The Effect of Conformation on UV-vis Absorption Spectra of
Disazo Reactive Red Dyes
XIE XiaoMei, LI XiaoLei, LUO Hanhan, LI Wei 
College of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan 430073, China.
ABSTRACT: Due to the characteristics of bright color, complete color system and powerful applicability,
reactive dyes have become the dyes of heavy usage. In this study, twenty disazo reactive red dyes with J
acid as the coupling components were selected, and their ground state geometry were studied by BLYP
functional and TZVP basis set. Disazo reactive red dye has cis-, trans- and azo three conformations,
cis-conformation has the lowest energy and is considered as the most stable conformation. The UV-vis
absorption spectra were calculated by TDDFT employing B3LYP and PBE0 hybrid functionals and TZVP
basis set, and the mean errors are 0.094 eV and 0.133 eV for B3LYP and PBE0, respectively. Comparing
the calculated max of cis-, trans- and azo- conformations with experimental one, it can be found that
conformation plays an important role on UV-Vis absorption. Dyes 6 and 8 exist in azo-conformation not in
cis-conformation. “hole-electron” distribution analysis reveals that although these max arises from
different electron transitions, these electron excitations have the same character of local excitation (LE).
Keywords: Reactive dye, Uv-vis absorption spectrum, TDDFT
0. Introduction
Reactive dyes were initially commercially introduced for application to cellulose fibers since 1956,
until now, it is still their most important usage. Due to the characteristics of bright color, complete color
schemes and powerful applicability, reactive dyes have become heavily used. The matrix of reactive dyes
includes azo, quinonehydrazone, phthalocyanine, etc, and that azo dyes are more than 75% in the

Foundation item: Supported by Natural Science Foundation of Hubei Province (2010CDA089); Foundation of Hubei Provincial Education
Department (D20131605); Discipline Innovation Team Project of Wuhan Textile University (NO.201401020).
Biography: XIE Xiaomei, female, Master, research direction: supramolecular chemistry.
1
production and in varieties. As one of three primary colors, reactive red dyes are used as primary color in
triadic color schemes for the dyeing of cellulose fiber. Its coupling component mainly includes H acid, J
acid and γ acid series. H acid series dyes present a brilliant blue light red. Their synthesis processes are
simple and the costs are low, however, they have low affinities and moderate fixation rate, and the colors
of dyed fabrics are not full and sunlight fastness are only about 3-4 degree [1-3]. Reactive red dyes with γ
acid as the coupling component has satisfactory sunlight fastness [4-6], but its colored light is not as bright
as that of H acid series, and the fixation rate is not ideal. With J acid as the coupling component monoazo
dyes show an orange color and has high sunlight fastness and bright-colored light. Azo dye is
donor-acceptor chromogen and has two tautomers: azo isomer and quinonehydrazone isomer. Different
substituents on diazo group will make dyes present different colors, because the electron transfers of
chromogen and substituent are in different direction [7].
As the development of quantum mechanics theory and computer technology, molecular orbital theory
is more and more widely applied in the research of dye molecular structures, color prediction as well as the
designs and development of new dye molecules and chromogens [8-10]. Time-dependent density
functional theory (TDDFT) having the characteristics of high accuracy in predicting electronic spectra,
wide applicable scope and low-cost computational resources, has been successfully applied to the
prediction of electronic spectra of dyes [11-16]. However, some important issues of azo dyes have not been
studied. For example, which conformation is the most stable among cis-, trans- and azo-conformation?
What is the effect of conformational dynamics on UV-vis absorption?
In this study, twenty disazo reactive red dyes with J acid as the coupling components were selected as
target molecules. The ground state geometries of cis-, trans- and azo-conformation were studied by DFT
with BLYP functional, the UV-vis absorption were calculated by TDDFT method employing B3LYP and
PBE0 hybrid functionals and the effect of conformation on UV-vis spectrum was discussed.
1. COMPUTATIONAL MODEL AND MEHTODS
The structures and partial atom number of twenty disazo reactive red dyes are shown in Fig1. All dyes
have the same chromophore structural framework but with different substituent on rings A, B, C and amino.
2
The local energy lowest point on potential energy surface was obtained by geometry optimization with
DFT employing BLYP [17-20] exchange-correlation functional and TZVP basis set with small frozen core.
The relativistic effects were taken into account by scalar zero order regular approximation (ZORA). The
UV-vis absorption spectra were calculated at optimal geometry by TDDFT employing B3LYP and PBE0
[21-23] hybrid functionals and TZVP all electron basis set. The solvent effects were evaluated by
self-consistent reaction field (SCRF) method and ethanol was selected as solvent. Five lowest-lying
singlet-singlet excitations were calculated and the electron excitations were analyzed by Multiwfn program
[24]. All calculations were performed with ADF 2013 program suite [25-27].
2. RESULTS AND DISCUSSION
2.1 Conformation
The disazo reactive red dyes have azo and quinonehydrazone two tautomers. The quinonehydrazone
also has two conformers, one is the cis-conformer, in which the N-H is on the same side with carbonyl; the
other is trans-conformer, in which the N-H is on the opposite side with carbonyl (see Fig.2). The rings A
and C can freely rotate around ring B through C2-N1 or C17-N4 single bonds. (see Fig.1).
The optimized geometries at BLYP/TZVP level are presented in Fig.2. The bond energies are
-18.8495 Hartree, -18.8227 Hartree and -18.8132 Hartree for cis- and trans-conformation of
quinonehydrazone and azo conformation, respectively. The cis-conformation has the lowest energy and is
considered as the most stable geometry. In following UV-vis section, the cis-conformation was used to
calculate UV-vis absorption spectra.
Fig.1 The structures of disazo reactive red dyes.
3
A
B
C
Fig.2 The optimized geometries of cis-conformation (A), trans-conformation (B) and azo-conformation (C).
2.2 UV-Vis absorption spectrum
Five lowest-lying singlet excited states were calculated by B3LYP and PBE0 hybrid functionals and
TZVP basis set. The excitation energies (/nm), oscillator strengths (f), configurations and main
contributions of all dyes are listed in Table 1. The maximum absorption wavelength (λmax) were assigned
according to the principle of oscillator strength precedence. The errors of excitation energy calculated with
different functionals are plotted in Fig.3. The absolute mean errors are 0.092 eV and 0.133 eV for B3LYP
and PBE0, respectively.
0.5
0.4
B3LYP
PBE0
0.3
Error/eV
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
0
2
4
6
8
10
12
14
16
18
20
Number of dyes
Fig.3 The absolute error of calculated excitation energy (in eV) by B3LYP and PBE0.
Table 1 Excited energy (λ/nm), oscillator strength (f) and main configuration of dyes in cis-conformation calculated with SCRF-TDDFT at
B3LYP/TZVP and PBE0/TZVP level in ethanol solution
4
B3LYP/TZVP
dyes
Exp/
Excited
λ/nm
f
PBE0/TZVP
Main Configuration
nm
state
1
515
11A
514.1
1.412
HOMO→LUMO (96%)
2
505
21A
503.3
0.2821
H-3→LUMO(34%);
λ/nm
f
Main Configuration
Attribution
11A
489.3
1.454
HOMO→LUMO (93%)
π→π*
31A
471.4
0.919
H-1→LUMO (61%)
π→π*
Excited
state
H-1→LUMO (27%);
HOMO→LUMO (34%)
HOMO→LUMO(18%)
3
496
31A
488.9
0.592
H-2→LUMO (37%)
11A
513.9
0.512
HOMO→LUMO (81%)
π→π*
11A
584.6
0.592
HOMO→LUMO (71%)
π→π*
11A
507.2
0.622
HOMO→LUMO (46%)
π→π*
H-1→LUMO (32%)
HOMO→LUMO (14%)
H-2→L+1 (13%)
4
518
11A
5
529
11A
513.9
0.259
HOMO→LUMO (46%)
523.6
0.574
HOMO→LUMO (47%)
H-1→LUMO (46%)
H-1→LUMO (30%)
H-2→LUMO (23%)
H-2→LUMO (13%)
H-2→L+1 (13%)
H-1→LUMO (11%)
6
518
1A
604.1
0.763
HOMO→LUMO (83%)
1A
560.3
0.812
HOMO→LUMO (75%)
π→π*
7
496
21A
487.1
1.128
HOMO→LUMO (75%)
21A
471.7
1.475
HOMO→LUMO (94%)
π→π*
8
510
11A
508.2
1.224
HOMO→LUMO (91%)
11A
491.9
1.138
HOMO→LUMO (77%)
π→π*
9
518
21A
538.5
0.983
HOMO→LUMO (82%)
21A
519.2
1.136
HOMO→LUMO (93%)
π→π*
10
501
31A
519.1
0.989
HOMO→LUMO (53%)
31A
499.7
0.869
H-1→LUMO (60%)
π→π*
11
492
31A
522.4
0.685
HOMO→LUMO (48%)
21A
514.1
0.773
1
1
H-1→LUMO (44%)
HOMO→LUMO (38%)
H-1→LUMO (47%)
516
21A
13
524
11A
559.2
0.812
14
521
11A
530.9
1.059
12
486
0.427
HOMO→LUMO (76%)
π→π*
H-1→LUMO (18%)
11A
587.3
0.713
HOMO→LUMO (86%)
π→π*
HOMO→LUMO (94%)
11A
536.0
1.030
HOMO→LUMO (95%)
π→π*
HOMO→LUMO (78%)
21A
506.0
0.909
HOMO→LUMO (61%)
π→π*
H-1→LUMO(59%)
HOMO→LUMO(28%)
H-2→LUMO (13%)
H-2→L+1 (15%)
15
524
11A
16
518
11A
523.7
1.068
489.1
0.880
HOMO→LUMO (91%)
11A
505.1
1.100
HOMO→LUMO (86%)
π→π*
H-1→LUMO (61%)
31A
473.1
0.910
H-1→LUMO (72%)
π→π*
HOMO→LUMO (16%)
HOMO→LUMO (13%)
H-2→LUMO (15%)
17
503
31A
543
11A
503.3
0.932
H-1→LUMO (77%),
31A
487.5
0.873
H-1→LUMO (83%)
π→π*
460.3
0.651
HOMO→L+1 (52%)
π→π*
HOMO→LUMO (10%)
18
473.3
0.643
HOMO→LUMO (60%);
31A
H-5→LUMO (15%)
H-2→LUMO (16%)
H-2→LUMO (15%)
5
19
552
31A
528
31A
512.2
0.899
H-1→LUMO (79%)
11A
516.1
0.417
HOMO→LUMO (81%)
π→π*
11A
527.1
0.596
HOMO→LUMO (88%)
π→π*
HOMO→LUMO (13%)
20
533.9
0.834
H-1→LUMO (89%)
HOMO→LUMO (8%)
B3LYP functional presents a higher accuracy. The absolute errors of all dyes are less than 0.2 eV with
the exception of dyes 6 and 18 whose errors are 0.341 eV and 0.338eV, respectively. PBE0 presents a
slightly lower accuracy. There are four dyes whose errors are larger than 0.2 eV, they are dyes 4, 12, 16 and
18 whose errors are 0.273 eV, 0.292 eV, 0.227 eV and 0.410 eV, respectively. Conformation plays an
important role on these large errors. In B3LYP calculations, Dyes 6 and 18 have the largest error among all
dyes and were selected as model molecules to study the effect of conformation on the UV-vis absorption.
The UV-vis absorption spectra of cis- and trans-conformation of quinonehydrazone and azo-conformation
for dyes 6 and 18 were calculated at B3LYP/TZVP level of theory. The excitation energy, oscillator
strengths (f) and main configuration for different conformations are included in Table 2 and the simulated
spectra are presented in Fig.4. As for dye 6, the experimental max is 518 nm, the calculated max are
604.1 nm, 619.8 nm and 532.6 nm for cis- and trans-conformation of quinonehydrazone and azo
conformation, respectively. The calculated max of azo conformation is closest to experimental one and the
error is 14.6 nm. In above discussion, the cis-conformation has the lowest energy and is considered as the
most stable conformer, however, according to the comparison of max, dye 6 exists actually in azo
conformation not in cis-conformation of quinonehydrazone tautomer.
6
Fig.4
Table 2
The calculated UV-vis absorption spectrum of dyes 6 and 18 in cis-, trans- and azo-conformation
Calculated excited energy (λ/nm), oscillator strength (f) and main configuration of cis and trans conformation of quinonehydrazone
and azo tautomer of dye 6 and 18 at B3LYP/TZVP level
Exp/
nm
Excited
λ/nm
f
Main Configuration
state
dye 6
Cis-conformation of quinonehydrazone
518
11A
604.1
0.763
HOMO→LUMO (83%)
Trans-conformation of quinonehydrazone
518
11A
518
11A
619.8
0.569
HOMO→LUMO (80%)
Azo conformation
532.6
0.913
HOMO→LUMO (92%)
dye18
Cis-conformation of quinonehydrazone
543
31A
473.3
0.643
HOMO→L+1 (60%)
H-2→LUMO (16%)
Trans-conformation of quinonehydrazone
543
21A
497.8
0.520
H-1 H-2→LUMO(64%)
Azo conformation
543
21A
541.9
0.582
HOMO→LUMO(72%)
As for dye 18, the experimental max of dye 18 is 543 nm, the calculated max are 473.3 nm, 497.8 nm
and 541.9 nm for cis- and trans-conformation of quinonehydrazone and azo conformation, respectively.
The calculated max of azo conformation is almost equal to the experimental value, therefore, dye 18 exists
actually in azo-conformation not in cis-conformation.
From B3LYP calculations, it can be found that the maximum absorption bands of all twenty dyes
7
primarily arise from three types of electron transitions. The maximum absorption bands of dyes
1,6,7,8,9,13,14 and 15 mainly arise from the HOMOLUMO transitions; The maximum absorption bands
of dyes 2,4,10,11,12,17,18,19 and 20 mainly arise from the HOMO→LUMO and H-1→LUMO combined
transitions; The maximum absorption bands of dye 3,5 and 16 mainly arise from H-2→LUMO,
H-1→LUMO, HOMO→LUMO and H-2→L+1 combined transitions.
In single-electron excitation process, hole and electron distributions respectively denote the region
where an electron leaves and goes to. There are three well-known types of excitations [24]: (1) Local
excitation (LE): The hole and electron significantly share the same spatial range. (2) Charge-transfer
excitation (CT): The spatial separation of hole and electron is large, leading to an evident movement of
charge density from one place to another place. (3) Rydberg excitation (R): Electron mainly consists of
high-lying MOs, therefore the overlap between electron and hole is small. Rydberg excitation in general
does not lead to a prominent long-range movement of charge density; in other words, the interval between
the centroid of hole and electron is small. The r index is a quantitative indicator of electron excitation
mode. The smaller the r index is, the more likely the excitation is a local excitation mode.
All electron transitions were analyzed with “hole-electron” distribution, the hole (a) and electron (b)
distribution and charge density difference (C) of different electron excitation are presented in Fig. 5-7( in
accessory), and the r index of electron transition of dye 1, 2 and 3 are listed in Table 3.
Table 3 The r index of electron excitation for dyes 1,2 and 3
dye
Excited state
Configuration
r (Å)
1
11A
HOMO→LUMO (96%)
1.213
2
31A
H-1→LUMO (41%)
0.817
HOMO→LUMO (51%)
0.547
H-2→LUMO(37%),
1.978
H-1→LUMO (32%)
0.214
H-2→L+1 (13%),
0.095
HOMO→LUMO (14%)
0.515
3
31A
8
As for dye 1, The hole mainly distributes on C2,C3,C5 and C6 of ring A, C7,C13 and C15 of ring B,
N1 and N4 of azo, O of OCH3, N of NH2. (Fig.5a); the electron has main contribution from N1-N2 and
N3-N4 of azo, C7, C8, C9, C14, C15, C16 of ring B (Fig.5b). Fig. 5d presents the isosurface of charge
density difference, the blue region denotes the decrease of electron density, and the green region denotes
the increase of electron density. The HOMO→LUMO transition results in the decrease of electron density
of C2,C3, C5,C6 of ring A, C7,C13 and C15 of ring B, N1 of azo, O of OCH3, N of NH2, and results in the
increase of electron density of C8,C14,C16 of ring B, N2,N3,N4 of azo, O of carbonyl. The r index of
HOMO→LUMO excitation is 1.213 Å, which is smaller than 2 Å, reveals that the HOMO→LUMO
transition is a local excitation (LE).
As for dye 2, the max mainly arises from the HOMO→LUMO and H-1→LUMO combined
transitions. The isosurface of hole, electron, overlap of hole-electron and charge density difference of dye 2
are presented in Fig.6. Comparing with that of dye1, the hole and electron have the similar distribution.
The HOMO→LUMO and H-1→LUMO combined excitation make the same effect on the increase or
decrease of electron density. The r index of HOMO→LUMO and H-1→LUMO excitation are 0.817Å
and 0.547 Å, respectively, reveals that HOMO→LUMO and H-1→LUMO combined transitions have the
character of local excitation (LE).
As for dye 3, the max mainly arises from H-2→LUMO, H-1→LUMO, H-2→L+1 and
HOMO→LUMO combined transitions. The distribution of hole and electron is slightly different from
those of dyes 1 and 2. The excitation results in the increase of electron density of N2, C16,C8 and C14 and
the decrease of electron density of N1, C7, C15 and C13 (see Fig.7 ). The r index of H-2→LUMO,
H-1→LUMO, H-2→L+1 and HOMO→LUMO excitation are 1.978 Å, 0.214 Å, 0.09 Å and 0.515 Å,
respectively. These r indexes are all smaller than 2.0 Å, shows these electron transitions are local
excitations (LE).
From the discussion above, we can find that although the max of twenty dyes arises from different
types of electron transition, all these electron excitation modes have the character of local excitation.
9
a
b
c
Fig.5 The hole (a) and electron (b) distribution and charge density difference (c) of 11A excitation (HOMOLUMO)
of dye1.
A
b
c
Fig.6 The hole (a) and electron (b) distribution and charge density difference (c) of 31A excitation (H-1→LUMO (41%), HOMO→LUMO
(51%)) of dye 2.
10
A
b
c
Fig.7 The hole (a) and electron (b) distribution and charge density difference (c) of 31A excitation
3. CONCLUSION
Disazo reactive red dyes have cis-, trans- and azo three conformations; the cis-conformation has the
lowest energy and is considered as the most stable geometry. B3LYP and PBE0 functional accurately
predict the  max and the absolute mean errors are 0.092 eV and 0.133 eV, respectively. Comparing the
calculated max of cis-, trans- and azo-conformations with experimental ones, it can be found that
conformation plays an important role on UV-vis absorption spectrum. Dyes 6 and 8 exist in azo
conformation not in cis-conformation. “hole-electron” distribution analysis reveals that although the max
arises from different electron transition, these electron excitations have the same character of local
excitation (LE).
REFERENCES
[1]Austin
P.
Reactive
dyestuffs
containing
the
residue
of
l-hydroxy-7-amino-8-(5-amino-2,4-
-isulphopheny-lazo)-naphthalen-3,6-disulphonic acid[P]. US 4191687,1980.
[2] Yutaka K, Kokusei O, Yasuo T. Dyeing of cellulosic fiber[P]. JP57199877,1982.
[3] Toshiko M, Sadanobu Y, Takashi O, Monoazo compound and dyeing or printing method using same[P]. JP62167364,
11
1987.
[4] Pdemond R, Stekoburge J. Black dyestuff mixture of active azo dyes for fibers, production and use thereof[P].
CN1376741,2002.
[5] Frank R, Manfred P. Reactive dyes with a combination anchor[P]. US6410698, 2002.
[6] Ruang HC,Yu CH. Monoazo reactive dyestuff[P]. EP1411090, 2004.
[7] Griffiths J. Colour and constitution of organic molecules[M]. Academic Press, New York; 1976.
[8] Gupta VD, Tathe AB, Padalkar VS, Umape PG, Sekar N. Red emitting solid state fluorescent triphenylamine dyes:
Synthesis, photo-physical property and DFT study[J]. Dyes Pigm. 2013,97:429-439.
[9] Umape PG, Patil VS, Padalkar VS, Phatangare KR, Gupta VD, Thate AB, Sekar N. Synthesis and characterization of
novel yellow azo dyes from 2- morpholin-4-yl-1,3-thiazol-4(5H)-one and study of their azoe hydrazone tautomerism[J].
Dyes Pigm. 2013, 99:291-298.
[10] Zanjanchi F, Hadipour NL, Sabzyan H, Beheshtian J. Photo-oxidation of phenylazonaphthol dyes and their reactivity
analysis in the gas phase and adsorbed on cellulose fibers states using DFT and TD-DFT[J]. Dyes Pigm. 2011,89:16-22.
[11] Fabian J. TDDFT-calculations of Vis/NIR absorbing compounds[J]. Dyes Pigm. 2010,84:36–53.
[12] Ebead YH. Spectrophotometric investigations and computational calculations of prototropic tautomerism and acide base
properties of some new azo dyes[J]. Dyes Pigm. 2011,92:705-713.
[13] Solomon RV, Jagadeesan R, Vedha SA, Venuvanalingam P. A DFT/TDDFT modelling of bithiophene azo
chromophores for optoelectronic applications[J]. Dyes Pigm. 2014,100:261-268.
[14] Meyer TD, Hemelsoet K, Speybroeck VV, Clerck KD. Substituent effects on absorption spectra of pH indicators: An
experimental and computational study of sulfonphthaleine dyes[J]. Dyes Pigm. 2014,102:241-250.
[15] Le Y, An WW, Chen JF.The preparation and TD-DFT studies of 1-[(4-methyl-2-nitrophenyl) azo]-2-naphthalenol (C.I.
Pigment Red 3) nanoparticles[J]. Dyes Pigm. 2010,87:119-124.
[16] Azpiroz JM, Angelis FD. DFT/TDDFT Study of the Adsorption of N3 and N719 Dyes on ZnO(1010) Surfaces[J]. J.
Phys. Chem. A 2014,118:5885−5893.
[17] Becke AD. Density-functional exchange-energy approximation with correct asymptotic-behavior[J]. Phys. Rev. A
1998,38:3098-3100.
12
[18] Lee C, Yang W, Parr RG. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron
density[J]. Phys. Rev. B 1988,37:785-89.
[19] Johnson BG, Gill PMW, Pople JA. The performance of a family of density functional methods[J]. J. Chem. Phys.
1993,98:5612.
[20] Russo TV, Martin RL, Hay PJ. Density Functional calculations on first-row transition metals[J]. J. Chem. Phys.
1994,101:7729.
[21] Stephens PJ, Devlin FJ, Chabalowski CF, Frisch MJ. Ab Initio Calculation of Vibrational Absorption and Circular
Dichroism Spectra Using Density Functional Force Fields[J]. J. Phys. Chem. 1994,98:11623-11627.
[22] Grimme S. Accurate description of van der Waals complexes by density functional theory including empirical
corrections[J]. J. Comput. Chem. 2004,25:1463-1473.
[23] Ernzerhof M, Scuseria G. Assessment of the Perdew.Burke.Ernzerhof exchange-correlation functional[J]. J. Chem.
Phys. 1999, 110:5029.
[24] Lu T, Chen FW. Multiwfn: A Multifunctional Wavefunction Analyzer[J]. J. Comp. Chem. 2012,33:580-592.
[25] Velde G, Bickelhaupt FM, Gisbergen SJAV, Guerra CF, Baerends EJ, Snijders JG, Ziegler T. Chemistry with ADF[J]. J.
Comput. Chem. 2001, 22:931-967.
[26] Guerra CF, Snijders JG, Velde G, Baerends EJ. Towards an order-N DFT method[J]. Theor. Chem. Acc. 1998,
99:391-403.
[27] ADF2013, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com
13