Photo-fragmentation behavior of methyl- and methoxy-substituted derivatives of hexa-peri-hexabenzocoronene (HBC) cations

A systematic study, using ion trap time-of-flight mass spectrometry, is presented for the photo-fragmentation of methyl- and methoxy-substituted derivatives of HBC cations, (OCH₃)₆HBC${}^{+}$ and (CH₃)₄(OCH₃)₂HBC${}^{+}$. Both substituted HBC cations fragment through sequential loss of CH₃CO units upon laser (595nm) irradiation, resulting in a PAH-like derivative C₃₆H₁₂${}^{+}$ and a methyl-substituted PAH derivative C₄₄H₂₄${}^{+},$ respectively. Upon ongoing irradiation, these species further fragment. For lower laser energy C₄₄H₂₄${}^{+}$ dehydrogenates and photo-fragments through CH₃ and CHCH₂ unit losses; for higher laser energy isomerization takes place, yielding a regular PAH-like configuration, and both stepwise dehydrogenation and C₂/C₂H₂ loss pathways are found. C₃₆H₁₂${}^{+}$ follows largely this latter fragmentation scheme upon irradiation.

It is concluded that the photo-dissociation mechanism of the substituted PAH cations studied here is site selective in the substituted subunit. This work also shows experimental evidence that photo-fragmentation of substituted PAHs may contribute to the formation in space of smaller species that are normally considered to form by merging atoms and molecules.