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

Weiwei Zhang,Junfeng Zhen,Yubing Si,Harold Linnartz,Alexander G G M Tielens,Tao Chen

doi:10.3847/1538-4357/aafe10

Abstract

Abstract The fullerene C60, one of the largest molecules identified in the interstellar medium (ISM), has been proposed to form top-down through the photochemical processing of large (more than 60 C atoms) 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—[C13H9−C13H9]+—and the fluorene trimer cation—[C13H9−C13H8−C13H9]+—undergo photodehydrogenation and photoisomerization, resulting in bowl-structured aromatic cluster ions, C26H12 + and C39H20 +, 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 dissociation energies for hydrogen loss, 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, because 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 one to search for such species using radio astronomy.

Highlights

Engineering, Pennsylvania State University, University Park, PA 16802, United States.formation and destruction processes has attracted much attention in the field of molecular astrophysics (Tielens 2013).Based on IR observations of interstellar reflection nebulae, Berne et al proposed that polycyclic aromatic hydrocarbon (PAH) can be converted into graphene and subsequently to C60 by photochemical processing combining the effects of dehydrogenation, fragmentation and isomerization (Berne & Tielens 2012; Berne et al 2015)
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 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
We observed a series of m/z as shown in Figures 1 and 2, and we employ quantum chemistry to link these observations to their structures and identify possible reaction pathways

Summary

Introduction

Based on IR observations of interstellar reflection nebulae, Berne et al proposed that PAHs can be converted into graphene and subsequently to C60 by photochemical processing combining the effects of dehydrogenation, fragmentation and isomerization (Berne & Tielens 2012; Berne et al 2015) This idea is supported by laboratory studies, which demonstrate that C66H22+ can be transformed into C60+ upon irradiation, following full dehydrogenation, graphene flake folding and C2 losing chemical pathways (Zhen et al 2014b). Experimental and ab-initio molecular dynamics studies on ionization of van der Waals bonded acetylene clusters reveal a reaction channel towards the benzene cation. In this route, the excess photon and chemical energy is taken away by an H-atom or by one of the spectator acetylenes in the cluster (Stein et al 2017). We point towards experimental studies on the interaction of energetic ions with PAH clusters which lead to covalently bonded PAH-multimers (Zettergren et al 2013; Gatchell et al 2015; Gatchell & Zettergren 2016)

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Conclusion