Direct Imaging of Photoswitching Molecular Conformations Using Individual Metal Atom Markers

Photoswitching behavior of individual organic molecules was imaged by annular dark-field scanning transmission electron microscopy (ADF-STEM) using a highly electron beam transparent graphene support. Photoswitching azobenzene derivatives with ligands at each end containing single transition-metal atoms (Pt) were designed (Pt-complex), and the distance between the strong ADF-STEM contrast from the two Pt atoms in each Pt-complex is used to track molecular length changes. UV irradiation was used to induce photoswitching of the Pt complex on graphene, and we show that the measured Pt-Pt distances within isolated molecules decrease from similar to 2.1 nm to similar to 1.4 nm, indicative of a trans-to-cis isomerization. Light illumination of the Pt-complex on the graphene support also caused their diffusion out from initial clusters to the surrounding area of graphene, indicating that the light-activated mobilization overcomes the intermolecular van der Waals interactions. This approach shows how individual isolated heavy metal atoms can be included as markers into complex molecules to track their structural changes using ADF-STEM on graphene supports, providing an effective method to study a diverse range of complex organic materials at the single molecule level.