Aryl radical addition-dehydrocyclization drives rapid polycyclic aromatic hydrocarbon growth.

Polycyclic aromatic hydrocarbons are ubiquitous in space, yet astrochemical models fail to reproduce their observed abundances because known growth mechanisms are far too slow. Here, we report a plausible pathway for rapid polycyclic aromatic hydrocarbon mass growth mediated by multi-ring aryl radicals via the aryl addition-dehydrocyclization mechanism. High-temperature reactions of 1- and 2-naphthyl radicals with biphenyl and naphthalene serve as proof-of-concept, demonstrating the gas-phase formation of five-ring polycyclic aromatic hydrocarbons including benzo[b]triphenylene, benzo[4]helicene, benzo[k]fluoranthene, benzo[j]fluoranthene, and perylene through [3 + 2] zig-zag edge and [4 + 2] carbon bay closures. Products are identified isomer-selectively using tunable vacuum-ultraviolet photoionization coupled with i²PEPICO spectroscopy. Initiated by a single collision, the aryl addition-dehydrocyclization mechanism forms multiple carbon-carbon bonds, possibly accelerating polycyclic aromatic hydrocarbon growth compared with sequential pathways such as hydrogen abstraction-acetylene addition. This pathway provides a route from small aromatics to extended two- and three-dimensional carbon nanostructures, reshaping our understanding of aromatic carbon evolution in high-temperature astrophysical environments, from circumstellar envelopes to planetary nebulae, offering insights into the formation of complex carbonaceous molecules in space.