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

Hunan Key Laboratory for Stellar and Interstellar Physics and School of Physics and Optoelectronics, Xiangtan University, Xiangtan, Hunan, 411105, China

^★ Corresponding authors: jfzhen@xtu.edu.cn; xjyang@xtu.edu.cn

Received: 8 April 2025
Accepted: 19 June 2025

Abstract

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)₂C_n]⁺, 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.