Manfred Parschau

Electrically driven directional motion of a four-wheeled molecule on a metal surface

Propelling single molecules in a controlled manner along an unmodified surface remains extremely challenging because it requires molecules that can use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates motion. Nature’s motor proteins have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems such as DNA walkers and light- and redox-driven molecular motors. But although controlled movement of single molecules along a surface has been reported, the molecules in these examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM tip. Here we present a molecule with four functional units—our previously reported rotary motors—that undergo continuous and defined conformational changes upon sequential electronic and vibrational excitation. Scanning tunnelling microscopy confirms that activation of the conformational changes of the rotors through inelastic electron tunnelling propels the molecule unidirectionally across a Cu(111) surface. The system can be adapted to follow either linear or random surface trajectories or to remain stationary, by tuning the chirality of the individual motor units. Our design provides a starting point for the exploration of more sophisticated molecular mechanical systems with directionally controlled motion.

Amplification of chirality in two-dimensional enantiomorphous lattices

The concept of chirality dates back to 1848, when Pasteur manually separated left-handed from right-handed sodium ammonium tartrate crystals. Crystallization is still an important means for separating chiral molecules into their two different mirror-image isomers (enantiomers), yet remains poorly understood. For example, there are no firm rules to predict whether a particular pair of chiral partners will follow the behaviour of the vast majority of chiral molecules and crystallize together as racemic crystals, or as separate enantiomers. A somewhat simpler and more tractable version of this phenomenon is crystallization in two dimensions, such as the formation of surface structures by adsorbed molecules. The relatively simple spatial molecular arrangement of these systems makes it easier to study the effects of specific chiral interactions; moreover, chiral assembly and recognition processes can be observed directly and with molecular resolution using scanning tunnelling microscopy. The enantioseparation of chiral molecules in two dimensions is expected to occur more readily because planar confinement excludes some bulk crystal symmetry elements and enhances chiral interactions; however, many surface structures have been found to be racemic. Here we show that the chiral hydrocarbon heptahelicene on a Cu(111) surface does not undergo two-dimensional spontaneous resolution into enantiomers, but still shows enantiomorphism on a mesoscopic length scale that is readily amplified. That is, we observe formation of racemic heptahelicene domains with non-superimposable mirror-like lattice structures, with a small excess of one of the heptahelicene enantiomers suppressing the formation of one domain type. Similar to the induction of homochirality in achiral enantiomorphous monolayers by a chiral modifier, a small enantiomeric excess suffices to ensure that the entire molecular monolayer consists of domains having only one of two possible, non-superimposable, mirror-like lattice structures.

Switching the chirality of single adsorbate complexes.

Pumped up: Propene molecules form chiral complexes when adsorbed on a copper surface. Inelastically scattered tunneling electrons from the tip of a scanning tunneling microscope induce rotation or diffusion of the adsorbate on the surface. Higher tunneling currents can lead to conversion of the adsorbate into the opposite enantiomer.

Building 2D crystals from 5-fold-symmetric molecules

Concepts of close packing in monolayers of 5-fold-symmetric buckybowls are discussed. When the symmetry of lattice and molecular building blocks are incompatible, new strategies evolve. Corannulene forms a hexagonal lattice on Cu(111) by tilting away from the C(5) symmetry and aligning one hexagonal ring parallel to the surface. The chiral 5-fold-substituted chloro and methyl derivatives do not show this tilt and maintain the 5-fold symmetry as adsorbates. Consequently, a nonperfect tiling is observed. Their lattices are quasi-hexagonal: one in an antiparallel fashion with almost pm symmetry and the other with azimuthal and positional disorder on the hexagonal grid. Our results are in remarkable agreement with computational and mechanical modeling experiments of close packing of hard pentagonal discs in macroscopic two-dimensional systems and prove the validity of such modeling strategies.

Reversible phase transitions in a buckybowl monolayer.

Like penguins on ice, buckybowl molecules move closer together when cooled on a copper surface (see model of a corannulene molecule adsorbed on Cu(111)). Upon heating, the molecules spread out into the original crystal phase again. The lower density at room temperature can be explained by the increase in entropy owing to the excitation of bowl vibrations at the surface.

Single-molecule chemistry and analysis: mode-specific dehydrogenation of adsorbed propene by inelastic electron tunneling.

A single propene molecule, located in the junction between the tip of a scanning tunneling microscope (STM) and a Cu(211) surface can be dehydrogenated by inelastic electron tunneling. This reaction requires excitation of the asymmetric C-H stretching vibration of the ═CH(2) group. The product is then identified by inelastic electron tunneling action spectroscopy (IETAS).

Chiral Reconstruction of a Metal Surface by Adsorption of Racemic Malic Acid

The adsorption of racemic malic acid on Cu(110) has been studied in ultrahigh vacuum (UHV) by means of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and reflection-absorption infrared spectroscopy (RAIRS). In contrast to enantiopure malic acid, which forms eleven different ordered phases on Cu(110), only four structures are observed for the racemate. Three of them are superpositions of enantiomorphous phases that have not been observed for the pure enantiomers. Only the non-enantiomorphous c(2×4) saturation structure was found for pure enantiomers and for the racemate, but also shows differences at short-range order. This suggests that heterochiral two-dimensional (2D) phases are present in all cases. A restructuring of the copper surface is clearly identified in STM for some phases after careful annealing, causing chirality transfer via the metal substrate.

From Homochiral Clusters to Racemate Crystals: Viable Nuclei in 2D Chiral Crystallization.

The quest for enantiopure compounds raises the question of which factors favor conglomerate crystallization over racemate crystallization. Studying nucleation and crystal growth at surfaces with submolecular-resolution scanning tunneling microscopy is a suitable approach to better understand intermolecular chiral recognition. Racemic heptahelicene on the Ag(100) surface shows a transition from homochiral nuclei to larger racemic motifs, although the extended homochiral phase exhibits higher density. The homochiral-heterochiral transition is explained by the higher stability of growing nuclei due to a better match of the molecular lattice to the substrate surface. Our observations are direct visual proof of viable nuclei.

Disappearing Enantiomorphs: Single Handedness in Racemate Crystals

Although crystallization is the most important method for the separation of enantiomers of chiral molecules in the chemical industry, the chiral recognition involved in this process is poorly understood at the molecular level. We report on the initial steps in the formation of layered racemate crystals from a racemic mixture, as observed by STM at submolecular resolution. Grown on a copper single-crystal surface, the chiral hydrocarbon heptahelicene formed chiral racemic lattice structures within the first layer. In the second layer, enantiomerically pure domains were observed, underneath which the first layer contained exclusively the other enantiomer. Hence, the system changed from a 2D racemate into a 3D racemate with enantiomerically pure layers after exceeding monolayer-saturation coverage. A chiral bias in form of a small enantiomeric excess suppressed the crystallization of one double-layer enantiomorph so that the pure minor enantiomer crystallized only in the second layer.

Heterochiral to Homochiral Transition in Pentahelicene 2D Crystallization Induced by Second-Layer Nucleation

Gaining insight into molecular recognition at the molecular level, in particular, during nucleation of crystallites, is challenging and calls for studying well-defined model systems. Investigated by means of submolecular resolution scanning tunneling microscopy and theoretical molecular modeling, we report chiral recognition phenomena in the 2D crystallization of the helical chiral aromatic hydrocarbon pentahelicene on a Cu(111) surface. Homochiral, van der Waals bonded dimers constitute building blocks for self-assembly but form heterochiral as well as homochiral long-range-ordered structures. 2D racemate crystals, built up by homochiral dimers of both enantiomers, are observed at coverages close to a full monolayer. As soon as the coverage leads to second-layer nucleation, the dense racemate phase in the first layer disappears and a homochiral dimer conglomerate phase of lower 2D density appears. Our results show that, at the onset of second-layer nucleation, a local change of enantiomeric composition in the first layer occurs, causing the transition from a 2D racemate to a 2D conglomerate.