STM Imaging ortho- and para-Fluorothiophenol Self-Assembled Monolayers on Au(111)

Self-assembled monolayers (SAMs) of para- and ortho-fluorothiophenol (p- and o-FTP) spontaneously formed on Au(111) substrate have been contrasted through investigation by a scanning tunneling microscope (STM) at room temperature. High-resolution STM imaging reveals that p-FTP adopts a 6 x root 3R30 degrees molecule arrangement containing six molecules. Two different kinds of p-FTP molecule dimer line structures have been formed on Au(111) by intermolecular pi-pi stacking along < 112 > substrate directions, besides a single p-FTP molecule line. In contrast, o-FTP molecules self-assemble into a much looser wave-like SAM, which can be described as a 5 x 3 root 3R30 degrees structure containing two molecules. Periodic density functional theory (DFT) calculations for the two systems suggest that these kinds of FTP molecules preferentially take the asymmetrical positions between 3-fold face-centered cubic (fcc) hollow and bridge sites on Au(111), tilting from the substrate surface. Theoretical simulation gives apparent average tilted angles of 58 degrees and 68 degrees for p-FTP and o-FTP with respect to the surface normal, respectively. This simulation shows that o-FTP is more inclined to lie down toward the Au(111) surface compared to p-FTP. The difference between p-FTP and o-FTP SAM structures can be qualitatively understood in terms of the variation of intermolecular dipole-dipole orientation. This suggests that, besides well-known Au-S and pi-pi interactions, electrostatic interactions including dipole-dipole, quadrupole-quadrupole, and dipole-quadrupole interactions might also play an important role in influencing the SAM structures formed by aromatic thiols with a permanent dipole moment.