Noncovalent intramolecular interactions in the monomers and oligomers of the acceptor and donor type of low band gap conducting polymers

Intramolecular interactions of noncovalent nature in the conjugated oligomers of co-thiophene-thieno[3,4-b]pyrazine-thiophene and co-thiophene-benzo[c]1,2,5-thiadiazole-thiophene are studied quantum mechanically. S center dot center dot center dot N contacts are identified by the existence of topological bond critical points (bcp's) along the bond path by means of atoms-in-molecules (AIM) theory. The planarity of the oligomers is attributed to the observed ring critical points among the thiophene and pyrazine or thiadiazole rings. Similar systems, obtained by replacing the thiophene by pyrrole and pyrazine by ethylenedioxy, are further studied with AIM analysis. The extent of the contribution of intramolecular interactions to the intrinsic conductivity of the conjugated polymers is investigated by extrapolating the oligomer band gap values from the time-dependent density functional theory excitation energies and density functional theory HOMO-LUMO gaps and is found to be in excellent agreement with measured experimental values, where values are present. Pyrrole substitution leads to the lowest band gap value (<1 eV) among the oligomers studied. A rule of thumb for designing and engineering new low band gap polymers, which focuses on the interplay between the HOMO of the donor and LUMO of the acceptor moieties, is posed.