2, Harvard University, Cambridge, Massachusetts, United States
The direct (hetero)arylation polymerization (DHAP) reaction harnesses the single-step activation and arylation of aromatic carbon-hydrogen bonds for the efficient synthesis of conjugated polymers. This palladium-catalyzed reaction allows for the formation of a bond between two sp2-hybridized carbons via the coupling of the carbon-halogen (C–X) bond of one arene (or heteroarene) with the carbon-hydrogen (C–H) bond of another. By avoiding the need for transmetalating agents used in other polymerization techniques, the number of synthesis steps is reduced, the need for expensive and often unstable reagents is minimized and the production of toxic organometallic by-products is altogether eliminated. These factors contribute to a reaction which is more favourable than traditional methods for the preparation of conjugated polymers from an industrial and an environmental perspective.
Most high-performing conjugated polymers for organic photovoltaic applications contain thiophene-based repeating units. These heterocycles possess desirable electronic features and are easily functionalized with electron-accepting or -donating moieties or with solubilizing side-chains in order to tune their electronic and physical properties. However, the issue has arisen over the selectivity of the concerted metalation-deprotonation (CMD) transition state, the key step of the direct arylation mechanism which determines the selectivity of C-H bond activation. There are often multiple reactive C–H bonds on a given monomer, and if the undesired bond (the “β”-C–H bond) were to be activated, it would generate a β-defect in the resulting polymer. This may lead to a disruption in both the π-conjugation of the polymer and the supramolecular organization of the material in the solid state, factors which can contribute to reduced performance in organic electronic devices.
Given the ubiquity of thiophene-based units in conjugated polymers and the assumed issues regarding selectivity, we endeavored to study using computational techniques the direct arylation mechanism on model thiophene substrates possessing various electronic features. Using density functional theory and coupled-cluster methods, activation barriers for the CMD transition states of various C-H bonds were calculated and analyzed using the distortion/interaction model. The activating effect of a halide (C–X) on thiophene derivatives was also studied. The results suggest that there are inherent features of selectivity for electron-rich or electron-poor thiophenes, and that the location of the halide moiety greatly influences coupling selectivity by activating the undesirable β-C-H bond. We expect that these findings could guide the design of monomers amenable to high-selectivity DHAP polymerizations.
 Pouliot, J.-R.; Grenier, F.; Blaskovits, J. T.; Beaupré, S.; Leclerc, M., Chem. Rev. 2016, 116 (22), 14225-14274.
 Gorelsky, S. I.; Lapointe, D.; Fagnou, K., J. Am. Chem. Soc. 2008, 130 (33), 10848-10849.