Photodissociation processes of Bisanthenquinone cation

A systematic study, using ion trap time-of-flight mass spectrometry, is presented for the photo-dissociation processes of Bisanthenquinone (Bq) cations, C₂₈H₁₂O₂⁺, a ketone substituted Polycyclic Aromatic Hydrocarbon (PAH). The Bq cation fragments through sequential loss of the two neutral carbonyl (CO) units upon laser (626nm) irradiation, resulting in a PAH-like derivative C₂₆H₁₂⁺. Upon further irradiation, C₂₆H₁₂⁺ exhibits both stepwise dehydrogenation and C₂/C₂H₂ loss fragmentation channels. Quantum chemistry calculations reveal a detailed picture for the first CO-loss, which involves a transition state with a barrier of ~ 3.4 eV, which is lower than the energy required for the lowest H-loss pathway (~ 5.0 eV). The barrier for the second CO-loss is higher (~ 4.9 eV). The subsequent loss of this unit changes the Bq geometry from a planar to a bent one. It is concluded that the photodissociation mechanism of the substituted PAH cations studied here is site selective in the substituted subunit. This work also shows that an acetone substituted PAH cation is not photo-stable upon irradiation.