Ethynylene-Linked Donor-Acceptor Alternating Copolymers

Controlling steric interactions between neighboring repeat units in donor acceptor (D-A) alternating copolymers can positively impact morphologies and intermolecular electronic interactions necessary to obtain high performances in organic photovoltaic (OPV) devices. Herein, we design and synthesize 12 new conjugated D A copolymers, employing ethynylene linkages for this control. We explore D A combinations of fluorene, benzodithiophene, and diketopyrrolopyrrole with analogues of pyromellitic diimide, thienoisoindoledione, isothianaphthene, thienopyrazine, and thienopyrroledione. Computational modeling suggests the ethynylene-containing polymers can adopt virtually planar conformations, while many of the analogous polyarylenes lacking the ethynylene linkage are predicted to have quite twisted backbone (>35 degrees). The introduction of ethynylene linkages into these D A systems universally results in a significant blue-shift in the absorbance spectra (by as much as 100 nm) and a deeper HOMO value (similar to 0.1 eV) as compared to the polyarylene analogues. The contactless time-resolved microwave conductivity technique is used to measure the photoconductance of polymer/fullerene blends and is further discussed as a tool for screening potential active layer materials for OPV devices. Finally, we demonstrate that an ethynylene-linked alternating copolymer of diketopyrrolopyrrole and thienopyrroledione, with a rather deep LUMO estimated at -4.2 eV, shows increased photoconductance when blended with a perfluoroalkyl fullerene C-60(CF3)(2) as compared to the standard PC61BM. We attribute the change in increased free carrier generation to the higher electron affinity of C-60(CF3)(2) that is more appropriately matched with the deeper LUMO of the polymer.