The influence of extending the molecular length of donor acceptor chromophores on properties relevant to organic optoelectronic devices has been studied by using two new narrow-band-gap systems. Most significantly, we find that the higher molecular weight systems exhibit higher thermal stabilities (beyond 200 degrees C) when introduced into field effect transistor devices. It is also possible to fabricate bulk heterojunction solar cells using PC61BM with power conversion efficiencies >6%. These high values are not heavily influenced by the blend composition and are achieved without the influence of solvent additives or postdeposition thermal annealing.