Influence of Solvent on the Chiral Resolution of Organic Molecules on Au(111): EC-STM Study of Biphenyl Dicarboxylic Acid on Au(111) in Aqueous Environment
Adsorption induced chiral resolution of organic molecules is important due to its potential applications in stereo-selective catalysis. We studied the adsorption induced chiral resolution using a model achiral molecule of 4,4’ biphenyl dicarboxylic acid (BPDA) on Au(111) in 0.1 M perchloric acid (HClO4) by scanning tunneling microscopy (STM). The BPDA molecules are known to have chiral resolution on Au(111) in ultra-high vacuum conditions (UHV). However, our experimental data show that the molecules form island structures with distinctive preferred orientations at the length scale of the molecular size, but they have no orientation order at a larger length. We calculated angle-dependent binding energy between the substrate and a BPDA molecule, the intermolecular interactions between the BPDA molecules, and their interactions with water molecules. The calculations suggest that the absence of chiral resolution in the aqueous environment may originate from the increase in the effective rotational energy barrier of the BPDA molecules due to its hydrogen bonds with the surrounding water molecules. The strength of the hydrogen bonding between BPDA molecules is sufficient to overcome the energy barrier for chiral resolution through rotational motion in UHV, but not in an aqueous environment.