Solvent and central metal effects on the photophysical and photochemical properties of 4-benzyloxybenzoxy substituted phthalocyanines

Article abstract of DOI:10.1016/j.jorganchem.2010.10.024

The synthesis and characterization of new peripherally tetra-4- benzyloxybenzoxy substituted metal-free, zinc and lead phthalocyanines are described for the first time in this study. The influence of various organic solvents and the nature of the central

Full text of DOI:10.1016/j.jorganchem.2010.10.024

Journal of Organometallic Chemistry 696 (2011) 913e924
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Solvent and central metal effects on the photophysical and photochemical
properties of 4-benzyloxybenzoxy substituted phthalocyanines
a
b
,
a
*
ꢀ
Ece Tugba Saka , Mahmut Durmus¸ , Halit Kantekin
a Department of Chemistry, Faculty of Arts & Sciences, Karadeniz Technical University, 61080 Trabzon, Turkey
b Gebze Institute of Technology, Department of Chemistry, PO Box 141, Gebze 41400, Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of new peripherally tetra-4-benzyloxybenzoxy substituted metal-
free, zinc and lead phthalocyanines are described for the first time in this study. The influence of various
organic solvents and the nature of the central metal ion on the spectroscopic, photophysical and
photochemical properties has been investigated. General trends are described for photodegradation,
singlet oxygen and fluorescence quantum yields, and fluorescence lifetimes of these compounds in
different solvents. Photophysical and photochemical properties of phthalocyanine compounds are very
useful for photodynamic therapy applications. Especially high singlet oxygen quantum yields are very
important for Type II mechanism. The studied phthalocyanine compounds showed good singlet oxygen
generation and these compounds show potential as Type II photosensitizers. The fluorescences of the
studied compounds are effectively quenched by 1,4-benzoquinone in different solvents.
Ó 2010 Elsevier B.V. All rights reserved.
Received 16 August 2010
Received in revised form
28 September 2010
Accepted 12 October 2010
Available online 17 November 2010
Keywords:
Phthalocyanine
Solvent effect
Fluorescence
Quantum yields
Singlet oxygen
Photodegradation
1. Introduction
aqueous medium results in highly aggregated complexes. However,
many phthalocyanine complexes remain aggregated even in non-
Phthalocyanines (Pcs) have great interest due to their scientific
and technological importance in many fields. In recent years,
metallophthalocyanines (MPcs) have been extensively studied in
non-linear optic applications, chemical sensors, catalysis, liquid
crystals, photovoltaic cells, semiconductive materials and photo-
dynamic therapy (PDT) [1e4]. The chemical and physical charac-
teristics of these compounds vary with the central metal ion,
peripheral substitution and ligands attached to the central metal
atom [5].
aqueous solutions [10e12]. Aromatic solvents such as benzene or
toluene are known to give narrow Q bands in the absorption
spectra for phthalocyanine complexes whereas broadening is
observed in non-aromatic solvents [13].
PDT is the novel treatment in cancer therapies and this tech-
nique is based on a photochemical reaction, which is initiated by
light activation of a photosensitizing drug causing tumor cell death.
Light is mainly applied by superficial illumination of the tumor and
the surrounding tissue causing drug activation followed by tumor
cell death. Pcs and MPcs are useful photosensitizers due to their
intense absorption in red region of the visible light [14,15]. Espe-
cially zinc phthalocyanine complexes are often used due to their
long triplet lifetimes [16]. Such long lifetimes constitute a great
advantage since the number of diffusional encounters between the
triplet excited state and ground state molecular oxygen increases
with the lifetime of the excited state [17].
The aim of our research is to synthesize new peripherally
substituted metal-free (H2Pc), zinc (ZnPc) and lead (PbPc) phtha-
locyanines as potential PDT agents. Our previous studies have
already reported synthesis, photophysical and photochemical
properties of various substituted phthalocyanines [18e23]. These
phthalocyanine complexes show interesting photophysical and
photochemical properties especially high singlet oxygen quantum
The wide planar organic surface of the phthalocyanine molecule
causes aggregation of the complexes both in solution and in solid
state due to the
pe
p* stacking interactions [6,7]. Because aggre-
gation of the molecules affects solubility of the phthalocyanines,
many phthalocyanines are poorly soluble in common organic
solvents and water. The solubility of these molecules increases by
changing peripheral substituents since the substituents increase
the distance between the stacked phthalocyanines and enable their
solvation [8,9].
Solvents also affect aggregation in phthalocyanine complexes.
Organic solvents are known to reduce aggregation whereas
* Corresponding author. Tel.: þ90 262 6053077; fax: þ90 262 6053101.
E-mail address: durmus@gyte.edu.tr (M. Durmus¸ ).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.024

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E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
yields which are very important for PDT of cancer. In this study, we
report the effects of the peripherally 4-benzyloxybenzoxy
substituted metal-free (H2Pc), zinc (ZnPc) and lead (PbPc) phtha-
locyanines on the photophysical and photochemical parameters.
Aggregation behaviour, photophysical (fluorescence lifetime and
quantum yields) and photochemical (singlet oxygen and photo-
degradation quantum yields) properties of newly synthesized
compounds (Scheme 1) are investigated in different solvents. Since
PDT activity is mainly based on singlet oxygen, its production is
determined by the dye-sensitized photooxidation of 1,3-dipheny-
lisobenzofuran (DPBF), a specific scavenger of this species [24]. The
studies of the photostability of H2Pc, ZnPc and PbPc complexes
during photosensitized reactions are also immense importance.
chemical shifts (d) were reported relative to Me4Si as internal
standard. Mass spectra were measured on a Micromass Quatro
LC/ULTIMA LCeMS/MS spectrometer. UVevisible spectra were
recorded with a Shimadzu 2001 UV spectrophotometer. Fluores-
cence excitation and emission spectra were recorded on a Varian
Eclipse spectrofluoremeter using 1 cm pathlength cuvettes at room
temperature.
Photo-irradiations were done 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.
2. Experimental
2.3. Photophysical parameters
2.1. Materials
2.3.1. Fluorescence quantum yields and lifetimes
Fluorescence quantum yields (FF) were determined by the
comparative method (Eq. (1)) [27,28],
4-Benzyloxybenzyl-alcohol (1) was purchased from Aldrich.
1,3-diphenylisobenzofuran (DPBF) was purchased from Fluka.
4-Nitrophthalonitrile (2) [25] was prepared according to the liter-
ature. All reagents and solvents were of reagent grade quality and
were obtained from commercial suppliers. All solvents were dried
and purified as described by Perin and Armarego [26].
F$AStd$n2
FF
¼
FFðStdÞ
(1)
F
Std$A$n2Std
where F and FStd are the areas under the fluorescence emission
curves of the samples (H2Pc, ZnPc and PbPc) and the standard,
respectively. A and AStd are the respective absorbance of the
samples and standard at the excitation wavelength, respectively. n2
and n2Std are the refractive indices of solvents used for the sample
and standard, respectively. Unsubstituted ZnPc (Std-ZnPc) (in
DMSO) (FF ¼ 0.20) [29] was employed as the standard. Both the
2.2. Equipment
The IR spectra were recorded on a Perkin Elmer 1600 FT-IR
spectrophotometer using KBr pellets. 1H and 13C NMR spectra were
recorded on a Varian Mercury 200 MHz spectrometer in CDCl3, and
O
CN
CN
CN
CN
OH
i
+
O
O
O2N
2
3
1
ii
O
O
O
O
N
N
N
N
M
N
N
N
N
O
O
O
O
Compound H2Pc ZnPc PbPc
2H Zn Pb
M
Scheme 1. Synthetic pathway for the preparation of phthalocyanine complexes (H2Pc, ZnPc and PbPc): (i) anhydrous K2CO3, dry DMF, 50 ꢀC, (ii) 160 ꢀC, n-pentanol, DBU for H2Pc;
anhydrous Zn(CH3COO)2 and PbO for ZnPc and PbPc, respectively.

E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
915
samples and standard were excited at the same wavelength. The
absorbance of the solutions at the excitation wavelength ranged
between 0.04 and 0.05.
fluorescence intensity related to BQ concentration by the
SterneVolmer (S-V) equation [37] were shown in Eq. (5):
I0
Natural radiative lifetimes (s0) were determined using Photo-
¼ 1 þ KSV½BQꢃ
(5)
I
chemCAD program which uses the StricklereBerg equation [30].
The fluorescence lifetimes (sF) were evaluated using Eq. (2).
where I0 and I are the fluorescence intensities of fluorophore in the
absence and presence of quencher, respectively. [BQ] is the
concentration of the quencher and KSV is the SterneVolmer
constant which is the product of the bimolecular quenching
sF
FF
¼
(2)
s0
constant (kq) and the
sF and is expressed in Eq. (6).
2.4. Photochemical parameters
KSV ¼ kq$sF
(6)
2.4.1. Singlet oxygen quantum yields
The ratios of Io/I were calculated and plotted against [BQ]
according to equation (5), and KSV is determined from the slope.
Singlet oxygen quantum yield (FD) determinations were carried
out using the experimental set-up described in literature [31,32].
Typically, 2 cm3 portions of the respective metal-free, zinc and lead
phthalocyanine (H2Pc, ZnPc and PbPc) solutions containing the
singlet oxygen quencher were irradiated in the Q band region with
the photo-irradiation set-up described in Refs. [31,32]. FD values
were determined in air using the relative method with DPBF as
singlet oxygen chemical quencher in different solvents (Eq. (3)):
2.5. Synthesis
2.5.1. 4-(4-Benzyloxybenzoxy)phthalonitrile (3)
4-(Benzyloxybenzyl)alcohol (1) (5 g, 23.33 mmol) was dis-
solved in dry DMF (40 cm3) under an inert nitrogen atmosphere
and 4-nitrophthalonitrile (2) (4.66 g, 23.33 mmol) was added to
the solution. After stirring 10 min, finely ground anhydrous K2CO3
(10.0 g, 69.25 mmol) was added portion wise within 2 h with
efficient stirring. The reaction mixture was stirred under nitrogen
atmosphere at 50 ꢀC for 3 days. Then the solution was poured into
ice-water (100 cm3). The precipitate formed was filtered off,
washed with water until the filtrate was neutral and then washed
with diethyl ether and dried in vacuum over P2O5. The crude
product was crystallized from ethanol. Yield: 6.35 g (80%), mp:
140e142 ꢀC. Anal. Calcd for C22H16N2O2: C, 77.64; H, 4.70; N, 8.23%.
Found: C, 77.67; H, 4.71; N, 8.22%. IR (KBr tablet) nmax/cmꢂ1: 3076
(Ar-H), 2941, 2875 (Aliph. CeH), 2235 (C^N), 1587 (C]C), 1174
Std
R$I
RStd$Iabs
FSDtd
(3)
abs
FD
¼
where FSDtd is the singlet oxygen quantum yield for the standard,
ZnPc (F ZDnPc ¼ 0.67 in DMSO [33], 0.56 in DMF [34], 0.58 in toluene
[35] and 0.53 in THF [36]). R and RStd are the DPBF photobleaching
rates in the presence of the respective samples (H2Pc, ZnPc and
PbPc) and standard, respectively. Iabs and I are the rates of light
absorption by the samples (H2Pc, ZnPc and PbPc) and standard,
respectively. To avoid chain reactions induced by DPBF in the
presence of singlet oxygen [34], the concentration of DPBF was
lowered to w3 ꢁ10ꢂ5 M. Solutions of sensitizer (absorbance w 1.5
at the irradiation wavelength) containing DPBF were prepared in
the dark and irradiated in the Q band region. DPBF degradation at
417 nm was monitored. The light intensity used for FD determi-
nations was found to be 6.48 ꢁ 1015 photons sꢂ1 cmꢂ2. The error in
the determination of FD was w10% (determined from several FD
values).
Std
abs
(AreOeC). 1H NMR. (CDCl3), (
(m, 5H, Ar-H), 7.30e7.25 (m, 2H, Ar-H), 6.99e7.03 (m, 4H, Ar-H)
5.01 (s, 4H, CH2eO). 13C NMR. (CDCl3), (
: ppm): 162.69, 156.83,
136.53, 135.73, 130.07, 128.61, 127.82, 127.05, 126.65, 119.73, 118.56,
117.26, 116.38, 115.71, 109.18, 69.87, 69.73. MS (ESþ), (m/z):
340 [M]þ.
d: ppm): 7.68 (d, 1H, Ar-H), 7.37e7.45
d
2.5.2. 2,(3)-Tetrakis(4-benzyloxybenzoxy)phthalocyanine (H2Pc)
4-(4-Benzyloxybenzoxy)phthalonitrile (3) (1.50 g, 4.41 mmol)
was dissolved in dried n-pentanol (5 cm3) under an inert nitrogen
atmosphere. 1.8-diazabicyclo [5.4.0]undec-7-ene (DBU) (0.583 cm3,
3.91 mmol) was added to the solution and the mixture was heated
to 160 ꢀC for 16 h. After cooling, the solution was dropped in
ethanol. The green solid product was precipitated and collected by
filtration and washed with ethanol. The crude product was purified
by passing through a silica gel column, using THF as eluting solvent.
Furthermore this product was purified with preparative thin layer
chromatography (silica gel) using CHCl3:MeOH (95:5) as a solvent
system. Yield: 0.540 g (36%). Anal. Calcd for C88H66N8O8: C, 77.53;
H, 4.84; N, 8.22%. Found: C, 77.76; H, 4.85; N, 8.31%. IR (KBr tablet)
nmax/cmꢂ1: 3294 (NeH), 3060e3027 (Ar-H), 2924e2851 (Aliph.
CeH), 1602 (NeH bending), 1569 (C]C), 1174 (AreOeC). 1H NMR
2.4.2. Photodegradation quantum yields
Photodegradation quantum yield (Fd) determinations were
carried out using the experimental set-up described in literature
[31,32]. Photodegradation quantum yields were determined using
Eq. (4),
ðC0 ꢂ CtÞ$V$NA
Fd
¼
(4)
I
abs$S$t
where C0 and Ct are the samples (H2Pc, ZnPc and PbPc) concen-
trations before and after irradiation respectively, V is the reaction
volume, NA is the Avogadro’s constant, S is the irradiated cell area
and t is the irradiation time. Iabs is the overlap integral of the
radiation source light intensity and the absorption of the samples
(H2Pc, ZnPc and PbPc). A light intensity of 2.16 ꢁ 1016 photo-
ns sꢂ1 cmꢂ2 was employed for Fd determinations.
(CDCl3), (
Benzyl-H), 4.88 (s, 16H, CH2-O), e6.66 (s, 2H, NeH). 13C NMR
(CDCl3), ( : ppm) 162.36, 156.45, 154.88, 139.12, 137.70, 136.46,
d: ppm): 7.87e7.26 (m, 4H, Pc-H), 7.11e6.63 (m, 44H, Pc-H,
2.4.3. Fluorescence quenching by 1,4-benzoquinone (BQ)
d
Fluorescence quenching experiments of the substituted phtha-
locyanines (H2Pc, ZnPc and PbPc) were carried out by the addition
of different concentrations of BQ to a fixed concentration of the
samples, and the concentrations of BQ in the resulting mixtures
were 0, 0.008, 0.016, 0.024, 0.032 and 0.040 M. The fluorescence
spectra of substituted phthalocyanines (H2Pc, ZnPc and PbPc)
at each BQ concentration were recorded, and the changes in
134.67, 132.48, 131.09, 129.02, 127.67, 124.78, 124.34, 119.92, 108.86,
108.26, 70.21, 69.89. MS (ESþ), (m/z): 1363 [M þ H]þ.
2.5.3. 2,(3)-Tetrakis[(4-benzyloxybenzoxy)phthalocyaninato] zinc
(II) (ZnPc)
A mixture of 4-(4-benzyloxybenzoxy)phthalonitrile (3) (1.50 g,
4.41 mmol), anhydrous Zn(CH3COO)2 (1.51 g, 8.82 mmol) and dried

916
E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
n-pentanol (5 cm3) was heated at 160 ꢀC for 16 h. After cooling to
room temperature the reaction mixture was dropped in hot ethanol
(40 cm3) to precipitate the product which was filtered off. The
green solid product was purified by passing through a silica gel
column, using CHCl3:MeOH (95:5) as eluting solvent system. Yield:
0.597 g (38%). Anal. Calcd for C88H64N8O8Zn: C, 74.00; H, 4.48; N,
spectrum of metal-free phthalocyanine (H2Pc) showed new
aromatic protons at
¼ 7.87e7.26 (m, 4H, Pc-H), 7.11e6.63 (m, 44H,
Pc-H, Benzyl-H) and aliphatic ether protons at
¼ 4.88 (s,16H, CH2).
The inner core protons (NeH) of this compound (H2Pc) were
d
d
observed at
d
¼ ꢂ6.66 (br s, 2H, NeH). The peak for the nitrile
carbon atoms in the 13C NMR (CDCl3) spectrum of H2Pc was not
seen. The mass spectrum of compound H2Pc exhibited an intense
peak at 1363 m/z for [M þ H]þ.
7.85%. Found: C, 74.06; H, 4.46; N, 7.88%. IR (KBr tablet) nmax/cmꢂ1
3064 (Ar-H), 2922e2862 (Aliph. CeH), 1596 (C]C), 1172 (AreOeC).
1H NMR (CDCl3), (
: ppm): 7.90e7.72 (m, 4H, Pc-H), 7.26e6.79 (m,
44H, Pc-H, Benzyl-H), 4.92 (s, 16H, CH2). 13C NMR (CDCl3), (
: ppm)
:
d
After conversion into zinc or lead phthalocyanine, the charac-
teristic C^N stretch at 2235 cmꢂ1 of phthalonitrile 3 disappeared,
indicative of metallophthalocyanine formation. The 1H NMR spec-
d
159.74, 156.45, 153.67, 152.45, 138.79, 136.62, 135.52, 132.90, 130.88,
129.23, 128.84, 124.88, 124.53, 120.68, 119.12, 110.24, 69.80, 68.21.
MS (ESþ), (m/z): 1427 [M þ H]þ.
trum of ZnPc showed new signals at
d
¼ 7.90e7.72 (m, 4H, Pc-H),
7.26e6.79 (m, 44H, Pc-H, Benzyl-H), 4.92 (s, 16H, CH2). 13C NMR
spectrum of ZnPc showed 16 aromatic carbon atoms signals and 2
aliphatic carbon atoms signals. These signals were observed at
2.5.4. 2,(3)-Tetrakis[(4-benzyloxybenzoxy)phthalocyaninato] lead
(II) (PbPc)
(d: ppm): 159.74, 156.45, 153.67, 152.45, 138.79, 136.62, 135.52,
A mixture of 4-(4-benzyloxybenzoxy)phthalonitrile (3) (1.50 g,
4.41 mmol), anhydrous PbO (1.96 g, 8.82 mmol) and dried n-pen-
tanol (5 cm3) was heated at 160 ꢀC for 16 h. After cooling to room
temperature the reaction mixture was dropped in hot ethanol
(40 cm3) to precipitate the product which was filtered off. The
green solid product was purified by passing through a silica gel
column, using CHCl3:MeOH (95:5) as eluting solvent system. Yield:
0.530 g (32%). Anal. Calcd for C88H64N8O8Pb: C, 67.34; H, 4.08; N,
132.90, 130.88, 129.23, 128.84, 124.88, 124.53, 120.68, 119.12, 110.24,
69.80, 68.21. In the 1H NMR spectrum of PbPc complex, signals
were observed at (
44H, Pc-H, Benzyl-H), 4.86 (s, 16H, CH2eO). The new signals were
observed at ( : ppm): 160.74, 158.72, 154.45, 138.52, 136.74, 135.90,
d: ppm): 7.92e7.36 (m, 4H, Pc-H), 7.28e7.12 (m,
d
133.56, 131.23, 128.84, 125.88, 124.53, 123.68, 122.80, 120.61, 119.72,
108.24, 69.80, 68.21 in the 13C NMR spectrum of PbPc. The mass
spectra of compounds ZnPc and PbPc exhibited intense peaks at
(m/z): 1427 [M þ H]þ and 1570 [M þ 2H]þ, respectively and
confirmed the proposed structures.
7.14%. Found: C, 67.30; H, 4.06; N, 7.16%. IR (KBr tablet) nmax/cmꢂ1
3060 (Ar-H), 2924e2862 (Aliph. CeH), 1508 (C]C), 1174 (AreOeC).
1H NMR (CDCl3), (
: ppm): 7.92e7.36 (m, 4H, Pc-H), 7.28e7.12 (m,
44H, Pc-H, Benzyl-H), 4.86 (s, 16H, CH2-O). 13C NMR (CDCl3), (
:
d
d:
3.2. Ground state electronic absorption and fluorescence spectra
ppm) 160.74, 158.72, 154.45, 138.52, 136.74, 135.90, 133.56, 131.23,
128.84, 125.88, 124.53, 123.68, 122.80, 120.61, 119.72, 108.24, 69.80,
68.21. MS (ESþ), (m/z): 1570 [M þ 2H]þ.
The ground state electronic absorption spectra showed mono-
meric behaviour evidenced by a single (narrow) Q band, typical of
metallated phthalocyanine complexes in different solvents except
for ZnPc showed a little aggregation in toluene due to broadness in
w650 nm regions, Fig. 1 [38]. The metal-free phthalocyanine
complex H2Pc gives a doublet Q band in different solvents as
a result of the D2h symmetry [38].
3. Results and discussion
3.1. Synthesis and characterization
4-(4-benzyloxybenzoxy)phthalonitrile (3) was prepared by
reaction of 4-(benzyloxybenzyl)alcohol (1) with 4-nitrophthalo-
nitrile (2) in dried DMF as a solvent at 50 ꢀC in the presence of solid
The Q band values of the studied phthalocyanine complexes in
different solvents are given in Table 1. The Q bands of the lead
phthalocyanine complex (PbPc) are red-shifted when compared to
the corresponding metal-free (H2Pc) and zinc (ZnPc) phthalocya-
nine complexes due to the effect of the lead atom as central atom
(Table 1). The B-bands are broad due to the superimposition of the
B1 and B2 bands in the 340e360 nm region [39].
anhydrous K2CO3 as
a base. Therefore, 4-benzyloxybenzoxy
substituted metal-free phthalocyanine compound (H2Pc) was
obtained directly from the reaction of 4-(4-benzyloxybenzoxy)
phthalonitrile (3) and DBU in n-pentanol at reflux temperature.
Also, cyclotetramerization of 4-(4-benzyloxybenzoxy)phthaloni-
trile (3) in the presence of anhydrous Zn(CH3COO)2 or PbO and DBU
in n-pentanol gave the 4-benzyloxybenzoxy substituted zinc or
lead phthalocyanine derivatives ZnPc and PbPc, respectively.
All new synthesized compounds (H2Pc, ZnPc and PbPc) were
characterized by elemental analyses, IR, 1H NMR, 13C NMR and mass
spectra. In the IR spectrum, the formation of compound 3 was
clearly confirmed by the disappearance of the eOH and eNO2
bands at 3335 and 1530e1357 cmꢂ1 and appearance of eC^N band
at 2235 cmꢂ1. In the 1H NMR spectrum of 3, OH group of compound
1 disappeared as expected. 1H NMR (in CDCl3) spectrum of
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, nature of the substituents, complexed metal ions and
temperature [40,41]. In this study, the aggregation behaviour of the
phthalocyanine complexes (H2Pc, ZnPc and PbPc) was investi-
gated in different solvents (DMSO, DMF, toluene and THF), Fig. 1.
While the metal-free phthalocyanine complex (H2Pc) showed
a little aggregation in DMSO, the zinc phthalocyanine complex
(ZnPc) showed a little aggregation in toluene. Lead phthalocyanine
complex (PbPc) did not show aggregation in all studied solvents.
The aggregation behaviour of the studied phthalocyanine
complexes (H2Pc, ZnPc and PbPc) was also studied at different
concentration in DMSO. In DMSO, as the concentration was
increased, the intensity of absorption of the Q band also increased
and there were no new bands (normally blue shifted) due to the
aggregated species for the metallated complexes (ZnPc and PbPc)
(Fig. 2 as an example for complex ZnPc). The metal-free phthalo-
cyanine complex (H2Pc) was showed a little aggregation in DMSO.
BeereLambert law was obeyed for metallated phthalocyanine
complexes (ZnPc and PbPc) in the concentrations ranging from
1.4 ꢁ10ꢂ5 to 4 ꢁ10ꢂ6 M.
compound 3 showed new signals at
d
¼ 7.68 (d,1H, Ar-H), 7.37e7.45
(m, 5H, Ar-H), 7.30e7.25 (m, 2H, Ar-H), 6.99e7.03 (m, 4H, Ar-H) and
5.01 (s, 4H, CH2eO) belonging to aromatic protons and aliphatic
ether protons, respectively. The 13C NMR (in CDCl3) spectrum of
compound 3 indicated the presence of nitrile carbon atoms
d
¼ 115.71 ppm. The mass spectrum of compound 3 showed an
intense peak at m/z ¼ 340 [M]þ support the proposed formula for
this compound.
The sharp vibration for the eC^N group in the IR spectrum
of phthalonitrile compound 3 at 2235 cmꢂ1 disappeared after
formation of metal-free phthalocyanine (H2Pc). The 1H NMR

E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
917
Fig. 1. Absorption spectra of: (a) H2Pc; (b) ZnPc and (c) PbPc in different solvents. Concentration ¼ 1.2 ꢁ 10ꢂ5 M.

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E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
Table 1
Absorption, excitation and emission spectral data for tetra substituted phthalocyanine complexes (H2Pc, ZnPc and PbPc) in different solvents.
Comp.
Solvent
Q band, lmax (nm)
(log
3
)
Excitation, lEx (nm)
Emission, lEm (nm)
Stokes shift, DStokes (nm)
H2Pc
DMSO
DMF
Toluene
THF
673, 705
670, 702
667, 704
665, 702
4.33, 4.13
4.58, 4.56
4.45, 4.41
4.65, 4.68
676, 708
673, 707
668, 707
668, 705
715
711
710
709
10
9
6
7
ZnPc
DMSO
DMF
Toluene
THF
681
678
681
678
5.03
5.03
4.68
5.16
686
679
682
679
697
692
691
689
16
14
10
10
PbPc
DMSO
DMF
Toluene
THF
712
710
716
707
4.85
4.64
4.59
4.79
676, 706
672, 706
669, 706
669, 706
716
712
718
710
4
2
2
3
Std-ZnPc
DMSO
DMF
Toluene
THF
672a
670b
672c
666
5.14a
5.37b
5.14c
5.19
672a
670b
672
682a
676b
676d
673
10a
6b
4d
7
666
a
Ref. [19].
Ref. [48].
Ref. [23].
Ref. [35].
b
c
d
The fluorescence behaviour of the phthalocyanine complexes was
studiedindifferentsolvents.Fig. 3 showstheabsorption, fluorescence
emission and excitation spectra of the phthalocyanine complexes
(H2Pc, ZnPc and PbPc) in DMF. The zinc phthalocyanine complex is
more fluorescent than other studied phthalocyanine complexes. The
lead phthalocyanine complex showed that lowest emission could be
due to the largerlead metal being more displaced from the core of the
Pc ring, and the displacement being more pronounced on excitation
hence a loss of symmetry.
The shapes of the excitation spectra were similar to absorption
spectra for the metal-free (H2Pc) and zinc phthalocyanine (ZnPc)
complexes (Fig. 3a and b) suggesting that the proximity of the
wavelength of each component of the Q band absorption to the Q
band maxima of the excitation spectra for phthalocyanines H2Pc
and ZnPc suggests that the nuclear configurations of the ground
and excited states are similar and not affected by excitation. For
the lead phthalocyanine complex (PbPc), the shape of excitation
spectra was different from the absorption spectra in that the Q
band of the former showed two peaks in the Q band region,
Fig. 3c, unlike the narrow Q band of the latter. This suggests that
there are changes in the molecule following excitation most likely
due to the larger lead metal being out of the plane of the phtha-
locyanine ring.
Fluorescence emission and excitation peaks in different solvents
are listed in Table 1. While the observed Stokes shift of the
substituted zinc phthalocyanine complex (ZnPc) is higher, the lead
phthalocyanine complex (PbPc) is lower than standard ZnPc (Std-
ZnPc) (Table 1). The observed Stokes shift of metal-free phthalo-
cyanine complex (H2Pc) is similar for standard ZnPc (Std-ZnPc)
(Table 1).
2
1.8
1.6
y = 107800x + 0.0971
1.6
R
= 0.9995
1.2
0.8
0.4
0
A
B
C
D
1.4
1.2
1
0.00E+00 2.00E-06 4.00E-06 6.00E-06 8.00E-06 1.00E-05 1.20E-05 1.40E-05
Concentration
E
F
0.8
0.6
0.4
0.2
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Absorption spectra of ZnPc in DMSO at different concentrations. 14 ꢁ10ꢂ6 M (A), 12 ꢁ 10ꢂ6 M (B), 10 ꢁ 10ꢂ6 M (C), 8 ꢁ 10ꢂ6 M (D), 6 ꢁ 10ꢂ6 M (E), 4 ꢁ 10ꢂ6 M (F) (Inset: Plot
of absorbance versus concentration).

E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
919
Fig. 3. Absorption, fluorescence emission and excitation spectra of (a) H2Pc, (b) ZnPc and PbPc in DMF. Excitation wavelength ¼ 650 nm for H2Pc and ZnPc, 680 nm for PbPc.

920
E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
Table 2
presence of ligands which decreasing of the fluorescence quench-
ing. Thus the increasing in the FF value for substituted phthalocy-
anine complexes in the presence of the ring substituents suggests
that the substituents less quench the excited singlet state, hence
the fluorescence. The substituted H2Pc complex shows larger FF
values in all studies solvents (Table 2). All phthalocyanine
complexes (H2Pc, ZnPc and PbPc) showed largest FF values in THF
among the studied solvents.
Photophysical and photochemical parameters of tetra substituted phthalocyanine
complexes (H2Pc, ZnPc and PbPc) in different solvents.
sF (ns) s0 (ns) kF (sꢂ1
)
Fd (ꢁ10ꢂ3
)
FD
a
Comp.
Solvent FF
(ꢁ107)
2.34
4.40
3.19
4.86
H2Pc
DMSO
DMF
0.21
0.32
8.12
7.27
9.40
7.19
42.75
22.71
31.33
20.55
0.34
1.14
0.69
0.01
0.34
0.28
0.49
0.43
Toluene 0.30
THF
0.35
Fluorescence lifetime (sF) refers to the average time a molecule
ZnPc
PbPc
DMSO
DMF
Toluene 0.20
0.19
0.29
2.40
3.56
5.19
1.95
11.42
12.28
25.96
8.89
8.75
8.14
3.85
0.009
0.19
0.74
0.74
0.65
0.79
0.46
stays in its excited state before fluorescing, and its value is directly
related to that of FF; i.e. the longer the lifetime, the higher the
quantum yield of fluorescence. Any factor that shortens the fluo-
THF
0.22
11.23
0.008
rescence lifetime of a fluorophore indirectly reduces the value of FF
.
DMSO
DMF
Toluene 0.24
THF 0.29
0.043 0.64
0.063 1.75
15.08
27.79
26.08
18.25
6.63
3.59
3.83
5.47
1.92
29.32
61.26
25.73
0.16
0.55
0.72
0.22
Such factors include internal conversion and intersystem crossing.
As a result, the nature and the environment of a fluorophore
determine its fluorescence lifetime.
6.26
5.29
Std-ZnPc DMSO
0.20b 1.22c
0.17d 1.03d
6.80c
6.05d
1.47c
1.65d
7.70d
9.17
0.26c
0.67c
0.56d
0.58e
0.53g
Lifetimes of fluorescence (sF, Table 2) were calculated using the
DMF
0.023d
0.062f
0.002
StricklereBerg equation. Using this equation, a good correlation
has been [28] found between experimentally and the theoretically
determined lifetimes for the unaggregated molecules as is the case
in this work. Thus we suggest that the values obtained using this
equation are an appropriate measure of fluorescence lifetimes.
Generally, the sF values are within the range reported for MPc
complexes [42]. sF values are higher for substituted complexes
H2Pc, ZnPc and PbPc when compared to Std-ZnPc, except
for PbPc in DMSO (Table 2), again suggesting less quenching
of fluorescence by the ring substituents. For the substituted
complexes, longer sF values are obtained for the H2Pc complex
compared to other studied phthalocyanine complexes (ZnPc
and PbPb).
Toluene 0.07e 0.90d
12.97d
10.90
THF
0.25
2.72
a
b
c
kF is the rate constant for fluorescence. Values calculated using kF ¼ FF/sF.
Ref. [29].
Ref. [19].
Ref. [48].
Ref. [35].
Ref. [22].
Ref. [36].
d
e
f
g
3.3. Photophysical properties
3.3.1. Fluorescence lifetimes and quantum yields
The natural radiative lifetime (s0) and the rate constants for
The fluorescence quantum yields
(FF) of phthalocyanine
fluorescence (kF) values are also given in Table 2. s0 values of the
substituted phthalocyanine complexes (H2Pc, ZnPc and PbPc) are
higher than Std-ZnPc in all studied solvents except for complex
ZnPc in THF. The substituted H2Pc complex showed the highest s0
values in all studied solvents when compared to other substituted
complexes (ZnPc and PbPc). There was no clear trend found for the
rate constants for fluorescence (kF) values among the substituted
H2Pc, ZnPc and PbPc complexes.
complexes H2Pc, ZnPc and PbPc are typical of MPc complexes in
different solvents, but the FF values of PbPc in DMSO and DMF are
lower than for the Std-ZnPc (Table 2) suggesting that there are
more changes in the molecule following excitation in DMSO and
DMF for complex PbPc most likely due to the larger lead metal
being out of the plane of the phthalocyanine ring. Generally, the FF
values of the substituted complexes are higher than Std-ZnPc. An
increase in fluorescence intensity (hence yield) may occur in the
1.2
1.2
1
y = -0.027x + 0.9953
2
R = 0.9776
0.8
0.6
1
0.4
0.2
0
0.8
0
5
10
15
20
25
Time (sec)
0 sec
5 sec
0.6
0.4
0.2
0
10 sec
15 sec
20 sec
25 sec
300
400
500
Wavelength (nm)
600
700
800
Fig. 4. A typical spectrum for the determination of singlet oxygen quantum yield. This determination was for compound ZnPc in DMSO at a concentration of 1.2 ꢁ 10ꢂ5 M (Inset:
Plots of DPBF absorbance versus time).

E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
921
3.4. Photochemical properties
state energy, ability of substituents and solvents to quench the
generated 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 decrease in the Q
band or formation of new bands during FD determinations (Fig. 4 as
an example for ZnPc in DMSO): this indicates that the phthalocy-
anines are not degradated during singlet oxygen studies. It is
believed that during photosensitization, the phthalocyanine is
firstly excited to the singlet state and through intersystem crossing
reaches the triplet state, then transfers its energy to ground state
oxygen, O2(3Sg), itself converted into its excited state (singlet
oxygen), O2(1Dg). This singlet oxygen is the chief cytotoxic species,
which subsequently oxidizes the surrounding substrates: this
oxidation is the key of the Type II mechanism.
The values of FD are higher for substituted ZnPc complex when
compared to respective Std-ZnPc complex in all studied solvents
except for THF. The FD values of substituted H2Pc and PbPc are
lower than Std-ZnPc in all studied solvents except for complex
PbPc in toluene (Table 2). When compared to the FD values among
the substituted complexes (H2Pc, ZnPc and PbPc), substituted
ZnPc complex shows highest FD values in all studied complexes.
3.4.1. Singlet oxygen quantum yields
A good photosensitizer must be very efficient in generating
singlet oxygen, the active species of a photodynamic therapy
treatment. Energy transfer between the triplet state of a photo-
sensitizer (such as phthalocyanines) and the ground state of
molecular oxygen leads to the production of singlet oxygen and
must be highly efficient to generate large amounts of singlet
oxygen. This can be quantified by the singlet oxygen quantum yield
(FD), a parameter giving an indication of the potential of molecules
to be used as photosensitizers in applications where singlet oxygen
is required (e.g. for Type II mechanism). In this study, FD values of
the substituted phthalocyanines (H2Pc, ZnPc and PbPc) were
determined in different solvents (DMSO, DMF, toluene and THF)
using a chemical method. 1,3-Diphenylisobenzofuran (DPBF) used
as a singlet oxygen quencher. The disappearance of DPBF absor-
bance was monitored using UVeVis spectrophotometer (Fig. 4 as an
example for ZnPc in DMSO).
Many factors are responsible for the magnitude of the deter-
mined quantum yield of singlet oxygen including: triplet excited
1
1
a
0.8
y = -0.0009x + 0.8693
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
2
R
= 0.9784
0 sec
60 sec
120 sec
0
50
100
150
200
250
300
180 sec
240 sec
300 sec
Time (sec)
300
400
500
600
700
800
Wavelength (nm)
0.8
0.6
0.4
0.2
0
b
0 sec
5 sec
300
400
500
600
700
800
Wavelength (nm)
Fig. 5. The photodegradation of compound: (a) ZnPc in toluene (b) PbPc in THF showing the disappearance of the Q-band at sixty seconds intervals for ZnPc and five seconds
intervals for PbPc (Inset: Plot of Q band absorbance versus time for ZnPc in toluene).

922
E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
700
600
500
400
300
200
100
0
[BQ]=0
[BQ]=saturated
660
680
700
720
740
760
780
800
Wavelength (nm)
Fig. 6. Fluorescence emission spectral changes of H2Pc (1.00 ꢁ 10ꢂ5 M) on addition of different concentrations of BQ in toluene. [BQ] ¼ 0, 0.008, 0.016, 0.024, 0.032, 0.040 M and
saturated with BQ.
The FD values of the substituted complexes are higher in toluene
than other studied solvents.
3.4.3. Fluorescence quenching studies by 1,4-benzoquinone [BQ]
The fluorescence quenching of substituted phthalocyanine
complexes (H2Pc, ZnPc and PbPc) by BQ in different solvents was
found to obey SterneVolmer kinetics. Fig. 6 shows the quenching
of H2Pc complex by BQ in toluene as an example. The slope of the
plots shown at Fig. 7 gave KSV values. The SterneVolmer plots for
all studied complexes (H2Pc, ZnPc and PbPc) gave straight lines,
depicting diffusion-controlled quenching mechanisms. Quinones
have high electron affinities, and their involvement in electron
transfer processes is well documented [44]. The energy of the
lowest excited state for quinones is greater than the energy of the
excited singlet state of MPc complexes [45], hence, energy transfer
from the excited MPc to BQ is not likely to occur. Moreover, MPcs
are known to be easily reduced. Therefore MPc fluorescence
quenching by BQ is via excited state electron transfer, from the
MPc to the BQ [46]. The KSV and bimolecular quenching constant
(kq) values for the BQ quenching of phthalocyanine complexes in
different solvents are listed in Table 3. The KSV values of the
substituted phthalocyanine complexes (H2Pc, ZnPc and PbPc) are
lower than Std-ZnPc in all studied solvents. When compared the
substituted complexes, H2Pc complex showed the highest KSV
values, while complex PbPc showed the lowest KSV values in all
studied solvents. The substitution with 4-benzyloxybenzoxy group
seems to decrease the KSV values of the complexes. The order of
KSV values for substituted complexes among the studied solvents
was as follows: toluene > THF > DMF > DMSO. In different
solvents, the KSV values for BQ quenching of phthalocyanine
complexes vary directly with the solvents’ polarity. The bimolec-
ular quenching constant (kq) values of the substituted phthalocy-
anine complexes (H2Pc, ZnPc and PbPc) were also lower than for
Std-ZnPc in studied solvents except for ZnPc complex in THF, thus
substitution with 4-benzyloxybenzoxy group seems to decrease
the kq values of the complexes. The kq values were found to be
close to the diffusion-controlled limits, w1010 Mꢂ1 sꢂ1, which is in
agreement with the EinsteineSmoluchowski approximation at
room temperature for diffusion-controlled bimolecular interac-
tions [47].
3.4.2. Photodegradation studies
Degradation of the molecules under irradiation can be used to
study their stability and this is important for their application as
photocatalysts (such as photosensitizers). Photodegradation is the
oxidative degradation of a photosensitizer molecule with time
into lower molecular weight fragments under light [34]. Photo-
degradation generally depends on the structure of the molecule,
concentration, solvent and light intensity. It is believed that
photodegradation is a singlet oxygen mediated process, since
singlet oxygen is highly reactive and it can react with phthalo-
cyanines [17]. Generally, phthalimide residue was found to be the
photooxidation product following degradation of phthalocya-
nines [43].
The spectral changes observed for all the complexes (H2Pc, ZnPc
and PbPc) during irradiation are as shown in Fig. 5 (using complex
ZnPc in toluene and complex PbPc in THF as examples). The
collapse of the absorption spectra without any distortion of the
shape confirms clean photodegradation not associated with pho-
totransformation for complexes H2Pc and ZnPc (Fig. 5a as an
example for complex ZnPc in toluene). For substituted PbPc
complex, the shape of the absorption spectra changed during the
photodegradation studies in all studied solvents (Fig. 5b as an
example for complex PbPc in THF). The single narrow Q band of the
PbPc split two bands during photodegradation under light irradi-
ation. This suggests that there are changes in the molecule
following photodegradation most likely due to the larger lead metal
being out of the plane of the phthalocyanine ring.
The photodegradation quantum yield (Fd) values of the phtha-
locyanine complexes in different solvents are given in Table 2.
Stable phthalocyanine molecules show Fd values as low as 10ꢂ6 and
for unstable molecules, values of the order of 10ꢂ3 have been
reported [42]. While the substituted H2Pc and ZnPc complexes
have an intermediate photostability, the substituted PbPc is less
stable than other studied phthalocyanine complexes.

E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
923
4
3.5
3
a
Toluene
THF
DMF
2.5
2
DMSO
1.5
1
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
[BQ]
3.5
b
Toluene
3
THF
DMF
2.5
2
DMSO
1.5
1
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
[BQ]
c
Toluene
3
2.5
2
THF
DMF
DMSO
1.5
1
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
[BQ]
Fig. 7. SterneVolmer plots for benzoquinone (BQ) quenching of substituted phthalocyanine complexes in different solvents (a) for H2Pc (b) for ZnPc and for PbPc. [MPc] w
1.00 ꢁ 10ꢂ5 M. [BQ] ¼ 0, 0.008, 0.016, 0.024, 0.032, 0.040 M in all studied solvents.

924
E.T. Saka et al. / Journal of Organometallic Chemistry 696 (2011) 913e924
Table 3
Acknowledgements
Fluorescence quenching data for tetra substituted phthalocyanine complexes (H2Pc,
ZnPc and PbPc) in different solvents.
This study was supported by the Research Fund of Karadeniz
Technical University (project no: 2006.111.002.1).
Comp.
Solvent
KSV (Mꢂ1
)
kq/1010 (Mꢂ1 sꢂ1
)
H2Pc
DMSO
DMF
Toluene
THF
23.26
40.10
60.38
46.42
0.28
0.55
0.71
0.64
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In the presented work, the syntheses of new peripherally tetra-
4-benzyloxybenzoxy substituted metal-free (H2Pc), zinc (ZnPc)
and lead (PbPc) phthalocyanines were described and the
compounds were characterized by elemental analyses, IR, 1H and
13C NMR spectroscopy, electronic spectroscopy and mass spec-
trometry. The photophysical and photochemical properties of these
phthalocyanine complexes (H2Pc, ZnPc and PbPc) were also
described in different solvents (DMSO, DMF, toluene and THF) for
comparison. All the compounds are soluble in most solvents such as
chloroform, toluene, DMSO etc. In solution, the absorption spectra
showed monomeric behaviour evidenced by a single (narrow) Q
band, for ZnPc and PbPc in different solvents typical of metallated
phthalocyanine complexes except for ZnPc in toluene. The metal-
free phthalocyanine complex H2Pc showed a doublet Q band in
different solvents. The substitution of the 4-benzyloxybenzoxy
substituents on the phthalocyanine ring increased the wavelength
of the Q band. The fluorescence behaviour of the phthalocyanine
complexes was studied in different solvents. The zinc phthalocya-
nine complex is more fluorescent than other studied phthalocya-
nine complexes. The lead phthalocyanine complex showed that
lowest emission could be due to the larger lead metal ion. Gener-
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ZnPc. The substituted complexes (H2Pc, ZnPc and PbPc) have good
singlet oxygen quantum yields (FD), especially complex ZnPc
results in the highest values. The value of FD ranged from 0.16 (for
complex PbPc in DMSO) to 0.79 (for complex ZnPc in toluene) gives
an indication of the potential of the complexes as photosensitizers
in applications where singlet oxygen is required (Type II mecha-
nism). While the substituted H2Pc and ZnPc complexes showed
similar stability, the substituted PbPc is less stable than other
studied phthalocyanine complexes most likely due to the larger
lead metal being out of the plane of the phthalocyanine ring during
photodegradation studies. The substituted complexes showed
lower KSV values when compared to the Std-ZnPc in all studied
solvents.
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