Stepwise Lateral Extension of Phenyl-Substituted Linear Polyphenylenes

Polyphenylenes (PPs) are unique polymers showing high mechanical strength and chemical stability, and having potential applications, for example, in proton transfer and gas-separation membranes. Moreover, phenyl-substituted linear PPs can serve as precursors for bottom-up syntheses of graphene nanoribbons (GNRs), a new class of nanoscale carbon materials that appear promising for nanoelectronics. Notably, lateral extensions of linear PPs with appropriate branched phenyl substituents, that is, avoiding spatial overlap of benzene rings in their projections into a plane, can lead to wider GNRs with modulated electronic and optical properties. GNRs with widths up to approximate to 2 nm are obtained, but synthetic methods to further expand PPs laterally so as to achieve even wider GNRs have not yet been developed. Here, phenyl-substituted linear PPs bearing two ethynyl groups at outer positions in each repeating unit, together with 3,4-diphenylcyclopentadienones bearing a trimethylsilyl-protected ethynyl group are used for stepwise lateral extension of the PPs, based on a sequence of Diels-Alder cycloadditions and deprotections. Each successive reaction step is corroborated by H-1 NMR and IR spectroscopy, as well as gel permeation chromatography, providing a new pathway toward branched polyphenylenes with linear-PP-backbones that can potentially serve as precursors of wider GNRs with tunable electronic bandgaps.