Photo-physical and structural studies of some synthesized arylazoquinoline dyes

Article abstract of DOI:10.1016/j.saa.2017.05.035

This study presents the spectral and structure characteristics of seven azoquinoline dyes with different substituents and their new methylated counterparts for the first time, where some compounds are newly synthesized. The solvatochromic, tautomeric, hal

Full text of DOI:10.1016/j.saa.2017.05.035

Accepted Manuscript
Photo-physical and structural studies of some synthesized
arylazoquinoline dyes
A. Ghanadzadeh Gilani, V. Taghvaei, E. Moradi Rufchahi, M.
Mirzaei
PII:
S1386-1425(17)30411-0
doi: 10.1016/j.saa.2017.05.035
SAA 15177
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Revised date:
Accepted date:
22 February 2017
10 April 2017
16 May 2017
Please cite this article as: A. Ghanadzadeh Gilani, V. Taghvaei, E. Moradi Rufchahi,
M. Mirzaei , Photo-physical and structural studies of some synthesized arylazoquinoline
dyes, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2016), doi:
1
0.1016/j.saa.2017.05.035
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ACCEPTED MANUSCRIPT
Photo-physical and structural studies of some synthesized
arylazoquinoline dyes
1
,2
2
3
4
A. Ghanadzadeh Gilani* , V. Taghvaei , E. Moradi Rufchahi , M. Mirzaei
1
) Department of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran
) Department of Chemistry, University Campus 2, University of Guilan, Rasht, Iran
) Department of Chemistry, Faculty of Science, Lahijan Branch, Islamic Azad University, Lahijan, Iran
) School of Pharmacy and Pharmaceutical Sciences, Isfahan University of Medical Sciences, Isfahan, Iran
2
3
4
Address:
Prof. A. Ghanadzadeh Gilani,
Department of Chemistry, University of Guilan, 41335 Rasht, Iran
Tel: 0098 13 33333262, Fax: 0098 13 33233262
E-mail address: aggilani@guilan.ac.ir (A. Ghanadzadeh Gilani)
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Abstract
This study presents the spectral and structure characteristics of seven azoquinoline dyes
with different substituents and their new methylated counterparts for the first time, where
some compounds are newly synthesized. The solvatochromic, tautomeric, halochromic, and
dichroic behavior of the compounds were studied by electronic spectroscopy in various
media. The different types of media were ordinary, multifunctional, and ordered liquids.
The experiments were extended to include under acidic or basic conditions. The
orientational behavior of the azo dye-doped liquid crystals was studied, and it was
established that the azo form is the main species in high polar anisotropic media. The multi-
parameter polarity scales were used to correlate the spectral data. Influence of acid and base
on the absorption spectra of the dyes was also examined. Ionization constants for these dyes
were determined in ethanol-water media. As a result, at the high dye concentrations,
the intermolecular hydrogen bonding is more stable than the intra-molecular hydrogen
bond, and therefore, the azo form is the main species in concentrated solutions. In order to
provide more details, time-dependent density functional theory (TD-DFT) calculations
were carried out for the representative models.
Keywords: Azo-hydrazone tautomerism; Solvatochromism; Halochromism; Dichroism;
TD-DFT calculation
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Introduction
Aromatic azo dyes are among the most important classes of colorants with enormous
applications in several fields of science and technology [14]. Synthesis and photophysical
behavior of this class of dyes have extremely been investigated [519]. Among the
aromatic azo dyes, hydroxy aryl azo dyes attract considerable interest and display
interesting optical and non-linear optical properties. These materials are applied in many
technical applications such as optical storage capacity, optical switching and non-linear
optics. Therefore, the design and investigation of new aryl azo compounds having
interesting photo-physical and photo-chemical properties is always needed in the wide
range of technical applications [1,2].
The azo-hydrazone tautomerism of this group of the compounds has been known for many
years and was reviewed several times [2022]. Study of azo-hydrazone tautomerism and
intra-molecular hydrogen transfer in various aryl azo dyes have been of particular interest
[
2329]. In particular, investigation of absorption spectra in various media is valuable for
better understanding of the photophysical behavior of these compounds. In general, various
important factors such as solvent nature, pH, and substituents structure, greatly influence on
the spectral and tautomeric properties of these compounds [30]. The solvent characteristic
is very important factor in formation of the tautomers and their stability in solutions [18].
In fact, the state of azo-hydrazone equilibrium in solution is closely related to the nature of
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intermolecular interactions. It is well reported that hydrazone form is favored by polar
media, and by electron-withdrawing substituents in the conjugated phenyl ring [3132].
Moreover, it is well reported that the spectral characteristics of azo dyes in ordered liquid
crystals are strongly influenced by anisotropic intermolecular interaction [3336]. Due to
the large molecular anisotropy and intermolecular ordering, nematic liquid crystals are
highly anisotropic materials. Nematic liquid crystals are known to form partially ordered
structure and are able to orient dissolved dye molecules parallel to their long axes. They are
suited as anisotropic solvents for polarized spectroscopy. The addition of the dyes to the
nematic solvents affects various solvent and solute behaviors, which is due to the strong
interactions between the dye and anisotropic molecules. The spectra behavior and polarized
absorption spectra of some arylazoquinoline dyes with different substituents in nematic
liquid crystals have been reported in our previous publication [37].
In this work, a comparative study on the spectral performance of hydroxyquinoline dyes
and their methylated counterparts (Figure 1a) in various environments were studied. This
report concerns the effect of electron withdrawing groups (  CF3 and  Cl ) on the
tautomeric behavior of the dyes as well. It is well reported that the position and nature of
substituents play an essential role in determining the state of the tautomeric equilibrium of
dyes [24]. The electron withdrawing substituents especially  CF3 [23] can reduce the
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electron density of the aromatic rings and stabilize the hydrazone tautomer. In contrast, azo
tautomer is favored by electron donating groups such as (  OCH3 and  OC H5 ).
2
Moreover, a detailed quantitative study was carried out on the nature and extent of solvent-
solute interactions using multi-parameter solvent polarity relationships [38]. Effect of the
dye concentration was also investigated and the result was discussed based on
intermolecular and intra-molecular hydrogen bonding competition. Effect of acid/base on
the absorption spectra of the dyes was reported. Ionization constants, pK , for these dyes
a
were determined in ethanol-water media.
In order to have a deeper understanding of the photo-physical properties of the dyes in
anisotropic environments, the polarized absorption spectra of the compounds in nematic
solvent were investigated and interpreted. The spectral results also provide information
about effect of anisotropic media on azo-hydrazone tautomerism. Moreover, in order to
assign the azo and hydrazone spectra and a deep explanation and interpretation of the
photophysical behavior of the studied hydroxy azoquinoline compounds, their methylated
counterparts were prepared and their spectral data were analyzed. Finally, time-dependent
density functional theory (TD-DFT) calculations were performed for the representative p-
chloro hydroxy azoquinoline to provide further details at the molecular scale.
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2
.
Experimental
2
.1. Materials
All the organic solvents with reagent or spectroscopic grade were purchased from Merck,
Fluka, and AppliChem and their physical properties along with their polarity parameters are
taken from Ref 30. The organic solvents listed in the Supplementary data Table 1.
Diethanolamine, DEA, (purity > 0.98), aminoethyl ethanolamine, AEEA, (purity > 0.98),
1
,5-pentadiol, 1,5PD (purity > 0.98), and Triton X-100 (TX100, surfactant) were used as
multifunctional solvents (Figure 1b), and were obtained from Merck. In this work, 5CB and
a nematic liquid crystal of 1751 were used as anisotropic solvents. The chemical
composition and structure of the nematic mixture is given in (Figure 1c). The liquid crystals
were obtained from Prof. R. Dabrowski (the military technical academy, Warsaw, Poland).
2
.2. Preparation of test and buffer solutions
The stock solutions of the dyes (110 M) were prepared in 25 ml volumetric flasks by
dye dissolving in spectroscopy grade acetone (Merck). The sample solutions with proper
concentrations were made by micropipetting and aliquot of the stock solutions. An
analytical balance (AND, accuracy ±0.1 mg) was used to weighting the samples.
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The pH constancy was made by appropriate buffer solutions (Supplementary data Table 2).
The chemicals employed for the buffer preparation were analytical grade and supplied from
Merck and Aldrich. The buffer solutions were prepared as explained elsewhere [39]. In the
most cases, the pH needed to be adjusted using a pH meter and the drop wise addition of
either 1 M HCl or 1 M NaOH to 1 L of the solution. The pH measurements were carried out
by a digital pH meter Genway model 3505.
2
.3. Absorption spectra measurement
A Cary 100 UV–Vis double-beam spectrophotometer was used for running the
absorption spectra of the compounds over a wavelength range between 350 and 750 nm at
room temperature. The sample holders were a series of rectangular optical cell with 1–10
mm path lengths. The absorption maximum of the azo and hydrazone tautomers was
estimated using a peak deconvolution spectral analysis (Origin’s program). Deconvolution
of the spectra allowed a more precise evaluation of the peak profile. Prior to the
measurement of the spectra of the dyes, the spectrophotometer was zeroed with the solvent.
2
.4. Liquid crystal cell preparation and linear dichroism measurement
The dye-doped liquid crystal cells with parallel alignment were prepared using two parallel
optical glass plates. The cells separated by 50 µm Mylar spacers and sealed by epoxy resin
glue. The parallel orientation of the liquid crystal molecules was achieved using surface
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treatment of polyvinyl alcohol by the rubbing method. In this work, the dye-
doped nematic liquid crystals were studied up to a concentration about 1% w/w.
Polarized absorption measurements [36] were performed for the studied dyes in nematic
liquid crystals using a Cary 100 UV–Vis equipped with polarizers. Linear dichroism (LD)
and dichroic ratio ( All / A ) of the dyes were determined based on the measurements of
parallel ( All ) and perpendicular ( A ) absorbances to nematic orientation.
2
.5.
Synthesis of the azoquinoline dyes
The studied compounds were synthesized, purified and dried in our laboratory according to
the route outlined in Supplementary Figure 1 [40,41]. These compounds were five hydroxy
azoquinoline dyes (i.e. p-methoxy, p-ethoxy, p-chloro, o,p-dichloro, and m,m-
di(trifluoromethyl) hydroxy azoquinolines and their two methylated counterparts. All the
original and non-original compounds were synthesized in our laboratory. The prepared
1
compounds were characterized by the UV–Vis (Cary 100), FT-IR (Shimadzu 8400), and H
NMR (FT-NMR 400 MHz, Brucker) spectral analysis. Elemental analyses were performed
on a Heracus CHN-O-Rapid analyzer and the results were within the accepted range (±
0
.40) of the calculated values (Appendix A). The values in elemental analysis of the
compounds are in agreements with the assigned structures.
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Typically, to examine of the existence of azo-hydrazone tautomerism, we also measured
1
1
the H NMR of p-Cl HAQ in CDCl (as a neutral aprotic solvent). The H NMR spectrum
3
of p-Cl HAQ (Supplementary Figure 2) in CDCl3 clearly demonstrates the azo-hydrazone
tautomerism. Whereas both tautomers are simultaneously present in the solution, the sum
of integrals is equal to the number of protons of dye in azo and hydrazone structures. It can
be concluded that the azo dyes exist as equilibrium of azo
solution.
hydrazone tautomerism in
In addition, in order to specify the azo and hydrazone spectral bands in visible region, the
two methylated counterparts, 5-(4-methoxyphenylazo)-8-methoxy quinoline (p-M-MAQ)
and 5-(4-chlorophenylazo)-8-methoxy quinoline (p-Cl-MAQ) were synthesized for the first
time. The compounds were prepared using the procedures described in Appendix A.
3
.
Results and discussion
3
.1. Environment effect on absorption spectra of the dyes
Three classes of solvents based on their chemical and physical nature (i.e. ordinary
isotropic, multifunctional, and anisotropic solvents) were used in order to investigate the
effect of environment on spectral and structural parameters of the synthesized azoquinoline
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dyes. The selected ordinary solvents can be classified into three major groups of non-polar,
dipolar aprotic, and polar protic solvents.
The spectral and solvatochromic behavior of the prepared aryazoquinoline dyes (HAQs and
MAQs) were investigated in various organic solvents with different polarity and
polarizability. The spectral data were determined in dilute dye solutions from 2´10-6M to
´10-5M. In this range of concentrations, the dye molecules exist almost totally in
2
monomeric forms, and thus, the solvent-solute interaction has a main contribution to
control the shaping of the dye absorption spectra in dilute solutions. Stock solutions of each
dye were accurately prepared in acetone (110 M) and dilutions of these stocks were used
3
for absorption measurements. The spectral data are summarized in Tables 1, 2, and 3.
In order to assign the azo and hydrazone peaks, the hydroxyl groups of p-Cl-HAQ and p-
methoxy-HAQ were methylated by dimethyl sulfate in basic solution for the first time. It
can be seen that the new synthesized methoxy compounds showed only a single band in the
same region of the corresponding band of azo form, and the hydroxyl compounds show a
double maximum. Figures 2a and 2b represents selected absorption spectra of p-Cl-MAQ
and p-M-MAQ dyes, respectively. The two dyes (MAQs) show the same azo spectral
characteristics, as it expected.
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Typically, Figure 3ad presents visible absorption spectra of the hydroxy azoquinoline dyes
(HAQs) in selected organic solvents. As it can be observed, the spectral features of the dyes
are affected by the solvent characteristics. As similar absorption profiles in the spectra of p-
ethoxy-hydroxy azoquinoline (p-ethoxy-HAQ) and the p-methoxy-HAQ were observed, the
spectra of p-ethoxy-HAQ are not shown here. The absorption spectrum of each dye in
cyclohexane ( 2´10-5M) is also shown in the figures. All the dyes show simple transition
band at shorter wavelengths in this non-polar solvent. The choice of cyclohexane as a
reference solvent was motivated by the following reasons. It is a non-HBD/HBA solvent
with very low dielectric constant and its solvatochromic parameter values are equal zero.
The visible spectra of the dyes (HAQs) in a number of used solvents exhibit dual
absorption bands (a band in the region about 380–400 and a band in 450–500 nm). This
characteristic can be due to the azo-hydrazone tautomerism [10,11], where the absorption
bands at longer wavelengths are due to the hydrazone formation (Figure 3ad). The
tautomerism is more obvious for the HAQ-dyes dissolved in the polar solvents, where, the
hydrazone form is dominated in the dipolar aprotic solvents and multifunctional with basic
nature. This group of solvents such as DMF and DEA lead to enhance medium basicity,
and therefore, the azo form should be converted into its hydrazone form. However, despite
of the strong structural similarity of the investigated dyes, their spectral data are different in
a common medium.
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As it is obvious from the spectral data, m,m-CF
3
-HAQ and o,p-Cl-HAQ exist almost in
hydrazone forms in the most polar solvents as compared to the others. Azo-hydrazone
tautomerism is more obvious for fluorinated azoquinoline compound (m,m-CF -HAQ) and
3
o,p-Cl-HAQ, where the hydrazone form is dominated in the dipolar aprotic solvents such as
DMSO and DMF (Table 1). Due to the high basicities and high polarity of the dipolar
aprotic solvents, the –NH group in the hydrazone form should be more stabilized in HBA
solvents. As a result, the hydrazone formation becomes more favor in the order of the
solutions containing m,m-CF -HAQ > o,p-Cl-HAQ > p-Cl-HAQ > p-methoxy-HAQ (p-
3
ethoxy-HAQ). The introduction of the second electron acceptor substituent (Cl ) in the
orto-position of the tautomeric system (o,p-Cl-HAQ), increases the acidic character of the
OH proton, and therefore, we can expect a shift of the tautomeric equilibrium toward the
hydrazone form. Also, the fluorinated hydroxy azoquinoline compound possess two
electron withdrawing groups, CF3 , attached to the aromatic skeleton, which leads to form
the stronger azo/hydrazone tautomerism. In contrast, the electron donor substituent (-OCH
or -OC ) attached to the aromatic ring of the tautomeric system shifts the equilibrium
toward the azo form.
3
2
H
5
The organic multifunctional solvents used in this study were diethanolamine (DEA),
aminoethyl ethanol amine (AEEA), 1,5-pentadiol (1,5PD), and Triton X-100 (TX-100,
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surfactant). These liquids have different polar functional groups and can interact with the
dye molecules. DEA has a basic chemical nature with both amine and alcohol groups.
Figure 4 demonstrate typical absorption spectra of the dyes in the multifunctional solvent,
5
TX-100, 1,5PD, and DEA at a dye concentration of 210 M. The spectra of the dyes in
AEEA are not shown here, but the absorption spectra are very similar to that of DEA. As
can be seen, the spectral characteristics of the compounds, in particular m,m-CF -HAQ and
3
o,p-Cl-HAQ are strongly affected by the multifunctional media. The obvious spectral
performance is the disappearance of the azo spectral band and the occurrence of a
hydrazone peak at longer wavelengths.
The spectral feature of the dyes in TX-100 is similar to that of 1-octanol. As can be seen,
the spectral profiles of the dyes in 1,5PD are similar to that of 1-pentanol. The present of
second -OH group in 1,5PD does not make a markedly variation on the spectral data of the
dyes. In diols molecules, the existence of two non-adjacent primary hydroxyl groups leads
to the formation of a network structure. Formation of network structure in diols, due to the
molecular association, is a well-known phenomenon [42].
Despite of the structural similarity of 1,5PD and DEA (Figure 1b), however, they belong to
two different chemical classifications. The obvious difference between these two solvents is
that DEA molecule has a -NH group in middle of the carbon skeleton instead of whole
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carbon structure in 1,5PD molecules. Moreover, AEEA is a diamine with one secondary
and one primary amine groups. DEA and AEEA have a basic chemical nature. They are
aliphatic bifunctional molecules with both amine and alcohol groups. This difference makes
a remarkable change in the spectral characteristics of the azo dyes. The similarities of
absorption spectra of the dye in DEA (or AEEA) and in the solution of water-ethanol +
NaOH 0.1 M (Figure 5) suggest that the presence of -NH group in center of the molecule,
and consequently, alkaline nature of DEA (AEEA) has an important effect on the
absorption spectra rather than the existence of the two non adjacent –OH groups. The
alkaline medium of DEA or AEEA is mainly responsible for the spectra changes observed
for the dyes, specifically for the short-wavelength band, where the neutral form of the dye
may be formed on alkaline media. Thus, several factors such as polarity, polarizability and
alkaline nature of DEA (AEEA) may give many possibilities to form different type of
interactions. As can be seen from Figure 5, the hydrazone form is entirely dominated in
basic solutions. In contrast, the azo form is totally dominated in acidic medium.
The studied dyes are able to accept or donate hydrogen bonds. However, the multiple
solvent effects such as solvent dipolarity, polarizability and H-bonding on the dye spectral
characteristics in primary aliphatic alcohols make it difficult to determining of the H-
bonding contribution. The changes in solvent hydrogen bonding strength are at least
partially responsible for the spectra shifts and azo-hydrazone tautomerism observed in
alcohols (see Table 1). For instant, Figure 6 typically shows the absorption spectra of the p-
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Cl-HAQ dye dissolved in the primary aliphatic alcohols ( 2´10-5M). As can be seen from
the figure, on going from methanol to 1-octanol, i.e. with decreasing solvent polarity but
increasing solvent polarizability, the tautomeric equilibrium is shifted to the high polar
tautomer. Similar trend was observed for the other hydroxy azoquinoline dyes. It could be
due to the formation of inter-molecular H-bonding between the HBD solvents and –NH
group in the hydrazone form. It is more evident for o,p-Cl-HAQ dye, where the hydrazone
form is dominated in the heavy alcohols such as 1-hexanol, 1-heptanol, and 1-octanol.
From the observed data, it can be seen that the dye–solvent dispersion interactions are
important in the solvents with high refractive indices and hence large polarizabilities.
Therefore, the solvent polarizability effect through the van der Waals interaction (steric and
dispersive) has also an important effect on the dyes absorption spectra in the solvent
environments. Noted that, for the primary alkanols series (from CH OH toC H OH) the
3
8
17
addition of a methylene group increases the overall polarizability of the molecules, as the
chain length develop.
The solvent polarity and polarizability values as expressed by the solvent polarity and
2
2
polarizability functions ([f()  (1) / (21)] and [f(n)  (n 1) / (2n 1) ]). The π* is
solvent dipolarity/polarizability scale. The solvent polarity parameters together with
selected physical properties are summarized in the Supplementary data Table 1. Typically,
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Figure 7a–c show the variation of maximum wave number (max ) of the o,p-Cl-HAQ as a
function of solvent polarity and polarizability functions [f() , f(n) and *] for both the
azo and hydrazone forms. As it can be seen, poor correlations were observed between
max values and the solvent polarity parameters for both the azo and hydrazone forms. The
regressions indicate that the solvent polarity has almost an irregular effect on the position of
absorption spectral of the azo-hydrazone dyes. Nevertheless, in the case of the hydrazone
form, *-scale shows a relatively good linear correlation between the observed data. The
regression of about 0.78 was obtained for this scale. However, there is no good correlation
between empirical parameters reflecting the solvent polarity and the position of the azo-
hydrazone tautomeric equilibrium as reported by Reeves and Kaiser [9]. However, for the
investigated dyes dependable correlations might be obtained between multiparameter
polarity scales and the absorption band positions.
3
.2.
Multi-parameter solvent polarity relationships
The effect of three groups of isotropic ordinary solvents was studied at room temperature.
The first group is composed of the polar protic solvents such as primary linear alcohols
(from methanol to 1-octanol), which exhibit strong hydrogen bond donor capabilities. The
second group composed of the dipolar aprotic solvents such as acetone and DMSO, which
are very good hydrogen bond acceptors, and poor hydrogen bond donors. Cyclohexane,
toluene and 1,4-dioxane were used as non-polar solvents.
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In this work, the linear solvation energy relationships (LSER) correlations such as Kamlet-
Abboud-Taft (KAT) and Katritzky (KTZ) multiparameter equations [38] were used for
quantitative estimation of the solvent effects. The KAT equation [30]
(
   a b  c *) evaluates hydrogen bonding donor/acceptor interactions
o
between solute and solvent.

is the scale of solvent H-bond donor ability,
measure of solvent
is the band maximum in cm . The coefficients ( a,band ) are
the relative susceptibilities of the solute property. The KTZ equation

is the scale
of solvent H-bond acceptor ability, and   is
a
1
dipolarity/polarizability.

c
2
2
(
   aE (30)b( 1/2 1)c(n 1/2n 1) combined Reichardt polarity
o
T
 n
function ( ET (30) ) with the dielectric constant ( ) and index of refraction function ( ).
The correlated data were determined using the values listed in Supplementary data Table 1
and multi-linear regression analyses. The regression analyses were listed in the
Supplementary data Tables 3 to 8. In order to have comparable coefficients values, the
parameters for all the used solvents were re-normalized and re-scaled and summarized in
Table 4 and Table 5. In general, both the polarity scales correlate the dyes spectral data
acceptably. According to the LSER correlations such as KAT equation, the overall
contribution of the intermolecular interactions between the dye and the solvents was
considered to be 100%. It was divided for the different types of the interactions (different
types of the correlation parameters) for each correlation.
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Regarding to the Kamlet-Taft correlation, the solvent hydrogen–bond acceptor ability (β)
possesses the main contribution into spectral feature in azo region (Table 4). In contrast,
this contribution irregularly decreases in hydrazone regions. As can be seen from the table,
 *
has the main contribution in the hydrazone region. Although, for o,p-Cl-HAQ, the
solvent hydrogen–bond acceptor ability (β) has also at least partially contribution into
spectral characteristic in hydrazone region. The multiple solvent effects have cooperative
contributions on the position of the absorption band of the dyes in hydrazone region.
However, in the hydrazone region, these contributions alter differently for different dyes.
As it can be seen from Table 5, the KTZ multi-parameter correlation shows that the value
connected with Reichardt function, ET (30), and polarity function ( 1/ 2 1) have 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 spectra of the dyes in anisotropic media
The azo-hydrazone tautomerism of the dye-doped nematic liquid crystals were also studied
at room temperature. In order to elucidate the spectral behavior of the dyes in the polar
nematic liquid crystals, the nematic mean dielectric constants, 1/3(  2 ) , and
ll

refractive indices, n 1/3(n  2n ) were used [33], where ll and  are dielectric
e
o
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constants parallel and perpendicular to the molecular axis, respectively. no and ne are the
ordinary and extraordinary refractive indices, respectively. These mean data can be
considered as measure of the liquid crystal polarities and polarizabilities (Supplementary
data Table 9).
The selective spectral data in the anisotropic media are also summarized in Table 1. For this
dye, the azo form is almost completely dominated in the nematic hosts. The same behavior
was observed for the other dyes. These observations indicate that the anisotropic solvation
is complicated. The spectral features in the nematic hosts might depend on several factors
such as anisotropic interaction. However, overall contributions prevent the formation of the
more polar tautomers. Perhaps, it can be attributed to the relatively ordered and packed
structure of the host molecules. The result of current work verify our previously report [37].
The dichroic ratio directly relates to the orientational order parameter Sdye  (R 1)/(R  2)
for the transition moment of dye [43]. For these dyes, the angle between the absorption
oscillator and the long molecular axis is not zero, thus, the term of “degree of anisotropy”
was applied instead of order parameter. Typically, Figure 8a-c shows the polarized
absorption spectra of the dyes in 5CB. The dichroic ratios and degree of anisotropies of the
compounds were complied in Table 6. The low values of these parameters for the
investigated dyes might be related to the dye molecular structure, the intermolecular
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interactions between the dye and liquid crystal molecules, and the angle between the
transition dipole moment and the long molecular axis. The Sdye -value is in the order of the
nematic solutions containing p-Cl-HAQ
 
p-methoxy-HAQ p-ethoxy-HAQ > o,p-Cl-
HAQ. The presence of the second group chlorine in the o-position of the (o,p-Cl-HAQ)
molecule causes the direction of transition moment to deviate from the long molecular axis.
3
.4
Concentration effect on the photophysical behavior
In order to investigate the effect of concentration on the photophysical and tautomeric
properties of the dyes, the spectral characteristics of the compounds in water-ethanol
solutions at the different dye concentrations were studied. In this purpose, the absorption
spectra of each dyes in aqueous-ethanolic solutions (50:50), at different concentrations
6
3
ranging from 110 M to110 M, were recorded. Typically, Figure 9 shows the spectra
of p-chloro hydroxy azoquinoline (p-Cl-HAQ) in water-ethanol mixture as function of the
dye concentration. As it is expected, the spectra of p-Cl-HAQ show the existence of both
the tautomeric in water-ethanol solutions. As can be seen from the figure, the band at about
3
70 nm is assigned to the azo form. Similar spectral results were observed for the other
studied dyes.
As can be seen from Figure 9, no new band is observed when concentration of the dye is
5
3
increased (i.e. from 110 M to 110 M). However, it was observed that the
concentration of the dyes plays a noticeable role in the dye spectral characteristics as a
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change in their concentration varies the intensity of the azo-hydrazone spectra. In the high
concentrations, the tautomeric equilibrium is shifted to the less polar tautomer. The
concentration dependence of the dye spectra is related to the molecular interactions
between the dye and solvents molecules as well as the dye molecular association (dye
aggregation) at high dye concentration [22].
It can be concluded that at the high dye concentrations, the molecular association might be
appeared as the number of interacting molecules increases. The aggregate species are held
together by different kinds of intermolecular interactions such as hydrogen bonding and
dispersion interactions. At the high dye concentrations, the intermolecular hydrogen
bonding is more stable than the intramolecular hydrogen bonding, and therefore, the azo
form is the main species in concentrated solutions.
3
.5
Determination of pKa value of the dyes in ethanol-aqueous buffer solution
In order to investigate the role of pH on the spectral properties of the studied dyes, their
absorption characteristics in 50 vo1% ethanol-water buffer solutions were studied over the
pH range 1.1-13.2. The presence of the neutral and ionic species in solutions caused by the
alkaline media might be involved in the spectral data of the dyes.
The pH meter readings (B) recorded in 50 vo1% ethanol-water solutions were changed to

hydrogen ion concentration [
H
] using the Van Uitert and Haas equation
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
[
log[H ]  BlogUH ] [44], where logU H is the correction factor. The readings were
carried out at room temperature on a series of solutions with known quantities of
HCland NaOH in which the ionic strength was equal to 0.1 in 50 vol.% ethanol-water.
The acid dissociation constants for the hydroxy azoquinoline dyes (HAQs) were
5
determined by spectroscopic analysis at a constant dye concentration ( 210 M) and in
5
3
0% ethanol-water mixture (v/v). Each compound exhibits two bands in the region of
80410 nm and 483–520 nm for molecular and the anionic species, respectively. As the
pH value of the solution increases, the height of the former band decreases and
simultaneously that of the latter band increases. From the visible absorption spectra, the
isosbestic points indicate that two species are in equilibrium. Figure 10ad demonstrates a
comparison of the absorption spectra of the hydroxy azoquinoline dyes in different pH
buffer solutions.
As can be seen, the absorption bands are pH dependent and display distinguishable spectral
characteristics, in particular at pH 8. Figure 11 shows a typical curve of absorbance-pH
for m,m-CF -HAQ. These spectral variations supposed to be caused by deprotonation of the
3
dye molecules. However, at the highest pH tested (pH = 13.2), only the anionic forms exist.
The absorption spectra of the methylated azo hydroxyquinolines (MAQ's) in different pH
buffer solutions were also investigated. No remarkable changes were observed in
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absorption bands of the dyes, as consequence of the lack of -OH group in the MAQ's
molecules
The value of pK  pH +log[(A -A )/(A A )] was calculated from each measurement:
a
i
b
i
i
a
where Ai is the absorbance of the solution at intermediate pHi and Aa and Ab are the
absorbance values of the strongest acidic and alkaline buffer solutions of each compound.
The pKa values of the dyes were estimated by analysis of the spectral changes and were
listed in Table 7. Since the electron density on the aromatic rings can be reduced by the
electron withdrawing groups such as Cl, the pK
hydroxyl group increases (7.89 for p-Cl-HAQ, 7.28 for o,p-Cl-HAQ, and 7.10 for m,m-
CF -HAQ). While  OCH3  OC H ) group attached to phenyl ring of p-M-HAQ dye
a
values show that the acidity of the
3
(
2
5
decrease the acidic character, due to its electron donating ability which destabilizes anionic
forms of the dyes.
3
.6. DFT and TD-DFT Calculations:
From the presented spectra data in different solvents and acid-base spectra, a question then
arises as whether only one tautomeric form and corresponding anion exist or more? Based
on the position of absorption spectra (the band around 500nm), is there an existence of two
different particles (hydrazone and anion forms)?
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3

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To have a better description for our experimental data, we performed DFT and TD-DFT
calculations employing the B3LYP/6-31+G* methodology as implemented in the Gaussian
0
9 package [45]. First, DFT calculations were performed to optimize the representative
structure of p-Cl-HAQ to show possibilities of formations of tautomeric azo and hydrazone
forms. Both the neutral and anoionic forms were investigated. All three models of p-Cl-
HAQ (Figure 12) including azo, hydrazone, and anionic forms were optimized first.
Subsequently, TD-DFT calculations were performed to evaluate wavelengths for the
maximum absorptions. All data are evaluated for the gas-phase system. The obtained data
are visualized by Chemcraft package [46].
The HOMO and LUMO distribution patterns (Figure 12) demonstrate that  * electron
transfer could be possible between these two MO levels as the patterns are distributed on
both parts of molecules. However, for the tautomeric forms, the major electron transfer
could be done for the fused rings. From the values of Table 8, it could be mentioned that
the largest required energy of transition is for the neutral structure and the smallest required
energy is for the negative-charged structure. This trend is approved by the calculated
wavelengths of maximum absorptions (Fig. 13), in which the neutral azo structure has the
lowest wavelength but the negative-charged structure has the highest wavelength of
maximum absorptions. Therefore, it could be mentioned that the effects of structural
changes are observed for the electronic properties of p-Cl-HAQ. The tautomeric structures
are between the wavelength ranges of neutral azo and negative-charged structures. The
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4

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obtained values of calculated wavelengths are in good agreements with the experimental
data. Based on our calculated results, it could be mentioned that the absorptions for
hydrazone form are close to the experimental data in higher regions in comparison with azo
form. For the anionic form, the values are also higher than both of azo and hydrazone
forms. In this case, it could be concluded that both hydrazone and anionic forms could be
detected by absorptions in higher wavelengths than those of azo form.
4
.
Conclusions
In this work, environment, concentration, and substituent effects on spectral, photophysical
behavior of some arylazo-8-hydroxyquinoline dyes and their new methylated counterparts
(HAQs and MAQs) were studied. The spectral data show the importance of substituent
attached to the aromatic skeleton in these compounds. The spectral behavior of the dyes
and the ratio of azo/hydrazone mixture are largely changed by the substituents structure and
nature. Because of tautomerism and solvent effect, three major regions including azo
region, hydrazone region and an intermediate region can be identified through the
absorption data analyses. The experimental result indicated that the strong azo-hydrazone
tautomerism occurred in some dipolar aprotic solvents such as DMF and THF and solvent
with basic nature.
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Interestingly, in high polar anisotropic hosts, the studied dyes mainly adopt an azo form.
The result of current work verify our previously report. Positive dichroic ratios (R > 1) are
found for all the dyes in liquid crystalline solvents. According to the LD measurements
(
A  A  0 ) all the absorption bands are due to the * transitions. Noted that, the
ll


* is a parallel transition, i.e. during the transition a net displacement of charge occurs
parallel to the molecular plane.
The hydrazone form is also dominated in the heavy primary alcohols such 1-octanol. The
dye spectra in the alcoholic solutions show an alternation in the dye absorbances. The odd-
even variation of the dye absorbances in the alkanols series (C OH andC OH ) could
1
8
be due to the influence of the chain length and/or chain orientation on the dye–solvent
interactions. The spectral characteristics of the dyes in 1,5-pentanediol (a diol) are very
similar to that of 1-pentanol. It means that the present of second -OH group in this diol does
not make a noticeably variation on the spectral data of the dyes.
The ionization constants for the studied dyes were determined using their spectral data. The
presence of the neutral and anionic species in the buffer solutions was involved in the
spectral data of the dyes. Each compound exhibits two bands. As the pH value of the
solution increases, the intensity of the short wavelength band decreases and simultaneously
the long wavelength band increases.
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For the studied dyes in the high concentrations (50/50 v% water–ethanol) the azo form is
dominated. As a result, at the concentrated solutions, the intermolecular hydrogen bonding
is more stable than the intra-molecular hydrogen bond. The TD-DFT calculations confirm
the possibility of existence of two different particles (one tautomeric form and the
corresponding anion) at around 500 nm.
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