The photophysical studies of a mixture of CdTe quantum dots and negatively charged zinc phthalocyanines

Article abstract of DOI:10.1016/j.poly.2008.03.007

Fluorescence resonance energy transfer (FRET) studies were carried out with quantum dots capped with thioglycolic acid (TGA) and 2-mercaptoethanol (2-ME) and negatively charged phthalocyanines {Zn tetracarboxy (ZnTCPc), Zn octacarboxy (ZnOCPc) and Zn tetrasulfo (ZnTSPc) phthalocyinines} in a 0.1 NaOH:EtOH (50:50) solvent mixture. The best overlap between emission spectra of the donor (QDs) and the absorption spectra of the acceptor (ZnPc derivatives) was observed for TGA capped QDs, very little overlap was obtained for 2-ME QDs. ZnTSPc gave the highest FRET efficiency (0.3), with ZnOCPc and ZnTCPc giving a FRET efficiency of 0.2. The Φ_T values of the MPcs generally decreased in the presence of the QD whereas the triplet lifetimes (τ_T) of the ZnPc derivatives were higher in the presence of QDs.

Full text of DOI:10.1016/j.poly.2008.03.007

Polyhedron 27 (2008) 1953–1958
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
The photophysical studies of a mixture of CdTe quantum dots and negatively
charged zinc phthalocyanines
Sharon Moeno, Tebello Nyokong *
Department of Chemistry, Rhodes University, P.O. Box 94, 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:
Fluorescence resonance energy transfer (FRET) studies were carried out with quantum dots capped with
thioglycolic acid (TGA) and 2-mercaptoethanol (2-ME) and negatively charged phthalocyanines {Zn tet-
racarboxy (ZnTCPc), Zn octacarboxy (ZnOCPc) and Zn tetrasulfo (ZnTSPc) phthalocyinines} in a 0.1 NaO-
H:EtOH (50:50) solvent mixture. The best overlap between emission spectra of the donor (QDs) and the
absorption spectra of the acceptor (ZnPc derivatives) was observed for TGA capped QDs, very little over-
lap was obtained for 2-ME QDs. ZnTSPc gave the highest FRET efficiency (0.3), with ZnOCPc and ZnTCPc
Received 12 February 2008
Accepted 4 March 2008
Available online 21 April 2008
Keywords:
Quantum dots
giving a FRET efficiency of 0.2. The U
whereas the triplet lifetimes (s ) of the ZnPc derivatives were higher in the presence of QDs.
Ó 2008 Elsevier Ltd. All rights reserved.
T
values of the MPcs generally decreased in the presence of the QD
2
-Mercaptoethanol
T
Thioglycolic acid
Phthalocyanine
Fluorescence resonance energy transfer
1
. Introduction
Quantum dots (QDs) are a relatively new class of compounds
we studied the behaviour of CdTe nanoparticles capped with
thioglycolic acid (TGA) and 2-mercaptoethanol (2-ME), in the pres-
ence of negatively charged phthalocyanines: zinc tetracarboxy
(ZnTCPc), octacarboxy (ZnOCPc) and tetrasulpho (ZnTSPc) phthalo-
cyanines, Fig. 1. ZnTCPc and ZnTSPc are tetrasubstituted and con-
sist of a mixture of isomers which are difficult to separate,
whereas ZnOCPc is octasubstituted and is isomerically pure [27–
30].
with unique properties which have fueled increased research inter-
est in a variety of disciplines [1–6], including applications in tag-
ging of biological molecules, polymer electronics, as phosphors,
in light emitting diodes and in photodynamic therapy (PDT) [7–
1
9]. QDs are inorganic semiconductors which exhibit size depen-
dent optical and electronic properties [1,4–6]. Among some of
the unique optical properties exhibited by QDs are the narrow
emission spectra and the excellent photochemical and chemical
stability and [1–6,20–24]. QDs have a larger band gap as opposed
to the bulk materials [1–6,20–22], and behave differently from
the bulk material because of the difference in electronic makeup
It has been shown in a number of studies [18,31,32] that QDs
can serve as near infra-red (IR) energy donors and phthalocyanines
(Pcs) can accept this energy in a process known as fluorescence
resonance energy transfer (FRET). Most of the studies used non-
water soluble MPc complexes. The use of water soluble QDs and
MPc complexes is desirable for many applications. FRET is a photo-
physical process based on a non-radiative energy transfer between
a donor and a suitable energy acceptor [33–35]. Moreover, the effi-
ciency of energy transfer depends upon the amount of spectral
overlap between the emission spectrum of the donor and the
absorption spectrum of the acceptor [34]. In this work, FRET effi-
ciency between the CdTe QDs and the negatively charged phthalo-
cyanines was studied.
[
5,11,25].
In this study, CdTe QDs were synthesized from aqueous solution
and were stabilized by using water soluble thiols, 2-mercap-
toethanol and thioglycolic acid. In addition to serving as stabilizers,
these thiol cappings serve to passivate the surface of the QDs and
thereby removing the surface traps which lower the photolumi-
nescence (PL) efficiency of the quantum dots [1,3,5,20,22]. The cap-
ping agents are also a means by which QDs interact with other
molecules and their environment [1,3,20–22]. Thiol cappings of
QDs were found to suppress intermittence of emission known as
blinking [26].
2. Experimental
2.1. Materials
QDs can be linked to other molecules through covalent attach-
ments, electrostatic forces [19] and hydrogen bonding. In this work
Thioglycolic acid (TGA), 2-mercaptoethanol (2-ME), tellurium
powder (200 mesh, 99.8%), 4-sulphophthalic acid, nitrobenzene,
trimellitic anhydride, pyromellitic anhydride and zinc acetate
dihydrate were obtained from Sigma–Aldrich. Cadmium chloride
*
Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
0
277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.03.007

1
954
S. Moeno, T. Nyokong / Polyhedron 27 (2008) 1953–1958
R
HOOC
COOH
N
N
N
N
N
N
HOOC
HOOC
COOH
COOH
N
N
Zn
N
Zn
R
N
N
R
N
N
N
N
N
COOH
HOOC
(c)
R
(
a) R = SO Na; (b) R = COOH
3
Fig. 1. Molecular Structures of (a) zinc tetrasulfo phthalocyanine (ZnTSPc), (b) zinc tetracarboxy phthalocyanine (ZnTCPc) and (c) zinc octacarboxyphthalocyanine (ZnOCPc).
monohydrate, sodium borohydride, ammonium chloride, ammo-
nium molybdate, urea, sodium hydroxide, and sulphuric acid were
obtained from Saarchem. 1,8-Diazabicyclo[5,4,0]undec-7-ene
where F and FStd are the areas under the fluorescence curves of the
MPc derivatives 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
(
DBU) was obtained from Fluka. Ultra pure water was obtained
from a Milli-Q Water System (Millipore Corp., Bedford, MA, USA).
Solvent mixture: (1:1, 0.1 M NaOH, ethanol) was employed.
as a standard, U
quantum yields of the ZnPc derivatives in the 0.1 M NaOH:ethanol
solvent mixture. Rhodamine 6G in ethanol with U = 0.94 was em-
F
= 0.20 [38] for the determination of fluorescence
2.2. Synthesis of ZnPc derivatives and quantum dots
F
ployed as the standard for the quantum dots [39,40]. Both the sam-
ple and the standard were excited at the same relevant wavelength.
The fluorescence quantum yields of the QDs are represented as
Zinc tetracarboxy (ZnTCPc), octacarboxy (ZnOCPc) and tetra-
sulphonated (ZnTSPc) phthalocyanines were synthesized and char-
acterized according to reported methods [27–30]. CdTe quantum
dots were synthesized using wet chemical routes with thioglycolic
acid and 2-mercaptoethanol cappings, using reported methods [4].
UF(QD), where QD represents either 2-ME or TGA capped QDs. And
for ZnPc complexes the U are represented as UF(ZnPc) (where ZnPc
F
represents ZnTSPc, ZnOCPc, ZnTCPc). The determined fluorescence
quantum yield values of the QDs were employed in determining
Briefly, CdCl
ized water and 0.26 g (2.86 mmol) TGA or 2-ME was added to the
CdCl solution. The pH of this precursor solution was adjusted to
1.5 with 1 M NaOH and left under a flow of nitrogen gas for half
an hour. The solution of NaHTe was prepared by modified litera-
ture method [4] as follows: 0.05 M NaOH was reacted with H Te
gas (the latter generated by reaction of NaBH with Te powder in
the presence of 0.5 M H SO under a flow of nitrogen gas). Then
2
0.22 g (1.18 mmol) was dissolved in 65 mL of deion-
their fluorescence quantum yields in the mixture with ZnPc deriva-
Mix
tives ðU
Þ using a modified form of Eq. (2) as shown by
2
FðQDÞ
1
Mix
F
Mix
QD
U
¼ UFðQDÞ F
ð3Þ
FðQDÞ
QD
2
4
where UF(QD) is the fluorescence quantum yield of the QDs alone,
Mix
2
4
and was used as standard, FQD
is the fluorescence intensity of
the freshly prepared NaHTe 0.18 g (1.2 mmol) was added to the
precursor solution. The solution was then refluxed under air at
00 °C for different times to control the size of the CdTe QDs. Pre-
cipitation of the QDs from the aqueous colloidal solution was
achieved by using propan-2-ol yielding a yellow–green and or-
ange–maroon colour of CdTe quantum dots powder for 2-ME and
TGA cappings, respectively.
The quantum dots sizes were estimated using a polynomial fit-
ting function (Eq. (1)) derived in the literature [36]
QDs in the mixture (with ZnPc derivatives) when excited at the
excitation wavelength of the QDs (500 nm) and FQD is the fluores-
cence intensity of the QD alone at the same excitation wavelength.
Triplet quantum yields were determined using a comparative
method based on triplet decay, using the following equation:
1
Sample
Std
T
Std
DAT
ꢂ e
Sample
UT
¼ UT
ð4Þ
Std
Sample
DAT ꢂ eT
Sample
Std
where DAT
and DAT are the changes in the triplet state absor-
bance of the MPc derivatives and the standard, respectively. e
and eT are the triplet state extinction coefficients for the ZnPc
derivative and standard, respectively, and were determined as pre-
viously reported [41]. UT is the triplet state quantum yield for the
3
2
Sample
D ¼ ð9:8127 ꢀ 10 ꢁ 7Þk ꢁ ð1:7147 ꢀ 10 ꢁ 3Þk þ ð1:0064Þk
ð194:84Þ
T
Std
ꢁ
ð1Þ
Std
where k is the absorption maxima of the QDs. The fitting function is
not valid for sizes of quantum dots outside the size range of 1–9 nm
36]. It is valid in this work since as reported before [32] the quan-
tum dots sizes are within this range.
Std
standard, ZnTSPc in aqueous solution, UT ¼ 0:56 [42]. U was also
T
[
determined for the mixture of QDs and ZnPc derivatives and is rep-
Mix
Mix
resented as U
and the corresponding triplet lifetime as s
The error in the determination of triplet yield was ±10%.
.
TðZnPcÞ
TðZnPcÞ
2.3. Photophysical studies
2.4. Determination of FRET parameters
F
Fluorescence quantum yields (U ) were determined by compar-
ative method [37] using the following equation:
Fluorescence resonance energy transfer (FRET) involves energy
transfer from a photoexcited donor fluorophore to an acceptor
fluorophore of a different species which is in close proximity. FRET
is dependent on the center-to-center separation distance between
2
F ꢂ AStd ꢂ n
U
F
¼ UFðStdÞ
ð2Þ
2
Std
F
Std ꢂ A ꢂ n

S. Moeno, T. Nyokong / Polyhedron 27 (2008) 1953–1958
1955
donor and acceptor (r), the degree of spectral overlap of the donor’s
fluorescence emission spectrum and the acceptor’s absorption
spectrum [33,40]. There is a decrease of the donor photoemission
and an increase in the acceptor’s fluorescence when FRET has
occurred. FRET efficiency (Eff) is determined experimentally from
literature methods, gave characteristic spectral behavior [4]. As
heating is prolonged during the formation of QDs, both the absorp-
tion and the emission spectra continue to get red shifted as the
quantum dots grow [44]. The 2-ME capped CdTe QDs had their first
emission peak at 524 nm after 15 min of heating under reflux, and
their growth ceased when they reached 555 nm (Fig. 2a) after heat-
ing for 64 h. The first emission peak was observed around 525 nm
after 15 min of heating for TGA capped CdTe quantum dots and the
peak shifted to 645 nm (Fig. 2b) after 56 h of reflux, at which time
they stopped growing. It was observed that the 2-ME capped CdTe
nanoparticles grow at a much slower rate than those that were
capped with TGA as was mentioned in literature [44]. The esti-
mated full width at half maximum (FWHM) was 60 nm for the
2-ME capped CdTe nanoparticles (at 555 nm) and 83 nm for the
TGA capped counterparts (at 645 nm). The sizes (diameter) of
QDs were used for this study are 3.3 and 4.2 nm for 2-ME and
TGA capped QDs, respectively.
the fluorescence quantum yields of the donor in the absence
Mix
FðQDÞ
(
U
F(QD)) and presence ðU
Þ of the acceptor using Eq. (5) [33,40]
Mix
U
FðQDÞ
Eff ¼ 1 ꢁ U
ð5Þ
FðQDÞ
FRET efficiency (Eff) is related to r (Å) by Eq. (6) [40]
6
0
R
Eff ¼
ð6Þ
6
6
R þ r
0
0
where R (The Förster distance, Å) is the critical distance between
the donor and the acceptor molecules for which efficiency of energy
transfer is 50% and depends on the quantum yield of the donor,
Eq. (7) [40]
Absorbance and fluorescence emission spectra of the MPc com-
plexes are shown in Fig. 3 (for ZnTSPc and ZnOCPc) in a mixture of
0.1 M NaOH and ethanol (50:50). In this solvent mixture, the ZnPc
derivatives are monomeric. Aggregation in MPc complexes is due
to a coplanar association of rings, resulting in splitting and broad-
ening of spectra, with a blue shifted peak at ꢃ630 nm being due to
the aggregate. Addition of organic solvents normally breaks the
aggregates, resulting in monomeric species, hence the NaOH:etha-
nol solvent mixture was employed in this work. The ZnPc deri-
vatives obeyed Beer’s law for concentrations less than 1 ꢀ
R ¼ 8:8 ꢀ 10 j2nꢁ4
6
23
U
FðQDÞ
J
ð7Þ
0
where j2 is the dipole orientation factor; n, the refractive index of
F
the medium; U , the fluorescence quantum yield of the donor in
6
the absence of the acceptor; and J (cm ) is the Förster overlap inte-
gral, defined by
Z
4
J ¼
f
QDðkÞeZnPcðkÞk dk
ð8Þ
ꢁ5
ꢁ3
1
0
mol dm in this solvent mixture. For all the ZnPc complexes
where fQD is the normalized QD emission spectrum and eZnPc, is the
(Fig. 3), the fluorescence emission spectra were mirror images of
molar extinction coefficient of ZnPc derivatives. In this case, it is
the absorption spectra which in turn were similar to excitation
spectra. At the high pH employed for the synthesis of the QDs
and also used for the photophysical studies, the 2-ME and TGA cap-
ping will be ionized. It is known that the thiol moiety donates elec-
trons to the surface of QDs, hence reducing the so called ‘‘blinking”
2
assumed that j is 2/3; such assumption is often made for donor–
acceptor pairs in a liquid medium, which are considered to be iso-
tropically oriented during their fluorescence lifetimes. k is the
wavelength of the acceptor, the Q band. FRET parameters were
computed using the programme PhotochemCAD [43]. The overlap
[
26]. The MPcs were not chemically linked to the ligands on the
integral (J) has been defined in several ways with different units
QDs surface, and thus the MPcs are most likely to adsorb onto
the surface of the QDs.
6
[
40]. The program PhotochemCAD gives the units as cm using e
ꢁ1
ꢁ1
in M cm and the wavelength in nm.
2.5. Equipment
1
0
.5
1
a
Fluorescence excitation and emission spectra were recorded on
a Varian Eclipse spectrofluoremeter. UV–Vis spectra were recorded
on a Varian 500 UV–Vis/NIR spectrophotometer.
Laser flash photolysis experiments were performed with light
pulses produced by a Quanta-Ray Nd: YAG laser providing
00 mJ, 90 ns pulses of laser light at 10 Hz, pumping a Lambda-
(i)
(ii)
.5
4
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 oscil-
loscope (Tektronix TDS 360). The triplet life times were determined
by exponential fitting of the kinetic curves using the program Orig-
in Pro 7.5.
0
3
50
400
450
500
550
600
650
700
750
800
Wavelength (nm)
1
.5
1
b
(
i)
(ii)
3
. Results and discussion
3
.1. Absorption and emission spectra
0
.5
0
The synthesis of the ZnPc complexes (ZnTCPc, ZnOCPc and
ZnTSPc) is well established, and in this work literature methods
of synthesis and characterization were employed and gave exp-
ected results [27–30]. The quantum dots (QDs) were also synthe-
sized using literature methods [4]. QDs are characterized by their
absorption and photoluminescence spectra which are also related
to their size. The CdTe QDs synthesized in this work according to
4
00
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 2. (i) Emission and (ii) absorbance spectra of (a) CdTe–2-ME and (b) CdTe-TGA
and in 50:50, NaOH:EtOH solution.

1
956
S. Moeno, T. Nyokong / Polyhedron 27 (2008) 1953–1958
Excitation of the QDs at 500 nm in the presence of ZnPc deriv-
1
.2
1
a
(
ii)
(i)
atives, where Pcs do not absorb, resulted in significant fluorescence
of the ZnPc derivatives as observed in Fig. 6. The 2-ME-QDs have
been left out from Fig. 6 due to weak overlap between the emission
spectra of 2-ME-QDs and absorption spectra of ZnPc derivatives.
There was clear transfer of energy (FRET) from the TGA QDs to ZnPc
derivatives (for ZnTSPc and ZnOCPc), Fig. 6a and b. This is judged
by the increase in the emission spectra of ZnPc derivatives in the
presence of QDs. There was insignificant emission by these two
complexes (in the absence of QDs) on excitation at 500 nm,
Fig. 6a–c (curves ii). For ZnTCPc there was no increase in the emis-
sion spectra of ZnTCPc probably due to a small fraction of fluoresc-
0
0
0
0
.8
.6
.4
.2
0
3
50
450
550
650
750
Wavelength (nm)
1
.2
1
b
(ii)
(i)
ing molecules of the Pc (Fig. 6c). This complex has a very low U
<0.01, Table 1), hence no fluorescence peak is observed, but there
was a decrease in the fluorescence intensity of the QDs.
The overlap integral (J) values were computed using Photo-
F
(
0
0
0
0
.8
.6
.4
.2
0
ꢁ14
6
chemCAD [43]. The J values are usually of the order 10
cm
for porphyrin based molecules and the values obtained in this
ꢁ
13
6
work were of the order 10
cm for the overlap between the ZnPc
derivatives and CdTe QDs, Table 2. The overlap integral was largest
for ZnTSPc in the presence of CdTe-TGA QDs and decreasing as fol-
lows: ZnTSPc > ZnOCPc > ZnTCPc. It would be reasonable to assume
that the QD–Pc mixture having the highest J value would also have
the highest efficiency of energy transfer (Eff, calculated using Eq.
3
50
450
550
650
750
Wavelength (nm)
Fig. 3. Fluorescence (i) and absorbance (ii) spectra of (a) ZnTSPc (excita-
tion = 610 nm) and (b) ZnOCPc (excitation = 620 nm) in 50:50, NaOH:EtOH solution.
(
5)) from the QD to the Pc, and this was observed in Table 2 for
ZnTSPc.
0
The values of the Förster distance, R (Å) which is the critical
3.2. Fluorescence quantum yields and energy transfer studies
distance between the donor and the acceptor molecules for which
efficiency of energy transfer is 50% [43,44], were computed with
PhotochemCAD and the values are shown in Table 2. The center-
to-center separation distance (r, Å) between donor and acceptor
chromophores, were calculated using Eq. (6) and the values are
shown in Table 2, ZnOCPc–CdTe-TGA QDs mixture showed the
lowest value of r compared to the others. This indicates that the
ZnOCPc is in close proximity to the donor (QDs) and thus there
should be an ease of energy transfer (Eff) between the excited
TGA capped QDs and the ZnOCPc. The center to center distance
(r) increased with MPc in the order ZnTSPc > ZnTCPc > ZnOCPc for
TGA capped QDs–MPc mixture. However it is ZnTSPc with the larg-
Fluorescence quantum yield (U
in the absence of QDs) are shown in Table 1 for excitation at
30 nm. ZnTSPc and ZnOCPc gave U values of 0.21 and a very
low value for ZnTCPc (U values <0.01) was obtained. The U values
for the CdTe QDs were calculated in 0.1 M NaOH:ethanol solvent
mixture using Eq. (2) and are listed in Table 1. The fluorescence
F
) values of the ZnPc derivatives
(
6
F
F
F
quantum yields (U
F
) of the QDs were observed to be fairly low
(
0.037 for 2-ME capped and 0.11 for TGA capped) and they de-
creased further in the presence of Pc’s. The lower fluorescence
quantum yields of QDs was expected because as the QD (donor)
particles fluoresce there is a transfer of energy to the MPc mole-
cules (acceptor) and this reduces the fluorescence intensity of the
QDs in the QD–MPc mixture through FRET (discussed below),
hence the reduced quantum yields of the QDs in the QD–MPc
mixture.
6
est r (Å) and J (cm ), which had the largest FRET efficiency and not
ZnOCPc.
T T
3.2.1. Triplet quantum yields (U ) and lifetime (s ) studies
As stated above the efficiency of energy transfer depends upon
the amount of spectral overlap between the emission spectrum of
the donor and the absorption spectrum of the acceptor. A poor
overlap was obtained for 2-ME capped QDs (Fig. 4). CdTe-TGA
QDs had a good overlap with all the MPcs (Fig. 5), hence FRET
parameters were calculated only for the mixture of CdTe-TGA
capped QDs and ZnPc derivatives.
The transient absorption spectra were recorded in argon de-
gassed solutions by exciting the ZnPc photosensitizer (in NaOH:E-
tOH mixture) in the Q band region and recording the transient
absorption spectra point by point from 400 to 820 nm (Fig. 7),
showing a triplet absorption near 530 nm. A representative decay
profile is shown in Fig. 8. The triplet decay curves for ZnPc deriva-
tives alone and in the presence of QDs were similar. The triplet
Table 1
Fluorescence quantum yields of QDs and photophysical parameters of ZnPc derivatives in the presence of QD’s in 0.1 M NaOH/ethanol (1:1)
ZnPc derivativeb
U
Mix
FðQDÞ
k
exc = 500 nma
U
k
exc = 630 nm
U
Mix
TðZnPcÞ
b
s
Mix
ðlsÞ
TðZnPcÞ
b
F(ZnPc)
ZnTSPc
ZnTSPc + CdTe-QD (TGA)
ZnTSPc + CdTe-QD (2-ME)
0.014
0.011
0.21
0.21
0.25 (0.56)
0.44 (0.56)
230 (170)
160 (170)
ZnTCPc
ZnTCPc + CdTe-QD (TGA)
ZnTCPc + CdTe-QD (2-ME)
0.06
0.017
<0.01
<0.01
0.53 (0.50)
0.34 (0.50)
1300 (600)
730 (600)
ZnOCPc
ZnOCPc + CdTe-QD (TGA)
ZnOCPc + CdTe-QD (2-ME)
0.018
0.012
0.21
0.21
0.63 (0.75)
0.72 (0.75)
190 (140)
130 (140)
a
F
QDs size: 2-ME = 3.3 nm and TGA = 4.2 nm. TGA = thioglycolic acid, 2ME = 2-mercaptoethanol. U values for quantum dots alone are: CdTe-2ME = 0.037; CdTe-TGA = 0.11.
Values in brackets are for ZnPc derivatives in the absence of QDs.
b

S. Moeno, T. Nyokong / Polyhedron 27 (2008) 1953–1958
1957
3
2
2
1
1
0
5
0
5
0
5
0
(i)
(ii)
0.5
0.4
140
120
100
80
a
a
(
i)
0
0
0
0
.3
.2
.1
(iii)
60
4
2
0
0
0
(ii)
3
50
400
450
500
550
600
650
700
750
800
Wavelength(nm)
5
00
550
600
650
700
750
800
Wavelength (nm)
3
2
2
1
1
0
(i)
0.5
0.4
(
ii)
b
140
5
0
5
0
5
0
b
(i)
1
1
20
00
80
60
0
0
0
.3
.2
.1
(
iii)
4
2
0
0
0
0
800
3
50
400
450
500
550
600
650
700
750
(ii)
Wavelength(nm)
5
00
550
600
650
700
750
800
Wavelength (nm)
5
0
0.3
c
4
3
2
1
0
0
0
0
(
ii)
140
120
c
(i)
0
0
0
.2
.1
(i)
(iii)
1
00
80
60
40
20
0
(ii)
0
3
50
400
450
500
550
600
650
700
750
800
5
00
550
600
650
700
750
800
Wavelength(nm)
Wavelength (nm)
Fig. 4. Emission of CdTe (2-ME) (i) and absorbance (ii) of (a) ZnTSPc, (b) ZnOCPc and
c) ZnTCPc in 50:50, NaOH:EtOH solution. kexcitation = 500 nm.
Fig. 6. Emission of (i) CdTe (TGA) alone, (ii) ZnPc derivative alone and (iii) a mixture
of ZnPc derivative and QDs. (a) ZnOCPc, (b) ZnTSPc and (c) ZnTCPc in 50:50, NaO-
H:EtOH solution. kexcitation = 500 nm.
(
1
00
0.5
0.4
0.3
0.2
0.1
0
a
b
(ii)
80
60
40
20
0
Table 2
(
i)
Energy transfer parameters for ZnPc–CdTe-TGA QDs interactions in 0.1 M NaOH:eth-
anol (1:1)
Capping thiol
J (1013 cm6)
R0 (Å)
r (Å)
Eff
3
50
400
450
500
550
600
650
700
750
800
ZnTSPc + CdTe-QD (TGA)
ZnTCPc + CdTe-QD (TGA)
ZnOCPc + CdTe-QD (TGA)
18
7.2
7.9
57
49
50
67
63
60
0.3
0.2
0.2
Wavelength (nm)
1
00
0.5
0.4
0.3
0.2
0.1
0
(
ii)
80
60
40
20
0
(
i)
0
.2
0
Wavelength (nm)
600 700
3
50
400
450
500
550
600
650
700
750
800
Wavelength (nm)
400
500
800
-
-
-
-
0.2
0.4
0.6
0.8
1
00
0.3
c
8
0
0
0
0
(
ii)
0
0
0
.2
.1
6
4
2
(
i)
0
3
-1
50
400
450
500
550
600
650
700
750
800
Wavelength (nm)
-1.2
Fig. 5. Emission of CdTe (TGA) (i) and absorbance (ii) of (a) ZnTSPc, (b) ZnOCPc and
c) ZnTCPc in 50:50, NaOH:EtOH solution. kexcitation = 500 nm.
-
-
-
1.4
1.6
1.8
(
curve in Fig. 8 obeyed second order kinetics. This is typical of MPc
4
complexes at high concentrations (>1 ꢀ 10 M) [45] due to the
Fig. 7. Transient differential spectrum of complex ZnOCPc (excitation wave-
length = 690 nm), in 50:50, NaOH:EtOH solution.
triplet–triplet recombination. The concentrations employed in this

1
958
S. Moeno, T. Nyokong / Polyhedron 27 (2008) 1953–1958
decreased in the presence of QDs but the lifetimes increased, hence
making the combination of ZnPc derivatives and QDs potential
candidates for applications as photosensitisers in PDT.
0
0
0
0
0
0
.10
.08
.06
.04
.02
.00
Acknowledgements
This work has been supported by the National Research Foun-
dation (NRF GUN 2053657) of South Africa as well as Rhodes Uni-
versity. SM thanks DAAD foundation for a scholarship.
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0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
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The interaction of CdTe quantum dots with ZnPc derivatives
shows that a strong overlap occurs between the emission of the
TGA capped QDs and the absorption spectra of the ZnPc derivatives
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[
[
(