Fused ring structures such as cyclopentadithiophenes (CPDT), fluorenes and dithienopyrroles have been shown to have low band gaps and extended conjugation lengths. Both of these properties make them interesting materials for organic semiconductor applications. It has been shown that increasing the planarity of building blocks, and in turn the polymer or molecule they are contained within, can improve charge transport by increasing π-π stacking. While the homo-polymers of dithienopyrrole, and alkene bridged CPDT and fluorene have low band gaps, it is possible to further reduce them by forming co-polymers, either by combination with an acceptor unit or by increasing the planarity of the polymer. Three families of small molecules and corresponding co-polymers were therefore synthesised using three main building blocks - alkene bridged CPDT, alkylidene fluorene and dithienopyrrole - combined with a series of co-monomers. In the cases of CPDT and dithienopyrrole two different alkyl chains were used with the aim of influencing solubility and solid-state packing. The optical and electrochemical properties were studied and trends determined. In addition physical and thermal properties were studied and for several of the small molecules, crystal structures and theoretical structures were also evaluated to probe intermolecular interactions in the solid state.During the synthesis of one alkylidene fluorene small molecule an unexpected impurity was isolated in which the bridging alkene of the fluorene had rearranged. Possible reasons for the formation were investigated and the properties of the compound studied.Several small molecules and one co-polymer were incorporated into preliminary OFET devices, although performance was limited by poor film formation. Film forming studies were also undertaken on the co-polymer.