Polyfluorenes without monoalkylfluorene defects

Article abstract of DOI:10.1021/ja074634i

A synthetic route to 9,9-dioctyl-9H-fluorene (free of monoalkyl-substituted fluorene defects) by alkylative cyclization of 9-biphen-2-ylheptadecan-9-ol is described. Polyfluorenes prepared by Yamamoto and Suzuki polymerization of the defect-free monomers, when incorporated in light-emitting devices, exhibited minimal green emission attributable to fluorenone formation. By contrast, devices fabricated with polyfluorene arising from conventionally synthesized 9,9-dioctyl-9H-fluorene showed significant emission at 533 nm, even at the turn-on voltage. Incorporation of as little as 0.06 mol % of 9-octyl-9H-fluorene comonomer in polyfluorene led to evolution of green fluorescence in the photoluminescence (PL) and electroluminescence (EL) emission spectra. Copyright

Full text of DOI:10.1021/ja074634i

Published on Web 09/07/2007
Polyfluorenes without Monoalkylfluorene Defects
Sung Yong Cho,†,‡ Andrew C. Grimsdale,‡,§ David J. Jones, Scott E. Watkins, and
‡
‡
,
‡,#
Andrew B. Holmes*
MelVille Laboratory for Polymer Synthesis, Department of Chemistry, UniVersity of Cambridge, Lensfield Road,
Cambridge CB2 1EW, United Kingdom, School of Chemistry, Bio21 Institute, UniVersity of Melbourne, Victoria
3010, Australia, and Department of Chemistry, Imperial College, South Kensington, London SW7 2AZ, United
Kingdom
Received June 25, 2007; E-mail: aholmes@unimelb.edu.au
Scheme 1. Synthesis of Monomers 6a and 6ba
Polymer-based light-emitting diodes (PLEDs) are one of the most
important emerging technologies in the field of organic electronics.1
Poly(dialkylfluorene)s have played a leading part in the evolution
of the commercial technology, although the presence of emissive
fluorenone defects formed by oxidation of monoalkylfluorene
2
3
impurities is a disadvantage. In this paper, an alkylative cyclization
route to defect-free 9,9-dioctyl-9H-fluorene is described. Polyfluo-
renes prepared by Yamamoto and Suzuki polymerization of the
defect-free monomers, when incorporated in light-emitting devices,
exhibited minimal green emission attributable to fluorenone forma-
tion. By contrast, devices fabricated with polyfluorene arising from
conventionally synthesized 9,9-dioctyl-9H-fluorene showed sig-
nificant emission at 533 nm, even at the turn-on voltage. Incorpora-
tion of as little as 0.06 mol % of 9-octyl-9H-fluorene comonomer
in polyfluorene led to evolution of green fluorescence in the
photoluminescence (PL) and electroluminescence (EL) emission
spectra.
a
Reagents and conditions: (a) C8H17Br, Mg, ether, 0-25 °C, 36%; (b)
n-BuLi, THF, -78 °C, 4, then 25 °C; (c) BF3‚Et2O, CH2Cl2, 25 °C, 12 h,
9
7% (5a), 80% (5b over two steps); (d) Br2, I2, CH2Cl2, 0-25 °C, 3 h,
In this work, two routes to the fluorene 5 were developed
74% (see Supporting Information).
(
Scheme 1). Both involved cyclization of the tertiary alcohol 2,
Scheme 2. Synthesis of Polyfluorenes 8a-da
with Route 2 being preferred as it guarantees the incorporation of
both octyl substituents before the alkylative cyclization step. The
dibromofluorenes 6a and 6b (Scheme 1) and the bis(boronate) 7b
(Scheme 2) were prepared by standard methods.
The effect of complete dialkylation in the fluorene monomers
6a and 6b compared with conventionally prepared 9,9-dioctyl-9H-
fluorene 6c was evaluated by preparation and luminescence studies
of the corresponding polymers. Poly(9,9-dialkyl-9H-fluorene)s
8a-c were prepared from fluorene monomers 6a-c, respectively,
by Yamamoto coupling reactions with phenyl group end-capping
4
using bromobenzene. Polymer 8d was prepared by Suzuki poly-
condensation of the dibromide 6b and the bis(boronate) 7b using
Pd(PPh
and was also end-capped with phenyl substituents. Polymers (M
20 000-40 000) were obtained with polydispersities ranging from
.7 to 3.5. The optical properties of polymer films 8a-d are
3 4 4
) and 20% aqueous Et NOH in toluene as catalyst system
5
w
a
)
1
Reagents and conditions: (a) Ni(COD)2, 2,2′-bipyridyl, COD, DMF,
toluene, 75 °C, 24 h, bromobenzene, 12 h; (b) Pd(PPh3)4, toluene, 20% aq
Et4NOH, 90 °C, 24 h, bromobenzene, 12 h, phenylboronic acid, 12 h.
a-series, prepared from 1 (Route 1, Scheme 1); b-series, prepared from 3
described in Figure S1 (see Supporting Information).
Polymers 8a, 8b, and 8d, prepared from the fully dialkylated
monomers, exhibited striking improvements over the conventional
material 8c in simple electroluminescent devices which were
fabricated with the configuration ITO/PEDOT:PSS (75 nm)/polymer
(
Route 2, Scheme 1); c-series, prepared by conventional dioctylation of
2,7-dibromo-9H-fluorene.
polymer-coated substrates were transferred to a glovebox for
deposition of the cathode and testing. All devices were made and
tested in parallel such that the only variable between devices was
the synthetic origin of the emissive layer. In all devices, electrolu-
minescence arose from polymer 8 aligned in the â-phase. All
devices showed turn-on voltages of about 4 V and maximum
brightnesses of between 80 and 140 cd/m . The EL spectra of 8a,
8b, and 8d are very similar with all devices appearing blue (Figure
1
8
(65 ( 5 nm)/LiF (0.2 nm)/Al (100 nm). Deposition of the polymer
layers was deliberately carried out in air to accelerate the possible
degradation of the devices and to emphasize the differences between
the fully dialkylated and conventional material. Once annealed, the
2
†
University of Cambridge.
‡
University of Melbourne.
§
Present address: School of Materials Science and Engineering, Nanyang
a). The EL spectra of 8a, 8b, and 8d were all stable up to a drive
Technological University, Nanyang Avenue, 639798 Singapore.
#
Imperial College.
voltage of 10 V. By contrast, the conventionally synthesized
11910
9
J. AM. CHEM. SOC. 2007, 129, 11910-11911
10.1021/ja074634i CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Synthesis of Statistical Copolymers 10 and 11a
a
Reagents and conditions: (a) Ni(COD)2, 2,2′-bipyridyl, COD, DMF,
toluene, 90 °C, 24 h, bromobenzene, 12 h.
related to the electrochemically generated defects discussed by
Montilla and Mallavia.9
In summary, fully dialkylated polyfluorenes suppress the emis-
sion attributed to ketone defects in PL and EL of polyfluorenes.
The present work has shown that as little as 0.06 mol % of
monoalkylated fluorene comonomer can lead eventually to the
emergence of long wavelength emission, and there is no evidence
of ketone formation from fully 9,9-dialkylated polyfluorenes. These
findings support the previous report by the Eindhoven group that
treatment with base and subsequent chromatography can remove
6
monoalkylfluorene impurities from the dialkyl compound. The
Figure 1. (a) Electroluminescence spectra of polymers 8a, 8b, 8c, and 8d.
latter is a pragmatic approach to removing putative monoalkyl
impurities from dialkylfluorenes (as are conventional purification
techniques) but can only be validated in the final polymeric product,
whereas Grignard coupling of the iodobiphenyl 3 with the ketone
(
b) Photoluminescence spectra of polymer 11: pristine (-), 12 h at 110 °C
(
+), 24 h at 110 °C (0). (c) Electroluminescence spectrum of polymer 11.
Device configuration of ITO/PEDOT:PSS (70 nm)/polymer (65 nm)/LiF
0.2 nm)/Al (100 nm). EL spectra are at their turn-on voltage (4 V). For
(
PL and EL data for 10, see Figures S3 and S4.
4
unambiguously guarantees the incorporation of two octyl groups
in the 9-position of the fluorene 5b.
polymer 8c exhibited significant emission from a low-energy band
at 533 nm in a device, even at its turn-on voltage. This behavior,
which became even more significant with time and at higher drive
voltages, is fully consistent with the emergence of ketone defects
attributed to incomplete alkylation at the 9-position in a fraction
Acknowledgment. We thank the Australian Research Council,
CSIRO, and the Victorian Endowment for Science, Knowledge and
Innovation (VESKI) for generous financial support. We thank the
EPSRC National Mass Spectrometry Service (UK) for provision
of some mass spectra. This Communication is dedicated to the
memory of Dr. Carl R. Towns, Cambridge Display Technology
Limited.
6
of fluorene units in the polymer chain and is distinct from the
7
underlying band in pristine polyfluorene at around 530 nm. The
photoluminescence emission spectra of the polymers 8a-c when
thermally annealed at 110 °C for 12 and 24 h (Figure S1) mirrored
the observations reported in the electroluminescent devices. In other
words, complete dialkylation in polyfluorenes drastically suppresses
the green emission in both PL and EL.
In order to confirm unequivocally that the long wavelength green
emission in polyfluorene-based devices correlated with the presence
of monoalkylfluorene defects and to provide a qualitative assessment
of the proportion sufficient to cause this effect, we prepared the
statistical copolymers 10 and 11 containing, respectively, 0.06 and
Supporting Information Available: General experimental methods
for the preparation and characterization of 1-11 as well as UV-vis
spectra and TGA data (8a-d, 10, 11) and PL and EL spectra for
polymers 8a-d and 10. This material is available free of charge via
the Internet at http://pubs.acs.org.
References
(
1) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998,
37, 403. See Supporting Information for further references.
2) List, E. J. W.; G u¨ ntner, R.; Scandiucci de Freitas, P.; Scherf, U. AdV.
Mater. 2002, 14, 374.
(
0.6 mol % of monooctylfluorene (Scheme 3).
Monooctylfluorene monomer 9 was prepared in good yield by
(3) For alkylative cyclization approaches to ladder polymers and fluorenes,
8
see: (a) Scherf, U.; Bohnen, A.; M u¨ llen, K. Makromol. Chem. 1992, 193,
selective monoalkylation using octan-1-ol and base, followed by
1
127. (b) Scherf, U. J. Mater. Chem. 1999, 9, 1853. (c) Elmahdy, M. M.;
bromination. The copolymers 10 (M
w
) 71 000) and 11 (M
w
)
Floudas, G.; Oldridge, L.; Grimsdale, A. C.; M u¨ llen, K. ChemPhysChem
2
006, 7, 1431. (d) Amara, J. P.; Swager, T. M. Macromolecules 2006,
64 600) were prepared by controlling the feed ratios of the
3
9, 5753 and references cited therein.
monomers 6b and 9 in Yamamoto polymerizations. The PL spectra
of the pristine films indicated that the polymer changed from the
amorphous phase to the â-phase after thermal treatment.
After the polymers had been thermally annealed at 110 °C in
air for 12 and 24 h respectively (Figure 1b, 11; Figure S3, 10),
they both showed clear evidence in the PL of the presence of the
low-energy band at 533 nm. The long wavelength emission in EL
devices fabricated with polymers 10 and 11 was not as prominent
as that of 8c but evolved over time and with increasing drive
voltages (see Figure 1c and Figure S4). This evolution may be
(
(
4) Yamamoto, T. Prog. Polym. Sci. 1992, 17, 1153.
5) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby, C.
E.; K o¨ hler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc. 2004,
126, 7041.
(6) Craig, M. R.; de Kok, M. M.; Hofstraat, J. W.; Schenning, A. P. H. J.;
Meijer, E. W. J. Mater. Chem. 2003, 13, 2861.
7) Ryu, G.; Xia, R.; Bradley, D. D. C. J. Phys.: Condens. Matter 2007, 19,
056205.
(
(
8) Evans, N. R.; Devi, L. S.; Mak, C. S. K.; Watkins, S. E.; Pascu, S. I.;
K o¨ hler, A.; Friend, R. H.; Williams, C. K.; Holmes, A. B. J. Am. Chem.
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