Synthesis of phthalimide disperse dyes and study on the interaction energy

Article abstract of DOI:10.1016/j.dyepig.2017.07.013

Three azo dyes had been synthesized using N-ethyl substituted, dibromo-substituted and dicyano-substituted phthalimides as diazo components. All of the synthesized intermediates and dyes have been characterized by MS, ¹H NMR, IR and elemental analyses. The dyeing behavior and fastness properties of these dyes have been investigated. Modeling the interaction energy of benzene-benzene, phthalimide-benzene and phthalimide-phthalimide rings, molecule optimization and the interaction energy researching have been done with density functional theory ωB97XD, 6-311G++(d,p) basis set. The results showed that: phthalimide molecule has better coplanarity. Molecule energy of benzene-benzene, phthalimide-benzene and phthalimide-phthalimide rings decreased first and then increased with the increase of the distance between the rings, the lowest energy happened when the distance was about 3.5 ?. Electrostatic force affected the interaction energy most. Substituent groups mainly altered the dispersion force and reduced the energy of the system. The optimization interaction energy of phthalimide-phthalimide, phthalimide-benzene and benzene-benzene was ?47.78 kJ/mol, ?34.66 kJ/mol and ?17.59 kJ/mol. Substituted dibromo group reduced the dipole interaction of the dye.

Full text of DOI:10.1016/j.dyepig.2017.07.013

Accepted Manuscript
Synthesis of phthalimide disperse dyes and study on the interaction energy
Yizhen Zhan, Xue Zhao, Wei Wang
PII:
S0143-7208(17)31174-9
10.1016/j.dyepig.2017.07.013
DYPI 6102
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 May 2017
Revised Date: 3 July 2017
Accepted Date: 7 July 2017
Please cite this article as: Zhan Y, Zhao X, Wang W, Synthesis of phthalimide disperse dyes and study
on the interaction energy, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.07.013.
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Graphical Abstract

ACCEPTED MANUSCRIPT
Synthesis of phthalimide disperse dyes and study on the
interaction energy
Yizhen Zhana, Xue Zhaob, Wei Wanga,*
aCollege of Chem-Bio Engineering, Donghua University, Shanghai 201620, PR China
bCollege of Textile&Garment, Shaoxing University, Shaoxing 312000, PR China
*Corresponding author: Wei Wang, Professor; E-mail: wangv@dhu.edu.cn
Abstract: Three azo dyes had been synthesized using N-ethyl substituted,
dibromo-substituted and dicyano-substituted phthalimides as diazo components. All of the
1
synthesized intermediates and dyes have been characterized by MS, H-NMR, IR and
elemental analyses. The dyeing behavior and fastness properties of these dyes have
been investigated. Modeling the interaction energy of benzene-benzene,
phthalimide-benzene and phthalimide-phthalimide rings, molecule optimization and the
interaction energy researching have been done with density functional theory ωB97XD,
6-311G++(d,p) basis set. The results showed that: phthalimide molecule has better
coplanarity. Molecule energy of benzene-benzene, phthalimide-benzene and
phthalimide-phthalimide rings decreased first and then increased with the increase of the
distance between the rings, the lowest energy happened when the distance was about
3.5Å. Electrostatic force affected the interaction energy most. Substituent groups mainly
altered the dispersion force and reduced the energy of the system. The optimization
interaction energy of phthalimide-phthalimide, phthalimide-benzene and benzene-
benzene was -47.78kJ/mol, -34.66kJ/mol and -17.59 kJ/mol. Substituted dibromo group
reduced the dipole interaction of the dye.
Key words:
Phthalimide
Interaction energy
Density functional theory
Structural optimization
Planarity
1. Introduction
Due to the smooth hand feel, drape property and anti-crease resistance,
polyester/spandex fabric was widely used in high end women's fabric. However, spandex
would be stained heavily when dyeing polyester/spandex fabrics with disperse dyes, and

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the stained dyes is difficult to clean away, which causes polyester/spandex fabrics to have
poor wash fastness [1,2]. Especially, when there are functional finishing requirements,
auxiliaries such as softeners, soil release agents, hydrophilic agents, anti-UV agents and
antistatic agents which can be added to the fabrics [3,4], which may easily migrate dyes
from the inside of the fiber to the fiber surface because of the solvent effect, and thus the
wash fastness will be significantly reduced.
So it’s an urgent task to find good wash fastness disperse dyes to resolve the
problem of polyester/spandex in dyeing and garment industry. Phthalimide-based azo
disperse dyes can be eliminated from the surface of the dyed polyester fiber through alkali
treatment and meet the increasingly demanding for wash fastness of textiles. Wash
fastness rating of phthalimide disperse dyes could reach to 4-5 grade according to the
testing methods AATCC 2A wash fastness to washing 49 ℃and ISO 105-C06 C2S wash
fastness to washing 60 ℃ [5,6]. Researchers mainly investigated the analysis of the
alkali-hydrolysis mechanism, under relatively mild alkaline conditions, phthalimide-based
azo disperse dyes undergo ring opening and are converted to water-soluble phthalate
species [7,8].
The application of phthalimide structure mainly as diazo component or coupling
component in disperse dyes [9-11]. Phthalimide disperse dyes also have better coplanar
aromatic rings, and one of best way to improve the washing fastness of dyes, is to
improve the planarity of dye molecules, better planarity aromatic molecules produce
greater intermolecule interaction energy, lower heat migration and excellent washing
fastness [12,13]. Most of azo disperse dyes containing benzene ring structure, when
benzo heterocyclic structure was introduced into the Molecule, and heterocyclic structure
was coplanar with the aromatic ring, the interaction energy(such as π-π interactions and
dipole interaction) between the dyes can affect dyes-dyes forces and dye-fiber forces, but
there are few relevant research reports to support this.
In order to understanding the good wash fastness mechanism of phthalimide-based
azo disperse dyes, the general need for a basic research of the relation between aromatic
molecule structure and intermolecule interaction energy has become vital.
More and more researchers pay attention to the study of quantum theory in

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intermolecule forces. Due to the high calculation efficiency, density functional theory (DFT)
in quantum theory calculation has been widely applied in electronic structure and energy
[14-17]. Long-range correction functional ωB97XD of DFT theory belongs to dispersion
corrected density functional theory (DFT-D), ωB97XD has been used to calculate
electrostatic properties, investigate the structure and interaction energy, the stacking
interactions between organic aromatic molecules and the hydrogen bond strength of the
phenol-phenolate anionic complex [18-21].
So density functional theory is a good theoretic computation tool to reveal the
disperse dyes structure-performance relationship. In this paper, the synthesis of N-ethyl
substituted (D1), dibromo N-ethyl substituted (D2) and dicyano N-ethyl substituted (D3)
and dyeing behaviour and fastness properties of these dyes have been investigated.
Phenyl group and phthalimide were optimized and the molecule interaction energy
between rings researched at ωB97XD/6-311G++(d, p) level.
2. Experimental section
2.1. Materials
Bromoethane, phthalimide, N,N-diethylaniline, copper(I) cyanide and zinc cyanide were
obtained from Aladdin Industrial Corporation (China). Nitric acid, sulfuric acid,
Tin( )chloride dehydrate, hydrochloric acid, ethanol, toluene,sodium hydroxide were
supplied by Xilong Chemical Co. (China), while ammonia, sodium nitrite, acetic acid,
sodium acetate, bromine were and triethylamine provided by Sinopharm Chemical
Reagent Co.,Ltd (China).
2.2. Dye application
Dyeing was carried out with a laboratory dyeing machine (Yabo, China). 5.0 g
polyester/spandex blends fabric (95:5) was dyed at 130 ℃×50 min with a liquor ratio of
15:1. The dye concentration was 2% (o.w.f) and the dyebath was adjusted to pH 4.5 using
acetic acid/sodium acetate. For the dyeing rate curve, the dyeing process was interrupted
at different temperatures during the heating stage (80 ℃, 90 ℃, 100 ℃, 110 ℃, 120 ℃and
130 ℃) and at different times during the isothermal step at 130 ℃(10 min, 20 min, 30 min,
40 min and 50 min).

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After dyeing, the samples were washed with warm water of 50 ℃ and then clearing at
80 ℃for 15 min (2 g/L NaOH). The reduction cleared samples were rinsed with water and
dried at 190 ℃× 40 s.
2.3. Measurement
2.3.1. Color evaluation
Color depth of disperse dyes on PET fabric is obtained from dyed sample at the maximum
wavelength by using a spectrophotometer Datacolor SF 600 PLUS-CT (Datacolor, USA).
Color measurement conditions: CIE standard illuminant D65, 10o observer, ultraviolet
included, specular component included, large aperture. The samples were measured at
four readings rotated 90o and the color intensity results averaged.
Color-depthbatch
Color intensity(%) =
×100%
Color-depthsample
700nm
Color-depth =
f (x)k /sdx
∫
380nm
Where f(x) is the function of K/S curve.
The colour strength (K/S) was calculated on the basis of the Kubelka–Munk equation
(1-R)2
K / S =
2R
where K is the adsorption coefficient, R is the reflectance of the dyed sample, and S is the
scattering coefficient.
Color intensity is the ratio of color-depthbatch-sample and color-depthstandard-sample. Standard
sample was randomly selected in parallel experiment. Color intensity results were
obtained from the spectrophotometer directly.
2.3.2. Spectrophotometry
1H-NMR spectra were obtained using an Bruker Advance Digital 400 (Swistzerland) at
400 MHz for solutions in solvent CDCl3. Mass spectrometry (MS) was carried out using a
Shimadzu GCMS-QP2010Ultra spectrometer (Japan). A Nicolet 6700 (Thermo Fisher,
USA) equipped with an attenuated total internal reflectance (ATR) accessory was used to
analyse chemical functionalities of samples. Elemental analyses were carried out on an
Elmentar Vario EL Ⅲ(Germany) for C, H and N.
2.3.3. Colorfastness

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The color fastness to washing was determined in accordance with the ISO 105-C06:
2010 (B1S) using rotawash washing machine (SDL-Atlas, USA). The color fastness to
perspiration and dye transfer was determined in accordance with the ISO 105-E04: 2013
and AATCC 163: 2013 using Binder oven(Binder, Germany).The degree of color staining
on the multifiber was assessed using Datacolor SF 600 PLUS-CT(USA).
2.4. Density functional theory calculations
Geometry optimization calculation and the stable ground state configuration and unit
cell parameter of synthesized dyes were obtained using the DFT/B3LYP method,
combined with the 6-311G (d) basis set. HOMO and LUMO molecule orbital and energy
were obtained using the DFT/B3LYP method, combined with the 6-311G++(d,p) basis set.
Molecule ring interaction energy studied using density functional ωB97XD function, basis
set selection 6-311G++(d,p).
2.5. Synthesis
Phthalimid disperse dyes(D1, D2 and D3) were synthesized according to Fig. 1, and
characterized using IR analysis as well as mass, elemental analyses and 1H-NMR
spectrophotometry.

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O
O
O
NO
NO
2
2
HNO
2
C H Br
2
5
HN
HN
N
H SO
2
DMF
4
O
O
O
MB1
MA1
O
O
N
N
N
NH
SnCl
2
N
N
N
2
HCl
O
N
D1
O
MB2
Br
HOAC
2
O
Br
Br
O
N
NH
N
N
2
N
N
O
Br
D2
Br
O
MB3
CuCN /Zn(CN)2
DMF
CN
O
N
N
N
N
O
D3
CN
Scheme 1. Synthetic scheme of Dyes
2.5.1. Synthesis of phthalimide intermediates
2.5.1.1. synthesis of 4-Nitrophthalimide (MA1)
Phthalimide (80 g) was added into mixed acid which prepared from fuming nitric acid
(65%, 100 g) and concentrated sulphuric acid (98%, 500 g) at 10 ℃. The reaction mixture
was then allowed to warm slowly to 25 ℃. After 12 h, the reaction product was
precipitated by pouring into ice (1500 g), filtered and washed with water until free from
acid, and purified by crystallisation from ethanol. Yield: 94%.
2.5.1.2. synthesis of 4-nitro-N-Ethylphthalimide (MB1)
4-Nitrophthalimide (30 g) was dissolved in DMF (180 ml), then potassium carbonate (20 g)
was added to the mixture, followed by stirring at room temperature for 6 h.
1-bromopropane (18 g) was added into the mixture and stirred for 6 h at 60 ℃. Following
drowning onto ice, the resultant precipitate was isolated by filtration, washed with water
and dried. The crude product was purified by recrystallisation from methanol. Yield: 81%.
2.5.1.3. 4-Amino-N- Ethylphthalimide (MB2)
N- Ethyl-4-nitrophthalimide (22 g) was stirred into a solution of stannous chloride

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dihydrate (100 g) in hydrochloric acid (37%, 100 ml) and water (100 ml). After 2.5 h, the
precipitate was collected by drowning onto ice. Filtered and washed with cold water until
free from acid. The product was purified byrecrystallisation from toluene. Yield: 92%.
2.5.1.4. synthesis of 4-Amino-3,5-dibromo-N-ethylphthalimide (MB3)
A mixture of 4-amino-N-ethylphthalimide (9.6 g), sodium acetate (10 g) and acetic
acid(150 ml) were stirred together at room temperature. Bromine (16 g) in acetic acid (25
ml) was added dropwise with thorough stirring, the reaction mixture was then drowned
onto ice and the resulting product was collected by filtration, washed with water and
recrystallised from ethanol. Yield: 67%.
2.5.2. Synthesis of Dyes
2.5.2.1. synthesis of dye D1
0.05 mol of 4-aminophthalimide was diazotized in 40 ml of 37 % HCl and 240 ml of
water, by adding 0.06 mol of NaNO2 at a temperature of 0 ~ 5 ℃. Then stirred for 5 h at 5
~ 10 ℃. The resulting solution was used immediately in the following coupling reaction.
The coupling component (0.05 mol) was dissolved in water (200 ml) and hydrochloric
acid (37%, 20 ml) and then cooled to 0 ～ 5 ℃ by the addition of ice. The diazonium
solution previously prepared was added dropwise over 30 min at the same temperature.
The mixture was stirred for a further 5 h at 5 ～ 10 ℃ and then aqueous sodium
hydroxide solution (10%) was added slowly below 10 ℃ until the pH rose to within the
range of 3 ～ 4. The precipitated product was filtered off, washed with water until the
filtrate was pH 6 ～ 7 and dried. The crude dyes were purified by recrystallisation from
ethanol.
2.5.2.2. synthesis of dye D2
D2 was prepared using the same procedure as the preparation of D1.
2.5.2.2. synthesis of dye D3
D3
A mixture of D2 (0.05 mol), zinc cyanide (0.055 mol) and copper(I) cyanide (0.025 mol) in
DMF (125 ml) was stirred at 90 ℃. After 3 h, the reaction mixture was added onto ice to
precipitate the product, which was then filtered off, washed with water and dried at 70 ～
75 ℃. The crude dyes were purified by recrystallisation from ethanol.

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Intermediates and dyes nuclear magnetic, elemental analyses and ms analysis were as
follows. 1HNMR spectrum of intermediates and dyes were shown in Fig. 1.
MA1
MB1
MB2
MB3
D1
D2

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D3
Fig.1. 1HNMR spectrum of intermediates and dyes
MA1: m/z : 192. 1H-NMR(CDCl3): δ=8.08-8.10(1H , d , Ar-H)，8.64-8.71(2H , t , Ar-H)，
11.77 (1H , s , -NH)ppm.
MB1: m/z : 220. 1H-NMR(CDCl3): δ=1.31(3H , t , -CH3)，3.80 (2H , m , N-CH2)，5.13(2H,
s,-NH2)，8.06-8.63 (3H ,m , Ar-H)ppm.
MB2: m/z : 190. 1H-NMR(CDCl3): δ=1.22(3H , t , -CH3)，3.70 (2H , m , N-CH2)，5.13(2H,
s,-NH2)，6.80-7.80 (3H ,m , Ar-H)ppm.
MB3: m/z : 348. 1H-NMR(CDCl3): δ=1.27(3H , t , -CH3)，3.69 (2H , m , N-CH2)，6.15(2H,
s,-NH2)，7.85 (1H ,s , Ar-H)ppm.
D1: m/z : 350. 1H-NMR(CDCl3): δ=1.23(9H , m, -CH3)，3.48(4H , q, 2×CH2)，3.75（2H，
q，N-CH2），6.90 (2H ,m , Ar-H)，7.88-8.23 (5H ,m , Ar-H)ppm. Calculated for C20H22N4O2:
C, 68.55; H, 6.33; N, 15.99; found: C, 66.26; H, 5.98; N, 14.78. Calculated for C18H18N4O2:
C, 68.55; H, 6.33; N, 15.99; found: C, 66.26; H, 5.98; N, 14.78.
D2: m/z : 508. 1H-NMR(CDCl3): δ=1.27(9H , m, -CH3)，3.51(4H , q, 2×CH2)，3.74（2H，
q，N-CH2），6.90 (2H ,d , Ar-H)，7.85 (2H ,d , Ar-H)，5.12(H ,s , Ar-H)ppm. Calculated for
C20H20Br2N4O2: C, 47.27; H, 3.97; N, 11.02; found: C, 42.09; H, 3.18; N, 9.73. Calculated
for C19H20N4O2: C, 47.27; H, 3.97; N, 11.02; found: C, 42.09; H, 3.18; N, 9.73.
D3: MS, m/z: 400. 1H-NMR (CDCl3): δ= 0.96 (3H, t, CH3)，1.25(6H, t, 2×CH3)，1.68 (2H,
m, CH2)， 3.68 (4H, q, 2×CH2)， 3.88 (2H, t, N-CH2)， 6.88 (2H, d, Ar-H)， 8.00 (H, d,
Ar-H)，8.30 (H, s, Ar-H) ppm. Calculated for C21H24N4O2: C, 65.99; H, 5.03; N, 20.99 ;
found: C, 39.94; H, 3.10; N, 13.20.

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2.6. Molecule interaction energy model setting up
One of influencing factors associated with heat migration and wash fastness is the
molecule interaction energy of dye-dye and dye-fiber. Disperse dyes aggregated inside
the fiber after dyeing, if the molecule interaction energy of dye-dye and dye-fiber is great,
the dye will be difficulty to migrate to the fiber surface.
O
C
O
O
CH CH
O
2
C O
2
(a) Polyester repeat unit
O
O
C
NH C H
NH
C
NH
CH
NH
2
4
2
(b) Spandex repeat unit
Fig.2 Polyester and spandex repeat unit
Polyester molecule is polyethylene terephthalate; spandex molecule structure is more
complicated, spandex is a polymer composed of organic units joined by carbamate
(urethane) links and formed by reacting a di- or polyisocyanate with a polyol, as shown in
Fig. 2. Most of disperse dyes applied are single azo. All of the three molecules above
contain single benzene ring structure, the inter-rings forces is benzene-benzene
interaction energy. Phthalimide disperse dyes containing phthalimide group, the
aggregated dye-dye inter-rings forces is phthalimide – phthalimide interaction energy, the
dye-fiber inter-rings forces is phthalimide – benzene interaction energy.
In order to investigate the influence of phthalimide in aromatic interaction energy,
benzene and phthalimide optimized at ωB97XD/6-311G++(d, p) level firstly, then several
typical dimeric molecule energy model were set up with optimized configuration, as shown
the dimer model a1 ~ d4 in Fig. 3. Five- or six- ring center overlapped in the direction of
the dotted line in each model.
a1
a2
a3

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b1
b2
c1
c2
c3
c4
c5
d1
d2
D3
d4
Fig.3. Molecule interaction energy model
3. Results and discussion
3.1. Infrared spectra analysis
D3
D2
2225
D1
2944
1595
MB3
MB2
MB1
MA1
3237
3449
3353
2990
2948
1731
1698
2000
wavenumbers(cm-1)
3319
3500
1545
1500
1346
1000
4000
3000
2500
500
Fig. 4. IR spectrum of intermediates and dyes

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The FTIR spectra of intermediates and dyes recorded in the range of 500 - 4000 cm−1
are presented in Fig. 4. FTIR spectrum of MA1 shows a broad absorption band centered
at 3319 cm-1 is attributed to stretching vibrations of –NH- of phthalimide group. The two
absorption bands near 1731 cm-1 and 1698 cm-1 are identified as C=O stretching
vibration. The FTIR spectrum of intermediates and dyes shows a number of characteristic
absorption bands. The peak at 1545 cm-1, 1346 cm-1 is associated with the stretching
vibrations of -NO2. The lower peak at 2990 cm-1, 2948 cm-1 and 2944 cm-1 were the
contribution of -CH3, -CH2 stretching vibration from ethyl group. The main peaks appear at
3449 cm-1，3353 cm-1 and 3237 cm-1 of MB2 and MB3 are related with the formation of
-NH2 group. The peaks located at 1595 cm-1 are attributed to stretching vibration of –N=N-.
The peak appeared at 2225 cm-1 is associated with the stretching vibrations of -CN.
3.2. Dyeing rate curve
120
100
80
60
40
20
0
D1
D2
D3
C.I.Disperse Red 167
C.I.Disperse Orange 30
C.I.Disperse Blue 79
T
t
80 90 100 110 120 130 130 130 130 130 130
10 20 30
40 50
temperture ( )×time (min)
℃
Fig. 5. Dyes time-temperature curve
Reasonable dyeing process could be set up according to dyeing curve. Fig. 5 showed
that dyeing rate of synthesized dyes at temperature 80 ℃ ～ 130 ℃was slower than azo
or anthraquinone disperse dye (C.I. Disperse Red 167, C.I. Disperse Orange 30 and C.I.
Disperse Blue 79). D1, D2 and D3 could not achieve maximum uptake until 130 ℃×10
min and reach dyeing equilibrium until 130 ℃×30 min. Different from equilibrium dyeing
temperature at 110 ℃ ～ 130 ℃ of azo or anthraquinone disperse dye, the dyeing
temperature of D1, D2 and D3 may be 135 ℃ in order to ensure dyeing levelness, and
heating rate will be strictly controlled.
The different time-temperature curve between synthesized dyes D1, D2 and D3 and

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C.I. 167, C.I. 30, and C.I. 79 showed that there was larger dye interaction energy in
synthesized dyes than S-type dyeing disperse dyes. Disperse dyeing of polyester fibers
follows a solid–solid interaction where a solid dye is solubilised in another solid fiber
phase. The disperse dyes in solid form or in micelle form dissolved into single dye
molecule in water and dyed on the fibers. Slow dyeing rate means more energy was
needed to damage dye-dye interaction energy. Phthalimide group has better coplanarity.
Meanwhile phthalimide group contains electron-rich hetero-atoms N,
O
and
electron-deficient phenyl ring, which will generated electrostatic potential energy and
affect the dye-dye, dye-fiber interaction.
3.3. Colorfastness
Table 1 Colorfastness of synthesized dyes
wool
acrylic polyester
3-4
nylon cotton diacetate
WI
4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3-4
4
5
5
5
5
5
5
5
5
5
5
5
5
3-4
4
Acid
5
5
PI
PI
PI
D1
D2
D3
Alkali
4
4
DT
WI
5
4-5
4
4-5
4
4-5
5
Acid
4-5
4-5
5
4-5
4-5
5
Alkali
5
DT
WI
5
4-5
5
4
4
Acid
4-5
4-5
5
4-5
4-5
5
Alkali
5
DT
5
WI, washing fastness; PI, perspiration fastness; DT, dye transfer fastness.
Usually, the nylon, diacetate, polyester and wool components of multifiber adjacent
fabrics are particularly prone to staining while acrylic and cotton suffered less staining
during colorfastness testing of disperse-dyed polyester fabrics.

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Fastness values for the staining of these multifibers are presented in Table 1. The
synthesized phthalimide-base dyes showed excellent levels of colorfastness to washing,
perspiration and dye transfer. The tested samples only were washed with 2 g/L NaOH and
dried at 190 ℃ × 40 s. Alkali washed fabrics exhibited high washing and perspiration
fastness, presumably because the phthalimide ring of the dyes is hydrolyzed to
solubilising groups. These hydrolyzed dyes are readily removed by alkaline wash-offs and
exhibit good wet fastness.
In addition, the good dye transfer fastness might be explained by interaction energy
between the dye-dye and the dye-fiber, which instructed that a bigger energy was needed
to destroy the dye-fiber and dye-dye interaction energy.
3.4. Phthalimide molecule optimization
Phthalimide molecule is optimized at ωB97XD/6-311G++(d, p) level and obtained
bond angles and dihedral angle parameters, as shown in Fig.6 and Table 2.
Fig.6. Optimized phthalimide structure
Table 2 Bond angle and dihedral angle parameter of optimized phthalimide
Bond angle /0
121.6143
108.484
Dihedral angle /0
A(2C,1C,6C)
A(2C,1C,13C)
A(6C,1C,13C)
A(1C,2C,3C)
A(1C,2C,11C)
A(3C,2C,11C)
A(2C,11C,12O)
D(6C,1C,2C,11C)
D(13C,1C,2C,3C)
179.9927
180.0082
129.9017
121.6143
108.4841
129.9016
129.1844
D(13C,1C,2C,11C) 0.0007
D(2C,1C,13C,14O) -180.0022
D(2C,1C,13C,15N) 0.0008
D(6C,1C,13C,14O) 0.0067
D(6C,1C,13C,15N) -179.9904

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A(2C,11C,15N)
A(12O,11C,15N)
A(1C,13C,14O)
A(1C,13C,15N)
A(14O,13C,15N)
104.7071
D(1C,2C,11C,12O) 180.0029
D(1C,2C,11C,15N) -0.0018
D(3C,2C,11C,12O) -0.0054
D(3C,2C,11C,15N) 179.9898
126.1084
129.1843
104.7072
126.1085
The ground-state geometries of optimized phthalimide structure were shown in Fig.6.
The optimized molecules structural parameters of dihedral angles and bond angles listed
in Table 2 which are in accordance with the atom numbering molecule given in Fig.6.
Fig. 6 and Table 2 shows that the sum of associated three bond angles of
phthalimide molecule are almost 360o. Such as A (2C, 1C, 6C), A (2C, 1C, 13C) and A
( 6C, 1C, 13C) bond angles were 121.6143o, 108.484o and 129.9017o respectively, and
the sum of them is 360o. And almost all the dihedral angles are 180o, both of bond angles
and dihedral angles described that six- and five- rings of phthalimide molecule had a
planar arrangement. Better planarity of dye molecules provides better molecule charge
distributions and larger π- π stacking interaction [22,23]. Van der Waals forces and
dispersion force also relate to the overlapped area between the molecules. planar
geometry favoring more efficient intra-molecule charge transfer, Most of the auxiliary
donors had well coplanar with the donor, acceptor and electron bridges, which could
effectively improve the density and flow capacity of the conjugate electron cloud [24].
3.5. Benzene - benzene interaction energy
Change the distance of benzene - benzene configurations(a1 is face to face distance
(the distance between the vertical ring center and parallel benzene center), a2 is edge to
face distance (the distance between the two H atom at the lowest position of the vertical
ring and parallel benzene center), a3 is point to face distance (the distance between H
atom at the lowest position of the vertical ring and parallel benzene center)), benzene -
benzene interaction energy is studied atωB97XD/6-311G++(d, p) level, shown in Fig. 7.

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50
40
30
20
10
0
a1
a2
a3
-10
-20
3
3.25 3.5 3.75
d (Å)
4
4.5
5
Fig.7. Benzene - benzene interaction energy
Fig. 7. shows that the system interaction energy reduced with the increase of the
distance between the rings, benzene - benzene three configurations reached minimum
energy when the distance was 3.5 Å ~ 3.75 Å, system energy then increased slowly with
increasing distance between the rings. This indicates that interaction energy between the
rings belongs to long range interactions. The a3 configuration of three model molecules
has lowest interaction energy. a3 configuration is the most stable structure, a3
configuration interaction energy is -13.13 kJ/mol at 3.5 Å distance. But overall, benzene -
benzene interaction energy is small.
3.6. Phthalimide - benzene interaction energy
Change the distance of phthalimide - benzene configurations (b1 is benzene ring to
phthalimide five- ring distance, b2 is benzene ring to phthalimide six- ring distance),
phthalimide - benzene interaction energy is shown in Fig. 8.
30
20
b2
b1
10
0
-10
-20
-30
-40
3
3.25 3.5 3.75
d (Å)
4
4.5
5
Fig. 8. Phthalimide- benzene interaction energy
Fig. 8 shows that, phthalimide - benzene interaction energy increased first and then

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decreased with increasing distance between the rings, phthalimide - benzene system
reached minimum energy output at 3.5 Å , this indicates that phthalimide - benzene
interaction energy between the rings is still belonging to long-range effect. Phthalimide -
benzene interaction energy is greater than that of benzene - benzene, b1 and b2
configuration interaction energy were -31.50 kJ/mol and -22.32 kJ/mol at the 3.5 Å
distance, proved there is a higher interaction energy between phthalimide disperse dyes
and fibers. On the other hand, b1 configuration has lower interaction energy than b2, the
five- rings containing nitrogen atoms and an oxygen atom, is an electronegativity center,
and benzene rings are electropositive, and they have electrostatic attractive force.
Mulliken charge analysis are essential analysis for study the intermolecule interactions,
and furthermore as suitable basis for examination the interactions energy in molecule
systems[15]. Some atoms are electron donor and others are acceptors, the energy
differentiation between the atoms makes the molecule vulnerable for interactions.
Calculated atomic charge in Fig.9 reveals that in contrast to the relative positive value
of the benzene group, the phthalimide group is relative negative, which indicates that
there is a concentration of charge density between the phthalimide and benzene rings, as
shown in Fig. 10, as expected for a electrostatic interaction, indicated that electrostatic
interaction energy plays an important role in phthalimide - benzene interaction energy.
Fig. 9. Mulliken charge of phthalimide and benzene rings
b1
b2
Fig. 10. Phthalimide - benzene electrostatic potential surface

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3.7. Phthalimide- phthalimide interaction energy
Change the distance of phthalimide - phthalimide configurations(c1 ~ c5 is face to
face distance, d1 ~ d4 is point to face distance), phthalimide - phthalimide interaction
energy is studied at ωB97XD/6-311G++(d, p) level, shown in Fig. 11.
40
25
c1
c3
c5
c2
c4
10
-5
-20
-35
-50
3 3.25 3.5 3.75
d (Å)
4
4.5
5
10
5
0
d1
d3
d2
d4
-5
-10
-15
-20
-25
-30
2.25 2.5 2.75
3
3.25 3.5 3.75
4
d (Å)
Fig.11. Phthalimide - phthalimide interaction energy
Fig.11 shows, phthalimide - phthalimide interaction energy increased first and then
decreased with increasing distance between the rings, overall, C-configuration is more
stable than D-configuration, both of the configuration system reached lowest energy at
3.5Å, the lowest c3 and d3 were -45.42 kJ/mol and -27.83 kJ/mol.
c3 has the greatest interaction energy and most stable in C-configuration, Fig.12
electrostatic potential energy shows that c3 is trans structure, the electrostatic attraction
between the rings, C- 1 is cis structure, the electrostatic repulsion between the rings,
indicated that electrostatic interaction energy plays an important role in phthalimide -
phthalimide interaction energy. d3 has the greatest interaction energy in D-configuration,

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mainly because of forming of weak non-covalent bond -N-H...π interactions between -NH-
and benzene rings, which played crucial role in chromophore alignment and poling
process.The degree of the orbital overlap between the five - imide ring and six - benzene
ring is suitable for the mobility of the electronic cloud. This molecule configuration
generates π – π interactions with the centroid - centroid bond distance between the
antiparallel configuration [25].
c1
c2
c3
c4
c5
Fig. 12. Phthalimide rings electrostatic potential energy diagram
3.8. Optimized configuration interaction energy
Optimized benzene - benzene (B-B), phthalimide - benzene (PLM-B) and phthalimide
- phthalimide (PLM-PLM) dimers based on the above results, optimized interaction energy
shown in Fig. 13.
-60
-50
-40
-30
-20
-10
0
PLM-PLM
PLM-B
B-B
Optimized dimer
Fig. 13. Optimized dimer interaction energy

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Fig. 13 shows that, optimized phthalimide - phthalimide, phthalimide - benzene and
benzene - benzene dimer interaction energy were -47.78 kJ/mol, -34.66 kJ/mol and
-17.59 kJ/mol, respectively. phthalimide - phthalimide and phthalimide - benzene
interaction energy are much larger than that of benzene – benzene. As for disperse dyes,
preferably coplanarity phthalimide structure can improve interaction energy between the
dye-dye and the dye-fiber. That’s to say phthalimide disperse dyes are more difficult to
migrate to the surface which improves the dye color fastness to washing.
3.9. Influence of substituents on interaction energy
Substituted position of phthalimide usually happened at 15N, 3C, 5C, for example,
alkly substitute on 15N, Br- and –CN substitute on 3C, 5C. Interaction energy was
affected by static electricity and spatial hindrance of substituents. c3 and b1 are the most
stable configuration in phthalimide - phthalimide and phthalimide – benzene system,
select these two type configuration, and substituted phthalimide with CH3-, Br- and –CN,
set up e1, e2, e3, e4, e5, e5 model, as shown in Fig.14.
O
O
O
Br
H C
CN
CN
3
HN
O
N
HN
O
O
Br
e1
e2
e5
e3
e4
e5
Fig.14. CH3-, Br- and CN- substituted molecule energy model

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0
-10
-20
-30
-40
-50
-60
e6
e4
e2
e5
e3
e1
3
3.25
3.5
3.75
d (Å)
4
4.5
Fig.15. Influence of substituents on interaction energy
Fig.15 shows that the introduction of a substituent does not change the trend with
the changing distance, the lowest substituted e1, e2, e3, e4, e5, e5 interaction energy at
3.5 Å are -34.13 kJ/mol, -39.38 kJ/mol, -38.86 kJ/mol, -52.24 kJ/mol, -51.72 kJ/mol and
-49.40 kJ/mol, so the substituted dimers are more stable than the unsubstituted dimmers,
both the electron-accepting and electron-donating substituted group would reduce the
energy of the system. The introduction of substituents mainly changes spatial hindrance,
dispersion forces and electrostatic forces. Spatial hindrance will increase system energy
while dispersion forces will reduce the system energy; -CH3 with its electron-donating
nature will increase electrostatic attractive force and reduce the interaction energy, while
the Br- and –CN with its electron- accepting nature will decrease electrostatic attractive
force and increase the interaction energy. But the e2 configuration energy is less than e1
configuration, e3 and E4 configuration energy is almost the same. These results suggest
that the electrostatic force does not greatly affect and is not a major influential factor, and
that the dispersion force may play an important role in interaction energy.
The synthesized dyes contained electron donor group (aniline), electronic bridge
(Nitrogen-nitrogen double bonds) and electron acceptor (phthalimide). Charge transfer
can take place in presence of acceptor-π-donor (A-π-D) configurations. Intramolecule
electron transfer caused the dipole interactions between dye molecules. Individual dye
molecules are also attracted to each other through dipole interaction, with the result that
many dyes exist in dyeing solution and fibers as aggregates. The influence of substituted
group on dyes structure and electron transfer also important to mention here.

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Table 3 substituent effects on dipole moment and ∆EHOMO→LUMO
dipole moment
(Debye)
8.454
EHOMO
(eV)
ELUMO
(eV)
∆EHOMO→LUMO
(eV)
-5.611
-5.751
-5.828
-2.702
-2.675
-3.301
2.909
D1
D2
D3
8.114
3.076
8.673
2.527
DFT computational results given in Table 3 showed that the HOMO–LUMO bandgap
∆EHOMO→LUMO of D1, D2 and D3 were 2.909 eV, 3.076 eV and 2.527 eV respectively. It is
generally believed that low-lying HOMO-LUMO energy gap might be helpful for electron
transition. In addition, the charge transfer from donor to acceptor can be clearly observed
[26]. Substituted changes in ∆EHOMO → LUMO associated with the electron density transition
from HOMO to LUMO were shown in Fig. 16. The introduced ethyl substituent groups did
not influence the dye molecule conjugated system. The Br- substituent in D2 damaged the
conjugated system. The torsion between acceptor phthalimide ring and benzene ring will
hinder the effective π-conjugation and was confirmed by electron-donor and acceptor in
chromophore [16]. HOMO and LUMO orbital electron distribution was relatively
concentrated which would restrict the movement of electrons in the dye. Compared with
D2, HOMO and LUMO orbital electron distribution of D3 did not change obviously and the
rod-shaped -CN group was also involved in frontier orbitals of conjugated system, which
caused electron dispersion and migration in the molecule more effortlessly.
HOMO orbital
LUMO orbital
D1

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D2
D3
Fig. 16. Dyes HOMO and LUMO orbital
Dihedrals of phthalimide structure and coupling components benzene ring in D1 and
D3 were almost situated at 180o, which described the good coplanarity of phthalimide
structure, coupling components benzene ring and the azo unit. However, in D2 the
phthalimide structure was not on the plane of coupling component benzene ring. The
dihedral in D2 was 50.99o, as shown in Fig. 16.
Planarity of dyes ensured the efficient intramolecule charge-transfer which would
affect intermolecule forces. As dipole moment shown in Table 3, dipole moment that can
be presented as the sum of three terms, namely, the orientation force, induction force and
the dispersion force [25]. DFT calculations were carried out to determine the dipole
feature of the synthesized dyes, and the results showed that D3 possessed the largest
dipole moment (8.673 Debye) which can provide strong dipole-dipole interactions
between dyes. On the contrast, D2 had the lowest dipole moment (8.114 Debye).
Introduction of ortho-substituted dibromo atoms at positions of the phthalimide phenyl ring
produced steric hindrance into dye backbone hindered electron transfer and reduced the
dipole-dipole interaction.
4. Conclusion
Phthalimide molecule is optimized at ωB97XD/6-311G++(d, p) level and the results

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described show that rings which contain six-and five-rings of phthalimide molecules have
better coplanarity. The system interaction energy increased first and then decreased with
increasing distance between the rings, phthalimide - phthalimide, phthalimide - benzene
and benzene - benzene configurations reached minimum energy when the distance was
at 3.5 Å. This indicates interaction energy between the rings belong to long range
interactions. Electrostatic interaction energy plays an important role in phthalimide –
phthalimide and phthalimide - benzene interaction energy.
Optimized phthalimide - phthalimide, phthalimide - benzene and benzene - benzene dimer
interaction energy were -47.78 kJ/mol, -34.66 kJ/mol and -17.59 kJ/mol, respectively.
phthalimide - phthalimide and phthalimide - benzene interaction energy are much larger
than that of benzene – benzene. As for disperse dyes, preferably coplanarity phthalimide
structure can improve interaction energy between the dye-dye and the dye-fiber. The
substituted dimers are more stable than the unsubstituted dimmers, adding a substituted
group would reduce the energy of the system. The electrostatic force is not a major
influential factor and the dispersion force play an important role in interaction energy. DFT
computational results given in D2 reveal that the resultant ∆EHOMO→LUMO for twisted
dibromo substituted azo dye is larger than the untwisted dicyano substituted D3, which
are consistent with their dipole moment.
N-ethyl substituted, dibromo-substituted and dicyano-substituted phthalimides dispese
dyes were synthesized through nitration, alkylation, amination, bromination, diazotization
coupling reaction.All of the synthesised intermediates and dyes have been characterised
by MS, 1H-NMR, IR and elemental analyses.
Dyeing rate of prepared dyes at temperature 80
~ 130
was slower than azo or
anthraquinone disperse dye (C.I. Disperse Red 167, C.I. Disperse Orange 30 and C.I.
Disperse Blue 79). In order to ensure dyeing levelness, heating rate of prepared dyes will
be strictly controlled. Phthalimide - phthalimide and phthalimide - benzene interaction
energy were much larger than that of benzene – benzene. Due to interaction energy of
dye-fiber and dye-dye, the dyes have good colorfastness.
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Highlights
·Three azo dyes had been synthesized using N-ethyl substituted, dibromo-substituted
and dicyano-substituted phthalimides as diazo components.
· Dimer molecular optimization and the interaction energy researching had been done with
density functional theory.
·Electrostatic force affected the interaction energy of phthalimide-benzene and
phthalimide-phthalimide rings most.
· Substituent groups mainly altered the dispersion force and reduced the energy of the
system.