Conjugated systems built by connecting two electron-donor side-chains to a diketopyrrolopyrrole (DPP) core have been synthesized and evaluated as donor material in heterojunction organic solar cells. The effects of composition of the side-chain on the electronic properties of the conjugated system have been analyzed on first series of compounds containing various combinations of benzofuran, benzothiophene, thiophene and furan units. In a second series of compounds, the keto groups of DPP have been replaced by one or two thioketo groups. Results of UV-vis absorption spectroscopy, fluorescence emission spectroscopy and cyclic voltammetry show that the composition of the side-chain has little effect on the HOMO and LUMO levels of the system, but strongly affects the sensitivity of the material toward thermal treatment and thus indirectly the performances of the resulting solar cells. On the other hand, replacement of the keto groups of DPP by thioketo ones leads at the same time to significant reduction of the band gap due to a decrease of the LUMO level, to a quenching of fluorescence and to dramatic decrease of the photovoltaic activity of the molecule. (C) 2012 Elsevier Ltd. All rights reserved.