Oriented Surface-Anchored Metal-Organic Frameworks (SurMOFs) as Electrochromic Active Thin Films

Nowadays, electrochromic thin-films are mostly based on inorganic oxides (WO₃, V₂O₅…) or organic compounds (viologens, polymers…). Hybrid materials, combining inorganic and organic building blocks, are ideal candidates to expand the range of compounds for the development of original and performant EC thin films. Herein, we report the synthesis of novel hybrid EC thin films based upon highly oriented porous materials. The unusual property of the films lead to the design of an original electrochromic device.<br/>Over the past two decades, porous coordination polymers and Metal-Organic Frameworks (MOFs) have emerged as an important family of crystalline materials with a broad range of applications^[1]. These unique class of hybrid materials assembled from metal ions or clusters and organic ligands as building units allow the combination of functional ligands in a highly ordered three-dimensional fashion^[2]. In these materials, the EC property can either originate from the organic or inorganic component. Recently, a strategy based on the sequential and controlled immobilization of the key components of the MOFs led to the fabrication of highly homogenous materials where the ligands and the metals present a preferential orientation. These new MOF thin films are called Surface-Anchored Metal-Organic Frameworks (SurMOFs)^[3]. Besides the generation of highly oriented and non-interpenetrated materials, this method is compatible with almost every substrates and uses a very small amount of precursors (few mg/cm*cm) making SurMOFs ideal candidates for the integration of highly sophisticated ligands into functional materials.<br/>In this work, we developed novel electrochromic thin films based on oriented SurMOFs. Thin films were synthesized via spray-coating by using zinc acetate and a ligand from the perylene diimide (PDI) family. The electrochromic activity of the SurMOFs was recorded in imidazolium-based ionic liquids. Interestingly, two strikingly different behaviors were observed depending on the electrolyte composition. The as-deposited films switch reversibly from orange to dark blue in a lithium-based electrolyte while they adopt a cyan reduced state in salt free electrolytes. Taking advantages of this unusual property, a bi-color device with an original configuration was designed offering a uniform orange oxidized state and a bi-color cyan and dark blue reduced state. Clearly, two different mechanisms took place and the reduced species responsible of the color were identified. To our knowledge, this work represents the first one regarding the use of SurMOFs for electrochromic applications and opens the doors of a new category of EC materials as the combination between ligands and metals are infinite.<br/><b>References :</b><br/>[1] H.-C. Zhou, S. Kitagawa, <i>Chem. Soc. Rev., </i><b>2014, </b>8, 5415-5418.<br/>[2] Special Issue, Metal-Organic Frameworks,<i> Chem. Soc. Rev., </i><b>2009, </b>38.<br/>[3] J.-L. Zhuang, A. Terfort, C. Wöll, <i>Coord. </i><i>Chem. Rev.</i>, <b>2016, </b>307, 391-424.