Molecular-Scale Observation of Flexibility and Thermal Motions in Surface-Coordinated Polymers

Abstract

We report a molecular-scale study of a one-dimensional coordination polymer formed on a Cu(111) surface. A tailored ligand, 2,7-dicyano-9,9-dimethylfluorene (DCF), was designed to reduce interactions with the surface while enabling directional coordination with Cu adatoms. Scanning tunneling microscopy reveals that the resulting DCF–Cu polymer exhibits pronounced flexibility dominated by the CN–Cu–CN coordination angle, which constitutes the primary internal degree of freedom governing the overall chain geometry. Temperature-dependent measurements show the onset of thermal motion above ～40 K for linear chain segments, whereas three-coordinated branches remain immobile up to 71 K, functioning as molecular-scale cross-links. These results directly connect polymer physics concepts with single-molecule motions on surfaces.