Time-resolved imaging and analysis of the electron beam-induced formation of an open-cage metallo-azafullerene

Abstract

The bottom-up synthesis of strained hollow cage structures, such as fullerenes, using traditional thermal or photochemical methods remains one of the most challenging tasks in organic chemistry. Here, we demonstrate the synthetic use of an electron beam by in-depth single-molecule atomic resolution time-resolved transmission electron microscopy studies to induce the formation of an elusive doubly-holed fullerene-porphyrin cage structure from a well-defined benzoporphyrin precursor deposited on graphene. Through real-time imaging, we analyze the hybrid's peculiar ability to host up to two Pb atoms and subsequently gain insights into the dynamics of the Pb-Pb binding motif in this exotic metallo-organic cage structure. Through density functional theory calculations and image simulations, we conclude that the much slower secondary electrons, which accumulate in the irradiated area's periphery, can also initiate chemical reactions. Consequently, designing advanced carbon nanostructures by e-beam lithography will depend on the understanding and limitations of molecular radiation chemistry.

Electron beam-induced demetallation of Fe, Co, Ni, Cu, Zn, Pd, and Pt metalloporphyrins: insights in e-beam chemistry and metal cluster formations

“…Further, at the employed electron flux, a metal release due to critical fragmentations of the meso or α -carbons in the TPP ligand is earliest expected to proceed after ∼30 s; however, the metal atoms were immediately detected upon scanning the surface, rendering a fragmentation-induced metal release less likely ( vide infra ). 1…”

“…This pathway explains why metal atoms are already found to be displaced when screening the surface without being exposed to prolonged e-beams, as SEs are ejected into the periphery of the primary electron beam (beam shadow). 1 At the same time, this effect is enhanced in thicker samples as more SEs are generated. Consequently, using (S)TEM for molecular structure determination, using a low electron flux, thin samples to reduce the formation of SEs, and a conductive specimen to facilitate charge recombination processes, is critical.…”

“…Recently, we published findings on the unexpected electron beam-induced demetallation of custom-made metallobenzoporphyrins deposited on graphene as a substrate. 1 While previous studies reported on the use of heavy transition metal contrast markers in molecular imaging by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), 2–6 our operando transmission electron microscopy (TEM) observations on metallobenzoporphyrins suggested that the metal atoms exhibited deficient complexation behavior by the macrocycle, once subjected to the electron beam (e-beam). This, in turn, is an opportunity for analyzing single metal atoms.…”

“…The intersection of the slopes for the binding potential (equilibrium −2.8 Å) and the non-binding potential (4–7 Å) resulted in the determined threshold energies for the homolytic bond scissions at different redox states. 1,17…”

Open-cage fullerenes as ligands for metals

“…Proceeding along the opposite direction, a macrocyclic ligand has been used to construct an open-cage metalloazafullerene. 62 As monitored by electron microscopy, the lead porphyrin shown in Scheme 10 was subjected to an electron beam that caused the formation of 24 new carbon–carbon bonds and the creation of a new bowl-shaped molecule. It will be interesting to see if more traditional chemical or thermal methods can be used to make macroscopic amounts of this open-cage metalloazafullerene.…”