Nanoscale motion of organic π-conjugated molecules: exploring van der Waals forces, friction, and quantum effects.

The single-molecule dynamics of π-conjugated organic molecules on surfaces is fundamental for applications ranging from catalysis to molecular electronics. Adsorption and diffusion, in particular of organic aromatics, are typically driven by van der Waals forces, energy dissipation in terms of friction, and quantum effects, making them ideal for probing surface energy landscapes. However, their fast motion at thermal equilibrium poses experimental challenges. Recent advances have provided unprecedented insights into the diffusion mechanisms of several organic molecules on metallic and graphitic surfaces. These studies reveal a spectrum of motion, from ballistic transport to Brownian diffusion, influenced by surface symmetry, molecular size, charge transfer, and molecular degrees of freedom. Notably, friction at 2D material interfaces can be exceptionally low, leading to superlubricity - a phenomenon which highlights the role of atomic-scale interactions in determining energy dissipation and molecular mobility. We review experimental and computational techniques capturing diffusion at atomic length scales, highlighting how density functional theory and molecular dynamics complement experimental findings. Despite recent advances, key questions remain, such as how friction varies across different surfaces and how external factors affect mobility. Understanding these interactions is essential for controlling molecular assembly and surface functionalisation: controlling diffusion and dissipation at the nanoscale may enable self-assembled nanostructures, where controlled molecular motion drives highly ordered surface architectures. Finally, beyond technological applications, surface diffusion is also critical in astrochemistry, where it influences the formation of complex organic molecules.