Synthesis, photophysical and photochemical properties of poly(oxyethylene)-substituted zinc phthalocyanines

Article abstract of DOI:10.1039/b617773e

The synthesis, photophysical and photochemical properties of the tetra- and octa-poly(oxyethylene)substituted zinc (ii) phthalocyanines are reported for the first time. The new compounds have been characterized by elemental analysis, IR, ¹H and

Full text of DOI:10.1039/b617773e

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www.rsc.org/dalton | Dalton Transactions
Synthesis, photophysical and photochemical properties of
poly(oxyethylene)-substituted zinc phthalocyanines†
Devrim Atilla,a Mahmut Durmus¸,a,b Ays¸e Gu¨l Gu¨rek,a Vefa Ahsena,c and Tebello Nyokong*b
Received 6th December 2006, Accepted 1st February 2007
First published as an Advance Article on the web 16th February 2007
DOI: 10.1039/b617773e
The synthesis, photophysical and photochemical properties of the tetra- and
octa-poly(oxyethylene)substituted zinc (II) phthalocyanines are reported for the first time. The new
compounds have been characterized by elemental analysis, IR, 1H and 13C NMR spectroscopy,
electronic spectroscopy and mass spectra. General trends are described for photodegradation, singlet
oxygen, triplet state and fluorescence quantum yields, and triplet and fluorescence lifetimes of these
compounds in dimethylsulfoxide (DMSO). Photophysical and photochemical properties of
phthalocyanine complexes are very useful for PDT applications. The effects of the substituents on the
photophysical and photochemical parameters of the zinc(II) phthalocyanines (3a, 5a and 6a) are also
reported. The singlet oxygen quantum yields (UD), which give an indication of the potential of the
complexes as photosensitizers in applications where singlet oxygen is required (Type II mechanism)
ranged from 0.60 to 0.72. Thus, these complexes show potential as Type II photosensitizers. The
fluorescence of the complexes was quenched by benzoquinone (BQ).
been considerable research on the syntheses of soluble metal
Introduction
phthalocyanine derivatives.2 In order to be soluble in an aqueous
media, sulfo groups are introduced on to the MPc ring.2,21–23 For
example, sulfonated chloroaluminium and zinc phthalocyanine
derivatives have been studied as potential sensitizers for PDT.21–23
The great interest in the development of these macromolecules
has led us to investigate the synthesis and properties of phthalo-
cyanines substituted with amphiphilic groups in order to evaluate
their influence on the solubility and photophysical properties.
Thiol-derivatized metallophthalocyanine complexes show rich
spectroscopic and photochemical properties. For example, they are
known to absorb at longer wavelengths24–28 than other substituted
metallophthalocyanine complexes, which is a very useful feature
for application in PDT, optoelectronics and near-IR devices. In
addition to this feature, the function of the poly(oxyethylene)
groups is to enhance the solubility of the phthalocyanines in water,
rendering them easier to administer in medical applications. Thus,
in this work we describe the synthesis of new sterically hindered
thia-bridged tetra- and octa-poly(oxyethylene)-substituted zinc(II)
phthalocyanine derivatives (Fig. 1). The influence of the presence
of the thia bridges in the poly(oxyethylene) side chains on the
photophysical and photochemical properties is presented.
Phthalocyanines (Pcs) play a major role in modern photochem-
istry. Complexation of phthalocyanines with metal ions has an
influence on their photophysical properties. These compounds
are used as catalysts as well as photoreceptors in electrographic
printing.1,2 Pcs have found applications as photosensitizers in
photodynamic therapy of cancer (PDT).3,4 All these applications
require compounds of various solubility and a high degree of
purity. Porphyrin derivatives such as hematoporphyrin derivatives
R
and Photofrinꢀ have been used as photosensitizers for PDT.5,6
Aluminium and zinc phthalocyanine derivatives have, in particu-
lar, found use as photosensitizers for PDT.6–13
In PDT, singlet oxygen is produced through transfer of energy
from the excited triplet state of the photosensitizer to the ground
state (triplet) of oxygen.14 Singlet oxygen does not only cause
tumor necrosis15 but it also induces several biological reactions.16,17
It has been found that phthalocyanines are better photosensitizers
for PDT than porphyrins, since the former exhibit effective
tissue penetration18 due to their chemical stability, photodynamic
activity, and efficient light absorption in the red region of the
electromagnetic radiation.15 Phthalocyanines have long triplet
lifetimes, and high singlet oxygen quantum yields.19 Aggregation
in phthalocyanines has a strong influence on the bioavailability,
the in vivo distribution and singlet oxygen production efficiency.20
In spite of the above properties of phthalocyanines, there
are fewer reports involving their use as photosensitizers for
PDT (in comparison with porphyrins) because of their low level
of solubility in common organic solvents. Naturally, there has
Experimental
Materials
All solvents were purified as described in Perrin and Armarego.29
1,3-Di[2-(2-ethoxyethoxy)ethoxy]-2-propanethiol (1),30 4-nitro-
phthalonitrile (2),31 4-{2-[2-(2-ethoxyethoxy) ethoxy]-1-[2-(2-
ethoxyethoxy)) ethoxymethyl] ethylsulfanyl} phthalonitrile (3),32
1,2-dichloro-4,5-dicyano benzene(4),33 4-chloro-5{2-[2-(2-ethoxy-
ethoxy)ethoxy]-1-[2-(2-ethoxyethoxy) ethoxy) ethoxymethyl]
aGebze Institute of Technology, Department of Chemistry, PO Box 141,
Gebze, 41400, Kocaeli, Turkey
bDepartment of Chemistry, Rhodes University, Grahamstown, 6140, South
Africa. E-mail: t.nyokong@ru.ac.za; Fax: 27 46 603 8260
ethylsulfanyl}
phthalonitrile
(5)32
and
4,5-di{2-[2-(2-
cTUBITAK-Marmara Research Center, Materials Institute, PO Box 21,
Gebze, 41470, Kocaeli, Turkey
ethoxyethoxy)ethoxy]-1-[2-((2-ethoxy ethoxy) ethoxy)ethoxy-
† The HTML version of this article has been enhanced with colour images.
methyl]ethyl sulfanyl} phthalonitrile (6)32 were prepared
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Photophysical parameters
Fluorescence quantum yields. Fluorescence quantum yields
(UF) were determined by the comparative method (eqn 1),34,35
2
FAStdg
FStdAg
UF = UF(Std)
(1)
2
Std
where F and FStd are the areas under the fluorescence curves of the
samples (3a, 5a and 6a) and the standard respectively. A and AStd
are the respective absorbances of the samples and standard at the
excitation wavelengths (absorbance ∼0.05), and g and gStd are the
refractive indices of solvents used for the sample and standard,
respectively. Unsubstituted ZnPc in DMSO (UF = 0.18)36 was
employed as the standard. Both the sample and standard were
excited at the same wavelength.
Fluorescence (sF) and natural radiative (s0) life times were
determined using PhotochemCAD Program37 which uses the
Strickler–Berg equation.37
Triplet quantum yields and lifetimes
The de-aerated solutions of the respective octa and tetra substi-
tuted ZnPc complexes (3a, 5a and 6a) were introduced into a 1 cm
pathlength spectrophotometric cell and irradiated at the Q band
maxima with the laser system described above. Triplet quantum
yields (UT) were determined by a comparative method using triplet
decay,38 eqn 2:
Fig. 1 Structure of the substituted zinc(II) phthalocyanines.
according to the reported procedures. Benzoquinone and 1,3-
diphenylisobenzofuran (DPBF) were purchased from Aldrich.
All other reagents and solvents were of reagent grade quality.
DASTampleeSTtd
USTample = USTtd
(2)
DASTtdeSTample
Equipment
where DATSample and DATStd are the changes in the triplet state ab-
sorbances of the samples (3a, 5a and 6a) and standard, respectively.
eSTample and eSTtd are the triplet state extinction coefficients for the
samples (3a, 5a and 6a) and standard, respectively. The standard
employed was zinc phthalocyanine (ZnPc) in DMSO. The triplet
quantum yield (UTStd) for the standard is USTtd = 0.65 for ZnPc39 in
DMSO.
Elemental analyses were obtained from a Carlo Erba 1106 Instru-
ment. Infrared spectrum in a KBr pellet was recorded on a Perkin-
Elmer 983 spectrophotometer. Absorption spectra in the UV-
visible region were recorded with a Shimadzu 2001 UV Pc spec-
trophotometer and Varian 500 UV-Vis/NIR spectrophotometer.
Fluorescence excitation and emission spectra were recorded on a
Varian Eclipse spectrofluorometer using 1 cm pathlength cuvettes
at room temperature. Mass spectra were recorded using a VG Zab-
Spec GC-MS spectrometer by fast atom bombardment techniques
Quantum yields of internal conversion (UIC) were obtained
from eqn 3, which assumes that only three processes (fluorescence,
intersystem crossing and internal conversion) jointly deactivate the
excited singlet state of octa and tetra substituted ZnPc complexes.
1
using m-nitrobenzyl alcohol (MNBA) as the matrix. H and 13C
NMR spectra were recorded in CDCl3 solutions on a Bruker
200 MHz and Varian 500 MHz spectrometers using TMS as an
internal reference.
UIC = 1 − (UF + UT)
Triplet lifetimes were determined by exponential fitting of
the kinetic curves using OriginPro 7.5 software supplied by
(3)
Photo-irradiations were performed using a General Electric
Quartz line lamp (300 W). A 600 nm glass cut off filter (Schott)
and a water filter were used to filter off ultraviolet and infrared
radiations respectively. An interference filter (Intor, 670 nm with
a band width of 40 nm) was additionally placed in the light
path before the sample. Light intensities were measured with a
POWER MAX5100 (Molelectron detector incorporated) power
meter. Triplet absorption and decay kinetics were recorded on a
laser flash photolysis system, the excitation pulses were produced
by a Quanta-Ray Nd:YAG laser providing 400 mJ, 90 ns pulses
of laser light at 10 Hz, pumping a Lambda-Physik FL3002 dye
(Pyridine 1 dye in methanol). Single pulse energy was 2 mJ. The
analyzing beam source was from a Thermo Oriel xenon arc
lamp, and a photomultiplier tube was used as a detector. Signals
were recorded with a two-channel digital real-time oscilloscope
(Tektronix TDS 360); the kinetic curves were averaged over 256
laser pulses.
R
OriginLabꢀ.
Singlet oxygen and photodegradation quantum yields
Singlet oxygen (UD) and photodegradation (Ud) quantum yield
determinations were carried out using the experimental set-up
described above.40,41 Typically, a 2 ml portion of the respective
octa and tetra substituted ZnPc (3a, 5a and 6a) solutions
(absorbance ∼1 at the irradiation wavelength) containing the
singlet oxygen quencher was irradiated in the Q band region
with the photo-irradiation set-up described in references.40,41 UD
values were determined in air using the relative method with
1,3-diphenylisobenzofuran (DPBF) as singlet oxygen chemical
quencher in DMSO (eqn 4):
Std
RI
UD = UStd
(4)
abs
D
Std
R
Iabs
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where USDtd is the singlet oxygen quantum yield for the standard
ZnPc (USDtd = 0.67 in DMSO42). R and RStd are the DPBF
photobleaching rates in the presence of the respective (3a, 5a
system. Yield : 0.06 g (18%). (Found C 57.35, H 7.05, N 5.53%.
C92H136N8O24S4Zn (1930) requires C 57.20, H 7.10, N 5.80%);
IR mmax/cm−1 (KBr cell): 3040 (ArCH), 2980-2850 (CH2), 1600
and 6a) and standard, respectively. Iabs and IStd are the rates
(ArC C), 1529, 1450, 1400, 1381, 1350, 1310, 1260, 1240,
=
abs
of light absorption by the samples (3a, 5a and 6a) and standard,
respectively. The concentrations of DPBF in the solutions were
calculated using the determined values of log e = 4.36 at 417 nm
(DPBF in DMSO). The light intensity used for UD determinations
was found to be 9.27 × 1015 photons s−1 cm−2. The error in the
determination of UD was ∼10% (determined from several UD
values).
1120-1080 (C–O–C), 930, 880; MS (FAB)m/z (%): 1931(100)
[M + 1]+, 1624 (20) [M–(CH(CH2 (OCH2CH2)2OC2H5)2)]+, 1315
(7) [M − 2(CH(CH2(OCH2CH2)2 OC2H5)2)]+, 705 (5) [M −
4(CH(CH2(OCH2CH2)2OC2H5)2)]+.
Tetrakis (2, 9, 16, 23-{2-[2-(2-ethoxyethoxy)ethoxy]-1-[2-(2-
ethoxyethoxy)) ethoxymethyl] ethylsulfanyl} 3,10,17,24-chloro) ph-
thalocyaninato zinc (II)(5a). 4-Chloro-5{2-[2-(2-ethoxyethoxy)-
ethoxy]-1-[2-(2-ethoxy ethoxy)ethoxy) ethoxy methyl] ethylsul-
fanyl} phthalonitrile (5) (0.64 g, 1.27 mmol), DBU (0.19 g,
1.27 mmol), anhydrous zinc acetate (0.079 g, 0.43 mmol) and
dried 1-hexanol (4 ml) were refluxed with stirring under an
argon atmosphere for 48 h. Then 1-hexanol was removed under
reduced pressure, and the crude green product was purified by
column chromatography (Al2O3, CH2Cl2 : methanol/100 : 1
(RF = 0.81) as eluent). Furthermore, this product was purified
with preparative TLC (silica gel) using CH2Cl2 : MeOH/20 : 1
solvent system. Yield: 0.120 g (18%). (Found C 53.80, H 6.22, N
5.60%. C92Cl4H132N8O24S4Zn (2068) requires C 53.39, H 6.43, N
5.41%); IR mmax/cm−1 (KBr cell): 3040 (ArCH), 2980-2850 (CH2),
Photodegradation quantum yields were determined using eqn 5,
(C0 − Ct)VNA
Ud =
(5)
IabsSt
where C0 and Ct are the concentrations of complexes 3a, 5a and 6a
before and after irradiation respectively, V is the reaction volume,
NA the Avogadro’s constant, S the irradiated cell area and t the
irradiation time. Iabs is the overlap integral of the radiation source
light intensity and the absorption of the samples (3a, 5a and 6a).
A light intensity of 3.09 × 1016 photons s−1 cm−2 was employed for
Ud determinations.
Fluorescence quenching by benzoquinone (BQ)
=
1600(ArC C), 1450, 1410, 1380, 1340, 1280, 1240, 1120-1080(C–
Fluorescence quenching experiments on the tetra- and octasubsti-
tuted zinc phthalocyanine complexes (3a, 5a or 6a) were carried
out by the addition of different concentrations of BQ to a fixed
concentration of the complex, and the concentrations of BQ in
the resulting mixtures were 0, 0.008, 0.016, 0.024, 0.032, 0.040
and 0.048 mol dm−3. The fluorescence spectra of tetra- and octa-
substituted zinc phthalocyanine complexes (3a, 5a or 6a) at each
BQ concentration were recorded, and the changes in fluorescence
intensity related to BQ concentration by the Stern–Volmer (SV)
equation:43
O–C), 960, 880; MS (FAB) m/z (%): 2069 (75) [M + 1]+, 1927 (10)
[M–O(CH2CH2O)2C2H5]+, 1763 (35) [M–(CH(CH2(OCH2CH2)2
OC2H5)2)]+, 1455 (8) [M–2(CH(CH2(OCH2CH2)2OC2H5)2)]+, 842
(10) [M − 4(CH(CH2(OCH2CH2)2OC2H5)2)]+.
Octakis {2-[2-(2-ethoxyethoxy)ethoxy]-1-[2-(2-ethoxyethoxy))
ethoxymethyl] ethylsulfanyl} phthalocyaninato zinc (II) (6a).
A
mixture of 4,5-di{2-[2-(2-ethoxyethoxy)ethoxy]-1-[2-((2-
ethoxyethoxy) ethoxy)ethoxymethyl]ethyl sulfanyl} phthalonitrile
(6) (0.80 g, 1 mmol), DBU (0.15 g, 1 mmol), anhydrous zinc
acetate (0.062 g, 0.34 mmol) and dried 1-hexanol (2 ml) were
refluxed with stirring under an argon atmosphere for 48 h. Then
1-hexanol was removed under reduced pressure, and the crude
green product was purified by column chromatography over
Al2O3 with a mixture of dichloromethane : methanol/100 : 1
(RF = 0.31) as eluent. Furthermore this product was purified
with preparative TLC (silica gel) using CH2Cl2 : MeOH/20 : 1
solvent system. Yield: 0.110 g (13%). (Found C 55.72, H 7.71,
N 3.14. C152H256N8O48S8Zn (3285) requires C 55.56, H 7.85,
N 3.41%); IR mmax/cm−1(KBr cell): 3040 (ArCH), 2980-2850
I0
I
= 1 + KSV [Q]
(6)
where I0 and I are the fluorescence intensities of fluorophore in the
absence and presence of quencher, respectively. KSV is the Stern–
Volmer constant; and is the product of the bimolecular quenching
constant (kq) and the fluorescence lifetime sF, i.e.
KSV = kq.sF
(7)
The ratios I0/I were calculated and plotted against [BQ]
according to eqn 6, and KSV determined from the slope.
=
(CH2), 1600 (ArC C), 1520, 1450, 1410, 1380, 1340, 1280, 1240,
1120-1080(C–O–C), 960, 880, 780, 750, 710; MS (FAB) m/z (%):
3285 (100) [M]+, 2977 (25) [M–(CH(CH2(OCH2CH2)2OC2H5)2)]+,
2670 (20) [M − 2(CH(CH2 (OCH2CH2)2OC2H5)2)]+, 830 (15)
[M − 8(CH(CH2(OCH2CH2)2OC2H5)2)]+.
Synthesis
Tetrakis {2-[2-(2-ethoxyethoxy)ethoxy]-1-[2-(2-ethoxyethoxy))
ethoxymethyl] ethylsulfanyl} phthalocyaninato zinc (II) (3a).
A
mixture
of
4-{2-[2-(2-ethoxyethoxy)ethoxy]-1-[2-(2-ethoxy-
ethoxy)) ethoxymethyl] ethylsulfanyl} phthalonitrile (3) (0.326,
0.7 mmol), 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU) (0.10 ml,
0.7 mmol) and anhydrous zinc acetate (0.044 g, 0.24 mmol) in
1-hexanol (2 ml) were refluxed under an argon atmosphere for
48 h. Then 1-hexanol was removed under reduced pressure and
finally the crude product was purified by column chromatography
Results and discussion
Synthesis and characterization
Depending on the reaction conditions, the syntheses of phthalo-
cyanine derivatives from aromatic nitriles result in different yields.
The reaction of substituted dinitriles in the presence of strong
non-nucleophilic bases such as 1,8-diazabicyclo [5.4.0]undec-7-
ene (DBU) or 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) either in
over Al2O3 using a mixture of CH2Cl2 : MeOH/100 : 1(RF
0.80) as eluent. Furthermore, this product was purified with
preparative TLC (silica gel) using CH2Cl2 : MeOH/20 : 1 solvent
=
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Scheme 1 Synthetic pathway for the preparation of compounds 3a, 5a and 6a (i) DMSO, K2CO3, 50 ◦C, 48 h; (ii) 1-hexanol, DBU, Zn(OAc)2, reflux,
48 h.
1-pentanol or in a bulk reaction (without solvent) results in
yields of phthalocyanines as high as 90%; this is more efficient in
comparison with other methods.44–46 In addition, these reactions
are easy to perform, work under relatively mild conditions and
yield pure phthalocyanines. Therefore, cyclotetramerization of
dinitrile derivatives 3, 5 and 6 in the presence of anhydrous zinc(II)
acetate and DBU in 1-pentanol gave the zinc(II) phthalocyanines
3a, 5a and 6a, respectively (Scheme 1). We expect that 3a and 5a
were prepared as a statistical mixture of four regioisomers due to
the various possible positions of the poly(oxyethylene) side chains
relative to one another. Four possible isomers can be obtained,
with molecular symmetries D4h, C4h, C2v and Cs in a ratio of 1 : 1 :
2 : 4.47 No attempt was made to separate the isomers of 3a and 5a.
The thia-bridged tetra- and octa-poly(oxyethylene)-substituted
phthalocyanines (3a, 5a, 6a) were purified in each case by
column chromatography (Al2O3) and the preparative Thin-Layer
Chromatography (TLC) (silica gel) using a mixed solvent system
of dichloromethane/methanol as eluent. The intense green waxy
products are very soluble in polar and apolar solvents such
as chloroform, benzene, diethylether, carbontetrachloride, N, N-
dimethylformamide, ethanol and acetone.
A close investigation of the mass spectra of the tetra- and
octasubstituted phthalocyanines (3a, 5a, 6a) confirmed the pro-
posed structures. The mass spectra were obtained by FAB
techniques using a m-nitro benzyl alcohol (MNBA) as matrix.
Fig. 2 shows the region of the molecular ion and of the other
fragment ions together with corresponding leaving groups for
6a. The mass spectrometry permitted us to characterize 6a
unambiguously. In addition to the [M]+ peak at 3285, fragment
ions corresponding to the loss of CH[CH2O(CH2CH2O)2C2H5]2
Fig. 2 Mass spectrum of octasubstituted phthalocyanine (6a).
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Table 1 1H-NMR spectral data for the phthalocyanine complexes in
Complex 5a showed a broad band around 650 nm due to
the aggregation in DMSO. Complex 3a was also aggregated in
methanol. As expected, aggregation was not detectable for any
complex in less polar tetrahydrofuran and carbon tetrachloride.
In DMSO, the Q bands were observed at: 672 (ZnPc), 692 (3a), 696
(5a) and 705 (6a) nm, Table 3. Thus, substitution of the ZnPc with
branched polyoxy substituents increased the wavelength of the Q
band. The extinction coefficients of the substituted complexes (3a,
5a and 6a) decreased with increase in the number of substituents,
Table 3.
Aggregation is usually depicted as a coplanar association of
rings progressing from monomer to dimer and higher order
complexes. It is dependent on the concentration, nature of
the solvent, peripheral substituents, complexed metal ions and
temperature.49,50 In the aggregated state the electronic structure
of the complexed phthalocyanine rings is perturbed, resulting in
alternation of the ground and excited state electronic structure.51
In this study, the aggregation behaviour of the phthalocyanine
complexes (3a, 5a and 6a) are investigated at different solvents
(chloroform, THF, CCl4 and MeOH) (Fig. 4 for complex 3a).
CDCl3
Compound CH3
OCH2 and CH
Aromatic H
3a
1.07 (t, 24 H) 3.22–4.01 (m, 100H) 7.82–8.24 (br, m, 4H)
8.80–9.20 (br, m, 8H)
5a
6a
1.06 (t, 24H) 3.29–4.14 (m, 100H) 8.31–9.05 (br, m, 8H)
1.11 (t, 48H) 3.20–4.21 (m, 200H) 9.58 (s, 8H)
([M-307]+), 2{CH[CH2O(CH2CH2O)2C2H5]2} ([M-614]+) and
8{CH[CH2O(CH2CH2O)2C2H5]2} ([M-2455]+) were easily identi-
fied.
The IR spectra showed characteristic vibrations corresponding
to ether groups (C–O–C) at 1120–1100 cm−1 and CH2 stretching
at ca. 2950-2850 cm−1, which are common for the phthalocyanine
compounds (3a, 5a, 6a). NMR investigation of all compounds gave
characteristic chemical shifts for the expected structures (Table 1
and 2).
Electronic absorption spectroscopic properties
The electronic spectra of the tetra- and octa-substituted phthalo-
cyanine complexes (3a, 5a and 6a) showed intense Q absorption
bands around 690–700 nm, Fig. 3. The spectra showed monomeric
behaviour evidenced by a single (narrow) Q band, typical of
metallated phthalocyanine complexes for 3a and 6a in DMSO.48
Fig. 4 UV-Vis Spectrum of 3a in different solvents in: (a) chloroform, (b)
THF, (c) CCl4 (d) MeOH Concentration ∼3.5 × 10−6 mol dm−3
.
The aggregation behaviour of the phthalocyanine complexes
(3a and 6a) was investigated at different concentrations in DMSO
(Fig. 5 for complex 3a). As the concentration was increased, the
intensity of the Q band absorption also increased and there were no
new bands (normally blue shifted) associated with the aggregated
species for complexes 3a and 6a (Fig. 5 for complex 3a). Thus, the
Fig. 3 Absorption spectra of substituted zinc phthalocyanine complexes
(3a, 5a and 6a) in DMSO. Concentration = 4 × 10−6 mol dm−3
.
Table 2 13C-NMR spectral data for the phthalocyanine complexes in CDCl3
Compound
CH3
CH
OCH2
Aromatic CH
Aromatic C
3a
5a
6a
15.25
15.32
15.55
48.55
48.65
49.40
50.20
48.05
66.72, 69.98, 70.69
70.96, 71.05
66.59, 69.85, 70.82
71.15, 72.32
65.57, 68.89, 70.68
70.89, 71.09
122.41, 124.32
125.75
122.72, 125.13
132.70, 136.15, 137.36139.80
154.08
136.47, 137.12
138.10, 152.73
136.72, 141.54, 152.55
126.60
Table 3 Absorption, excitation and emission spectral data for unsubstituted, tetra- and octasubstituted zinc phthalocyanine complexes in DMSO
Compound
Q band kmax/nm
log e
Excitation kEx/nm
Emission kEm/nm
Stokes shift DStokes/nm
ZnPc
3a
672
692
696
705
5.14
5.42
5.21
5.12
672
692
697
706
682
702
707
716
10
10
11
11
5a
6a
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Fig. 5 Aggregation behaviour of 3a in DMSO at different concentrations:
14 × 10−6(A), 12 × 10−6(B), 10 × 10−6(C), 8 × 10−6(D), 6 × 10−6(E), 4 ×
10−6(F) mol dm−3
.
phthalocyanine derivatives (3a and 6a) did not show aggregation
in DMSO at different concentrations. The Beer–Lambert law was
obeyed for all of these compounds in concentrations ranging from
1.4 × 10−5 to 4 × 10−6 mol dm−3.
Fluorescence quantum yields and lifetimes
Complexes 3a and 6a showed similar fluorescence behaviour in
DMSO. Fig. 6 shows the absorption, fluorescence emission and
excitation spectra for complexes (3a, 5a and 6a) in DMSO. Com-
plex 5a was aggregated in DMSO, but the fluorescence spectra
shows it was only the monomer that fluoresced, as judged by the
narrowing of the fluorescence peak. Fluorescence emission peaks
were observed at: 702 nm for 3a, 707 nm for 5a and 716 nm for
6a in DMSO (Table 3). The excitation spectra were similar to
absorption spectra and both were mirror images of the fluorescent
spectra in DMSO (Fig. 6) for 3a and 6a. The proximity of the
wavelength of each component of the Q-band absorption to the Q
band maxima of the excitation spectra for all complexes suggests
that the nuclear configurations of the ground and excited states
are similar and not affected by excitation in DMSO. The observed
Stokes shifts (Table 3) were typical of MPc complexes in DMSO.
The fluorescence quantum yields (UF) of all complexes were
similar and typical of MPc complexes, Table 4. Although the
UF value of the tetrasubstituted complex (3a) is larger than
for unsubstituted ZnPc, the UF values of the tetra- (5a) and
octasubstituted complexes (6a) are lower than unsubstituted
ZnPc in DMSO. The tetrasubstituted complex (3a) shows the
largest (UF) value in DMSO compared to the other complexes.
The tetrasubstituted complex (5a) shows the lowest UF value
in DMSO, which can be ascribed to the aggregation tendencies
of the molecules in this solvent (Fig. 2). Aggregation reduces
the likelihood of radiative deactivation (fluorescence) through
dissipation of energy by the aggregates.
Fig. 6 Absorption, excitation and emission spectra of the compound 3a
(a), 5a (b) and 6a (c) in DMSO. Excitation wavelength = 665 nm.
and 6a when compared to unsubstituted ZnPc in DMSO. Thus,
the substitution on the phthalocyanine ring increased the sF
values. The sF value for 5a is lower than unsubstituted ZnPc
in DMSO because of the aggregation in this solvent, Table 5.
The natural radiative lifetime (s0) values of substituted complexes
(3a, 5a and 6a) also increased with the number of substituents in
DMSO. The rate constants for fluorescence (kF) decreased with
the number of the substituents in DMSO. kF values for tetra- (5a)
and octasubstituted complexes (6a) are lower than unsubstituted
complex (ZnPc), while longer for tetrasubstituted complex (3a) in
DMSO. The substituted complexes (3a and 6a) showed lower rate
constant (kIC) and quantum yield for internal conversion (UIC)
in DMSO when compared to unsubstituted ZnPc, Table 4 and
Table 5. Complex 5a showed higher kIC and UIC values when
compared to unsubstituted ZnPc in DMSO, again due to the
aggregation. In addition, complexes 3a and 6a, and ZnPc showed
almost the same rate constants for intersystem crossing (kISC
)
Lifetimes of fluorescence (sF) were calculated using the
Strickler–Berg equation. Using this equation, a good correlation
has been found35 for the experimentally and theoretically deter-
mined fluorescence lifetimes for the unaggregated molecules, as is
the case in this work for 3a and 6a in DMSO. Thus, we believe
that the values obtained using this equation are a good measure
of fluorescence lifetimes. sF values are longer for complexes 3a
values, while complex 5a showed larger kISC in DMSO when
compared to unsubstituted ZnPc.
Triplet lifetimes and quantum yields
Transient spectrum for complex 3a in DMSO is shown in Fig. 7,
and shows a maximum at 540 nm, hence the triplet lifetimes and
1240 | Dalton Trans., 2007, 1235–1243
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Table 4 Photophysical and photochemical parameters of unsubstituted, tetra- and octasubstituted zinc phthalocyanines in DMSO. Triplet absorption
wavelength used = 510 nm for ZnPc and 540 nm for 3a, 5a and 6aa
Compound
UF
sT/ls
UT
UIC
Ud(x10−5
)
UD
SD
ZnPc
3a
0.18 [36]
0.22
0.10
350
220
210
300
0.65 [39]
0.73
0.71
0.17
0.05
0.19
0.10
2.61
0.26
0.19
1.94
0.67 [42]
0.72
0.62
1.03
0.98
0.87
0.80
5a
6a
0.15
0.75
0.60
a References in brackets
Table 5 Rate and quenching constants for various excited state deactivation processes of unsubstituted, tetra- and octasubstituted zinc phthalocyanines
in DMSO
kq/dm3
d
Compound
sF (ns)
s0 (ns)
kF/s−1(x108)a
k
ISC/s−1(x108)b
kIC/s−1(x107)c
kd/s−1(x10−2
)
K
SV/M−1
mol−1 s−1(x1010)
ZnPc
3a
1.22
1.34
0.83
1.57
6.80
5.95
8.29
10.47
1.47
1.68
1.20
0.95
5.32
5.44
8.55
4.77
13.9
3.73
22.89
6.37
7.4
1.2
0.9
6.4
31.90
21.07
14.76
11.77
2.61
1.57
1.78
0.75
5a
6a
a kF is the rate constant for fluorescence. Values calculated using kF = UF/sF. b kISC is the rate constant for intersystem crossing. Values calculated using
kISC = UT/sF. c
k
IC is the rate constant for internal conversion. Values calculated using kIC = UIC/sF. d kd is the rate constant for photodegradation. Values
calculated using kd = Ud/sT.
for substituted complexes (3a, 5a and 6a), resulting in high UT
values. Complex 6a showed larger UT value when compared to the
other substituted complexes (3a and 5a) in DMSO.
Singlet oxygen quantum yields
Singlet oxygen quantum yields (UD) were determined in DMSO
using a chemical method with DPBF as a quencher. The dis-
appearance of DPBF spectra was monitored using UV-vis spec-
trophotometer. Many factors are responsible for the magnitude
of the determined quantum yield of singlet oxygen, including
triplet excited state energy, ability of substituents and solvents
to quench the singlet oxygen, the triplet excited state lifetime and
the efficiency of the energy transfer between the triplet excited
state and the ground state of oxygen.
There was no change in the Q band intensity during the
UD determinations, confirming that complexes are not degraded
during singlet oxygen studies. The UD values of tetrasubstituted
complex 3a were higher when compared to unsubstituted ZnPc in
DMSO. The UD values of tetrasubstituted (5a) and octasubstituted
(6a) complexes were lower when compared to unsubstituted ZnPc
in DMSO (Table 4). The magnitude of the SD ( = UD/UT)
represents the efficiency of quenching of the triplet excited state
by singlet oxygen. All the complexes 3a, 5a and 6a showed values
of SD of near unity (Table 4), suggesting efficient quenching of
the triplet state by singlet oxygen. Complex 6a showed lower SD
when compared to the other substituted complexes (3a and 5a) in
DMSO, suggesting a less efficient quenching of oxygen compared
to the rest of the complexes.
Fig. 7 Transient differential spectrum of complex 3a in DMSO.
yields were determined at this wavelength for complexes 3a, 5a and
6a. Fig. 8 shows the triplet decay curves of the complexes (using
complex 3a in DMSO as an example). Complex 6a showed longer
triplet lifetime when compared to the other substituted complexes
(3a and 5a), suggesting that it is not the plurality of the substituents
that results in more quenching. The triplet lifetime (sT) values were
lower for substituted complexes (3a, 5a and 6a) when compared
to unsubstituted ZnPc, Table 4.
Fig. 8 Triplet decay curve of 3a in DMSO. Excitation wavelength =
540 nm.
Photodegradation studies
The triplet quantum yields (UT) for substituted complexes (3a,
5a and 6a) in DMSO are higher when compared to unsubstituted
ZnPc in DMSO. The higher values of UT in DMSO suggest effi-
cient intersystem crossing (ISC) in the presence of the substituents
Degradation of the molecules under irradiation can be used to
study their stability, and this is especially important for those
molecules intended for use as photocatalysts. The collapse of the
absorption spectra without any distortion of the shape confirms
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clean photodegradation not associated with phototransformation.
The spectral changes observed for all the complexes 3a, 5a and 6a
during photodegradation occurred without phototransformation.
Table 4 shows that all substituted complexes (3a, 5a and 6a)
were more stable to degradation compared to unsubstituted ZnPc
in DMSO. Thus, the substitution of ZnPc with a branched
polyoxy group seem to increase the stability of the complexes
in DMSO. The octasubstituted complex 6a was less stable when
compared to the other substituted complexes (3a and 5a). The
stability order among the substituted complexes was 5a > 3a >
6a > ZnPc in DMSO, which does not follow the trend in
UD, suggesting that the degradation of the ring is not caused
only by singlet oxygen. The rate constants for photodegradation
(kd) of substituted complexes (3a, 5a and 6a) were lower than
unsubstituted ZnPc in DMSO.
unsubstituted ZnPc in DMSO, thus substitution with branched
polyoxy group seems to decrease the kq values of the complexes.
The order in kq values among the substituted complexes was also
as follows: 5a > 3a > 6a in DMSO.
Conclusions
In the present work, the syntheses of new tetra- and octa
thia(polyoxyethylene) substituted Zn(II) phthalocyanines are de-
scribed andthe compounds are characterized bystandard methods
(1H- and 13C-NMR, elemental analysis, IR, UV/Vis and mass
spectrometry). In solution, the spectra showed monomeric be-
haviour evidenced by a single (narrow) Q band, typical of metal-
lated phthalocyanine complexes for 3a and 6a in DMSO. Complex
5a showed a broad band around 650 nm due to the aggregation in
DMSO. The substitution of the branched polyoxy substituents on
the phthalocyanine ring increased the wavelength of the Q band.
The fluorescence quantum yields of substituted complexes were
typical for MPcs. The substituted complexes (3a, 5a and 6a) have
good singlet oxygen quantum yields (UD), especially complex 3a,
which has the highest value. The singlet oxygen quantum yields,
which give an indication of the potential of the complexes as
photosensitizers in applications where singlet oxygen is required
(Type II mechanism), ranged from 0.60 to 0.72. Thus, these
complexes show potential as Type II photosensitizers. Substitution
of ZnPc with a branched polyoxy group seem to increase the
stability of the complexes in DMSO. The stability order among the
substituted complexes was 5a > 3a > 6a in DMSO. The substituted
complexes showed lower KSV and kq values when compared to the
unsubstituted ZnPc in DMSO.
Fluorescence quenching studies by benzoquinone [BQ]
The fluorescence quenching of zinc phthalocyanine complexes by
BQ in DMSO was found to obey Stern–Volmer kinetics, which is
consistent with diffusion-controlled bimolecular reactions. Fig. 9
shows the quenching of complex 5a by BQ in DMSO as an
example. The slope of the plots shown at Fig. 10 gave KSV
values, listed in Table 5. The KSV values of the substituted
zinc phthalocyanine complexes (3a, 5a and 6a) are lower than
unsubstituted ZnPc in DMSO. Of the substituted complexes, 3a
has highest KSV value, while complex 6a has the lowest KSV in
DMSO. The substitution with a branched polyoxy group seems
to decrease the KSV values of the complexes in DMSO. The
bimolecular quenching constant (kq) values of the substituted zinc
phthalocyanine complexes (3a, 5a and 6a) are also lower than for
Acknowledgements
This work was supported by the National Research Foundation
of South Africa (NRF GUN # 2053657) as well as Rhodes
University. The authors wish to thank Mine Tir Bilsel and Go¨khan
¨
˙
Bilsel (TUBITAK-UME) for the FAB mass spectra.
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