para-Aminoazobenzenes-Bipolar Redox-Active Molecules.

Azobenzenes (ABs) are versatile compounds featured in numerous applications for energy storage systems, such as solar thermal storages or phase change materials. Additionally, the reversible one-electron reduction of these diazenes to the nitrogen-based radical anion has been used in battery applications. Although the oxidation of ABs is normally irreversible, 4,4'-diamino substitution allows a reversible 2e- oxidation, which is attributed to the formation of a stable bis-quinoidal structure. Herein, we present a system that shows a bipolar redox behaviour. In this way, ABs can serve not only as anolytes, but also as catholytes. The resulting redox potentials can be tailored by suitable amine- and ring-substitution. For the first time, the solid-state structure of the oxidized form could be characterized by X-ray diffraction.

Multiple Stabilization Effects of Benzylhydrazine on Scalable Perovskite Precursor Inks for Improved Perovskite Solar Cell Production.

Perovskite precursor inks suffer various forms of degradation, such as iodide anion oxidation and organic cation breakdown, hindering reliable perovskite solar cell manufacturing. Here we report that benzylhydrazine hydrochloride (BHC) not only retards the buildup of iodine as previously reported but also prevents the breakdown of organic cations. Through investigating BHC and iodine chemical reactions, we elucidate protonation and dehydration mechanisms, converting BHC to harmless volatile compounds, thus preserving perovskite film crystallization and solar cell performance. This inhibition effect lasts nearly a month with minimal BHC, contrasting control inks without BHC where organic cations fully react in less than a week. This enhanced understanding, from additive stabilization to end products, promises improved perovskite solar cell production reliability.

High-Performance Azo Cathodes Enabled by N-Heteroatomic Substitution for Zinc Batteries with a Self-Charging Capability.

Redox-active azo compounds are emerging as promising cathode materials due to their multi-electron redox capacity and fast redox response. However, their practical application is often limited by low output voltage and poor thermal stability. Herein, we use a heteroatomic substitution strategy to develop 4,4'-azopyridine. This modification results in a 350 mV increase in reduction potential compared to traditional azobenzene, increasing the energy density at the material level from 187 to 291 Wh kg-1. The introduced heteroatoms not only raise the melting point of azo compounds from 68 °C to 112 °C by forming an intermolecular hydrogen-bond network but also improves electrode kinetics by reducing energy band gaps. Moreover, 4,4'-azopyridine forms metal-ligand complexes with Zn2+ ions, which further self-assemble into a robust superstructure, acting as a molecular conductor to facilitate charge transfer. Consequently, the batteries display a good rate performance (192 mAh g-1 at 20 C) and an ultra-long lifespan of 60,000 cycles. Notably, we disclose that the depleted batteries spontaneously self-charge when exposed to air, marking a significant advancement in the development of self-powered aqueous systems.

Modular Cascade of Flow Reactors: Continuous Flow Synthesis of Water-Insoluble Diazo Dyes in Aqueous System.

Continuous flow synthesis is pivotal in dye production to address batch-to-batch variations. However, synthesizing water-insoluble dyes in an aqueous system poses a challenge that can lead to clogging. This study successfully achieved the safe and efficient synthesis of azo dyes by selecting and optimizing flow reactor modules for different reaction types in the two-step reaction and implementing cascade cooperation. Integrating continuous flow microreactor with continuous stirred tank reactor (CSTR) enabled the continuous flow synthesis of Sudan Yellow 3G without introducing water-soluble functional groups or using organic solvents to enhance solubility. Optimizing conditions (acidity/alkalinity, temperature, residence time) within the initial modular continuous flow reactor resulted in a remarkable 99.5% isolated yield, 98.6 % purity, and a production rate of 2.90 g h-1. Scaling-up based on different reactor module characteristics further increased the production rate to 74.4 g h-1 while maintaining high yield and purity. The construction of this small 3D-printing modular cascaded reactor and process scaling-up provide technical support for continuous flow synthesis of water-insoluble dyes, particularly high-market-share azo dyes. Moreover, this versatile methodology proves applicable to continuous flow processes involving various homogeneous and heterogeneous reaction cascades.

Construction of fluorescent sensor array with nitrogen-doped carbon dots for sensing Sudan Orange G and identification of various azo compounds.

In this study, nitrogen-doped carbon dots (N-CDs) were facilely fabricated by one-pot hydrothermal method with levulinic acid and triethanolamine. A fluorescent sensor array was established for identifying azo compounds including Sudan Orange G (SOG), p-diaminoazobenzene, p-aminoazobenzene, azobenzene and quantitative detection of SOG. Experimental results revealed that azo compounds could quench the fluorescent intensity of N-CDs. Owing to various azo compounds showing different affinities to N-CDs, the sensor array exhibited different fluorescence quenching changes, which were further analyzed with principal component analysis to discriminate azo compounds. The sensor array was able to differentiate and recognize diverse concentrations of azo compounds from 0.25 to 2 mg/L. Simultaneously, a variety of factors affecting the detection of SOG were optimized. Under the optimized conditions, the sensor showed excellent stability and sensitivity. The sensor possessed marvelous linearity in the range of 0.1-1 mg/L and 1-4 mg/L and the detection limit was 27.82 µg/L. Spiked recoveries of 90.8-98.2 % were attained at spiked levels of 0.2 mg/L and 1 mg/L, demonstrating that the constructed fluorescence sensor was dependable and feasible for sensing SOG in environmental water samples.

Synthesis of Naphthalimide Azocarboxylates Showing Turn-On Fluorescence by Substitution Reaction with Sulfinates.

The synthesis and characterization of sulfinate addition-responsive fluorescent molecules are described. We found that addition reaction of sulfinates to naphthalimide-substituted azocarboxylates afforded the corresponding sulfonyl hydrazides with high fluorescence quantum yields (up to 0.91 in THF and 0.54 in methanol), which exhibited a large Stokes shift (105 nm) in protic methanol solvent, while the unsubstituted hydrazide and the sulfonyl-position isomer showed no fluorescence in polar solvents.

Accelerated Dynamic Reconstruction in Metal-Organic Frameworks with Ligand Defects for Selective Electrooxidation of Amines to Azos Coupling with Hydrogen Production.

Electrosynthesis coupled hydrogen production (ESHP) mostly involves catalyst reconstruction in aqueous phase, but accurately identifying and controlling the process is still a challenge. Herein, we modulated the electronic structure and exposed unsaturated sites of metal-organic frameworks (MOFs) via ligand defect to promote the reconstruction of catalyst for azo electrosynthesis (ESA) coupled with hydrogen production overall reaction. The monolayer Ni-MOFs achieved 89.8 % Faraday efficiency and 90.8 % selectivity for the electrooxidation of 1-methyl-1H-pyrazol-3-amine (Pyr-NH2) to azo, and an 18.5-fold increase in H2 production compared to overall water splitting. Operando X-ray absorption fine spectroscopy (XAFS) and various in situ spectroscopy confirm that the ligand defect promotes the potential dependent dynamic reconstruction of Ni(OH)2 and NiOOH, and the reabsorption of ligand significantly lowers the energy barrier of rate-determining step (*Pyr-NH to *Pyr-N). This work provides theoretical guidance for modulation of electrocatalyst reconstruction to achieve highly selective ESHP.

Crystal structure, Hirshfeld surface analysis and energy frameworks of 1-[(E)-2-(2-fluoro-phen-yl)diazan-1-yl-idene]naphthalen-2(1H)-one.

The title compound, C16H11N2OF, is a member of the azo dye family. The dihedral angle subtended by the benzene ring and the naphthalene ring system measures 18.75 (7)°, indicating that the compound is not perfectly planar. An intra-molecular N-H⋯O hydrogen bond occurs between the imino and carbonyl groups. In the crystal, the mol-ecules are linked into inversion dimers by C-H⋯O inter-actions. Aromatic π-π stacking between the naphthalene ring systems lead to the formation of chains along [001]. A Hirshfeld surface analysis was undertaken to investigate and qu-antify the inter-molecular inter-actions. In addition, energy frameworks were used to examine the cooperative effect of these inter-molecular inter-actions across the crystal, showing dispersion energy to be the most influential factor in the crystal organization of the compound.

Theoretical Design and Synthesis of Caged Compounds Using X-Ray-Triggered Azo Bond Cleavage.

Caged compounds are frequently used in life science research. However, the light used to activate them is commonly absorbed and scattered by biological materials, limiting their use to basic research in cells or small animals. In contrast, hard X-rays exhibit high bio-permeability due to the difficulty of interacting with biological molecules. With the main goal of developing X-ray activatable caged compounds, azo compounds are designed and synthesized with a positive charge and long π-conjugated system to increase the reaction efficiency with hydrated electrons. The azo bonds in the designed compounds are selectively cleaved by X-ray, and the fluorescent substance Diethyl Rhodamine is released. Based on the results of experiments and quantum chemical calculations, azo bond cleavage is assumed to occur via a two-step process: a two-electron reduction of the azo bond followed by N─N bond cleavage. Cellular experiments also demonstrate that the azo bonds can be cleaved intracellularly. Thus, caged compounds that can be activated by an azo bond cleavage reaction promoted by X-ray are successfully generated.

A Synthetic Approach for Oxadiazole-Decorated Azobenzene Photoswitches.

This work reports the design and synthesis of novel oxadiazole-decorated azobenzenes, structural analysis of the resulting compounds and behavior under light irradiation. The synthetic strategy involved constructing amino functionalized heterocyclic key intermediates which were used either to yield electrophilic diazonium salts able to react with phenol moieties or as nucleophilic partners in Bayer-Mills reaction with nitroso-substituted derivatives. The amino-derived oxadiazole intermediates were investigated by absorption and emission spectroscopy providing blue and green emitted light. The target oxadiazole-decorated azobenzenes were structurally characterized, including solid-state structures, and subsequently used in irradiation experiments in order to take advantage of the azo group known to provide photoswitching abilities. We noticed quenching of the emissive properties in presence of the azo group; however, all compounds were very stable to repeated cycles of light irradiation. In addition, according to structural diversification we could obtain half-lives of the meta stable isomers within hours to hundreds of hours range. The experimental results were very well correlated with DFT calculations.

Photomodulation of the Mechanical Properties and Photo-Actuation of Chitosan-Based Thin Films Modified with an Azobenzene-Derivative.

A sophisticated comprehension of the impacts of photoisomerization and photothermal phenomena on biogenic and responsive materials can provide a guiding framework for future applications. Herein, the procedure to manufacture homogeneous chitosan-based smart thin films are reported by incorporating the light-responsive azobenzene-derivative Sodium-4-[(4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)diazen-yl]-benzenesulfonate (TEGABS) in the biopolymer through electrostatic interactions. When irradiated with UV-light the TEGABS/chitosan films show a biresponse, comprising the E→Z photoisomerization with a half-life of 13 - 20 h and the light-induced evaporation of residual moisture leading to an increase in the reduced indentation modulus (up to 49%) and hardness. Freestanding films of TEGABS/chitosan show actuation up to 13° while irradiated with UV-light. This work shows the potential of biogenic polysaccharides in the design of biresponsive materials with photomodulated mechanical properties and unveils the link between the humidity of the environment, residual moisture, and the photomodulation of the mechanical properties.

A Photomechanical Film in which Liquid Crystal Design Shifts the Absorption into the Visible Light Range.

Liquid crystalline polymer networks (LCN) with azobenzene monomers bend reversibly under UV-light irradiation, combining photomechanical and photothermal effects. However, the harmful nature of UV-light limits their use in biology and soft robotics. Although visible light-absorbing tetra-ortho-fluoro-substituted azobenzenes exist, liquid crystalline monomers have never been prepared. Previously, such azobenzenes were added as photoactive additives (up to 10%) to otherwise passive liquid crystalline polymer networks. In this work, a molecular design of a liquid crystalline, polymerizable azobenzene switchable by visible light is presented. The monomer assembles in a highly fluid nematic phase, but polymerizes in a layered smectic C phase. The films are produced solely from the monomer without additional liquid crystalline components and are actuated with visible light. Bending experiments in air and under water differentiate photomechanical and photothermal effects. Remarkably, a 60 µm splay aligned film maintains its deformation for hours, slowly reverting over days. Monomer liquid crystallinity is characterized using differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and polarized optical microscopy (POM); polymer films are analyzed using WAXS and DSC on a homogeneously aligned film. The synthetic procedure is high yielding and polymer film fabrication is scalable, which enables the use of safe and efficient photomechanical LCNs in soft robotics, engineering and biology.

Recent Advances in the Synthesis of Aromatic Azo Compounds.

Aromatic azo compounds have -N=N- double bonds as well as a larger π electron conjugation system, which endows aromatic azo compounds with wide applications in the fields of functional materials. The properties of aromatic azo compounds are closely related to the substituents on their aromatic rings. However, traditional synthesis methods, such as the coupling of diazo salts, have a significant limitation with respect to the structural design of aromatic azo compounds. Therefore, many scientists have devoted their efforts to developing new synthetic methods. Moreover, recent advances in the synthesis of aromatic azo compounds have led to improvements in the design and preparation of light-response materials at the molecular level. This review summarizes the important synthetic progress of aromatic azo compounds in recent years, with an emphasis on the pioneering contribution of functional nanomaterials to the field.

Crystal structures and Hirshfeld surface analyses of (E)-1-[1-(4-tert-butyl-phen-yl)-2,2-di-chloro-ethen-yl]-2-phenyl-diazene, (E)-1-[1-(4-tert-butyl-phen-yl)-2,2-di-chloro-ethen-yl]-2-(4-methyl-phen-yl)diazene, (E)-1-[1-(4-tert-butyl-phen-yl)-2,2-di-chloro-ethen-yl]-2-(4-meth-oxy-phen-yl)diazene and (E)-1-[1-(4-tert-butyl-phen-yl)-2,2-di-chloro-ethen-yl]-2-(3-methyl-phen-yl)diazene.

The crystal structures and Hirshfeld surface analyses of four similar azo compounds are reported. (E)-1-[1-(4-tert-Butyl-phen-yl)-2,2-di-chloro-ethen-yl]-2-phenyl-diazene, C18H18Cl2N2, (I), and (E)-1-[1-(4-tert-butyl-phen-yl)-2,2-di-chloro-ethen-yl]-2-(4-methyl-phen-yl)diazene, C19H20Cl2N2, (II), crystallize in the monoclinic space group C2/c with Z = 8, and (E)-1-[1-(4-tert-butyl-phen-yl)-2,2-di-chloro-ethen-yl]-2-(4-meth-oxy-phen-yl)diazene, C19H20Cl2N2O, (III), in the monoclinic space group P21/c with Z = 4. (E)-1-[1-(4-tert-Butyl-phen-yl)-2,2-di-chloro-ethen-yl]-2-(3-methyl-phen-yl)diazene, C19H20Cl2N2, (IV), crystallizes in the triclinic space group P with Z = 4 and comprises two mol-ecules (A and B) in the asymmetric unit. In the crystal structures of (I) and (II), mol-ecules are linked by C-H⋯π and C-Cl⋯π inter-actions, forming layers parallel to (02), while mol-ecules of (III) are linked by C-H⋯O contacts, C-H⋯π and C-Cl⋯π inter-actions forming layers parallel to (02). The stability of the mol-ecular packing is ensured by van der Waals forces between these layers. In the crystal structure of (IV), mol-ecules are linked by C-H⋯π and C-Cl⋯π inter-actions, forming a tri-periodic network.

Direct blue 53, a biological dye, inhibits SARS-CoV-2 infection by blocking ACE2 and spike interaction in vitro and in vivo.

COVID-19 is a global health problem caused by SARS-CoV-2, which has led to over 600 million infections and 6 million deaths. Developing novel antiviral drugs is of pivotal importance to slow down the epidemic swiftly. In this study, we identified five azo compounds as effective antiviral drugs to SARS-CoV-2, and mechanism study revealed their targets for impeding viral particles' ability to bind to host receptors. Direct Blue 53, which displayed the strongest inhibitory impact, inhibited five mutant strains at micromole. In vitro, mechanism study demonstrated Direct Blue 53 inhibited viral infection through interaction with the spike of SARS-CoV-2. And 25 mg/kg/d compound treatment showed 50% or 60% survival protection against lethal Delta or Omicron BA.2 infection in vivo. Taken together, our results demonstrate that azo compounds with dimethyl-biphenyl-diyl-bis(azo)bis structure may be promising anti-SARS-CoV-2 drug candidates, which provide practicable therapies with the aid of structural optimizations and further research.

Site-Selective Aryl Diazonium Installation onto Protein Surfaces at Neutral pH using a Maleimide-Functionalized Triazabutadiene.

Aryl diazonium cations are versatile bioconjugation reagents due to their reactivity towards electron-rich aryl residues and secondary amines, but historically their usage has been hampered by both their short lifespan in aqueous solution and the harsh conditions required to generate them in situ. Triazabutadienes address many of these issues as they are stable enough to endure multiple-step chemical syntheses and can persist for several hours in aqueous solution, yet upon UV-exposure rapidly release aryl diazonium cations under biologically-relevant conditions. This paper describes the synthesis of a novel maleimide-functionalized triazabutadiene suitable for site-selectively installing aryl diazonium cations into proteins at neutral pH; we show reaction with this molecule and a surface-cysteine of a thiol disulfide oxidoreductase. Through photoactivation of the site-selectively installed triazabutadiene motifs, we generate aryl diazonium functionality, which we further derivatize via azo-bond formation to electron-rich aryl species, showcasing the potential utility of this strategy for the generation of photoswitches or protein-drug conjugates.

Light-induced Photoswitching of 4-(Phenylazo)benzoic Acid on Au(111).

Photochromic molecules can undergo a reversible conversion between two isomeric forms upon exposure to external stimuli such as electromagnetic radiation. A significant physical transformation accompanying the photoisomerization process defines them as photoswitches, with potential applications in various molecular electronic devices. As such, a detailed understanding of the photoisomerization process on surfaces and the influence of the local chemical environment on switching efficiency is essential. Herein, we use scanning tunneling microscopy to observe the photoisomerization of 4-(phenylazo)benzoic acid (PABA) assembled on Au(111) in kinetically constrained metastable states guided by pulse deposition. Photoswitching is observed at low molecular density and is absent in tight-packed islands. Furthermore, switching events were noted in PABA molecules coadsorbed in a host octanethiol monolayer, suggesting an influence of the surrounding chemical environment on photoswitching efficiency.

Colossal Anisotropic Thermal Expansion in a Diazo-Functionalized Compound with Switchable Solid-State Behavior.

Achieving substantial anisotropic thermal expansion (TE) in solid-state materials is challenging as most materials undergo volumetric expansion upon heating. Here, we describe colossal, anisotropic TE in crystals of an organic compound functionalized with two azo groups. Interestingly, the material exhibits distinct and switchable TE behaviors within different temperature regions. At high temperature, two-dimensional, area zero TE and colossal, positive linear TE (α=211 MK-1 ) are attained due to dynamic motion, while at low temperature, moderate positive TE occurs in all directions. Investigation of the solid-state motion showed the change in enthalpy and entropy are quite different in the two temperature regions and solid-state NMR experiments support motion in the solid. Cycling experiments demonstrate that the solid-state motions and TE behaviors are completely reversible. These results reveal strategies for designing significant anisotropic and switchable behaviors in solid-state materials.

In-situ Monitoring of Photocontrollable Wrinkle Erasure in Azobenzene-based Supramolecular Systems.

In this contribution, dynamic photoinduced wrinkle erasure enabled by photomechanical changes in supramolecular polymer-azo complexes was characterized via confocal microscopy. Different photoactive molecules, disperse yellow 7 (DY7) and 4,4'-dihydroxyazobenzene (DHAB), were compared to 4-hydroxy-4'-dimethylaminoazobenzene (OH-azo-DMA). The characteristic erasure times of wrinkles were quickly assessed by using an image processing algorithm. The results confirm that the photoinduced movement on the topmost layer can be successfully transferred to the substrate. Furthermore, the chosen supramolecular strategy allows decoupling the effect of molecular weight of the polymer and photochemistry of the chromophore, allowing quantitative comparison of wrinkling erasure efficiency of different materials and providing a facile way to optimize the system for specific applications.

Light-Triggered Reversible Swelling of Polymeric Spheres via Surfactant-Free RAFT Emulsion Polymerization-Induced Self-Assembly.

Responsive photonic crystals (RPCs) assembled by monodisperse colloidal particles have attracted enormous interest recently due to their tremendous applications in smart devices. Their structural colors can be determined by particle sizes. However, the lack of a reliable way to tune the sizes in situ limits their development. Herein, we present an efficient route to solve this problem through the fabrication of spherical polymeric particles with light-triggered reversible swelling behavior via surfactant-free reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization-induced self-assembly (PISA). Amphiphilic macro-RAFT agents containing azobenzene groups were synthesized and subsequently employed to mediate the polymerization of methyl methacrylate. Uniform submicron spheres were obtained by modulating solid contents and other parameters. Benefiting from the photoisomerization of azobenzene moieties, the particle sizes expanded and contracted upon alternative ultraviolet/visible-light irradiation accordingly. This strategy will be a supplement to the emulsion PISA and especially give aid to the progress of the RPC materials.