Tautomeric behavior of some azoquinoline dyes in liquid and liquid crystalline media

Article abstract of DOI:10.1016/j.saa.2011.11.020

The absorption spectra of three azoquinolin-8-ol derivatives (o-, m-, p-cyano hydroxy azoquinolin dyes) were investigated in liquid and liquid crystalline solutions as a function of the solvent polarity. The spectral data of the dyes were compared in both ordinary liquid solvents and liquid crystalline media. Analysis of the spectral data was used to determine the azo and hydrazone forms in both the environments. The spectral shifts were correlated by Kamlet-Taft and Katritzky multi-parameter polarity scales. For the azoquinoline dyes, the azo form is almost entirely dominated in polar anisotropic hosts. In contrast, the compounds remain dominantly in hydrazone form in some polar solvents such as DMF. The polarized absorption spectra of the compounds in the anisotropic media were measured and their degree of anisotropies was determined.

Full text of DOI:10.1016/j.saa.2011.11.020

Spectrochimica Acta Part A 87 (2012) 112–118
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Tautomeric behavior of some azoquinoline dyes in liquid and liquid crystalline
media
A. Ghanadzadeh Gilania,∗, E. Moradib, S. Binaya, M. Moghadama
a Department of Chemistry, Faculty of Science, University of Guilan, 41335 Rasht, Iran
b Department of Chemistry, Faculty of Science, Lahijan Branch, Islamic Azad University, Lahijan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The absorption spectra of three azoquinolin-8-ol derivatives (o-, m-, p-cyano hydroxy azoquinolin dyes)
were investigated in liquid and liquid crystalline solutions as a function of the solvent polarity. The spec-
tral data of the dyes were compared in both ordinary liquid solvents and liquid crystalline media. Analysis
of the spectral data was used to determine the azo and hydrazone forms in both the environments. The
spectral shifts were correlated by Kamlet–Taft and Katritzky multi-parameter polarity scales. For the
azoquinoline dyes, the azo form is almost entirely dominated in polar anisotropic hosts. In contrast,
the compounds remain dominantly in hydrazone form in some polar solvents such as DMF. The polar-
ized absorption spectra of the compounds in the anisotropic media were measured and their degree of
anisotropies was determined.
Received 14 August 2011
Received in revised form 5 November 2011
Accepted 9 November 2011
Keywords:
Azoquinolin dye
Azo-hydrazone tautomerism
Solvatochromism
Anisotropic media
© 2011 Elsevier B.V. All rights reserved.
Multi-parameter polarity scales
1. Introduction
properties of the azo compounds in isotropic and anisotropic
nematic solvents is important for better understanding their photo-
Azo-hydrazone tautomerism in azo compounds is quite inter-
esting both from theoretical and practical point of view and
has been of particular interest [1–5]. The tautomeric behavior
of a number of hydroxy azo dyes has been identified in var-
ious media [6–8]. It is described by the intramolecular proton
transfer between nitrogen and oxygen atoms. The presence of
azo/hydrazone tautomerism in these compounds influence con-
siderably in their unique photo-physical properties, which is in
turn strongly influenced by several factors including temperature,
substituents structure and solvent polarity [9,10].
physical behavior in isotropic and anisotropic surrounding.
In this work, the spectroscopic properties of o-, m-, p-cyano
hydroxy azoquinolin dyes (see Supplementary Fig. 1a) in various
organic solvents and in some anisotropic hosts with different polar-
ities and polarizabilities were studied at room temperature. The
hydroxy azoquinoline compounds investigated in this work pos-
sess an electron-withdrawing group, –C N, attached to the phenyl
ring. One of the principal aims of the investigation was to see how
the location of –C N group affects the spectral and tautomeric
properties of the dyes.
Azo-hydrazone tautomerism has been studied by various optical
techniques, including electronic method. Due to the character-
istic differences between the optical spectra of the dyes in azo
and hydrazone forms, azo/hydrazone tautomeric behavior can be
investigated from the electronic spectra. Therefore, electronic spec-
troscopy is one of the most applicable and successful techniques for
investigating the azo/hydrazone tautomerism. Therefore, the both
tautomers can be detected from the absorption spectra in the visible
region.
Spectroscopic methods, based on absorption spectra, give
valuable information on contribution of different types of
solute–solvent interactions using multi-parameter solvent polar-
ity scales [11–13]. Knowledge of the spectroscopic and solvation
The addition of a dye to a nematic host affects several host
parameters, which is due to the strong mutual interactions between
the dye and nematic molecules [14]. Therefore, the investigations of
various kinds of guest–host systems are among the most important
and needed in order to understand and provide further information
about the intermolecular forces in anisotropic matrix. Among the
many interesting properties of dye doped in liquid crystals, absorp-
tion anisotropy is one of the most important [15]. The guest–host
interactions of azo dyes in liquid crystalline (LC) host have been
the subject of much interest in recent years [16–18]. Such inter-
est may arise from their optically switchable properties, due to
photochromic cis–trans isomerization, in LC host.
As far as the authors are aware, no investigation on the
azo/hydrazone tautomerism for the investigated dyes in various
isotropic and anisotropic environments has been attempted. The
most significance of this research is using polarized absorption
method for investigation of the role of anisotropic media on the
∗
Corresponding author. Tel.: +98 131 3233262; fax: +98 131 3233262.
E-mail address: aggilani@guilan.ac.ir (A. Ghanadzadeh Gilani).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.11.020

A. Ghanadzadeh Gilani et al. / Spectrochimica Acta Part A 87 (2012) 112–118
113
NH2
N2HSO4
azo/hydrazone tautomerism. Therefore, this paper describes a com-
parative study on solvatochromic measurements in anisotropic and
isotropic media. Moreover, this work is characterized by detailed
quantitative studies on the nature and extent of solvent–solute
interactions using correlations with multi-parameter solvent
polarity scales.
H2SO4
+
NaNO2
N
60 oC (45 min)
NC
NC
NC
HN
N2HSO4
pH = 10-11
0-5 oC (l h)
+
HO
N
HO
2. Experimental
N
CN
2.1. Materials
Scheme 1. Synthesis procedure used for preparation of the azoquinoline dyes.
All the isotropic organic solvents with high purity were pur-
chased from Merck and Fluka. Three pure liquid crystals were
used in our experiments as anisotropic hosts. They are the
pentyl cyanobiphenyl (5CB), octyl cyanobiphenyl (8CB), 4-trans-4-
n-hexyl-cyclohexyl-isothiocyanato-benzene (6CHBT), respectively
(see Supplementary Fig. 1b). 8CB exhibits a smectic phase below
the nematic to isotropic phase transition. In this work, the nematic
mixtures of E7 (see Supplementary Fig. 1c) and 1751 with high
and positive dielectric anisotropy were also used as anisotropic sol-
vents. All the liquid crystalline materials were synthesized in the
Institute of Chemistry of the Military Technical Academy, Warsaw,
Poland.
FT-IR-8400. 1HNMR spectra were taken on a FT-NMR (500 MHz)
Brucker versus TMS in CDCl3 (Supplementary data Table 1). Melting
points were recorded with an electro-thermal apparatus.
2.3. Liquid crystal cell preparation
The guest–host cell was made by sandwiching the solution
between two optical glass plates (2 cm × 1.2 cm). The planar ori-
entation of the guest and host molecules was achieved by surface
treatment of a cast film of polyvinyl alcohol (Sigma) followed by
the rubbing process. The spacing between the electrodes surfaces
was 50 ␮m and was set by use of a Mylar. The introduction of the
dissolved dye in nematic liquid crystal solvent was achieved by
capillary action.
2.2. Synthesis of the azoquinoline dyes
The azoquinoline dyes were successfully synthesized in our lab-
oratory according to the route outlined in Scheme 1.
2.4. LD measurement
The purifying and drying of the compounds were performed
according to the common procedure. The synthesized materi-
als were characterized by conventional spectroscopic methods.
The detail of the synthesis of the materials has already been
reported [19]. Spectroscopic characterization utilized the follow-
ing instrumentations: The absorption spectra of the compounds
were scanned on a Cary UV–vis double-beam spectrophotometer
(Model 100). FT-IR spectra were recorded on a Shimadzu Model
The polarized absorption spectra of the compounds in the liq-
uid crystalline hosts were scanned on a Cary UV–vis double-beam
spectrophotometer (Model 100) equipped with the sheet polariz-
ers. The sample with homogeneous orientation of the guest and
host molecules was mounted in a thermostated holder. Dichroic
ratio R of the dissolved compound was obtained based on polarized
absorption measurements [14,15]. The polarizer was rotated by 90◦
Table 1
The maximum absorption wavelengths and the ratio of hydrazone/azo absorbances (Ah/Aa) of the azoquinoline dyes in isotropic and anisotropic solvents.
Solvent
o-Azoquinoline
m-Azoquinoline
p-Azoquinoline
ꢀazo
ꢀhydrazone
Ah/Aa
ꢀazo
ꢀhydrazone
Ah/Aa
ꢀazo
ꢀhydrazone
Ah/Aa
Isotropic solvent
Cyclohexane
1,4-Dioxane
CCl4
Benzene
Toluene
Chloroform
2-Ethyl-1-hexanol
Dichloromethane
Benzyl alcohol
2-Butanol
1-Butanol
2-Propanol
Acetone
402
406
403
403
404
402
409
402
411
409
402
408
402
406
404
405
401
437
416
404
–
–
–
–
–
–
–
–
–
–
511
513
511
–
–
–
510
514
511
–
–
513
∼0
389
396
393
393
393
392
400
393
400
400
403
398
392
403
394
397
389
405
409
392
454
–
502
494
509
508
502
505
497
432
461
478
478
484
471
496
468
504
505
497
520
494
∼0
395
404
400
401
400
406
421
382
416
434
414
412
406
409
406
413
407
419
417
404
–
–
–
–
–
–
479
476
–
427
469
491
469
522
458
522
442
517
538
–
∼0
∼0
0.06
0.08
0.08
0.07
0.12
0.17
0.07
0.58
0.42
0.58
0.35
0.15
0.69
0.22
0.49
0.15
1.15
0.14
0.14
1.48
∼0
∼0
∼0
∼0
∼0
∼0
∼0
∼0
0.52
1.66
∼0
∼0
∼0
∼0
0.96
1.15
0.60
0.59
1.26
0.55
1.35
0.86
1.66
1.90
∼0
0.09
0.16
0.12
∼0
Ethanol
∼0
Benzonitrile
Methanol
Acetonitrile
DMF
∼0
0.17
0.33
1.55
∼0
DMSO
Acetic acid
NaOH (1 M)
Anisotropic solvents
5CB
8CB
6CHBT
∼0
480
514
0.39
∼0
∼0
404
409
409
411
408
–
–
–
–
–
∼0
∼0
∼0
∼0
∼0
392
399
392
396
393
–
–
–
–
–
∼0
∼0
∼0
∼0
∼0
416
410
410
400
395
–
–
–
–
–
∼0
∼0
∼0
∼0
∼0
E7
1751

114
A. Ghanadzadeh Gilani et al. / Spectrochimica Acta Part A 87 (2012) 112–118
1.5
1.5
1.0
0.5
0.0
Azo
Hydrazone
c
1
2
a
b
5
1
3
4
0.5
0
a
350
450
450
450
550
650
650
650
λ
(nm)
350
450
550
650
λ
(nm)
Fig. 2. Absorption spectra of the azoquinoline dyes in DMF: (a) o-azoquinoline, (b)
m-azoquinoline, and (c) p-azoquinoline.
1.5
1.0
0.5
0.0
1
2
5
b
3
4
organic solvents used in this work. The Kamlet–Taft parameters (˛,
ˇ, and ꢁ*) are summarized in Supplementary Table 2. ˛ is the scale
of solvent hydrogen bond donor ability (HBD), ˇ is the scale of sol-
vent hydrogen-bond acceptor ability (HBA). The ꢁ* is a measure
of solvent dipolarity/polarizability, which measures the ability of
the solvent to stabilize a charge or a dipole by virtue of its dielec-
tric effect and correlated with the solvatochromic behavior values
[13].
The visible absorption spectra of the hydroxy azoquinolin com-
pounds (ca. 1 × 10−5 M) were obtained at room temperature in
various organic solvents with different polarity. The selective spec-
tral data are summarized in Table 1. Typically, Fig. 1(a–c) presents
electronic absorption spectra of the azoquinoline compounds in
five selected solvents. The absorption spectra of the compounds in
cyclohexane, which is used as reference solvent, are also shown in
the figures. The choice of cyclohexane as a base for solvatochromic
behavior measurement was motivated by the following consid-
erations. Cyclohexane is a non-polar and non-HBD/HBA solvent
with very low dielectric constant (ε = 2.02) and its solvatochromic
parameter values of cyclohexane are equal zero. Therefore, this
solvent is a good reference in the solvatochromism investigations.
It can be seen that the spectral profile considerably changed
with the solvent polarity. The electronic spectra of the compounds
in some organic solvents exhibit dual absorption bands. This feature
can be attributed to the formation of azo/hydrazone tautomerism,
where the absorption band at longer wavelength region is due to
the hydrazone formation. The azo/hydrazone tautomerism is more
obvious for the dyes dissolved in dimethylformamide (DMF), where
the hydrazone form is dominated in this polar solvent (Fig. 2). DMF
with high dielectric constant is known to enhance the medium
basicity, and therefore, the azo form should be converted into its
hydrazone form; i.e. the –NH group in the hydrazone form should
be more stabilized in this solvent.
350
550
λ
(nm)
1.5
1.0
0.5
0.0
1
c
5
2
3
4
350
550
λ
(nm)
Fig. 1. Absorption spectra of (a) o-azoquinoline, (b) m-azoquinoline, and (c) p-
azoquinoline in various solvents: (1) cyclohexane, (2) EtOH, (3) ACN, (4) benzene,
(5) DMF.
to record the absorbances in parallel and perpendicular directions
Table 2
to the rubbing direction of the cell, A and A , respectively.
ꢀ
⊥
Percentage of different parameters contributions for Kamlet–Taft multiparameter
scale.
3. Results and discussion
Dye
Absorbance
˛
ˇ
ꢁ*
Azo
Hydrazone
Azo
Hydrazone
Azo
Hydrazone
34 ( 3)
19 ( 9)
19 ( 4)
57 ( 5)
1 (–)
62 ( 4)
4 ( 3)
o-Azoquinoline
3.1. Solvent effect on the absorption spectra of the hydroxy
azoquinolin-8-ol dyes
40 ( 16)
69 ( 6)
7 ( 6)
71 ( 17)
22 ( 5)
41 ( 19)
12 ( 5)
36 ( 9)
28 ( 13)
33 ( 6)
m-Azoquinoline
p-Azoquinoline
The Kamlet–Taft solvent parameters ꢁ*, ˛, and ˇ were used
to indicate the polarity and hydrogen bonding properties of the
45 ( 4)

A. Ghanadzadeh Gilani et al. / Spectrochimica Acta Part A 87 (2012) 112–118
115
1.0
1.0
0.5
0.0
cyclohexane
a
0.8
0.6
0.4
0.2
0.0
1
1
2
3
2
3
4
5
4
6
350
450
550
650
370
370
370
420
470
λ (nm)
520
520
520
570
570
570
λ
(nm)
Fig. 3. Normalized absorption spectra of m-azoquinoline in alcohols: (1) EtOH, (2)
1-BuOH, (3) MeOH, (4) 2-BuOH and 2-PrOH, (5) 2EH, (6) BzOH.
1.0
0.8
0.6
0.4
0.2
0.0
Table 3
cyclohexane
Percentage of different parameters contributions for Katritzky multiparameter scale.
b
1
2
Dye
Absorbance
PET (30) (%)
Pε (%)
Pn (%)
r
Azo
Hydrazone
Azo
Hydrazone
Azo
Hydrazone
11 ( 3)
24 ( 0)
67 ( 10)
66 ( 8)
52 ( 11)
48 ( 4)
26 ( 5)
34 ( 0)
18 ( 9)
19 ( 4)
23 ( 10)
45 ( 6)
63 ( 32)
42 ( 0)
15 ( 8)
15 ( 5)
25 ( 6)
7 ( 2)
o-Azoquinoline
3
4
m-Azoquinoline
p-Azoquinoline
In this work, the effect of acid and base media on the visi-
ble absorption maxima of the dyes was also investigated. It was
observed that the absorption spectra of the compounds were very
sensitive to basic solution, and the hydrazone form is entirely dom-
inated in basic aqueous solution (0.1 M NaOH). In contrast, the azo
form is totally dominated in acidic medium (acetic acid). A typical
example for the dyes in basic solution is shown in Supplementary
Fig. 2.
The spectral features of the compounds in polar protic solvents
(alcohols) were obtained. As an example, Fig. 3 shows the absorp-
tion spectra of m-azoquinoline in the alcoholic solutions. In the
protic solvents, the tautomeric equilibrium is shifted to the less
polar tautomer. It could be due to the formation of inter-molecular
H-bonding between the HBD solvents and –OH group in the azo
form, which should mainly stabilize the less polar.
420
470
λ (nm)
1.0
0.8
0.6
0.4
0.2
0.0
cyclohexane
c
1
2
3
4
3.2. Correlation with multi-parameter solvent polarity scales
In this work, the Kamlet–Taft [13] and Katritzky [12] multi-
parameter equations were used for quantitative evaluation of the
solvent effects:
420
470
λ (nm)
ꢂ¯ = ꢂ¯o + aꢁ × ˛ + bꢁ × ˇ + cꢁ × ꢁ∗
(1)
ꢂ
ꢃ
ꢀ
ꢁ
ε − 1
n2 − 1
Fig. 4. Absorption spectra of the azoquinoline dyes ((a) o-azoquinoline, (b) m-
azoquinoline, and (c) p-azoquinoline) in the liquid crystalline solvents: (1) 8CB, (2)
5CB, (3) 6CHBT, (4) E7.
ꢂ¯ = ꢂ¯o + a × ET(30) + b ×
2ε + 1
+ c ×
(2)
2n2 + 1
Table 4
The mean dielectric constants (ε¯) and refractive indices (n¯ ) of the liquid crystals,
taken from Ref. [18].
where ꢂ¯ is the solute property (band maximum in cm−1) and the
coefficients a, aꢁ, b, bꢁ and c, cꢁ are the relative susceptibilities
of ꢂ¯ to the indicated solvent parameters. The Kamlet–Taft equa-
tion originally evaluates specific hydrogen bonding donor/acceptor
interactions between solute and solvent, while the Katritzky equa-
tion essentially evaluates dipolarity/polarizability interactions
between solute and solvent. The Kamlet–Taft equation combines
spectroscopic polarity scales including hydrogen bonding donor
ability (˛), hydrogen bonding accepting ability (ˇ) and dipolarity/
Anisotropic host
ε¯
n¯
6CHBT
5CB
6CB
6.80
10.10
9.80
7.93
9.80
1.566
1.587
1.583
1.570
1.592
1.560
8CB
E7a
1751a
21.00
a
Nematic mixture.

116
A. Ghanadzadeh Gilani et al. / Spectrochimica Acta Part A 87 (2012) 112–118
Table 5
The dichroic ratios (R) and the order parameters (Sdye) of the azoquinoline dyes in
the anisotropic media.
LC
o-Azoquinoline
m-Azoquinoline
p-Azoquinoline
R
Sdye
R
Sdye
R
Sdye
5CB
8CB
6CHBT
E7
1751
1.49
1.47
1.69
1.37
1.75
0.14
0.13
0.19
0.11
0.20
1.51
1.67
1.62
1.53
1.32
0.14
0.18
0.17
0.15
0.10
1.32
2.31
2.42
2.38
1.65
0.10
0.30
0.32
0.31
0.18
polarizability (ꢁ*) as a multiparameter scale. It was found that gen-
erally both Kamlet–Taft and Katritzky polarity scales correlate the
spectral data of each synthesized dye acceptably. The correlated
results for both the scales (Supplementary data Tables 3 and 4) were
obtained using the data quoted in Table 1 and multi-linear regres-
sion analysis. The obtained result for the solvents were transformed
into percentage contributions and summarized in Tables 2 and 3 for
clarification.
According to Kamlet–Taft correlations, for the dyes, the solvent
hydrogen-bond acceptor ability (ˇ) possesses the main contribu-
tion into spectral feature in azo region. In contrast, this contribution
decreases in hydrazone regions, irregularly. As it can be seen from
Table 3, except for o-azoquinoline, the Katritzky multi-parameter
correlation shows that the value connected with ET(30) has the
major effectiveness. However, contributions assigned to dipolarity
and polarizability of the solvent cannot be ignored. In hydrazone
region, these contributions alter differently for different dyes.
3.3. Absorption spectrum of the azoquinoline dyes in anisotropic
hosts
The mean dielectric constant and refractive index in the nematic
phase can be calculated using:
1
3
ε¯ = (ε + 2ε )
(3)
(4)
ꢀ
⊥
1
n¯ = (ne + 2no)
3
where ε and ε are dielectric constants parallel and perpendicu-
ꢀ
⊥
lar to the molecular axis, respectively. no and ne are the ordinary
and extraordinary refractive indices, respectively. As it can be seen,
the investigated compounds experience a wide range of polar-
ity and polarizability in anisotropic media (Table 4). Therefore,
in this investigation we might be able to determine the role of
dipole–dipole interactions between the dye and the nematic media.
The anisotropic solvation characteristics of the investigated
dyes in liquid crystals with various polarity and polarizability were
investigated at room temperature. The selective spectral data in
the anisotropic media are also summarized in Table 1. Fig. 4(a–c)
shows the visible absorption spectra of the dyes in various liquid
crystalline hosts (0.1%, w/w). The absorption spectra of the com-
pounds in cyclohexane, which is used as reference solvent, are also
shown in the figures. As it can be seen, the compounds appear to
exist almost in its azo form in the anisotropic solution. Interest-
ingly, the similarities of the spectra behavior (azo form) observed
in the polar nematic hosts and cyclohexane with remarkable dis-
similar physical properties cannot be explained based on regular
solvatochromic behavior.
Fig. 5. Polarized absorption spectra of the azoquinoline dyes ((a) o-azoquinoline,
(b) m-azoquinoline, and (c) p-azoquinoline) in 5CB.
viscosity, molecular packing, host rigidity and polarizability. There-
fore, the anisotropic medium provides a relatively rigid, packed
and polarizable environment for the solute molecules, and thus
prevents azo/hydrazone tautomerism.
The spectral behavior of the investigated compounds in nematic
solvents should have multiple origins, and several interactions
contribute to some extent. Therefore, the influence of anisotropic
medium on the solvation of the compound should be complicated.
It should be concluded that, the spectral features in the nematic
hosts might depends on several factors such as solvent polarity,
3.4. Linear dichroism of the hydroxy azoquinolin-8-ol dyes
The polarized absorptions spectra of the compounds were
recorded in parallel-aligned liquid crystal cells and their dichroic

A. Ghanadzadeh Gilani et al. / Spectrochimica Acta Part A 87 (2012) 112–118
117
dissolved in oriented nematic host can be determined from the
expression:
ꢂ
ꢃ
R − 1
R + 2
1
Sdye
=
(5)
(1/2)(3 cos2 ˇ − 1)
This equation can be used to evaluate the order parameter of the
dye if the angle ˇ is known [14]. If the transition moment vector
between two electronic states of the dye is directed along its long
molecular axis (i.e. ˇ = 0), Eq. (5) reduces to:
R − 1
Sdye
=
(6)
R + 2
The angle between the absorption oscillator and the long molec-
ular axis (ˇ) is unknown for the dyes under study. If we assume
ˇ = 0, and estimate the order parameter of the dye dissolved in the
nematic host by using Eq. (6). However, for these dyes ˇ =/ 0, so
the term of “degree of anisotropy” could be applied instead of order
parameter.
Polarized absorption spectra of the compounds dissolved in
parallel-aligned nematic hosts were recorded at room temperature.
Typically, Figs. 5 and 6 show the polarized absorption spectra of
the compounds in 5CB and 8CB, respectively. The dichroic ratios
and degree of anisotropies of the compounds were complied in
Table 5. The low values of these parameters for the investigated
dyes in the ordered anisotropic host might be related to the several
factors, i.e. the dye molecular structure, the intermolecular interac-
tions between the dye and liquid crystal molecules, and the angle
between the transition dipole moment and the long molecular axis.
Moreover, the alignment properties of the guest–host systems can
be affected by the molecular structures of the dyes. In other words,
the director orientation can be altered by shape, size and structure
of the dye over long distances.
4. Conclusions
The synthesized hydroxy azoquinolin-8-ol dyes were used in
this research. The spectral data show the importance of location
of CN group attached to the aromatic skeleton in these com-
pounds. The spectral behavior of the compounds and the ratio of
azo/hydrazone mixture are largely changed by the position of this
electron-attracting group.
The experimental result indicated that the strong
azo/hydrazone tautomerism occurred in some polar solvents such
as DMF. This phenomenon is more evidence for m-azoquinoline.
However, in solvents of low polarity, the azo form is dominated.
Interestingly, the compounds appear to exist almost entirely in its
azo form in the anisotropic media (both the polar and low polar).
In conclusion, the anisotropic solvents provide relatively rigid,
packed and polarizable media for the dye molecules, and therefore,
prevent azo/hydrazone tautomerism.
According to the dichroism results obtained in the present work
the dichroic ratios R of the dyes are larger than one. This means that
the absorption bands for these dyes could be considered as parallel
transition (i.e. ␲–␲*).
Fig. 6. Polarized absorption spectra of the azoquinoline dyes ((a) o-azoquinoline,
(b) m-azoquinoline, and (c) p-azoquinoline) in 8CB.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.11.020.
ratios were obtained. The dichroic ratios (R = A /A ) of the dyes
ꢀ
⊥
dissolved in the nematic host were calculated from the absorp-
tion of light polarized parallel (A ) and perpendicular (A ) to the
References
ꢀ
⊥
[1] P. Gregory, Dyes Pigm. 7 (1986) 45–56.
[2] G. Pavlovic, L. Racane, H. Cicak, V. Tralic-Kulenovic, Dyes Pigm. 83 (2009)
354–362.
[3] K.S. Cheon, Y.S. Park, P.M. Kazmaier, E. Buncel, Dyes Pigm. 53 (2002) 3–14.
[4] R.L. Reeves, R.S. Kaiser, J. Org. Chem. 35 (1970) 3670–3675.
liquid crystal alignment (rubbing direction). The dichroic ratio
directly relates to the orientational order parameter, Sdye, for the
transition moment of dye absorption which is responsible for the
color of the dye. The orientational order parameter of the dye

118
A. Ghanadzadeh Gilani et al. / Spectrochimica Acta Part A 87 (2012) 112–118
[5] Q. Peng, M. Li, K. Gao, L. Cheng, Dyes Pigm. 18 (1992) 271–286.
[14] D. Bauman, H. Moryson, J. Mol. Struct. 404 (1997) 113–120.
[15] J. Michl, E.W. Thulstrup, Spectroscopy with Polarized Light, Wiley-VCH Book,
1995.
[16] N.K. Viswanathan, D.Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J.
Kumar, S.K. Tripathy, J. Mater. Chem. 9 (1999) 1941–1955.
[17] A. Ghanadzadeh, M.A. Shahzamanian, S. Shoarinejad, M.S. Zakerhamidi, M.
Moghadam, J. Mol. Liq. 136 (2007) 22–28.
[6] B.R. Hsieh, D. Desilets, P.M. Kazmaier, Dyes Pigm. 14 (1990) 165–189.
[7] A. Ghanadzadeh Gilani, M.R. Yazdanbakhsh, N. Mahmoodi, M. Moghadam, E.
Moradi, J. Mol. Liq. 139 (2008) 72–79.
[8] S. Tauro, E. Coutinho, J. Mol. Struct. Theochem. 532 (1–3) (2000) 23–29.
[9] L. Antonov, S. Kawauchi, M. Satoh, J. Komiyama, Dyes Pigm. 40 (1999) 163–170.
[10] J.O. Morley, J. Phys. Chem. 98 (1994) 13177–13181.
[11] M.S. Masouda, A.E. Ali, M.A. Shaker, M. Abdul Ghani, Spectrochim. Acta A 60
(2004) 3155–3159.
[18] A. Ghanadzadeh Gilani, M. Moghadam, M.S. Zakerhamidi, E. Moradi, Dyes Pigm.
92 (2012) 1320–1330.
[12] A.R. Katritzky, D.C. Fara, H. Yang, K. Tamm, T. Tamm, M. Karelson, Chem. Rev.
104 (2004) 175–198.
[19] M.R. Yazdanbakhsh, N.O. Mahmoodi, S. Dabiry, Mendeleev Commun. 3 (2006)
192–194.
[13] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, 2nd ed., Wiley-
VCH, 1988.