A Covalent Organic Framework Film for Three-State Near-Infrared Electrochromism and a Molecular Logic Gate

Manipulating p-π Resonance through Methoxy Group Engineering in Covalent Organic Frameworks for an Efficient Photocatalytic Hydrogen Evolution

Kinetic factors frequently emerge as the primary constraints in photocatalysis, exerting a critical influence on the efficacy of polymeric photocatalysts. The diverse conjugation systems within covalent organic frameworks (COFs) can significantly impact photon absorption, energy level structures, charge separation and migration kinetics. Consequently, these limitations often manifest as unsatisfactory kinetic behavior, which adversely affects the photocatalytic activity of COFs. To address these challenges, we propose a methoxy (-OMe) molecular engineering strategy designed to enhance charge carrier kinetics and mitigate mass transfer resistance. Through strategic modulation of the position and quantity of -OMe units, we can effectively manipulate the p-π conjugation, thereby enhancing charge separation and migration. Moreover, COFs enriched with -OMe moieties exhibit enhanced mass transfer dynamics due to the hydrophilic nature of methoxy groups, which facilitate the diffusion of reactants and products within the porous structure. This approach is hypothesized to drive an efficient photocatalytic hydrogen evolution reaction.

Emerging Mixed-Valence Porous Materials

The development and applications of porous materials have been revolutionized over the past decades. To fine-tune their fascinating behaviors such as charge-transport, magnetic, and catalytic properties, one of the most effective strategies is incorporating mixed valency into the structures. This Concept provides recent progress on emerging crystalline porous materials, namely, metal-organic frameworks, metal-organic polyhedra, covalent organic frameworks, and hydrogen-bonded organic frameworks, where their mixed valency is either intrinsic or postsynthetically induced via an external stimulus. We further highlight the investigation of mixed valency and potential applications, which sheds light on the design and exploitation of mixed-valence crystalline porous materials.

Redox-Active Triphenylamine-Triazine Covalent Organic Framework: A High Surface Area Microporous Material for Electrochromism and Electrofluorochromism

Porous covalent organic frameworks (COFs) offer significant advantages in electrochromic (EC) applications due to their high surface area and porosity, which facilitate faradaic redox-mediated diffusion-controlled processes. However, COFs remain underexplored as EC materials due to challenges such as limited solubility and poor film-forming ability. In this study, we synthesized a redox-active triphenylamine-containing COF (TPA-TCIF) on ITO glass via a solvothermal method tailored for electrochromism and electrofluorochromism. The COF, formed through the condensation of tris(4-formyl phenyl)amine and 2,4,6-tris-(4-aminophenyl)-1,3,5-triazine, features a high surface area of 1136.8 m2/g and a pore diameter of 1.18 nm. This structure promotes efficient ion transport, reducing switching times and enhancing coloration efficiency (CE). Reversible redox reactions induce a color change from yellow to red, with an optical contrast (ΔT) of 37 % and a CE of 47.7 cm2/C. The film exhibited stable electrofluorochromic (EFC) behavior, switching from yellow emissive to dark black upon voltage application, with stability over 40 cycles. A quasi-solid-state electrochromic device (ECD) based on TPA-TCIF demonstrated reversible color transitions, showcasing its potential for future display technologies and highlighting the promise of COFs in advanced EC applications.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible well-defined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.

In Situ Electrochemical Interfacial Polymerization for Covalent Organic Frameworks with Tunable Electrochromism

A rapid in situ synthesis of electrochromic covalent organic frameworks (EC-COFs) was proposed by using green electrochemical interface polymerization of N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine (TPDA) and 2,5-dihydroxyterephthalaldehyde (DHBD). The synthetized TPDA-DHBD films exhibit stable polymorphic color variations under different applied potentials, which can be attributed to the redox state changes of bis(triphenylamine) and imine electroactive functional groups within the COFs skeleton. TPDA-DHBD represents markedly different electrochromisms from red to cyan due to the steric hindrance effect caused by the presence of UO2 2+, demonstrating the unique tunability of COFs materials. This work offers a new feasible idea for rapid EC-COFs synthesis and tunable EC-COFs realization.

A Wurster-Type Covalent Organic Framework with Internal Electron Transfer-Enhanced Catalytic Capacity for Tumor Therapy

The low immunogenicity of tumors, along with the abnormal structural and biochemical barriers of tumor-associated vasculature, impedes the infiltration and function of effector T cells at the tumor site, severely inhibiting the efficacy of antitumor immunotherapy. In this study, a cobaloxime catalyst and STING agonist (MSA-2)-coloaded Wurster-type covalent organic framework (Co-TB COF-M) with internal electron transfer-enhanced catalytic capacity was developed as a COF-based immune activator. The covalently anchored cobaloxime adjusts the energy band structure of TB COF and provides it with good substrate adsorption sites, enabling it to act as an electron transmission bridge between the COF and substrate in proton reduction catalytic reactions. This property significantly enhances the sonodynamic catalytic performance. Under sono-irradiation, Co-TB COF-M can produce a substantial amount of reactive oxygen species (ROS) to induce Gasdermin D-mediated pro-inflammatory pyroptosis, thereby effectively enhancing the immunogenicity of tumors. Furthermore, MSA-2 is specifically released in response to ROS at the tumor site, minimizing the off-target side effects. More importantly, Co-TB COF-induced STING activation normalizes tumor vasculature and increases the expression of endothelial T cell adhesion molecules, which greatly enhance the infiltration and function of effector T cells. Thus, Co-TB COF-M as an immune activator could remold the tumor microenvironment, leading to increased infiltration and an improved function of T cells for immunotherapy.

Regulating the Porosity and Bipolarity of Polyimide-Based Covalent Organic Framework for Advanced Aqueous Dual-Ion Symmetric Batteries

The all-organic aqueous dual-ion batteries (ADIBs) have attracted increasing attention due to the low cost and high safety. However, the solubility and unstable activity of organic electrodes restrict the synergistic storage of anions and cations in the symmetric ADIBs. Herein, a novel polyimide-based covalent organic framework (labeled as NTPI-COF) is constructed, featured with the boosted structure stability and electronic conductivity. Through regulating the porosity and bipolarity integrally, the NTPI-COF possesses hierarchical porous structure (mesopore and micropore) and abundant bipolar active centers (C═O and C─N), which exhibits rapid dual-ion transport and storage effects. As a result, the NTPI-COF as the electrodes for ADIBs deliver a high reversible capacity of 109.7 mA h g-1 for Na+ storage and that of 74.8 mA h g-1 for Cl- storage at 1 A g-1, respectively, and with a capacity retention of 93.2% over 10 000 cycles at 10 A g-1. Additionally, the all-organic ADIBs with symmetric NTPI-COF electrodes achieve an impressive energy density of up to 148 W h kg-1 and a high power density of 2600 W kg-1. Coupling the bipolarity and porosity of the all-organic electrodes applied in ADIBs will further advance the development of low-cost and large-scale energy storage.

Deep learning assisted logic gates for real-time identification of natural tetracycline antibiotics

The overuse and misuse of tetracycline (TCs) antibiotics, including tetracycline (TTC), oxytetracycline (OTC), doxycycline (DC), and chlortetracycline (CTC), pose a serious threat to human health. However, current rapid sensing platforms for tetracyclines can only quantify the total amount of TCs mixture, lacking real-time identification of individual components. To address this challenge, we integrated a deep learning strategy with fluorescence and colorimetry-based multi-mode logic gates in our self-designed smartphone-integrated toolbox for the real-time identification of natural TCs. Our ratiometric fluorescent probe (CD-Au NCs@ZIF-8) encapsulated carbon dots and Au NCs in ZIF-8 to prevent false negative or positive results. Additionally, our independently developed WeChat app enabled linear quantification of the four natural TCs using the fluorescence channels. The colorimetric channels were also utilized as outputs of logic gates to achieve real-time identification of the four individual natural tetracyclines. We anticipate this strategy could provide a new perspective for effective control of antibiotics.

Triphenylamine-Based Covalent Organic Framework Films for Dopamine-Responsive Electrofluorochromism

Covalent organic framework (COF) film with electrofluorochromic (EFC) and electrochromic (EC) properties has been synthesized by using triphenylamine-based monomers. The film exhibited a high maximum fluorescence contrast of 151 when subjected to a drive voltage of 0.75 V vs the Ag/AgCl electrode, causing the fluorescence to be quenched, which resulted in the EFC process's "fluorescence off" state. The switching times for the fluorescence on and off states were 0.51 and 7.79 s, respectively. Over the same voltage range, the COF film also displayed EC properties, achieving a contrast of 50.23% and a coloration efficiency of 297.4 cm2 C-1 at 532 nm, with switching times of 18.6 s for coloration and 0.7 s for bleaching. Notably, the quenched fluorescence of the COF film could be restored by adding dopamine as a reductant. This phenomenon enabled the implementation of a NAND logic gate using the applied potential as a physical input and dopamine addition as a chemical input. This study demonstrates the successful development of COF films with bifunctional EFC and EC properties, showcasing their potential for use in constructing advanced optoelectronic devices.

High-Performance Complementary Electrochromic Batteries using Nb18W16O93 by the Synergistic Effects of Aqueous Al3+/K+ Dual-Ion

Multivalent ions, especially Al3+ in aqueous electrolyte contributes to higher capacity and color contrast for more sustainable post-lithium electrochromism and energy storages. However, the lack of suitable cathodic and anodic electrochromic materials is a major challenge for Al-ion electrochromic batteries, which limits their optical contrast and lifespan. Herein, we report that Wadsley-Roth phase Nb18W16O93 with open structure achieves Al3+ intercalation/extraction reversibly. The complementary electrochromic energy storage devices based on Nb18W16O93 coupled with Prussian blue using hybrid Al3+/K+ aqueous electrolytes show a fast response, a high capacity and a large coloring efficiency. The superior performances are due to the cations of Al3+ and K+ selectively insert/extract in the electrode of Nb18W16O93 and Prussian blue, respectively. This work provides an effective strategy for high-performance and low-cost electrochromic batteries with higher sustainability.

Four-State Electrochromism in Tris(4-aminophenyl)amine- terephthalaldehyde-based Covalent Organic Framework

Covalent organic frameworks (COFs) are of massive interest due to their potential application spanning diverse fields such as gas storage and separation, catalysis, drug delivery systems, sensing, and organic electronics. In view of their application-oriented quest, the field of electrochromism marked a significant stride with the reporting of the first electrochromic COF in 2019 [J. Am. Chem. Soc. 2019, 141, 19831-19838]. Since then, new and novel COF structures with electrochromic features (denoted as ecCOFs) have been searched continuously. Yet, only a handful of ecCOFs have been constructed to date. A closer look at these reports suggests that multielectrochromism (showing at least three redox color states) in a COF assembly has only been achieved once, manifested through three-state electrochromism [Angew. Chem. 2021, 133, 12606-1261]. Herein, we report four-state electrochromism in tris(4-aminophenyl)amine-terephthalaldehyde (TAPA-PDA)-based COF constructed through the metal-catalyst free Schiff base approach. The four-state (orange, pear, green, and cyan) electrochromism demonstrated by the TAPA-PDA ecCOF opens several futuristic avenues for ecCOF's end use in flip-flop logic gates, intelligent windows, decorative displays, and energy-saving devices.

2D Conjugated Polymer Thin Films for Organic Electronics: Opportunities and Challenges

2D conjugated polymers (2DCPs) possess extended in-plane π-conjugated lattice and out-of-plane π-π stacking, which results in enhanced electronic performance and potentially unique band structures. These properties, along with predesignability, well-defined channels, easy postmodification, and order structure attract extensive attention from material science to organic electronics. In this review, the recent advance in the interfacial synthesis and conductivity tuning strategies of 2DCP thin films, as well as their application in organic electronics is summarized. Furthermore, it is shown that, by combining topology structure design and targeted conductivity adjustment, researchers have fabricated 2DCP thin films with predesigned active groups, highly ordered structures, and enhanced conductivity. These films exhibit great potential for various thin-film organic electronics, such as organic transistors, memristors, electrochromism, chemiresistors, and photodetectors. Finally, the future research directions and perspectives of 2DCPs are discussed in terms of the interfacial synthetic design and structure engineering for the fabrication of fully conjugated 2DCP thin films, as well as the functional manipulation of conductivity to advance their applications in future organic electronics.

Simultaneous Protonation and Metalation of a Porphyrin Covalent Organic Framework Enhance Photodynamic Therapy

Covalent organic frameworks (COFs) have been explored for photodynamic therapy (PDT) of cancer, but their antitumor efficacy is limited by excited state quenching and low reactive oxygen species generation efficiency. Herein, we report a simultaneous protonation and metalation strategy to significantly enhance the PDT efficacy of a nanoscale two-dimensional imine-linked porphyrin-COF. The neutral and unmetalated porphyrin-COF (Ptp) and the protonated and metalated porphyrin-COF (Ptp-Fe) were synthesized via imine condensation between 5,10,15,20-tetrakis(4-aminophenyl)porphyrin and terephthalaldehyde in the absence and presence of ferric chloride, respectively. The presence of ferric chloride generated both doubly protonated and Fe3+-coordinated porphyrin units, which red-shifted and increased the Q-band absorption and disrupted exciton migration to prevent excited state quenching, respectively. Under light irradiation, rapid energy transfer from protonated porphyrins to Fe3+-coordinated porphyrins in Ptp-Fe enabled 1O2 and hydroxyl radical generation via type II and type I PDT processes. Ptp-Fe also catalyzed the conversion of hydrogen peroxide to hydroxy radical through a photoenhanced Fenton-like reaction under slightly acidic conditions and light illumination. As a result, Ptp-Fe-mediated PDT exhibited much higher cytotoxicity than Ptp-mediated PDT on CT26 and 4T1 cancer cells. Ptp-Fe-mediated PDT afforded potent antitumor efficacy in subcutaneous CT26 murine colon cancer and orthotopic 4T1 murine triple-negative breast tumors and prevented metastasis of 4T1 breast cancer to the lungs. This work underscores the role of fine-tuning the molecular structures of COFs in significantly enhancing their PDT efficacy.

Viologen-based solution-processable ionic porous polymers for electrochromic applications

Electrochromic porous thin films are promising for applications in smart windows and energy-efficient optical displays. However, their generally poor processing ability and excessive processing times remain grand challenges. Herein, we report the design and convenient synthesis of core-altered N-arylated viologens with aldehyde groups (πV-CHO) as new building blocks to prepare soluble, viologen-embedded ionic porous polymers. We also demonstrate that these polymers can be easily solution-processed by drop-coating to fabricate high-quality electrochromic films with tunable optoelectronic properties in a cost-effective fashion. The prepared films exhibit excellent electrochromic performance, including a low driving voltage (1.2-1.4 V), fast switching times (0.8-1.7 s), great coloration efficiency (73-268 cm2 C-1), remarkably high optical contrast up to 95.6%, long cycling stability, and tunable oxidation and reduction colors. This work sheds important light on a new molecular engineering approach to produce redox-active polymers with combined properties of intrinsic porosity, reversible and tunable redox activity, and solution processability. This provides the materials with an inherently broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

Molecular-Potential and Redox Coregulated Cathodic Electrosynthesis toward Ionic Azulene-Based Thin Films for Organic Memristors

Organic memristors as promising electronic units are attracting significant attention owing to their simplicity of molecular structure design. However, fabricating high-quality organic films via novel synthetic technologies and exploring unprecedented chemical structures to achieve excellent memory performance in organic memristor devices are highly challenging. In this work, we report a cathodic electropolymerization to synthesize an ionic azulene-based memristive film (PPMAz-Py+Br-) under the molecular-potential and redox coregulation. During the cathodic electropolymerization process, electropositive pyridinium salts migrate to the cathode under an electric field, undergo a reduction-coupling deprotonation reaction, and polymerize into a uniform film with a controllable thickness on the electrode surface. The prepared Al/PPMAz-Py+Br-/ITO devices not only exhibit a high ON/OFF ratio of 1.8 × 103, high stability, long memory retention, and endurance under a wide range of voltage scans, but also achieve excellent multilevel storage and history-dependent memristive performance. In addition, the devices can mimic important biosynaptic functions, such as learning/forgetting function, synaptic enhancement/inhibition, paired-pulse facilitation/depression, and spiking-rate-dependent plasticity. The tunable memristive performances are attributed to the capture of free electrons on pyridinium cations, the migration of the aluminum ions (Al3+), and the form of Al conductive filaments under voltage scans.

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.

Smart Home Sleep Respiratory Monitoring System Based on a Breath-Responsive Covalent Organic Framework

A smart home sleep respiratory monitoring system based on a breath-responsive covalent organic framework (COF) was developed and utilized to monitor the sleep respiratory behavior of real sleep apnea patients in this work. The capacitance of the interdigital electrode chip coated with COFTPDA-TFPy exhibits thousands-level reversible responses to breath humidity gases, with subsecond response time and robustness against environmental humidity. A miniaturized printed circuit board, an open-face-mask-based respiratory sensor, and a smartphone app were constructed for the wearable wireless smart home sleep respiratory monitoring system. Leveraging the sensitive and rapid reversible response of COFs, the COF-based respiratory monitoring system can effectively record normal breath, rapid breath, and breath apnea, enabling over a thousand cycles of hour-level continuous monitoring during daily wear. Next, we took the groundbreaking step of advancing the humidity sensor to the clinical trial stage. In clinical experiments on real sleep apnea patients, the COF-based respiratory monitoring system successfully recorded hour-level sleep respiratory data and differentiated the breathing behavior characteristics and severity of sleep apnea patients and subjects with normal sleep function and primary snoring patients. This work successfully advanced humidity sensors into clinical research for real patients and demonstrated the enormous application potential of COF materials in clinical diagnosis.

2D Covalent Organic Frameworks with Kagome Lattice: Synthesis and Applications

2D covalent organic frameworks with Kagome (kgm) topology are a promising class of crystalline frameworks that possess both triangular and hexagonal pores. These dual-pore structures enable kgm COFs to exhibit unique advantages in selective separation, mass transfer, and targeted drug release. However, the synthesis of 2D kgm COFs has been hindered by the reliance on empirical methods. This review systematically summarizes the conventional macrocycle-to-framework strategy, typical [4+2] co-polymerization synthetic strategy, and bifunctional molecules self-condensation approach for constructing 2D kgm COFs. Factors influencing the formation of kgm lattice are surveyed, such as monomer type, solvent polarity, substrate concentration, etc., and highlight the representative examples on targeted synthesis. Additionally, applications of 2D kgm COFs and relationships between structure and performances are summarized. Finally, key fundamental perspectives are proposed to accelerate the further development of this intriguing material.

Kagome-topology 2D covalent organic frameworks assembled from D2h-symmetric and non-centrosymmetric C2-symmetric blocks for photothermal imaging

In this study, we synthesized two new two-dimensional (2D) covalent organic frameworks (COFs), COF-TA and COF-DP, by combining 4-connected D2h-symmetric and 2-connected non-centrosymmetric C2-symmetric building blocks. Unlike the typical sql topology, these COFs exhibit an unconventional kgm topology characterized by a favorable anti-parallel stacking arrangement, which results in a lower energy configuration. Notably, COF-DP, with its unique D-A-D structural motif and photosensitive properties, demonstrates a narrow band gap and excellent photothermal conversion capabilities, making it a promising material for photothermal imaging applications.

A Large-Area Patterned Hydrogen-Bonded Organic Framework Electrochromic Film and Device

Fabrication of a patterned hydrogen-bonded organic framework (HOF) films on a large scale is an extreme challenge. In this work, a large area HOF film (30 × 30 cm2 ) is prepared via an efficient and low-cost electrostatic spray deposition (ESD) approach on the un-modified conductive substrates directly. Combining the ESD with a template method, variously patterned HOF films can be easily produced, including deer- and horse-shaped films. The obtained films exhibit excellent electrochromic performance with multicolor change from yellow to green and violet, and two-band regulation at 550 and 830 nm. Benefiting from the inherently present channels of HOF materials and the additional film porosity created by ESD, the PFC-1 film could quickly change color (within 10 s). Furthermore, the large-area patterned EC device is constructed based on the above film to prove practical potential application. The presented ESD method can be extended to other HOF materials; thus, this work paves a feasible path for constructing large-area patterned HOF films for practical optoelectronic applications.

Cobaloxime-Integrated Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution Coupled with Alcohol Oxidation

We report an azide-functionalized cobaloxime proton-reduction catalyst covalently tethered into the Wurster-type covalent organic frameworks (COFs). The cobaloxime-modified COF photocatalysts exhibit enhanced photocatalytic activity for hydrogen evolution reaction (HER) in alcohol-containing solution with no presence of a typical sacrificial agent. The best performing cobaloxime-modified COF hybrid catalyzes hydrogen production with an average HER rate up to 38 μmol h-1 in ethanol/phosphate buffer solution under 4 h illumination. Ultrafast transient optical spectroscopy characterizations and charge carrier analysis reveal that the alcohol contents functioning as hole scavengers could be oxidized by the photogenerated holes of COFs to form aldehydes and protons. The consumption of the photogenerated holes thus suppresses exciton recombination of COFs and improves the ratio of free electrons that were effectively utilized to drive catalytic reaction for HER. This work demonstrates a great potential of COF-catalyzed HER using alcohol solvents as hole scavengers and provides an example toward realizing the accessibility to the scope of reaction conditions and a greener route for energy conversion.

Synthesis of Near-Infrared-Absorbing Anionic Heptamethine Cyanine Dyes with Trifluoromethyl Groups

A novel anionic heptamethine cyanine (HMC) dye with two trifluoromethyl groups that selectively absorb near-infrared light is synthesized. When contrasted with previously studied anionic HMC dyes with substituents such as methyl, phenyl, and pentafluorophenyl groups, the trifluoromethylated dye displays a red-shifted maximum absorption wavelength (for instance, 948 nm in CH₂Cl₂) along with enhanced photostability. Furthermore, HMC dyes with broad absorption in the near-infrared region are synthesized by combining a trifluoromethylated anionic HMC dye with a cationic HMC dye as a counterion.

Survey of Recent Advances in Molecular Fluorophores, Unconjugated Polymers, and Emerging Functional Materials Designed for Electrofluorochromic Use

In this review, recent advances that exploit the intrinsic emission of organic materials for reversibly modulating their intensity with applied potential are surveyed. Key design strategies that have been adopted during the past five years for developing such electrofluorochromic materials are presented, focusing on molecular fluorophores that are coupled with redox-active moieties, intrinsically electroactive molecular fluorophores, and unconjugated emissive organic polymers. The structural effects, main challenges, and strides toward addressing the limitations of emerging fluorescent materials that are electrochemically responsive are surveyed, along with how these can be adapted for their use in electrofluorochromic devices.

Two-Dimensional Optical Waveguides at Telecom Wavelengths Based on Organic Single-Crystal Microsheets of a Charge Transfer Complex

Organic charge transfer (CT) cocrystals open a new door for the exploitation of low-dimensional near-infrared (NIR) emitters by a convenient self-assembly approach. However, research about the fabrication of sheet-like NIR-emitting microstructures that are significant for structural construction and integrated application is limited by the unidirectional molecular packing mode. Herein, via regulation of the biaxial intermolecular CT interaction, single-crystalline microsheets with remarkable NIR emission from 720 to 960 nm were synthesized via the solution self-assembly process of dithieno[3,2-b:2',3'-d]thiophene and 7,7,8,8-tetracyanoquinodimethane. The expected sheet-like structure is conducive to achieving a two-dimensional (2D) optical waveguide with an ultralow optical loss rate of 0.250 dB/μm at 860 nm. More significantly, these as-prepared organic microsheets with tunable thicknesses (h) from 100 to 1100 nm exhibit thickness-dependent NIR optical transportation performance. These findings could pave the way to a new class of low-dimensional NIR emitters for 2D photonics at telecom wavelengths.

Electrically Conductive Carbazole and Thienoisoindigo-Based COFs Showing Fast and Stable Electrochromism

Thienothiophene thienoisoindigo (ttTII)-based covalent organic frameworks (COFs) have been shown to offer low band gaps and intriguing optical and electrochromic properties. So far, only one tetragonal thienothiophene thienoisoindigo-based COF has been reported showing stable and fast electrochromism and good coloration efficiencies. We have developed two novel COFs using this versatile and nearly linear ttTII building block in a tetragonal and a hexagonal framework geometry to demonstrate their attractive features for optoelectronic applications of thienoisoindigo-based COFs. Both COFs exhibit good electrical conductivities, show promising optical absorption features, are redox-active, and exhibit a strong electrochromic behavior when applying an external electrical stimulus, shifting the optical absorption even farther into the NIR region of the electromagnetic spectrum and achieving absorbance changes of up to 2.5 OD. Cycle-stable cyclic voltammograms with distinct oxidation and reduction waves reveal excellent reversibility and electrochromic switching over 200 cycles and confirm the high stability of the frameworks. Furthermore, high coloration efficiencies in the NIR region and fast switching speeds for coloration/decoloration as fast as 0.75 s/0.37 s for the Cz-ttTII COF and 0.61 s/0.29 s for the TAPB-ttTII COF at 550 nm excitation were observed, outperforming many known electrochromic materials, and offering options for a great variety of applications, such as stimuli-responsive coatings, optical information processing, or thermal control.

A sweat-responsive covalent organic framework film for material-based liveness detection and sweat pore analysis

Covalent organic frameworks have shown considerable application potential and exceptional properties in the construction of stimulus-responsive materials. Here, we designed a sweat-responsive covalent organic framework film for material-based fingerprint liveness detection. When exposed to human sweat, the COF_TPDA-TFPy film can transform from yellow to red. The COF_TPDA-TFPy film, when touched by living fingers, can produce the naked-eye-identified fingerprint pattern through the sweat-induced color change, while artificial fake fingerprints cannot. This technique, which we named material-based liveness detection, can thus intuitively discern living fingers from fake fingerprints with a 100% accuracy rate. Additionally, the distribution of sweat pores on human skin can also be collected and analyzed by shortening the contact time. By merely washing them with ethanol, all the samples can be utilized again. This work inventively accomplished material-based liveness detection and naked-eye-identified sweat pore analysis and highlighted their potential for use in clinical research and personal identification.

Covalent Organic Framework Thin-Film Photodetectors from Solution-Processable Porous Nanospheres

The synthesis of homogeneous covalent organic framework (COF) thin films on a desired substrate with decent crystallinity, porosity, and uniform thickness has great potential for optoelectronic applications. We have used a solution-processable sphere transmutation process to synthesize 300 ± 20 nm uniform COF thin films on a 2 × 2 cm² TiO₂-coated fluorine-doped tin oxide (FTO) surface. This process controls the nucleation of COF crystallites and molecular morphology that helps the nanospheres to arrange periodically to form homogeneous COF thin films. We have synthesized four COF thin films (TpDPP, TpEtBt, TpTab, and TpTta) with different functional backbones. In a close agreement between the experiment and density functional theory, the TpEtBr COF film showed the lowest optical band gap (2.26 eV) and highest excited-state lifetime (8.52 ns) among all four COF films. Hence, the TpEtBr COF film can participate in efficient charge generation and separation. We constructed optoelectronic devices having a glass/FTO/TiO₂/COF-film/Au architecture, which serves as a model system to study the optoelectronic charge transport properties of COF thin films under dark and illuminated conditions. Visible light with a calibrated intensity of 100 mW cm^-2 was used for the excitation of COF thin films. All of the COF thin films exhibit significant photocurrent after illumination with visible light in comparison to the dark. Hence, all of the COF films behave as good photoactive substrates with minimal pinhole defects. The fabricated out-of-plane photodetector device based on the TpEtBr COF thin film exhibits high photocurrent density (2.65 ± 0.24 mA cm^-2 at 0.5 V) and hole mobility (8.15 ± 0.64 ×10^-3 cm² V^-1 S^-1) compared to other as-synthesized films, indicating the best photoactive characteristics.

Highly Crystalline Polyimide Covalent Organic Framework as Dual-Active-Center Cathode for High-Performance Lithium-Ion Batteries

Polyimide covalent organic framework (PI-COF) materials that can realize intrinsic redox reactions by changing the charge state of their electroactive sites are considered as emerging electrode materials for rechargeable devices. However, the highly crystalline PI-COFs with hierarchical porosity are less reported due to the rapid reaction between monomers and the poor reversibility of the polyimidization reaction. Here, we developed a water-assistant synthetic strategy to adjust the reaction rate of polyimidization, and PI-COF (COF_TPDA-PMDA) with kgm topology consisting of dual active centers of N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine (TPDA) and pyromellitic dianhydride (PMDA) ligands was successfully synthesized with high crystallinity and porosity. The COF_TPDA-PMDA possesses hierarchical micro-/mesoporous channels with the largest surface area (2669 m²/g) in PI-COFs, which can promote the Li⁺ ions and bulky bis(trifluoromethanesulfonyl)imide (TFSI^-) ions in organic electrolyte to sufficiently interact with the dual active sites on COF skeleton to increase the specific capacity of cathode materials. As a cathode material for lithium-ion batteries, COF_TPDA-PMDA@50%CNT which integrated high surface area and dual active center of COF_TPDA-PMDA with carbon nanotubes via π-π interactions gave a high initial charge capacity of 233 mAh/g (0.5 A/g) and maintains at 80 mAh/g even at a high current density of 5.0 A/g after 1800 cycles.

Moisture-Insensitive and Highly Selective Detection of NO2 by Ion-in-Conjugation Covalent Organic Frameworks

As a common toxic gas, nitrogen dioxide (NO₂) seriously threatens the environment and human respiratory system even at part per billion (ppb) level. Covalent organic frameworks (COFs) have gained widespread attention in sensing applications because of the benefits of designability, environmental stability, and a large number of active sites. However, the competitive adsorption of water molecules and the target gas molecules at room temperature as well as the weak interaction between COFs and gas molecules hinder their practical applications. Here, we introduce ion-in-conjugation (IIC) into a covalent organic framework (COF) by preparing a condensate of squaraine (SA) with 1,3,5-tris(4-aminophenyl)benzene (TAPB) to form a mesoporous macrocyclic material (SA-TAPB). Layers of SA-TAPB, drop cast onto interdigitated Ag-Pd alloy electrodes, show a statistically significant conductivity response to NO₂ at concentrations as low as 30 ppb and a theoretical detection limit of 10.9 ppb. The sensor displays a lower sensitivity to variations in humidity when operated at 80 °C compared to room temperature. The density functional theory (DFT) calculations indicated that the main adsorption site of NO₂ is dual hydrogen bonds formed between two amide hydrogen atoms of SA-TAPB and the NO₂ molecule. Gas adsorption experiments revealed that SA-TAPB has the largest adsorption capacity of NO₂ versus other interference gases, which were responsible for the excellent selectivity toward NO₂.

Emerging Electrochromic Materials and Devices for Future Displays

With the rapid development of optoelectronic fields, electrochromic (EC) materials and devices have received remarkable attention and have shown attractive potential for use in emerging wearable and portable electronics, electronic papers/billboards, see-through displays, and other new-generation displays, due to the advantages of low power consumption, easy viewing, flexibility, stretchability, etc. Despite continuous progress in related fields, determining how to make electrochromics truly meet the requirements of mature displays (e.g., ideal overall performance) has been a long-term problem. Therefore, the commercialization of relevant high-quality products is still in its infancy. In this review, we will focus on the progress in emerging EC materials and devices for potential displays, including two mainstream EC display prototypes (segmented displays and pixel displays) and their commercial applications. Among these topics, the related materials/devices, EC performance, construction approaches, and processing techniques are comprehensively disscussed and reviewed. We also outline the current barriers with possible solutions and discuss the future of this field.

The Flexible Electrochromic Device That Turns Blue Under Both Positive and Negative Application Voltages

Using the photocurable electrolyte solution containing viologen derivative 1,1′-bis- (4-vinylbenzyl)-4,4′-bipyridinium dihexafluorophosphate (EV2+•2PF6−) and its composite of EV2+•2PF6− and triphenylamine derivative N,N′-di(4-((3-(triethoxysilyl)-propyl)imino)methyl)phenyl-N,N′-diphenyl-4,4′-biphenyldiamine (TPB-PSSO) as the electrochromic active layer, respectively, the flexible electrochromic devices (FECDs) were prepared by photocurable technology. The device structure was PET-ITO/photocurable electrolyte solution/PET-ITO. The electrochromic properties of FECDs were investigated. The results show that the FECDs can all undergo a reversible color change, especially the PET-ITO/photocurable electrolyte solution-composite/PET-ITO can undergo polychromatic transition. The FECD based on EV2+•2PF6− can reversibly change between colorless and deep blue, and the FECD based on the composite of TPB-PSSO-EV2+•2PF6− can reversibly change between black blue, light yellow, and sky blue.

Anion and Temperature Responsive Molecular Switches Based on Trimetallic Complexes of Ru(II) and Os(II) That Demonstrate Advanced Boolean and Fuzzy Logic Functions

Anion- and temperature-induced alteration of the photoredox behaviors of our recently reported trimetallic complexes was carried out to fabricate potential molecular switches. The triads [(phen)₂Ru(d-HIm-t)M(t-HIm-d)Ru(phen)₂]⁶⁺ (phen = 1,10-phenanthroline, d-HIm-t = heterotopic bipyridine-terpyridne type bridging ligand, and M = Ru^II and Os^II) possess two acidic imidazole NH protons that upon interaction with anions give rise to considerable changes in their photophysical and electrochemical behaviors. Red-shifts of the absorption and emission maximum and a decrease in the M³⁺/M²⁺ potential occur when the triads are treated with basic anions. In fact, the triads can function as triple-channel sensors for F^-, AcO^-, CN^-, OH^-, and H₂PO₄^- in acetonitrile and as selective probes for CN^- and SCN^- in water. The equilibrium constants for the receptor-anion interaction are on the order of 10⁶ M^-1, while the limit of detection was on the order of 10^-9 M. Temperature plays a key role in the luminescence properties of the triads by adjusting the energy barrier between the emitting triplet metal-to-ligand charge transfer and non-emitting triplet metal-centered levels. A decrease in temperature leads to increases in the emission intensity and lifetime of the triads displaying the "on state", whereas an increase in temperature leads to a decrease in their emission characteristics and thus indicates the "off state" and that the process is fully reversible. In essence, the triads could function as potential switches on the basis of the reversible change in their spectral and redox behaviors under the influence of anion, acid, and temperature. The most important outcome of this study is mimicking several advanced Boolean and fuzzy logic functions by utilizing the spectral response of the triads upon the action of said external stimuli.

Grafting Hollow Covalent Organic Framework Nanoparticles with Thermal-Responsive Polymers for the Controlled Release of Preservatives

Covalent organic frameworks (COFs) hold great potential in various applications because of their well-defined pore structures and morphologies. However, most COF materials demonstrate poor dispersibility in solvents that significantly limits their processing and applications. Herein, we report the synthesis of COF-based hollow nanoparticles (h-NPs) with good water dispersibility, high capacity, and thermal responsiveness to load essential oil molecules for longer-term preservation of fruits. Imine-based COF h-NPs possessing a pore width of 1.3 nm, inner/outer diameters of ∼150/239 nm, and high crystallinity were synthesized and grafted with water-soluble polymers such as polyethylene glycol or poly(N-isopropylacrylamide) (PNIPAM) with molecular weights of 1-3 kDa. The h-NP products with grafting densities of 0.6-2.1 nm-2 can be well dispersed in water at room temperature. PNIPAM-grafted ones are temperature-responsive in that they can precipitate out from the dispersion at 40 °C and redisperse at 25 °C for at least 15 cycles. The h-NPs are used as nanocarriers to load essential oils such as hexanal and trans-2-hexenal with a high capacity of 1.1 g/g for fruit fresh-keeping, and the encapsulated preservatives can be released controllably at 25-40 °C as regulated by the grafted polymers. As a result, the storage time of cherry tomatoes can be prolonged by 4 days compared to the control run. Moreover, these h-NPs can be recycled and reused. Our work highlights the potential of COF nanomaterials grafting with stimuli-responsive polymers for controlled release application in various food preservation.

Covalent organic frameworks based on tetraphenyl-p-phenylenediamine and metalloporphyrin for electrochemical conversion of CO2 to CO

The as-prepared TPPDA-CoPor-COF shows high CO faradic efficiencies of 87–90% from −0.6 to −0.9 V vs. RHE, and the largest CO partial current density of TPPDA-CoPor-COF exceeds most of reported COF-based electrocatalysts.

Redox-Active Heteroatom-Functionalized Polyacetylenes

The discovery of metallic conductivity in polyacetylene [-HC=CH-]_n upon doping represents a landmark achievement. However, the insolubility of polyacetylene and a dearth of methods for its chemical modification have limited its widespread use. Here, we employ a ring-opening metathesis polymerization (ROMP) protocol to prepare functionalized polyacetylenes (fPAs) bearing: (1) electron-deficient boryl (-BR₂ ) and phosphoryl (-P(O)R₂ ) side chains; (2) electron-donating amino (-NR₂ ) groups, and (3) ring-fused 1,2,3-triazolium units via strain-promoted Click chemistry. These functional groups render most of the fPAs soluble and can lead to intense light absorption across the visible to near-IR region. Also, the presence of redox-active boryl and amino groups leads to opposing near-IR optical responses upon (electro)chemical reduction or oxidation. Some of the resulting fPAs show greatly enhanced air stability when compared to known polyacetylenes. Lastly, these fPAs can be cross-linked to yield network materials with the full retention of optical properties.

Two-dimensional Covalent Organic Frameworks for Electrochromic Switching

The electrochromic materials have received immense attention for the fabrication of smart optoelectronic devices. The alteration of the redox states of the electroactive functionalities results in the color change in response to electrochemical potential. Even though transition metal oxides, redox-active small organic molecules, conducting polymers, and metallopolymers are known for electrochromism, advanced materials demonstrating multicolor switching with fast response time and high durability are of increasing demand. Recently, two-dimensional covalent organic frameworks (2D COFs) have been demonstrated as electrochromic materials due to their tunable redox functionalities with highly ordered structure and large specific surface area facilitating fast ion transport. Herein, we have discussed the mechanistic insights of electrochromism in 2D COFs and their structure-property relationship in electrochromic performance. Furthermore, the state-of-the-art knowledge for developing the electrochromic 2D COFs and their potential application in next-generation display devices are highlighted.

Light and Cation-Driven Optical Switch based on a Stilbene-Appended Terpyridine System for the Design of Molecular-Scale Logic Devices

A molecular system comprising a terpyridine moiety capable of coordinating with different cations and a photoswitchable stilbene unit has been utilized here for the fabrication of multiply configurable logic systems. Incorporation of a substituted stilbene unit into the terpyridine motif generates an intraligand charge-transfer-sensitive module whose absorption and emission spectral properties are highly sensitive to light as well as cations. On the basis of the optical response profile of the receptor in the presence of selected cations as well as light of a specific wavelength, we are able to demonstrate multiple Boolean logic functions such as INHIBIT, IMPLICATION, OR, NOR, and NAND, as well as various combinations of them. Of particular interest, we utilized the present system for the construction of security keypad locks and memory devices by maintaining a proper sequence of the stimuli and monitoring either absorption or emission spectral response at a specific wavelength as the output signal. In addition to various Boolean logic functions, the present system has also the ability to mimic fuzzy logic operations for generating an infinite-valued logic scheme depending on its emission spectral responses upon varying the concentration of cationic (Fe²⁺ and/or Zn²⁺) and anionic (CN^-) inputs.