Synthesis and photophysical properties of a novel zinc photosensitizer and its gold nanoparticle conjugate

Article abstract of DOI:10.1016/j.jphotochem.2011.07.007

The peripherally tetra substituted zinc phthalocyanine with 1,6-hexanedithiol as substituent (THdTZnPc, 3) was synthesized and is reported for the first time in this work. The potential of this zinc complex as a suitable photosensitizer for use in photodynamic therapy was determined through the investigation of the photophysical and photochemical properties. In this work complex 3 is attached to gold nanoparticles through the terminal thiol groups of the phthalocyanine resulting in a 3-AuNP conjugate whose photophysicochemical properties are investigated. Fluorescence lifetimes were determined using time correlated single photon counting and they show an increase in the abundance of the monomeric species (τ₂) for the Pc in the 3-AuNP conjugate: ～1 and 0.71 (with respective lifetimes 2.69 ns and 2.86 ns) compared to the free complex 3 with abundances of 0.12 and 0.13 (with respective lifetimes 3.36 ns and 3.28 ns) in DMSO and DMF, respectively.

Full text of DOI:10.1016/j.jphotochem.2011.07.007

Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 343–350
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and photophysical properties of a novel zinc photosensitizer and its
gold nanoparticle conjugate
Sharon Moeno, Edith Antunes, Tebello Nyokong∗
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The peripherally tetra substituted zinc phthalocyanine with 1,6-hexanedithiol as substituent (THdTZnPc,
3) was synthesized and is reported for the first time in this work. The potential of this zinc complex as
a suitable photosensitizer for use in photodynamic therapy was determined through the investigation
of the photophysical and photochemical properties. In this work complex 3 is attached to gold nanopar-
ticles through the terminal thiol groups of the phthalocyanine resulting in a 3-AuNP conjugate whose
photophysicochemical properties are investigated. Fluorescence lifetimes were determined using time
correlated single photon counting and they show an increase in the abundance of the monomeric species
(ꢀ2) for the Pc in the 3-AuNP conjugate: ∼1 and 0.71 (with respective lifetimes 2.69 ns and 2.86 ns) com-
pared to the free complex 3 with abundances of 0.12 and 0.13 (with respective lifetimes 3.36 ns and
3.28 ns) in DMSO and DMF, respectively.
Received 10 February 2011
Received in revised form 28 May 2011
Accepted 12 July 2011
Available online 23 July 2011
Keywords:
Phthalocyanine
1,6-Hexanedithiol
Gold nanoparticles
Triplet state quantum yields
Fluorescence lifetimes
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
able changes in the photophysicochemical and spectral properties
of phthalocyanine complexes [14–18].
Gold nanoparticles (AuNPs) have found application in various
fields such as in catalysis, imaging, chemical recognition and pho-
tothermal therapy [1–4]. The prominence of these AuNPs results
from their intrinsic size tunable optical properties. These optical
properties are influenced by the size and shape of the nanoparticle
[5,6]. Functionalization of AuNPs with photoactive complexes has
been reported [7]. There are many varied photoactive molecules
that may be employed for AuNP modification, however in this work
the emphasis is on phthalocyanine (Pc) complexes. Phthalocya-
nines are adaptable macrocyclic complexes that find use as dyes,
sensors, in non linear optics and in photodynamic therapy (PDT) of
cancer [8–11].
The photophysicochemical properties of Pcs may be altered by
simply varying the central metal ion, the axial ligand on the metal
ion and the substituents on the periphery of the Pc [11,12]. Dia-
magnetic ions with a closed shell such as Zn2+ and Si4+ result in
both high triplet quantum yields (˚T) and long triplet lifetimes (ꢀT)
[9,12,13]. The inherent insolubility of Pcs can be improved by intro-
ducing a variety of substituents that have the capacity to enhance
the solubility of these complexes [14–16]. In addition, introducing
substituents onto the periphery of the Pc ring results in consider-
The formation of self assembled monolayers (SAMs) of metal-
lophthalocyanine (MPc) molecules onto bulk gold or AuNP surfaces
has been reported [19,20]. In this work AuNPs functionalized with
alkyl thio substituted ZnPc are investigated for their potential use
in PDT since reports have shown that the AuNPs enhance singlet
oxygen production by the Pc [19].
The stabilization of AuNPs by a mono thiol substituted zinc
phthalocyanine has been reported [21], and were shown to gener-
ate singlet oxygen with enhanced quantum yields as compared to
the free Pc. The current work reports on the stabilization of AuNPs
using tetrakis-2,(3)-[(1,6-hexanedithiol)phthalocyaninato]zinc(II),
(THdTZnPc, 3, Scheme 1). The complex is tetrasubstituted with thiol
groups and also bridged with sulfur. The photophysicochemical
behaviour of the complex in the presence and absence of AuNPs
is discussed. It has been reported before that for MPc complexes
linked to other nanoparticles such as quantum dots [22,23], both
the triplet quantum yield and lifetime increase in the presence of
nanoparticles, this is very beneficial for use of MPc complexes as
photosensitizers. These types of studies have not been reported for
AuNP–MPc conjugates, hence the current work. Studies on the abil-
ity of noble metal nanoparticles to absorb light in the near-infrared
(NIR) region and initiate cancer cell death via a photothermal effects
are of current interest [24]. Thus the study of phthalocyanines (as
photosensitizers for PDT) in the presence AuNPs is part of efforts
in developing combination therapy agents where two or more sub-
stances are applied for curing one disease, in this case cancer.
∗
Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.07.007

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S. Moeno et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 343–350
SR
N
N
CN
R-SH, Solvent
CN
N
Metal salt, Catalyst
Solvent , Δ
SR
, RT
K2CO3
Zn
N
N
O2N
CN
RS
CN
RS
N
N
N
(1)
(2)
SR
(3)
R =
SH
Scheme 1. Synthetic route of 1,6-hexanedithiol (R-SH) tetra substituted zinc phthalocyanine (THdTZnPc, 3) complex.
2. Experimental
Baseline correction was performed on each diffraction pattern by
subtracting a spline fitted to the curved background and the full-
width at half-maximum values used in this study were obtained
from the fitted curves. Transmission electron microscope (TEM)
images were obtained using a JEOL JEM 1210 transmission elec-
tron microscope at 100 kV accelerating voltage. A few drops of the
solutions of the samples were placed on carbon coated 300 mesh
grids and were left to dry for about 30 s.
2.1. Materials
Zinc phthalocyanine (ZnPc), 1,3-diphenylisobenzofuran (DPBF),
1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), 1,6-hexanedithiol and
gold(III) chloride trihydrate (99.9%) were obtained from Sigma
Aldrich. Ethanol, chloroform, dichloromethane (DCM), tetrahydro-
furan (THF), toluene, dimethyl formamide (DMF), diethylether,
4-nitrophthalonitrile and acetone were obtained from Saarchem.
Tetraoctyl ammonium bromide (TOAB) (98%), sodium borohydride
and dimethyl sulfoxide (DMSO) were obtained from Fluka. Bio-
beads S-X1 were obtained from Bio-Rad. AuNPs were synthesized,
purified and characterized according to literature [25]. Ultra pure
water was obtained from a Milli-Q Water System (Millipore Corp,
Bedford, MA, USA).
Fluorescence lifetimes were measured using a time correlated
single photon counting (TCSPC) setup (FluoTime 200, Picoquant
GmbH). The excitation source was a diode laser (LDH-P-C-485
with 10 MHz repetition rate, 88 ps pulse width). Fluorescence was
detected under the magic angle with a peltier cooled photomul-
tiplier tube (PMT) (PMA-C 192-N-M, Picoquant) 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 about 300 ps. The
ratio of stop to start pulses was kept low (below 0.05) to ensure
good statistics. All luminescence decay curves were measured at
the maximum of the emission peak. The data were analysed with
the program FluoFit (Picoquant). The support plane approach was
used to estimate the errors of the decay times [26]. Mass spec-
tra data were collected with a Bruker AutoFLEX III Smartbeam
TOF/TOF Mass spectrometer. The instrument was operated in pos-
itive ion mode using an m/z range of 400–3000. The voltage 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 354 nm
nitrogen laser.
2.2. Equipment
Fluorescence excitation and emission spectra were recorded
on a Varian Eclipse spectrofluorimeter. UV–visible spectra were
recorded on a Varian 500 UV–Vis/NIR spectrophotometer. IR data
was obtained by using the Perkin-Elmer spectrum 2000 FTIR Spec-
trometer. 1H NMR spectra were recorded using a Bruker AMX
400 MHz spectrometer. Elemental analyses were carried out on a
Vario EL III MicroCube CHNS Analyzer.
Laser flash photolysis experiments were performed with light
pulses produced by a Quanta-Ray Nd:YAG laser providing 400 mJ,
9 ns pulses of laser light at 10 Hz, pumping a Lambda-Physik FL3002
dye (Pyridin 1 dye in methanol). Single pulse energy ranged from 2
to 7 mJ. The analyzing beam source was from a Thermo Oriel xenon
arc lamp, and a photomultiplier tube was used as a detector. Signals
were recorded with a digital real-time oscilloscope (Tektronix TDS
360). The triplet life times were determined by exponential fitting
of the kinetic curves using the program OriginPro 7.5.
2.3. Photophysical studies
X-ray powder diffraction patterns were recorded on a Bruker D8,
Discover equipped with a proportional counter, using Cu-K␣ radi-
2.3.1. Fluorescence quantum yield determinations
Fluorescence quantum yields (˚F) were determined by a com-
parative method using Eq. (1) [27]:
˚
ation (ꢁ = 1.5405 A, nickel filter). Data were collected in the range
from 2ꢂ = 5◦ to 60◦, 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 silicon wafer slide. The X-ray diffraction data were treated
using the freely-available Eva (evaluation curve fitting) software.
FAStdn2
˚
F
= ˚F(Std)
(1)
F
StdAn2Std

S. Moeno et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 343–350
345
where F and FStd are the areas under the fluorescence curves of the
ZnPc derivative and the reference, respectively. A and AStd are the
absorbances of the sample and reference at the excitation wave-
length, and n and nStd are the refractive indices of solvents used for
the sample and standard, respectively. ZnPc in DMSO was used as a
standard, ˚F = 0.20, for the determination of fluorescence quantum
yields [28]. The sample and the standard were both excited at the
same relevant wavelength. The fluorescence quantum yield for the
MPc complex is represented as ˚F(MPc) (where MPc represents com-
plex 3). Solution for fluorescence quantum yield determinations
were not deaerated.
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 an ethanol. Yield: 2.7 g (90%) IR [(KBr)
ꢄmax/cm−1]: 3007 (C–H), 2230 (C N), 1668, 1583 (C C), 1437,
1408, 1387 (C–H) 884, 666 (C–S–C). 1H NMR (600 MHz, DMSO-d6)
ı ppm: 7.64–7.62 (m, 1H, Ar–H), 7.53 (d, 1H, Ar–H), 7.48–7.47 (dd,
1H, Ar–H), 3.02–3.00 (m, 12H, methine–H).
2.4.2. Tetrakis-2,(3)-[(1,6-hexanedithiol)
phthalocyaninato]zinc(II) (THdTZnPc, 3, Scheme 1)
A mixture of anhydrous zinc(II) acetate (0.51 g, 2.8 mmol),
4-(1,6-hexanedithiol)phthalonitrile (2) (0.5 g, 1.6 mmol), DBU
(0.55 mL, 4 mmol) and pentanol (15 mL) was stirred at 160 ◦C for
5 h. After cooling, the solution was mixed with n-hexane. The
green solid product was precipitated and collected by filtration
and washed with n-hexane. The crude product was purified by
column chromatography using silica gel and eluting with chloro-
form. Yield: 0.048 g (12%) UV/vis (DMSO): ꢁmax nm; 706 (4.64), 638
(4.08), 364 (4.33). IR (KBr): ꢄmax/cm−1; 2997 (C–H), 1436, 1406
(C C), 1307 (C–H), 667 (C–S–C). 1H NMR (600 MHz, DMSO-d6) ı
ppm: 7.70–7.69 (dd, 4H, Ar–H), 7.53–7.51 (dd, 4H, Ar–H), 6.97 (s,
4H, Ar–H), 4.30–4.11 (m, 48H, Methyl–H), not observed (s, 4H, S–H).
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: 1170.56
[M]+ for C56H64N8S8Zn.
2.3.2. Triplet state quantum yields and lifetimes
Triplet quantum yields were determined using a comparative
method based on triplet decay, using Eq. (2) [29]:
ꢃASTampleεTSample
ꢃASTtdεSTample
Sample
T
˚
= ˚STtd
(2)
where ꢃATSample and ꢃATStd are the changes in the triplet state
absorbance of the ZnPc derivative and the standard, respectively.
εSTample and εSTtd are the triplet state extinction coefficients for com-
plex 3 and standard (ZnPc), respectively. ˚STtd is the triplet state
quantum yield for the standard, ZnPc in DMSO, ˚STtd = 0.65 [30]
and in DMF, ˚STtd = 0.58 [31]. ˚T values were determined for com-
plex 3 and are represented as ˚T(MPc) and the corresponding triplet
lifetime as ꢀT(MPc)
.
Quantum yields of internal conversion (˚IC) were obtained from
Eq. (3). This equation assumes that only three processes (fluo-
rescence, intersystem crossing and internal conversion), jointly
deactivate the excited singlet state of the ZnPc derivative.
2.4.3. Tetrakis-2,(3)-[(1,6-hexanedithiol)
phthalocyaninato]zinc(II) functionalized AuNPs (3-AuNP,
Scheme 2)
The THdTZnPc (3) (0.020 g, 0.016 mmol) and TOAB-AuNPs
(0.050 g) in toluene (5 mL) were mixed and stirred for 72 h for the
ligand exchange to take place so as to attach the Pc onto AuNPs.
After the ligand exchange, the unreacted MPc was separated in a
size exclusion column: Biobeads S-X1 using toluene as an eluent.
The MPc–AuNP conjugate comes off first, followed by unreacted
AuNPs and finally the unreacted MPc. IR [(KBr) ꢄmax/cm−1]: 3018
(N–H), 2932 (C–H), 1517 (C C), 1213 (C–N), 1039 (C–C), 749, 667
(N–H).
˚
IC
= 1 − (˚F + ˚T )
(3)
2.3.3. Singlet oxygen quantum yields
The singlet oxygen quantum yield (˚ꢃ) determinations for 3
were carried out using an experimental set-up that is described
in detail elsewhere [32]. In this work ˚ꢃ values were determined
using DPBF as a singlet oxygen quencher in organic media employ-
ing Eq. (4) [33,34]:
WDPBF · IStd
abs
˚
= ˚Std
(4)
2.5. AuNP and 3-AuNP characterization
ꢃ
ꢃ
Std
W
· Iabs
DPBF
where ˚Sꢃtd is the singlet oxygen quantum yield for the standard,
The synthesis of AuNPs and their characterization has been
reported before [21]. The sizes of the TOAB AuNPs and the MPc func-
tionalized AuNPs were determined using X-ray powder diffraction
(XRD) [36].
XRD allows the determination of the particle-diameter (d)
through the use of the Debye–Scherrer Eq. (6):
ZnPc ˚ꢃStd = 0.67 in DMSO [34] and ZnPc ˚Sꢃtd = 0.56 in DMF [35].
Std
DPBF
WDPBF and W
are the DPBF photobleaching rates in the pres-
ence of the ZnPc derivative under investigation and the standard,
Std
abs
respectively. Iabs and I are the rates of light absorption by the ZnPc
derivative and the standard, respectively. The initial DPBF concen-
trations used were kept the same for both the ZnPc derivative and
the standard.
The values of the fraction of the excited triplet state quenched by
ground state molecular oxygen (Sꢃ) were determined by employ-
ing Eq. (5):
kꢁ
˚
d(A) =
(6)
ˇ Cos ꢂ
where k is an empirical constant equal to 0.9, ꢁ is the wavelength
˚
of the X-ray source, (1.5405 A), ˇ is the full width at half maximum
of the diffraction peak, and ꢂ is the angular position of the peak.
˚
˚
ꢃ
Sꢃ
=
(5)
ꢃ
3. Results and discussion
2.4. Synthesis
3.1. Characterization of complex 3
2.4.1. 4-(1,6-Hexanedithiol) phthalonitrile (2, Scheme 1)
1,6-Hexanedithiol (1.6 mL, 11 mmol) and 4-nitrophthalonitrile
(1.68 g, 9 mmol) were dissolved in DMF (80 mL) under a stream of
nitrogen and the mixture stirred at room temperature for 15 min.
Thereafter, finely ground K2CO3 (2.12 g, 15 mmol) was added por-
tion wise over a period of 4 h and the reaction mixture left to stir for
The new complex (3) was characterized using UV–vis, IR, NMR
spectroscopies, MALDI-TOF mass spectra and elemental analysis.
The analyses are all consistent with the expected as shown in the
experimental section. The complex exhibited moderate solubility
in organic solvents such as CHCl3, DCM, THF, DMF and DMSO.

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S. Moeno et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 343–350
Scheme 2. A schematic representation of the functionalization of TOAB AuNPs with complex 3 (THdTZnPc, 3, b), not a true representation of the loading capacity of Pc onto
the AuNPs.
The mass spectra of the phthalocyanine was obtained by the
relatively soft ionization MALDI-TOF technique with the molecular
ion peak observed at 1170.56 for (3).
most likely due to protonation in acid solvents such as chloroform
[37]. The Q band is observed at 706 nm in DMSO and 702 in DMF.
The excitation spectra are mirror images of the fluorescence emis-
sion spectra, Fig. 2. The absorption spectra is however broad due to
aggregation. Table 1 shows the spectral data with Stokes shifts for
complex 3 that are typical of Pc complexes in general [38].
The fluorescence quantum yield (˚F) values of complex 3 in
DMSO and DMF are shown in Table 1 for excitation at 620 nm in
both DMSO and DMF. The values are low compared to MPc com-
plexes in general in both DMSO and DMF due to aggregation [38].
Within experimental error, the ˚F values are not very different in
DMF and DMSO.
The 1H NMR spectra of the ZnPc derivative (3) shows complex
patterns due to the mixed isomer character of tetra substituted MPc
derivatives and also due to aggregation. Complex 3 was found to
be pure by 1H NMR with all the substituents and ring protons were
observed in their respective regions.
The newly synthesized complex 3 showed slight aggregation in
the absorbance spectra, Fig. 1a(i) and b(i), with Q band maxima at
702 in chloroform. The broad feature at 770 nm in chloroform is
a
1.2
0.8
0.4
0
1.2
0.8
0.4
0
(i)
(ii)
(iii)
600
650
700
750
800
Wavelength (nm)
b
1.2
0.8
0.4
0
1.2
0.8
0.4
0
(i)
(ii)
(iii)
600
650
700
750
800
Wavelength (nm)
Fig. 1. (a) Absorption spectra of (i) complex 3 alone, (ii) TOAB AuNPs, (iii) mixture of
3 and AuNPs and (iv) 3-AuNP conjugate in CHCl3. (b) Normalized Q band absorption
spectra for (i) complex 3 alone, (ii) mixture of 3 and AuNPs and (iii) 3-AuNP conjugate
in CHCl3.
Fig. 2. Absorption (i), excitation (ii) and emission (iii) spectra of complex 3 in (a)
DMF and (b) DMSO (ꢁexcitation = 620 nm).

S. Moeno et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 343–350
347
Table 1
Absorbance and fluorescence data for complex 3 and 3-AuNP conjugate.
Sample
Solvent
Q band ꢁmax (nm)
Emission ꢁmax (nm)
Stokes shift ꢃꢁstokes (nm)
Log ε
˚
F(MPc)
0.01
3
DMSO
DMSO
DMF
706
697
702
696
718
706
712
706
12
9
10
10
4.63
–
4.88
–
0.06
0.07
0.08
0.06
3-AuNP
3
3-AuNP
DMF
3.2. Characterization of complex 3-AuNP conjugates
more of the thiol groups to be attached to the AuNPs. It is however
more feasible that the Pcs are perpendicularly oriented on the AuNP
surface, since the Pc units displace the TOAB stabilizer which could
not have easily been displaced to allow for planar orientation of the
Pc units across the surface of the AuNP.
The absorbance spectra of complex 3 alone, the mixture of com-
plex 3 and AuNPs and the 3-AuNP conjugate at the Q band position
are shown as normalized spectra in Fig. 1b in CHCl3. The normal-
ized superimposed spectra show that there is a slight shift of the
Q-band position from 702 nm for complex 3 alone to 698 nm for
complex 3 in the mixture with AuNPs and to 692 nm for complex
3 in the 3-AuNP conjugate. All the spectral shifts of the Q band
position of the Pc are towards the blue in the presence of AuNPs.
The absorption spectrum of the Pc in the conjugate (3-AuNP) is
narrower compared to 3 alone (Fig. 1b). The large blue shifting for
3-AuNP conjugate shows that the sulfur groups are engaged in the
linking to AuNPs.
As observed in Fig. 1b, the absorption spectrum of complex 3
alone (curve i) was rather broad and this behaviour is attributed to
aggregation. However, when the solution of AuNPs was added to
the solution of complex 3, the Pc was disaggregated by the AuNPs
as judged by the narrowing of the Q band (curve ii). This effect of
monomerization of the Pc Q-band is also observed in the conjugate
where there is no sign of aggregation of complex 3 in the 3-AuNP
conjugate, Fig. 1b(curve iii). The monomerizing effect of the Pc was
shown to be by TOAB. It is suggested that complex 3 attaches to the
AuNPs via a coordinate covalent bond.
XRD diffractograms obtained for this work showed the expected
Bragg reflections at 2ꢂ values of 39.1, 45.3 and 65.2 which are
assigned to the (1 1 1), (2 0 0), (2 2 0) reflections of cubic face centred
nanoparticles. However many other reflections were observed and
these may be due to the TOAB stabilizer used on the parent AuNPs
as well as local structural disorders and defects of the AuNPs, Fig. 3
[39,40]. The Pc functionalized AuNPs show less Bragg reflections
and these as expected are slightly shifted in comparison to those of
the parent AuNPs. The sizes were determined for TOAB AuNPs to
be 4.96 nm while that of the complex 3 functionalized AuNPs was
found to be around 5.96 nm. The functionalization of the AuNPs
with the Pc increases the size of the AuNP in the conjugate by 1 nm.
The TOAB AuNPs and the AuNPs functionalized with self assem-
bled monolayers of complex 3 (represented as 5 in Scheme 2)
were also characterized with transmission electron microscopy
(TEM) [36,41], Fig. 4A and B. These TEM pictures at the same mag-
nification (2000 nm) clearly show the distinction between TOAB
AuNPs and 3-AuNPs, where TOAB stabilized AuNPs are clustered.
The 3-AuNP conjugate particles are monodisperse unlike their non
MPc-functionalized counterparts indicative that complex 3 has
been successfully attached (assembled) onto the AuNPs.
The MPc substituents in this work each terminate with a thiol
moiety. It is this thiol group that allows for attachment onto the
AuNPs since they are known to have a high affinity for gold surfaces.
Complex 3 has four thiol groups, suggesting a possibility of one or
Fig. 3. Overlaid X-ray diffractograms of TOAB stabilized AuNPs and the 3-AuNP conjugate.

348
S. Moeno et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 343–350
Fig. 4. TEM images of (A) TOAB stabilized AuNPs (4.96 nm) and (B) 3-AuNPs (5.96 nm) (the scale bar for A and B is 2000 nm).
Table 2
The emission of complex 3 attached to the AuNPs was slightly
Fluorescence lifetime measurements for complex 3 and 3-AuNP conjugate in DMSO
and DMF.
blue-shifted compared to that of free complex 3 (shown in Fig. 5) in
DMSO and DMF. The ˚F values of complex 3 in the 3-AuNP conju-
gate in DMSO and DMF are shown in Table 1. It is expected that for
the MPc molecules in the 3-AuNP conjugate, the AuNPs will quench
the emission of the Pc hence lowering the ˚F values, but Table 1
shows no significant decrease in these values in the presence of
AuNPs.
In Fig. 1a the surface plasmon band for TOAB AuNPs is shown
to be around 389 nm in CHCl3. However this band is merged with
the B band of phthalocyanines with a maximum at 426 nm for the
3-AuNP conjugate also in CHCl3. The change in the surface plasmon
absorption band is indicative of modification of NPs. The observed
spectral changes for the gold nanoparticle surface plasmon band
in the 3-AuNP conjugate (Fig. 1a) indicate that the attachment of
the complex 3 to the AuNPs was successful and that it affects the
b
b
Sample
ꢀ1 (ns)a
˛
1
ꢀ2 (ns)a
˛
2
3 (DMSO)
3-AuNP (DMSO)
3 (DMF)
1.88 0.01
0.001 0.0
2.04 0.01
0.95 0.01
0.88
0.002
0.87
0.29
3.36 0.04
2.69 0.03
3.28 0.03
2.86 0.03
0.12
0.998
0.13
0.71
3-AuNP (DMF)
a
Fluorescence lifetimes at MPc emission.
˛ denotes the amplitude fraction.
b
size of the nanoparticles as proven with XRD. The absorbance of
the mixture of AuNPs with complex 3 shows the resulting merger
of the B-band of the Pc with the surface plasmon band of the AuNPs
with the peak from this combination appearing at 384 nm in CHCl3.
3.3. Fluorescence lifetimes
a
30
25
20
15
10
5
Time correlated single photon counting (TCSPC) measurements
were carried out at the MPc emission maxima. The measurements
reveal biexponential decay kinetics for complex 3, Fig. 6, which
is common for substituted Pc macrocycles especially those hav-
ing monomeric and aggregated species. The results are shown in
Table 2, where the aggregated specie of the Pc displays shorter flu-
orescence lifetime ꢀ1 while the monomeric specie exhibits longer
fluorescence lifetime ꢀ2. Complex 3 showed a trend where the
aggregated specie (with ꢀ1) was in larger abundance in both DMSO
and DMF for the MPc alone. The abundance changed though, in
the 3-AuNP conjugate because as seen from Table 2 the aggregated
specie (ꢀ1) was now in less abundance in DMF and DMSO. The
fact that the abundance of the monomeric species of complex 3 is
increased in the 3-AuNP conjugate solutions (compared to complex
3 alone) confirms that the AuNPs and the associated TOAB stabi-
lizer have a monomerizing effect on the phthalocyanine complex
in solution. It is reasonable to expect disaggregation of complex 3 to
be a cause for the increased abundances of the monomeric species
(ꢀ2) in the conjugate as was clearly observed in Fig. 1b.
There was a lowering of the MPc lifetimes in the conjugate com-
pared to those of complex 3 alone in solution. This observation of
lowered ꢀ2 values may be due to the interaction of the AuNPs and
complex 3 which could possibly result in energy transfer from the
AuNPs to complex 3, but this is tentative since TCSPC measure-
ments were carried out at the excitation wavelength where only
complex 3 absorbs. Two main mechanisms for energy transfer are
well known with one being that of the Förster resonance energy
transfer. This mechanism, however would mainly involve excita-
3
3-AuNP
0
600
650
700
750
800
Wavelength (nm)
30
25
20
15
10
5
b
3
3-AuNP
0
600
650
700
750
800
Wavelength (nm)
Fig. 5. Emission spectra of complex 3 and 3-AuNP conjugate in (a) DMSO and (b)
DMF (ꢁexcitation = 620 nm).

S. Moeno et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 343–350
349
10000
3
3-AuNP
8000
6000
4000
2000
0
4
2
0
-2
-4
0
3
6
9
12
15
Lifetime (ns)
Fig. 6. Photoluminescence decay curves of (i) complex 3 alone and (ii) 3-AuNP conjugate in DMF (ꢁexcitation = 712 nm, ꢀ1/e = 3.28 and 2.86 ns, respectively).
Table 3
tion at the AuNP emission wavelength and thus is not possible in
Photophysical parameters for 3 alone and in the 3-AuNP conjugate.
this work. Another principal mechanism for energy transfer is the
Dexter energy transfer which requires very short distance ranges
(<1 nm) which are possible for complex 3 assembled onto the
AuNPs (conjugate) [26]. Dexter energy transfer proceeds through
electron transfer and is most likely to take precedence in this work
considering the fact that electron and charge transfer are prone to
occur in the complex 3 and AuNP conjugates [20,26].
Sample
Solvent
˚
ꢀT(MPc)/␮s
˚
˚
Sꢃ
IC
T(MPc)
ꢃ
3
DMSO
DMSO
DMF
0.67
0.80
0.63
0.75
76
84
210
304
0.27
0.13
0.29
0.19
0.59
0.74
0.59
0.71
0.88
0.93
0.94
0.95
3-AuNP
3
3-AuNP
DMF
in DMF compared to DMSO could be related to lower viscosity of
the former. A decrease in viscosity of the solvent increases the pos-
sibility of deactivation of the excited state by external conversion,
and DMF is less viscous than DMSO.
3.4. Triplet state quantum yields (˚T) and lifetime (ꢀT) studies
Eq. (2) was used to calculate for ˚T values. The ˚T values quan-
tify the fraction of absorbing molecules that undergo intersystem
crossing (ISC) to the triplet state. In Fig. 7 a representative triplet
state decay curve for complex 3 in DMF is shown. A variation of ˚T
values of complex 3 and for 3 in the 3-AuNP conjugate in DMSO
and DMF is shown in Table 3. An increase of the ˚T values for
complex 3 in the 3-AuNP conjugate was observed. This observation
results from the heavy atoms of AuNPs which encourage intersys-
tem crossing (to the triplet state) of the MPc complex through spin
orbit coupling. The increase in ˚T values of MPcs associated with
heavy metal ions constituting nanoparticles (CdTe QDs) has been
previously reported by our group [42,43]. The slightly lower values
Triplet state lifetime (ꢀT) values of the complex 3 alone were
slightly lower than those of complex 3 assembled onto AuNPs,
Table 3. This observation however does not correspond well to the
increased ˚T values in the 3-AuNP conjugate, where an increase
in ˚T values is expected to result in a decrease in the triplet state
lifetime, this behaviour has been encountered with the use of other
nanoparticles [22,23,42]. The values of ꢀT are much larger in DMF
than in DMSO in Table 3; this is not expected based on the above
discussion on differences in viscosity between the two solvents.
However the high ꢀT values correspond to slightly lower ˚T values
for 3 and its conjugate.
The ˚IC values were calculated with the use of Eq. (3). As shown
in Table 3, the values of ˚IC for complex 3 were moderate indicative
of the fact that radiationless transitions have an important role in
the deactivation of the excited state. The combined effect of radia-
tionless transitions and the high ˚T values account for deactivation
of the excited state thereby resulting in the low ˚F values obtained
for complex 3.
0.0060
0.0055
0.0050
0.0045
0.0040
0.0035
0.0030
3.5. Singlet oxygen quantum yields
Singlet oxygen quantum yield (˚ꢃ) values reflect the amount of
singlet oxygen produced. The ˚ꢃ values were obtained through the
use of Eq. (4). The singlet oxygen generation efficiency depends on
the triplet state quantum yield ˚T [23] and the triplet state lifetime
ꢀT. Table 3 shows that the ˚ꢃ values for 3 are relatively high in
both DMSO and DMF, corresponding to the large ˚T values. The
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
Lifetime (sec)
Fig. 7. A representative triplet decay curve for complex 3 (ꢁexcitation = 685 nm in
˚ꢃ values of complex 3 in Table 3 are: ˚ꢃ = 0.59 (DMSO), ˚ꢃ = 0.59
DMF).

350
S. Moeno et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 343–350
(DMF) for 3 alone and ˚ꢃ = 0.74 (DMSO), ˚ꢃ = 0.71 (DMF) for 3 in
the 3-AuNP conjugate. Thus ˚ꢃ values for 3 are the same in DMSO
and DMF and do not show the trend observed for ˚T values as
expected. The trend for ˚ꢃ values for 3-AuNP follows that observed
for ˚T values when comparing DMSO with DMF.
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The synthesis and characterization of tetrakis-2,(3)-[(1,6-
hexanedithiol) zinc phthalocyanine] was carried out successfully.
The photophysicochemical properties of complex 3 were deter-
mined in DMSO and DMF. Complex 3 was successfully assembled
onto AuNPs by virtue of the free terminal thiol groups on the sub-
stituents of the Pc. The presence of the TOAB stabilizing agent
resulted in the monomerization of the complexes in DMSO and
DMF but not in chloroform. The photophysicochemical proper-
ties of the Pc in the 3-AuNP conjugate were enhanced. There
is an increase in both the triplet yield quantum yields and life-
times of complex 3 in the presence of AuNPs. The increase in both
parameters shows advantage of the conjugates for use as pho-
tosensitizers. The fluorescence quantum yields however showed
insignificant change in the presence of AuNPs. The fluorescence
lifetimes of complex 3 were slightly decreased in the 3-AuNP
conjugate.
Acknowledgements
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF), South
Africa through DST/NRF South African Research Chairs Initiative
for Professor of Medicinal Chemistry and Nanotechnology as well
as Rhodes University and Medical Research Council of South Africa.
SM thanks DAAD foundation for a scholarship.
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