Spontaneous chiral resolution in supramolecular assembly of 2,4,6-tris(2-pyridyl)-1,3,5-triazine on Au(111)

We have investigated the self-assembly of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPTZ) molecule on Au(111) surface using an ultrahigh vacuum low-temperature scanning tunneling microscope. The TPTZ molecules form enantiomorphous domains composed of rhombic supercells with various periods depending on the coverage of molecules, that is, "1 x 1" and "2 x 2" structures at low coverages, and "6 x 6", "7 x 7", and "8 x 8" structures at higher coverages. In a unit cell of a certain enantiomer, the two triangular half-unit cells, consisting of adsorbed TPTZ molecules, are centrosymmetric to each other. The molecules inside each half-unit cell are bound to each other through a single -CH...N- hydrogen bond, while the molecules at the boundaries between half-unit cells are bound through double -CH...N- hydrogen bonds. The STM images and the DFT calculations reveal that the molecules in an enantiomorphous domain adopt the same adsorption orientation of either R-TPTZ or L-TPTZ, which indicates that the adsorbed TPTZ molecules on Au(111) undergo spontaneous chiral resolution. The subtle balance between the intermolecular interaction and the molecule-substrate interaction tunes the period of the superstructure. The total interaction energy densities obtained from the DFT calculations explain the experimental observations quantitatively.