Synthesis and characterization of thermally stable blue light-emitting polyfluorenes containing siloxane bridges

Article abstract of DOI:10.1021/ma034622r

The first novel polyfluorene copolymers containing siloxane bridges were synthesized by Ni(0)-mediated copolymerization between 2,7-dibromo-9,9′-dihexylfluorene and a bridged fluorene monomer containing siloxane linkages. Two such bridged copolymers were prepared: PSiloBg1 (containing 1 mol % siloxane-bridged fluorene unit) and PSiloBg3 (containing 3 mol % siloxane-bridged fluorene unit). PSiloBg1 and PSiloBg3 exhibited good solubility in common organic solvents, thermal stability up to 420°C, and facile film formation. The glass transition temperatures of the bridged polymers (106 and 110°C, respectively) were higher than that of the homo poly(dihexylfluorene) (PDHF). In particular, PSiloBg3 exhibited a polymerization yield (96%) and a molecular weight (MW = 185 000) higher than those of PDHF (polymerization yield = 62%, MW = 82 000). Interestingly, after the bridged polymers had been annealed at 150 °C for 4 h in air, their PL spectra showed no significant increase in vibronic structures at 450 and 475 nm and no evidence of aggregation formation and excimers at wavelengths of >500 nm. In addition, the full width at half-maximum (fwhm) of the bridged polymers was very small (fwhm = 51-52 nm) compared to that of PDHF (fwhm = 85 nm). Collectively, the results show that the polyfluorenes with siloxane bridges exhibit thermally stable almost pure blue emission, making these polymers promising candidate materials for device applications.

Full text of DOI:10.1021/ma034622r

6704
Macromolecules 2003, 36, 6704-6710
Synthesis and Characterization of Thermally Stable Blue
Light-Emitting Polyfluorenes Containing Siloxane Bridges
Hoon -J e Ch o, Byu n g-J u n J u n g, Na m Su n g Ch o, J a em in Lee, a n d Hon g-Ku Sh im *
Center for Advanced Functional Polymers, Department of Chemistry and School of Molecular Science
(BK21), Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
Received May 12, 2003; Revised Manuscript Received J uly 2, 2003
ABSTRACT: The first novel polyfluorene copolymers containing siloxane bridges were synthesized by
Ni(0)-mediated copolymerization between 2,7-dibromo-9,9′-dihexylfluorene and a bridged fluorene
monomer containing siloxane linkages. Two such bridged copolymers were prepared: PSiloBg1 (containing
1 mol % siloxane-bridged fluorene unit) and PSiloBg3 (containing 3 mol % siloxane-bridged fluorene unit).
PSiloBg1 and PSiloBg3 exhibited good solubility in common organic solvents, thermal stability up to 420
°C, and facile film formation. The glass transition temperatures of the bridged polymers (106 and 110
°C, respectively) were higher than that of the homo poly(dihexylfluorene) (PDHF). In particular, PSiloBg3
exhibited a polymerization yield (96%) and a molecular weight (MW ) 185 000) higher than those of
PDHF (polymerization yield ) 62%, MW ) 82 000). Interestingly, after the bridged polymers had been
annealed at 150 °C for 4 h in air, their PL spectra showed no significant increase in vibronic structures
at 450 and 475 nm and no evidence of aggregation formation and excimers at wavelengths of >500 nm.
In addition, the full width at half-maximum (fwhm) of the bridged polymers was very small (fwhm )
51-52 nm) compared to that of PDHF (fwhm ) 85 nm). Collectively, the results show that the
polyfluorenes with siloxane bridges exhibit thermally stable almost pure blue emission, making these
polymers promising candidate materials for device applications.
In tr od u ction
involve the suppression of interchain aggregation and
therefore of excimer formation. These solutions, outlined
below, have led to the synthesis of polymers whose
optical properties are more thermally stable.8,9,13 Miller
et al.12,13 demonstrated that the introduction of bulky
groups (e.g., substituted fluorene or anthracene) as end
cappers or introduction of cross-linkable moieties such
as the styryl group tends to suppress excimer emission
and to enhance color stability. Huang et al.14 reported
that the inclusion of spirofluorene-functionalized bif-
luorene moieties into polyfluorenes prevented the poly-
fluorene from undergoing molecular aggregation, and
that the spectral properties of these polyfluorenes were
not degraded by treatment at higher temperatures.
Introduction of dendron side groups into the C-9 position
of polyfluorenes is also known to inhibit interchain
aggregation of polyfluorenes and to improve the thermal
stability in PL spectra.15 Other trials of cross-linking
or networking polyfluorenes were carried out by the
groups of Carter and Advincula.12 Carter et al. synthe-
sized amorphous networked polyfluorenes using a spiro-
bifluorene with a tetrareactive site and a cross-linkable
styryl end group. These networked polyfluorenes ex-
hibited good color stability and luminescence stability
even after being heated to 150 °C.12b Advincula et al.
prepared partially cross-linked polyfluorenes by reacting
precursor-type polymers with tethered fluorene units.
The PL spectra of these polyfluorenes from the oxidative
cross-linking reaction with FeCl3 were more or less
broadened compared to the spectra of other poly-
(dialkylfluorene)s due to the presence of the partially
oligomerized fluorene units in the polymer networks.12c
As shown above, it is important to prepare polyfluorene
derivatives that, even after thermal treatment, stably
emit blue light without an increased level of excimer
formation or a spectral tail at long wavelengths.
Since the first use of conjugated polymers in elec-
troluminescence devices by Friend and co-workers,1
such polymers have attracted great attention as candi-
date light-emitting materials for display applications.2-4
π-Conjugated polymers such as poly(p-phenylenevi-
nylene), polythiophene, poly(p-phenylene), and their
derivatives are easily fabricated as light-emitting diode
(LED) devices via solution processing, and can be used
in large area light-emitting displays.2-6,11-13 In 1989,
Fukuda et al.7 reported the synthesis and optical
properties of novel polyfluorenes in which the two
benzene rings of the biphenyl in each monomer are
made coplanar by linking them with a methylene group.
Subsequent work has established polyfluorenes as
promising candidates for applications requiring blue
light-emitting materials. Polyfluorenes have many prop-
erties that favor their use over other conjugated poly-
mers in blue light-emitting applications.6,8-10 They
exhibit high photoluminescence (PL) and electrolumi-
nescence efficiencies, offer facile functionalization at the
C-9 position of fluorene, and have good solubility in
common organic solvents such as chloroform, toluene,
and tetrahydrofuran (THF).13-18
The utility of polyfluorenes for polymer light-emitting
diode (PLED) applications has been limited because of
their tendency to undergo interchain aggregation when
the solid-state films of the polymers are maintained for
several hours at temperatures above Tg. This aggrega-
tion gives rise to various undesirable characteristics,
including excimer formation and a tailed emission band
at long wavelengths (>500 nm). A number of solutions
to this problem have been proposed, most of which
* To whom correspondence should be addressed. Telephone:
+82-42-869-2827. Fax: +82-42-869-2810. E-mail: hkshim@
mail.kaist.ac.kr.
10.1021/ma034622r CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/15/2003

Macromolecules, Vol. 36, No. 18, 2003
Thermally Stable Blue Light-Emitting Polyfluorenes 6705
a
Sch em e 1. Syn th etic Rou tes to th e Mon om er s a n d P olym er s22
a
(i) NaOH (50 wt % aqueous solution), toluene, TBAB, bromohexane, 2-(2-bromoethoxy)tetrahydro-2H-pyran, 100 °C, 8 h; (ii)
HCl (several drops), ethanol, 80 °C, 20 min; (iii) NaOH (50 wt % aqueous solution), toluene, TBAB, allyl bromide; (iv) hydride-
terminated siloxane (MW ∼ 580), toluene, Pt on activated carbon, 60 °C, 24 h; (v) (a) Ni(COD)2, COD, DMF, 2,2′-bipyridyl, toluene,
80 °C, 3 days; (b) bromopentafluorobenzene, 80 °C, 24 h.
Aldrich Chemical Co. and used without further purification.
Bis(1,5-cyclooctadienyl)nickel(0) was purchased from STREM
Chemicals, and hydride-terminated siloxane (MW ∼ 580) was
obtained from ShinEtsu Chemical Co. Sodium hydroxide was
purchased from J unsei Chemical Co., and all other reagents
and solvents were purchased commercially, were analytical-
grade quality, and were used without further purification.
In str u m en ta tion . 1H NMR and 13C NMR spectra were
recorded on Bruker AVANCE 300 and 400 spectrometers,
respectively, with tetramethylsilane as an internal reference.
For the NMR measurements, chloroform-d (CDCl3) was used
as the solvent. FTIR spectra were obtained using a Nicolet
model 800 spectrometer with samples prepared as KBr pellets.
The number- and weight-average molecular weights of the
polymers were determined by gel permeation chromatography
(GPC) on a Waters GPC-150C instrument, using THF as the
eluent and polystyrene as the standard. Thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
In this paper, we report a novel method for preparing
soluble, bridged polyfluorenes in which poly(dihexyl-
fluorene) chains are interconnected by siloxane linkages.
To our knowledge, this is the first report of polyfluorenes
containing siloxane moieties. Siloxane moieties are well-
known as the materials which show the oxidative,
thermal, and chemical stability and exhibit low dielec-
tric constants. In addition, the dimethylsiloxane unit
of poly(dimethylsiloxane) (PDMS) linkages occupies a
larger volume than the methylene unit of the hydro-
carbon linkage, and sufficiently long siloxane linkages
separate the interchains of the polymers at some
distance. These properties of siloxane linkages can
effectively hinder the aggregation and excimer forma-
tion of polyfluorene chains. The PL spectra of these
bridged polyfluorenes showed no evidence of excimer
formation at ambient temperature, indicating that the
polymers had not aggregated with the siloxane linkage.
In addition, after thermal treatment in the solid state,
the PL spectra of the bridged polyfluorenes showed no
significant increase in excimer formation at wavelengths
of >500 nm or the appearance of strong vibronic
structures; as a result, the polyfluorenes continued to
show almost pure blue emission even after being an-
nealed at temperatures well above Tg. In the study
presented here, the preparation of a bridged fluorene
monomer provides a facile method for asymmetric
substitution of functional groups at the C-9 positions
of the fluorene units in the polyfluorene backbones. The
molecular structures and synthetic procedures for the
novel bridged polyfluorenes containing siloxane inter-
chain bridges are shown in Scheme 1. In a few of the
fluorene units of 9,9′-dihexylfluorene polymers, one
hexyl chain at the C-9 position of dihexylfluorene was
replaced with a siloxane bridge, which then connected
the polyfluorene chains.
measurements of the polymers were performed under
a
nitrogen atmosphere at a heating rate of 10 °C/min using a
Dupont 9900 analyzer. UV-vis spectra were recorded on a
J asco V-530 UV-vis spectrometer, and PL spectra of the
polymers were recorded at room temperature on a Spex
Fluorolog-3 spectrofluorometer (model FL3-11) using spin-
coated films. Cyclic voltammetry was performed on an AU-
TOLAB/PG-STAT12 model system with a three-electrode cell
in a solution of Bu4NBF4 (0.10 M) in acetonitrile at a scan rate
of 50 mV/s. A film of each polymer was coated onto a Pt wire
electrode by dipping the electrode into a solution of the polymer
(0.5 wt % in chloroform). A Pt wire was used as the counter
electrode, and an Ag/AgNO3 (0.10 M in acetonitrile) electrode
was used as the reference electrode.
Syn th eses of Mon om er s a n d P olym er s. (1) P r ep a r a -
tion of 2-[2-(2,7-Dibr om o-9-h exylflu or en -9-yl)eth oxy]p e-
r h yd r o-2H-p yr a n (2). To a solution of 29.18 g (90 mmol) of
2,7-dibromofluorene, 11.95 g (72 mmol) of 1-bromohexane,
24.39 g (117 mmol) of 2-(2-bromoethoxy)tetrahydro-2H-pyran,
50 mL of toluene, and 1.2 g of tetrabutylammonium bromide
(TBAB) as a phase-transfer catalyst was added 50 mL of a 50
wt % NaOH aqueous solution. The reaction mixture was then
refluxed at 100 °C for 10 h, after which the reaction mixture
was cooled to ambient temperature. The resulting solution was
extracted with 200 mL of ethyl acetate from 200 mL of
saturated sodium bicarbonate, washed with water, dried over
anhydrous magnesium sulfate, and then concentrated in
Exp er im en ta l Section
Mea su r em en ts. 2,7-Dibromofluorene, 1-bromohexane, 2-(2-
bromoethoxy)tetrahydro-2H-pyran, tetrabutylammonium bro-
mide, 2,2′-dipyridyl, and 1,5-cyclooctadiene were obtained from

6706 Cho et al.
Macromolecules, Vol. 36, No. 18, 2003
Ta ble 1. P h ysica l P r op er ties of th e P olym er s
vacuo. The crude oily product was chromatographed on silica
gel with a mixture of ethyl acetate and n-hexane (1/30) as the
eluent to give 27.85 g (57.7% yield) of the product as a viscous
oil: 1H NMR (CDCl3) δ 0.58 (br s, 2H), 0.74 (t, 3H), 0.98-1.18
(m, 6H), 1.20-1.65 (m, 6H), 1.92 (m, 2H), 2.32 (t, 2H), 2.73-
3.15 (m, 2H), 3.23-3.47 (m, 2H), 4.10 (t, 1H), 7.41-7.50 (m,
6H); 13C NMR (CDCl3) δ 13.94, 19.29, 22.53, 23.32, 25.30,
29.44, 30.42, 31.41, 39.38, 40.68, 53.89, 61.88, 63.60, 98.68,
121.15, 121.47, 126.49, 130.35, 138.86, 151.71. Anal. Calcd for
feed
ratio
(x/y)
content
ratio
yield
(%)
MW
Tg
Td
polymer
PDHF
(x/y)a
(×104) PDI (°C) (°C)b
100/0 100/0
PSiloBg1 99/1
PSiloBg3 97/3
62
78
96
8.2
7.5
18.5
2.3
75c 421
2.2 106
2.4 110
98.7/1.3
96.9/3.1
425
427
a
Content ratios were calculated by NMR assignment of the
polymers, and x/y indicates the ratio of 2,7-dibromo-9,9′-dihexy-
lfluorene to the siloxane-bridged fluorene.21 b Temperature result-
ing in a 5% weight loss based on the initial weight. c From ref 12b.
C
26H32Br2O2: C, 58.22; H, 6.01. Found: C, 58.34; H, 6.17.
(2) P r ep a r a tion of 2-(2,7-Dibr om o-9-h exylflu or en -9-yl)-
eth a n -1-ol (3). To 16.21 g (30 mmol) of the THP-protected
fluorene compound, 2, in a single-necked 250-mL round-bottom
flask was added 100 mL of acidified ethanol (containing 5 mL
of a 10% HCl aqueous solution). The reaction mixture was then
refluxed for 30 min, after which it was cooled to room
temperature, and the ethanol was removed by rotoevaporation
in vacuo at 60 °C. The reaction solution was extracted with
chloroform and washed with a saturated aqueous sodium
bicarbonate solution and two 100 mL portions of water. The
combined chloroform solutions were then dried over anhydrous
magnesium sulfate and concentrated in vacuo. The crude
product was column chromatographed on silica gel using a
mixture of ethyl acetate and n-hexane (1/10) as the eluent,
yielding 10.34 g (87.6% yield) of the viscous oily product: 1H
NMR (CDCl3) δ 0.52 (br s, 2H), 0.75 (t, 3H), 1.00-1.10 (m,
6H), 1.92 (t, 2H), 2.27 (t, 2H), 2.96 (t, 2H), 7.42-7.51 (m, 6H);
13C NMR (CDCl3) δ 14.09, 22.61, 23.03, 29.49, 31.45, 40.43,
42.25, 53.76, 58.46, 121.38, 121.77, 126.33, 130.56, 138.71,
151.67. Anal. Calcd for C21H24Br2O: C, 55.77; H, 5.35. Found:
C, 55.91; H, 5.38.
(5) P r ep a r a t ion of 2,7-Dib r om o-9,9′-d ih exylflu or en e
(Mon om er -2). To a solution of 24.92 g (76.9 mmol) of 2,7-
dibromofluorene, 26.05 g (157.8 mmol) of 1-bromohexane, 50
mL of toluene, and 1.4 g of tetrabutylammonium bromide as
a phase-transfer catalyst was added 50 mL of a 50 wt % NaOH
aqueous solution. The reaction mixture was then refluxed at
100 °C for 10 h, after which it was cooled to ambient
temperature. The resulting solution was extracted with 200
mL of methylene chloride from 200 mL of saturated sodium
bicarbonate, washed with water, dried over anhydrous mag-
nesium sulfate, and then concentrated in vacuo. The crude
product was purified by recrystallization with n-hexane several
times, yielding 34.75 g (91.8% yield) of the solid product: 1H
NMR (CDCl3) δ 0.58 (br s, 4H), 0.74 (t, 6H), 1.02-1.12 (m,
12H), 1.90 (m, 4H), 7.42-7.51 (m, 6H); 13C NMR (CDCl3) δ
14.00, 22.58, 23.66, 29.59, 31.46, 40.21, 55.70, 121.13, 121.49,
126.20, 130.17, 139.08, 152.58. Anal. Calcd for C25H32Br2: C,
60.99; H, 6.55. Found: C, 61.05; H, 6.64.
(3) P r ep a r a tion of 1-[2-(2,7-Dibr om o-9-h exylflu or en -
9-yl)eth oxy]p r op -2-en e (4). The deprotected compound (3)
(9.05 g, 20 mmol), allyl bromide (12 g), and tetrabutylammo-
nium bromide (1.4 g) were dissolved in 30 mL of toluene over
the course of 10 min. To this solution was added 100 mL of a
50 wt % NaOH aqueous solution, and the reaction mixture
was stirred at 120-140 °C for 24 h. After being cooled to room
temperature, the resulting solution was extracted with chlo-
roform from brine and a saturated aqueous sodium bicarbonate
solution, and finally distilled water. The organic layer was
dried over anhydrous magnesium sulfate and concentrated in
vacuo to give an oily product. The crude product was purified
by column chromatography on silica gel using a mixture of
ethyl acetate and n-hexane (1/30) as the eluent, yielding 8.64
g (87.7% yield) of the oily product: 1H NMR (CDCl3) δ 0.63
(br s, 2H), 0.76 (t, 3H), 1.02-1.09 (m, 6H), 1.93 (m, 2H), 2.30
(t, 2H), 2.73 (t, 2H), 3.58 (d, 2H), 5.04 (m, 2H), 5.63 (m, 1H),
7.43-7.51 (m, 6H); 13C NMR (CDCl3) δ 14.00, 22.58, 23.33,
29.47, 31.46, 39.30, 40.55, 53.71, 66.20, 71.63, 116.71, 121.23,
121.61, 126.39, 130.46, 134.44, 138.77, 151.68. Anal. Calcd for
All the copolymers were synthesized by the nickel(0)-
mediated polymerization method (commonly called the Yama-
moto coupling reaction).13
(6) P r ep a r a tion of P SiloBg-3. A mixture of 0.735 g of bis-
(1,5-cyclooctadienyl)nickel(0), 0.417 g of 2,2′-dipyridyl, 0.2 mL
of 1,5-cyclooctadiene, and 5 mL of anhydrous DMF was kept
at 80 °C for 30 min under an argon atmosphere. To this
solution was added dropwise a total of 1.8 mmol of monomers,
comprising a mixture of 2,7-dibromo-9,9′-dihexylfluorene (mono-
mer-2) and siloxane containing bridged fluorene (Monomer-1)
in a molar ratio of 97/3, dissolved in 15 mL of toluene. The
resulting solution was stirred at 80 °C for 3 days. Then, 0.1 g
of bromopentafluorobenzene (the end capper) in 5 mL of
anhydrous toluene was added to the polymer solution, and the
resulting solution was stirred at 80 °C for a further 24 h. The
polymer was precipitated in 400 mL of a mixture of HCl,
acetone, and methanol (1/1/2, v/v). The filtered crude polymer
was extracted with chloroform from an NaHCO3 aqueous
solution, washed with distilled water, and dried over anhy-
drous magnesium sulfate. The organic layer was concentrated
in vacuo, dissolved in chloroform, and precipitated in methanol
again. The resulting polymer was purified by Soxhlet extrac-
tion in methanol (80 °C, 3 days) and dried under vacuum.
Finally, the dried polymer was dissolved in chloroform,
precipitated in methanol, and dried in vacuo to give 0.617 g
(96% yield) of a slightly yellow polymer: 1H NMR (CDCl3) δ 0
(m), 0.58 (m), 0.78 (t), 1.12 (m), 2.09 (m), 7.66-7.83 (m); FTIR
(KBr) 2954, 2927, 2856, 1458, 1402, 1377, 1250, 1107, 1033,
999, 885, 813, 758 cm-1. Anal. Calcd for PSiloBg-3: C, 89.44;
H, 9.67. Found: C, 87.67; H, 9.84.
C
24H28Br2O: C, 58.55; H, 5.73. Found: C, 58.46; H, 5.89.
(4) P r ep a r a tion of 1-[2-(2,7-Dibr om o-9-h exylflu or en -
9-yl)et h oxy]-3-{1-[1-(1-{1-[1-(1-{4-[2-(2,7-d ib r om o-9-h ex-
ylflu or en -9-yl)eth oxy]-1,1-dim eth yl-1-silabu toxy}-1-m eth -
yl-1-sila et h oxy)-1-m et h yl-1-sila et h oxy]-1-m et h yl-1-sila -
eth oxy}-1-m eth yl-1-sila eth oxy)-1-m eth yl-1-sila eth oxy]-1-
m eth yl-1-sila eth oxy}-3-m eth yl-3-sila bu ta n e (Mon om er -
1). A total of 0.87 g (2.0 mmol) of the vinylfluorene compound
(4) and 0.56 g (0.98 mmol) of hydride-terminated siloxane (MW
∼ 580) were dissolved in 30 mL of toluene, and 0.2 g of Pt on
activated carbon was dispersed in the solution. The resulting
solution was stirred at 60 °C for 24 h and then filtered through
a medium-size glass filter packed with Celite-545. After the
mixture had been filtered, the toluene was evaporated in vacuo
and the concentrated crude product was purified by column
chromatography using a mixture of ethyl acetate and n-hexane
(1/30) as the eluent, yielding 1.21 g (86.25% yield) of the oily
product: 1H NMR (CDCl3) δ -0.10 to 0.16 (m, 48H), 0.35 (m,
4H), 0.60 (b, 4H), 0.76 (t, 6H), 0.93-1.19 (m, 12H), 1.35 (m,
4H), 1.93 (m, 4H), 2.28 (t, 4H), 2.73 (t, 4H), 2.98 (t, 4H), 7.37-
7.56 (m, 12H); 13C NMR (CDCl3) δ 0.11, 1.1, 1.2, 14.0, 14.1,
22.6, 23.3, 23.4, 29.5, 31.4, 39.4, 40.5, 53.8, 66.5, 73.6, 121.2,
121.6, 126.4, 130.4, 138.8, 151.8. Anal. Calcd for C63H104Br4O9-
Si8: C, 48.82; H, 6.76. Found: C, 48.27; H, 6.49.
(7) P r ep a r a tion of P SiloBg-1 a n d P oly(d ih exylflu o-
r en e) (P DHF ). The polymerization and purification proce-
dures for PSiloBg-1 and PDHF were the same as those used
for PSiloBg-3; the polymerization yields are listed in Table 1.
1
The H NMR and FTIR spectra of PSiloBg-1 and PDHF were
similar to those of PSiloBg-3. Anal. Calcd for PSiloBg-1: C,
90.01; H, 9.69. Found: C, 89.47; H, 9.83. Anal. Calcd for
PDHF: C, 90.30; H, 9.70. Found: C, 90.42; H, 9.96.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . We first synthe-
sized 2-[2-(2,7-dibromo-9-hexylfluoren-9-yl)ethoxy]per-
hydro-2H-pyran (2) having different substituents at the

Macromolecules, Vol. 36, No. 18, 2003
Thermally Stable Blue Light-Emitting Polyfluorenes 6707
9 position of fluorene by alkylation of 2,7-dibromofluo-
rene, and then deprotected 2 under acidic conditions to
obtain 2-(2,7-dibromo-9-hexylfluoren-9-yl)ethan-1-ol (3).
To synthesize compound 2, at first we mixed three
starting reagents [2,7-dibromofluorene, 1-bromohexane,
and 2-(2-bromoethoxy)tetrahydro-2H-pyran] at a feeding
ratio of 1/1/1. With regard to the feeding ratio of the
reagents, the products were predicted to be in a ratio
of 1/1/1 (33.3/33.3/33.3, mol %) for 2,7-dibromo-9,9′-
dihexylfluorene, 2-{2-[2,7-dibromo-9-(2-perhydro-2H-
pyran-2-yloxyethyl)fluoren-9-yl]ethoxy}perhydro-2H-
pyran, and 2-[2-(2,7-dibromo-9-hexylfluoren-9-yl)ethoxy]-
perhydro-2H-pyran, respectively. But, unexpectedly the
yield of 2-[2-(2,7-dibromo-9-hexylfluoren-9-yl)ethoxy]-
perhydro-2H-pyran (compound 2) was 43 mol %. The
higher yield of compound 2, compared to the yields of
the other two products, probably occurs because the
reaction is accomplished in a two-phase solution of
water and toluene, and the phase-transfer reaction is
advantageous for the substitution of 2-(2-bromoethoxy)-
tetrahydro-2H-pyran which has both a hydrophilic
ethoxytetrahydro-2H-pyran moiety and a hydrophobic
bromoethoxy moiety compared to 1-bromohexane which
is a very hydrophobic material. Also, the substitution
of two 2-(2-bromoethoxy)tetrahydro-2H-pyrans into the
9 position of a fluorene compound is shown to be difficult
because of the sterically bulky size of 2-(2-bromoethoxy)-
tetrahydro-2H-pyran compared to 1-bromohexane.
F igu r e 1. 1H NMR spectrum of Monomer-1 in the range of
-1 to 8 ppm (in CDCl3).22
Therefore, we used the higher feeding ratio of 2-(2-
bromoethoxy)tetrahydro-2H-pyran to obtain the higher
yield of compound 2. Moreover, we used the overdose
of two alkyl bromides on the whole to prevent the
formation of 9-monoalkylated fluorene derivatives which
generate the keto defect sites of fluorene derivatives.20
The resulting yield of compound 2 was ∼58 mol %.
Compounds 2 and 3 were clearly isolated from the
reactants and other byproducts at the respective syn-
thetic steps by column chromatography, as judged from
the evident separation of their spots on the TLC plate.
After allylation of 3 under basic conditions, the siloxane
oligomer-bridged fluorene monomer (Monomer-1) was
prepared by hydrosilation between 1-[2-(2,7-dibromo-
9-hexylfluoren-9-yl)ethoxy]prop-2-ene (4) and the H-
terminated siloxane (MW ∼ 580). 2,7-Dibromo-9,9′-
dihexylfluorene (Monomer-2) was synthesized by alkyla-
tion of 2,7-dibromofluorene. The new siloxane-bridged
fluorene monomer (Monomer-1) was successfully syn-
F igu r e 2. 1H NMR spectra of the polymers (in CDCl3): (a)
1H NMR spectra of the polymers in the range of -1 to 10 ppm
and (b) 1H NMR spectra of the polymers in the range of -1 to
3 ppm.
1
thesized and confirmed by elemental analysis, the H
NMR spectrum, and the 13C NMR spectrum, and the
1H NMR spectrum of Monomer-1 is shown in Figure 1.
Monomer-1 and Monomer-2 were polymerized via the
Ni(0)-mediated coupling reaction to give PSiloBg1 and
PSiloBg3. The crude polymers were purified using a
previously specified procedure.11a The polymerization
yields are reported in Table 1. The polymerization yield
of PSiloBg3 (96%) was higher than that of PSiloBg1
(78%), but the polymerization yield of PSiloBg1 exceeded
that of PDHF (62%). The molecular structures of PSi-
loBg1 and PSiloBg3, shown in Scheme 1, were con-
firmed by 1H NMR and FTIR spectroscopy. Both the 1H
NMR and FTIR spectra of the copolymers (Figures 2
and 3, respectively) exhibited characteristics of PDHF
due to the similarity of the molecular structures of the
copolymers and PDHF, and additionally exhibited peaks
due to the siloxane groups. For PSiloBg1 and PSiloBg3,
proton peaks characteristic of Si-CH3 groups in silox-
F igu r e 3. FTIR spectra of the polymers: first spectrum,
homo-PF; second spectrum, PSiloBg1; and third spectrum,
PSiloBg3 in a descending series.
(Figure 2), and the FTIR spectra contain Si-O stretch-
ing bands at 1107 and 1033 cm-1 and a Si-C stretching
band at 1250 cm-1 (Figure 3). The spectral features
associated with the siloxane groups, proton peaks near
0 ppm in the 1H NMR spectra and Si-O stretching
bands near 1107 and 1033 cm-1 in the FTIR spectra,
were more intense in the PSiloBg3 spectra than in those
of PSiloBg1. The content ratio of a siloxane-bridged
1
ane units appear in the H NMR spectrum near 0 ppm

6708 Cho et al.
Macromolecules, Vol. 36, No. 18, 2003
Ta ble 2. Op tica l P r op er ties of th e P olym er s
λmax (UV, nm)
solution film solution film UV/nm)a ΦPL
λmax (PL, nm)
Eg (eV,
b
polymer
PDHF
PSiloBg1
PSiloBg3
385
385
386
391
392
391
415
415
415
425
425
424
2.91
2.91
2.91
0.82
0.83
0.86
a
Eg, calculated from the onset values of the absorption spectra
b
of the spin-coated film on quartz. Solution fluorescence quantum
yields measured in chloroform relative to quinine sulfate (ca. 1 ×
10-5 M) in 0.10 M H2SO4 as a standard.
F igu r e 4. TGA traces and DSC thermograms of the polymers.
fluorene monomer in the polymers was calculated from
the ratio of the integrals of the 1H NMR spectral
features corresponding to aromatic protons and siloxane
unit protons (near 0 ppm).22 The calculated content
ratios were similar to the polymerization feed ratios; the
content ratios of the monomers in the polymers and the
higher polymerization yields of the bridged polymers
compared to those of PDHF show that the network
systems of the bridged polymers were successfully
fabricated.12b
F igu r e 5. UV-visible absorption and photoluminescence
spectra of the polymers as spin-coated films (2000 rpm,
p-xylene).
PSiloBg1 and PSiloBg3 exhibited good solubility in
common organic solvents such as chloroform, tetra-
hydrofuran, toluene, p-xylene, and chlorobenzene with-
out any gel formation. It seems that the low levels of
the flexible siloxane bridges do not reduce the solubility
of the copolymers. All the copolymers easily formed films
when solutions of the polymers in chloroform were cast
on a glass substrate and the solvent was evaporated
under air. The molecular weights of the synthesized
polymers were determined by gel permeation chroma-
tography (GPC) against polystyrene standards with
tetrahydrofuran (THF) as the eluent. The molecular
weights and polydispersity indices of the polymers are
reported in Table 1. The molecular weight of PSiloBg3
(Mn ) 76 000, MW ) 185 000) was higher than that of
PSiloBg1 (Mn ) 34 000, MW ) 75 000) or PDHF (Mn )
35 000, MW ) 82 000). Moreover, PSiloBg3 exhibited
molecular weights higher than those previously reported
for networked or cross-linked polyfluorenes.12 The rea-
son behind the higher molecular weights of PSiloBg3
is likely that this polymer contains the highest siloxane
bridge content of the polymers, which presumably leads
to the creation of larger network systems in PSiloBg3
than in PSiloBg1 and PDHF. The polydispersity indices
of the polymers equaled 2.2-2.4.
The glass transition temperatures (Tg) of the bridged
polyfluorenes were determined by DSC (heating rate of
10 °C/min) and are reported in Table 1. The glass
transition temperatures increase with increasing silox-
ane content, and the Tg values for PSiloBg1 and PSi-
loBg3 (106 and 110 °C, respectively) are higher than
that of PDHF (75 °C). The observation of a higher Tg
for PSiloBg3 is attributed to that polymer having a more
extensive network system. In addition, the observation
of a higher Tg for the bridged polymers than for PDHF
provides further evidence that PSiloBg1 and PSiloBg3
are the network systems of bridged polymers. The
thermal stabilities of the polymers were measured by
TGA (heating rate of 10 °C/min). The copolymers
exhibited good thermal stability. As shown in Figure 4,
the decomposition temperatures for 5% weight loss (Td)
of the copolymers were above 420 °C,17 and the copoly-
mers exhibited no significant weight loss upon being
heated to ∼400 °C (see Table 1 for Td values).
Op tica l a n d P h otolu m in escen ce P r op er ties. The
key features of the UV-vis and PL spectra of the
bridged polyfluorenes in p-xylene are listed in Table 2.
The maximum absorption wavelengths of PSiloBg1 and
PSiloBg3 in p-xylene were 385 and 386 nm, respectively.
The maximum absorption wavelengths and spectral
patterns of the polymers in p-xylene are not significantly
different from those of PDHF.14 The PL spectra of the
polymers in p-xylene exhibit a maximum emission
wavelength of 415 nm for both PSiloBg1 and PSiloBg3.
Moreover, the onsets and shapes of the emission spectra
in p-xylene are the same for PSiloBg1, PSiloBg3, and
PDHF. This result is consistent with the findings of
Huang et al.,14 who reported that substitution of the 9
position of fluorene units of a polyfluorene does not
remarkably change the electronic structure of the poly-
fluorene.
The UV-vis and PL spectra of thin films of PSiloBg1
and PSiloBg3 on quartz plates prepared by spin casting
at 2000 rpm of a 1 wt % polymer solution in p-xylene
are shown in Figure 5. The UV-vis absorption patterns
of the two bridged polyfluorenes are similar, showing
absorption maxima in the range of 391-392 nm, and
these absorption patterns are the same as that of PDHF.
The PL spectra of the polymer thin films (Figure 5) are
also similar to each other and to those of PDHF in onset
and shape, with a PL maximum of ∼425 nm. In previous
studies, the aggregation peak of PDHF was observed
at ∼525 nm;16 however, in our experiments, this peak
was indistinct. This discrepancy is attributed to the end
capping of the copolymers and the higher molecular
weight (MW ) 8.4 × 104), compared to that of PDHF,
used in the study presented here.13 Miller et al.13a found
that high-molecular weight fluorene copolymers may not

Macromolecules, Vol. 36, No. 18, 2003
Thermally Stable Blue Light-Emitting Polyfluorenes 6709
F igu r e 7. Photoluminescence spectra of PDHF (‚‚‚), PSiloBg1
(- - -), and PSiloBg3 (s) as spin-coated films (1500 rpm,
p-xylene) and annealed at 150 °C for 4 h.
F igu r e 6. Photoluminescence spectra of PDHF (‚‚‚), PSiloBg1
(- - -), and PSiloBg3 (s) as spin-coated films (1500 rpm,
p-xylene) and annealed at 150 °C for 30 min.
Ta ble 3. Nor m a lized P ea k In ten sity a n d F u ll Wid th a t
Ha lf-Ma xim u m of th e P olym er s
exhibit any emission at longer wavelengths. The photo-
luminescent quantum yields of PSiloBg1 and PSiloBg3,
which are listed in Table 2, are similar to those of
PDHF.17 The PL quantum yields of PSiloBg1 and
PSiloBg3 in solution were 0.83 and 0.86, respectively.17
annealed at
150 °C, 30 min
annealed at
150 °C, 4 h
fwhmb
fwhmb
polymer 0-0a 0-1a 0-2a (nm) 0-0a 0-1a 0-2a (nm)
Spin-coated thin films of PSiloBg1 and PSiloBg3 on
quartz plates were used to examine the thermal stability
of the optical properties of the bridged copolymers. The
films were maintained at 150 °C for 30 min in air, and
then cooled to room temperature.12,14 The PL spectra of
the annealed copolymer films are shown in Figure 6.
The PL spectrum of the annealed PSiloBg3 film exhibits
no additional peaks at wavelengths of >500 nm due to
aggregation or excimer formation, and no increased
vibronic peak intensities. In contrast, under the same
experimental conditions, the PL spectrum of PDHF
annealed at 150 °C exhibits peaks characteristic of
aggregation and excimer formation at wavelengths of
>500 nm. Thus, the introduction of siloxane bridges into
polyfluorenes seems to restrict aggregation of the poly-
mer chains. Previous studies have found that the
polymer chains of polyfluorenes heated to a temperature
above their Tg pack more closely with each other
because of increased diffusive motion and that PDHF
annealed at >100 °C shows an additional emission peak
above 500 nm due to the formation of interchain
excimers.14 The reduced levels of excimer formation and
aggregation in the bridged copolymers are probably due
to the siloxane bridges preventing the free rearrange-
ment of the polyfluorene chains. This property of silox-
ane-bridged polyfluorenes can potentially be exploited
to create polyfluorene derivatives with thermally stable
spectral features. For example, the copolymer PSiloBg3
prepared in this work does not exhibit an increase in
the magnitudes of vibronic peaks and aggregation peaks
after annealing at temperatures well above its glass
transition temperature, most likely because the bridging
system in this polymer lowers the mobility of the
polymer chains and the flexible siloxane linkages occupy
sufficient volume to prevent efficiently the aggregation
of the polymer chains.
PDHF
1.00 0.65 0.28
43
43
44
1.00 0.96 0.65
1.00 0.82 0.41
1.00 0.81 0.39
85
52
51
PSiloBg1 1.00 0.68 0.27
PSiloBg3 1.00 0.66 0.27
a
Columns denoted 0-1 and 0-2 give the relative peak intensity
of the resolved peaks at 448 and 476 nm, respectively, in the
standards of the peak at 424 nm (0-0).18 fwhm means full width
b
at half-maximum.
PSiloBg3 exhibited remarkably suppressed vibronic
peaks and aggregation peaks, and a small fwhm (51-
52 nm) compared to that of PDHF. Thus, the inclusion
of a bridging system in the polyfluorene structure seems
to restrict the various vibronic bands of the polymer
chains, aggregation, and the formation of excimers, even
after thermal treatment at high temperatures, thereby
ensuring the spectral stability of the bridged polymers.14
For purposes of comparison, the fwhm values and
relative peak intensity around 425, 450, and 475 nm of
the thin films of PSiloBg1, PSiloBg3, and PDHF an-
nealed at 150 °C for 30 min and 4 h are listed in Table
3.19
Electr och em ica l P r op er ties. To investigate the
electrochemical properties of the copolymers and esti-
mate the HOMO and LUMO energy levels, cyclic
voltammetry (CV) was carried out using a copolymer-
coated platinum electrode as the working electrode, a
platinum wire as the counter electrode, and an Ag/
AgNO3 (0.10 M) electrode as the reference electrode. CV
was performed with these three electrodes immersed in
a solution of 0.1 M tetrabutylammonium tetrafluorobo-
rate (Bu4NBF3) in anhydrous acetonitrile at room tem-
perature under nitrogen with a scan rate of 50 mV/s.
The measurements were calibrated using ferrocene as
the standard. Figure 8 exhibits the oxidation wave of
the copolymers induced by p-type doping. From the first
oxidation process, the HOMO energy levels of PSilo1Bg1
and PSilo3Bg3 were estimated to be 5.85 and 5.86 eV,
respectively; these values coincide with the HOMO
energy level (5.85 eV) of PDHF.17 The LUMO energy
levels of the polymers were estimated from the onset of
the absorption spectra of the copolymer films. The
LUMO energy levels of PSiloBg1, PSiloBg3, and PDHF
were estimated to be 2.94, 2.95, and 2.94 eV, respec-
tively. The band gap energies of the bridged copolymers
Additional experiments in which the polymer films
were annealed at 150 °C for 4 h yielded very interesting
results. As shown in Figure 7, after PDHF had been
annealed for 4 h, its PL spectra exhibited a very broad
fwhm (full width at half-maximum) (85 nm) as well as
more intense vibronic peaks at 450 and 478 nm, ag-
gregation peaks, and excimer formation at >500 nm.
In contrast, after 4 h at 150 °C the PL spectrum of

6710 Cho et al.
Macromolecules, Vol. 36, No. 18, 2003
blue light-emitting materials. In future work, we intend
to focus on the improvement of the electroluminescent
properties of bridged polyfluorenes.
Ack n ow led gm en t. We gratefully acknowledge the
support of the Center for Advanced Functional Polymers
(CAFPoly) through KOSEF.
Refer en ces a n d Notes
(1) Bourroughes, J . H.; Bradley, D. D. C.; Brown, A. R.; Marks,
R. N.; Mackey, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B.
Nature 1990, 347, 539.
(2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Eng. 1998, 37, 402.
(3) Shim, H. K.; J in, J . I. Adv. Polym. Sci. 2002, 158, 194.
(4) (a) Hwang, D. H.; Shim, H. K.; Lee, J . I.; Lee, K. S. Chem.
Commun. 1994, 2461. (b) Kim, S. T.; Hwang, D.; Li, X. C.;
Gruner, J .; Friend, R. H.; Holmes, A. B.; Shim, H. K. Adv.
Mater. 1996, 8, 879. (c) Ko, S. W.; J ung, B. J .; Ahn, T.; Shim,
H. K. Macromolecules 2002, 35, 6217.
F igu r e 8. Cyclic voltammograms of PSiloBg1 and PSiloBg3.
Ta ble 4. Electr och em ica l P r op er ties a n d En er gy Levels
of th e P olym er s
(5) Pei, Q.; Yang, Y. J . Am. Chem. Soc. 1996, 118, 7416.
(6) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997,
30, 7686.
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(8) Yu, W.-L.; Pei, J .; Cao, Y.; Huang, W.; Heeger, A. J . Chem.
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(9) Pei, J .; Yu, W.-L.; Huang, W.; Heeger, A. J . Chem. Commun.
2000, 1631.
(10) 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.
(11) (a) Cho, N. S.; Hwang, D. H.; Lee, J . I.; J ung, B. J .; Shim, H.
K. Macromolecules 2002, 35, 1224. (b) Hwang, D. H.; Cho,
N. S.; Lee, J . I.; Shim, H. K. Opt. Mater. 2003, 21, 199. (c)
J ung, B. J .; Lee, J . I.; Chu, H. Y.; Do, L. M.; Shim, H. K.
Macromolecules 2002, 35, 2282. (d) Lim, E. H.; J ung, B. J .;
Shim, H. K. Macromolecules 2003, 36, 4288.
(12) (a) Kla¨rner, G.; Lee, J . I.; Lee, V. Y.; Chan, E.; Chen, J . P.;
Nelson, A.; Markiewicz, D.; Scott, J . C.; Miller, R. D. Chem.
Mater. 1999, 11, 1800. (b) Marsitzky, D.; Murray, J .; Scott,
J . C.; Carter, K. R. Chem. Mater. 2001, 13, 4285. (c) Inaoka,
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(13) (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.
p-doping (V vs SCE)
Eg
HOMO LUMO
polymer
PDHF
Epa
Epc
E1/2 (eV, UV/nm) (eV)a
(eV)b
1.14
PSiloBg1 1.10
PSiloBg3 1.15
0.89
0.93
0.90
1.02
1.02
1.03
2.91
2.91
2.91
5.85
5.85
5.86
2.94
2.94
2.95
a
Determined from E1/2 (energy level of ferrocene of 4.8 eV under
b
vacuum).17 Calculated from the HOMO and Eg.
and PDHF were obtained from the onset of the UV
spectrum of each polymer; all the polymers exhibited a
band gap energy of 2.91 eV.17 Thus, within the
errors of the experimental measurements, PSilo1Bg1,
PSilo3Bg3, and PDHF have the same HOMO and
LUMO energy levels and band gap energies. This result
indicates that the siloxane linkages of the bridged
copolymers are not involved in the oxidation of the
polymers, and that the siloxane linkages do not affect
the redox properties of the fluorene copolymers com-
pared to those of PDHF. The HOMO-LUMO energy
levels and band gap energies of the polymers are listed
in Table 4.
(14) (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.
Su m m a r y
(15) (a) 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. (b) Marsitzky, D.; Vestberg, R.;
Blainey, P.; Tang, B. T.; Hawker, C. J .; Carter, K. R. J . Am.
Chem. Soc. 2001, 123, 6965. (c) Tang, H. J .; Fujiki, M.; Zhang,
Z. B.; Torimitsu, K.; Motonaga, M. Chem. Commun. 2001,
2426. (d) Chou, C. H.; Shu, C. F. Macromolecules 2002, 35,
9673.
(16) Xia, C.; Advincula, R. Macromolecules 2001, 34, 5854.
(17) Suh, Y. S.; Ko, S. W.; J ung, B. J .; Shim, H. K. Opt. Mater.
2002, 21, 109.
(18) Wu, F. I.; Reddy, S.; Shu, C. F.; Liu, M. S.; J en, A. K. Y. Chem.
Mater. 2003, 15, 269.
(19) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365.
(20) Scherf, U.; List, E. J . W. Adv. Mater. 2002, 14, 477.
(21) The calculation equation of x/y is x/y ) (48A - 12B)/6B, where
A denotes the integral of peaks at 7.5-8.0 ppm of 1H NMR
spectra of the copolymers, B denotes the integral of near 0
ppm of 1H NMR spectra of the copolymers, 48 is derived from
the H number of the siloxane linkages in Monomer-1, 12 is
derived from the H number of the fluorene moieties in
Monomer-1, and 6 is derived from the H number of the
fluorene moieties in Monomer-2.
We have synthesized a novel type of bridged fluorene
copolymer containing siloxane linkages via Ni-mediated
polymerization. This work is related to the first reported
polyfluorene derivatives containing siloxane units. The
bridged polyfluorenes exhibited higher polymerization
yields, molecular weights (for PSiloBg3), and glass
transition temperatures than PDHF. Additionally, in
contrast to PDHF, the bridged polyfluorenes exhibited
almost pure blue PL emission. Importantly, even after
being annealed at 150 °C for 30 min, the bridged
polyfluorenes exhibited structureless PL spectra and a
narrow fwhm (43-44 nm). Moreover, the PL spectra of
the bridged polyfluorenes showed no increase in the
magnitudes of vibronic peaks and excimer peaks above
500 nm and exhibited very small fwhm values (51 nm)
even after being annealed at 150 °C for 4 h in air. In
contrast, the PL spectra of PDHF annealed at 150 °C
for 4 h exhibited strong vibronic bands at ∼450 and
∼478 nm and more intense excimer peaks above 500
nm, and the PL spectral fwhm of PDHF was very large
(85 nm). These results highlight the promise of poly-
fluorenes with siloxane bridges as candidates for pure
(22) Because the siloxane linkage is a polydisperse material, the
accurate expression of the siloxane linkage is [Si(CH3)2O]7-
Si(CH3)2.
MA034622R