Greater Porosity with Redox Reaction Speeds Up MOF Color Change

Abstract

In this issue of Chem, Dincă and colleagues show that immobilization of redox-active naphthalene diimide moiety into a mesoporous metal-organic framework structure (MOF-74) allows the rapid and reversible transparent-to-dark electrochromism and has a potential application in smart windows. In this issue of Chem, Dincă and colleagues show that immobilization of redox-active naphthalene diimide moiety into a mesoporous metal-organic framework structure (MOF-74) allows the rapid and reversible transparent-to-dark electrochromism and has a potential application in smart windows. As our brand-new Boeing 787 Dreamliner rose into the Tokyo skies en route to Paris, the pilot banked smoothly over the capital, and morning sunlight shot dazzlingly into the cabin. Passengers who had never before flown on a Dreamliner began to search for the pull-down blinds, but they sought in vain, for Boeing’s flagship airplane has replaced them with a discrete series of switches. The Dreamliner employs smart windows dependent on the ability to darken or lighten on command through the utilization of electrochromic materials able to change color and transparency through electron injection. Thanks to modern technology, fumbling with window shades could now be a thing of the past, yet the system is clearly not without problems, given that the relatively slow response time of the electrochromic materials means that it takes a good 10 s for the Dreamliner’s large windows to fully darken. Recent findings by Dincă and coworkers look set to change this situation.1Alkaabi K. Wade C.R. Dincă M. Chem. 2016; 1: 264-272Abstract Full Text Full Text PDF Scopus (118) Google Scholar Currently, most studies on electrochromism are focused on metal oxides, viologens, conjugated conducting polymers, coordination complexes, and the pigment Prussian blue.2Mortimer R.J. Annu. Rev. Mater. Res. 2011; 41: 241-268Crossref Scopus (473) Google Scholar Although these materials are promising for industrial applications and some are already commercially available, another class of materials with a faster response time is being awaited. The Dincă group developed new electrochromic materials based on mesoporous metal-organic frameworks (MOFs), which are crystalline materials assembled from inorganic and organic building blocks that possess inherent mesoporosity. The key to success was to introduce into the scaffold of MOFs a redox-active organic moiety, which changes color by electron reduction, and to simultaneously make a large hole in the materials, which allows counter ions or electrolytes to rapidly diffuse therein. In particular, because electrochromic performance such as switching rate relies on the transport rate of ions during the redox reaction, ion transport through the large pore, as found in mesopores, is expected to enhance its kinetic performance. Indeed, the same research group previously reported the electrochromic behavior of MOFs based on naphthalene diimide (NDI) linkers with smaller microporosity.3Wade C.R. Li M. Dincă M. Angew. Chem. Int. Ed. Engl. 2013; 52: 13377-13381Crossref PubMed Scopus (221) Google Scholar Several substituted NDI linkers with pyrazolate moieties for further coordination were used to react with zinc ions, leading to the formation of MOFs with a two-dimensional distorted square grid structure with a microporous cavity. In order to charge electrons from the electrode, the authors deposited the MOF microcrystals as a thin film on fluorine-doped tin oxide (FTO)-coated glass substrates. Reducing the NDI moiety to the corresponding monoradical anion form, [NDI]⋅−, and further reducing it to the dianion form, [NDI]2−, changed the color of the film drastically; for instance, the non-substituted NDI-based MOFs changed color from pale yellow to pale green via two-electron reduction. Although this study was the first example of electrochromic behavior of MOFs, the performance was poor in comparison with that of other materials. Another example of electrochromic MOFs was reported by Farha, Hupp, and coworkers, who used pyrene as the chromophere.4Kung C.W. Wang T.C. Mondloch J.E. Fairen-Jimenez D. Gardner D.M. Bury W. Klingsporn J.M. Barnes J.C. Van Duyne R. Stoddart J.F. et al.Chem. Mater. 2013; 25: 5012-5017Crossref Scopus (205) Google Scholar The pyrene moiety functionalized with four benzoates working as linkers connected hexa-zirconium clusters and formed three-dimensional MOFs. Although it appears as though no bulk sample was obtained, Farha and colleagues were able to characterize the thin-film formation of NU-901 on FTO substrates by comparing the X-ray diffraction data with the corresponding simulated structure. Removing a single electron from pyrene changed the color of the thin film from yellow to deep blue, which was attributed to the formation of the corresponding cation radical. Because of the large porosity of NU-901, the transportation of counter anions occurred relatively quickly, and the coloration time was estimated to be 12 s. However, the single chromophere system made it difficult to absorb all visible light. The present manuscript by Dincă and coworkers elegantly overcomes this shortcoming.1Alkaabi K. Wade C.R. Dincă M. Chem. 2016; 1: 264-272Abstract Full Text Full Text PDF Scopus (118) Google Scholar The authors reacted the NDI moiety anchoring two salicylates with either magnesium (Mg) ions or nickel (Ni) ions to construct large hexagonal tubular mesoporous frameworks, known as MOF-745Rosi N.L. Kim J. Eddaoudi M. Chen B. O’Keeffe M. Yaghi O.M. J. Am. Chem. Soc. 2005; 127: 1504-1518Crossref Scopus (2061) Google Scholar or CPO-276Dietzel P.D.C. Morita Y. Blom R. Fjellvåg H. Angew. Chem. Int. Ed. Engl. 2005; 44: 6354-6358Crossref Scopus (485) Google Scholar isoreticular frameworks. The resulting structures opened the one-dimensional pores over 3 nm in diameter without interpenetration. The successful mesoporous construction was unambiguously confirmed by the measurement of nitrogen adsorption. The thin-film growth of both materials was achieved with the use of FTO substrates; however, the fiber-like morphology of the Mg framework scattered light and was not appropriate for electrochromic devices, and therefore, the smooth thin film of the Ni framework was investigated for its suitability in electrochromic studies. Thanks to the large porosity of the Ni framework, electrolytes were rapidly transferred therein during the redox reaction, and color change occurred within 7 s. The advantage of NDI relies on its characteristic absorption based on the strong π–π* transition, which highly depends on the redox state. The neutral NDI absorbed only the ultraviolet spectrum, leading to a colorless appearance. On the other hand, the single-electron reduced form, [NDI]⋅−, produced with an applied voltage of −1.6 V (versus ferrocene/ferrocenium), absorbed the region of visible light with λmax = 475, 700, and 772 nm, thus giving a brown color. Further electron reduction to the dianion form, [NDI]2−, at −2.3 V provided a green color with the intense absorption spectrum centered at λmax = 397, 423, and 560 nm. Importantly, these two redox states, [NDI]⋅− and [NDI]2−, have almost complementary absorption profiles across the region of visible light, and the film becomes almost black or dark gray at the applied potential of −2.0 V, where the ratio between [NDI]⋅− and [NDI]2− is approximately 1:1. When a potential of −0.7 V was applied, a colorless state returned, confirming the reversibility of transparent-to-dark electrochromic behavior (Figure 1). Indeed, the authors demonstrated that at least ten consecutive cycles did not destroy the thin-film structures. The prerequisite criteria for developing electrochromic materials for, say, smart windows are most likely satisfied with this rapid change in mesoporous MOF materials from a colorless state to dark gray. Note that it is still difficult to reasonably compare this electrochromic performance with that of other materials because performance also strongly depends on device configuration and thin-film growth conditions. Collaboration with specialist electric and electronics engineers would certainly lead to an improvement in performance by optimizing device configuration. This research also encourages synthetic chemists and physical chemists to participate in the development of new MOFs. Besides other known MOF systems based on dicarboxylates, the MOF-74 system allows the usage of a wider variety of metal ions rather than only Mg and Ni ions and the maintenance of large mesoporosity without interpenetration; for instance, unsaturated coordination sites of Mg ions help low-energy cost removal of CO27McDonald T.M. Mason J.A. Kong X. Bloch E.D. Gygi D. Dani A. Crocellà V. Giordanino F. Odoh S.O. Drisdell W.S. et al.Nature. 2015; 519: 303-308Crossref PubMed Scopus (836) Google Scholar and the expansion of pore aperture up to ∼10 nm demonstrates the capturing of GFPs.8Deng H. Grunder S. Cordova K.E. Valente C. Furukawa H. Hmadeh M. Gándara F. Whalley A.C. Liu Z. Asahina S. et al.Science. 2012; 336: 1018-1023Crossref PubMed Scopus (1493) Google Scholar Introducing redox-active moiety, such as NDI, into the wall of well-defined mesoporous MOFs will pave the way for an MOF as more than just a box, whereby a MOF works not only as a space for capturing and releasing molecules or ions therein but also as a pathway for electrons or energy. In particular, the integration of ligand-centered and metal-centered redox-active sites into the scaffold would give new applications in energy conversion and energy-storage devices.9Miyasaka H. Acc. Chem. Res. 2013; 46: 248-257Crossref Scopus (177) Google Scholar, 10D’Alessandro D.M. Chem. Commun. (Camb.). 2016; https://doi.org/10.1039/C6CC00805DCrossref Google Scholar Transparent-to-Dark Electrochromic Behavior in Naphthalene-Diimide-Based Mesoporous MOF-74 AnalogsAlKaabi et al.ChemAugust 11, 2016In BriefDincă and colleagues show that incorporating a naphthalene-diimide-based ligand into a MOF affords the synthesis of redox-active, mesoporous MOF-74 analogs that display fast and reversible electrochromic switching from transparent to dark. The authors devised techniques that allow for the deposition of these new materials as thin films, whose morphology affects the electrochromic response. Full-Text PDF Open Archive