From planes to bowls: photodissociation of the bisanthenequinone cation

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.