Facile Deposition of Multicolored Electrochromic Metal–Organic Framework Thin Films

Abstract

The construction of well-defined organic architectures on solid surfaces is a key step toward the development of more efficient and robust electronic devices. Indeed, a number of self-assembly strategies have been applied to building supramolecular structures for use in devices such as organic lightemitting diodes (OLEDs), organic transistors, and solar cells. Recently, self-assembled coordination polymers, namely crystalline, microporous metal-organic frameworks (MOFs), have received considerable attention for their potential in a number of applications, including electronic devices. The solvothermal methods used to synthesize MOF materials generally afford bulk single crystals or microcrystalline powders and, as a result, a number of methods have been developed to process MOFs as thin films, which are generally more desirable for use in devices. However, MOFs still face a number of challenges in this area, particularly with regard to their insulating nature. The incorporation of redox-active organic linkers represents one strategy toward improving the charge transport properties of MOF materials as well as imparting a response to electrochemical stimuli. In the latter case, the use of colorswitching, redox-active organic linkers might be considered for the design of new, color-tunable electrochromic materials. Inorganic materials such as metal oxides and Prussian blue analogs have been extensively studied for electrochromic applications. More recently, conducting organic polymers such as polythiophenes have received considerable attention owing to their greater color tunability and efficiency, and faster switching times. Despite significant advances, the need still exists for more easily synthesized and processable electrochromic materials with the same or greater degree of tunability. In this sense, the well-defined structures, selfassembly synthesis, and guest-accessible microporosity of MOFs might prove useful for their application in this field. Herein, we describe the solvothermal deposition and electrochemical properties of porous, micron-thick Zn-pyrazolate MOF films containing redox-active naphthalene diimide (NDI) linkers. NDIs have been extensively studied for their electron accepting properties which make them attractive as n-type semiconductors. Furthermore, core substitution of NDIs with electron donor groups results in the appearance of charge-transfer transitions in the visible spectrum, affording color-tunable and sometimes fluorescent dyes with applications in optoelectronics and sensing. A handful of MOFs containing NDI-based linkers have been reported and studied as gas sorption, photochromic, and fluorescent sensor materials. However, studies of their electrochemical properties remain scarce. Here, we show that processing them as homogeneously dispersed thin films enables their use as multicolored electrochromic devices. Our group has been concerned with developing modular syntheses for water-stable pyrazolate-based MOFs, and recently we described a series of core-substituted NDIbased materials. These MOFs (Zn(NDI-X), Figure 1) were obtained as microcrystalline powders by heating 1.1:1