Toward stable interfaces in conjugated polymers: Microporous poly(p-phenylene) and poly(phenyleneethynylene) based on a spirobifluorene building block

Article abstract of DOI:10.1021/ja801691x

Conjugated polymer networks based on spirobifluorene building units exhibit defined photoluminescence as well as pronounced microporosity, that is, large, stable interfaces. Copyright

Full text of DOI:10.1021/ja801691x

Published on Web 04/24/2008
Toward Stable Interfaces in Conjugated Polymers: Microporous
Poly(p-phenylene) and Poly(phenyleneethynylene) Based on a Spirobifluorene
Building Block
Jens Weber* and Arne Thomas*
Max Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry,
Research Campus Golm, D-14424 Potsdam, Germany
Received March 6, 2008; E-mail: jens.weber@mpikg.mpg.de; arne.thomas@mpikg.mpg.de
Organic electronics is an important field of modern technology.
Organic light emitting diodes (OLED) and organic solar cells are
the most prominent among the various applications. Often, their
performance relies on an intimate contact between a charge transport
material and an optoelectronic active polymer.1 Recent attempts
toward the control of the morphology included blending, block
copolymer approaches, solvent, and thermal annealing, as well as
hard templating approaches.1–7
assisted synthesis turned out to be crucial for the successful
quantitative and reproducible synthesis of the p-phenylene type
networks.20
P3 was synthesized in quantitative yield from 2 and 1,4-
diethynylbenzene (5), employing known reaction conditions.15
The purified polymer networks were analyzed by means of IR
spectroscopy, gas sorption, X-ray scattering, electron microscopy,
and photoluminescence (PL) measurements.
Characterization of the networks by IR spectroscopy, elemental
analysis, and energy dispersive X-ray spectroscopy (EDX) con-
firmed the chemical identity of the samples.
Microporous materials have shown to be beneficial in a broad
variety of both industrial and academic applications. However, the
synthesis of purely organic, noncarbonaceous, microporous materi-
alswasuptonowmainlyrestrictedtohypercrosslinkedpolystyrenes.8,9
During the last years, a broader variety of microporous polymers
(e.g., polybenzodioxanes,10 polyimides, and11,12 polyanilines13) and
crystalline organic frameworks14 was introduced by various groups.
Recently, also the first microporous conjugated polymers, that is,
microporous poly(aryleneethynylene)s, were synthesized.15
No palladium residues could be observed by EDX. If the
electrooptic performance of such polymers would suffer from
undetected Pd traces, known chemical methods for the removal of
Pd should be applicable due to the microporosity of the polymers.21
The condensation reactions were not fully complete. Traces of
bromine and oxygen, originating from unreacted end groups, were
detected. This fact is not surprising, as a full condensation is not
possible if the reaction is performed under kinetic control. Wide-
angle X-ray scattering revealed the amorphous nature of the
networks. Scanning electron microscopy showed that P1 and P2
are produced as small spheres (ca. 200 nm), which agglomerate
and react to larger structures. P3 shows a fiber-like structure on
the micrometer scale.
Poly(p-phenylene)-type polymers are another class of conjugated
polymers that have promising properties regarding their application
as OLED material. It is known that blending of them with charge
transport materials can enhance the electroluminescence efficiency
if phase separation can be excluded.1 A potential pathway is,
therefore, the mixing of a microporous host material with dopants
to form stable interpenetrating network (IPN).
The analysis of the networks by nitrogen sorption confirmed their
microporous nature. Figure 1c shows the nitrogen sorption iso-
therms. For all polymers, a hysteresis is observed at low relative
pressures. This feature is due to elastic deformations in the course
of nitrogen sorption, that is, the networks are swelling.12 Therefore,
the networks can be regarded as soft matter in comparison to stiff,
inorganic materials like zeolites. The hysteresis is largest for P2,
indicating a lower degree of cross-linking. This is in accordance
with the low surface area of P2 (SBET: 210 m2 g-1). P1 and P3
show a smaller hysteresis and can, therefore, be regarded as more
rigid. (SBET: 450 m2 g-1 and 510 m2 g-1, respectively.)
Although the microporous nature of the networks is confirmed
by the steep uptake of nitrogen at very low relative pressures, it is
not possible to determine reliable or comparable pore size distribu-
tions (PSD). The swelling effects are neither considered in state-
of-the-art methods like nonlinear density functional theory nor in
traditional methods like Horvarth-Kawazoe, rendering these
methods physically untrustworthy.22
Here we present the synthesis of microporous poly(p-phenylene)s
based on the concept of intrinsic microporosity. These networks
can be regarded as promising hosts for IPNs.
9,9′-Spirobifluorene (1) is a well-known monomer for the
synthesis of materials for organic electronics,16 as well as for
microporous polymers.11,12 It incorporates a 90° kink in every
repeating unit, which prevents the otherwise stiff polymer chains
from space efficient packing. Therefore, a very high, accessible
free volume is obtained. Polymers based on this concept are,
therefore, named polymers of intrinsic microporosity (PIM).10,17
2,2′,7,7′-Tetrabromo-9,9′-spirobifluorene (2), which is readily
available from 1,18,19 was used as the structure directing monomer
in the synthesis of three different conjugated polymers. Figure 1
shows the chemical structure of the monomers as well as two 3D
views of the spatial structure of 2. The tetrahedral carbon atom
directs the two fluorene moieties into a cross-like shape, giving
rise to a contorted polymer structure.
Specific surface areas of microporous polymers can also be
determined by small-angle X-ray scattering (SAXS).12 The mea-
surements give specific surface areas of 1230 m2 g-1 (P1), 860
m2 g-1 (P2), and 1030 m2 g-1 (P3), respectively (see Supporting
Information). The mismatch between the surface areas determined
by different methods points to the presence of closed or nonac-
cessible porosity.
Reaction of 2 with benzene-1,4-diboronic acid (3) in a mixture
of N,N-dimethylformamide (DMF) and water (20 vol %) under the
presence of palladium acetate, triphenyl phosphine, and sodium
bicarbonate gave P1. Reaction of 2 with 4,4′-biphenyldiboronic acid
(4) under the same conditions gave P2. The synthesis of P1 and
P2 was performed under microwave heating. The microwave-
9
6334 J. AM. CHEM. SOC. 2008, 130, 6334–6335
10.1021/ja801691x CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. (a) Synthetic pathway towards the conjugated polymer networks P1-P3, (i) Br2, cat. FeCl3, CHCl3, room temperature, (ii) cat. Pd(OAc)2, PPh3,
NaHCO3, DMF/water (4/1), µ-wave, 150 °C, (iii) cat. Pd(OAc)2, PPh3, CuI, toluene/Et3N (1/1), 80 °C; (b) two 3D views on the structure of 2,2′,7,7′-
tetrabromospirobifluorene (2); (c) nitrogen sorption isotherms of the networks P1-P3.
Acknowledgment. We are grateful to J. Schmidt, Dr. P. Kuhn,
and R. Rothe for helpful comments. Financial support was provided
by the ENERCHEM project (Max Planck Society).
Supporting Information Available: Synthetic procedures, analytical
data, photographs, SAXS, and PSD analysis. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Figure 2 summarizes the optical properties of the materials. The
photoluminescence (PL) emission spectra of P1 and P2 show a
maximum at λ ) 460 nm (blue emission, Figure S3).
No secondary emission peak could be observed, indicating an
effective prevention of aggregation. This in agreement with earlier
experiments on spirobifluorene-based light-emitting materials.16
It is worth mentioning that there is nearly no influence of the
comonomer on the optical properties. The correlation length in both
polymers is therefore higher than the effective correlation length,
indicating a rather high degree of condensation.
The PL spectrum of P3 is dominated by a peak at 400 nm,
originating from the spirobifluorene building block. A shoulder is
observed at 525 nm, which is in accordance with spectra reported
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Detailed information regarding the applied models is also given there.
In conclusion, we have shown that conjugated polymers based
on a spirobifluorene building unit possess microporosity, unifying
two important concepts of material science in one material. This
approach allows the introduction of a large, stable interface into
materials with high potential for use in organic electronics. Current
work is performed on the application of the concept on film forming
materials, allowing the fabrication of advanced devices.
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