  We present a combined experimental and theoretical study of the
photodissociation of the bisanthenequinone (C28H12O2) cation, Bq+. The
experiments show that, upon photolysis, the Bq+ cation does not dehydrogenate,
but instead fragments through the sequential loss of the two neutral carbonyl
groups, causing the formation of five-membered carbon cycles. Quantum chemical
calculations confirm this Bq+ -> [Bq - CO]+ -> [Bq - 2CO]+ sequence as the
energetically most favorable reaction pathway. For the first CO loss, a
transition state with a barrier of ~3.2 eV is found, substantially lower than
the lowest calculated H loss dissociation pathway (~ 4.9 eV). A similar
situation applies for the second CO loss channel (~3.8 eV vs. ~4.7 eV), but
where the first dissociation step does not strongly alter the planar PAH
geometry, the second step transforms the molecule into a bowl-shaped one.
