Chiral Monolayers with Achiral Tetrapod Molecules on Highly Oriented Pyrolytic Graphite

The self-assembly of organic molecules at the surface of highly oriented pyrolytic graphite (HOPG) is a promising process for constructing molecular-scale architectures. However, selectable organic molecules are generally restricted to two-dimensional molecules with planar Jr-conjugated structures and alkyl chains. We herein present the formation of self-assembled monolayers of tetrakis(4-ethynylphenyl)methane (TEPM) having a three-dimensional (3D) tetrapod geometry on HOPG, which was achieved by utilizing a simple spin-coating method. The arrangements of TEPM molecules in the monolayers were investigated using frequency-modulation atomic force microscopy (FM-AFM). The resulting subnanometer-resolution FM-AFM images revealed that the TEPM molecules formed linear rows with a periodicity of 0.85 nm oriented in a parallel configuration but with two alternating intervals of 0.7 and 1.0 nm. Moreover, the TEPM monolayers were classified into two chiral types with a relationship of mutual mirror-image symmetry, according to the observed molecular arrangements. Our results demonstrate the capability of TEPM molecules to act as 3D building blocks for the design of molecular-scale architectures at interfaces.