Stop-FrameFilming and Discovery of Reactions at theSingle-Molecule Level by Transmission Electron Microscopy

摘要

We report an approach, named chemTEM, to follow chemical transformations at the single-molecule level with the electron beam of a transmission electron microscope (TEM) applied as both a tunable source of energy and a sub-angstrom imaging probe. Deposited on graphene, disk-shaped perchlorocoronene molecules are precluded from intermolecular interactions. This allows monomolecular transformations to be studied at the single-molecule level in real time and reveals chlorine elimination and reactive aryne formation as a key initial stage of multistep reactions initiated by the 80 keV e-beam. Under the same conditions, perchlorocoronene confined within a nanotube cavity, where the molecules are situated in very close proximity to each other, enables imaging of intermolecular reactions, starting with the Diels–Alder cycloaddition of a generated aryne, followed by rearrangement of the angular adduct to a planar polyaromatic structure and the formation of a perchlorinated zigzag nanoribbon of graphene as the final product. ChemTEM enables the entire process of polycondensation, including the formation ofmetastable intermediates, to be captured in a one-shot “movie”.A molecule with a similar size and shape but with a different chemicalcomposition, octathio[8]circulene, under the same conditions undergoesanother type of polycondensation via thiyl biradicalgeneration and subsequent reaction leading to polythiophene nanoribbonswith irregular edges incorporating bridging sulfur atoms. Grapheneor carbon nanotubes supporting the individual molecules during chemTEMstudies ensure that the elastic interactions of the molecules withthe e-beam are the dominant forces that initiate and drive the reactionswe image. Our ab initio DFT calculations explicitlyincorporating the e-beam in the theoretical model correlate with thechemTEM observations and give a mechanism for direct control not onlyof the type of the reaction but also of the reaction rate. Selectionof the appropriate e-beam energy and control of the dose rate in chemTEMenabled imaging of reactions on a time frame commensurate with TEMimage capture rates, revealing atomistic mechanisms of previouslyunknown processes.