A combined experimental and TD-DFT investigation of three disperse azo dyes having the nitroterephthalate skeleton

Article abstract of DOI:10.1016/j.dyepig.2013.10.035

Abstract Three disperse azo dyes were synthesized using diazotized dimethyl 2-amino-5-nitroterephthalate (5) followed by the diazo coupling with different N-substituted aromatic amines. The structures of the dyes were confirmed using FT-IR, ¹H

Full text of DOI:10.1016/j.dyepig.2013.10.035

Dyes and Pigments 103 (2014) 25e33
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Dyes and Pigments
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A combined experimental and TD-DFT investigation of three disperse
azo dyes having the nitroterephthalate skeleton
*
Mininath S. Deshmukh, Nagaiyan Sekar
Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai,
Maharashtra 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2013
Received in revised form
23 October 2013
Accepted 25 October 2013
Available online 10 November 2013
Three disperse azo dyes were synthesized using diazotized dimethyl 2-amino-5-nitroterephthalate (5)
followed by the diazo coupling with different N-substituted aromatic amines. The structures of the dyes
were confirmed using FT-IR, 1H NMR, 13C NMR, MS and HRMS 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 computed absorp-
tion spectral data of the azo derivatives are in good agreement with the experiment, thus allowing an
assignment of the UVevis spectra. The dyes displayed a broad absorption maximum in the visible region
between 498 and 561 nm. The synthesized azo disperse dyes were applied on polyester and nylon fiber
and they show very good light fastness and washing fastness properties.
Keywords:
Nitroterephthalate azo dyes
Photo-physical properties
Light fastness
Ó 2013 Elsevier Ltd. All rights reserved.
TD-DFT
Solvatochromism
Dyeing
1. Introduction
relative to the azo bridge is attributed to the hydrogen bonding
with azo groups [8]. Also, acetamido and ester moieties are helpful
Azobenzenes have versatile applicability ranging from textile
dyeing [1], leather dyeing [2], coloring of plastics and polymer [3] to
advanced applications such as liquid crystal displays [4], biological
and medical studies [5]. In addition to the above features, azo-
benzenes have interesting properties like the reversible cis-trans
to increases their light fastness, color strength and bright hue as
well as photostability properties [9,10].
In this paper, we report synthesis of disperse azo dyes by using
traditional azocouplingbetween N,N-diethylsubstitutedaniline as a
coupler and diazonium salt dimethyl 2-amino-5-nitroterephthalate
[11]. Density functional theory computations [B3LYP/6-31G(d)]
were used to study the geometrical and electronic properties of
the synthesized molecules. The solvent effect on absorbance char-
acteristics of the synthesized azo dispersed dyes were studied in
solvents of different polarities.
photoisomerization about the azo
p-bond when heated or irradi-
ated in UV [6]. Also, they contribute to the greatest production
volume of the dyestuff industry due to simplistic mode of their
synthesis with a high atom economy.
The compounds having an intramolecular charge transfer
properties are usually functionalized by electron-donating (D) and
electron-accepting (A) groups through an azo p-conjugated bridge
2. Experimental section
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 elec-
trochemical properties [7]. The electron-rich N,N-diethylaniline
unit is one of the promising donor moiety of donoreacceptor type
of functional molecules because of its good electron-donating
properties. The presence of acetamido groups at ortho position
2.1. Materials and equipments
Dimethyl 2-aminoterephthalate, N,N-diethylaniline, N-(3-(dieth-
ylamino)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
further purification. Liquid reagents distilled at their boiling points
and used thereafter. Solvents were used after distillation at their
boiling point and drying according to standard processes.
* Corresponding author. Tel.: þ91 22 3361 1111/2222/2707 (direct); fax: þ91 22
3361 1020.
E-mail addresses: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com (N. Sekar).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.10.035

26
M.S. Deshmukh, N. Sekar / Dyes and Pigments 103 (2014) 25e33
All the reactions were monitored on precoated silica gel
were carried out using the Self-Consistent Reaction Field (SCRF)
under the Polarizable Continuum Model (PCM) [17].
aluminum based plates kisel gel 60 F254 Merck, India. Purification of
all the compounds was achieved by recrystallization. Melting
points were recorded on instrument from Sunder Industrial Prod-
uct Mumbai by using open capillary and are uncorrected. The ab-
sorption spectra of the compounds were recorded on a Spectronic
Genesys 2 UVevisible 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 stan-
dard. Mass spectra were recorded on Finnigan Mass spectrometer.
2.3. Synthesis and characterization
The synthetic scheme for the preparation of dyes 7ae7c is
shown in Scheme 1. Dimethyl 2-amino-5-nitroterephthalate (5)
was prepared by the reported procedure [18] from dimethyl 2-
aminoterephthalate (1).
2.3.1. Preparation of azo dyes (7ae7c)
The chemical shift values are expressed in
d ppm using CDCl3 as a
2.3.1.1. Preparation of diazotization salt (6). A solution of dimethyl
2-amino-5-nitroterephthalate 5 (2.5 g, 0.01 mol) in concentrated
sulfuric acid (10 ml) was slowly added to 1.56 M nitrosylsulfuric
acid at 0e5 ꢀC within 1 h. Diazotization reaction was monitored
using starch iodide paper. 0.02 g of urea was added to consume
excess of nitrous acid. The resulting diazonium solution 6 was
further used immediately for coupling.
solvent and TMS as an internal standard. DFT calculations were
performed on a HP workstation XW 8600 with Xeon processor,
4 GB RAM and Windows Vista as operating system. The software
package used was Gaussian 09W. The ground state geometry was
optimized at B3LYP level of theory and 6-31G (d) as basis set.
2.2. Computational methods
2.3.1.2. General procedure for coupling. The coupler aec (0.01 mol)
were dissolved in 150 ml ethanol at 0e5 ꢀC. Diazonium solution was
added dropwise to coupler at 0e5 ꢀC and 5e6 pH adjusted by using
10% sodium hydroxide cold solution. The reaction mixture was
further stirred overnight and monitor by using H-acid and starch
iodide paper. Precipitated product was filtered and washed with
water, crystallized from ethanol to give dyes 7ae7c respectively.
(E)-Dimethyl 2-((4-(diethylamino)phenyl)diazenyl)-5-nitrotereph
thalate (7a)
Gaussian 09 program package was used to optimize geome-
try and to study the synthesized azo dyes in their azo and
hydrazone tautomeric forms [12]. Ground state (S0) geometry of
the dyes in gas 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
atom. Same method was used for vibrational analysis to verify
that the optimized structures correspond to local minima on the
energy surface. Time Dependent Density Functional Theory (TD-
DFT) computations were used to obtained the vertical excitation
energies and oscillator strengths at the optimized ground state
equilibrium geometries at the same hybrid functional and basis
set [16]. All the computations in solvents of different polarities
Yield: 72%, Melting Point: 40e42 ꢀC.
FAB-MS, m/z: 415.67 [M þ H]þ,
HRMS (FABD) m/z: calcd for [M þ H]þ, 415.1618; found,
415.1617.
1H NMR (CDCl3, 600 MHz): 8.26 (s, 1H, AreH), 7.99 (s, 1H,
AreH), 7.86 (d, 2H, AreH, J ¼ 9 Hz), 6.74 (d 2H, AreH, J ¼ 9 Hz),
Scheme 1. Synthesis of azo disperse dyes 7a, 7b and 7c.

M.S. Deshmukh, N. Sekar / Dyes and Pigments 103 (2014) 25e33
27
Fig. 1. Structures of the dyes 7a, 7b and 7c.
3.96 (s, 3H, eOCH3), 3.95 (s, 3H, eOCH3), 3.47 (q, 4H, eNCH2),
groups an azo conjugated
p
-bridge. They were synthesized by azo
1.25 (t, 6H, eCH3).
coupling reaction [11]. The diazotisation was carried out using
nitrosylsulfuric acid because of its high reactivity which helps for
protonation of the N-atom [19]. The resulting diazonium solution 6
were further coupled with the different N,N-diethyl substituted
aniline (aec) to give the target azo disperse dyes (7ae7c) Fig. 1. In
the first step, acylation of 1 in acetic anhydride in toluene to give 2,
which on further nitration in fuming HNO3 and H2SO4 gives in-
termediates 3 (8%) and 4 (92%). The deacylation of 4 in concen-
trated sulfuric acid and methanol gives 5 (Scheme 1).
13C NMR (CDCl3, 600 MHz): 12.91(s), 39.47(s), 40.47(s), 44.88,
53.55, 112.05, 118.62, 125.48, 127.62, 129.72, 136.34, 143.08, 146.40,
147.47, 152.66, 165.26, 166.57.
FT-IR: 1254, 1526, 1723, 1593 cmꢁ1
(E)-Dimethyl 2-((2-acetamido-4-(diethylamino)phenyl)diazenyl)-
5-nitroterephthalate (7b)
Yield: 78%, Melting Point: 118e120 ꢀC.
FAB-MS, m/z: 472.20 [M þ H]þ,
HRMS (FABD) m/z: calcd for [M þ H]þ, 472.1832; found,
472.1832.
The structure of the compounds was confirmed by FTIR, 1H
NMR, 13C NMR, FAB-MS and HRMS spectral analysis. The 1H NMR
spectra of compound 7c amide eNH proton exchange with solvent
and their HRMS in positive [M þ H]þ and negative [M ꢁ H]ꢁ mode
was found to be at 502.1938 and 500.1782 respectively, while
compound 7a and 7b FAB-MS [M þ H]þ and HRMS [M þ H]þ is in
good agreement with the molecular weight of the compounds.
1H NMR (CDCl3, 400 MHz): 12.22 (s, 1H, NH), 8.33 (s, 1H, AreH),
8.27 (s, 1H, AreH), 8.10 (s, 1H, AreH), 7.71 (d, 1H, AreH, J ¼ 9.2 Hz),
6.55 (d, 1H, AreH, J ¼ 8.8 Hz), 3.95 (s, 6H, eOCH3), 3.50 (q, 4H, e
NCH2), 2.29 (s, 3H, eCH3), 1.28 (t, 6H, eCH3).
13C NMR (CDCl3, 600 MHz): 13.01(s), 25.33, 39.47(s), 40.47(s),
44.35, 53.49, 100.96, 109.06, 118.71, 125.83, 127.88, 132.25, 137.08,
146.26, 146.75, 154.00, 165.04, 167.05, 170.16.
FT-IR: 1251, 1522, 1713, 1590, 2968 cmꢁ1
3. Result and discussion
E)-Dimethyl
2-((2-acetamido-4-(diethylamino)-5-methoxyphenyl)
The absorption properties of the newly synthesized ester con-
taining mono azo disperse dyes are correlated with an analogous
reported dye in DMF. The synthesized nitro substituted tere-
phthalate mono azo disperse dyes 7ae7c are red shifted compared
to the reported dyes devoid of nitro group 8ae8c (Fig. 2) because of
the strong electron withdrawing effect of the nitro para to the azo
group [20]. The dye 7ae7c have blue shifted absorption as
compared to the reported dye 8d [21] which may be attributed to
the strong electron withdrawing effects of acetonitrile ester (Fig. 2).
diazenyl)-5-nitroterephthalate (7c)
Yield: 74%, Melting Point: 133e135 ꢀC.
FAB-MS, m/z: 502.60 [M þ H]þ,
HRMS (FABD) m/z: calcd for [M ꢁ H]ꢁ and [M þ H]þ 500.1781
and 502.1938; found 500.1782 and 502.1938 respectively.
1H NMR (CDCl3, 600 MHz): 8.07 (s,1H, AreH), 7.68 (s,1H, AreH),
7.64 (s, 1H, AreH), 6.73 (s, 1H, AreH), 4.07 (m, 11H), 2.35 (s, 3H, e
CH3), 1.38 (s, 6H, eCH3).
13C NMR (CDCl3, 600 MHz): 13.50(s), 25.06, 39.64(s), 40.31(s),
46.99, 53.68, 54.07, 100.96, 106.27, 121.22, 127.71, 128.40, 133.15,
137.05, 146.51, 146.90, 148.24, 164.76, 167.87, 169.45.
3.1. Photo-physical properties
FT-IR: 1240, 1251, 1522, 1713, 1590, 2968 cmꢁ1
.
The UVeVis absorption spectra of 1 ꢂ 10ꢁ6 mol Lꢁ1 solution of
dyes 7ae7c were measured in solvent of different polarity, dielec-
tric constant, refractive indices in (Tables 1e3). These newly syn-
thesized terephthalate azo derivatives with DepeA system consist
of an electron-donating N,N-diethylaniline unit and electron-
2.4. Synthetic strategy
Three De eA chromophoric dyes have been synthesized by
p
conventional methods. The dyes contain an electron donor N,N-
withdrawing nitro or carbmethoxy groups conjugated through
diethylaniline group, electron acceptor nitro and carbmethoxy
azo p-bonding exhibited strong red-shifted absorption (Fig. 3). The

28
M.S. Deshmukh, N. Sekar / Dyes and Pigments 103 (2014) 25e33
Fig. 2. Correlation of synthesized mono azo disperse dye (A) with an analogous reported dye (B) in DMF.
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 favors the intra-
molecular charge transfer (ICT) effect [7]. Red shifted absorption of
7b compared to 7a may be attributed to the planarity associated
with the hydrogen bonding between the hydrogen of acetamido
and azo nitrogen atom. Such a hydrogen bonding is absent in 7a.
The absorption spectra of dye 7a showed red shift with the in-
crease in the solvent polarity. The compound 7a shows absorption
maxima at 492 nm in dichloromethane and 516 nm in DMF. Simi-
larly, dyes 7b and 7c show absorption maxima at 522 and 537 nm in
dichloromethane and ethanol and at 549 in dichloromethane and
561 nm in DMF respectively (Tables 1e3 and Fig. 3). The dyes 7a, 7b
and 7c show a shift of 24, 27 and 24 nm respectively, shift in the
absorption maxima in different solvents of varying polarities
(Fig. 4). The comparison of 7a (516 nm) with the compounds 7b and
7c, the compounds 7b and 7c show a red shifted absorption 549 nm
for 7b and 561 nm for 7c in DMF respectively. The red shift of CT
band of 7be7c indicates that acetamido and methoxy as well as
acetamido unit present on donor unit maintained planarity and
hydrogen bonding effect than unsubstituted N,N-diethylaniline
unit present in 7a. This fact is supported by DFT. Pushepull charge
transfer mechanism in DepeA type chromophore 7b is illustrated
in Fig. 5.
Table 1
Observed UVevisible absorption and computed absorption spectra at B3LYP/6-
31G(d) for dyes 7a in different solvent.
3.2. Optimized geometries of dyes 7a, 7b and 7c
Medium
Experimental
Computed (TD-DFT)
%Dd
a
Ground state geometries of the dyes were optimized at B3LYP/6-
31G(d) levels for dyes 7ae7c. A small twisting was observed be-
tween C2eC1eC28eC29 (7a: 27.20, 7b: 20.60 and 7c: 21.51) in DMF
(Figs. S1eS5). The variation of twist dihedral angle of 7a, 7b and 7c
from non-polar to polar solvents are 0.477, 0.408 and 0.143
respectively. It is concluded that resulting optimized geometry of
dyes has little small twist dihedral angle in polar than non-polar
solvent (Table 4). Optimized bond lengths of the synthesized dyes
7ae7c in DMF are tabulated in (Table S1). The N]N bond lengths
are 1.279, 1.291, 1.294 for dyes 7ae7c. The NeH bond lengths are
1.024, 1.025 for 7be7c, the increased NeH bond length is due to the
hydrogen bonding between N27 and H40 atom [8]. While the
hydrogen bonding between N27/H40 for 7be7c in DMF chromo-
phores are 1.880 and 1.875 at B3LYP/6-31G(d) levels of calculations
respectively. From the computational data (Calculated energies (E/
lmax
Ɛ Molar
Verticalb
excitation
(nm)
fc
Orbital
contribution
band gap (eV)
(nm) absorptivity
(dm3
molꢁ1 cmꢁ1
)
THF
DCM
Acetone
Ethanol
Methanol
510
492
510
504
498
91,660
71,829
76,383
78,205
74,768
67,979
91,949
82,759
534
535
537
538
538
538
542
542
0.640 H / L (56%)
0.777 H / L (67%)
0.852 H / L (79%)
0.870 H / L (85%)
0.850 H / L (82%)
0.873 H / L (82%)
0.940 H / L (90%)
0.937 H/L (90%)
4.7
8.7
5.3
6.7
8.0
8.0
5.0
5.0
Acetonitrile 498
DMF
DMSO
516
516
a
Experimental absorption wavelength.
Computed absorption wavelength.
Oscillator strength.
b
c
d
(%D) Deviation between experimental absorption and vertical excitation
computed by DFT.

M.S. Deshmukh, N. Sekar / Dyes and Pigments 103 (2014) 25e33
29
Table 2
Observed UVevisible absorption and computed absorption spectra at B3LYP/6-31G(d) for dye 7b in different solvent.
Experimental
Computed (TD-DFT)
Azo
Hydrazone
a
lmax
Ɛ Molar
Verticalb
excitation
(nm)
fc
Orbital contribution
band gap (eV)
%Dd
Verticalb
excitation (nm)
f c
Orbital contribution
band gap (eV)
%Dd
(nm)
absorptivity
(dm3 molꢁ1 cmꢁ1
)
a
b
c
d
e
f
540
522
534
528
525
525
549
543
30,380
26,235
27,648
27,742
27,554
22,938
23,927
26,093
518
520
521
521
520
521
525
524
1.125
1.131
1.106
1.102
1.093
1.093
0.909
0.929
H / L (98%)
H / L (98%)
H / L (98%)
H / L (98%)
H / L (98%)
H / L (98%)
H / L (85%)
H / L (85%)
4.1
0.4
2.4
1.3
1.0
0.8
4.4
3.5
459
460
459
460
459
459
462
462
1.206
1.213
1.189
1.190
1.177
1.183
1.214
1.209
H-1 / L (95%)
H-1 / L (95%)
H-1 / L (95%)
H-1 / L (95%)
H-1/L (95%)
H-1 / L (95%)
H-1 / L (95%)
H-1 / L (95%)
15
12
14
13
13
13
16
15
g
h
Where, a ¼ Tetrahydrofuran, b ¼ Dichloromethane, c ¼ Acetone, d ¼ Ethanol e ¼ Methanol, f ¼ Acetonitrile, g ¼ N,N-Dimethylformamide, h ¼ Dimethyl sulphoxide.
a
Experimental absorption wavelength.
Computed absorption wavelength.
Oscillator strength.
b
c
d
(%D) Deviation between experimental absorption and vertical excitation computed by DFT.
hartree), Gibbs free energies (
D
G/hartree), Relative energies (
DE/
level in gas phase. The calculated energies (E/(
DG/hartree) and
k J molꢁ1), Electronic vertical excitation spectra (TD-DFT) and
Interatomic distance) it is observed that compounds exit in only azo
form (Tables S2eS3).
relative energies (
D
E/kJ molꢁ1) in different solvents also show that
the azo forms are slightly more stable in non-polar solvents as
compared to polar solvents [24].
The Mulliken charge distribution in ground state (DMF sol-
vent) on selected atoms of the dyes 7a, 7b and 7c are shown in
Table S4. In the ground state, charge on the atom N25 and C7 was
found to be nearly same but atom N27 and N39 observed some
changes for azo and hydrazone form of dyes 7be7c. The negative
charge in azo form is more located on the amide nitrogen N39
compared to the nitrogen N27. The Mulliken charge distribution
results suggest that azo form is more stable than hydrazone form
of dyes 7be7c (see Figs. S6eS10) [22]. Mulliken charge distribu-
tion of dyes 7ae7c is summarized in Table S4 by using Gauss
View 5.0 software [23].
3.4. Electronic vertical excitation spectra (TD-DFT)
Electronic vertical excitations were calculated using TD-B3LYP/
6-31G(d) method in gas phase as well as in tetrahydrofuran,
dichloromethane, acetone, ethanol, methanol, acetonitrile, n,n-
dimethylformamide, dimethyl sulphoxide solvent of different po-
larities. Computed vertical excitation spectra associated with their
oscillator strength (f), orbital contribution and band gap as well as
their experimental absorbance spectra of the dyes 7ae7c are
shown in (Tables 1e3). The absorption band at lower energy with
higher oscillator strength is due to intramolecular charge transfer
3.3. Calculated energies of azo-hydrazone tautomeric forms
(ICT) and is characteristic of donorepeacceptor pushepull dyes.
These ICT bands for all dyes mainly occurred due to the electronic
transition from highest occupied molecular orbital (HOMO) to
lowest unoccupied molecular orbital (LUMO).
The dye 7a shows blue shifted absorption in dichloromethane
(492 nm) and red shifted absorption in DMF (516 nm). The
vertical excitation of the dye 7a were computed and it shows
blue shift absorption in THF (534 nm) and red shift in DMF
To find out stability of azo and azo-hydrazone forms calculated
energies (E/hartree), Gibbs free energies (
DG/hartree) and relative
energies (
D
E/kJ molꢁ1) of the chromophores 7be7c in their azo and
hydrazone tautomeric forms calculated at B3LYP/6-31G(d) level
(Table S2eS3). It is clear that the azo form is relatively more stable
than the corresponding hydrazone form by 48.00, 47.67 kJ molꢁ1
Table 3
Observed UVevisible absorption and computed absorption spectra at B3LYP/6-31G(d) for dye 7c in different solvent.
Experimental
Computed (TD-DFT)
Azo
Hydrazone
Verticalb
excitation (nm)
fc
Orbital contribution
band gap (eV)
%Dd
Verticalb
excitation (nm)
fc
Orbital contribution
band gap (eV)
%Dd
a
lmax
Ɛ Molar absorptivity
(nm)
(dm3 molꢁ1 cmꢁ1
)
a
b
c
d
e
f
540
540
543
537
537
543
561
561
5862
6613
5611
6012
6914
6112
6764
7415
579
581
582
583
582
583
587
587
0.850
0.860
0.844
0.846
0.835
0.841
0.874
0.870
H / L (95%)
H / L (95%)
H / L (95%)
H / L (95%)
H / L (95%)
H / L (95%)
H / L (95%)
H / L (95%)
7.2
7.6
7.2
8.6
8.4
7.4
4.6
4.6
470
470
483
483
482
482
484
484
0.710
0.644
0.482
0.517
0.526
0.553
0.638
0.647
H-1 / L (67%)
H-1 / L (61%)
H-1 / L (50%)
H-1/L (54%)
H-1 / L (54%)
H-1 / L (56%)
H-1 / L (65%)
H-1 / L (65%)
13
13
11
10
10
11
14
14
g
h
Where, a ¼ Tetrahydrofuran, b ¼ Dichloromethane, c ¼ Acetone, d ¼ Ethanol, e ¼ Methanol, f ¼ Acetonitrile, g ¼ N,N-Dimethylformamide, h ¼ Dimethyl sulphoxide.
a
Experimental absorption wavelength.
Computed absorption wavelength.
Oscillator strength.
b
c
d
(%D) Deviation between experimental absorption and vertical excitation computed by DFT.

30
M.S. Deshmukh, N. Sekar / Dyes and Pigments 103 (2014) 25e33
DMF (0.4 nm dichloromethane) and 8.6 nm ethanol (4.6 nm
DMF) for dyes 7a, 7b and 7c respectively.
3.5. Frontier molecular orbitals
The different frontier molecular orbitals were studied to un-
derstand the electronic transition and charge delocalization
within these pushepull chromophores. The comparative increase
and decrease in the energy of the occupied (HOMO’s) and virtual
orbitals (LUMO’s) gives a qualitative idea of the excitation prop-
erties and the ability of hole or electron injection [25]. First
allowed and the strongest electron transitions with largest
oscillator strength usually correspond almost exclusively to the
transfer of an electron from HOMO / LUMO (Tables S5eS7) and
shows the energies of different molecular orbitals involved in the
electronic transitions of these pushepull dyes in different sol-
vents. It was observed that electronic transition in each case
included HOMO / LUMO transition. In the case of all synthe-
sized azo disperse dye 7ae7c, the energy gap of HOMO and
LUMO orbitals were decreased as the solvent polarity was
increased (Fig. 6, and Figs. S11eS12, Tables S5eS7). The LUMO
energy level of the compound 7ae7c are ꢁ3.04, ꢁ3.07,
and ꢁ3.07 eV in DMF remained same, and it may be due to the
very similar reduction potential (Fig. 7) of the dyes 7ae7c. This is
understandable because of the reduction site, assuming the
carbonyl of ester and nitro has the same chemical structure in
three molecules [26].
Fig. 3. Absorption spectra of dyes 7a, 7b and 7c (in DMF).
(542 nm). Similar, solvatochromic results were obtained for dyes
7b and 7c. The chromophores 7b and 7c experimental (vertical
excitation) absorption show blue shift absorption in dichloro-
methane 522 nm, (THF 518 nm) and methanol 537 nm (THF
579 nm), while red shift absorption in DMF at 549 nm (525 nm)
and 561 nm (587 nm) respectively. The vertical excitations of
azo form having less percent of deviation with experimental
data as compared to hydrazone form summarized in Tables 1e3
The largest (minimum) percentage deviation between the
experimental absorption maxima and computed vertical excita
tion is 8.7 nm in dichloromethane (4.7 nm in THF), 4.4 nm in
Molecular orbital diagrams of the dyes 7ae7c are shown in
(Fig. 7), from the pictorial diagram, it is clear that the electron
densities in the HOMOs of all these three dyes were largely located
on donor N,N-diethylaniline moiety, and electron densities on the
Fig. 4. Absorption spectra of dyes 7a, 7b and 7c in different solvent. Where, a¼Tetrahydrofuran, b¼Dichloromethane, c¼ Acetone, d¼Ethanol e¼Methanol, f¼ Acetonitrile, g¼ N,N-
Dimethylformamide, h¼Dimethyl sulphoxide.

M.S. Deshmukh, N. Sekar / Dyes and Pigments 103 (2014) 25e33
31
Fig. 5. Charge transfer mechanism in pushepull chromophore of DepeA (dye 7b).
LUMOs were found localized on acceptor through azo
p
-bridge. The
of 1 g Lꢁ1 sodium hydrosulfite and 1 g Lꢁ1 sodium hydroxide using
1:50 liquor to goods ratio at 80 ꢀC for 30 min. The treated fabrics
were rinsed by cold water and allowed to dry in the open air.
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 in-
teractions within the dye.
5. Fastness properties
4. General procedure of dyeing [27,28]
Washing, sublimation, light fastness and color match properties
were done by using standard procedure [27,28].
Disperse dyeing of polyester and nylon fabric was carried out
using high temperature high pressure method in Rossari Labtech
Flexi Dyer dyeing machine with a material to liquor ratio of 1:20. 2%
Dye was using for fabric dying (calculated on weight of the fabric).
All synthesized azo disperse dyes are having very less solubility in
water. Firstly 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. Fine dispersion of the dye in water
was obtained after ultrasonication for 15 min. Metamol was used as
a dispersant. Polyester fabric and nylon were dyed using the above
solution. Dyeing was commenced at room temperature. The dye
bath temperature was raised at a rate of 3 ꢀC minꢁ1e130 ꢀC and 80 ꢀC
respectively, maintained at this temperature for 60 min, and rapidly
cooled to 50 ꢀC as depicted in Fig. 8. The dyed fabrics were rinsed
under cold water and then reduction cleared in an aqueous solution
5.1. 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 g Lꢁ1 Na2CO3, 1 g Lꢁ1NaOH)
at 60 ꢀC for 30 min. 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 (Table 5).
Table 4
Dihedral angle between C2eC1eC28eC29 of dyes 7a, 7b and 7c in different solvent.
Solvents
7a
7b
7c
Dihedral angle
Dihedral angle
Dihedral angle
THF
DCM
27.64
27.56
27.33
27.28
27.23
27.21
27.20
27.16
21.02
20.92
20.66
20.63
20.61
20.61
20.61
20.61
21.64
21.63
21.57
21.55
21.52
21.52
21.51
21.50
Acetone
Ethanol
Methanol
Acetonitrile
DMF
DMSO
Fig. 6. Energy gap between HOMO / LUMO of the dye 7a in different solvent.

32
M.S. Deshmukh, N. Sekar / Dyes and Pigments 103 (2014) 25e33
Fig. 7. Frontier molecular orbitals of dyes 7a, 7b and 7c in the ground state.
damage which fully depends on molecular structure. According to
the AATCC test method all dye showed good light fastness on both
polyester and nylon fabrics.
6. Color assessment
The colorimetric parameters of the dyed polyester and nylon
fabrics using the synthesized azo disperse dyes 7ae7c 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 7ae7c on both the
polyester and nylon fabrics in terms of L*, a* and b* (Table 6) All the
dyes have good affinity towards the polyester fabrics at high tem-
perature and gave red shades on polyester fabrics. The values of
color coordinates suggested that the color hue of dyes 7a of poly-
ester and nylon shifted towards the redder direction on the red-
green axis as well as towards the yellowish direction on yellow-
blue axis as positive values of a* and b* respectively. While color
hue of dyes 7b and 7c of polyester and nylon shifted towards bluish
direction on the yellow-blue axis as negative values of b* (Table 6).
Color strength of all dyes applied on polyester and nylon fabrics are
expressed as K/S values which are dependent on the type of sub-
stituent present on the aromatic ring. K/S values of these dyes
showed polyester have eight to ten time good dyeing properties as
compared to nylon.
Fig. 8. Dyeing profile of polyester/Nylon used in this study.
5.2. Fastness to sublimation
Sublimation fastness is the most significant requirement of dyed
fabrics, as migration of dye molecules and wet fastness of azo
disperse dyes on dyed fabrics are totally depended on heat treat-
ment. Sublimation fastness and staining of 7ae7c azo disperse dyes
on the undyed polyester-cotton and nylon-cotton for polyester and
nylon respectively showed poor to moderate ratings because of
presence of ester moiety. The ratings were done by the interna-
tional geometric gray scale.
5.3. 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
Table 5
Fastness properties of azo disperse dyes 7ae7c on polyester and nylon fabrics.
Fabrics
Polyester
Nylon
Dye no.
Light fastness
(1e8)
Wash fastness
(1e5)
Staining on fabric after washing (1e5)
Sublimation
fastness (1e5)
Staining on fabric after sublimation (1e5)
Polyester/Nylon
Cotton
Polyester/Nylon
Cotton
7a
7b
7c
7a
7b
7c
7
7
6
6
7
6
5
5
4e5
4e5
4e5
4
5
5
3
3
4
4e5
4
2
2
5
5
1e2
4
4
3e4
3e4
1e2
3e4
3e4
3
4e5
4e5
5
5
4e5
4
4
4e5
4
3
Light fastness: (Grading: 1- poor, 2- slight, 3- moderate, 4- fair, 5- good, 6- very good, 7- excellent).
Wash fastness: (Grading: 1- poor, 2- fair, 3- good, 4- very good, 5- excellent).

M.S. Deshmukh, N. Sekar / Dyes and Pigments 103 (2014) 25e33
33
Table 6
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Color values of terephthalate azo disperse dyes 7ae7c on polyester and nylon
fabrics.
Fabrics
Dye no.
L*
a*
b*
11.88
ꢁ13.92
ꢁ7.02
4.68
c*
Ho
21.65
332.73
320.87
9.17
K/S
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Polyester
7a
7b
7c
7a
7b
7c
57.65
56.62
72.70
59.52
56.32
72.20
29.90
26.99
8.62
29
13.28
8.20
32.18
30.37
11.11
29.38
19.19
10.77
14.72
16.46
4.14
1.97
1.61
0.44
Nylon
ꢁ13.85
313.8
319.62
ꢁ6.98
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7. Conclusion
In this paper, we report the synthesis of three De
peA mono azo
dyes containing N,N-diethylaniline as the electron donor and the
electron withdrawing nitro/carbmethoxy group. These synthesized
1
dyes were confirmed by FT-IR, H NMR, 13C NMR, Mass and HRMS
spectral analysis. All the disperse azo dyes show very good fastness
properties except sublimation which can be attributed to the presence
of the ester group. The vertical excitation spectra were computed at
B3LYP/6-31G(d) level and comparedwith the experimentalvalues. The
results clearly indicate the dyes 7b and 7c only exist in the azo. Frontier
molecular orbital diagram shows electron density of chromophores
is slightly more concentrated on the donor moiety in the ground state
HUMO and also the observed absorbance at longer wavelength is due
to the HOMO / LUMO with high oscillator strength.
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Acknowledgments
The authors are greatly thankful to TIFR, SAIF-I.I.T. Mumbai for
recording the 1H NMR and Mass spectra. One of the authors Min-
inath S. Deshmukh is grateful to CSIR for financial support.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.10.035.
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