Laboratory formation and photochemistry of covalently bonded polycyclic aromatic nitrogen heterocycle (PANH) clusters in the gas phase

Abstract

ABSTRACT To examine the evolution processes of the nitrogen-containing polycyclic aromatic hydrocarbon (PAH) molecules occurring in interstellar environments, in this work we focus on the formation of large covalently bonded N-substituted polyaromatic species and their photochemistry behaviour in the gas phase. The experimental results show large PANH (e.g. DC/acridine and DC/phenazine) cluster cations formed in a chemical reaction between large PAH (e.g. dicoronylene, DC, C48H20) cations and small PANHs (e.g. acridine, C13H9N, or phenazine, C12H8N2) by gas-phase condensation through ion–molecule reactions. With laser irradiation, PANH cluster cations are involved in a complex photofragmentation process (e.g. dehydrogenation, HCN/CN, C2 or N2 units lost) and then form large PANH/PAH or multiple dehydrogenated molecules; in particular, the dehydrogenation of PANH clusters provides a possible way to synthesize large nitrogen-containing graphene species (e.g. C59N+ and C61N+). Also, we perform quantum-theoretical calculations on the formation and photochemistry of DC/acridine and DC/phenazine cluster cations: two types of molecular cluster are considered (C–C and C–N bond type) and the formation pathway and dissociation energy for each isomer are determined. The experimental and theoretical findings obtained give a general molecular growth pathway toward all-benzenoid aromatic species with size (> 60 C atoms) in the astrophysically relevant range, during a ground-up formation process, and offer understanding of the nitrogen element effect on their chemical-evolutionary behaviour. Also, studies of DC/acridine and DC/phenazine clusters (89–112 atoms, ∼2 nm in size) offer a feasible means of explanation for the formation of nanoscale dust grains (nitrogen element included) in space.