  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.
