Gas-phase preparation of azulene (CH) and naphthalene (CH) the reaction of the resonantly stabilized fulvenallenyl and propargyl radicals.

Synthetic routes to the 10π Hückel aromatic azulene (CH) molecule, the simplest polycyclic aromatic hydrocarbon carrying an adjacent five- and seven-membered ring, have been of fundamental importance due to the role of azulene - a structural isomer of naphthalene - as an essential molecular building block of saddle-shaped carbonaceous nanostructures such as curved nanographenes and nanoribbons. Here, we report on the very first gas phase preparation of azulene by probing the gas-phase reaction between two resonantly stabilized radicals, fulvenallenyl and propargyl , in a molecular beam through isomer-resolved vacuum ultraviolet photoionization mass spectrometry. Augmented by electronic structure calculations, the novel Fulvenallenyl Addition Cyclization Aromatization (FACA) reaction mechanism affords a versatile concept for introducing the azulene moiety into polycyclic aromatic systems thus facilitating an understanding of barrierless molecular mass growth processes of saddle-shaped aromatics and eventually carbonaceous nanoparticles (soot, interstellar grains) in our universe.