Geometrical structures, thermal, optical and electrical properties of azo quinoline derivatives

Article abstract of DOI:10.1016/j.molliq.2015.07.050

Azo quinoline derivatives (AQ_x) were synthesized by coupling of 8-hydroxyquinoline with aniline derivatives. The optimized bond lengths, bond angles and the quantum chemical parameters for the ligands (AQ_x) were calculated. The dielectric constants (ε_r and ε_i) and ac conductivity (σ_ac) were studied as a function of both temperature and frequency in the temperature range 293-509 K and frequency range 10²-10⁵ Hz. The thermal activation energies ΔE₁ and ΔE₂ were calculated and found to be in the range of 0.03-0.26 and 0.2-1.31 eV, respectively, depending on the substituent and frequency. The conduction mechanism was investigated for all the derivatives under investigation. The ligands (AQ₁, AQ₂ and AQ₄) were found to be controlled by correlated barrier hopping model and the ligands (AQ₃ and AQ₅) were controlled by small polaron tunneling mechanism. The optical absorption properties of the ligands thin films were investigated. The absorption coefficient (α) spectra reveals two absorption peaks which are assigned as π-π^? and n-π^? transitions. The optical energy gap (E_g) was investigated near the absorption edge and found to be in the range of 1.34-2.26 and 1.47-1.69 eV for direct and indirect optical transitions, respectively.

Full text of DOI:10.1016/j.molliq.2015.07.050

Journal of Molecular Liquids 211 (2015) 628–639
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Geometrical structures, thermal, optical and electrical properties of azo
quinoline derivatives
b
b
N.A. El-Ghamaz a, A.A. El-Bindary b, , A.Z. El-Sonbati , N.M. Beshry
⁎
a
Physics Department, Faculty of Science, Damietta University, Damietta 34517, Egypt
Chemistry Department, Faculty of Science, Damietta University, Damietta 34517, Egypt
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Azo quinoline derivatives (AQx) were synthesized by coupling of 8-hydroxyquinoline with aniline derivatives.
The optimized bond lengths, bond angles and the quantum chemical parameters for the ligands (AQx) were
calculated. The dielectric constants (εr and εi) and ac conductivity (σac) were studied as a function of both
temperature and frequency in the temperature range 293–509 K and frequency range 102–105 Hz. The thermal
activation energies ΔE1 and ΔE2 were calculated and found to be in the range of 0.03–0.26 and 0.2–1.31 eV,
respectively, depending on the substituent and frequency. The conduction mechanism was investigated for all
the derivatives under investigation. The ligands (AQ1, AQ2 and AQ 4) were found to be controlled by correlated
barrier hopping model and the ligands (AQ 3 and AQ5) were controlled by small polaron tunneling mechanism.
The optical absorption properties of the ligands thin films were investigated. The absorption coefficient (α) spec-
Received 25 June 2015
Received in revised form 14 July 2015
Accepted 20 July 2015
Available online xxxx
Keywords:
Azo quinolines
Thermogravimetric analysis
Optical properties
Electrical properties
⁎
⁎
tra reveals two absorption peaks which are assigned as π–π and n–π transitions. The optical energy gap (Eg)
was investigated near the absorption edge and found to be in the range of 1.34–2.26 and 1.47–1.69 eV for direct
and indirect optical transitions, respectively.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
and the dominant conduction mechanism is Variable Range Hopping
mechanism (VRH). The ac conductivity and conduction mechanism of
The study of azo dyes has been an active area of research
and attracted the attention of many researchers in recent years [1,2].
These compounds have technological, industrial and practical applica-
tions such as coloring fibers [3], optical storage technology [4] and tex-
tile dyes [5]. 8-Hydroxyquinoline derivatives and their complexes
with transition metals have high antibacterial activities [6]. Also, the
chemical properties of quinoline and its derivatives have been widely
discussed because of their biological relevance, coordination capacity
and their use as metal extracting agent [7]. They have attracted special
interest due to their therapeutic properties. Quinoline azodye and
its derivatives are very important compounds and have attracted
much attention in both academic and applied research used in many
applications such as chromophoric and metallochromic indicators in
analytical chemistry [8].
The optical, electrical, thermal and structural properties of many
8-hydroxyquinoline derivatives and their metal complexes have been
studied [9–14]. The dc electrical properties and conduction mechanism
of azo sulfonyl quinoline ligands and their uranyl complexes have been
investigated by El-Ghamaz et al. [9]. They concluded that the dc electri-
cal conductivity (σdc) increases with increasing temperature. The
thermal activation energy was found to be in the range 0.44–0.90 eV
5-(4′-derivatives phenyl azo)-8-hydroxy-7-quinolinecarboxaldehyde
(AQLn) and their metal complexes [Cu(AQLn)2] ∙ 5H2O showed semicon-
ductor behavior with variation of temperature and according to the
data, the conduction mechanisms were small polaron tunneling (SPT),
quantum mechanical tunneling (QMT) and correlated barrier hopping
(CBH) depending on the temperature range and type of substituent
[10]. Also the thermal property investigations for AQLn and
[Cu(AQLn)2] ∙ 5H2O showed that all the complexes are thermally stable
in wider temperature range than their azo ligands. The optical proper-
ties of many quinoline derivatives have been investigated by many
authors [11–14]. El-Ghamaz et al. investigated the absorption and dis-
persion properties of different quinoline azodyes derivatives [12–14].
The electronic transition near the absorption edge of 5-(4′-derivatives
phenyl azo)-8-hydroxy-7-quinolinecarboxaldehyde and Cu(II) com-
plexes of (4-alkylphenylazo)-5-sulfo-8-hydroxyquinoline was found
to be indirect allowed and the values of optical energy gap (Eg) were
found in the ranges 1.33–1.92 and 2.48–3.51 eV, respectively [12,13].
The effect of substituent group variation on the optical functions, dielec-
tric constant, optical conductivity, surface energy loss (SEL) and volume
energy loss (VEL) of 5-sulfono-7-(4-x phenyl azo)-8-hydroxyquinoline
(SAHQ-x) has been studied. It was found that the substitution group
variation has no effect on the transmittance properties of the films in
the absorption region of the spectra. Substituent variation influences
the refractive index (n), the oscillator energy (Eo), the dispersion energy
⁎
Corresponding author.
E-mail address: abindary@yahoo.com (A.A. El-Bindary).
http://dx.doi.org/10.1016/j.molliq.2015.07.050
0167-7322/© 2015 Elsevier B.V. All rights reserved.

N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
629
(Ed) and the bond length of 5-Sulfono-7-(4-phenyl azo)-8-hydroxy
quinoline compound [14]. M. Saçmacı et al. [11] reported that the
optical transition for 2-[(E)-(8-hydroxyquinolin-5yl)-diazenyl]-4,5-
dimethoxybenzoic acid was direct allowed and has the value of
1.95 eV for energy gap.
We are interested in studying the physical properties of many
quinoline ligands and their complexes because of their wide technolog-
ical and industrial applications. In continuation of these studies on azo
quinoline derivatives, we aimed, in present work, to investigate the op-
tical, thermal and dielectrical properties of 5-(2-aryldiazenyl)quinolin-
8-ol (AQx) as one of the promising azo derivatives of quinoline for
many industrial and technological applications.
with heating rates of 10–15 °C/min over a temperature range
from room temperature up to 1073 K. The molecular structures
of the investigated compounds are optimized by HF method with
3–21 G basis set. The molecules are built with the Perkin Elmer
ChemBio Draw and optimized using Perkin Elmer ChemBio3D
software [20,21].
The ac conductivity measurements are performed on the samples in
the form of discs of thickness 0.3–0.8 mm and compressed under a pres-
sure of 4 t cm−2 using hydraulic press. The ac conductivity measure-
ments of samples are measured as a function of temperature in the
range 293–509 K and frequency range 102–105 Hz using Stanford re-
search systems Model SR 720 LCR METER. The capacitance (CP) and
the loss tangent (tan δ) are measured in parallel mode. The temperature
is measured by NiCr–NiAl thermocouple. The potential across the heater
varied gradually through a variac transformer to produce a slow rate of
increase of temperature in order to obtain accurate temperature
measurements. The range of temperature for electrical measurements
is chosen according to TGA measurements.
2. Experimental techniques
2.1. Preparation of 5-(2-aryldiazenyl)quinolin-8-ol (AQ x)
5-(2-Aryldiazenyl)quinolin-8-ol (AQ x) ligands are prepared accord-
ing to El-Sonbati et al. [15,16]. In a typical preparation, 25 ml of distilled
water containing 0.01 mol hydrochloric acid is added to aniline
(0.01 mol) or its p-derivatives. A solution of 0.01 mol sodium nitrite in
20 ml of water is added dropwise to the resulting mixture then stirred
and cooled to 0 °C. The formed diazonium chloride is consecutively
coupled with an alkaline solution of 0.01 mol quinolin-8-ol, in 10 ml
of pyridine. The preparation of ligands (AQ x) is summarized in Scheme
S1 in the supplementary. The colored precipitate, which formed imme-
diately, is filtered through sintered glass crucible and washed several
times with water. The crude products are purified by recrystallization
from hot ethanol and dried in vacuum desiccator over P2O5. Yield per-
cent was 65–81%. The ligands are also characterized by IR spectroscopy.
The resulting formed ligands (AQx) are:
3. Results and discussion
3.1. Molecular structures of the ligands
The calculated molecular structures for ligands (AQ x) are shown in
Fig. 1. Primary calculations reveal that the azo form (B) is more stable
and reactive than azo form (A) (Scheme S1 in the supplementary).
The calculated quantum chemical parameters are listed in Table 1.
Molecular structures (HOMO & LUMO) are presented in Fig. S1 in the
supplementary for ligands (AQ x) [azo form (B)]. Selected geometric
parameter bond lengths and bond angles of AQ x are tabulated in
Tables S1–S5 in the supplementary for ligands (AQ x) [azo form (B)].
The HOMO–LUMO energy gap (Et) is an important stability index
which is applied to develop theoretical models for explaining the
structure and conformation barriers in many molecular systems. As
the value of Et decreases the stability of the compound increases [17,
20,21]. Additional parameters such as separation energies (Et), absolute
electronegativities (χ), chemical potentials (Pi), absolute hardness (η),
absolute softness (σ), global electrophilicity (δ), global softness (Sg)
and additional electronic charge (ΔNmax) are calculated according to
the following equations [17,20,21]:
AQ 1: 5-(2-(4-methoxyphenyl)diazenyl)quinolin-8-ol.
AQ 2: 5-(2-(4-methylphenyl)diazenyl)quinolin-8-ol.
AQ 3: 5-(2-phenyldiazenyl)quinolin-8-ol.
AQ 4: 5-(2-(4-chlorophenyl)diazenyl)quinolin-8-ol.
AQ 5: 5-(2-(4-nitrophenyl)diazenyl)quinolin-8-ol.
2.2. Preparation of azo thin films
Homogenous thin films of (AQ x) are prepared by a conventional
spin coating technique onto pre-cleaned optical flat quartz substrates
[17]. The prepared ligands are dissolved in chloroform until saturation.
The rotating speed of the spin coating system is controlled to be about
1600 rps. The thicknesses of the prepared thin films are then measured
optically by the Michelson interferometric method [18]. The measured
thicknesses of the prepared thin films are found to be in the range
400–500 nm.
Et ¼ ELUMO − EHOMO
ð1Þ
ð2Þ
−ðEHOMO þ ELUMO
Þ
χ ¼
2
ELUMO − EHOMO
η ¼
ð3Þ
2
σ ¼ 1=η
Pi ¼ −χ
ð4Þ
ð5Þ
2.3. Analytical techniques
The infrared spectra are recorded as KBr discs using a Jasco
FTIR-4100 spectrophotometer. The 1H-NMR spectra by Bruker WP
300 MHz using DMSO-d6 as a solvent containing TMS as the internal
standard. Mass spectra were recorded by the EI technique at 70 eV
using MS-5988 GS–MS Hewlett–Packard. Ultraviolet–visible (UV–vis)
spectra of the compounds are recorded in the wavelength range
200–900 nm using a Perkin–Elmer AA800 spectrophotometer
Model AAS. X-ray diffraction measurement (XRD) is recorded on
X-ray diffractometer in the range of diffraction angle 2θ° = 5–80°.
This analysis is carried out using CuKα1 radiation (λ = 1.540598 Å).
The applied voltage and the tube current are 40 KV and 30 mA,
respectively. The diffraction peaks are indexed and the lattice
parameters are determined with the aid of CRYSFIRE computer
program [19]. Thermal analysis of the ligands are carried out using
a Shimadzu thermogravimetric analyzer under nitrogen atmosphere
1
Sg ¼
2η
ð6Þ
δ ¼ Pi2=2η
ð7Þ
ð8Þ
ΔNmax ¼ −Pi=η:
The values of Et for AQ 1, AQ 2, AQ 3, AQ 4 and AQ 5 ligands were found
2.386, 2.576, 2.696, 2.327 and 3.135 eV, respectively.
3.2. IR spectra of the ligands
All of the free ligands showed IR characteristic bands at 1555,
1540 and 3200 cm−1 assigned to υ(C_N)py, υ(–_N–) and υ(OH)

630
N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
frequencies, respectively. El-Sonbati and co-workers [22–24] found
three kinds of OH structures on the basis of the IR frequencies:
(i) Only oxygen is in the bridge while the hydrogen is free.
(ii) Polymer chains are formed in which both hydrogen and oxygen
atoms are precipitating in the hydrogen bond.
AQ1
(iii) Dimer associated structures are formed.
On the basis of these, AQ x exists on a five-membered chelate
skeleton and the hydrogen bonding could be classified into two types:
(a) Intramolecular hydrogen bonding O–H…..N (Scheme S1(B) in
the supplementary) resulted from hydrogen (C8–OH) and
nitrogen of azomethine group (C_Npy).
(b) Intermolecular hydrogen bonding O–H….O (Scheme S1(C) in
the supplementary) and (O–H..N) (Scheme S1(D) in the
supplementary) resulted from the C8–OH group through two
molecules and/or C8–OH with C_N through two molecules.
AQ2
The assignment of the C_N and C–C stretching modes on our previ-
ous work [23,24] and investigations by Henry et al. has been reported
[25]. Upon comparing the position of these bands for different ligands,
it was found that ring substitution resulted in a shift to lower frequen-
cies. This behavior can probably be explained by the lowering in the
electronic density caused by electronegative groups.
These comparisons showed that the electronegativity of the ring
substituents produces not only a decrease in the electronic density
over the ring, geometrating a diminution in the υ(C = N) stretching
frequency, but also causes a plane deformation, doubly degenerate
stretch and doubly degenerate in plane bending in D3h symmetry [26].
3.3. Mass spectra
AQ3
The electron impact mass spectrum of ligand (AQ3) is recorded
and investigated at 70 eV of electron energy. The mass spectrum
fragmentation mode of ligand (AQ3) shows the exact mass of 249 cor-
responding to the formula C15H11N3O. The ion of m/z = 249 undergoes
fragmentation to a stable peak at m/z = 172 by losing C6H5 atoms
(structure I) as shown in Scheme 1. The loss of N2 leads to the fragmen-
tation with m/z = 144 (structure II). The loss of CHO atoms leads to the
fragmentation with m/z = 115 (structure III). A breakdown of the
backbone of AQ3 ligand gives the fragment (IV).
3.4. 1H NMR spectra
AQ4
The 1HNMR spectra of the free ligands (AQ x) in DMSO-d6
showed two signals in the range 8.95–9.30 ppm. These signals are
probably assigned to the azomethine group and hydroxy group of
8-hydroxyquninoline derivatives [23]. The presence of these
signals at high values downfield of TMS is due to hydrogen bonding
between this hydroxy group and the nitrogen of azo group. We also
found that upon deutration of the solution the signal disappeared.
3.5. X-ray diffraction analysis
AQ5
Single crystals of the ligands could not be prepared to get the XRD
and hence the powder diffraction data were obtained for structural
characterization. Structure determination by X-ray powder diffraction
data has gone through a recent surge since it has become important to
get to the structural information of materials, which do not yield good
quality single crystals. The X-ray diffraction (XRD) pattern of AQ3 ligand
in powder form is shown in Fig. 2. The XRD pattern shows that AQ3
ligand has a polycrystalline nature. The calculated crystal system is
found to be monoclinic with space group P21/A. The estimated lattice
parameters are found to be 20.4710 Å, 18.8150 Å, 19.9590 Å, 90.0°,
92.7° and 90.0° for a, b, c, α, β and γ, respectively. The inter-planar
Fig. 1. The calculated molecular structures of the investigated compounds (AQx).

N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
631
Table 1
The calculated quantum chemical parameters for the investigated compounds (AQx).
Compound
EHOMO (eV)
ELUMO (eV)
Et (eV)
Χ (eV)
η (eV)
σ (eV)−1
Pi (eV)
S (eV)−1
ω (eV)
ΔNmax
AQ 1
Form A
Form B
−4.694
−4.079
−2.038
−1.695
2.656
2.386
3.366
2.887
1.328
1.192
0.753
0.839
−3.366
−2.887
0.377
0.419
4.266
3.497
2.535
2.422
AQ 2
Form A
Form B
−4.897
−4.266
−2.035
−1.689
2.862
2.576
3.466
2.978
1.432
1.288
0.699
0.776
−3.466
−2.978
0.349
0.388
4.197
3.442
2.421
2.312
AQ 3
Form A
Form B
−5.021
−4.384
−2.030
−1.684
2.991
2.696
3.525
3.034
1.495
1.349
0.669
0.741
−3.525
−3.034
0.334
0.370
4.156
3.410
2.358
2.248
AQ 4
Form A
Form B
−4.615
−4.011
−2.029
−1.684
2.586
2.327
3.322
2.847
1.293
1.164
0.773
0.859
−3.322
−2.847
0.387
0.429
4.267
3.484
2.569
2.447
AQ 5
Form A
Form B
−6.381
−5.573
−2.735
−2.438
3.646
3.135
4.558
4.006
1.823
1.567
0.549
0.638
−4.558
−4.006
0.274
0.319
5.698
5.118
2.500
2.556
spacing (d) and Miller indices (hkl) which are estimated by CRYSFIRE
are listed in Table 2. The average crystallite size (S) is calculated
according to Scherer's equation [27,28] as follows:
The TGA is used for the determination of rates of decomposition
of AQx. The rate constant of the thermal decomposition is plotted
according to the Arrhenius relationship [21]:
Eꢀ
RT
0:95λ
ψ cos θ
lnKꢀ ¼ ln β −
;
ð10Þ
S ¼
ð9Þ
⁎
where ψ is the width measured in radians of the half-maximum peak
intensity, λ is the X-ray wavelength and θ is the Bragg's angle. The
estimated crystallite size (S) is found to be about 24.3 nm.
where K is the rate constant of the thermal decomposition, β is a pre
⁎
exponential constant, E the thermal activation energies of decomposi-
tion, R is a gas constant (8.314 J ∙ mol−1 ∙ K−1) and T is the absolute tem-
⁎
perature. The values of E are calculated from the slope of the straight
line obtained from the plot ln K versus 1/T as shown in Fig. 3. The values
of E are found to be 76.09, 73.27, 78.61, 85.68 and 88.86 kJ/mol for AQ1,
⁎
3.6. Thermal analysis
⁎
The TGA data for AQ2, AQ3 and AQ5 shows two steps of the loss of
masses, while for AQ1 and AQ4 shows three steps. The loss of about
~4% of AQ5 at the first heating stage up to 50 °C may be due to loss of
moisture. The temperature intervals and the percentage of loss of
masses corresponding to the probable decomposition processes are
listed in Table 3.
AQ2, AQ3, AQ4 and AQ5, respectively. The high values of the activation
energies (E ) reflect the thermal stability of the compounds. So, it is
clear from TGA analysis that AQ5 ligand is more stable than the other li-
gands [27]. This can be attributed to the fact that the effective charge ex-
perienced increases by the electron withdrawing p-substituent –Cl and
–NO2 (AQ4 and AQ5) while it decreases by the electron donating
⁎
N
N
N
N
- C6H5
-N2
N
N
N
OH
OH
OH
m/z = 172
Structure I
m/z = 144
m/z = 249
Structure II
- CHO
- C3H3
N
N
m/z = 115
m/z = 76
Structure IV
Structure III
Scheme 1. Fragmentation patterns of AQ3 ligand.

632
N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
4
Table 3
The thermal analysis data for ligands (AQx).
4000
3000
2000
1000
0
Compound
AQ 1
Temp. range (°C)
50–136
136–300
300–800
N800
Found mass loss (calc.)%
Assignment
1
11.8 (11.11)
44.5 (43.01)
40.3 (41.57)
3.39 (4.30)
Loss of OCH3
Loss of C6H4N2O
Loss of C8H6N
Loss of C atom
Loss of C9H6N3O
Loss of C5H7
Loss of C atom
Loss of C6H5N2O
Loss of C8H6N
Loss of C atom
Loss of C10H6NOCl
Loss of N2
5
9
AQ 2
AQ 3
AQ 4
90–300
300–800
N800
100–300
300–800
N800
110–300
300–430
430–800
N800
63.64 (65.40)
27.50 (25.48)
8.86 (4.56)
48.10 (48.59)
47.40 (46.59)
4.50 (4.82)
66.55 (67.55)
10.96 (9.88)
18.04 (18.34)
4.45 (4.23)
11
3
7
12
13
6
14
8
2
10
15
17
16
Loss of C4H4
Loss of C atom
Loss of C6H4N3O3
Loss of C8H6N
Loss of C atom
0
10
20
30
40
θ (ο)
50
60
70
80
AQ 5
120–270
270–800
N800
56.51 (56.46)
36.79 (39.46)
6.70 (4.08)
2
Fig. 2. X-ray diffraction pattern of AQ3 in the powder form.
⁎
assigned to π–π transitions between orbitals largely localized in
phenyl ring [30].
The energy gap value and the type of optical transition can be ob-
tained from the data of α near the absorption edge using the following
relation [31,32]:
character of –OCH3 and –CH3 (AQ1 and AQ2). This is in accordance with
that expected from Hammett's constant (σR) as shown in Fig. 4.
3.7. Optical properties of 5-(2-(aryldiazenyl)quinolin)-8-ol (AQ x)
thin films
ꢀ
ꢁ
x
αhv ¼ B hv − Eg ꢁ Eph
;
ð12Þ
Valuable information about the energy band structure and the most
probable electronic transition can be extracted from the absorption
spectra of a material in accordance to the Orbital Molecular Theory.
The absorption coefficient (α) is calculated from the measured
absorbance (Abs.) according to the following equation [29]:
where B is a constant, which depends on the electronic transition
probability [33], Eg is the optical energy gap of the material and Eph is
theenergy of phonon accompanyingtheindirect transition(thefactor
allows for the possibility of photon absorption or emission, with
the +ve sign corresponding to absorption and the −ve sign to emis-
sion) and x is the power which characterizes the transition process,
where x = 1/2 and 3/2 for direct allowed and forbidden transitions,
respectively, and x = 2 and 3 for indirect allowed and forbidden transi-
tions, respectively. The dependence of the optical absorption of an inci-
dent phonon energy is examined in terms of Eq. (12) and we found that
the best fit of the experimental data of α near the absorption edge is for
the value of x = 3/2 for all ligands and x = 2 for AQ1 and AQ4. This in-
dicates that the forbidden direct transition is the probable electronic
transition for all ligands, on the other hand, the indirect allowed transi-
tion is also probable for the ligands AQ1 and AQ4. The dependence
of (αhν)1/2 and (αhν)2/3 on incident photon energy is presented in
2:3Abs:
α ¼
;
ð11Þ
L
where L is the thickness of the film.
The absorption coefficient (α) of AQx thin films as a function of inci-
dent photon energy (hν) is shown in Fig. 5. The absorption coefficient
spectrum revealed two peaks indicating transitions of charge carriers
from bonding to antibonding molecular orbitals. The bands appearing
⁎
at 3.0–3.5 eV are assigned to n–π transitions of azodye derivatives
function group N_N [30]. The bands appearing at 5.0–5.2 eV are
Table 2
AQ
1
Crystallographic data of AQ3 ligand.
AQ
2
Peak no.
2θobs. (°)
dobs. (Å)
dcal. (Å)
(h k l)
-3
AQ
3
AQ
4
1
2
3
6.514
7.917
9.788
13.56162
11.16377
9.030481
13.70641
11.21252
8.988331
0 1 1
1 1 1
AQ
2 1 0
5
4
14.586
6.070134
6.070134
1 1 3
2 2 2
0 3 3
4 1 2
3 2 3
5
6
7
8
9
15.772
19.490
20.388
21.288
23.871
5.614431
4.5526
4.354438
4.170547
3.724855
5.606259
4.563389
4.344614
4.170547
3.730266
-4
4 3 2
2 3 4
5 2 2
10
11
12
24.110
25.052
26.359
3.689176
3.553044
3.378509
3.692713
3.562905
3.385934
3 4 3
0 5 3
3 2 5
3 4 4
-5
13
14
15
16
27.205
28.082
29.584
44.022
3.276478
3.175185
3.017235
2.055778
3.275085
3.176495
3.0196
0.0019 0.0020 0.0021 0.0022 0.0023 0.0024
1/T (K-1)
2.055778
7 6 3
5 8 4
17
47.760
1.902915
1.902915
⁎
Fig. 3. The relation between ln K and 1/T for AQx.

N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
633
95
90
85
80
75
70
100
AQ
1
AQ
4
80
AQ
5
AQ
4
60
40
AQ
AQ
1
3
AQ
2
20
1.2
1.6
2.0
2.4
2.8
3.2
hν
,
eV
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
R
Hammett's substituent coefficients (σ )
½
Fig. 6. Plot of (αhν) vs. hν for AQn thin films.
⁎
Fig. 4. The relation between thermal activation energies of decomposition (E ) and
Hammett's substituent coefficients (σR) for AQx.
where εr is the real part of the dielectric constant and εi is the imaginary
part of the dielectric constant. The real dielectric constant (εr) can
be calculated from the parallel mode measured capacitance by the
equation [36,37]:
Figs. 6 and 7, respectively. The intersection of extrapolation of the linear
part to zero absorption with photon energy axes is taken as the funda-
mental energy gap. The determined values of Eg and Eph are listed in
Table 4. The variation of the obtained values of Eg and the type of the
electronic transition may be attributed to either withdrawing or
donating function group [13]. The effect of methoxy group is reduc-
ing the Eg value this is because of the methoxy group is a strong
donating function group, while the effect of halogen group as
withdrawing function group is to raise the Eg. A similar effect of the
substituent OCH3 is reported by Leontie and Danac [34]. The values
of HOMO–LUMO energy gap (Et) are found to exceed the optical
energy gap (Eg). Similar results were obtained for antipyrine Schiff
base derivatives [17] and other π-conjugated organic molecules
such as α-sixithiophene (α-6 T), N,N′-diphenyl-N,N′-bis(L-naphthyl)-l,l′
biphenyl-4,4″ diamine (α-NPD), tris(8-hydroxy-quinoline)aluminum
(Alq3) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) [35].
The value of (Et–Eg) is considered as the binding energy of excites [35]
and found to be in the range 0.3–1.0 eV for AQx.
CpL
εoΑ
εr ¼
;
ð14Þ
where Cp is the capacitance of the samples measured in parallel
mode, εo is the permittivity of free space, A is the cross sectional
area and L is the thickness of the samples. The imaginary part of
the dielectric constant (εi) can be calculated from the calculated
values of εr and the measured values of loss tangent (tan δ) using
the equation [36,37]:
εi ¼ εr tan δ:
ð15Þ
The value of εr and εi are calculated. The temperature dependence
of εr and εi at different frequencies is shown in Figs. 8 and 9, respectively.
In general, both of εr and εi, decrease with increasing frequency and
increase with increasing temperature for all ligands (AQ x). This
behavior is in agreement with the expected behavior of the organic
materials [12,36].
3.8. AC measurements analysis
The dielectric constant (ε) is considered as a complex quantity and
The ac conductivity (σac) for AQx derivatives were calculated from
the calculated values of εi according to the following equation [36]:
given by the equation:
ε ¼ εr − iεi;
ð13Þ
σac ¼ ωε0εi;
ð16Þ
1.5x104
AQ
4
770
700
630
560
490
420
350
280
210
140
70
AQ
3
1.2x104
9.0x103
6.0x103
AQ
1
AQ
2
AQ
2
AQ
1
AQ
3
AQ
4
3.0x103
0.0
1
2
3
4
5
6
0
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
hν
,
eV
hν,
eV
Fig. 5. The absorption coefficient (α) as a function of the incident photon energy hν
for AQx thin films.
Fig. 7. Plot of (αhν)2/3 vs. hν for AQx thin films.

634
N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
Table 4
temperature and frequency ranges considered before and are
Optical energy gaps of the investigated ligands AQx.
shown in Fig. 10. The values of σac increases with increasing frequen-
cies as expected, while the behavior of σac with increasing tempera-
ture depends on the chemical composition of each ligand and
the decomposition stages observed in the TGA measurements.
The conductivity of AQ 1, AQ 2, AQ 4 and AQ 5 ligands, shown in
Fig. 10, increases gradually with increasing temperature until the
decomposition process starts.
Compound
Eignd. (eV)
EPh (eV)
Edgir.(eV)
AQ 1
AQ 2
AQ 3
AQ 4
1.47
–
–
0.16
–
–
1.34
2.21
2.26
1.64
1.69
0.18
The ac electrical conductivity (σac) as a function of temperature is
given by equation [12,36,37]:
where ω is the angular frequency of applied field (ω = 2πf (where
f is the frequency)). The calculated values of σac for the AQx under
consideration as a function of temperature are investigated in the
ꢂ
ꢃ
−ΔE
kBΤ
σac ¼ σ0 exp
;
ð17Þ
90
0.1 kHz
1 kHz
10 kHz
100 kHz
0.1 kHz
1 kHz
10 kHz
100 kHz
720
600
AQ
2
AQ
80
70
60
50
40
30
20
1
480
εr
εr 360
240
120
0
320
360
400
440
AQ
320
360
400
440
AQ
T (K)
T (K)
120
100
80
0.1 kHz
1 kHz
10 kHz
100 kHz
0.1 kHz
1 kHz
10 kHz
100 kHz
3
2000
1500
4
εr
1000
εr
60
500
0
40
320
360
400
440
480
520
360
400
440
480
T (K)
T (K)
3500
0.1 kHz
1 kHz
10 kHz
100 kHz
AQ
5
3000
2500
2000
1500
1000
500
εr
0
320
360
400
440
480
520
T (K)
Fig. 8. Temperature and frequency dependence of the real dielectric constant (εr) for ligands (AQx).

N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
635
2000
1600
1200
800
400
0
0.1 kHz
1 kHz
10 kHz
100 kHz
0.1 kHz
1 kHz
10 kHz
100 kHz
4500
AQ
AQ
1
2
3600
2700
1800
900
0
εi
εi
320
360
400
440
320
360
400
440
T (K)
T (K)
100
0.1 kHz
1 kHz
10 kHz
100 kHz
10800
9000
7200
5400
3600
1800
0
0.1 kHz
1 kHz
10 kHz
100 kHz
AQ
3
AQ
4
80
60
40
20
0
εi
εi
320
360
400
440
480
320
360
400
440
480
520
T (K)
T (K)
10800
0.1 kHz
1 kHz
10 kHz
AQ
5
9000
7200
5400
3600
1800
0
100 kHz
εi
320
360
400
440
480
520
T (K)
Fig. 9. Temperature and frequency dependence of the imaginary dielectric constant (εi) for ligands (AQx).
where σo is the pre-exponential constant, ΔE is the thermal activa-
tion energy and kB is Boltzmann's constant. The relation between
ln σac as a function of (1/T) for AQ x is shown in Fig. 11 in the
temperature and frequency ranges under consideration. The semi-
conductor behavior observed in these figures is characterized by
two thermal activation energies (ΔE1 and ΔE2) depending on the
temperature range. The values of the thermal activation energy,
ΔE1 and ΔE2, for AQ x at different frequencies are determined in
the low and high temperature regions, respectively, and listed in
Table 5. The variation in the values of ΔE1 and ΔE2 is due to the
effect of the substituents [12,36]. The relatively low values of ΔE1
for all ligands at all test frequencies suggested that the operating
conduction mechanism may be tunneling and/or hopping. The
relatively high values of ΔE2 in the higher region of temperature
suggest the conduction mechanism to be thermally activated
process. It is found that AQ5 has the lowest values for ΔE1 and
ΔE2. The lower and upper limits of ΔE1 at test frequency 0.1 kHz
are found to be 0.03, 0.19 eV for AQ5 and AQ1, respectively,
while those of ΔE2 are found to be 0.33, 1.31 eV for AQ5 and
AQ2, respectively.
To investigate the suitable operating conduction mechanism, theo-
retical model has been reported to correlate the conduction mechanism
with the behavior of σac with the frequency [12,36,37]. The frequency
dependence of σac at certain temperature can be investigated according
to the equation [36–38]:
σac ¼ Aꢀωs;
ð18Þ

636
N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
-6
10
-6
x
x
10
0.1 kHz
1 kHz
10 kHz
100 kHz
25
20
15
10
5
0.1 kHz
1 kHz
10 kHz
100 kHz
AQ
AQ
1
2
20
15
10
5
0
0
320
360
400
440
320
360
400
440
T (K)
T (K)
-6
10
-6
x10
x
16
14
12
10
8
0.1 kHz
1 kHz
10 kHz
100 kHz
0.1 kHz
1 kHz
10 kHz
100 kHz
14
12
10
8
AQ
AQ4
3
6
6
4
4
2
2
0
0
360
400
T (K)
440
320
360
400
T (K)
440
480
-6
x
10
36
0.1 kHz
1 kHz
10 kHz
100 kHz
AQ5
30
24
18
12
6
0
320
360
400
440
480
520
T (K)
Fig. 10. The conductivity (σac) as a function of temperature for ligands (AQx) at different test frequencies.
⁎
where A is a constant depending on temperature, ω is the angular
frequency and S is an exponent in which its value and its behavior
with temperature determines the dominant conduction mechanism.
The value of S is the slope of the straight line obtained from the
relation between log σac and log ω. The values of S as a function of
temperature for AQx are shown in Fig. 12. The general behavior of S
with temperature for AQ 1, AQ2 and AQ4 appears to be consistent
with the hopping process of charge carriers between localized sites
and suggested that the correlated barrier hopping (CBH) model is
the best suitable model for conduction mechanism [12,36,37],
while small polaron tunneling (SPT) model is found to be more
suitable with AQ 3 and AQ5. In SPT model the ac conductivity is
given by following equation [39,40]:
2
π
24
4 e2KBT½NðEF Þꢂ ωR4W
σacðSPTÞ
¼
;
ð19Þ
α

N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
637
-12
-11
-12
-13
-14
-15
-16
AQ1
AQ2
-14
-16
-18
0.1 kHz
1 kHz
10 kHz
100 kHz
0.1 kHz
1 kHz
10 kHz
-20
100 kHz
0.0024
0.0026
0.0028
0.0030
0.0032
0.0024
0.0027
0.0030
0.0033
(1/T (K))
(1/T (K))
-11
AQ3
AQ4
-12
-14
-16
-18
-12
-13
-14
-15
-16
-17
0.1 kHz
1 kHz
10 kHz
100 kHz
0.1 kHz
1 kHz
10 kHz
100 kHz
0.0021
0.0024
0.0027
0.0030
0.0022
0.0024
0.0026
0.0028
0.0030
0.0032
(1/T (K))
(1/T (K))
-10
AQ5
-12
-14
-16
0.1 kHz
1 kHz
10 kHz
100 kHz
0.0021
0.0024
0.0027
0.0030
0.0033
(1/T (K))
Fig. 11. ln σac vs. 1/T for ligands (AQx) at different test frequencies.
where α is the spatial extent of polaron, N(EF) the density of states at
the Fermi level and Rw is the tunneling distance. According to this
model, the frequency exponent (S) is evaluated as,
Table 5
Thermal activation energies (ΔE1 and ΔE2) for the ligands (AQx).
4
S ¼ 1−
;
ð20Þ
Compound
ΔE1 (eV)
ΔE2 (eV)
ln½1=ωτoꢂ − WHT
0.1
kHz
1
kHz
10
kHz
100
kHz
0.1
kHz
1
kHz
10
kHz
100
Hz
0.1
kHz
where WH is the barrier height for infinite site separation.
In CBH model the ac conductivity is given by following equa-
tion [39,40]:
AQ 1
AQ 2
AQ 3
AQ 4
AQ 5
0.19
0.05
–
0.15
0.03
0.15
0.15
–
0.12
0.06
0.1
0.26
–
0.04
0.08
0.06
0.22
–
0.03
0.09
0.58
1.31
1.13
0.49
0.33
0.48
0.82
0.75
0.43
0.35
0.38
0.54
0.57
0.33
0.26
0.31
0.27
0.31
0.25
0.2
0.58
1.31
1.13
0.49
0.33
nπ3
24
σacðCBHÞ
¼
N2εεoωR6 ;
ð21Þ
H

638
N.A. El-Ghamaz et al. / Journal of Molecular Liquids 211 (2015) 628–639
1.0
0.8
0.6
0.4
0.2
0.0
states. The maximum barrier height (Wm) as a function of temperature
for AQ1, AQ2 and AQ4 is plotted in Fig. 13. The value Wm decreases with
increasing temperature for AQ1, AQ2 and AQ4.
4. Conclusion
5-(2-Aryldiazenyl)quinolin-8-ol (AQx) ligands are synthesized
by azo coupling reaction. The alternating current conductivity (σac
)
and dielectric properties of AQx are investigated in the frequency
range 102–105 Hz. The real and imaginary parts of the dielectric con-
stant εi and εr increase with increasing the temperature and decrease
with increasing frequency for all AQx. The calculated values of σac in-
crease with increasing frequencies for all AQx. The correlated barrier
hopping (CBH) is the dominant conduction mechanism for ligands
AQ1, AQ2 and AQ4, while for ligands AQ3 and AQ5 the small polaron
tunneling (SPT) is the dominant conduction mechanism. The probable
electronic transitions were found to be direct forbidden transition for
all ligands and indirect allowed transition for the AQ1 and AQ4 ligands.
The optical values of energy gap (Eg) for all derivatives were in the range
1.34–2.26 eV for direct transitions and 1.47–1.69 eV for indirect transi-
tions. It was found that the change of substituents affects the thermal
properties, dielectrical properties, ac conductivity, optical properties
and the values of the thermal activation energy of AQx.
AQ1
AQ2
AQ3
AQ4
AQ5
300
320
340
360
380
400
T (K)
Fig. 12. Plot of the frequency exponent (S) vs. T for ligands (AQx).
where N is the density of a pair of sites. The hopping length, RH,
is expressed as:
ꢄ
ꢅ
ne2
πεεo
1
ωτo
RH
¼
wm − kBT ln
:
ð22Þ
Appendix A. Supplementary data
The frequency exponent (S) in this model is given by the
equation [36]:
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.molliq.2015.07.050.
6kBT
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