Layer-by-Layer Assemblies of Coordinative Surface-Confined Electroactive Multilayers: Zigzag vs Orthogonal Molecular Wires with Linear vs Molecular Sponge Type of Growth

Surface-anchored coordination-based molecular assemblies (CBMA) are very powerful tools for the design of modern materials. The CBMAs were created using bis-terpyridine--iron coordination chemistry by the alternation of linear or bent bisterpyridine ligands and Fe(II) metal centers. The coordination of the bent ligand results in the formation of zigzag structures that experience linear growth with each successive deposition step. Interestingly, the deposition of the linear ligand has two distinct steps. During the first four deposition cycles, the thickness and iron uptake (surface coverage) of the multilayer change similarly to ones of the bent ligand based multilayer. However, during the following deposition cycles, the linear ligand based demonstrates significantly higher growth rate. This unusual behavior might be attributed with the reorganizational transformations leading to a higher organizational level of the assembly. As a result, a part of templating layer molecules, which due to sterical reasons cannot seed a molecular wire growth, and successfully formed molecular wires begin acting as by holders or molecular sponges able to trap extra metal ions and provide these ions to the system during the next deposition step, thus speeding up the growth of the molecular wires. Indeed, incubation of linear assemblies in solutions of Zn(II) or Cd(II) demonstrates significant trapping abilities for the ions. During electrochemical cycling, both assemblies show high stability, fast response times, and sufficient coloration efficiencies that make them valuable building blocks for the design of novel electrochromic materials.