Synthesis and optical properties of a rod-coil diblock copolymer with polyoxometalate clusters covalently attached to the coil block

Article abstract of DOI:10.1021/cm1007075

A rod-coil diblock copolymer (DCP) containing an oligo(phenylene vinylene) (OPV) rigid block and a polystyryl-type flexible block with polyoxometalate (POM) clusters as side chain pendants has been successfully synthesized. The coil block of 2,6-dimethyl-

Full text of DOI:10.1021/cm1007075

Chem. Mater. 2010, 22, 3995–4006 3995
DOI:10.1021/cm1007075
Synthesis and Optical Properties of a Rod-Coil Diblock Copolymer with
Polyoxometalate Clusters Covalently Attached to the Coil Block
Sanjiban Chakraborty,† Andrew Keightley,‡ Vladimir Dusevich,§ Yong Wang,§ and
Zhonghua Peng*,†
†Department of Chemistry, ‡School of Biological Sciences, and §Department of Oral Biology, University of
Missouri-Kansas City, Kansas City, Missouri 64110
Received March 10, 2010. Revised Manuscript Received June 4, 2010
A rod-coil diblock copolymer (DCP) containing an oligo(phenylene vinylene) (OPV) rigid block
and a polystyryl-type flexible block with polyoxometalate (POM) clusters as side chain pendants has
been successfully synthesized. The coil block of 2,6-dimethyl-4-vinyl aniline protected by phthalic
anhydride was prepared by atom transfer radical polymerization (ATRP). The terminal bromo-end
group was then converted to an azide which subsequently coupled with an ethynyl terminated OPV rod
block by “click” chemistry. After removing the phthalimide protection groups in the coil block to give
free aryl amines, POM clusters were finally attached to the coil block covalently to yield the first POM-
containing DCP. The hybrid DCP shows two absorption bands, one at 370 nm which is attributed to
the ligand-to-metal charge transfer transition associated with imido-functionalized POM clusters, and
the other at 450 nm which is due to the π-π* transition of the OPV backbone. Cyclic voltammetry
measurements show a distinct reversible reduction process at -1.1 eV, significantly cathodically shifted
compared to that of free hexamolybdate cluster. These results indicate that the POM clusters are indeed
covalently linked to the DCP. Although little fluorescence quenching is observed in solution, the POM
cluster is found to quench 74% of the OPV fluorescence in films. Such a significant fluorescence
quenching is likely due to a photoinduced electron-transfer process from the OPV donor to the POM
cluster, making it a potential candidate as an efficient photovoltaic material. Preliminary film
morphology studies show that the hybrid DCP aggregates very differently from its precursor DCPs.
Introduction
variations but also because of their often superior mate-
rials properties resulting from the realization of the desired
phase-separated morphologies.1 While all organic DCPs
continue to dominate the field, organic-inorganic hybrid
DCPs have drawn increasing attention.2 In particular,
Diblock copolymers (DCPs) are perhaps the most ex-
tensively studied polymer architectures in the past couple
of decades not only because of their unlimited combina-
tions of different blocks with compositional and structural
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*To whom correspondence should be addressed. E-mail: pengz@
umkc.edu.
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2010 American Chemical Society
Published on Web 06/15/2010
pubs.acs.org/cm

3996 Chem. Mater., Vol. 22, No. 13, 2010
Chakraborty et al.
metal-containing DCPs have been actively pursued in
recent years. For example, Manners has studied a variety
of DCPs with one block containing ferrocenes.3 Chan,4
Schubert,5 and others6 have explored DCPs containing
transition metal complexes. While these studies are ex-
emplary, they include only discrete metal centers; block
copolymers containing metal clusters, as far as we know,
have not yet been reported. Among various metal-oxygen
clusters, polyoxometalates (POM) are most attractive not
only because of their structural versatility and rich optoe-
lectronic properties but also because of their discrete
molecule structures which allow surface functionalization
in a controlled and rational way.7 Indeed, a number of
POM-containing organic-inorganic hybrids have recently
Figure 1. Structure of a hybrid rod-coil diblock copolymer.
been reported,8-26 among which a few examples involve
POM-containing polymers.27 While most POM-contain-
ing polymers are based on an insulating polymer back-
bone, we have previously reported both main chain and
side chain POM-containing conjugated polymers and have
demonstrated that POM clusters as electron acceptors, in
conjunction with organic π-conjugated segments as elec-
tron donors, may find applications as novel photovoltaic
(PV) materials.28 The PV performances of our reported
hybrid conjugated polymers are, however, not spectacular.
This is not surprising though as an efficient PV device
requires not only efficient photoinduced charge transfer
but also efficient and separate charge transporting path-
ways for positive and negative charge carriers. A conju-
gated polymer with POM clusters uniformly distributed
either in the main chain or in side chains may exhibit
efficient photoinduced charge separation, but possess no
separate charge transporting channels for electrons and
holes. Since electrons transport through hopping among
POM clusters while holes transport through aggregated
conjugated segments, a polymer system which can segre-
gate the two structural components may be able to provide
the separate domains for different charge transport. With
these considerations in mind, we have set out to synthesize
a rod-coil DCP with POM clusters linked to the coil
block. The rod block is a conjugated polymer which serves
as the electron donor, while the POM cluster in the coil
block acts as the electron acceptor. If bicontinuous phase
separated domains can be formed through DCP self-
assembly, such a hybrid DCP may possess significantly
improved PV performance over our previously reported
hybrid conjugated polymers. In this paper, we report the
synthesis and optical properties of the first such POM-
containing hybrid DCP.
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Results and Discussion
Synthesis. Figure 1 shows the structure of the POM-
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Article
Chem. Mater., Vol. 22, No. 13, 2010 3997
Scheme 1. Synthesis of an Azide Terminated PS Block
as the rod block which is attached to the coil block
through the triazole functional group whereas the hex-
amolybdate cluster is connected to the coil block via an
imido linkage. The hexamolybdate cluster has a molecu-
lar formula of [Mo6O19]2- with tetrabutylammonium as
the counterion.
was then subjected to atom transfer radical polymerization
(ATRP).30 With CuBr, N,N,N,N,N-pentamethyldiethyl-
enetriamine (PMDETA) and methyl-2-bromopropionate
(MBP) as the catalyst, ligand and initiator, respectively,
monomer 2was polymerizedtoformthe flexible coilblock.
It is noted that in the course of the reaction, active chain
ends with the bromo group could gradually convert to dead
chains, possibly by the elimination of bromine because of
chain transfer to the aliphatic amine ligand31 or by the β-H
elimination reaction.32 To ensure that all chain ends are
active, the ATRP of 2 was quenched prior to complete
monomer-to-polymer conversion. The bromo-end func-
tional group of the coil block obtained by ATRP was then
converted to an azide group by nucleophilic substitution
reaction with sodium azide.33
The 1H NMR spectra of monomer 2, PS-Br and PS-N3
are shown in Figure 2. One observes that after polymer-
ization, the vinyl protons of monomer 2, observed at 5.27,
5.75, and 6.67 ppm, completely disappeared, while new
broad signals around 1.7 and 2.2 ppm corresponding to
methylene protons in the polymer backbone appeared.
Three broad aromatic signals are observed at 6.65, 7.71,
and 7.80 ppm, among which the first can be assigned to
aromatic protons in the bridging phenyl ring (proton e in
Scheme 1) while the other two are attributed to protons of
the phthalimide unit (proton g). The dominant alkyl
signal at 2.03 ppm is assigned to the two phenyl-bonding
methyl groups.
Polymer hybrids can be realized, in principle, by two
different pathways: the post-polymerization hybridiza-
tion approach and the post-hybridization polymerization
(hybridization first) approach. The first route involves
the synthesis of a precursor DCP with functional pen-
dants on one block followed by the covalent linkage of the
POM cluster. The second route initiates from the synthe-
sis of hybrid monomers, which are in turn used directly
for polymerization. Both avenues while enjoying some
advantages, at the same time are inflicted with some
drawbacks. The former approach has better control over
the degree of polymerization of each block with the
flexibility of synthesizing polymer hybrids with varying
clusters. There may be, however, a lack of control over the
extent of cluster attachment. The latter approach, while
ensuring complete cluster functionalization on the coil
block, demands the development of new chemistry: the
living polymerization of POM containing monomers has
yet to be demonstrated though living polymerization of
styrene monomer using trivanadium substituted poly-
tungstate type POM initiator has been reported.27c While
we have made efforts on both fronts concomitantly and
our preliminary results indicate that vinyl functionalized
POM clusters can indeed undergo polymerization under
ATRP conditions, this paper instead will focus on the first
route, that is, the post-polymerization hybridization
pathway to prepare our targeted hybrid DCP. In other
words, a DCP with pendant free aryl amine is synthesized
first, followed by cluster attachment to yield the targeted
hybrid.
End groups give small but clear signals. The initiating
end gives methoxy signals at 3.42 ppm, and methyl signals
(signal b) around 0.9 ppm. Signals corresponding to the
propagating end are not as clear. If one expands the
region with chemical shifts from 4.0 to 5.0 ppm, one sees
a clear signal around 4.43 ppm which can be assigned to
the proton associated with the Br-bonding carbon. After
converting to an azide, this signal is shifted to 3.95 ppm.
Using the integration ratio of a signal distinctively attrib-
uted to the repeating unit versus one to the end groups,
To prepare the rod-coil DCP, we set to synthesize an
azide terminated coil block and an ethynyl functionalized
PPV block. The two blocks can then be joined together
through “click” chemistry. Scheme1 showsthe synthesisof
the coil block. Compound 1 was prepared by iodinating
2,6-dimethylaniline according to a literature procedure,29
followed by protection of the free amine group by phthalic
anhydride. Stille coupling reaction of 1 with tributyl vinyl-
tin gave the vinyl monomer 2 in 60% yield. Monomer 2
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3998 Chem. Mater., Vol. 22, No. 13, 2010
Chakraborty et al.
Figure 2. 1H NMR spectra of monomer 2, PS-Br and PS-N3 in CDCl3.
Scheme 2. Synthesis of an Ethynyl-Terminated PPV Block
such as g versus b, one may calculate the degree of
polymerization to be around 22.
alkyne signal 10 (3.12) are clear. Signals at the other end,
such as 3, 5 are again clear with no change in chemical shift
observed. Using the integration of signals 7 and 6 (7:6 =
60:2.5), a degree of polymerization of 11 is again obtained.
The azide terminated flexible coil block (PS-N3) and
ethynyl terminated rigid block (PPV-ꢀ) were joined
together through modified Huisgen 1,3-dipolar cycload-
dition or simply “click” chemistry (Scheme 3). This reac-
tion has gained a considerable amount of attention over
the years because of its mild conditions, high yields, easy
experimental set up, minimal synthetic workup, and
tolerance of a wide variety of functional groups.35 While
PS-N3 is soluble in common organic solvents such as
chloroform, dichloromethane, dimethylformamide (DMF),
dimethyl sulfoxide (DMSO), acetone, tetrahydrofuran
The synthesis of an ethynyl-functionalized PPV block
is shown in Scheme 2. A PPV block with one terminal
aldehyde group was synthesized in one step via the Siegrist
polycondensation of monomer 3, an approach demon-
strated previously by Kretzschmann et al. and Hadziioan-
nou et al.34 The PPV-CHO was converted to the ethynyl
terminated PPV by Horner-Wittig-Emmons reaction
with compound 4.
Figure 3 shows the 1H spectra of compound 3 and the
two PPV polymers. After polymerization, two major peaks
at 7.48 (8) and 7.15 (9) appeared, which are assigned to
aromatic and vinyl protons, respectively. The methyl and
aldehyde end groups (1 and 6) are clear. Other end group
signals such as 2, 3, 4, 5, and so forth are also visible. Using
the integration ratio of signal 7 versus 1 (7:1 = 76:1), one
obtains the degree of polymerization around 11. After
converting the aldehyde to alkyne, signals associated with
the -CHO end such as 1, 2, 4, and so forth disappeared.
Signals corresponding to the new end group, such as
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H.; Meier, J. Tetrahedron Lett. 1991, 32, 5059.

Article
Chem. Mater., Vol. 22, No. 13, 2010 3999
Figure 3. 1H NMR spectra of monomer 3 and resulting PPV blocks in CDCl3.
Scheme 3. Synthesis of the PS-PPV DCP and the Targeted Hybrid DCP
(THF), and so forth the PPV-ꢀ block is soluble only in
chloroform, dichloromethane, and THF. The “click” reaction
is thus carried out in THF. The resulting diblock copolymer
PS-PPV is found to exhibit similar solubility to that of the PPV
block, making the separation of any unreacted PPV block
difficult. To ensure complete reaction of the PPV block, more
than one equivalent of PS-N3 block was used. The unreacted
excess PS-N3 block can be easily removed by precipitating the
mixture from acetone: PS-N3 is soluble in acetone while PS-
PPV is not. As shown in Figure 4, the 1H NMR spectrum of
the “clicked-together” diblock copolymer is essentially a com-
bination of the spectra of the two polymer blocks, except for
the missing terminal alkyne proton signal (signals 10). When
the spectral region of 5-6 ppm is expanded, one can note a
signal around 5.15 ppm which can be assigned to the proton
associated with triazole-binding carbon in the PS block.35h
Using the integration of signals attributed distinctively to either
block, one can calculate the n/mratio (nand mare the degree of
polymerizations of the PS and PPV block, respectively). For
example, using the integration of signals g (34) and 8 (8), the
n/mratio is calculated to be 2.1 to 1, consistent with the degrees
of polymerization of the two blocks.
The protected amine group was made free with hydra-
zine hydrate according to the Ing-Manske procedure.36
The complete removal of the protecting phthalimide
1
groups is confirmed by the H NMR spectrum of the
resulting polymer (see Figure 4) where signal g is com-
pletely missing. The final cluster attachment was carried
out in 1-methyl-2-pyrrolidinone (NMP) with excess hex-
amolybdate cluster and dicyclohexylcarbodiimide (DCC).
Both the cluster and the PS-PPV DCP are soluble in NMP,
and the cluster attachment reaction was followed by gel
permeation chromatography (GPC). Excess free clusters
after the reaction were removed by precipitating the poly-
mer from hot acetonitrile. The resulting cluster-attached
DCP is soluble in NMP, but only slightly soluble in
chloroform and THF, and insoluble in most other organic
solvents.
1
The H NMR spectrum of the cluster-attached DCP
was taken in CDCl3. Because of its limited solubility, the
spectrum reflects only the structures of those soluble
DCPs. As shown in Figure 4, the spectrum shows rather
weak aromatic signals. The broad signals e and f seen in
1
the H NMR spectrum of the deprotected DCP have
disappeared. Weak but discernible signals seen at 6.92
and 2.80 ppm are likely downfield shifted signals e and f,
(36) (a) Khan, M. N. J. Org. Chem. 1996, 61, 8063. (b) Meuer, S.; Zentel,
R. Macromol. Chem. Phys. 2008, 209, 158.

4000 Chem. Mater., Vol. 22, No. 13, 2010
Chakraborty et al.
Figure 4. 1H NMR of the PS-PPV DCP, the deprotected DCP, and the cluster attached DCP in CDCl3.
respectively. The hybrid DCPs which are soluble in
chloroform are likely to have only short PS segments
which may account for the unusually low intensity of
proton signals associated with the PS segment. New
signals corresponding to the tetrabutylammonium coun-
1
terion, such as the one at 3.42 ppm, appeared in the H
NMR spectrum of the cluster-attached DCP. While
signals overlap extensively in the 1.0 to 2.0 region and
the aromatic region, in the 3.0 to 5.0 ppm range, one
observes two strong well-resolved broad signals at 4.05
and 3.42 ppm, which can be assigned to the -OCH2-
proton of the PPV block and the -NCH2- protons of the
tetrabutylammonium counterion, respectively. On the
basis of their 1:1 integration ratio, one can estimate that
the number of attached clusters is about one-fourth of the
Figure 5. GPC traces of the PS-Br block, the PPV-ꢀ block, the PS-PPV
degree of polymerization (the m value shown in Figure 4)
of the PPV block, which yields about 3 POM clusters per
PS block.
DCP, and the hybrid DCP using THF as the eluent.
shortened retention time. No peaks corresponding to the
unreacted DCP and the free [Mo6] cluster are observed,
indicating thatall DCPs havereacted and freeclustershave
completely washed out. While one cannot conclude that all
amines have reacted and most likely that will not happen
because of steric reasons, the narrow PD of the resulting
hybridDCP indicatesthatthe extent of functionalizationis
fairly consistent among different DCP chains.
Realizing that GPC produces only a relative mole-
cular weight (in relation to polystyrene standards),
matrix-assisted laser desorption/ionization-Time-of-
Flight (MALDI-TOF) mass spectrometry (MS) measure-
ments were also carried out on all polymers to yield infor-
mation on their absolute molecular weights and molecular
weight distributions. As shown in Figure 6, MALDI-TOF
spectra of PS-Br and PS-N3 show a regularly distributed
set of peaks with mass difference of one repeating unit
(277.11). On the basis of the molecule weights and the
Molecular Weights. Figure 5 shows the gel-permeation
chromatography traces of the rigid PPV block, the PS
block, the PS-PPV DCP, free cluster [Mo6], and the
hydrid DCP. All measurements were run at 30 °C using
THF as the eluent. The coil PS block has a number
average molecular weight (Mn) of 2732 and a polydisper-
sity (PD) of 1.10. The relatively narrow molecular weight
distribution indicates the living nature of the polymeri-
zation. The PPV block has a Mn of 2208 and a slightly
larger PD of 1.27. Such a PD is consistent with literature
reports.34b After “click” coupling, the PS-PPV DCP shows
a Mn of 5900 and a PD of 1.22. No uncoupled PPV block
or PS block is seen in the DCP’s GPC trace, indicating a
complete coupling reaction. Such a result also confirms
that all the PS coil blocks prepared from the ATRP process
possess the active Br end group. After cluster attachment,
the resulting hybrid DCP shows a peak with a significantly

Article
Chem. Mater., Vol. 22, No. 13, 2010 4001
Figure 7. TGA thermograms of PS-Br, PPV-ꢀ, PS-PPV DCP, [Mo6O19]
3
2NBu4 cluster, and the hybrid DCP.
While MALDI-TOF measurements yield molecular
weight information for the PS block, PPV block and the
PS-PPV DCP. The MALDI-TOF spectra of the cluster-
attached DCP revealed no peaks beyond a mass/charge
ratio of 3000, whether using a positive or negative detec-
tion mode. The lack of high mass/charge signals may be
due to the difficulty in vaporizing cluster-attached DCPs
under the MALDI conditions or due to the simple fact
that the hybrid DCP is highly charged (as each attached
cluster carried a -2 charge).
Figure 6. MALDI-TOF spectra of the PS block, the PPV block, and the
PS-PPV DCP. The theoretical MW distribution is calculated based on the
random statistical reaction of the two blocks with the shown MW
distribution.
Thermal Properties. Thermal properties of the two
blocks, PS-PPV DCP and the hybrid DCP were studied
by thermogravimetric analysis (TGA), and the results are
shown in Figure 7. While both the cluster and the PS-PPV
DCP are thermally stable up to 300 °C, the hybrid DCP
started to decompose at a much lower temperature of
200 °C. Weight loss continued until 530 °C when no
further loss is observed up to 700 °C. A residual weight
of 34% remains. For the free Mo6 cluster, a similar no
weight loss region from 530 to 700 °C is observed, and the
residual weight is 64%, which is consistent with its cluster
anion content of 64%. The PS block, the PPV block, and
PS-PPV, on the other hand, all started to degrade around
300 °C and continued to lose weight up to 700 °C. All
these polymers have less than 8% residual weight at
700 °C. It is thus reasonable to assume that at 700 °C,
the thermally decomposed residue of the hybrid DCP is
MoO3, from which one can calculate the weight percen-
tage of Mo in the hybrid DCP to be 22%. Using the DPs
of 18 and 7, obtained from the above MALDI-TOF
measurements, for the PS and PPV blocks, respectively,
and assuming a complete phthalimide deprotection, a
22% Mo content in the hybrid DCP indicates that the
number of attached clusters per PS block is between 3
(20% Mo) and 4 (23% Mo), quite consistent with the
result obtained from 1 H NMR studies.
intensities of peaks, one can calculate the Mn of PS-Br and
PS-N3 to be 4494 and 4596, respectively, which corresponds
to an average degree of polymerization (DP) of 18, reason-
1
ably consistent with the DP obtained by H NMR. Both
polymers show narrow molecular weight distributions with
PD of 1.10 and 1.11, respectively, confirming the living
nature of the polymerization. The MALDI-TOF spectrum
of the PPV block shows one major peak at a mass/charge
ratio of 2209.8, corresponding to 7 repeating units. The Mn
and PD of PPV are calculated to be 2116 and 1.03, res-
pectively. After being clicked together, the resulting DCP
shows a Mn of 6644 and a PD of 1.04. The molecular weight
of PS-PPV DCP matches very well with the theoretical value
of 6712 (4596 þ 2116) calculated based on the Mn’s of the
two blocks. Taking the distributions of the PS-N3 block and
the PPV block and assuming a random reaction between
them, the theoretical statistical molecular weight distribu-
tion of the PS-PPV DCP is calculated and shown in Figure 6
as well. One sees a nice match between the theoretical
prediction and the actual experimental results, confirming
the diblock copolymer formation. It is noted that a lower
molecular weight distribution in the 1000-3000 range is
observed, which was initially attributed to any unreacted
PPV block. Repeated attempts in chromatography separa-
tion failed to remove these low molecular weight compo-
nents. Since GPC trace of PS-PPV does not show a corres-
ponding low molecular distribution, it is possible that those
low molecular weight components are fragments of PS-
PPV, generated during the MS measurements.
IR Spectra. Both 1H NMR and TGA analysis confirm
the existence of clusters in the hybrid DCP, and GPC
indicates that those clusters are attached covalently to the
DCP polymer. The cluster attachment is also confirmed

4002 Chem. Mater., Vol. 22, No. 13, 2010
Chakraborty et al.
Figure 9. Cyclic voltammograms of the PS-PPV DCP, free Mo6 cluster,
and the hybrid DCP.
Figure 8. IR spectra of the PS-PPV DCP, free Mo6 cluster, and the
hybrid DCP.
by IR measurements. As shown in Figure 8, the PS-PPV
DCP shows a sharp and intense peak at 1725 cm-1 which
can be assigned to the carbonyl stretching of the phtha-
limide protecting group. After deprotection and cluster
attachment, this peak is significantly weakened. On the
other hand, new peaks at 798 and 953 cm-1, which are
characteristic Mo-O stretching vibrations, are shown
clearly in the IR spectra of the hybrid DCP. One also
notes a side/shoulder peak next to 953 cm-1, which can be
seen more clearly when expanded. This peak is attributed
to the Mo-N stretching vibration.28b The observation of
this peak is a good indication that Mo6 clusters are
attached to the coil block.
Electrochemistry. Cyclic voltammetry measurements of
the DCP and hybrid DCP were carried out in acetontrile
using a Pt-disk working electrode coated with the respec-
tive polymer film. Under identical conditions, a reversible
oxidation wave at 0.32 eV is observed for the ferrocene/
ferrocenium couple. As shown in Figure 9, the cyclovol-
tammogram of the hybrid DCP shows clearly a reduction
process around -1.1 eV which does not exist in the
precursor DCP, and is cathodically shifted compared to
that of free [Mo6]. This reduction process can be attrib-
uted to the imido-POM clusters, which are known to
exhibit higher reduction potentials (more difficult to be
reduced) than the free Mo6 cluster.7,37 The clear observa-
tion of this reduction wave and the absence of reduction
process of free clusters indicate that all POM clusters in
the hybrid DCP are covalently bonded to the coil block
through the imido Mo-N bond. Both the PS-PPV DCP
and the hybrid DCP show a semi-reversible reduction
process at around -1.5 eV, which is due to the reduction
of the PPV block.
Figure 10. UV/vis absorption spectra of PS-Br, PPV-ꢀ, a 1:1 PS/PPV
mixture, and the PS-PPV DCP in dilute chloroform solutions.
Optical Properties. Figure 10 shows the absorption
spectra of the PS block, PPV block, a 1:1 PS/PPV mixture
and the “click” coupled DCP. The absorption spectrum
of the DCP closely matches that of the 1:1 PS/PPV
mixture in both the UV and the visible region, indicating
again a complete coupling between the two blocks.
It also implies that there are minimal electronic interac-
tions between the two blocks in their ground states. After
cluster attachment, a new peak at around 370 nm ap-
peared while the λmax in the visible region is blue-shifted
(Figure 11). The new peak at 370 nm matches well with
the absorption spectrum of the imido-POM derivative of
2,6-dimethylaniline,38 and can thus be assigned to the
ligand-to-metal charge-transfer (LMCT) transition of the
pendant imido-POM component. The observation of this
peak indicates clearly the formation of imido-functiona-
lized POM clusters, again confirming the covalent cluster
attachment. The absorption band in the visible region of
the hybrid DCP is due to the π-π* transition of the PPV
block. The slight blue shift of this band, compared to that
of PS-PPV DCP, indicates that the attachment of POM
clusters to the coil block has some effect on the effective
conjugation length of the PPV block. It is possible that the
(37) Strong, J. B.; Yap, G. P. A.; Ostrander, R.; Liable-Sands, L. M.;
Rheingold, A. L.; Thouvenot, R.; Gouzerh, P.; Maatta, E. A.
J. Am. Chem. Soc. 2000, 122, 639.
(38) Li, Q.; Wu, P.; Wei, Y.; Wang, Y.; Wang, P.; Guo, H. Inorg. Chem.
Commun. 2004, 7, 524.

Article
Chem. Mater., Vol. 22, No. 13, 2010 4003
Figure 11. UV/vis spectra of the PS-PPV DCP, the hybrid DCP, and the
2,6-dimethylaniline functionalized hexamolybdate cluster in dilute
chloroform solutions.
bulky cluster-containing PS block may twist the bridging
phenylene vinylene segment and thus decreases the effec-
tive conjugation length of the PPV block.
Both the DCP and hybrid DCP exhibit strong fluores-
cence in dilute solutions. Their fluorescence quantum
yields are 0.21 and 0.19, respectively, indicating negligible
fluorescence quenching in solution after cluster attach-
ment. This observation is consistent with our previous
report on side chain POM-containing conjugated poly-
mers where POM clusters exhibit inefficient fluorescence
quenching when they are linked to the conjugated back-
bone through flexible alkyl bridges.28b In the hybrid
DCP, the POMs are further away from the PPV donor
and thus photoinduced electron-transfer from PPV to
POM is kinetically hampered. In the solid film, however,
hybrid DCP does show much weaker fluorescence. Com-
pared to PPV and PS-PPV, whose films show FL quan-
tum yields of 0.48 and 0.34, respectively, hybrid DCP
shows a fluorescence quantum yield of only 0.09, indicat-
ing the quenching of 74% of the PPV fluorescence when
compared to PS-PPV. It is noted that while PS-PPV
shows identical fluorescence to that of PPV in dilute
solution, the emission of PS-PPV films is red-shifted by
15 nm while the hybrid DCP blue-shifted by 10 nm, as
shown in Figure 12. Clearly, the PPV segments aggregate
quite differently after the cluster attachment. Since the
PPV emission in the hybrid DCP is not as red-shifted as
that in PS-PPV, the observed fluorescence quenching
of hybrid DCP may not be attributed to a better PPV
π-stacking. The more plausible cause is the photoinduced
electron transfer from the PPV exciton to the POM clus-
ters. Such an electron transfer becomes facile in the solid
state because of the close proximity of the donor PPV of
one DCP and the acceptor POM clusters of another
DCP. In other words, the inefficient intrapolymer photo-
induced electron transfer in a dilute solution is replaced
with efficient interpolymer electron transfer in the solid
state, which accounts for the observation that the hybrid
DCP is highly fluorescent in dilute solutions but exhibits
much weaker fluorescence in the solid state.
Figure 12. Fluorescence emission spectra of PPV (black), PS-PPV (red),
and the hybrid DCP (PS-Mo6-PPV, blue) in dilute chloroform solutions
and as solid films.
and 1500 rpm. After drying in a vacuum oven overnight,
the films were sputter-coated with a thin layer of gold/
palladium. Field-emission scanning electron microscope
(SEM) XL30 (FEI Company, Hillsboro, OR) was used
with an accelerating voltage of 15.0 kV. The SEM images
(Figure 13) of the PS-PPV films show interesting micro-
porous structures. At a high spin rate, the pores have irre-
gular shape and varied sizes. At a spin rate of 1500 rpm,
the pores show a regular spherical shape and much more
uniform size. The average diameter of the pores is around
500 nm. While a number of rod-coil diblock copolymers
have been shown to give microporous films,42 such mor-
phologies are usually obtained by drop-casting with slow
evaporation of the solvent. The formation of microporous
films by spin-coating indicates that the PS-PPV DCP self-
assembles rather quickly as solvent evaporates. Insights
into the self-assembly process and the micropore forma-
tion mechanism require a detailed and systematic study of
how the block sizes, solvent, polymer concentration, spin
rate, thermal treatment, and so forth affect the film mor-
phologies. Such studies are currently in progress. After
cluster attachment, the resulting hybrid DCP yields films
with very different morphologies. As shown in Figure 13
(c and d), spin-coating ofa hybridDCP solution gives films
with an uneven surface and non-uniform coverage. While
one sees pits in the film and some areas of aggregated
lumps, the widespread pores seen in the DCP film are
clearly missing. When the smooth film area is magnified,
one sees no phase separated domains. It should be noted
Film Morphologies. Chloroform solutions of PPV, PS-
PPV, and the hybrid DCP (1 mg in 1 mL chloroform)
were spin-coated onto silicon wafer at a spin rate of 3000

4004 Chem. Mater., Vol. 22, No. 13, 2010
Chakraborty et al.
Figure 13. SEM images of PS-PPV films (a,b) and the hybrid DCP fims (c, d), spin-coated at spin rates of 3000 rpm (a and c) and 1500 rpm (b and d).
that the morphology of a spin-coated pristine film is
formed through a kinetic process rather than a thermo-
dynamic one, and thus it may be altered with time and
temperature. The actual morphology depends on several
factors like choice of solvent, annealing temperature, dry-
ing speed, and, above all, the degree of polymerization
of each block. In our preliminary morphology studies,
chloroform, agoodsolventfor both blocks, wasused asthe
solvent. It remains to be seen whether a good solvent for
only one of the blocks will be able to create any new type of
morphology. While no phase-separated morphologies were
observed for either the PS-PPV DCP or the hybrid DCP
films without prior thermal treatment, we are currently
exploring the film morphologies spin-coated from differ-
ent solvents, under different spin rates and after thermal
annealing at various temperatures.
To realize the desired morphology for photovoltaic ap-
plications, we are currently synthesizing POM-containing
DCPs with different block structures, longer block lengths,
and different volume fractions of each block.
Experimental Section
Materials. Triethylamine was distilled over CaH2. THF was
purified by distillation over sodium pellets and benzophenone. CuBr
was purified by washing consecutively with glacial acetic acid,
absolute ethanol and ethyl ether, and then dried under vacuum.
N,N,N0,N0,N00-pentamethyldiethylenetriamine (PMDETA, 99%),
methyl-2-bromopropionate, aniline, 1-methyl-2-pyrrolidinone, and
p-xylene were distilled in vacuo before use. All other chemicals were
purchased either from Aldrich or Acros and were used as received
unless otherwise stated. Compound 128b and 339 were prepared
according to literature procedures. Compound 4 was prepared in
three steps by benzylic bromination of 4-iodotoluene40 followed
by Arbuzov reaction and Sonogashira coupling reaction with
trimethylsilylacetylene.41
Conclusions
Instrumentation. All reactions were conducted under the
protection of nitrogen. The 1H and 13C NMR spectra were
collected on a Varian INOVA 400 MHz FT NMR spectrometer.
All samples were referenced to the deuterated solvents. GPC
measurements were performed at 30 °C on a Waters system
equipped with a Waters 410 differential refractometer, Waters
515 HPLC pump, and a styragel HR 4E column with THF as the
eluent. The calibration curve was determined by the use of five
polystyrene standards from 800 to 90,000. FT-IR spectra were
obtained from samples dispersed on KBr pellets on an IR100
FT-IR spectrometer (Thermo-Nicolet Co.). UV-vis absorption
spectra were measured on a Hewlett-Packard 8452A diode array
spectrophotometer, and photoluminescence spectra were mea-
sured using a Shimadzu RF-5301PC spectrofluorophotometer.
Fluorescence quantum yields for solution were determined
In conclusion, we have successfully prepared the first
POM-containing DCP through the post-polymerization
functionalization approach. Click chemistry was used to
join together a rigid PPV block and a flexible PS block to
form the precursor DCP. Both GPC and MALDI-TOF
measurements have confirmed the formation of the DCP
with a narrow molecular weight distribution. POM clus-
ter attachment was carried out in NMP, a solvent in
which both the precursor DCP and the Mo6 cluster are
soluble. The cluster attachment was confirmed by GPC,
IR, CV, and optical spectroscopy. The hybrid DCP shows
two absorption bands, one at 450 nm due to π-π*
transition of the PPV block, and the other at 370 nm
attributable to the ligand-to-metal charge transfer transi-
tion of functionalized Mo6 clusters. Spin-coated PS-PPV
films show microporous structures which are not observed
in the hybrid DCP films. Significant fluorescence quench-
ing is observed for the hybrid DCP because of photo-
induced electron transfer from the PPV exciton to POM
cluster, indicating its potential as photovoltaic materials.
(39) Gu, T.; Nierengarten, J.-F. Tetrahedron Lett. 2001, 42, 3175.
(40) Wilson, A. A.; Dannals, R. F.; Ravert, H. T.; Frost, J. J.; Wagner,
H. N. J. Med. Chem. 1989, 32, 1057.
(41) Xiao, J.; Li, J.; Li, C.; Huang, C.; Li, Y.; Cui, S.; Wang, S.; Liu, H.
Tetrahedron Lett. 2008, 49, 2656.
(42) (a) Lee, M.; Cho, B. -K.; Zin, W.-C. Chem. Rev. 2001, 201, 3869.
(b) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (c) Lin, C. L.;
Tung, P. H.; Chang, F. C. Polymer 2005, 46, 9304.

Article
Chem. Mater., Vol. 22, No. 13, 2010 4005
using quinine sulfate in 1 N H2SO4 (jfl ≈ 0.55) as the standard.
For films, diphenylanthracene (dispersed in PMMA film with
concentration less than 10-3 M, assuming PL efficiency of 0.83)
was used as the standard. Cyclic Voltammetry (CV) studies were
carried out in acetonitrile at room temperature under argon
protection using a BAS Epsilon EC electrochemical station
employing a 1 mm2 Pt disk as the working electrode, silver wire
as the reference electrode, and a Pt wire as the counter electrode.
[Bu4N]PF6 was the supporting electrolyte, and the scan rate was
50 mV/s. Polymer films were cast from chloroform onto the
Pt disk working electrode. Ferrocene was used as an internal
standard. Thermal analyses were performed on Shimadzu
TGA-60 at the heating rate of 10 °C/min. A Voyager DE Pro
(Perceptive Biosystems/ABI) MALDI-TOF mass spectrometer
was used for mass measurement, operating in both linear and
reflector mode. A mixture of silver trifluoroacetate/dithranol
(1, 8-dihydroxyanthrone) (1:25 w/w) was used as the matrix.
Sample Preparation for SEM. One mg of each of the three
polymer samples was dissolved separately in 1 mL of chloroform
solution inside a glovebox. Though PPV and PS-PPV were
readily soluble in chloroform, hybrid DCP were heated at
50 °C and stirred overnight under nitrogen protection to make
a homogeneous solution. The solutions were filtered through
0.45 μm PTFE filter prior to spin coating on silicon wafers at
spin rates of 3000 or 1500 rpm. All the films were dried in a
vacuum oven overnight and then sputter-coated with a thin
layer of gold.
dried at 50 °C under vacuum to give 1.2 g of product (yield 55%).
1H NMR (400 MHz, CDCl3, 25 °C): δ 10.43 (s, 1H), 7.48 (m,
16H), 7.15 (m, 14H), 6.71 (s, 2H), 4.04 (br), 2.22 (s, 3H), 1.84
(br), 1.53 (br), 1.35 (br), 0.89 (br). 13C NMR (CDCl3, 400 MHz):
δ 189.2, 156.3, 151.1, 127.1, 123.0, 110.4, 69.5, 31.7, 29.5,
26.0, 22.7, 14.1. Anal. Calcd. For C162H244O17: C, 78.91; H,
9.98. Found: C, 77.63; H, 9.92. GPC: Mn = 2208, PDI = 1.27.
MALDI MS: Mn = 2116, PDI = 1.03.
PPV-ꢀ. PPV-CHO (0.600 g) was added into a two-neck
flask, and the flask was evacuated and then backfilled with
nitrogen three times. Dry THF (20 mL) was then added,
followed by sodium hydride (0.08 g, 3.33 mmol). After stirring
at room temperature for 5 min, compound 4 (0.075 g, 0.297
mmol) in dry THF (5 mL) was then added dropwise to the
above solution, and the resulting reaction mixture was refluxed
for 16 h. The solution was then poured into water (50 mL) and
extracted with chloroform. The organic layer was washed with
water (100 mL) and dried over magnesium sulfate. After
stripping the majority of the solvent, the concentrated solution
was poured into methanol to yield the polymer as red solids,
1
which were dried at 50 °C under vacuum (0.700 g, 90%). H
NMR (400 MHz, CDCl3, 25 °C): δ 7.48 (m, 19H), 7.15 (m,
12H), 6.71 (s, 2H), 4.04 (br), 3.12 (s, 1H), 2.22 (s, 3H), 1.86 (br),
1.53 (br), 1.36 (br), 0.90 (br). 13C NMR (CDCl3, 400 MHz): δ
151.1, 132.4, 127.6, 126.3, 124.8, 123.2, 110.4, 69.5, 31.7, 29.5,
26.0, 22.7, 14.1. Anal. Calcd. For C169H248O16: C, 79.97; H,
9.85. Found: C, 76.40; H, 9.51. GPC: Mn = 2208, PDI = 1.27.
MALDI MS: Mn = 2110, PDI = 1.03.
Compound 2. Compound 1 (8.95 g, 23.7 mmol) and tetrakis
(triphenylphosphine) palladium (0) (0.522 g, 0.435 mmol) were
added into a 100 mL two-neck flask. The flask was evacuated
and backfilled with nitrogen three times. Tributyl (vinyl) tin,
(97%, 7.75 g, 24.4 mmol) and DMF were then added dropwise
to the flask with a syringe. The resulting mixture was stirred at
100 °C for 3 h and then poured into water. The solution was
extracted with methylene chloride (3 ꢁ 300 mL). The organic
layer was collected, washed with water (3 ꢁ 300 mL), and dried
over magnesium sulfate. The solvent was evaporated to yield a
yellow crude product, which was purified by column chroma-
tography (silica gel, hexane/methylene chloride 4:1 as the eluent)
to afford compound 2 as a white solid (4.00 g, 60%, mp 186-
PS-Br Block (ATRP of Compound 2). Compound 2 (0.500 g,
1.803 mmol) and CuBr (0.006 g, 0.041 mmol) were added to
flame-dried two-neck round-bottom flask. The flask was de-
gassed and backfilled with nitrogen three times, and left under
nitrogen. Then PMDETA (0.009 mL, 0.008 g, 0.041 mmol) was
added via a gastight syringe, followed by p-xylene (0.900 mL).
After addition, three freeze-pump-thaw cycles were per-
formed, and the mixture was allowed to stir at room tempera-
ture for 45 min under nitrogen. The solution became light green.
The flask was then placed into an oil bath at 110 °C, and
deoxygenated methyl-2-bromopropionate (0.005 mL, 0.007 g,
0.041 mmol) was added through a gastight micro liter syringe to
initiate the polymerization. After 11 h, the contents of the flask
were exposed to air to quench the polymerization. THF (30 mL)
was then added to the flask to dissolve the polymer. The solution
was then filtered through neutral alumina to remove the copper
catalyst. The resulting colorless polymer solution was concen-
trated and twice precipitated into 10-fold excess of methanol.
The white polymer was collected by vacuum filtration and dried
under vacuum for 12 h (0.600 g). 1H NMR (400 MHz, CDCl3,
25 °C): δ 7.80 (br), 7.71 (br), 6.69 (br), 3.42 (br), 2.03 (br), 1.7
(br), 1.00 (br). 13C NMR (CDCl3, 400 MHz): δ 167.0, 145.9,
136.2, 133.9, 132.2, 127.8, 123.4, 40.4, 18.0. Anal. Calcd. For
1
188 °C). H NMR (400 MHz, CDCl3, 25 °C): δ 7.95 (m, 2H),
7.79 (m, 2H), 7.21 (s, 2H), 6.67 (dd, J = 10.8 Hz, 17.6 Hz, 1 H),
5.75 (dd, J = 0.8 Hz, 17.6 Hz, 1H), 5.27 (dd, J = 0.8 Hz, 10.8 Hz,
1H), 2.14 (s, 6 H). 13C NMR (CDCl3, 400 MHz): δ 167.2, 138.6,
136.9, 136.1, 134.3, 131.9, 129.1, 126.3, 123.8, 115.0, 18.1. Anal.
Calcd. For C18H15NO2: C, 77.90; H, 5.45; N, 5.05. Found: C,
77.65; H, 5.33; N, 5.17.
PPV-CHO. Synthesis of Poly(2,5-Dihexyloxy-1,4-phenylene-
vinylene). Compound 3 (1.46 g, 4.68 mmol) was added into a
two-neck flask. The flask was evacuated and backfilled with
nitrogen three times. To the flask was added a sample of aniline
(0.480 g, 5.15 mmol) with a nitrogen-filled syringe. The reaction
mixture was stirred at 60 °C for 1.5 h. Potassium tert-butoxide
(1.05 g, 9.36 mmol) was then added into the above mixture,
followed by dry DMF (36 mL). The resulting reaction mixture
was stirred at 80 °C under nitrogen for 3 h. After being cooled
down to room temperature, the solution was poured into a
mixture of 150 mL of 1 N HCl and 200 mL of CHCl3 to
hydrolyze the aldimine end group. After the mixture was stirred
for 1 h, the organic layer was collected and washed with water
until the aqueous phase was neutral. The organic layer was dried
over sodium sulfate. After stripping off most of the solvent, the
concentrated residue solution was poured into a large excess of
acetone. The resulting dark red precipitates were filtered and
C
328H277N18O38Br: C, 76.36; H, 5.41; N, 4.88. Found: C, 74.44;
H, 4.67; N, 4.75. GPC: Mn = 2732, PD = 1.10. MALDI-TOF
MS: Mn = 4494, PD = 1.10.
PS-N3. PS-Br block (0.500 g, 0.164 mmol), sodium azide (0.032 g,
0.492 mmol), and DMF (4.00 mL) were added in a two-neck flask.
The resulting solution was stirred at room temperature for 14 h.
The reaction mixture was then precipitated into cold methanol
(50 mL). The white solid was collected by vacuum filtration and
washed with water. The solid was dried under vacuum. (0.450 g).
1H NMR (400 MHz, CDCl3, 25 °C): δ 7.80 (br), 7.71 (br), 6.65
(br), 3.37 (br), 2.03 (br), 1.70 (br), 0.93 (br). 13C NMR (CDCl3,
400 MHz): δ 167.0, 146.5, 136.3, 133.9, 132.2, 127.7, 123.5, 40.6,
18.1. Anal. Calcd. For C328H277N21O38: C, 76.90; H, 5.45; N, 5.75.

4006 Chem. Mater., Vol. 22, No. 13, 2010
Chakraborty et al.
Found: C, 75.40; H, 4.89; N, 5.21. GPC: Mn = 2700, PDI = 1.10.
MALDI MS: Mn = 4537, PDI = 1.11.
filtered off. The orange color filtrate was stripped off the solvent
and precipitated from methanol to yield an orange-red colored
powder. (0.040 g, 91%). 1H NMR (400 MHz, CDCl3, 25 °C): δ
7.41 (br), 7.11 (br), 6.25 (br), 4.04 (br), 3.45 (br), 2.06 (br), 1.85
(br), 1.54 (br), 1.35 (br), 0.89 (br).
PS-PPV. PS-N3 (0.092 g, 0.031 mmol) and CuBr (0.005 g,
0.035 mmol) were charged to a two-neck flask. The flask was
then evacuated and backfilled with nitrogen three times.
PMDETA (0.007 mL, 0.006 g, 0.035 mmol) and freshly distilled
THF (15 mL) were then added to the above mixture. To the
resulting light yellow homogeneous solution was added drop-
wise a THF solution of PPV-ꢀ (0.060 g, 0.024 mmol in 15 mL
of THF). The resulting red solution was stirred at 45 °C for 24 h.
The reaction mixture was filtered through neutral alumina to
remove the copper catalyst. The resulting red solution was
concentrated and precipitated into methanol (100 mL). The
crude red solid was collected by vacuum filtration and washed
with acetone to remove excess PS-N3. The red solid was dried
Hybrid DCP (PS-Mo6-PPV). In a two-neck flask were added
deprotected DCP (0.030 g) and DCC (0.090 g, 0.436 mmol). The
flask was degassed and backfilled with nitrogen. Hexamolybdate
2-
cluster, [Mo6O19] 2N(C4H9)4þ (0.900 g, 0.659 mmol) dissolved
3
in NMP (4 mL) was then added into the flask, and the reaction
mixture was stirred at 100 °C for 12 h. The resulting dark red
solution was cooled to room temperature, and the white solid
precipitate (urea) was filtered off. The solvent was distilled off
from the dark filtrate solution under vacuum, and the resulting
concentrated solutionwas pouredintohotacetonitrile toyieldthe
crude hybrid DCP, which was further washed with hot acetoni-
trile to get rid of any remaining free cluster. The resulting hybrid
DCP is a brownish red solid (0.035 g, 89%). 1H NMR (400 MHz,
CDCl3, 25 °C): δ 7.48 (br), 7.15 (br), 6.71, (br), 4.05 (br), 3.42(br),
1.85 (br), 1.66 (br), 1.54 (br), 1.30 (br), 1.05 (br), 0.89 (br).
1
under vacuum at 50 °C (0.182 g, 90%). H NMR (400 MHz,
CDCl3, 25 °C): δ 7.78 (br), 7.65 (br), 7.43 (br), 7.11 (br), 6.66
(br), 3.99 (br), 2.17 (br), 1.99 (br), 1.81 (br), 1.60 (br), 1.54 (br),
1.30 (br), 0.85 (br). 13C NMR (CDCl3, 400 MHz): δ 167.0, 151.0,
136.3, 133.9, 132.2, 127.6, 123.5, 110.4, 69.4, 40.6, 31.7, 29.5,
26.0, 22.7, 18.1, 14.1. Anal. Calcd. For C497H525N21O54: C,
77.80; H, 6.91; N, 3.44. Found: C, 77.44; H, 6.96; N, 2.80. GPC:
Mn = 5900, PDI = 1.22. MALDI MS: Mn = 6644, PDI = 1.04.
Deprotection of Phthalimide Group. PS-PPV DCP (0.050 g,
0.008 mmol) was dissolved in freshly distilled THF in a two-neck
flask. Then hydrazine monohydrate (0.200 g, 3.99 mmol) was
added dropwise into the above red solution with constant
stirring, and the entire reaction mixture was refluxed for 13 h
under nitrogen protection. The resulting solution was cooled to
room temperature, and the light orange-white precipitate was
Acknowledgment. This work is supported by the National
Science Foundation (DMR 0804158).
Supporting Information Available: 13C NMR spectra of
monomers and polymers, additional optical spectra (absorption,
fluorescence emission) of the polymers in both dilute solutions
and as solid films, additional SEM and AFM images. This
material is available free of charge via the Internet at http://
pubs.acs.org.