Synthesis and optical properties of two new PPV derivatives embedded on the surface of PbS nanocrystals

Article abstract of DOI:10.1016/j.jphotochem.2010.07.022

In this work we present the optical characteristics of three PPV derivatives, pure and conjugated with PbS nanocrystals (NCs). Our results exhibit strong dependence of the fluorescence lifetime of the polymers on the PbS concentration. It appears that the

Full text of DOI:10.1016/j.jphotochem.2010.07.022

Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 69–75
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and optical properties of two new PPV derivatives embedded on the
surface of PbS nanocrystals
Piotr Piatkowski∗, Wojciech Gadomski, Pawel Przybylski, Boz˙ ena Ratajska-Gadomska
Laboratory of Physicochemistry of Dielectrics and Magnetics, Department of Chemistry, University of Warsaw, ul. Zwirki i Wigury 101, 02-089 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work we present the optical characteristics of three PPV derivatives, pure and conjugated with PbS
nanocrystals (NCs). Our results exhibit strong dependence of the fluorescence lifetime of the polymers on
the PbS concentration. It appears that the fluorescence lifetimes increase with the PbS NCs concentration.
This is a surprising result because the presence of the quantum dots, which create new nonradiative
pathways, should cause a shortening of the exciton lifetime. The observed phenomenon indicates that
the surface of the PbS NCs strongly affects the excited polymer, leading to the stabilization of the emerging
exciton.
Received 15 April 2010
Received in revised form 13 July 2010
Accepted 20 July 2010
Available online 30 July 2010
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
charge separation between the nanocrystal and the polymer
[7].
In this paper we present the results of our study on the
complexes of conductive polymers linked to PbS quantum dots.
Nowadays conductive polymers are the subject of intense research
by many authors [1–10], because these materials couple the elec-
trical and optical properties of traditional semiconductors with
mechanical advantages of polymers [1]. Conjugated polymers are
used as thin films or fibers as well as organic solutions. The produc-
tion of polymer mixtures with various materials is a simple method
for the creation of the composite materials for light processing
[2–5]. These materials are attractive due to their potential appli-
cations in low-cost electronic and optoelectronic devices (LEDs,
photovoltaic cells, sensors) working at lower currents and higher
temperatures [6,7].
A great attention is being paid to the polymer–nanoparticle
hybrid materials, especially to the materials including nanopar-
ticles having specific spectral properties in infrared region [7,8].
Sensitizing conjugated polymers with infrared-active nanocrys-
tal quantum dots provides a spectrally tunable means providing
access to the infrared, while maintaining the advantageous prop-
erties of polymers [7]. In the last few years a lot of research
groups examined photovoltaic properties of hybrid compos-
ites consisting of PbS nanocrystals (PbS NCs) and conductive
polymers [7–9]. Especially poly[2-methoxy-5-(2ꢀ-ethyl-hexyloxy)-
1,4-phenylenevinylene] (MEH-PPV) attracted the attention [7–9]
due to its low ionization potential laying close to the ioniza-
tion potential of PbS QDs. This property is critical for achieving
Composite materials including PbS QDs have better electroopti-
cal properties than organic solar cells working in the visible spectral
region and achieving 6.5% solar conversion efficiency [10]. It is
possible to obtain solar cells achieving 40% conversion efficiency
[4,11] in the infrared region for QD-conductive polymer hybrid
materials. The light conversion efficiency of composite depends on
the interactions between polymer and nanocrystals, which in turn
depend on the distance, type and strength of bonds between the
ingredients. These factors influence also the structure of polymers,
interaction between polymer chains and optical properties of the
composite, so that changes in the optical properties are the measure
of interactions in hybrid material.
Measurements of luminescence in conductive polymers and
conductive polymer–nanocrystal composites are important for
understanding the nature of their excited states, which are respon-
sible for the operation of electroluminescent devices [12,13]. The
photo-excitation leads to generation of exciton, which then decays
by radiative or nonradiative pathways. The rates of radiative and
nonradiative deactivation of the excited state determine the quan-
tum efficiency of fluorescence and the fluorescence lifetime. The
longer is the lifetime of the exciton, the longer is the distance it
can cover before relaxation and, thus, the possibility of polymer-
quantum dot charge transfer is higher.
The modification of conjugated polymers through the side
chains gives a chance to obtain more complex composite materi-
als with, for instance, semiconductor nanocrystals. The functional
groups of the lateral chains ensure better and stronger contact
with the surface of nanocrystals. Polar functional groups cause the
conjugated polymer to be more compatible with quantum dots.
The length of the side chain, which separates the polymer from
∗
Corresponding author. Tel.: +48 22 8223032; fax: +48 22 8225996.
E-mail addresses: piatekpiatek@wp.pl, piatek@chem.uw.edu.pl (P. Piatkowski).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.07.022

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P. Piatkowski et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 69–75
the nanocrystal, can modify the strengths of interactions between
components of the composite material. We can expect that optical
properties of the polymers depend on the lengths of the side chains
separating the main polymer chain from quantum dots.
have been carried out in the presence of potassium carbonate and
potassium iodide. The mixture has been heated under reflux for
6 h. Next, the resulted 3-(4-methoxyphenyl)propanoic acid (2) or
5-(4-methoxyphenoxy)pentanoic acid (3) has been extracted with
water and chloroform.
The reactants (2) or (3) received in the first step, paraformalde-
hyde, acetic acid and 33% HBr in acetic acid (100 ml) have
been mixed and heated to 70 ◦C for 4 h at established N2
atmosphere. The reaction product, 3-[2,5-bis(bromomethyl)-
The aim of our study on optical properties of three poly(p-
phenylenevinylene) (PPV) derivatives covering PbS QDs is to
establish the relation between the polymer structure and the range
of the mutual polymer–nanocrystal coupling. The promising results
obtained for the MEH-PPV has stimulated us to look for modi-
fied PPV with shorter lateral chains, which are responsible for the
reduction of the distance between the main polymer chain and
the quantum dot. Thus, the ends of the chains have been modi-
fied with carboxylic group to obtain stronger interaction between
PbS QDs and polymer. It is shown below that this interaction sig-
nificantly influences optical properties of the polymer, especially
the fluorescence lifetime.
4-methoxyphenoxy]
propanoic
acid
(4)
and
5-[2,5-
bis(bromomethyl)-4-methoxyphenoxy]pentanoic acid (5), has
been diluted with chloroform followed by extraction with water
and NaHCO3. After that the chloroform has been removed under
the reduced pressure.
The last step depends on the polymerization of (4) or (5) with
potassium tert-butoxide in tetrahydrofuran. The reaction mixture
has been stirred for 16 h. The obtained polymers have been purified
by washing with ethyl acetate and dried under reduced pressure.
The final polymers are presented in Fig. 2.
2. Experimental
In order to verify the structure and purity of the synthesized
materials we have performed the NMR spectra (Varian Unity Plus
spectrometer operating at 700 MHz for 1H NMR), using tetram-
ethylsilane as an internal standard. The results confirm that the
synthesized materials are consistent with the assumed structures
without any sign of additives or impurities. Moreover, we have
applied the TLC analyses performed on Merck 60 silica gel glass
plate and visualized using iodine vapor, and the column chromatog-
raphy carried out at atmospheric pressure using silica gel (100–200
mesh, Merck).
2.1. The polymer synthesis
The first polymer MEH-PPV has been obtained using
method described in [14]. Next two polymers poly[2-methoxy-
5-(2ꢀ-carboxyethoxy)-1,4-phenylenevinylene]
(MCE-PPV),
a
poly[2-methoxy-5-(4ꢀ-carboxybuthoxy)-1,4-phenylenevinylene]
(MCB-PPV) have been synthesized for the first time according
to the modified method [14]. The three-step synthesis of the
MCB-PPV and MCE-PPV is shortly outlined in Fig. 1. The com-
pounds (2,3) containing alkyl chains of different lengths were
obtained by the Williamson reaction between commercially avail-
able 4-methoxyphenol and appropriate bromo carboxylic acid
derivatives. The key-step of the synthesis is the polymerization, at
room temperature, catalyzed by potassium tert-butoxide to build
up final compounds (6,7).
2.2. PbS NCs synthesis
We have synthesized PbS nanocrystals using the method
described in [15]. PbO in OA (oleic acid) has been heated to obtain
lead oleate precursor under nitrogen at 150 ◦C. After 1 h the solu-
tion was cooled to 100 ◦C and a solution of bis(trimethylsilyl)sulfide
First, the reactions of 4-methoxyphenol (1) with 3-
bromopropionic acid or 5-bromopentanoic acid in acetone
Fig. 1. Scheme of the synthesis of the polymers.

P. Piatkowski et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 69–75
71
Fig. 2. (a) Poly[2-methoxy-5-(2ꢀ-carboxyethoxy)-1,4-phenylenevinylene] (MCE-PPV), (b) poly[2-methoxy-5-(4ꢀ-carboxybuthoxy)-1,4-phenylenevinylene] (MCB-PPV) and
(c) poly[2-methoxy-5-(2ꢀ-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV).
TMS in octadecene was added. The resulted spherical nanocrystals
were isolated from the mixture by precipitation with methanol and
redispersed in chloroform. The size of spherical QDs was measured
by SAXS (Small-angle X-Ray Scattering) technique and was found
to be about 5 nm.
2.3. Preparing of PbS/polymer mixture
The 10−4 g/l solutions of three different polymers in CHCl3 were
prepared, then mixed with PbS NCs at the following mass ratios of
polymer/PbS = 1:0.1; 1:0.2; 1:0.4; 1:0.8; 1:1.6.
2.4. Spectroscopic measurement
In order to characterize the obtained mixtures the following
spectroscopic methods were used: UV–vis absorption spectroscopy
(Shimadzu UV-3101PC), emission spectroscopy (FluoroLog HORIBA
Jobin Yvon), time resolved emission spectroscopy (FluoroLog
HORIBA Jobin Yvon with FluoroHub) and Fourier-transform IR spec-
troscopy (Nicolet 6700).
3. Results and discussion
3.1. IR spectroscopy
We have measured IR spectra for pure polymers, polymers
mixed with PbS nanocrystals, oleic acid and PbS covered by oleic
acid. The samples of polymers mixed with quantum dots were
prepared as follows. First the solutions of MEH-PPV, MCE-PPV
and MCB-PPV in chloroform were prepared and then the PbS NCs
were added. The samples were mixed and heated at 61 ◦C for 5 h
under nitrogen. The prepared nanocrystals were precipitated with
methanol, dried and redispersed in chloroform. Next the solutions
were spotted on a KBr tablets and measured by FTIR spectrometer.
Fig. 3 shows FTIR spectra of MEH-PPV (2a), MCE-PPV (2b) and
MCB-PPV (2c) with and without PbS QDs and the spectra of oleic
acid with embedded PbS nanocrystals.
There is an apparent difference between spectra of a pure oleic
acid and oleic acid capping PbS quantum dots [16]. When oleic acid
covers the surface of the quantum dots, the band of carboxyl at
1710 cm−1 in carboxyl acid and a wide OH band between 3500 and
2500 cm−1 disappear, whereas a band at 1544 cm−1, characteristic
for carboxylic acid salts, appears. This second band is characteristic
Fig. 3. FTIR spectra of: (a) MEH-PPV, (b) MCE-PPV and (c) MCB-PPV with and without
PbS NCs in chloroform. All pictures contain spectra of OA covering PbS quantum dots.

72
P. Piatkowski et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 69–75
for carboxylic acid salts. Oleic acid reacts with the surface of PbS
nanocrystals.
As we can see in Fig. 3a–c the spectra of polymers with QDs
include bands characteristic for both PPV derivatives and OA, i.e.
1710 cm−1 corresponding to carboxylic group of both oleic acid
and polymers, 1544 cm−1 attributed to the COO− group vibrations,
1464 cm−1 characteristic for CH3, CH2 bending vibrations. The band
at 722 cm−1 corresponds to CH2 rocking vibration in oleic acid
[16]. In our case, as Fig. 2b and c shows, two bands at 760 and
800 cm−1 appear. These bands are attributed to the methylene
rocking vibrations in the lateral chains of MCE-PPV and MCB-PPV.
The polymers, which contain lateral chains with carboxylic groups,
interact strongly with each other by hydrogen bonds. This is the rea-
son why the chains of polymers are strongly bound. These mutual
interactions influence the rocking vibrations of methylene groups
[17,18]. In the composite with the PbS the band at 750 cm−1, char-
acteristic for both polymers, disappears, whereas the weak band at
800 cm−1 grows. First band can be assigned to the methylene group
vibration in self-organized structure while the second band is asso-
ciated with amorphous fraction of polymers. It suggests that the
MCE-PPV and MCB-PPV interact with PbS, which results in destruc-
tion of their self-organized structure. The sharp bands between
2700 and 3100 cm−1 is characteristic for stretching vibrations of
CH groups, whereas the OH groups vibrations are seen in the area
Fig. 4. Absorption and emission spectra (solid line) and Gaussians fits (dashed lines),
of: (a) MEH-PPV, (b) MCE-PPV and (c) MCB-PPV in chloroform.
2500–3600 cm−1. The wide band between 3100 and 3600 cm−1
,
seen in Fig. 3b and c for pure polymers, is attributed to the symmet-
ric and asymmetric modes of H-bonds created between polymer
chains [19–21]. The appearance of this band proves the evidence
of interchain interactions and self-organization in polymer solu-
tions, since there are no OH groups in a single chain. In the case of
MEC-PPV and MCB-PPV with embedded PbS nanocrystals the OH
band becomes much weaker. It proves that the strong interactions
between carboxylic groups of polymer molecules are destroyed in
the presence of nanocrystals. It can be seen, in Fig. 3b and c, that at
the same time the band between 2500 and 2700 cm−1 appears. This
second band is attributed to local resonance interactions between
vibrations of C–O–H and C O groups from carboxylic group [19].
This resonance is amplified by the polymer–PbS surface interaction.
The strong band at 1202 cm−1 corresponds to C–O–C– stretch-
ing vibrations in MEH-PPV [22]. As we can see in Fig. 2b and c,
the similar band, located at 1237 cm−1, is present in the FTIR spec-
trum of MCE-PPV and MCB-PPV. This band becomes significantly
weaker in MCE-PPV mixed with PbS but no apparent changes can
be observed for other polymers (Fig. 3a and c). Thus, we can con-
clude that the vibrations of C–O–C– are probably damped by the
interaction of a short lateral chain in MCE-PPV with a surface of the
nanocrystal.
The presence of oleic acid peaks in the spectra of polymers with
embedded nanocrystals (Fig. 3a–c) suggests that there is still OA on
a surface of nanocrystals. It is possible that the band at 1544 cm−1
derives not only from COO− created from OA carboxyl group but
also from carboxyl groups included in the polymer lateral chains.
Fig. 3 shows that MEH-PPV covers PbS more efficiently than the
next two polymers. The concentration of this polymer on the sur-
face of the nanocrystal relative to the OA concentration is high, so
that we cannot see the bands of OA in the spectra of MEH-PPV/PbS
(Fig. 2a). The MCE-PPV:OA and MCB-PPV:OA concentration ratios
on the surface of the PbS QDs are lower than for MEH-PPV, so that
the OA bands in Fig. 3b and c are still present in a spectra of these
polymers with nanocrystals.
actions between polymer chains on the relaxation processes of the
exciton. The absorption and emission spectra of MEH-PPV, MCE-
PPV, and MCB-PPV are shown in Fig. 4. The broad absorption band
resulting from PPV derivatives appears around 320–560 nm, which
is due to ␲–␲* transitions of polyconjugated systems. Photolumi-
nescence (PL) was excited at the wavelength 430 nm. We have fitted
PL spectra with Gaussians (Fig. 3). The emission bands of these
polymers consist of two bands for MCE-PPV and three bands for
MEH-PPV and MCB-PPV. The distances between the components
correspond to frequencies of phonon modes of polymers. We found
the fitted bands to be the zero phonon line 0–0, the first 0–1 and
the second 0–2 vibronic bands [23], respectively.
Optical properties of the polymers depend on the structure of
the polymer chain, which influences the energy transfer between
chromophores. Other authors [24] describe three ways of energy
migration in polymer systems. One way depends on the elec-
tron hoping from one chromophore to another, which is typical
for pendant-type polymers. Next way is the energy transfer
between loops in flexible polymers. The last way is the energy
migration along the polymer chain, which is characteristic for poly-
mers consisting of carbon–carbon conjugated double bonds. We
can neglect the first and the second mechanism of the energy
transfer due to the structures of the polymers used in our experi-
ments.
The absorption of light by conductive polymers causes the for-
mation of excited states (excitons) that undergo rapid relaxation
[25]. A part of excitons diffuse through the polymer, before they
emit photons. Another part of excitons recombines without diffu-
sion. When the polymer is dispersed in the solution new pathways
of relaxation appear. This is due to high probability of interchain
exciton delocalization. Both processes lower the coupling to the
ground state and increase the radiative lifetime [25,26].
3.2. VIS spectroscopy
3.3. The time resolved fluorescence
We have performed the absorption and emission measurements
in pure polymer solutions in chloroform. The applied polymer
concentrations are low in order to reduce the influence of the inter-
The created excitons decay due to the radiative and nonradiative
processes. The lifetime of each excited state ꢀ, measured in the
experiment, is expressed by both the radiative kR and nonradiative

P. Piatkowski et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 69–75
73
fitted by double exponential function. It turns out that in the case of
MEH-PPV the fluorescence lifetimes hardly depend on the concen-
tration of the PbS NCs. The lifetimes shown in table in Fig. 5a have
almost the same values for various concentrations of PbS. The inter-
action between the surface of a nanocrystal and MEH-PPV is weak,
because the polymer is well separated from quantum dots with a
long, unpolar chain of OA and a lateral chain of MEH-PPV. This pre-
vents the influence of PbS on the exciton in a polymer. Fig. 5b and
c shows the time decay curves for MCE-PPV and MCB-PPV and the
values of lifetimes estimated for those curves. As it can be seen, the
lifetimes of both polymers are strongly dependent on the concen-
tration of PbS nanocrystals. It proves a strong interaction between
quantum dots and polymers. Both lifetimes increase with the con-
centration of the QDs (Fig. 5b and c), but they start to decrease when
the concentration of PbS QDs is too large. Probably the nonradiative
pathways of deactivation become more efficient than the stabiliz-
ing effect of nanocrystals. This effect is seen for all polymers used
in our experiment.
Changes of the fluorescence lifetime are much larger for MCE-
PPV than for MCB-PPV. The PbS NCs influence on the fluorescence
lifetime is stronger for the shorter lateral chains of the polymer,
because in this case the polymer chain is closer to the surface
of the PbS QDs. The increase of the time decay is attributed to
more efficient charge separation in the PbS/polymer system. Then
the fraction of excitons, which diffuse through the polymer chain,
migrate over larger region before recombination.
3.4. Fluorescence anisotropy decay
The excitation wavelength as well as the emission wavelengths
established in the fluorescence anisotropy decay experiment, for
each of the polymer samples, have been the same as these used for
the fluorescence decay measurements (Section 3.3). Fig. 6 shows
anisotropy decays measured for all polymers and double exponen-
tial fits to the experimental data. The fits has been made according
to the formula [27]:
−t/ꢀ
+ r02e−t/ꢀ
(2)
01
02
r(t) = r
e
01
where r01, r02 denote the zero time anisotropies and ꢀ01, ꢀ02
denote decay times for the first and the second component of the
anisotropy decay, respectively. The values of r01, r02, ꢀ01, ꢀ02 are
presented in Table 1. As we can see r01 has a value close to 0.4
for all polymers, which means that both the absorption and the
emission transition moments are parallel [28]. This result suggests
that the same excited states take part in both processes, i.e. absorp-
tion and emission of light. The r02 values, which are close to −0.1
(the absorption and emission transition moments are perpendicu-
lar [28]), demonstrate that the emission of light takes place from
another electronic state than the state reached by the exciton due
to the primary excitation. The anisotropy decay times correspond-
ing to ꢀ01 and ꢀ02 can be attributed to longer and shorter lifetimes
of the fluorescence, respectively. The longer fluorescence lifetime
has been assigned to this part of excitons, which migrate along
the chain before recombination. Thus, the emission connected with
these excitons should be depolarized with respect to the absorbed
light. It is in fact observed in our experiment, as can be seen in
Fig. 6. This fraction of excitons is responsible for the fluorescence
Fig. 5. Time decay curves and the measured lifetimes of the polymers excitons for
different PbS NCs concentrations. The spectra contain calculated values of the life-
times for various molar ratios of polymer to PbS. Colors of the plots correspond to
the colors of the calculated values of lifetimes.
kNR decay constants:
1
ꢀ
= kR + kNR
(1)
In order to find the lifetimes of excitons, we performed the time-
domain fluorescence measurements. The excitation wavelength
was 440 nm, whereas the data were collected at the wavelength
500 nm for MCE-PPV and 570 nm for MEH-PPV and MCB-PPV. The
results of measurements for the polymers are presented in Fig. 5.
Two different time decays were observed for polymers used in the
experiment. The longer lifetime of an excited state is attributed to
the part of excitons, which migrate along a polymer chain before
recombination [23–26]. The time decay curves in Fig. 5 have been
Table 1
Emission anisotropy decay parameters of the PPV derivatives obtained from a double
exponential fits to the experimental data.
ꢀ01 (ns)
r01
ꢀ02 (ns)
r01
MEH-PPV
MCB-PPV
MCE-PPV
0.34
0.48
0.48
0.36
0.39
0.40
1.11
1.21
1.32
−0.11
−0.10
−0.11

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P. Piatkowski et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 69–75
Fig. 6. Emission anisotropy decay (dots) and fitted curves (red line) of three PPV derivatives. (For interpretation of the references to colour in this figure legend, the reader
is referred to the web version of the article.)
anisotropy decay described by r02 and ꢀ02 in Eq. (2). For all poly-
mers considered in this paper the longer anisotropy lifetimes are
almost the same as the fluorescence lifetimes, whereas the shorter
anisotropy decay times are significantly smaller than the corre-
sponding fluorescence lifetimes. The existence of two components
of anisotropy decay connected with two completely different val-
ues of zero time anisotropy supports our previous conclusion that
two observed fluorescence lifetimes attribute to different pathways
of relaxation.
of polymers, thereby it can destroy the nonradiative ways of exciton
deactivation, for instance interchain energy migration [25,26]. The
fluorescence lifetimes strongly depend on the distance between the
nanocrystal and the polymer. The shorter is the side chain of poly-
mer the stronger is the nanocrystal influence on the exciton and
the longer is fluorescence decay time. Thus, the final conclusion is
that embedding quantum dots into the matrix of conducting poly-
mers of the appropriate structure may significantly improve the
efficiency of the charge transport along the polymer chain.
4. Conclusions
Acknowledgment
For the first time we have synthesized two new derivatives of
PPV, MCE-PPV and MCB-PPV and we have used it to the PbS sur-
face passivation. The shortening of side chains reduces the distance
between the main chain of the polymer and the quantum dot, while
the carboxylic group of lateral chains enables the MCB-PPV and
MCE-PPV to bind stronger to the surface of nanocrystal. The FTIR
spectra show that the presence of quantum dots strongly affects
the structure of both polymers and the interactions between poly-
mer chains. The presence of PbS QDs destroys the hydrogen bonds
between carboxylic groups of polymers, in expense of the bonds
created between the polymers and the quantum dots.
This work was supported by the Ministry of Scientific
Research and Information Technology in 2008–2010, Project No.
N N20423984.
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