Partially π-exposed 3D carbohelicene for mechanical tuning of conductance and thermopower in single-molecule...

Helicenes, composed of ortho-fused benzene rings, possess a spring-like π-backbone, enabling electron delocalization through both bonds and space. Theoretical predictions suggest that mechanical stretching or compression of molecular junctions in higher-order diazahelicenes could increase conductance and thermopower. However, this theory has not been experimentally verified, likely due to the difficulty of forming robust higher-order helicene molecular junctions using existing heteroatom anchors and the high electrical resistance of metal-heteroatom bonds. Thus, constructing mechanically stable single-molecule junctions through multipoint metal-π interactions would be ideal. Here we verify this theory using a 3D carbohelicene with a shielded π-face bearing bulky tert-butyl groups and an exposed π-face lacking tert-butyl groups. This partially π-exposed 3D carbohelicene forms a robust monolayer on the Au(111) substrate at room temperature through multipoint metal-π interactions. Using the break-junction technique, we demonstrate a compression-induced increase in both peak conductance (10^-3-10^-2 G₀) and thermopower (-44 μV/K).