Azobenzene Derivatives Research Articles

Second harmonic responses of clickable azobenzenes in solution: Comparative Hyper-Rayleigh scattering and density functional theory studies

The linear and second-order nonlinear optical (NLO) properties of clickable azobenzene derivatives are investigated in chloroform solution by means of UV/Visible and Hyper-Rayleigh scattering (HRS) spectroscopies. Their absorption properties are also monitored along time in order to characterize the rate and kinetics of the E-Z photoswitching reaction. The magnitude and symmetry of the first hyperpolarizability of the compounds are rationalized with the help of DFT calculations, which support well the experimental data. We demonstrate that the second harmonic generation (SHG) intensity of the E form, as well as its variation upon photoisomerization, can be tuned by varying the nature of the peripheral substituents. A fully characterized library of ready-to-click switchable compounds displaying large hyperpolarizability contrasts, and differing in the amplitude and orientation in their dipole moments is provided. This library expands the database of photoswitching compounds for further anchorage into polymer matrices or nano- or micro-sized solid substrates.

Singlet and Triplet Pathways Determine the Thermal Z/E Isomerization of an Arylazopyrazole-Based Photoswitch.

Understanding the thermal isomerization mechanism of azobenzene derivatives is essential to designing photoswitches with tunable half-lives. Herein, we employ quantum chemical calculations, nonadiabatic transition state theory, and photosensitized experiments to unravel the thermal Z/E isomerization of a heteroaromatic azoswitch, the phenylazo-1,3,5-trimethylpyrazole. In contrast to the parent azobenzene, we predict two pathways to be operative at room temperature. One is a conventional ground-state reaction occurring via inversion of the aryl group, and the other is a nonadiabatic process involving intersystem crossing to the lowest-lying triplet state and back to the ground state, accompanied by a torsional motion around the azo bond. Our results illustrate that the fastest reaction rate is not controlled by the mechanism involving the lowest activation energy, but the size of the spin-orbit couplings at the crossing between the singlet and the triplet potential energy surfaces is also determinant. It is therefore mandatory to consider all of the multiple reaction pathways in azoswitches in order to predict experimental half-lives.

Four - Electron Oxidative Addition of an N=N Double Bond at a Chromium Metallocyclopropene.

This report describes the synthesis of a pseudo-tetrahedral chromium alkyne complex supported by a bidentate phosphinimide ligand and its reactivity with azobenzene. Characterization of the former by structural and computational methods reveals an unprecedented extent of alkyne activation by a formal chromium(II) center, suggesting that this complex is best described as a chromium(IV)-metallocyclopropene. Exposure of this compound to an azobenzene derivative results in the formation of a chromium(VI) diimido complex, which constitutes a rare 4-electron oxidative addition of an N=N double bond. The isolation of a chromium(IV)-hydrazido intermediate enabled mechanistic investigations of this challenging bond cleavage process. This work substantiates the notion that terminal phosphinimide ligands can engender first-row transition metal ions with exceptional reactivity.

Light-Sensitive Open Channel Block of Ionotropic Glutamate Receptors by Quaternary Ammonium Azobenzene Derivatives.

Glutamate ionotropic receptors mediate fast excitation processes in the central nervous system of vertebrates and play an important role in synaptic plasticity, learning, and memory. Here, we describe the action of two azobenene-containing compounds, AAQ (acrylamide-azobenzene-quaternary ammonium) and QAQ (quaternary ammonium-azobenzene-quaternary ammonium), which produced rapid and fully reversible light-dependent inhibition of glutamate ionotropic receptors. The compounds demonstrated voltage-dependent inhibition with only minor voltage-independent allosteric action. Calcium-impermeable AMPA receptors had weaker sensitivity compared to NMDA and calcium-permeable AMPA receptors. We further revealed that the compounds bound to NMDA and calcium-permeable AMPA receptors in different modes. They were able to enter the wide selectivity filter of AMPA receptors, and strong negative voltages caused permeation into the cytoplasm. The narrow selectivity filter of the NMDA receptors did not allow the molecules to bypass them; therefore, QAQ and AAQ bound to the shallow channel site and prevented channel closure by a foot-in-the-door mechanism. Computer simulations employing available AMPA and NMDA receptor structures readily reproduced the experimental findings, allowing for the structure-based design of more potent and selective drugs in the future. Thus, our work creates a framework for the development of light-sensitive blockers of calcium-permeable AMPA receptors, which are desirable tools for neuroscience.

Construction of emissive azobenzene derivatives by hydroxylation and aggregation

Azobenzene derivatives are well-known for their photoisomerization. Due to E-Z isomerization, azobenzene derivatives are seldom fluorescent, which hinders their high-tech applications. Herein, we report two azobenzene derivatives, TPE-Azo-OH and TPE-Azo-2OH, with photoisomerization properties and solid-state fluorescence. Theoretical investigations revealed that the unusual fluorescence contributed from the S2 (π, π*) by state-mixing in solid state. TPE-Azo-2OH exhibits even stronger fluorescence due to the suppression of photoisomerization by intramolecular hydrogen bond. Furthermore, upon reduction, these azobenzene derivatives exhibit fluorescence transformation from red to green. These new azobenzene derivatives with AIE attributes and stimuli-responsiveness are expected to provide a new platform for developing photo-responsive luminescent materials.

Synthesis and cis-trans kinetics of an azobenzene-based molecular switch for light-responsive silica surfaces

Light-responsive materials design is an attractive research field that stimulates demand for the synthesis of novel molecular photoswitches. The novel photoswitches should be capable of being attached to a desired surface in order to provide it with light-responsive properties. The switchback to its ground state is expected to be gradual, with the possibility of acceleration. This study presents a comprehensive description of a chemical route to a molecular photoswitch based on an azobenzene derivative with a trifluoromethoxy group and a long alkyl chain. The photoswitch is capable of modifying submicrometer silica particles and silicon wafers as models of curved and flat surfaces, respectively. The thermal relaxation times of the synthesized azobenzene were studied in three different solvents: ethanol, chloroform, and toluene. The results reveal a significantly delayed relaxation process lasting for several days. This observation is found to be related to the polarity of the solvents used. To characterize the extensive thermal relaxation process, a methodology including the use of white-light pulses to accelerate cis-trans isomerization is suggested. The white light spectrum used in pulses affects thermal relaxation but does not alter the relaxation time extrapolated to zero. The density of the photoswitch molecules attached to particles varies from 0.7 to 1.7 molecules/nm2 as the initial amount of azobenzene in the reaction increases. Surface-attached photoswitches demonstrate reversible cis-trans isomerization behavior when exposed to UV and white light irradiation. A 14° contact angle change is observed when flat surfaces modified with photoswitches are irradiated with UV light.

Tuning the Switching Probability of Azobenzene Derivatives on Graphite Surface through Chemical Functions

Tuning the Switching Probability of Azobenzene Derivatives on Graphite Surface through Chemical Functions

Spectroscopy, electrochemistry and liquid crystal properties of monosubstituted ferrocene based azobenzene derivatives with 1,2,3-triazole heterocycle

Monosubstituted ferrocene based azobenzene liquid crystals were synthesized, in which ferrocene was flexibly attached to the mesomorphic core by click reaction. The counterpart compounds without ferrocene unit were also prepared. Two series of compounds displayed liquid crystal behaviours during heating and cooling, and had wide mesophase range. In the process of cooling, the compounds underwent a transformation from nematic phase to smectic C phase. When ferrocene was introduced into the molecule, the melting point of most compounds decreased by 30–40 °C, and the mesophase range was still as wide as 71–97 °C. These compounds emitted fluorescence at 400–550 nm and had strong absorption near 280 nm and 350 nm. Under the irradiation of 365 nm ultraviolet light, these compounds transformed rapidly from E to Z isomers, while the reverse reaction occurred slowly in the dark, especially the molecules containing ferrocene. Cyclic voltammetry studies showed that ferrocene based azobenzene derivatives exhibited one electron transfer processes with low formal redox potentials, indicating that these compounds are easily oxidized.

A robust photo-responsive zirconium metal-organic framework for tailorable CO2 capture

Photo-responsive adsorbents (PRAs) have attracted much attention due to their energy efficiency for controllable gas capture. However, the construction of robust PRAs is still full of challenges. In this project, we report the construction of azobenzene-modified UiO-67, namely Azo-U, consisted of hexa-nuclear zirconium clusters and azobenzene derivative ligand Azo-H2BPDC. The single crystal X-ray diffraction (SCXRD) analysis demonstrates that Azo-U shows analogous structures to UiO-67 and the specific surface area is up to 1327 m2·g−1 derived from N2 uptake property at 77 K. The incorporation of Azo-H2BPDC imparts Azo-U with reversible photo-responsive properties. Considering its high porosity and photo-responsive properties, Azo-U was evaluated for controllable CO2 capture and the regulation efficiency can be up to 25.5%. Such a photo-controllable CO2 capacity of Azo-U is further confirmed by density functional theory (DFT) and Grand Canonical Monte Carlo calculation. Besides, the regulation efficiency can be well-preserved upon alternative ultraviolet and visible light irradiation. Moreover, Azo-U possesses good stability under aqueous solutions of different pH values (1.0 to 11.0). This study might shed light on the design and fabrication of target-specific PRAs to meet the various demands.

In silico investigation of the photoisomerization mechanism of push-push azoderivatives

The design of novel chromophores showing specific photophysical traits is nowadays crucial for the development of new dyes and optical devices. Azobenzene derivatives comprise the largest fraction of industrial dyes and remain one of the main functions to be employed in the design of general-purpose photoactivated switches. In this context, we have investigated the optical and photoisomerization properties of two push-push azo derivatives, from a purely theoretical perspective, to outline a mechanistic landscape that can contribute to setting the grounds for the future goal of formulating dyes with specific photophysical properties. Both derivatives show downhill excited-state potential energy surfaces, hinting at fast photoactivated isomerization. In addition, both the trans and cis forms show nearly complementary absorption spectra leading to discriminatory color and differential excitation possibilities. Additionally, the reverse cis to trans ground state thermal isomerization reactions show higher energy barriers than the parent azobenzene molecule, supporting their potential use in different technological applications.

Controlled assembly of perovskite nanoparticles by photoswitchable functional ligands

Organic ligands play a crucial role in the properties and functionality of nanostructures. Functional ligands are an interesting research direction that can be utilized to influence the properties and functionality of nanoparticles (NPs). In this work, we demonstrate controlled assembly of CsPbBr3 perovskite NPs as a result of light. Azobenzene derivative molecules were used as the photoswitchable ligands for the NPs. The assembly and disassembly of the NPs were achieved by cis–trans isomerization. By utilizing polarization-modulated, infrared reflection–absorption spectroscopy and diffusion-ordered nuclear magnetic resonance, we were able to track the attachment of the ligands to the surface of the NPs. Absorbance, photoluminescence and high-resolution, transmission electron microscopy followed the assembly and disassembly of the NPs. This work demonstrates functional ligands paired to perovskite nanostructures through controlling their assembly and disassembly, which opens the way for sensing and photodetection applications.

Tuning Crystal Packing and Magnetic Properties in a Series of [Dy12] Metallocubanes Based on Azobenzene Derivatives of Salicylic Acid.

A series of four new Dy12 dodecanuclear clusters based on azobenzene derivative ligands of salicylic acid (L1-L4) has been synthesized and characterized in the crystalline phase using X-ray diffraction on single crystal and powder, IR spectroscopy, elemental analysis, and DSC-TGA methods. It was revealed that all obtained clusters exhibit the formation of the similar metallic cluster nodes, as vertex-sharing heterocubanes, obtained from four Dy3+ cations, three bridging hydroxyl groups, and O atoms from the salicylic ligands. The coordination geometry around the Dy(III) centers has been carefully analyzed. Whereas Dy12-L1 and Dy12-L2 with L1 and L2 containing Me and OMe groups in para positions of the phenyl rings, respectively, form similar porous 3D diamond-like molecular networks due to CH-π interactions, for Dy12-L3 with L3 bearing NO2-electron-withdrawing group, the generation of 2D molecular grids assembled by π-π staking is observed, and for Dy12-L4 with L4 bearing phenyl substituent, 3D hexagonal channels have been generated. The complexes Dy12-L1, Dy12-L2, and Dy12-L3 exhibit a zero-field slow magnetic relaxation effect. After UV irradiation of Dy12-L1, a decrease of the magnetic anisotropy energy barrier displaying the possibility of control over magnetic properties by the external stimulus has been observed.

Kinetics of the dark cis–trans isomerization of azobenzene and azo pyridine derivatives in ethanol and chloroform solutions

The paper presents a series of azobenzene and azo pyridine compounds containing hydroxyl or 6-hydroxyhexyloxyl substituents. The nitrogen atom is located at the ortho-, meta- or para-position in the structure of azo pyridine derivatives. The impact of the structural features of azo chromophores on their thermal (DSC, TGA) and optical (UV–Vis) properties was investigated. The dark cis–trans isomerization kinetics was studied in ethanol and chloroform at 298 K. The formation of noncovalent hydrogen bonds between azo molecules and/or chromophore-solvent was monitored using the 1H NMR spectroscopy and estimated by density functional theory (DFT) calculations. Based on experimental and computational studies we proposed the mechanism of the dark cis–trans isomerization for hydroxy-substituted azo compounds.

Synthesis of Pyrene-Substituted Azobenzene Derivatives and Their Assembly into Flat Structures for Stimuli-Induced Switching

Synthesis of Pyrene-Substituted Azobenzene Derivatives and Their Assembly into Flat Structures for Stimuli-Induced Switching

To Isomerize or not to Isomerize? E /ZIsomers of Cyclic Azobenzene Derivatives and their Reactivity upon One-Electron Reduction.

Azo compounds are efficient electron acceptors. Upon one-electron reduction they generally isomerize forming the thermodynamically most stable radical anion. Here we show that the size of the central ring in 1,2-diazocines and diazonines has a ruling influence on the configuration of the one-electron reduced species. Markedly, diazonines, which bear a central nine membered heterocycle, show light inducedE/Zisomerization, but retain the configuration of the diazene N=N moiety upon one-electron reduction. Accordingly,E/Zisomerization is not induced by reduction.

G-quadruplex-based CRISPR photoswitch for spatiotemporal control of genomic modulation.

CRISPR (clustered regularly interspaced short palindromic repeats) technology holds tremendous promise for gene regulation and editing. However, precise control of CRISPR editing is essential to overcome its uncontrollable reaction process and excessive activity that leads to off-target editing. To overcome this problem, we engineered a photoswitch on G-quadruplex gRNA (GqRNA) for precisely controlled gene editing and expression by embedding dicationic azobenzene derivatives (AZD++). Our results demonstrated that rational design of the G-quadruplex onto crRNA conferred higher stability and sequence recognition specificity than unmodified single guide (sgRNA). Light-induced isomerization of AZD++ quickly transformed the on state of GqRNA, which facilitated rapid activation of ribonucleoprotein activity for genome editing of on-target sites in cells with excellent editing efficiency. In turn, AZD++-GqRNA promptly refolded to an off state to inhibit genomic cleavage, and limited the generation of off-target effects and by-products. Therefore, the proposed strategy of a photo-reversible modality presents a new opportunity for CRISPR-Cas9 modulation to improve its safety and applicability.

Reversible photo-controllable surface wettability of epoxy-polyurethane smart coating

Light-responsive materials, in which a rapid and reversible response can be achieved, are of particular interest for several technological applications. The transformation of external stimuli into mechanical energy is very promising in the field of molecular motors and energy convertors. Azobenzene chromophores undergo photon-driven reversible cis-trans isomerization. In these stimuli-responsive materials, reversible molecular motion can be responsible for switching surface properties, such as surface wettability. In this work, the authors synthesized a fluorinated epoxy - isocyanate thermosetting coating (EPUF) modified with an azobenzene derivative (AZO). The increase in AZO content on the surface by migration during synthesis, as a consequence fluorine presence, leads to changes in the surface hydrophobicity, when UV light is used as a stimulus. First, polyurethane precursors (PUs) based on isophorone diisocyanate (IPDI), a fluorinated diol, and AZO were synthesized. Size-Exclusion Chromatography (SEC), Fourier Transform Infrared Spectroscopy (FTIR), and Ultraviolet-Visible Spectroscopy (UV) techniques were used to characterize the precursors. The polyurethane – epoxy reaction obtained allowed to prepare AZO modified epoxy networks. The resulting materials were characterized using different techniques. Fluorinated and pristine networks were compared. The changes in contact angle, roughness, and mechanical behavior by UV irradiation were analyzed. The reversible photo-induced switching of the surface properties of the fluorine-AZO modified coatings renders them potentially useful for multiple technological applications, in which cycling surface properties are of great use.

Supramolecular Self-Assembly and Photo-Induced Transition of a Halogenated Azo-Benzene Molecule on Au(111) Surface

Azobenzene derivatives are a unique class of photo-switch molecules with promising potential for nanoscale optical applications. We have studied the self-assembly and photo-induced mechanical switching of azobenzene derivatives on Au(111) at the single-molecule level by using scanning tunneling microscope (STM). 4,4′-Dibromo-azobenzene (Br-AB) molecules are assembled into two types of well-ordered structures on Au(111) surfaces in the trans-isomer configuration. Br-AB molecules experienced configurational changes from trans-to-cis photo-isomerization upon the exposure to the UV light. This photo-isomerization of Br-AB molecules was observed to occur at random sites of ordered structure, evidenced by the appearance of bright protrusions with the height increment. Our results may open up new routes to engineer nanoscale photo-switch molecular devices.

Sensor to Electronics Applications of Graphene Oxide through AZO Grafting.

Graphene is a two-dimensional (2D) material with a single atomic crystal structure of carbon that has the potential to create next-generation devices for photonic, optoelectronic, thermoelectric, sensing, wearable electronics, etc., owing to its excellent electron mobility, large surface-to-volume ratio, adjustable optics, and high mechanical strength. In contrast, owing to their light-induced conformations, fast response, photochemical stability, and surface-relief structures, azobenzene (AZO) polymers have been used as temperature sensors and photo-switchable molecules and are recognized as excellent candidates for a new generation of light-controllable molecular electronics. They can withstand trans-cis isomerization by conducting light irradiation or heating but have poor photon lifetime and energy density and are prone to agglomeration even at mild doping levels, reducing their optical sensitivity. Graphene derivatives, including graphene oxide (GO) and reduced graphene oxide (RGO), are an excellent platform that, combined with AZO-based polymers, could generate a new type of hybrid structure with interesting properties of ordered molecules. AZO derivatives may modify the energy density, optical responsiveness, and photon storage capacity, potentially preventing aggregation and strengthening the AZO complexes. They are potential candidates for sensors, photocatalysts, photodetectors, photocurrent switching, and other optical applications. This review aimed to provide an overview of the recent progress in graphene-related 2D materials (Gr2MS) and AZO polymer AZO-GO/RGO hybrid structures and their synthesis and applications. The review concludes with remarks based on the findings of this study.

Synthesis and characterization of dual light/temperature-responsive supramolecular injectable hydrogel based on host-guest interaction between azobenzene and starch-grafted β-cyclodextrin: Melanoma therapy with paclitaxel

Injectable stimuli-responsive hydrogels could offer an opportunity for local administration at the tumor site and a sustained drug release. In this paper, a copolymer of azobenzene derivative and N-isopropyl acrylamide (NIPAM) was synthesized, which are performed as light- and thermo-sensitive parts, respectively. The DAS@SCD/NIPAZO hydrogel was prepared upon the establishment of host-guest interactions between the hydrophobic core of CD and azobenzene moiety. The LCST of the synthesized copolymer was modified from 31.3 °C to 36.5 °C by the incorporation of the hydrophilic host moieties of the modified starch into the NIPAM copolymer structure. The LCST-based property of the hydrogel made it syringable in low temperatures and switch to a gel state after local injection. The drug release profile of the hydrogel was explored in four different conditions involving two distinct temperatures combined with two different light wavelengths to examine the light- and thermo-sensitivity of the hydrogel. Moreover, a Paclitaxel-loaded hydrogel was prepared to study the in vitro efficiency of the sample and was investigated by MTT assay against the cancerous fibroblastic cells (A-431), which revealed a sharp decline in cell viability under 365 nm light irradiation; furthermore, to evaluate the in vivo effects of the PTX-loaded hydrogel, histological studies based on staining techniques were carried out.