Laboratory formation of fullerenes from PAHs: Top-down interstellar chemistry

TL;DR

This study demonstrates experimentally that large PAHs can transform into C60 fullerenes through photochemical processes, supporting a top-down formation pathway relevant to interstellar chemistry.

Contribution

It provides the first experimental evidence that PAHs larger than 60 carbons can efficiently convert into C60 fullerenes via photochemical evolution, highlighting a top-down synthesis route in space.

Findings

Large PAHs undergo sequential dehydrogenation and C2 loss to form cages.

Photo-isomerization of PAHs leads to Buckminsterfullerene formation.

Supports top-down formation pathway for interstellar C60.

Abstract

Interstellar molecules are thought to build up in the shielded environment of molecular clouds or in the envelope of evolved stars. This follows many sequential reaction steps of atoms and simple molecules in the gas phase and/or on (icy) grain surfaces. However, these chemical routes are highly inefficient for larger species in the tenuous environment of space as many steps are involved and, indeed, models fail to explain the observed high abundances. This is definitely the case for the C$_{60}$ fullerene, recently identified as one of the most complex molecules in the interstellar medium. Observations have shown that, in some PDRs, its abundance increases close to strong UV-sources. In this letter we report laboratory findings in which C$_{60}$ formation can be explained by characterizing the photochemical evolution of large PAHs. Sequential H losses lead to fully dehydrogenated PAHs and subsequent losses of C$_{2}$ units convert graphene into cages. Our results present for the first time experimental evidence that PAHs in excess of 60 C-atoms efficiently photo-isomerize to Buckminsterfullerene, C$_{60}$. These laboratory studies also attest to the importance of top-down synthesis routes for chemical complexity in space.