The Effect of Conformation on UV-vis Absorption Spectra ofDisazo ReactiveRedDyes
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. Disazoreactivereddye hascis-, trans- and azo three conformations, cis-conformation has the lowest energy and is considered as the most stable conformation. The UV-vis absorption spectrawere 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 production and in varieties.As one of three primary colors, reactive reddyes 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 dyespresent 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 reddyes 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 dyespresent 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 inpredicting electronic spectra, wideapplicable 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 dyeshave 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. Theground state geometries of cis-, trans- and azo-conformation were studied by DFT with BLYPfunctional, 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 disazoreactive reddyes are shown in Fig1. All dyes have the same chromophore structural framework but with different substituent on rings A, B, C and amino. Thelocal energy lowest point on potential energy surface wasobtained 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 accountby scalar zero order regular approximation (ZORA). The UV-vis absorption spectrawere calculated at optimal geometry by TDDFT employing B3LYP and PBE0 [21-23]hybrid functionals and TZVPall electron basis set. The solvent effects were evaluated by self-consistent reaction field (SCRF) method and ethanolwas selected as solvent. Five lowest-lying singlet-singletexcitations were calculated and the electron excitations were analyzedby 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 quinonehydrazonetwotautomers. 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 rotatearound ring B through C2-N1 or C17-N4 single bonds. (see Fig.1).
The optimized geometriesat 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 energyand is considered as the most stablegeometry. 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.
A B
C
Fig.2 The optimized geometries of cis-conformation (A), trans-conformation (B) and azo-conformation (C).
2.2UV-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 functionalsare plotted in Fig.3. The absolute mean errors are 0.092 eV and 0.133 eV for B3LYP and PBE0, respectively.
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
B3LYP/TZVP / PBE0/TZVPdyes / Exp/
nm / Excited
state / λ/nm / f / Main Configuration / Excited
state / λ/nm / f / Main Configuration / Attribution
1 / 515 / 11A / 514.1 / 1.412 / HOMO→LUMO (96%) / 11A / 489.3 / 1.454 / HOMO→LUMO (93%) / π→π*
2 / 505 / 21A / 503.3 / 0.2821 / H-3→LUMO(34%); H-1→LUMO (27%); HOMO→LUMO(18%) / 31A / 471.4 / 0.919 / H-1→LUMO (61%) HOMO→LUMO (34%) / π→π*
3 / 496 / 31A / 488.9 / 0.592 / H-2→LUMO (37%)
H-1→LUMO (32%)
HOMO→LUMO (14%)
H-2→L+1 (13%) / 11A / 513.9 / 0.512 / HOMO→LUMO (81%) / π→π*
4 / 518 / 11A / 513.9 / 0.259 / HOMO→LUMO (46%)
H-1→LUMO (46%) / 11A / 584.6 / 0.592 / HOMO→LUMO (71%) / π→π*
5 / 529 / 11A / 523.6 / 0.574 / HOMO→LUMO (47%)
H-1→LUMO (30%)
H-2→LUMO (13%) / 11A / 507.2 / 0.622 / HOMO→LUMO (46%)
H-2→LUMO (23%)
H-2→L+1 (13%)
H-1→LUMO (11%) / π→π*
6 / 518 / 11A / 604.1 / 0.763 / HOMO→LUMO (83%) / 11A / 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%) H-1→LUMO (44%) / 31A / 499.7 / 0.869 / H-1→LUMO (60%)
HOMO→LUMO (38%) / π→π*
11 / 492 / 31A / 522.4 / 0.685 / HOMO→LUMO (48%) H-1→LUMO (47%) / 21A / 514.1 / 0.773 / HOMO→LUMO (76%)
H-1→LUMO (18%) / π→π*
12 / 516 / 21A / 486 / 0.427 / H-1→LUMO(59%)
HOMO→LUMO(28%) / 11A / 587.3 / 0.713 / HOMO→LUMO (86%) / π→π*
13 / 524 / 11A / 559.2 / 0.812 / HOMO→LUMO (94%) / 11A / 536.0 / 1.030 / HOMO→LUMO (95%) / π→π*
14 / 521 / 11A / 530.9 / 1.059 / HOMO→LUMO (78%) / 21A / 506.0 / 0.909 / HOMO→LUMO (61%)
H-2→LUMO (13%)
H-2→L+1 (15%) / π→π*
15 / 524 / 11A / 523.7 / 1.068 / HOMO→LUMO (91%) / 11A / 505.1 / 1.100 / HOMO→LUMO (86%) / π→π*
16 / 518 / 11A / 489.1 / 0.880 / H-1→LUMO (61%)
HOMO→LUMO (16%)
H-2→LUMO (15%) / 31A / 473.1 / 0.910 / H-1→LUMO (72%)
HOMO→LUMO (13%) / π→π*
17 / 503 / 31A / 503.3 / 0.932 / H-1→LUMO (77%), HOMO→LUMO (10%) / 31A / 487.5 / 0.873 / H-1→LUMO (83%) / π→π*
18
19
20 / 543
552
528 / 11A
31A
31A / 473.3
512.2
533.9 / 0.643
0.899
0.834 / HOMO→LUMO (60%); H-2→LUMO (16%)
H-1→LUMO (79%)
HOMO→LUMO (13%)
H-1→LUMO (89%)
HOMO→LUMO (8%) / 31A
11A
11A / 460.3
516.1
527.1 / 0.651
0.417
0.596 / HOMO→L+1 (52%)
H-5→LUMO (15%)
H-2→LUMO (15%)
HOMO→LUMO (81%)
HOMO→LUMO (88%) / π→π*
π→π*
π→π*
B3LYP functional presents a higher accuracy. The absolute errors of all dyes are less than 0.2 eV with the exception of dyes 6and 18 whose errors are 0.341 eVand 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 whoseerrors are 0.273 eV, 0.292 eV, 0.227 eV and 0.410 eV, respectively.Conformation playsan 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 conformationon the UV-vis absorption. The UV-vis absorption spectra of cis- and trans-conformationof quinonehydrazone and azo-conformationfor dyes 6 and 18 were calculated at B3LYP/TZVP level of theory. The excitation energy, oscillator strengths (f) and main configuration for different conformationsare included in Table 2and thesimulated 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 quinonehydrazoneand 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 azoconformation not in cis-conformation of quinonehydrazone tautomer.
Fig.4 The calculated UV-vis absorption spectrum of dyes 6 and 18 incis-, trans- and azo-conformation
Table 2 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
state / λ/nm / f / Main Configuration
dye 6
Cis-conformation of quinonehydrazone
518 / 11A / 604.1 / 0.763 / HOMO→LUMO (83%)
Trans-conformation of quinonehydrazone
518 / 11A / 619.8 / 0.569 / HOMO→LUMO (80%)
Azo conformation
518 / 11A / 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 maxof dye 18 is 543 nm, the calculated maxare 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 dyesprimarily 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 HOMOLUMO transitions; The maximum absorption bands of dyes 2,4,10,11,12,17,18,19 and 20mainly 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) Rydbergexcitation (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 rindex 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) distributionand 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
3 / 31A / H-2→LUMO(37%), / 1.978
H-1→LUMO (32%) / 0.214
H-2→L+1 (13%), / 0.095
HOMO→LUMO (14%) / 0.515
As for dye1,The holemainly distributes onC2,C3,C5 and C6 of ring A,C7,C13and C15 of ring B, N1 andN4 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, C16of ring B (Fig.5b). Fig. 5d presents the isosurface of charge density difference, the blue region denotesthe decrease of electron density, and the green region denotesthe 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 C15of 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 rindex 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 dye3, the maxmainly arises from H-2→LUMO, H-1→LUMO, H-2→L+1 and HOMO→LUMOcombined 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 rindex 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. Theserindexes are all smaller than 2.0 Å, shows these electron transitionsare 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 thecharacter of local excitation.
a b
c
Fig.5 The hole (a) and electron (b) distributionand charge density difference (c) of 11A excitation (HOMOLUMO) 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.
A b
c
Fig.7 The hole (a) and electron (b) distributionand charge density difference (c) of 31A excitation
3. CONCLUSION
Disazoreactivereddyes havecis-, trans- and azo three conformations;the cis-conformation has the lowest energy and is considered as the most stable geometry. B3LYP and PBE0 functionalaccurately 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-conformationswith 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).
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1
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.