Cryogenic Bottom-up Formation of the Benzene Cation from Acetylene in Helium Nanodroplets

Bottom-up ion-molecule reaction networks are thought to drive molecular complexity in space. Nevertheless, even the formation of the first aromatic ring (benzene) remains poorly understood. In this publication, we report the formation of the benzene radical cation from an acetylene cation and two acetylene molecules via the sequential two-step reaction C₂H₂^•+ + C₂H₂ → C₄H₄^•+ followed by C₄H₄^•+ + C₂H₂ → C₆H₆^•+ inside cryogenic helium nanodroplets. The superfluid helium-environment, used as a model system to study chemical reactivity in an environment with efficient energy dissipation such as interstellar dust and ice, stabilizes reaction intermediates and products and enables direct spectroscopic investigation. Helium-tagging spectroscopy in the visible wavelength range unambiguously identifies the C₆H₆^•+ product as the benzene radical cation. This is consistent with quantum-chemical calculations that predict a reaction pathway that is barrierless in both steps. These results establish a low-temperature route to the formation of the first aromatic ring in very cold condensed phases, which may impact our understanding of polycyclic aromatic hydrocarbon formation in the interstellar medium.