Synthesis, characterization and electrochemical studies of azo dyes derived from barbituric acid

Dyes and Pigments 136 (2017) 742e753

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Synthesis, characterization and electrochemical studies of azo dyes derived from barbituric acid S. Harisha a, Jathi Keshavayya a, *, B.E. Kumara Swamy b, C.C. Viswanath b a

Department of PG Studies and Research in Chemistry, School of Chemical Sciences, Kuvempu University, Jnana Sahyadri, Shankaraghatta 577451, Karnataka, India b Department of PG Studies and Research in Industrial Chemistry, Kuvempu University, Jnana Sahyadri, Shankaraghatta 577451, Karnataka, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 July 2016 Received in revised form 31 August 2016 Accepted 1 September 2016 Available online 7 September 2016

We have made an effort to synthesize three heterocyclic azo dyes 1(a-c) by means of diazotization of 6methoxy-1,3-benzothiazol-2-amine, 4-(4-nitrophenyl)-1,3-thiazol-2-amine and 6-methyl-1,3-benzothiazol-2-amine by coupling with barbituric acid with high yield in basic media (pH ¼ 9e10). Structural confirmation of the synthesized azo dyes has been accomplished by Ultra Violet-Visible, Fourier Transform Infrared, Proton Nuclear Magnetic Resonance and Mass spectrometric techniques. The electrochemical properties of above azo dyes are investigated by cyclic voltammetric technique. The effects of scan rate and concentration of sulphuric acid was studied at glassy carbon electrode. The overall electrode process is diffusion controlled. The electrode reaction mechanism was proposed. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Azo dye Benzothiazole Disperse dye Cyclic voltammetry Glassy carbon electrode

1. Introduction In recent decades, organic color chemistry has undergone rapid and very exciting developments as a result of the opportunities presented by dye applications in various high technology fields. The design and synthesis of various organic chromophores as nonlinear optical (NLO) materials have attracted a great deal of attention in recent years [1]. Azo dyes of organic NLO materials have many advantages over inorganic materials including large nonlinear optical co-efficient, greater ease of synthetic design, easy preparation & lower cost. Azo dyes are of particular interest because they can be readily prepared with a wide range of donor & acceptor group in it. They are promising candidates for photoconductivity [2,3]. Development of azo dyes based on heterocyclic compounds of pharmacological and industrial importance is an important task for heterocyclic chemists. Nitrogen and sulphur containing five membered heterocyclic compounds are the platform to produce a substance of interest in numerous therapeutic areas. Their biological importance is well known for their use as antidiabetics [4], antiseptics [5], and other useful chemotherapeutic agents [6e8]. It

* Corresponding author. E-mail addresses: [email protected] (J. Keshavayya), kumaraswamy21@ yahoo.com (B.E. Kumara Swamy). http://dx.doi.org/10.1016/j.dyepig.2016.09.004 0143-7208/© 2016 Elsevier Ltd. All rights reserved.

has been found that the activity of azo linkage increases on the incorporation of suitable heterocyclic moiety, amino benzothiazoles are a class of heterocycles and they are of significant interest in medicine and pesticide chemistry in a number of biological targets including anti-inflammatory agents, antibacterial, tuberculostatic and antimicrobial activity. Recently, the reduction of azo dyes has considerable interest area because, in order to obtain a deeper insight into the ground state properties and more specially the mutual donor-acceptor electronic influence having applications in catalysis [9e11], material science [12], sensing, chromophoric and metallochromic reagents [13], photochromic materials [14], colorants and photosensitizers [15] have attracted the interest of many electrochemists. Therefore the electrochemical investigations of various heterocyclic azo dyes were reported. Because SeN containing ring compounds possess very fascinating properties and the most promising of all these is their ability to be redox-active; and show good conducting and magnetic properties. In this regard, the electrochemical behavior and electrode reaction pathways of numerous azo-dyes in various supporting electrolytes were studied and discussed [16e22]. Chemical reduction of azo compounds leads to saturate the azo group giving a hydrazo derivative and then breaks the eNe¼Ne linkage to form two primary amine molecules. Therefore, in most of the studies, electrochemical reduction was utilized for the study of the azo dyes because electrochemistry

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Fig. 1. UVeVisible spectrum of compound 1a in DMSO. Fig. 4. UVeVisible spectrum of compound 1b in DMF.

Fig. 2. UVeVisible spectrum of compound 1a in DMF.

provides convenient methods for studying their mechanism and kinetics. The introduction of substituent groups into the aromatic ring can completely change the nature of the electrode reaction because of their different sizes and orientations [23e27]. In this work, a systematic study was undertaken to investigate the electro reduction of three heterocyclic azo dyes derived from barbituric acid at different concentrations of sulphuric acid media and the effect of scan rate was also studied at glassy carbon electrode by using cyclic voltammetric technique. The results showed

Fig. 5. UVeVisible spectrum of compound 1c in DMSO.

that reduction of azo dye occurs through two irreversible reduction peaks and the reduction mechanism was proposed and discussed. 2. Method and materials 2.1. General All the chemicals, reagents and solvents used for the synthesis are purchased from Sigma Aldrich, SD-fine, Hi-Media Company of

Fig. 3. UVeVisible spectrum of compound 1b in DMSO.

Fig. 6. UVeVisible spectrum of compound 1c in DMF.

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Fig. 7. FTIR spectrum of compound 1a.

synthetic grade. Solvents were purified before use. Diazotization of various heterocyclic amines was effected with glacial acetic acid: propionic acid mixture and NaNO2. A typical procedure used was described in section 2.2.1. The obtained compounds were purified by recrystallization and then analyzed. Ultravioletevisible (UVeVis) absorption spectra were recorded on basic Eppendorf Bio-Spectrometer at the wavelength of maximum absorption (lmax) in a range of solvents, i.e. dimethyl sulphoxide (DMSO) and dimethylformamide (DMF) (Figs. 1e6). FTIR spectra were recorded on a Bruker alpha-T FTIR Spectrophotometer in KBr (Figs. 7e9). 1HNMR spectra were recorded on

Bruker 400 MHz in DMSO-d6 with TMS as internal reference (Figs. 10e12). Chemical shifts are expressed in d units (ppm). LCMS were recorded in LCMS:waters (aquity) 2777C(Figs. 13e15). 2.2. Synthesis of azo dyes Azo dyes were synthesized by conventional method The diazotization of 2-amino 6-methoxy benzothiazole, 4-(4-nitrophenyl)1,3-thiazol-2-amine and 6-methyl-1,3-benzothiazole-2-amine were carried out in acidic media followed by coupling with barbituric acid (see Table 1).

Fig. 8. FTIR spectrum of compound 1b.

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Fig. 9. FTIR spectrum of compound 1c.

2.2.1. Preparation of benzothiazole azo dyes 1(a-c) 2.2.1.1. General procedure. Amines (2.0 mmol) were dissolved in glacial acetic acid and propionic acid mixture (2:1, 6.0 mL) and were quickly cooled in an ice-salt bath at 0e5  C. The mixture was added in portions during to a cold solution of nitrosyl sulfuric acid [prepared from sodium nitrite (2.2 mmol, 0.15 g) and concentrated sulfuric acid (3 mL at 60  C)]. The mixture was stirred for an additional 3 h at the same temperature. After completion of

diazotization procedure, the diazonium salt solution was added drop wise to the solution of barbituric acid (2.0 mmol) in KOH. The resulting solution was vigorously stirred at 0e4  C for 2 h, while the pH of the reaction mixture was maintained at 9e10 with simultaneous addition of sodium carbonate solution (0.5 M). The progress of the reaction was monitored by TLC and then crude dyes were filtered, washed with hot water for several times and purified by recrystallization method Scheme 1.

Fig. 10. 1HNMR spectrum of compound 1a.

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Fig. 11. 1HNMR spectrum of compound 1b.

3. Experimental 3.1. Preparation of 5-[(E)-(6-methoxy-1,3-benzothiazol-2-yl) diazenyl]pyrimidine-2,4,6 (1H, 3H,5H)-trione; (1a) This dye was obtained by diazotization of 2-Amino 6methoxybenzothiazole and coupling with barbituric acid as red

 C), crystals (yield:74%; mp:308e309 FT-IR (KBr)vmax: eNH(3571 cm1),eC]O(1742 cm1), eC]N(1669 cm1), eN]N(1511 cm1); 1HNMR (DMSO-d6) d values are 10.1 (brs, 2H,NH), 7.6 (d, 1H,ArH,J ¼ 8.8 Hz), 7.4 (d,1H, ArH, J ¼ 2.4 Hz), 6.9 (dd,1H, ArH, J ¼ 11.2 Hz), 3.7 (s,3H, OCH3), 2.50 (s, 1H, CH). Anal.Calc (%) for C12H9N5O4S: C, 45.14; H, 2.84; N, 21.93; O, 20.04; S, 10.04. Found (%): C, 45.19; H, 2.79; N, 21.75; O, 20.08; S, 10.06. LCMS m/z¼320.1 (MþH).

Fig. 12. 1HNMR spectrum of compound 1c.

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Fig. 13. LCMS spectrum of compound 1a.

3.2. Preparation of 5-{(E)-[4-(4-nitrophenyl)-1,3-thiazol-2-yl] diazenyl}pyrimidine-2,4,6(1H, 3H,5H)-trione; (1b) This dye was obtained by diazotization of 4-(4-nitrophenyl)-1,3thiazol-2-amineand coupling with barbituric acid as red crystals (yield:77%; mp:303e305  C). FT-IR (KBr)vmax:Ar-CH(2835 cm1), eC]O (1737 cm1), eC]N(1631 cm1), eN]N-(1496 cm1); 1 HNMR (DMSO-d6) d values are 10.23 (brs,2H,eNH), 7.60 (d, 2H,ArH, J ¼ 6 Hz), 7.42 (m, 2H,ArH), 6.94 (s,1H,ArH),3.80 (s,1H,CH). Anal.Calc (%) for C13H8N6O5S: C, 43.34; H, 2.24; N, 23.32; O, 22.20; S, 8.90. Found (%): C, 43.31; H, 2.29; N, 23.35; O, 22.17; S, 8.87. LCMS m/z¼361.1 (MþH). 3.3. Preparation of 5-[(E)-(6-methyl-1,3-benzothiazol-2-yl) diazenyl]pyrimidine-2,4,6(1H, 3H, 5H)-trione; (1c) This dye was obtained by diazotization of 6-methyl-1,3benzothiazole-2-amine and coupling with barbituric acid as red crystals (yield:72%; mp:312e314  C). FT-IR(KBr)vmax:eNH (3431 cm1), Ar-CH(3010 cm1), eC]O(1702 cm1), eC] N(1625 cm1), eN]N-(1484 cm1); 1HNMR(DMSO-d6) d values are 10.18 (brs,2H,eNH),7.57 (m,2H,ArH), 7.12 (d,1H,ArH,J ¼ 8 Hz), 2.376

(s,1H,CH),2.50 (s, 3H, eCH3). Anal.Calc (%) for C13H8N6O5S: C, 47.52; H, 2.99; N, 23.09; O, 15.83; S, 10.57. Found (%): C, 47.58; H, 2.96; N, 23.08; O, 15.85; S, 10.52. LCMS m/z¼304.1(MþH) (see Table 2). 4. Cyclic voltammetry In cyclic voltammetric study, all synthesized azo dyes were dissolved in dimethyl formamide (DMF) and sulphuric acid was used in different concentration as supporting electrolyte. The cyclic voltammetric (CV) experiments were performed using a model CHI-660c (CH Instrument-660 electrochemical work station) at room temperature. All experiments were carried out in a conventional three electrochemical cell. The electrode system contained a working Glassy carbon electrode, a platinum wire as counter electrode and saturated calomel as reference electrode for the electrochemical measurements. 5. Results and discussion Structural characterizations of synthesized dyes were carried out using the different spectral studies such as 1HNMR, FT-IR and UVeVisible spectroscopy.

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Fig. 14. LCMS spectrum of compound 1b.

5.1. Electronic absorption spectra and substituent effect The electronic absorption spectra of synthesized heterocyclic azo dyes 1(a-c) was recorded in DMSO and DMF at room temperature by Eppendorf Bio-Spectrometer in the l range from 270 to 800 nm at a concentration of 2  106 M (Figs. 1e6). All compounds displayed an absorption band in the range between 375 and 510 nm (see Table 3). Examination of the results reported in Table 3, reveals that the introduction of electron-donating methoxy group, methyl group into benzothiazole ring (Compound 1a and 1c) results bathochromic shift in DMSO and DMF when compared with compound 1b. These bathochromic shifts can be attributed due to interaction H atom of amino proton of dyes with basic solvents such as DMSO or DMF because of increased polarity of the dye system, especially in the excited state. Moreover, the lmax values of the hydrazone tautomeric form of an azo dyes will show a general shift to shorter wavelengths when substituent's of increasing electron

withdrawing strength are introduced into the ring of the diazo component. In contrast, electron donor substituent's produce strong bathochromic shifts. However, the dyes showed a bathochromic shift with the introduction of electron donating substituents in the phenyl ring in the sequence of H < CH3 < OCH3 < OC2H5 [28]. The electron-accepting nitro group into the benzothiazole ring (Compound 1b) results in slight bathochromic shifts in DMSO and DMF. The results presented in Table 3 are in agreement with these conclusions. 5.2. Infrared spectra IR spectra were recorded in the region of 4000 cm1 to 400 cm1 on a FTIR-ALPHA BRUKER IR spectrometer in KBr pellets. The important infrared bands exhibited by all the prepared azo dyes are given in Table 4. A strong and broad absorption band within the region 3500-3437 cm1 is due to eNH group. The other nmax values at 3098-3010 cm1 (aromatic CeH) and at 1750-

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Fig. 15. LCMS spectrum of compound 1b.

1600 cm1 from its high intensity the band is easily recognized as n C]O, and from its position it is evident that the band is due to conjugated carbonyl. In addition, the FTIR spectrum of azo dyes revealed a weak and sharp band at ~1631-1625 cm1 due to n C]N of the thiazole ring.

Table 1 List of Amines used. Amines used

Structure O

6-methoxy-1,3-benzothiazol-2-amine Het - NH2

+

O O

O

4-(4-nitrophenyl)-1,3-thiazol-2-amine

H N

1. Acetic acid:Proponoic acid ( 0- 5o C) 2. Coupling

N H

Het - N=N

H3CO

NH2

S

Het = O2N

N S

6-methyl-1,3-benzothiazol-2-amine

NH2

N

Het = H3C

O O

N Het =

S

H N

NH2

Scheme 1. Synthesis of Benzothiazole Azo Dyes 1(aec).

N H

750

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Table 2 Physical data of synthesized Azo dyes. Sl no.

Name of the dye

Melting point ( C)

Yield (%)

Solubility

1. 2. 3.

5-[(E)-(6-methoxy-1,3-benzothiazol-2-yl)diazenyl]pyrimidine-2,4,6(1H,3H,5H)-trione (1a) 5-{(E)-[4-(4-nitrophenyl)-1,3-thiazol-2-yl]diazenyl}pyrimidine-2,4,6(1H,3H,5H)-trione (1b) 5-[(E)-(6-methyl-1,3-benzothiazol-2-yl)diazenyl]pyrimidine-2,4,6(1H,3H,5H)-trione (1c)

324e326 303e305 312e314

74% 77% 72%

DMSO/DMF DMSO/DMF DMSO/DMF

that it confirms their formula weights are equal to their molecular weight.

Table 3 UVeVisible and Molar absorptivity (Ɛ) data of compounds 1(a-c). Compound

lmax (nm) DMSO

DMF

DMSO

DMF

6. Voltammetric behavior

Compound 1a Compound 1b Compound 1c

490 390 460

450 380 410

4.89 3.86 4.59

4.47 3.75 4.06

6.1. Irreversible electrochemical character of azo dye at glassy carbon electrode (GCE)

5.3.

Logε

1

H NMR spectra

1 H NMR spectra of Compound 1a, 1b and 1c obtained at ambient temperature in DMSO-d6, display signals that are consistent with the proposed structures using TMS as an internal standard. 1H NMR spectra of all the synthesized compounds were in accordance with the expected theoretical values of the compounds shown important signals at their positions, confirms their structures, as shown in Table 4. The aromatic protons were observed in the range of d 6.90e7.62 ppm as a multiplet. The hydrogen atom of the imide group (eNH) of all synthesized azo compounds was observed in the range of d 10.16e10.24 ppm as broad singlet. Methoxy protons and methyl protons in compound 1a and compound 1c were assigned to a singlet peak at d 3.79 ppm and d 3.34 ppm respectively. Additionally, a methine proton of all synthesized azo compounds was observed in the range of d 2.37e3.80 ppm.

5.4. Mass spectral studies Mass spectra of azo dyes, confirms the proposed formula by showing peaks at 320.1 m/z, 360.1 m/z, 304.1 m/z for Compound 1a, 1b and 1c corresponding to their [mþH] ion peaks, is in good agreement with the suggested molecular formula respectively. So

The cyclic voltammetry of synthesized azo dyes [Compound 1(ac)] was recorded in DMF and also studied their reduction behavior at different concentration of sulphuric acid media as supporting electrolyte. The voltammograms were recorded at different scan rates (50 mV s1e250 mV s1). It shows two reduction peaks (Figs. 16e18) corresponds to cleavage of azo linkage. The irreversibility was confirmed by the absence of anodic peak during the electrode process. The electrochemical reduction has been reported to occur in 2e/2Hþ process to give hydrazo products [29]. However in the presence of strong electron donating groups, reduction has been found to occur in 4e-/4H þ reaction to give amino compounds as final products. The synthesized azo dyes reduced in two-step involving 4e/4Hþ during the electrode process [30]. The reduction mechanism of synthesized azo dyes was illustrated in Scheme 2. 6.2. The electrode reaction The electrochemical behavior of synthesized azo dyes based on the reduction of the eN]N- double bond, the consumption of electrons and protons during reduction process is shown in Scheme 2. Considering the results obtained and previously reported, the electrochemical behavior of various azo dyes in neutral and basic media reduction takes place in one step and

Table 4 1 HNMR, FTIR and LCMS Spectral data of Synthesized Azo dyes. Compound

1a

1

H NMR

IR (nmaxin cm1) 1

LCMS m/z (MþH)

10.1 (brs,2H,e NH) 7.6 (d,1H,ArH) 7.4 (d,1H,ArH) 6.9 (dd,1H,ArH) 3.7 (s,3H,e OCH3) 2.50 (s,1H,e CH)

e e e e e

NH (3571 cm ) Are H(3098 cm1) C]O(1742 cm1) C]N (1669 cm1) N]N(1511 cm1)

320.1

10.23 (brs,2H,e NH) 7.605 (d,2H, ArH) 7.42 (m, 2H, ArH) 6.94 (s, 1H) 3.80 (s, 1H, eCH)

e e e e e e

NH (3451 cm1) C]N-(1631 cm1) C]O (1737 cm1) N]N- (1495 cm1) Ce Ne (1266 cm1) Ce Se (731 cm1)

361.1

10.18 (brs,2H,e NH) 7.57 (m,2H, ArH) 7.12 (d,1H, ArH) 2.50 (s,3H,e CH3) 2.37 (s,1H,e CH)

e e e e e

NH (3431 cm1) Are H(3010 cm1) C]O (1702 cm1) C]N (1625 cm1) N]Ne (1484 cm1)

304.1

1b

1c

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Fig. 16. Cyclic voltammograms of compound 1a at different scan rate. (50 mV s1 to 250 mV s1) in 1 M H2SO4 at GCE.

one irreversible cathodic peak was observed in cyclic voltammetry and leads to formation of stable hydrazo compound via this is a 2e/2Hþ electrode process. Therefore, in compound 1b presence of nitro group is usually reduced prior to an azo group in alkaline medium. However, result shows that the azo group is more easily reduced than the nitro group in strongly acidic media. Therefore, in acidic media electrochemical reduction takes place at two stages i.e two irreversible cathodic peaks were obtained (see Fig. 19) and it involves 2e/2Hþelectrode process leads to stable hydrazo compound. Further the hydrazo compound reduces in next step by consumption of 2e/2Hþ. The whole process involves 4e/4Hþ [30]. The reduction mechanism may be deduced, so that the electroreduction reaction pathway of the examined azo dyes at glassy carbon electrode can be expressed as follows.

751

Fig. 18. Cyclic voltammograms of compound 1c at different scan rate (50 mV s1 to 250 mV s1) in 1 M H2SO4 at GCE.

R N N

O

O

H O

R NH NH

O

N H

H N

H N O N H

O

+

-

2e

+

R NH NH

+

H

H N O

O

N H

O O

O

H

O

O

H N O

O

R N + N

Fast

N H

O

R N + N

H+

H N

N H

+

-

2e

+

+

2H

R NH2

+ H2N

O O

N R=

S

H3CO 1a

O 2N 1b

N H

N

N S

H N

S

H3C 1c

Scheme 2. Reduction mechanism of a Compound (1a), Compound (1b) and Compound (1c).

Fig. 17. Cyclic voltammograms of compound 1b at different scan rate (50 mV s1 to 250 mV s1) in 1 M H2SO4 at GCE.

Fig. 19. Electrochemical reduction of synthesized azo dyes 1(aec) at Glassy Carbon. Electrode (GCE) in 1 M H2SO4 at scan rate 100 mV s1.

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Fig. 20. Graph of Current (Ipc) v/s Different Scan rate (50 mV s1 to 250 mV s1), Compound 1a.

6.3. Effect of scan rate The cyclic voltammetric behavior of the synthesized azo dyes was studied by varying the sweep rate from 50 mV s1 to 250 mV s1. For the azo group, peak current (Ip) at glassy carbon electrode changes linearly with scan rate (n) (see Figs. 20e22). On the other hand, the plots of peak current (Ipc) v/s square root of scan rate (n1/2), linear correlations were obtained, which confirmed that the process was diffusion controlled (see Figs. 23e25). 6.4. Effect of concentration of sulphuric acid The effect of concentration of sulphuric acid studied by cyclic voltammetric technique and it has been seen that increasing in the concentration of sulphuric acid ranges from 0.066 M, 0.133 M, 0.266 M, 0.4 M, 0.533 M. The cathodic peak potentials of two peaks were positively shifted thereby showing the involvement of protons in the reduction process [31]. These results illustrates the reduction process becomes easier at higher concentration of sulfuric acid. This type of electrode process is further confirmed by linear nature of Ipc v/s n 1/2plots.

Fig. 21. Graph of Current (Ipc) v/s Different Scan rate (50 mV s1 to 250 mV s1), Compound 1b.

Fig. 22. Graph of Current (Ipc) v/s Different Scan rate (50 mV s1 to 250 mV s1), Compound 1c.

7. Conclusion In conclusion, we have synthesized three heterocyclic azo dyes and characterized by various analytical techniques (UVeVisible, FTIR, 1HNMR, Mass). By comparing the derivatives synthesized, it can be noticed that the electron donating and withdrawing group on phenyl azo moiety have significant influence on the UVeVisible absorption properties. Unfortunately the effect of varying solvent on the absorption ability of these dyes could not be examined due to insolubility of azo compounds in common organic solvents. Further, the electrochemical behavior of a synthesized azo dyes was studied at glassy carbon electrode by cyclic voltammetric technique was elucidated. From the results obtained, it has been concluded that reduction of the azo group in all the synthesized dyes does not stop at hydrazo stage but further reduction leads to cleavage of eNHeNH- linkage to give amino compounds as the final products. Therefore, it can be said that two cathodic peaks belong to reduction of azo group to hydrazo (first peak) and hydrazo to alkyl amines (second peak). According to results obtained, it can be claimed that the reduction of azo group takes place irreversibly via four electrons to amino compounds in acidic media. The

Fig. 23. Graph of Current (Ipc) v/s Square root of scan rate (n1/2), Compound 1a.

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Fig. 24. Graph of Current (Ipc) v/s Square root of scan rate (n1/2), Compound 1b.

Fig. 25. Graph of Current (Ipc) v/s Square root of scan rate (n1/2), Compound 1c.

electrochemical process is diffusion controlled and the reduction peak potentials decreases with increase in concentration of sulphuric acid. All prepared compounds having extraordinary colouring properties and their high molar extinction coefficient it may have an interesting properties for the future as electro-optical materials. We expect that, this study will be used to evaluate their redox properties would be a good strategy for the electrochemical treatment of these azo dyes. Acknowledgement One of the authors, Mr. Harisha S thankful to the UGC, New Delhi, India for providing BSR Fellowship to carry out the Research work and also to the Department of Post Graduate Studies and Research in Chemistry, Kuvempu University, Shimoga for providing laboratory facilities. References [1] Peters AT, Yang SS. Mono azo disperse dyes derived from mononitro-dichloro-

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