Study of stereochemical crystallization of racemic mixtures of [5] and [7]thiaheterohelicene molecules on Ag(111) surface by scanning tunneling microscopy and Raman scattering spectroscopy

Abstract

The crystallization of a racemic mixture of [5] and [7]thiaheterohelicene molecules into two-dimensional highly ordered molecular domains on a Ag(111) surface was investigated by scanning tunneling microscopy (STM) at 80 K. We observed that the crystallization of both thiaheterohelicene molecules on a Ag(111) surface was strongly affected by stereospecific interactions resulting from different numbers of aromatic rings incorporated into the helical skeletons of molecules. [5]Thiaheterohelicene molecules crystallized predominant into a few racemate polymorphic molecular domains, allowing us to obtain high-resolution STM images with the unambiguous identification of the absolute handedness of enantiomeric species. In contrast, the stereochemical chiral interactions among [7]thiaheterohelicene molecules led to the formation of one type of racemate molecular domain consisting of self-assembled zigzagged twin rows. X-ray photoelectron spectroscopy investigation strongly suggests weak interaction between thiaheterohelicene molecules and Ag(111) surface and lack of formation the S-Ag chemical bond. Surface-enhanced Raman spectroscopy combined with theoretical calculation based on a density functional theory was applied to the chemical verification of molecules adsorbed on a plasmonic silver surface for future tip-enhanced Raman spectroscopy.