Synthesis and characterization of luminescent polyfluorenes incorporating side-chain-tethered polyhedral oligomeric silsesquioxane units

Article abstract of DOI:10.1021/ma0479520

Using Suzuki polycondensation, we have synthesized polyhedral silseaquioxane-tethered polyfluorene copolymere, poly(9,9'-dioctylfluorene-co-9,9'-bis[4-(N,N-dipolysilsesquioxane)aminophenyl] -fluorene) (PFO - POSS), that have well-defined architectures. Th

Full text of DOI:10.1021/ma0479520

Macromolecules 2005, 38, 745-751
745
Synthesis and Characterization of Luminescent Polyfluorenes
Incorporating Side-Chain-Tethered Polyhedral Oligomeric
Silsesquioxane Units
Chia-Hung Chou, So-Lin Hsu, K. Dinakaran, Mao-Yuan Chiu, and Kung-Hwa Wei*
Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu, Taiwan 30049 R.O.C.
Received October 4, 2004; Revised Manuscript Received November 8, 2004
ABSTRACT: Using Suzuki polycondensation, we have synthesized polyhedral silsesquioxane-tethered
polyfluorene copolymers, poly(9,9′-dioctylfluorene-co-9,9′-bis[4-(N,N-dipolysilsesquioxane)aminophenyl]-
fluorene) (PFO)POSS), that have well-defined architectures. This particular PFO)POSS molecular
architecture increases the quantum yield of polyfluorene significantly by reducing the degree of interchain
aggregation; in addition, these copolymers exhibit a purer and stronger blue light by preventing the
formation of keto defects.
Introduction
as the molecular weight of the polymer increases, which
limits the amount of POSS that can be attached (ca.
1.2%). Another study involved a synthesis of a bridged
polyfluorene copolymer by using fluorene tetrabromide
monomers featuring a siloxane bridge. The thermal
stability of a device made from this siloxane-bridged
polyfluorene appears to be better than that prepared
from pure PFO.18d A third related study involved
attaching polyfluorene to the functionalized vertexes on
the cubic polyhedral silsesquioxane core units to form
starlike structures; these polymers possessed improved
thermal and optoelectronic characteristics. The amount
of POSS content in the polyfluorene was ca. 3.8%.18e
The enhanced electroluminescence characteristics in
these polyfluorenes have been attributed to POSS
imparting a reduction in either the degree of aggrega-
tion and excimer formation or the number of keto
defects. The molecular architecture that polyfluorene
possessed in the latter two studies cannot be defined
easily. Previously, we synthesized a polyimide-side-
chain-tethered POSS as an approach to lowering its
dielectric constant.19a In this present study, we took a
copolymer approach by synthesizing polyfluorene-
tethered POSS in a well-defined architecture. To the
best of our knowledge, the introduction of an inorganic
side group, such as POSS, into the C-9 position of
polyfluorene has not been explored previously. We
believe that by developing a POSS/polyfluorene copoly-
mer having well-defined architecture we will be able to
tailor its luminescence properties more precisely by
modifying the molecular structure. In this paper, we
report the synthesis and characterization of fluorene-
based random copolymers featuring tethered polyhedral
oligomeric silsesquioxanes (POSS) units.
Semiconducting polymers have been studied exten-
sively for their potential applications in electrolumines-
cent displays,1 solar cells,2 and thin film organic tran-
sistors.3 One of the most promising conjugated polymers
is polyfluorene, which has been applied widely for its
characteristic of emitting blue light.4-7 Polyfluorenes
may be functionalized readily by modifying the C-9
position of the fluorene monomer; such modification can,
for instance, provide good solubility in common organic
solvents that allows further processing. In the solid
state, polyfluorene and its derivatives exhibit adequate
photoluminescence (PL) and electroluminescence (EL)
efficiencies.8-10 The application of polyfluorenes in light
emitting diodes, however, has been hampered by the
appearance of green electroluminescence (ca. 530 nm)
in addition to the desired peak at 425 nm; this green
electroluminescence has been attributed to either in-
termolecular interactions, which lead to the formation
of aggregates,11-17 or to the presence of emissive keto
defect sites that arise as a result of thermo- or electro-
oxidative degradation of the polyfluorene backbone.18a,b
A detailed discussion of this phenomenon has been
presented elsewhere.18c As a result, the expected blue
emission from polyfluorene becomes an undesired blue-
green color in LED applications.
Several approaches have been adopted to reduce the
formation of aggregation or keto defect in polyfluorenes,
including the introduction of bulky side chains, using
cross-linked structures, improving the oxidative stability
of pendant groups or chain ends,15 and limiting chain
mobility by blending it with a high-Tg polymer.17 One
of the most recent approaches involves incorporation of
polyhedral oligomeric silsesquioxane (POSS) into the
conjugated polymer. The first such study was under-
taken by attaching POSS covalently to the chain ends
of poly(2-methoxy-5-[2-ethylhexyloxy]-1,4-phenylenevi-
nylene) (MEHPPV) and poly(9,9′-dioctylfluorene) (PFO),
which resulted in enhanced thermal stability of the
devices prepared from these modified polymers.15 The
density of the polymer chain ends, however, decreases
Scheme 1 displays the synthetic procedure we
used to prepare the POSS)dibromide monomer
(D-POSS)diAF), in which cyclopentyl-POSS was co-
valently bonded to the C-9 carbon atom of the fluor-
ene unit through a 4-aminophenyl spacer. We used
POSS)dibromide as a comonomer, which, together
with 2,7-dibromo-9,9′-dioctylfluorenone (5), were reacted
with 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)fluorene (6) through Suzuki coupling followed by end
capping. Scheme 2 presents the complete synthetic
* Corresponding author. Telephone: 886-35-731871. Fax:
886-35-724727. E-mail: khwei@cc.nctu.edu.tw.
10.1021/ma0479520 CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/13/2005

746 Chou et al.
Scheme 1. Synthesis of D-POSS-diAF (4)a
Macromolecules, Vol. 38, No. 3, 2005
Anal. Calcd for C25H18Br2N2: C, 59.31; H, 3.58; N, 5.53.
Found: C, 59.38; H, 3.62; N, 5.47.
Synthesis of 9,9′-Bis[4-(N,N-dipolysilsesquioxane)ami-
nophenyl]fluorene (D-POSS-diAF) (4). 9,9-Bis(4-amino-
phenyl)-2,7-dibromofluorene (200 mg, 0.395 mmol) was stirred
with K2CO3 (764 mg, 5.53 mmol) and KI (262 mg, 1.58 mmol)
in DMF (5 mL) and THF (4 mL) at room temperature for 1 h.
A small amount of Cl)POSS (888 mg, 0.832 mmol) was added
and then the whole mixture was heated at 70 °C for 3 h. The
reaction mixture was then slowly poured into water (150 mL)
and extracted with chloroform (3 × 30 mL). The combined
extracts were dried (MgSO4), the solvents were evaporated,
and the residue was purified by column chromatography
1
(hexane/ chloroform, 1:10) to afford 4 (0.69 g, 68%). H NMR
(CDCl3): δ 7.62-7.63 (m, 14H), 7.01-6.84 (m, 8H), 4.81 (s,
2H), 4.27 (s, 4H), 2.06-1.18 (m, 112H), 1.14-0.79 (m, 14H)
ppm. Anal. Calcd for C109H154Br2O24N2Si16 (%): C, 52.67; H,
6.24; N, 1.13. Found: C, 52.11; H, 6.21; N, 1.07.
General Procedure for the Synthesis of Alternating
Copolymers PFO)POSS. Aqueous potassium carbonate (2
M) and aliquate 336 were added to a solution of the POSS-
appended fluorene dibromide monomer 4, dibromide 5, and
diboronate 6 in toluene. The mixture was degassed and purged
with nitrogen three times. The catalyst, tetrakis(triphenylphos-
phine)palladium (3.0 mol %), was added in one portion under
a nitrogen atmosphere. The solution was then heated at 90
°C and vigorously stirred under nitrogen for 5 days. End group
capping was performed by heating the solution under reflux
for 6 h sequentially with phenylboronic acid and bromoben-
zene. After cooling, the polymer was recovered by precipitating
it into a mixture of methanol and acetone (4:1). The crude
polymer was collected, purified twice by reprecipitation from
THF into methanol, and subsequently dried under vacuum at
50 °C for 24 h. The 1H and 13C NMR spectra of PFO and
PFO)POSS appear to be identical because of the low content
of POSS in the latter polymer.
Characterization. 1H, 13C, and 29Si nuclear magnetic
resonance (NMR) spectra of the compounds were obtained
using a Bruker DRX 300 MHz spectrometer. Mass spectra of
the samples were obtained on a JEOL JMS-SX 102A spec-
trometer. Fourier transform infrared (FTIR) spectra of the
synthesized materials were acquired using a Nicolet 360
FT-IR spectrometer. Gel permeation chromatographic analyses
were performed on a Waters 410 Differential refractometer
and a Waters 600 controller (Waters Styragel column). All GPC
analyses of polymers in THF solutions were performed at a
flow rate of 1 mL/min at 40 °C; the samples were calibrated
using polystyrene standards. Thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) measure-
ments were performed under a nitrogen atmosphere at heating
rates of 20 and 10 °C/min, respectively, using Du Pont TGA-
2950 and TA-2000 instruments, respectively. UV-vis absorp-
tion and photoluminescence (PL) spectra were recorded on a
HP 8453 spectrophotometer and a Hitachi F-4500 lumines-
cence spectrometer, respectively. Before investigating the
thermal stability of the synthesized polymers, their polymer
films were annealed in air at 200 °C for 2 h.
Device Fabrication and Testing. The electroluminescent
(EL) devices were fabricated on an ITO-coated glass substrate
that was precleaned and then treated with oxygen plasma
before use. A layer of poly(ethylene dioxythiophene):poly-
(styrenesulfonate) (PEDOT:PSS, Baytron P from Bayer Co.;
ca. 40 nm thick) was formed by spin-coating from its aqueous
solution (1.3 wt %). The EL layer was spin-coated at 1500 rpm
from the corresponding toluene solution (15 mg mL-1) on top
of the vacuum-dried PEDOT:PSS layer. The nominal thickness
of the EL layer was 65 nm. Using a base pressure below 1 ×
10-6 Torr, a layer of Ca (30 nm) was vacuum deposited as the
cathode and a thick layer of Al was deposited subsequently
as the protecting layer. The current-voltage characteristics
were measured using a Hewlett-Packard 4155B semiconductor
parameter analyzer. The power of the EL emission was
measured using a Newport 2835-C multifunction optical meter.
The brightness was calculated using the forward output power
a (i) CrO3, acetic anhydride, HClaq. (ii) aniline, aniline
hydrochloride. (iii) K2CO3, KI, DMF/THF (5:4).
procedure for the preparation of POSS-tethered poly-
fluorene. The extended 9,9′-bis(4-aminophenyl)fluorenyl
core, which is readily accessible, offers the following
additional advantages: (1) Direct attachment of the
benzyl groups of POSS to the C-9 carbon atom of the
fluorene unit is avoided; such benzyl linkages are
potentially susceptible to photooxidation, which may
cause degradation and failure of the polymer LEDs.19b,20
(2) The introduction of the 4-aminophenyl spacer re-
duces the steric hindrance imposed by POSS.21 The
insertion of a rigid phenylene spacer between the POSS
side chain and the polymer backbone may lead to a
shielding effect on the polyfluorene main chain, while
leaving the reaction sites of the macromonomer acces-
sible for the palladium-catalyzed polymerization reac-
tion. (3) Through this copolymerization approach, the
amount of POSS incorporated into the polyfluorene
may be tuned by controlling the amount of POSS)
dibromide monomer used in the polymerization.
Experimental Section
Materials. 2,7-Dibromo-9,9′-dioctylfluorenone (5),21 2,7-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)fluorene (6),22 and
the chlorobenzylcyclopentyl-POSS23 were synthesized accord-
ing to literature procedures. THF was distilled under nitrogen
from sodium benzophenone ketyl; other solvents were dried
using standard procedures. All other reagents were used as
received from commercial sources unless otherwise stated.
Chlorobenzylcyclopentyl)POSS. 1H NMR (300 MHz,
CDCl3): δ 7.64 (d, J ) 8.1 Hz, 2H), 7.37 (d, J ) 8.1 Hz, 2H),
4.57 (s, 2H), 2.26-1.21 (m, 56H), 1.16-0.81 (m, 7H) ppm. 29Si
NMR (600 MHz, THF): δ -67.8, -68.2, -79.6 ppm.
Synthesis of 9,9′-Bis(4-aminophenyl)-2,7-dibromofluo-
rene (3). A mixture of 2,7-dibromo-9-fluorenone (2) (3.0 g, 8.88
mmol), aniline (4.0 g, 4.30 mmol), and aniline hydrochloride
(1.15 g, 8.88 mmol) was heated at 150 °C under nitrogen for 6
h. The reaction mixture was then slowly added into water (150
mL) and extracted with ethyl acetate (3 × 50 mL). The
combined extracts were dried (MgSO4), the solvent was
evaporated, and the residue was purified by column chroma-
tography (hexane/ethyl acetate, 4:1) to afford 3 (3.23 g, 72%).
1H NMR (CDCl3): δ 7.52 (d, J ) 8.1 Hz, 2H), 7.44 (4H), 6.90
(d, J ) 8.1 Hz, 4H), 6.53 (d, J ) 8.1 Hz, 4H), 3.58 (s, 4H) ppm.

Macromolecules, Vol. 38, No. 3, 2005
Luminescent Polyfluorenes 747
Scheme 2. Synthesis of PFO-POSS Copolymers
Table 1. Physical Properties of the PFO-POSS
and the EL spectra of the devices; a Lambertian distribution
of the EL emission was assumed.
Copolymers
Tda (°C)
Mn
Mw
PDI yield (%)
Results and Discussion
PFO
372
382
381
397
416
27000 51000 1.86
21000 42000 1.96
20000 37000 1.85
16000 31000 1.93
12000 24000 1.97
96
78
68
79
65
1
PFO)POSS-1%
PFO)POSS-3%
PFO)POSS-5%
PFO)POSS-10%
Figure 1 displays the H NMR spectra of 9,9′-bis(4-
aminophenyl)-2,7-dibromofluorene (3), Cl)POSS, and
D-POSS)diAF (4). The peak for the NH protons
shifted downfield from 3.59 ppm for 3 to 4.48 ppm for
4, while the CH2 peak of Cl)POSS shifted upfield from
4.47 to 4.21 ppm in D-POSS)diAF. The ratio of the
peak areas of the benzylic CH2 and NH protons is ca.
2:1. Taken together, all of these data suggest that
Cl)POSS had reacted with 4-aminophenyl fluorene
to form D-POSS)diAF. Table 1 lists the thermal
properties and molecular weight distributions of the
PFO)POSS copolymers. Both the thermal degradation
and glass transition temperature increased as the
amount of POSS in PFO increased, presumably be-
cause the tethered POSS enhanced the thermal stabil-
ity and retarded the polymer chain mobility. The
molecular weights of the PFO)POSS copolymers
decreased upon increasing the POSS content; this
phenomenon can be attributed to the steric hindrance
caused by POSS during the polymerization process.
a Temperature at which 5% weight loss occurred, based on the
initial weight.
Figure 2. FTIR spectra of (a) PFO, (b) PFO)POSS-1%, (c)
PFO)POSS-3%, (d) PFO)POSS-5%, and (e) PFO)POSS-
10%.
Figure 2 displays FTIR spectra of PFO copolymers
containing different amounts of POSS. The FTIR
spectrum of POSS displays two major characteristic
peaks in the range 1000-1180 cm-1 (Si-O-Si stretch-
ing). The Si-C band at 1074 cm-1, however, overlaps
with the Si-O-Si band and, thus, could not be observed
clearly.
Figure 3 displays the X-ray diffraction curves of
Cl)POSS, PFO, and PFO)POSS. We observed no
Figure 1. 1H NMR spectra of (a) 3, (b) Cl)POSS, and (c) 4.

748 Chou et al.
Macromolecules, Vol. 38, No. 3, 2005
Figure 3. X-ray diffraction curves of PFO)POSS nanocom-
posite films: (a) PFO, (b) PFO)POSS-1%, (c) PFO)POSS-
3%, (d) PFO)POSS-5%, (e) PFO)POSS-10%, and (f) pure
Cl)POSS.
Table 2. Optical Properties of the PFO-POSS
Nanocomposites
quantum
λmax (UV, nm)
solutionb film solutionb
λmax (PL, nm)a
film
yield
filmc
PFO
384
374
374
372
362
390 418 (441) 425 (448)
383 417 (440) 423 (447)
381 417 (437) 423 (447)
381 417 (437) 423 (447)
380 416 (436) 422 (446)
0.55
0.57
0.64
0.67
0.86
PFO)POSS-1%
PFO)POSS-3%
PFO)POSS-5%
PFO)POSS-10%
a The data in parentheses are the wavelengths of shoulders and
subpeaks. b The absorption and emission measured in THF. c PL
quantum yield estimated relative to a sample of poly(2,7-(9,9′-
dioctylfluorene)) (ΦFL ) 0.55).
X-ray diffraction peaks for pure PFO. There are three
distinct diffraction peaks at 2θ ) 8.3, 19.1, and 26.1°
observed for Cl)POSS (Figure 3f), which correspond
to d-spacings of 10.5, 4.6, and 3.3 Å, respectively. The
d-spacing of 10.5 Å reflects the size of the Cl)POSS
molecules; the other two spacings reflect the rhombo-
hedral crystal structure of POSS molecules.28 The
presence of a small crystal peak of POSS in PFO
indicates a mild degree of aggregation of POSS mol-
ecules. The DSC curves of pure PFO24 indicate a value
of Tg of 62 °C, a crystallization exothermal peak (Tc) at
98 °C, and a melting endothermal peak (Tm) at 155 °C
(see the Supporting Information). Only a small Tm at
166 °C appears in the curve for PFO)POSS-10%; the
disappearance of the Tg and Tc peaks in this case (PFO
containing 10% POSS) can be explained by the fact that
the mobility of the PFO main chains, including chain
folding, is severely retarded by the steric hindrance
imposed by the bulky side-chain-tethered POSS units,
as has been reported in the literature.25 The UV-vis
and photoluminescence spectra of PFO and PFO-POSS
recorded in THF are presented in the Supporting
Information. Table 2 lists the wavelengths of the
absorption and PL maxima and the quantum yields of
PFO)POSS. The absorption and emission peak maxima
of PFO occur at 384 and 418 nm, respectively; these
values are close to those reported in the literature.26 We
observed no aggregation band in these spectra because
THF is a good solvent for PFO. The absortion and
emission peaks for PFO and PFO)POSS are almost
identical. For each polymer, the absorption peak maxi-
mum in solution (THF) is located between 384 and
362 nm, with a slight blue shift caused by the presence
of POSS; the PL maxima occur at similar wavelengths
for all of the polymers. The quantum yields of the
Figure 4. (a) PL spectra of PFO films before annealing
(dotted line) and after annealing at 200 °C for 1 h (dashed
line) and 2 h (solid line) under a nitrogen atmosphere. (b) FTIR
spectra of (i) PFO, (ii) PFO)POSS-5%, and (iii) PFO)POSS-
10% before (solid line) and after (dashed line) baking at 200
°C for 6 h.
PFO)POSS copolymers increased substantially as the
amount of tethered POSS increased. In particular, the
quantum yield of PFO containing 10% POSS was 54%
higher than that of pure PFO (0.86 vs 0.55). This
finding can be attributed to the steric hindrance caused
by the POSS units preventing aggregation of the PFO
main chains, which, in turn, reduces the degree of dimer
formation after excitation. This phenomenon is a result
of the particular side-chain-tethered POSS architecture
that we have employed: it has not been reported in
previous studies of POSS/PFO copolymers. The other
significant effect that the incorporation of the silses-
quioxane into PFO side chain causes is the persistence
of luminescence behavior after thermal treatment of
PFO)POSS. The stability of both color and lumines-
cence at elevated temperatures are critical for polymer
LEDs because sometimes the operating temperature of
these devices exceeds 86 °C. Next, we investigated in
detail the effect that thermal treatment has on POSS-
incorporated PFO. Figure 4a displays the photolumi-
nescence spectra of PFO as a solid film after heat
treatment at 200 °C; in all cases, the absorption peaks
at 386 nm remain. Other than the main peak at 425
nm, we observe an additional small peak, at ca. 530 nm,
in the spectrum of PFO film exposed at 200 °C for 1 h.
The green emission peak at 530 nm increased in
intensity, while the intensity of the main 425 nm peak

Macromolecules, Vol. 38, No. 3, 2005
Luminescent Polyfluorenes 749
Figure 6. Transmission electron micrographs of (a)
PFO)POSS-1%, (b) PFO)POSS-3%, (c) PFO)POSS-5%,
and (d) PFO)POSS-10%.
in the spectrum of the sample containing 10% POSS.
Undesired fluorescence emission resulting from the
oxidation defects in polymer chains can be reduced by
improving the thermal-oxidation properties of the
polymers.18b,28 In this case, the bulky siloxane units
attached to 4-aminophenyl spacer appear to provide a
good shield for the neighboring dialkyl groups from
thermal oxidation because the thermal degradation
temperature of siloxane units is higher than that of
dialkyl chains, as evidenced in the TGA results of
POSS/PFO copolymer in Table 1, where the thermal
degradation temperature of the copolymer increases
with the amount of POSS.
Figure 5 presents the normalized absorption and PL
emission spectra of the annealed PFO)POSS films
with respect to that of their fresh films. The fact that
the absorption λmax of PFO)POSS in Figure 5, parts
c and d appears at the same wavelength before and after
heating suggests that the thermal treatment did not
disrupt the conjugation in these PFO)POSS samples.
The PL spectrum of PFO containing 1% POSS exhibits
a much smaller shoulder at 530 nm than that for the
pure PFO after the same thermal treatment. This peak
was reduced further when 3% POSS (Figure 5b) was
present and became an insignificant shoulder at ca. 500
nm when g5% POSS was incorporated into PFO
(Figure 5c,d). These results suggest that the tethered
POSS units not only reduced the aggregation of PFO
molecular chains but also prevented keto defects from
forming upon thermal treatment. Figure 6 presents
transmission electron microscopy (TEM) images of dif-
ferent PFO)POSS samples; these images clearly
reveal that no large aggregates have formed, but small
domains of POSS are present, and the POSS domains
are dispersed well in the polymer matrix.
Figure 5. UV-vis absorption and PL spectra of (a)
PFO)POSS-1%, (b) PFO)POSS-3%, (c) PFO)POSS-5%,
and (d) PFO)POSS-10% films before (dotted line) and after
(solid line) annealing at 200 °C for 2 h under a nitrogen
atmosphere.
reduced quite dramatically, after annealing the PFO
film for 2 h. Figure 4b indicates that, after thermal
treatment, the intensity of the keto peak (CdO, 1713
cm-1) in the FTIR spectra decreases as the amount of
POSS in PFO increases: almost no keto peak appears
Electroluminescence (EL) Characteristics. Fig-
ure 7 displays the electroluminescence (EL) spectra of

750 Chou et al.
Macromolecules, Vol. 38, No. 3, 2005
improvements might be due to a combination of a lesser
degree of aggregation and fewer keto defects being
formed upon the incorporation of POSS into PFO.
Summary
We have synthesized a novel polyfluorene side-chain-
tethered polyhedral silsesquioxane that has a well-
defined architecture. This particular molecular archi-
tecture of PFO)POSS increases the quantum yield of
polyfluorene significantly by reducing the degree of
interchain aggregation; it also results in a purer and
stronger blue light being emitted from the EL device
by preventing the formation of keto defects.
Acknowledgment. The authors thank the National
Science Council, Taiwan, and the US Air Force Office
of Scientific Research for funding this work through
Grants NSC 91-2120-M-009-001 and AOARD-03-4018,
respectively. Mr. Yao-Te Chang is also acknowledged
for experimental assistance and helpful discussions.
Figure 7. Electroluminescence spectra of the devices pre-
pared from PFO)POSS and PFO in the configuration ITO/
PEDOT/polymer/Ca/Al.
Supporting Information Available: Figures showing
normalized UV-vis absorption, photoluminescence spectra,
DSC, and TGA of PFO and PFO)POSS. This material is
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) (a) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend,
R. H.; Holmes, A. B. Nature 1993, 365, 628. (b) Gustafsson,
G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger,
A. J. Nature 1992, 357, 477. (c) Burroughes, J. H.; Bradley,
D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R.
H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (d)
Jenekhe, S. A.; Osaheni, J. A. Science 1994, 620, 765.
(2) (a) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia,
E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature
(London) 1995, 376, 498. (b) Yu, G.; Gao, J.; Hummelen, J.
C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (c)
Granstro¨m, M.; Petritsch, K.; Arias, A. C.; Lux, A.; Andersson,
M. R.; Friend, R. H. Nature 1998, 395, 257.
Figure 8. I-V curves of the devices prepared from PFO)
POSS and PFO in the configuration ITO/PEDOT/polymer/
Ca/Al.
(3) (a) Yang, Y.; Heeger, A. J. Nature 1994, 372, 344. (b) Brown,
A. R.; Pomp, A.; Hart, C. M.; de Leeuw, D. M. Science 1995,
270, 972. (c) Sirringhaus, H.; Tessler, N.; Friend, R. H.
Science 1998, 280, 1741. (d) Sirringhaus, H.; Brown, P. J.;
Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss,
B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.;
Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (e) Babel,
A.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13656.
(4) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997,
30, 7686.
PFO)POSS devices. The EL device prepared from
PFO emits a weak blue signal at 425 nm and a more
intense green signal in the range 470-600 nm, which
is presumably due to the aggregation and keto defects
discussed previously. When 5% POSS was included in
PFO, the intensity of the peak at 425 nm increased,
but the intensity of the green emission remained ap-
proximately the same. In the case where 10% POSS was
incorporated in PFO, the green emission in the range
470-600 nm was reduced sharply, while the peak at
425 nm became the major emission peak and had an
intensity much larger than that exhibited by the device
prepared from pure PFO. The reduction in the green
emission is apparently due to the presence of the
siloxane units. The introduction of bulky siloxane units
into polyfluorene side chain presumably serves a dual
function; it not only hinders oxidation of fluorenes but
also increases the interchain distance, thereby retarding
the interchain interactions and leading to a reduction
of excitons migration to defect sites as discussed in a
previous study.27
(5) Yu, W.-L.; Pei, J.; Cao, Y.; Huang, W.; Heeger, A. J. Chem.
Commun. 1999, 1837.
(6) Pei, J.; Yu, W.-L.; Huang, W.; Heeger, A. J. Chem. Commun.
2000, 1631.
(7) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A.
C.; Mu¨llen, K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. J.
Am. Chem. Soc. 2003, 125, 437.
(8) (a) Lee, J. I.; Kla¨rner, G.; Miller, R. D. Chem. Mater. 1999,
11, 1083. (b) Kla¨rner, G.; Lee, J. I.; Davey, M. H.; Miller, R.
D. Adv. Mater. 1999, 11, 115.
(9) (a) Yu, W.-L.; Pei, J.; Huang, W.; Heeger, A. J. Adv. Mater.
2000, 12, 828. (b) Zeng, G.; Yu, W.-L.; Chua, S.-J.; Huang,
W. Macromolecules 2002, 35, 6907.
(10) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.;
Mu¨llen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am.
Chem. Soc. 2001, 123, 946.
(11) (a) Kreyenschmidt, M.; Kla¨rner, G.; Fuhrer, T.; Ashenhurst,
J.; Karg, S.; Chen, W. D.; Lee, V. Y.; Scott, J. C.; Miller, R.
D. Macromolecules 1998, 31, 1099. (b) Kla¨rner, G.; Lee, J. I.;
Lee, V. Y.; Chan, E.; Chen, J. P.; Nelson, A.; Markiewicz, D.;
Siemens, R.; Scott, J. C.; Miller, R. D. Chem. Mater. 1999,
11, 1800. (c) Lee, J. I.; Kla¨rner, G.; Miller, R. D. Chem. Mater.
1999, 11, 1083.
Figure 8 displays the variations of the current density
and brightness of the EL devices. The turn-on voltage
increased to 5.8 V forPFO containing 10% POSS from
5.4 V for the pure-PFO EL device. A significant increase
(68%) in the maximum brightness of the PFO)POSS-
10%-based device occurred relative to that of the pure-
PFO EL device (392 vs 234 cd/m2) at a drive voltage of
10 V and a current density of 576 mA/cm2. These
(12) (a) Marsitzky, D.; Klapper, M.; Mullen, K. Macromolecules
1999, 32, 8685. (b) Marsitzky, D.; Murray, J.; Scott, J. C.;
Carter, K. R. Chem. Mater. 2001, 13, 4285. (c) Marsitzky,
D.; Vestberg, R.; Blainey, P.; Tang, B. T.; Hawker, C. J.;

Macromolecules, Vol. 38, No. 3, 2005
Luminescent Polyfluorenes 751
Chem., Int. Ed. Engl. 1998, 37, 402.
(20) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.;
Mu¨llen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am.
Chem. Soc. 2001, 123, 946.
Carter, K. R. J. Am. Chem. Soc. 2001, 123, 6965. (d) Setayesh,
S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mu¨llen, K.;
Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc.
2001, 123, 946. (e) Ego, C.; Grimsdale, A. C.; Uckert, F.; Yu,
G.; Srdanov, G.; Mu¨llen, K. Adv. Mater. 2002, 14, 809. (f)
Pogantsch, A.; Wenzl, F. P.; List, E. J. W.; Leising, G.;
Grimsdale, A. C.; Mu¨llen, K. Adv. Mater. 2002, 14, 1061. (g)
Lupton, J. M.; Schouwink, P.; Keivanidis, P. E.; Grimsdale,
A. C.; Mu¨llen, K. Adv. Funct. Mater. 2003, 13, 154.
(13) Shu, C. F.; Dodda, R.; Wu, F. I.; Liu, M. S.; Jen, A. K. Y.
Macromolecules 2003, 36, 6698.
(21) Miller, T. M.; Neenan, T. X.; Zayas, R.; Bair, H. E. J. Am.
Chem. Soc. 1992, 114, 1018.
(22) Ranger, M.; Leclerc, M. Macromolecules 1999, 32, 3306.
(23) Leu, C. M.; Chang, Y. T.; Wei, K. H. Chem. Mater. 2003, 15,
3721.
(24) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Adv.
Mater. 1997, 9, 798.
(14) Zeng, G.; Yu, W. L.; Chua, S. J.; Huang, W. Macromolecules
2002, 35, 6907.
(25) Wu, F. I.; Reddy, D. S.; Shu, C. F.; Liu, M. S.; Jen, A. K. Y.
(15) Xiao, S.; Nguyen, M.; Gong, X.; Cao, Y.; Wu, H.; Moses, D.;
Heeger, A. J. Adv. Funct. Mater. 2003, 13, 25.
Chem. Mater. 2003, 15, 269.
(26) (a) Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.;
Inbasekaran, M.; Woo, E. P.; Soliman, M. Acta Polym. 1998,
49, 439. (b) Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.;
Inbasekaran, M.; Wu, W. W.; Woo, E. P. Appl. Phys. Lett.
1998, 73, 629. (c) Friend, R. H.; Gymer, R. W.; Holmes, A.
B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D.
D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.;
Salaneck, W. R. Nature 1999, 397, 121.
(16) (a) Cho, H. J.; Jung, B. J.; Cho, N. S.; Lee, J.; Shim, H. K.
Macromolecules 2003, 36, 6704. (b) Lim, E.; Jung, B. J.; Shim,
H. K. Macromolecules 2003, 36, 4288.
(17) Kulkarni, A. P.; Jenekhe, S. A. Macromolecules 2003, 36,
5285.
(18) (a) List, E. J. W.; Guentner, R.; Freitas, P. S.; Scherf, U. Adv.
Mater. 2002, 14, 374. (b) Romaner, L.; Pogantsch, A.; Freitas,
P. S.; Scherf, U.; Gaal, M.; Zojer, E.; List, J. W. Adv. Funct.
Mater. 2003, 13, 597. (c) Kulkarni, A. P.; Kong, X.; Jenekhe,
S. A. J. Phys. Chem. B. 2004, 108, 8689. (d) Cho, H. J.; Jung,
B. J.; Cho, N. S.; Lee, J.; Shim, H. K. Macromolecules 2003,
36, 6704. (e) Lin, W. J.; Chen, W. C.; Wu, W. C.; Niu, Y. H.;
Jen, A. K. Y. Macromolecules 2004, 37, 2335.
(27) Grimsdale, A. C.; Lecle`re, P.; Lazzaroni, R.; MacKenzie, J.
D.; Murphy, C.; Setayesh, S.; Silva, C.; Friend, R. H.; Mu¨llen,
K. Adv. Funct. Mater. 2002, 12, 729.
(28) Zheng, L.; Waddon, A. J.; Farris, R. J.; Coughlin, E. B.
Macromolecules 2002, 35, 2375.
(19) (a) Leu, C. M.; Chang, Y. T.; Wei, K. H. Macromolecules 2003,
36, 9122. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew.
MA0479520