Formation and photochemistry of covalently bonded large functional PAH clusters

¹ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
e-mail: jfzhen@ustc.edu.cn
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
³ CAS Center for Excellence in Quantum Information and Quantum Physics, Hefei National Laboratory for Physical Sciences at the Microscale, and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, PR China
⁴ Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA 16802, USA

Received: 2 May 2019
Accepted: 13 July 2019

Abstract

Polycyclic aromatic hydrocarbon (PAH) molecules belong to a large and diverse chemical family in the interstellar medium (ISM). We study the formation and photochemistry of covalently bonded large functional PAH clusters, dicoronylene (DC, C₄₈H₂₀)/9-vinylanthracene (C₁₆H₁₂) and dicoronylene/9-methylanthracene (C₁₅H₁₂) cluster cations, in the gas phase, and we offer an approach to the evolution of different types of large (covalently bonded) PAH clusters in the ISM. The experiments, which we combined with a quadrupole ion trap and time-of-flight mass spectrometry, show that large functional PAH cluster cations can form by gas-phase condensation through molecular-ion reactions. One group of functional PAH cluster cations contain the vinyl group (−CHCH₂), that is, from C₁₆H₁₂DDC⁺ (e.g., C₁₆H₁₂C₄₈H₁₉⁺, m/z = 799) to (C₁₆H₁₂)₂DDC⁺ (e.g., (C₁₆H₁₂)₂C₄₈H₁₈⁺, m/z = 1002). The other group of functional PAH cluster cations contain the methyl group (−CH₃), that is, from C₁₅H₁₂DDC⁺ (e.g., C₁₅H₁₂C₄₈H₁₉⁺, m/z = 787) to (C₁₅H₁₂)₂DDC⁺ (e.g., (C₁₅H₁₂)₂C₄₈H₁₈⁺, m/z = 990). With laser irradiation, the DC/9-vinylanthracene and DC/9-methylanthracene cluster cations show a very complicated dissociation process (e.g., dehydrogenation, −CH₃ or −CHCH₂ unit losses). We investigate the structure of newly formed PAH cluster cations, the bond energy, and the photodissociation energy for these reaction pathways with quantumchemical calculations. The obtained results provide a general molecular growth route toward large PAH cluster cations (e.g., functional PAH clusters) in a bottom-up formation process and the insight of the functional group (e.g., vinyl, −C₂H₃ and methyl, −CH₃) effect on their evolutionary behavior. In addition, the studies of DC/9-vinylanthracene and DC/9-methylanthracene clusters (94–123 atoms, ∼2 nm in size) also provide a possible way of interpreting the formation processes of nanometer-sized grains in the ISM, especially when functional PAHs are included.