Novel azo disperse dyes derived from N-benzyl-N-ethyl-aniline: Synthesis, solvatochromic and optical properties

Article abstract of DOI:10.1016/j.molliq.2009.11.010

Seven new azo disperse dyes based on N-benzyl-N-ethyl-aniline were synthesized and characterized by UV-Vis, FT-IR and ¹H NMR spectroscopic techniques. The UV-visible studies and solvatochromic behavior of all dyes in 15 solvents with different polarity were examined and a meaningful correlation was observed. In the optical characterization of dyes using Z-Scan experiment, thin films of polymethyl metacrylate doped with guest synthesized chromophores 2, 3, 4 and 7 were investigated.

Full text of DOI:10.1016/j.molliq.2009.11.010

Journal of Molecular Liquids 151 (2010) 107–112
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Novel azo disperse dyes derived from N-benzyl–N-ethyl–aniline: Synthesis,
solvatochromic and optical properties
a
b
b
b
M.R. Yazdanbakhsh a, , A. Mohammadi , E. Mohajerani , H. Nemati , N. Hosain Nataj ,
⁎
A. Moheghi b, E. Naeemikhah a
a
Department of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran
Laser Research Institute, University of Shahid Beheshti (G.C.) , Tehran, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven new azo disperse dyes based on N-benzyl–N-ethyl–aniline were synthesized and characterized by
UV–Vis, FT-IR and 1H NMR spectroscopic techniques. The UV–visible studies and solvatochromic behavior of
all dyes in 15 solvents with different polarity were examined and a meaningful correlation was observed. In
the optical characterization of dyes using Z-Scan experiment, thin films of polymethyl metacrylate doped
with guest synthesized chromophores 2, 3, 4 and 7 were investigated.
Received 21 September 2009
Received in revised form 20 November 2009
Accepted 25 November 2009
Available online 4 December 2009
© 2009 Elsevier B.V. All rights reserved.
Keywords:
Azo dyes
Solvatochromic behavior
Optical materials
Z-scan
1. Introduction
benzyl–N-ethyl–aniline as a coupling component and evaluation of
their solvatochromic behavior in 15 solvents with different polarities.
The azo dyes are by far the most important class, accounting for
over 50% of all commercial dyes, and have been widely studied [1].
Amongst the azo dyes, hetero-cyclic diazo compounds have attracted
much interest recently, especially those including sulfur and/or
nitrogen. These S or S/N heterocyclic azo dyes provide bright, strong
shades that range from red through green and blue, which com-
plement the yellow–orange colors of the nitrogen heterocyclic azo
dyes to provide a complete coverage of the entire shade range [2–6].
In addition, polymers containing azo dyes have received considerable
attention over the last two decades due to their anisotropic responses
to polarized laser light [7,8] and their applications in reversible
holographic data storage, image processing and switching [9,10]. For
such applications it is important to characterize the mechanisms
involved in the interaction of laser light with the material. Numerous
articles have tried to explain the mechanisms, but more detailed
studies are still needed. Photo isomerization followed by molecular
re-orientation is believed to be mainly responsible for the anisotropic
response of this group of material [11].
Also, we have investigated optical properties of a number of polymer
films doped with newly synthesized azo dyes using photo induced
birefringence experiment. The nonlinear response of these materials
in 5CB liquid crystal host is also measured using Z-Scan technique
[14,15].
2. Results and discussion
2.1. Synthesis and characterization
For the synthesis of dyes 1–7 (Scheme 1), carbocyclic and hetero-
cyclic amines were first diazotized in nitrosyl sulfuric acid, which then
coupled with N-benzyl–N-ethyl–aniline dissolved in acetonitrile
containing several drops of glacial acetic acid in an ice bath. The
structure of dyes was unambiguously confirmed by their spectral data,
as shown in Tables 1 and 2. The 1H NMR spectra of dyes were recorded
in CDCl3. 1H NMR spectrum of all synthesized dyes showed a clear
signal at δ=4.7 ppm, which can be attributed to the benzylic protons
in N-benzyl–N-ethyl–aniline as coupling component. Other signifi-
cant signals that were detected in 1H NMR spectrum of dyes are a
triplet peak at δ=1.3 ppm and a quartet peak about δ=3.6–3.7 ppm.
These signals were attributed to the adjacent methyl and methylene
protons, respectively. The aromatic protons were observed from 6.4 to
8.7 ppm in the 1H NMR spectrum. All chemical shifts were assignable
in the spectrum and clearly showed the formation of these dyes.
In continuation of our previous studies [12,13], we report here the
synthesis of some novel azo dyes 1–7 resulting from the use of N-
⁎
Corresponding author. Tel.: +98 131 7723677; fax: +98 131 7720733.
E-mail address: mr-yazdan@guilan.ac.ir (M.R. Yazdanbakhsh).
0167-7322/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2009.11.010

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M.R. Yazdanbakhsh et al. / Journal of Molecular Liquids 151 (2010) 107–112
Scheme 1. Synthesis of dyes 1–7.
isoxazole and triazole derivatives. This bathocromic shift in absorption
maximum of thiazolyl dyes has been attributed to the existence of
sulfur, either alone or in combination with nitrogen atoms in the
2.2. UV–visible and solvatochromic studies of dyes
For the study electronic absorption spectra, we recorded wavelength
of synthesized dyes in 15 solvents with different polarity at a
concentration of 10−5–10−6 M in the range of 300 to 700 nm. The
results are given in Table 3. The absorption maximum of all dyes except
for dye 6 showed bathochromic shift (positive solvatochromic) in more
polarity solvents such as DMSO and DMF in comparison to solvents with
less polarity characteristics. This effect has been attributed to interaction
of basic solvents with non-bonding electron pair on nitrogen atom of N-
benzyl–N-ethyl–aniline as coupler component that causes extended
conjugation resonance system of dyes. All dyes, specially dye 3 with
electron-donating group on the benzothiazole ring showed a significant
bathocromic shift in acidic solvent (glacial acetic acid) relative to other
investigated solvents in this study. This shift in absorption maximum
can be attributed to the presence of these dyes in hydrazone form as
observed in our previous study on azobenzene dyes [13]. The spectral
shifts of compound 3 in various solvents are shown in Fig. 1. Maximum
wavelength of synthesized dyes showed a regular variation with the
polarity of solvents. As it is apparent in Table 3, all dyes except for dye 6
(r=0.367) exhibit an excellent correlation coefficient (r) for the linear
salvation energy relationship with π* values calculated by Kamlet et al.
in investigated solvents [16]. Therefore, these dyes can be considered as
solvent polarity indicator dyes.
Table 2
Spectral data for dyes (1–7).
Dye IR (cm−1
No.
)
1H NMR (ppm)a
1
2
2880–2980(Aro.–H),
1508(N=N)
1.33(3H, t, J=7.1 Hz, CH3) 3.63(2H, q, J=7.1 Hz,
CH2) 4.70(2H, s, CH2) 6.80(2H, d, J=9.22 Hz, Ar–
H) 7.25(2H, d, J=7.21 Hz, Ar–H) 7.28(1H, d,
J=3.38 Hz, Ar–H) 7.32(1H, t, J=7.31 Hz, Ar–H)
7.38(2H, t, J=7.28 Hz, Ar–H) 7.94(1H, d,
J=3.38 Hz, Ar–H) 7.95(2H, d, J=9.22 Hz, Ar–H)
1.34(3H, t, J=7.1 Hz, CH3) 3.65(2H, q, J=7.1 Hz,
CH2) 4.71(2H, s, CH2) 6.82(2H, d, J=9.25 Hz, Ar–
H) 7.24(2H, d, J=7.24 Hz, Ar–H) 7.32(1H, t,
J=7.28 Hz, Ar–H) 7.39(2H, t, J=7.63 Hz, Ar–H)
7.42(1H, t, J=7.25 Hz, Ar–H) 7.50(1H, t,
J=7.33 Hz, Ar–H) 7.87(1H, d, J=7.75 Hz, Ar–H)
8.0(2H, d, J=9.25 Hz, Ar–H) 8.12(1H, d,
J=7.99 Hz, Ar–H)
2900–2985(Aro.–H),
1510(N=N)
3
4
2885–2995 (Aro.–H),
1495(N=N)
1.34(3H, t, J=7.1 Hz, CH3) 3.64(2H, q, J=7.1 Hz,
CH2) 3.93(3H, s, CH3) 4.73(2H, s, CH2) 6.82(2H, d,
J=9.28 Hz, Ar–H) 7.11(1H, d, J=8.90 Hz, Ar–H)
7.25(2H, d, J=7.25 Hz, Ar–H) 7.30(1H, t,
J=7.26 Hz, Ar–H) 7.34(1H, d, J=8.90 Hz, Ar–H)
7.38(2H, t, J=7.60 Hz, Ar–H) 7.97(2H, d,
J=9.28 Hz, Ar–H) 8.00(1H, s, Ar–H)
1.38(3H, t, J=7.1 Hz, CH3) 3.68(2H, q, J=7.1 Hz,
CH2) 4.77(2H, s, CH2) 6.86(2H, d, J=9.15 Hz, Ar–
H) 7.25(2H, d, J=7.46 Hz, Ar–H) 7.34(1H, t,
J=7.31 Hz, Ar–H) 7.40(2H, t, J=7.20 Hz, Ar–H)
8.02(2H, d, J=9.15 Hz, Ar–H) 8.14(1H, d,
J=8.91 Hz, Ar–H) 8.35(1H, d, J=8.91 Hz, Ar–H)
8.78(1H, s, Ar–H)
In our previous works, we exactly evaluated the substituent effects
on absorption maxima of azobenzene dyes [12,13]. Here, we evaluated
the substituent and extension of resonance system effects on absorption
maximum of synthesized heterocyclic dyes (Table 3). Apparently, in the
series of synthesized heterocyclic azo dyes, thiazole derivatives showed
a higher maximum wavelength relative to the dyes produced from
2900–2995(Aro.–H),
1512(N=N)
5
6
7
2860–2980(Aro.–H),
1505(N=N)
1.20(3H, t, J=7.00 Hz, CH3) 2.45(3H, s, CH3) 3.62
(2H, q, J=7.00 Hz, CH2) 4.73(2H, s, CH2) 6.44(1H,
s, Ar–H) 6.86(2H, d, J=9.24 Hz, Ar–H) 7.24(2H, d,
J=7.25 Hz, Ar–H) 7.26(1H, t, J=7.32 Hz, Ar–H)
7.35(2H, t, J=7.6 Hz, Ar–H) 7.78(2H, d,
J=9.24 Hz, Ar–H)
Table 1
Yield, physical properties and UV–Vis absorption spectra of dyes 1–7.
a
Compounds Color
Mp
(°C)
λmax
υmax
Purification Yield
(nm)
(cm−1
)
(%)
1
2
3
4
5
6
7
Red
134–
136
158–
160
154–
156
160–
162
142–
144
178–
180
126–
128
487
508
512
544
428
441
480
20,533 EtOH/H2O
19,685 EtOH
85
82
81
72
85
91
96
(NH), 1850–1930
1.32(3H, t, J=7.10 Hz, CH3) 3.63(2H, q,
(Aro.–H), 1508(N=N) J=7.10 Hz, CH2) 4.70(2H, s, CH2) 6.80(2H, d,
J=9.13 Hz, Ar–H) 7.25(2H, d, J=7.25 Hz, Ar–H)
7.32(1H, t, J=7.29 Hz, Ar–H) 7.39(2H, t,
Red
Red–violet
19,531 Acetone/
H2O
18,382 Acetone/
H2O
J=7.64 Hz, Ar–H) 7.96(2H, d, J=9.13 Hz, Ar–H)
8.25(1H, s, Ar–H) 13.74(1H, br, NH)
Violet–
green
Orange
2865–2990(Aro.–H),
1508(N=N)
1.35(3H, t, J=7.12 Hz, CH3) 3.65(2H, q,
J=7.12 Hz, CH2) 4.72(2H, s, CH2) 6.83(2H, d,
J=9.18 Hz, Ar–H) 7.26(2H, d, J=7.17 Hz, Ar–H)
7.32(1H, t, J=7.22 Hz, Ar–H) 7.39(2H, t,
J=7.13 Hz, Ar–H) 7.92(2H, d, J=9.18 Hz, Ar–H)
7.95(2H, d, J=9.02 Hz, Ar–H) 8.36(2H, d,
J=9.02 Hz, Ar–H)
23,364 EtOH
Orange
Red
22,675 EtOH
20,833 EtOH/H2O
a
Apparent multiplicity is given and the J coupling constant values have been determined
a
All the UV–Vis spectra were recorded in ethanol.
by first order analysis of the spectra.

M.R. Yazdanbakhsh et al. / Journal of Molecular Liquids 151 (2010) 107–112
109
Table 3
chemical structures of such dyes. The influence of the substituent on the
6 position of benzothiazole derivatives is demonstrated by the
comparison of λmax of dyes 2, 3 and 4. We found that dye 4, having a
strong electron acceptor group (NO2), showed bathochromic shift
relative to dye 3, which contains an electron rich group (OCH3). Dye 2,
which includes benzothiazolyl rings, showed bathochromic shift due to
extended resonance system in comparison with dye 1 containing
thiazolyl ring as shown in Table 3 and Fig. 2.
Solvatochromic data [λmax (nm)] for all synthesized dyes in 14 solvents with π* values
by Kamlet et al [16].
Solvents
π*a
Dye 1 Dye 2 Dye 3 Dye 4 Dye 5 Dye 6 Dye 7
λmax λmax λmax λmax λmax λmax λmax
(nm) (nm) (nm) (nm) (nm) (nm) (nm)
Cyclohexane
Tetrachloromethane 0.21 456
Diethyl ether
Ethyl acetate
Dioxane
Ethanol
Tetrahydrofuran
Methanol
0.00 438
465
477
484
490
493
508
494
510
491
602
503
499
500
511
520
472
483
489
494
497
512
498
513
494
614
506
503
505
515
522
499
510
521
527
528
540
531
542
537
563
538
535
536
549
557
398
404
406
411
414
428
414
432
419
469
424
420
422
437
443
414
437
419
423
427
441
427
445
423
503
428
457
447
427
432
451
456
461
468
469
480
473
476
478
516
478
481
481
491
500
0.24 465
0.45 469
0.49 475
0.54 487
0.55 473
0.60 489
0.62 474
0.64 582
0.66 483
0.69 481
0.73 482
2.3. Optical properties of synthesized azo dyes
Thin films of polymethyl metacrylate (PMMA from Merck) doped
with different guest chromophores (thickness: 1.95 μm, dye concen-
tration: 7 wt.%) are studied. In photo induced birefringence experi-
ment, Nd: Yag laser (wavelength 532 nm, power 6 mw) and He–Ne
Laser (wavelength 633 nm, power b2 mw) were used as pump and
probe beams, respectively. A photo detector translates the transmit-
ted light intensity of the probe beam as a measure of induced
birefringence. Fig. 3 shows the experimental results of the induced
birefringence for samples 2, 3, 4 and 7 as a function of time.
Acetone
Glacial Acetic acid
Acetonitrile
Chloroform
Dichloromethane
Dimethylformamide 0.88 491
Dimethylsulfoxide
1.00 497
a
The correlation coefficient r obtained for the linear salvation energy relationship
with π* values by Kamlet et al. for solvents was г=0.929 for Dye1, г=0.927 for Dye2,
г=0.918 for Dye3, г=0.971 for Dye4, г=0.935 for Dye5, г=0.367 for Dye6 and
г=0.981 for Dye7. These values obtained without acetic acid, chloroform and
dichloromethane, which deviate slightly from the regression line.
The anisotropic response of the samples is due to the fast trans-cis
photo isomerization following slow reorientation.
In Z-Scan experiment, the nematic liquid crystal, 5CB from Merck,
was used. The LC was doped with the four synthesized azo dyes (dye
Fig. 1. Absorption spectra of dye 3 in various solvents.
Fig. 2. Absorption spectra of dyes 1, 2, 3 and 4 in ethanol.

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M.R. Yazdanbakhsh et al. / Journal of Molecular Liquids 151 (2010) 107–112
Fig. 3. Photo induced birefringence versus time for four samples.
concentration: 0.3 wt.%). Standard cells (thickness 7.9 μm, parallel
alignments) were filled with the prepared mixtures. In this experi-
ment, the polarized light of second harmonic Nd: Yag laser (70 mw)
was focused with a lens and the samples were moved about its focal
point. The transmitted intensity was measured with a photo detector.
Fig. 4 shows the normalized intensity as a function of sample position.
Table 4 shows the optical nonlinear refractive indices, n2, which were
calculated by the equations found in Ref. [14].
These experiments demonstrate that the newly synthesized azo
dyes show trans-cis isomerization and reorientation which is required

M.R. Yazdanbakhsh et al. / Journal of Molecular Liquids 151 (2010) 107–112
111
Fig. 4. Normalized Z-Scan experimental curves. The inlet exhibits the unusual action of sample 4.
for optical and electro optical applications. As it is evident from Figs. 3
and 4, samples 2 and 3 which are classified as heterocyclic dyes, show
relatively good anisotropic response and sample 4, unexpectedly,
shows an unusual behavior. Sample 7, as an azobenzene dye shows
weaker optical response than those of samples 2 and 3.
3 mmol of heterocyclic amines (3 mmol) in an ice bath. The mixture
was stirred for an additional 1 h at 0–5 °C. After completion of
diazotization procedure, the diazonium salt solution was added
dropwise to the solution of N-benzyl–N-ethyl–aniline (3 mmol) in
acetonitrile (20 mL) and some drops of acetic acid. The resulting
solution was vigorously stirred at 0–4 °C for 2 h, while the pH of the
reaction mixture was maintained at 6–7 by simultaneous addition of
sodium hydroxide solution (0.5 M). The progress of the reaction was
evaluated by thin layer chromatography (TLC) and then crude dyes
were filtered, washed with excess water and purified using recrys-
tallization method. The method of preparation of diazonium salt from
4-nitroaniline for synthesis of dye 7 was described elsewhere [12,13].
The physical and spectral data of the purified dyes are listed in
Tables 1 and 2, respectively.
3. Experimental
3.1. Material and apparatus
The chemicals used in this work were obtained from Merck and
Aldrich Chemical Companies and were used without further purifica-
tion. All melting points were determined on an electrothermal melting
point apparatus and are uncorrected. Infrared spectra were recorded on
a Shimadzu 8400 FT-IR spectrophotometer. The 1H NMR spectra were
obtained on a FT-NMR (500 MHz) Bruker apparatus spectrometer, and
the chemical shifts are expressed in δppm using TMS as an internal
standard. UV–Vis absorption spectra were obtained using a Pharmacia
Biotech Spectrophotometer.
4. Conclusions
In this work, 7 new azo disperse dyes with metallic luster were
synthesized by a classical method of diazotizing-coupling. Their
structures were confirmed by 1H NMR, FT-IR and UV–visible spectra.
The solvatochromic properties of synthesized dyes in 15 selected
solvents with different salvation characters were investigated
thoroughly and a meaningful correlation between the maximum
absorption (λmax) of dyes and solvent polarity parameter (π*) was
observed. The study of substituent effects showed that the withdraw-
ing group on the diazo moiety has significant influence (bathochromic
shift) on the electron absorption spectra of these dyes.
Finally, the optical properties of the chromophores 2, 3, 4, and 7 were
evaluated carefully. As apparent in Figs. 3 and 4, heterocyclic azo dyes 2
and 3 showed good optical response in comparison to synthesized
azobenzene dye 7 in this study and commercially available DR1.
However, heterocyclic azo dye 4 containing donor and acceptor
terminal groups showed the unusual behavior.
3.2. The general procedure for the synthesis and purification of disperse
azo dyes
For the preparation of dyes 1–6, the diazonium coupling reaction
was employed. The route of synthesis of dyes is presented in
Scheme 1. A typical procedure used for preparation of dyes is
described below.
Nitrosyl sulfuric acid solution prepared from concentrated sulfuric
acid (8 mL) and sodium nitrite (0.21 g, 3 mmol) at 70 °C, and then
cooled to 5 °C. The solution was added dropwise, with stirring, to
30 mL of acetic acid and propionic acid mixture (2:1) containing
Table 4
Nonlinear refractive index n2 for four samples.
Sample no.
n2 (cm2/w) 10−5
Acknowledgment
2
3
4
7
4.8349
4.8384
N/A
We thank the Guilan University and Laser Research Institute of
Shahid Beheshti University for financial support of this work.
3.6427

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[9] A. Natansohn, P. Rochon, Chem. Rev. 102 (2002) 4139.
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