Designing highly delocalized solitons by harnessing the structural parity of π-conjugated polymers

π-Conjugated polymers are a class of materials featuring an alternation of single and double bonds along their backbone, a configuration that can result in delocalized π-electrons. The unique electronic structure of these polymers makes them vital in applications such as organic electronics, solar cells and light-emitting diodes. A key feature in such materials is the emergence of topological quasiparticles, termed solitons, which are crucial for their observed high electrical conductivity. By using on-surface synthesis, we present a chemical reaction based on the regio- and stereoselective coupling of indenyl moieties for fabricating π-conjugated acenoindenylidene polymers, which feature a longitudinal polyacetylene backbone, on a Au(111) surface. The relationship between structural parity and electronic properties is investigated. We discover that odd-membered polymers exhibit an in-gap soliton state, which, due to their low bandgaps, spatially extends several nanometres along the longitudinal polyacetylene backbone. Our findings pave the way for the design of π-conjugated polymers that are able to host intrinsic solitons through chemical design by exploiting structural parity, without the need for external doping. An on-surface synthetic route for the regio- and stereoselective coupling of indenyl moieties, affording the design of π-conjugated acenoindenylidene polymers on Au(111), is reported. The relationship between the structural parity of the polymers and their electronic properties reveals the emergence of highly delocalized soliton quasiparticles in odd-membered polymers.