Fullerenes and Caps By Sequential HF Elimination from F-PAHs

Appropriately functionalized planar fluorinated polycyclic aromatic hydrocarbons (FPAHs) can be efficiently converted into curved carbon structures via a sequence of thermally activated, regioselective HF elimination steps. [1] On surfaces, such HF elimination is associated with a “planar-to-cap transition” which has been studied by scanning probe methods. [2] The resulting caps have recently been applied to the controlled growth of chirality-selected SWCNTs. [3]  We have studied the mechanism of planar-to-curved conversion  in a variety of active model FPAHs (C₂₆H₁₄F₂, C₃₆H₁₅F₃ and C₆₀H₂₁F₉) using both laser induced desorption (LID) as well electron-impact ionization/heating/fragmentation (EIIHF). For all precursors, we observe sequential HF elimination to be the dominant fragmentation channel. Experiments performed on deposited products of the fragmentation cascade (e.g. C₂₆H₁₂), suggest that the pentagon-hexagon structure of the original FPAH carbon network rearranges during fragmentation. The energy flux deposited into the FPAHs is larger in LID than for EIIHF.  As a result, the sequence of regioselective HF elimination steps can to be taken to completion - to form closed fullerene cages and/or fluorine-free carbon caps depending on the precursor FPAH. By combining LID with ion mobility measurements supported by DFT calculations, we have been able to determine the molecular structure of the (intermediate) fragments along the reaction path thus obtaining insight into the fragmentation mechanism.

[1] M. Kabdulov, et al. Chemistry - A European Journal 19 (2013) 17262

[2] K. Amsharov, et al. Angew. Chem. Int. Ed. 49 (2010) 9392

[3] J.R. Sanchez-Valencia, et al. Nature 61 (2014) 512