Gas-phase formation and photochemistry of large cyano-containing polycyclic aromatic hydrocarbon clusters

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Cyano (CN)-containing polyaromatic species (e.g., cyano polycyclic aromatic hydrocarbons; CN-PAHs) have been observed in interstellar space, and cyano units play an important role in their chemical evolution processes. We investigate the gas-phase formation of large cyano-containing PAH clusters and their photochemistry behavior. The experimental results show large cyano-containing PAH cluster cations that formed through ion-molecule reaction between large PAH (e.g., dicoronylene; DC; C48H20) cations and small cyano-PAHs (e.g., 9-cyanoanthracene; C15H9N) in the gas phase. Under laser irradiation, the types of formed products are more diverse. Two main types of large cluster cations are observed: [PAH–PAH]+ (e.g., [(C15H9N)2C48H(1–19)]+) and [carbon clusters–PAH]+ (e.g., [(C15H9N)2Cn]+, n = [36, 48]). With irradiation, cyano-containing PAH cluster cations involve a complex photo-fragmentation process, dehydrogenation, CN-loss, and C2-loss channels are identified. The photo-fragmentation process in particular provides a potential way to synthesize large nitrogen-containing carbon clusters (e.g., [C61N]+ and [C63N]+). With theoretical calculations, we obtained the structure and bonding energy for the formation reactions, as well as the photodissociation pathways for the newly formed PAH cluster cations. The adduct processes and the newly formed molecular structure depend on the carbon reaction sites and connection pathways (C-C and C-N bonding type). This results in many reactions and very complex molecular clusters. We infer that small cyano-PAHs (e.g., 9-cyanoanthracene) can effectively aggregate on the large PAH species (e.g., (dehydrogenated) DC cations or carbon clusters), which provides a general molecular evolution route for the formation of nitrogen or cyano containing nanometer-sized polyaromatic species in a bottom-up process in the interstellar medium.