Synthesis of a soluble poly(fluorenone)

Article abstract of DOI:10.1039/b516078b

The synthesis and characterisation of a soluble poly(fluorenone) is presented, a polymer with high electron affinity with potential for use in plastic electronic devices as an n-type material. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b516078b

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Synthesis of a soluble poly(fluorenone){
Luke Oldridge, Marcel Kastler and Klaus M u¨ llen*
Received (in Cambridge, UK) 14th November 2005, Accepted 15th December 2005
First published as an Advance Article on the web 19th January 2006
DOI: 10.1039/b516078b
The synthesis and characterisation of a soluble poly(fluorenone)
is presented, a polymer with high electron affinity with potential
for use in plastic electronic devices as an n-type material.
Our approach is to attach a bulky alkylated tetra-phenyl benzene
unit to the 4-position to increase solubility and processability. The
attached alkyl substituents have been previously shown to enhance
the solubility of strongly aggregating materials. The synthesis of a
soluble 2,7-poly(9-fluorenone), which can be performed on a large
scale, is shown in Scheme 1.
The field of polymer optoelectronics has witnessed a great deal of
progress since the discovery of the conducting properties of
1
polyacetylene in 1977. Derivatives such as poly(9,9-dialkyl-
Commercially available 2,7-dibromo-fluorene was nitrated
24
fluorene)s have received considerable attention as candidates for
the active material in optoelectronic devices such as organic light-
emitting diodes (OLEDs), due to their good chemical stability and
according to literature procedures to generate 1.
The
Clemmensen reduction of 1 followed by a Sandmeyer reaction
gave the 2,7-dibromo-4-iodo-fluorene 3. 3, together with tetra-
butylammonium hydroxide as a phase transfer catalyst, was
dissolved in pyridine and oxidized with oxygen gas to the tri-halo
fluorenone. A Hagihara–Sonogashira cross coupling reaction
allowed the coupling of 2,7-dibromo-4-iodo-fluorenone with
trimethylsilyl acetylene, which was deprotected to yield 5. Diels–
Alder reaction of 5 with a dialkyl substituted tetraphenyl
cyclopentadienone gave the tetraphenyl benzene dendrimer sub-
stituted fluorenone monomer, 6. 2,7-Poly(9-fluorenone) (7) was
obtained by Yamamoto polymerization of 6 according to the
standard procedure.
2,3
high luminescence quantum efficiency. The majority of emitting
polymers currently in use are largely hole-transporting in
character, i.e. p-type materials. In comparison, there are relatively
few reports on electron-deficient conjugated polymers, in other
words conjugated polymers with low lying conduction bands and
4
,5
high electron affinity (EA). Polymers of this nature would not
only be useful in OLEDs as electron transport materials (ETM)
for increased electron injection from the cathode and as hole
blockers, but could also serve as an n-type material for use in
photovoltaic devices.
The properties that govern the effectiveness of a polymer for use
as an ETM include its EA, ionization potential (IP), electron
mobility, and electrochemical and thermal stabilities. The most
widely studied organic structural types among ETMs are those
7
was soluble in common organic solvents such as toluene and
THF. The gel permeation chromatography (GPC) of 7 showed a
monomodal distribution with a number-average molecular weight
4
M ) of 2.6 6 10 g mol (poly-para-phenylene standard). This
n
corresponds to a number average degree of polymerization of
approximately 20 repeat units. Thermogravimetric analysis (TGA)
indicated that 7 has good thermal stability up to 400 uC. The
differential scanning calorimetry (DSC) analysis demonstrated that
21
(
6–10
containing oxadiazoles,
11–15
matics,
various N-containing heteroaro-
16,17
and perfluorinated aromatic materials.
One
promising class of polymers that has received less attention,
mainly due to the inability to synthesize readily processable
materials, are aromatic polyketones. Fluorenone has been
7
has an amorphous structure with no phase transition between
18,19
incorporated in various copolymers in the past.
The
homopolymer 2,7-poly(9-fluorenone) was first reported as an
unprocessable film deposited on an electrode after electrochemical
2
0
polymerization. The preparation of 2,7-poly(9-fluorenone) films
from a spin cast soluble precursor (polyfluorene ketal) resulted in a
promising ETM, however it involved a polymer analogous
21,22
deprotection to give the insoluble ketone.
Fluorenone is
known to act as a charge-trapping, low energy defect in
polyfluorene based materials; however, this is not the issue for
2
3
the fluorenone homopolymer. Herein, we describe the synthesis
of the first highly soluble 2,7-poly(9-fluorenone) 7, which is easily
processable and has good film forming properties.
As polyfluorene is typically solubilized with substituents at the 9
position, an alternative substitution is required for 9-fluorenone.
Max-Planck-Institute for Polymer Research, Postfach 3148, D-55021
Mainz, Germany. E-mail: muellen@mpip-mainz.mpg.de;
Fax: +49 6131 379100; Tel: +49 6131 3790
{ Electronic supplementary information (ESI) available: concentration
dependent photoluminescence spectroscopy, experimental procedures and
characterization for all described compounds. See DOI: 10.1039/b516078b
Scheme 1 Synthesis of a dendronised 2,7-poly(9-fluorenone). i) HCl, Sn,
EtOH reflux; ii) H SO , NaNO , 0 uC, then KI, 100 uC; iii) (C NOH,
, pyridine, r.t.; iv) TMS-acetylene, CuI, PPh , Pd(PPh , r.t.; v) K CO
THF, water, r.t.; vi) substituted tetraphenyl cyclopentadienone, o-xylene,
150 uC; vii) Ni(COD) , COD, bipy, toluene, DMF, 80 uC.
2
4
2
4 9 4
H )
O
2
3
3
)
4
2
3
,
2
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2100 uC and 350 uC, due to the bulky tetraphenyl benzene
substituents.
The UV/vis absorption and fluorescence spectra of 7 in a dilute
THF solution and a thin film drop cast from a toluene solution are
shown in Fig. 1. The polymer showed two maxima in the
absorption spectrum at 367 nm and 456 nm in solution. The higher
energy maximum at 367 nm corresponds to the p–p* transition of
the polyphenylene backbone, while the weaker absorption at lower
energies corresponds to the symmetry forbidden n–p* transition of
2
1
the carbonyl function. The hypsochromic shift of the p–p*
transition suggested that the sterically hindered polymer is highly
twisted. Although this negatively effects the conjugation of the
polymer, the interchain charge transport would not be adversely
affected. The thin film absorption spectrum showed a typical
bathochromic shift relative to the solution absorption spectrum.
The fluorescence emission maxima of 7 in solution and thin film
were 524 nm and 554 nm, respectively (excitation wavelengths
Fig. 2 Differential pulse voltammogram (reduction) of 7 vs. AgQRE
4
(film on gold, 0.1 M TBAClO in acetonitrile).
for reduction. In comparison with a prevalent ETM, 2-(4-
biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), with a
EA of 22.4 eV, the EA of 23.0 eV of 7 results in a reduced
energy barrier to electron injection from the cathode, e.g. Mg (W =
365 nm and 360 nm, respectively). The emission spectrum of 7 in
solution showed a shoulder at 550 nm, which we attribute to
excimer formation as the shoulder emission is at the same position
as the thin film emission maximum. This is supported by
concentration dependent photoluminescence spectroscopy, which
demonstrated the relative increase of the intensity of the emission
at 550 nm with increasing concentration (see ESI{). There is no
observed emission from the polyphenylene backbone, indicating
complete energy transfer to the carbonyl chromophore. The
spectroscopic properties of 7 were comparable to the previously
published spectra of fluorenone containing polyfluorene materials.
In order for 7 to be considered an ETM, it must show a
2
5
23.7 eV). Other electron deficient polymers that have been used
in the past as the ETM lack redox stability. Cyano substituted
PPV, CN-PPV, has been successfully used as the ETM in an
26,27
OLED, but shows an irreversible reduction.
In summary, we present the synthesis of a soluble poly-
fluorenone. By attaching a bulky tetraphenylenebenzene at the
4-position of fluorenone, a soluble polymer can be prepared with
good film forming properties. This negates the need for any
undesirable protection and deprotection steps in the synthesis. The
differential pulse voltammogram of the polymer demonstrated
that the reduction of the carbonyl group is fully reversible.
Investigations into the charge carrier mobility of 7, and its
incorporation into photovoltaic devices, are currently in progress.
The authors acknowledge financial support from the Zentrum
5
reversible reduction. The differential pulse voltammogram of a
film of 7 is presented in Fig. 2. The differential pulse
voltammogram exhibited a reversible reduction with a reduction
potential of 21.29 V vs. AgQRE for 7. An estimation of the EA is
possible from the reduction potential by calibration with ferrocene/
ferrocenium standard, which has a vacuum energy level of 24.8 eV.
This resulted in an EA of 23.0 eV for 7. The EA of
polyfluorenone prepared from the ketal precursor was found to
f u¨ r
Funktionseinheiten (BMBF 03N 6500), EU-TMR project
SISITOMAS, the Deutsche Forschungsgemeinschaft
Schwerpunkt Feldeffekttransistoren). M.K. thanks the Fond der
Multifunktionelle
Werkstoffe
und
Miniaturisierte
(
2
1
be 23.3 eV. Polymer 7 may not be as easy to reduce compared to
the insoluble polyfluorenone due to the tetraphenylbenzene side
chain, which shields the ketone moiety, making it less susceptible
Chemischen Industrie and the Bundesministerium f u¨ r Bildung und
Forschung for financial support. We thank Dr J. Robertson for
recording the differential pulse voltammogram.
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