Two functionalized dithieno[3,2-b:2',3'-d]phospholes with solubilizing groups have been synthesized that allow for the generation of a series of pi-conjugated AB- and ABC-copolymers. The polymers obtained show notable optoelectronic properties with red-shifted absorption and emission in the orange to red section of the optical solar spectrum. Although combination of dithienophosphole units with fluorene building blocks gives access to processable polymers with band gaps between 2.2 and 2.3 eV in solution and 2.0 eV in the solid state, an ABC copolymer based on dithienophosphole, fluorene, and bis(thienyl)benzothiadiazole units was found to not only exhibit a suitable band gap for solar cell applications (solution: 2.0 eV; solid state: 1.7 eV) but also showed good solubility as well as good electron transfer properties in the presence of fullerene (C-60). (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8179-8190, 2008