Polyfluorenes with polyphenylene dendron side chains: Toward non-aggregating, light-emitting polymers

Article abstract of DOI:10.1021/ja0031220

A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption-and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been demonstrated to produce blue emission with onset voltages below 4 V.

Full text of DOI:10.1021/ja0031220

946
J. Am. Chem. Soc. 2001, 123, 946-953
Polyfluorenes with Polyphenylene Dendron Side Chains: Toward
Non-Aggregating, Light-Emitting Polymers
Sepas Setayesh,† Andrew C. Grimsdale,† Tanja Weil,† Volker Enkelmann,† Klaus Mu1llen,*,†
Farideh Meghdadi,‡ Emil J. W. List,‡ and Gu1nther Leising‡
Contribution from the Max-Planck-Institute for Polymer Research, Ackermannweg 10,
D-55128 Mainz, Germany, and Institute for Solid State Physics, Technical UniVersity Graz,
Petergasse 16, A-8010 Graz, Austria
ReceiVed August 21, 2000
Abstract: A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress
formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption-
and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra
torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to
determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been
demonstrated to produce blue emission with onset voltages below 4 V.
Introduction
Scheme 1. Blue-Emitting Materials: Poly-p-phenylene (PPP)
1, Polytetrahydrophenanthrene (PTHP) 2, Polyfluorene (PF)
3, 2,8-Polyindenofluorene (2,8-PIF) 4, Ladder-Type
Poly-p-phenylene (LPPP) 5
The development of blue-light emitting polymers has been
the subject of intense academic and industrial research directed
toward the fabrication of full color organic displays. Poly-p-
phenylene (PPP) 1 and its derivatives have the large HOMO-
LUMO energy gaps required for obtaining blue emission.1,2
However, as yet, no organic polymer fully meets the desired
criteria of long-term stability, high efficiency, and bright blue
emission that are essential for commercially viable light emitting
diodes (LEDs). The realization of electrically pumped organic
laser devices is becoming feasible with conjugated materials2b
if they are intrinsically stable and able to carry high electrical
current densities. To synthesize well-defined PPPs, solubilizing
groups have been attached to the phenylene repeating units.3
The introduction of such side chains however, dramatically
reduces the conjugation between the phenylene units with a
resultant hypsochromic shift of the emisssion wavelength. One
of the great advantages of polytetrahydropyrene (PTHP) 2,4
polyfluorene (PF) 3,5,6 poly-2,8-indenofluorene (PIF) 4,7 and
ladder-type PPP (LPPP) 58 in comparison to soluble PPPs is
their ability to introduce solubilizing side chains without
hampering the conjugation within the aromatic π-system
(Scheme 1).
Although these polymers exhibit remarkably high photo-
luminescence quantum efficiencies as well as good thermal and
chemical stability, their utility in electroluminescence (EL)
devices is restricted by their tendency to form aggregates either
during annealing or passage of current, leading to red-shifted
and less efficient emission.9-11 To minimize their tendency to
aggregate, several statistical copolymers of 212 and 59 with
various conjugated and nonconjugated monomers have been
prepared. However, aggregate formation could not be fully
prevented in these copolymers.
† Max-Planck-Institute for Polymer Research.
‡ Technical University Graz.
(1) (a) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl.
Phys. 1991, 30, L1941. (b) Grem, G.; Leditzky, G.; Ulrich, B.; Leising, G.
AdV. Mater. 1992, 4, 36. (c) Marsitzky, D.; Mu¨llen, K. In AdVances in
Synthetic Metals, Twenty Years of Progress in Science and Technology;
Bernier, P., Lefrant S., Bidan G., Eds.; Elsevier: New York, 1999; p 1.
(2) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402. (b) Kranzelbinder, G.; Leising, G. Rep. Prog. Phys.
2000, 63, 729.
Miller et al.13 have used semiflexible poly(benzyl ether)
(Fre´chet-type)14 dendrimers as endcapping reagents in the Ni-
(8) Scherf, U.; Mu¨llen, K. Macromol. Chem., Rapid Commun. 1991, 12,
489.
(3) Rehahn, M.; Schlu¨ter, A.-D.; Wegner, G. Macromol. Chem.1990,
191, 1991.
(4) Kreyenschmidt, M.; Uckert, F.; Mu¨llen, K. Macromolecules 1995,
28, 4577.
(5) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. AdV. Mater.
1997, 9, 798.
(6) Fukuda, M.; Sawada, K.; Yoshino, K. J. Polym. Sci. Polym. Chem.
1993, 31, 2465.
(7) Setayesh, S.; Marsitzky, D.; Mu¨llen, K. Macromolecules 2000, 33,
2016.
(9) Huber, J.; Mu¨llen, K.; Salbeck, J.; Schenk, H.; Scherf, U.; Stehlin,
T.; Stern, R. Acta Polym. 1994, 45, 244.
(10) Lemmer, U.; Heun, S.; Mahrt, R. F.; Scherf, U.; Hopmeier, M.;
Sieger, U.; Go¨bel, E. O.; Mu¨llen, K.; Ba¨ssler, H. Chem. Phys. Lett. 1995,
240, 373.
(11) Grell, M.; Bradley, D. D. C.; Ungar, G.; Hill, J.; Whitehead, K. S.
Macromolecules 1999, 32, 5810.
(12) Kreyenschmidt, M.; Kla¨rner, G.; Fuhrer, T.; Aschenhurst, J.; Karg,
S.; Chen, W. D.; Lee, V. Y.; Scott, J. C.; Miller, R. D. Macromolecules
1998, 31, 1099.
10.1021/ja0031220 CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/16/2001

Polyfluorenes with Polyphenylene Dendron Side Chains
J. Am. Chem. Soc., Vol. 123, No. 5, 2001 947
Scheme 2. Synthesis of the Precursor Polymer 9a
(0)-mediated polymerization of 2,7-dibromo-9,9-dihexylfluorene
to suppress aggregate formation in the resultant polymers. It
was demonstrated that for polymers carrying fourth generation
dendrimers intermolecular π-stacking and consequent aggregate
formation were inhibited, but as yet there have been no devices
reported using these polymers. In contrast to Fre´chet-type
dendrimers, polyphenylene (Mu¨llen-type) dendrimers15,16 are
both shape-persistent and chemically stable. Therefore, they
exclude π-stacking at lower dendritic generations than the
Fre´chet type. Their three-dimensional shielding ability has been
demonstrated using perylene dyes as the core molecule.17
The idea of combining the encapsulation of a linear polymeric
emitter with a dendritic structure led us to synthesize 12, a 2,7-
PF which carries in the side chain polyphenylene dendrons that
not only serve as solubilizing groups but also permit spatial
control of the polymer chain and hinder aggregation. To
determine the minimal steric requirements for prevention of
aggregation, we also prepared materials bearing side chains with
varying degrees of bulk.
a a) NaOH (50%), 1-bromomethyl-4-iodo-benzene, benzyl trimethyl-
ammonium chloride, DMSO, 2 h, rt, 73 %; b) RC2H, Pd(PPh3)2Cl2,
Cu(I)I, THF, triethylamine, rt, 4 d, 8a: 69 %, 8b: 72 %, 8c: 75%; c)
Ni(cod)2, cyclooctadiene (cod), 2,2′-bipyridyl, DMF, toluene, 3 d, 80°.
Results and Discussion
To synthesize the polymer 11 two different approaches were
envisaged. Since the Diels-Alder reaction of tetracyclopenta-
dienone with acetylene proceeds quantitatively, one might
synthesize the precursor polymer 9 that could then be den-
dronized in a polymer-analogous step, although complexation
of Ni(0) with the ethynyl groups might limit the reaction.18,19
Alternatively, one could first synthesize the dendritic monomer
11 via the reaction of tetraphenylcyclopentadienone with the
diacetylene 10 and then polymerize it under Ni(0)-mediated
(Yamamoto) conditions. In this case the bulky substituents at
the 9-position might limit the degree of polymerization.20
The reaction of 2,7-dibromofluorene 6 with 4-iodobenzyl
bromide with sodium hydoxide as base and benzyltrimethyl-
ammonium chloride as phase-transfer catalyst21 produced the
diaddition product 7 in 73% yield. A selective Pd(0)-mediated
Hagihara-Sonogashira coupling22 of 7 with trimethyl- or
triisopropylsilylacetylene afforded the desired 2,7-dibromo-9,9-
di(4-trimethylsilylethynylbenzyl)fluorenes 8 in good yields.
Unfortunately, due to the interaction of the acetylene groups
with the Ni(0), neither polymers nor oligomers of 8a could be
obtained from the Yamamoto polymerization. Using the much
bulkier triisopropyl-substituted 8b gave only oligomers with up
to five repeat units in very low yield (Scheme 2).
Scheme 3. Synthesis of the Shielded Polyfluorene 12aa
a a) K2CO3, THF, MeOH, 3 h, rt, 98 %; b) tetraphenylcyclopenta-
dienone, o-xylene, 12 h, reflux, 82 %; c) tetraphenylcyclopentadienone,
o-xylene, 5 d, reflux, 85 %; d) Ni(cod)2, cod, 2,2′-bipyridyl, DMF,
toluene, 3 d, 80 °C, bromobezene, 12 h, 80°, 75%.
Consequently the dendrimer forming Diels-Alder reaction
had to be performed before the polymerization step. Deprotec-
tion of 8a with potassium carbonate in methanol-THF produced
an almost quantitative yield of the diacetylene 10. The Diels-
Alder reaction of 10 with tetraphenylcyclopentadienone in
refluxing o-xylene gave the monomer 11a in 82% yield (Scheme
3). Coupling of 7 with phenylacetylene gave the bisdiarylacetyl-
ene 8c (72%), which reacted with tetraphenylcyclopentadienone
to give the hexaphenyl monomer 11b in 85% yield. The
polymerization of 11a occurred with Ni(0) under Yamamoto
conditions for 3 days. To remove any remaining bromine
endgroups, bromobenzene was added, and the mixture was
allowed to react for an additional 12 h.. The resulting polymer
12 exhibits good solubility in toluene, benzene, and chlorinated
organic solvents and shows excellent film-forming properties.
Polymerization of 11b did not proceed under these conditions.
Gel permeation chromatography (GPC) analysis (toluene,
polystyrene standards) of 12 shows a monomodal distribution
with a weight-average molecular weight (Mw) of 1.6 × 105
g/mmol and a number-average molecular weight (Mn) of 4.6 ×
104 g/mmol (PD ) 3.6), corresponding to 42 repeat units.
However, it is well established that GPC misrepresents the Mn
value of rigid rod polymers.4 Bradley et al. determined an
(13) Kla¨rner, G.; Miller, R. D.; Hawker, C. J. Polym. Prepr. 1998, 39-
(2), 1006.
(14) Fre´chet, J. M. J. Science 1994, 263, 1710.
(15) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99,
1747.
(16) Morgenroth, F.; Reuter, E.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl.
1997, 36, 631.
(17) Herrmann, A.; Mu¨llen, K., unpublished results.
(18) Bonrath, W.; Po¨rschke, K. R.; Wilke, G.; Angermund, K.; Kru¨ger,
C. Angew. Chem., Int. Ed. Engl. 1988, 27, 833.
(19) Ferrara, J. D.; Tessier-Youngs, C.; Youngs, W. J. J. Am. Chem.
Soc. 1985, 107, 6719.
(20) Yamamoto, T. Prog. Polym. Sci. 1992, 17, 1153.
(21) Woo, E. P.; Inbasekaran, M.; Shiang, W.; Roof, G. R. Int. Pat. Appl.
WO 97/05184, 1995.
(22) (a) Sonogashira, K.; Tohda, Y., Hagihara, N. Tetrahedron Lett. 1975,
50, 4467. (b) Takahashi, S.; Kunoyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627. (c) Stephens, E. B.; Tour, J. M. Macromolecules 1993,
26, 2420.

948 J. Am. Chem. Soc., Vol. 123, No. 5, 2001
Setayesh et al.
Figure 1. Absorption and emission spectra of polymer 12.
overestimation factor of 2.7 for poly(9,9-dioctylfluorene) by
comparing the Mn values of light scattering with those of the
GPC.23
Thermogravimetric analysis (TGA) shows the excellent
thermal stability of the polymer 12. Initial weight loss (5%)
was observed at 400 °C, with the main decomposition occurring
at 426 °C (50%). Differential scanning calorimetry (DSC)
exhibits only a phase transition at 248 °C. As polarizing optical
microscopy shows no evidence for a thermotropic liquid
crystalline phase, we attribute this to a glass transition. This
result stands in contrast to the behavior of the octyl and
ethylhexyl substituted polyfluorenes which show liquid crystal-
line phases with transition temperatures at 156 and 167 °C,
respectively.24,25 The absence of a thermotropic phase and the
high Tg are caused by the rigid pentaphenylbenzene groups and
indicate the effective supression of the orientation in the polymer
main chain.
The UV-vis spectrum of the polymer 12 in chloroform
solution exhibits an absorption maximum at 387 nm, while the
fluorescence spectrum (λexc ) 380 nm) displays a vibronic fine-
structure with two sharp bands at 419 and 443 nm (Figure 1).
The value for absorption and emission maxima of the shielded
PF 12 are similar to those for the alkyl substituted PF. This
implies that the bulky groups in the 9-position do not increase
the torsion angle in the conjugated polymer backbone. Such a
limitation in conjugation would cause a hypsochromic shift in
the absorption and emission spectra as has been demonstrated
for copolymers of unsubstituted and 2,5-dihexyl substituted
PPP.26-28 The emission maximum (λexc ) 380 nm) of a drop-
casted film of the polymer 12 is only 6 nm bathochromically
shifted in comparison to the wavelength of the dissolved
polymer and does not show any sign of aggregate formation.
In comparison the films prepared via spin coating of poly(9,9-
dioctylfluorene) solution in chloroform by Bradly et al. exhibit
two vibronic peaks at 427 and 448 nm, followed by a long
featureless tail extending into the red.11 The emission spectra
from the film of polymer 12 did not change even after thermal
Figure 2. ORTEP diagram of 11b determined by X-ray.
treatment at 100 °C for 24 h. Thermal treatment of poly(9,9-
dioctylfluorene) causes an even higher bathochromic shift of
the emission wavelengths (430, 452, 487, 520, and a shoulder
at 560 nm).11 The absorption and emission spectra of 12 did
not change even after addition of methanol to the chloroform
solution (chloroform/methanol: 1/1). These results indicate that
the steric impact of the dendritic substituents suppresses the
intermolecular π-stacking of the polymer without undesired
blue-shifts in the emission spectra.
By contrast, in the first reported combination of dendrimers
with conjugated polymers by Schlu¨ter et al.,29 who employed
Fre´chet-dendrons as side chains on PPP to prepare macrocyl-
inders that formed micelles, vesicles, or membranes, the
sterically demanding dendritic side chains led to strong torsion
about the phenyl-phenyl bonds. Such increased torsional strains
have been avoided by Aida et al.30 and by Bao et al.31 who
made copolymers of poly(phenylene ethynylene) (PPE) and
poly(phenylene vinylene) (PPV), respectively, with the dendritic
side chains attached only to every second phenylene unit. In
these cases, although no evidence of increased phenylene-
phenylene torsion was seen, the aggregation was also not totally
suppressed.
The packing of polymer 12 in the solid state was found to
be amorphous by X-ray diffraction experiments, with two broad
correlation peaks being observed at d ) 4.4 Å (average C-C
intermolecular distance) and 7.3 Å (average interchain dis-
tance).32 To better examine the steric effects of the dendrimer
side chains in the solid state, the crystal structure of the
hexaphenyl monomer 11b was obtained. Single-crystals of 11b
were obtained by slow evaporation of chloroform from methanol-
chloroform solution. The monomer builds up cocrystals with
chloroform. Figure 2 shows the ORTEP plot of 11 accomplished
by X-ray diffraction at 150 K.32 The hexaphenylbenzene groups
stand perpendicular to the fluorene unit. Therefore, the conceived
polymer backbone is efficiently shielded by the dendritic side
groups (Figure 3).
(23) Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.; Inbaseka-
ran, M.; Woo, E. P.; Soliman, M. Acta Polym. 1998, 49, 439.
(24) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.;
Nothhofer, H.-G.; Scherf, U.; Yasuda, A. AdV. Mater. 1999, 11, 671.
(25) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. AdV.
Mater. 1997, 9, 798.
(26) Rehahn, M.; Schlu¨ter, A.-D.; Wegner, G. Macromol. Chem. 1990,
191, 1991.
(27) Elsenbaumer, R. L.; Shacklette L. W. Handbook of Conducting
Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1,
p 213.
(28) Grimme, J.; Kreyenschmidt, M.; Uckert, F.; Mu¨llen, K.; Scherf, U.
AdV. Mater. 1995, 7, 292.
(29) Schlu¨ter, A. D.; Rabe, J. P. Angew. Chem., Int. Ed. 2000, 39, 864.
(30) Sato, T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10658.
(31) (a) Bao, Z.; Amundson, K. R.; Lovinger, A. J. Macromolecules 1998,
31, 8647; Jakubiak, R.; Bao, Z.; Rothberg, L. J. Synth. Met. 2000, 114, 61.
(32) See Supporting Information.

Polyfluorenes with Polyphenylene Dendron Side Chains
J. Am. Chem. Soc., Vol. 123, No. 5, 2001 949
Furthermore, MM+ minimization performed on an alkyl-
substituted PF dimer and trimer indicated torsion angles of 15°
along the polymer backbone. By contrast, reported calculations
on the quantum mechanics level HF, 6-31G for unsubstituted
fluorene dimer and trimer revealed a torsion angle in the range
of 40°,33a but these results cannot be directly compared with
ours, as different methods were used. Our calculated torsion
angles are consistent with the observed absorption and emission
spectra, as larger torsion angles would lead to a blue-shift in
the absorption and emission maxima of 12 compared with other
polydialkylfluorenes.33b
To further model the effects of the dendron side chains on
the packing of the polymer chains, the properties of well-defined
oligomers will need to be studied. To this end we have
synthesized a dimer 19 (Scheme 4). Reaction of 2-bromofluo-
rene 13 and 4-iodobenzylbromide under phase-transfer condi-
tions gave the dialkylated product 14 in 83% yield. Hagihara-
Sonogashira coupling with trimethylsilylacetylene followed by
hydrolysis with potassium carbonate gave a mixture of the
desired diacetylene 16 (60%) and the triacetylene 17 (15%)
which were readily separated by chromatography on silica. The
Diels-Alder reaction of 16 with tetraphenylcyclopentadiene
proceeded smoothly to give the bisdendronised-mono-bromo-
fluorene 18 in 85% yield. This material should be useful as a
bulky endcapping reagent and in the synthesis of oligomers.
Nickel(0) coupling of 18 gave the dimer 19 in 81% yield.
Attempts are underway to obtain a crystal structure of 19 for
comparison with the calculated structure.
Figure 3. Crystal structure of 11b determined by X-ray.
The benzyl linkages in polymer 12 are potentially susceptable
to photooxidation, thus introducing oxygenated functionality into
the polymer, which is known to be the major cause of polymer
degradation and device failure in conjugated polymer based
LEDs.2a Also, it is not clear that the effective shielding of the
backbone requires so bulky a substituent. To this end we have
investigated the effects of introducing smaller substituents
lacking benzyl protons into the 9-position. It was decided to
prepare the poly(9,9-diarylfluorene)s 20. The synthetic route
to 20 is shown in Scheme 5. Treatment of methyl 4,4′-
dibromobiphenyl-2-carboxylate 2234 with 2 equiv of phenyl-
lithium gave the carbinol 23a (70%) which was ring-closed with
sulfuric acid in acetic acid to give 2,7-dibromo-9,9-diphenyl-
fluorene 24a in 89% yield. Yamamoto nickel(0) coupling of
24a gave the desired poly(9,9-diphenyl-2,7-fluorene) 20a in high
yield. However, the polymer proved to be totally insoluble in
all organic solvents tried. Soxhlet extraction over 5 days with
toluene gave a small (10% of total mass) soluble fraction which
from the mass spectrum was a mixture of the pentamer and
tetramer. The UV-vis and fluorescence spectra showed maxima
at 368 nm for absorption and at 415 and 435 for emission in
toluene solution. Kla¨rner and Miller35 report emission maxima
in THF solution of 425 and 434 nm for the tetramer and
pentamer of 9,9-dihexylfluorene, while Lee and Tsutsui36 report
an emission maximum of 435 for the tetramer in chloroform
solution. A MALDI-TOF analysis of the insoluble bulk material
(details to be published separately37) shows a mass range from
1900 to 8000 Da, corresponding to degrees of polymerization
Figure 4. Calculated structure of the dimer.
The structure of the monomeric fluorene building unit (12, n
) 1) has been optimized using the MM2 (MM+) force field,
as implemented in HyperChem 5.1 (Hypercube Inc.)*. A force
field method was preferred over the application of quantum
mechanics methods due to the prohibitively large number of
atoms in the dendritic molecule. The minimized structure of
the monomer was employed to build up the dimer of 11a. After
rotation around the bond connecting both fluorene units, several
conformers were obtained. Figure 4 represents the conformer
with the lowest energy, containing two twisted fluorene units
with a torsion angle of 12° and dendrons in anti position from
each another. The lowest-energy conformers for higher oligo-
mers (tetramer, octamer) were obtained by connecting the
required numbers of the lowest-energy dimer units and subse-
quent energy minimization of the resulting oligomers. The
calculated structure for the octamer is shown in Figure 5.
(33) (a) Lieser, G.; Oda, M.; Miteva, T.; Meisel, A.; Nothofer, H.-G.;
Scherf, U.; Neher, D. Macromolecules, to be published. (b) Shkunov, M.
N.; O¨ sterbacka, R.; Fujii, A.; Yoshino, K.; Vardeny, Z. V. Appl. Phys. Lett.
1999, 74, 1648.
(34) Bradsher, C. K.; Beavers, L. G.; Tokura, N. J. Am. Chem. Soc. 1956,
78, 3196.
(35) Klaerner, G.; Miller, R. D. Macromolecules 1998, 31, 2007.
(36) Lee, S. H.; Tsutsui, T. Thin Solid Films 2000, 363, 76.
(37) Trimpin, S.; Rouhanipour, A.; Przybilla, L.; Keune, S.; Grimsdale,
A. C.; Ra¨der, H.-J.; Mu¨llen, K. Anal. Chem., to be submitted.

950 J. Am. Chem. Soc., Vol. 123, No. 5, 2001
Setayesh et al.
Figure 5. Calculated structure of the octamer.
Scheme 4. Synthesis of the Dimer 19a
Scheme 5. Synthesis of Poly(diarylfluorene)s 20a
a a) 2 ArLi, THF, -78° to rt, 18 g, 23a (94%); 23b (79%); 23c
(69%); b) HOAc, H2SO4, 100°, 1 h, 24a (89%); 24b (73%); 24c (82%);
c) Ni(cod)2, cyclooctadiene (cod), 2,2′-bipyridyl, DMF, toluene, 1-3
d, 80°; d) PhBr, 1 d, 80°.
5% weight loss up to 500° and major weight loss (10%)
occurring only at 520°. DSC performed up to 250° showed no
phase transitions. Obviously, introduction of solubilizing sub-
stituents onto the aromatic rings is required to give solubility.
Reaction of 22 with 2 equiv of the lithium derived from 3,5-
bis(tert-butyl)bromobenzene gave the carbinol 23b (77%) which
was ring-closed with acid to give 2,7-dibromo-9,9-di(3,5-bis-
tert-butylphenyl)fluorene 24b in 73% yield. Polymerization of
24b by the Yamamoto method for 24 h, followed by endcapping
with bromobenzene, gave the desired polymer 20b which was
a a) NaOH (50%), 1-bromomethyl-4-iodo-benzene, tetrabutylammo-
nium chloride, DMSO, 19 h, rt, 83 %; b) TMS-acetylene, Pd(PPh3)2Cl2,
Cu(I)I, THF, triethylamine, rt, 70 h; c) K2CO3, THF, MeOH, 4 h, rt,
16 (60%), 17 (15%); d) tetraphenylcyclopentadienone, o-xylene, 39 h,
reflux, 85 %; e) Ni(cod)2, cyclooctadiene (cod), 2,2′-bipyridyl, DMF,
toluene, 46 h, 80°, 81%.
of up to 25 units. This suggests that the end of the polymer
chain continued to be reactive even when it was no longer in
solution. TGA showed 20a to be extremely stable with less than

Polyfluorenes with Polyphenylene Dendron Side Chains
J. Am. Chem. Soc., Vol. 123, No. 5, 2001 951
Scheme 6. Synthesis of the Bromodendron 28a
a a) TMS-acetylene, Pd(PPh3)2Cl2, Cu(I)I, THF, triethylamine, rt,
18 h, 25 (91%), 26 (7%); b) K2CO3, THF, MeOH, 5 h, rt, 78%; c)
tetraphenylcyclopentadienone, o-xylene, 19 h, reflux, 78 %.
soluble in chloroform, dichloromethane, toluene, and THF. GPC
showed a weight-averaged molecular weight of 11 300 Da with
a polydispersity of 1.84. This material was also very thermally
stable with a 10% mass loss between 250 and 490 °C, followed
by a loss of 30% between 490 and 550 °C. DSC performed up
to 250 °C showed no phase transitions. The absorption
maximum for 20b in chloroform solution was at 382 nm, while
the fluorescence spectrum (λexc ) 370 nm) shows two sharp
bands at 416 and 439 nm. The fluorescence spectrum for a film
dropcast from chloroform showed a slight bathochromic shift,
with maxima at 423 and 445 nm and a small tail into the yellow.
After annealing at 100 °C for 24 h, however, the intensity of
emission from the longer-wavelength region (500-600 nm) of
the spectrum increased. Although the aggregation appears to
be much weaker than for poly(9,9-dialkylfluorene)s, it is clear
that the bis(di-tert-butylphenyl) substituents are insufficiently
bulky to completely suppress aggregation. Consequently, we
prepared the polymer 20c with dendronised substituents. Hagi-
hara-Sonogashira coupling of 1-bromo-4-iodobenzene with a
slight excess of trimethylsilylacetylene for 18 h, gave largely
the monoaddition product 2522c (91%), which was separated
from the diadduct 2622b (7%) by chromatography on silica.
Hydrolysis of 25 with potassium carbonate in methanol gave
4-bromophenylacetylene 2722c (78%). The Diels-Alder reaction
with tetraphenylcyclopentadienone produced the bromodendron
28 in 78% yield (Scheme 6). Lithiation and addition to 22
afforded the carbinol 23c (69%) which was ring-closed to the
desired monomer 24c in 82% yield. Polymerization of this gave
the desired polymer 20c which was soluble in dichloromethane,
chloroform, toluene, and THF. GPC in toluene revealed a
weight-averaged molecular mass of 63 000, with a polydispersity
of 1.28. This material showed exceptional themal stability with
a less than 1% mass loss prior to the primary mass loss of 16%
at 570°. DSC performed up to 250° again showed no phase
transitions. The UV-vis spectrum in toluene exhibited an
absorption maximum at 384 nm, while in the emission spectrum
(λexc ) 320 nm) there appeared a single maximum at 414 nm,
with a shoulder at 432 nm. The solid-state fluorescence spectrum
showed a small bathochromic shift with distinct maxima at 421
and 445 nm. No long wavelength emission was detected after
annealing at 100 °C for 24 h. These results suggest that the
minimum degree of side chain bulk required to avoid aggrega-
tion is intermediate between the bis(di-tert-butylphenyl) and bis-
dendron substituents of 20b and 20c respectively. A more
precise determination of the critical size criteria for aggregation
suppression will require synthesis of polymers with side chains
of intermediate size.
Figure 6. Photoluminescence emission (dotted line) and excitation
(dashed line) spectra (T ) 300 K) of a solid film of polymer 12 together
with the photoduced absorption spectrum of polymer 12 (laser excitation
) 350 nm) and the photoinduced absorption spectrum of a solid film
of polymer 5 taken at 77 K.
nm) and 2.55 eV (487 nm). The absorption spectrum does not
exhibit any vibronic replica but is composed of only one broad
feature with a maximum at 3.2 eV (388 nm). Furthermore, it
was found that the onset of the absorption feature is less steep
than that of the emission feature. This is characteristic of many
π-conjugated polymers and arises from the distribution of the
effective conjugation lengths. The energy of the π-π* transition
depends on the conjugation length; the distribution of energies
broadens the absorption and obscures the vibronic structure of
any particular segment.38,39 By contrast PL emission only probes
the lowest energetic sites. Hence, the combination of a large
Stokes shift as observed for the polymer 12 and vibronic
structure in the emission spectrum arises from exciton migration
prior to fluorescence.40
The PIA spectrum of polymer 12 (Figure 6) reveals one broad
feature at 1.6 eV (775 nm) which extends up to 2.5 eV (496
nm). By comparison, the PIA spectrum of polymer 5 (R1 )
C10H21, R2 ) C6H13, R3 ) H), which represents the planar limit
of a polyfluorene system and thus has a very well-defined
electronic structure due to the absence of rotational disorder,
exhibits two distinct features at 1.3 eV (956 nm) and 1.9 eV
(654 nm). From a comparison with doping and charge induced
absorption measurements the feature at 1.3 eV was attributed
to excited-state absorption of triplet excitonssa triplet-triplet
transition (T1 f Tn)sand the band at 1.9 eV to a polaron P2
bands.41a Two bands are also seen in the PIA spectra of
polyindenofluorenes 441b (1.5 and 2.1 eVs829 and 592 nm)
and poly(9,9-dialkylfluorene)s 341c (1.65 and 2.1 eVs753 and
592 nm). Owing to its similarity we therefore attribute the PIA
band at 1.6 eV for polymer 12 to a T1 f Tn transition. The
absence of a distinct strong polaronic absorption feature in the
(38) Wohlgenannt, M.; Graupner, W.; Wenzl, F. P.; Tasch, S.; List, E.
J. W.; Leising, G.; Graupner, M.; Hermetter, A.; Rohr, U.; Schlichting, P.;
Geerts, Y.; Scherf, U.; Mu¨llen, K. Chem. Phys. 1998, 227, 99.
(39) den Hartog, F. T. H.; van Papendrecht, C.; Silbey, R. J.; Vo¨lker, S.
J. Chem. Phys, 1999, 110, 1010.
(40) (a) Kersting, R.; Lemmer, U.; Mahrt, R. F.; Leo, K.; Kurz, H.;
Ba¨ssler, H.; Go¨bel, E. O. Phys. ReV. Lett., 1993, 70, 3820. (b) List, E. J.
W.; Creely, C.; Leising, G.; Schulte, N.; Schlu¨ter, A. D.; Scherf, U.; Mu¨llen,
K.; Graupner, W. Chem. Phys. Lett. 2000, 325, 132. (c) List, E. J. W.;
Kim, C. H.; Shinar, J.; Pogantch, A.; Leising, G.; Graupner, M. Appl. Phys.
Lett. 2000, 76, 2083.
Photoinduced Absorption and Electroluminescence Mea-
surements on the Polymer 12. Figure 6 shows the PL emission
and PL excitation spectrum of poymer 12 in the solid state.
The PL emision spectrum is characterized by a maximum at
2.9 eV (425 nm), and two resolved vibronic features at 2.7 (460
(41) (a) Graupner, W.; Jost, T.; Petritsch, K.; Tasch, S.; Leising, G.;
Graupner, M.; Hermetter, A. Tech. Pap. - Soc. Plast. Eng. 1997, XLIII,
1339. (b) Silva, C.; Russell, D. M.; Stevens, M. A.; Mackenzie, J. D.;
Setayesh, S.; Mu¨llen, K.; Friend, R. H. Chem. Phys. Lett., 2000, 319, 494.
(c) Cadby, A. J.; Lane, P. A.; Wohlgenannt, M.; An, C.; Vadreny, Z. V.;
Bradley, D. D. C. Synth. Met. 2000, 111-112, 515.

952 J. Am. Chem. Soc., Vol. 123, No. 5, 2001
Setayesh et al.
minescent device fabricated with polymer 12 as the active layer,
ITO/PEDT:PSS for the anode and calcium (70 nm)/aluminum
(300 nm) for the cathode. The average thickness of the polymer
films was 100 nm, and of the PEDT:PSS layer was 60-70 nm,
and the active area of the LEDs was 3 × 3 mm. The
electroluminescence spectrum of 12 closely matches the PL
spectrum (Figure 7), with an emission maximum at 420 nm
(2.95 eV); some minor differences may be ascribed to the
presence of the PEDT layer and consequent morphological
differences. All measurements on the devices were undertaken
in an argon atmosphere. The device stability and lifetimes have
not been studied systematically, but all devices showed repro-
ducible current/voltage characteristics and electroluminescence
spectra with no sign of aggregation provided the driving voltage
did not exceed ∼12 V. To test the effect of molecular weight
on the efficiency of devices using 12, we extracted the powder
with cyclohexane for 100 h at room temperature to obtain a
low-molecular weight fraction. Both this and the remaining high-
molecular weight fraction were dissolved in toluene for spin-
coating films. The devices using the high-molecular weight
fraction were found to give more reproducible results. AFM
pictures (Figure 8) of films of the two fractions cast from
solution onto polished silicon wafers show that while the low-
molecular weight fraction forms isolated particles of height about
40 nm embedded in polymer film islands (Figure 8a), the high-
molecular weight fraction produces reproducibly homogeneous
films with an average surface roughness of about 2 nm (Figure
8b). Due to the high ionization potential of polyfluorenes (Ip ≈
5.8 eV) and thus a significant energy barrier of about 0.8 eV
with ITO (Ip ≈ 5 eV) it is quite difficult to inject holes directly
from ITO into polyfluorenes (PFs). Therefore a level matching
Figure 7. EL emission (inset), current-voltage (solid circles), and
luminance-voltage (open circles) characteristics of a typical electrolu-
minescence device [device area: 4 mm2, device structure: glass/ITO/
PEDT:PSS/polymer12/Ca/Al]; ITO: indium-tin-oxide, PEDT/PSS:
poly(3,4-ethylenedioxythiophene/polystyrene sulfuric acid.
tail of the T1 f Tn feature for 12 is yet another indication for
the isolation of the polymer chains due to the bulky dendrimer
side chains. In contrast to triplet excitons, which are intrinsically
long-lived metastable states due to the forbidden transition to
the ground state, polarons require extrinsic stabilization or
trapping as provided by nonisolated polymer chains to be
observed in continuous wave PIA experiments. We plan further
thermally stimulated current experiments and photoluminescence
detected magnetic resonance41c on 12 to determine the defect
density and their energy level positions, as well as the interaction
of these trapped states with singlet excitons.
Figure 7 depicts the EL spectrum (inset) and the current-
voltage and luminance-voltage characteristics of an electrolu-
Figure 8. AFM pictures of (a) low-molecular weight, (b) high-molecular weight fractions of polymer 12 cast on polished silicon wafers.

Polyfluorenes with Polyphenylene Dendron Side Chains
J. Am. Chem. Soc., Vol. 123, No. 5, 2001 953
layer (PEDT:PSS, Bayer BAYTRON P) was inserted between
the ITO and the layer of 12. Injecting electrons into PFs is much
easier, since the work function of metals such as calcium differs
by only a few tenths of an eV from the LUMO energy level.
The onset of the electroluminescence emision was observed
between 3 and 4 V. The maximum luminance for for bright
blue emission as depicted in the inset of Figure 7 was found to
be 400cd/m2 at a driving voltage of ∼10 V. For this device we
found the maximum efficiency to be ∼0.2 cd/A at a driving
voltage of 7.5 V wih a current density of 22 mA/cm2. This
corresponded to a maximum power efficiency of 0.16 lm/W.
These device related parameters are very similar to those found
for other devices fabricated from polyfluorenes (PFO),42 except
that in our devices we typically found lower onset voltages.
The onset voltage for EL in ITO/HTL/PFO/Ca devices is
generally observed above 5 V.42,43 However, if polyfluorene
with sufficently high molecular weight is applied as the emission
layer in ITO/PEDT:PSS/PFO/Ca-LEDs, onset voltages below
3 V can be obtained.44 This is in agreement with the performance
parameters of EL devices fabricated with our polymer 12,
proving that attaching the dendrimer side chains reduces the
aggregation of the polymer chains without altering their
electronic properties. Recently, Meerholz45 has reported that
poly(4,4′-methoxybithiophene) is an even better polymeric hole-
injecting anode for polyfluorene-based LEDs than PEDT, and
it is possible devices using this anode would show even better
performances.
Conclusions
In this paper we have established synthetic approaches to
nonaggregating polyfluorenes 12 and 20c using pentaphenylene
dendrons as shielding and solubilizing groups and made some
initial studies into their electronic properties. The emission
spectra of the polymer films reveal the efficiency of the side
chains in preventing π-stacking without causing undesirable
blue-shifts in the absorption or emission spectra. Comparison
of these polymers with others bearing slightly less bulky side
chains has given some indication as to the minimum side chain
size requirements for complete aggregation inhibition in py-
fluorenes. The synthetic routes developed are capable of
extension to give other materials with bulky, possibly function-
alized, side chains, and to the synthesis of other polymers, for
example, 2,8-PIF 4 with dendritic side chains.
Electroluminescent devices have been constructed using the
dendronised polymer 12, and their luminance and electrical
characteristics have been found to be comparable with those of
alkylated polyfluorenes. This further indicates that the suppres-
sion of aggregation has been acheived without sacrificing the
desirable properties of the polyfluorene backbone.
Acknowledgment. We acknowledge the support of this work
by the European Commission (BRITE EURAM BE 97-4438
OSCA) and by the Austrian Science Foundation (Project No.
P12806PHY). We thank Mr N. Koch for recording the AFM
pictures.
Supporting Information Available: Experimental section;
UV-vis absorption and PL fluorescence spectra for polymers
20b and 20c; X-ray diffraction spectra for polymer 12 (PDF).
An X-ray crystallographic file (CIF) for compound 11b. This
material is available free of charge via the Internet at
http.//pubs.acs.org.
(42) Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.; Inbasekaran, M.;
Wu, W. W.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 629.
(43) Yu, W. L.; Pei, J.; Huang, W.; Heeger, A. J. AdV. Mater. 2000, 12,
828.
(44) Weinfurter, K. H.; Fujikawa, H.; Tokito, S.; Taga, Y. Appl. Phys.
Lett. 2000, 76, 2502.
(45) Gross, M.; Mu¨ller, D. C.; Nothofer, H.-G.; Scherf, U.; Neher, D.;
Bra¨uchle, C.; Meerholz, K. Nature 2000, 405, 661.
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