Synthesis, photophysical and photochemical properties of amphiphilic carboxyl phthalocyanine oligomers

Article abstract of DOI:10.1039/b904569d

The synthesis and photochemical properties are reported for a series of novel amphiphilic carboxyl polymeric phthalocyanines, with zinc (3a), aluminum (3b), ytterbium (3c) and hydrogen (metal-free, 3d) as the substituted central atom, respectively. The sy

Full text of DOI:10.1039/b904569d

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Synthesis, photophysical and photochemical properties of amphiphilic
carboxyl phthalocyanine oligomers
Peng Zhao,a,c Yenan Song,b Sida Dong,a Lihong Niua and Fushi Zhang*a
Received 11th March 2009, Accepted 14th May 2009
First published as an Advance Article on the web 30th June 2009
DOI: 10.1039/b904569d
The synthesis and photochemical properties are reported for a series of novel amphiphilic carboxyl
polymeric phthalocyanines, with zinc (3a), aluminum (3b), ytterbium (3c) and hydrogen (metal-free, 3d)
as the substituted central atom, respectively. The synthesis routes included cyclotetramerization of
tetra-phthalonitriles and subsequent hydrolysis of cyano to carboxyl in alkaline solution. All four
molecules were verified to be oligomers by viscosity method. Specifically, 3c showed strongest
fluorescent emission, which can be elevated by Triton X 100 and CTAB. The singlet oxygen quantum
yield (UD) for 3a and 3b were larger than that of 3c and 3d. In the presence of surfactants, nearly 2 folds
enhancement of UD was observed as 0.76 and 0.70 for 3a and 3b, respectively. The characteristic
photochemical properties of these oligomers suggested potential applications in photodynamic therapy,
photocatalysis and photodynamic diagnoses.
Good solubility and bio-compatibility are crucial to many
photosensitive processes. The PDT and fluorescent imaging
1. Introduction
diagnosis were actualized in neutral tissue media,10,11 and they
needed the photosensitizer with amphiphilic solubility, both in
water and fat, but common Pcs are hardly soluble in water and
organic solutions. An effective way to improve their solubility is
to substitute with hydrophilic groups (sulfonate or carboxyl) and
hydrophobic alkyl groups in the molecule. Nyokong12 reported
octa-triethylene-oxysulfonyl-zinc-phthalocyanine with a quantum
yield of singlet oxygen (UD) of 0.70 in DMSO. Ng13 synthe-
sized asymmetric phthalocyanine-substituted with polyethylene
glycol groups with good in vitro photocytotoxicity. Masilela14
investigated the octacarboxyl zinc phthalocyanine with UD of
0.52 in pH 10 solution. The carboxyl group can facilitate the
couple of Pcs with bioactive substance (such as antibodies and
DNAs) through amide bonds. Several carboxyl Pcs have already
been verified to be compatible with Albumin Bovine.15,16 This
bio-conjugative capacity is important to the preparation of the
third generation photosensitizer.7 Meanwhile, we noticed that
recently the synthesis of phthalocyanine tended to be large-scale or
supramolecular.13,17,18 Therefore, the phthalocyanine produced on
a large scale, with enough active sites and amphiphilic solubility,
is valuable for the investigation of a novel photosensitizer.
The polymeric phthalocyanines are other important groups of
large-scale Pcs but with little research as photosensitizers. In fact,
the first generation photosensitizer drug PhotofrinTM was a mix-
ture of porphyrin oligomers. However, the polymerization made
the solubility of these compounds even worse than monomeric
Pcs. In a recent work, our lab found that the hydrolysis of cyano
groups to carboxyl in the polymer can improve the solubility of
polymeric phthalocyanine in aqueous solution.19 In this study,
our target is to prepare novel phthalocyanine photosensitizer with
amphiphilic solubility using the above method. The correlated
photophysical and photochemical properties of synthesized pho-
tosensitizer, such as quantum yields of fluorescence, singlet oxygen
and photobleaching were investigated in detail. The influence of
The phthalocyanine molecule has the structure of tetrabenzo-
tetraaza-porphyrin. The phthalocyanine compounds (Pcs) are
provided with unique properties in physics, photochemistry and
electricity, which derive from their planar 18 p-electron conjugated
orbit.1 In the latest 30 years, Pcs have been the most important
photosensitive compounds following with the development of
photochemical technology. These compounds are widely used
in photocatalysis, photolithography, solar cells, non-linear optics
and photodynamic therapy (PDT).2–6 The excellent photosensitive
properties of Pcs originate from several advantages: their strong
absorption to visible light and high yield of singlet excited state;
isosystem conversion from the singlet state to the triplet state;
the triplet state energy of them is higher (ET>1.1 eV) than that
of the singlet state of oxygen (0.98 eV).7 These characteristics
facilitate the energy transfer from the triplet state of Pcs to the
oxygen at the ground state and produce singlet oxygen effectively
under visible light irradiation. Singlet oxygen is an unstable and
non-selective strong oxidant, which takes an important role in
further reactions, such as damaging the cancer cell or oxidizing
the environmental hazard. This photo-functional process is named
as the photosensitization through type II mechanism;7 common
phthalocyanine photosensitizers, with zinc, aluminum, cadmium
or phosphorus5,8–9 as central atoms, usually perform as this
mechanism. The solvent, photosensitizer, and exited light are key
factors in the type II mechanism.8
aThe Key Lab of Organic Photoelectrons & Molecular Engineering of Min-
istry of Education, Department of Chemistry, Tsinghua University, Beijing,
100084, P. R. China. E-mail: zhangfs@mail.tsinghua.edu.cn; Fax: +86-10-
62770304; Tel: +86-10-62782456
bDepartment of Applied Chemistry, School of Science, Harbin Insistitute of
Technology, Harbin, 150001, P. R. China
cDepartment of Industry Chemistry, College of Natural Science, Sangmyung
University, Seoul, 110-743, R. Korea
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different surfactants on their photophysical and photochemical
properties is also discussed in the pH 7.4 buffer solution for further
understanding.
where UD,std is the singlet oxygen quantum yield for the standard
ZnPcS2 (0.4923), Iads,std and Iabs are the rates of light absorp-
tion by the testing samples and the standard, respectively. The
light intensity was determined as 1.2 ¥1015 photons s-1 cm-2.
The photo bleaching rate of RNO was already investigated by
Mosinger,24 and the uncertainty was estimated to be ca. 5% in this
measurement.
2. Experimental
2.1 Materials and methods
2.1.3 Quantum yields of photobleaching (UP). The quantum
yield of photobleaching were determined under the same light
source without the addition of a singlet oxygen scavenger. The
singlet oxygen fully participates in the photobleaching in this
condition. The first order kinetics of photobleaching was assumed
here. The UP was calculated by eqn (3)12:
3-Nitro-phthalonitrle, pentaerythritol, n-octan-1-ol, dimethyl for-
mamide (DMF), 1,8-diaza bicycle[5,4,0]-undec-7-ene (DBU),
p-nitroso-N,N’-dimethylaniline (RNO), imidiazole, cetyl tri-
methyl ammonium bromide (CTAB) and Triton X 100TM in
analytical or chemical grade were purchased from commercial
sources. Aluminum trichloride, zinc acetate and ytterbium acetate
were anhydrous salts. Zinc phthalocyanine disulfonate (ZnPcS2)
was synthesized in our lab.20
UP = (C0 - Ct)VNA/IabsSt
(3)
where V is the reaction volume (m3). C0 and Ct are the concentra-
tion of Pcs before and after irradiation. S is the illuminated area
(m2) and t is irradiated time. Iabs is the overlap integral of radiation
source light intensity and the adsorption of test samples (1.3 ¥ 1016
photons s-1 cm-2).12 NA is Avogadro’s number. In both experiments,
the solid surfactants (CTAB and Triton X 100) were added in the
test solutions at the concentration of 0.1% (by weight).
NMR was performed on a JOEL JNM-ECA300 spectrometer.
MALDI-TOF MS test was conducted in methanol on a BIFLEX
III spectrometer. Electronic spectra were recorded on a HP
5324A UV-vis array diode spectrometer. Fluorescence spectra
were recorded by a Hitachi F4500 spectrometer with a setting of
5 nm grating slit and 700 V amplifier of the photomultiplier. The
fluorescence decay curves were obtained by an Edinburgh FLS920
transient/steady state spectrometer. FT-IR was obtained from an
Avata 360A spectrometer using solid KBr pellets. Ionic coupling
plasma-atomic emission spectrometry (ICP-AES) of the metallic
content were obtained with an IRIS Intrepid II XS spectrometer.
The element analysis was performed on a Vario EL 3 element
analyzer. The average viscometric molecular weight (Mn) was
measured on an Ubbelohde viscometer at 298 K in water using
the method reported in the literature21 with PVP-K30 (Mn 44 000–
54 000) as standard.
2.2 Synthesis of tetrakis-[(2,3-dicyanophenoxy)-methyl]methane
(1a)25
3-Nitro-phthalonitrile (17.3 g, 0.1 mol) and pentaerythritol (3.4 g,
0.025 mol) were stirred in 200 mL DMF for 72 h at room
temperature using 21.2 g (0.2 mol) anhydrous potassium carbonate
(added in portions) as desiccant. The product was then poured
into water (1 L), filtrated and washed with acetone and THF. The
filter cake was stirred in 200 mL ethyl acetate for 1 h and
filtrated to get the solid. The raw product was recrystallized
in acetonitrile to be purified. The light pink colored prod-
uct was tetrakis-[(2,3-dicyano-phenoxy)methyl]methane (10.0 g,
62.5%).
2.1.1 Fluorescence quantum yield (UF). The fluorescence
quantum yields were determined by comparative methods as
described elsewhere22 with ZnPcS2 as the standard (UF = 0.1823,
in DMF) in pH 7.4 buffer. The value was calculated by:
dH (300 MHz; DMSO-d6; Me4Si): 7.92 (1 H, t, J = 3 Hz, Ph),
7.80 (1 H, d, J = 2.6 Hz, Ph), 7.25 (1 H, d, J = 2.5 Hz, Ph), 4.65
(2 H, s, CH2) ppm. dC (300 MHz; DMF): 56.3 (s), 67.4 (s), 135◦
DEPT: 56.3 (CH2) ppm. MS (in DMSO): m/z 640 (M+, 67%).
Elemental analysis, found: C 69.30, H 3.21, N 17.53, O 10.07.
Calcd for C37H20N8O4: C 69.37, H 3.15, N 17.49, O 9.99%.
UF = UF,std (SAstdn2/SstdAn2
)
(1)
std
where S and Sstd are the areas under the emission curves of
the sample and standard; A and Astd are the absorbance of the
sample and standard; and n and nstd are the refractive indices
of the solvents used for sample and standard, respectively. The
absorbance of the test solutions at the excitation wavelength was
about 0.3. The lifetime tF was determined by fitting the decay
curves with the Strichler–Berg equation. Overall uncertainty in
the measurement was estimated to be ca. 5–8%.
2.3 Synthesis of polymeric phthalocyanine (2a, 2b, 2c)
1a (1.28 g, 0.002 mol), 0.004 mol metal salt (anhydrous AlCl3,
Zn(Ac)2, Yb(Ac)3) and 1 mL DBU were pre-mixed in 15 mL
n-octan-1-ol for 1 h by stirring and then heated to reflux for 5 h
under argon. The green solid product was filtered and washed by
hydrochloric acid (6 M) to remove the residual metallic salt. The
product was then stirred in 100 mL DMF for 2 h. After filtering
and washing with acetone several times, a dark green powder (2a,
2b, 2c) was obtained. See (Scheme 1).
2.1.2 Quantum yield of singlet oxygen (UD). The quantum
yield of singlet oxygen was measured using the relative method
reported by Mosinger24 with ZnPcS2 as the standard in solution
at pH 7.4. The imidazole with RNO was used as the singlet
oxygen scavenger. A Philips halogen lamp (30 W) with a 600 nm
glass cut off filter (Beijing Optics Factory) was used as the light
source. Testing solution was bubbled with air using a Scorpion
I pump. Sampling times were chosen from 3 to 8 min based on
different reaction rates. The following equation was employed in
the calculation:
2.4 Carboxyl polymeric phthalocyanine (3a, 3b, 3c)
For each synthesis, 0.5 g of the corresponding precursor (2a,
2b, and 2c) was stirred with 50 mL 15% NaOH solution at
80 ◦C for 12 h, after which a green solution was obtained.
UD = UD,std(Iabs, std/Iabs
)
(2)
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Scheme 1 The synthesis of polymeric carboxyl phthalocyanine 3a, 3b, 3c and 3d.
The insoluble residue was removed via centrifuge, and then
3. Results and discussions
6 M HCl was used to adjust the acidity of the solution to
pH = 1 and a green product precipitated. The precipitate was
then dissolved back into the NaOH solution. This process was
repeated three times and the resulting green solid product (3a,
3b, 3c) (Scheme 1) was gathered from the acidic solution. Fine
purification was achieved by chromatography on an acidic alumina
column using DMF as eluent, the first light green component was
discarded and the successive dark green solution was collected
containing the bulk product. The purified product (3a, 3b, 3c)
was acidified out by pouring the DMF solution into a water
solution at pH 1. The yields varied from 52 to 66% with different
compounds.
3.1 Synthesis and characterization
The formation of polymeric phthalocyanine needs a hydrogen-
abundant solvent and a higher temperature than normal
phthalocyanine.26 Generally used solvents in the synthesis of
monomeric phthalocyanine, such as n-pentan-1-ol, ethylene glycol
and DMF, were not qualified for this reaction due to their
low boiling points (around 130–140 ◦C). Solvents with boiling
point near 200 ◦C, such as n-octan-1-ol, tetralin, nitrobenzene
and quinoline, were better for the polymerization. In this study,
n-octan-1-ol was selected as solvent due to its low polluted hazard.
The strong base DBU was used as catalyst to accelerate the cyclic-
condensation in the reaction.
In the hydrolysis step to form water soluble carboxyl polymeric
phthalocyanine, the eight cyano groups of 1a was a critical factor.
The cyano groups can not fully react in the polymerization due
to steric hindrance. Some of them were left as residue groups in
the polymer 2a, 2b, 2c, 2d (Scheme 1), which can be hydrolyzed to
produce adequate carboxyl groups. These hydrophilic carboxyl
groups were the key points to make compounds 3a, 3b, 3c,
3d soluble in alkaline solution. In comparison, the previously
reported27 phthalocyanine polymers synthesized from bi-phthalo
nitrile did not have enough cyano residues to form carboxyl groups
to be soluble in water.
Metal-free carboxyl polymeric phthalocyanine 3d was obtained
by heating 1 g of 1a in n-octan-1-ol without any metal salt and
hydrolyzed the product 2d in NaOH solution. The product was
then purified as described by the above processes.
3a: FT-IR (KBr pellet): nmax/cm-1 1735 s (C O), 3300–3600 br
=
(COOH), 1100 s, 1350 s, 1455 s, 1557 s (Ph). dH (300 MHz;
DMSO-d6; Me4Si): 7.6–8.3 (m, Ph), 11.2–13.5 (br COOH)), 4.3–
4.6 (CH2, m). dC (300 MHz; DMF) 175–178 (COOH), 104.3,
116.0, 118.1, 126.8, 136.3, 160.6 (Ph), 48.3 (CH2), 68.2 (quaternary
C). Elemental analysis: found: C 70.23, H 4.53, N 17.13, O 7.82,
COOH 5.1, Zn (atom) 5.5%.
3b: FT-IR (KBr pellet): nmax/cm-1 1737 s (C O), 3250–3550 br
=
(COOH), 1102 s, 1350 s, 1453 s, 1567 s (Ph). dH (300 MHz;
DMSO-d6; Me4Si): 7.7–8.3 (m, Ph), 11.5–13.1 (br, COOH), 4.4–
4.7 (m, CH2). dC (300 MHz; DMF): 175–178 (COOH), 104.7,
116.3, 118.3, 126.1, 135.2, 160.6 (Ph), 48.1 (CH2), 68.3 (quaternary
C). Elemental analysis: found: C 70.65, H 4.13, N 17.54, O 7.57,
COOH 4.7, Al (atom) 4.9%.
The results of solubility tests for the four compounds proved
the effectiveness of hydrolysis. The data in Table 1 showed
compatible solubility of 3a, 3b, 3c and 3d, which were better
than sulfonate or alkyl Pcs. They can dissolve in polar solu-
tions (C2H5OH), low-polar acetate (AcOEt, acetic acetate) and
aqueous (pH 5.85 and 7.4) buffers. The amphiphilic solubility
3c: FT-IR (KBr pellet): nmax/cm-1 1740 s (C O), 3300–3600 br
=
(COOH), 1150 s, 1320 s, 1485 s, 1650 s (Ph). dC (300 MHz;
DMF): 176–178 (COOH), 102.3, 116.7, 118.3, 126.5, 136.1, 158.6
(Ph) 48.2 (C H2), 68.2 (quaternary C). Elemental analysis: found:
C 68.93, H 4.23, N 16.33, O 7.22, COOH 4.8, Yb (atom)
5.1%.
Table 1 The solubility test for 3a, 3b, 3c and 3d in different solutions
3a
3b
3c
3d
b
b
b
b
b
a
b
b
a
a
b
AcOEt
C2H5OH
DMSO
DMF
CHCl3
pH 5.85
pH 7.4
3d: FT-IR (KBr pellet): nmax/cm-1 1720 s (C O), 3100–3600 br
=
12.3 g L-1
(COOH), 1153 s, 1320 s, 1475 s, 1640 s (Ph). dH (300 MHz;
DMSO-d6; Me4Si): 7.5–8.3 (m, Ph), 10.3–13.8 (br, COOH)),
3.7–4.2 (m, CH2), 2.6–2.9 (br, N–H). dC (in DMF): 175–179
(carboxyl C), 101.3, 114.3, 116.3, 127.5, 133.1, 155.6 (aromatic
C) 48.3 (C H2), 68.2 (quaternary C) (alkyl C). Elemental analysis:
found: C 72.32, H 4.69, N 17.33, O 7.99, COOH 4.5%.
10.3 g L-1
9.5 g L-1
13.2 g L-1
6.3 g L-1
a
a
a
a
a
a
a
a
1.6 g L-1
1.8 g L-1
1.5 g L-1
1.2 g L-1
a Solubility < 0.5 g L-1. b 0.5 g L-1 < solubility < 1.0 g L-1.
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was preference in the evaluation of novel photosensitizer
in PDT.
The FT-IR spectra of 2b and 3b before and after hydrolysis
(shown in Fig. 1) clearly monitored the formation of the carboxyl
species.
hydrogens, respectively. No signal was obtained for the ytterbium
phthalocyanine due to the possible paramagnetic effect.
The molecular weight of the polymeric phthalocyanines 3a,
3b, 3c and 3d were identified by viscosity method. The average
viscometric molecular weight (Mn) of 3a, 3c, 3d and 3d was
measured to be 8000–12 000 in 10 parallel experiments. We
also found that 2b and 2d had a Mn of 7000–15 000 using
the same method. This indicated that these polymers can be
classified as oligomers consisting of 10–20 phthalocyanine units.
The molecular weight of the polymeric units (one phthalocyanine
unit) was estimated to be about 600–800 based on the structure in
Scheme 1.
In the synthetic procedure (Scheme 1), the mass conservation
of C and N consists of 1a to 2a, 2b, 2c and 2d; while only C
element is balanced to form 3a, 3b, 3c and 3d for the difficulty
of complete hydrolysis of the cyano groups. We checked the
content of C and N by elemental analysis. The proportion of
C : N (about 3.9) of 2a, 2b, 2c and 2d was consistent with 1a
(C37H20N8O4, C : N 3.96), indicating no by-product formation
during the cyclotetramerization. The carboxyl content of 3a, 3b,
3c3d is all around 4.5%, showing similar hydrolysis degree of
them. It is about 10 carboxyl groups in one oligomer (10–20
phthalocyanine units). The aqueous solubility of the oligomer
originated form here. The metallic content of 3a, 3b and 3c is about
5% (at%) by ICP-AES, indicating no remarkable demetallization
during the alkaline hydrolysis, comparing to the value of 2a, 2b
and 2c (about 6%).
Fig. 1 The FT-IR spectrum of 2b and 3b.
The disappearance of the peak at 2230 cm-1 indicated the loss
of cyano groups during hydrolysis. Corresponding peaks of the
newly formed –COOH groups were seen at 3400–3600 cm-1 and
1700–1750 cm-1. The intensity of the peaks at 1200–1250 cm-1
should be the CO–OH stretching in COOH. The characteristic
peaks of phthalocyanine at 1200–1600 cm-127 did not change
significantly, which implied that there was no decomposition of
the phthalocyanine during hydrolysis in alkaline solution.
The results the of 13C NMR experiment (in Fig. 2 for 3b)
supported the conclusion from the FT-IR as well. The peak of
COOH was found at the chemical shift of d 175–180 ppm, which
illustrated the hydrolysis of cyano group to carboxyl group. The
chemical shift of quaternary carbon (d 68–69 ppm) and CH2
(d 48–49 ppm) were similar to that of 1a, indicated the preservation
of tetrahedral structure after synthesis. The chemical shifts of
the corresponding carbon atoms in all the compounds (3a, 3b,
3c and 3d) did not fluctuate on a large scale, which suggested
similar carbon skeletons and high repeat units. The signals of
2a, 2b, 2c and 2d were not obtained due to their poor solubility in
DMF. In the 1H NMR spectra, chemical shifts, at d 11–13, 7–9 and
4.2–4.7 ppm, were also assigned to the COOH, aromatic and alkyl
3.2 Electronic spectra
The electronic spectra of these compounds in different solvents
are shown in Fig. 3. Absorption intensity and peak positions are
listed in Table 2.
Aggregation is a common phenomenon for phthalocyanine
compounds, which can be reflected from the shapes of their Q
bands. Here, the aggregation tendency of these compounds was
Table 2 Absorption, excitation and emission data for 3a, 3b, 3c and 3d
in pH 7.4 buffer
Q band
max/nm
Ex.
Em.
Stokes shift,
D/nm
l
l
max/nm
l
max/nm
3a
690.6
695.7
683.3
697.3
680.0
625.2
635.6
625.3
650.2
—
712.2
715.3
721.1
720.0
695.7
21.6
19.6
37.8
22.7
15.7
3b
3c
3d
31
ZnPc·(COOH)16
Fig. 2 The 13C-NMR spectra of 3b.
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Fig. 3 (a) Electronic spectra for 3a, 3b, 3c and 3d in pH 7.4 buffer (c = 1 g L-1). (b) 3a in pH 7.4 buffer with and without surfactant. (c) 3b in pH 7.4
buffer with and without surfactant, (inset: the fitting to Beer’s law). (d) 3c in different solvent.
different from their central atoms. In Fig. 3(c), the sharp Q bands
of 3b did not change after addition of surfactant, implying a
lower aggregation tendency; while there were many reports about
monomeric aluminum phthalocyanines that aggregated in water.9
A good fitting to Beer’s law was obtained for the Q bands intensity
in pH 7.4 solution from 0.1–1.0 g L-1, which also proved the low
aggregation of 3b. On the contrary, 3a, 3c and 3d showed broad Q
bands due to their aggregation in pH 7.4 buffer.28 The supposed
reason was that the oligomer has a large number of conjugated
p orbitals in the Pcs moieties and may aggregate between the
molecules. The aggregation of the Pcs photosensitizer can enhance
the non-radiative dissipation of excited states and reduce the
singlet oxygen quantum yield.9 As we known, the addition of
surfactants can effectively disassociate the aggregation. For 3a,
after the addition of CTAB, the intensity of Q band increased and
the aggregation peak at 630 nm disappeared (Fig. 3(b)), indicating
that disaggregation occurred. But the Triton X 100 had a lesser
effect than that of CTAB, and the Q bands of 3a were minimally
changed. The detailed spectra for 3c in different solvents were
shown in Fig. 3(d). The broad peak indicated strong aggregation
in pH 7.4 buffer solution even with Triton X 100 and methanol.
Two split Q bands were found at 675 nm and 705 nm for 3c after
adding CTAB, which indicated the CTAB attributed to reduce the
aggregation.
ever discussed by Woehrle.30 The Stokes shift of 3a, 3b and 3d
were found to be about 20 nm, while 3c topped at 37.8 nm. It
suggested that the singlet state of 3c had larger polarity than the
others, while ZnPc(COOH)16 had a Stokes shift about 15.7 nm
only.31
3.3 Photophysical properties
The fluorescence spectra of 3c under different conditions were
shown in Fig. 4, the related photophysical parameters were listed
in Table 3.
Fig. 4 Fluorescence spectra of 3c under different conditions.
In Table 2, the Q bands position of 3a, 3b and 3d located around
690 nm. 3c had a hypsochromic shift to 683.3 nm caused by the
heavy rare earth atom.29 The usual Q band of monomeric carboxyl
phthalocyanines are at about 680 nm. The bathochromic shift of
3a, 3b and 3d were characteristics of polymeric phthalocyanine
Generally, 3b and 3c showed larger UF than 3a and 3d with
or without surfactants. For each compound the UF decreased
as the following sequence, 3b > 3d > 3c > 3a. The 3a with
strong aggregation has the smallest fluorescence due to only
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Table 3 Photophysical properties for 3a, 3b, 3c and 3d at pH 7.4 in different conditions
UF
tF/ns
+CTAB
+Triton
UF MeOH
+CTAB
+Triton
3a
0.18
0.35
0.23
0.27
0.60
0.44
0.28
0.40
0.71
0.45
—
0.36
0.50
0.87
0.53
—
<0.01
<0.01
0.02
<0.01
—
—
—
—
3b
5.3
7.8
—
5.25
—
10.6
12.7
—
—
2.0
82.2
130.1
—
—
—
3c
3d
H2PcS4 in DMF32
AlPc·(COOH)8 in pH 7.433
—
—
—
Table 4 Photochemical properties for 3a, 3b, 3c and 3d in pH 7.4 buffer solution with and without surfactant
UD
UD
UP 10-5
UP 10-5
UP 10-5
pH 7.4 buffer
UD
0.55
0.30
<0.01
0.11
+CTAB
+Triton
+CTAB
+Triton
3a
3b
3c
3d
0.64
0.69
0.05
0.15
—
0.76
0.70
0.06
0.16
0.54
—
3.3
0.6
—
—
2.6
—
38.6
3.4
—
—
—
25.2
1.3
—
—
7.0
—
34
ZnPcSmix
0.45
0.52
ZnPc·(COOH)8, pH 1014
—
—
non-aggregation ZnPc that fluoresced.34 The 3b has the largest
thalocyanine photosensitizer. The addition of surfactants can
effectively dissociate the self-aggregation of Pcs and enhance their
photophysical and photochemical parameters. Here, we also found
a similar phenomena affected by surfactant with several different
characters.
In Fig. 2, the addition of CTAB and Triton X 100 showed
common enhancement of fluorescent emission. Four-fold eleva-
tion of UF from 0.23 to 0.87 was observed for 3c in pH 7.4
buffer after the addition of Triton X 100; methanol can quench
this enhanced fluorescence. The remarkable enhancement and
quenching of fluorescence of 3c presented a meaningful method
to control fluorescence emission by the addition of surfactant.
Meanwhile, in Table 3, the Triton X 100 can increase the lifetime
of fluorescence (tF) about 20 fold for 3b and 3c. We noticed in
Fig. 3, the CTAB can effectively dissociate the aggregation; here
it affect weaker on tF than that of Triton X 100.
On the photochemical aspect, the enhancement by Triton X 100
behaved more effectively than that of CTAB on 3a and 3b. The
UD of 3b is 0.30 in pH 7.4 buffer, and two-fold enhancement to
0.70 was achieved with the addition of Triton X 100. Similarly,
the UD of 3a increased from 0.55 to 0.64 and 0.76, with the
presence of CTAB and Triton X 100, respectively. As Daraio35
reported, the CTAC can increase UD to 0.7 for tetracarboxyl zinc
phthalocyanine ZnPc(COOH)4. With increasing UD, the UP of
3a showed a tenfold increasing from 3.3 ¥ 10-5 to 38.6 ¥10-5
and 25.2 ¥ 10-5, in the presence of CATB and Triton X 100,
respectively, while no so large difference was observed for 3b.
It suggested that the aggregation zinc phthalocyanine was more
sensitive to photodegradation than its aggregated form. Similar
phenomena was observed by Spiller36 on tetra sulfonate zinc
phthalocyanine (ZnPcS4) under the same condition. The rapid
photobleaching of 3a in presence of surfactants facilitated its ap-
plication in homogenous photodegradation of organic pollutant
in water solution. However, the sensitizer in PDT needs more
balanced photobleaching capacity between residue photocytotox-
icity and effective curative time. 3b performed more stably to
quantum yield of fluorescence as 0.35; while the carboxyl phthalo-
33
cyanine AlPc(COOH)8 also had a UF of 0.44. The lifetime of
fluorescence for 3b and 3c is 5.3 and 7.8 ns, respectively. The value
is equivalent with normal fluorescent phthalocyanine compound
(H2PcS4, 5.25 ns).32
3.4 Photochemical properties
The capacity of the four polymers to generate singlet oxygen
under various conditions is listed in Table 4. Al3+ and Zn2+ are
diamagnetic ions, which usually gave phthalocyanines a large
UD.9 Generally, 3a and 3b have a larger UD than 3c and 3d
under these conditions. 3a showed the largest UD under both
conditions, while the sequence of the other three are in the
order of 3b > 3d >> 3c. The UD of 3a was 0.55 in pH 7.4
buffer solution; while zinc phthalocyanine ZnPc(COOH)8 had
a UD about 0.52 at pH 10.14 The large UD of 3a indicated it
can produce singlet oxygen in high efficiency, which facilitated to
strong cytotoxicity in photodynamic therapy; or high oxidization
capacity in photocatalysis. The rare-earth ytterbium ion quenched
the triplet state due to its paramagnetism (similar to Cu2+, Co2+).
It resulted in the UD decreasing; and 3c showed negligible singlet
oxygen production.
The photobleaching property illustrated the stability and dura-
bility of photosensitizer during the photosensitive process. The
result in Table 4 showed that the quantum yield of photo-
bleaching UP is at 10-5 magnitude, which is similar to reported
phthalocyanines.9 The 3b had a weaker photodegradation in
pH 7.4 buffers than 3a.
It also noticed that both the singlet oxygen and photobleaching
were influenced largely by the surfactants as shown in Table 4.
3.5 Surfactant effects
As commonly known,7,9,10 the aggregation is the main factor
for the quenching of fluorescence and singlet oxygen of ph-
6332 | Dalton Trans., 2009, 6327–6334
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photobleaching than 3a both with and without surfactants,
which means it is more suitable to be a photosensitizer in PDT
than 3a.
4. Conclusion
Four amphiphilic phthalocyanine carboxyl oligomers, 3a (zinc as
centric atom), 3b (aluminum), 3c (ytterbium) and 3d (metal-free)
have been synthesized, and their photophysical and photochemical
properties investigated.
The existed researches usually illustrated Triton X 100 can
increases both of the UF and UD by dissociating the aggregation of
phthalocyanines. In this study, it was inserting that the Triton
X 100 only enhanced the UF and UD of polymeric Pcs, but
had puny effect on their aggregation; while the behaviour of
CTAB was similar to what has been reported in the literature.9,36
It indicated that the two surfactants worked in quite differ-
ent ways on these unique polymeric phthalocyanine carboxyl
molecules.
In summary, 3a and 3b had higher singlet oxygen quantum
yields; while 3c and 3d were better at fluorescence. The electronic
spectra showed that 3b had a lower aggregation tendency than
the other three in pH 7.4 buffer solution. The anionic surfactant
CTAB can dissociate the aggregates of 3a, 3c and 3d, but the
non-ionic surfactant Triton X 100 could not. The photophysical
and photochemical properties of the four compounds were more
sensitive to Triton X100. The quantum yield of fluorescence was
observed from 0.23 to 0.87 for 3c; with an increasing of UD (0.55
to 0.76) being recorded for 3a as well, both in the presence of
Triton X 100. Additionally, 3b showed a stronger photobleaching
resistance than 3a.
The experimental results indicate that polymeric carboxyl
phthalocyanines can be used as a photosensitizer with more
specific advantages than the monomeric phthalocyanines. 3a
showed rapid photobleaching and a high UD, which may be used
as photocatalyst in co-degradation of homogeneous systems. The
non-aggregation of 3b was considered as a valuable amphiphilic
photosensitizer in PDT applications. The fluorescence emission of
3c can be enhanced by Triton X 100 and quenched by methanol
in water solution, which was desired as fluorescent probe or
photodiagnostic imaging materials.
As shown in Fig. 5, the carboxyl groups of the polymeric
Pcs mainly exist as carboxylic anion in pH 7.4 solution; while
CTAB has a quaternary ammonium ion. We suggested that
the cationic CTAB may wrap the oligomer molecules by an
electrostatic effect between quaternary ammonium cation and
carboxylic anion. This enveloping function of CTAB resulted in
the effective disaggregation of 3a, 3c and 3d (shown in Fig. 5).
While non-ionic Triton X 100 was only functional by being
inserted into the interspace between the adjacent molecules as
Ng37 reported, thus the aggregates of 3a, 3c and 3d still existed.
Moreover, the oxygen atoms provided the long chain of Triton
X 100 with a high electronic density distribution; alternatively it
was low density alkyl chain for CTAB at the similar position.
The singlet excited state of these polymeric molecules had larger
polarity than the ground state (Table 2, Stoke shift). Triton X 100
had better capacity to disperse the polarity and stabilize the singlet
state through conjugated effect. We propose that the enhancement
of UF and UD by the stable singlet state was stronger than the
reducing of them by dissipation effect from aggregation; while
CTAB only reduced the aggregation and so affected the excited
state less effective. Thus, CTAB presented a weaker effect than
that of Triton X 100. The above hypothesis was partly proved by
the long lifetime of fluorescence (tF = 130.1 ns) for 3c recorded
in presence of Triton X 100 but not for CTAB (tF = 12.7 ns,
Table 4). In addition, 3b had a lower aggregation and a weaker
response to the surfactants than that of 3a, 3c and 3d. In common
phthalocyanines, AlPc also behave more monomeric than other
metallic Pc in solution.
Acknowledgements
We are pleased to acknowledge the financial support provided
by the National Science Foundation of China (no. 20773077 and
50872149) and the National Basic Research Program of China
(no. 2007CB808000). The author also thank Dr Cheng Zhong
(Chem. Dept., Tsinghua University) for his contribution to the
work.
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