Direct and Versatile Synthesis of Red-Shifted Azobenzenes

Starazo triple switches - synthesis of unsymmetrical 1,3,5-tris(arylazo)benzenes

Multistate switches allow to drastically increase the information storage capacity and complexity of smart materials. In this context, unsymmetrical 1,3,5-tris(arylazo)benzenes - 'starazos' - which merge three photoswitches on one benzene ring, were successfully prepared. Two different synthetic strategies, one based on Baeyer-Mills reactions and the other based on Pd-catalyzed coupling reactions of arylhydrazides and aryl halides, followed by oxidation, were investigated. The Pd-catalyzed route efficiently led to the target compounds, unsymmetrical tris(arylazo)benzenes. These triple switches were preliminarily characterized in terms of their isomerization behavior using UV-vis and 1H NMR spectroscopy. The efficient synthesis of this new class of unsymmetrical tris(arylazo)benzenes opened new avenues to novel multistate switching materials.

Thiophenylazobenzene: An Alternative Photoisomerization Controlled by Lone-Pair⋅⋅⋅π Interaction

Azoheteroarene photoswitches have attracted attention due to their unique properties. We present the stationary photochromism and ultrafast photoisomerization mechanism of thiophenylazobenzene (TphAB). It demonstrates impressive fatigue resistance and photoisomerization efficiency, and shows favorably separated (E)- and (Z)-isomer absorption bands, allowing for highly selective photoconversion. The (Z)-isomer of TphAB adopts an unusual orthogonal geometry where the thiophenyl group is perfectly perpendicular to the phenyl group. This geometry is stabilized by a rare lone-pair⋅⋅⋅π interaction between the S atom and the phenyl group. The photoisomerization of TphAB occurs on the sub-ps to ps timescale and is governed by this interaction. Therefore, the adoption and disruption of the orthogonal geometry requires significant movement along the inversion reaction coordinates (CNN and NNC angles). Our results establish TphAB as an excellent photoswitch with versatile properties that expand the application possibilities of AB derivatives.

Reversible Spatiotemporal Control of Induced Protein Degradation by Bistable PhotoPROTACs

Off-tissue effects are persistent issues of modern inhibition-based therapies. By merging the strategies of photopharmacology and small-molecule degraders, we introduce a novel concept for persistent spatiotemporal control of induced protein degradation that potentially prevents off-tissue toxicity. Building on the successful principle of bifunctional all-small-molecule Proteolysis Targeting Chimeras (PROTACs), we designed photoswitchable PROTACs (photoPROTACs) by including ortho-F4-azobenzene linkers between both warhead ligands. This highly bistable yet photoswitchable structural component leads to reversible control over the topological distance between both ligands. The azo-cis-isomer is observed to be inactive because the distance defined by the linker is prohibitively short to permit complex formation between the protein binding partners. By contrast, the azo-trans-isomer is active since it can engage both protein partners to form the necessary and productive ternary complex. Importantly, due to the bistable nature of the ortho-F4-azobenzene moiety employed, the photostationary state of the photoPROTAC is persistent, with no need for continuous irradiation. This technique offers reversible on/off switching of protein degradation that is compatible with an intracellular environment and, therefore, could be useful in experimental exploration of biological signaling pathways-such as those crucial for oncogenic signal transduction. Additionally, this strategy may be suitable for therapeutic intervention to address a variety of diseases. By enabling reversible activation and deactivation of protein degradation, photoPROTACs offer advantages over conventional photocaging strategies that irreversibly release active agents.

Photoswitchable peptides for spatiotemporal control of biological functions

Light is unsurpassed in its ability to modulate biological interactions. Since their discovery, chemists have been fascinated by photosensitive molecules capable of switching between isomeric forms, known as photoswitches. Photoswitchable peptides have been recognized for many years; however, their functional implementation in biological systems has only recently been achieved. Peptides are now acknowledged as excellent protein-protein interaction modulators and have been important in the emergence of photopharmacology. In this review, we briefly explain the different classes of photoswitches and summarize structural studies when they are incorporated into peptides. Importantly, we provide a detailed overview of the rapidly increasing number of examples, where biological modulation is driven by the structural changes. Furthermore, we discuss some of the remaining challenges faced in this field. These exciting proof-of-principle studies highlight the tremendous potential of photocontrollable peptides as optochemical tools for chemical biology and biomedicine.

2-Azo-, 2-diazocine-thiazols and 2-azo-imidazoles as photoswitchable kinase inhibitors: limitations and pitfalls of the photoswitchable inhibitor approach

In photopharmacology, photoswitchable compounds including azobenzene or other diarylazo moieties exhibit bioactivity against a target protein typically in the slender E-configuration, whereas the rather bulky Z-configuration usually is pharmacologically less potent. Herein we report the design, synthesis and photochemical/inhibitory characterization of new photoswitchable kinase inhibitors targeting p38α MAPK and CK1δ. A well characterized inhibitor scaffold was used to attach arylazo- and diazocine moieties. When the isolated isomers, or the photostationary state (PSS) of isomers, were tested in commonly used in vitro kinase assays, however, only small differences in activity were observed. X-ray analyses of ligand-bound p38α MAPK and CK1δ complexes revealed dynamic conformational adaptations of the protein with respect to both isomers. More importantly, irreversible reduction of the azo group to the corresponding hydrazine was observed. Independent experiments revealed that reducing agents such as DTT (dithiothreitol) and GSH (glutathione) that are typically used for protein stabilization in biological assays were responsible. Two further sources of error are the concentration dependence of the E-Z-switching efficiency and artefacts due to incomplete exclusion of light during testing. Our findings may also apply to a number of previously investigated azobenzene-based photoswitchable inhibitors.

Designing azobenzene-based tools for controlling neurotransmission

Chemical and electrical signaling at the synapse is a dynamic process that is crucial to neurotransmission and pathology. Traditional pharmacotherapy has found countless applications in both academic labs and the clinic; however, diffusible drugs lack spatial and temporal precision when employed in heterogeneous tissues such as the brain. In the field of photopharmacology, chemical attachment of a synthetic photoswitch to a bioactive ligand allows cellular signaling to be controlled with light. Azobenzenes have remained the go-to photoswitch for biological applications due to their tunable photophysical properties, and can be leveraged to achieve reversible optical control of numerous receptors and ion channels. Here, we discuss the most recent advances in photopharmacology which will improve the use of azobenzene-based probes for neuroscience applications.

Rationally designed azobenzene photoswitches for efficient two-photon neuronal excitation

Manipulation of neuronal activity using two-photon excitation of azobenzene photoswitches with near-infrared light has been recently demonstrated, but their practical use in neuronal tissue to photostimulate individual neurons with three-dimensional precision has been hampered by firstly, the low efficacy and reliability of NIR-induced azobenzene photoisomerization compared to one-photon excitation, and secondly, the short cis state lifetime of the two-photon responsive azo switches. Here we report the rational design based on theoretical calculations and the synthesis of azobenzene photoswitches endowed with both high two-photon absorption cross section and slow thermal back-isomerization. These compounds provide optimized and sustained two-photon neuronal stimulation both in light-scattering brain tissue and in Caenorhabditis elegans nematodes, displaying photoresponse intensities that are comparable to those achieved under one-photon excitation. This finding opens the way to use both genetically targeted and pharmacologically selective azobenzene photoswitches to dissect intact neuronal circuits in three dimensions.

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

Light is a nearly ideal stimulus for molecular systems. It delivers information encoded in the form of wavelengths and their intensities with high precision in space and time. Light is a mild trigger that does not permanently contaminate targeted samples. Its energy can be reversibly transformed into molecular motion, polarity, or flexibility changes. This leads to sophisticated functions at the supramolecular and macroscopic levels, from light-triggered nanomaterials to photocontrol over biological systems. New methods and molecular adapters of light are reported almost daily. Recently reported applications of photoresponsive systems, particularly azobenzenes, spiropyrans, diarylethenes, and indigoids, for smart materials and photocontrol of biological setups are described herein with the aim to demonstrate that the 21st century has become the Age of Enlightenment-"Le siècle des Lumières"-in molecular sciences.

Non-adiabatic molecular dynamics with ΔSCF excited states

Accurate modelling of nonadiabatic transitions and electron-phonon interactions in extended systems is essential for understanding the charge and energy transfer in photovoltaic and photocatalytic materials. The extensive computational costs of the advanced excited state methods have stimulated the development of many approximations to study the nonadiabatic molecular dynamics (NA-MD) in solid-state and molecular materials. In this work, we present a novel ▵SCF-NA-MD methodology that aims to account for electron-hole interactions and electron-phonon back-reaction critical in modelling photoinduced nuclear dynamics. The excited states dynamics is described using the delta self-consistent field (▵SCF) technique within the density functional formalism and the trajectory surface hopping. The technique is implemented in the open-source Libra-X package freely available on the Internet (https://github.com/Quantum-Dynamics-Hub/Libra-X). This work illustrates the general utility of the developed ▵SCF-NA-MD methodology by characterizing the excited state energies and lifetimes, reorganization energies, photoisomerization quantum yields, and by providing the mechanistic details of reactive processes in a number of organic molecules.

Aryldiazonium Salts as Nitrogen-Based Lewis Acids: Facile Synthesis of Tuneable Azophosphonium Salts

Inspired by the commercially available azoimidazolium dyes (e.g., Basic Red 51) that can be obtained from aryldiazonium salts and N-heterocyclic carbenes, we developed the synthesis of a unique set of arylazophosphonium salts. A range of colours were obtained by applying readily tuneable phosphine donor ligands and para-substituted aryldiazonium salts as nitrogen-based Lewis acids. With cyclic voltammetry, a general procedure was designed to establish whether the reaction between a Lewis acid and a Lewis base occurs by single-electron transfer or electron-pair transfer.

Synthesis and Photophysical Characterization of Azoheteroarenes

A set of azoheteroarenes have been synthesized with Buchwald-Hartwig coupling and microwave-assisted O₂ oxidation as the key steps. Several compounds exhibit good to excellent photoswitching properties (high switching efficiency, good fatigue resistance, and thermal stability of Z-isomer) relevant for photocontrolled applications, which pave the way for use in photopharmacology.

Synthesis of Functionalized 1,3-Butadienes via Pd-Catalyzed Cross-Couplings of Substituted Allenic Esters in Water at Room Temperature

An environmentally responsible, mild method for the synthesis of functionalized 1,3-butadienes is presented. It utilizes allenic esters of varying substitution patterns, as well as a wide range of boron-based nucleophiles under palladium catalysis, generating sp-sp², sp²-sp², and sp²-sp³ bonds. Functional group tolerance measured via robustness screening, along with room temperature and aqueous reaction conditions highlight the methodology's breadth and potential utility in synthesis.

Reversible photoswitching of encapsulated azobenzenes in water

Efficient molecular switching in confined spaces is critical for the successful development of artificial molecular machines. However, molecular switching events often entail large structural changes and therefore require conformational freedom, which is typically limited under confinement conditions. Here, we investigated the behavior of azobenzene-the key building block of light-controlled molecular machines-in a confined environment that is flexible and can adapt its shape to that of the bound guest. To this end, we encapsulated several structurally diverse azobenzenes within the cavity of a flexible, water-soluble coordination cage, and investigated their light-responsive behavior. Using UV/Vis absorption spectroscopy and a combination of NMR methods, we showed that each of the encapsulated azobenzenes exhibited distinct switching properties. An azobenzene forming a 1:1 host-guest inclusion complex could be efficiently photoisomerized in a reversible fashion. In contrast, successful switching in inclusion complexes incorporating two azobenzene guests was dependent on the availability of free cages in the system, and it involved reversible trafficking of azobenzene between the cages. In the absence of extra cages, photoswitching was either suppressed or it involved expulsion of azobenzene from the cage and consequently its precipitation from the solution. This finding was utilized to develop an information storage medium in which messages could be written and erased in a reversible fashion using light.

Design, Synthesis, and Biological Evaluation of Axitinib Derivatives

Axitinib is an approved kinase inhibitor for the therapy of advanced metastatic renal cell carcinoma (RCC). It prevents angiogenesis, cellular adhesion, and induces apoptosis of cancer cells. Here, nine axitinib derivatives were designed by replacing the C=C moiety with the N=N group, and the substituted benzene or pyrrole analogs were considered to replace the pyridine ring. Biological activity results showed that most of nascent derivatives exhibited favorable VEGFR-2 kinase inhibitory activities, and TM6, 7, 9, and 11 behaved more potent anti-proliferative activities than axitinib. This novel series of compounds shows a potential for the treatment of solid tumors and other diseases where angiogenesis plays an important role.

Rh(iii)-Catalyzed regioselective mono- and di-iodination of azobenzenes using alkyl iodide

We developed a highly chemoselective oxidative access to ortho-iodinated azobenzenes by Rh(iii)-catalysis with alkyl iodide as the iodinating reagent.

Photocontrol of Antibacterial Activity: Shifting from UV to Red Light Activation

The field of photopharmacology aims to introduce smart drugs that, through the incorporation of molecular photoswitches, allow for the remote spatial and temporal control of bioactivity by light. This concept could be particularly beneficial in the treatment of bacterial infections, by reducing the systemic and environmental side effects of antibiotics. A major concern in the realization of such light-responsive drugs is the wavelength of the light that is applied. Studies on the photocontrol of biologically active agents mostly rely on UV light, which is cytotoxic and poorly suited for tissue penetration. In our efforts to develop photoswitchable antibiotics, we introduce here antibacterial agents whose activity can be controlled by visible light, while getting into the therapeutic window. For that purpose, a UV-light-responsive core structure based on diaminopyrimidines with suitable antibacterial properties was identified. Subsequent modification of an azobenzene photoswitch moiety led to structures that allowed us to control their activity against Escherichia coli in both directions with light in the visible region. For the first time, full in situ photocontrol of antibacterial activity in the presence of bacteria was attained with green and violet light. Most remarkably, one of the diaminopyrimidines revealed an at least 8-fold difference in activity before and after irradiation with red light at 652 nm, showcasing the effective "activation" of a biological agent otherwise inactive within the investigated concentration range, and doing so with red light in the therapeutic window.

Near-Infrared Photoswitching of Azobenzenes under Physiological Conditions

Biological tissue exhibits an absorbance minimum in the near-infrared between 700 and 900 nm that permits deep penetration of light. Molecules that undergo photoisomerization in this bio-optical window are highly desirable as core structures for the development of photopharmaceuticals and as components of chemical-biological tools. We report the systematic design, synthesis, and testing of an azobenzene derivative tailored to undergo single-photon photoswitching with near-infrared light under physiological conditions. A fused dioxane ring and a methoxy substituent were used to place oxygen atoms in all four ortho positions, as well as two meta positions, relative to the azobenzene N═N double bond. This substitution pattern, together with a para pyrrolidine group, raises the pK_a of the molecule so that it is protonated at physiological pH and absorbs at wavelengths >700 nm. This azobenzene derivative, termed DOM-azo, is stable for months in neutral aqueous solutions, undergoes trans-to-cis photoswitching with 720 nm light, and thermally reverts to the stable trans isomer with a half-life near 1 s.

Let there be light: how to use photoswitchable cross-linker to reprogram proteins

Azobenzene is a photo-isomerizing molecule whose end-to-end distance changes upon external illumination. When combined with site-specific reactive groups, it can be used as molecular tweezers to remote-control the structure and function of protein targets. The present study gives a brief overview over the rational design strategies that use an azobenzene-based photoswitchable cross-linker to engineer ON/OFF switches into functional proteins or to reprogram proteins for novel functions. The re-engineered proteins may be used as remote controls for cellular pathways, as light-gated drug delivery platforms or as light-powered machinery of synthetic cells and micro-scaled factories.

The Art of Building Small: From Molecular Switches to Motors (Nobel Lecture)

A journey into the nano-world: The ability to design, use and control motor-like functions at the molecular level sets the stage for numerous dynamic molecular systems. In his Nobel Lecture, B. L. Feringa describes the evolution of the field of molecular motors and explains how to program and control molecules by incorporating responsive and adaptive properties.

Visible-Light Excitation of a Molecular Motor with an Extended Aromatic Core

Exploring routes to visible-light-driven rotary motors, the possibility of red-shifting the excitation wavelength of molecular motors by extension of the aromatic core is studied. Introducing a dibenzofluorenyl moiety in a standard molecular motor resulted in red-shifting of the absorption spectrum. UV/vis and 1H NMR spectroscopy showed that these motors could be isomerized with light of wavelengths up to 490 nm and that the structural modification did not impair the anticipated rotary behavior. Extension of the aromatic core is therefore a suitable strategy to apply in pursuit of visible-light-driven molecular motors.

Arylazoindazole Photoswitches: Facile Synthesis and Functionalization via SNAr Substitution

A straightforward synthetic route to arylazoindazoles via nucleophilic aromatic substitution is presented. Upon deprotonation of the NH group, a C₆F₅-substituted formazan undergoes facile cyclization as a result of intermolecular nucleophilic substitution (S_NAr). This new class of azo photoswitches containing an indazole five-membered heterocycle shows photochemical isomerization with high fatigue resistance. In addition, the Z-isomers have long thermal half-lives in the dark of up to several days at room temperature. The fluorinated indazole group offers a handle for further functionalization and tuning of its properties, as it is shown to be susceptible to a subsequent, highly selective nucleophilic displacement reaction.

Controlling azobenzene photoswitching through combined ortho-fluorination and -amination

We present a series of visible-light-absorbing azobenzenes through a combination of ortho-fluorination and –amination showing a wide range of cis-lifetimes.

Copper(i)-mediated carboamination of vinyl azides by aryldiazonium salts: synthesis of N2-substituted 1,2,3-triazoles

Copper(i)-mediated carboamination reaction of vinyl azides with sole aryldiazonium salt for N²-substituted 1,2,3-triazoles.