Self-Assembly of Chiral and Pseudochiral Molecules at Interfaces
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Chirality is a subtle molecular property that has a profound impact in nature. A chiral object or enantiomer is defined as one that has a nonsuperimposable mirror image, in the same way that a left hand is related to a right hand. Although this geometric distinction may seem trivial, it plays a very significant role in a molecule's functionality and has especially important consequences for many biological molecules and pharmaceutical drugs. One of the more infamous and tragic examples of the role chirality plays in a molecule's functionality is the drug thalidomide, administered in the 1950s to pregnant women to alleviate nausea: It was discovered that only one enantiomer of thalidomide was safe whereas the other enantiomer caused severe birth defects. Pharmaceutical companies invest tremendous resources into synthesizing enantiomerically pure compounds or separating racemic compounds (where equal amounts of left- and right-handed molecules are present) into enantiomerically pure forms. Not only are enantiomers extremely difficult to separate, but they are also difficult to identify because all of their chemical properties—except for the direction in which they rotate plane-polarized light—are identical. Therefore classical chemical techniques such as nuclear magnetic resonance (NMR), circular dichroism (CD), and other forms of spectroscopy cannot identify the absolute chirality of a given molecule, and many techniques require some sort of chemical modification of the enantiomer (e.g., forming a diastereomer) to achieve even a relative differentiation between enantiomers.
Scanning tunneling microscopy (STM) has become an obvious and highly relevant technique with which to study chiral systems. The ability of the scanning tunneling microscope to observe molecules on an atom-by-atom basis with angstrom-level resolution makes it a natural choice for detailed, high-resolution studies of chiral molecules and chiral surface structures. The scope of STM studies on chiral systems is too large to be covered in a single article, and so here we focus on surface studies of physisorbed self-assembled monolayers at the liquid–solid interface (i.e., ambient conditions).