The design and study of π-conjugated polymers has received great attention along the last decades. The relevant optical and electronic properties stemming from their delocalised π-electrons allow for a number of applications in the emerging field of organic electronics. However, the inherent limited solubility of planar π-conjugated systems hinders their development, forcing chemists to introduce ancillary solubilising side-chains. On the other hand, ultrahigh-vacuum on-surface synthesis has become a powerful discipline that enables designing with atomistic precision a new plethora of molecular compounds, polymers, and nanomaterials that otherwise are unachievable by conventional organic chemistry.

Herein we present a novel on-surface chemical transformation that allows obtaining π-conjugated acene polymers from simple aromatic molecules carrying =CBr₂ functionalities. The deposition of such precursors on an Au(111) surface gives rise to close-packed assemblies. Thermal annealing promotes the debromination of the species that thereafter homocouple and give rise to long anthracene wires linked by acetylene bridges, featuring a bandgap of 1.5 eV (see figure below).

When larger acenes or periacenes are used (i.e. pentacene, bisanthene, peripentacene) the resulting polymers undergo dramatic structural and electronic changes. Non-contact-AFM evince that the benzoid subunits evolve from aromatic (anthracene) to quinoid (pentacene, bisanthene...), while the alkyne linkers turn into cumulenic. The STM images allow witnessing the HOMO-LUMO levels crossing from anthracene to pentacene. This swap destabilises the aromatic structure and enables a biradical-quinoid one, that permit almost vanishing bandgaps below 0.35 eV. These findings can also be rationalised by topological band gap theory: DFT, tight binding and GW calculations predict that polymers these quasi-metallic polymers exhibit a topologically non-trivial electronic structure. Our results herald novel pathways to engineer π-conjugated polymers on solid surfaces, addressing the relevant family of acenes and, thus, contributing to develop the field of on-surface chemistry and to steer the design of modern low bandgap polymers.