Synthesis of Azobenzenes Using N-Chlorosuccinimide and 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)

Photoswitchable Catalysis by a Self-Assembled Molecular Cage

A heteroleptic [Pd2L2L'2]4+ coordination cage containing a photoswitchable azobenzene-derived ligand catalyzes the Michael addition reaction between methyl vinyl ketone and benzoyl nitromethane within its cavity. The corresponding homoleptic cages are catalytically inactive. The heteroleptic cage can be reversibly disassembled and reassembled using 530 and 405 nm light, respectively, allowing catalysis within the cage to be switched OFF and ON at will.

Photoswitchable luminescent lanthanide complexes controlled and interrogated by four orthogonal wavelengths of light

Optical information storage requires careful control of excitation and emission wavelengths in a reversible and orthogonal manner to enable efficient reading, writing, and erasing of information. Photochromic systems, in which a photoswitch is typcially coupled to an emissive organic fluorophore, have much promise in this regard. However, these suffer from considerable spectral overlap between the switch and fluorophore, such that their emissive and photoswitchable properties are not orthogonal. Here, we overcome this limitation by coupling visible/NIR emissive lanthanide complexes with molecular photoswitches, enabling reversible and orthogonal photoswitching with visible light. Crucially, photoswitching does not lead to sensitised emission from the lanthanide, while excitation of the lanthanide does not induce photoswitching, enabling the state of the system to be probed without perturbation of the switch. This opens up the possibility of developing multi-colour read-write methods for information storage using emissive photoswitches.

Red-light modulated ortho-chloro azobenzene photoswitch for peptide stapling via aromatic substitution

The application of peptide stapling using photoswitchable linkers has gained notable interest for potential therapeutic applications. However, many existing methodologies of photoswitching still rely on the use of tissue-damaging and weakly skin-penetrating UV light. Herein, we describe the development of a tetra-ortho-chloro azobenzene linker that was successfully used for cysteine-selective peptide stapling via SNAr. This linker facilitates precise photocontrol of peptide structure via trans to cis isomerisation under red light irradiation. As a proof-of-concept, we applied the developed peptide stapling platform to a modified PMI peptide, targeting the inhibition of MDM2/p53 protein-protein interaction (PPI). Biophysical characterisation of the photoswitchable peptide by competitive fluorescence polarisation showed a significant difference in affinity between the trans and cis isomer for the p53-interacting domain of the human MDM2. Remarkably, the cis isomer displayed a >240-fold higher potency. To the best of our knowledge, this is the highest reported difference in binding affinity between isoforms of a photoswitchable therapeutic peptide. Overall, our findings demonstrate the potential of this novel photoswitchable peptide stapling system for tuneable, selective modulation of PPIs via visible-light isomerisation with deeply-tissue penetrating red light.

Three-in-One: Dye-Volatile Cocrystals Exhibiting Intensity-Dependent Photochromic, Photomechanical, and Photocarving Response

Cocrystallization of a cis-azobenzene dye with volatile molecules, such as pyrazine and dioxane, leads to materials that exhibit at least three different light-intensity-dependent responses upon irradiation with low-power visible light. The halogen-bond-driven assembly of the dye cis-(p-iodoperfluorophenyl)azobenzene with volatile halogen bond acceptors produces cocrystals whose light-induced behavior varies significantly depending on the intensity of the light applied. Low-intensity (<1 mW·cm-2) light irradiation leads to a color change associated with low levels of cis → trans isomerization. Irradiation at higher intensities (150 mW·mm-2) produces photomechanical bending, caused by more extensive isomerization of the dye. At still higher irradiation intensities (2.25 W·mm-2) the cocrystals undergo cold photocarving; i.e., they can be cut and written on with micrometer precision using laser light without a major thermal effect. Real-time Raman spectroscopy shows that this novel photochemical behavior differs from what would be expected from thermal energy input alone. Overall, this work introduces a rational blueprint, based on supramolecular chemistry in the solid state, for new types of crystalline light-responsive materials, which not only respond to being exposed to light but also change their response based on the light intensity.

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.

Copper-catalysed dehydrogenative self-coupling/cyclization of 5-aminopyrazoles: synthesis and photophysical study of pyridazines

An interesting self-coupling/cyclization of 5-aminopyrazoles is revealed, which provides a variety of pyridazine cores in reasonable yields. In this reaction, C(sp²)-C(sp²) and N-N bond formation occurs simultaneously in one reaction vessel. The photophysical properties of the synthesized compounds were also studied and some of them exhibited fluorescence properties with good quantum yields. A radical mediated reaction mechanism is proposed with the help of control experiments.

Emerging trends in the sustainable synthesis of N-N bond bearing organic scaffolds

N-N bond bearing organic frameworks such as azos, hydrazines, indazoles, triazoles and their structural moieties have piqued the interest of organic chemists due to the intrinsic nitrogen electronegativity. Recent methodologies with atom efficacy and a greener approach have overcome the synthetic obstacles of N-N bond construction from N-H. As a result, a wide range of amine oxidation methods have been reported early on. This review's vision emphasizes the emerging methods of N-N bond formation, particularly photo, electro, organo and transition metal free chemical methods.

Amber Light Control of Peptide Secondary Structure by a Perfluoroaromatic Azobenzene Photoswitch

The incorporation of photoswitches into the molecular structure of peptides and proteins enables their dynamic photocontrol in complex biological systems. Here, a perfluorinated azobenzene derivative triggered by amber light was site-specifically conjugated to cysteines in a helical peptide by perfluoroarylation chemistry. In response to the photoisomerization (trans→cis) of the conjugated azobenzene with amber light, the secondary structure of the peptide was modulated from a disorganized into an amphiphilic helical structure.

Photo-switchable anion binding and catalysis with a visible light responsive halogen bonding receptor

Photo-switchable receptors allow for photo-control over guest binding and release with spatial and temporal precision. Here we report the first halogen bonding photo-switchable anion receptors in which chloride binding may be reversibly modulated by irradiation with red and blue light, with over a 50-fold enhancement in chloride binding affinity observed for the Z isomer. We demonstrate that this switchable binding enables unprecedented photo-controlled catalysis of XB-mediated halide abstractions and a Mukaiyama Aldol reaction.

Design and Synthesis of Visible-Light-Responsive Azobenzene Building Blocks for Chemical Biology

Tetra-ortho-fluoro-azobenzenes are a class of photoswitches useful for the construction of visible-light-controlled molecular systems. They can be used to achieve spatio-temporal control over the properties of a chosen bioactive molecule. However, the introduction of different substituents to the tetra-fluoro-azobenzene core can significantly affect the photochemical properties of the switch and compromise biocompatibility. Herein, we explored the effect of useful substituents, such as functionalization points, attachment handles, and water-solubilizing groups, on the photochemical properties of this photochromic system. In general, all the tested fluorinated azobenzenes exhibited favorable photochemical properties, such as high photostationary state distribution and long half-lives, both in organic solvents and in water. One of the azobenzene building blocks was functionalized with a trehalose group to enable the uptake of the photoswitch into mycobacteria. Following metabolic uptake and incorporation of the trehalose-based azobenzene in the mycobacterial cell wall, we demonstrated photoswitching of the azobenzene in the isolated total lipid extract.

Synthesis, derivatization and photochemical control of an ortho-functionalized tetrafluorinated azoben---zene-modified siRNA

We report the synthesis of an ortho -functionalized tetrafluorinated azobenzene phosphoramidite for its site-specific incorporation into RNA. The tetrafluorinated azobenzene is embedded within the antisense strand of an siRNA duplex to form an ortho -functionalized tetrafluorinated azobenzene-containing siRNA (F-siRNAzo). The F-siRNAzo is inactivated via trans to cis conversion with green light (530 nm), and reactivated with blue light (470 nm) via cis to trans conversion in cell culture. The long half-life and stability of the tetrafluorinated azobenzene unit allows for reversible control of the F-siRNAzo in cell culture for up 72 hours.

Sulfur and Azobenzenes, a Profitable Liaison: Straightforward Synthesis of Photoswitchable Thioglycosides with Tunable Properties

Azobenzene photoswitches are valuable tools for controlling properties of molecular systems with light. We have been investigating azobenzene glycoconjugates to probe carbohydrate‐protein interactions and to design glycoazobenzene macrocycles with chiroptical and physicochemical properties modulated by light irradiation. To date, direct conjugation of glycosides to azobenzenes was performed by reactions providing target compounds in limited yields. We therefore sought a more effective and reliable coupling method. In this paper, we report on a straightforward thioarylation of azobenzene derivatives with glycosyl thiols as well as other thiols, thereby increasing the scope of azobenzene conjugation. Even challenging unsymmetrical conjugates can be achieved in good yields via sequential or one‐pot procedures. Importantly, red‐shifted azoswitches, which are addressed with visible light, were easily functionalized. Additionally, by oxidation of the sulfide bridge to the respective sulfones, both the photochromic and the thermal relaxation properties of the core azobenzene can be tuned. Utilizing this option, we realized orthogonal three‐state photoswitching in mixtures containing two distinct azobenzene thioglycosides.

Sulfonimidamides by Sequential Mechanochemical Chlorinations and Aminations of Sulfinamides

Here, we report the first mechanochemical synthesis of sulfonimidamides. The one-pot, two-step method requires neither a solvent nor inert conditions. In a mixer mill, sulfinamides are rapidly converted to sulfonimidoyl chlorides by oxidative chlorination with N-chlorosuccinimide (NCS). Subsequent substitutions with amines provides a wide range of diversely substituted sulfonimidamides.

Cold photo-carving of halogen-bonded co-crystals of a dye and a volatile co-former using visible light

The formation of co-crystals by the assembly of molecules with complementary molecular recognition functionalities is a popular strategy to design or improve a range of solid-state properties, including those relevant for pharmaceuticals, photo- or thermoresponsive materials and organic electronics. Here, we report halogen-bonded co-crystals of a fluorinated azobenzene derivative with a volatile component-either dioxane or pyrazine-that can be cut, carved or engraved with low-power visible light. This cold photo-carving process is enabled by the co-crystallization of a light-absorbing azo dye with a volatile component, which gives rise to materials that can be selectively disassembled with micrometre precision using low-power, non-burning laser irradiation or a commercial confocal microscope. The ability to shape co-crystals in three dimensions using laser powers of 0.5-20 mW-substantially lower than those used for metals, ceramics or polymers-is rationalized by photo-carving that targets the disruption of weak supramolecular interactions, rather than the covalent bonds or ionic structures targeted by conventional laser beam or focused ion beam machining processes.

Visible-Light Switching of Metallosupramolecular Assemblies

A photoswitchable ligand and palladium(II) ions form a dynamic mixture of self-assembled metallosupramolecular structures. The photoswitching ligand is an ortho-fluoroazobenzene with appended pyridyl groups. Combining the E-isomer with palladium(II) salts affords a double-walled triangle with composition [Pd3 L6 ]6+ and a distorted tetrahedron [Pd4 L8 ]8+ (1 : 2 ratio at 298 K). Irradiation with 410 nm light generates a photostationary state with approximately 80 % of the E-isomer of the ligand and results in the selective disassembly of the tetrahedron, the more thermodynamically stable structure, and the formation of the triangle, the more kinetically inert product. The triangle is then slowly transformed back into the tetrahedron over 2 days at 333 K. The Z-isomer of the ligand does not form any well-defined structures and has a thermal half-life of 25 days at 298 K. This approach shows how a thermodynamically preferred self-assembled structure can be reversibly pumped to a kinetic trap by small perturbations of the isomer distribution using non-destructive visible light.

Red-shifted tetra-ortho-halo-azobenzenes for photo-regulated transmembrane anion transport

Photo-responsive synthetic ion transporters are of interest as tools for studying transmembrane transport processes and have potential applications as targeted therapeutics, due to the possibility of spatiotemporal control and wavelength-dependent function. Here we report the synthesis of novel symmetric and non-symmetric red-shifted tetra-ortho-chloro- and tetra-ortho-fluoro azobenzenes, bearing pendant amine functionality. Functionalisation of the photo-switchable scaffolds with squaramide hydrogen bond donors enabled the preparation of a family of anion receptors, which act as photo-regulated transmembrane chloride transporters in response to green or red light. The subtle effects of chlorine/fluorine substitution, meta/para positioning of the anion receptors, and the use of more flexible linkers are explored. NMR titration experiments on the structurally diverse photo-switchable receptors reveal cooperative binding of chloride in the Z, but not E isomer, by the two squaramide binding sites. These results are supported by molecular dynamics simulations in explicit solvent and model membranes. We show that this intramolecular anion recognition leads to effective switching of transport activity in lipid bilayer membranes, in which optimal Z isomer activity is achieved using a combination of fluorine substitution and para-methylene spacer units.

Synthesis and Investigations of Building Blocks with Dibenzo[b,f] Oxepine for Use in Photopharmacology

The synthesis of photoswitchable azo-dibenzo[b,f]oxepine derivatives and microtubule inhibitors were described. Subsequently, we examined the reaction of methoxy derivative 3-nitrodibenzo[b,f]oxepine with different aldehydes and in the presence of BF3·OEt2 as a catalyst. Our study provided a very concise method for the construction of the azo-dibenzo[b,f]oxepine skeleton. The analysis of products was run using experimental and theoretical methods. Next, we evaluated the E/Z isomerization of azo-dibenzo[b,f]oxepine derivatives, which could be photochemically controlled using visible-wavelength light.

Electrochemical Heteroatom-Heteroatom Bond Construction

Heteroatom-heteroatom linkage, with S-S bond as a presentative motif, served a crucial role in biochemicals, pharmaceuticals, pesticides, and material sciences. Thus, preparation of the privileged scaffold has always been attracting tremendous attention from the synthetic community. However, classic protocols suffered from several drawbacks, such as toxic and unstable agents, poor functional group tolerance, multiple steps, and explosive oxidizing regents as well as the transitional metal catalysts. Electrochemical organic synthesis exhibited a promising alternative to the traditional chemical reaction due to the sustainable electricity can be employed as the traceless redox agents. Hence, toxic and explosive oxidants and/or transitional metals could be discarded under mild reaction with high efficiency. In this context, a series of electrochemical approaches for the construction of heteroatom-heteroatom bond were reviewed. Notably, most of the cases illustrated the dehydrogenative feature with the clean energy molecules hydrogen as the sole by-product.

Synthesis and properties of photoswitchable diphosphines and gold(I) complexes derived from azobenzenes

Diphosphines displaying azobenzene scaffolds and the corresponding bis-gold chloride complexes have been prepared and fully characterized by photophysical, spectroscopic and X-ray diffraction studies. DFT calculations provide complementary information on their electronic, structural and spectroscopic properties. Comparative investigations have been carried out on compounds featuring phosphorus functions in the meta- and para-positions, respectively, with respect to the azo functions, as well as on diphosphines with an ortho-tetrafluoro substituted azobenzene core. The effects of the substitution patterns on structural and spectroscopic properties are discussed.

Recent advances in the synthesis of organic chloramines and their insights into health care

Organic nitrogen–chlorine compounds and their derivatives are important heterocyclic motifs, exhibiting applications such as N-chlorinating agents, analytical reagents, disinfectants, antipathogens, and as synthetic intermediates for drugs, polymers, and natural products.

Divergent synthesis of unsymmetrical azobenzenes via Cu-catalyzed C–N coupling

Efficient, one-pot, and low-cost metal-catalyzed tandem Chan–Lam coupling/deprotection and Ullmann/Chan–Lam/deprotection reactions to access unsymmetrical azobenzenes are disclosed.

Oxidative Dehydrogenation of Hydrazobenzenes toward Azo Compounds Catalyzed by tert-Butyl Nitrite in EtOH

We describe a tert-butyl nitrite-catalyzed oxidative dehydrogenation of hydrazobenzenes for producing azobenzenes. This method proceeds at ambient temperature and under an atmospheric environment by employing eco-friendly EtOH as the medium, representing a mild, general route to the synthesis of various symmetrical and nonsymmetrical azobenzenes in excellent yields with broad functional group tolerance.

Reversible photo-control over transmembrane anion transport using visible-light responsive supramolecular carriers

Supramolecular anion carriers responsive to visible light enable reversible two-colour photo-control over transmembrane anion transport.

Ion transport across lipid bilayer membranes in biology is controlled by membrane proteins, which in turn are regulated in response to chemical-, physical- and photo-stimuli. The design of synthetic supramolecular ion transporters able to be precisely controlled by external signals, in particular bio-compatible wavelengths of visible light, is key for achieving spatio-temporal control over function. Here we report two-colour responsive molecular photo-switches that act as supramolecular transmembrane anion carriers. Reversible switching of the photo-switch within the lipid bilayer membrane is achieved using biocompatible visible wavelengths of light, such that temporal control over transmembrane anion transport is achieved through alternating irradiation with red and blue light.

TEMPO catalyzed oxidative dehydrogenation of hydrazobenzenes to azobenzenes

A metal-free direct oxidative dehydrogenation approach for the synthesis of azobenzenes from hydrazobenzenes has been developed by using TEMPO as an organocatalyst for the first time. The reaction proceeded in open air under mild reaction conditions. A wide range of hydrazobenzenes readily undergo dehydrogenation to give the corresponding azobenzenes in excellent yields.

Tubular Perylene Bisimide Macrocycles for the Recognition of Geometrical Isomers of Azobenzenes

Perylene bisimide-based materials are good candidates for photosensitive applications. Herein, we report synthesis, characterization, and complexation studies of perylene bisimide macrocycles obtained through bayside coupling. The isomeric macrocycles incorporated with interesting optical properties and tubular-shaped cavities are able to recognize geometric isomers of azobenzenes and aromatic amines. Such selective recognition is useful toward developing potential sensors for interesting isomeric pairs in the future.

A practical route to azo compounds by metal-free aerobic oxidation of arylhydrazides using an NOx system

Metal-free aerobic oxidation of aryl hydrazides catalyzed by NO_x was developed for the practical and environment friendly synthesis of azo compounds.

Iodine-mediated oxidative N-N coupling of secondary amines to hydrazines

An I2-mediated N-N coupling reaction has been established for oxidative dimerization of N-aryl aminopyridines to a variety of novel hydrazine derivatives under mild conditions. This synthetic method does not require use of transition metals and can be conveniently carried out on a gram scale. It is also applicable to diphenylamine and N-alkyl aniline substrates.

tBuOK-triggered bond formation reactions

Recently, inexpensive and readily available tBuOK has seen widespread use in transition-metal-free reactions. Herein, we report the use of tBuOK for S–S, S–Se, N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N and CN bond formations, which significantly extends the scope of tBuOK in chemical synthesis. Compared with traditional methods, we have realized mild and general methods for disulfide, azobenzenes imine etc. synthesis.

Inexpensive and readily available ^tBuOK can trigger a series of bond formation reactions, including S–S, S–Se, Se–Se, and NN and CN bonds.

Experimental and Theoretical Investigations of an Electrochromic Azobenzene and 3,4-Ethylenedioxythiophene-based Electrochemically Formed Polymeric Semiconductor

An electrochromic material based on azobenzene and 3,4-ethylenedioxythiophene (EDOT) semiconducting layer was electrochemically deposited on an indium tin oxide coated glass electrode. Chemical synthesis of the azobenzene and EDOT-based chromophore (DAE) and electrochemical formation of its corresponding polymer (pDAE) are reported. The electrochromic properties of the synthesized polymer pDAE were investigated by electrochemical and spectroelectrochemical methods. pDAE exhibited an optical bandgap of 1.82 eV and three distinct colored states in its reduced, neutral, and oxidized forms. The pDAE polymer showed 44 % optical contrast at 710 nm between its reduced and oxidized states and a fast electrochromic switching time of 1.0 s. The frontier molecular orbitals, Raman shifts, and semiconducting properties of this electrochromic polymer were evaluated by density functional theory calculations. The optical absorption bands of the polymer charged states were assigned and investigated.