In this paper we analyze, with the help of density-functional theory calculations, the relationship between the molecular structure and the optical and vibrational properties of two narrow band gap pi-conjugated co-oligomers containing an alternating sequence of aromatic donor and o-quinoid acceptor units. The optimized molecular geometries of these co-oligomers reveal that short inter-ring S...N contacts occur in their minimum-energy structure between the two types of constituiting units and that the resulting rigid coplanar arrangement of the rings enhances the degree of pi conjugation and lowers the band gap.