Indenyl-Benzyl Radical Recombination Leads to the Formation of Fluoranthene, a Non-Alternant Polycyclic Aromatic Hydrocarbon

Nonalternant polycyclic aromatic hydrocarbons (PAHs) incorporating five-membered rings are fundamental intermediates linking planar aromatics to curved carbon nanostructures including fullerenes and nanotubes, yet their formation mechanisms are not fully understood. The detection of fluoranthene (C16H10) in samples from the near-Earth carbonaceous asteroid 162,173 Ryugu and 101,955 Bennu highlights the need for chemically viable pathways under both combustion and astrochemical conditions. Here, we demonstrate that fluoranthene forms efficiently commencing with barrierless recombination of resonance-stabilized indenyl (C9H7•) and benzyl (C7H7•) radicals. Microreactor experiments combined with electronic structure calculations reveal rapid carbon–carbon bond formation, cyclization, and dehydrogenation to the nonalternant framework. This mechanism represents an alternative to stepwise acetylene addition and enables early incorporation of pentagonal motifs during molecular growth. These findings establish radical–radical recombination as a versatile route to nonalternant PAHs, providing a unified framework for their formation in high-temperature combustion systems and carbon-rich astrophysical environments like circumstellar envelopes of carbon stars and planetary nebulae as their descendants.