Reports: GB5
47226-GB5 Chiral Recognition of 4,4' Biphenyl Dicarboxylic Acid (BPDA) on fcc(111) Surfaces in Electrochemical Solutions
The goal of this project is to understand the influence of molecular symmetry of the composite structure of 4,4' Biphenyl Dicarboxylic Acid (BPDA) /fcc(111) on organic chiral recognition in the electrochemical environment. The electrochemical scanning tunneling microscopy (EC-STM) was employed to determine if there existed any kind of chiral recognition of BPDA on both Pd(111) and Au(111) in the perchloric acid HClO4 in contrast to the previous ultra-high vacuum (UHV) based studies of 4-trans-2-(pyrid-4-yl-vynyl) benzoic acid (PVBA) on Pd(111) and Au(111). The BPDA molecule is similar to PVBA, except that all bond angles between the carbon atoms that connect the benzene rings are straight (180 degrees) rather than bent, meaning there is no difference between mirror images of BPDA on a 2D surface. Although there is only a single enantiomer of BPDA on the flat surface (unlike the two of PVBA), the BPDA can become chiral when it forms a composite structure of BPDA/fcc(111) with a fcc(111) surface. The PI and his undergraduate students have made the following achievement during the two years of ACS-PRF funding:
1. Molecular Recognition of BPDA on Au(111) and Pd(111).
The molecular structures of the BPDA molecules were clearly resolved on both Pd(111) and Au(111) in the perchloric acid HClO4 by EC-STM. The surface has been cleaned by repeated cycles of cyclovoltammetry (CV) and purging of the 1mM perchloric acid HClO4 before deposition of BPDA to a substrate. The EC-STM images show BPDA molecules form self-assembled hydrogen bonding networks on both Au(111) and Pd(111). However, they also show different orientational ordering and packing density between Au(111) and Pd(111). The result indicates the difference in molecular recognition between two substrates in the perchloric acid HClO4. There was no chiral recognition present on Au(111) (in other words, both heterochiral and homochiral dimers were predicted to exist), unlike on Pd(111) where chiral recognition is present (only homochiral dimers were predicted to exist).
2. Theory and Modeling
Extending the previous work to this system meant that the geometry within the program for PVBA had to be changed to match BPDA. The PI's original C code as a template programming from scratch minimized BPDA energy within a unit cell of fcc(111) produced the following numerical results: the molecules form on the metal surface at angles of 19 degrees or 41 degrees from the atomic row on Au(111), whereas they form 30 degrees from the atomic row on Au(111). These conclusions are all supported by the experimental STM data discussed above.
3. Impacts on the PI's Career and the students
The ACS-PRF support increased the research and education
facilities and opportunities for the interdisciplinary undergraduate students,
which is an important addition to the PI's research
program at
Undergraduate students participating in this project have learned to design sophisticated scientific instrumentation, to collect EC-STM images, and to analyze EC-STM data through the numerical modeling. So far, the undergraduate students have presented their results at the four state-wide and on-campus undergraduate conferences.
