Jordan Journal of Chemistry Vol. 2 No.2, 2007, pp. 133-144

JJC

Characterization and Thermal Decomposition of Indolylidene Aniline Azo-Dyes Derivatives

Salem A. Hameed*

King Abdulaziz University, Faculty of Science, Chemistry Department

P.O.Box 80203, Jeddah 21589, Saudi Arabia Received on May 31, 2007 Accepted on Sep. 30, 2007

Abstract New heterocyclic azo-dyes derived from aniline derivatives using indolyl aldehyde as a

coupler have been prepared and investigated by elemental analysis, IR,1H-NMR and Uv- vis

spectra furthermore, the spectral properties of the dyes, solvent effect and buffer solution pH

effect have been studied for determination of pKa values (9.62-12.25) for all compounds.

Kinetics of differential scanning calorimetry (DSC) of these compounds were measured. The

activation energy and also critical temperature (Tc), glassy transition temperature (Tg) and

melting point (Tm) temperatures were calculated.

Keywords: Azo-dyes; Spectroscopy; pKa-Value; Differential Scanning Calorimetry

(DSC).

Introduction Azo-dyes are one of the most important and versatile classes of synthetic organic

compounds,with an enormous variety of applications.[1,2] Technological applications of

these dyes result from the combination of the properties of the azo-group and several

types of aromatic-substituted ligands that confer to them intense color over the whole

visible range, thermal and photochemical stability, non-complex synthetic

methodologies and low costs of production. More recently, some of these dyes have

been studied as materials for non-linear optical applications when they contain

substituents on the aromatic rings with push-pull electron ability.[3,4]

Azo-dyes as guests or attached to the polymer chains can undergo photo and

thermal isomerization reactions from the most stable anti-form to the syn. In general,

these reactions produce remarkable changes of the dipole moments of these

molecules leading to photo induced optical birefringency.[5-7] The sequential reversible

isomerization anti-syn-anti leads to the possibility of optical storage, photo induced

switching,[8] formation of surface gratings.[9-11] In the present investigation concerned

with preparation of new dyes, determined ionization constant, solvent effect and kinetic

thermal analysis.

* Corresponding author. E-mail : sahameed2@hotmail.com.

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Experimental Materials:

All chemicals used in the synthesis of all dyes were obtained from BDH chemical

company and were used without further purification . The solvents used were

spectroscopic grade .

Apparatus:

The elemental analysis (C,H,N) were carried out on a microanalysis unit of

Perkin Elmer model 2400 and given in table 1.

Table (1): Elemental analysis of azo compounds.

%C %H %N

Calc(found) Calc(found) Calc(found)

Molecular Formula (Mol.Wt.)

m.p oC)(

Comp.

16.86

14.47)(

4.42

4.86)(

72.28

(72.72)

C15H11N3O

(249)

190-195 I

15.85

(15.39)

4.15

4.72)(

67.92

68.95)(

C15H11N3O2

(265)

195-199 II

15.96

15.53)(

4.94

5.07)(

73.00

73.24)(

C16H13N3O

(263)

130-135 III

12.80

12.45)(

3.04

3.99)(

54.89

54.75)(

C15H10N3OBr

(328)

162-163 IV

14.33

13.48)(

3.75

4.22)(

65.52

66.49)(

C16H11N3O3

293)(

190-195 V

The infrared spectra of the dyes were recorded as KBr pellets on a Perkin Elmer

FT-IR spectrophotometer. 1H-NMR spectra of the dyes were recorded on a Brucker 400MHz.

UV/Vis. absorption spectra of the dyes in different solvents and different buffer

solutions at room temperature were recorded using Perkin Elmer Lampda EZ 210

UV/Vis. spectrophotometer.

Differential scanning calorimetry (DSC) curves were obtained using the Mettler

TA3000 DSC apparatus . Samples were heated at 5, 10, 15 and 20 oC / min.

Synthesis of dyes:

NaNO2 (0.025 mol) was slowly added with stirring to distilled water while allowing

the temperature to -5.0oC.[6-8] The solution of indolyl aldehyde was then cooled to -

5.0oC. NaOH (10%) was added drop by drop with stirring, allowing the temperature to -

5.0oC. The reaction mixture was then cooled to-5.0oC, and the diazo component

indolyl-3-aldehde (0.025 mol) was added drop by drop and stirring was continued at

this temperature for 3 h. The clear diazonium salt solution thus obtained was used

immediately in the coupling reaction (see scheme 1). Aniline (0.025 mol), was

dissolved in 20 cm3 of HCl (1:1), then cooled in an ice-bath at a -5.0oC. The diazonium

solution previously prepared was added drop by drop over 1 h with vigorous stirring at

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-5.0oC. The reaction mixture was stirred for further 3 h at -5.0oC. The product was then

filtered off, washed with water until acid-free, dried at 50.0oC in an oven to give azo

dye .

NH2

X

NaNO2

H2 OH Cl N=N-Cl

X

N

C=O

H

cool

N

C=O

N=N

X

H

H

H

where x = H , p-OH , p-CH3 , p- Br , 0-COOH

Scheme1 Results and discussion

The relevant IR spectra bands that can provide diagnostic structural evidence for

the existence of azo (enol) or hydrazone (keto) tautomers are given in table2. The

data of this table reveals that the coupled moiety (specifically the position of the OH

relative to the azo one ) plays an important rule in detecting which form predominant in

the solid (figure 1).

It is evident from the data of table2 identify the predominant present compounds

in solid state carbonyl and both azo N=N and C-N stretching vibrations are shown,

unfortunately, ν C-N can not be used for diagnostic purpose because of the interference

with ring C=N vibration of the hetero ring moiety. On the other hand, the bands

observed in the range 3500-3000 cm-1 are little use for diagnostic purpose due to the

possible mixing between the chelate hydrogen – bonding vibration with that of the N-H

group[6].

136

Table(2): IR- Spectra of azo compounds. CHAr υ C-N υ N=N υ C=O υ C=C υ

Comp.

3160.5

1134.6

1447.9

1806.6

1614.5

I

3155.1

1134.4

1446.7

1806.6

1625.2

II

3178.6

1131.9

1446.6

1806.7

1615.0

III

3187.6

1277.7

1490.8

1806.7

1603.3

IV

3177.3

1134.1

1446.0

1806.7

1622.0

V

Figure 1: IR spectra of I,II,III,IV and V.

137

The structure of the compounds under study are investigated on the basis of 1H-

NMR spectra (table 3). The data shows two signals at 7.19 and 8.17 ppm which can

assigned to the protons of benzene ring,[10] compounds ( figure 2) , shows signal at

low field ( δ ≈10.5ppm) which can be assigned to the proton of NH or OH, since the

proton signal of the hydrogen bonded NH must appear at low field. This supports the

IR suggestion of the possibility for the structure of these compounds.

Table(3): ¹H-NMR- Spectra of azo compounds.

CH [Ar]δ (ppm)

CHOδ (ppm)

NHδ (ppm)

Comp.

7.19 - 8.17 9.97 11.99 I

7.21 - 8.21 9.98 - 10.06 11.99 II

6.54 - 8.67 9.983 11.99 III

6.55 - 8.20 9.97 - 9.98 11.99 IV

7.21 - 8.21 9.99 11.99 V

Figure 2: 1H-NMR spectra of I .

Electronic absorption spectra:

The electronic absorption spectra of the ethanolic solutions of the compounds

under investigation are studied, λmax (nm) and εmax (dm3 mol-1cm-1) values of the different

bands obtained in various solvents are summarized in table 4. The data indicate that the

138

spectra of the compounds comprise mainly three bands in the Uv-visible regions. The

bands are influence by the nature of the substituted (X) and these are due to excitation

of π-electrons of aromatic ring, one composed of double head band and shoulder, the

maximum of these bands are slightly influenced by changing the solvent polarity. This

behaviour characteristic of such type of electronic transition. For the aryl derivatives the

π–π* band is red shifted compared to other azo compounds this is attributed to a

longitudinal of the π system in aryl and indolyl moieties. The data show that the

observed red shift in the CT shoulder is in a little accordance with the following

sequence which is in harmony with the increase in the solvent polarity : CCl4 < EtOH <

Acetone < DMF. Table (4): Molar extinction coefficient of azo compounds at λ ~300 nm in different

solvents.

E (dm3.mol-1.cm-1)

Acetone CCl4 DMF EtOH

Colour

Comp.

1084.6

251.6

232.7

826.2

Dark brown

I

1034.8

686.6

658.2

1159.3

Orange Light

II

1010.1

667.4

511.6

1230.6

Yellow

III

519.0

712.0

183.8

522.2

Golden

IV

1284.0

608.0

623.4

1212.5

Brown

V

Spectra in buffer solutions:

Generally, the spectra of these compounds in buffer solutions are very similar

and show two or three bands due to the existence of different species as

monocationic, neutral, monoanionic and /or dianionic. The effect of buffer solutions on

the absorption spectra of I as representative (figure 3) are explained, the pk,s are

calculated from absorbance–pH, (figure 4) and discussed , the mean pk,s values are

sited in table 5 .

139

Figure 3: Absorption spectra of I in universal buffer solutions.

140

Figure 4: Limiting modified absorption method of I .

From table 5, it is concluded that the difference between the pka values of

compounds I,II,III,IV and V is attributed to the substituent groups in the phenyl ring.

This is mainly due to electron–donating character of the substituted P–OH and P–

COOH groups which increasing the pka values, the pka values decrease as a result of

electron–drawing character from these groups, increasing the electron–donating

character leads to the easier charge–transfer toward the hetero ring.

Table (5): The pKa,s values of the azo compounds obtained by the modified

limiting method.

AAA

−maxmax

pH = pKa+log min

maxAA

AA−

−pH = pKa+log

pka pka

( nm)λ

Com.

11.75 11.10

12.10 11.85

440 595

I

12.15 12.25

12.24 11.88

475 520

II

11.90 11.78

11.75 9.62

480 580

III

11.82 11.98

11.86 11.76

480 580

IV

10.98 11.74

11.42 9.93

480 520

V

141

Thermal analysis: The fraction of crystallized material, calculated using the partial area analysis,

the crystallization activation energy (Ec) were calculated from the differential scanning

calorimetry (DSC) using different methods.

The DSC curves of the investigated compounds were indicated by one

exothermic crystallization peak, Tp (figure 5) corresponding to the crystallization

process, the onset temperature of crystallization,Tc has been defined as a temperature

corresponding to interaction of two linear portions of the DSC trace in the exothermic

direction[12]. The peak temperature of crystallization,Tp is the temperature at which the

overall crystallization rate attains its maximum value, the values of Tc and Tp for the

investigated (I,II) are given in table 6 as a function of heating rates α. The table

reveals that these values are shifted to higher values by increasing the heating rates.

The activation energy of crystallization ,Ec for the investigated compounds has been

estimated using the following methods :

Ozawa- Chen method :

ln (α / T2 ) = _ Ec / RT + const.

Plot of ln (α / T2 ) versus 1/T yields a straight line, then Ec can be evaluated .

Table(6): The Physical constants and activation energy of I and II.

Ec , x102 kcal mol-1

Comp.

heating

rate oC/min.

Tc (0C)

Tg (0C)

Tm (0C)

Ozawa-Chen

method

The Coats- Redferm –

Sestak method

I

5

10

15

20

183

184

185

185

195

196

198

199

199

201

202

208

75.69

73.07

72.54

79.33

3.13

2.77

2.61

2.10

II

5

10

15

20

184

191

192

192

199

200

201

206

204

205

206

206

77.57

76.84

77.57

76.08

2.61

2.54

2.77

1.96

142

Figure 5: DSC curves of I at different heating rates.

Before the second method can be applied to evaluate the activation

energy of crystallization (Ec), the order of the crystallization reaction (n), must

be determined using Ozawa method,[13] the value of n can be determined at

any fixed temperature as the slope of the reaction :

log [_ ln (1 _ x) ] = const. _ n ln α

The plot of log [_ln (1_x)] versus lnα for the investigated compounds at

four different temperatures, the average deduced value of n is equal (1≈2)[14].

The Coats- Redferm – Sestak method :

determines the influence of temperature on the crystallization fraction

(x) for at a particular heating rate in this method the following equation is

used:

ln [_ ln (1 _ x) / T 2n] = _ n Ec / RT + const.

The plot of ln [_ ln (1 _ x) / T 2n] versus 1/ Tof I,II at a heating rate 10oC /min is

shown in figure 6 and the Ec value was determined 2.77x102 and 2.54x102 kcal/mol

respectively.

143

Figure 6: The relation of ln[-ln(1-x)] versus 1/T of I(a) andII(b) at heating rate 10oC/

min.

The straight different in the value of Ec evaluated by different formulations may

be attributed to the different approximations that have been adopted while arriving at

the final equation of the various formalisms. This is due to the fact that the activation

energy in this method has been derived from the variation of the temperature that

scans the whole curves starting from the beginning of the crystallization process till

approximately its ends. Besides, it allows the determination of the dimensionality of

growth and the crystallization mechanism involves in amorphous materials. Table 6

shows the values of the effective activation energies of crystallization Ec calculated by

means of different models.

144

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[3] Clemlaand, D.S.; Zyss, J. eds . In Nonlinear Optical Properties of Organic Materials and Crystals, Academic Press: New York, 1987.

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