Azo−Hydrazone Tautomerism in Protonated Aminoazobenzenes: Resonance Raman Spectroscopy and Quantum-Chemical Calculations

Hydrazo Pyrazole-Pyridone Fluorescent tag for NLO, Live cell imaging, LFPs visualization, Photophysical probing, and Electrochemical sensor for Dopamine detection

The present communication reports on the synthesis of a novel methyl-pyridone azo fluorescent tag (MPAFT) were proven through 1H (NMR), FT-IR, UV-vis, and high-resolution mass spectrometry. The quantum chemical parameters of MPAFT were evaluated using density functional theory (DFT) analysis. It was further investigated for its latent fingerprint (LFPs) in various surfaces and anticounterfeiting applications. By exposing Level I-Level III, ridge features to UV light with a wavelength of 365 nm, a bioimaging investigation has also demonstrated the potential of MPAFT's emission behaviour. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) at MPAFT/MGCE (modified glassy carbon electrode) were used to explore the electrochemical sensitivity and reliable detection of dopamine (DA) in neutral PBS (pH 7) electrolyte solution, and the results show good sensitivity and detection. The lower detection limit for LSV was 0.81 μM under optimum conditions.

On the Simulation of Thermal Isomerization of Molecular Photoswitches in Biological Systems

Molecular photoswitches offer precise, reversible photocontrol over biomolecular functions and are promising light-regulated drug candidates with minimal side effects. Quantifying thermal isomerization rates of photoswitches in their target biomolecules is essential for fine-tuning their light-controlled drug activity. However, the effects of protein binding on isomerization kinetics remain poorly understood, and simulations are crucial for filling this gap. Challenges in the simulation include describing multireference electronic structures near transition states, disentangling competing reaction pathways, and sampling protein-ligand interactions. To overcome these challenges, we used multiscale simulations to characterize the thermal isomerization of photostatins (PSTs), which are light-regulated microtubule inhibitors for potential cancer phototherapy. We employed a new ab initio multireference electronic structure method in a quantum mechanics/molecular mechanics setting and combined it with enhanced sampling techniques to characterize the cis to trans free-energy profiles of three PSTs in a vacuum, aqueous solution, and tubulin dimer. The significant advantage of our novel approach is the efficient treatment of the multireference character in PSTs' electronic wavefunction throughout the conformational sampling of protein-ligand interactions along their isomerization pathways. We also benchmarked our calculations using high-level ab initio multireference electronic structure methods and explored the competing isomerization pathways. Notably, calculations in a vacuum and implicit solvent models cannot predict the order of the PSTs' thermal half-lives in the aqueous solution observed in the experiment. Only by explicitly treating the solvent molecules can the correct order of isomerization kinetics be reproduced. Protein binding perturbs free-energy barriers due to hydrogen bonding between PSTs and nearby polar residues. Our work generates comprehensive, high-quality benchmark data and offers guidance for selecting computational methods to study the thermal isomerization of photoswitches. Ab initio multireference free-energy calculations in explicit molecular environments are crucial for predicting the effects of substituents on the thermal half-lives of photoswitches in biological systems.

Supramolecular Complexation of Azobenzene Dyes by Cucurbit[7]uril

This report describes cucurbit[7]uril (CB7) complexation of azobenzene dyes that have a 4-(N,N'-dimethylamino) or 4-amino substituent. Absorption and NMR data show that CB7 encapsulates the protonated form of the azobenzene and that the complexed dye exists as its azonium tautomer with a trans azo conformation and substantial quinoid resonance character. Because CB7 complexation stabilizes the dye conjugate acid, there is an upward shift in its pKa, and in one specific case, the pKa of the protonated azobenzene is increased from 3.09 to 4.47. Molecular modeling indicates that the CB7/azobenzene complex is stabilized by three major noncovalent factors: (i) ion-dipole interactions between the partially cationic 4-(N,N'-dimethylamino) or 4-amino group on the encapsulated protonated azobenzene and the electronegative carbonyl oxygens on CB7, (ii) inclusion of the upper aryl ring of the azobenzene within the hydrophobic CB7 cavity, and (iii) a hydrogen bond between the proton on the azo nitrogen and CB7 carbonyls. CB7 complexation enhances azobenzene stability and increases azobenzene hydrophilicity; thus, it is a promising way to improve azobenzene performance as a pigment or prodrug. In addition, the striking yellow/pink color change that accompanies CB7 complexation can be exploited to create azobenzene dye displacement assays with naked eye detection.

Nonreducing terminal chimeric isomaltomegalosaccharide and its integration with azoreductase for the remediation of soil-contaminated lipophilic azo dyes

Lipophilic azo dyes are practically water-insoluble, and their dissolution by organic solvents and surfactants is harmful to biological treatment with living cells and enzymes. This study aimed to evaluate the feasibility of a newly synthesized nonreducing terminal chimeric isomaltomegalosaccharide (N-IMS) as a nontoxic solubilizer of four simulated lipophilic azo dye wastes for enzymatic degradation. N-IMS bearing a helical α-(1 → 4)-glucosidic segment derived from a donor substrate α-cyclodextrin was produced by a coupling reaction of cyclodextrin glucanotransferase. Inclusion complexing by N-IMS overcame the solubility issue with equilibrium constants of 1786-242 M^-1 (methyl yellow > ethyl red > methyl red > azo violet). Circular dichroism spectra revealed the axial alignment of the aromatic rings in the N-IMS cavity, while UV-visible absorption quenching revealed that the azo bond of methyl yellow was particularly induced. Desorption of the dyes from acidic and neutral soils was specific to aqueous organic over alkali extraction. The dissolution kinetics of the incorporated dyes followed a sigmoid pattern facilitating the subsequent decolorization process with azoreductase. It was demonstrated that after soil extraction, the solid dyes dissolved with N-IMS assistance and spontaneously digested by coupled azoreductase/glucose dehydrogenase (for a cofactor regeneration system) with the liberation of the corresponding aromatic amine.

Thermal Half-Lives of Azobenzene Derivatives: Virtual Screening Based on Intersystem Crossing Using a Machine Learning Potential

Molecular photoswitches are the foundation of light-activated drugs. A key photoswitch is azobenzene, which exhibits trans-cis isomerism in response to light. The thermal half-life of the cis isomer is of crucial importance, since it controls the duration of the light-induced biological effect. Here we introduce a computational tool for predicting the thermal half-lives of azobenzene derivatives. Our automated approach uses a fast and accurate machine learning potential trained on quantum chemistry data. Building on well-established earlier evidence, we argue that thermal isomerization proceeds through rotation mediated by intersystem crossing, and incorporate this mechanism into our automated workflow. We use our approach to predict the thermal half-lives of 19,000 azobenzene derivatives. We explore trends and trade-offs between barriers and absorption wavelengths, and open-source our data and software to accelerate research in photopharmacology.

Synthesis, characterization and application of a novel polyazo dye as a universal acid-base indicator

A novel organic polyazo dye is synthesized by the diazotization of aromatic aniline, followed by coupling it with sulfanilic acid and N,N-dimethylaniline. Characterization was done by ¹H-NMR, ¹³C-NMR, and FTIR spectroscopy. Differential scanning calorimetry (DSC) reveals that phase transition for this molecule is exothermic. The optical band gap is estimated from the absorption cutoff point using UV-Visible spectroscopy. Thermal gravimetric analysis (TGA) addresses the thermal stability of the molecule and is found to be at ∼250 °C. The structure of the synthesized molecule is analogous to that of methyl orange and contains three azo groups. These three azo groups help accept more than two protons and provide two pK _a values when diprotic acid or a mixture of acids is used in different titrations. Specifically, when a polybasic acid is in strong base titration, the pK _a values were found to be 3.5 and 9.1. Moreover, for strong base and (strong + weak) acid mixture titration, the pK _a values are found to be 9.2 and 3.3. Furthermore, the pK _a values are found to be 8.6 and 2.8 for (strong and weak) base mixture and (strong and weak) acid mixture titration, respectively. Owing to its increased proton accepting capacity, it can be found in the two pH ranges of 2.1-3.8 for orange color and 8.2-9.8 for yellow color, thus indicating a unique property as a universal indicator for acid-base titration. The dissociation constant of this dye is found to be 3.4 × 10^-6, determined in a mixed aqueous solution of 10 wt% ethanol, and a linear relationship between pK _a and pH is observed in this solvent system.

Cucurbit[7]uril Complexation of Near-Infrared Fluorescent Azobenzene-Cyanine Conjugates

Two new azobenzene heptamethine cyanine conjugates exist as dispersed monomeric molecules in methanol solution and exhibit near-infrared (NIR) cyanine absorption and fluorescence. Both conjugates form non-emissive cyanine H-aggregates in water, but the addition of cucurbit[7]uril (CB7) induces dye deaggregation and a large increase in cyanine NIR fluorescence emission intensity. CB7 encapsulates the protonated azonium tautomer of the 4-(N,N-dimethylamino)azobenzene component of each azobenzene-cyanine conjugate and produces a distinctive new absorption band at 534 nm. The complex is quite hydrophilic, which suggests that CB7 can be used as a supramolecular additive to solubilize this new family of NIR azobenzene-cyanine conjugates for future biomedical applications. Since many azobenzene compounds are themselves potential drug candidates or theranostic agents, it should be possible to formulate many of them as CB7 inclusion complexes with improved solubility, stability, and pharmaceutical profile.

Reversible, Red-Shifted Photoisomerization in Protonated Azobenzenes

Azobenzenes are among the best-studied molecular photoswitches and play a key role in the search for red-shifted photoresponsive materials for extended applications. Currently, most approaches deal with aromatic substitution patterns to achieve visible light application, on occasion paired with protonation to yield red-shifted absorption of the azonium species. Appropriate substitution patterns are essential to stabilize the latter approach, as conventional acids are known to induce a fast Z- to E-conversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the pK_a of the acid is low enough, yielding both the Z- and E-azonium as supported by UV–vis- and ¹H NMR spectroscopy as well as density functional theory calculations. Moreover, the steady-state protonation of para-methoxyazobenzene, specifically, yields photoisomerizable azonium ions in which the direction of switching is essentially reversed, that is, visible light produces the out-of-equilibrium Z-azonium. Although the current conditions render the visible light photoswitch unsuitable for in vivo and material application, the demonstrated understanding of simple azobenzenes paves the way for a great range of further work on this already widely studied photoswitch.

Multiple Responsiveness of Polymer Actuators

To improve photosensitivity of polymer materials, an effective protocol is to increase the content of photosensitive moieties. However, most of photosensitive units are toxic. The high content is not acceptable for real-world applications. Therefore, achievement of photosensitive polymers with low content of photosensitive moieties but maintaining their photosensitivity remains a challenge. We herein report a protocol to address this challenge by combination of photosensitive monomers with hygroscopic monomers, where the synergistic action of two types of functional moieties can improve the photosensitivity of polymer network. Upon exposure to light irradiation, the polymer can be driven by not only the structural isomerization of photosensitive units but also the photothermal effects. This synergistic effect results in the polymer-based soft actuators capable of showing rapid response to light even at the extremely-low content of photosensitive moieties of 2.6 mol%. Importantly, the combination of hygroscopic and photosensitive moieties provides polymer with multiple responsiveness including acidochromism, humidity responsiveness, photo-hardening, shape memory, photochromism and in-situ swelling, making it useful in sensing systems, information transmission and artificial muscles. This article is protected by copyright. All rights reserved.

Enhanced photoinduced mass migration in supramolecular azopolymers by H-bond driven positional constraint

Here we investigated the role of hydrogen bonding in the design of supramolecular azopolymers with a highly directional and constrained azobenzene-chain interaction involving the aromatic ring of the photoactive molecule, by exploiting the 2-aminopyrimidine/carboxylic acid supramolecular synthon as the tool for molecular recognition. We have shown that this approach is advantageous for producing affordable and versatile photopatternable azomaterials by complexation with polyacrylic acid (PAA). Molecular model complexes were successfully prepared and characterized by X-ray diffraction analysis and FTIR spectroscopy to reveal the multiple, non-ionic interaction occurring between the azobenzene units and the polymer chains. Surface photopatterning of thin films, driven by the typical mass migration phenomenon occurring in azopolymers, resulted strongly enhanced with increasing azobenzene content until equimolar composition. Results show that polymers with synthon-based azobenzenes markedly outperform single H-bonded systems bearing azomolecules with similar structure and electronic properties. We finally demonstrated that the azobenzene units can be easily extracted from a photopatterned film by a simple solvent rinse and without any chemical pre-treatment, leaving the periodicity of the inscribed surface relief gratings unaltered. This result was enabled by the orthogonal solubility of the components in the supramolecular system.

o-Azophenylboronic Acid-Based Colorimetric Sensors for d-Fructose: o-Azophenylboronic Acids with Inserted Protic Solvent Are the Key Species for a Large Color Change

Many boronic acid-based chemosensors for saccharides have been developed; however, their detection mechanisms have seldom been studied. In this study, we synthesized 10 o-azophenylboronic acid derivatives (azoBs) and conducted a fundamental study on the reactivity and the sensing mechanism of azoBs, which undergoes a large color change, e.g., from red to yellow, upon a reaction with saccharides. Their pH-independent formation constants were determined by spectrophotometric titration and then converted to the conditional formation constant K' at pH 7.4. A linear free energy relationship was established between log K' and the pK_a of azoB. ¹¹B NMR measurements indicate that in aprotic solvents, azoB forms a trigonal planar structure, while in protic solvents, it forms a quasi-tetrahedral structure (azoB-ROH) with a protic solvent molecule (ROH) inserted between the boronic acid moiety and the azo group. In addition, UV-vis spectroscopic studies showed that the color change during the reaction between azoB and d-fructose in ROH was caused by the release of the ROH from azoB-ROH by d-fructose. Based on the findings in this study, we proposed a guideline for designing an azoB-based chemosensor that exhibits high reactivity toward saccharides and a sufficient color change to allow for the visual detection of saccharides.

Spectroscopic study of the pH dependence of the optical properties of a water-soluble molecular photo-switch

In this report we present a UV-Vis spectroscopic study of the pH dependent optical properties of 4,4'-azobenzene dicarboxylic acid in aqueous solution. A combination of chemical (acid-base) and light stimuli is combined to demonstrate that the system undergoes two types of optical switching cycles. At neutral pH the azobenzene undergoes photo-induced cis-trans isomerisation. Upon pH reduction the UV-Vis spectra show changes consistent with aggregation of the azobenzene photo-switch. The photo-responsive behaviour is dependent on the pH and conformation of the azobenzene. The optical properties of the system are dependent on the mode of pH reduction and the isomeric cis/trans composition of the photo-switch, showing hierarchical control of self-assembly.

Synergy of Ionic and Dipolar Effects by Molecular Design for pH Sensing beyond the Nernstian Limit

Knowledge of interfacial interactions between analytes and functionalized sensor surfaces, from where the signal originates, is key to the development and application of electronic sensors. The present work explores the tunability of pH sensitivity by the synergy of surface charge and molecular dipole moment induced by interfacial proton interactions. This synergy is demonstrated on a silicon-nanoribbon field-effect transistor (SiNR-FET) by functionalizing the sensor surface with properly designed chromophore molecules. The chromophore molecules can interact with protons and lead to appreciable changes in interface dipole moment as well as in surface charge state. In addition, the dipole moment can be tuned not only by the substituent on the chromophore but also by the anion in the electrolyte interacting with the protonated chromophore. By designing surface molecules to enhance the surface dipole moment upon protonation, an above-Nernstian pH sensitivity is achieved on the SiNR-FET sensor. This finding may bring an innovative strategy for tailoring the sensitivity of the SiNR-FET-based pH sensor toward a wide range of applications.

Unraveling the Thermal Isomerization Mechanisms of Heteroaryl Azoswitches: Phenylazoindoles as Case Study

The research on heteroaromatic azoswitches has been blossoming in recent years due to their astonishingly broad range of properties. Minimal chemical modifications can drastically change the demeanor of these switches, regarding photophysical and (photo)chemical properties, promoting them as ideal scaffolds for a vast variety of applications based on bistable light-addressable systems. However, most of the characteristics exhibited by heteroaryl azoswitches were found empirically, and only a few works focus on their rationalization. Herein we report on a mechanistic study employing phenylazoindoles as a model reference, combining spectroscopic experiments with comprehensive computational analysis. This approach will elucidate the intrinsic correlations between the molecular structure of the switch and its thermal behavior, allowing a more rational design transferable to various heteroaryl azoswitches.

Tautomeric photoswitches: anion-assisted azo/azine-to-hydrazone photochromism

The photoswitching behaviour of isatin 4-nitrophenylhydrazones in presence of anions was investigated.

Photoswitchable and pH responsive organoplatinum(ii) complexes with azopyridine ligands

Several platinum(ii) complexes with ligands containing azo groups have been prepared and structurally characterised, and their photoswitching between trans and cis azo group isomers has been studied. The azo groups in the cationic complexes [PtMe(bipy)(4-NC₅H₄-N[double bond, length as m-dash]N-4-C₆H₄X)][PF₆], X = H, OH or NMe₂, and in the dicationic complex [Pt(bipy)(4-H₂NC₆H₄-N[double bond, length as m-dash]N-C₆H₅)₂][OTf]₂ undergo trans to cis photoswitching on irradiation at 365 nm. The complex [PtMe(bipy)(4-NC₅H₄-N[double bond, length as m-dash]N-4-C₆H₄NMe)₂][PF₆] also exhibits a reversible halochromic effect on protonation to give the dicationic complex [PtMe(bipy)(4-NC₅H₄-NH[double bond, length as m-dash]N-4-C₆H₄NMe₂]²⁺. The nature of the frontier orbitals in the platinum(ii) complexes depends on the charge on the complex and on the degree of metal-ligand π-bonding.

Water-Soluble Colorimetric Amino[bis(ethanesulfonate)] Azobenzene pH Indicators: A UV-Vis Absorption, DFT, and 1H-15N NMR Spectroscopy Study

Water-soluble azobenzene derivatives containing amino[bis(ethanesulfonate)] groups are demonstrated as colorful pH indicators in water and on filter paper. Vibrant color changes were observed from yellow/orange to pink between pH 1 and 4, which are attributed to an intramolecular charge-transfer mechanism. The pK _as of the indicators range from 2.1 to 2.6. ¹H/¹H-¹⁵N NMR studies in deuterium oxide reveal that the protonation of the azobenzene pH indicators occurs predominantly at the β-azo nitrogen atom, in agreement with the density functional theory calculations. Excellent selectivity for protons was confirmed in water over common biologically relevant metal ions. Studies in methanol, however, indicate that the pH indicator with a methoxy group ortho to the amino[bis(ethanesulfonate)] group facilitates the selective coordination of Cu²⁺ with a binding constant pβ_Cu2+ of 4.6 ± 0.1. The indicators complement the existing library of azobenzene indicator dyes and may be useful for measuring the environmental pH at higher proton concentrations.

Synthesis, spectroscopic and computational characterization of the tautomerism of pyrazoline derivatives from chalcones

In the present study a series of novel pyrazolines derivatives has been synthesized, and their structures assigned on the basis of FT-Raman, (1)H and (13)C NMR spectral data and computational DFT calculations. A joint computational study using B3LYP/6-311G(2d,2p) density functional theory and FT-Raman investigation on the tautomerism of 3-(4-substituted-phenyl)-4,5-dihydro-5-(4-substituted-phenyl)pyrazole-1-carbothioamide and 3-(4-substituted-phenyl)-4,5-dihydro-5-(4-substituted-phenyl)pyrazole-1-carboxamide are presented. The structures were characterized as a minimum in the potential energy surface using DFT. The calculated Raman and NMR spectra were of such remarkable agreement to the experimental results that the equilibrium between tautomeric forms has been discussed in detail. Our study suggests the existence of tautomers, the carboxamide/carbothioamide group may tautomerize, in the solid state or in solution. Thermodynamic data calculated suggests that the R(CS)NH2 and R(CO)NH2 species are more stable than the R(CNH)SH and R(CNH)OH species. Additionally, results found for the (1)H NMR shifting, pointed out to which structure is present.

Vibrational study of adsorption of Congo red onto TiO2 and the LSPR effect on its photocatalytic degradation process

Chemisorption of Congo red on a P25 yielded hydrazone tautomer caused distinctions on the photodegradation preference in comparison to another TiO₂ catalyst based on anatase. Photocatalytic degradation in the presence of Ag NPs demonstrated considerable improvement on kinetic parameters due to the near field enhancement.

Thermally stable water insoluble azo-azomethine dyes: synthesis, characterization and solvatochromic properties

Six new water insoluble azo-azomethine dyes have been synthesized via condensation reaction of α,α'-bis(o-aminophenylthio)-1,2-xylene with substituted azo-coupled salicylaldehyde. The condensation reaction provides the expected bis-iminated azo-azomethine dyes in good yields, ranging from 59% to 90%. The dyes have been characterized by IR, UV-Vis and (1)H NMR spectroscopic methods as well as elemental analysis. The thermal behavior of the prepared dyes has been determined using thermogravimetry technique. Furthermore, the effect of various organic solvents with different polarities on the UV-Vis spectra of the dyes has been also studied.

Light-controlled real time information transmitting systems based on nanosecond thermally-isomerising amino-azopyridinium salts

Aminoazopyridines are valuable molecules for stable information transmitting systems as well as for light-controlled optical oscillators. Amino-substituted azopyridinium methyl iodide salts transmit optical information within the time scale of nanoseconds, and moreover, show oscillation frequencies up to 1 MHz at room temperature.

A novel mechanism for azoreduction

Azoreductases are important due to their ability to activate anti-inflammatory azo pro-drugs and to detoxify azo dyes. Three genes encoding azoreductases have been identified in Pseudomonas aeruginosa. We describe here a comparison of the three enzymes. The pure recombinant proteins each have a distinct substrate specificity profile against a range of azo substrates. Using the structure of P. aeruginosa azoreductase (paAzoR) 1 and the homology models of paAzoR2 and paAzoR3, we have identified residues important for substrate specificity. We have defined a novel flavin mononucleotide binding cradle, which is a recurrent motif in many flavodoxin-like proteins. A novel structure of paAzoR1 with the azo pro-drug balsalazide bound within the active site was determined by X-ray crystallography and demonstrates that the substrate is present in a hydrazone tautomer conformation. We propose that the structure with balsalazide bound represents an enzyme intermediate and, together with the flavin mononucleotide binding cradle, we propose a novel catalytic mechanism.

pH-Dependent cis --> trans isomerization rates for azobenzene dyes in aqueous solution

Azobenzenes can function as molecular switches driven by their unusual cis <--> trans photoisomerization properties. The stability of an azobenzene-based switch depends on its rate of thermal relaxation, which is known to depend on the solvent environment, but few kinetic studies in aqueous media have been reported. We use nanosecond UV laser flash photolysis-transient absorption spectroscopy to measure thermal cis --> trans isomerization rates for mono- and disubstituted p-aminoazobenzenes and p-hydroxyazobenzenes in water at 23 degrees C over the pH range of 4 to 11. Observed absorption transients are fit to first-order relaxation rate constants between 10(5) and 10(1) s(-1), which is generally much faster than in nonpolar solvents, and the relaxation rates vary systematically and predictably with pH as the equilibrium shifts to ionized forms of the dyes that isomerize much more rapidly. Acid ionization constants for these dyes determined from our kinetic mechanism are compared with the pH dependence of their equilibrium UV-vis spectra. New kinetics results may enable pH control of azobenzene-based molecular switching times.

Benzothiazolylazo derivatives of some β-dicarbonyl compounds and their Cu(II), Ni(II) and Zn(II) complexes

The coupling of diazotized 2-aminobenzothiazole with 1,3- dicarbonyl compounds (benzoylacetone, methylacetoacetate and acetoacetanilide) yielded a new series of tridentate ligand systems (HL). Analytical, IR, 1H-NMR, 13C-NMR and mass spectral data indicated that the compounds exist in the intramolecularly hydrogen bonded azo-enol tautomeric form in which one of the carbonyl groups of the dicarbonyl moiety had enolised and hydrogen bonded to one of the azo nitrogen atoms. The compounds formed stable complexes with Ni(II), Cu(II) and Zn(II) ions. The Cu(II) complexes conform to [CuL(OAc)] stoichiometry while the Ni(II) and Zn(II) complexes are in agreement with [ML2] stoichiometry. Analytical, IR, 1H-NMR, 13C-NMR and mass spectral data of the complexes are consistent with the replacement of the chelated enol proton of the ligand with a metal ion, thus leading to a stable sixmembered chelate ring involving a cyclic nitrogen, one of the azo nitrogens and the enolate oxygen. The Zn(II) chelates are diamagnetic while Cu(II) and Ni(II) complexes showed a normal paramagnetic moment.