We have uncovered how added charges can strengthen the stability of pi-stacking. We refer to specific pi-stacking nano-size aggregation mode of polycyclic aromatic hydrocarbons (PAHs) occurring in practically all carbon nanomaterials, including carbon dots, soot, and asphaltenes [1]. This interaction type involves electron sharing and it has been referred to as “pimerization” or “pancake bonding” and features specific preferred pi-pi stacking associations in many pi-radicals[2]. We have observed in computational modeling of the prototypical PAH radical, phenalenyl (PLY), and the related olympicenyl (OPY) [3], that pancake bonding can be maintained, even strengthened, when a dimer is charged, in combining a neutral radical (R) with their counterpart closed shell anion (R-) or cation (R+). The respective (R…R)+ dimers show strong pancake bonding and so do the perfluorinated (PF-R…PF-R)- dimers. We also address the reasons why the pancake bonding is weakened for the (R…R)- dimers and the perfluorinated (PF-R…PF-R)+ dimers. We trace the effect and its charge-asymmetry to the charge distribution within the four-substituted and unsubstituted PAHs. The competition between sigma- vs. pi-aggregation is well known to be shifted toward pi-stacking with bulky side groups. [4] Design strategies are discussed that yield pi-stacking without large side groups. We are also describing our search for further examples of this effect among the recently discovered asphaltene PAHs by non-contact Atomic Force Microscopy using the pancake bonding concept. [1]

Partial charging drastically modulates the stability of pi-stacked radical polycyclic aromatic hydrocarbons

ACS Spring Meeting 2021

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