Photophysical, photochemical and aggregation behavior of novel peripherally tetra-substituted phthalocyanine derivatives

Article abstract of DOI:10.1016/j.jphotochem.2012.05.023

In this study, the novel metal-free (4) and zinc (II) (5) phthalocyanine compounds substituted with four 1-(2-oxyethyl)-4-piperidone ethylene ketal functional groups at peripheral positions have been prepared. These new phthalocyanine compounds have been

Full text of DOI:10.1016/j.jphotochem.2012.05.023

Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photophysical, photochemical and aggregation behavior of novel peripherally
tetra-substituted phthalocyanine derivatives
Ece Tug˘ba Sakaa, Cem Gölb, Mahmut Durmus¸ b, Halit Kantekina,∗, Zekeriya Bıyıklıog˘lua
a Department of Chemistry, Faculty of 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:
In this study, the novel metal-free (4) and zinc (II) (5) phthalocyanine compounds substituted with four 1-
(2-oxyethyl)-4-piperidone ethylene ketal functional groups at peripheral positions have been prepared.
These new phthalocyanine compounds have been characterized by IR, 1H NMR, 13C NMR spectroscopy,
MS spectral data and elemental analysis. The synthesized phthalocyanine compounds exhibited excel-
lent solubility in common organic solvents and the zinc (II) phthalocyanine complex (5) showed J-type
aggregation in chloroform. The photophysical and photochemical properties of metal-free (4) and zinc
(II) (5) phthalocyanine complexes were also investigated in DMSO. The investigation of the photophys-
ical and photochemical properties of photosensitizers is very useful for photodynamic therapy (PDT)
applications.
Received 27 February 2012
Received in revised form 26 April 2012
Accepted 23 May 2012
Available online 1 June 2012
Keywords:
Phthalocyanine
Zinc
J-aggregation
Photophysical
Photochemical
Singlet oxygen
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
side-to-side J-aggregation [1,2]. Typically, phthalocyanine aggrega-
tion results in a decrease in intensity of the Q band corresponding
Metallophthalocyanine (MPc) complexes are being extensively
researched because of their diverse applications. These include
their uses in electronic devices, non-linear optics, electrochromic
devices, Langmuir–Blodgett films, as gas sensors and photo-
sensitizers in photodynamic therapy (PDT) of cancer [1,2]. The
exceptional chemical and physical properties of phthalocyanines
can be because of various substituents on the phthalocyanine
framework. Over 70 elements can be included into the phthalo-
cyanine core and its chemical versatility allows the introduction of
many different substituents at peripheral positions [3,4].
Metallophthalocyanines are known to have low solubility in
most organic solvents. The solubility of these compounds can
be improved via substitution of different groups on the phthalo-
cyanine skeleton. It has been documented that tetra-substituted
phthalocyanines are more soluble than their octa-substituted
counterparts due to formation of constitutional isomers and their
high dipole moments [5,6].
to the monomeric species, meanwhile a new, broader and blue-
shifted band is seen to increase in intensity. This shift to
lower wavelengths indicates to H-type aggregation among the
phthalocyanine molecules. Rare cases red-shifted bands have been
observed corresponding to J-type aggregation of the phthalocya-
nine molecules. Generally, J-aggregates of Pc occurred by utilizing
the coordination of the side substituent from one Pc molecule
to the central metal ion in a neighbor [7–11]. The substituted
zinc Pcs in non-coordinated organic solvents, e.g. chloroform and
dichloromethane exhibit J-aggregation [12,13]. The addition of
coordinating solvents such as methanol or ethanol caused dissoci-
ation of the dimers, which implies that the absence of coordinating
solvents is essential for J-aggregation of Pc [14]. UV–vis and MALDI-
TOF-MS spectra could be used for determination presence of
J-aggregation for Pc compounds [15].
Furthermore, owing to their extensively planar aromatic ␲ sys-
tem, phthalocyanines exhibit a high aggregation tendency which
leads to insolubility in the case of unsubstituted parent deriva-
tives or hinders purification and characterization of compounds in
many respects together with a lower efficiency in their use in PDT
[16,17]. PDT is a binary therapy that involves the combination of
visible light and a photosensitizer [18]. Diamagnetic ions such as
Zn2+, Al3+, Ga3+ and Ti4+ give phthalocyanine complexes comprising
both high triplet yields and long triplet lifetimes which are suit-
able for photodynamic therapy (PDT) applications [19]. Due to the
intense absorption in the visible region, high efficiency to generate
Aggregation of the phthalocyanine compounds is an impor-
tant phenomenon. The substituted metallophthalocyanines could
be form two types of aggregations which affect on electronic
and optical properties, namely face-to-face H-aggregation and
∗
Corresponding author at: Department of Chemistry, Karadeniz Technical Uni-
versity, 61080 Trabzon, Turkey. Tel.: +90 462 3772599; fax: +90 462 3253196.
E-mail address: halit@ktu.edu.tr (H. Kantekin).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.05.023

68
E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
reactive oxygen species (such as singlet oxygen), and low dark
toxicity, phthalocyanines have been used in this avenue for the
treatment of various cancers and photoinactivation of viruses
[18,20].
2.3. Photophysical parameters
2.3.1. Fluorescence quantum yields and lifetimes
Fluorescence quantum yields (˚F) were determined in DMSO
by the comparative method using equation 1 [29,30].
Our previous studies have already reported synthesis, pho-
tophysical and photochemical properties of various substituted
phthalocyanines [21–25]. These phthalocyanine complexes show
interesting photophysical and photochemical properties especially
high singlet oxygen quantum yields which are very important
for PDT of cancer. In this work, we have been synthesized new
metal-free (4) and zinc (5) phthalocyanines substituted with four 1-
(2-oxyethyl)-4-piperidone ethylene ketal groups as potential PDT
agents. Aggregation behavior, photophysical (fluorescence lifetime
and quantum yields) and photochemical (singlet oxygen and pho-
todegradation quantum yields) properties were investigated. This
work has also been reported the effects of the substituents and
the nature of the metal on the photophysical and photochemical
parameters of 1-(2-oxyethyl)-4-piperidone ethylene ketal substi-
tuted phthalocyanine derivatives in DMSO. This work also explores
the effects of substituents and nature of the central metal ions
on the fluorescence properties of the phthalocyanines and on the
quenching of the phthalocyanines by 1,4-benzoquinone (BQ) using
the Stern–Volmer relationship.
F · AStd · n2
˚
= ˚F (Std)
,
(1)
F
FStd · A · n2
Std
where F and FStd are the areas under the fluorescence emission
curves of the samples (4 and 5) and the standard, respectively. A
and AStd are the respective absorbances of the samples and stan-
dard at the excitation wavelengths, respectively. n2 and nS2td are
the refractive indices of solvents used for the sample and standard,
respectively. Unsubstituted ZnPc (in DMSO) (˚F = 0.20) [31] was
employed as the standard. The absorbance of the solutions at the
excitation wavelength ranged between 0.04 and 0.05.
Natural radiative life times (ꢀ0) were determined using Pho-
tochemCAD program [32] which uses the Strickler–Berg equation.
The fluorescence lifetimes (ꢀF) were evaluated using Eq. (2).
ꢀF
˚
F
=
.
(2)
ꢀ0
2.4. Photochemical parameters
2.4.1. Singlet oxygen quantum yields
2. Experimental
Singlet oxygen quantum yield (˚ꢁ) determinations were car-
ried out using the experimental set-up described in the literature
[33–35]. Typically, a 3 ml portion of the respective unsubstituted,
metal-free Pc (4) and zinc Pc complex (5) solutions (C = 1 × 10−5 M)
containing the singlet oxygen quencher was irradiated in the
Q band region with the photo-irradiation set-up described in
references [33–35]. Singlet oxygen quantum yields (˚ꢁ) were
determined in DMSO using the relative method with unsubstituted
ZnPc as reference. DPBF was used as chemical quencher for singlet
oxygen in DMSO. Eq. (3) was employed for the calculations:
2.1. Materials
All
under
techniques.
phthalocyanine
nitrogen atmosphere
1,3-Diphenylisobenzofuran
reactions
were
standard
(DPBF)
carried
Schlenk
and 1,8-
out
using
diazabicyclo[5.4.0]undec-7-ene (DBU) were purchased from Fluka.
All solvents were dried and purified as described by reported
procedure [26]. 1-(2-Hydroxyethyl)4-piperidone ethylene ketal
(1) [27], 4-nitrophthalonitrile (2) [28] were prepared according to
the literature procedure.
Std
R · I
abs
RStd · Iabs
˚
= ˚Std
,
(3)
ꢁ
ꢁ
2.2. Equipment
where ˚Sꢁtd is the singlet oxygen quantum yield for the stan-
dard unsubstituted ZnPc (˚Sꢁtd = 0.67 in DMSO)[36]. R and RStd
FT-IR spectra were obtained on a Perkin Elmer 1600 FTIR spec-
trophotometer with the samples prepared as KBr pellets. NMR
spectra were recorded on a Varian Mercury 200 MHz spectrometer
in CDCl3, and chemical shifts were reported (ı) relative to TMS as an
internal standard. Mass spectra were recorded on a Micromass Qua-
tro LC/ULTIMA LC–MS/MS spectrometer. The elemental analyses
were performed on a Costech ECS 4010 instrument. The forma-
tion of J-aggregation for zinc (II) Pc complex (5) was determined
by positive ion and linear mode MALDI-MS in dihydroxybenzoic
acid as MALDI matrix using nitrogen laser accumulating 50 laser
shots using Bruker Microflex LT MALDI-TOF mass spectrometer.
Melting points were measured on an electrothermal apparatus.
Domestic microwave oven was used synthesis of zinc (II) Pc com-
plex (5). Absorption spectra in the UV–vis region were recorded
with a Shimadzu 2001 UV spectrophotometer. Fluorescence exci-
tation and emission spectra were recorded on a Varian Eclipse
spectrofluorometer using 1 cm pathlength cuvettes at room tem-
perature. 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.
are the DPBF photobleaching rates in the presence of the sam-
Std
abs
ples (4 and 5) and standard, respectively. Iabs and I
are the
rates of light absorption by the samples (4 and 5) and standard,
respectively. To avoid chain reactions induced by DPBF in the pres-
ence of singlet oxygen, the concentration of quenchers (DPBF)
was lowered to ∼3 × 10−5 mol dm−3 [37]. Solutions of sensitizer
(C = 1 × 10−5 M) containing DPBF were prepared in the dark and
irradiated in the Q band region using the photoirradiation setup.
DPBF degradation at 417 nm was monitored. The light intensity
7.3 × 1015 photons s−1 cm−2 was used for ˚ꢁ determinations.
2.4.2. Photodegradation quantum yields
Photodegradation quantum yield (˚d) determinations were car-
ried out using the experimental set-up described in the literature
[33–35]. Photodegradation quantum yields were determined using
Eq. (4),
(C0 − Ct) · V · NA
˚
=
,
(4)
d
Iabs · S · t
where C0 and Ct are the samples (4 and 5) concentrations 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 (4 and 5). A

E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
69
light intensity of 2.19 × 1016 photons s−1 cm−2 was employed for
(KBr tablet) ꢂmax/cm−1: 3290 (N H), 3065 (Ar H), 2926–2875
˚
determinations.
(Aliph. C H), 1611, 1468, 1425, 1389, 1365, 1341, 1310, 1240,
d
1146 (C
O
C), 1096, 1010, 962, 944, 911, 822, 745, 716. 1H NMR
(CDCl3) (ı:ppm): 7.90–7.71 (m, 4H, ArH), 7.09–6.98 (m, 4H, ArH),
6.79 (m, 4H, ArH), 4.10 (m, 24H, CH2 O), 3.06 (bs, 8H, CH2 N),
2.91 (bs, 16H, CH2 N), 1.99 (bs, 16H, CH2 N), −5.34 (s, 2H, NH).
13C-NMR. (CDCl3), (ı:ppm): 160.04, 136.99, 129.04, 128.18, 122.89,
118.28, 107.47, 104.54, 66.51, 64.61, 57.41, 52.41, 35.25. MS (ES+),
(m/z): 1255 [M]+. C68H78N12O12: calcd. C 65.07, H 6.21, N 13.39%;
found: C 65.40, H 6.39, N 13.18.
2.5. Fluorescence quenching by 1,4-benzoquinone (BQ)
Fluorescence quenching experiments on the metal-free (4) and
zinc (II) (5) Pc compounds were carried out by the addition of differ-
ent concentrations of BQ to a fixed concentration of the compounds,
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 the substituted metal-free (4) and zinc (II) (5) Pc compounds at
each BQ concentration were recorded, and the changes in fluores-
cence intensity related to BQ concentration by the Stern–Volmer
(SV) equation [38] using Eq. (5):
2.6.3. Tetrakis-[2-(1,4-dioxa-8-azaspiro[4.5]dec-8-yl)ethoxy]
phthalocyaninato zinc(II) (5)
A
mixture
of
4-[2-(1,4-dioxa-8-azaspiro[4.5]dec-8-
yl)ethoxy]phthalonitrile (3) (0.6 g, 1.91 × 10−3 mol), Zn(CH3COO)2
(0.086 g, 0.47 × 10−3 mol) and 2-(dimethylamino)ethanol (0.006 L)
was irradiated in a microwave oven at 175 ◦C, 350 W for 12 min.
After the mixture being cooled to the room temperature, it was
stirred by EtOH (0.025 L) addition for overnight, and then filtered
off. The product was refluxed by EtOH (0.04 L) for 4 h. The green
product was filtered off, washed with hot EtOH–MeOH. The crude
product was purified by column chromatography which is placed
aluminum oxide using CHCl3:CH3OH (9:1) as solvent system and
gave pure green solid product. This product was dried under
vacuum over P2O5. Yield: 0.264 g (42%), mp >300 ◦C. IR (KBr tablet)
I0
= 1 + KSV[BQ],
(5)
I
where I0 and I are the fluorescence intensities of samples in the
absence and presence of BQ, respectively. KSV is the Stern–Volmer
constant; and this is the product of the bimolecular quenching con-
stant (kq) and the fluorescence lifetime ꢀF (Eq. (6)):
KSV = kqꢀF .
(6)
The ratios I0/I were calculated and plotted against [BQ] accord-
ing to Eq. (5), and KSV determined from the slope.
␯
max/cm−1: 3060 (Ar H), 2950–2884 (Aliph. C H), 1717, 1606,
2.6. Synthesis
1489, 1390, 1365, 1337, 1282, 1237, 1096, 1054, 963, 946, 913,
855, 827, 748, 665. 1H NMR (CDCl3) (ı:ppm): 8.87 (bs, 4H, ArH),
8.42 (bs, 4H, ArH), 7.44 (bs, 4H, ArH), 4.49 (bs, 8H, CH2 O), 3.98
(bs, 16H, CH2 O), 3.09 (bs, 8H, ArH), 2.83 (m, 16H, CH2 N), 1.88
(m, 16H, CH2 N). 13C-NMR. (CDCl3), (ı:ppm): 160.80, 141.07,
132.09, 125.46, 124.21, 120.98, 108.86, 106.18, 67.35, 64.47, 56.58,
52.05, 34.95. MS (ES+), (m/z): 1318 [M]+. C68H76N12O12Zn:
2.6.1.
4-[2-(1,4-dioxa-8-azaspiro[4.5]dec-8-yl)ethoxy]phthalonitrile (3)
1-(2-Hydroxyethyl)4-piperidone ethylene ketal (1) (2 g,
0.010 mol) was dissolved in dry DMF (0.04 L) under N2 atmosphere
and 4-nitrophthalonitrile (2) (2.13 g, 0.010 mol) was added to the
solution. After stirring 10 min, finely ground anhydrous K2CO3
(4.64 g, 0.032 mol) was added portion wise within 2 h with efficient
stirring. The reaction mixture was stirred under N2 at 50 ◦C for
2 days. Then the solution was poured into ice-water (0.1 L). The
precipitate formed was filtered off, washed with water firstly until
the filtrate was neutral and then diethyl ether and dried in vacuum
over P2O5. The crude product was recrystallized from methanol.
Yield: 2.67 g (80%), mp: 85–86 ◦C. IR (KBr tablet) ꢂmax/cm−1: 3082
(Ar H), 2956–2888 (Aliph. C H), 2231 (C N), 1598, 1561, 1493,
calcd.
C 61.90, H 5.77, N 12.75%; found: C 62.14, H 6.09,
N 12.88.
3. Results and discussion
3.1. Synthesis and characterization
The general synthetic route was used for the new substituted
phthalonitrile compound (3) starting from 1-(2-hydroxyethyl)4-
piperidone ethylene ketal (1) and 4-nitrophthalonitrile (2) in the
presence of dry K2CO3 as a base at 50 ◦C in dry DMF (Scheme 1). The
tetramerization of the substituted phthalonitrile compound (3) in a
high-boiling solvent in the presence of a few drops DBU as a strong
base at reflux temperature under a nitrogen atmosphere afforded
the metal-free phthalocyanine (4) as a green solid after purification
by column chromatography which is placed aluminum oxide using
CHCl3:CH3OH (9:1) as solvent system. Zinc (II) phthalocyanine
complex (5) was obtained using microwave irradiation in the pres-
ence of the anhydrous Zn(CH3COO)2 in 2-(dimethylamino)ethanol
as a green solid after purification by column chromatography which
is placed aluminum oxide using CHCl3:CH3OH (9:1) as solvent sys-
tem.
1470, 1424, 1366, 1311, 1255, 1212, 1147 (C
O C), 1097, 1039,
964, 946, 912, 839, 762. 1H-NMR. (CDCl3), (ı:ppm): 7.72 (d, 1H,
ArH), 7.28 (s, 1H, ArH), 7.22 (d, 1H, ArH), 4.16 (t, 2H, CH2 O), 3.95
(t, 4H, CH2 O), 2.86 (t, 2H, CH2 N), 2.65 (t, 4H, CH2 N), 1.75 (t,
4H, CH2 N). 13C-NMR. (CDCl3), (ı:ppm): 162.12, 135.47, 119.99,
119.67, 117.59, 115.95, 115.51, 107.00, 67.58, 64.51, 56.44, 52.11,
34.96. MS (ES+), (m/z): 314 [M+H]+. C17H19N3O3: calcd. C 65.14, H
6.06, N 13.40%; found: C 65.42, H 6.35, N 13.20.
2.6.2. Tetrakis[2-(1,4-dioxa-8-azaspiro[4.5]dec-8-
yl)ethoxy]phthalocyanine (4)
4-[2-(1,4-Dioxa-8-azaspiro[4.5]dec-8-yl)ethoxy]phthalonitrile
(3) (0.3 g, 0.96 × 10−3 mol), n-pentanol (0.003 L) and 1,8-
diazabicyclo[5.4.0]
undec-7-ene
(DBU)
(0.07 × 10−3 L,
0.48 × 10−3 mol) was placed in a standard Schlenk tube under
nitrogen atmosphere and degassed three times. Then temperature
was increased up to 160 ◦C. The reaction system was stirred at
160 ◦C for 16 h. After the reaction mixture was cooled to room
temperature, 0.06 L n-hexane was added and mixed for half an
hour. Precipitate was filtered and washed with diethyl ether. The
crude product was purified by column chromatography which
is placed aluminum oxide using CHCl3:CH3OH (9:1) as solvent
system and gave pure green solid product. This product was
dried under vacuum over P2O5. Yield: 0.12 g (40%), mp >300 ◦C.
In the IR spectrum, the formation of phthalonitrile compound
(3) was clearly confirmed by the disappearance of the–OH and–NO2
bands and appearance of
C
N band at 2231 cm−1. In the 1H NMR
spectrum of compound 3, OH group of compound 1 disappeared
as expected. The signals were observed at ı = 7.72 ppm, 7.28 ppm
and 7.22 ppm for the aromatic protons and 4.16 ppm, 3.95 ppm,
2.86 ppm, 2.65 ppm, 1.75 ppm for aliphatic CH2 protons. The 13C-
NMR spectrum of 3 showed signals at ı = 162.12, 135.47, 119.99,
119.67, 117.59, 115.95, 115.51, 107.00, 67.58, 64.51, 56.44, 52.11,
34.96 ppm. The MS spectrum of compound 3, which showed a peak

70
E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
CN
O
OH
N
O2N
CN
O
2
1
i
CN
CN
O
O
O
N
ii
3
iii
RO
RO
OR
OR
N
N
N
HN
N
N
N
N
N
Zn
N
N
N
NH
N
N
N
4
OR
5
RO
OR
RO
O
N
R =
O
Scheme 1. The synthetic route of the phthalonitrile, metal-free and zinc phthalocyanines. Reagents and conditions: (i) dry DMF, K2CO3, 50 ◦C, 48 h; (ii) n-pentanol, DBU,
160 ◦C, (iii) Zn(CH3COO)2, DMAE, 175 ◦C, 350 W.
at m/z = 314 [M+H]+ support the proposed formula for this com-
pound.
The sharp peak in the IR spectrum for the C≡N vibration
of phthalonitrile compound (3) at 2231 cm−1 disappeared after
conversion into metal-free phthalocyanine, indicative of phthalo-
and 2.83 ppm, 1.88 ppm as multiplets. The 13C-NMR spectrum of 5
showed signals at ı = 160.80, 141.07, 132.09, 125.46, 124.21, 120.98,
108.86, 106.18, 67.35, 64.47, 56.58, 52.05, 34.95 ppm. In the mass
spectrum of compound 5, the presence of molecular ion peaks at
m/z = 1318 [M]+ confirmed the proposed structure.
cyanine formation. The characteristic IR band for the inner N
H
stretching of metal-free phthalocyanine (4) was observed at
3290 cm−1. The IR spectra of metal-free (4) and zinc (II) (5) phthalo-
cyanine compounds are very similar, except these ꢃ (NH) vibrations
of the inner phthalocyanine core in the metal-free molecule. The
3.2. Ground state electronic absorption spectra
Phthalocyanine compounds exhibit typical electronic spectra
with two strong absorption regions, one around ca. 300 nm is called
as the “B” or Soret band because of electronic transitions from
deeper ␲-HOMO to n*-LUMO energy levels, while the other one at
600–750 nm is called as the “Q” band, due to electronic transitions
from ␲-HOMO to ␲*-LUMO energy levels [41]. One of the best indi-
cators of the formation of phthalocyanines is their UV–vis spectra
in dilute solution. The UV–vis spectra of the studied metal-free (4)
and zinc (II) (5) phthalocyanine compounds in DMSO were given in
Fig. 2. The Q band of the metal-free phthalocyanine was observed as
splitted two bands due to D2h symmetry [42]. The splitting Q band
was observed at ꢄmax 704 and 676 nm in DMSO (Table 1), indicating
the structure with non-degenerate D2h symmetry [42]. The ground
state electronic absorption spectrum of zinc (II) phthalocyanine
compound (5) showed monomeric behavior evidenced by a single
(narrow) Q band, typical of metallated phthalocyanine complexes,
Fig. 2 [43]. The Q band of this complex was observed at 684 nm
in DMSO, Table 1. The B bands of studied phthalocyanine com-
pounds (4 and 5) were observed at around 340–360 nm in DMSO
(Fig. 2).
N
H protons of metal-free phthalocyanine (4) were also iden-
tified in the 1H NMR spectrum (Fig. 1) with a singlet peak at
ı = −5.34 ppm, and the signals disappeared after the addition of D2O
[39,40]. The 1H NMR spectrum of compound 4 indicated signals
were observed at ı = 7.79–7.71 ppm, 7.09–6.98 ppm and 6.79 ppm
for the aromatic protons and 4.10 ppm, 3.06 ppm, 2.91 ppm,
1.99 ppm for aliphatic CH2 protons. The 13C-NMR spectrum of
4 showed signals at ı = 160.04, 136.99, 129.04, 128.18, 122.89,
118.28, 107.47, 104.54, 66.51, 64.61, 57.41, 52.41, 35.25 ppm. In
the mass spectrum of compound 4, the presence of the characteris-
tic molecular ion peak at m/z = 1255 [M]+ confirmed the proposed
structure.
In the IR spectrum of zinc(II) phthalocyanine compound (5)
the sharp vibration for the
C N groups in the IR spectrum of
phthalonitrile compound (3) at 2231 cm−1 disappeared after con-
version into zinc (II) phthalocyanine. The 1H NMR spectrum of
zinc (II) phthalocyanine showed aromatic ring protons at 8.87 ppm,
8.42 ppm and 7.44 ppm as broad peaks, and the aliphatic protons
were observed at 4.49 ppm, 3.98 ppm and 3.09 ppm as broad peaks

E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
71
Fig. 1. 1H-NMR spectrum of metal-free phthalocyanine 4.
1.8
1.5
1.2
0.9
0.6
0.3
0
are perpendicular to the line connecting their centers. In contrast,
in J-aggregates, the component monomers adopt a side-by-side
conformation, and their transition dipoles are parallel to the line
connecting their centers [47]. Aggregation is highly depends on
some parameters such as concentration, temperature, nature of the
substituents, nature of solvents and complexed metal ions [48].
Aggregation behavior of metal-free (4) and zinc (II) (5) Pc com-
pounds was investigated in different solvents (Fig. 3). Compound
4 did not show any aggregation in chloroform, dichloromethane
and THF, but this compound showed a little bit H-type aggre-
gation (Fig. 3a) in both DMF and DMSO due to coordinating
ability of these solvents. On the other hand the zinc (II) Pc com-
pound (5) did not show aggregation in DMSO, DMF and THF,
but this compound showed splitting Q bands at 678 and 698 nm
in both chloroform and dichloromethane (Fig. 3b). The observed
red-shifted splitting band at 698 nm could be due to protonation
of nitrogen atoms on phthalocyanine skeleton or formation of J-
aggregates between phthalocyanine molecules in these solvents.
Chloroform (or dichloromethane) in fact usually contains traces
of acids which are probably protonating the Pcs giving rise to
the peak at 698 nm which would be the result of a protonation
process. Trifluoroacetic acid (TFA) was added to chloroform solu-
tion of complex 5 for understanding the reason of the formation
of splitting Q band (protonation or formation of J-aggregates). A
new band was observed at 713 nm instead of 698 nm by addi-
tion of TFA (Fig. 4). It is suggesting that the splitting of Q band in
chloroform (or dichloromethane) is not due to protonation of com-
plex 5. The phthalocyanine compounds can be form J-aggregates in
non-coordinating solvents such as chloroform or dichloromethane
and J-aggregation can be deducible exhibition of red-shifted
4
5
300
400
500
600
700
800
Wavelenght (nm)
Fig. 2. Absorption spectra of compounds
tion = 1 × 10−5 M.
4 and 5 in DMSO. Concentra-
3.3. Aggregation studies
High aggregation tendency of phthalocyanine compounds due
to the interactions between their 18 ␲-electron systems often cause
weak solubility or insolubility in many solvents. It also affects
seriously their spectroscopic, photophysical, photochemical and
electrochemical properties. It has been established that phthalo-
cyanines can form H- and J-type aggregates depending on the
orientation of the induced transition dipoles of their constituent
monomers [44–46]. In H-aggregates, the component monomers are
arranged into a face-to-face conformation, and transition dipoles
Table 1
Absorption, excitation and emission spectral data for unsubstituted zinc (ZnPc), substituted metal-free (4) and substituted zinc (II) (5) phthalocyanine compounds in DMSO.
Compound
Q band ꢄmax, (nm)
log ε
Excitation ꢄEx, (nm)
Emission ꢄEm, (nm)
Stokes shift ꢁStokes, (nm)
4
5
676,704
684
4.65,4.54
5.17
5.14
675,710
687
714
695
682
10
11
10
ZnPca
672
672
a
Data from Ref. [54].

72
E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
1,2
0,9
0,6
0,3
0
solvents. It is suggesting that complementary coordination
(a)
between the nitrogen atoms on the substituted groups in one
phthalocyanine molecule and zinc (II) metal ion of another phthalo-
cyanine molecule could be responsible for the formation of
J-aggregation. On the other hand, the linker oxygen atoms may
also interact with the central zinc (II) metal ion. Generally, oxy-
gen atoms can be coordinated with zinc (II) when they attached to
phthalocyanine framework at non-peripheral position (not periph-
eral position as in this study). It could not be possible interaction of
the oxygen atoms on the substituents with zinc (II) metal ion due
to distance between these oxygen atoms and zinc (II) metal ion. As
a result, the J-aggregation was formed among the zinc (II) phthalo-
cyanine molecules due to interaction between nitrogen atoms on
the substituents and zinc (II) metal ions in the phthalocyanine
cavity in this study. Fig. 5 shows the slipped-cofacial structure of
J-aggregates of zinc (II) phthalocyanine compound (5). The stud-
ied tetra-substituted phthalocyanine compounds were obtained
as a mixture of four regioisomers of C4h, D2h, C2v and Cs symme-
try, but only the C4h symmetrical structure is shown in Fig. 5 for
complex 5. It should be noted that unfortunately, we were not
able to separate the isomeric mixtures of studied tetra-substituted
phthalocyanines. On the other hand, only monomer band at 678 nm
was appeared and J-aggregation band at 698 nm disappeared by the
addition of methanol to the J-aggregated solution (Fig. 6). Thus, the
addition of a coordinating solvent breaks J-aggregation completely
as a result of the competitive coordination of MeOH molecules to
central Zn atoms [10,15,49]. The formation of J-aggregation among
the zinc (II) phthalocyanine molecules instead of protonation was
also confirmed by the appearance of peaks in its MALDI-TOF mass
spectrum (Fig. 7) at 2636.717 Da for dimer formation, 3954.802 Da
for trimer formation and 5272.496 Da for tetramer formation of the
molecules.
DMSO
DMF
THF
DCM
Chloroform
300
400
500
600
700
800
Wavelength (nm)
1,8
1,5
1,2
0,9
0,6
0,3
0
(b)
DMSO
DMF
THF
DCM
Chloroform
300
400
500
600
700
800
Beer–Lambert law could be obeyed for studied metal free (4)
and zinc (II) (5) Pc compounds in non-aggregated solvents in the
concentrations ranging from 1.2 × 10−5 to 2 × 10−6 M (Fig. 8 for
compound 5 in DMSO).
Wavelength (nm)
Fig. 3. Absorption spectra of metal-free Pc compound 4(a) and zinc (II) Pc compound
5(b) in different solvents.
splitting Q band in these solvents. It is suggesting that the zinc (II)
Pc complex (5) exhibits J-type aggregates instead of protonation
in these solvents as evidenced by the appearance of a red-
shifted band at 698 nm due to non-coordinating behavior of these
3.4. Fluorescence studies
The fluorescence behavior of the metal-free (4) and zinc (II) (5)
phthalocyanine compounds was studied in DMSO. Fig. 9 shows
0.8
0.6
0.4
0.2
0
5 in CHCl 3
5 in CHCl3+TFA
300
400
500
600
700
800
Wavelength (nm)
Fig. 4. Absorption spectra of zinc Pc compound 5 in chloroform and changing by addition of trifluoroacetic acid (TFA).

E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
73
O
O
O
N
O
O
N
O
N
N
Zn
N
N
N
N
O
N
N
O
N
O
O
N
O
O
N
O
O
N
O
O
O
N
O
N
N
N
Zn
N
N
N
N
O
N
O
O
N
O
O
O
Fig. 5. The slipped-cofacial structure of J-aggregates of substituted zinc (II) Pc compound (5). This figure only shows the C4h isomer of zinc (II) Pc compound (5).
fluorescence emission, absorption and excitation spectra of com-
pound 5 in DMSO as an example. Metal-free Pc 4 and zinc Pc
complex 5 showed similar fluorescence behavior in DMSO. The
excitation spectra were similar to absorption spectra and both
were mirror images of the fluorescent spectra for phthalocyanine
compounds 4 and 5 in DMSO. The proximity of the wavelength
of each component of the Q-band absorption to the Q band max-
ima of the excitation spectra for all studied compound suggested
that the nuclear configurations of the ground and excited states are
similar and not affected by excitation. The observed Stokes shifts
were within the region (∼10–20 nm) observed for Pc complexes
(Table 1).
1.2
0.9
3.5. Fluorescence quantum yields and lifetimes
The fluorescence quantum yields (˚F) of metal-free (4) and zinc
(II) (5) phthalocyanine compounds in DMSO are given in Table 2.
The ˚F values of the studied phthalocyanine compounds (4 and 5)
are slightly higher than unsubstituted zinc Pc in DMSO. The increas-
ing in the (˚F) value for substituted phthalocyanine complexes in
the presence of the ring substituents suggests that the substituents
less quench the excited singlet state then the fluorescence.
0.6
Chloroform
Chloroform+Methanol
Fluorescence lifetime (ꢀF) refers to the average time a molecule
stays in its excited state before emission, and its value is directly
related to that of ˚F. Any factor that shortens the fluorescence
lifetime of a fluorophore indirectly reduces the value of ˚F. Such
factors include internal conversion and intersystem crossing. As a
result, the nature and the environment of a fluorophore determine
its fluorescence lifetime. Lifetimes of fluorescence were calculated
using the Strickler–Berg equation. Using this equation, a good
correlation has been found [50] between experimentally and the
theoretically determined lifetimes for the unaggregated molecules
0.3
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 6. Absorption spectra of compound 5 in chloroform and changing by addition
of methanol.

74
E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
Fig. 7. MALDI-TOF mass spectrum of zinc (II) phthalocyanine compound (5) with dihydroxybenzoic acid matrix.
as is the case for DMSO in this work. While the ꢀF values of
metal-free Pc (4) and zinc (II) Pc (5) compounds are higher than
unsubstituted zinc Pc, compound 4 has also the highest value in
DMSO. The natural radiative lifetime (ꢀ0) and the rate constants for
fluorescence (kF) values are also given in Table 2. The ꢀ0 values of
metal-free Pc (4) and zinc Pc (5) compounds are higher while the kF
values of these compounds (4 and 5) are lower than unsubstituted
zinc Pc complex in DMSO.
Table 2
Photophysical and photochemical parameters of unsubstituted zinc (ZnPc), substituted metal-free (4) and substituted zinc (II) (5) phthalocyanine compounds in DMSO.
a
Compound
˚
F
ꢀF (ns)
kF (s−1) (× 108)
ꢀ0 (ns)
˚
d
(× 10−5
)
˚
ꢁ
4
5
ZnPc
0.21
0.23
0.20b
3.38
1.78
1.22c
0.62
1.29
1.47c
16.32
7.72
2.16
2.01
2.61c
0.49
0.71
0.67c
6.80c
a
kF is the rate constant for fluorescence. Values calculated using kF = ˚F/ꢀF.
Data from Ref. [32].
Data from Ref. [54].
b
c

E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
75
2
1.6
1.2
0.8
0.4
0
energy between excited triplet state of photosensitizer and ground
state of oxygen to generate large amounts of singlet oxygen, essen-
tial for PDT. The singlet oxygen quantum yield (˚ꢁ) value gives
the amount of the singlet oxygen generation and this value is an
indication of the potential of the complexes as photosensitizers
in applications where singlet oxygen is required. The ˚ꢁ values
were determined using a chemical method (using DPBF in DMSO
as quenchers). The disappearance of DPBF was monitored using
UV–vis spectrophotometer (Fig. 10 as an example for compound
5 in DMSO) Many factors are responsible for the magnitude of
the determined quantum yield of singlet oxygen including such
as 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 decrease in the
Q band of formation of the studied metal-free (4) and zinc (II) (5)
phthalocyanine derivatives during ˚ꢁ determinations (Fig. 10 as
an example for compound 5 in DMSO).
2.0
1.5
1.0
0.5
0.0
y = 146801x + 0.030
= 0.999
R
2.00E-06
4.00E-06
6.00E-06
8.00E-06
1.00E-05
1.20E-05
0.00E+00 3.00E-06 6.00E-06 9.00E-06 1.20E-05
Concentration
300
400
500
600
700
800
Wavelength (nm)
Fig. 8. Absorption spectra of compound 5 in DMSO at different concentrations.
(Inset: Plot of absorbance versus concentration.)
Table 2 gives the ˚ꢁ values of substituted metal-free (4),
substituted zinc (II) (5) phthalocyanines and unsubstituted zinc
phthalocyanine which is used as standard for singlet oxygen studies
of phthalocyanine compounds. While the ˚ꢁ value of substituted
zinc (II) phthalocyanine compound (5) is higher, the ˚ꢁ value of
substituted metal-free (4) phthalocyanine compound is lower than
unsubstitued zinc (II) phthalocyanine in DMSO. The ˚ꢁ value of
3.6. Singlet oxygen quantum yields
Energy transfer from the triplet state of a photosensitizer to
ground state molecular oxygen leads to the production of sin-
glet oxygen. There is a necessity of high efficiency of transfer of
1000
750
500
250
0
1,6
1,2
Excitation
Emission
Absorbance
0,8
0,4
0
500
550
600
650
700
750
800
850
Wavelength (nm)
Fig. 9. Absorption, excitation and emission spectra of 5 in DMSO. Excitation wavelength: 650 nm.
1
0,9
0,6
0,3
0,8
0
0
10
20
30
Time (sec)
0,6
0,4
0,2
0
0 sec
10 sec
20 sec
30 sec
300
400
500
600
700
800
Wavelength (nm)
Fig. 10. Absorbance changes during the determination of singlet oxygen quantum yield of 5 in DMSO at a concentration of 1.0 × 10−5 M. (Inset: plots of DPBF absorbance
versus time.)

76
E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
1,6
1,2
0,8
0,4
0
substituted zinc (II) phthalocyanine compound (5) is also higher
than the ˚ꢁ value of substituted metal-free (4) phthalocyanine
compound in DMSO. Generally, zinc phthalocyanine compounds
possess high triplet yields and they can be generated highly sin-
glet oxygen since the d10 configuration of the central Zn2+ ion,
which make them more appropriate photosensitizers for PDT appli-
cations. ˚ꢁ value of employed metal-free (4) phthalocyanine
compound higher than the metal-free phthalocyanine derivatives
having different substituents on the phthalocyanine ring ranging
from 0.01 to 0.27 in the literature [51]. Generally, the ˚ꢁ value
of studied zinc (II) phthalocyanine compound (5) is also higher
than zinc (II) phthalocyanines having different substituents on the
phthalocyanine ring ranging from 0.07 to 0.88 in the literature
[52]. Only a few zinc (II) phthalocyanine complexes have higher
1,6
1,2
0,8
0,4
0
y = -8.73E-05x + 1.47
=0.995
R
0 min
0
600 1200 1800 2400 3000 3600
Time (sec)
10 min
20 min
30 min
40 min
50 min
60 min
300
400
500
600
700
800
˚ꢁ yields than studied zinc (II) phthalocyanine complex (5) in this
Wavelenght (nm)
study [52]. Although, the ˚ꢁ value of standard unsubstituted zinc
(II) phthalocyanine (ZnPc) is quite good, its cell penetration is low
and unsufficient for PDT applications.
The possible use of phthalocyanine derivatives in PDT can
not be overemphasized; considering the fact that the effective-
ness of photodynamic action is chiefly based on the generation
of singlet oxygen, suffice is it to conclude that phthalocyanine
derivatives studied in this work are potential candidates as regards
consideration in PDT. Singlet oxygen quantum yields of studied
Fig. 11. Absorbance changes during the photodegradation study of 5 in DMSO show-
ing the disappearance of the Q-band at 10 min intervals. (Inset: plot of Q band
absorbance versus time.)
800
(a)
2,2
2,0
1,8
1,6
700
[BQ]=0
600
1,4
y = 21.66x +0.97
R = 0.993
1,2
500
400
1,0
0,000
0,010
0,020
0,030
0,040
0,050
[BQ]
300
[BQ]=saturated
200
100
0
650
700
750
800
850
Wavelength (nm)
600
(b)
2,2
2,0
1,8
1,6
1,4
1,2
1,0
[BQ]=0
500
400
300
200
100
0
y = 24.81x + 1
R = 0.997
0,000
0,010
0,020
0,030
0,040
0,050
[BQ]
[BQ]=saturated
650
700
750
800
850
Wavelength (nm)
Fig. 12. Fluorescence emission spectral changes of: (a) compound 4 and (b) compound 5 (C = 1.00 × 10−5 M) on addition of different concentrations of BQ in DMSO. [BQ] = 0,
0.008, 0.016, 0.024, 0.032, 0.040 M and saturated with BQ. (Insets: Stern–Volmer plots for benzoquinone (BQ) quenching compounds 4 and 5 in DMSO.)

E.T. Saka et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 67–78
77
Table 3
4. Conclusion
Fluorescence quenching data for unsubstituted zinc (ZnPc), substituted metal-free
(4) and substituted zinc (II) (5) phthalocyanine compounds in DMSO.
The novel metal-free (4) and zinc (II) (5) phthalocyanines
bearing four 1-(2-oxyethyl)-4-piperidone ethylene ketal groups at
peripheral position have been synthesized for the first time in this
study. The studied compounds were characterized by UV–vis, IR,
1H-NMR, ESI mass spectroscopies and elemental analysis. It was
concluded from the investigation of the spectroscopic behavior
of these novel compounds in various solvents that the zinc (II)
phthalocyanine compound (5) formed J-aggregates in chloroform
and DCM as a result of the complementary coordination of the
nitrogen atom in the substituents of one molecule to the central
zinc metal atom of another molecule through hydrogen bonding.
The formation of J-aggregates among the zinc (II) Pc molecules was
determined by UV–vis and MALDI-TOF spectroscopic techniques.
The J-aggregates among the zinc (II) Pc molecules were broken
up by the addition of a coordinating solvent such as methanol.
The photophysical (fluorescence quantum yields and lifetimes)
and photochemical properties (singlet oxygen and photodegrada-
tion quantum yields) of metal-free (4) and zinc (5) phthalocyanine
compounds were investigated in DMSO. The substituted zinc Pc
compound (5) shows good singlet oxygen generation and gives
an indication of the potential of this complex as photosensi-
tizer in photocatalytic applications such as photodynamic therapy
of cancer. The fluorescence quenching behavior of the studied
phthalocyanine compounds (4 and 5) have also been studied by
BQ in DMSO.
Compound
KSBVQ/(M−1
)
kq/1010 (M−1 s−1
)
4
5
21.66
24.81
31.90
0.64
1.39
2.61
ZnPca
a
Data from Ref. [54].
phthalocyanine derivatives ranged from 0.49 to 0.71 in DMSO.
Photosens®, which has been in use for PDT has a singlet oxygen
quantum yield of 0.42, which is about the least value obtained for
the studied phthalocyanine derivatives studied.
3.7. Photodegradation studies
Degradation of the molecules under irradiation can be used
to study their stability and this is especially important for those
molecules intended for use in photocatalysis. The collapse of the
absorption spectra without any distortion of the shape confirms
photodegradation not associated with phototransformation into
different forms of photosensitizers absorbing in the visible region.
The spectral changes observed for metal-free (4) and zinc (II) (5) Pc
complexes during confirmed photodegradation occurred without
phototransformation (Fig. 11 as an example for compound 5).
The photodegradation quantum yield (˚d) values of metal-free
(4) and zinc (II) (5) Pc complexes as well as unsubstituted zinc (II)
phthalocyanine in DMSO are given in Table 2. All the studied com-
Acknowledgement
plexes showed about the same stability with ˚d of the order of 10−5
.
The ˚d values, found in this study, are similar with zinc Pc com-
plexes having different substituents on the phthalocyanine ring in
literature [52]. Stable zinc phthalocyanine complexes show ˚d val-
ues as low as 10−6 and for unstable molecules, values of the order of
10−3 have been reported [52]. The metal-free (4) and zinc (II) (5) Pc
complexes showed slightly lower ˚d values when compared to the
unsubstituted ZnPc in DMSO. The substitution of the phthalocya-
nine framework with 1-(2-oxyethyl)-4-piperidone ethylene ketal
groups slightly increased the stability of studies phthalocyanine
compounds. The substituted zinc (II) phthalocyanine compound
(5) is a little bit more stable than metal-free phthalocyanine (4)
compound in DMSO.
This study was supported by the Research Fund of Karadeniz
Technical University, Project no: 2010.111.002.5 (Trabzon-Turkey).
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