Spectroscopic behavior of cationic metallophthalocyanines in the presence of anionic quantum dots

Article abstract of DOI:10.1016/j.saa.2009.10.050

The interactions and spectroscopic properties between cationic zinc phthalocyanine derivatives (peripherally and non-peripherally tetrasubstituted and peripherally octa substituted with 2-diethylmethylaminoethylsulfanyl (βTZnPc, αTZnPc and βOZnPc)) and CdTe core quantum dots (QDs) capped with mercaptopropionic acid or thioglycolic acid (represented as CdTe@MPA and CdTe@TGA, respectively) have been studied in methanol:water mixture. Strong coupling of MPcs was deduced from the interaction since the UV-vis spectroscopic studies of the ground state complex formed on mixing both components showed loss of the phthalocyanine monomeric band with the formation of a dimeric band (spectrum of aggregated species). The dimerization constants were of the order of 10⁴ M^-1.

Full text of DOI:10.1016/j.saa.2009.10.050

Spectrochimica Acta Part A 75 (2010) 411–416
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic behavior of cationic metallophthalocyanines in the presence of
anionic quantum dots
∗
Mopelola Idowu, 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:
Received 8 April 2009
Received in revised form 14 October 2009
Accepted 28 October 2009
The interactions and spectroscopic properties between cationic zinc phthalocyanine derivatives
(peripherally and non-peripherally tetrasubstituted and peripherally octa substituted with 2-
diethylmethylaminoethylsulfanyl (␤TZnPc, ␣TZnPc and ␤OZnPc)) and CdTe core quantum dots (QDs)
capped with mercaptopropionic acid or thioglycolic acid (represented as CdTe@MPA and CdTe@TGA,
respectively) have been studied in methanol:water mixture. Strong coupling of MPcs was deduced from
the interaction since the UV–vis spectroscopic studies of the ground state complex formed on mixing
both components showed loss of the phthalocyanine monomeric band with the formation of a dimeric
Keywords:
Zinc phthalocyanine
2
-Diethylmethylaminoethylsulfanyl
4
−1
.
band (spectrum of aggregated species). The dimerization constants were of the order of 10 M
2009 Elsevier B.V. All rights reserved.
Quantum dots
Cadmium telluride
Dimerization constants
©
1
. Introduction
[23]. Their ionic charges enables them to form stable ion pairs with
complexes of opposite charges in solution. In most cases, the prop-
erties of the mixed complexes may differ from those of the parent
compounds [24,25].
The use of metallophthalocyanines (MPcs) in visible light-driven
processes as light harvesters or molecular catalysts cannot be over-
emphasized. Their intense absorption in the red and near infra-red
regions of the solar spectrum, has made them attractive as chro-
mophores for light-driven processes. MPcs have found applications
in non-linear optics [1], catalysis [2], liquid crystals [3] and as
chemical sensors [4]. Their applications in oncology [5] and in pho-
tovoltaic cells [6–8] are also well documented.
Quantum dots (QDs) are semiconductor nanoparticles and are
well suited for applications as light sensitizers [9,10]. They have
broad, size-controlled absorption spectra, large extinction coeffi-
cients, and their electronic structure can be size-tuned to optimize
the driving force for electron transfer processes in QD/catalyst
assemblies, where large yields of charge carriers useful for driv-
ing multielectron chemical reactions can be extracted rapidly
The interactions of cationic MPcs grafted with quater-
nary ammonium groups (non-peripherally and peripherally
2-diethylmethylaminoethylsulfanyl tetrasubstituted (␣TZnPc (1)
and ␤TZnPc (2), respectively) and peripherally octasubsti-
tuted zinc phthalocyanine (␤OZnPc (3)) Fig. 1) with thiol
capped CdTe core QDs (capped with thioglycolic acid (TGA)
or mercaptopropionic acid (MPA) Fig. 1) are investigated spec-
troscopically in this study. We have recently reported on
the interaction of positively charged zinc tetramethyl-tetra-2,3-
pyridinoporphyrazine with QDs. The former was reduced in the
presence of QDs [24]. We also reported that [tetramethyl-2,(3)-
[tetra-(2-mercaptopyridinephthalocyaninato)] zinc(II)]4+, linked
to the Pc ring with a sulfur and [tetramethyl-2,(3)-[tetra-(2-
pyridyloxyphthalocyaninato)] zinc(II)]4+, linked to the Pc ring with
an oxygen, showed a different behavior in the presence of quantum
dots [25]. The former showed spectra typical of aggregation in MPc
complexes, while the latter showed no spectral changes. Thus in
this work we investigate further the effects of substituents on the
spectra of MPc complexes in the presence of quantum dots.
[
11–19]. The fluorescence of functionalized QDs can be quenched
or enhanced, depending on the photoactive molecule or nanopar-
ticles they are associated with and there are different mechanisms
behind these various responses that are yet to be fully understood
[
20].
MPcs can be grafted with ionic substituents [21,22] which are
known to make them water soluble and important as sensitizers
2. Experimental and method
2.1. Materials
∗ Corresponding author at: Department of Chemistry, Rhodes University, P.O. Box
9
4
4, Grahamstown, Eastern Cape 6140, South Africa. Tel.: +27 46 6038260; fax: +27
6 6225109.
The
syntheses
and
characterization
of
2-
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
diethylmethylaminoethylsulfanyl tetra and octa substituted
1
386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.10.050

4
12
M. Idowu, T. Nyokong / Spectrochimica Acta Part A 75 (2010) 411–416
Fig. 1. Molecular structures of 2-diethylmethylaminoethylsulfanyl substituted zinc(II) phthalocyanine tetra substituted at the ␣ position (␣TZnPc) (1), ␤ position (␤TZnPc)
2) and octa substituted at the ␤ position (␤OZnPc) (3) and the structural representation of thioglycolic acid capped CdTe quantum dots (CdTe@TGA) and mercaptopropionic
(
acid capped CdTe quantum dots (CdTe@MPA), respectively.
◦
zinc(II) phthalocyanines (␣TZnPc (1), ␤TZnPc (2) and ␤OZnPc (3))
100 C for different times (8 h for MPA capped QDs and 29 h for the
have recently been reported [22,26]. NaBH , NaOH and H SO
TGA capped QDs) to control the size of the CdTe core QDs. On cool-
ing, the CdTe core QDs were precipitated out from solution using
excess ethanol; the solutions were then centrifuged to harvest the
CdTe core QDs. The CdTe core QDs are represented in Fig. 1.
The quantum dots sizes were estimated using a polynomial fit-
ting function (Eq. (1)) derived in literature [27].
4
2
4
were obtained from SAARCHEM. CdCl ·H O, tellurium powder
2
2
(
200 mesh), thioglycolic acid and 3-mercaptopropionic acid were
obtained from Aldrich. Ultra pure water was from a Milli-Q Water
System (Millipore Corp, Bedford, MA, USA).
2.2. Synthesis of CdTe core QDs
D = (9.8127 × 10− )ꢀ − (1.7147 × 10 )ꢀ2
7
3
−3
The preparation of thiol capped QD was via a modified method
+(1.0064)ꢀ − (194.84)
(1)
adapted from literature [11]. Briefly, 2.35 mmol of CdCl ·H O was
2
2
dissolved in 125 mL of water and 5.7 mmol of the respective car-
boxylic acid thiol (thioglycolic acid (TGA) or 3-mercaptopropionic
acid (MPA)) was added under stirring. The solution was adjusted
to pH 12 with 1 M NaOH. Nitrogen gas was bubbled through the
where ꢀ is the absorption maxima of the CdTe core QDs. The fitting
function is not valid for sizes of quantum dots outside the size range
of 1–9 nm [27]. It is valid in this work since as reported before [10]
the quantum dots sizes are within this range.
solution for about 1 h. The aqueous solution was reacted with H Te
2
gas. The H Te gas was generated by the reaction of NaBH4 with Te
2
2.3. Equipment
powder in the presence of 0.5 M H SO4 under a flow of nitrogen
2
gas, and passed through the aqueous solution of CdCl2 with the
respective carboxylic acid thiol as it is been generated. A change of
colour of the solution containing CdCl2 and the thiol was observed
Fluorescence excitation and emission spectra were recorded
on a Varian Eclipse spectrofluorometer. UV–visible spectra were
recorded on a Varian 500 UV–vis/NIR spectrophotometer.
on addition of H Te gas. The solution was then refluxed under air at
2

M. Idowu, T. Nyokong / Spectrochimica Acta Part A 75 (2010) 411–416
413
Table 1
2.4. Interaction studies between CdTe core QDs and MPcs
Spectral and fluorescence data of Zn phthalocyanine derivatives in water and in
methanol:water solvent mixture.
The interaction between MPcs and CdTe core QDs was studied
spectrophotometrically, by gradual addition of aliquots of (5 ␮L)
Sample
Solventa
ꢀQ (nm)
ꢀF (nm)
˚F
0.10
−
6
−5
of [1 × 10 M] solution of CdTe core QDs to 5 mL of 10 M solu-
tion of complexes 1–3 (Fig. 1). Measurements were performed in
a 1 cm quartz cell at room temperature. Methanol: water (60:40)
solvent mixture was employed. In this mixture MPc derivatives are
monomeric while aggregated in water alone.
␣TZnPc
CH OH–H O
696
653, 692
709
710
3
2
H2O
<0.01
␤TZnPc
CH3OH–H2O
H2O
684
643, 682
697
702
0.15
<0.01
␤OZnPc
CH3OH–H2O
H2O
702
661, 700
715
716
0.11
<0.01
2.5. Fluorescence studies
a
CH3OH–H2O = methanol:water (60:40, v/v).
Fluorescence quantum yields (˚F) were determined by compar-
ative method [28] (Eq. (2)),
ous media in the absence and presence of cremophore EL [26]. In
this work we study the interaction of QDs with complexes 1–3 in
methanol:water solvent mixture hence the spectral behavior of the
complexes in this solvent mixture is reported.
FAStdn2
˚F = ˚F(Std)
.
(2)
FStdAn2
Std
The ground state electronic absorption spectra of the com-
plexes (1–3) in aqueous medium and methanol:water (60:40, v/v)
are shown in Fig. 2. As reported before, the complexes are highly
soluble but are aggregated in aqueous medium, Fig. 2a, with the for-
mation of two main bands at ∼680–700 nm due to the monomeric
species and ∼640–660 nm due to the aggregated species, respec-
tively. The complexes however exhibit monomeric behavior in
methanol:water, Fig. 2b (with Q band maxima listed in Table 1)
compared to aqueous medium alone since it is known that organic
solvents tend to favour the monomeric nature of complexes [30,31].
The absorption and excitation spectra of the complexes in
methanol:water medium are similar and are mirror images of the
emission spectra (Fig. 3) which is usual for monomeric MPc deriva-
tives. The Stokes’ shifts were ∼13 nm.
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
wavelength, and n and nStd are the refractive indices of solvents
used for the sample and standard, respectively. ZnPc in DMSO was
employed as standard, ꢁF = 0.2 [29]. At least three independent
experiments were performed for the quantum yield determina-
tions. Both the sample and the standard were excited at the same
relevant wavelength.
3
. Results and discussion
3.1. Spectrophotometric studies
The fluorescence quantum yield (˚F) values of the complexes
in methanol:water medium are typical of phthalocyanines [31,32]
but the values in aqueous medium (in the absence of methanol)
were low (Table 1) compared to MPcs in general [32]. Aggregation
is known to dissipate the electronic energy of the excited singlet
state, thereby leading to fluorescence quenching.
The absorption and emission spectra of complexes 1–3 have
recently been reported in dimethylsulfoxide (DMSO) and in aque-
Fig. 4 shows the absorption and emission spectra of the quan-
tum dots (QDs) used in this study in methanol:water solvent
mixture. The diameters of the QDs employed in the work were
3
.6 nm for CdTe@MPA and 3.7 nm for CdTe@TGA. These sizes were
chosen since they gave good spectral overlap with the MPc com-
plexes 1–3. The absorption spectra show a typical broad peak in
the visible region indicating an absorption peak maxima at about
5
95 and 600 nm for mercaptopropionic acid capped quantum dots
(CdTe@MPA) and thioglycolic capped quantum dots (CdTe@TGA),
respectively. The emission spectrum is characterized by good sym-
metry and sufficiently narrow with full width at half maximum
Fig. 2. Ground state electronic absorption spectra of (i) ␣TZnPc, (ii) ␤TZnPc and
−
6
−3
(
iii) ␤OZnPc in (a) aqueous medium (concentration ∼4 × 10 mol dm ) and (b)
Fig. 3. Fluorescence excitation (i) and emission (ii) spectra of ␤TZnPc (2) in
methanol:water mixture (60:40). ꢀexc = 620 nm.
−
6
−3
methanol:water mixture (60:40) (concentration ∼5 × 10 mol dm ).

4
14
M. Idowu, T. Nyokong / Spectrochimica Acta Part A 75 (2010) 411–416
Fig. 4. Absorption (i) and emission (ii) spectra of CdTe@TGA (a) (dotted line) and
CdTe@MPA (b) (solid line). ꢀexc = 500 nm. Solvent: methanol:water mixture (60:40).
(
FWHM) of ∼60 nm. The QDs were photoluminescent in the visible
region with emission maxima of 632 and 650 nm for CdTe@TGA
and CdTe@MPA, respectively.
3.2. Interaction of MPcs with CdTe core QDs
Overlap exists between emission spectra of the CdTe core QDs
and the MPcs absorption spectra, as shown in Fig. 5, and this is a
good indication that energy transfer is possible from the CdTe core
QDs to the MPcs.
−
6
Titration of aliquots of CdTe core QDs [1 × 10 M] to a fixed
−5
concentration of MPc [1 × 10 M] in methanol:water mixture was
carried out. Spectral changes observed on addition of CdTe@MPA
to ␣TZnPc or ␤TZnPc are shown in Fig. 6a and b, respectively. For
␣
TZnPc, Fig. 6a, a reduction of intensity and broadening of the Q
band of the MPc was observed; the intensity of the vibronic band
at 630 nm also decreased with the formation of a new band at about
6
50 nm and increased intensity at around 730 nm region. There
were isosbestic points at 717, 662, 636 and 608 nm. Similar spec-
tral changes were observed on addition of CdTe@TGA to ␤TZnPc,
Fig. 6b (or to ␤OZnPc). Addition of CdTe@MPA to any of the com-
plexes also resulted in similar spectral changes. Further addition of
the CdTe core QDs solution did not change the spectra in Fig. 6a and
b any further. The spectral changes shown in Fig. 6a and b occurred
instantly, allowing time for the reaction to occur did not change the
spectra. The change only occurred when the concentration of QDs
was changed.
Fig. 6. Gradual absorption spectral changes (a) in ␣TZnPc on addition of aliquots of
CdTe@MPA, (b) in ␤TZnPc on addition of aliquots of CdTe@TGA and (c) on addition
of aliquots of ␣TZnPc to CdTe@MPA, (i) is before and (ii) is after addition of QDs (for
a and b).
MPc to the CdTe core QD solution. The typical spectral changes in
these series are shown in Fig. 6c for titration of CdTe@MPA with
In the reverse titration, the broadening of the Q band of the
MPc was observed immediately on addition of the first aliquot of
␣TZnPc. There is a decrease in QDs absorption (hence of emission,
figure not shown) in Fig. 6c in the presence of QDs. The quench-
ing of QDs emission in the presence of MPc complexes has been
documented [10,25]. The quenching results in lowering of the fluo-
rescence quantum yields of QDs in the presence of MPc complexes
[
10].
Aggregation in MPc complexes is due to a coplanar association
of rings, resulting in splitting and broadening of spectra, with a
blue-shifted peak due to the aggregate. Spectral changes observed
in Fig. 6 suggest aggregation of the MPc complexes, with a broad
blue-shifted band near 650 nm, typical of H aggregates. However,
adding excess methanol, hence increasing the ratio of methanol
to water or addition of surfactants did not break the aggregates,
suggesting these are not H aggregates. Dilution of the solution
containing aggregates did not show evidence of monomerization,
expected for H aggregates. Metal ion (e.g. Ag(I) or Pd(II)) binding
to MPc complexes bearing thio functionality also shows spectral
changes similar to those observed in Fig. 6a and b [33]. The aggre-
gates formed by metal binding are not easily monomerized. This
Fig. 5. Normalized absorption spectra of MPcs and emission spectra of CdTe QD
solutions showing overlap of the emission spectra of the CdTe core QDs with the
absorption spectra of MPcs. (i) ␣TZnPc, (ii) ␤TZnPc and (iii) ␤OZnPc.

M. Idowu, T. Nyokong / Spectrochimica Acta Part A 75 (2010) 411–416
415
Table 2
type of aggregation was not observed for positively charged por-
phyrazine complexes containing pyridine groups instead benzene
rings [24], where charge transfer and reduction of the porphyrazine
molecule occurred. The observed spectral changes could be due to
salting out phenomena which results in the aggregation of MPc
complexes in the presence of QDs. The aggregates may then pre-
cipitate out since a precipitate was observed when the solution of
complexes 1–3 was left standing for several hours (∼35 h). Thus it
seems first there is aggregation on addition of QDs to solutions of
MPc and with time there is precipitation of the aggregates out of
solution.
Estimated association and equilibrium dimerization constants of the Zn phthalo-
cyanine derivatives in methanol:water medium.
5
−1
4
−1
)
Log εD (complex)a
Complex
Ka (×10
M
)
KD (×10
M
␣TZnPc:CdTe@MPA
␤TZnPc:CdTe@MPA
0.67
0.61
0.78
1.20
2.12
6.43
1.10
1.41
1.09
1.05
1.84
1.79
4.25
4.99
4.27
4.14
4.91
4.08
␤
␣
␤
␤
OZnPc:CdTe@MPA
TZnPc:CdTe@TGA
TZnPc:CdTe@TGA
OZnPc:CdTe@TGA
a
Log ε: ␣TZnPc, ␤TZnPc and ␤OZnPc = 5.29, 5.18 and 5.13, respectively, in
methanol:water (60:40, v/v).
In order to confirm the possibility of salting out in the pres-
ence of an electrolyte, we added salts such as KCl to the solution
of the complexes and similar spectral changes as observed in Fig. 6
were observed. This shows that the interaction with QDs results
in salting out which leads to aggregation of the MPcs. The spectra
in Fig. 6 are typical of aggregation in MPc complexes. The fact that
the salting out phenomena was not observed for positively charged
porphyrazine complexes may be due to ease of reduction of these
complexes. There is a possibility of association of MPc complexes
In order to investigate the aggregation state of the MPcs solu-
tion in the presence of the CdTe core QDs and to determine the
equilibrium dimerization constant (KD) of MPcs the absorbance
spectra of the MPc solutions were monitored with increasing QD
concentrations in methanol:water medium.
The equilibrium constant for 2M ↔ D is defined by Eq. (4):
[
D]
KD =
(4)
1–3 with QDs via the formation of a sulfide bond.
M]2
[
Association constants of the MPc molecules, due to aggregation
in the presence of QDs were determined from the spectrophoto-
metric titrations using Eq. (3) [34].
where [M] and [D] are the concentrations of the monomer and the
dimer, respectively, the total concentration of the MPcs being given
by Eq. (5):
A0
A0
A0
1 1
=
+
(3)
ꢂA
ꢂAmax
ꢂAmax Ka Ct
C = [M] + 2[D]
(5)
where ꢂA = A − A, A and A are the absorbances of the respective
0
0
The absorbance of a solution containing both the monomer and the
dimer is given by Eq. (6) [36,37].
MPc in the absence and presence of total CdTe QD concentra-
tion (Ct), respectively, and ꢂAmax is the maximum change in
absorbance. The association constant Ka is obtained from the ratio
of A /ꢂAmax and the slope of the plot of A /ꢂA versus 1/Ct as shown
ꢀ
[1 − (1 + 8K C)[ε /2 − ε ] + 2ε K C]
D
D
M
D D
Abs =
(6)
0
0
4lK
D
in Fig. 7a and b. The association constant Ka obtained for MPc com-
plexes in the presence of CdTe core QDs was high and within the
range expected for MPc complexes [35] as shown in Table 2. We
observed that Ka increased by one order of magnitude when the
MPcs are in the presence of CdTe@TGA compared to CdTe@MPA.
where C is the total concentration of the MPcs (monomer and
dimer), εM and εD are the extinction coefficients of the monomer
and dimer, respectively, and l the path length.
An estimated value of εM obtained from the measurements in
methanol:water solution within the concentration range at which
the Beer–Lambert’s law holds for each MPc were substituted in Eq.
(6) and the absorbance data were fitted to obtain εD and KD by a
non-linear least square procedure using MATLAB 6.1 software.
The values of KD are of the same order of magnitude for the com-
plexes in Table 2 and are within the range that have been observed
for some MPc complexes [37] indicating a strong association
between the aggregates. KD for photosensitizers such as porphyrins
4
7
−1
and phthalocyanines ranges from 10 to 10 M depending on the
solvent and temperature [37,38]. The values of the dimerization
constants in this work will be affected by the possibility of higher
aggregates. The values of εD (Table 2) are within the range reported
for some MPc complexes [36].
4. Conclusions
This study has shown that mixing of anionic CdTe core QDs and
cationic MPcs results in aggregation of MPc complexes, probably
due to the salting out effect. The aggregation of MPcs occurred with
4
dimerization constants of the order of 10 M. Higher dimerization
constants were observed for TGA capped CdTe core QDs compared
to MPA capped ones due to the differences in electrolytic behavior.
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
Fig. 7. Plots for the determination of association constants in the interaction of MPc
complexes ((i) ␤OZnPc, (ii) ␣TZnPc and (iii) ␤TZnPc) with (a) CdTe@TGA and (b)
CdTe@MPA.

4
16
M. Idowu, T. Nyokong / Spectrochimica Acta Part A 75 (2010) 411–416
as Rhodes University and Medical Research Council of South Africa.
MI thanks the Andrew Mellon Foundation for scholarship.
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