Synthesis and photophysical studies of phthalocyanine-gold nanoparticle conjugates

Article abstract of DOI:10.1039/c1dt11151e

This work reports on the synthesis, characterization and photophysical studies of phthalocyanine-gold nanoparticle conjugates. The phthalocyanine complexes are: tris-(5-trifluoromethyl-2-mercaptopyridine)-2-(carboxy) phthalocyanine (3), 2,9,17,23-tetrakis

Full text of DOI:10.1039/c1dt11151e

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PAPER
Synthesis and photophysical studies of phthalocyanine–gold nanoparticle
conjugates
Nolwazi Nombona, Edith Antunes, Christian Litwinski and Tebello Nyokong*
Received 19th June 2011, Accepted 26th August 2011
DOI: 10.1039/c1dt11151e
This work reports on the synthesis, characterization and photophysical studies of phthalocyanine–gold
nanoparticle conjugates. The phthalocyanine complexes are: tris-(5-trifluoromethyl-
2-mercaptopyridine)-2-(carboxy)phthalocyanine (3), 2,9,17,23-tetrakis-[(1, 6-hexanedithiol)
phthalocyaninato]zinc(II) (8) and [8,15,22-tris-(naptho)-2(amidoethanethiol) phthalocyanato]
zinc(II)(10). The gold nanoparticles were characterized using transmission electron microscopy, X-ray
diffraction, atomic force microscopy and UV-vis spectroscopy where the size was confirmed to be
~5 nm. The phthalocyanine Au nanoparticle conjugates showed lower fluorescence quantum yield
values with similar fluorescence lifetimes compared to the free phthalocyanines. The Au nanoparticle
conjugates of 3 and 10 also showed higher triplet quantum yields of 0.69 to 0.71, respectively. A lower
triplet quantum yield was obtained for the conjugate compared to free phthalocyanine for complex 8.
The triplet lifetimes ranged from 70 to 92 ms for the conjugates and from 110 to 304 ms for unbound Pc
complexes.
suggesting energy transfer from the Pc to the AuNP, though the
phase transfer agent (tetraoctylammonium bromide, TOABr) is
Introduction
also known to decrease the lifetimes of free Pcs.22 Energy transfer
from the photoexcited phthalocyanines to the gold nanoparticles
occurs in the picosecond time scale of ~3 ps.24 In another study,25
gold nanoparticles were found not to quench the fluorescence of
cobalt tetraminophthalocyanine, whereas silver nanoparticles did.
Thus it is clear from the literature that the fluorescence
behaviour of phthalocyanines alone or in the presence of AuNPs
depends on the nature of the macrocycle. In this work, we
compare the effects of AuNP on the photophysical behaviour of
phthalocyanines containing SR groups (3), four SH groups (8) or
one SH group (10), see Schemes 1 to 3 for structures. The rest of
the substituents (for 3 and 10) were chosen to enhance solubility
of the Pc complexes. Both unmetallated and Zn phthalocyanine
complexes are employed. Ligand exchange was used as a
method for phthalocyanine attachment on the nanoparticles.
The phthalocyanine complexes employed are: 9,16,23-tris-(5-
trifluoromethyl-2-mercaptopyridine)-2-(carboxy)phthalocyanine
(3, containing sulfur bridges), 2,9,17,23-tetrakis-[(1, 6-
hexanedithiol)phthalocyaninato]zinc(II) (8) and [8,15,22-tris-
(naptho)-2(amidoethanethiol)phthalocyanato]zinc(II), containing
a terminal SH group (10).
Photodynamic therapy (PDT) is a well known alternative for
cancer treatment currently in clinical trials.1–4 This therapy is
based on the principle that upon irradiation, a photosensitiser
will get excited and transfer its energy to the surrounding oxygen
generating singlet oxygen,5,6 which causes cellular damage to
surrounding cancerous cells via necrosis or apoptosis.7–9 Due to
the poor drainage in tumour cells, the photosensitiser efficiently
accumulates in tumour cells, making PDT a more efficient method
for cancer treatment.10
Nanotechnology has become a thriving area of research that has
significantly transformed the health care system by fighting deadly
diseases more competently.11,12 The properties of nanoparticles
differ from the bulk and individual atoms they are comprised
of.13–17
Ligands such as phthalocyanines can be attached to nanoparti-
cles in order to improve their properties with regards to better
drug specificity.18 Phthalocyanines conjugated to nanoparticles
have demonstrated improved necrosis and/or apoptosis.19 The
gratifying toxicity profile of gold and platinum nanoparticles
makes them favourable for medicinal application.20,21
Conjugates of phthalocyanines and Au nanoparticles (AuNPs)
have been reported for drug delivery applications.19,22,23 The ph-
thalocyanine complexes employed were either disulfide derivatives
or those substituted with one SH terminal group. Gold nanopar-
ticles decreased the fluorescence lifetime of the Pc molecule,
Results and Discussion
Synthesis and characterization
Phthalocyanine complexes. The syntheses and characteriza-
tion of complex 9 (Scheme 3) has been previously described.26
Department of Chemistry, Rhodes University, Grahamstown, 6140, South
Africa. E-mail: t.nyokong@ru.ac.za; Fax: +27 46 622 5109
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Scheme 1 Synthetic pathway of 9,16,23-tris-(5-trifluoromethyl-2-mercaptopyridine)-2-(carboxy)phthalocyanine (3).
Scheme 2 Synthetic route of 4-(6-mercaptohexan-1-ol) phthalonitrile (5) and 2,9,17,23-tetrakis-[(1, 6-hexanedithiol)phthalocyaninato]zinc(II) (8).
Compounds 1, 2 and 4 were synthesized according to literature27–29
and the synthetic strategy used for the synthesis of 3 (Scheme
1) was similar to that reported for the pyridyloxy derivative.30
The synthesis of unsymmetrical phthalocyanines is complicated
compared to symmetrically substituted Pcs as they often require
extensive purification methods to obtain the desired product. To
increase the product yield for complex 3, compounds 1 and 2 were
ground together. The powder was added to a refluxing mixture of
dry pentanol containing dissolved lithium. A relatively good yield
for the unsymmetrical phthalocyanine (3) was obtained (13%)
after continuous washing in hot methanol and purification by
column chromatography. The different lipophilicities of 1 and 2
assisted in the purification of 3 from by-products due to different
silica-binding properties in the solvent mixture employed. The
phthalocyanine showed partial solubility in most organic solvents
including dimethylsulfoxide (DMSO), dichloromethane (DCM),
methanol, acetone and toluene. However it showed good solubility
in chloroform dimethylformamide (DMF) and tetrahydrofuran
(THF).
Scheme 2 shows the chemical structure and synthetic route of
compound 8. The synthesis of complex 8 starts with the synthesis
of 5. The base catalyzed nucleophilic substitution of the nitro
functional group on phthalonitrile 4 with mercaptohexanol was
achieved in dry DMSO at room temperature under inert nitrogen
atmosphere with good yields obtained for phthalonitrile 5.27–29
The conversion of 5 into the corresponding hydroxyl substituted
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Scheme 3 Synthetic route of [8,15,22-tris-(naptho)-2-(amidoethanethiol) phthalocyanato] zinc(II) (10) and 10-AuNP conjugate.
zinc phthalocyanine (6) was accomplished in pentanol in the
presence of a catalyst and zinc acetate. The terminal hydroxyl
groups on complex 6 were converted into their mesylate coun-
terparts using triethanolamine (TEA) followed by the addition
of methanesulfonyl chloride at low temperature under standard
reaction conditions to give the substituted phthalocyanine 7.
Finally, the terminal thiol functional group was obtained by
refluxing thiourea, under an inert atmosphere in the absence of
light, in a previously degassed THF/ethanol solvent mixture. The
resultant product was then hydrolysed using 20% NaOH (also
previously degassed). The inert atmosphere inhibits the formation
of disulfides via aerobic oxidations and ensures the formation of
complex 8 at a relatively low yield.
Bulky napthol non-peripheral substituents were purposely
chosen for complex 9 to reduce the typical aggregation of the
planar macrocyclic system and to allow for simple determination
of the photophysical properties of the complex in the monomeric
form. Complex 9 was covalently linked to cysteamine in the
presence of an amide coupling agent dicyclohexylcarbodiimide
(DCC) in a DMSO/pyridine mixture to form complex 10.
DCC reacts with the carboxylic acid to form the O-acylisourea
anhydride intermediate. This intermediate is more reactive than
the resonance stabilised carboxylic centre and can directly react
with the amine to yield complex 10. The formation of possible by-
products was diminished ◦by reacting 9 with DCC in the presence
of a base (pyridine) at 0 C prior to the addition of cysteamine.
Purification of the complex was achieved using BioBeads with
THF as the eluent.
Characterization of complexes 3, 8 and 10 were achieved using
infra-red (IR), ultraviolet-visible (UV-Vis), matrix-assisted laser
desorption/ionization-time of flight (MALDI-TOF) mass spectra
and proton nuclear magnetic resonance (1H NMR) spectroscopies
as well as elemental analyses.
The absence of the sharp C N vibration (~2230 cm-1) for
compounds 1, 2, 4 and 5 in the IR spectra of 3 and 8 confirmed
phthalonitrile cyclisation. For 3 the typical N–H band at 1079
cm-1 further confirmed unmetallated Pc formation and the IR
spectroscopy data for complex 10 confirmed amide bond forma-
tion with a peak observed at 1693 cm-1.
1
The H NMR spectrum for complex 3 showed 21 aromatic
protons between 6.35 to 8.21 ppm. MALDI-TOF MS further
confirmed the formation of complex (3) with a molecular ion peak
observed at 1090 amu.
Complex 8 was found to be pure by 1H NMR. Aromatic protons
were observed between 6.97 and 7.70 ppm integrating for 12 and
CH2 protons were between 4.22 and 1.24 ppm, integrating for 48
protons, in total, as expected. The mass spectra of the complex
showed a molecular ion peak at 1170.56 amu.
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Table 1 UV-Vis spectral parameters of phthalocyanines and
phthalocyanine-AuNP conjugates in chloroform, unless otherwise
stated
a
a
a
Pc
l
abs /nm
l
ems/nm
l
exc/nm
UF
bUT
tT/ms
3
667, 703
667, 704
704
698
691
708
709
716
716
708
704
701
669, 701
670, 703
705
708
697
696
694
0.04
0.02
0.15
0.07
0.1
0.05
0.04
0.68
0.71
0.75
0.69
0.62
0.63
0.71
110
92
304
87
240
140
70
3-AuNP
8
8-AuNP
9
10
692
10-AuNP 689
a
labs = Q band absorption maximum, lems = Q band emission maximum,
lexc = Q band excitation maximum. b UT = values were obtained in DMF.
1
For complex 10, H NMR showed aromatic protons between
6.72 and 8.20 ppm integrating for 34 protons and CH2 protons
being observed between 0.82 and 1.50 integrating for 4 protons as
expected. MALDI-TOF MS further confirmed the formation of
complex 10 with a mass peak observed at 1108.24 amu.
The UV-vis spectrum of 3 in chloroform is shown in Fig. 1A
(spectrum i). A split Q-band was observed for 3 with maxima at
667 and 703 nm, Table 1. The splitting of the Q band is typical for
metal-free Pc and is caused by reduced symmetry of the molecule.
The B band was observed at 340 nm for 3.
Complex 3 is aggregated in DMF and DMSO, and less
aggregated in chloroform. Thus chloroform was chosen as a
solvent to allow for comparison of 3 with other complexes (8
and 10, which are not aggregated in chloroform). For complex 3
in chloroform, there is a slight broadening of the absorption bands
between 550 and 650 nm, typical of aggregation in phthalocyanine
complexes.31 However, there was no observed disaggregation on
addition of Triton X or chromophore EL as surfactants. Complex
3 obeys Beer’s law at concentrations lower than 1.5 ¥ 10-6 M in
chloroform. The absorbance spectra of complex 8 showed slight
aggregation in DMSO and DMF with no aggregation observed
in chloroform, Fig. 1B, with Q band maxima at 704 nm in
chloroform.
Fig. 1C shows that complexes 9 and 10 display similar spectra
in terms of Q band maxima, showing the presence of one SH
band does not affect the spectra. MPc complexes containing sulfur
groups are usually red-shifted. Complex 10 shows a split in the Q
band which could be due to the loss of symmetry as a result
of unsymmetric substitution (which should also affect 3 and 9)
or due to protonation of the aza nitrogens caused by the acidic
chloroform.32 The protonation would be substituent specific since
it depends on the basicity of the ring, hence we suggest that the
split in the Q band is due to loss of symmetry as a result of
unsymmetrical protonation of the inner nitrogen atom.
Fig. 1 Absorption spectra (concentration= ~1 ¥ 10-5 M) of (A) compound
3 (i), 3-AuNP (ii) and TOABr-AuNP (iii), (B) compound 8 (i), 8-AuNP (ii)
and TOABr-AuNP (iii), (C) compound 9 (i), 10 (ii), 10-AuNP (iii) and
TOABr-AuNP (iv) in chloroform.
of the phthalocyanines will be by linkage with this one group,
resulting in a perpendicular arrangement shown in Scheme 3. For
complexes 3 and 8, it is possible that one or more of the SR or SH
groups are attached.
The Au nanoparticles were characterised using UV-Vis and
X-ray diffraction (XRD) spectroscopies as well as atomic force
microscopy (AFM) and transmission electron microscopy (TEM).
To determine the size of the gold nanoparticles and the Pc-AuNP
conjugates, XRD technique was employed. The Debye-Scherrer
equation33 (eqn (1)) was used to calculate the size of the gold
nanoparticles and Pc conjugates.
AuNPs
and
Pc-AuNP
conjugates. Phthalocyanine-
functionalised gold nanoparticles were synthesized using a
ligand exchange process, where the loosely bound TOABr ligands
were partially exchanged by Pcs that bind covalently to the
nanoparticle surface. The phthalocyanine complexes were most
likely attached to the AuNPs surface via Au–S bond due to
the sulfur groups on the periphery of the phthalocyanine ring,
for complexes 3, 8 and 10. A schematic representation of the
conjugation of 10 with the TOABr-AuNP is shown in Scheme
3. For complex 10 containing one thiol group, the arrangement
ꢀ
kl
bCosq
d(A) =
(1)
where k is an empirical constant equal to 0.9, l is the wavelength of
˚
the X-ray source, (1.5405 A), b is the full width at half maximum
of the diffraction peak, and q is the angular position of the
peak. Fig. 2 shows the XRD pattern for the AuNPs and the
corresponding TOABr pattern employed in this work, the average
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Fig. 2 XRD of TOABr-AuNP and TOABr.
Fig. 4 TEM images of TOABr-AuNP (a), 8-AuNP (b) and 10-AuNP (c).
size for the AuNPs was calculated to be 5.37 nm when the peak
at 64.63◦ corresponding to gold was fitted. This size was further
confirmed by atomic force microscopy where the average size of
the nanoparticles ranged between 3.5 and 4.5 nm. Fig. 3 shows
the size distribution of the gold nanoparticles as determined by
AFM.
(Fig. 1). This is consistent with the reported SPR peak for gold
nanoparticles of sizes less than 50 nm.16,34
The UV-vis spectra of the Pc-AuNP conjugates are shown in
Fig. 1 for 3-AuNP, 8-AuNP and 10-AuNP and together with
their unconjugated phthalocyanine derivatives. It is known that
the absorption spectrum of the Pc-AuNP shows a broadening of
the phthalocyanine Q-band absorption, this was attributed to a
tight packing of the phthalocyanines on the gold.24 In Fig. 1A for
3-AuNP this broadening is observed, in addition there is a small
blue shift since the SR groups are now engaged with the AuNPs
reducing their electron donating abilities. Similarly, a slight blue
shift is observed for 8-AuNP in Fig. 1B and for 10-AuNP in Fig. 1C.
For 10-AuNP, there is an increase in a split in the Q band, which
suggests that the splitting due to loss of symmetry is enhanced by
attachment of AuNPs. A closer look at Fig. 1B for 8-AuNP, shows
the SPR peak in the conjugate to be larger than for 3-AuNP or
10-AuNP, suggesting more coordinated AuNPs, in the latter two
conjugates. The diminished and broad surface plasmon band has
been found to be indicative of surface complexation; hence the
SPR band serves as a probe to monitor the interaction between
AuNPs with surface bound molecules.35
Fig. 3 Size distribution histogram (from AFM) of synthesized gold
nanoparticles.
Photophysical properties
Fluorescence spectra, quantum yields and lifetimes. Fig. 5
shows the absorption, emission and excitation spectra of 3
Fig. 4a shows the TEM image of the TOABr-AuNP and Fig.
4b and 4c show the TEM images of the 8-AuNP and 10-AuNP,
respectively. Fig. 4a shows the AuNPs as spheres with different
sizes dispersed in a disorderly manner, these are seen as dark spots
on the TEM image.
On conjugation with Pc complexes TEM images (using 8-AuNP
and 10-AuNP as examples) show a highly ordered arrangement,
Fig. 4b and 4c. The size of the nanoparticles determined from
the TEM image was found to range between 5.97–7.87 nm. The
size of AuNPs (~5 nm) determined using XRD is expected to be
more reliable, unlike TEM which is an estimation due to partial
aggregation. The size determined using AFM is closer to that
determined using XRD.
The surface plasmon resonance (SPR) band for AuNP ab-
sorption was determined from UV-vis spectrum to be 518 nm
Fig. 5 Absorption (i), excitation (ii) and emission (iii) spectra of 3.
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Table 2 UV-Vis spectral parameters of phthalocyanines and
in chloroform. The excitation spectra showed the typical split
Q-band at the same wavelength positions as the absorption spec-
tra. The lack of agreement between excitation and absorption spec-
tra, is due to the absorption of the latter with broad bands between
600 and 650 nm due to aggregates. These bands are not observed
in the excitation spectra since aggregated species do not fluoresce.
The observation of a single band in the emission spectrum is
typical of low symmetry phthalocyanines such as unmetallated
ones.36 Fluorescence quantum yield of 3 was determined to be 0.04,
Table 1.
phthalocyanine-AuNP conjugates in chloroform
Pc
t
F1/ns Relative amplitude
tF2/ns Relative amplitude
3
2.5
2.6
2.5
2.5
4.5
2.7
1
0.7
1
1
1
1
0.8
—
1.1
—
—
—
—
0.8
—
0.3
—
—
—
—
0.2
3-AuNP
8
8-AuNP
9
10
10-AuNP 2.5
The absorption, emission and excitation spectra of 3-AuNP in
chloroform showed similar behaviour to complex 3 alone. The
nearness of the absorption Q band wavelength (especially the low
energy component) with the excitation Q band wavelength (for 3
or 3-AuNP) implies that the nuclear configurations of the ground
and excited states are similar and not affected by excitation, Table
1. The fluorescence quantum yield of 3 was slightly higher than for
3-AuNP (Table 1) confirming that the presence of AuNPs lowers
the UF, and that AuNPs quench fluorescence as has been reported
before,24 contrary to when cobalt tetraamino phthalocyanine
interacts with AuNPs.25 Comparing 9 and 10 shows that the
presence of the thiol group in complex 10 lowered the quantum
yield value from 0.1 (9) to 0.05 for 10, however the quantum yield
value for 10-AuNP was similar to that of the free phthalocyanine.
The conjugation of AuNPs did not have a significant effect on the
UF value for 10-AuNP.
The fluorescence emission spectra were mirror images of the
absorption spectra for the metalated complexes. A decrease
in the fluorescence quantum yield of 8-AuNP compared to
8 was rather significant suggesting that the presence of the
AuNPs strongly quenches fluorescence as was also the case for
3-AuNP.
Fig. 6 Fluorescence decay curve of complex 3 (red) and 3-AuNP (black)
in chloroform.
Typically, Pcs in close proximity with AuNPs experience
emission quenching which can originate either from enhanced
intersystem crossing due to the heavy atom effect or through
direct energy transfer from the Pc to the AuNPs. This energy
transfer is called surface energy transfer (SET) as the photoexcited
Pc transfers its energy to the AuNP, upon relaxation back to
the ground state. This phenomenon generally occurs for gold
nanoparticles with small diameters37 and that are larger than for
Foster resonance energy transfer (FRET).38 It is possible that the
slight aggregation observed for phthalocyanines when conjugated
to AuNPs can result in self-quenching decreasing the fluorescence
quantum yields further.
Even though both the free and bound Pc were reported to
have a double exponential function,22 time-resolved fluorescence
measurements for complex 3 indicate a mono-exponential decay
(Fig. 6, Table 2), with fluorescence lifetimes (tF1) of 2.5 ns
indicating that there is only one species in the solution. Complex
3-AuNP showed a bi-exponential decay where the first component
(with tF1 of 2.6) may be due to the presence of free phthalocyanines
and the second component (with tF2 of 1.1 ns, Table 2) is due to
derivatives conjugated to the nanoparticles. 10-AuNP also showed
bi-exponential decay with tF1 = 2.5 ns, and tF2 = 0.8 ns, Table 2. The
second components for 3-AuNP and 10-AuNP were short-lived
with low abundance. Complex 8-AuNP, however, showed a mono-
exponential decay suggesting that the pthalocyanines attached on
the gold surface are highly quenched to the point where they are
not fluorescent.
Triplet quantum yields and lifetimes
The triplet quantum yield (UT) values could not be determined in
CHCl3 due to the lack of standard in this solvent, hence DMF was
employed for these studies. Triplet quantum yields and lifetimes
of the phthalocyanines and the conjugates were determined using
laser flash photolysis. The triplet decay curves (figure not shown) of
all complexes in DMF obeyed second order kinetics. This is typical
of Pc complexes at high concentrations (>1 ¥ 10-5 M) due to the
triplet–triplet recombination.25 The concentrations employed in
this work were in this range; hence triplet–triplet recombination is
expected. Intersystem crossing to the triplet state and fluorescence
are competing deactivating processes for a molecule in the excited
singlet state. Therefore their values are complementary. Table 1
show high triplet quantum yield (UT) values for the conjugates
of 3 or 10 with AuNPs: 0.71 (for both 3-AuNP and 10-AuNP) in
DMF compared to the free phthalocyanines where the UT values
are 0.68 for 3 and 0.63 for 10. This is due to the presence of
the AuNPs that encourage intersystem crossing due to the heavy
atom effect. For complex 8 containing four thiol groups, there is
a surprising decrease in UT value in the presence of AuNPs. This
could be due to the slight aggregation of 8 in DMF which was
discussed above.
All the conjugates showed a decrease in triplet lifetimes in
the conjugates compared to free phthalocyanines. The speedy
deactivation of the triplet sate for the conjugates may be explained
by the heavy atom effect.
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with a diode laser (LDH-P-670 with PDL 800-B, Picoquant
GmbH, 670 nm, 20 MHz repetition rate, 44 ps pulse width).
Fluorescence was detected under the magic angle with a peltier
cooled photomultiplier tube (PMT) (PMA-C 192-N-M, Pico-
quant) and integrated electronics (PicoHarp 300E, Picoquant
GmbH). A monochromator with a spectral width of about 8 nm
was used to select the required emission wavelength band. The
response function of the system, which was measured with a
scattering Ludox solution (DuPont), had a full width at half-
maximum (FWHM) of 300 ps. All luminescence decay curves
were measured at the maximum of the emission peak and lifetimes
were obtained by deconvolution of the decay curves using the
FluorFit Software program (PicoQuant GmbH, Germany). The
support plane approach40 was used to estimate the errors of the
decay times.
A laser flash photolysis system was used for the determination
of triplet decay kinetics. The excitation pulses were produced by
a Quanta-Ray Nd:YAG laser (1.5 J/9 ns), pumping a Lambda
Physik FL 3002 dye laser (Pyridin 1 in methanol). The analyzing
beam source was from a Thermo Oriel 66902 xenon arc lamp, and
a Kratos Lis Projekte MLIS-X3 photomultiplier tube was used
as the detector. Signals were recorded with a two-channel, 300
MHz digital real time oscilloscope (Tektronix TDS 3032C); the
kinetic curves were averaged over 256 laser pulses. Triplet lifetimes
were determined by exponential fitting of the kinetic curves using
OriginPro 8 software.
Experimental section
Materials
Chloroform (CHCl3), dimethyl sulfoxide (DMSO), tetrahydrofu-
ran (THF) dimethylformamide (DMF), dichloromethane (DCM),
methanol (MeOH), ethanol (EtOH), toluene, glacial acetic
acid, acetone, pentanol, pyridine, hydrochloric acid, sodium
hydroxide and sodium borohydride (95%) were purchased from
SAARCHEM. Tetraoctylammonium bromide (TOABr) (98%),
gold(III) chloride trihydrate (99.9%), dicyclohexylcarbodiimide
(DCC) (99%), cysteamine hydrochloride, magnesium sulphate,
methanesulfonylchoride, thiourea, triethanolamine (TEA), 6-
mercaptohexan-1-ol and lithium metal were purchased form
Sigma-Aldrich. Silica gel for column chromatography was pur-
chased from Merck. Phthalonitriles 1, 2 and 4 were synthesized
as reported in the literature.27–29 Tetraoctylammonium bromide
gold nanoparticles (TOABr-AuNP) were synthesised according
to literature.24,39 The sizes of AuNPs were confirmed by atomic
force microscopy, transmission electron microscopy and X-ray
diffraction. Column chromatography was performed on silica
gel 60 (0.04–0.063 mm) or on Bio-beads S-X1 200-400 mesh
purchased from BioRad.
Equipment
Ground state electronic absorption spectra were performed on
a Shimadzu UV-2550 spectrophotometer; Infra-red spectra on a
Perkin Elmer Spectrum 100 FT-IR Spectrometer and 1H nuclear
magnetic resonance signals on a Bruker AMX 400 MHz NMR
spectrometer or with a Bruker AMX 600 MHz NMR spec-
trometer. Elemental analysis was done using a Vario-Elementar
Microcube ELIII. Mass spectral data were collected with a Bruker
AutoFLEX III Smartbeam TOF/TOF Mass spectrometer. The
instrument was operated in positive ion mode using a m/z range
of 400–3000 amu. The voltages of the ion sources were set at 19
and 16.7 kV for ion sources 1 and 2 respectively, while the lens was
set at 8.50 kV. The reflector 1 and 2 voltages were set at 21 and
9.7 kV respectively. The spectra were acquired using dithranol as
the MALDI matrix, using a 337 nm nitrogen laser. Transmission
electron microscope (TEM) images were obtained using a JEOL
JEM 1210 transmission electron microscope at 100 kV accelerating
voltage. X-ray powder diffraction patterns were recorded on a
Bruker D8 Discover equipped with a LynxEye detector, using Cu-
Synthesis
Syntheses of [8,15,22-tris-(naphtho)-2-(carboxy)phthalocyanato]
zinc(II) (9) has been reported.26
[9,16,23-Tris-(5-trifluoromethyl-2-mercaptopyridine)-2-(carbo-
xy)phthalocyanine] (3, Scheme 1). Using a mortar and pestle,
0.109 g (3 eq.) of 1 and 0.0204 g (1 eq.) of 2 were mixed and
ground, and the powder was added in a round bottom flask
containing a refluxing solution of dry pentanol (5 mL) with
dissolved lithium (0.8 mmol). The reaction was stirred under reflux
in an argon atmosphere at 140 ◦C for 2 h. Thereafter the reaction
mixture was left to cool to room temperature. Glacial acetic acid
(20 mL) was added and the precipitate of the crude complex 3
was centrifuged and washed with water. The purification of the
product was carried out by repetitive washing in hot methanol;
the product was thereafter dissolved in chloroform and column
chromatography was carried out using a chloroform/DMF (9 : 1)
mixture as the eluent. Multiple fractions were collected with the
second fraction being the desired product. The product obtained
was highly aggregated in numerous solvents including THF and
DMSO, but less aggregated in chloroform. Yield: ~13%. UV-vis
(CHCl3): lmax nm (log e): 699 (4.34), 664 (4.31), 635 (4.03), 603
(3.86), 340 (4.27). IR nmax/cm-1: 1595 (C–O coupled to O–H),
1323 (Aryl NH), 742 (C–S). Calc. for: C51H24N11S3O2F9: C, 60.80:
H, 4.51: N, 9.45: S, 8.12%; found C, 60.40: H, 3.96: N, 9.42: S,
8.60%.1H NMR (CDCl3): d, ppm 8.21–6.35 (21H, m, Ar–H), (1H,
s, OH) not observed, (2H, s, Pc–H) not observed. MALDI-TOF-
MS m/z Calc: 1089.98; found [M]-: 1090.20
˚
Ka radiation (l = 1.5405 A, nickel filter). Data were collected in
the range from 2q = 5◦ to 65◦, scanning at 1◦ min-1 with a filter
time-constant of 2.5 s per step and a slit width of 6.0 mm. Samples
were placed on a zero background silicon wafer slide. The X-ray
diffraction data were treated using Eva (evaluation curve fitting)
software. Baseline correction was performed on each diffraction
pattern. Atomic force microscopy (AFM) images were recorded
in the non-contact mode in air with a CP-11 Scanning Probe
Microscope from Veeco Instruments (Carl Zeiss, South Africa)
at a scan rate of 1 Hz. The images were obtained using a spring
constant range of 20–80 N m-1, and resonant frequency range of
217–276 Hz.
Fluorescence excitation and emission spectra were recorded on
a Varian Eclipse fluorescence spectrofluorimeter. Fluorescence
lifetimes were measured using a time correlated single photon
counting setup (TCSPC) (FluoTime 200, Picoquant GmbH)
4-(6-Mercaptohexan-1-ol)
phthalonitrile
(5,
Scheme
2). 6-Mercaptohexan-1-ol (1.48 mL, 11.22 mmol) and
4-nitrophthalonitrile (4) (1.68 g, 9.35 mmol) were dissolved
11882 | Dalton Trans., 2011, 40, 11876–11884
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1
in DMSO (80 mL) under a stream of nitrogen and the mixture
stirred at room temperature for 15 min. Thereafter, finely ground
K2CO3 (2.11 g, 15.31 mmol) was added portion wise over a period
of 4 h and the reaction mixture left to stir for a further 24 h at
room temperature. The mixture was then added to water (150 mL)
and stirred for 30 min. The resulting precipitate was filtered off,
thoroughly washed with diethyl ether and acetone, dried and
recrystallized from ethanol. Yield: 90%. IR [(KBr) nmax/cm-1]:
3007 (C–H), 2230 (C N), 1668, 1583 (C C), 1437, 1408, 1387
2.66. H NMR (600 MHz, CDCl3) d (ppm): 8.20–6.72 (m, 34H,
Ar–H, N–H), 1.50–0.82 (m, 4H, methylene). MALDI-TOF-MS
m/z Calc: 1107.51; found (M–H)-: 1108.24.
Gold nanoparticles (TOABr-AuNP). The gold nanoparticles
were synthesized according to a procedure previously reported
in literature.25,39 Briefly, a gold solution was prepared by stirring
HAuCl4·3(H2O)3 (0.019 g, 25 mM) in toluene (2 mL). A solution
of TOABr (0.139 g, 85 mM) in 3 mL toluene was added to the gold
solution and the mixture was vigorously stirred until the colour
of the mixture changed from yellow to a brownish-yellow colour.
Thereafter NaBH4 (0.002 g, 36 mM) in 2 mL of water was added
dropwise to the gold solution until the solution changed colour
from brownish-yellow to milky then scarlet and finally to a purple
colour. The mixture was left to stir for a further 30 min. The
gold nanoparticles were washed repeatedly with water to remove
the reducing agent and were stored in a minimum amount of
toluene. The concentration of the TOABr-AuNPs was estimated
to be 4.99 ¥ 10-5 M-1 L-1. After a couple of days the solution turned
colourless and additional amounts of NaBH4 (36 mM, in water)
was added to refurbish the nanoparticles.
1
(C–H) 884, 666 (C–S–C). H NMR (600 MHz, DMSO-d6: d,
ppm: 7.65 (d, 1H, Ar–H), 7.57 (d, 1H, Ar–H), 7.50 (dd, 1H,
Ar–H), 3.65 (t, 2H, O–CH2), 3.03 (t, 2H, S–CH2), 1.75 (m, 2H,
CH2), 1.61–1.43 (m, 6H, CH2).
[2,9,17,23-Tetrakis-(1,6-hexanedithiol)phthalocyaninato] zinc(II)
(8, Scheme 2). The synthetic route followed for the synthesis
of complex 8 was the same as described by Chambrier et al.41
Firstly, complex 6 was prepared by refuxing a mixture of anhy-
drous zinc(II) acetate (0.51 g, 2.8 mmol), 4-(6-mercaptohexan-1-
ol)phthalonitrile (5) (0.5 g, 1.6 mmol), DBU (0.55 mL, 4 mmol)
◦
and pentanol (15 mL) at 160 C for 5 h. The formed complex 6
(111 mg) was then dissolved in dry DCM (20 mL) in the presence
of dry TEA (10 eq. 0.92 mmol). Methanesulfonyl chloride (10 eq.
0.92 mmol) was added to the solution with cooling to 10 ◦C.
The solution was then stirred and allowed to warm to room
temperature (10 min). The product in DCM was then washed
with water, dried using MgSO4, filtered and the DCM removed
under reduced pressure. This mesylate salt (complex 7) was then
dissolved in a THF (10 mL)/EtOH (3 mL) mixture and degassed
with sonication for 30 min. The solution was brought to reflux
under N2 in the dark. Thiourea (30 mg, excess) was added and the
reflux continued for a further 8 h. N2 gas was then flushed through
the reaction mixture, degassed aqueous NaOH (20%, 6 mL, 30 min
sonication) was added and the reflux continued for a further 2 h.
This mixture was subsequently poured into a dilute HCl/ice
mixture, extracted with DCM (2 ¥ 40 mL) and dried over MgSO4,
filtered and the solvent evaporated. The final phthalocyanine
complex 8 was then purified twice with BioBeads using DCM
as the eluent. Yield: 12%. UV/vis (DMSO): lmax nm (log e); 706
(4.64), 638 (4.08), 364 (4.33). IR (KBr): nmax/cm-1; 2997 (C–H),
Self assembly of phthalocyanines onto gold nanoparticles
(Scheme 3). In 1 mL THF or chloroform, complexes 3, 8 or 10
(1 mM) and TOABr-AuNP (0.5 mL in toluene) were stirred at room
temperature for 48 h under Ar atmosphere and protection from
light. Purification of the Pc-AuNP conjugates was performed using
column chromatography on Bio-Beads with THF or chloroform
as the eluting solvent.
Photophysical properties
Flourescence quantum yields and lifetimes. The comparative
method was used to determine the fluorescence quantum yields
(UF) of the phthalocyanine complexes using eqn (2).42
FAStdn2
(2)
Φ = Φ
F
F(Std) F AnS2td
Std
where F and Fstd are the areas under the fluorescence curves of
the complexes (3, 8 and 10 and their AuNP conjugates) and the
standard respectively. A and Astd are the respective absorbances
of the sample and the standard at the excitation wavelength and
n and nstd are the refractive indices of the solvents used for the
sample and standard respectively. ZnPc was used as a standard in
DMSO where UF = 0.20.43
1
1436, 1406 (C C), 1307 (C–H), 667 (C–S–C). H NMR (600
MHz, DMSO-d6) d (ppm): 7.70–7.69 (dd, 4H, Ar–H), 7.53–7.51
(dd, 4H, Ar–H), 6.97 (s, 4H, Ar–H), 4.22 (m, 8H, CH2–S–Ar),
1.67 (m, 8H, CH2–S), 1.43–1.24 (m, 32H, CH2). SH protons not
observed. Calc. for C56H64N8S8Zn: C; 57.38, H; 5.59, N; 9.56,
Found: C; 57.35, H; 5.44, N; 9.65. MALDI-TOF-MS m/z: Calc:
1171.08; Found [M]+: 1170.56.
Triplet quantum yields and lifetimes. The triplet quantum
yields were determined using eqn (3):44
[8,15,22-Tris-(naptho)-2-(amidoethanethiol)phthalocyanato]
zinc(II)(10 Scheme 3). Under an Ar atmosphere, complex 9
(2 mg, 2 mmol) and DCC (0.4 mg, 2 mmol) were dissolved in a
mixture of dry DMSO (2 mL) and pyridine (0.8 mL). After 10
min of stirring at 0 ◦C, cysteamine was added to the mixture and
the reaction was left to stir at room temperature for 48 h. The
product was purified using column chromatography on BioBeads.
THF was used as an eluent. Yield: ~10%. UV-vis (THF): lmax nm
(log e): 733 (4.57), 687 (4.31), 655 (4.03), 615 (3.86), 353 (4.27). IR
DATeSTtd
Φ = ΦStd
(3)
T
T
DATStdeT
where DAT and DAStd are the changes in the triplet state
T
absorbances of complexes 3, 8 or 10 and the standard, respectively.
eT and eStd are the triplet state molar extinction coefficients for
T
complexes 3, 8 or 10 (and their conjugates) and the standard,
n
max/cm-1: 3000 (amide N–H), 2562 (S–H), 1693 (C O coupled
respectively. UStd is the triplet quantum yield for the standard,
T
to N–H), 1323 (Aryl NH). Calc. for: C65H39N9SO4Zn: C, 70.49:
H, 3.55: N, 11.36: S, 2.89%; found C, 69.97: H, 3.81: N, 11.77: S,
ZnPc (UStdT = 0.58 in DMF).45 Triplet lifetimes were determined by
exponential fitting of the kinetic curves using OriginPro 8 software.
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 11876–11884 | 11883
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The synthesis and characterization of 9,16,23-tris-(5-
trifluoromethyl-2-mercaptopyridine)-2-(carboxy)phthalocyanine
(3), 2,9,17,23-tetrakis-[(1, 6-hexanedithiol)phthalocyaninato]
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The Royal Society of Chemistry 2011
©