Side chain mediated electronic contact between a tetrahydro-4H-thiopyran-4- ylidene-appended polythiophene and CdTe quantum dots

Article abstract of DOI:10.1002/chem.200501480

The properties of a mixed CdTe quantum dot/tetrahydro-4H-thiopyran-4- ylidene-functionalized polythiophene system are reported. This system was prepared by exposing trioctylphosphine (TOP)-capped CdTe quantum dots to the polythiophene in solution. Strong fluorescence emission quenching and shortening of the fluorescence emission lifetimes of both the polythiophene and the quantum dots occur when they are mixed, indicating the occurrence of photoinduced charge separation. Photoinduced absorption spectroscopy reveals a considerable decrease in the population of the polythiophene triplet excited state in the mixed system, These results demonstrate that between the quantum dots and the polythiophene there is both physical and electronic contact, which is mediated by the tetrahydro-4H-thiopyran-4-ylidene side chains.

Full text of DOI:10.1002/chem.200501480

FULL PAPER
DOI: 10.1002/chem.200501480
Side Chain Mediated Electronic Contact between a Tetrahydro-4H-
thiopyran-4-ylidene-Appended Polythiophene and CdTe Quantum Dots
[
a]
[a]
[a]
Rick van Beek, Arjan P. Zoombelt, Leonardus W. Jenneskens,
[
a]
[b]
[c]
Cornelis A. van Walree,* Celso de Mello Donegµ, Dirk Veldman, and
RenØ A. J. Janssen
[
c]
Abstract: The properties of a mixed
CdTe quantum dot/tetrahydro-4H-thio-
polythiophene and the quantum dots
occur when they are mixed, indicating
the occurrence of photoinduced charge
separation. Photoinduced absorption
spectroscopy reveals a considerable de-
crease in the population of the poly-
thiophene triplet excited state in the
mixed system. These results demon-
strate that between the quantum dots
and the polythiophene there is both
physical and electronic contact, which
is mediated by the tetrahydro-4H-thio-
pyran-4-ylidene side chains.
pyran-4-ylidene-functionalized
poly-
thiophene system are reported. This
system was prepared by exposing trioc-
tylphosphine
(TOP)-capped
CdTe
Keywords: electron
photophysics polythiophenes
quantum dots · semiconductors
transfer
·
·
quantum dots to the polythiophene in
solution. Strong fluorescence emission
quenching and shortening of the fluo-
rescence emission lifetimes of both the
·
Introduction
Quantum dots are usually capped with a layer of organic
molecules to prevent clustering of particles and to suppress
nonradiative decay of excited states at the quantum dot sur-
Hybrid inorganic/organic semiconducting systems are prom-
ising materials for application in optoelectronic devices,
[12,19,20,24,27–29]
face.
Unfortunately, this capping layer forms an
[1–5]
such as photovoltaic systems
and light-emitting diodes
obstacle to the application of quantum dots in hybrid inor-
ganic/organic semiconducting systems. Owing to its electron-
ically insulating character, it prevents electronic contact be-
tween quantum dots and other electroactive materials. Nev-
ertheless, hybrid quantum dot/organic systems for optoelec-
[
5–10]
(
LEDs).
As inorganic constituent, nanocrystalline inor-
ganic semiconductors (quantum dots) form an interesting
class of materials because of their size-tunable photophysical
[11–17]
properties
and
high
luminescence
quantum
[
13,14,18–22]
yields.
easily prepared by wet-chemical methods,
the synthesis of bulk inorganic semiconductors often re-
In addition, quantum dots can be relatively
tronic applications have been obtained by casting from solu-
[
12,19,23,24]
[1–3,30]
whereas
tions of mixtures of both constituents
techniques.
or by deposition
[7–10]
Although encouraging results, such as im-
[25,26]
quires high temperatures or very low pressures.
proved electroluminescence efficiencies compared with one-
component systems and the occurrence of photoinduced
charge separation, have been reported, the development of
hybrid quantum dot/organic systems with a substantial elec-
tronic coupling between the constituents is desired. This re-
quires optimized contact, both physical and electronic, be-
tween the components. One way to achieve this is to func-
tionalize a polymer with side chains possessing a high affini-
[
a] Dr. R. van Beek, A. P. Zoombelt, Prof. Dr. L. W. Jenneskens,
Dr. C. A. van Walree
Debye Institute, Organic Chemistry and Catalysis, Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
Fax : (+31)30-253-4533
E-mail: c.a.vanwalree@chem.uu.nl
[31,32]
[
b] Dr. C. de Mello Donegµ
ty for inorganic materials.
Thus the polymer can substi-
Debye Institute, Condensed Matter and Interfaces, Utrecht Universi-
ty
Princetonplein 1, 3584 CC Utrecht (The Netherlands)
tute for the original capping layer on quantum dots and
passivate the surface of quantum dots. More importantly,
the side chain must also be able to mediate charge transport
between the quantum dots and the polymer backbone.
It has been shown that tetrahydro-4H-thiopyran-4-yli-
dene-functionalized compounds make electronic contact
[
c] D. Veldman, Prof. Dr. R. A. J. Janssen
Laboratory for Macromolecular and Organic Chemistry, Eindhoven
University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
Chem. Eur. J. 2006, 12, 8075 – 8083
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8075

3
with CdSe quantum dots and mediate charge transport be-
weight-average molecular weight M of poly(4) was 2.7”10
w
[33]
tween the quantum dots and a gold electrode or between
with a polydispersity D of 1.4. The presence of two olefinic
H NMR signals of similar intensity indicates that poly(4)
[
34,35]
1
two gold electrodes.
This charge transport is relayed by
[36,37]
s–p through-bond interactions.
Consequently, side
has a highly random regioregularity.
chains based on this moiety are promising candidates to ach-
ieve both physical and electronic contact between a semi-
conducting polymer and inorganic semiconductor quantum
dots. Therefore, we have investigated wheth-
er it is possible to substitute (part of) the cap-
ping layer of CdTe quantum dots with a poly-
thiophene (poly(4)) bearing a tetrahydro-4H-
thiopyran-4-ylidene group in the side chain
and whether this side chain induces substan-
tial electronic contact between the polymer
and the quantum dot. CdTe quantum dots
were used particularly because of their rela-
tively low bandgap (1.5 eV for bulk materi-
CdTe quantum dots were obtained by a procedure based
[12,20]
on that developed by Wuister et al.
To facilitate com-
plexation with poly(4), the excess trioctylphosphine/dodecyl-
amine (TOP/DDA) capping agent was removed by precipi-
1
tation into methanol. H NMR spectroscopy revealed almost
complete removal of DDA, implying that the precipitated
quantum dots were covered predominantly with TOP.
Properties of poly(4): The UV/Vis absorption maximum of
poly(4) in chloroform solution is situated at 477 nm (Fig-
ure 1A). It is shifted bathochromically with respect to the
solution absorption maxima generally observed for regioran-
[41,42]
dom poly(3-alkylthiophene)s, which lie near 430 nm.
[
38]
al ), which enables the separation of the fluorescence emis-
sion bands of the quantum dots and the polythiophene. The
complexation between the quantum dots and the polymer,
and the optical properties of the hybrid systems, were inves-
tigated with UV/Vis absorption, fluorescence, and photoin-
duced absorption (PIA) spectroscopy.
The relatively low energy of the absorption maximum of
poly(4) is explained by the extension of the conjugated
system of the repeating unit with the olefinic bond of the
tetrahydro-4H-thiopyran-4-ylidene substituent. The absorp-
tion maximum of poly(4) as a film on quartz (Figure 1A) is
at a somewhat longer wavelength (500 nm) than in solution.
Results and Discussion
Synthesis: Monomer 4 was synthesized in a two-step proce-
dure (Scheme 1). Tetrahydro-4H-thiopyran-4-carboxylic acid
1
was doubly deprotonated and subsequently coupled to 2,5-
dibromothiophene carboxaldehyde 2 to yield b-hydroxy acid
Scheme 1. Synthetic route to poly(4).
[
39]
3
.
Decarboxylative dehydration of 3 with N,N-dimethyl-
[39]
formamide dineopentyl acetal furnished alkene 4.
Al-
though 3 was obtained with a purity of only 35%, this did
not prevent the formation and isolation of pure 4 in the
next step; it was obtained in 19% yield relative to the start-
ing aldehyde 2. Monomer 4 was polymerized using [Ni-
Figure 1. A) UV/Vis absorption spectra and B) fluorescence emission
spectra of poly(4) in chloroform solution (solid line) and as a film on
quartz, cast from chloroform solution (broken line). In B, lexc = 460 nm
for the spectrum in solution and lexc = 500 nm for the spectrum of the
film.
[40]
A
C
H
T
R
E
U
N
G
(cod) ], furnishing poly(4), which was soluble in chloro-
2
form and partially soluble in dichloromethane and THF. As
determined by size-exclusion chromatography (SEC) the
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Electronic Contact between Polythiophene and Quantum Dots
FULL PAPER
This is the consequence of a better ordering of the polymer
chains in the solid state, which leads to a longer conjugation
[41,43]
length.
The fluorescence emission spectrum of poly(4) in chloro-
form solution (Figure 1B) has a maximum at 582 nm and a
shoulder near 630 nm, representing a vibronic transition.
The emission spectrum of the film on quartz is red-shifted,
with the maximum now at 665 nm (Figure 1B). The red shift
ꢀ
1
upon going from solution to the solid state (2200 cm ) is
considerably stronger than that found with absorption spec-
ꢀ
1
troscopy (800 cm ). This is explained by the occurrence of
interchain energy transfer in the solid state; in the solid
state longer chain segments are involved in the fluorescence
process than in the absorption process.
The properties of poly(4) were investigated further with
cyclic voltammetry (CV) in THF solution. Anodic scans re-
vealed a single irreversible oxidation wave centered at 1.0 V
versus SCE, whereas cathodic scans exhibited a single irre-
versible reduction wave centered at ꢀ1.7 V versus SCE. The
oxidation potential is more positive than that found for
poly[3-(2,5,8-trioxanonyl)thiophene] (PTT) in THF (0.8 V
[43,44]
versus SCE
). The relatively high oxidation potential of
poly(4), which may be the consequence of a higher degree
of distortion of the polythiophene backbone by the bulky
ring-containing substituents, seems to be in disagreement
with the observation that the absorption maximum of
poly(4) (465 nm for a saturated solution in THF) is situated
at a longer wavelength than that of PTT (440 nm in
[43]
THF ). This apparent contradiction can be explained only
by assuming that the effect of the tetrahydro-4H-thiopyran-
4
-ylidene substituent on the electronic spectrum of poly(4)
Figure 2. A) UV/Vis absorption spectra and B) emission spectra (lexc
=
is based predominantly on a stabilization of the conduction
band.
4
06 nm) of pristine poly(4) (solid line), CdTe quantum dots with TOP
capping (broken line) and in the presence of 40% poly(4) (dotted line),
recorded in chloroform solution. A also shows a difference spectrum
(dash-dotted line) obtained by subtraction of the spectrum of the TOP-
capped CdTe quantum dots from that of the mixed system.
Mixed CdTe quantum dot/poly(4) systems: Hybrid CdTe
quantum dot/poly(4) systems were prepared in such a way
that at most about 40% of the quantum dot surface could
be occupied by poly(4). This corresponds with a CdTe unit/
poly(4) repeating unit ratio of approximately 9:1 (see Exper-
imental Section). The CdTe quantum dots used in these
studies had an average particle diameter of 4.0 nm, as de-
occurred upon mixing with the quantum dots. It is very pos-
sible that the change in backbone conformation is induced
by adhesion of poly(4) to the surface of the quantum dots.
A similar phenomenon has been found for PTT deposited
[11]
[43]
duced from the first absorption maximum near 620 nm.
on a ZnSe single-crystal surface.
Figure 2A shows the UV/Vis absorption spectrum of the
mixed system in chloroform, together with the spectra of
pristine poly(4) and bare TOP-capped CdTe quantum dots.
The emission spectrum of the mixture, along with the
spectra of the individual components, is shown in Figure 2B.
The spectra were recorded upon excitation at 406 nm, which
implies that in the mixed sample both components were ex-
cited. As with absorption measurements, the concentrations
of both components in the mixed sample equalled those in
the samples containing the separate components. They were
ꢀ
5
The concentrations of CdTe quantum dots (9.8”10 mol
ꢀ
1
ꢀ5
CdTe unitsL ) and poly(4) (1.1”10 mol repeating
ꢀ
1
unitsL ) in the mixed sample were equal to those in the
solutions of the separate components. It is readily seen from
the region above 600 nm that the CdTe quantum dot absorp-
tion remains unaffected by exposure to poly(4). However, a
difference spectrum, resulting from subtraction of the spec-
trum of the bare CdTe quantum dots from that of the mixed
system, shows a poly(4) absorption that is slightly red-shift-
ed (with the maximum now at 490 nm) and broadened com-
pared with that of pristine poly(4). This could be an indica-
tion that a change in backbone conformation of poly(4) has
ꢀ
5
ꢀ1
ꢀ6
1.6”10 mol CdTe unitsL and 1.9”10 mol poly(4) re-
ꢀ
1
peating unitsL . The emission maximum of pristine poly(4)
is at 582 nm while the bare CdTe quantum dots give an exci-
ton emission maximum at 645 nm and a trap emission cen-
tered at 720 nm. The latter originates from defect states that
are populated by nonradiative decay from the exciton state
(that is, exciton trapping). It is conspicuous that in the
mixed system the poly(4) emission has almost completely
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8077

C. A. van Walree et al.
disappeared (Figure 2B). Furthermore, the intensity at the
maximum of the CdTe quantum dot exciton emission has
decreased by a factor of approximately 4.5, while that at the
maximum of the defect emission has decreased by a factor
of about 2. This strong fluorescence quenching shows that a
new, efficient decay channel has been created upon mixing
poly(4) with CdTe quantum dots. It is likely that this new
decay channel involves formation of a charge-separated
(
CS) state. Thus, after population of an excited state, which
can be localized either on the quantum dots or on poly(4),
electron transfer from poly(4) to the CdTe quantum dots, or
vice versa, may occur, causing quenching of the local
poly(4) and CdTe quantum dot fluorescence. The observa-
tion that quenching of the defect emission of the quantum
dots is less pronounced than that of the exciton emission
shows that the radiationless transition to the defect state suf-
fers less from introduction of the charge separation pathway
than radiative decay from the exciton state. This is consis-
tent with the trapping process being faster than the radiative
decay (vide infra). Most importantly, the strong fluorescence
quenching implies that attachment of poly(4) at the quan-
tum dot surface has taken place.
To substantiate the findings from the steady-state fluores-
cence spectra, fluorescence decay curves were measured for
the mixed system and the individual components. The fluo-
rescence decay was monitored at lem = 582 and 645 nm, the
respective maxima of the poly(4) and CdTe quantum dot ex-
citon emission. The fluorescence decay of pristine poly(4) is
mono-exponential (Figure 3A) and has a lifetime of 0.48 ns,
Figure 3. A) Fluorescence decay curves (chloroform solution) of pristine
poly(4) (solid line) and a poly(4)/CdTe quantum dot mixture (broken
line), monitored at lexc = 406 nm and lem = 582 nm. B) Fluorescence
decay curves (chloroform solution) of pristine CdTe quantum dots (solid
line) and CdTe quantum dots capped with 40% poly(4) (broken line),
monitored at lexc = 406 nm and lem = 645 nm. The traces are not decon-
voluted from the instrumental response function.
[45,46]
a common value for a polythiophene in solution.
In the
mixed sample the decay is multi-exponential and much
faster than for pristine poly(4). The decay time of the fastest
component is estimated to be 70 ps or faster (limited by the
temporal resolution of the experimental setup at 10 MHz
repetition rate; see Experimental Section). The considerably
faster fluorescence decay in the mixed sample confirms the
findings from the fluorescence emission spectra: in presence
of CdTe quantum dots a new route for depopulation of the
singlet excited state of poly(4) is created, causing quenching
of the local fluorescence. The decay curve of the mixed
system also reveals a slower component with a decay time
of 5.0 ns and an amplitude that is approximately two orders
of magnitude lower than that of the fast component (vide
infra).
radiative decay prevails. Bi-exponential fitting of the decay
curve of the mixed system yields a main component with a
very fast decay time (ꢁ120 ps, limited by the temporal re-
sponse of the instrumental setup at 2.5 MHz repetition rate;
see Experimental Section) and a component with a longer
average decay time of 7.1 ns. The short component again
shows that the exciton decay in the mixed system is much
faster than in the pristine quantum dots. This implies that
9
ꢀ1
the charge transfer process (rateꢂ7”10 s as derived from
the fast decay component in the mixed system) competes
with the trapping process in most of the quantum dots (rate
9
ꢀ1
In Figure 3B fluorescence decay curves of the mixed
sample and of the pristine CdTe quantum dots, monitored at
the maximum exciton emission (645 nm), are depicted. The
decay curves are multi-exponential, reflecting a distribution
of fluorescence lifetimes as a result of sample inhomogenei-
ties (both in quantum dot size and in surface defect struc-
ꢃ1”10 s , derived from the fast decay component of the
pristine quantum dots) and is much faster than the exciton
radiative decay. Similar fast charge transfer rates can be de-
rived from the fluorescence decay curves of poly(4) in the
1
0
ꢀ1
mixed system (ꢂ1”10 s ). The 7.1 ns lifetime is compara-
ble to the long component of pristine quantum dots and
probably originates from a small fraction of free quantum
dots. The 5.0 ns component of the mixed system measured
at lem = 582 nm (Figure 3A) is also thought to originate
from a small fraction of free quantum dots which were excit-
ed at 406 nm and emit at 582 nm.
[12]
ture). Despite the multi-exponential nature of the decay
curve, for the pristine CdTe quantum dots it could be fitted
with a bi-exponential function, giving an average fast decay
component of 0.9 ns and an average slower decay compo-
nent of 11 ns. The short exciton lifetimes reflect the decay in
[12]
nanocrystals where exciton trapping predominates, while
long exciton lifetimes originate from quantum dots in which
Photoinduced absorption spectroscopy was performed to
gain further insight into the photophysical behavior of the
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Electronic Contact between Polythiophene and Quantum Dots
FULL PAPER
[
1,47,48,50–53]
mixed CdTe quantum dot/poly(4) system. The PIA spectrum
of pristine poly(4) (Figure 4, solid line) shows one positive
band centered at 1.42 eV (l = 875 nm), which corresponds
1.5 eV and another near 0.5 eV.
It is likely that po-
laron states decay to the ground state on a shorter timescale
than monitored with the PIA setup.
to a transition from a triplet exciton (T ) to a higher triplet
Whether photoinduced charge separation occurs can be
assessed by considering the thermodynamics of the process
1
[54]
using Equation (1).
Moreover, this provides insight into
which components of the hybrid CdTe quantum dot/poly(4)
system function as the electron donor and as the electron
acceptor.
0
DG ¼ E ðDÞꢀEredðAÞꢀE00 þ C
ð1Þ
ox
0
In Equation (1) DG is the driving force for photoinduced
charge separation. E (D) represents the oxidation potential
ox
of the donor, whereas Ered(A) is the reduction potential of
the acceptor. E00 stands for the zero–zero excitation energy,
that is, the smallest bandgap in the hybrid system, and C is a
[54]
solvent-dependent term.
CdTe quantum dots (Figure 2A) E00 was determined to be
From the absorption onset of
1
.9 eV. For poly(4) Eox = +1.0 V and Ered = ꢀ1.7 V versus
Figure 4. Photoinduced absorption spectra of pristine poly(4) (solid line)
and poly(4) deposited onto CdTe quantum dots (broken line), measured
in chloroform solution. The poly(4) concentration is equal in both the
pristine poly(4) sample and in the mixed system. Inset: normalized spec-
tra.
SCE, as found with CV in THF (vide supra). For CdTe
quantum dots various values of Eox and Ered have been re-
ported in the literature. In Table 1, literature redox poten-
0
Table 1. DG of charge separation calculated using different sets of CdTe
[
a]
quantum dot redox potentials.
state (T ). Similar triplet-associated PIA signals have been
0
0
n
Ref.
E
ox, CdTe quan-
E
red, CdTe quan- DG
DG (poly(4)
[42,47–50]
observed for poly(3-alkylthiophene)s.
At energies
tum dots [V vs.
SCE]
tum dots [V vs.
SCE]
(poly(4) =
donor) [eV] [eV]
= acceptor)
above 2.3 eV the spectra show photoinduced ground-state
[42,50]
bleaching.
The PIA spectrum of the mixed system
[38] ꢀ0.2
ꢀ2.1
ꢀ1.7
ꢀ1.1
+1.2 + C
+0.8 + C
+0.2 + C
ꢀ0.4 + C
shows a T –T signal that is slightly broadened compared
[55] +0.45
+0.25 + C
+0.1 + C
1
n
[
56] +0.3
with the signal of pristine poly(4), but is situated at similar
energies (see insert in Figure 4, which shows the PIA spectra
normalized at 1.4 eV). More importantly, the signal of the
mixed system has a considerably (approximately 30 times)
[a] The redox potentials listed refer to quantum dots with diameters of
.6–4.0 nm.
3
lower intensity. This implies that population of the T state
tials of CdTe particles with diameters similar to that of the
1
[38,55,56]
of poly(4) is largely prevented, which is in line with the exis-
tence of an extra decay channel in the mixed system. The
marked decrease in poly(4) ground-state bleaching in the
mixed system implies that almost no excited state species
with a lifetime of 10 ms–10 ms (which are typically moni-
tored with the PIA setup) are present and that the system
must have decayed back to the ground state in a shorter
period. It is not very probable that the disappearance of the
poly(4) T –T absorption is caused by increased spin–orbit
particles used in this study are given.
CdTe quantum dot redox potentials is due to differences in
The spread in
[38]
measurement method (pulse radiolysis and cyclic voltam-
[55]
metry in either a 5:1 (v/v) benzene/acetonitrile mixture or
[56]
aqueous solution ). Furthermore, different capping agents
[
38]
[55]
(thioglycerol,
trioctylphosphine oxide,
or thioglycolic
[56]
0
acid ) were used. The values of DG with the different sets
of CdTe quantum dot redox potentials were calculated both
when poly(4) was assumed to be the electron donor and
1
n
coupling due to the presence of heavy elements in the CdTe
quantum dots. In that case quenching of the CdTe quantum
dot emission upon mixing with poly(4) should not occur.
Note that if the quenching of the T –T absorption were the
when it was taken to be the electron acceptor (Table 1).
0
It can be seen that DG is negative when using E for
ox
CdTe quantum dots with a thioglycerol capping from pulse
[38]
radiolysis and when taking poly(4) as acceptor. However,
use of this Eox value is not entirely realistic since the nega-
tively charged thiolate capping will shift the potential
toward a value which is too negative for a (neutral) dialkyl
sulfide capping. With the cyclic voltammetry data a slightly
1
n
consequence of a CdTe-induced heavy atom effect, this
would also present compelling evidence for the deposition
of poly(4) at the quantum dot surface. In the PIA spectrum
of the mixed system no signals belonging to polaron states
of poly(4), which might be expected in the case of photoin-
duced charge separation in a mixed quantum dot/polymer
0
positive DG value has been calculated, again under the as-
sumption that CdTe is the electron donor. For the quantum
dots with trioctylphosphine oxide ligands the Eox value may
however be somewhat too positive, since the organic shells
[1,50]
system,
can be discerned. Such polaron-associated PIA
signals usually occur as two broad bands: one centered near
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8079

C. A. van Walree et al.
form a kinetic barrier to electron removal from the quantum
[
56]
0
dot. We therefore expect that the actual DG will be close
to zero (the constant C is expected not to exceed a few
tenths of an electronvolt in the present systems). This does
not confirm unambiguously that photoinduced charge sepa-
ration is allowed thermodynamically, but it also does not
0
render it unlikely. It is conspicuous that DG is in general
more negative when poly(4) is assumed to be the electron
acceptor than when it is taken to be the donor. This result is
surprising since in inorganic semiconductor/semiconducting
polymer mixtures the polymer typically functions as an elec-
[1,50]
tron donor.
The possibly reversed donor–acceptor be-
havior of the CdTe quantum dot/poly(4) system is caused by
the very high energies of the band edges of the CdTe quan-
[
16,57]
tum dots compared with other semiconductors.
Figure 5. Schematic of photophysical transitions in the mixed CdTe quan-
tum dot/poly(4) system. CS = charge separation; CR = charge recombi-
nation; ISC = intersystem crossing. Arrows for nonradiative transitions
to the ground state from the quantum dot exciton and defect states and
General Discussion and Conclusions
1 1
the polymer S and T states have been omitted.
From the fluorescence spectra and decay times of the hybrid
CdTe quantum dot/poly(4) systems it is evident that poly(4)
has affinity for the surface of CdTe quantum dots. This
shows that it is possible to deposit a semiconducting poly-
mer on a quantum dot through interaction with the side
chain. The observed fluorescence quenching and fluores-
cence lifetime shortening indicate that photoinduced charge
transfer takes place. Thermodynamic considerations do not
fully confirm the occurrence of photoinduced charge separa-
tion in mixed CdTe quantum dot/poly(4) systems, but they
also do not contradict it. Surprisingly, the data tend to sug-
gest that, if charge transfer takes place in the mixed system,
CdTe quantum dots act as the electron donor and poly(4)
acts as the electron acceptor.
PIA spectroscopy did not reveal polaron states in the
mixed CdTe quantum dot/poly(4) system. This implies that,
if polarons are present, their lifetime is short. In solution at
room temperature, the lifetime of positively charged polar-
ons may indeed be shorter than the microsecond–millisec-
ond range, while negatively charged polarons (the formation
of which is suggested by thermodynamic investigations) are
expected to possess even shorter lifetimes, since these are
less stable in a polythiophene. In addition, it cannot be ex-
cluded that trapping of charge carriers at defect sites on the
quantum dots also contributes to their fast decay. This could
be in line with the observation that the intensity of the
CdTe quantum dot defect emission relative to the exciton
emission intensity increases upon mixing with poly(4) (Fig-
ure 2B).
ing pathways are strongly suppressed. This is explained by
rapid decay to a charge-separated state, which presumably is
formed by transfer of an electron from the quantum dot to
[58]
poly(4). In view of the observation that it is faster than
the temporal resolution of the equipment used in the decay
time measurements, the charge separation occurs with a rate
1
0
ꢀ1
of at least 10 s . Rapid photoinduced charge separation in
the mixed system also takes place upon excitation of the
CdTe quantum dots, thereby largely preventing both CdTe
exciton and defect emission. The most likely origin of the
residual exciton emission and the 5–7 ns decay component is
free (that is, TOP-capped) quantum dots. This would not be
expected in view of the maximum coverage of 40%, but can
be explained by attachment of one polymer chain to a quan-
tum dot facilitating attachment of other polymer chains.
Greater insight into the surface coverage and the dynamics
of the aggregation process can be obtained by Stern–
Volmer-like studies.
In conclusion, both physical and electronic contact with
CdTe quantum dots has been achieved by functionalizing a
polythiophene with tetrahydro-4H-thiopyran-4-ylidene side
chains. It is noteworthy that fluorescence quenching was
also found in blends of CdSe quantum dots and some
[1–3]
poly(p-phenylene vinylene) (PPV) derivatives.
However,
in these cases the inorganic and organic components were
brought into physical contact by casting from a solution con-
taining the two components. The particular significance of
the results reported here is that electronic contact was creat-
ed in solution, which shows that the polymer and the quan-
tum dots have a real affinity for each other, created by the
presence of the tetrahydro-4H-thiopyran-functionalized side
chains.
A schematic representation of the possible possible pho-
tophysical events in the mixed CdTe quantum dot/poly(4)
system is shown in (Figure 5). Excitation of pristine TOP-
capped quantum dots yields local exciton or defect emission.
When isolated poly(4) is excited, S –S fluorescence is ob-
1
0
served. Intersystem crossing to the first triplet state T1
occurs as well, giving rise to the T –T triplet–triplet absorp-
1
n
tion. When poly(4) is brought into contact with the CdTe
quantum dots, both the fluorescence and intersystem cross-
8080
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 8075 – 8083

Electronic Contact between Polythiophene and Quantum Dots
FULL PAPER
The Ag/AgCl reference electrode was prepared as described in reference
Experimental Section
[
60].
2
,5-Dibromo-3-thiophenecarboxaldehyde (2): This synthesis was carried
General: Unless specified otherwise, all reactions were carried out in a
out as much as possible in the absence of light. To a solution of 3-thio-
phenecarboxaldehyde (4.50 g, 40.1 mmol) in chloroform/acetic acid (1:1
v/v; 140 mL) N-bromosuccinimide (NBS) (15.65 g, 88.0 mmol) was
added, after which the reaction mixture was stirred at room temperature
for four days. To complete bromination at both thiophene a positions, ad-
ditional aliquots of NBS (17.73 g, 99.6 mmol in total) and chloroform/
acetic acid (1:1 v/v; 330 mL in total) were added in five parts. Subse-
quently, water (75 mL) was added. After extraction with chloroform (3”
2
N atmosphere. Before use, THF was distilled from sodium/benzophe-
none ketyl radical, chloroform from calcium chloride and 4 Š molecular
sieves, and acetonitrile from calcium hydride. Methanol and N,N-dime-
thylformamide (DMF) were dried on molecular sieves (3 Š for methanol
and 4 Š for DMF). All solvents used in experiments involving CdTe
quantum dots were degassed in advance by boiling them in vacuo for 3”
5
min.
1
NMR spectra were recorded on a Bruker AC300 spectrometer ( H NMR
7
1
5 mL) the combined organic layers were washed with NaOH (1m; 2”
00 mL) and water (2”100 mL). The organic phase was dried over
1
3
3
3
00.133 MHz; C NMR 75.47 MHz) in CDCl . Chemical shifts (ppm) are
given downfield from TMS. FT-IR spectra (neat samples) were acquired
with a Perkin-Elmer System 2000 spectrometer by the ATR technique.
Analytical size-exclusion chromatography (SEC) was performed with a
Jordi Gel DVB Mixed Bed column equipped with a Thermo Separation
Product Series 200 pump and a UV/Vis detector (l = 460 nm). Molecu-
lar weight distributions are given relative to polystyrene standards. Solu-
tion UV/Vis absorption spectra were taken on a Cary 1 UV/Vis spectro-
photometer in transmission mode. UV/Vis absorption spectra of polymer
films, cast from chloroform solution onto quartz substrates, were meas-
ured on a Cary 5 UV/Vis-NIR spectrophotometer in transmission mode.
MgSO
4
and the remaining solvent was removed under reduced pressure.
The crude product was recrystallized from hexane, yielding brown crys-
tals (3.93 g, 14.6 mmol, 36%). H NMR: d
1
=
9.80 (s, 1H; CHO),
13
7
1
1
6
.34 ppm (s, 1H; aromatic H); C NMR: d = 183.1, 139.2, 128.6, 124.2,
13.3 ppm; IR: n˜ = 3308, 3099, 2871, 2857, 2116, 1821, 1749, 1666, 1644,
520, 1435, 1370, 1353, 1252, 1174, 1139, 1013, 971, 912, 832, 735, 684,
67 cm
ꢀ1
.
4
-[(2,5-Dibromo-3-thienyl)hydroxymethyl]tetrahydro-4H-thiopyran-4-
carboxylic acid (3): A slightly modified literature procedure was em-
ployed. To a solution of diisopropylamine (2.72 g, 26.9 mmol) in THF
[
39]
Fluorescence spectra were taken on
a Spex Fluorolog 2 apparatus.
(
150 mL), cooled to ꢀ408C, n-butyllithium (16.6 mL of a 1.6m solution in
Chloroform solutions were measured in right-angle and films on quartz
cast from chloroform solution were measured in front-face geometry. For
all measurements in chloroform solution the absorption at lexc was below
hexanes, 26.6 mmol) was added during 20 min. After this mixture had
been stirred at ꢀ408C for 30 min, tetrahydro-4H-thiopyran-4-carboxylic
[
61]
acid 1
(1.98 g, 13.5 mmol) was added as a solid. The mixture was
0
.1.
heated at 508C for 2 h and then cooled to ꢀ408C. Subsequently, 2
3.66 g, 13.6 mmol) was added. The reaction mixture was stirred at 508C
for 2 h and at room temperature overnight. After quenching with water
250 mL) the aqueous phase was washed with diethyl ether (3”200 mL),
Fluorescence decay time curves were measured with a Pico Quant
PDL 800-B laser as excitation source (lexc = 406 nm, 55 ps pulse width,
(
2
.5–10 MHz repetition rate). A combination of an iris and neutral density
(
filters was used to attenuate the laser power as needed. The lumines-
cence was collected through a focusing lens, filtered through a crossed
polarizer and a combination of suitable optical cut-off filters, dispersed
then acidified with HCl (2m, 200 mL). The aqueous phase was subse-
quently extracted with dichloromethane (3”150 mL). The combined or-
ganic layers were dried (MgSO ), and filtered, after which the filtrate
4
was concentrated in vacuo. The crude product was subjected to Kugel-
rohr distillation (T = 1208C, p = 1.1 mbar), which furnished a brown,
glasslike solid. The product contained 3 (as identified by H NMR signals
at d = 6.95 (s, 1H; aromatic H), 4.88 ppm (s, 1H; CH a to thiophene
ring) contaminated with remnants of 1 and some unidentified side-prod-
ucts. Yield 3.39 g of 35% purity (2.9 mmol 3, 21%) as estimated from the
ꢀ
1
by a 0.1 m monochromator (1350 linesmm grating, blazed at 500 nm),
and detected by a fast Hamamatsu photomultiplier tube (H5738P-01).
The PMT signal was amplified by an inverting preamplifier (PAM-102-T;
PicoQuant) and used as the start input for a Time Harp 200 multi-chan-
nel computer card, which was synchronized with the laser pulse by means
of the stop input. The decay curves were obtained by time-correlated
single-photon counting through time-to-amplitude conversion. The ratio
of stop to start pulses was kept low (below 0.04:1) to ensure good statis-
tics. The acquisition time for a full time trace was typically shorter than
1
1
integrals of the H NMR signals of 3.
2
(
,5-Dibromo-3-[(tetrahydro-4H-thiopyran-4-ylidene)methyl]thiophene
4): A solution of 3 (2.31 g of 35% purity, 1.9 mmol 3) and N,N-dimethyl-
formamide dineopentyl acetal (2.70 g, 11.7 mmol) in acetonitrile
125 mL) was stirred at room temperature for 45 min and then at reflux
6
0 s. The instrument response function (IRF) was determined with a di-
luted suspension of silica particles (Ludox) as the scattering medium and
the same experimental conditions as for the fluorescence decay measure-
ments. The raw data were analyzed by the fluorescence decay analysis
software program Fluofit 3.3 (PicoQuant). The instrumental resolution
was 70 ps and 120 ps at 10 MHz and 2.5 MHz repetition rate, respectively
(
[39]
temperature overnight. After removal of the solvent in vacuo, the resi-
due was dissolved in dichloromethane (100 mL) and washed with water
(
4
4”150 mL). The remaining organic phase was dried (MgSO ) and fil-
tered. Subsequently the filtrate was concentrated in vacuo and subjected
to Kugelrohr distillation (T = 1308C, p = 0.11 mbar). The distillate was
recrystallized from methanol furnishing white crystals (0.61 g, 1.7 mmol,
(
10% of the IRF FWHM).
Photoinduced absorption spectra were recorded as described in refer-
+
1
ence [50].
A
mechanically modulated CW Ar laser pump beam
89%). H NMR: d = 6.80 (s, 1H; aromatic H), 5.92 (s, 1H; =CꢀH), 2.75
1
3
(
488 nm, 275 Hz) was used as excitation source and the probe light (with
(m, 2H), 2.68 (m, 2H), 2.59 ppm (m, 4H); C NMR: d = 143.2, 138.4,
131.1, 117.0, 110.6, 110.1, 38.6, 32.0, 30.9, 30.0 ppm; IR: n˜ = 3084, 2948,
2924, 2904, 2882, 2819, 1681, 1649, 1519, 1423, 1408, 1339, 1321, 1303,
1290, 1268, 1223, 1205, 1187, 1167, 1133, 1120, 1011, 992, 973, 946, 937,
energies between 0.35 and 3.0 eV) was produced by a tungsten–halogen
lamp. The change in transmission through the sample (DT) was recorded
with a phase-sensitive lock-in amplifier after dispersion by a triple-gra-
ting monochromator and detection with Si, InGaAs, and cooled InSb de-
tectors. The photoinduced absorption ꢀDT/T was calculated from the
change in transmission and was corrected for the photoluminescence,
which was recorded in a separate scan. Samples were held in quartz cuv-
ettes with a path length of 1 mm and measurements were performed at
room temperature.
ꢀ
1
915, 852, 831, 768, 727, 690, 670, 656 cm
.
Poly{3-[(tetrahydro-4H-thiopyran-4-ylidene)methyl]thiophene} (poly(4)):
The preparation was based on a procedure developed by Yamamoto
[62]
et al.
A
H
R
U
G
2
A mixture of Ni(cod) (0.21 g, 0.76 mmol), 1,5-cyclooctadiene
(
(
70 mL, 0.57 mmol), triphenylphosphine (0.18 g, 0.69 mmol), and
0.1942 g, 0.55 mmol) in DMF (5 mL) was stirred at 608C for 16 h and
4
Cyclic voltammetry was performed using an EG&G Potentiostat/Galva-
nostat Model 263 A in THF containing 0.1m tetrabutylammonium hexa-
subsequently at 1008C for 20 h. The reaction mixture was quenched with
water (75 mL) and extracted with chloroform (4”100 mL). The com-
bined organic layers were washed with water (100 mL), dried (MgSO4)
and filtered. After concentration of the filtrate in vacuo the residue was
fluorophosphate as supporting electrolyte at
1
a
scanning rate of
00 mVs . Redox potentials were determined relative to an Ag/AgCl
reference electrode and were referenced to SCEby measuring the oxida-
ꢀ
1
washed with diethyl ether (7”50 mL).
A purple powder (0.0827 g,
+
[59]
3
tion potential of the [FeCp
2
]/
A
H
R
U
G
2
]C couple (0.54 V versus SCE ).
0.43 mmol repeating units, 78%) with M = 2.7”10 and D = 1.4 was
w
Chem. Eur. J. 2006, 12, 8075 – 8083
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8081

C. A. van Walree et al.
obtained. The product was soluble in chloroform and partially soluble in
dichloromethane and THF. H NMR: d = 6.95 (br, 1H; aromatic H),
[6] A. Shik, S. Yu, E. Johnson, H. Ruda, E. H. Sargent, Solid-State Elec-
tron. 2002, 46, 61.
1
6
2
(
.21 (s, ca. 0.4H; =CꢀH), 6.11 (s, ca. 0.6H; =CꢀH), 2.80 (br, 2H),
[7] M. C. Schlamp, X. Peng, A. P. Alivisatos, J. Appl. Phys. 1997, 82,
5837.
[8] V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Nature 1994, 370, 354.
[9] S. Coe, W.-K. Woo, M. Bawendi, V. Bulovic, Nature 2002, 420, 800.
[10] S. Chaudhary, M. Ozkan, W. C. W. Chan, Appl. Phys. Lett. 2004, 84,
2925.
[11] W. W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 2003, 15, 2854.
[12] S. F. Wuister, F. van Driel, A. Meijerink, Phys. Chem. Chem. Phys.
2003, 5, 1253.
[13] D. V. Talapin, S. Haubold, A. L. Rogach, A. Kornowski, M. Haase,
H. Weller, J. Phys. Chem. B 2001, 105, 2260.
[14] P. Reiss, J. Bleuse, A. Pron, Nano Lett. 2002, 2, 781.
[15] C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc. 1993,
115, 8706.
[16] E. Kucur, J. Friegler, G. A. Urban, T. Nann, J. Chem. Phys. 2003,
119, 2333.
[17] L. Bakueva, S. Musikhin, M. A. Hines, T.-W. F. Chang, M. Tzolov,
G. D. Scholes, E. H. Sargent, Appl. Phys. Lett. 2003, 82, 2895.
[18] A. A. Bol, A. Meijerink, J. Phys. Chem. B 2001, 105, 10203.
[19] A. A. Bol, A. Meijerink, J. Phys. Chem. B 2001, 105, 10197.
1
3
.63 ppm (br, 6H); C NMR: d = 119.1, 118.8, 38.7, 32.1, 30.8, 30.2 ppm
1
3
because of a poor signal-to-noise ratio the backbone C NMR signals
could not be detected); IR: n˜ = 3053, 2937, 2900, 2824, 1647, 1588, 1550,
1
8
C
496, 1423, 1321, 1300, 1287, 1267, 1225, 1183, 1119, 1052, 1015, 966, 936,
ꢀ
1
47, 828, 771, 736, 721, 691, 653 cm ; elemental analysis calcd (%) for
(194.3): C 61.81, H 5.19, S 33.00; found: C 61.80, H 5.10, S
10
H
10
S
2
3
3.11.
CdTe quantum dots: These were synthesized in a glove-box under argon
[
12,20]
by a modification of the procedure reported by Wuister et al.
Te
powder (<250 mm particle size, 0.16 g, 1.3 mmol) was added to a solution
of dimethylcadmium (0.26 g, 1.8 mmol) in TOP (20 mL) and DDA (15 g),
heated to 508C. The reaction mixture was stirred at 1808C and the reac-
tion progress was monitored by measuring the fluorescence of samples
consisting of a few drops of reaction mixture diluted in toluene (10 mL)
under UV irradiation (365 nm). The synthesis was stopped at the
moment that a sample taken from the reaction mixture showed bright
orange to red fluorescence. The quantum dots had an average diameter
of 4.0 nm, as estimated from the first absorption maximum of the CdTe
[
11]
quantum dots, which is near 620 nm.
[
20] S. F. Wuister, I. Swart, F. van Driel, S. G. Hickey, C. de Mello
Complexation experiments: The maximum coverage of CdTe quantum
dots by poly(4) was estimated from the area occupied by a repeating unit
Donegµ, Nano Lett. 2003, 3, 503.
2
[63]
[21] J. Li, X. Hong, Y. Liu, D. Li, Y. A. Wang, J. Li, Y. Bai, T. Li, Adv.
Mater. 2005, 17, 163.
(
27.7 Š as estimated from an MM2 calculation in Chem3D Pro ) and
the area of a completely stripped quantum dot of known radius. The
number of quantum dots in a sample was calculated under the assump-
[
22] H. Bao, Y. Gong, Z. Li, M. Gao, Chem. Mater. 2004, 16, 3853.
2
ꢀ
2+
[23] A. A. Bol, R. van Beek, A. Meijerink, Chem. Mater. 2002, 14, 1121.
[24] S. F. Wuister, A. Meijerink, J. Lumin. 2003, 102–103, 338.
[25] K. Prabakar, S. K. Narayandass, D. Mangalaraj, Physica B 2003, 328,
355.
tion that a quantum dot consists of a Te lattice with the Cd ions occu-
2
ꢀ
[64]
pying the voids. This is justified by the ionic radius of Te (2.11 Š
being much larger than that of Cd (0.97 Š ). Thus, from the number
of Te ions fitting in a quantum dot of known radius and the amount of
CdTe quantum dot material used (by taking a given amount from a dis-
persion of known concentration), the numbers of quantum dots, and of
CdTe units, were obtained.
)
2
+
[64]
2
ꢀ
[
26] V. I. Levchenko, V. N. Yakimovich, L. I. Postnova, V. I. Konstanti-
nov, V. P. Mikhailov, N. V. Kuleshov, J. Cryst. Growth 1999, 198/199,
9
80.
[
[
27] N. Gaponik, D. V. Talapin, A. L. Rogach, A. Eychmüller, H. Weller,
Nano Lett. 2002, 2, 803.
28] N. Gaponik, D. V. Talapin, A. L. Rogach, K. Hoppe, E. V. Shevchen-
ko, A. Kornowski, A. Eychmüller, H. Weller, J. Phys. Chem. B 2002,
Before exposure to poly(4), an aliquot of CdTe quantum dots (8 mL of a
dispersion in TOP/DDA with a CdTe unit concentration of 0.036m) was
precipitated into methanol (70 mL). When the precipitate had settled,
the supernatant was decanted. The precipitate was dispersed in chloro-
form (8 mL) and again precipitated into methanol (70 mL). After centri-
fugation (2400 rpm for 15 min) the supernatant was decanted and the
precipitate was dispersed in chloroform (8 mL). For photophysical inves-
tigation of a CdTe/poly(4) mixture, a system was prepared containing
CdTe quantum dots and poly(4) in a ratio of 9:1 (mol CdTe units per mol
poly(4) repeating units). This implies that the occupation of the quantum
dot surface by poly(4) was approximately 40% at most. It should be real-
ized that the actual coverage by poly(4) was less than the maximum,
since TOP remained attached to the quantum dot surface. Mixed systems
were stored in an inert atmosphere for at least 2 h before measurement
to allow full equilibration of the complexation process.
1
06, 7177.
29] T. Tsuruoka, K. Akamatsu, H. Nawafune, Langmuir 2004, 20,
1169.
30] A. van Dijken, J. J. A. M. Bastiaansen, N. M. M. Kiggen, B. M. W.
Langeveld, C. Rothe, A. Monkman, I. Bach, P. Stçssel, K. Brunner,
J. Am. Chem. Soc. 2004, 126, 7718.
[
[
1
[
[
[
31] J. Liu, E. N. Kadnikova, Y. Liu, M. D. McGehee, J. M. J. FrØchet, J.
Am. Chem. Soc. 2004, 126, 9486.
32] J. Liu, T. Tanaka, K. Sivula, A. P. Alivisatos, J. M. J. FrØchet, J. Am.
Chem. Soc. 2004, 126, 6550.
33] E. P. A. M. Bakkers, A. W. Marsman, L. W. Jenneskens, D. Vanmae-
kelbergh, Angew. Chem. 2000, 112, 2385; Angew. Chem. Int. Ed.
2
000, 39, 2297.
[
34] Y.-V. Kervennic, J. M. Thijssen, D. Vanmaekelbergh, R. Dabirian,
L. W. Jenneskens, C. A. van Walree, H. S. J. van der Zant, Angew.
Chem. 2006, 118, 2602; Angew. Chem. Int. Ed. 2006, 45, 2540.
35] M. Poot, E. Osorio, K. OꢁNeill, J. M. Thijssen, D. Vanmaekelbergh,
C. A. van Walree, L. W. Jenneskens, H. S. J. van der Zant, Nano Lett.
2006, 6, 1031.
Acknowledgements
[
The authors gratefully acknowledge the Netherlands Organization for
Scientific Research, Priority Program of Materials (NWO-PPM), for fi-
nancial support of this work.
[36] A. W. Marsman, R. W. A. Havenith, S. Bethke, L. W. Jenneskens, R.
Gleiter, J. H. van Lenthe, Eur. J. Org. Chem. 2000, 2629.
[
37] A. W. Marsman, R. W. A. Havenith, S. Bethke, L. W. Jenneskens, R.
Gleiter, J. H. van Lenthe, M. Lutz, A. L. Spek, J. Org. Chem. 2000,
65, 4584.
[
[
1] D. S. Ginger, N. C. Greenham, Phys. Rev. B 1999, 59, 10622.
2] N. C. Greenham, X. Peng, A. P. Alivisatos, Phys. Rev. B 1996, 54,
1
7628.
[38] T. Rajh, O. I. Micic, A. J. Nozik, J. Phys. Chem. 1993, 97, 11999.
[39] F. J. Hoogesteger, R. W. A. Havenith, J. W. Zwikker, L. W. Jennesk-
ens, L. W. Kooijman, N. Veldman, A. L. Spek, J. Org. Chem. 1995,
60, 4375.
[
[
3] N. C. Greenham, X. Peng, A. P. Alivisatos, Synth. Met. 1997, 84, 545.
4] B. S. Ong, Y. Wu, P. Liu, S. Gardner, J. Am. Chem. Soc. 2004, 126,
3
378.
[
5] K. Yoshino, Y. Kawagishi, S. Tatsuhara, H. Kajii, S. Lee, M. Ozaki,
Z. V. Vardeny, A. A. Zakhidov, Superlattices Microstruct. 1999, 25,
[40] Compound 4 was also polymerized in the presence of CdTe quan-
tum dots. However, as indicated by photophysical studies, the quan-
tum dots did not survive the polymerization. CdTe quantum dots
3
25.
8082
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 8075 – 8083

Electronic Contact between Polythiophene and Quantum Dots
FULL PAPER
2
+
can be neither oxidized nor reduced by [Ni
A
H
R
U
G
2
] or Ni (which is
[50] P. A. van Hal, M. P. T. Christiaans, M. M. Wienk, J. M. Kroon,
R. A. J. Janssen, J. Phys. Chem. B 1999, 103, 4352.
[51] M. Westerling, R. Österbꢂcka, H. Stubb, Phys. Rev. B 2002, 66,
165220.
[52] M. Westerling, R. Österbacka, H. Stubb, Thin Solid Films 2002,
403–404, 510.
[53] E. M. Conwell, H. A. Mizes, Phys. Rev. B 1995, 51, 6953.
[54] A. Weller, Z. Phys. Chem. (Muenchen Ger.) 1982, 133, 93.
[55] Y. Bae, N. Myung, A. J. Bard, Nano Lett. 2004, 4, 1153.
[56] S. K. Poznyak, N. P. Osipovich, A. Shavel, D. V. Talapin, M. Gao, A.
Eychmüller, N. Gaponik, J. Phys. Chem. B 2005, 109, 1094.
[57] J. W. Thackeray, M. J. Natan, P. Ng, M. S. Wrighton, J. Am. Chem.
Soc. 1986, 108, 3570.
formed during the polymerization process), as the redox potential of
2
+
[64]
the Ni /Ni couple (ꢀ0.23 V vs. NHE, ꢀ0.47 V vs. SCE ) is situat-
ed between the oxidation and reduction potentials of CdTe quantum
[
38, 55, 56]
2+
dots.
However, oxidation by Ni can occur after excitation
of an electron in the quantum dots under the influence of light,
since the conduction band edge of the quantum dots is at a higher
energy level than the redox potential of the Ni /Ni couple (photo-
oxidation). See: M. Wijtmans, PhD thesis, Radboud University, Nij-
megen (The Netherlands), 2004.
[
41] R. D. McCullough, P. C. Ewbank, in Handbook of Conducting Poly-
mers, 2 ed. (Ed.: J. R. Reynolds), Marcel Dekker, New York, 1998,
p. 225.
[
[
[
42] M. Westerling, C. Vijila, R. Österbacka, H. Stubb, Chem. Phys.
[58] It is not excluded that in solution ultrafast energy transfer from
poly(4) to the quantum dots takes place before the charge separa-
tion. However, energy transfer was not resolved in our experiments.
[59] A. Facchetti, M.-H. Yoon, C. L. Stern, G. R. Hutchison, M. A.
Ratner, T. J. Marks, J. Am. Chem. Soc. 2004, 126, 13480.
[60] J. Friedrich, M. Baumgarten, Appl. Magn. Reson. 1997, 13, 393.
[61] S. Strꢂssler, A. Linden, Helv. Chim. Acta 1997, 80, 1528.
[62] T. Yamamoto, A. Morita, Y. Miyazaki, T. Maruyama, H. Wakayama,
Z. Zhou, Y. Nakamura, T. Kanbara, Macromolecules 1992, 25, 1214.
[63] Chem3D Pro 8, Version 8.0.3, CambridgeSoft, Cambridge (USA),
2004.
2
003, 286, 315.
43] R. van Beek, L. W. Jenneskens, A. N. Zdravkova, J. P. J. M. van der
Eerden, C. A. van Walree, Macromol. Chem. Phys. 2005, 206, 1006.
44] The comparison with PTT is made because this is one of the few
polythiophenes for which cyclic voltammetry in THF solution has
been reported. The photophysical properties and redox behavior of
PTT resemble those of poly(3-alkylthiophene)s. See also refer-
ence [43].
[
[
[
[
[
45] M. BelletÞte, L. Mazerolle, N. Desrosiers, M. Leclerc, G. Durocher,
Macromolecules 1995, 28, 8587.
46] J. Seixas de Melo, H. D. Burrows, M. Svensson, M. R. Andersson,
A. P. Monkman, J. Chem. Phys. 2003, 118, 1550.
[64] Handbook of Chemistry and Physics, 59th ed. (Eds.: R. J. Weast,
M. J. Astle), CRC Press, Boca Raton, 1979.
47] X. M. Jiang, R. Österbacka, C. P. An, Z. V. Vardeny, Synth. Met.
2
003, 137, 1465.
48] X. M. Jiang, R. Österbacka, O. Korovyanko, C. P. An, B. Horovitz,
R. A. J. Janssen, Z. V. Vardeny, Adv. Funct. Mater. 2002, 12, 587.
49] B. Kraabel, D. Moses, A. J. Heeger, J. Chem. Phys. 1995, 103, 5102.
Received: November 29, 2005
Revised: May 30, 2006
Published online: August 10, 2006
Chem. Eur. J. 2006, 12, 8075 – 8083
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8083