Azobenzenes—synthesis and carbohydrate applications

Tuning the Thermal Stability of Tetra-o-chloroazobenzene Derivatives by Transforming Push-Pull to Push-Push Systems

Molecular photoswitches provide interesting tools to reversibly control various biological functions with light. Thanks to its small size and easy introduction into the biomolecules, azobenzene derivatives have been widely employed in the field of photopharmacology. All visible-light switchable azobenzenes with controllable thermostability are highly demanded. Based on the reported tetra-o-chloroazobenzenes, we synthesized push-pull systems, by introducing dialkyl amine and nitro groups as strong electron-donating and electron-withdrawing groups on the para-positions, and then transformed to push-push systems by a simple reduction step. The developed push-pull and push-push tetra-o-chloroazobenzene derivatives displayed excellent photoswitching properties, as previously reported. The half-life of the Z-isomers can be tuned from milliseconds for the push-pull system to several hours for the push-push system. The n-π* and π-π* transitions have better resolution in the push-push molecules, and excitation at different wavelengths can tune the E/Z ratio at the photostationary state. For one push-pull molecule, structure and absorption spectra obtained from theoretical calculations are compared with experimental data, along with data on the push-push counterpart.

Synthesis of Mono-, Di-, Tri-, and Tetra-cationic Pyridinium and Vinylpyridinium Modified [2.2]Paracyclophanes: Modular Receptors for Supramolecular Systems

In this report, a new series of mono-, di-, tri-, and tetra-cationic pyridinium and vinyl pyridinium-modified [2.2]paracyclophanes as useful molecular tectons for supramolecular systems are described. Regioselective functionalization at specific positions, followed by resolution step and successive transformations through Pd-catalyzed Suzuki-Miyaura and Mizoroki-Heck cross-coupling chemistry furnish a series of modular PCP scaffolds. In our proof-of-concept study, on N-methylation, the PCPs bearing (cationic) pyridyl functionalities were demonstrated as useful molecular receptors in host-guest supramolecular assays. The PCPs on grafting with light-responsive azobenzene (-N=N-) functional core as side-groups impart photosensitivity that can be remotely transformed on irradiation, offering photo-controlled smart molecular functions. Furthermore, the symmetrical PCPs bearing bi-, and tetra-pyridyl functionalities at the peripheries have enormous potential to serve as ditopic and tetratopic 3D molecular tectons for engineering non-covalent supramolecular assemblies with new structural and functional attributes.

Photopharmacology of Protease Inhibitors: Current Status and Perspectives

Proteases are involved in many essential biological processes. Dysregulation of their activity underlies a wide variety of human diseases. Photopharmacology, as applied on various classes of proteins, has the potential to assist protease research by enabling spatiotemporal control of protease activity. Moreover, it may be used to decrease side-effects of protease-targeting drugs. In this review, we discuss the current status of the chemical design of photoactivatable proteases inhibitors and their biological application. Additionally, we give insight into future possibilities for further development of this field of research.

Design rules for optimization of photophysical and kinetic properties of azoarene photoswitches

Azoarenes are an important class of molecular photoswitches that often undergo E → Z isomerization with ultraviolet light and have short Z-isomer lifetimes. Azobenzene has been a widely studied photoswitch for decades but can be poorly suited for photopharmacological applications due to its UV-light absorption and short-lived Z-isomer half-life (t1/2). Recently, diazo photoswitches with one or more thiophene rings in place of a phenyl ring have emerged as promising candidates, as they exhibit a stable photostationary state (98% E → Z conversion) and E-isomer absorption (λmax) in the visible light range (405 nm). In this work, we performed density functional theory calculations [PBE0-D3BJ/6-31+G(d,p)] on 26 hemi-azothiophenes, substituted with one phenyl ring and one thiophene ring on the diazo bond. We calculated the E-isomer absorption (λmax) and Z-isomer t1/2 for a set of 26 hemi-azothiophenes. We compared their properties to thiophene-based photoswitches that have been studied previously. We separated the 26 proposed photoswitches into four quadrants based on their λmax and t1/2 relative to past generations of hemi-azothiophene photoswitches. We note 8 hemi-azothiophenes with redshifted λmax and longer t1/2 than previous systems. Our top candidate has λmax and a t1/2 approaching 360 nm and 279 years, respectively. The results here present a pathway towards leveraging and optimizing two properties of photoswitches previously thought to be inversely related.

Pd(II)-catalyzed coupling of C-H bonds of carboxamides with iodoazobenzenes toward modified azobenzenes

In this paper, we report a synthetic protocol for the construction of biaryl motif-based or π-extended azobenzene and alkylated azobenzene derivatives via the Pd(II)-catalyzed bidentate directing group (DG)-aided C-H activation and functionalization strategy. In the past, the synthesis of biaryl motif-based azobenzenes was accomplished through the traditional cross-coupling reaction involving organometallic reagents and aryl halides or equivalent coupling partners. We have shown the direct coupling of C-H bonds of aromatic/aliphatic carboxamides (possessing a DG) with iodoazobenzenes as the coupling partners through the Pd(II)-catalyzed bidentate DG-aided, site-selective C-H functionalization method. Azobenzene-containing compounds are a versatile class of photo-responsive molecules that have found applications across branches of chemical, biological and materials sciences and are prevalent in medicinally relevant molecules. Accordingly, the synthesis of new and functionalized azobenzene-based scaffolds has been an attractive topic of research. Although the classical methods are efficient, they need pre-functionalized starting materials. This protocol involving the Pd(II)-catalyzed, directing group-aided site-selective C-H arylation of aromatic and aliphatic carboxamides using iodoazobenzene as the coupling partner affording azobenzene-based carboxamides is an additional route and also a contribution towards enriching the library of modified azobenzenes. We have also shown the photoswitching properties of representative compounds synthesized via the Pd(II)-catalyzed directing group-aided site-selective C-H functionalization method.

Protocol for microwave-assisted synthesis of unsymmetrical azo dyes

Aromatic azo dyes bear immense commercial significance because of their extensive usage in the textile, paint, and food industries. With growing environmental concerns, developing alternative greener approaches for the synthesis of azo dyes is crucial. Herein, we describe a metal-free, microwave (MW)-assisted protocol for rapid access to a large variety of unsymmetrical azo dyes by coupling nitroarenes and aromatic amines. After MW-assisted coupling, the azo dyes are then isolated by precipitation followed by recrystallization to obtain pure azo dyes. For complete details on the use and execution of this protocol, please refer to Thakuri et al. (2022).¹.

Design, Synthesis, and Photo-Responsive Properties of a Collagen Model Peptide Bearing an Azobenzene

Collagen is a vital component of the extracellular matrix in animals. Collagen forms a characteristic triple helical structure and plays a key role in supporting connective tissues and cell adhesion. The ability to control the collagen triple helix structure is useful for medical and conformational studies because the physicochemical properties of the collagen rely on its conformation. Although some photo-controllable collagen model peptides (CMPs) have been reported, satisfactory photo-control has not yet been achieved. To achieve this objective, detailed investigation of the isomerization behavior of the azobenzene moiety in CMPs is required. Herein, two CMPs were attached via an azobenzene linker to control collagen triple helix formation by light irradiation. Azo-(PPG)10 with two (Pro-Pro-Gly)10 CMPs linked via a photo-responsive azobenzene moiety was designed and synthesized. Conformational changes were evaluated by circular dichroism and the cis-to-trans isomerization rate calculated from the absorption of the azobenzene moiety indicated that the collagen triple helix structure was partially disrupted by isomerization of the internal azobenzene.

Direct photochemical route to azoxybenzenes via nitroarene homocoupling

We report on a direct photochemical method for the one-pot, catalyst- and additive-free synthesis of azoxybenzene and substituted azoxy derivatives from nitrobenzene building blocks. This reaction is conducted at room temperature and under air, and can be applied to substrates with a wide range of substituents. Yields of products derived from para- and meta-substituted nitrobenzenes are typically good, while sterically encumbered ortho-substituted substrates are not as fruitful. Photochemical Wallach rearrangement of generated azoxybenzenes to ortho-hydroxyazoxybenzenes was observed in some cases, most markedly in selected ortho-halogenated nitrobenzenes. Overall, this method provides an efficient, green pathway to highly value-added azoxybenzene products.

Development of an Indole-Amide-Based Photoswitchable Cannabinoid Receptor Subtype 1 (CB1R) "Cis-On" Agonist

Activation of the human cannabinoid receptor type 1 (hCB₁R) with high spatiotemporal control is useful to study processes involved in different pathologies related to nociception, metabolic alterations, and neurological disorders. To synthesize new agonist ligands for hCB₁R, we have designed different classes of photoswitchable molecules based on an indole core. The modifications made to the central core have allowed us to understand the molecular characteristics necessary to design an agonist with optimal pharmacological properties. Compound 27a shows high affinity for CB₁R (K_i (cis-form) = 0.18 μM), with a marked difference in affinity with respect to its inactive "trans-off" form (CB₁R K_i trans/cis ratio = 5.4). The novel compounds were evaluated by radioligand binding studies, receptor internalization, sensor receptor activation (GRABeCB2.0), Western blots for analysis of ERK1/2 activation, NanoBiT βarr2 recruitment, and calcium mobilization assays, respectively. The data show that the novel agonist 27a is a candidate for studying the optical modulation of cannabinoid receptors (CBRs), serving as a new molecular tool for investigating the involvement of hCB₁R in disorders associated with the endocannabinoid system.

Systematic modifications of substitution patterns for property tuning of photoswitchable asymmetric azobenzenes

Suitable designed photoswitches based on azobenzenes are essential structural features for photopharmacological compounds. Optimized azobenzenes are important for serving as building blocks in "azo extension" strategies, and for designing photodrugs with tailored properties. Herein we present the synthesis and characterization of a variety of asymmetric azobenzenes by addressing selected structural features of the diazene core, such as polarity, steric demand, and electronic properties. Systematic exploration led to photoswitches with a relaxation half-life of seconds, minutes, hours, or days. Furthermore, the influence of different substitution patterns on the photophysical properties was charted. For analysis of all switches, robust characterization as well as examination under near-to physiological conditions was established, in order to assist with photoswitch choice for specific biological applications.

Photoswitching Affinity and Mechanism of Multivalent Lectin Ligands

Multivalent receptor–ligand binding is a key principle in a plethora of biological recognition processes. Immense binding affinities can be achieved with the correct spatial orientation of the ligands. Accordingly, the incorporation of photoswitches, which can be used to reversibly change the spatial orientation of molecules, into multivalent ligands is a means to alter the binding affinity and possibly also the binding mode of such ligands. We report a divalent ligand for the model lectin wheat germ agglutinin (WGA) containing an arylazopyrazole photoswitch. This switch, which has recently been introduced as an alternative to the more commonly used azobenzene moiety, is characterized by almost quantitative E/Z photoswitching in both directions, high quantum yields, and high thermal stability of the Z isomer. The ligand was designed in a way that only one of the isomers is able to bridge adjacent binding sites of WGA leading to a chelating binding mode. Photoswitching induces an unprecedentedly high change in lectin binding affinity as determined by isothermal titration calorimetry (ITC). Furthermore, additional dynamic light scattering (DLS) data suggest that the binding mode of the ligand changes from chelating binding of the E isomer to crosslinking binding of the Z isomer.

Microwave Assisted Rapid and Sustainable Synthesis of Unsymmetrical Azo Dyes by Coupling of Nitroarenes with Aniline Derivatives

Aromatic azo dyes are of immense commercial importance, and the development of greener routes for their synthesis is imperative due to current environmental concerns. In the present study, a microwave-assisted route has been developed for rapid and convenient synthesis of unsymmetrical azo dyes in a single step. In a metal-catalyst-free approach, an aromatic amine was used as an in situ reductant to affect its direct cross-condensation with nitroarenes to afford a variety of dispersed and water-soluble azo dyes. The electronic and substituent effects were thoroughly understood by placing suitable substituents in both nitroarenes and aniline derivatives in competitive reactions. The microwave (MW) method worked better with aniline or electron-rich aromatic amines to prepare a range of unsymmetrical azo dyes in up to 97% yields within a few minutes. The method worked well in the gram-scale synthesis of commercial dye, solvent yellow 7.

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Microwave-based green synthesis of unsymmetrical azo dyes

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Catalyst-free, rapid synthesis

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Gram-scale synthesis of commercial dyes

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Efficient synthesis of water-soluble dyes

Cross-conjugation controls the stabilities and photophysical properties of heteroazoarene photoswitches

Azoarene photoswitches are versatile molecules that interconvert from their E-isomer to their Z-isomer with light. Azobenzene is a prototypical photoswitch but its derivatives can be poorly suited for in vivo applications such as photopharmacology due to undesired photochemical reactions promoted by ultraviolet light and the relatively short half-life (t_1/2) of the Z-isomer (2 days). Experimental and computational studies suggest that these properties (λ^max of the E isomer and t_1/2 of the Z-isomer) are inversely related. We identified isomeric azobisthiophenes and azobisfurans from a high-throughput screening study of 1540 azoarenes as photoswitch candidates with improved λ^max and t_1/2 values relative to azobenzene. We used density functional theory to predict the activation free energies and vertical excitation energies of the E- and Z-isomers of 2,2- and 3,3-substituted azobisthiophenes and azobisfurans. The half-lives depend on whether the heterocycles are π-conjugated or cross-conjugated with the diazo π-bond. The 2,2-substituted azoarenes both have t_1/2 values on the scale of 1 hour, while the 3,3-analogues have computed half-lives of 40 and 230 years (thiophene and furan, respectively). The 2,2-substituted heteroazoarenes have significantly higher λ^max absorptions than their 3,3-substituted analogues: 76 nm for azofuran and 77 nm for azothiophene.

Advances and opportunities in the exciting world of azobenzenes

Azobenzenes are archetypal molecules that have a central role in fundamental and applied research. Over the course of almost two centuries, the area of azobenzenes has witnessed great achievements; azobenzenes have evolved from simple dyes to 'little engines' and have become ubiquitous in many aspects of our lives, ranging from textiles, cosmetics, food and medicine to energy and photonics. Despite their long history, azobenzenes continue to arouse academic interest, while being intensively produced for industrial purposes, owing to their rich chemistry, versatile and straightforward design, robust photoswitching process and biodegradability. The development of azobenzenes has stimulated the production of new coloured and light-responsive materials with various applications, and their use continues to expand towards new high-tech applications. In this Review, we highlight the latest achievements in the synthesis of red-light-responsive azobenzenes and the emerging application areas of photopharmacology, photoswitchable adhesives and biodegradable materials for drug delivery. We show how the synthetic versatility and adaptive properties of azobenzenes continue to inspire new research directions, with limits imposed only by one's imagination.

Substituent and Solvent Polarity on the Spectroscopic Properties in Azo Derivatives of 2-Hydroxynaphthalene and Their Difluoroboranes Complexes

Novel fluorescent dyes such as difluoroborane complexes of 1-phenylazonaphthalen-2-ol derivatives were successfully synthesized and characterized with a focus on the influence of a substituent and a solvent on the basic photophysical properties. ¹H, ¹¹B, ¹³C, ¹⁵N, and ¹⁹F nuclear magnetic resonance (NMR) spectra of substituted 1-phenylazonaphthalen-2-ol difluoroboranes and their parent azo dyes were recorded and discussed. The absorption and emission properties of synthesized compounds were investigated in solvents of varying polarity. They were found to be fluorescent despite the presence of the azo group. The azo group rotation was blocked by complexing with -BF₂ to get a red shift in absorption. Solvent-dependent spectral properties of compounds were investigated using Lipper-Mataga and Bakhshiev plot. The calculated DFT energies and Frontier Molecular Orbitals calculations of the studied compounds were proved to be consistent with the experimental observations.

Synthesis of Glycodendrimers with Antiviral and Antibacterial Activity

Glycodendrimers are an important class of synthetic macromolecules that can be used to mimic many structural and functional features of cell-surface glycoconjugates. Their carbohydrate moieties perform key important functions in bacterial and viral infections, often regulated by carbohydrate-protein interactions. Several studies have shown that the molecular structure, valency and spatial organisation of carbohydrate epitopes in glycoconjugates are key factors in the specificity and avidity of carbohydrate-protein interactions. Choosing the right glycodendrimers almost always helps to interfere with such interactions and blocks bacterial or viral adhesion and entry into host cells as an effective strategy to inhibit bacterial or viral infections. Herein, the state of the art in the design and synthesis of glycodendrimers employed for the development of anti-adhesion therapy against bacterial and viral infections is described.

"Photo-Rimonabant": Synthesis and Biological Evaluation of Novel Photoswitchable Molecules Derived from Rimonabant Lead to a Highly Selective and Nanomolar "Cis-On" CB1R Antagonist

Human cannabinoid receptor type 1 (hCB₁R) plays important roles in the regulation of appetite and development of addictive behaviors. Herein, we describe the design, synthesis, photocharacterization, molecular docking, and in vitro characterization of "photo-rimonabant", i.e., azo-derivatives of the selective hCB₁R antagonist SR1411716A (rimonabant). By applying azo-extension strategies, we yielded compound 16a, which shows marked affinity for CB₁R (K_{i (cis form)} = 29 nM), whose potency increases by illumination with ultraviolet light (CB₁R K_itrans/cis ratio = 15.3). Through radioligand binding, calcium mobilization, and cell luminescence assays, we established that 16a is highly selective for hCB₁R over hCB₂R. These selective antagonists can be valuable molecular tools for optical modulation of CBRs and better understanding of disorders associated with the endocannabinoid system.

Direct and Divergent Solid-Phase Synthesis of Azobenzene and Spiropyran Derivatives

Here, we report a solid-phase approach to synthesize azobenzene and spiropyran derivatives. The divergent synthesis process requires no purification steps to obtain the desired product with a 28-55% yield, depending on the specific compound. For the spiropyran compounds, solid-phase resin cleavage is performed under mild conditions to minimize spiropyran ring opening. The solid-phase method enables the synthesis of a library of azobenzene and spiropyran derivatives without the need to develop purification strategies for each derivative.

Catalytic Synthesis of Conjugated Azopolymers from Aromatic Diazides

Conjugated polymers containing main chain azoarene repeat units are synthesized by a dinickel catalyzed N=N coupling reaction of aromatic diazides. The polymerization exhibits broad substrate scope and is compatible with heterocycles commonly featured in high performance organic materials, including carbazole, thiophene, propylenedioxythiophene (ProDOT), diketopyrrolopyrrole (DPP), and isoindigo. Copolymerizations can be carried out using monomer mixtures, and monoazide chain stoppers can be used to install well-defined end groups. Azopolymers possess unique properties owing to the functionality of the azo linkages. For example, protonation at nitrogen results in LUMO lowering and red-shifted absorption bands. Additionally, N=N bonds possess low-lying π* levels, allowing azopolymers to be reversibly reduced under mild conditions.

Photocatalytic Oxidative Coupling of Arylamines for the Synthesis of Azoaromatics and the Role of O2 in the Mechanism

The photocatalytic oxidative coupling of aryl amines to selectively synthesize azoaromatic compounds has been realized. Multiple different photocatalysts can be used to perform the general reaction; however, Ir(dF-CF₃-ppy)₂(dtbpy)⁺, where dF-CF₃-ppy is 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine and dtpby is 4,4'-tert-butyl-2,2'-bipyridine, showed the greatest range of reactivity with various amine substrates. Both electron-rich and -deficient amines can be coupled with yields up to 95% under an ambient air atmosphere. Oxygen was deemed to be essential for the reaction and is utilized in the regeneration of the photocatalyst. Fluorescence quenching and radical trap experiments indicate an amine radical coupling mechanism that proceeds through a hydrazoaromatic intermediate before further oxidation occurs to form the desired azoaromatic products.

Theoretical investigation of a novel xylene-based light-driven unidirectional molecular motor

In this study, the working mechanism of the first light-driven rotary molecular motors used to control an eight-base-pair DNA hairpin has been investigated. In particular, this linker was reported to have promising photophysical properties under physiological conditions, which motivated our work at the quantum mechanical level. Cis-trans isomerization is triggered by photon absorption at wavelengths ranging 300 nm-400 nm, promoting the rotor to the first excited state, and it is mediated by an energy-accessible conical intersection from which the ground state is reached back. The interconversion between the resulting unstable isomer and its stable form occurs at physiological conditions in the ground state and is thermally activated. Here, we compare three theoretical frameworks, generally used in the quantum description of medium-size chemical systems: Linear-Response Time-Dependent Density Functional Theory (LR-TDDFT), Spin-Flip TDDFT (SF-TDDFT), and multistate complete active space second-order perturbation theory on state-averaged complete active space self consistent field wavefunctions (MS-CASPT2//SA-CASSCF). In particular, we show the importance of resorting to a multireference approach to study the rotational cycle of light-driven molecular motors due to the occurrence of geometries described by several configurations. We also assess the accuracy and computational cost of the SF-TDDFT method when compared to MS-CASPT2 and LR-TDDFT.

Copper-Catalyzed Aerobic Oxidative Cyclization of 2-Alkynylanilines with Nitrosoarenes: Synthesis of Organic Solid Mechanoluminescence Compounds of 4-Oxo-4H-cinnolin-2-ium-1-ide

An efficient Cu(I)/DMAP/air system for the one-pot synthesis of 4-oxo-4H-cinnolin-2-ium-1-ides, which are often difficult to prepare by traditional routes from substituted 2-alkynylanilines and nitrosoarenes, was developed. These 4-oxo-4H-cinnolin-2-ium-1-ides have practical applications as mechanoluminescent materials. Preliminary mechanistic experiments were performed, and a plausible mechanism for this tandem process is proposed. The use of an inexpensive copper catalyst and molecular oxygen as the oxygen source and the oxidant make this an attractive green protocol with potential synthetic applications.

N–C bond formation between two anilines coordinated to a ruthenium center in cis-form affording a 3,5-cyclohexadiene-1,2-diimine moiety

Four-electron oxidation of two anilines coordinated to a ruthenium(ii) center in a cis-form affords N1-phenylcyclohexa-3,5-diene-1,2-diimine through an N–C bond formation with N–H and C–H bond activation.

The modulation effect of electron-rich solvents on the supramolecular networks and photochromic properties of naphthalene diimide molecules

The electron-rich solvents can effectively regulate the interfacial contacts of electron donors/acceptors and the photochromic properties of naphthalene diimide molecules.

Pd/β-cyclodextrin-catalyzed C–H functionalization in water: a greener approach to regioselective arylation of (NH)-indoles with aryl bromides

A greener and more practical approach to the regioselective C3/C2-arylation of (NH)-indoles with (hetero)aryl bromides in water is developed via the Na2PdCl4-catalyzed and β-cyclodextrin ligand-mediated cross-coupling reactions.

One-pot synthesis of azo compounds in the absence of acidic or alkaline additives

A one-pot method for the synthesis of azo compounds by the reaction of β-naphthol with aryl amines using t-BuONO as the nitrosonium source in DCM at room temperature was developed. This method features mild reaction conditions, a simple experimental procedure, and is free of acidic or alkaline additives.

An Efficient and Robust Method for Selective Conversion of Aniline to Azobenzene Using nano-TiO2 -P25-SO3 H, under Visible Light Irradiation

Nano-TiO₂ -P25-SO₃ H as our previous report has successfully been utilized to synthesize azobenzene through the selective conversion of aniline under visible light irradiation. According to PL emission spectra, the immobilizing a solid Brønsted acid of -SO₃ H groups on the pure-TiO₂ -P25 surface with a close interface is an approach to amplify the nano-TiO₂ -P25 response to visible light, which can productively hinder the recombination rate of photogenerated electrons and holes as carriers. Therefore, the photocatalytic activity of the semiconductor is highly likely to increase. Photooxidation of aniline to azobenzene was achieved by applying nano-TiO₂ -P25-SO₃ H (E_g  = 2.6 eV) that activated by blue photons (λ_max  = 460 nm), green photons (λ_max  = 510 nm) and red photons (λ_max  = 630 nm) which is introducing as a sustainable procedure. Central composite design (CCD) was employed for evaluating the effects of photocatalyst amount, oxidant concentration and irradiation time on the synthesis of azobenzene by this approach. Easily synthesizing, recyclability of the photocatalyst, mild reaction condition and short reaction time could be considered as plus points of this process.

Contrasting Photo-Switching Rates in Azobenzene Derivatives: How the Nature of the Substituent Plays a Role

A molecular design approach was used to create asymmetrical visible light-triggered azo-derivatives that can be good candidates for polymer functionalization. The specific electron-donor substituted molecules were characterized and studied by means of NMR analyses and UV-visible spectroscopy, comparing the results with Time Dependent Density Functional (TD-DFT) calculations. A slow rate of isomerization (ki = 1.5 × 10-4 s-1) was discovered for 4-((2-hydroxy-5methylphenyl) diazenyl)-3-methoxybenzoic acid (AZO1). By methylating this moiety, it was possible to unlock the isomerization mechanism for the second molecule, methyl 3-methoxy-4-((2-methoxy-5-methylphenyl) diazenyl)benzoate (AZO2), reaching promising isomerization rates with visible light irradiation in different solvents. It was discovered that this rate was heightened by one order of magnitude (ki = 3.1 × 10-3 s-1) for AZO2. A computational analysis using density functional (DFT/PBE0) and wavefunction (QD-NEVPT2) methodologies provided insight into the photodynamics of these systems. Both molecules require excitation to the second (S2) excited state situated in the visible region to initiate the isomerization. Two classic mechanisms were considered, namely rotation and inversion, with the former being energetically more favorable. These azo-derivatives show potential that paves the way for future applications as building blocks of functional polymers. Likewise, they could be really effective for the modification of existing commercial polymers, thus transferring their stimuli responsive properties to polymeric bulky structures, converting them into smart materials.

Metal-free cascade reactions of aziridines with arylalkynes and aryldiazoniums: facile access to arylazopyrrolines

A novel and facile approach to synthesize arylazopyrroline scaffolds via metal-free cascade reactions of aziridines with arylalkynes and aryldiazoniums has been developed, providing access to a variety of 4-arylazo-2-pyrrolines in a highly concise fashion. This efficient process, which can be performed at the gram scale, enjoys operational simplicity and mild and metal-free conditions.

New cobalt(ii) coordination designs and the influence of varying chelate characters, ligand charges and incorporated group I metal ions on enzyme-like oxidative coupling activity

In transition-metal-mediated catalysis, design of new, well defined coordination architectures and subjecting them to catalysis testing under the same reaction conditions is a necessity tool for improved understanding of desirable active site geometries and characteristics.

A compatibility study on the glycosylation of 4,4′-dihydroxyazobenzene

Photoresponsive glycoconjugates based on the azobenzene photoswitch are valuable molecules which can be used as tools for the investigation of carbohydrate–protein interactions or as precursors of shape-switchable molecular architectures, for example. To access such compounds, glycosylation of 4,4′-dihydroxyazobenzene (DHAB) is a critical step, frequently giving heterogeneous results because DHAB is a challenging glycosyl acceptor. Therefore, DHAB glucosylation was studied using nine different glycosyl donors, and reaction conditions were systematically varied in order to find a reliable procedure, especially towards the preparation of azobenzene bis-glucosides. Particular emphasis was put on glucosyl donors which were differentiated at the primary 6-position (N₃, OAc) for further functionalisation. The present study allowed us to identify suitable glycosyl donors and reaction conditions matching with DHAB, affording the bis-glycosylated products in fair yields and good stereocontrol.

The glycosylation of 4,4′-dihydroxyazobenzene was investigated to identify suitable conditions providing access to valuable photoswitchable glycoconjugates.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Synthesis of 5-Alkyl- and 5-Phenylamino-Substituted Azothiazole Dyes with Solvatochromic and DNA-Binding Properties

A series of new 5-mono- and 5,5'-bisamino-substituted azothiazole derivatives was synthesized from the readily available diethyl azothiazole-4,4'-dicarboxylate. This reaction most likely comprises an initial Michael-type addition by the respective primary alkyl and aromatic amines at the carbon atom C5 of the substrate. Subsequently, the resulting intermediates are readily oxidized by molecular oxygen to afford the amino-substituted azothiazole derivatives. The latter exhibit remarkably red-shifted absorption bands (λabs =507-661 nm) with high molar extinction coefficients and show a strong positive solvatochromism. As revealed by spectrometric titrations and circular and linear dichroism studies, the water-soluble, bis-(dimethylaminopropylamino)-substituted azo dye associates with duplex DNA by formation of aggregates along the phosphate backbone at high ligand-DNA ratios (LDR) and by intercalation at low LDR, which also leads to a significant increase of the otherwise low emission intensity at 671 nm.

Strain-Dependent Kinetics in the Cis-to-Trans Isomerization of Azobenzene in Bulk Elastomers

The cis-to-trans isomerization of azobenzene is accelerated in a bulk PDMS elastomer under uniaxial tension. The kinetics are cleanly described by a single-exponential first-order process (k = 2.7 × 10^-5 s^-1) in the absence of tension but become multiexponential under constant strains of 40-90%. The complex kinetics can be reasonably modeled as a two-component process. The majority (∼92%) process is slower and occurs with a rate constant that is similar to that of the unstrained system (k = 2.3-2.7 × 10^-5 s^-1), whereas the rate constant of the minority (∼8%) process increases from k = 10.1 × 10^-5 s^-1 at 40% strain to k = 21.3 × 10^-5 s^-1 at 90% strain. Simple models of expected force-rate relationships suggest that the average force of tension per strand in the minority component ranges from 28 to 44 pN across strains of 40-90%.