Photofragmentation of corannulene (C20H10) and sumanene (C21H12) cations in the gas phase and their astrophysical implications

Abstract

Aromatic infrared bands (AIBs) dominate the mid-infrared spectra of many galactic and extragalactic sources. These AIBs are generally attributed to fluorescence emission from aromatic molecules. Unified efforts from experimentalists and theoreticians to assign these AIB features have recently received additional impetus with the launch of the James Webb Space Telescope (JWST) as the Mid-InfraRed Instrument (MIRI) delivers a mid-IR spectrum with greatly increased sensitivity and spectral resolution. PAHs in space can exist in either neutral or ionic form, absorb UV photons and undergo fragmentation, becoming a rich source of small hydrocarbons. This top-down mechanism of larger PAHs fragmenting into smaller species is of utmost importance in photo-dissociation regions (PDR) in space. In this work, we experimentally and theoretically investigate the photo-fragmentation pathways of two astronomically significant PAH cations – corannulene (C₂₀H₁₀) and sumanene (C₂₁H₁₂), which are structural motifs of fullerene C₆₀, to understand their sequential fragmentation pathways. The photo-fragmentation experiments exhibit channels that are significantly different from planar PAHs. The breakdown of the carbon skeleton is found to follow different pathways for C₂₀H₁₀ and C₂₁H₁₂ because of the number and positioning of pentagon rings, yet the most abundant low mass cations produced by these two species are found to be similar. The low mass cations showcased in this work could be of interest due to their possible astronomical detections. For completeness, the qualitative photofragmentation behaviour of dicationic corannulene and sumanene has also been investigated, but the potential energy surface of these dications is beyond the scope of this paper.