Highly Stable and Rapid Switching Electrochromic Thin Films Based on Metal-Organic Frameworks with Redox-Active Triphenylamine Ligands

Efficiently regulating the electrochromic behavior of naphthalene-diimide-based zirconium-organic frameworks through linker installation

Redox-active metal-organic frameworks (MOFs) have already demonstrated their unique advantages in the field of electrochromism over the past decade. However, controlling and modulating the electrochromic behaviors in specific MOFs remains a significant challenge, as most of the cases reported so far are achieved by designing and constructing new MOFs. Herein, we design a redox-active Zr-MOF with coordination unsaturated pockets, termed NKM-908, based on a naphthalene diimide containing tetratopic carboxylate ligand. The fabricated NKM-908 thin film exhibits a reversible electrochromic behavior, showing a color change from basically colorless to light yellow and then to green. Remarkably, two forms of distinct and precise modulations to the electrochromic performance of NKM-908 are achieved through installations of linear auxiliary bitopic carboxylate linkers (TPDC-X) into the coordination unsaturated pockets, i.e. deepening the color and deriving new colors/shifting modes. This work is a successful attempt in tailoring the color change of electrochromic MOFs as required without synthesizing them from scratch. Considering the high efficiency and conveniency of this post-synthetic modification, such strategy opens up a versatile route in designing electrochromic materials, as well as facilely extending their potential towards practical applications.

Analyte-initiated disassembly of electrochromic metal-organic framework-based nanocomposites for smart colorimetric sensing

An electrochromic (EC) sensing strategy was presented through analyte-induced disassembly of an EC metal-organic framework-based nanocomposite. Carbon dots underwent charge reversion to separate from the entity stimulated by analytes, acting as a nanoswitch of the EC activity. We achieved colorimetric detection of glucose via the EC activity recovery mediated by released protons during glucose oxidation.

Alkali-Stable Metal-Organic Frameworks with Enhanced Electroconductivity for Black-Brown Electrochromic Energy Storage Smart Window

Metal-organic frameworks (MOFs) deliver potential applications in electrochromism and energy storage. However, the poor intrinsic conductivity of MOFs in electrolytes seriously hampers the development of the above-mentioned electrochemical applications, especially in one MOF electrode. Herein, a new Ni-based MOF (denoted Ni-DPNDI) is proposed with enhanced conductivity by π-delocalized DPNDI connectors. Predictably, the obtained Ni-DPNDI MOF achieves a conductivity of up to 4.63 S∙m-1 at 300 K. Profiting from its unique electronic structure, the Ni-DPNDI MOF delivers excellent electrochromic and energy storage performance with a great optical modulation (60.8%), a fast switching speed (tc = 7.9 s and tb = 6.4 s), a moderate specific capacitance (25.3 mAh·g-1) and good cycle stability over 2000 times. Meanwhile, energy storage capacity is visual by the coloration states of Ni-DPNDI film. As a proof of the potential application, a large-area (100 cm2) electrochromic energy storage smart window is further designed and displayed. The strategy provides an interesting alternative to porous multifunctional materials for the new generation of electronic devices with diverse applications.

Linker Conformation Controls Oxidation Potentials and Electrochromism in Highly Stable Zr-Based Metal-Organic Frameworks

The development of tailor-made electrochromic (EC) materials requires a large variety of available substances with properties that precisely match the task. Since the inception of electrochromic metal-organic frameworks (MOFs), the field relies only on a limited set of building blocks, providing the desired electrochromic effect. Herein, we demonstrate for the first time the implementation of a Piccard-type system (N,N,N',N'-benzidinetetrabenzoate) into Zr-MOFs to obtain electrochromic materials. With fast switching rates, high contrast ratio, long-life stability, and exceptional chemical and physical stability, the novel material is on par with inorganic EC material. The new EC system exhibits an ultrahigh contrast from the bleaching state, with transmittance in the visible region >53%, to the colored state with a transmittance of ca. 3%. The 5 μm thick film attained up to 90% of the coloring in 12.5 s and exhibited high electrochemical reversibility. Moreover, the conformational lability of the electrochromic ligand chosen is locked via the topology design of the framework, which is not attainable in the solution. Locked conformations of the redox active linker in distinct polymorphous frameworks (DUT-65 and DUT-66) feature different redox characteristics and opens the door to the overarching control of the oxidation pathway in the Piccard-type systems.

Gold Nanoparticles with N-Heterocyclic Carbene/Triphenylamine Surface Ligands: Stable and Electrochromically Active Hybrid Materials for Optoelectronics

Organic-hybrid particle-based materials are increasingly important in (opto)electronics, sensing, and catalysis due to their printability and stretchability as well as their potential for unique synergistic functional effects. However, these functional properties are often limited due to poor electronic coupling between the organic shell and the nanoparticle. N-heterocyclic carbenes (NHCs) belong to the most promising anchors to achieve electronic delocalization across the interface, as they form robust and highly conductive bonds with metals and offer a plethora of functionalization possibilities. Despite the outstanding potential of the conductive NHC-metal bond, synthetic challenges have so far limited its application to the improvement of colloidal stabilities, disregarding the potential of the conductive anchor. Here, NHC anchors are used to modify redox-active gold nanoparticles (AuNPs) with conjugated triphenylamines (TPA). The resulting AuNPs exhibit excellent thermal and redox stability benefiting from the robust NHC-gold bond. As electrochromic materials, the hybrid materials show pronounced color changes from red to dark green, a highly stable cycling stability (1000 cycles), and a fast response speed (5.6 s/2.1 s). Furthermore, TPA-NHC@AuNP exhibits an ionization potential of 5.3 eV and a distinct out-of-plane conductivity, making them a promising candidate for application as hole transport layers in optoelectronic devices.

Transparent-to-Brown-Black Patterned Electrochromic Metal-Organic Frameworks

Metal-organic frameworks (MOFs) exhibit promising electrochromic (EC) performance owing to their porous structure, regular channel, and tunable component characteristics. However, few reports focus on MOF materials with the EC performance of a transparent to brown-black (neutral colored state) change that is more suitable for smart windows. In this work, we proposed a strategy for synthesizing MOF (named Ni-BPY) EC materials and corresponding films fabricated via a low-cost electrostatic spray deposition technique. The obtained film exhibits excellent EC performance with a neutral color change from transparent to brown-black, a large optical modulation of 70% at 430 nm, and a fast response within 10 s. Benefiting from good electrical and chemical stability, the Ni-BPY film can be cycled over 500 times. Notably, the Ni-BPY MOF film also delivers a stepwise-controlled process during the bleached state due to its porous characteristics. In addition, the unique color variation of the Ni-BPY film derives from the redox reaction of the Ni metal node between Ni2+ and Ni3+, which is verified by the in situ potential-dependent Raman and X-ray photoelectron spectroscopy (XPS) measurement. As a proof of application, the patterned Ni-BPY EC films and devices are additionally constructed to demonstrate their potential application in electronic tags and logo displays.

Resonant-Cavity-Enhanced Electrochromic Materials and Devices

With rapid advances in optoelectronics, electrochromic materials and devices have received tremendous attentions from both industry and academia for their strong potentials in wearable and portable electronics, displays/billboards, adaptive camouflage, tunable optics, and intelligent devices, etc. However, conventional electrochromic materials and devices typically present some serious limitations such as undesirable dull colors, and long switching time, hindering their deeper development. Optical resonators have been proven to be the most powerful platform for providing strong optical confinement and controllable lightmatter interactions. They generate locally enhanced electromagnetic near-fields that can convert small refractive index changes in electrochromic materials into high-contrast color variations, enabling multicolor or even panchromatic tuning of electrochromic materials. Here, resonant-cavity-enhanced electrochromic materials and devices, an advanced and emerging trend in electrochromics, are reviewed. In this review, w e will focus on the progress in multicolor electrochromic materials and devices based on different types of optical resonators and their advanced and emerging applications, including multichromatic displays, adaptive visible camouflage, visualized energy storage, and applications of multispectral tunability. Among these topics, principles of optical resonators, related materials/devices and multicolor electrochromic properties are comprehensively discussed and summarized. Finally, the challenges and prospects for resonant-cavity-enhanced electrochromic materials and devices are presented.

Soluble polymer facilely self-grown in situ on conducting substrates at room temperature towards electrochromic applications

Electrochromic polymer film preparation methods such as spin coating, spray coating, and electrochemical polymerization, are commonly used. At present, developing new film preparation technology is an important aspect in the field of electrochromics. Herein, a continuous in situ self-growing method based on the chemical reaction occurring on the surface of an ITO glass between a metal oxide and organic acid groups was successfully applied to prepare electrochromic polymer films at a mild room temperature. SEM, FT-IR spectroscopy, XPS, and XRD characterization methods were combined to reveal the process and mechanism of film formation. The following notable electrochromic properties were observed: switching time within 6 s, contrast reached 35%, and minimal decrease of stability after 600 cycles. Finally, the patterned films were obtained through the directional growth of polymers in solution. This study provides an effective strategy for designing and preparing electrochromic films by self-growing methods in future applications.

A Stable Triphenylamine-Based Zn(II)-MOF for Photocatalytic H2 Evolution and Photooxidative Carbon-Carbon Coupling Reaction

Efficient charge transfer has always been a challenge in heterogeneous MOF-based photoredox catalysis due to the poor electrical conductivity of the MOF photocatalyst, the toilless electron-hole recombination, and the uncontrollable host-guest interactions. Herein, a propeller-like tris(3'-carboxybiphenyl)amine (H₃TCBA) ligand was synthesized to fabricate a 3D Zn₃O cluster-based Zn(II)-MOF photocatalyst, Zn₃(TCBA)₂(μ₃-H₂O)H₂O (Zn-TCBA), which was applied to efficient photoreductive H₂ evolution and photooxidative aerobic cross-dehydrogenation coupling reactions of N-aryl-tetrahydroisoquinolines and nitromethane. In Zn-TCBA, the ingenious introduction of the meta-position benzene carboxylates on the triphenylamine motif not only promotes Zn-TCBA to exhibit a broad visible-light absorption with a maximum absorption edge of 480 nm but also causes special phenyl plane twists with dihedral angles of 27.8-45.8° through the coordination to Zn nodes. The semiconductor-like Zn clusters and the twisted TCBA^3- antenna with multidimensional π interaction sites facilitate photoinduced electron transfer to render Zn-TCBA a good photocatalytic H₂ evolution efficiency of 27.104 mmol·g^-1·h^-1 in the presence of [Co(bpy)₃]Cl₂ under visible-light illumination, surpassing many non-noble-metal MOF systems. Moreover, the positive enough excited-state potential of 2.03 V and the semiconductor-like characteristics of Zn-TCBA endow Zn-TCBA with double oxygen activation ability for photocatalytic oxidation of N-aryl-tetrahydroisoquinoline substrates with a yield up to 98.7% over 6 h. The durability of Zn-TCBA and the possible catalytic mechanisms were also investigated by a series of experiments including PXRD, IR, EPR, and fluorescence analyses.

A functional cobalt-organic framework constructed by triphenylamine tricarboxylate: Detect nitroaromatics by fluorescence sensing and UV-shielding

Luminescent metal-organic frameworks (LMOF) with ligand-modified are a promising strategy to be applied to fabricate chemical sensors. Herein, a novel Co (II) metal-organic framework (Co-MOF), namely Co [(NTB) bpy] (NTB = 4,4'4″-tricarboxylic acid triphenylamine, bpy = 4,4 '-bipyridyl), was successfully synthesized with excellent water stability and fluorescence properties. Due to the propeller structure of NTB ligands, a special topological structure of Co-MOF was shown: {2⁴.4¹⁶.6⁸}{2}4. It was proved that Co-MOF has great stability by soaking in different solvents for two weeks. Remarkably, the fluorescence quenching experiment verified that Co-MOF has excellent fluorescence sensor performance. Trinitrophenol, 2,4-dinitrophenol, and 2-amino-4-nitrotoluene (10^-5 M) with LOD of 9.00 × 10^-5, 5.40 × 10^-5 and 5.07 × 10^-6 M can be detected via the process of fluorescence enhancement and quenching. Throughout the investigation, the mechanics of fluorescence quenching was performed. Due to the excellent UV absorption capacity of Co-MOF, it was a promising application to combine low-dimensional nanomaterials with sustainable biomass materials. A hybrid films of Co-MOF and cellulose acetate (CA) was generated. The hybrid films had highly transparency in the visible wavelength range and excellent UV-shielding ability owing to the CA/Co-MOF hybrid films enhanced the UV absorption capacity of Co-MOF.

Versatile Photo/Electricity Responsive Properties of a Coordination Polymer Based on Extended Viologen Ligands

Responsive chromogenic materials have attracted increasing interest among researchers; however, up until now, few materials have exhibited multifunctional chromogenic properties. The coordination polymers (CPs) provide intriguing platforms to design and construct multifunctional materials. Here, a multifunctional photo/electricity responsive CP named Zn−Oxv, which is based on the “extended viologen” (ExV) ligand, was synthesized. The Zn−Oxv exhibited reversible photochromism, photomodulated fluorescence, electrochromism and electrofluorochromism. Furthermore, we prepared Zn−Oxv thin films and investigated electrochromic (EC) properties of viologen−based CPs for the first time. Zn−Oxv thin films showed excellent EC performance with a rapid switching speed (both coloring and bleaching time within 1 s), high coloration efficiency (102.9 cm²/C) and transmittance change (exceeding 40%). Notably, the Zn−Oxv is by far the fastest CP EC material based on redox−active ligands ever reported, indicating that the viologen−based CPs could open up a new field of materials for EC applications. Therefore, viologen−based CPs are attractive candidates for the design of novel multi−responsive chromogenic materials and EC materials that could promise creative applications in intelligent technology, dynamic displays and smart sensors.

High-Performance Electrochromic Covalent Hybrid Framework Membranes via a Facile One-Pot Synthesis

Porous framework materials have sparked enormous interest in the electrochromic field, as they possess intrinsic high porosity and a large surface area that are beneficial for electron and ion transport. However, the fabrication of these porous framework materials often requires multiple processing steps or harsh reaction conditions, which significantly limit large-scale fabrication of such materials. In this work, we report a one-pot in situ polycondensation method to construct electrochromic covalent hybrid framework membranes via nucleophilic substitutions between hexachlorocyclotriphosphazene (HCCP) and triphenylamine (TPA) in an ambient environment. With the high transparency of polyphosphazene in a wide optical range, the constructed phosphazene-triphenylamine (PPTA) covalent hybrid framework membranes can be reversibly switched between light gray and dark blue, with a high transmittance change of up to 79.8%@668 nm and fast switching time (<4 s). Owing to the easy one-pot fabrication and good electrochromic properties, the PPTA covalent hybrid framework membrane has great potential in various fields such as displays and dynamic optical windows.

Electrochromic behavior of fac-tricarbonyl rhenium complexes

Tricarbonyl rhenium complex shows good electrochromic performance with a colored stage of green, rapid response and good switching stability.

Enhancement of Electrochromic Switching Properties with Tröger's Base-Derived Intrinsic Microporous Polyamide Films

The formation of Tröger's Base (TB) configuration is a useful approach to synthesize polymers of intrinsic microporosity (PIM). Herein, the V-shaped TB scaffold is incorporated to prepare electrochromic (EC) polyamide with electroactive triphenylamine (TPA) moiety. The presence of intrinsic microporosity derived from inefficient packing of TB scaffolds can facilitate the counterions diffusion between electroactive species and electrolytes. Consequently, the resulting TB-based polyamide exhibits enhanced EC behaviors, such as a lower driving potential, reduced the difference of redox potentials ΔE, and shorter switching response time compared to the corresponding EC counterpart polyamide.

A thin film of naphthalenediimide-based metal-organic framework with electrochromic properties

A metal-organic framework (MOF) thin film constructed from Zn nodes and naphthalenediimide (NDI) linkers was grown in-situ uniformly on a transparent conducting glass substrate. This transparent thin film exhibits intriguingly high-contrast electrochromic (EC) switching between canary yellow and dark brown by means of a one-electron redox reaction at its NDI linkers. The findings provide a basic comprehension of the relations between redox state and electrochromism and enrich the application of MOF in the field of optoelectronic materials.

Heteroatom-Doped Nickel Oxide Hybrids Derived from Metal-Organic Frameworks Based on Novel Schiff Base Ligands toward High-Performance Electrochromism

The tenets of coordination chemistry enable researchers to design and develop nanostructured materials based on metal-organic frameworks (MOFs). Herein, for the first time, we applied the Schiff base system to MOF derivatives as a strategy for the heteroatom introduction into carbon-based metal oxides toward electrochromic applications. The presented Ni-MOF thin films based on Schiff base ligands were prepared by a facile and economical reductive electrosynthesis approach, facilitating the scalable fabrication of large-size electrochromic films derived from MOFs. After the pyrolysis, the desired N-doped NiO@C (N-C@NiO) films can achieve a high cycling stability (500 cycles with 7% contrast attenuation) and coloration efficiency (80.18 cm²/C) via different pyrolysis procedures. In addition, the one-step fabricated N-C@NiO shows an excellent ability of contrast modulation (68%@580 nm) with merely 3.6% transmittance at the colored state. These improvements in electrochromic properties are attributed to hierarchical porous heterostructures and influenced by the N/C ratio and C-N bonding configuration, indicating that N-C@NiO systems derived from Schiff base MOFs are promising for low-transmittance displays.

Advances in nanomaterials for electrochromic devices

This review article systematically highlights the recent advances regarding the design, preparation, performance and application of new and unique nanomaterials for electrochromic devices.