On-Surface Synthesis and Cryogenic Exfoliation of Sterically Frustrated Atropisomers

On-surface synthesis provides exceptional control over nanostructure and material composition, enabling the creation of molecular compounds that are difficult or impossible to obtain with other synthesis methods. In this work, we demonstrate the possibility of synthesizing atropisomeric molecules made of chains of polyaromatic hydrocarbon units via on-surface synthesis. Scanning probe microscopy reveals that molecules adsorbed on Au(111) surfaces adopt a planar structure, with adjacent monomeric units aligning either in parallel or antiparallel configurations, influencing the alignment of the molecule on the surface. Cryo-force spectroscopy peeling experiments show that metastable conformers can be mechanically stabilized during the lifting-redeposition process of the polymer from the surface. In this process, periodic drops in frequency shift are observed, corresponding to monomer detachment-readsorption. Interestingly, this periodicity is independent of the parallel/antiparallel configuration but is counterintuitively smaller than the monomer size. Molecular dynamics simulations relate this effective reduction in unit length to a tethering effect between the chain and the surface. This, in turn, allowed us to test and validate Silva’s analytical phenomenological power law model for peeling. Our findings not only provide a method for studying the elusive class 1 atropisomeric molecules but also offer deeper insight into the peeling phenomenon at the nanoscale.