Decorating nickel phthalocyanine periphery by aryl-1,3,4-oxadiazole pendants: Synthesis, characterization, and conductivity studies

Article abstract of DOI:10.1080/00958972.2010.529902

We have decorated nickel phthalocyanine (NiPc) periphery by four different aryl-[1,3,4]-oxadiazole pendants. Introduction of aryl-[1,3,4]-oxadiazole pendants into the NiPc core results in improved thermal stability, fine-tuning of the position of the Q-ba

Full text of DOI:10.1080/00958972.2010.529902

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Decorating nickel phthalocyanine
periphery by aryl-,,-oxadiazole
pendants: synthesis, characterization,
and conductivity studies
M.N.K. Harish a , J. Keshavayya a a , K.R. Venugopala Reddy b ,
H.R. Mallikarjuna c , R.A. Shoukat Ali a & T. Rajesh a
a Department of Chemistry, School of Chemical Sciences, Jnana
Sahyadri, Kuvempu University, Shankaraghatta – 577 451,
Shimoga, Karnataka, India
b Department of Industrial Chemistry, School of Chemical
Sciences, Jnana Sahyadri, Kuvempu University, Shankaraghatta –
577 451, Shimoga, Karnataka, India
c Department of Chemistry, Rajiv Institute of Technology, Hassan –
573 201, Karnataka, India
Published online: 20 Nov 2010.
To cite this article: M.N.K. Harish , J. Keshavayya , K.R. Venugopala Reddy , H.R. Mallikarjuna ,
R.A. Shoukat Ali & T. Rajesh (2010) Decorating nickel phthalocyanine periphery by aryl-,,-
oxadiazole pendants: synthesis, characterization, and conductivity studies, Journal of Coordination
Chemistry, 63:23, 4050-4060, DOI: 10.1080/00958972.2010.529902
To link to this article: http://dx.doi.org/10.1080/00958972.2010.529902
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Journal of Coordination Chemistry
Vol. 63, No. 23, 10 December 2010, 4050–4060
Decorating nickel phthalocyanine periphery by
aryl-[1,3,4]-oxadiazole pendants: synthesis,
characterization, and conductivity studies
M.N.K. HARISHy, J. KESHAVAYYA*y, K.R. VENUGOPALA REDDYz,
H.R. MALLIKARJUNAx, R.A. SHOUKAT ALIy and T. RAJESHy
yDepartment of Chemistry, School of Chemical Sciences, Jnana Sahyadri, Kuvempu
University, Shankaraghatta – 577 451, Shimoga, Karnataka, India
zDepartment of Industrial Chemistry, School of Chemical Sciences, Jnana Sahyadri,
Kuvempu University, Shankaraghatta – 577 451, Shimoga, Karnataka, India
xDepartment of Chemistry, Rajiv Institute of Technology, Hassan – 573 201,
Karnataka, India
(Received 24 June 2010; in final form 1 September 2010)
We have decorated nickel phthalocyanine (NiPc) periphery by four different aryl-[1,3,4]-
oxadiazole pendants. Introduction of aryl-[1,3,4]-oxadiazole pendants into the NiPc core results
in improved thermal stability, fine-tuning of the position of the Q-band with decrease in band
gap, indicating effective electronic communication between the two different ring systems with
NiPc core. The magnitude of DC electrical conductivity for aryl-[1,3,4]-oxadiazole-substituted
NiPcs 3a–3d is found to be ꢀ105 times higher than the parent NiPc (1). The temperature-
dependent DC conductivity studies reveal semiconducting nature of the newly synthesized NiPc
compounds with significant decrease in thermal activation energy (DE) compared to parent
NiPc.
Keywords: Phthalocyanine; [1,3,4]-Oxadiazole; Conducting; Solid-state electronic absorption
spectra; Solid-state 13C-NMR
1. Introduction
Peripheral functionalization of phthalocyanines (Pcs) and their metal Pcs (MPcs)
complexes have attracted attention because of their electronic structures, extreme
thermal, and chemical stabilities, giving rise to applications in materials science [1–4].
In addition to their intrinsic use as dyes and pigments, MPcs have demonstrated
potential in emerging technologies including sensors [5], photovoltaic cell elements [6],
photoconducting agents [7], nonlinear optics [8], liquid crystals [9], electrocatalysis, and
fuel cells [10, 11]. Research in the recent years has focused on tuning the properties of
related macrocycles via changes in their chemical structure.
In 1990, [1,3,4]-oxadiazole functionalized organic molecules were found to
serve as excellent electron-transporting [12–16] and hole-blocking materials [17].
*Corresponding author. Email: jkeshavayya@gmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.529902

Nickel phthalocyanine
4051
As planar organic semiconducting material, oxadiazole is an attractive building block
for organic light emitting devices because of its electron affinity and ease in building a
stable ꢀ-conjugated system [18, 19]. Introduction of an oxadiazole ring is a promising
approach to attain high charge-carrier mobility, making them prime candidates for
applications in organic electronics [20–22].
Thus, functionalization of Pc with oxadiazole and an aryl system is desirable. Prior
to our study, there was only one report pertaining to the synthesis of Pc bearing
[1,3,4]-oxadiazole moieties with phenyl ether linkage, with improved solubility and
electrical conductivity [23]. This encouraged us to synthesize Pc macrocycles with aryl-
1,3,4-oxadiazole subunits. This initial study to develop a new peripheral modified Pcs
involves incorporation of heteroaromatic and aromatic moieties. The synthetic route in
this study involves direct association of aryl oxadiazole pendants to Pc allowing
extensive electronic interaction of Pc with oxadiazole and aryl substituent. Investigation
on these new Pcs may be regarded as an important step in developing viable sensor and
semiconducting materials with high chemical and thermal stabilities. Further, the
synthetic scheme is very general and the possibilities are only highlighted by this
preliminary study.
All the newly synthesized 2,9,16,23-tetra-aryl-[1,3,4]-oxadiazole-substituted nickel
phthalocyanines (NiPcs) 3a–3d are bottle green in color. Characterization of these Pc
complexes was difficult because of insolubility in almost all organic solvents.
Nevertheless, the elemental analysis, FT-IR, solid-state UV-Vis, and solid-state
13C-NMR spectroscopy, in addition to X-ray diffraction (XRD), scanning electron
microscopy (SEM, spectral and thermal analyses provided sufficient evidence for the
proposed structures.
2. Experimental
2.1. Materials
1,2,4-Benzene tricarboxylic anhydride was purchased from Aldrich and other chemicals
were obtained from Merck (India) and used without purification. Tetracarboxy nickel
phthalocyanine (TCPNi) [24] and aryl hydrazides 2a–2d [25] were prepared according to
the described procedure.
2.2. Instrumentation
Elemental analyses were obtained from a Carlo-Erba 1106 instrument. Infrared (IR)
spectra were recorded on a FT-IR 8400s Shimadzu spectrometer in KBr pellets. Solid-
state electronic absorption spectra were recorded on a Perkin-Elmer UV-Vis spectro-
meter, model UV/VIS-35. Solid-state 13C-NMR spectra were recorded on a Bruker
DSX-300 solid-state NMR spectrometer with a magnetic field of 7.04 T and carbon
frequency of 75.47 MHz (internal standard was glycine). Powdered XRD measurements
were carried out on a Bruker D8 Advance X-ray diffractrometer. DC electrical
conductivity measurements were carried out by the two-probe method using a Keithley
source meter (Model-2400). A calibrated Chromel–Alumel thermocouple was used to
measure the temperature of the sample. For DC electrical conductivity measurements,

4052
M.N.K. Harish et al.
powder samples were pressed uniaxially into a pellet of thickness ꢁ2 mm and of
diameter 10 mm by applying a pressure of 100 Mpa for 3 min. The pellets were sintered
at 100ꢂC for 3 h to eliminate trapped oxygen and to get thermal stability. Fine quality
silver paint was applied on both sides of the pellets for good electrical contacts. The
pellet was sandwiched tightly between two steel rods in a sample holder with the help of
screws and kept in a cell. Then, the cell was placed in a muffle furnace. A suitable
constant and low voltage (5 V) was applied across the pellet and the corresponding
current through the sample was measured from 300 to 500 K; conductivity readings
were recorded during the heating cycle.
2.3. General method for the synthesis of 2,9,16,23-tetra-aryl-[1,3,4]-oxadiazole-
substituted NiPc
TCPNi (0.001 mol) and aromatic hydrazide (2a–2d) (0.006 mol) were stirred into
preheated polyphosphoric acid (PPA) (100 g) containing 10 g of P2O5 at 100ꢂC in a
three-necked round bottom flask containing a mechanical stirrer, condenser, and
thermometer for 1 h and then maintained at 150ꢂC for 20 h. The reaction mixture was
allowed to cool to 100ꢂC and quenched with ice cold water and filtered. The product
obtained was repeatedly treated with 0.1N sodium hydroxide solution followed
by water, hot acetic acid, 10% sodium bicarbonate solution, water, and acetone to
get 3a–3d.
2.3.1. 2,9,16,23-Tetra-{5-(phenyl)[1,3,4]-oxadiazole}-nickel phthalocyanine (3a). Yield:
80%. Calcd: C (66.97), H (2.81), N (19.53). Found: C (66.03), H (2.12), N (18.94). IR
[(KBr) ꢁmax/cmꢃ1]: 3065 (Ar–CH), 2922 (Ar–CH), 2848 (Ar–CH), 1609 (C¼N), 1530
(C¼C), 1092, 930, 824, 724 (Pc skeleton vibration). 13C-NMR (solid state) ꢂ/ppm 217,
163, 150–120. UV-Vis (solid state) ꢃmax/nm 497, 602, 666, 768.
2.3.2. 2,9,16,23-Tetra-{5-(4-methyl-phenyl)[1,3,4]-oxadiazole}-nickel phthalocyanine (3b).
Yield: 82%. Calcd: C (67.84), H (3.35), N (18.62). Found: C (67.61), H (2.98),
N (18.13). IR [(KBr) ꢁmax/cmꢃ1]: 3067 (Ar–CH), 2919 (Ar–CH), 2854 (Ar–CH), 1609
(C¼N), 1528 (C¼C), 1138, 1089, 920, 829, 727 cmꢃ1 (Pc skeleton vibration). 13C-NMR
(solid state) ꢂ/ppm 219, 162, 150–120, 20. UV-Vis (solid state) ꢃmax/nm 484, 603, 667,
785.
2.3.3. 2,9,16,23-Tetra-{5-(4-chloro-phenyl)[1,3,4]-oxadiazole}-nickel phthalocyanine (3c).
Yield: 86%. Calcd: C (59.80), H (2.20), N (17.43). Found: C (59.73), H (2.03),
N (17.04). IR [(KBr) ꢁmax/cmꢃ1]: 3067 (Ar–CH), 2923 (Ar–CH), 2848 (Ar–CH), 1603
(C¼N), 1530 (C¼C), 1092, 926, 837, 734 (Pc skeleton vibration). 13C-NMR (solid state)
ꢂ/ppm 213, 162, 150–120. UV-Vis (solid state) ꢃmax/nm 482, 604, 668, 784.
2.3.4. 2,9,16,23-Tetra-{5-(4-nitro-phenyl)[1,3,4]-oxadiazole}-nickel phthalocyanine (3d).
Yield: 80%. Calcd: C (57.89), H (2.13), N (21.10). Found C (57.8), H (1.97), N (20.04).
IR [(KBr) ꢁmax/cmꢃ1]: 3067 (Ar–CH), 2923 (Ar–CH), 2854 (Ar–CH), 1606 (C¼N), 1523

Nickel phthalocyanine
4053
(C¼C), 1338, 1094, 926, 851, 725 (Pc skeleton vibration). 13C-NMR (solid state) ꢂ/ppm
223, 162, 150–120. UV-Vis (solid state) ꢃmax/nm 481, 603, 667, 787.
3. Results and discussion
3.1. Synthesis
The target 2,9,16,23-tetraaryl 1,3,4-oxadiazole-substituted NiPcs (3a–3d) were obtained
in good yield through the melt condensation of TCPNi with four different aromatic
hydrazides 2a–2d in the presence of PPA as a solvent and condensing agent
(scheme 1). A similar reaction employing POCl3 for cyclization resulted in very poor
yield (528%).
3.2. IR absorption spectra
FT-IR spectra for 3a–3d exhibited a series of absorptions at 725–735, 840–851, 920–926,
1089–1094, and 1142–1138 cmꢃ1, attributed to the Pc skeleton. Peaks at 1603–1614 and
1528–1530 cmꢃ1 for 3a–3d can be assigned to aromatic –C¼N– and –C¼C– in plane
skeletal vibrations of the Pc core. Peaks at 3030–3070 and 2930–2850 cmꢃ1 are associated
with aromatic –C–H stretching vibrations; a peak in the region 1088–1095 cmꢃ1 is due to
aromatic –C–H bending vibrations. Comparison of FT-IR spectra of TCPNi and 3a–3d
reveals some marked differences; the characteristic intense absorption due to –C¼O
(–COOH) of TCPNi at 1697 cmꢃl disappear in IR spectra of 3a–3d, indicating
involvement of carboxy in condensation followed by cyclization with acid hydrazides.
The characteristic stretching vibrations due to –C¼N– and –C–O– of the oxadiazole ring
in 3a–3d is coupled to skeletal vibrational peaks of –C¼N– and –C¼C– of the Pc core.
Emergence of two intense bands at 1523 and 1338 cmꢃ1 for 3d was assigned for NO2
Scheme 1. Synthesis of tetraaryl-[1,3,4]-oxadiazole-substituted NiPc.

4054
M.N.K. Harish et al.
asymmetric and symmetric stretches, respectively. Characteristic –CH3 and –C–Cl
functionality in 3b and 3d is shown by peaks at 2919 and 514 cmꢃ1, respectively.
3.3. Solid-state 13C-NMR spectra
Solid-state 13C-NMR spectra of 3a–3d exhibit the expected broad signal at
120–150 ppm and an additional peak at 162–165 ppm for aromatic carbons and
aromatic carbons attached to nitrogen, respectively. Peaks at 213–221 ppm were
assigned to carbons attached to the electron deficient oxadiazole in 3a–3d. A peak at
20 ppm confirms the presence of –CH3 in 3b (Supplementary material).
3.4. Electronic absorption spectra
Marked difference is apparent in representative solid-state electronic absorption spectra
of unsubstituted NiPc and 3a–3d (figure 1) and results are summarized in table 1.
Electronic spectra recorded from 3a–3d in solid state exhibit characteristic Q-band in
the region ꢃ ꢁ 600 nm and a second band of lower intensity in near-IR region at
ꢃ ꢁ 780 nm. The electron-acceptor oxadiazole pendants induced redshift and splitting of
Figure 1. Solid-state electronic absorption spectra for NiPc and 3a–3d.
Table 1. Solid-state electronic absorption data for
NiPc (1) and 3a–3d.
NiPc
Peaks ꢃmax (nm)
1
3a
3b
3c
3d
577
497, 602, 666, 768
484, 603, 667, 785
482, 604, 668, 784
481, 603, 667, 787

Nickel phthalocyanine
4055
Table 2. Optical band gap data for NiPc (1) and
3a–3d.
Peaks
ꢃmax (nm)
Band
gap (eV)
NiPc
1
3a
577
602
768
603
785
604
784
603
787
2.04
1.974
1.461
1.974
1.444
1.974
1.446
1.974
1.461
3b
3c
3d
the Q-band due to a2u-to-eg and b2u-to-eg transitions, indicating effective electronic
communication between the two different ring systems of the Pc core [26, 27].
Optical band gaps for 3a–3d were determined by the absorption edge of the
absorption spectrum of each compound in the solid state, and the data are summarized
in table 2.
Since interactions of MPc molecules are of van der Waals type, rearrangements of
molecule alter the energy band gap between valence and conduction bands. A decrease
in optical band gap for NiPcs 3a–3d is observed by the introduction of aryl oxadiazole
pendants to the NiPc periphery as compared to NiPc. In 3a–3d, the Ni is in a position of
nearest approach to nitrogens in parallel molecules, and the presence of heteroaromatic
oxadiazole and phenyl rings in the Pc periphery results in extended conjugation and
increased orbital overlap between parallel pairs of molecules influence the stacking
arrangements and crystal phase of the Pc molecules [1, 2, 28, 29].
3.5. Scanning electron microscopy
Morphology of the synthesized NiPcs was studied by SEM analysis. The particulates of
3a–3d are not uniform in shape (figure 2); the diameter of particulates was in the range
400–5000 nm. NiPc attached to the aryl oxadiazole pendant can act as a nucleation
center, which further induces growth of Pc crystallite. Since the aryl oxadiazole
pendants were distributed along the periphery of the NiPc core, the size of crystallite
was restricted by the accessibility of adjacent Pc molecules. On the contrary,
aggregation of oxadiazole-substituted NiPc can hardly be prevented.
3.6. Thermal analysis
Analysis of TG curves of 3a–3d indicate three-step decomposition. The first step at
30–105ꢂC may be accounted for by loss of water. The second degradation at 200–450ꢂC
can be accounted for by loss of peripheral phenyl. The third step from 400 to 900ꢂC
may be due to the Pc core [30].

4056
M.N.K. Harish et al.
Figure 2. SEM micrographs for 3a–3d.
Figure 3. Powder XRD patterns for NiPc and 3a–3d.
3.7. XRD studies
The powdered XRD pattern of NiPc and 3a–3d are shown in figure 3. The diffraction
pattern of the oxadiazole-substituted Pc exhibited characteristic broad peaks.

Nickel phthalocyanine
4057
Table 3. Powdered XRD data for NiPc (1) and
3a–3d.
Lattice
spacing
˚
(d) (A)
2ꢄ
(degrees)
Relative
intensity
NiPc
1
5.70
27.01
26.23
5.76
6.53
7.87
25.20
5.10
6.00
7.62
26.48
5.16
6.91
7.34
27.38
5.15
6.32
7.34
15.4860
3.2975
3.1427
15.3036
13.5151
11.495
100
74.3
100
98.7
86.7
75.4
100
73.8
93.8
76.4
100
89.1
88
3a
3b
3c
3d
3.4902
17.3135
14.7060
11.5925
3.3626
17.1024
12.7669
12.0285
3.2538
86.8
100
79
56.9
49.6
17.1315
13.9668
12.0221
The heteroaromatic oxadiazole and phenyl rings in the Pc result in extended
conjugation and the greater ꢀ-electron region influences the stacking of the Pcs in
parallel molecules. The interplanar distances for 3a–3d are less than the parent NiPc,
due to the aryl oxadiazole pendants. Data (2ꢄ, lattice spacing, and relative intensity) are
provided in table 3.
3.8. DC electrical conductivity studies
The values of room temperature DC electrical conductivity observed for 3a–3d are ꢀ105
times higher than the parent NiPc [31], in the order 3d 4 3c 4 3b 4 3a.
Increase in conductivity for 3a–3d can be attributed to substitution at the Pc
periphery, resulting in extended conjugation. The oxadiazole as an electron-deficient
ring system can act as a ꢀ-electron acceptor. The presence of 12 heteroatoms in the Pc
periphery influences the stacking of the Pc molecules increasing the orbital overlap
between parallel pairs of molecules. This results in decrease in the intersegment spacing
and provides a pathway for charge carriers.
The temperature dependence of conductivity can be expressed as
ꢀ
ꢁ
ꢃDE
ꢅ ¼ ꢅ0 exp
,
kT
where ꢅ is the DC electrical conductivity at temperature T, ꢅ0 is the pre-exponential
factor, k is Boltzmann’s constant, and DE is the activation energy.
Figure 4(a) shows variation of resistivity with temperature. For 3a–3d, the resistivity
decreases with increase in temperature. Further, figure 4(b) represents the Arrhenius

4058
M.N.K. Harish et al.
Figure 4. (a) Variation of resistivity with temperature for 3a–3d and (b) Arrhenius plots for 3a–3d.
Table 4. Activation energies for 3a–3d.
Conductivity
ꢅDC at 300 K
DE (300–500 K)
(eV)
NiPc
(mho cmꢃ1
)
3a
3b
3c
3d
2.14046 ꢄ 10ꢃ5
2.67069 ꢄ 10ꢃ5
3.11583 ꢄ 10ꢃ5
4.52733 ꢄ 10ꢃ5
0.108
0.105
0.102
0.101
plots of log ꢅDC (DC conductivity) versus 1/T (reciprocal of temperature) for 3a–3d
from 300 to 500 K. The thermal activation energies (DE) obtained from Arrhenius plots
are shown in table 4; DE for 3a–3d is considerably less than the parent NiPc [31]. The
plots reveal that temperature has pronounced effect on DC conductivity. All the newly
synthesized Pc molecules are semiconducting in nature. The increase in DC electrical
conductivity of 3a–3d with increase in temperature can be explained for conduction
occurs by electron transfer across insulating gaps in the conducting pathway by

Nickel phthalocyanine
4059
fluctuation-induced tunneling model, proposed by Sheng [32] and hopping at the
localized states in the amorphous regions between the small metallic regions [33].
4. Conclusion
We have shown that NiPc decorated with aryl-[1,3,4]-oxadiazole pendants, 3a–3d,
exhibit improved thermal stability and could absorb visible light from 400 to 800 nm.
The presence of electron-acceptor oxadiazole pendants resulted in fine-tuning of the
position of the Q-band with decreased optical band gap. The room temperature DC
electrical conductivity for 3a–3d is ꢀ105 times higher than the parent NiPc and it
increases with increase in temperature with significant decrease in thermal activation
energy (DE). This study has revealed that these new Pcs can serve as an excellent optical,
sensor, and charge-carrier semiconducting materials with high chemical and thermal
stabilities [34]. Further, investigation on electrocatalytic properties of aryl-[1,3,4]-
oxadiazole-substituted NiPcs is in progress.
Acknowledgments
The authors thank Prof. S. Natrajan, IISc, Bangalore for providing solid-state
electronic absorption spectra, as well as NMR Research Center, IISc, Bangalore for
providing solid-state 13C-NMR spectra.
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