Thermal, dielectrical properties and conduction mechanism of Cu(II) complexes of azo rhodanine derivatives

Article abstract of DOI:10.1016/j.materresbull.2015.01.063

A novel series of Cu(II) complexes of azo rhodanine derivatives [CuL_n(OAc)(OH₂)]2H₂O (n = 1, R = OCH₃; n = 2, CH₃; n = 3, H; and n = 4, NO₂) have been synthesized. The alternating current conductivity (σ_ac) and dielectric properties of Cu(II) complexes of azo rhodanine derivatives were investigated in the frequency range 0.1-100 kHz and temperature range 303-600 K. The values of the thermal activation energies of electrical conductivity ΔE₁ and ΔE₂ for all Cu(II) complexes [CuL_n(OAc)(OH₂)]2H₂O were calculated at different frequencies. The conductivity depends on the substituents of the complexes. The correlated barrier hopping (CBH) is the dominant conduction mechanism for complexes [CuL_n(OAc)(OH₂)]2H₂O (n = 1, 2 and 4) while for complex [CuL₃(OAc)(OH₂)]2H₂O the small polarons tunneling (SPT) is the dominant conduction mechanism.

Full text of DOI:10.1016/j.materresbull.2015.01.063

Materials Research Bulletin 65 (2015) 293–301
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Thermal, dielectrical properties and conduction mechanism of Cu(II)
complexes of azo rhodanine derivatives
b
b
b,1
N.A. El-Ghamaz a, A.Z. El-Sonbati b, , M.A. Diab , A.A. El-Bindary , Sh.M. Morgan
*
a
Physics Department, Faculty of Science, Damietta University, Damietta, Egypt
Chemistry Department, Faculty of Science, Damietta University, Damietta, Egypt
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 4 February 2014
Received in revised form 11 November 2014
Accepted 30 January 2015
Available online 31 January 2015
A novel series of Cu(II) complexes of azo rhodanine derivatives [CuLn(OAc)(OH2)]2H2O (n = 1, R = OCH3;
n = 2, CH3; n = 3, H; and n = 4, NO2) have been synthesized. The alternating current conductivity (sac) and
dielectric properties of Cu(II) complexes of azo rhodanine derivatives were investigated in the frequency
range 0.1–100 kHz and temperature range 303–600 K. The values of the thermal activation energies of
electrical conductivity
DE1 and DE2 for all Cu(II) complexes [CuLn(OAc)(OH2)]2H2O were calculated at
different frequencies. The conductivity depends on the substituents of the complexes. The correlated
barrier hopping (CBH) is the dominant conduction mechanism for complexes [CuLn(OAc)(OH2)]2H2O
(n = 1, 2 and 4) while for complex [CuL3(OAc)(OH2)]2H2O the small polarons tunneling (SPT) is the
dominant conduction mechanism.
Keywords:
A. Inorganic compounds
B. Chemical synthesis
C. Thermogravimetric analysis (TGA)
D. Dielectric properties
D. Electrical properties
ã 2015 Elsevier Ltd. All rights reserved.
1. Introduction
and exocyclic electron-donating atoms, are interesting as ligands
because they can exist either as neutral molecules or as mono-
Azo rhodanine and its derivatives have wide industrial
applications as intermediates in the synthesis of dyes, antioxidants
and brightening additives in silver electroplating, as well as
pharmacological [1], and biological activities [2–6]. Another
interesting aspect of the chemistry of these compounds is their
donating power to metal ions, which makes them strong ligands in
coordination compounds [7–10]. These compounds are also used
in analytical chemistry as highly sensitive reagents for heavy
metals [11,12].
charged anions and, in solution, are also involved in tautomeric
equilibria [18]. Azo rhodanine appear to be very important as far as
biocidal potency is concerned, probably via incorporation of the
N—C S moiety, the importance of which has been stressed in
¼
many fungicides and bactericides [19,20].
The alternating current conductivity
(sac) and dielectric
properties of ligands (5-(40-derivatives phenylazo)-2-thioxothia-
zolidin-4-one) were investigated [21]. The thermal activation
energies of electrical conductivity, DE1 and DE2, increase according
Azo rhodanine and its derivatives contain heteroatoms which
are considered to be adsorption centers (nitrogen, sulphur, oxygen)
and could be used as anti-corrosion agents for protection of metals.
Azo rhodanine and its some derivatives were found to be good
corrosion inhibitors for stainless steel [13], mild steel, [14] and
carbon steel [15]. Rhodanine derivatives have been proven to be
attractive compounds due to their outstanding biological activities
and have undergone rapid development as anticonvulsant,
antibacterial, antiviral and antidiabetic agents [16,17]. Heterocyclic
compounds such as rhodanine and 2-thiohydantoin, which in spite
of having only five-membered rings possess several different endo-
to the following order p-(NO2 > H > CH3 > OCH3). This is in
accordance with that expected from Hammett’s substituent
coefficients (s
R). The conductivities are found to be dependent
on the structure of the ligands. The correlated barrier hopping
(CBH) is the dominant conduction mechanism for the ligands.
Moreover, only a few reports are available on Pd(II) and VO(IV)
complexes with 5-(40-alkyl phenylazo)-2-thioxo-4-thiazolidinone
[7,8]. These papers describe the synthesis, characterization and
their electronic absorption spectra of mixed ligand (azo rhodanine
and methylacetoacetate) and metal complexes. In addition to the
novel complexes of Ru(III) with 5-(40-alkyl phenylazo)-2-thioxo-4-
thiazolidinone have been prepared and characterized [10] by
different techniques.
The objectives of the present work are the synthesis of Cu(II)
complexes of 5-(40-derivatives phenylazo)-2-thioxo-4-thiazolidi-
none (HLn). The thermogravimetric analysis (TGA) studies, the ac
* Corresponding author Tel.: +20 1060081581; fax: +20 572403868.
E-mail address: elsonbatisch@yahoo.com (A.Z. El-Sonbati).
1
Abstracted from her Ph.D. thesis.
http://dx.doi.org/10.1016/j.materresbull.2015.01.063
0025-5408/ã 2015 Elsevier Ltd. All rights reserved.

294
N.A. El-Ghamaz et al. / Materials Research Bulletin 65 (2015) 293–301
S
R
N a N O 2
H C l
S
0 -5 o C
C
S
N
+
R
N H 2
N
S
alk alin e so lu tion
N H
C
O
N H
O
K e to fo rm
R
R
S
S
N
N
N
N
H
S
S
C
C
N H
N H
H O
O
H y d ra zo n fo r m
E n o l fo r m
n=1, R = OCH3; n=2, R = CH3; n=3, R = H; and n=4, R = NO2
Scheme 1. The formation mechanism of azo rhodanine derivatives.
conductivity, dielectric properties and conduction mechanism
of Cu(II) complexes of azo rhodanine derivatives, as well as the
effect of substituents of complexes on these properties were
discussed.
2.2. Preparation of complexes
A
hot ethanolic solution containing the 5-(40-derivatives
phenylazo)-2-thioxothiazolidin-4-one (HLn) was mixed with a
hot ethanolic solution of Cu(OAc)2 H2O (1 mmol) as shown in
ꢂ
2. Experimental
Scheme 2. The mixture was then refluxed on a water bath for ꢁ10 h
and allowed to cool whereby the solid complexes were separated,
which were filtered off, washed several times with ethanol, dried
and kept in a desiccator over dried CaCl2.
All the chemicals used were of British Drug House (BDH)
quality.
2.1. Preparation of 5-(40-derivatives phenylazo)-2-thioxothiazolidin-
4-one (HLn)
2.3. Measurements
Elemental microanalyses of the compounds for C, H and N were
determined on automatic analyzer CHNS Vario ELIII, Germany. The
infrared spectra were recorded as KBr discs using a PerkinElmer
1340 spectrophotometer. The analyses were repeated twice to
check the accuracy of the analyzed data. The metal content in the
complexes was estimated by standard methods [8]. The magnetic
moment of the prepared solid complexes was determined at room
temperature using the Gouy’s method. Mercury(II) (tetrathiocya-
nato) cobalt(II), [Hg{Co(SCN)4}], was used for the calibration of the
Gouy’s tubes. Thermogravimetric analysis (TGA) measurements
were investigated using Simultaneous Thermal Analyzer (STA)
6000 with scan rate 15 ꢀC/min under dynamic nitrogen
atmosphere in the temperature range 40–800 ꢀC. Ac conductivity
measurements are performed on the samples in the form of discs
In a typical preparation, 25 ml of distilled water containing
0.01 mol hydrochloric acid were added to aniline (0.01 mol) or
p-derivatives. To the resulting mixture stirred and cooled to 0 ꢀC, a
solution of 0.01 mol sodium nitrite in 20 ml of water was added
dropwise. The formed diazonium chloride was consecutively
coupled with an alkaline solution of 0.01 mol 2-thioxo-4-thiazo-
lidinone, in 10 ml of pyridine as shown in Scheme 1. The colored
precipitate, which formed immediately, was filtered through
sintered glass crucible, washed several times with water and
ether. The crude products was purified by recrystallization from
hot ethanol, yield ꢁ65% then dried in a vacuum desiccator over
P2O5 [21].
R
R
S
N
S
N
S
N
C
N
C u (O A c)2 .H 2
E th an o l
O
S
C
2 H 2 O
H 2
O
N H
C u
O
N H
H O
O A c
E n o l fo r m
Scheme 2. The structure of Cu(II) complexes.

N.A. El-Ghamaz et al. / Materials Research Bulletin 65 (2015) 293–301
295
Table 1
Analytical and magnetic moments of Cu(II) complexes.
Complex
meff. B.M.
Calc. (exp)%
C
H
N
S
M
[Cu(L1)(OAc)(OH2)]2H2O (1)
[Cu(L2)(OAc)(OH2)]2H2O (2)
[Cu(L3)(OAc)(OH2)]2H2O (3)
[Cu(L4)(OAc)(OH2)]2H2O (4)
1.83
1.84
1.85
1.87
32.54 (32.47)
28.51 (28.40)
33.76 (33.60)
32.00 (31.86)
3.84 (3.72)
3.02 (2.89)
3.99 (3.82)
3.64 (3.44)
9.49 (9.13)
9.07 (8.78)
9.85 (9.46)
10.18 (9.87)
14.46 (14.07)
13.82 (13.65)
15.01 (14.86)
15.51 (15.27)
14.36 (14.06)
13.72 (13.47)
14.90 (14.72)
15.40 (15.17)
of thickness 0.4–0.8 mm and compressed at a pressure of 12 t cmꢃ2
.
In the IR spectra of all Cu(II) complexes (1–4) a number of
The surface of each sample was covered by a layer of silver, then
was held between two copper electrodes and inserted vertically
changes are observed:
The appearance of a new bands around ꢁ 3380 cmꢃ1 and two
1)
into a cylindrical electric furnace. The ac conductivity (sac
)
measurements of samples are measured as function
a
sharp bands at ꢁ715 and 420 cmꢃ1, the latter two can be
assigned to the wagging and rocking modes of vibration of the
water molecule, respectively [23] in the prepared Cu(II)
complexes (1–4) may be taken as a strong evidence for the
presence of coordinated water. Such region, however, is not
initially present in the free ligands. This is confirmed by the
elemental analysis of these complexes (Table 1).
of temperature range from 303 to 600 K and frequency range
0.1–100 kHz using Stanford research systems Model SR 720 LCR
METER. Ac conductivity was calculated from the measured values
of capacitance (Cp) and loss tangent (tand) in parallel mode. The
temperature is measured by NiCr–NiAl thermocouple. The range of
temperature for electrical measurements is chosen according to
TGA measurements.
2)
The N N stretching frequency of the azo groups is shifted to
¼
lower frequency by ꢁ15–25 cmꢃ1 due to the involvement of one
of the azo nitrogen atoms in coordination with metal ion [7,24].
This lowering of frequency can be explained by the transfer of
electrons from nitrogen atom to the Cu(II) ion due to
coordination.
3. Results and discussion
3.1. Structure of the metal complexes
3)
4)
The complexes were found to be insoluble in common organic
solvents but soluble in coordinated solvents. The analytical data
are given in Table 1. Magnetic measurements were conducted in
order to obtain information about the geometry of the complexes.
The magnetic susceptibility values (1.83–1.87 B.M.) suggesting the
square planar geometry for the Cu(II) complexes of azo rhodanine
derivatives (Table 1) [22].
To confirm the presence of coordinating H2O molecules in the
complex, we carried out thermogravimetric analysis of all the
complexes. This study shows loss of weight corresponding to
one water molecule in the temperature range above 190 ꢀC,
indicating that the water molecule in these complexes is
coordinated to the metal ion.
The participation of the OH group in Cu(II) complexes is
confirmed by the appearance of new bands in Cu(II) complexes
at 555–525 cmꢃ1 for complexes related to the Cu—O vibration
[24]. The acetate complexes show two new bands at 1600 and
1390 cmꢃ1 attributed to yas and ys of the acetate group. The
difference between the two bands indicates the monodentate
nature of the acetate group [22,25].
3.2. Infrared spectra of complexes and nature of coordination
The broad/strong absorption bands in the ligands (HLn) located
at ꢁ3450–3434 and at ꢁ1710–1695 cmꢃ1 region assigned to the
y
(NH) stretching vibrations mode and carbonyl stretching
vibrations. The three bands in the 1600–1500 cmꢃ1 region are
characteristic for most six-membered aromatic ring system.
On the basis of all these data, the molecular structure of the Cu
The frequencies for the
N
¼N
stretching lie in the region
(II) complexes could be suggested based on: (i) the presence
of anion, (ii) the disappearance of C O, (iii) the coordination of
azo-group and (iv) the presence of water.
1440–1435 cmꢃ1. The region between 1500 and 1900 cmꢃ1 is
due C—N stretching, N—H in plane or out of plane bending and out-
of-plane C—H bending vibrations [7,8,10].
¼
Table 2
The thermal analysis data of the Cu(II) complexes.
Complexa
Temp. range (ꢀC) Found mass loss (calc.) % Assignment
(1)
45–331
331–506
506–709
45–316
316–533
533–670
45–273
273–514
514–640
45–288
288–400
400–592
592–670
23.91 (25.52)
24.43 (26.88)
14.38 (16.27)
23.22 (26.49)
20.47 (21.33)
15.45 (16.87)
26.52 (27.39)
18.06 (17.94)
22.04 (21.57)
20.87 (24.7)
10.46 (10.05)
13.72 (15.74)
22.44 (22.51)
Loss of water molecules and one coordinated acetate group
Further decomposition of a part of the ligand (C3H7N2SO)
Decomposition of a part of the ligand (C2H2NS) leaving CuO residue with contaminated carbon atoms
Loss of water molecules and one coordinated acetate group
Decomposition of a part of the ligand (C2H7N2S)
Decomposition of a part of the ligand (C2H2NS) leaving CuO residue with contaminated carbon atoms
Loss of water molecules and one coordinated acetate group
Decomposition of a part of the ligand (CH2N2S)
Decomposition of a part of the ligand (C3H7NS) leaving CuO residue with contaminated carbon atoms
Loss of water molecules and one coordinated acetate group
Loss of NO2 group
(2)
(3)
(4)
Decomposition of a part of the ligand (C2H2NS)
Decomposition of a part of the ligand (C3H7N2S) leaving CuO residue with contaminated carbon atoms
a
Numbers as given in Table 1.

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N.A. El-Ghamaz et al. / Materials Research Bulletin 65 (2015) 293–301
1.0
(1)
100
90
80
70
60
50
40
30
(2)
46
45
44
43
42
41
40
39
38
(4)
0.6 0.8
0.0025
0.5
(3)
(4)
0.0
(1)
-0.5
-1.0
(3)
(2)
-0.4 -0.2 0.0
0.2R0.4
σ
(1)
-1.5
(2)
(3)
-2.0
(4)
-2.5
0-.01023
0.0020
0.0021
0.0022
0.0024
100
200
300
400
500
600
700
800
1/T (K )
Temperature (°C)
Fig. 2. The relation between ln K* versus 1/T for Cu(II) complexes (1–4) (inset: the
Fig. 1. TGA curves for Cu(II) complexes (1–4).
relation between Ea and Hammett’s substituent coefficients (s
R)).
(1)
(2)
313 K
343 K
373 K
403 K
313 K
343 K
373 K
403 K
80
80
70
60
50
72
εr
64
56
48
εr
6
8
10
12
14
6
8
10
12
14
lnω
lnω
600
500
400
300
200
100
(3)
(4)
313 K
343 K
373 K
403 K
313 K
343 K
373 K
403 K
2000
1600
1200
800
400
0
εr
εr
6
8
10
12
14
6
8
10
12
14
lnω
lnω
Fig. 3. Variation of real dielectric constant, er, with frequency for Cu(II) complexes (1–4) at different temperatures.

N.A. El-Ghamaz et al. / Materials Research Bulletin 65 (2015) 293–301
297
ω
ω
ω
ω
Fig. 4. Variation of imaginary dielectric constant, ei, with frequency for Cu(II) complexes (1–4) at different temperatures.
According to the structure shown in Scheme 1 the ligands (HLn)
takes its usual anionic form (Ln) to chelate Cu(II) through N- of azo
group and oxygen of hydroxy group (Scheme 2).
the gas constant (=8.314 JKꢃ1 molꢃ1
)
and
T
is the absolute
temperature. The thermal activation energies of decomposition
(Ea) of Cu(II) complexes (1–4) are calculated from the slope of the
straight line obtained from the plot ln K* versus 1/T as shown in
Fig. 2, and the values found to be 40.98, 39.28, 38.88 and 45.13 kJ/
mol for complexes (1–4), respectively. The effect of the sub-
stituents of complexes on the thermal activation energies of
decomposition (Ea) can be confirmed in terms of Hammett’s
3.3. Thermogravimetric analysis of the complexes
The TGA of the isolated complexes was taken as a proof for the
existence of water molecules as well as the anions in the
coordination sphere. The thermal analyses data for the Cu(II)
complexes (1–4) are summarized in Table 2. In the temperature
range ꢁ45–273 ꢀC, the loss of water molecules as well as the loss of
one coordinated acetate group occurs as shown in Fig. 1. According
to the literature, the azo bonds in the azo metal complexes
breakdown when the temperature is higher than 260 ꢀC [22,
26–28]. The weight loss stages (ꢁ273–500 ꢀC) are due to the
decomposition of a part of the ligand. The final weight losses
(ꢁ500–640 ꢀC) are largely attributed to complete decomposition of
other part of the ligand molecule. The remaining final product is
CuO with contaminated carbon atoms.
substituent coefficients (s
R) are shown in the inset of Fig. 2. The
effect of methoxy and methyl groups is to decrease the Ea, while the
effect of nitro group is to increase Ea. The complex (4) is the highest
value of Ea. This can be attributed to the fact that the effective
charge experienced by the d-electrons increases due to the
electron withdrawing p-substituent NO2 while it decreases by
the electron donating character of OCH3 and CH3. This indicates
that the complex (4) is more thermally stable than the other
complexes. This can be attributed to the strong withdrawing
nature of the NO2 as a function group [21,29]. It is important to note
that the existence of a methyl and/or methoxy group enhances the
electron density on the coordination sites.
The rate constant of the thermal degradation is plotted in Fig. 2
according to the Arrhenius relationship [21]:
Ea
RT
ln Kꢄ ¼ ln Ao ꢃ
(1)
3.4. Analysis of ac measurements
where K* is the rate constant of the thermal degradation, Ao is a
constant, Ea the thermal activation energy of decomposition, R is
The ac conductivity of a material can be determined in terms of
its dielectric constant. The dielectric constant is an important

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N.A. El-Ghamaz et al. / Materials Research Bulletin 65 (2015) 293–301
Fig. 5. The relation of conductivity (sac) as a function of temperature for Cu(II) complexes (1–4) at different frequencies.
parameter which gives valuable information about the electric
properties of the material which is given by [21,30,31]:
decrease with increasing frequency and increase with increasing
temperature.
The ac conductivity, sac, for complexes can be calculated from
Eq. (5) [21]:
eꢄ
¼
er ꢃ iei
where er is the real dielectric constant and ei is the imaginary
dielectric constant. The real dielectric constant, can be
;
(2)
s
¼
veoei
;
(5)
is the angular frequency. The calculated values of sac of Cu
(II) complexes (1–4) under consideration as function of
e
r,
calculated from the measured capacitance in parallel mode by
Eq. (3) [21,30,31]:
where v
a
temperature are investigated in the temperature range 303–
600 K and in frequency range 0.1–100 kHz and are shown in Fig. 5.
The calculated values of sac are found to increase with increasing
temperature and frequency. The values of conductivity at temper-
atures higher than ꢁ470 K show a remarkable increase. This
behavior can be attributed to the decomposition process.
The ac electrical conductivity, sac, for Cu(II) complexes (1–4) as
a function of temperature which is given by [21,32]:
Cpd
er
¼
;
(3)
e
oA
where Cp is the capacitance of the samples measured in parallel
mode, eo is the permittivity of free space, A is the cross sectional
area and d is the thickness of the samples. The imaginary part of the
dielectric constant,
[21,30,31]:
e
i, can be calculated using the equation below
ꢀ
ꢁ
ꢃ
DE
kBT
ei
¼
ertan
d
(4)
s
¼
soexp
;
(6)
where so is the pre-exponential constant,
D
E is the thermal
where tan
d
is the measured values of loss tangent. The value of er
activation energy of electrical conductivity and kB is Boltzmann’s
constant. The relation between ln sac as a function of (1/T) for
complexes is shown in Fig. 6 in the temperature range 303–470 K
and frequency ranges under consideration. The temperature
interval is chosen according to the TGA analysis. The data show a
thermally activated behavior and are characterized by two thermal
and ei are calculated in the frequency range 0.1–100 kHz and
temperature range 303–600 K. Both of er and ei in general, are
found to decrease with increasing frequency and increase with
increasing temperature for all Cu(II) complexes (1–4) (The
calculated data of er and ei, not presented her). Figs. 3 and 4 show
the frequency dependence of real and imaginary dielectric
constants at different temperatures. It is evident that er and ei

N.A. El-Ghamaz et al. / Materials Research Bulletin 65 (2015) 293–301
299
Fig. 6. Dependence of ln sac versus 1/T for Cu(II) complexes (1–4) at different frequencies.
coefficients (
increase with increasing
s
R) are shown in Fig. 7. It is clear that the values of
R. This can be attributed to the fact that
DE1
activation energies of electrical conductivity
regions I and II, respectively, depending on the temperature range.
DE1 and DE2 in the
s
the effective charge experienced by the d-electrons increases due
to the electron withdrawing p-substituent NO2 while it decreases
by the electron donating character of OCH3. This is in accordance
The values of the thermal activation energies of electrical
conductivity,
frequencies are determined and listed in Table 3. The relative
low values of E1 ( E1 = 0.032–0.344 eV) in the lower temperature
region proposes hopping and/or tunneling conduction mechanism.
The variations in both E1 and E2 are attributed to the effect of
DE1 and DE2, for all complexes at different
with that expected from Hammett’s substituent coefficients (
shown in Fig. 7. On the other hand the frequency dependence of the
E1 and E2 for Cu(II) complexes (1–4) show that both of E1 and
E2 tends to decrease with the increase of frequency. Thus, the
s
R) as
D
D
D
D
D
D
D
D
the substituents [21]. This behavior has been observed from the ac
and dc electrical conductivity of some rhodanine azodye
compounds and their complexes [21,33]. The effect of the
substituents on the thermal activation energies of electrical
conductivity can be confirmed in terms of Hammett’s substituent
increase of the applied frequency enhances the electronic jumps
between the localized states [34,35].
To operating conduction mechanism for Cu(II) complexes (1–4)
can be investigated by studying the behavior of sac with frequency
Table 3
The values of the thermal activation energies of electrical conductivity for Cu(II) complexesa at different frequencies.
F (kHz)
(1)
(2)
(3)
(4)
DE1 (eV)
DE2 (eV)
DE1 (eV)
D
E2 (eV)
D
E1 (eV)
D
E2 (eV)
DE1 (eV)
DE2 (eV)
0.1
1
10
0.107
0.037
0.043
0.032
0.488
0.362
0.262
0.194
0.117
0.109
0.074
0.067
0.344
0.314
0.282
0.188
0.092
0.082
0.055
0.042
0.211
0.174
0.176
0.169
0.164
0.097
0.086
0.057
0.240
0.234
0.225
0.186
100
a
Numbers as given in Table 1.

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0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
(1)
(2)
(4)
(4)
0.1 kHz
1 kHz
10 kHz
100 kHz
0.8
0.6
0.4
0.2
(1)
(3)
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
300
330
360
390
420
450
R
T (K)
Hammett's substituent coefficients (σ )
E1 (eV) and Hammett’s substituent coefficients (sR
)
Fig. 9. The maximum barrier height, Wm, as a function of temperature for Cu(II)
complexes (1, 2 and 4) according to CBH conduction model.
Fig. 7. The relation between
for Cu(II) complexes (1,3 and 4).
D
4
S ¼ 1 ꢃ
;
(8)
according to Eq. (7) [21,36]:
ln½1=vtoꢅ ꢃ WH=KBT
where WH is the barrier height for infinite site separation, to the
relaxation time and the angular frequency. According to this
model, the ac conductivity is given by equation [38]:
s
¼ Aꢄvs
;
(7)
where A* is a constant depending on temperature and S is the
frequency exponent in which its behavior with temperature
determines the operating conduction mechanism. In order to
determine the conduction mechanism for a material, theoretical
model has been reported to correlate the conduction mechanism
with the behavior of the frequency exponent S [21,37]. The value of
S is the slope of the straight line obtained from the relation
v
p
24
4e2KBT½NðEFÞꢅ2vRw04
sacðSPTÞ
¼
;
(9)
a
where e is the electronic charge,
a
is the spatial extent of polaron, N
(EF) the density of states at the Fermi level and R0w is the tunneling
distance.
between log sac and log v. The values of S as a function of
In CBH model, S is given by Eq. (10) [21,32]:
temperature for Cu(II) complexes (1–4) are shown in Fig. 8. The
behavior of S with temperature for complexes (1), (2) and (4)
appear to be consistent with the hopping process of charge carriers
between localized sites and suggests that the correlated barrier
hopping (CBH) model is the best suitable model for conduction
mechanism [21,30,32]. The value of S for complex (3) was found to
increase with increasing temperature. This behavior can be
explained in terms of small polaron tunneling (SPT) model for
ac conduction [38]. The frequency exponent S based on this model
is evaluated as,
6kBT
wm ꢃ kBTln
Â
À
ÁÃ
S ¼ 1 ꢃ
;
(10)
1
=
vto
where Wm is the maximum barrier height (the energy required to
move the electron from a site to infinity).
The dependence of sac(CBH) on temperature and frequency is
given by Eq. (11) [37,39]:
p3
24
sacðCBHÞ
¼
½NðEFÞꢅ2eeovR6w;
(11)
where Rw is the distance between hopping states.
First approximation to Eq. (10) gives the expression:
6kBT
1 ꢃ S
wm
¼
(12)
(1)
(2)
0.8
The maximum barrier height, Wm, as a function of temperature for
complexes (1), (2) and (4) is presented in Fig. 9. The value of
maximum barrier height, Wm, for complex (1) decreases
with increasing the temperature and have values in the range
0.35–0.80 eV. The values of Wm for complexes (2) and (4) are
slightly increased with increasing temperature. This behavior may
be attributed to probable increase of the hopping distance, Rw, of
charge carrier from one site to another with heating.
(3)
(4)
0.6
0.4
0.2
0.0
4. Conclusions
In this paper, synthesis and characterization of a series of Cu(II)
complexes of azo rhodanine derivatives. The complexes character-
ized by elemental analysis, IR spectra, magnetic measurements
and thermogravimetric analysis (TGA). The complexes behave as
monobasic bidentate ligands when react with Cu(II) salt and
undergo coordination through azodye nitrogen, enolic oxygen
320
360
400
440
480
T (K)
Fig. 8. Plot of the frequency exponent, S, versus temperature, T, for Cu(II) complexes
(1–4).

N.A. El-Ghamaz et al. / Materials Research Bulletin 65 (2015) 293–301
301
atom. It is found that the change of substituent affects the thermal
properties of the complexes. The values of the thermal activation
energies of decomposition (Ea) of complexes (1–4) are found to be
40.98, 39.28, 38.88 and 45.13 kJ/mol, respectively. The values of the
thermal activation energies of electrical conductivity for Cu(II)
complexes (1–4) under investigation were calculated at different
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DE1
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