Abstract
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.