Chain-growth polymerization of unusual anion-radical monomers based on naphthalene diimide: A new route to well-defined n-type conjugated copolymers

Article abstract of DOI:10.1021/ja208710x

Strongly electron-deficient (n-type) main-chain π-conjugated polymers are commonly prepared via well-established step-growth polycondensation protocols which enable limited control over polymerization. Here we demonstrate that activated Zn and electron-deficient brominated thiophene-naphthalene diimide oligomers form anion-radical complexes instead of conventional Zn-organic derivatives. These highly unusual zinc complexes undergo Ni-catalyzed chain-growth polymerization leading to n-type conjugated polymers with controlled molecular weight, relatively narrow polydispersities, and specific end-functions.

Full text of DOI:10.1021/ja208710x

ARTICLE
pubs.acs.org/JACS
Chain-Growth Polymerization of Unusual Anion-Radical Monomers
Based on Naphthalene Diimide: A New Route to Well-Defined n-Type
Conjugated Copolymers
Volodymyr Senkovskyy,*,† Roman Tkachov,† Hartmut Komber,† Michael Sommer,‡ Maria Heuken,†
Brigitte Voit,† Wilhelm T. S. Huck,‡ Vladislav Kataev,§ Andreas Petr,§ and Anton Kiriy*,†
†Leibniz-Institut f€ur Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany
‡University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
§Leibniz Institute for Solid State and Materials Research Dresden, Helmholtzstrasse 20, 01069, Germany
S Supporting Information
b
ABSTRACT: Strongly electron-deficient (n-type) main-chain
π-conjugated polymers are commonly prepared via well-established
step-growth polycondensation protocols which enable limited
control over polymerization. Here we demonstrate that activated
Zn and electron-deficient brominated thiophene-naphthalene
diimide oligomers form anion-radical complexes instead of
conventional Znꢀorganic derivatives. These highly unusual zinc
complexes undergo Ni-catalyzed chain-growth polymerization leading to n-type conjugated polymers with controlled molecular
weight, relatively narrow polydispersities, and specific end-functions.
onjugated polymers are ideal candidates for printed organic
electronics, as they combine solution-processability with
utilizes highly toxic organostannyl derivatives. New synthetic
protocols that allow high performance n-type conjugated poly-
mers to be reproducibly prepared from nontoxic monomers via
controlled polymerization methods are therefore highly desirable.
We report here a novel nickel-catalyzed chain-growth poly-
merization of an unusual anion-radical monomer based on the
symmetric building block 2,6-bis(2-bromothien-5-yl)naphthalene-1,
4,5,8-tetracarboxylic-N,N0-bis(2-octyldodecyl) diimide (Br-TNDIT-Br),
polymerization of which leads to the corresponding bithiophene-
naphthalene diimide conjugated polymer P(TNDT) with low
polydispersity and specific end-functions (Scheme 1). Our initial
intention was to extend the scope of the chain-growth Kumada
catalyst transfer polycondensation (KCTP)9 toward electron-
acceptor monomers. This requires the design and preparation of
asymmetric AB-type monomers with a Grignard- and a halide-
function in the same molecule. Such monomers are commonly
prepared via magnesiumꢀhalogen exchange from the corre-
sponding dihalide precursors and alkyl magnesium halides.
Although electron-deficient aryl halides are usually more reactive
in halogenꢀmagnesium exchange reactions than electron-rich
ones,10 we found that this method fails to give the Grignard
monomer Br-TNDIT-MgBr upon the reaction of Br-TNDIT-Br
with Grignard compounds. Another method to prepare mono-
mers for the chain-growth polymerization of regioregular poly-
(3-alkylthiophene)s is Negishi polymerization (also referred to
as the “Rieke method”).11 Especially its variation which utilizes
C
film-forming properties.1 N-Type (or electron-conducting) poly-
mers are essential components in organic devices such as
ambipolar and n-channel field-effect transistors or organic
photovoltaics.2 In contrast to p-type (or hole-conducting) con-
jugated polymers, where a large variety of chemical structures has
been presented,3 a considerably smaller number of n-type
polymers is known to date. Within the limited number of reports
on n-type polymers,4 rylene diimide alternating main chain
copolymers are currently evolving as an intriguing class of
electron-conducting materials. Especially π-conjugated polymers
based on naphthalene diimide5,6 and perylene diimide7,8 have
demonstrated impressive progress in n-channel OFETs and all-
polymer OPV devices. For example, a bithiophene-naphthalene
diimide copolymer was reported to yield high electron mobilities
up to 0.85 cm2/(V s) under ambient conditions.5 Extending the
length of the rylene diimide aromatic core at the peri-positions
thus progressing from naphthalene diimide to perylene diimide
seems to be beneficial for all-polymer solar cells. Very recently,
Hashimoto et al. reported a record power conversion efficiency
of 2.2% for a polythiophene/carbazole-perylene diimide copoly-
mer blend.8 Although all these materials have shown outstanding
electronic properties, drawbacks with respect to control over
polymer architecture remain. Typically, conventional step-growth
polycondensations are used to synthesize rylene diimide-based
alternating copolymers, which result in broad polydispersities
(approximately 2ꢀ5) and limited control over polymer molec-
ular weight. Moreover, the often used Stille-coupling method
Received: September 15, 2011
Published: October 30, 2011
r
2011 American Chemical Society
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Scheme 1. Preparation of the Br-TNDIT-Br/Zn Radical-
Anion Monomer and Its Catalyst-Transfer Polymerization to
Ph/H P(TNDT)
deficient Br-TNDIT-Br occurs which leads to a radical-anion
(Scheme 1).
Electron paramagnetic resonance (EPR) measurements of the
Br-TNDIT-Br/Zn complex confirmed our hypothesis. The pre-
sence of the intense electron paramagnetic resonance signal at
around g = 2.0035 corroborated the formation of the Br-TNDIT-
Br anion-radical (Figure S3, Supporting Information).12 The
fine-structure of the EPR spectrum showed features consistent
with the coupling of the unpaired electron and the naphthalene
diimide nitrogen. The formation of a paramagnetic species also
corroborates the fact that addition of Zn to Br-TNDIT-Br leads
to the complete disappearance of the aromatic signals in the 1H
NMR spectrum, while the signals corresponding to the alkyl
chains are still observed (not shown).
Titration experiments with iodine confirmed the 1/1 stoichi-
ometry of the Br-TNDIT-Br/Zn complex, irrespective of
whether an equimolar amount or an excess of Zn was added.
Such a stoichiometry is highly surprising since it formally
corresponds to the unusual oxidation state +1 of Zn. Unlike
mercury, which has an extensive +1 oxidation state chemistry,
zinc usually adopts +2. We found only a single report in the
literature describing decamethyldizincocene, Zn2(η5-C5Me5)2,
in which Zn formally adopts +1 due to the formation of
zincꢀzinc bonds.13 By analogy, we hypothesize that the Br-
TNDIT-Br/Zn complex dimerizes to form a zincꢀzinc bond,
which yields [(Br-TNDIT-Br)1ꢀ]2[ZnꢀZn]2+. More studies are
necessary to investigate the structure and properties of this highly
unusual complex. It is also worthy to mention that the prepara-
tion of the charge-transfer complex Br-TNDIT-Br/Zn via mixing
Br-TNDIT-Br with an excess of active Zn followed by filtration is
very convenient because it eliminates the need to adjust the
precise 1/1 stoichiometry, which otherwise is experimentally
difficult.14
The polymerization of the Br-TNDIT-Br/Zn complex was
attempted next using various nickel complexes. Surprisingly
enough, the addition of Ni(dppe)Br2 or Ph-Ni(dppe)Br to Br-
TNDIT-Br/Zn resulted in its rapid polymerization (a few hours
until complete conversion) at room temperature. The resulting
dark-blue polymer (for UVꢀvis, see Figure S4, Supporting
Information) was readily soluble in chloroform. Detailed analysis
activated Zn for the generation of organozinc monomers from
aryl dihalides (i.e., BrꢀArꢀZnX) is interesting here and was
applied in order to generate polymers from Br-TNDIT-Br.
Reacting Br-TNDIT-Br with equimolar amounts of activated
Zn prepared by the reduction of ZnCl2 with sodium naphthale-
nide resulted in an immediate color change from red-orange
(inherent to Br-TNDIT-Br) to deep-green, accompanied by the
immediate disappearance of the solid phase. The resulting Br-
TNDIT-Br/Zn complex was completely soluble in THF (it passed
through a 200 nm membrane filter whereas active Zn suspension
did not). It is remarkable that the acidic workup of the thus-
prepared Br-TNDIT-Br/Zn complex resulted in quantitative
recovering of Br-TNDIT-Br but not of Br-TNDIT-H. This
indicates that the organo-zinc compound Br-TNDIT-ZnBr was
not formed under these conditions because otherwise hydrolysis
of Br-TNDIT-ZnBr should lead to Br-TNDIT-H. Thus, the
reaction of Br-TNDIT-Br with active Zn did not lead to the
usual monomer for Negishi/Rieke polymerizations. The green
Br-TNDIT-Br/Zn complex remained stable under oxygen- and
moisture-free conditions at least for several days at room
temperature and during prolonged heating (at 60 °C); neither
insertion of Zn into the CꢀBr bond nor the reduction
of the imide groups occurred under these conditions. We
propose that single electron transfer from Zn to the electron-
1
of H and 13C NMR spectra revealed a perfectly regioregular
connection of the monomer units in the polymer structure via the
2-position of the thiophene rings (Figures S5,6, Supporting
Information).5 To clarify whether this polymerization involves
the step-growth or the chain-growth mechanism, several experi-
ments were performed at different initiator/monomer ratios. It
was found that the polymerization allows a satisfactory control
over molecular weight (MW) in a broad range of the feed ratio
(i.e., from 1/10 to 1/70 (Figure 1). Although an accurate determi-
nation of MW for rigid-rod, yet prone to aggregation, polymers is
challenging, it is obvious from Figure 1 that lowering of the
Ni(dppe)Br2/monomer ratio leads to a substantial increase of
MW of the resulting polymer. Thus, polymers with Mn GPC = 25
kg/mol and 104 kg/mol and polydispersities of ∼1.3 and 1.7
were obtained for feed ratios of 1/10 and 1/50, respectively. A
small shoulder on the main polymeric peaks in GPC elution
curves can be explained either by a slow initiation process or by
concurrent chain-termination/reinitiation processes. The poly-
dispersity could be further narrowed by washing the raw products
with acetone and hexane.
The successful polymerization with high conversion further
corroborates the 1/1 Br-TNDIT-Br/Zn stoichiometry, because
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Figure 1. GPC elution curves of crude P(TNDT)s obtained upon
polymerization of Br-TNDIT-Br/Zn in the presence Ni(dppe)Br2 at the
following ratios: [initiator]/[monomer] = 1/10 (black); 1/30 (green);
1/50 (blue); 1/70 (red).
ZnBr2 is formed as leaving group.15 As no insertion of Zn into the
thiopheneꢀbromine bond of Br-TNDIT-Br was observed, the
discovered chain-growth polycondensation cannot be classified
as Negishi/Rieke-type.
1H NMR end-group analysis revealed the presence of a Ph-
starting group in P(TNDT) for polymerization initiated by Ph-
Ni(dppe)-Br (Figure S7, Supporting Information). Control
experiments demonstrated that the Ph-moiety is not incorpo-
rated into the polymer structure in Ni(dppe)Br2-initiated poly-
merizations, if Ph-Ni(dppe)Br is added after monomer consumption.
Thus, the presence of the Ph-moiety in the polymer structure is
an indication of the chain-growth polymerization mechanism. It
is also worthy to note that polymeric products with relatively
high MW are formed early in the polymerization reaction
(at low monomer conversion)16 which further supports the
chain-growth mechanism.17
Figure 2. (a) 31P NMR and (b) 1H NMR spectrum (aromatic region)
of the initiating Br-TNDIT-Ni(dppe)-Br complex (#, P(TNDIT)
signals; ꢁ, naphthalene from Zn monomer synthesis; see Figure S8,
for 2D spectra). (c) 1H NMR spectra of the reactants Br-TNDIT-Br and
(d) Ni(dppe)Br2 (solvent: THF-d8).
1
In situ 31P and H NMR studies are a powerful tool in the
investigation of KCTP.18 Such experiments were performed in
this work in order to get insight into the mechanism of
polymerization in more detail. Figure 2a depicts the 31P spectrum
after the addition of 1 equiv Br-TNDIT-Br/Zn to a solution of
Ni(dppe)Br2 in THF-d8. Two doublets at 61.3 and 45.8 ppm
(JPP ∼ 33 Hz) are characteristic for monoaryl Ni(dppe) com-
plexes.18,19 Obviously, the complex Br-TNDIT-Ni(dppe)-Br,
which is typical for the initiation of KCTP, was formed. Further
evidence is given by the comparison of the 1H NMR spectra of
the reaction product and the reactants (Figure 2bꢀd). While the
reactants could not be observed any more, the product showed
two proton signals for the NDI moiety (Hc, Hd), indicating its
nonsymmetric substitution. The two doublets at 7.13 and 7.04
ppm (Ha, Hb) correspond to the bromine-substituted thiophene
moiety. The second thiophene moiety results in two signals at
7.22 and 6.50 ppm (He, Hf) as approved by a COSY spectrum
(Figure S8a, Supporting Information). The signal at 6.50
ppm appears as a triplet, which is caused by PꢀH coupling as
doublets at 61 and 46 ppm (Figure S9b). As the latter doublets
were the only signals after the full consumption of Br-TNDIT-
Br/Zn, and because they appeared in the same region as the
structurally similar Br-TNDIT-Ni(dppe)-Br initiator complex,
we assign the doublets at 61 and 46 ppm to the propagating chain
end P(TNDT)-Ni(dppe)-Br. Signals at 49ꢀ46 ppm may be
attributed to either the Ni(0) complex between P(TNDT)-Br
and Ni(dppe) species or to P(TNDT)-Ni(dppe)-TNDIT-Br;
both are usual intermediates in catalyst-transfer polycondensa-
tions formed before and after the OA step, respectively. Further
identification efforts are still needed, and they are under way in
our lab.
A possible mechanism of the Ni(dppe)Br2-initiated polymer-
ization of Br-TNDIT-Br/Zn is given in Scheme 2. A catalytic
cycle includes oxidative addition (OA) and reductive elimination
(RE) elementary steps, the usual for metal-catalyzed coupling
reactions.20 Because the polymerization displays chain-growth
behavior, we propose that most of the Ni(0) species undergo
intramolecular OA (IVfV, Scheme 2), i.e., catalyst-transfer
previously observed in polycondensations of electron-rich
monomers.9,21 In catalyst-transfer polycondensations, trans-
metalation (TM) is an important step of the catalytic cycle
1
confirmed by a correlation peak in the 31Pꢀ H heterocorrelated
spectrum (Figure S8b). Assuming that 4JPH > 5JPH in this complex,
we assign this signal to Hf.
Adding several equivalents of Br-TNDIT-Br/Zn to Ni-
(dppe)Br2 resulted in the formation of closely located signals
in the 31P NMR spectrum between 49 and 46 ppm (Figure S9a,
Supporting Information). The intensity of these signals decreased
with polymerization time at the expense of two new distal
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Scheme 2. A Plausible Mechanism of Ni-Initiated Catalyst-
Transfer Polymerization of Br-TNDIT-Br/Zn (dppe ligand is
omitted)
the case here. The structure of the NDI-based anion-radical
monomer formed upon mixing Br-TNDIT-Br and active Zn is
equally intriguing, as it involves Zn in the highly unusual +1
oxidation state and because any unreacted complex monomer
can be recovered as the starting precursor Br-TNDIT-Br. Further
efforts will be directed to investigate the exact structure of the
anion-radical monomer and the details of the mechanism of
polymerization, the fine-tuning of which we expect to lead to a
plethora of novel and multifunctional conjugated polymer topol-
ogies when combined with additional other monomers.
’ ASSOCIATED CONTENT
S
Supporting Information. Description of instrumenta-
b
tion and materials used, experimental details for preparation of
the monomer and polymerization procedure, NMR spectro-
scopic data, and discussion of the polymerization mechanism.
This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
senkovskyy@ipfdd.de; kiriy@ipfdd.de
’ ACKNOWLEDGMENT
V.S., R.T., and A.K. thank the DFG for financial support (SPP
1355 “Elementary Processes of Organic Photovoltaics”, project
KI-1094/4-1). M.H. is thankful for funding from Saxon State
Excellence Cluster ECEMP supported by EFRE. W.H. and M.S.
acknowledge funding from the EPSRC (grant number RG51308).
responsible for the addition of monomers to the growing
polymer chain. Usually, TM involves a nucleophilic attack of
the carbanionic center of the monomer onto electrophilic Ni
along with elimination of ZnBr2. Because there is no well-defined
carbanionic center in the Br-TNDIT-Br/Zn, the TM here cannot
follow the usual mechanism. We tentatively propose that the first
step of a “quasi-transmetalation” process operative here involves
a single electron transfer (SET) from Zn+ to Ni(II) with a
concomitant elimination of Brꢀ leading to Ni(I) species (IfII,
Scheme 2). Ni(I) complexes are known as possible intermediates
in the Kumada coupling.22 The next step is an addition of
ArꢀNi(I) to the terminal carbon of the monomer (IIfIII).
Finally, the aromaticity of the system is recovered by eliminatioꢀn
of Brꢀ and Zn2+ (IIIfIV). Taking into account that one Br
was eliminated in a preceding step, the last elimination completes
the formation of ZnBr2, a usual byproduct in Negishi coupling.
However so far, we have no solid experimental evidence of the
proposed mechanism, and this is a subject of ongoing works in
our lab.23
In conclusion, we have presented a highly unusual Ni-cata-
lyzed chain-growth polymerization of NDI-based anion-radical
monomers leading to high-performance n-type conjugated poly-
mers P(TNDIT) with controlled molecular weight, relatively
narrow polydispersity, and specific end-functions. To the best of
our knowledge, this is the first report on the controlled chain-
growth polymerization of highly electron-deficient monomers
leading to high-performance n-type conjugated polymers. This
result is outstanding, taking into account the large length of the
monomer (approx 1.5 nm) and its highly polarized structure.
Intuitively, one could expect that both these factors hamper the
intramolecular catalyst-transfer process, which obviously is not
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