Viewing and Inducing Symmetry Breaking at the Single‐Molecule Limit

Abstract

Symmetry breaking by photons, electrons, and molecular interactions lies at the heart of many important problems as varied as the origin of homochiral life to enantioselective drug production. Herein we report a system in which symmetry breaking can be induced and measured in situ at the single-molecule level using scanning tunneling microscopy. We demonstrate that electrical excitation of a prochiral molecule on an achiral surface produces large enantiomeric excesses in the chiral adsorbed state of up to 39%. The degree of symmetry breaking was monitored as a function of scanning probe tip state, and the results revealed that enantiomeric excesses are correlated with the intrinsic chirality in scanning probe tips themselves, as evidenced by height differences between single molecule enantiomers. While this work has consequences for the study of two-dimensional chirality, more importantly, it offers a new method for interrogating the coupling of photons, electrons, and combinations of physical fields to achiral starting systems in a reproducible manner. This will allow the mechanism of chirality transfer to be studied in a system in which enantiomeric excesses are quantified accurately by counting individual molecules.

Development of an Electrically Driven Molecular Motor

“…This set of DFT calculations also revealed that the barrier to inversion between the energetically equivalent enantiomers was 0.24 eV. This relatively high barrier was also evident with STM experiments, as the rotors were not observed to switch chirality under normal scanning conditions at either 5 or 78 K. Only by supplying high‐energy electrons (>0.4 eV) with the STM tip was it possible to overcome the high barrier to inversion and switch the chirality of the adsorbed molecules …”

“…About 80% of all STM tips tested (both the etched W and cut Pt/Ir, the most commonly used STM probes) displayed this effect and hence are chiral. This result enabled us to differentiate between individual chiral molecules solely based on their apparent heights in STM . Perhaps most importantly we found that the tunneling electrons from a chiral tip have a stronger coupling to either right‐ or left‐handed molecules, resulting in rates of rotation of R and S rotors that differed by a factor of three under identical electrical excitation conditions .…”

“…Molecular asymmetry can lead to chirality in the adsorbed state and potentially yield an asymmetric rotational potential energy landscape. BuSMe is achiral in the gas phase but, due to its asymmetric alkyl tails, has two prochiral lone pairs on the central S atom that give rise to chirality in the surface‐bound molecules . The STM image in Figure reveals that the two enantiomers of BuSMe appear as pinwheels related by mirror symmetry.…”

“…Careful measurements of chiral molecular rotors revealed a particularly surprising result: different tunneling probabilities were observed through right‐ and left‐handed molecules . This indicated that the symmetry of the STM tip itself must have an influence on the tunneling process and, in cases in which right‐ and left‐handed molecules have different tunneling probabilities (i.e., apparent heights), the STM tip itself is chiral .…”

“…To focus on understanding and actuating the rotation of individual molecules on surfaces, we pioneered the use of a new, stable, and tunable molecular rotor system based on surface‐bound thioethers. We studied many aspects of molecular rotation, including the effect of temperature, molecular structure, surface, binding site, proximity of neighboring molecules, and electric fields/current, as well as the influence of molecular and probe chirality on directed rotation. Here we describe our scanning tunneling microscopy and density functional theory (DFT) work, which culminated in the first experimental demonstration of a single‐molecule electric motor …”