High frequency dielectric response in a branched phthalocyanine

Article abstract of DOI:10.1021/ja063796w

We describe the dielectric effects in a novel branched phthalocyanine system. The synthesis and characterization of the hyperbranched structure are provided. The dielectric constant was 45 over many decades of frequency, and the dispersion was small up to

Full text of DOI:10.1021/ja063796w

Published on Web 10/27/2006
High Frequency Dielectric Response in a Branched Phthalocyanine
Meng Guo,† Xingzhong Yan,† Young Kwon,‡ Teruaki Hayakawa,‡ Masa-aki Kakimoto,‡ and
Theodore Goodson III*,†
Department of Chemistry, The UniVersity of Michigan, 930 North UniVersity, Ann Arbor, Michigan 48109, and
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8550, Japan
Received May 30, 2006; E-mail: tgoodson@umich.edu
Scheme 1. Synthetic Route and Structure of the Branched Pc
Used in This Study
Recent interests in the use of novel polymeric materials for high
dielectric constant effects have increased owing to their possible
application in high-energy density and pulsed capacitors.1,2 In
particular, materials which demonstrate desired dielectric properties
at high operational frequencies (>1 kHz) are highly sought after.
Existing polymeric dielectrics, such as ceramic-polymers3 and
metal-polymeric composites,4 typically exhibit a strong interfacial
polarization (Maxwell-Wagner effect5), and this effect results in
a strong dispersion of the dielectric constant. Polymeric materials
mixed with phthalocyanine (Pc) tetramers have been reported with
good dielectric response which is heavily related to a space charge
effect.6 However, the dielectric loss of these percolative systems
is generally large,3,4,6 and efforts are still necessary to improve the
stability and flexibility of these systems. The use of Pc systems is
an intriguing approach, and Pc polymer coated nanoparticles have
been prepared and suggested as good candidates for dielectric
applications.7 Pc oligomers treated with base (KOH) have shown
a large dielectric response at lower frequency, and this was
explained under the Maxwell-Wagner-Sillars model.8 While many
of these reports present exciting avenues toward creating large
dielectric responses in primarily organic materials, there has not
been much attention to the intrinsic dielectric high-frequency
response of all-organic systems. In this Communication, we report
a strategy toward this aim utilizing an organic multichromophore
structure such as a Pc dendritic system. Because of the long-range
and fast polaron delocalization we find that a hyperelectronic
polarization dominates the large dielectric response.
Shown in Scheme 1 is the phthalocyanine dendritic system
investigated for strong dielectric response at high frequencies. The
novel hyperbranched Pc was synthesized by a copper fusion
technique consisting of 1,2-bis(3,4-dicyanophenoxy)benzene in
dimethylacetamide (DMAC) at 160 °C. The extent of the cyclo-
tetramerization reaction was monitored in real time by the increase
of the absorption extinction coefficient of the products in compari-
son to the absorption of copper phthalocyanine complex. After
extensive filtration and washing with MeOH, the products were
dried, and the resulting dark blue crystals were further analyzed.
The final structure of the branched Pc was determined by 1H-NMR
and UV-vis spectra (see Supporting Information).
in Figure 1 that the dispersion of the dielectric constant is very
small. This result suggests a major improvement over what has
been reported in the past with Pc related materials. For example,
in the case of the oligomers of Pc, the dielectric dispersion is very
large where the dielectric response drops by 3 orders of magnitude
over a similar frequency range as that seen in the result of Figure
1.8 Very small dispersion in an organic material has been also
observed in Co(acac)2-cured-expoy resin material; however, the
dielectric response at these higher frequencies in this material was
relatively small (∼6 at 1 MHz).3a It has been suggested that for
these systems such a small dispersion may be a result of the
formation of large polar moieties.3a From our measurements of the
previous similar systems, we find that this may also be the case
for the hyperbranched Pc system where a long-range polaron
delocalization in the branched structure gives rise to a small
dispersion at high frequencies.9,10
Also observed in Figure 1 is a very low loss even at higher
frequencies for this pristine dendritic Pc system. The loss is less
than 0.01 at 1 MHz. This is an unprecedented result for such a low
Measurements of the dielectric response up to 1 MHz were
carried out with the pressure compressed pellets of the pristine
material. Shown in Figure 1 is the dielectric response for a
compressed pellet of the hyperbranched Pc illustrated in Scheme
1. The thickness of the compressed pellet shown in Figure 1 was
∼76 µm. As it can be seen from Figure 1, with this pellet a dielectric
response of ∼46 is obtained even at frequencies of up to 1 MHz.
This value is significant and is larger than that observed for the
CuPc complex which was only 5.3 at 50 kHz.1 It can also be noted
† The University of Michigan.
‡ Tokyo Institute of Technology.
Figure 1. Dielectric dispersion curves of the hyperbranched Pc polymer
compressed pellet with a thickness of 76 µm.
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10.1021/ja063796w CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
(σAC) of this pristine film exhibited a behavior which can be fitted
by a power law with an s value in the range of ∼0.8 (Figure 2b).
This value for s suggests that the dominant mechanism of transport
is a polaron hopping mechanism and may be related to the
disordered dielectric effects in the system.15 From our results on
this and similar systems we have found that the polaron hopping
can be relatively fast.9 For example, with the use of time-resolved
spectroscopy we have found that the hopping time can be as fast
as ∼1 ps.9 Such a fast hopping would suggest a relatively strong
long range interaction of the branched structure and this is an
intrinsic effect. It is interesting to note that this was not the case
for the pressure compressed pellet samples of the hyperbranched
Pc, where a value of s greater than unity was obtained and this
may be related to the fabrication procedure used to make the
samples.
In conclusion, we have found a novel organic branched Pc
dendrimer system which shows impressive dielectric properties with
very low loss at high operational frequencies. Compared to
percolative systems, this dendritic system shows very low loss at
higher frequencies and a small dispersion, as well as improved
stability and flexibility. Measurements were carried out with both
pressure compressed pellets and thin films of the Pc dendrimer
system. The mechanism for this impressive high-frequency response
with relatively small dispersion is suggested to be due to a long-
range polaron hopping mechanism accompanied by strong intra-
molecular interactions in the branched system.
Figure 2. (a) Dielectric dispersion curves and (b) AC conductance of the
branched Pc film.
loss, as previous results for Pc related polymers and oligomers had
shown losses (tan δ) up to 0.7 for the polymer composite and above
1.0 for the Pc oligomers.6,8,11 For the purpose of comparison, we
also examined the dielectric properties of the Pc monomer: the
dielectric constant was relatively small (∼5) and the dielectric loss
was high (up to 0.3). Although other results have been reported
with the Pc oligomers and polymers, these results with the dendritic
Pc system present a major improvement which we attribute to the
long-range interactions in the hyperbranched structure.
To probe the transport mechanism which yields this impressive
dielectric response in the dendritic Pc system, we carried out the
analysis of the AC conductance. The AC conductance was analyzed
by the expression given as σAC ) ꢀ0ꢀr tan δ ω, where ꢀ0 is the
permittivity of the vacuum, ꢀr is the real part of the dielectric
constant, tan δ is the dielectric loss, and ω is the angular frequency.
Measurements with the AC conductivity of the Pc samples showed
a frequency dependent conductivity which could be fitted by a
power law given as σAC ) A + Bωs, where A and B are constants
related to the DC (zero frequency conductivity) and s is a fitting
parameter. The AC conductance of the pressure compressed pellet
with a thickness of 76 µm showed the super-linear power law
dependence with an s value of ∼1.70. It has been suggested that
the s parameter may give an indication of the charge transport
mechanism in the system. For example, a value of the s parameter
of 1.75 was obtained for a metallorgano Pc film.12 Also, low
temperature measurements of CuPc oligomers have found a
superlinear dependence as well.11 The explanation common to both
of these cases in regards to the transport mechanism was heavily
based on polaron tunneling in the system. However, for this situation
the tunneling of polarons occurs on a relatively short length scale
(short range interactions).12 For the case of the compressed pellet
sample of the dendritic Pc system, the contribution from a polaron
tunneling mechanism is reasonable in describing the transport
properties. It should be mentioned that in the case of the dendritic
structure we have found that the interactions may extend over a
number of repeat units and thus over larger length scales.13 This
effect may be sensitive to the macromolecular order exhibited in
the dendritic system which may be a function of the fabrication
(packing) of the solid-state system (compressed pellet or thin film).
To probe the importance of the morphology of the system once
fabricated in the solid-state, we also carried out measurements of
films of various thicknesses and concentrations of the branched Pc
system. Shown in Figure 2 is the result for a film of the branched
Pc coating on an Al substrate. This system also showed a very
small dispersion and the dielectric constant was close to ∼15 at 1
MHz. A very small dielectric loss (on the order of 0.001) was
obtained at high frequencies (Figure 2a). Compared to TiO2
functionalized polystyrene nanocomposites (ꢀr ) 6∼10),14 the
dielectric performance of the hyperbranched Pc films meets the
stiff requirements for gate dielectrics and of organic transistor
applications. Our measurements of different film’s AC conductance
showed no apparent thickness dependence. The AC conductance
Acknowledgment. This work is supported by the Office of
Naval Research.
Supporting Information Available: Synthesis and characterization
of the hyper-branched Pc polymer. This material is available free of
charge via the Internet at http://pubs.acs.org.
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