Direct and indirect band gap types in one-dimensional conjugated or stacked organic materials

Whether a semiconductor has a direct or an indirect band gap is important in determining physical properties such as photoconductivity and electroluminescence. For one-dimensional conjugated polymeric semiconductors, as well as organic molecular crystals, we show how the band gap type (direct or indirect) is determined by the shapes of the HOMO and the LUMO of constituent monomeric conjugated molecules. The connectivity of the monomer units, and the topology of orbital interaction determine the band gap type. Pairing symmetry in the π electronic system of even alternant hydrocarbons allows the immediate prediction of the band gap type, direct or indirect, by examining only the structures of the monomer units and their connectivity in polymers or molecular stacks.