Gas-phase formation and photochemistry of large amino-containing PAH clusters

To understand the chemical evolution processes of nitrogen-containing polyaromatic species in interstellar environments, the formation of large amino (NH₂)-containing polycyclic aromatic hydrocarbon (PAH) clusters and their photochemical behavior are studied here. Large amino-containing PAH cluster cations are efficiently formed through an ion–molecule reaction between large PAH cations [e.g. dicoronylene (DC, C₄₈H₂₀)] and small amino-substituted PAH molecules [e.g. 9-aminoanthracene (C₁₄H₁₁N) and 1-pyrenamine (C₁₆H₁₁N)]. Two types of large cluster cations are observed: [PAH–PAH]⁺ (e.g. [(C₁₄H₁₁N)₂C₄₈H₍₁₋₂₀₎]⁺ and [(C₁₆H₁₁N)₂C₄₈H₍₁₋₂₀₎]⁺) and [carbon clusters–PAH]⁺ (e.g. [(C₁₄H₁₁N)₂Cₙ]⁺ and [(C₁₆H₁₁N)₂Cₙ]⁺, with n = [34, 48]). Upon irradiation, the newly formed cluster cations undergo a complex photofragmentation process; both dehydrogenation and NH₂ unit loss channels are identified. Through theoretical calculations, the structure and bonding energy for the formation reactions, as well as the photodissociation pathways for the newly formed PAH cluster cations, are obtained. The adduct processes and the newly formed molecular structure depend on the carbon reaction sites and connection pathways. Gas-phase reactions between PAH species easily occur, resulting in many reactions and very complex molecular clusters. The NH₂ unit plays an important role in the formation and photochemistry processes, in which two molecular connection pathways are considered (C–C and C–N bonding types). We infer that small aminosubstituted PAHs (e.g. 9-aminoanthracene and 1-pyrenamine) can effectively aggregate on large PAH species (e.g. (dehydrogenated) DC cations or carbon clusters), which provides a general molecular evolution route for the formation of nitrogen-containing nanometre-sized dust grains through a bottom-up process in the interstellar medium.