The synthesis, photophysical and dielectric properties of ball-type dinuclear zinc phthalocyanine

Article abstract of DOI:10.1142/S1088424612500836

The synthesis of ball-type dinuclear Zn(II) phthalocyanine containing four 4,4′-(9H-fluorene-9,9-diyl)diphenol substituents at the non-peripheral position is presented. The structure of the synthesized compound was characterized using elemental analyzes,

Full text of DOI:10.1142/S1088424612500836

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2012; 16: 1–7
DOI: 10.1142/S1088424612500836
Published at http://www.worldscinet.com/jpp/
The synthesis, photophysical and dielectric properties
of ball-type dinuclear zinc phthalocyanine
a
b
c◊
Mevlüde Canlıca , Ahmet Altındal and Tebello Nyokong*
a
Yildiz Technical University, Chemistry Department, Inorganic Chemistry Division, Davutpasa Campus, 34220
Esenler, Istanbul-Türkiye
Yildiz Technical University, Physics Department, Davutpasa Campus, 34220 Esenler, Istanbul-Türkiye
b
c
Rhodes University, Chemistry Department, P.O. Box 6140, Grahamstown, South Africa
th
Dedicated to Professor Tebello Nyokong on the occasion of her 60 birthday
Received 13 January 2012
Accepted 22 March 2012
ABSTRACT: The synthesis of ball-type dinuclear Zn(II) phthalocyanine containing four
4
,4′-(9H-fluorene-9,9-diyl)diphenol substituents at the non-peripheral position is presented. The
structure of the synthesized compound was characterized using elemental analyzes, and UV-vis, FT-IR,
1
H NMR and mass spectroscopies. The F value was 0.16 and F value was 0.72. The complex showed
F
T
reasonably long triplet lifetimes with t 7210 ms in DMSO. The frequency and temperature dependence
of the dielectric properties of ZnPc were also investigated in the frequency range of 40–10 Hz and in
T
5
the temperature range of 300–440 °K. It has been observed that both dielectric constant e′ and dielectric
loss e′′ decrease with the rise in frequency as they increase with the rise in temperature. The decrease
in e′ with increasing frequency is attributed to the fact that as the frequency increases, the polarizability
contribution from orientation sources decreases and finally disappears.
KEYWORDS: Zn ball-type phthalocyanine, photodynamic therapy, non-peripheral phthalocyanine,
triplet quantum yield, dielectric properties.
1
forming excited singlet state oxygen ( O ). High triplet
INTRODUCTION
2
state quantum yields and long triplet lifetimes are required
Metallophthalocyanines (MPcs) have been studied as
materials for many applications including in electronics
for efficient photosensitization. Synthesis, electrochemical
and spectroelectrochemical behavior of ball-type Pc
derivatives have been extensively studied since the
complexes were published in the literature for the first
time in 2002 [15, 16]. Ball-type phthalocyanine derivatives
show spectroscopic and electrochemical properties which
differ significantly from the parent monomer [17–24].
However, the studies on non-peripherally substituted
ball-type MPc derivatives are still limited and the
photophysical behavior of ball-type molecules in general
have also not received much attention. Our interest in ball-
type complexes is due to the possibilities of intramolecular
interactions between the Pc rings and/or metal centers of
these compounds. The electronic properties of ball-type
Pcs can change dramatically depending on the bridging
compounds or the central metal. The distance between the
[
1], non-linear optics [2], liquid crystals [3], gas sensors [4],
photosensitizers [5, 6], electrocatalysis [7], semiconductors
8] and photovoltaic cells [9]. There is considerable interest
[
in MPc complexes containing non-transition metals for
use as photosensitizers in the relatively new method of
cancer treatment called photodynamic therapy (PDT) [5, 6,
10–14]. The mode of operation in PDT is based on visible
light excitation of a tumor-localized photosensitizer. After
excitation, energy is transferred from the photosensitizer
3
(in its triplet excited state) to ground state oxygen ( O ),
2

SPP full member in good standing
*
Correspondence to: Tebello Nyokong, email: t.nyokong@
ru.ac.za, tel: +27 46-6038260, fax: +27 46-6225109
Copyright © 2012 World Scientific Publishing Company

2
M. CANLICA ET AL.
two Pc units affects the degree of interactions between the
rings.
laser flash photolysis system, the excitation pulses were
produced by a Quanta-Ray Nd: YAG laser providing
400 mJ, 9 ns pulses of laser light at 10 Hz, pumping a
Lambda-Physic FL 3002, dye (Pyridin 1 in methanol).
The analyzing beam source was from a Thermo Oriel
xenon arc lamp, and a photomultiplier tube was used
as detector. Signals were recorded with a two-channel
300 MHz digital real-time oscilloscope (Tektronix
TDS 3032C); kinetic curves were averaged over 256
laser pulses. The triplet lifetimes were determined by
exponential fitting of the kinetic curves using the program
OriginPro 7.1.
The electrical conductivity and relaxation process in
phthalocyanine compounds play an important role and
these are often the deciding factors about the suitability of
the material for particular device applications. Therefore,
the extraction of true intrinsic nature of electrical
conduction and dielectric mechanism is essential for a
material.
In this paper, we describe the synthesis and
photophysicalpropertiesofnewsymmetricallysubstituted
ball-type ZnPc complex containing substituents at the
non-peripheral position (a). The dielectric behavior of
the compound was also investigated as a function of
frequency and temperature in the ITO/ZnPc/Au structure.
This is the first time that the dielectric behavior of a ball
type phthalocyanine is presented in the ITO/ZnPc/Au
configuration.
Synthesis
3
-Nitrophthalonitrile 1 was synthesized according
to literature [25]. 4,4′-(9H-fluorene-9,9-diyl)diphenol 2
was obtained from commercial suppliers. The synthesis
of compounds 3 has been reported in literature [22]. All
solvents were dried and purified as described by Perrin
and Armarego [26]. Complex 4 was prepared using solid
phase in presence a metal salt (zinc(II) acetate) and the
phthalonitrile derivative 3 (Scheme 1).
EXPERIMENTAL
Materials
Anhydrous zinc(II) acetate was pur-
chased from Sigma-Aldrich. All solvents
for example dimethylsulfoxide (DMSO)
and chloroform were from Saarchem.
Silica gel for column chromatography was
purchased from MERCK. All other reagents
were obtained from suppliers and used as
received.
NC
O N
CN
HO
HO
2
+
(
1)
(2)
(
i)
NC
CN
Equipments
O
UV-vis absorption spectra were obtained
using the Varian Cary 500 UV-vis/NIR
spectrometer. Fluorescence excitation and
emission spectra were recorded with Varian
Eclipse spectrophotometer. FT-IR data were
O
NC
CN
(
3)
ii)
recorded using the Perkin-Elmer spectrum
1
2
000 FTIR spectrometer. H NMR spectra
(
were obtained using a Bruker EMX 400
MHz spectrometer. Elemental analyses
were done on a Vario-Elementar Microcube
EL III. Mass spectral data were collected
with a Bruker AutoFLEX III Smartbeam
MALDITOF/TOF Mass spectrometer. The
instrumentwasoperatedinpositiveionmode
using a m/z range of 400–3000. The voltages
of the ion sources were set at 19 and 16.7 kV
for ion sources 1 and 2 respectively, while
the lens was set at 8.50 kV. The reflector 1
and 2 voltages were set at 21 and 9.7 kV
respectively. The spectra were acquired
using dithranol as the MALDI matrix, using
a 354 nm nitrogen laser. Triplet absorption
and decay kinetics were recorded on a
O
O
N
N
N
N
N
N
Zn
N
N
O
O
O
N
O
N
N
N
N
N
Zn
N
N
O
O
(
4)
Scheme 1. Non-peripherally substituted ball-type metallophthalocyanine. (i):
DMSO, rt, 5 days. (ii): Zn(CH COO) , reflux, n-pentanol, overnight for 4
3
2
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 2–7

THE SYNTHESIS, PHOTOPHYSICAL AND DIELECTRIC PROPERTIES
3
1
′,11′,15′,25′-[tetrakis(4,4′-(1,1′-binaphthyl-8,8′-
Quantum yields of internal conversion (F ) were
IC
diyl(oxy))diphenyl)]bis-phthalocyaninato dizinc(II)
4). A mixture of complex 3 (0.100 g) and excess of
obtained from Equation 4, which assumes that only
three processes (fluorescence, intersystem crossing and
internal conversion), jointly deactivate the excited singlet
state of complex 4.
(
zinc(II) acetate (0.100 g) was heated in sealed glass tube
for 24 h under argon atmosphere at reflux temperature in
n-pentanol. After cooling to room temperature, the blue-
green reaction product was washed with hot methanol
and hot water. The product was separated by column
chromatography on silica gel with chloroform and
gradient of chloroform-methanol up to 50%. Yield 0.013
ꢀ
F = 1 – (F + F )
(4)
IC
F
T
Dielectric measurements
In order to prepare thin film of the ZnPc organic
compound, firstly indium tin oxide (ITO) coated glass
substrate was washed successively with ethanol/acetone
mixture and de-ionized water, and then dried in a vacuum.
The compound was dissolved in DMF at concentrations
-
1
-1
g. UV-vis (CHCl ): l , nm (log e, M .cm ) 696 (5.69),
3
max
-1
6
3
(
27 (4.91), 322 (5.61), 309 (5.78). IR (KBr): lmax, cm
1
052 (Ar-CH), 1575 (C=C), 1255 (C-O-C). H NMR
DMSO-d ): d, ppm 8.02–6.99 (88H, Ar-H). Anal. calcd.
for C H N O Zn : C, 77.51; H, 3.49; N, 8.82. Found:
C, 77.49; H, 3.50; N, 8.85%. MALDI-TOF-MS: m/z
calcd. 2541.38, found [M + H] 2542.51.
6
−3
1
64 88 16
8
2
of 5 ×ꢀ10 M. Twenty microlitres of this solution was
added with a glass pipette onto the ITO substrate held onto
spinner (Speciality Coatings Systems Inc., Model P6700
Series). The substrate was spun at 2000 rpm for 80 s and
then the film was dried at 100 °C for 2 h to evaporate
the solvent. The substrate temperature was kept constant
at 300 °K during deposition of the Pc compounds. After
spinning process, circular dots of 1 mm in diameter and
+
Photophysical studies
Fluorescence quantum yields. Fluorescence quantum
yields (F ) were determined by the comparative method
27, 28], Equation 1:
F
[
1
500 Å thick Au contacts were deposited onto the Pc
2
F.A .n
Std
surface of the ITO substrate through a metal shadow mask
F = F
F
F(Std)
2
Std
(1)
-5
F .A.n
Std
in high vacuum system in the pressure of 1.5 × 10 mbar.
The device was connected to Keithley 3330 LCZ meter
for impedance measurement equipped with a suitable
coaxial cable. Impedance measurements were performed
where F and FStd are the areas under the fluorescence
curves of 4 and the standard, respectively. A and AStd are
the respective absorbances of the sample and standard
at the excitation wavelengths (which was ~0.05 in all
solvents used), and n and nstd are the refractive indices of
solvents used for the sample and standard, respectively.
between 300 °K and 450 °K and in the frequency range,
5
4
0–10 Hz. All measurements were performed under
-3
vacuum (≤10 mbar) and in the dark. Impedance data
were recorded using an IEEE-488 data acquisition system
incorporated into a personnel computer.
ZnPc (F = 0.20) was employed as the standard [29].
F
Triplet quantum yields and lifetimes. The solutions
for triplet quantum yields and lifetimes were introduced
into a 1.0 mm pathlength UV-visible spectrophotometric
cell, deaerated using nitrogen and irradiated at the Q-band
RESULTS AND DISCUSSION
maxima. Triplet state quantum yields (F ) of 4 were
determined by the triplet absorption method [30] using
zinc phthalocyanine (ZnPc) as a standard, Equation 2:
T
Synthesis and characterization
The ball-type binuclear Zn(II)Pc complex 4 was
prepared as a mixture of isomers by the reaction of
compound 3 with Zn(CH OO) . Complex 4 was prepared
Std
T
DA .e
Std
T
T
Std
3
2
F = F
.
(2)
T
DA .e
using solid phase method in the presence of Zn(CH
at 320 °C from the phthalonitrile derivative 3. Column
chromatography on silica gel using CHCI as mobile phase
was used twice to purify the complexes. The structure and
purity of the ZnPc derivative was confirmed by UV-vis, H
NMR, IR and mass spectral data and elemental analyses.
A diagnostic feature of the formation of 4 from the
phthalodinitrile derivatives 3, is the disappearance of
3 2
OO)
T
T
Std
where DA and DAT are the changes in the triplet state
3
T
absorbances of 4 and the standard, respectively. e and
T
Std
1
eS are the triplet state molar extinction coefficients for 4
Std
and the standard, respectively. FT is the triplet quantum
yield for the standard, ZnPc (F = 0.65 in DMSO) [31].
T
Std
e and e were determined from the molar extinction
T
S
-
1
coefficients of their respective ground singlet state (eS
the sharp CN vibration of the latter at 2228 cm . The
Std
1
and e ) and the changes in absorbances of the ground
H NMR spectra of 4, recorded in DMSO, shows the
S
Std
singlet states (DA and DA ), and excited triplet states
aromatic protons between 8.02–6.99 ppm, integrating for
a total of 88 for protons. Elemental analysis results are
also consistent with the theoretical values of proposed
structures of 4. The purified phthalocyanines were further
characterized by mass spectra. The expected mass values
S
S
Std
(
DA and DA
T
) according to Equation 3:
T
DAT
S
e = e .
(3)
T
DAS
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 3–7

4
M. CANLICA ET AL.
Fluorescence spectra and quantum yields
The absorption, fluorescence excitation and
emission spectra of complex 4 in DMSO are
shown in Fig. 2. The absorption and excitation
spectra show the same Q-band maxima. The
proximity of the wavelength of each component
of the Q-band absorption to the Q-band maxima
of the excitation spectra for the complex suggests
that the nuclear configurations of the ground and
excited states are similar and not affected by
excitation. The emission spectra are mirror images
of the excitation spectra. Stoke’s shifts were 9 nm,
typical of ZnPc complexes [21, 23, 24].
Fig. 1. Absorption spectra of complex 4 in CHCl at concentration
3
-
6
5
.3 × 10 M
The fluorescence quantum yield (F ) values are typical
F
corresponded with the found values for complex using
dithranol as the matrix. The results were repeated twice to
give the observed molecular peaks. Protonated molecular
ion peak ZnPc is found at 2542.51 amu.
of MPc complexes for the ZnPc derivative. The F values
F
was 0.16 in DMSO for 4. Face-to-face interaction of
the two monomers in dimers is expected to decrease the
energy gap between the singlet state and the triplet state
and enhance the formation of triplet state (i.e. intersystem
crossing increases) decreasing fluorescence [21, 23, 24].
The UV-vis spectrum of 4 in CHCl shown in Fig. 1.
3
The phthalocyanines show typical electronic spectra
with two absorption regions, one in the UV region at
about 300–400 nm (B-band) and the other in the visible
region at 600–700 nm (Q-band). The characteristic
Q-band transition of metallophthalocyanines with D4h
symmetry is observed as a single band of high intensity
in the visible region. Complex 4 show Q-band absorption
However, in this work the F values are similar to those
F
of monomeric phthalocyanines, showing the interaction
between the two rings in minimal.
Triplet quantum yields and lifetimes
Figure 3 shows a representative triplet decay curve
for the complex. Triplet quantum yields represent the
at 696 nm both in CHCl and DMSO and B-bands
3
absorptions at 309 and 322 nm in CHCl . The
3
Q-band for 4 is red shifted as is typical of
non-peripheral substitution. It is well-known
that a substitution result in red shifting of
the spectra in MPcs this is due to the electron
density enhancement caused by the substitution
at a position [20, 22]. However, complex 4 in
CHCl has an extra band near 742 nm, which
3
could be associated with loss of symmetry often
observed in chlorinated solvents. This band was
not observed in DMSO. The observation of the
extra band in chloroform is due to protonation
since this solvent may contain small amounts of
acid as observed before [24].
Fig. 2. Absorbance (a), excitation (b), emission (c) spectra of complex 4 in
DMSO, excitation lmax = 623 nm
Aggregation behavior of Pc is depicted as a
coplanar association of rings progressing from
monomer to dimer and higher order complexes
and it is dependent on concentration, nature
of solvent and substituents, metal ions and
temperature [32]. Aggregation in MPcs is
typified by broadened or split Q-band, with the
high energy band being due to the aggregate
and the low energy band due to the monomer. A
well defined band between 610 nm and 675 nm
in ball-type phthalocyanines is due to exciton
coupling between the two Pc rings [17–24].
The spectra of complexes 4 in DMSO shows
bands in this region, which may be attributed to
intermolecular interactions between the rings in
agreement with literature.
Fig. 3. Decay curve of complex 4 in DMSO
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 4–7

THE SYNTHESIS, PHOTOPHYSICAL AND DIELECTRIC PROPERTIES
5
fraction of absorbing molecules that undergo intersystem
crossing to the metastable triplet excited state. The triplet
quantum yields (F ) value for 4 is 0.72. High F values
T
T
and correspondingly low F values are observed for ZnPc
F
derivative, due to a more efficient intersystem crossing
(
(
ISC), for the complex. Complex 4 have low F values
IC
= 0.11). The complex showed reasonably long triplet
lifetime of tT = 7210 ms.
Dielectric behaviors
Dielectric properties represent the response of the
materialtothedisturbancecausedbytheexternallyapplied
electric field. The components of the complex dielectric
function, e*(w) = e′(w) - je′′(w), were calculated from the
measured impedance spectra using the expression [33],
Fig. 4. The variation of real part of dielectric function with
frequency at indicated temperatures
Y*
C
G
e* =
e′
(
w
)
- je′′
(
w
)
=
- j
(5)
jwC0
C0
wC0
where Y* is the measured admittance, C0 is the
capacitance of the empty measuring cell, C and G are the
measured capacitance and conductance of the Pc and w is
the angular frequency of the applied signal. The real part,
e′(w), of the complex dielectric function is usually called
the dielectric constant and the complex part, e′′(w), the
dielectric loss or dielectric loss factor.
The total polarizability of a material medium
is mainly due to the contribution of deformational
polarization (electronic and ionic) and relaxation
polarization (orientational and interfacial). The various
polarizabilities may be segregated from each other
because each contribution has its own characteristic
features which distinguishes it from the others. At low
frequencies, all four sources of polarization are important,
whereas at optical frequencies, i.e. above optical
phonon frequencies, only the electronic polarization
c omes into play. Dispersion curves of the real part of
the dielectric function for ITO/ZnPc/Au structure at
selected temperatures is shown in Fig. 4. As could be
noticed from Fig. 4, at low frequencies, e′ is strongly
frequency dependent, while at higher frequencies a
weak dependence is observed. A rapid decrease in e′ is
clear in the low frequencies for all temperatures. It is
also clear that the values of e′ approach a constant value
with further increase in frequency. The high values of e′
at lower frequencies can be attributed to the presence of
different types of polarizations (viz. electronic, dipolar,
interfacial, ionic, orientation, etc.) [34, 35]. A reasonable
explanation for the observed decrease in e′ with frequency
can be given as follows: When the frequency of the
applied signal is increased, the dipoles will no longer be
able to rotate sufficiently rapidly. So, their oscillations
begin to lag behind those of the field. As the frequency
is further increased, the dipole will be completely unable
to follow the field and the polarizability contribution
from orientation sources decreases and finally disappears
Fig. 5. Temperature dependence of e′ at selected fixed
frequencies
because of the frequency and temperature dependence
of orientation polarization, so e′ decreases at a higher
frequency approaching a constant value due to the
interfacial polarization [36].
Figure 5 shows the variation of e′ with temperature
at some fixed frequencies. As can be seen from Fig.
5 e′ increases with the increase in temperature and
it exhibits strong temperature dependence at higher
temperature and lower frequencies. This type of
temperature dependence of e′ reveals that ZnPc behaves
like polar dielectric in which the orientation of dipoles
is frozen at low temperatures and facilitated with rising
temperature and thereby the dielectric constant is
increased.
The dielectric loss, e′′(w), was also determined as
function of frequency and temperature. Figure 6 shows
the dependence of the imaginary part of dielectric
function, e′′, on the frequency. It is clear from the
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 5–7

6
M. CANLICA ET AL.
quantum yields and long triplet life are
required for efficient photosensitization.
From the dielectric measurements it was
found that the dielectric constant, e′,
and dielectric loss, e′′, decrease with the
increase in frequency and increase with
the increase in temperature. The observed
temperature dependence of e′ revealed that
the thin film ZnPc sample exists in the form
of molecular dipoles.
Acknowledgements
This work was supported by the
Department of Science and Technology
(
(
DST) and National Research Foundation
NRF), South Africa through DST/NRF
South African Research Chairs Initiative
for Professor of Medicinal Chemistry and
Nanotechnology and Rhodes University.
Fig. 6. Dependence of dielectric loss e′′ on frequency at indicated
temperatures. The inset shows temperature dependence of the parameter n
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