Seeing Is Believing: Directly Imaging Contractions of Artificial Molecular Muscles with Platinum Atom Markers

Artificial molecular muscles undergo well-controlled contractile and extensile motions upon external stimulation, leading to remarkable length changes. Evaluating such length changes at the molecular level is essential to the design of integrated artificial molecular muscles that mimic biological muscles. Taking advantage of the strong contrast of platinum (Pt) atoms in high-angle annular dark-field scanning transmission electron microscopy images, we imaged Pt-containing molecular [c2]daisy chains directly by employing metal atom markers. The length changes and associated conformational transformations of these newly developed artificial molecular muscles have been measured experimentally in combination with theoretical calculations. The contraction ratios of these two molecular muscles with the TEMPO or pyrene anchoring group were calculated to be 21.0% or 15.7% respectively, suggesting a substantial anchoring effect. This study demonstrates the experimental measurement of the length changes of artificial molecular muscles and provides a new avenue for investigating the motion of artificial molecular machines.