A one-step postpolymerization modification that converts three high bandgap poly(arylene ethynylene)s into low bandgap donor acceptor copolymers is described. The strategy relies on a palladium-catalyzed cyclopentannulation reaction between the main-chain ethynylene functionality and a small molecule aryl bromide (6-bromo-1,2-dimethylaceanthrylene). The reaction installs new cyclopenta[hi]aceanthrylene electronaccepting groups between the electron rich arylenes along the polymer backbone. The modified polymers include poly(9,9-didodecyl-fluorene-2,7-ethynylene), poly(9-dodecyl-carbazole-2,7-ethynylene), and poly(2,5-dioctyloxyphenylene-1,4-ethynylene). The functionalization efficiency was evaluated via isotopic C-13 labeling of the polymeric ethynylene carbons and then monitoring the chemical environment of those carbons via NMR spectroscopy. Near complete conversion of the sp carbon species to sp(2) carbon species was observed, which demonstrates the high efficiency of the modification strategy. Gel permeation chromatography shows that the hydrodynamic radius of the polymers is reduced considerably going from linear to kinked polymer morphology upon functionalization, and molecular dynamics simulations illustrate the underlying morphological change. The newly formed donor acceptor polymers showed dramatically different optical and electrochemical properties from the precursor poly(arylene ethynylene) polymers. A new absorption band centered at similar to 650 nm represents a red-shift of >300 nm for the onset of absorption compared with that of precursor polymers and cyclic voltammetry shows two new low-lying reduction peaks that coincide with the cyclopenta[hi]aceanthrylene moiety.