Conglomerate Aggregation of 7,12,17-Trioxa[11]helicene into Homochiral Two-Dimensional Crystals on the Cu(100) Surface

Abstract

Despite the vast development of stereochemistry it remains a great challenge to predict the fate of homochiral or heterochiral recognition in crystallization from racemic solution or melt. Using the well-defined model system of racemic trioxaundecahelicene on a Cu(100) surface in ultrahigh vacuum, the chiral recognition of this heterohelicene has been investigated by means of state-of-the-art scanning probe microscopy. The combination of scanning tunneling microscopy and tuning fork-based non-contact atomic force microscopy allowed assignment of the absolute configuration of single 7,12,17-trioxa[11]helicene molecules in their two-dimensional assemblies. At very low coverages, homochiral van der Waals-bonded tetramers are observed. These also constitute the building blocks of the completely filled monolayer, which is a 2D conglomerate of homochiral domains. The interpretation of the adsorption geometries was supported by density-functional theory calculations and molecular dynamics simulations, predicting a stronger interaction energy for homochiral structures over the heterochiral assembly. The striking similarity to heptahelicene on the same surface suggests that chiral recognition is dominated by the footprint interaction of the molecules with the metallic substrate.