An Atomic Scale View of Chirality at Surfaces

Chirality, the property of having left and right forms of the same object, plays a large role in many important areas of biology, chemistry and physics. Ones hands are the classic example of chiral objects. Chirality also occurs on the molecular level and many molecules, including those essential for life, can occur as left-handed or right-handed forms called enantiomers. Perhaps the best-known examples are the amino acids from which proteins are constructed, as well as sugars and DNA. In nature, amino acids appear almost exclusively in their left form, while sugars are present in their right form. This left-right distinction is particularly important in pharmacology, because certain chiral drugs must be only used in one of their forms, either the left or the right, as the “wrong” enantiomer can be toxic. For these reasons molecular chirality has long fascinated scientists, however, the question of how chirality is transferred from single molecules to larger structures is not well understood. For example, the biomineralization that leads to the unidirectional coiled shells of organisms is thought to stem from a fundamental interaction between peptides, proteins, and inorganic crystallite surfaces, but the exact mechanism by which this process occurs is not known. The advent of scanning probes has allowed chirality to be monitored at the single molecule or monolayer level and has opened up the possibility to track enantiospecific interactions and chiral self-assembly with molecular-scale detail.(1-4)

We have studied the self-assembly of simple, model molecules that are achiral in the gas phase, but become chiral when adsorbed on a surface.(1-3) For example, polyaromatic hydrocarbons form stable and reversibly ordered systems on Cu(111) in which the transmission of chirality from single surface-bound molecules to complex 2D chiral architectures can be monitored as a function of molecular packing density and surface temperature. In the case of naphthopyrene, in addition to the point chirality of the surface-bound molecule, the unit cell of the molecular domains was also found to be chiral due to the incommensurate alignment of the molecular rows with respect to the underlying metal lattice. These molecular domains always aggregated in groups of three, all of the same chirality, but with different rotational orientations, forming homochiral “tri-lobe” ensembles. At a larger length scale, these tri-lobe ensembles associated with nearest neighbor tri-lobe units of opposite chirality at lower packing densities before forming an extended array of homochiral tri-lobe ensembles at higher converges. This system displayed chirality at a variety of size scales from the molecular (~1 nm) and domain (~5 nm) to the tri-lobe ensemble (~10 nm) and extended array (>25 nm) levels. The chirality of the tri-lobe ensembles dictated how the overall surface packing occurred and both homo- and heterochiral arrays could be reproducibly and reversibly formed and interchanged as a function of surface coverage. Finally, these chirally templated surfaces displayed remarkable enantiospecificity for additional molecules adsorbed in the second layer. Given their simplicity, reversibility, and rich degree of order, these systems represent an ideal test bed for the investigation of symmetry breaking and the hierarchical transmission of chirality through multiple length scales.

We have also extended our previous work on symmetric thioether self-assembly to asymmetric thioethers and showed that these monolayers are chiral by virtue of the binding of one of the two pro-chiral lone pairs on the S atom to the Au surface.(3) We find that well ordered domains of butyl methyl sulfide form due to van der Waals interactions between molecules, and that the self-assembly is almost 100% enantiospecific, leading to the growth of large homochiral domains. With future applications in mind, we studied a partially fluorinated analogue and showed that despite this substitution, the same highly enantiospecific assembly leads to similar, well-ordered homochiral domains. We hope to eventually use this type of chiral, dipolar layer for studies of spin polarized electron transmission through organic layers. The fact that asymmetric thioethers are pro-chiral in the gas phase but form enantiomorphic domains when surface adsorbed offers possibilities for the interrogation of chirality at surfaces. Such experiments might involve symmetry breaking, in which small amounts of a chiral ‘‘seed’’ species are introduced to amplify the global single-handed organization of the molecules.

1. "Spontaneous Transmission of Chirality through Multiple Length Scales" E. V. Iski, H. L. Tierney, A. D. Jewell and E. C. H. Sykes - Chemistry - A European Journal 2011, 17, 7205 – 7212. [Cover Story]
2. "Visualization of Hydrogen Bonding and Associated Chirality in Methanol Hexamers" T. J. Lawton, J. Carrasco, A. E. Baber, A. Michaelides and E. C. H. Sykes - Physical Review Letters 2011, 107, 2561011-2561015.
3. "Asymmetric Thioethers as Building Blocks for Chiral Monolayers" A. D. Jewell, H. L. Tierney, O. Zenasni, T. R. Lee and E. C. H. Sykes - Topics in Catalysis 2011, 54, 1357-1367.
4. "The real structure of naturally chiral Cu{643}" A. E. Baber, A. J. Gellman, D. S. Sholl and E. C. H. Sykes Journal of Physical Chemistry C 2008, 112, 11086-11089.