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)(6)HBC+ and (CH3)(4) (OCH3)(2) HBC+. 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 C-2/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. (C) 2016 Elsevier B.V. All rights reserved.