Enantioselective Effects in the Electrical Excitation of Amine Single-Molecule Rotors

This paper describes a single-molecule study of N-methylbutylamine molecular rotors supported on a Cu(111) surface. It is first demonstrated that the chirality of the individual rotating molecules can be directly determined by scanning tunneling microscopy (STM) imaging and understood with density functional theory (DFT) simulations. Tunneling electrons from the STM tip are then utilized to excite vibrational modes of the molecule that drives the rotational motion. Experimental action spectra were used to demonstrate that the electrically induced rotational motion of N-methylbutylamine occurs above 360 meV, which coincides with C-H stretching vibrational modes. The measurements also reveal that, above this 360 meV threshold, the excitation occurs via a one-electron process. DFT calculations indicated that the rotation barrier is over an order of magnitude smaller, meaning that the rotor is excited via high-energy vibrational modes that then couple to the low energy rotational mode. Furthermore, by adjusting the electron flux, individual rotational motions between the six different stable orientations of the molecule on the Cu(111) surface were monitored in real time. It was found that, for most STM tips used to electrically excite the rotors, the rotation of one enantiomer is faster than the other. This confirms an earlier report that STM tips can themselves be chiral and illustrates the fact that diastereomerism arising from a chiral STM tip interacting with a chiral molecule can lead to significant physical differences in the rotation rates of R versus S molecular rotors. This result has ramifications for interpreting the data from experiments where nanoscale electrical contacts to chiral molecules are made in devices like break junctions and scanning probe experiments.

Automated summary

This paper presents a single-molecule study of N-methylbutylamine molecular rotors on a Cu(111) surface. The chirality of individual rotating molecules can be directly determined by STM imaging, and the rotational motion is driven by tunneling electrons exciting vibrational modes above 360 meV. Experimental action spectra show that the rotation occurs through a one-electron process. Adjusting the electron flux allows monitoring of individual rotational motions between stable orientations in real time. Furthermore, the study shows that the rotation rates of R versus S molecular rotors can differ significantly, indicating the importance of chiral STM tips in nanoscale experiments.