AVS2013 Session SS-ThM: Chirality & Enantioselectivity on Surfaces

Autocatalytic reactions occur in a variety of fields and are characterized by an initiation event causing the rate of a reaction to increase non-linearly until it ceases. In addition to occurring in ambient environments, this type of reaction has been observed in a growing number of systems on single crystal surfaces (sometimes called surface explosions) under ultra high vacuum conditions but the mechanism for this process has not been fully determined. These well-controlled model surfaces allowed us to study the progression of an autocatalytic reaction, the thermal decomposition of tartaric acid (TA) on a Cu(110) single crystal surface, with molecular resolution imaging using scanning tunneling microscopy (STM) and temperature programmed reaction spectroscopy (TPRS). Using STM we progressively anneal the tartaric acid system and obtain the first molecular-scale images during a surface explosion to determine the changes occurring as a function of temperature. From these images we determine that the density of molecules slightly decreases until a critical threshold is reached when the molecules can decompose then desorb. We also observe, for the first time, a new phase of TA complexed with Cu adatoms that may play a role in the surface explosion as it occurs at the onset of the explosion. One area in which an understanding of autocatalytic reactions could be exploited is the separation of enantiomers from a heterogeneous catalyst. In this example a more complete separation of enantiomers could be achieved compared to normal desorption kinetics.

The mode of operation of heterogeneous chiral modifiers can be classified into those operating as templates, where several modifier molecules act in concert to define a chiral adsorption site, or one-to-one modifiers that form a docking complex between the modifier and a prochiral reactant. Enantioselectivity is measured by adsorbing chiral probe molecules onto chirally modified surfaces. Templating is illustrated using aminoacids on Pd(111). Scanning tunneling microscopy (STM) reveals that some aminoacids form tetrameric units, and others form dimers. Only those aminoacids that form tetramers are enantioselective implying that the tetramers act as templates.

Naphthylethylamine (NEA) is proposed to acts as a one-to-one modifier. The interaction between NEA and a prochiral reactant, methyl pyruvate, is explored using STM. Possible docking complexes are identified using density functional theory and the simulated images are compared with experimental images.

Since more than a decade the field of spintronics deals with the manipulation of the electron spin to facilitate electronic operations. For such devices spin-dependent electron transfer processes (namely spin filters) are needed, which are usually realized using either magnetic materials or systems containing heavy atoms to make use of spin-orbit coupling. Here we present spin-selective electron transmission through organic molecules with helical symmetry of quite high filtering efficiencies.

The work is based on “electron dichroism”, an effect that describes different interactions of longitudinally spin-polarized electrons with chiral molecules. Experiments in the mid-90s showed that spin-polarized electron beams, guided through vapor of chiral molecules, are attenuated differently, depending on the longitudinal spin polarization of the electrons and the enantiomer of the molecules.

Studying the transmission of low-energy photoelectrons through ordered self-assembled monolayers of chiral molecules on gold, an intensity dichroism was observed between the excitation with circularly polarized light of opposite helicity, which has been interpreted as a spin-dependent transmission through these ordered layers. We extend these studies by directly measuring the electron spin polarization using a calibrated Mott detector [1]. The observed spin selectivity at room temperature is extremely high as compared to other known spin filters. A systematic study on DNA monolayers shows that the spin filtration efficiency depends on the length and organization of the adsorbed molecules: single-stranded DNA molecules, which form a rather floppy instead of an ordered layer, show almost no spin-filtering effect. Recently, the spin transmission studies have been extended to bacteriorhodopsin membrane protein physisorbed on gold and aluminum surfaces. Spin polarization measurements yield up 15% spin polarization for transmitted electron ensembles with a minor dependency on the preparation scheme. Results obtained using an Al substrate yield that the high spin-orbit coupling of the Au substrate used in former experiments does not influence the effect. Because of the low atomic numbers of the constituents (P, C, H) of the organic molecules adsorbed on the surface, spin-orbit-coupling is not sufficient to explain the observation of the quite large spin-specific interaction. Even though very recently different models were published aiming to rationalize the observed effect, it is not understood so far.

[1] Science, 331, 894 (2011)

The role of atomic surface structure in catalytic reactions requires a fundamental understanding to develop new catalysts. Surface Structure Spread Single Crystals (S⁴Cs) expose a continuous distribution of crystal planes across their surfaces. Each point on the S⁴Cs has a different local crystallographic orientation that can be determined from the shape of the S⁴Cs and the orientation of its bulk crystal lattice vectors. The majority of crystal planes on these S⁴Cs contains terraces, monatomic steps, and kinks and can be described as chiral with an R or an S orientation. When coupled with spatially resolved surface analysis techniques, S⁴Cs can be used to study the effects of surface chiralty on surface chemistry across a broad, continuous distribution of crystal planes. In this work, the enantioselectivity of the explosive decomposition of L and D tartaric acid was studied using Cu(100)±10°, Cu(110)±10°, and Cu(111)±10° S⁴Cs. We used Isothermal Temperature Programmed Desorption (TPD) in which each sample was held at a temperature >20 K below the temperature of peak decomposition observed in a standard (TPD) experiment until the decomposition reaction occurred. Spatially resolved X-ray Photoelectron Spectroscopy (XPS) was performed to determine which crystal planes on each S⁴C had undergone explosive decomposition after quenching the temperature of each sample at the decomposition peak during an isothermal TPD. It was found that D-tartaric acid decomposes before L-tartaric acid on R oriented surfaces and L-tartaric acid decomposes before D-tartaric acid on S oriented surfaces on all three Cu S⁴Cs.

The stereocontrol of chemical reactions on surfaces is a challenging task. STM imaging coupled with DFT calculations provides a powerful method for studying asymmetric induction on chirally modified catalysts. In combination, these techniques can be used to observe and define transient pre-organization complexes formed by binding the prochiral substrate to the chemisorbed chiral modifier. We present sub-molecularly resolved regiospecific and stereospecific data for isolated bimolecular and termolecular complexes formed at chiral sites on Pt(111). The measurements provide new insight on both the stereodirecting interactions involved and the dynamics of chiral selection.