A Combined Experimental and TD-DFT Investigation of Mono Azo

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Borderless Science Publishing 305

Canadian Chemical Transactions Year 2013 | Volume 1 | Issue 4 | Page 305-325

ISSN 2291-6458 (Print), ISSN 2291-6466 (Online)

Research Article DOI:10.13179/canchemtrans.2013.01.04.0052

A Combined Experimental and TD-DFT Investigation of

Mono Azo Disperse Dyes

Mininath S. Deshmukh and Nethi N. Sekar*

Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical Technology,

Nathalal Parekh Marg, Matunga, Mumbai - 400 019, India

*Corresponding Author: E-mail: [email protected], [email protected] Tel.: +91 22 3361

1111/2222, 2707(direct), Fax.: +91 22 3361 1020.

Received: October 21, 2013 Revised: November 1, 2013 Accepted: November1, 2013 Published: November 2, 2013

Abstract: Six mono azo disperse dyes were synthesized using diazotized dimethyl-4-amino-5-

nitrophthalate (5) and dimethyl-3-amino-4-nitrophthalate (5’) followed by the diazo coupling with

different N-substituted aromatic amines. The structures of the dyes were confirmed using FT-IR, 1H NMR

and mass spectral analysis. The geometries of the azo and hydrazone tautomeric forms of the dyes were

optimized at B3LYP/6-31G(d) level of theory, and their electronic excitation properties were evaluated

using density functional theory. The dyes displayed a broad absorption maximum in the visible region

between 502-550 nm. Synthesized azo disperse dyes were applied on polyester and nylon fiber. All the

dyes show very good light fastness and washing fastness properties.

Keywords: Phthalate Azo Dyes, Photo-Physical Properties, Light Fastness, Synthesis, TD-DFT

1. INTRODUCTION

Azobenzenes are most widely used in textile dyeing [1], leather dyeing [2] as well as high-tech

applications such as lasers [3], liquid crystal displays (LCD), non-linear optical devices [4], biological

and medical studies [5]. In addition to above features, azobenzenes have interesting properties such as the

reversible cis-trans photoisomerization of the azo π -bond when irradiated under UV light [6]. Also, they

contribute to the greatest production volume of the dyestuff industry due to their simplistic mode of

synthesis with a high atom economy.

Electron-donating (D) and electron-accepting (A) groups through an azo π-conjugated bridge

show intramolecular charge transfer properties, which makes it possible to reduce the gap between

HOMO and LUMO of the molecule for broadening the range of absorption and to study the relationship

between the variation of donor/acceptor chromophores and their corresponding photophysical and

electrochemical properties [7]. The electron-rich N,N-diethylaniline unit is one of the promising donor

moieties and nitro, ester are the acceptor moieties of the donor-acceptor type of functional chromophore

because of its good electron donating and accepting properties. The presence of acetamido groups at

ortho position relative to the azo link attributed to the hydrogen bonding with azo nitrogen as well as to

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Reaction condition: (i) H2SO4/ Methanol, Reflux,10 hr. (ii) H2/Pd-C, Methanol, rt. (iii) Toluene, Acetic anhydride, 80 oC, 4 hr. (iv) 90% fuming HNO3 , 0-5

oC. (v) Conc. H2SO4, 30 min, rt. (vi) Nitrosylsulfuric acid, 0-5

oC. (vii) Ethanol,

0-5 oC to rt.

Scheme 1: Synthesis of compound 6a-6c and 6’a-6’c

protect from reductive cleavage [8]. Furthermore, acetamido and ester moieties are helpful in increasing

their fastness properties, color strength and brightness as well as photostability [9-10].

In this paper, we report synthesis of mono azo disperse dyes by using azo coupling reaction

between N,N-diethyl substituted aniline as a coupler and diazonium salt of dimethyl-4-amino-5-

nitrophthalate and dimethyl-3-amino-4-nitrophthalate [11]. The Density Functional Theory (DFT) and

the time dependent density functional theory (TD-DFT) computations [B3LYP/6-31G(d)] were used to

study the geometrical and electronic properties of the synthesized molecules. The polarity effects of

different solvents on absorbance characteristics of the synthesized azo disperse dyes are also studied.

2. EXPERIMENTAL SECTION

2.1 Materials and equipments

4-Nitrophthalic acid, N,N-diethylaniline, N-(3-(diethylamino)phenyl)acetamide, N-(3-

(diethylamino)-4-methoxyphenyl)acetamide, sodium hydroxide, metamol (dispersing agent) and conc.

H2SO4 were purchased from s.d. fine chemicals Ltd, Mumbai, India. Solid reagents were characterized by

melting point and used without purification. Liquid reagents were purified by simple distillation at their

boiling points and used thereafter. Solvents were used after distillation at their boiling point and drying

according to standard processes.

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Figure 1: Correlation of synthesized mono azo disperse dye (A) with an analogous reported dye (B) in

DMF

All the reactions were monitored on precoated silica gel aluminum based plates kisel gel 60 F254

Merck, India. Purification of all the compounds was achieved by recrytallization. Melting points were

recorded on instrument from Sunder Industrial Product Mumbai by using open capillary and are

uncorrected. The absorption spectra of all the compounds were recorded on a Spectronic Genesys 2 UV-

visible spectrophotometer. The FT-IR spectra were recorded on a Jasco 4100 Fourier Transform IR

instrument (ATR accessories). 1H NMR spectra were recorded on a Varian Cary Eclipse Australia, USA

instrument using TMS as an internal standard. Mass spectra were recorded on Finnigan Mass

spectrometer. Chemical shifts are expressed in δ ppm using CDCl3 as a solvent and TMS as an internal

standard. Simultaneous DSC-TGA measurements were carried out on SDT Q600 v8.2 Build 100 model

of TA instruments Waters (India) Pvt. Ltd. DFT calculations were performed using Gaussian 09W

software package.

2.2 Synthetic Strategy

Six novel D-π-A azobenzenes have been synthesized by conventional methods. These dyes have

N,N-diethylaniline as an electron donor, nitro and ester group as the electron acceptors conjugated

through an azo π-bridge. They were synthesized by traditional azo coupling reaction [11] using

nitrosylsulfuric acid as the diazotizing agent because of its high reactivity which helps the protonation of

the N-atom [19]. The resulting diazonium salts were further coupled with different aromatic anilines (a-c)

to give the target azo disperse dyes (6a-6c and 6’a-6’c) Scheme 1.

In the first step, dimethyl-4-nitrophthalate 1 was synthesized from esterification of 4-

nitrophthalic acid. The compound 1 on reduction by using H2-10% Pd/C gave compound 2, which on

further acetylation using acetic anhydride in toluene yielded the intermediate 3. Nitration of the

intermediate 3 with fuming HNO3 and H2SO4 gave the intermediates 4 and 4’. Deacylation of 4 and 4’ in

concentrated H2SO4 gave the intermediates 5 and 5’ respectively.

The 1H NMR spectra of compounds 7c and 7’c recorded in both CDCl3 and DMSO solvents. In

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Figure 2: UV-Visible absorption spectra of synthesized azo disperse dyes 6a-6c and 6’a-6’c in ethanol

both solvents amide -NH proton peak was not observed but their Mass [M+H]+

peak was found to be at

502.60, while [M+H]+ peak of compounds 7a, 7b, 7’a and 7’b is in good agreement with their molecular

weight.

2.3 COMPUTATIONAL METHODS

Geometry of the synthesized azo dyes in their azo and hydrazone tautomeric forms was

optimized in the ground state at B3LYP/6-31G(d) level [12]. The ground state (S0) geometry of the dyes

in vacuum and solvent was optimized in their C1 symmetry using DFT [13]. The Becke’s three parameter

exchange functional (B3) [14] combining with nonlocal correlation functional by Lee, Yang and Parr

(LYP) [15] and basis set 6-31G(d) was used for all the atoms. The same method was used for vibrational

analysis to verify that the optimized structures correspond to local minima on the energy surface. TD-

DFT computations were used to obtain the vertical excitation energies and oscillator strengths at the

optimized ground state equilibrium geometries using the same hybrid functional and basis set [16]. The

geometry optimizations of all the molecules in different solvent environments were done using the Self-

Consistent Reaction Field (SCRF) method and the Polarizable Continuum Model (PCM) [17].

2.4 SYNTHESIS AND CHARACTERIZATION

The synthetic scheme for the preparation of the dyes 6a-6c and 6’a-6’c is shown in Scheme 1.

Dimethyl-4-amino-5-nitrophthalate (5) and dimethyl-4-amino-3-nitrophthalate (5’) were prepared by the

reported procedure [18] from 4-Nitrophthalic acid.

2.4.1 Preparation of azo dyes (6a-6c and 6’a-6’c)

2.4.1.1. Preparation of diazonium salts (6 and 6’)

A solution of the amine (5 or 5’) (2.5 g, 0.01 mol) in concentrated sulfuric acid (10 ml) was

slowly added to 1.56 M nitrosylsulfuric acid at 0-5 °C in 1 hr. Diazotization reaction was monitored using

starch iodide paper. Urea (0.02 g) was added to consume excess of nitrous acid on completion of

diazotization.

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General method for the preparation of nitrosylsulphuric acid

1.56 M Nitrosylsulfuric acid was prepared by adding 63 gm of dry sodium nitrite to 500 ml 98%

sulfuric acid at 0-5 °C. Addition of sodium nitrite to sulphuric acid was slow to avoid formation of brown

fumes. After complete addition the reaction mass was heated up to 80 °C till a clear solution was obtained

and cooled.

2.4.1.2 General method of Coupling

The coupler a-c (0.01 mol) was dissolved in 150 ml ethanol at 0-5 oC. The diazonium salt (6 &

6’) was added dropwise to the coupler solution at 0-5 oC over a period of 2 hr. After complete addition the

pH was adjusted between 5 and 6 by using cold sodium hydroxide (10% solution in water). The mixture

was stirred overnight and monitored by using H-acid as external indicator. The precipitated product was

filtered, washed with water and recrystallized from ethanol to give the dyes 6a-6c and 6’a-6’c

respectively.

2.4.2.1 (E)-Dimethyl-4-((4-(diethylamino)phenyl)diazenyl)-5-nitrophthalate 6a:

Yield: 75 %. m.p. 174-176 oC. MS: C20H22N4O6, m/z: 415.67 [M+H]

+. IR vmax cm

-1: 1252, 1525, 1723,

1593. 1H NMR (CDCl3, 400 MHz, δ ppm): 8.26 (s, 1H, Ar-H), 7.99 (s, 1H, Ar-H), 7.88 (d, 2H, Ar-H, J =

9 Hz), 6.76 (d 2H, Ar-H, J = 9 Hz), 3.96 (s, 3H, -OCH3), 3.95 (s, 3H, -OCH3), 3.51 (q, 4H, -NCH2), 1.27

(t, 6H, -CH3).

2.4.2.2 (E)-Dimethyl-4-((2-acetamido-4-(diethylamino)phenyl)diazenyl)-5-nitrophthalate 6b:


+. IR vmax cm

-1: 1251, 1522, 1713,

1590, 2968. 1H NMR (CDCl3, 400 MHz, δ ppm):

12.22 (s, 1H, NH), 8.33 (s, 1H, Ar-H), 8.27 (s, 1H, Ar-

H), 8.10 (s, 1H, Ar-H), 7.73 (d, 1H, Ar-H, J = 9.2 Hz), 6.52 (d, 1H, Ar-H, J = 8.8 Hz), 3.97 (s, 6H, -

OCH3), 3.54 (q, 4H, CH), 2.27 (s, 3H, CH), 1.29 (t, 6H, CH).

2.4.2.3 (E)-Dimethyl-4-((2-acetamido-4-(diethylamino)-5-methoxyphenyl)diazenyl)-5-nitro phthalate 6c:


+. IR vmax cm

-1: 1241, 1251, 1524,

1715, 1592, 2970. 1H NMR (CDCl3, 600 MHz, δ ppm): 8.07 (s, 1H, Ar-H), 7.68 (s, 1H, Ar-H), 7.64 (s,

1H, Ar-H), 6.73 (s, 1H, Ar-H), 4.07 (m, 13H), 2.35 (s, 3H, CH), 1.38 (s, 6H, CH3).

2.4.2.4 (E)-Dimethyl-4-((4-(diethylamino)phenyl)diazenyl)-3-nitrophthalate 6’a:


+. IR vmax cm

-1: 1254, 1528, 1725,

1590. 1H NMR (CDCl3, 600 MHz, δ ppm): 8.10 (d, 1H, Ar-H, J = 8.4 Hz), 7.92 (d, 1H, Ar-H, J = 8.4 Hz),

7.88 (d, 2H, Ar-H, J = 9 Hz), 6.85 (broad singlet 2H, Ar-H), 3.96 (s, 3H, -OCH3), 3.94 (s, 3H, -OCH3),

3.51 (q, 4H, -NCH2), 1.27 (t, 6H, -CH3).

2.4.2.5 (E)-Dimethyl-4-((2-acetamido-4-(diethylamino)phenyl)diazenyl)-3-nitrophthalate 6’b:


+. IR vmax cm

-1: 1250, 1522, 1712,

1589, 2966. 1H NMR (CDCl3, 400 MHz, δ ppm): 12.21 (s, 1H, NH), 8.21(s, 1H, Ar-H), 8.08 (d, 1H, Ar-

H, J = 8.4 Hz), 7.97 (d, 1H, Ar-H, J = 8 Hz), 7.76 (d, 1H, Ar-H, J = 9.2 Hz), 6.52 (d, 1H, Ar-H, J = 9.2

Hz), 3.95 (s, 6H, -OCH3), 3.52 (q, 4H, CH), 2.27 (s, 3H, CH), 1.28 (t, 6H, CH).

2.4.2.6 (E)-Dimethyl-4-((2-acetamido-4-(diethylamino)-5-methoxyphenyl)diazenyl)-3-nitro phthalate 6’c:


+. IR vmax cm

-1: 1242, 1254, 1522,

1715, 1590, 2972. 1H NMR (CDCl3, 600 MHz, δ ppm): 8.18 (d, 1H, Ar-H, J = 7.8 Hz), 7.77 (d, 1H, Ar-

H, J = 8.4 Hz), 7.43 (s, 1H, Ar-H), 7.08 (s, 1H, Ar-H), 3.98 (s, 3H, -OCH3), 3.96 (s, 3H -OCH3), 3.93 (s,

3H, -OCH3), 3.39 (s, 4H, CH), 2.37 (s, 3H, CH), 1.25 (s, 6H, CH).

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Where, a=Tetrahydrofuran, b=Dichloromethane, c= Acetone, d= Ethanol, e= Methanol, f=Acetonitrile, g= N,N-

Dimethylformamide, h=Dimethyl sulphoxide.

Figure 3: Absorption spectra of dyes 6a-6c and 6’a-6’c in different solvent

0.0

0.2

0.4

0.6

0.8

1.0

360 460 560

Ab

sorb

an

ce (

au

)

Wavelength (nm)

6a a

b

c

d

e

f

g

h

0.0

0.1

0.2

0.3

0.4

0.5

0.6

360 460 560

Ab

sorb

an

ce (

au

)

Wavelength (nm)

6b a b c d e f g

0.0

0.2

0.4

0.6

0.8

1.0

380 480 580

Ab

sorb

an

ce (

au

)

Wavelength (nm)

6c a b c d e f g h

0.0

0.2

0.4

0.6

0.8

1.0

400 500 600

Ab

sorb

an

ce (

au

)

Wavelength (nm)

6'a a b c d e f g h

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

400 500 600

Ab

sorb

an

ce (

au

)

Wavelength (nm)

6'b a b c d e f g h

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

400 500 600

Ab

sorb

an

ce (

au

)

Wavelength (nm)

6'c a b c d e f g h

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Table 1: Observed UV-visible absorption and computed absorption spectra at B3LYP/6-31G(d) for dye

6a in different solvents

Medium

Experimental

Computed (TD-DFT)

D%

λmaxa

(nm)

Ɛ

Molar

Absorptivity

(dm3mol

-1cm

-1)

Verticalb

Excitation

(nm)

f c

Orbital Contribution

Band gap (eV)

THF 489 37260 521 0.595 H→L (85 %) 6.5

DCM 495 30305 523 0.598 H→L (85 %) 5.7

Acetone 507 30802 528 0.566 H→L ( 87 % ) 4.1

Ethanol 495 33368 529 0.564 H→L (85 %) 6.9

Methanol 501 35935 529 0.551 H→L (82 %) 5.6

Acetonitrile 504 34196 529 0.555 H→L ( 87 % ) 5.0

DMF 519 29560 532 0.580 H→L ( 87 % ) 2.5

DMSO 516 35604 532 0.575 H→L ( 87 % ) 3.1 aExperimental absorption wavelength,

bComputed absorption wavelength,

cOscillator strength

Table 2: Observed UV-visible absorption and computed absorption spectra at B3LYP/6-31G(d) for dye

6’a in different solvents

Medium

Experimental

Computed (TD-DFT)

D%

λmaxa

(nm)

Ɛ

Molar

Absorptivity

(dm3mol

-1cm

-1)

Verticalb

Excitation

(nm)

f c

Orbital Contribution

Band gap (eV)

THF 507 33741 489 0.634 H→L (70 %) 3.6

DCM 498 32582 492 0.569 H→L (67 %) 1.2

Acetone 519 35770 501 0.420 H→L (50 %) 3.5

Ethanol 519 37053 502 0.450 H→L (56 %) 3.3

Methanol 501 35356 503 0.431 H→L (58 %) 0.4

Acetonitril

e 507 36929 503 0.440 H→L (58 %) 0.8

DMF 519 42269 505 0.493 H→L (63 %) 2.7

DMSO 519 40779 506 0.477 H→L (63 %) 2.5 aExperimental absorption wavelength,



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3. RESULTS AND DISCUSSION

The absorption properties of the newly synthesized ester containing mono azo disperse dyes are

correlated with an analogous reported dye in DMF. The synthesized ortho-nitro substituted phthalate

mono azo disperse dyes 6b, 6c, 6’b and 6’c are blue shifted compared to the para-nitro substituted

terephthalate mono azo disperse dyes 7b-7c (Figure 1) because of the effectively charge delocalized

(conjugation) length is more in the para nitro as compared to the ortho nitro[20].

3.1 Photo-physical properties

The effect of solvent polarities on the absorption properties of the azo disperse dyes 6a-6c and

6’a-6’c was examined by studying the absorption spectra of these dyes in eight solvents of differing

polarity, dielectric constant and refractive indices. The absorption spectral data are summarized in

(Tables 1-6). These newly synthesized phthalate azo dyes with D-π-A system consist of an electron-

donating N,N-diethylaniline unit and electron-withdrawing nitro and ester groups conjugated through the

azo π-bonding bridge. They exhibited a strong red-shifted absorption (Figure 2). The results showed that

these dyes exhibit strong solvatochromic properties. The presence of electron donor unit shows red shift

which may be due to the fact that electron favours the intramolecular charge transfer (ICT) effect. The

compounds 6b and 6c show red shift as compared to the compound 6a since there exists a hydrogen

bonding between acetamido and azo groups in 6b and 6c. Such a hydrogen bonding is absent in 6a.

Analogously the dyes 6’b, 6’c are red shifted compared to the dye 6’a.

The absorption spectra of the dye 6a showed red shift with the increase in the solvent polarity.

The compound 6a shows absorption maxima at 489 nm in tetrahydrofuran and 519 nm in DMF. Similarly

the dyes 6b and 6c show absorption maxima at 497 nm in dichloromethane and 528 nm in DMF and at

513 nm in acetone and 534 nm in DMSO respectively (Tables 1-6 and figure 3). The dyes 6a, 6b and 6c

show a red shift of 20, 31 and 21 nm respectively, in the absorption maxima from non-polar to polar

solvent (figure 3) but the dyes 6’a-6’c did not show such a correlation between the absorption maxima

and the solvent polarities. The red shift of the CT band of the dyes 6b-6’b and 6c-6’c indicates that

acetamido alone and methoxy as well as acetamido unit respectively present on the donor unit enhance

electron donating effect than the unsubstituted N,N-diethylaniline unit present in 6a-6’a. Push-pull charge

transfer mechanism in the D-π-A type chromophore 6b is illustrated in Figure 4.

Figure 4: Charge transfer mechanism in push-pull chromophore of D-π-A (dye 6b)

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Table 3: Observed UV-visible absorption and computed absorption spectra at B3LYP/6-31G(d) for

dye 6b in different solvents

Experimental

Computed (TD-DFT)

Azo Hydrazone

Solv

-

ents

λmaxa

(nm)

Ɛ

Molar

Absorptivity

(dm3mol-1cm-1)

Verticalb

Excitation

(nm)

f c

Orbital

Contribution

Band gap (eV)

D%

Verticalb

Excitatio

n

(nm)

f c

Orbital

Contribution

Band gap (eV)

D%

a 504 21693 536 0.491 H→L (90 %) 6.3 476 0.722 H-1→L (85 %) 5.6

b 519 20942 538 0.495 H→L (90 %) 3.7 478 0.725 H-1→L (85 %) 7.9

c 513 22946 541 0.467 H→L (90 %) 5.5 478 0.680 H-1→L (85 %) 6.8

d 504 25501 542 0.466 H→L (90 %) 7.5 479 0.678 H-1→L (85 %) 5.0

e 498 24549 541 0.454 H→L (90 %) 8.6 478 0.661 H-1→L (82 %) 4.0

f 498 24299 542 0.459 H→L (90 %) 8.8 479 0.667 H-1→L (82 %) 3.8

g 528 24198 544 0.487 H→L (90 %) 3.0 481 0.707 H-1→L (85 %) 8.9

h 501 26152 544 0.482 H→L (90 %) 8.6 481 0.696 H-1→L (85 %) 4.0

aExperimental absorption wavelength,




dye 6’b in different solvents

Experimental

Computed (TD-DFT)

Azo Hydrazone

Sol

v-

ents

λmaxa

(nm)

Ɛ

Molar

Absorptivit

y

(dm3mol-1cm-1)

Verticalb

Excitatio

n

(nm)

f c

Orbital

Contribution

Band gap (eV)

D%

Verticalb

Excitatio

n

(nm)

f c

Orbital

Contribution

Band gap (eV)

D%

a 516 60900 512 0.343 H→L (67 %) 0.8 480 0.511 H-1→L (72 %) 7.0

b 516 50821 515 0.342 H→L (70 %) 0.2 481 0.505 H-1→L (72 %) 6.8

c 516 49738 520 0.298 H→L (70 %) 0.8 484 0.441 H-1→L (72 %) 6.2

d 516 54824 521 0.296 H→L (70 %) 1.0 485 0.436 H-1→L (70 %) 6.0

e 516 56897 521 0.283 H→L (70 %) 1.0 485 0.418 H-1→L (70 %) 6.0

f 522 56803 522 0.287 H→L (70 %) 0.0 485 0.422 H-1→L (70 %) 7.1

g 516 51904 523 0.314 H→L (70 %) 1.4 487 0.450 H-1→L (72 %) 5.6

h 507 51622 523 0.308 H→L (72 %) 3.2 487 0.441 H-1→L (72 %) 3.9




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dye 6c in different solvents

Experimental Computed (TD-DFT)

Azo Hydrazone

Solv

-ents

λmaxa

(nm)

Ɛ

Molar

Absorptivity

(dm3mol-1cm-1)

Verticalb

Excitation

(nm)

f c

Orbital

Contribution

Band gap (eV)

D%

Verticalb

Excitation

(nm)

f c

Orbital

Contribution

Band gap (eV)

D%

a 522 39705 548 0.527 H→L (90 %) 5.0 497 0.426 H-1→L (65 %) 4.8

b 522 44604 549 0.532 H→L (90 %) 5.2 497 0.472 H-1→L (70 %) 4.8

c 513 37303 550 0.508 H→L (90 %) 7.2 497 0.519 H-1→L (79 %) 3.1

d 519 48183 551 0.508 H→L (90 %) 6.2 497 0.528 H-1→L (79 %) 4.2

e 522 46252 550 0.497 H→L (90 %) 5.4 496 0.523 H-1→L (82 %) 5.0

f 525 46817 551 0.502 H→L (90 %) 5.0 497 0.531 H-1→L (82 %) 5.3

g 525 42861 553 0.529 H→L (90 %) 5.3 498 0.560 H-1→L (85 %) 5.1

h 534 44651 553 0.525 H→L (90 %) 3.6 498 0.560 H-1→L (85 %) 6.7





dye 6’c in different solvents

Experimental Computed (TD-DFT)

Azo Hydrazone

Solv

-

ents

λmaxa

(nm)

Ɛ

Molar

Absorptivity

(dm3mol-1cm-1)

Verticalb

Excitation

(nm)

f c

Orbital

Contribution

Band gap (eV)

D%

Verticalb

Excitation

(nm)

f c

Orbital

Contribution

Band gap (eV)

D%

a 546 41733 567 0.340 H→L (72 %) 3.8 496 0.329 H-1→L (67 %) 9.2

b 555 42535 570 0.337 H→L (72 %) 2.7 497 0.325 H-1→L (67 %) 10.5

c 552 42936 576 0.294 H→L (70 %) 4.3 500 0.247 H-1→L (58 %) 9.4

d 552 42284 577 0.292 H→L (70 %) 4.5 500 0.218 H-1→L (52 %) 9.4

e 549 43387 578 0.278 H→L (70 %) 5.3 500 0.185 H-1→L (70 %) 8.9

f 555 43136 578 0.282 H→L (72 %) 4.1 501 0.164 H-1→L (58 %) 9.7

g 549 42535 580 0.308 H→L (72 %) 5.6 504 0.205 H-1→L (50 %) 8.2

h 564 42084 581 0.302 H→L (72 %) 3.0 504 0.222 H-1→L (54 %) 10.6




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Figure 5: Twist dihedral angle and interatomic distances of dyes 6c azo in DCM [B3LYP/6-31G(d)]

Table 7: Twist dihedral angle of dyes 6a-6c and 6’a-6’c in different solvent [B3LYP/6-31G(d)]

Solvent 6a 6b 6c 6’a 6’b 6’c

THF 28.83 16.16 16.90 12.26 13.44 11.18

DCM 28.90 16.28 17.01 12.27 13.57 11.27

Acetone 29.13 16.70 17.35 12.42 13.98 11.59

Ethanol 29.16 16.77 17.40 12.47 14.05 11.65

Methanol 29.20 16.85 17.45 12.53 14.13 11.73

Acetonitrile 29.21 16.87 17.46 12.55 14.15 11.75

DMF 29.22 16.88 17.47 12.56 14.17 11.76

DMSO 29.24 16.93 17.50 12.61 14.23 11.81

3.2 Optimized geometries of the dyes 6a-6c and 6’a-6’c

Ground state geometries of the dyes 6a-6c and 6’a-6’c were optimized at B3LYP/6-31G(d) level.

The small twisting was observed between the donor and acceptor residues: C1-C6-C19-C20 (6a: 28.90o, 6b:

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16.28o, 6c: 17.21

o, 6’a: 12.27

o, 6’b: 13.57

o and 6’c: 11.27

o) in DCM (Figure 5). The variation of twist

dihedral angle of 6a, 6b and 6c from non-polar to polar solvents are 0.41o, 0.77

o and 0.60

o respectively. It

is concluded that the resulting optimized geometries of the dyes have little smaller twist dihedral angle in

the non-polar solvents than in the polar solvents. Similar observation was seen for the dyes 6’a, 6’b and

6’c (Table 7). Optimized bond lengths of the synthesized dyes 6a-6c and 6’a-6’c in DCM are tabulated in

Table 8. The N=N bond lengths are 1.277, 1.288 and 1.284 for the dyes 6a-6c. For the dyes 6b, 6c, 6’b

and 6’c azo N57-H58 bond lengths are 1.023, 1.022, 1.022 and 1.022 Å while hydrazone N31-H58 bond

lengths are 1.040, 1.032, 1.050 and 1.041 Å in DCM respectively at B3LYP/6-31G(d) level of

calculations. It is thus evident that the dyes exist in azo form.

The Mulliken charge distributions on the selected atoms of the dyes in DCM solvent at ground

state are shown in Table 9. In the ground state, charge on the atom N36 and C25 was found to be nearly the

same but the atom N30 and N31 observed some changes for azo and hydrazone forms of the dyes 6b-6c

and 6’b-6’c. The negative charge in the azo form is more located on the amide nitrogen N57 compared to

the nitrogen N31. The Mulliken charge distribution suggests that the azo form is more stable than the

hydrazone form of the dyes 6b-6c and 6’b-6’c [21]. The Mullikan charge distribution of the dyes is

summarized in Figure 6 by using the Gauss View 5.0 software [22].

Table 8: Computed interatomic distances of azo-hydrazone tautomers of dyes 6a-6c and 6’a-6’c in Å

[B3LYP/6-31G(d)]

Atom No. 6a

6b

6c

6’a

6’b

6’c

Azo Hydra-

zone Azo

Hydra-

zone Azo

Hydra-

zone Azo

Hydra-

zone

C2-C14 1.499 1.501 1.507 1.504 1.507 1.510 1.511 1.516 1.511 1.516

C3-C9 1.498 1.496 1.489 1.494 1.487 1.493 1.493 1.493 1.493 1.492

C5-N27 1.469 1.462 1.454 1.463 1.449 1.473 1.469 1.463 1.468 1.458

C6-N31 1.400 1.396 1.381 1.399 1.375 1.403 1.399 1.384 1.397 1.379

C19-C21 1.410 1.446 1.483 1.440 1.483 1.415 1.446 1.480 1.438 1.479

C19-N30 1.382 1.370 1.319 1.370 1.317 1.382 1.365 1.322 1.364 1.319

C21-N57 ----- 1.393 1.347 1.392 1.327 ----- 1.393 1.351 1.394 1.345

C22-O63 ----- ----- ----- 1.381 1.356 ----- ----- ----- 1.366 1.358

C25-N36 1.368 1.364 1.360 1.381 1.377 1.367 1.364 1.360 1.369 1.362

N30-N31 1.277 1.288 1.326 1.281 1.334 1.277 1.288 1.322 1.290 1.331

N31-H58 ----- 1.864 1.040 1.869 1.032 ----- 1.868 1.050 1.871 1.041

N57-H58 ----- 1.023 1.826 1.022 1.866 ----- 1.022 1.738 1.022 1.788

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Table 9: Mulliken charge distribution for dyes 6a-6c and 6’a-6’c in the ground state (GS) optimized

geometry in DCM [B3LYP/6-31G(d)]

Atom

No. 6a

6b

6c

6’a

6’b

6’c

Azo Hydra-

zone Azo

Hydra-

zone Azo

Hydra-

zone Azo

Hydra-

zone

C6 0.330 0.342 0.398 0.340 0.403 0.335 0.358 0.419 0.359 0.417

C9 0.564 0.568 0.607 0.583 0.606 0.591 0.583 0.600 0.584 0.599

C14 0.582 0.578 0.553 0.564 0.553 0.572 0.574 0.581 0.574 0.583

C19 0.266 0.245 0.273 0.252 0.296 0.265 0.245 0.265 0.252 0.282

C21 -0.141 0.366 0.383 0.377 0.399 -0.141 0.365 0.394 0.372 0.395

C25 0.386 0.398 0.403 0.353 0.344 0.386 0.397 0.403 0.362 0.380

N27 0.365 0.348 0.381 0.349 0.382 0.336 0.329 0.331 0.329 0.325

N30 -0.336 -0.323 -0.338 -0.324 -0.358 -0.322 -0.324 -0.340 -0.331 -0.358

N31 -0.358 -0.423 -0.555 -0.479 -0.576 -0.372 -0.435 -0.546 -0.438 -0.557

N36 -0.474 -0.479 -0.473 -0.482 -0.467 -0.474 -0.486 -0.473 -0.479 -0.471

N57 ------ -0.742 -0.683 -0.743 -0.656 ------ -0.739 -0.683 -0.742 -0.688

H58 ------- -0.399 0.441 -0.399 -0.454 ------- -0.393 0.421 -0.393 -0.424

O63 ----- ----- ----- -0.544 -0.515 ----- ----- ----- -0.530 -0.520

Figure 6: Mulliken charge distribution on the dye 6b azo and hydrazone in the ground state optimized

geometry in DCM [B3LYP/6-31G(d)]

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3.3 Calculated energies of azo-hydrazone tautomeric forms

The calculated energies (E/hartree), the Gibbs free energies (ΔG/hartree) and the relative energies

(ΔE/kJmol-1

) of the chromophores 6b-6c and 6’b-6’c in their azo and hydrazone tautomeric forms at

B3LYP /6-31G(d) level of the azo and hydrazone form given in (Tables 10, S1-S3). It is clear that the

azo form is relatively more stable than the corresponding hydrazone form by 48.69, 34.20 kJ mol-1

and

59.68, 52.59 kJ mol-1

level in vacuum respectively [23]. The calculated energies (E/(ΔG/hartree) and

relative energies (ΔE/kJ mol-1) in different solvents also show that the azo forms are slightly more stable

in the non-polar solvents as compared to the polar solvents.

3.4 Frontier molecular orbitals

The different frontier molecular orbitals were studied to understand the electronic transitions and

the charge delocalization within these D-π-A chromophores. The relative increase and decrease in the

energy of the occupied (HOMO’s) and the virtual orbitals (LUMO’s) gives a qualitative idea about the

excitation properties and the ability of hole or electron injection [24]. First allowed and the strongest

electron transitions with the largest oscillator strength usually correspond almost exclusively to the

transfer of an electron from HOMO → LUMO. Tables S4-S9 show the energies of the different

molecular orbitals involved in the electronic transitions of these D-π-A dyes in different solvents. It was

observed that the electronic transition in each case included HOMO → LUMO transition. In the case of

all synthesized azo disperse dye 6a-6c and 6’a-6’c, the energy gap of HOMO and LUMO orbitals was

decreased as the solvent polarity was increased (Figure 7, Tables S4-S9). The compounds 6a-6b show a

red shift compared to 6’a-6’b. In the case of the compound 6’c a bathochromic shift is observed in DCM

as compared to 6c. HOMO → LUMO gap gives clear evidence for the experimental observation for the

compounds 6a, 6b and 6’c (Figure 8).

Table 10: Computed energy at B3LYP/6-31G(d) in gas phase and different solvent for dye 6b

6b

Medium

Azo

Hydrazone

E/Hartree

a

ΔE/kJb

ΔG/Hartreec

E/Hartree

a

ΔE/kJb

ΔG/Hartreec

mol-1

mol-1

Gas

-1653.63 0 -1653.23

-1653.61 48.69 -1653.21

THF

-1653.64 0 -1653.25

-1653.63 45.70 -1653.23

DCM

-1653.64 0 -1653.25

-1653.63 45.49 -1653.23

Acetone

-1653.65 0 -1653.25

-1653.63 44.86 -1653.24

Ethanol

-1653.65 0 -1653.25

-1653.63 44.77 -1653.24

Methanol

-1653.65 0 -1653.25

-1653.63 44.66 -1653.24

Acetonitrile

-1653.65 0 -1653.25

-1653.63 44.63 -1653.24

DMF

-1653.65 0 -1653.25

-1653.63 44.62 -1653.24

DMSO

-1653.65 0 -1653.25

-1653.63 44.56 -1653.24

1 Hartree = 2625.5 kJ/mol aCalculated energies,

bGibbs free energies,

crelative energies.

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Figure 7: Energy gap between HOMO → LUMO of the dye 6a in Different solvents

Figure 8: Energy gap between HOMO → LUMO of the dyes 6a-6c and 6’a-6’c in different solvents

Molecular orbital diagrams of the dyes 6a-6c and 6’a-6’c are shown in (Figure 9). From the

diagram, it is found that the electron densities in the HOMOs of all these dyes were largely located on the

donor N,N-diethylaniline moiety, and the electron densities on the LUMOs were found to be localized on

acceptor moiety through the azo π-bridge. The excitation from HOMO to LUMO mostly consists of

charge transfer from donor N,N-diethylaniline moiety to the acceptor end. The energy gap of HOMO →

LUMO explains the charge transfer interactions within the dye.

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Figure 9: Frontier molecular orbitals of dyes 6a-6c and 6’a-6’c in the ground state

3.5 Electronic vertical excitation spectra (TD-DFT)

The electronic vertical excitations were calculated using TD-B3LYP/6-31G(d) method in vacuum

as well as in tetrahydrofuran, dichloromethane, acetone, ethanol, methanol, acetonitrile, N,N-

dimethylformamide, dimethyl sulphoxide solvent of different polarities. The computed vertical excitation

spectra associated with their oscillator strength (f), orbital contribution and band gap as well as their

experimental absorption spectra of the dyes 6a-6c and 6’a-6’c are shown in (Tables 1-6). The absorption

band at lower energy with higher oscillator strength is due to the intramolecular charge transfer (ICT)

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which is characteristic of the donor-π-acceptor dyes. These ICT bands for all the dyes were mainly due to

the electronic transition from the HOMO to LUMO.

The dyes 6a and 6’a show blue shifted absorption in THF (489 nm) and DCM (498), red shifted

absorption in DMF (6a: 519 nm, 6b:519 nm). The vertical excitation of the dyes 6a and 6’a were

computed and it shows blue shifted vertical excitation absorption in THF (521 nm), DCM (492 nm) and

red shifted vertical excitation in DMF (532 nm), DMSO (506 nm) respectively. Similar observations were

made for the dyes 6b-6c and 6’b-6’c. The chromophores 6b, 6’b, 6c and 6’c show absorption at in DMF

(528 nm), acetonitrile (522 nm), DMSO (534 nm) and DMSO (564 nm) respectively. The computed

vertical excitation for the dyes 6b, 6’b, 6c and 6’c are 544, 523, 553 and 581 nm for the azo form and

481, 487, 498 and 504 nm for the hydrazone form respectively. The percent deviation of the vertical

excitation of azo is less compared to hydrazone form. The vertical excitations of azo form are in good

agreement with the experimental data as compared to the hydrazone form. The largest (minimum)

difference between the experimental absorption maxima and computed vertical excitation in ethanol is 34

nm (13 nm in DMF), THF 18 nm (2 nm in methanol), acetonitrile 44 nm (16 nm in DMF), DMSO 16 nm

(0 nm in acetonitrile), acetone 37 nm (19 nm in DMSO) and DMSO 41 nm (15 nm in DCM) for dyes 6a,

6’a, 6b, 6’b, 6c and 6’c respectively.

3.6 General procedure of dyeing

Disperse dyeing of polyester fabric was carried out using high temperature high pressure method

in Rossari Labtech Flexi Dyer dyeing machine at a material to liquor ratio of 1:20. 2% dye was using for

dying (calculated on weight of the fabric) [25-26]. All the azo disperse dyes are having less solubility in

water. Initially the dye was dissolved in 5 mL N,N-dimethylformamide and diluted with 15 mL buffered

solution of pH 5 made by using sodium acetate and acetic acid in water. The mixture was ultrasonicated

for 15 min to obtain fine dispersion. Metamol was used as a dispersant. Polyester fabric was dyed using

the above solution and metamol as dispersing agent. The dye bath temperature was raised at a rate of 3 °C

min-1

to 130 °C, maintained at this temperature for 60 minutes, and rapidly cooled to room temperature as

shown in Figure 10. The dyed fabrics were rinsed under cold water and then reduction cleared in an

aqueous solution of 1 gL-1

sodium hydrosulfite and 1 gL-1

sodium hydroxide using 1:50 liquor to goods

ratio at 80 °C for 30 minutes. The dyed fabrics were washed by cold water and allowed to dry in the open

air.

3.7 Dyeing and fastness properties of the dyes 6a-6c and 6’a-6’c on polyester fabric:

Washing, sublimation, light fastness and color match properties were done by using standard

procedure [25-26]. The azo disperse dyes are insoluble in water. An aqueous dispersion in DMF-water

mixture of the dyes is used to apply on polyester and nylon fabric to avoid loss of the dye. These readily

dispersible dyes 6a-6c and 6’a-6’c were dyed on hydrophobic fibers (polyester and nylon fabrics) at 2%

shade by using high temperature high pressure technique. The dyed fabrics were evaluated in terms of

their fastness properties. Various fastness properties such as light, wash and sublimation of the dyed

polyester fabrics were studied and the values are given in (Tables 14-15).

3.7.1 Fastness to washing

Wash fastness property depends upon the solubility of dye in water, size of dye molecules, charge

present on the dye and which type of linkage present in dye molecule and fabrics. A sample of dyed

fabric was washed in solution (3 gL-1

Na2CO3, 1 gL-1

NaOH) at 60 °C for 30 minutes. The change in tone

of washed fabrics was assessed by the international standard scale IS: 765-1979 (1 for poor and 5 for

excellent) given in (Tables 14-15)

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Figure 10. Dyeing profile of polyester/Nylon used in this study

Table 14: Fastness properties of azo disperse dyes 6a-6c and 6’a-6’c on polyester fabrics

Dye No.

Light

fastness

(1-8)

Wash

fastness

(1-5)

Staining on Fabric

after washing

(1-5)

Sublimation

fastness

(1-5)

Staining on Fabric

after sublimation

(1-5)

Polyester Cotton Polyester Nylon

6a 6 4-5 4-5 3-4 2 2 2

6b 8 4-5 4 4 2 1-2 1

6c 7 5 4-5 4-5 2 3-4 3

6’a 8 5 4-5 4 2 1-2 2

6’b 8 4-5 4 3-4 2-3 1-2 1

6’c 8 4-5 4 3 3 2-3 2

Table 15: Fastness properties of azo disperse dyes 6a-6c and 6’a-6’c on nylon fabrics

Dye No.

Light

fastness

(1-8)

Wash

fastness

(1-5)

Staining on Fabric

after washing

(1-5)

Sublimation

fastness

(1-5)

Staining on Fabric

after sublimation

(1-5)

Nylon Cotton Nylon Cotton

6a 6 4-5 4 3-4 2 2 2

6b 7 5 4-5 4 2 2 1-2

6c 7 5 4-5 3-4 2-3 2-3 2

6’a 7 4-5 4 3-4 2-3 2 2

6’b 7 4 3-4 3-4 2-3 2-3 2

6’c 8 4 4-5 4 2-3 2 2

3.7.2 Fastness to sublimation

Sublimation fastness is the most significant requirement of the dyed polyester fabrics, as the

migration of the dye molecules and wet fastness of the azo disperse dyes on polyester fabrics are totally

dependent on the heat treatment. Sublimation fastness and staining of the dyes 6a-6c and 6’a-6’c on the

undyed polyester-cotton and nylon-cotton respectively showed poor to moderate ratings according to the

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international geometric grey scale.

3.7.3 Fastness to light

Light fastness is the degree to which a dye resists fading due to light exposure. All synthesized

dyes have quite susceptible to light damage which fully depends on the molecular structure. According to

the AATCC test method, light fastness of the dyes on polyester was better than on nylon fabrics. All the

dye showed very good light fastness on both polyester and nylon except the dye 6a on nylon.

Table 16: Color values of disperse azo dyes 6a-6c and 6’a-6’c on Polyester fabric.

Dye No. L* a* b* c* ho K/S

6a 58.92 35.92 13.01 37.93 20.05 17.80

6b 56.10 28.80 -1.61 28.84 356.80 17.52

6c 58.1 15.9 -21.1 26.5 307.1 12.20

6’a 63.09 38.89 16.07 42.08 22.45 16.16

6’b 59.66 42.03 5.89 42.44 7.98 16.67

6’c 53.93 18.30 -17.99 25.66 315.50 16.81

Table 17: Color values of disperse azo dyes 6a-6c and 6’a-6’c on Nylon fabric.

Dye No. L* a* b* c* ho K/S

6a 59.89 29.59 4.90 29.99 9.404 1.98

6b 57.59 26.83 -5.30 27.35 348.83 1.83

6c 56.08 13.41 -14.09 19.45 313.59 1.64

6’a 64.4 33.02 7.35 33.83 12.54 1.73

6’b 60.40 36.46 -2.44 36.54 356.17 2.17

6’c 53.55 12.79 -8.01 15.09 327.97 1.86

3.8 COLOR ASSESSMENT

The colorimetric parameters of the dyed polyester and nylon fabrics using the synthesized azo

disperse dyes 6a-6c and 6’a-6’c were recorded on a reflectance spectrophotometer CE-7000A Gretag-

Macbeth. CIE 1976 Color Space method used to evaluate color values of the synthesized azo disperse

dyes 6a-6c and 6’a-6’c on polyester fabrics in terms of L*, a* and b* (Tables 16-17). All the dyes have

good affinity towards the polyester fabrics at high temperature and gave red shades on polyester fabrics.

The values of the color coordinates suggest that the color hue of the dyes 6a, 6’a, 6’b on polyester and

the dyes 6a, 6’a on nylon shifted towards the redder direction on the red-green axis as well as towards the

yellowish direction on the yellow-blue axis as positive values of a* and b* respectively. While color hue

of the dyes 6b, 6c, 6’c on polyester and 6b-6c, 6’b-6’c on nylon shifted towards bluish direction on the

yellow-blue axis as negative values of b*. (a*, b*values from Tables 16-17). Color strength of all the

dyes applied on polyester and nylon fabrics are expressed as K/S value which is dependent on type of the

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substituent present on aromatic ring. The K/S values of these dyes showed that polyester has good dyeing

properties as compared to nylon.

4. CONCLUSION

In this paper, we report the synthesis of six D-π-A mono azo disperse dyes containing N,N-

diethylaniline as electron donor and electron withdrawing nitro/carbomethoxy group. All the disperse azo

dyes show very good fastness properties except sublimation due to the presence of the ester group. The

vertical excitation spectra were computed at B3LYP/6-31G(d) level and compared with the experimental

values. The results clearly indicate the existence of the azo form in all the dyes. Frontier molecular orbital

diagram shows electron density of chromophores is mostly on the donor moiety at ground state HUMO

and also observed was absorbance at longer wavelength due to HOMO → LUMO with high oscillator

strength.

ACKNOWLEDGEMENT

The authors are greatly thankful to TIFR, SAIF-I.I.T. Mumbai for recording the 1H-NMR and

Mass spectra. One of the authors (Mininath Deshmukh) is grateful to CSIR for providing fellowship.

SUPPORTING INFORMATION

Energies, Cartesian Coordinates, MS and 1H NMR spectra of dyes are included. This document is

available at http://canchemtrans.ca/uploads/files/Supporting_Information_0052.pdf

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The authors declare no conflict of interest

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