Chiral Recognition of 4, 4’ Biphenyl-Dicarboxylic Acid on Pd(111) and Au(111) Studied by Electrochemical-Scanning Tunneling Microscopy
Dr. Byung Kim
Chirality, or the “handedness” of molecules, is a fundamental aspect of molecular biology and biochemistry, and is of central importance in pharmaceutics. Enantiomeric drugs are used to increase the chances of success in targeting enzymes, hormones, and receptors on cell surfaces, along with other compounds that are made up of chiral amino acids, carbohydrates, and lipids. Two-dimensional stereoselectivity using chiral surface processes and reactions has been conceived as an effective method to produce single-enantiomer compounds efficiently for pharmaceutical use as well as for a wide range of other applications. Although some enantioselective surface reactions were observed decades ago, researchers are just starting to address the fundamental questions of chiral surface physics. Here, we investigated the influence of molecular symmetry on chiral recognition using electrochemical scanning tunneling microscopy (EC-STM). An organic molecule, 4,4’ Biphenyl-dicarboxylic acid (BPDA) was used for this investigation because the BPDA molecules become chiral when they are deposited on a fcc(111) surface. The hydrogen bonding functional group of the BPDA molecule allows for the study of chiral recognition between two BPDA molecules on a fcc(111) surface. The molecular images taken by EC-STM in perchloric acid (HClO4) show that BPDA molecules form self-assembled hydrogen bonding networks on both Au(111) and Pd(111). Our further analysis shows that there is chiral recognition present on Pd(111), but not on Au(111). An angle dependent binding energy calculation suggests that the difference in chiral recognition of BPDA on a fcc(111) originates from the competition between adsorbate-adsorbate interaction and adsorbate- substrate interaction. This result is consistent with the previous report on chiral recognition of 4-trans-2-(pyrid-4-yl-vynyl) benzoic acid (PVBA) studied on Ag(111) and Pd(111). This finding not only improves our basic understanding of chiral recognition, but is also potentially applicable to the future development of heterogeneous catalytic surfaces.