Figure 1.

(a) Experimental AC-HRTEM images illustrating the transition of PCC from a face-on orientation (almost circular shape) to an edge-on orientation (line) on graphene observed under exposure to the 80 keV electron beam (dose rate ∼10⁷ e/nm²·s; an interval of a few seconds exists between each image). Uneven contrast around the edge of the molecule (third and second images in the top and bottom time series, respectively) indicates the loss of Cl-atoms. (b) Snapshots from DFT calculations illustrating structural perturbations in PCC caused by energy transfer from the e-beam to a carbon atom (highlighted in yellow). Significant displacement of atoms in PCC from their equilibrium positions takes place, including the elongation of C–Cl bonds up to 0.28 nm (equilibrium C–Cl distance in PCC is 0.174 nm). Structural diagrams showing PCC (c) lying face-on and (d, e) standing edge-on in two orthogonal orientations. (f) A proposed scheme of a monomolecular chemical reaction triggered by the 80 keV electron beam: Energy transfer from the e-beam to a carbon atom breaks the C–Cl bonds in PCC leading to aryne formation, which acts as a highly reactive dienophile and binds to the graphene covalently via Diels–Alder cycloaddition reaction. As a result of the cycloaddition to graphene, the molecule changes its orientation from face-on to edge-on. However, C–C bonds between the aryne and graphene are stretched, and the graphene structure is deformed, which means that the adduct has only limited stability, as under further exposure to the e-beam, the bonds with graphene dissociate, and the molecule returns back to the face-on orientation (the aryne also appears to be able to switch from one edge-on orientation to another within a single frame, for example, frame 4 in (a), thus appearing as two lines at an acute angle). After this cycle of transformations, the PCC molecule becomes damaged (i.e., missing two Cl atoms) and thus activated for further transformations, changing its shape and orientation and becoming progressively fragmented by the e-beam (Supporting Video 1).