Beyond the Solid Solution: Ordered Enantiomerically Unbalanced Packing in Surface-Confined Tetrahelicene Monolayers

The two-dimensional crystallization and on-surface chemistry of chiral polycyclic aromatic hydrocarbons provide access to surface-confined symmetry breaking and novel covalently bonded nanostructures. Here, we investigate the self-assembly of racemic 2,3-dicarbonitrile tetrahelicene on Ag(111). Using low-temperature scanning tunneling microscopy in combination with density functional theory calculations, we resolve the evolution of molecular organization as a function of coverage. After room-temperature deposition at submonolayer coverage, the tetrahelicene moiety spontaneously resolves into extended homochiral Kagomé domains upon cooling. The resulting monolayer consists of hexameric dimer motifs arranged in a porous network. These mirror-related supramolecular enantiomorphs are characteristic of two-dimensional conglomerate crystallization. As the surface coverage increases, the system undergoes a transition toward denser packing. At saturation, compact domains form that retains long-range lattice order while exhibiting a regular yet enantiomerically unbalanced distribution of both handednesses. The nonrandom incorporation of enantiomers distinguishes this phase from a racemic solid solution and instead indicates a compositionally biased, structurally ordered mixed phase.