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Journal of Molecular Structure 1146 (2017) 50e56

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Synthesis, spectroscopic and structural studies of new azo dyes metal chelates derivated from 1-phenil-azo-2-naphthol Gilson Rodrigues Ferreira a, b, *, Luiz Fernando C. de Oliveira b SUPREMA e Faculdade de Ci^ encias M edicas e da Saúde de Juiz de Fora, BR040, Km 796, Salvaterra, 36033-005, Juiz de Fora, MG, Brazil NEEM - Núcleo de Espectroscopia e Estrutura Molecular, Departamento de Química, Instituto de Ci^ encias Exatas, Universidade Federal de Juiz de Fora, Campus Martelos, 36036-330, Juiz de Fora, MG, Brazil a

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a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 September 2016 Received in revised form 24 May 2017 Accepted 25 May 2017 Available online 29 May 2017

In this study, experimental techniques such as Raman and infrared vibrational analysis and X-ray crystal diffraction were used to characterize three new azo chelate dyes derived from 1-phenyl-azo-2-naphthol (Sudan I) and its analogue 1-(xylylphenylazo)-2-naphthol (Sudan II) with metal ions. The Raman and infrared spectroscopic analysis have also provided useful information concerning the coordination and formation of the molecular complexes through their main bands. In the vibrational spectra, the fingerprint bands, such as the ones at 1369/1368/1359 cm1 assigned to [n(CC) þ d(CH)], n(C]N)], 1351/1352/ 1338 cm1 assigned to [d(CH)] and 816/824/813 cm1 assigned to [u(CH)] respectively for the SD1Cu, SD1Co and SD2Ni, can be used to characterize such compounds. © 2017 Published by Elsevier B.V.

Keywords: Azo dyes complexes Supramolecular chemistry Spectroscopy analysis and theoretical simulation

1. Introduction Azo compounds are an interesting class of organic compounds which have found wide application in the field of chemicals sciences. Dyes are also found in food, pharmaceutical, paints, polymers, and standard materials used in adsorption chromatography [1e3]. Another interesting aspect of the chemistry of azo dyes resides on the possibility of the tautomeric equilibrium between form azo/hydrazo [4e11]. In recent years, several studies have been developed on the synthesis and spectral properties of azo dyes [12,13]. So many works try to take advantage of the presence of the bridge azo (eN]Ne) and form new compounds with transition metal ions with the group (OH) [14e18]. These new compounds can have application since dyes, the potential drugs, are used in wastewater and nonlinear optical elements [12,19,20]. Azo metal chelate dyes have also been developed in view of their excellence in sensitivity and stability as optical recording medium [21,22]. The azo metal chelate compounds are a very important class of chemical compounds receiving attention in scientific research [23e28]. They are highly colored and have been used as dyes and pigments for a long time. Furthermore, they have

^ncias Me dicas e da Saúde * Corresponding author. SUPREMA e Faculdade de Cie de Juiz de Fora, BR040, Km 796, Salvaterra, 36033-005, Juiz de Fora, MG, Brazil. E-mail address: [email protected] (G.R. Ferreira). http://dx.doi.org/10.1016/j.molstruc.2017.05.121 0022-2860/© 2017 Published by Elsevier B.V.

been widely studied because of their excellent thermal and optical properties in applications such as toner, inkjet printing, and dye soluble in oil lightfast [21e23]. Recently, azo metal chelates have also attracted increasing attention due to their interesting electronic and geometrical features in connection with their application for molecular memory storage, nonlinear optical properties, and printing systems [23]. The variety of colors created by metal complexes dyes is large and several metal ions are commonly used including cobalt, cooper, chromium and nickel ions [28,29]. Another advantage for complexes that involved azo metal chelate dyes with transition metals is the possibility of new compounds with biological activity [29,30]. Transition metals have been used in treatment for several diseases, metal complexes that are capable of cleaving DNA under physiological conditions are of interest in the development of metal-based anticancer agents [31e33]. The searching of new strategies to develop more effective and specific target drugs has been motivation to inorganic chemists in new studies involving azo dyes [19,34e36]. As for the stability, absorption of light, electronic conjugation, thermal and magnetic properties of azo dyes can be increased by the simple coordination with metal ions, regardless of the form in question azo or hydrazo advantage due just to the site of coordination [30,37,38]. With this proposal, complexes of azo dyes with lanthanide in particular with gadolinium have been synthesized for use as contrast agents in magnetic resonance imaging [39]. Derived from of 1-phenyl-azo-2-naphthol have been synthesized aiming at

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the formation of new dyes or support for understanding of supramolecular interactions involving the chemistry of these systems [40,41]. Several metal complexes containing azo dyes ligands present aromatic sites with different groups which provide the formation of supramolecular systems presenting weak interactions such as p/p, CH/p, or even anion/p; all such interactions may have a crucial role in auto-assembly and recognition of the solid state structure. Hydrogen bonding interactions are the most reliable and widely used means of enforcing the molecular recognition of x-ray crystal structures, as it can be seen in literature [42e49]. In spite of the interest in the chemistry of azo dyes being largely and well explored in the literature on several aspects such as: variety of colors, tautomeric equilibrium between azo and hydrazo forms, toxicity of these compounds and good sites coordination, only few complexes between transition metals ions derived from 1phenyl-azo-2-naphthol were reported. In this work, we are presenting the synthesis of three new complexes of azo dyes coordinated with metallic cobalt, nickel and copper ion. Techniques such as Raman, UV/vis, infrared spectroscopy analysis, and X-ray Diffraction (single and polycrystal) were used in the structural characterization of these new compounds. 2. Experimental section 2.1. Chemical and reagents All azo dyes were purchased from Aldrich and the purity of 97% for Sudan I and 95% for the Sudan II were used without any further treatment and the solvents used were spectroscopic grade. We used these metal ions: cobalt chloride hexahydrate, copper chloride and nickel chloride hexahydrate purchased from Merck. 2.2. Synthesis 2.2.1. Synthesis of tris-1-(phenylazo)-2-naphtholatecobalt(III) (SD1Co) The synthesis for obtaining the metal complex tris-1-(phenylazo)-2-naphtholatecobalt(III) Scheme 1, was carried out by the addition of the 0.248 g of 1-phenyl-azo-2-naphthol dissolved in methanol, 60 ml and slowly adding 0.106 g of Na2CO3 in magnetic agitation and then 0.237 g of cobalt chloride hexahydrate dissolved in deionized water. The reaction occurred under reflux at 60 C for about 24 h, affording a black solid which was solubilized with dichloromethane and impurities extracted from mixing water and dichloromethane several times. The solid was filtered and washed with deionized water, and then drying the resulting complex was weighed 62% yield. The elemental analysis: Calculated (C48H33CoN6O3): C (72.00%), H (4.15%), N (10.50%). Found: C (69.49%), H (4.34%), N (9.76%) and the value measurements for 2q through powder diffraction are: (7.62 ; 9.84 ; 10.54 ; 14.18 ; 15.29 ). 2.2.2. Synthesis of bis-1-(phenyl-azo)-2-naphtholatecopper(II) (SD1Cu) Similarly, the synthesis was carried out to obtain the complex bis-1-(pheny-l-azo)-2-naphtholatecopper(II) used to be 0.248 g of 1-phenyl-azo-2-naphthol solubilized in 30 ml of methanol and 30 ml of dichloromethane, slowly adding 0.106 g of Na2CO3 in magnetic agitation and then 0.134 g of copper chloride obtained after 24 h in reflux at 60  C a precipitate of red color which was solubilized with dichloromethane and impurities extracted with water several times. The solid was filtered, purified and dried producing a product with 67% yield. The elemental analysis: Calculated (C32H22CuN4O2): C (61.67%), H (3.73%), N (8.72%). Found: C (59.67%), H (3.77%), N (8.51%) and the value measurements for 2q

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through powder diffraction are: (6.71 ; 7.59 ; 10.15 ; 15.51 ; 16.67 ; 23.42 ; 27.61 ). 2.2.3. Synthesis of bis-1-(2,4-xylylazo)-2-naphtholatenickel(II) (SD2Ni) To obtain the complex bis-1-(2,4-xylylazo)-2-naphtholatenickel(II) was used 0.276 g of 1-(2,4-xylylazo)-2-naphthol solubilized in 30 ml of methanol and 30 ml of dichloromethane, slowly adding 0.106 g of Na2CO3 in magnetic agitation and then 0.237 g of nickel chloride hexahydrate, after 24 h in reflux at 60  C a precipitate of brown color was obtained, solubilized with dichloromethane and impurities extracted with water several times. The solid was filtered, dried and purified producing a product with a yield of 78%. The elemental analysis: Calculated (C32H20NiN6O6): C (59.75%), H (3.13%), N (13.07%). Found: C (58.07%), H (3.04%), N (12.96%) and the value measurements for 2q through powder diffraction are: (9.80 ; 11.13 , 12.27 ; 18.15 ; 21.55 ). 2.3. Physical measurements 2.3.1. Experimental methods Raman spectra were obtained using a Bruker RFS 100 FT-Raman instrument equipped with germanium detector refrigerated by liquid nitrogen, with excitation at 1064 nm from a Nd:YAG laser, power 30 mW for sample in solid phase, in the range between 1800 and 200 cm1, and spectral resolution of 4 cm1, with an average of 1000 scans. All spectra were obtained at least twice to reproduce position and intensity of all the observed bands. Infrared (IR) spectra were recorded in an Alpha Bruker FT-IR spectrometer, in the region 1800e400 cm1 of the sample supported at KBr pellet, with 4 cm1 of spectral resolution, and average of 500 scans. The X-ray powder diffraction (XRP) experiments were obtained using Bruker D8 Advance diffractometer operating at 40 kV and 30 mA e X-ray 2q/q. The powder diffraction patterns were obtained with BreggBretano geometry using KaCu radiation (l ¼ 1.5406 Å). The scanning mode was taken from the 2q range of 5e65 with of 0.02 . The Single crystal X-ray data were collected using an Oxford GEMINI A Ultra diffractometer with mCuKa (l ¼ 1.547 Å) at room temperature (150 K) for tris-1-(pheny-l-azo)-2-naphtholatecobalt(III). Data collection, reduction and cell refinement were performed by CrysAlis RED, Oxford diffraction Ltd [50], Version 1.171.32.38. The structures were determined and refined using SHELXL-97 [51]. The empirical isotropic extinction parameter x was refined according to the method previously described by Larson [52], and a Multiscan absorption correction was applied [53]. The structure was drawn by ORTEP-3 for Windows [54] and Mercury [55] programs. The CCDC 882017 contains the supplementary crystallographic data for tris-1(phenylazo)-2-naphtholatecobalt(III). These data can be obtained free of charge at http://www.ccdc.cam.ac.uk or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 IEZ, UK [Fax: (internet.) 1 44-1223/336-033; Email: [email protected]. ac.uk]. 2.3.2. Calculations The structures of the metal complexes were fully optimized in the gas phase at B3LYP [56,57] level using the 6-311þþG(d,p) [58] triple-zeta basis-set with the inclusion of diffuse and polarization functions at heavy and hydrogen atoms (hereafter abbreviated as B3LYP/6-311þþG(d,p)) (the optimized structures are shown in Fig. S1 as Supplementary Materials). All of the geometries were considered as neutral species with the multiplicities set to singlet for the nickel complex and duplet for the copper complex. The infrared (IR) and Raman intensities were also calculated and the band spectra simulated by fitting a Lorentzian type function. All calculations were carried out with Gaussian 09 [59] program as

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G.R. Ferreira, L.F.C. de Oliveira / Journal of Molecular Structure 1146 (2017) 50e56

Scheme 1. Synthesis scheme for obtaining the complexes of metal ions with dyes Sudan I and Sudan II as ligand.

installed in the computers of the Núcleo de Estudos em Química Computational (NEQC-UFJF). 3. Results and discussion The crystal of the compound tris-1-(phenylazo)-2naphtholatecobalt(III), SD1Co, was obtained in a test tube from a methanol saturated solution through the slow evaporation of the solvent; the crystals were dried and separated for analysis of X-ray diffraction. The crystalline data for this compound can be seen in Table 1, and the most important bond distances, bond angles and dihedron angles are displayed in Table S1 Supplementary Materials. The complex tris-1-(Phenylazo)-2-naphtholatecobalt(III) (SD1Co) crystallizes in a triclinic system a 90.113(3), b (107.899(4) and g 94.978(4) and cell lengths a ¼ 12.4656(6), b ¼ 17.0356(8) and c ¼ 18.9232(6), presenting space group P-1 with Z equal to 2 repeat units per unit cell, which comprises a volume of 3807.84 Å3. Fig. 1 represents the repeat unit of the complex formed. For this compound the building block was formed through the coordination between three azo dyes (Sudan I) and the cobalt ion; the metallic site appears coordinated in an octahedral geometry slightly distorted, with the bond angles between the O1eCo1eO2, O3eCo1eO2, N1eCo1eN3, N5eCo1eN1, O2eCo1eN3 and O3eCo1eN3 atoms presenting values of 88.90(7) , 89.68(7) , 93.17(7) , 96.96(8) , 88.93(7) and 90.09(7) , respectively. The structure described here presents three bidentate ligands and the structure reported in literature has two tridentate ligands which may also explain the differences in such bond distances. The dihedral angles for C17eN4eN3eC27, C1eN2eN3eC11 and C33eN6eN5eC43, which form part of the azo bond (-N]N-), are 179.07, 176.75 and 178.21, respectively, which are very close to 180 . The dihedral angles that relate the phenyl to the azo group have different values: C12eC11eN1eN2/-142.16 , C16eC11e N1eN2/35.07, C32eC27eN3eN4/143.19 , C28eC27eN3eN4/-

Table 1 Main crystallographic parameters of the structures refinement for C48H33CoN6O3 complexes. Compound

SD1CO

Formula Formula weight/g mol1 Temperature/K Crystal system Space group a/Å b/Å c/Å

C48H33CoN6O3 800.73 150.1 (3) Triclinic P-1 12.4656 (6) 17.0356 (8) 18.9232 (6) 90.117 (3) 107.899 (4) 94.978 (4) 3807.8 (3) 2 0.33  0.23  0.14 2.60e66.39 1.397 1.54178 0.386/0.574 43620/13392 0.0505 10662 1045 0.0399 0.0924 1.028 0.047

a b g V/Å3 Z Crystal size/mm q Range/ dcalc/g cm1 m(Cu Ka)/mm1 Transmission factors (min/max) Reflections measured/unique Rint Observed reflections [F2o > 2s(F2o)] No. of parameters refined R[Fo > 2s(Fo)] wR[Fo2>2s(Fo)2] S RMS peak/

38.82 , C48eC43eN5eN6/-11.26 and C44eC43eN5eN6/67.80 . Such torsions affect the overall crystallographic arrangement by creating two adjacent planes and one perpendicular plane in relation to the naphthol and to the phenyl rings (Fig. S2). The solid state arrangement displays supramolecular interactions of the pstacking type between the two adjacent rings of the naphthol group (Fig. 2). The distance between the two centroids from the

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Fig. 1. Thermal ellipsoid representation of the C48H33CoN6O3 showing the atom labeling scheme. The thermal ellipsoids are scaled to the 50% probability level. Hydrogen atoms are drawn to an arbitrary scale.

naphthol groups (opposite rings) is 3.768 Å and 3.748 Å. An important interaction viewpoint of supramolecular chemistry was observed between CH from the phenyl group and the centroid of the naphthol group, with the average distances between the centroids/hydrogen in the order of 3.510 Å and 4.423 Å. These

distances suggest an arrangement, which is maintained united by

p-stacking and CH/p supramolecular interactions, which are observed between the centroids of the naphthol groups and CH/p, being notably important for the packaging of this crystalline system.

Fig. 2. Supramolecular bi-dimensional arrangement parallel to the bc plane for the tris-1-(Phenylazo)-2-naphtholatecobalt(III).

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For the compounds SD1Cu and SD2Ni the X-ray powder diffraction data indicate that they present exactly the same diffraction pattern as the supramolecular compounds bis-1-(phenylazo)-2-naphtholatenickel(II) and bis-1-(2,4-xylylazo)-2naphtholatecopper(II) [40] (Figs. S3 and S4 as Supplementary Materials) should provide square planar geometry and with its unit cell being formed by two repeat units and with the same supramolecular interactions. The vibrational spectra of the synthesized compounds are shown in Figs. 3 and 4 (infrared and Raman, respectively). The main vibrational modes and their respective assignments were based on similar chemicals systems [41,60] and theoretical calculations at

level B3LYP/6-311þþG(d,p) were performed (Figs. S5eS7 as Supplementary Materials) and the main vibrational assignments are shown in Table 2 (infrared) and Table 3 (Raman). Infrared spectroscopy was used in the elucidation of the structure of the all compounds, thus several bands fingerprint of the ligands was monitored through the intensity or the shift caused mainly due to the coordination by metal ion and the loss of the hydrogen atom present in the azo group before coordination (Fig. S7 as Supplementary Material). For the SD1Cu, SD1Co and SD2Ni complexes the bands related to the metal-ligand vibration modes were observed as low intensity bands at 657/658/667 cm1, 507/500/505 cm1 and 452/467/477 cm1, assigned to the [n(CuO)/n(CoO)/

Fig. 3. Experimental IR spectra (a) for tris-1-(Phenylazo)-2-naphtholatecobalt(III), for (b) bis-1-(Phenylazo)-2-naphtholatecopper(II) and (c) bis-1-(2,4-xylylazo)-2naphtholatenickel(II).

Fig. 4. Experimental Raman spectra (a) for tris-1-(Phenylazo)-2-naphtholatecobalt(III), for (b) bis-1-(Phenylazo)-2-naphtholatecopper(II) and (c) bis-1-(2,4-xylylazo)-2naphtholatenickel(II).

G.R. Ferreira, L.F.C. de Oliveira / Journal of Molecular Structure 1146 (2017) 50e56 Table 2 Wavenumber (in cm1) and assignments of the main IR absorption bands observed for the (a) tris-1-phenylazo-2-naphtholatecobalt(III), (b) bis-1-phenylazo-2naphtholatecopper(II) and (c) bis-1-(2,4-xylylazo)-2-naphtholatenickel(II). SD1Cu

SD1Co

SD2Ni

Assignments

1619 m 1596 w 1547 m 1500 vs 1461 w 1438 m 1406 w 1368 s 1351 vs 1300 vs 1253 w 1219 w 1186 m 1147 m 992 m 816 s 737 vs 690 s 657 m 507 w 452 w

1614 m 1596 m 1546 m 1500 vs 1474 s 1444 s 1407 w 1369 vs 1352 vs 1309 s 1256 w 1215 m 1181 vs 1147 vs 997 s 824 s 743 vs 693 s 658 m 500 w 467 w

1614 m 1595 w 1548 m 1502vs 1477 w 1377 w 1406 w 1359vs 1338 w 1307 m 1255 w 1215 w 1181 m 1148 m 1000 m 813 s 757 vs e 667 w 505 477

nCC naphthol nCC naphthol nCC þ dCH naphthol nCC þ dCH þ nCN nCO þ dCH þ nCN nCC nCO þ dCH þ nCN þnCC nCC þ dCH þ nCN dCH þ nsCN þnCC nCO þ dCH þ nCC nCC þ dCH naphthol nCC þ dCH nCC þ dCH nCC þ dCH dCCC uCH uCH naphthol uCH naphthol nCuO/nCoO/nNiO nCuO/nCoO nCuO/nCoO/nNiO

Abbreviations: vs: very strong, w: weak and m: medium.

Table 3 Wavenumber (in cm1) and assignments of the main Raman bands observed for the (a) tris-1-phenylazo-2-naphtholatecobalt(III), (b) bis-1-phenylazo-2naphtholatecopper(II) and (c) bis-1-(2,4-xylylazo)-2-naphtholatenickel(II). SD1Cu

SD1Co

SD2Ni

Assignments

1596 1548 1500 1466 1439 1360 1300 1252 1208 1170 1146 1000

1592 1547 1502 1474 1445 1372 1307 1259 1201 1171 1148 1002

1594 1547 1503 1469 1441 1367 1309 1256 1205 1172 1147 1000

nCC þ dCH nCC þ dCH þ nCN, nCO þ dCH þ nNN, nCO þ dCH þ nCN nCC þ dCH dCH þ nCO þ nCC nCC nCC þ dCH nCC þ dCH nCC þ dCH nCC þ dCH nCC þ dCH

m w w w w vs/1351 vs m w w m w w

m w w w w vs w w w w w w

m w w w w vs w m m m w w

Abbreviations: vs: very strong, w: weak and m: medium.

n(NiO) þ n(CC)], [n(CuO)/n(CoO)/n(NiO)] and [n(CuO)/n(CoO)/ n(NiO)] modes. Bands at 737/743/757 cm1 were attributed to the out of the plane angular deformation [u(CH) naphthol], whereas

the band at 690 cm1, which is also present in the SD1Co and SD1Cu complexes, but is not present in complex SD2Ni, has been attributed to the [u(CH) naphthol] mode. When comparing the spectra of the free ligands with the spectra of the complexes (Fig. S8 as Supplementary Material) the bands at 1389 cm1 [nsC ¼ N, dNH, nsCC, dCH] and 1340 cm1 [nsCC, dCH] can be used to confirm the coordination among the ligands and the metal ion, since such bands are increased in intensity as well change their wavenumber positions due to the coordination, ensuring that the nitrogen atom of the azo group is involved in the coordination. Raman spectroscopy was also used in the structural elucidation of these complexes and the spectra are shown in Fig. 4. An interesting information about this structure is the presence of ligands hydrazo group (NH) contributes to enhance several vibrational modes highlighting bands at 1595 cm1 intensified in the ligand SD1 and reduced intensity in the complexes (SD1Cu and SD1Co) (Fig. S9eS12 as Supplementary Material). The band at 1500 cm1 present in both ligands (SD1 and SD2) spectra with high intensity was attributed to the symmetrical stretching of the NH group,

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which is not seen in the complex spectrum. The medium intensity band at 1226 cm1 present in the SD1ligand spectrum, assigned to the [n(CC) þ d(CH) þ n(CeNH)] mode, is split into two new bands at 1206 cm1 and 1170 cm1 both assigned to the couple [n(CC) þ d(CH)] mode. All the spectroscopic data, in connection with the crystallographic data, point out to the prevalence of the hydrazo structure over the azo one, for all the investigated complexes involving Sudan I and II moieties. This is an important conclusion since some structural and electronic theoretical investigations done with azo dyes were pointing out exactly the same, i.e., the predominant structures in the solid state for these azo dyes are the hydrazo forms [1,40,41]. 4. Conclusion Three new coordination compounds derived from 1-phenyl-2azo-naphthol and 1-(2,4-xylylazo)-2-naphthol were synthetized with cobalt, copper and nickel ions present octahedron, square planar geometry. Powder diffraction, X-ray diffraction and Raman spectroscopy among other techniques experimental and theoretical were used for structural elucidation of these compounds. Supramolecular interactions based on p-stacking type between the two centroids from naphthol rings were observed at 3.768 Å and 3.748 Å; another important supramolecular interaction was observed between the CH from the phenyl group and the centroid of the naphthol group, with the average distance of 3.510 Å. Acknowledgements This work was supported by the CNPq, CAPES, FAPEMIG (PRONEX 04370/10, CEX-APQ-00617) and FINEP (PROINFRA 1124/06) for financial support and also LabCri (Departamento de Física e UFMG) for the X-ray facilities. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2017.05.121. References [1] G.R. Ferreira, H.C. Garcia, M.R.C. Couri, H.F. Dos Santos, L.F.C. de Oliveira, On the azo/hydrazo equilibrium in Sudan I azo dye derivatives, J. Phys. Chem. A 117 (2013) 642e649. [2] W. Peng, F. Ding, Y.-K. Peng, Y.-T. Jiang, L. Zhang, Binding patterns and structureeaffinity relationships of food azo dyes with lysozyme: a multitechnique approach, J. Agric. Food Chem. 61 (50) (2013) 12415e12428. [3] Z.F. Liu, K. Hashimoto, A. Fujishima, Photoelectrochemical information storage using an azobenzene derivative, Nature 347 (1990) 658e660. [4] A. Ünal, B. Eren, E. Eren, Investigation of the azo-hydrazone tautomeric equilibrium in an azo dye involving the naphthalene moiety by UVevis spectroscopy and quantum chemistry, J. Mol. Struct. 1049 (2013) 303e309. [5] S. Kawauchi, L. Antonov, Description of the tautomerism in some azonaphthols, J. Phys. Org. Chem. 26 (2013) 643e652. [6] M.A. Rauf, S. Hisaindee, N. Saleh, Spectroscopic studies of keto-enol tautomeric equilibrium of azo dyes, RSC Adv. 5 (2015) 18097e18110. [7] M.R. Almeida, R. Stephani, H.F. Dos Santos, L.F.C.D. Oliveira, Spectroscopic and theoretical study of the “Azo”-dye E124 in condensate phase: evidence of a dominant hydrazo form, J. Phys. Chem. A 114 (2010) 526e534. [8] G. Cui, P.-J. Guan, W.-H. Fang, Photoinduced proton transfer and isomerization in a hydrogen-bonded aromatic azo compound: a CASPT2//CASSCF study, J. Phys. Chem. A 118 (2014) 4732e4739. [9] D. Debnath, S. Roy, B.-H. Li, C.-H. Lin, T.K. Misra, Synthesis, structure and study of azo-hydrazone tautomeric equilibrium of 1,3-dimethyl-5-(arylazo)-6amino-uracil derivatives, Spectrochim. Acta Mol. 140 (2015) 185e197. [10] L. Antonov, V. Deneva, S. Simeonov, V. Kurteva, A. Crochet, K.M. Fromm, B. Shivachev, R. Nikolova, M. Savarese, C. Adamo, Controlled tautomeric switching in azonaphthols tuned by substituents on the phenyl ring, Chemphyschem 16 (2015) 649e657. [11] J.M. Mirkovic, G.S. Uscumlic, A.D. Marinkovic, D.Z. Mijin, Azo-hydrazone tautomerism of aryl azo pyridone dyes, Hem. Ind. 67 (2013) 1e15. [12] K. El-Baradie, R. El-Sharkawy, H. El-Ghamry, K. Sakai, Synthesis and

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