Temperature activated ionic conductivity in gallium and indium phthalocyanines

Article abstract of DOI:10.1016/j.poly.2010.12.047

The effects of introducing gallium and indium metals into phthalocyanine molecules were investigated via temperature and frequency dependent dielectric spectroscopy. The dielectric properties of Ga(III) and In(III) phthalocyanine pellets were measured at frequencies from 1 kHz to 1 MHz in the temperature range 300-530 K. The temperature dependence of the real part of the dielectric constant suggested that these compounds exhibit semiconductor behavior. The activation energy values were calculated from the Arrhenius plots at different frequencies. A distinct transition in these plots indicated the activation of ionic conductivity at higher temperatures.

Full text of DOI:10.1016/j.poly.2010.12.047

Polyhedron 30 (2011) 1023–1026
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Temperature activated ionic conductivity in gallium and indium phthalocyanines
a,
⇑
a
b
c
d
Sait Eren San , Mustafa Okutan , Tebello Nyokong , Mahmut Durmu ßs , Birol Ozturk
a
Organic Electronics Group, Department of Physics, Gebze Institute of Technology, Gebze 41400, Kocaeli, Turkey
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
Department of Chemistry, Gebze Institute of Technology, Gebze 41400, Turkey
Department of Physics, Syracuse University, Syracuse, NY 13244, USA
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
The effects of introducing gallium and indium metals into phthalocyanine molecules were investigated
via temperature and frequency dependent dielectric spectroscopy. The dielectric properties of Ga(III)
and In(III) phthalocyanine pellets were measured at frequencies from 1 kHz to 1 MHz in the temperature
range 300–530 K. The temperature dependence of the real part of the dielectric constant suggested that
these compounds exhibit semiconductor behavior. The activation energy values were calculated from the
Arrhenius plots at different frequencies. A distinct transition in these plots indicated the activation of
ionic conductivity at higher temperatures.
Received 18 November 2010
Accepted 31 December 2010
Available online 11 January 2011
Keywords:
Dielectric spectroscopy
Activation energy
Phthalocyanines
Semiconductors
Ó 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
have been reported to have good photosensitizing and optical lim-
iting properties [1,7–11].
Metallophthalocyanines (MPcs), a family of aromatic macrocy-
cles based on an extensive delocalized 18- electron system, are
In the scope of this work, chlorogallium (ClGaPc) and chloroindi-
um (ClInPc) phthalocyanine samples were examined with dielectric
spectroscopy (DS). This method is shown to be a reliable tool for
investigating molecular scale events and for the optimization of tai-
lored materials [12–14]. The temperature dependence of the real
part of their dielectric constants and dielectric loss were measured
and analyzed. Activation energies of ClGaPc and ClInPc samples
were also calculated at different frequencies. The conductivities
and the activation energies of the samples increased at elevated
temperatures, which were attributed to the activation of the ionic
conductivity with increasing temperature.
p
known not only as classical dyes in practical use but also as mod-
ern functional materials in scientific research. There is a growing
interest in the use of phthalocyanines (Pcs) in a variety of applica-
tions, including non-linear optics [1], semiconductor devices [2],
Langmuir–Blodgett films [3], electrochromic display devices [4], li-
quid crystals [5] and as photosensitizers in photodynamic therapy
(
PDT) [6]. For non-linear optical applications, MPcs have advanta-
ges over the currently used inorganic compounds due to their
small dielectric constants [7], fast response times, ease of process-
ability into optical components and their lower cost [1,7]. The
structures of MPcs can be modulated in many ways, by changing
the peripheral and non-peripheral substituents on the ring in addi-
tion to changing the central metal and the axial ligands.
Heavy metals, especially diamagnetic metals, play a major role
in photosensitizing and optical limiting mechanisms because they
enhance intersystem crossing through spin orbit coupling. This
enhancement is desirable as it improves the probability of forming
a large population in the triplet state. Axial ligands in MPcs play a
key role in preventing or minimizing intermolecular interactions,
which causes aggregation in solution. Aggregation can result in
the fast decay of excited states. Indium and gallium are useful cen-
tral metals in MPcs complexes since they are diamagnetic and are
able to host axial ligands. Gallium and indium phthalocyanines
2
. Experimental
2
2
.1. Sample preparation
.1.1. ClGaPc
This compound was synthesized and characterized according to
the method reported elsewhere [15]. Briefly, a mixture of phthalo-
nitrile (5 g, 0.04 mol), anhydrous gallium trichloride (5.5 g,
0
2
.03 mol), and 20 mL of quinoline (double distilled over CaH ,
deoxygenated) was refluxed for 1 h (particular attention was paid
to the exclusion of water during this step). After cooling the mix-
ture to approximately 273 K, the reaction mixture was filtered.
The mixture was washed with toluene and methanol and dried
at 383 K. The final solid compound had a purple color. Yield:
⇑
Corresponding author. Tel.: +90 262 605 13 13; fax: +90 262 653 84 97.
E-mail address: erens@gyte.edu.tr (S.E. San).
3.4 g (55%). Anal. Calc. for C32
18.14. Found: C, 62.75; H, 2.34; N, 18.89%.
16 8
H N GaC1: C, 62.22; H, 2.61; N,
0
277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.12.047

1
024
S.E. San et al. / Polyhedron 30 (2011) 1023–1026
Cl
M
dence of the real and imaginary parts of the dielectric constant were
recorded by the automated NTCS setup, shown in Fig. 2.
N
N
N
3
. Results and discussion
N
N
N
⁄
0
00
0
N
The complex dielectric expression,
e
=
e
ꢁ i
e , where e is the
N
00
real part, and
e
is the imaginary part of dielectric constant, was
utilized in the analysis of the DS results. The real part of the dielec-
0
tric constant was calculated from the equation
e
= C
p
d/(
e
0
A), where
M= Ga
ClGaPc
ClInPc
C
p
is the parallel plate capacitance, d is the inter electrode distance,
is the permittivity of free space and A is the area of the sample.
The dielectric loss (tand), also a critical parameter in DS, was
In
e
0
Fig. 1. Chemical structures of gallium(III) and indium(III) phthalocyanines.
0
0
derived from the ratio of the imaginary (
of the dielectric constant by using tan(d) =
and is the phase angle. The dielectric loss is also known as the
dissipation factor and is typically considered as a characteristic en-
e
) and the real (
e
) parts
0
0
0
e
/
e
, where d = 90 ꢁ
u
u
2
.1.2. ClInPc
Synthesis and purification was as outlined for ClGaPc, except
0
0
ergy loss quantity. The AC conductivity is given by
r = ue , where
that anhydrous indium trichloride was employed instead of anhy-
drous gallium trichloride. The employed reagents were: phthalo-
nitrile (5 g, 0.04 mol), indium(III) trichloride (6.6 g, 0.03 mol) in
quinoline (20 ml). A purple colored compound was obtained. Yield:
x
= 2
p
f is the angular frequency. The results of the temperature
measurements are shown in Fig. 3. Both samples
exhibited similar trends as the temperature and frequency were in-
0
dependent
e
0
creased;
e
had a constant value of ꢀ6 at low temperatures and
4
.1 g (62%). Anal. Calc. for C32
16 8
H N InCl: C, 57.99; H, 2.43; N, 16.91.
started to increase as the temperature was further raised at fre-
Found: C, 58.21; H, 2.68; N, 17.23%.
The chemical structures of the gallium(III) (ClGaPc) and indiu-
m(III) (ClInPc) phthalocyanines are shown in Fig. 1.
0
quencies of 1 kHz–1 MHz. The increase rate of
frequency was increased; it varied between approximately 9–59
for ClGaPc and 7.5–28.5 for ClInPc at T = 525 K for the reported fre-
e
declined as the
0
quencies. The onset temperature, where
e
started to increase, had
2
.2. Electrical measurements
varying values at different frequencies. As is shown in Fig. 3a, the
onset temperature values were approximately 370, 410, 450 and
490 K at 1 kHz, 11.6 kHz, 132 kHz and 1 MHz, respectively for the
ClGaPc sample. For the ClInPc sample (Fig. 2b), the onset tempera-
ture values were approximately 350, 390, 430 and 480 K at 1 kHz,
11.6 kHz, 132 kHz and 1 MHz, respectively. Pcs are known as func-
tional dielectric materials, and these results suggest that they have
After the synthesis and purification processes, the ClGaPc and
ClInPc samples were characterized with dielectric spectroscopy to
determine their dielectric properties and activation energies. Sam-
ples were pelletized with a hydraulic press at a pressure of 10 tons.
The resulting pellets had a 1 mm thickness and 13 mm diameter. A
HP 4194A impedance analyzer was employed in the DS measure-
ments. The temperature dependent dielectric response was mea-
sured at four different frequencies in the 1 kHz–1 MHz range and
the rms amplitude of the instrument was set to ꢀ495 mV. The tem-
perature was monitored by a PT 100 resistor that was in direct ther-
mal contact with the sample. The NOVOTHERM Temperature
Control System (NTCS) enabled the monitoring of the temperature
in the 300–530 K range with an accuracy >0.1 K. The duration of one
frequency sweep at a fixed temperature was approximately 1 min.
At each temperature point, the temperature and frequency depen-
acquired semiconductor properties with gallium and indium inser-
0
tion. The increasing
e
value with the decreasing frequency is
attributed to the blocking of the charge carriers at the electrodes.
Temperature scans of both samples for the dielectric loss were
also recorded at 1 kHz, 11.6 kHz, 132 kHz and 1 MHz frequencies
(Fig. 4). The magnitude of tand increased at elevated temperatures
for all the frequencies used. The samples showed a significant var-
iation in the temperature dependence of the dielectric loss (dissipa-
tion factor). The ClGaPc sample exhibited a strong intermediate
peak with a constant value of 0.6 at 1 kHz, 11.6 kHz and 132 kHz
Fig. 2. Experimental set up for temperature dependent dielectric spectroscopy. PC: computer, IA: impedance analyzer, NH: novotherm heating unit, NM: novotherm
microprocessor controller unit.

S.E. San et al. / Polyhedron 30 (2011) 1023–1026
1025
Fig. 3. Temperature dependence of the real part of the dielectric constant at
different frequencies, (a) gallium(III) phthalocyanine, (b) indium(III) phthalocya-
nine pellets.
Fig. 4. Temperature scans of the dielectric loss at different frequencies, (a)
gallium(III) phthalocyanine, (b) indium(III) phthalocyanine pellets.
state. This phenomenon is explained by the semiconductor struc-
ture acquired by centers involved in Pcs via the hopping charge
transport mechanism [18].
frequencies (Fig. 4a). The position of the peak moved to higher tem-
peratures as the frequency was increased and moved out of the
measured temperature range for the 1 MHz measurement. This
peak appeared as a shoulder in the 1 kHz measurement of the ClIn-
Pc sample and was absent in the higher frequency measurements
Fig. 5 shows the AC conductivity plots of the ClGaPc and ClInPc
samples at different frequencies. The conductivities of both sam-
ples increased with increasing temperature and frequency, indicat-
ing that the electronic conduction is dominant in the measured
range of temperatures and frequencies. The enhancement of the
conductivity with increasing frequency suggests that the hopping
mechanism is effective in electronic charge transport in both sam-
ples at all temperatures. Each curve in Fig. 4a and b had two dis-
tinctive regions (I and II) corresponding to different activation
energies, satisfying an Arrhenius law of the form:
(Fig. 4b). These results indicate the presence of dipoles in both sam-
ples; the deficiencies of the materials and imperfections lead to the
formation of dipoles. The increasing temperature causes faster rota-
tion of the dipoles, which in turn gives rise to the increasing tand
values and the movement of the intermediate tand peak to higher
temperatures with the increasing frequency, showing that both
samples are polar [16]. The overall temperature dependent dielec-
tric loss values of ClInPc were lower compared to those for ClGaPc
for the measured temperatures and the frequencies except the
r
¼
r
0
½expðꢁE
I
=ðkTÞÞ þ expðꢁEII=ðkTÞÞꢂ
ð1Þ
1
kHz measurement where the dielectric loss value raised to a high-
where
r
0
is the pre-exponential factor, E
I
and EII are the activation
er value than that for the ClGaPc sample at 525 K.
energies in units of electron volt (eV) and k is the Boltzmann con-
Charge conduction in MPcs is known to have both electronic
stant. E
I
and EII were calculated from the slopes of ln(
r
) versus
0
ꢁ1
and ionic components [17]. There is considerable
p–p
orbital over-
1000/T (K ) plots, shown in Fig. 5. The transition from region I to
region II occurs at 388 and 368 K for ClGaPc and ClInPc samples,
respectively. The frequency dependent activation energy values in
these regions are listed in Table 1. The activation energies of both
samples are proportional to the frequency at higher temperatures
and inversely proportional at lower temperatures. These results
can be understood in terms of the varying contribution of the ionic
component to the conductivity at increasing temperatures. The acti-
lapping between neighboring stacked MPc molecules which results
in the formation of a one dimensional electronic conduction path.
The metal cations also contribute to the conduction by traveling in
the channels formed by the stacked MPc molecules. It is shown
that, using the radius scales of synthesized Ga and In in combina-
tion with the dielectric behavior of Pcs, it is possible to evaluate the
ionic transport parameters. They acquire an intrinsic impurity

1
026
S.E. San et al. / Polyhedron 30 (2011) 1023–1026
Table 1
Temperature and frequency dependencies of activation energies of ClGaPc and ClInPc.
T (eV)
ClGaPc
ClInPc
Region I
Region II
Region I
Region II
(
388–530) K
0.55216
1.6 kHz 0.56965
32 kHz
MHz
(304–388) K
(368–530) K
(304–368) K
1
1
1
1
kHz
0.35806
0.35369
0.12245
0.07845
0.36724
0.41145
0.49863
0.56793
0.47241
0.43099
0.17008
0.09303
0.73686
0.85373
There are many serious works revealing the current advances in
phthalocyanines as newsworthy and living materials in technology
[
19–24]. Our attempts to exploit DS for the characterization of
these functional materials are presented in this work.
4
. Conclusions
Temperature dependent dielectric properties of ClGaPc and
ClInPc compounds were analyzed to determine the dielectric prop-
erties of these Pc complexes after the insertion of gallium and in-
dium metals. Ga and In embedded Pcs showed the standard
temperature dependence characteristics of conventional semicon-
ductors. The temperature dependence of the conductivities of both
samples exhibited Arrhenius behavior. Our results suggest that the
ionic conductivity is activated at elevated temperatures in both
samples. A detailed analysis of the dielectric properties of hybrid
Pcs with gallium and indium metals was performed for the first
time in the scope of this work. These metal embedded Pcs can be
utilized as building blocks in a variety of semiconductor based
new applications, such as solar cells and OLEDs. Our results can
be exploited in the design of these new devices, which could be
of critical importance in electronics.
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