From Multi-Switchable Self-Assemblies towards Surface Coordination Chemistry: An STM Investigation of Bipyridine-Terminated Ditopic Ligands

Abstract

A series of ligands, where ditopic bipyridines are connected by a variable central bridge (bpy-X-bpy), have been investigated by high-resolution scanning tunneling microscopy (STM). The bipyridines which are essential units to form different hydrogen bonds or to coordinate with transition metal ions at the solid/liquid interface. Their interactions have been observed to allow bpy-X-bpy molecules to self-assemble on HOPG. The bridges, varying between the fluorene and the biEDOT, provide redox-active or fluorescent functionalities also modify the intermolecular interactions with the self-assemblies. Consequently, supramolecular structural transitions or metal-molecule coordination are evidenced under STM: (i) Protonation induces a molecular cis- to trans-like isomerization where the bpy-X-bpy molecule changes from “U” to a “S” shape under STM ii) The molecular isomerization switches the nanoscale supramolecular organization on surface; (iii) Co(II) cations are controlled to react with the bipyridines where on-surface coordination complexes are obtained showing different coordination configuration; (iv) The different central bridge induces different self-organized structures on the surface due to the variable intermolecular central bridges interactions. Our work shows that the bpy-X-bpy is a multi-functional model system combining versatile molecular isomerization, structural phase transitions and metal-molecule coordination properties which have thus been systemically revealed by means of high-resolution STM.