Activating Elastic Conformational Switch at Single Molecule Level via Qplus Atomic Force Microscope

Mechanical properties of molecules adsorbed on material surfaces are increasingly vital for the applications of molecular thin films. Here, we induce molecule conformational change on a single molecule by mechanical load and quantify the force and energy required for such switch via a low temperature (∼ 5K) Scanning Tunneling Microscope (STM) and Qplus Atomic Force Microscope (Q+AFM). Molecule TBrPP-Co (a cobalt porphyrin) deposited on an atomically clean gold substrate typically has two of its pentagon rings tilted upward and the other two downward. An atomically sharp tip of the STM/Q+AFM, which vibrates with a high frequency (∼ 30kHz), is employed to run over a single TBrPP-Co molecule at different heights with 0.1 A˚ as increments and meanwhile to record tip-molecule interaction strength in the form of tip frequency change. When the tip approaches the threshold distance to the molecule, the mechanical load by the tip becomes large enough to switch the conformation of the molecule and cause pentagon rings to flip their direction. Due to the sensitive nature of tip-molecule interaction, the rings flipping can be directly visualized by STM, as rings tilting upward exhibit two bright protrusions in contrast to rings downward in the image. By processing frequency change, we obtain a three-dimensional mechanical potential and force map for the single molecule TBrPP-Co with the resolution of angstrom level in three dimensions. Our results indicate that an energy barrier of ∼ 48meV is needed to activate the elastic conformational switch responsible for inducing the ring flipping of TBrPP-Co.