Self-Assembly Formation of M‑Type Enantiomer of 2,13-Bis(hydroxymethyl)[7]-thiaheterohelicene Molecules on Au(111) Surface Investigated by STM/CITS

Adsorption of the M-type enantiomer of 2,13-bis­(hydroxymethyl)[7]-thiaheterohelicene molecules deposited on Au(111) was investigated by scanning tunneling microscopy (STM) and current imaging tunneling spectroscopy (CITS). We observed preferential adsorption at step edges, fcc regions, and fcc elbow regions of the herringbone reconstruction at a low coverage after molecular deposition onto a substrate maintained at 150 ± 20 K. Surprisingly, after molecular deposition onto a substrate maintained at 355 ± 20 K, highly ordered islands exhibiting molecular self-assembly surrounded by disordered areas were observed. The islands consist of self-assembled zigzagged twin rows with the preferential direction tilted clockwise and anticlockwise from the ⟨112̅⟩ direction of the Au(111) substrate by an angle of 12 ± 2°. A detailed statistical analysis of the twin rows indicated that the unit cell lattice vectors a and b have lengths of 2.4 and 4.4 nm, respectively, with an angle between them of 77°. We postulate an adsorption model without hydrogen bond formation between the hydroxyl groups of neighboring molecules or a strong π–π stacking interaction. We emphasize the advantages of this molecular system investigated for STM-induced luminescence experiments in the near future.