were synthesized for direct comparison. The resulting dimers were characterized by absorption spectroscopy, cyclic voltammetry, and computational modeling. Full comparison of the various electronic effects from each combination will be presented with discussion of the repercussions on the current DA models commonly applied on conjugated materials.