Synthesis, characterization and investigation of electrical and electrochemical properties of imidazole substituted phthalocyanines

Article abstract of DOI:10.1016/j.ica.2010.09.054

Compound 1 has been synthesized by the reaction of 4-nitrophthalonitrile and 4,5-diphenyl-1H-imidazolethiole in DMSO in the presence of K ₂CO₃. Compound 2 has been synthesized by heating compound 1 with metallic lithium in pentanol a

Full text of DOI:10.1016/j.ica.2010.09.054

Inorganica Chimica Acta 365 (2011) 340–348
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization and investigation of electrical and electrochemical
properties of imidazole substituted phthalocyanines
Ebru Yabasß a,b, Mustafa Sülü a, Sinan Saydam c, Fatih Dumludag˘ d, Bekir Salih e, Özer Bekarog˘lu f,
⇑
a
_
Department of Chemistry, Inönü University, 44280 Malatya, Turkey
b Department of Chemistry, Cumhuriyet University, 58140 Sivas, Turkey
c Department of Chemistry, Fırat University, 23119 Elazıg˘, Turkey
d
_
Department of Physics, Marmara University, 34722 Göztepe, Istanbul, Turkey
e Department of Chemistry, Hacettepe University, Beytepe, 06532 Ankara, Turkey
f
_
_
Department of Chemistry, Technical University of Istanbul, 34469 Maslak, Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Compound 1 has been synthesized by the reaction of 4-nitrophthalonitrile and 4,5-diphenyl-1H-imidazo-
lethiole in DMSO in the presence of K2CO3. Compound 2 has been synthesized by heating compound 1
with metallic lithium in pentanol and hydrolyzed with hydrochloric acid (10%) and then neutralized with
ammonia solution. Compound 3 and 4 have been synthesized by heating 1 with ZnCl2 or CoCl2 in DMF in
the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), respectively. The new compounds were
obtained in sufficient purity after successive washing with different solvents and were characterized
by elemental analysis, 1H NMR, 13C NMR, UV–vis, IR and mass spectra. We also studied aggregation
behavior, electrochemical and electrical properties of these phthalocyanines. Direct current (dc) and
alternating current (ac) conductivity (40–100 kHz) measurements of films of 2, 3 and 4 were performed
in the temperature range of 295–523 K. Impedance spectra (IS) of the 2, 3 and 4 were also studied. The
conduction processes were discussed. Temperature dependence of dc conductivity of the films revealed
that 2, 3 and 4 show a semiconductor behavior. Results of ac measurements showed that, conduction pro-
cesses of the films of the complex consisted of different mechanisms depending on temperature and
frequency.
Received 21 June 2010
Received in revised form 17 September
2010
Accepted 27 September 2010
Available online 8 October 2010
Keywords:
Imidazole
Phthalocyanines
Metal complexes
Aggregation
Electrical properties
Electrochemistry
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
have been reported in a few articles [25–29]. Some of these phthal-
ocyanines were used in photodynamic therapy [26], electron trans-
Phthalocyanines are the most studied compounds in supramo-
lecular chemistry [1–5]. They have been used in very different areas
of technology and medical applications, i.e., gas sensor [1,6,7], li-
quid crystal [2,8], solar cell [9,10], catalyst [2,5,11], electrochromic
display [3,12], photodynamic therapy of cancer [13,14]. Phthalocy-
anine has been studied with many metal ions and different substit-
fer processes [28] and synthesis of the phthalocyanine polymers
[27,29]. Therefore, synthesis and investigation some properties of
new 4,5-diphenyl-1H-imidazole substituted phthalocyanine could
be worthwhile.
In present work, we reported synthesis and characterization of
bisphenylimidazole substituted metal-free, zinc(II), cobalt(II) phthal-
ocyanines. We also investigated aggregation behavior, electrical and
electrochemical properties of these phthalocyanines.
uents on peripheral (b) and non-peripheral (a) positions [1–5]. The
synthesis and properties of binuclear double decker, binuclear
clamshell, polynuclear and dendritic phthalocyanine have also
been reported [4,15,16]. As a new class of phthalocyanine, ball-type
phthalocyanine has been studied during the last years [17].
On the other hand, imidazole substituted porphyrin have been
used and investigated optical, electronic and catalytic properties
[18–24]. Imidazole or benzimidazole substituted phthalocyanines
2. Experimental
All reactions were carried out under argon atmosphere and all
solvents were dried by molecular sieves or proper methods [30].
4-Nitrophthalonitrile was prepared according to the literature
[31]. IR and UV–vis spectra were recorded on an ATI Unicam-
Mattson 1000 spectrophotometer using KBr pellets and Shimadzu
UV–vis spectrometer, respectively. Elemental analysis were per-
formed on LECO CHNS 932 and at Tubitak Ankara Testing and Anal-
ysis Laboratory (ATAL). 1H NMR and 13C NMR spectra were
⇑
Corresponding author. Present address: Bilim Sokak, Kardesßler Apt., No. 6/9
_
Erenköy, Istanbul, Turkey. Tel.: +90 216 359 01 30; fax: +90 216 386 08 24.
E-mail addresses: eyabas@cumhuriyet.edu.tr (E. Yabasß), msulu@inonu.edu.tr (M.
Sülü), ssaydam@firat.edu.tr (S. Saydam), fatihdumludag@marmara.edu.tr (F. Dum-
ludag˘), bekir@hacettepe.edu.tr (B. Salih), obek@itu.edu.tr (Ö. Bekarog˘lu).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.09.054

E. Yabaßs et al. / Inorganica Chimica Acta 365 (2011) 340–348
341
obtained by using a Bruker 300 MHz spectrometer. Mass spectra
were acquired on a Voyager-DE™ PRO MALDI-TOF mass spectrom-
eter (Applied Biosystems, USA) equipped with a nitrogen UV-Laser
operating at 337 nm. Spectra were recorded in reflectron modes
with an average of 100 shots.
vacuum. The dark green compound was washed in a soxhlet appa-
ratus with acetone. Finally, the dark green compound was dis-
solved in a soxhlet apparatus with THF. The organic phase was
concentrated and precipitated by addition of diethyl ether and fil-
tered off. It was dried in vacuum. The dark green solid was soluble
in THF, DMF and DMSO. Yield 30.0 mg (37%). Mp >300 °C. 1H NMR
(300.13 MHz, DMSO-d6, 25 °C): d = 13.7 (br s, 4H, Im-NH, disap-
peared on D2O addition); 9.5–7.4 (br m, 52H, Ar-H). MS (MALDI-
2.1. Synthesis of compounds
2.1.1. 4[(4,5-Diphenyl-1H-imidazole)-2-yl-thio]phthalonitrile (1)
The 4-nitrophthalonitrile (653.0 mg, 3.77 mmol) and 4,5-diphe-
nyl-1H-imidazolethiole (1.0 g, 3.96 mmol) were dissolved in DMSO
(45 mL). Anhydrous potassium carbonate (782.0 mg, 5.66 mmol)
was added to the reaction solution over period of 2 h with efficient
stirring. The reaction mixture was stirred at room temperature for
5 days. Then the mixture was poured into solution of salt-water
(1%), and the precipitate was filtered off, washed with water and
dried in Vacuum oven at 40 °C. The crude product was dissolved
in diethyl ether (300 mL) and filtered off. The ether solution was
cooled at ꢀ18 °C, the precipitate was filtered off and dried in vac-
uum. The white solid was soluble in acetone, chloroform, EtOH,
MeOH and THF. Yield 600.0 mg (42%). Mp 131 °C. 1H NMR
(300.13 MHz, Aceton-d6, 25 °C): d = 12.5 (s, 1H, Im-NH, disap-
peared on D2O addition); 8.0–7.9 (m, 2H, Ar-H); 7.8–7.7 (dd, 1H,
Ar-H); 7.6–7.3 (m, 10H, Ar-H). 13C NMR (75.03 MHz, DMSO-d6,
25 °C): d = 145.31; 134.93; 132.34; 131.20; 131.16; 129.02;
128.12; 116.36; 115.95; 115.88; 111.95. UV–vis (DMF) kmax/nm
TOF) m/z: 1577 [M+H]+. UV–vis (DMF) kmax/nm (log
dm3 molꢀ1 cmꢀ1) 687 (5.16), 620 (4.52), 354 (4.81), 288 (4.99). IR
(KBr pellet)
(cmꢀ1) 3395; 3057; 3031; 1602; 1487; 1098; 765;
e,
t
697. Anal. Calc. for C92H56N16S4Zn: C, 69.97; H, 3.57; N, 14.19; S,
8.12%. Found: C, 70.18; H, 3.71; N, 14.33; S, 7.68%.
2.1.4. [2,9,16,23-tetra{(4,5-Diphenyl-1H-imidazole)-2-yl-
thio}phthalocyaninato-cobalt(II)] (4)
Mixtures of
1 (100.0 mg, 0.26 mmol) and CoCl2 (8.0 mg,
0.06 mmol) in DMF (3 mL) were heated at the presence of DBU at
180 °C for 12 h. After cooling, the green-blue product mixture
was precipitated and purified with used the same procedure ex-
plained for 3. The dark green solid was soluble in THF, DMF and
DMSO. Yield 25.0 mg (31%). Mp >300 °C. MS (MALDI-TOF) m/z:
1571 [M+H]+. UV–vis (DMF) kmax/nm (log
(5.00), 607 (4.50), 316 (4.98). IR (KBr pellet)
3059; 3028; 1603; 1446; 1103; 765; 696. Anal. Calc. for
92H56N16S4Co: C, 70.26; H, 3.59; N, 14.25; S, 8.16%. Found: C,
e
, dm3 molꢀ1 cmꢀ1) 674
t
(cmꢀ1) 3426;
C
(log
3022; 2974; 2861; 2233; 1676; 1583; 1072; 738. Anal. Calc. for
23H14N4S: C, 72.99; H, 3.73; N, 14.80; S, 8.47%. Found: C, 72.88;
e
, dm3 molꢀ1 cmꢀ1) 285 (5.23). IR (KBr pellet)
t
(cmꢀ1) 3057;
69.95; H, 3.70; N, 14.02; S, 8.57%.
C
2.2. MALDI sample preparation
H, 3.61; N, 14.40; S, 8.34%.
MALDI matrices, a-cyano-4-hydroxycinnamic acid (CHCA) were
2.1.2. [2,9,16,23-tetra{(4,5-Diphenyl-1H-imidazole)-2-yl-
thio}phthalocyanine] (2)
prepared in THF at a concentration of 10 mg/mL. MALDI samples
were prepared by mixing sample solutions (4 mg/mL in THF) with
the matrix solution (1:10 v/v) in a 0.5 mL EppendorfÒ micro tube.
Lithium metal (18.0 mg, 2.6 mmol) was heated to dissolve in
pentanol (4 mL) and allowed to cool to room temperature. Com-
pound 1 (100 mg, 0.26 mmol) was added to the above solution
and was heated at 180 °C for 4 h. After cooling down to room tem-
perature, the reaction mixture was diluted with THF (5 mL) and
hydrolyzed with the hydrochloric acid (10%) and stirred at room
temperature for 5 h and then neutralized with ammonia solution.
This mixture was precipitated by addition of MeOH (5 mL) and fil-
tered off and washed with water (3ꢁ 5 mL) and MeOH (3ꢁ 5 mL),
respectively, and dried in vacuum. The dark green compound was
washed in a soxhlet apparatus with acetone. Finally, the dark green
compound was dissolved in a soxhlet apparatus with chloroform.
The organic phase was concentrated and precipitated by addition
of diethyl ether, filtered off and dried in vacuum. The metal-free
phthalocyanine was soluble in chloroform, THF, DMF and DMSO.
Yield 50.0 mg (50%). Mp >300 °C. 1H NMR (300.13 MHz, DMSO-
d6, 25 °C): d = 14.0 (br s, 4H, Im-NH, disappeared on D2O addition);
9.0–7.0 (br m, 52H, Ar-H); ꢀ5.1 (br s, 2H, Pc-NH, disappeared on
D2O addition). MS (MALDI-TOF) m/z: 1515 [M+H]+. UV–vis (DMF)
Finally 1.0 lL of this mixture was deposited on the sample plate,
dried at room temperature and then analyzed.
2.3. Electrochemistry
Electrochemical grade tetrabutylammonium tetrafluoroborate
(TBAFB) (Fluka) was used as the supporting electrolyte in voltam-
metric measurements in non-aqueous solvents. Cyclic voltammo-
grams (CV) and square wave voltammograms (SWV) were carried
out using a Gamry Reference 600 Potentiostat/Galvanostat/ZRA. A
three electrode system was used for CV and SWV measurements
in DMF consisting of glassy carbon working electrode, and a plati-
num wire counter and platinum wire quasi-reference electrode.
The ferrocene/ferrocenium couple (Fc/Fc+) was used as an internal
standard and their potentials were reported with respect to Fc/Fc+
in non-aqueous solutions. Square wave voltammetric analysis
was carried out at the frequency of 5 or 10 Hz, amplitude of
40 mV and step potential of 4 mV. High purity argon is used for
the de-oxygenation of the cell at least 10 min prior to electrochem-
ical measurements and the solution was protected from air by a
blanket of argon during the experiments.
kmax/nm (log
e
,
dm3 molꢀ1 cmꢀ1
)
710 (4.76), 681 (4.77), 650
(4.57), 616 (4.39), 345 (4.67), 295 (4.75). IR (KBr pellet)
t
(cmꢀ1
)
3418; 3297; 3055; 3027; 1603; 1445; 1015; 764; 696. Anal. Calc.
for C92H58N16S4: C, 72.90; H, 3.86; N, 14.78; S, 8.46%. Found: C,
72.24; H, 3.75; N, 14.30; S, 8.51%.
2.4. Electrical characterization
2.1.3. [2,9,16,23-tetra{(4,5-Diphenyl-1H-imidazole)-2-yl-
2.4.1. Preparation of films
thio}phthalocyaninato-zinc(II)] (3)
Electrical measurements were performed using Interdigitated
electrodes (IDTs). IDTs were fabricated onto glass substrates. Gold
was selected as electrode material. Width and space between the
Mixtures of
1 (100.0 mg, 0.26 mmol) and ZnCl2 (8.3 mg,
0.06 mmol) in DMF (3 mL) was heated at the presence of 1,8-diaza-
bicyclo[5.4.0]undec-7-ene (DBU), at 180 °C for 8 h. After cooling,
the mixture was precipitated by addition of diethyl ether, filtered
off and dried in vacuum. The crude residue was washed with water
(3ꢁ 5 mL) and MeOH (3ꢁ 5 mL), respectively. It was dried in
electrodes were 100 lm. In order to fabricate IDTs with gold elec-
trodes, glass substrates were cleaned in acetone, isopropyl alcohol
and DI-water in ultrasonic cleaner and dried with pure nitrogen
blow. Cleaned glasses were placed in thermal evaporation system

342
E. Yabaßs et al. / Inorganica Chimica Acta 365 (2011) 340–348
and chromium and gold metals were evaporated through the glass
substrates in high vacuum ambient (<2 ꢁ 10ꢀ6 mbar) using Ed-
wards Auto 500 coater system. Thicknesses of the evaporated met-
als were 100 and 1000 Å for chromium and gold, respectively.
Interdigital transducer pattern was obtained using photolithogra-
phy techniques. In order to coat the synthesized molecules onto
IDT, the molecules were dissolved in THF. The homogeny solutions
were coated onto IDT using smearing method. The substrate tem-
perature was kept constant at 295 K during the deposition of solu-
tion onto IDT.
1). UV–vis, 1H NMR, 13C NMR, IR and mass spectra confirmed the
proposed structure of all new compounds. The elemental analysis
of the result of starting material and phthalocyanine complexes
shows good agreement with calculation values.
The characteristic vibration of the AC„N group appear at
2233 cmꢀ1 as a single peak in the IR spectrum of compound 1. This
peak disappeared in spectrum of 2, 3 and 4 after reaction of tetra-
merization. In addition, the IR spectrum of metal-free phthalocya-
nine, NH group inner was given an absorption at 3297 cmꢀ1 [3].
The vibration peak of imidazole NH in compound 1 appears be-
tween 3057 and 2861 cmꢀ1 which is attributed to the formation
of intermolecular hydrogen bonds [5,18,20]. The IR spectra of
phthalocyanine compounds 2, 3 and 4 are shown at 3055, 3027,
3057, 3031 and 3059, 3028 cmꢀ1, respectively, peaks for NH group.
1H NMR spectra were in good correlation with the structure of
the synthesized compounds. In the 1H NMR spectrum of compound
1 which was taken in Acetone-d6 at room temperature, the aro-
matic protons appeared at 8.0–7.9 ppm (m), 7.8–7.7 ppm (dd),
7.6–7.3 (m) and imidazole –NH proton appeared at 12.5 ppm (s).
NH peak disappeared on addition of D2O. 13C NMR spectra were
in good correlation with the structure of the compound 1.
1H NMR spectra of compounds 2 and 3 were taken in DMSO-d6
at room temperature. The spectra of the 2 and 3 were almost the
same, showed aromatic protons at 9.4–7.0 ppm as a broad multi-
plet. The NH protons of imidazole at compounds 2 and 3 appeared
at 14.0 and 13.7 ppm broad singlet which disappeared with D2O
exchange. 1H NMR measurement was precluded due to the para-
magnetic nature of compound 4 [3].
2.4.2. Electrical measurements
Electrical measurements of the films were performed in two
parts. (i) Direct current (dc) measurements. (ii) Alternating current
(ac) measurements. Dc and ac properties of the films were deter-
mined in temperature controlled measurement chamber under
vacuum ambient (<10ꢀ3 mbar) in dark. Dc conductivity measure-
ments were performed between the voltages of ꢀ1 and 1 V at dif-
ferent temperatures using Keithley model 617 Programmable
Electrometer. Ac conductivity and impedance spectra (IS) mea-
surements were performed using Keithley model 3330 LCZ meter
(40–105 Hz). Measurements were done between the temperatures
of 295–523 K. Both dc and ac conductivity data were recorded
using an IEEE-488 data acquisition system.
3. Results and discussion
3.1. Synthesis and characterization
The UV–vis spectra of 2–4 in DMF showed typical absorptions
between 600 and 700 nm in the Q band region (Fig. 1).
*
The synthesis route of compounds 1–4 is summarized in
Scheme 1. The starting compound, 1, was synthesized by the reac-
tion of 4-nitrophthalonitrile and 4,5-diphenyl-1H-imidazolethiole
in DMSO in the presence of K2CO3. The metal-free phthalocyanine,
2, was synthesized by heating compound 1, with metallic lithium
in pentanol and hydrolyzed with hydrochloric acid (10%) and then
neutralized with ammonia solution. Metal-containing phthalocya-
nines, 3 and 4, were synthesized by heating compound 1, with
ZnCl2 or CoCl2 in DMF at the presence DBU, respectively (Scheme
The Q band absorptions of the compounds refer to the
p ? p
transition from the highest occupied molecular orbital (HOMO)
to the lowest unoccupied molecular orbital (LUMO) of the phthalo-
cyanine ring. The B band absorptions of the compounds were ob-
served within the UV region between 300–350 nm due to the
transitions from the deeper
p levels to the LUMO [1]. The UV–vis
spectrum of 2 in DMF gives four absorptions at 710, 681, 650
and 616 nm (shoulder) in Q band region and at 295 and 345 nm
H
O2N
N
CN
CN
H
4
N
S
i
CN
CN
+
SH
N
N
1
ii
N
NH
S
N
N
S
H
N
N
N
N
M
N
N
N
N
H
S
S
N
N
N
HN
2 M: 2H
3 M: Zn
4 M: Co
Scheme 1. Synthesis of tetrasubstituted phthalocyanines, 2, 3, 4 (i) K2CO3, DMSO; (ii) (2) Li, pentanol, hydrolyzed with hydrochloric acid (10%) and then neutralized with
ammonia solution. (3, 4) ZnCl2 or CoCl2, DBU, DMF.

E. Yabaßs et al. / Inorganica Chimica Acta 365 (2011) 340–348
343
3.2. Electrochemistry
The voltammetric measurements of the 2, 3 and 4 were carried
out on glassy carbon (GC) electrode in DMF, TBAFB as supporting
electrolyte by cyclic voltammetry (CV) and Osteryoung square
wave voltammetry (SW). A wider potential range (ꢀ2.6 to +1.1 V
versus Fc/Fc+) was used to study electrochemical properties of all
the phthalocyanine compounds. The relevant data are given in
Table 1 which also includes selected data from the literature for
the related compounds. The phthalocyanine complexes can
undergo multiple one-electron reduction and oxidation of the con-
jugated macrocycle to yield anion and cation radicals, respectively.
In main group phthalocyanines the redox activity is directly asso-
ciated with the oxidation by the removal of the electrons from
HOMO, while up to four additional electrons can be added to LUMO
of phthalocyanines [3]. With the redox active metals, the oxidation
and the reduction of the metal occur between the oxidation and
reduction of the phthalocyanine ring such as cobalt, chromium,
manganese and iron phthalocyanines.
The cyclic and square wave voltammograms of 2, 3 and 4 are
shown in Figs. 3–5, respectively. The potential values and assign-
ments are summarized in Table 1. CV and SW voltammetric studies
showed that all the phthalocyanine compounds have similar well-
defined ring based redox couples up to four reductions and two
oxidations labeled R1, R2, R3, R4, O1 and O2.
The redox properties of 2 and 3 showed good electrochemical
processes with up to two well-separated oxidation and up to four
reduction waves (Figs. 3 and 4). The oxidation processes of 2 are
quasi-reversible as can be seen from Fig. 3. The ratio of cathodic
peak current to anodic peak current (ic/ia) is less than 1 in all of
the observed redox processes.
Fig. 1. UV–vis spectra of 2 (—), 3 (- - -) and 4 (. . ..) in DMF (C = 10ꢀ5 mol dmꢀ3).
in B band region. Compound 2 showed aggregation. Aggregation is
usually depicted as a coplanar association and is dependent of the
concentration [32]. In the aggregated state, the electronic structure
of phthalocyanine ring is perturbed resulting in alternation of the
ground and excited state electronic structures [33]. This causes
broadening and/or the split of the Q band, especially at high con-
centrations. The similar intensity of the Q and B band of 2
(Fig. 1) also due to the splitting of the Q band as a result of the low-
ering of molecular symmetry [34,35]. The aggregation behavior of
2–4 was investigated at different concentration (from 2 ꢁ 10ꢀ6 to
10 ꢁ 10ꢀ6 mol dmꢀ3) in DMF (Fig. 2).
At higher concentration the aggregation was observed in com-
pound 2. There is no aggregation of compounds 3 and 4 at different
concentrations in DMF.
4
A
µ
O
2
O
1
-3
-2
-1
0
1
E / V vs Fc/Fc+
R
1
5
A
µ
R
2
R
3
R
4
-3
-2
-1
0
+
E / VvsFc/Fc
Fig. 2. UV–vis spectra of 2 in different concentrations (10 ꢁ 10ꢀ6 (—), 8 ꢁ 10ꢀ6
(- - - -), 6 ꢁ 10ꢀ6 (- ꢂ -), 4 ꢁ 10ꢀ6 (- ꢂ ꢂ -), 2 ꢁ 10ꢀ6 (. . ..) mol dmꢀ3).
Fig. 3. Cyclic and square wave (inset) voltammograms of 2 in DMF containing 0.1 M
TBAFB, scan rate 50 mV sꢀ1
.
Table 1
Redox potentials of 2, 3, 4 and selected phthalocyanines compounds.
Compound
Solvent/supporting electrolyte
RE
O2
O1
R1
R2
R3
R4
D
E1
Ref.
2
3
4
H2Pc
ZnPc
CoPc
DMF/TBAFB
DMF/TBAFB
DMF/TBAFB
DCM/TBAP
DCM/TBAP
DMF/TBAFB
Fc/Fc+
Fc/Fc+
Fc/Fc+
SCE
0.55
0.34
0.42
0.10
0.82
0.85
0.01
ꢀ1.10
ꢀ1.13
ꢀ0.75
ꢀ0.72
ꢀ0.67
ꢀ0.88
ꢀ1.44
ꢀ1.51
ꢀ1.09
ꢀ0.90
ꢀ0.98
ꢀ2.02
ꢀ1.94
ꢀ1.94
ꢀ1.79
ꢀ1.76
ꢀ1.76
ꢀ2.43
ꢀ2.37
1.44
1.59
0.85
1.54
1.52
0.89
tw
tw
tw
[42]
[43]
[44]
0.51
1.14
ꢀ2.46
SCE
Fc/Fc+
0.44
RE, reference electrode,
D
E1 = O1 ꢀ R1, at 10 Hz, 40 mV amplitude and 4 mV step potential; tw, this work.

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E. Yabaßs et al. / Inorganica Chimica Acta 365 (2011) 340–348
complex. Thus the difference between the potentials of the first
oxidation and the first reduction processes E1 reflects the
HOMO-LUMO gap of the metal-free phthalocyanines and it is clo-
sely related to the HOMO–LUMO gap of MPc species involving a re-
D
10
A
µ
dox inactive metal center.
DE1 and the first and the second
reduction (R2 ꢀ R1 = 0.38 V) of 3 is comparable with the reported
literature [3].
O
1
-2
-1
0
1
The compound 4 showed (Fig. 5) both the oxidation and reduc-
tion waves of metal center and phthalocyanine ring, labeled O1 and
O2 and R1, R2, R3 and R4, with in the electrochemical window of
DMF containing 0.1 M TBAFB. The first reduction and first oxida-
tion processes are metal based in polar coordinating solvents,
which is a well-known electrochemical behavior of CoPc com-
plexes in coordinating solvents such as DMF and DMSO [36].
Therefore R1 (ꢀ0.75 V) and O1 (0.10 V) can be attributed to succes-
sive addition and removal of one electron to metal based orbitals of
the phthalocyanine complex [37–39]. Three reductions waves la-
beled R2, R3 and R4, at ꢀ1.09, ꢀ1.79 and ꢀ2.46 V with the oxidation
wave labeled O2 at 0.51 V are assigned to the ring based reduction
and oxidation of the phthalocyanine, respectively. The separation
between the reduction and oxidation peak potentials and the var-
iation of the peak potentials with the scan rate for all the processes
lies between 70 and 160 mV, showing their reversible or quasi-
reversible nature. The peak currents increased linearly with the
E / V vs Fc/Fc+
R
1
5
A
µ
R
2
R
3
-2
-1
0
1
+
E / VvsFc/Fc
Fig. 4. Cyclic and square wave (inset) voltammograms of 3 in DMF containing 0.1 M
TBAFB, scan rate 50 mV sꢀ1
.
square root of the scan rates ranging from 0.050 to 0.400 V sꢀ1
,
4
A
µ
indicating diffusion-controlled behavior [40]. Therefore the first
oxidation and the first reduction of 4 could be assigned to
CoII(Pcꢀ2)/CoIII(Pcꢀ2) (O1), CoII(Pcꢀ2)/CoI(Pcꢀ2) (R1), redox couples
and the second, third and fourth reductions and the second oxida-
tion processes are phthalocyanine ring based and assigned to
O
2
CoI(Pcꢀ2)/CoI(Pcꢀ3
)
(R2), CoI(Pcꢀ3)/CoI(Pcꢀ4
)
(R3), CoI(Pcꢀ4)/
-3
-2
-1
0
1
CoI(Pcꢀ5) (R4) and CoIII(Pcꢀ1)/CoIII(Pcꢀ2) (O2), respectively. The sep-
aration between the reduction and oxidation processes of metal
center (O1 ꢀ R1 = 0.85 V) which reflects the HOMO–LUMO gap of
the complex, is comparable with the reported cobalt phthalocya-
nine complexes [41]. The electrochemical reactions corresponding
to all waves in the voltammograms are stepwise one electron pro-
cess. The ratios of the anodic to cathodic peak currents (ia to ic) for
couples for all the waves except the first oxidation are close to
unity, suggesting a reversible/quasi-reversible redox process but
the first oxidation wave is an irreversible. There is an irreversible
oxidation wave before the first oxidation wave (labeled A) shown
in Fig. 5 which disappears with increasing scan speed, that could
be assigned to five or six coordinated phthalocyanine complex, ex-
tra ligand coming from donor solvent molecule DMF [3].
E / V vs Fc/Fc+
O
1
A
R
5
A
µ
1
R
2
R
3
R
4
-3
-2
-1
E / VvsFc/Fc
0
1
+
Fig. 5. Cyclic and square wave (inset) voltammograms of 4 in DMF containing 0.1 M
TBAFB, scan rate 50 mV sꢀ1
.
Fig. 3 represents the CV and SW voltammogram of 2 displaying
six redox processes, assigned to the phthalocyanine ring oxidation
and reduction, labeled O1, O2, R1 R2, R3 and R4. The first and the sec-
ond oxidation waves were observed at 0.34 and 0.55 V and the
reduction waves appeared at ꢀ1.10, ꢀ1.44, ꢀ1.94 and ꢀ2.37 V,
respectively, at .050 V sꢀ1 scan rate. SW voltammogram of 2 sug-
gest that all six redox processes have quasi-reversible character.
The separation of the first reduction and the first oxidation pro-
cesses (1.44 V) reflects the HOMO–LUMO gap of the metal-free
phthalocyanine complexes and is comparable to the literature.
The separation between the first reduction and the second ring
3.3. Electrical properties
3.3.1. Dc properties
Dc conductivity studies were done on the films of 2, 3 and 4 in
temperature controlled vacuum ambient (<10ꢀ3 mbar) in dark.
Measurements were done between the voltages of ꢀ1 and 1 V at
different temperatures between 295 and 523 K. Current values
passing through the films were recorded versus applied voltages
(I–V curves). Dc conductivity of the films was determined using
Eq. (1) and the slope of the I–V curves.
reductions (DE1) was found to be approximately 0.30–0.45 V and
is in agreement with the reported separation of the redox pro-
cesses of the reported phthalocyanine compounds [3].
ꢀ ꢁꢀ
ꢁ
I
V
d
rdc
¼
ð1Þ
The complex 3 gives (Fig. 4) three one-electron reduction and
one electron-oxidation processes labeled R1 = ꢀ1.13, R2 = ꢀ1.51,
R3 = ꢀ1.94 and O1 = 0.42 V versus Fc/Fc+ at 0.050 V sꢀ1 scan rate
(Table 1). Since Zn is a redox inactive metal in the working condi-
tion, all the observed oxidation and reduction processes are all
phthalocyanine ring based processes. The reduction of the phthalo-
cyanine ring is associated with the position of the LUMO and oxi-
dation is associated with the HOMO of the phthalocyanine
ð2n ꢀ 1Þlh
where (I/V) is slope of the I–V graph, d is distance between the fin-
ger pair, n is number of the finger pair, l is overlap length and h is
thickness of the electrodes .
Fig. 6 shows the variation of dc conductivities of the films of 2, 3
and 4 with temperature. Conductivity of the film 2 and 3 was found
as 6.7 ꢁ 10ꢀ11 and 1.0 ꢁ 10ꢀ10 S/cm at 295 K, respectively. The

E. Yabaßs et al. / Inorganica Chimica Acta 365 (2011) 340–348
345
der of conductivity values with other reports in different phthalocy-
anine films such as ꢃ10ꢀ12 S/cm at 300 K and ꢃ10ꢀ8 S/cm at 470 K
[47], and ‘10ꢀ12 to ꢃ10ꢀ6 S/cm between 300 < T < 400 K [48],
ꢃ10ꢀ7 S/cm between 370 < T < 470 K [49]. Our activation energy
values also exhibited same order of activation energy values with
other reports in different phthalocyanine films such as 0.6–0.8 eV
[47]. The activation energies EA1 corresponding to the region I is
associated with a short lived charge transfer between impurity
and film of complex. EA2 which is the activation energy correspond-
ing to the region III, is associated with the resonant energy involved
in short lived excited state. EA1, corresponds to impurity conduction
and EA2 corresponds to an intrinsic generation process [50]. The
intermediate region extends from the impurity exhaustion temper-
ature to intrinsic temperature. In this region, all impurity atoms are
ionized, but the intrinsic carrier are not yet excited to a marked de-
gree because the density of carriers remains approximately con-
stant and equal to the impurity concentration.
Fig. 6. Temperature dependence of the dc conductivity for the films of 2, 3 and 4
between the temperatures of 295–523 K.
conductivity values increased to 5.7 ꢁ 10ꢀ7 and 6.6 ꢁ 10ꢀ7 S/cm at
523 K, respectively. Dc conductivities of the 2 and 3 increase with
increasing temperature at the temperatures T < 378 K and
T P 438 K. Conductivity of the 4 was found as 9.6 ꢁ 10ꢀ11 S/cm
at 295 K and increased to 5.6 ꢁ 10ꢀ6 S/cm at 523 K. Generally, dc
conductivity of the film 4 increases with increasing temperature.
The results indicate that conductivity values of the 2 and 3 are
nearly equal at the same temperatures in the measured tempera-
ture range. Film of 4 has also nearly equal conductivity values up
to 378 K with 2 and 3. After this temperature 4 has higher conduc-
tivity than that of 2 and 3.
As seen from Fig. 6, dc conductivity behavior of the film 2 and 3
can be analyzed in three different temperature regions (I, II and III).
In the first region (T < 378 K), conductivity increases linearly with
increasing temperature then in the second region (378 K <
T 6 ꢃ438 K) it begins to decrease. In the third region (T P ꢃ438 K),
linear increase in the conductivity with temperature is observed
again. This behavior may be observed due to absorption of oxygen
by coated films during the deposition of the films onto IDT. Similar
behavior was reported for some organic molecules. The authors
have ascribed this behavior to exhaustion of absorbed oxygen
[45,46].
The linear relationship between the Ln dc conductivity and in-
verse of temperature (Arrhenius plot) shows that the films of 2, 3
and 4 behave as intrinsic semiconductor in the third region. For
the first region, behavior of Ln dc conductivity with inverse of tem-
perature can be interpreted as extrinsic semiconductor behavior
due to interpretations as mentioned above. The linear behavior
indicates that the temperature dependence of dc conductivity of
the film can be represented by the well-known expression (Eq.
(2)) for conductivity [16].
3.3.2. Ac properties
Ac measurements were performed by means of conductivity
and impedance measurements as a function of frequency between
the frequencies 40 Hz and 100 kHz at different temperatures (295–
523 K). Fig. 7 shows frequency dependence of ac conductivity for
the film of 2 at indicated temperatures. As seen from Fig. 7, ac con-
ductivity of the film increases with increasing temperature. Tem-
perature dependence of ac conductivity of the film can be
analyzed in two parts, (i) low temperature region (T < ꢃ438 K)
and (ii) high temperature region (T > ꢃ438 K). At low temperature
region, conductivity of the film strongly depends on frequency.
Conductivity of the film increases with increasing frequency. At
high temperature region, frequency dependence of ac conductivity
of the film decreases (except low frequency region above the tem-
perature of 438 K). The rac versus x curves of 3 and 4 were similar
to those in Fig. 7. At 295 < T < 438 K, temperature dependence of ac
conductivity of the 2 at different frequencies is given in Fig. 8. It
can be seen that conductivity of the film increases with increasing
frequency and temperature and also approaches to each other at
high temperatures in the measured frequency range.
At all temperatures frequency dependence of the measured ac
conductivity of the films follows a universal power law. Universal
power law is given by the Eq. (3) [16].
rac ¼ Axs
ð3Þ
where
x
is the angular frequency, A and s are material depended
constants. Investigation of the conduction mechanism involved
the variation of the exponent s with temperature. S values were
calculated from slope of straight portions of the rac versus
x plot.
ꢀ
ꢁ
ꢀEA
rdc
¼
r
0 exp
ð2Þ
kT
where EA is activation energy, T is temperature, k is Boltzmann’s
constant and r0 is a constant of proportionality. The activation en-
ergy values of the films which are determined from the slope of the
Ln conductivity versus 1/T graph were given in Table 2. Calculated
dc conductivity values of the films of 2, 3 and 4 exhibited same or-
Table 2
Activation energies of the films for two different regions (EA1 for T < 378 K, EA2 for
T ꢄ ꢃ438 K).
Film
EA1 (eV)
EA2 (eV)
2
3
4
0.86
0.86
0.80
0.83
0.75
0.77
Fig. 7. Frequency dependence of ac conductivity of the film
temperatures of 295–523 K.
2 between the

346
E. Yabaßs et al. / Inorganica Chimica Acta 365 (2011) 340–348
model [54], the exponent s is almost equal to 0.8 and increases
slightly with temperature or independent of temperature. Because
of this, QMT model is not applicable to our results. According to
the overlapping-large polaron tunneling (QLPT) model [55], the
exponent s is both frequency and temperature dependent and de-
creases with increasing temperature to a minimum value at a cer-
tain temperature, then it increases with increasing temperature.
This model is also not applicable to our results. According to the
small polaronquantum-mechanical tunneling (SP) model, the expo-
nent s is an increasing function of temperature [52]. It is also well
known that the low values of s indicate multihopping process, while
high values of s indicate single hopping process. According to these
models, charge transport mechanism can be interpreted in two
parts.
At low temperature region (T < ꢃ438 K) dominant conduction
mechanism can be modeled by single hopping over the measured
frequency range. At high temperature region (T > ꢃ438 K) conduc-
tion mechanism can be modeled depending on frequency region,
Fig. 8. Temperature dependence of ac conductivity of the film 2 at indicated
frequencies (T ꢅ 438 K).
(i) low frequency region (
gion (
mechanism can be modeled by small polaron tunneling (SP) and in
high frequency region by single hopping.
x
< ꢃ1 kHz) and (ii) high frequency re-
x
P ꢃ1 kHz). In low frequency region dominant conduction
Complex impedance spectroscopy is an important tool to ana-
lyze the electrical properties of a material. The complex impedance
Z(x), can be represented as a function of frequency as,
Zð
x
Þ ¼ Z0ð
x
Þ þ jZ00ð
x
Þ
ð5Þ
where Z0(
x
) and Z00(
x
) are the real and imaginary parts of imped-
ance. Fig. 10a and b show complex impedance spectrum (Nyquist
plot) of the film 3 recorded at indicated temperatures. It can be seen
from Fig. 10a that, the impedance spectra consist of a quasi-vertical
line for the film 3 up to 438 K. The existence of a semicircular
shaped curve in the Nyquist plot means that the impedance
Fig. 9. Temperature dependence of exponent s of the films 2, 3 and 4 for high
frequency region.
Derived values of s for the films 2, 3 and 4 are around 1.0 at 295 K
and decreases to 0.07 at 523 K. The variation of the s values of the
films with temperature for high frequency region (Pꢃ1 kHz) is gi-
ven in Fig. 9. As seen from Fig. 9, s values decrease with increasing
temperature. According to the band theory, the ac conductivity
should be frequency independent and drops at sufficiently high fre-
quency. Contrary to band model, in the hopping model the electrons
in charged defect states hop over the coulomb barrier. This model
predicts a frequency dependent ac conductivity, which increases
with the increasing frequency under the condition that, the fre-
quency exponent s 6 1 [51]. The Correlated Barrier Hopping (CBH)
model, which was developed by Elliott, [51], predicts a tempera-
ture-dependent s value which is given by
6kBT
Wm
s ¼ 1 ꢀ b ¼ 1 ꢀ
ð4Þ
where kB is Boltzmann’s constant and Wm is the binding energy. In
this model s values decreases with increasing temperature. Our re-
sults showed that s is a function of temperature for all the films and
shows a general tendency to decrease with increasing temperature
(for T < ꢃ438 K) over the measured frequency range. The variation
of ac conductivity and variation of s conductivity with temperature
(Figs. 7 and 9) are in agreement with the prediction of the CBH mod-
el. The same type of temperature dependence for s was observed in
other reports in different phthalocyanine films [52,53]. However, at
the temperatures T > ꢃ438 K, while s values decrease with increas-
ing temperature in high frequency region (
ꢃ438 K), it increases with increasing temperature in low frequency
region (
< ꢃ1 kHz). In the quantum-mechanical tunneling (QMT)
x
P ꢃ1 kHz) as in (T <
Fig. 10. The cole–cole plot at temperatures (a) T ꢅ 438 K; (b) T > 438 K for the film
of 3.
x

E. Yabaßs et al. / Inorganica Chimica Acta 365 (2011) 340–348
347
becomes capacitive even at relatively high frequency. The curves
can be modeled as resistor parallel with a capacitor in series with
another resistor [53]. Fig. 10b shows the complex impedance spec-
trum for the film 3 at the temperatures T > 438 K. As seen from
Fig. 10b impedance spectra consist of depressed semicircles with
different radius. With the examination of the Fig. 10b the effect of
the temperature on impedance spectra can be seen clearly. The ra-
dius of the semicircle decreases with increasing temperature. An
ideal semicircle in complex plane only appears in Debye dispersion
relations for single-relaxation time process. Most of the materials
show a pronounced deviation from Debye treatment as in our case.
In this case, the relaxation time is considered as a distribution of
values, rather than a single-relaxation time [56]. The depressed
semicircles with different radius indicate deviation from Debye dis-
persion relation. Therefore, the equivalent circuit is modified to in-
clude a constant phase element (CPE). The series resistance in the
equivalent circuit represents the ohmic losses in the test fixture
and electrode sheet resistance. The parallel resistance is of the coat-
ing material in parallel with that of the substrate. The intercepts of
the semicircular arcs with real axis give us an estimate of the bulk
resistance of the material. It is observed that bulk resistance de-
creases with increasing temperature indicating semiconductor
property. In addition, a straight line was observed in Fig. 10b in
low frequency range. The straight line in the low frequency range
indicates the presence of Warburg component [57]. The complex
impedance spectrum (Nyquist plot) of the film 2 and 4 were similar
to those in Fig. 10.
line in the low frequency range indicates the presence of Warburg
component.
Acknowledgements
The authors gratefully acknowledge the financial support by
Scientific and Technical Research Council of Turkey (TUBITAK, Pro-
ject Nos. 105T240 and 108T140) and the partial support by Turk-
isch Academy of Sciences (TUBA).
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< ꢃ1 kHz) and (ii) high frequency region (
x
P ꢃ1 kHz). In low
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