Observation of hierarchical chiral structures in 8-nitrospiropyran monolayers

The adsorption and self-organization of racemic mixture of 8-nitrospiropyran (SP8) molecules on Au(111) surfaces was studied by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). The SP8 enantiomers, in spite of their low-symmetric and nonplanar molecular structures, formed well-ordered monolayers on Au(111). In the monolayers, we found two types of enantiomorphous, i.e., mirror-imaged, 2D chiral domains, denoted as lambda and delta phases. Both phases consist of periodically packed chiral quatrefoils. In the lambda domain, the quatrefoils are counterclockwise folded, while in the delta domain, the quatrefoils are clockwise folded. High-resolution STM images revealed that each chiral quatrefoil contains four heterochiral dimers and that each dimer is composed of two antiparallelly packed homochiral SP8 molecules. Therefore both of the two mirror-imaged 2D chiral structures are not chirally pure but racemic 2D crystals. A domain boundary, which serves as the glide reflection line between a lambda domain and a delta domain, was also observed along the [11] direction of the Au(111) substrate.