Structural flexibility and mobility of coordination polymers on Cu(111)

Abstract

Coordination polymers on surfaces have been investigated at the atomic level via scanning tunneling microscopy (STM), revealing a variety of structures and electronic states. However, analysis of their dynamic behaviour at low temperatures has been hampered due to the strong adsorption of organic ligands on metal surfaces. In this study, we designed 2,7-dicyano-9,9-dimethyl-9H-fluorene (DCF) as a ligand for coordination polymers in order to reveal its mobility at low temperatures. The dimethyl group attached to the ligand reduces the interaction with the metal surface and serves as an indicator of the ligand orientation. This enables individual tracking of DCF in the DCF–Cu polymer on Cu(111) after their coordination with copper adatoms. Upon heating from 4 K to 78 K, branched structures were less mobile, while linear structures and short polymers with free ends exhibited higher mobility. Partial cleavage, recombination, and insertion of the polymer chains were observed in some sections of the polymer chains. Density functional theory (DFT) calculations suggest that the flexibility of the coordination angle (180° ± 20°) facilitates such transformations. This study provides a direct observation of the motion differences attributed to polymer chain structures as proposed in real materials.