AVS2012 Session SS-ThM: Molecular Films: Chirality & Electronic Features

Industrially, the selective conversion of prochiral reagents to chiral products is a crucial step in the production of a variety of asymmetric pharmaceuticals. While this feat is accomplished using either chiral catalysts or crystallization, many external influences have been shown to be capable of inducing such symmetry breaking including circularly polarized light, spin-polarized electrons, and combinations of unpolarized light and magnetic fields. Pioneering studies have made great strides towards explaining these various interactions, however many of the fundamental mechanisms by which chirality is transferred at the molecular-level are not yet fully understood. It is also a great challenge to design experimental setups with which to study these phenomena in a quantitative and reproducible manner. We report a simple thioether system in which symmetry breaking can be both induced and measured in situ at the single-molecule level. 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 40%, whereas thermal annealing produces racemic mixtures as expected. These effects arise from a previously unreported phenomenon that standard polycrystalline metal scanning probe tips can possess intrinsic chirality.

Thioethers also constitute a simple, robust system with which to study molecular rotation as a function of temperature, electron energy, applied fields, and proximity of neighboring molecules. In order for molecules to be used as components in molecular machines, methods are required to couple individual molecules to external energy sources and to selectively excite motion in a given direction. Studying the rotation of molecules bound to surfaces offers the advantage that a single layer can be assembled, monitored and manipulated using the tools of surface science. We report that a butyl methyl sulfide (BuSMe) molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunneling microscope are used to drive directional motion of the BuSMe molecule in a two terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular-scale in real time. The direction and rate of the rotation are related to the chiralities of the molecule and the tip of the microscope (which serves as the electrode), which again illustrates the importance of the symmetry of the metal contacts in atomic-scale electrical devices.

Stereoselective catalytic sites on achiral metallic surfaces may be prepared by adsorbing optically active compounds described as chiral modifiers. A fundamental understanding of the stereodirecting forces in such systems is necessary to develop more efficient enantioselective catalysts. We will present data for chirality transfer complexes formed by the chiral modifier (R)-(+)-1-(1-naphthyl)ethylamine ((R)-NEA) and pro-chiral α-phenylketone and α-ketoester substrates on Pt(111). Time-lapsed scanning tunneling microscopy allowed us to isolate individual chiral modifier/substrate complexes. The structure of the diastereomeric complexes were separately determined using DFT calculations. The extremely good convergence between the calculated structures and visual STM data, as well as supporting surface spectroscopy data, shows that prochiral steering on chirally modified Pt(111) can be followed with submolecular resolution at the reaction temperature (room temperature), thus enabling conformational, regiospecific and enantiospecific characterisation. The study reveals the contributions of steric repulsion, non-covalent attractive interactions and site-specific chemisorption to stereoinduction. We will conclude with a short description of the targeted design of new chiral modifiers.

A promising approach to study chiral molecular recognition is studying two-dimensional (2D) crystallization phenomena on well-defined surfaces via scanning tunneling microscopy (STM). We present studies on different two-dimensional chiral systems and discuss their tendency to undergo enantiomeric separation. A special surface enantiomorphism is observed via STM after adsorption of the enantiomers of a helical aromatic hydrocarbon on Cu(111). Instead of crystallization into homochiral 2D domains on the surface, racemic enantiomorphs are observed. In this situation, a small excess of one enantiomer is sufficient to create domains possessing single handedness throughout the entire surface layer. The induction of homochirality by chiral doping has also been observed for succinic acid and achiral (R,S)-tartaric acid. Our findings are explained by cooperative interactions between many chiral units, similar to the mechanism of chiral amplification observed in helical polymers and coined as "Sergeants-and-soldiers" principle. Another recently observed phenomenon is single enantiomorphism due to chiral conflict. Depending on the handedness of a chiral adduct to a racemic situation suppresses one enantiomorph during crystal growth, but supports the other by forming a quasiracemic solid solution. Finally, we present chirality aspects in single molecule surface dynamics, including conversion of adsorbate handedness and linear, unidirectional propulsion of a molecular car with chiral “wheels”.

Electrode materials for renewable energy applications are largely based on materials such as metal oxides and various forms of carbon because of their intrinsically high stability. However, the properties can be markedly enhanced through the integration of "smart" molecular functionalities. We have been investigating the development and application of new chemistry for fabricating novel types of electrochemically and photoelectrochemically active molecular structures on surfaces of metal oxides and on thin-film diamond. One area of interest has been the use of "click" chemistry as a versatile approach to functionalizing surfaces with redox-active molecules that can be used either as potential catalysts or as light-harvesting molecules. By using complementary functionalization on two different nanostructured oxides, it is also possible to make chemically-assembled oxide-oxide heterojunctions, such as TiO₂/SnO₂. In these cases the formation of a heterojunction can provide a built-in potential to enhance charge transfer at the interface.

A key question in these studies has been understanding how the presence of alkyl chain, ranging from ~4 atoms to ~ 12 atoms, impacts the electron transfer. While most previous work on molecular layers has been performed on densely packed layer on coinage metals such as gold and silver, when molecular layers are tethered to covalent materials such as diamond or metal oxides, the resulting layers have a high degree of disorder due to the mismatch between the native packing of the alkyl chains and the distribution of available surface sites. We have investigated the electron-transfer properties at these functionalized interfaces and find that the electron transfer rates are surprisingly high and only weakly dependent on the length of the alkyl chain, which we explain as a result of the increased conformational disorder. Our data suggest that the"best" molecular layers for electron-transfer applications are those that have a controlled degree of conformational disordered. We demonstrate these effects using recent measurements of electroactive Ru(bpy)-based complexes on diamond and on metal oxides.