Azo Form Research Articles

Structure Determination from Powder X-ray Diffraction Data of Black Azo (Hydrazone) Pigments

Abstract Crystal structures of two black monoazo (hydrazone) pigments have been determined from powder X-ray diffraction data combined with DFT calculations. The DFT calculations suggested the molecules are in hydrazone forms but not in azo forms in both crystal structures. The molecular arrangements in the crystal structures suggested Davydov splitting may occur by excitonic interactions and it causes the red absorption band shift in the crystalline state leading to the characteristic black colors.

Phenylazoindole dyes 3: Determination of azo-hydrazone tautomers of new phenylazoindole dyes in solution and solid state

A new two series of phenylazo indole dyes was synthesized and the structures of the dyes were confirmed by UV–vis, FT-IR, HRMS and 1H/13C NMR spectroscopic techniques. Five of these dyes (I, I′, II′, III and III′) were also characterized in solid state by using single crystal X-ray diffraction studies besides other spectroscopic techniques. The geometries of the azo and hydrazone tautomeric forms of the dyes were optimized by using Density Functional Theory (DFT). In addition, the effects of the donor and acceptor groups on the azo and hydrazone forms of the dyes were evaluated experimentally and theoretically. The results indicate that the phenylazoindole dyes derived from 2-phenyl indole as coupling component exist as azo form in solution, gas phase and solid state.

A theoretical study on tautomeric structures of 4′-nitroazobenzene-2,4-diol and 4-methyl-4′-nitroazobenzene-2,6-diol compounds

The goal of this study is to determine the most stable tautomeric forms, and their ground state conformers of 4′-nitroazobenzene-2,4-diol and 4-methyl-4′-nitroazobenzene-2,6-diol compounds. The calculations have shown that the most stable tautomeric forms of the compounds are hydrazo form for 4′-nitroazobenzene-2,4-diol and azo form for 4-methyl-4′-nitroazobenzene-2,6-diol. Besides, the vibrational frequencies, 1H and 13C NMR shifts, frontier molecular orbital’s energies for the tautomeric forms of the compounds calculated by using density functional theory-B3LYP method with 6-311G(d) basis set were interpreted. All the assignments of the theoretical frequencies were identified by potential energy distribution (PED) analysis. Generally, theoretical spectral results were seen to be in a good agreement with the corresponding experimental data.

Anion induced azo-hydrazone tautomerism for the selective colorimetric sensing of fluoride ion

The design, synthesis, characterization and their anion sensing properties of two receptors capable of exhibiting azo-hydrazone tautomerism are reported. The anion sensing properties have been investigated using electronic, fluorescence and nuclear magnetic spectral studies in addition to electrochemical and visual detection experiments. Both the receptors selectively bind fluoride ion with>100nm red-shift in the electronic spectrum and the color changes from yellow to red. The results of the spectral studies revealed that the sensing mechanism involves fluoride ion induced change of chromophore from CN (hydrazone form) to NN (azo form) in these receptors leading to the visible color change. Density Functional Theory calculations were conducted to rationalize the optical response of the receptors.

Cooperative intramolecular hydrogen bonding induced azo-hydrazone tautomerism of azopyrrole: Crystallographic and spectroscopic studies

The azo-hydrazone tautomerism of three azopyrrole compounds, which were synthesized by the reactions of 2-acylbenzenediazonium salt with pyrrole and meso-diethyl-2,2′-dipyrromethane, have been studied. In the solid state, 2-(2-acylphenyl)diazopyrrole adopted an azo tautomeric form, whereas 5,5′-bis(2-acylphenyl)diazo-dipyrromethane and 5-(2-acylphenyl)diazo-dipyrromethane crystallized in the hydrazone form. In polar solution, all of the compounds mainly adopt an azo form. In an apolar solution, however, 5,5′-bis(2-acylphenyl)diazo-dipyrromethane and 5-(2-acylphenyl)diazo-dipyrromethane mainly adopt hydrazone form. It is cooperative intramolecular hydrogen bonds that influence the azopyrrole tautomerisation from the azo to the hydrazone form.

Theoretical study of molecular structures, vibrational and NMR spectra on azobenzene derivatives

In this study, we have calculated the most stable tautomeric forms and their ground state conformers of 2,4-dihydroxyazobenzene and 2,4-dihydroxy-6-methyl-4′-nitroazobenzene molecules. Calculations show that the most preferential tautomeric forms of these molecules are azo form for 2,4-dihydroxyazobenzene and hydrazo form for 2,4-dihydroxy-6-methyl-4′-nitroazobenzene. Vibrational frequencies, 1H and 13C NMR shifts of ground state conformers of stable tautomeric forms of the molecules have been calculated by using density functional theory-B3LYP method with 6-311G(d,p) basis set. All assignments of theoretical frequencies have been performed by potential energy distribution analysis. Calculated spectral results are in a good agreement with the corresponding experimental data.

Investigation of the azo-hydrazone tautomeric equilibrium in an azo dye involving the naphthalene moiety by UV–vis spectroscopy and quantum chemistry

Photophysical properties of the azo-hydrazone tautomerism of Eriochrome Blue Black B (1-(1-hydroxy-2-naphthylazo)-2-naphthol-4-sulphonic acid) in DMF, MeCN and water were investigated using UV–visible spectroscopy and quantum chemical calculations. The optimized molecular structure parameters, relative energies, mole fractions, electronic absorption spectra and HOMO–LUMO energies for possible stable tautomeric forms of EBB were theoretically calculated by using hybrid density functional theory, (B3LYP) methods with 6-31G(d) basis set level and polarizable continuum model (PCM) for solvation effect. The effects of varying pH-, dye concentration-, solvent-, temperature-, and time-dependences on the UV–vis spectra of Eriochrome Blue Black B were also investigated experimentally. The calculations showed that the dye exhibited acid–base, azo-hydrazone and aggregate equilibria in DMF solution, while the most probably preferred form in MeCN solution was azo form. Thermodynamic parameters of dimerization reaction in DMF solution proved that entropy was the driving force of this reaction.

Synthesis and characterization of novel yellow azo dyes from 2-morpholin-4-yl-1,3-thiazol-4(5H)-one and study of their azo–hydrazone tautomerism

Novel yellow azo dyes were synthesized by diazotization of aromatic amines followed by coupling with 2-morpholin-4-yl-1,3-thiazol-4(5H)-one and fully characterized. The geometries of the synthesized dyes for azo and hydrazone tautomeric forms were optimized using B3LYP, CAM-B3LYP and M06 functional and 6-31G(d) and 6-311++G (d,p) basis sets, also their electronic excitation properties were evaluated using density functional theory. The optimized geometries reveal that the hydrazone for is more stable than the azo form. Photophysical properties of the synthesized dyes were evaluated by UV–Visible spectroscopy and compared with computed vertical excitation obtained from TDDFT. The results clearly illustrate existence of dye 8a, 8b and 8c in hydrazone tautomeric form, while 8d exist in both azo as well as hydrazone form. Thermal stabilities were estimated by using thermo gravimetric analysis, and results revels that the synthesized dyes have good thermal stability.

Post-translational Modifications of Recombinant Human Lysyl Oxidase-like 2 (rhLOXL2) Secreted from Drosophila S2 Cells*

Human lysyl oxidase-like 2 (hLOXL2) is highly up-regulated in metastatic breast cancer cells and tissues and induces epithelial-to-mesenchymal transition, the first step of metastasis/invasion. hloxl2 encodes four N-terminal scavenger receptor cysteine-rich domains and the highly conserved C-terminal lysyl oxidase (LOX) catalytic domain. Here, we assessed the extent of the post-translational modifications of hLOXL2 using truncated recombinant proteins produced in Drosophila S2 cells. The recombinant proteins are soluble, in contrast to LOX, which is consistently reported to require 2-6 m urea for solubilization. The recombinant proteins also show activity in tropoelastin oxidation. After phenylhydrazine derivatization and trypsin digestion, we used mass spectrometry to identify peptides containing the derivatized lysine tyrosylquinone cross-link at Lys-653 and Tyr-689, as well as N-linked glycans at Asn-455 and Asn-644. Disruption of N-glycosylation by site-directed mutagenesis or tunicamycin treatment completely inhibited secretion so that only small quantities of inclusion bodies were detected. The N-glycosylation site at Asn-644 in the LOX catalytic domain is not conserved in human LOX (hLOX), although the LOX catalytic domain of hLOX shares ∼50% identity and ∼70% homology with hLOXL2. The catalytic domain of hLOX was not secreted from S2 cells using the same expression system. These results suggest that the N-glycan at Asn-644 of hLOXL2 enhances the solubility and stability of the LOX catalytic domain.

Hyperchromisity and Molecular Recognition of a Novel Modified β-Cyclodextrin Tethering with Phenylaminoazobenzene

A modified β-cyclodextrin (1) tethered with a phenylaminoazobenzen moiety has been synthesized for the purpose of developing a new guest-responsive color-change chemosensor. When the solution became acidic, 1 changed color from yellow to purple due to a structural change of the dye moiety from the azo form to the azonium one by protonation on the azo group. However, the pH titration curve of 1 was affected by the presence of the guest, as shown by the fact that the curve in the acidic region shifted to the neutral side by the presence of 1-adamantanol. This suggests that 1 has a potential to show a guest-induced absorption variation under the acidic condition. Upon guest addition at pH 0.61 at the concentration of 1 with 1 × 10(-5) M, 1 showed a colorless-to-color change. Such hyperchromisity in the absorption spectrum of 1 is associated with a guest-induced conformational change of the dye moiety from inside to outside of the CD cavity, and is used for molecular sensing. Among various guests, 1 was found to be highly sensitive to a steroidal guest having a C12 hydroxyl group in the framework, such as ursodeoxycholic acid and lithocholic acid in an acidic aqueous solution.

Molecular-Level Insights into N–N π-Bond Rotation in the pH-Induced Interfacial Isomerization of 5-Octadecyloxy-2-(2-pyridylazo)phenol Monolayer Investigated by Sum Frequency Generation Vibrational Spectroscopy

In-situ and real-time characterization of molecular structure of pH stimuli-responsive assembling systems at interfaces is critical to understand the nature of interfacial driving force and weak molecular interaction behind such reactions and provide important clues to control them in a desired manner. In this study, sum frequency generation vibrational spectroscopy (SFG-VS) has been applied, supplemented by surface pressure (π)–area (A) isotherm measurements, and Brewster angle microscopy images, to investigate the interfacial tautomerism and isomerization reactions occurring in 5-octadecyloxy-2-(2-pyridylazo)phenol (PARC18) monolayer at air/buffer solution interface in situ. The isomerization mechanism was examined by measuring interfacial structure of PARC18 molecule at various subphase pH. Time-dependent change of the SFG intensity of the characteristic band was kinetically measured after spreading PARC18 chloroform solution onto different subphase pH buffer solutions. It was found that hydrazone form prevails on the air/water interface in acidic and neutral conditions while azo form dominates at subphase pH ≥ 11.6. The hydrazone form adopts a planar geometry at pH = 4.5 and 7.0, whereas the azo form adopts a nonplanar cis or cis-like conformation. It was indicated that the trans–cis isomerization processes follow a rotation mechanism. The deprotonation rate constant was deduced to be 0.20–0.42 M–1 s–1 at pH = 10.3–12.6. This is the first reported application of SFG-VS to elucidate the isomerization mechanism and deduce the deprotonation rate constant of azoaromatic compounds at interface. Resulting from this study will aid in a better understanding of the interfacial pH-controlled assembly processes.

New arylhydrazones of β-diketones and their optical and thermal properties

New arylhydrazones of β-diketones, 2-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl) benzoic acid (1) and (Z)-ethyl-2-(2-(2-nitrophenyl)hydrazono)-3-oxobutanoate (2), have been synthesized by reaction of activated β-diketones with the corresponding aryldiazonium salts, and characterized by IR, 1H and 13C NMR spectroscopies and elemental analysis. In DMSO solution, 1 is stable exclusively in the hydrazone form, while 2 exists in DMSO and H2O solutions as a mixture of enol–azo and hydrazone tautomeric forms. In the latter case, an increase in the solvent polarity shifts the tautomeric balance to the enol–azo form. Solvatochromic, thermal and photoluminescent properties of 1, 2 and the known related 2-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)benzoic acid (3), (2-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)phenyl)arsonic acid (4), (Z)-2-(2-(1,3-dioxo-1-phenylbutan-2-ylidene)hydrazinyl)benzoic acid (5) and 5-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)-2,3-dihydrophthalazine-1,4-dione (6) were studied. The obtained UV–vis absorption spectra confirm that εmах depends on the solvent and on the nature of AHBD, but no clear correlation was found. Upon heating, 1–3, 5 and 6 are stable up to 474K and decompose in two steps whereas 4 decomposes in three steps starting from 343K. The kinetic parameters of the thermal decomposition were estimated suggesting a low rate of the thermal decomposition. Compounds 2 and 3 exhibit luminescence with intense emission bands with maxima at λem of 466.0 (λex=318nm) and 532.5nm (λex=315nm), respectively, while compounds 4 and 6 show low-intensity bands at λem of 501 (λex=396nm) and 503nm (λex=389nm). Compounds 1 and 5 display no photoluminescence.

Synthesis, Characterization, and Tautomeric Properties of Some Azo-azomethine Compounds

The primary azo compound 1-(3-formyl-4-hydroxyphenylazo)-4-nitrobenzene reacts with some aliphatic and aromatic diamines and yields the corresponding azo-azomethine compounds. These compounds were characterized by elemental analysis, IR, UV/Vis, and NMR spectroscopy. The primary azo compound exists entirely in the azo form in solution as well as in the solid phase. The tautomeric structure of azo-azomethine compounds heavily depends on the solvent and the substituents. Aliphatic diamine-based compounds favor the enol-imine tautomer while aromatic diamine-based compounds have structures that lie between the two enol-imine and keto-amine tautomers due to a relatively strong intramolecular hydrogen bond. The compounds exhibit positive solvatochromism (bathochromic shift) so that their absorption bands move toward longer wavelengths as the polarity of the solvents increases. In addition, UV/Vis spectrophotometry has shown that the studied compounds have molar extinction coefficients larger than 40000.

Azo-hydrazone tautomerism observed from UV-vis spectra by pH control and metal-ion complexation for two heterocyclic disperse yellow dyes

The azo-hydrazone tautomerism of two pyridine-2,6-dione based Disperse Yellow dyes has been achieved by pH control and metal-ion complexation, respectively, which is evidenced by UV-visible spectra using pH-titration, (1)H NMR and X-ray single-crystal diffraction techniques for two dyes and one neutral dinuclear dye-metal complex. pH-titration experiments under strong and weak acidic conditions (HCl and HOAc) as well as strong and weak alkaline conditions (NaOH and ammonia) demonstrate that there is an equilibrium between the azo (HL(1-A) and HL(2-A)) and hydrazone (HL(1-H) and HL(2-H)) tautomers for two dyes in solution but the hydrazone form is dominant under conventional conditions. The hydrazone proton is also observed in the (1)H NMR spectra of HL(1-H) and HL(2-H) which can be verified by the hydrogen-deuterium exchange and the presence of cooperative six-membered intramolecular hydrogen rings involving the hydrazone proton in their X-ray single-crystal structures. Moreover, the azo-hydrazone tautomerism is evidenced by the formation of a novel neutral dinuclear dye-metal complex Cu(2)(L(2-A))(4), where all the ligands are in the azo form and two types of coordination modes are present for four L(2-A) ligands. Namely, the side two ligands serve as the bidentate capping ligands, while the middle ones act as the quadridentate bridging ligands linking adjacent Cu(II) centers in a reverse fashion.

Tautomeric behavior of some azoquinoline dyes in liquid and liquid crystalline media

The absorption spectra of three azoquinolin-8-ol derivatives ( o-, m-, p-cyano hydroxy azoquinolin dyes) were investigated in liquid and liquid crystalline solutions as a function of the solvent polarity. The spectral data of the dyes were compared in both ordinary liquid solvents and liquid crystalline media. Analysis of the spectral data was used to determine the azo and hydrazone forms in both the environments. The spectral shifts were correlated by Kamlet–Taft and Katritzky multi-parameter polarity scales. For the azoquinoline dyes, the azo form is almost entirely dominated in polar anisotropic hosts. In contrast, the compounds remain dominantly in hydrazone form in some polar solvents such as DMF. The polarized absorption spectra of the compounds in the anisotropic media were measured and their degree of anisotropies was determined.

Synthesis, characterization and crystal structures of 2-[(E)-2-(4-hydroxy-3,5-dimethylphenyl)-1-diazenyl]benzoic acid and its polymeric and monomeric triorganotin(IV) complexes

Three triorganotin(IV) complexes of composition R 3SnLH (R = Me, Bu and Ph and LH = 2-[(E)-2-(4-hydroxy-3,5-dimethylphenyl)-1-diazenyl]benzoate) have been synthesized and characterized by 1H, 13C, 119Sn NMR, and IR spectroscopic techniques in combination with elemental analysis. The crystal structures of the carboxylate ligand HO 2CC 6H 4{N N(C 6H 2-4-OH-3,5-(CH 3) 2)}- o in its neutral form and three triorganotin(IV) complexes, viz., polymeric (R 3Sn[O 2CC 6H 4{N–N(H)(C 6H 2-4-O-3,5-(CH 3) 2)}- o]) n (R = Me ( 1) and Bu ( 2)) and monomeric Ph 3Sn[O 2CC 6H 4{N–N(H)(C 6H 2-4-O-3,5-(CH 3) 2)}- o] ( 3) complexes are reported. The polymeric complexes 1 and 2 exist as extended chains in which the LH-bridged Sn-atoms adopt a trans-R 3SnO 2 trigonal bipyramidal configuration with R groups in the equatorial positions and the axial sites occupied by an oxygen atom from the carboxylate ligand and the phenoxide O atom of the next carboxylate ligand. The Sn atom in complex 3 has a distorted tetrahedral geometry. In all three complexes, the carboxylate ligand is in the zwitterionic form with the phenolic proton moved to the nearby azo nitrogen atom, in contrast to the free carboxylic acid ligand which is in the azo form.

Effect of Solvents and pH on β-Cyclodextrin Inclusion Complexation of 2,4-Dihydroxyazobenzene and 4-Hydroxyazobenzene

The spectral characteristics of 2,4-dihydroxyazobenzene (DHAB, sudan orange G) and 4-hydroxyazobenzene (HAB) have been studied in various solvents, different hydrogen ion and β-cyclodextrin (β-CD) concentrations, and are compared with azobenzene (AB). The inclusion complexes of the above molecules with β-CD were analyzed by UV-vis spectrometry, flourometry, FT-IR, 1H NMR, SEM and DFT methods. The solvent study shows that only the azo form is present in DHAB and HAB molecules. The unusually large red shift observed in acidic solutions indicates both molecules exhibit azo-hydrazo tautomerization. In the β-CD solutions, the increase in fluorescence intensity and large bathochromic shift in the S1 state indicates that DHAB and HAB form 2:2 inclusion complexes, whereas AB forms a 1:1 inclusion complex.

Azo or hydrazone structure in azo pigments

Azo dyes and pigments are typical of the classical pigments as characterized by the azo group (−N=N−). However, there are still controversial discussions on the azo form whether it exists as the azo form in the solid state or in the form of hydrazone structure (=N−NH−). We have tackled this problem from the aspects of X-ray structure analysis and the chromophoric theory in an attempt to construct a model that satisfies these requirements, using methyl orange derivative (MOH), Pigment Red 3 (PR3), and Pigment Yellow 3 (PY3). Then, we found a protonated azo structure (−N=N+H−) in these compounds as caused by NH…O “inter”- molecular hydrogen bonds in MOH, as well as by “intra”-molecular hydrogen bonds in PR3 and PY3. The present protonated azo model is borne out by the crystallographic azo bond-length as well as the chromophoric theory. These results lead us to conclude that the protonated azo structure is the right form in the solid state of hydrogen-bonded azo pigments as described above.

Spectroscopic and Theoretical Study of the “Azo”-Dye E124 in Condensate Phase: Evidence of a Dominant Hydrazo Form

Spectroscopic techniques, including Raman, IR, UV/vis, and NMR were used to characterize the samples of the azo dye Ponceau 4R (also known as E124, New Coccine; Cochineal Red; C.I. no. 16255; Food Red No. 102), which is 1,3-naphthalenedisulfonic acid, 7-hydroxy-8-[(4-sulfo-1-naphthalenyl) azo] trisodium salt in aqueous solution and solid state. In addition, first principle calculations were carried out for the azo (OH) and hydrazo (NH) tautomers in order to assist in the assignment of the experimental data. The two intense bands observed in the UV/vis spectrum, centered at 332 and 507 nm, can be compared to the calculated values at 296 and 474 nm for azo and 315 and 500 nm for hydrazo isomer, with the latter in closer agreement to the experiment. The Raman spectrum is quite sensitive to tautomeric equilibrium; in solid state and aqueous solution, three bands were observed around 1574, 1515, and 1364 cm(-1), assigned to mixed modes including deltaNH + betaCH + nuCC, deltaNH + nuC horizontal lineO + nuC horizontal lineN + betaCH and nuCC vibrations, respectively. These assignments are predicted only for the NH species centered at 1606, 1554, and 1375 cm(-1). The calculated Raman spectrum for the azo (OH) tautomer showed two strong bands at 1468 (nuN = N + deltaOH) and 1324 cm(-1) (nuCC + nuC-N), which were not obtained experimentally. The (13)C NMR spectrum showed a very characteristic peak at 192 ppm assigned to the carbon bound to oxygen in the naphthol ring; the predicted values were 165 ppm for OH and 187 for NH isomer, supporting once again the predominance of NH species in solution. Therefore, all of the experimental and theoretical results strongly suggest the food dye Ponceau 4R or E124 has a major contribution of the hydrazo structure instead of the azo form as the most abundant in condensate phase.

Synthesis, spectroscopic, and molecular structure characterizations of some azo derivatives of 2-hydroxyacetophenone

Some novel azo compounds were prepared by the reaction of 2-hydroxyacetophenone with aniline and its substituted derivatives. The structures of synthesized azo compounds were determined by IR, UV–Vis, 1H NMR and 13C NMR spectroscopic techniques and the structures of some of these compounds were also determined by X-ray diffraction studies. Structural analysis using IR in solid state shows that the azo form is favoured in the azo compounds whereas UV–Vis analysis of the azo compounds in solution has shown that there is a azo and ionic form. The azo compounds in the basic solvents dimethylformamide (DMF) and dimethylsulfoxide (DMSO) are both azo and ionic form while these compounds in ethyl alcohol (EtOH) and chloroform (CHCl 3) are only azo form.