Monolayer terpyridyl metal complexes for high-efficiency electrochromic materials

Abstract

Electrochromic materials (ECMs) can change their color with changes of applied electrochemical potential. Currently, the use of ECMs in the industry is limited to mainly inorganic metal oxides, conjugated conducting polymers, and liquid crystal-based materials. Hybrid metal-organic materials belong to a relatively new class of ECMs that offer exciting properties of extended stability, great color variability and the highest coloration efficiencies in the area. Metal–ligand coordination adducts offer reliable metal-centered redox behavior and intense redox-dependent colored states. So far, terpyridine (tpy) ligands were mainly explored as coordinating ligands for polymeric ECM design. In this work, tpy-based unit was applied as a coordinating moiety for ECMs design, and well-defined tpy-metal complexes were deposited on the porous conductive supports in a monolayer fashion. The resulting materials demonstrated superior performance, including redox stability, long-term switching durability, ultra-high coloration efficiencies, and high differences in optical density. The structural changes to the coordination complexes leads to multiple accessible colored states (purple, blue, green, red, pink, orange, and yellow). Furthermore, mixing together isostructural metal complexes with different metal centers provides access to materials featuring multiple color-to-color transitions in one single electrochromic layer. Therefore, this work highlights that the monolayers of tpy-based coordination complexes embedded into the transparent conductive oxide supports are viable candidates for ECM design, and the versatility of this approach to create ECMs with tunable colors, programmed properties and superior long-term durability is demonstrated.