Laboratory Photochemistry of Covalently Bonded Fluorene Clusters: Observation of an Interesting PAH Bowl-forming Mechanism

Abstract

The fullerene C 60 , one of the largest molecules identified in the interstellar medium (ISM), has been proposed to form top-down through the photo-chemical processing of large (more than 60 Catoms) polycyclic aromatic hydrocarbon (PAH) molecules. In this article, we focus on the opposite process, investigating the possibility that fullerenes form from small PAHs, in which bowl-forming plays a central role. We combine laboratory experiments and quantum chemical calculations to study the formation of larger PAHs from charged fluorene clusters. The experiments show that with visible laser irradiation, the fluorene dimer cation -[C 13 H 9 −C 13 H 9 ] + -and the fluorene trimer cation -[C 13 H 9 −C 13 H 8 −C 13 H 9 ] + -undergo photo-dehydrogenation and photo-isomerization resulting in bowl structured aromatic cluster-ions, C 26 H 12 + and C 39 H 20 + , respectively. To study the details of this chemical process, we employ quantum chemistry that allows us to determine the structures of the newly formed cluster-ions, to calculate the hydrogen loss dissociation energies, and to derive the underlying reaction pathways. These results demonstrate that smaller PAH clusters (with less than 60 C-atoms) can convert to larger bowled geometries that might act as building blocks for fullerenes, as the bowl-forming mechanism greatly facilitates the conversion from dehydrogenated PAHs to cages. Moreover, the bowl-forming induces a permanent dipole moment that -in principle -allows to search for such species using radio astronomy.

VUV photoionization dynamics of the C60 buckminsterfullerene: 2D-matrix photoelectron spectroscopy in an astrophysical context

“…25 For this reason, several recent studies have been focusing on PAH pentagon formation schemes, to further exploit bowling mechanisms under interstellar conditions. 26,27 With the tenuous connection between PAHs and fullerenes in space, it is worth noting that fullerenes may contribute to the ionization balance of the ISM and the photoelectric heating of PDRs in a similar way as PAHs have been found to be. 28 In and of itself C 60 has been proposed as a diagnostic tool to estimate the temperature and/or UV flux inside molecular clouds 29 but such data is dependent on measurements of solid C 60 rather than gas-phase measurements as those relative to DIBs studies.…”

Formation of Interstellar C60 from Silicon Carbide Circumstellar Grains

“…Also, Carpentier et al (2012) found disordered, "fullerene-like structures" in soot particles emulating HACs, while Duley & Hu (2012) suggested the presence of "protofullerenes" in Raman spectra of HAC analogs, but neither experimental study claimed evidence of C 60 in their data. Laboratory work by Tielens and collaborators (e.g., Berné & Tielens 2012;Zhen et al 2014;Zhang et al 2019) suggests that PAHs may be a route to C 60 .…”

Laboratory Formation and Photochemistry of Fullerene/Anthracene Cluster Cations

“…In our previous studies, we have reported laboratory experiments on van der Waals bonded, PAH clusters and their photochemical evolution towards large PAH molecules in a bottom-up process (Zhen et al 2018;Zhang et al 2019). Given the presence of C 60 in PDRs such as NGC 7023 (Sellgren et al 2010), studies of clusters involving C 60 have become of great interest as well.…”