Poly(3,6-silafluorene-co-2,7-fluorene)-based high-efficiency and color-pure blue light-emitting polymers with extremely narrow band-width and high spectral stability

Article abstract of DOI:10.1039/b609250k

A new series of copolymers (PSiFF) based on 3,6-silafluorene and 2,7-fluorene were synthesized via the Suzuki polycondensation. The influences of the 3,6-silafluorene content in the copolymers on the thermal, photophysical, electroluminescence and electrochemical properties were investigated. The incorporation of 3,6-silafluorene into the polyfluorene main chain not only suppressed the long-wavelength emission but significantly improved the efficiency and color purity of the copolymer-based devices. Both fluorene and 3,6-silafluorene took part in π-conjugation of the main chain and the electroluminescence spectra were remarkably blue-shifted with increasing wide bandgap 3,6-silafluorene content. The device based on PSiFF90 with the configuration of ITO/PEDOT: PSS/PVK/polymer/Ba/Al showed a quantum efficiency of 3.34% and a luminous efficiency of 2.02 cd A^-1 at a brightness of 326 cd m^-2 with CIE 1931 chromaticity coordinates of (0.16, 0.07), which almost match the NTSC standard blue pixel coordinates of (0.14, 0.08). Moreover, the incorporation of 3,6-silafluorene into the polyfluorene main chain significantly improved the spectral stability at annealing. The results indicate poly(3,6-silafluorene-co-2,7-fluorene) could be a promising candidate for blue-emitting polymers of high efficiency and good color purity. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b609250k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Poly(3,6-silafluorene-co-2,7-fluorene)-based high-efficiency and color-pure
blue light-emitting polymers with extremely narrow band-width and high
spectral stability{{
Ergang Wang, Chun Li, Yueqi Mo, Yong Zhang, Gang Ma, Wei Shi, Junbiao Peng,* Wei Yang and
Yong Cao*
Received 29th June 2006, Accepted 17th August 2006
First published as an Advance Article on the web 1st September 2006
DOI: 10.1039/b609250k
A new series of copolymers (PSiFF) based on 3,6-silafluorene and 2,7-fluorene were synthesized
via the Suzuki polycondensation. The influences of the 3,6-silafluorene content in the copolymers
on the thermal, photophysical, electroluminescence and electrochemical properties were
investigated. The incorporation of 3,6-silafluorene into the polyfluorene main chain not only
suppressed the long-wavelength emission but significantly improved the efficiency and color
purity of the copolymer-based devices. Both fluorene and 3,6-silafluorene took part in
p-conjugation of the main chain and the electroluminescence spectra were remarkably blue-shifted
with increasing wide bandgap 3,6-silafluorene content. The device based on PSiFF90 with the
configuration of ITO/PEDOT : PSS/PVK/polymer/Ba/Al showed a quantum efficiency of 3.34%
2
1
22
and a luminous efficiency of 2.02 cd A at a brightness of 326 cd m with CIE 1931
chromaticity coordinates of (0.16, 0.07), which almost match the NTSC standard blue pixel
coordinates of (0.14, 0.08). Moreover, the incorporation of 3,6-silafluorene into the polyfluorene
main chain significantly improved the spectral stability at annealing. The results indicate
poly(3,6-silafluorene-co-2,7-fluorene) could be a promising candidate for blue-emitting polymers
of high efficiency and good color purity.
long-wavelength emission was attributed to the formation of
5
excimer/aggregates of polymer chains and/or the presence of
Introduction
Polymer light-emitting diodes (PLEDs) have attracted sig-
nificant attention in both academic fields and industry because
of their potential applications in large-area, flat-panel displays
quenching defects such as ketone groups on the 9-carbon
6,7
of the fluorene ring formed upon oxidation.
Although
various approaches have been tried to suppress the long-
5,6,8
1
which can operate at relatively low driving voltages. For full-
wavelength emission,
color purity and efficiency remain the
color display applications, red, green and blue light-emitting
materials (polymers, oligomers or small organic molecules)
key problems for fluorene-based blue-emitting polymers.
Most of the blue polymer emitters with high efficiency
2
,3
with high efficiency, color purity and stability are required.
reported so far in the scientific literature show CIE coordinates
9,10
During the past decade, green and red (orange) light-emitting
3
polymers have already been developed. Compared with green
(
0.15, 0.20) and over.
A typical example of this category
10
is reported by Wu et al. from the Dow Chemical Company
and red (orange), a good blue polymer emitter with high
efficiency and good color purity remains a big challenge.
Polyfluorenes (PFs) and their derivatives have evolved as a
major class of blue-emitting materials for LEDs because of
their high photoluminescent quantum yield, good solubility
for blue-emitting polyfluorene copolymers with CIE coordi-
21
nates (0.17, 0.21) and a good efficiency of 3–4 cd A
22
at
1
00–1000 cd m , though no detailed chemical structure has
been reported. Besides the efficiency, the color saturation is
most important for display applications because in order to
display all colors of the visible spectrum by synthesizing them
from the three primary colors their respective colors have to be
as saturated as possible, that is the color coordinates have to
4
and acceptable charge carrier mobility. However, an
undesired emission appearing on the long-wavelength side
from 500 to 600 nm in both photoluminescence (PL) and
electroluminescence (EL) of PFs not only limits the
11
be positioned close to the borders of the color triangle. It is
4
emission efficiency but damages the blue color purity. This
ideal for the CIE coordinates of blue light-emitting polymers in
practical display applications to match with CIE coordinates
Institute of Polymer Optoelectronic Materials and Devices, Key
Laboratory of Specially Functional Materials, South China University of
Technology, Guangzhou 510640, P. R. China.
E-mail: poycao@scut.edu.cn; psjbpeng@scut.edu.cn;
Fax: +(86) 20 8711 0606; Tel: +(86) 20 8711 4609
(
0.14, 0.08) specified in the National Television System
Committee (NTSC) standard. Recently, several groups
reported deep-blue emitters, the CIE coordinates of which
matched the NTSC standard, yet the efficiencies of the devices
8c,12
based on such emitters were still low.
{
Electronic supplementary information (ESI) available: NMR, CV
and DSC data. See DOI: 10.1039/b609250k
The HTML version of this article has been enhanced with colour
For example, Miteva
et al. reported end-capped polyfluorenes of good deep-blue
{
images.
emission with CIE coordinates (0.15, 0.08) and an efficiency of
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 4133–4140 | 4133

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2
1 12a
.
1
.1 cd A
Hung et al. reported TAZ-end-capped poly-
Synthesis
,7-Dibromo-9,9-dioctylfluorene (8) and 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (9) were
lfluorenes showing blue emission peaked at 442 nm with a
2
maximum efficiency of 1.67 cd A and CIE coordinates of
2
1
8
c
(
0.165, 0.076).
In our previous work, we synthesized poly(3,6-silafluorene)
1
6
prepared according to the published procedures.
1
3
(
PSiF) homopolymers via the Yamamoto coupling reaction,
N,N9-Biphenyl-2,29-diylbiacetamide (3). When a mixture of
,29-dinitrobiphenyl (1) (20.0 g, 82.0 mmol), iron powder
Similar polymers were soon after reported via a different
2
1
4
15
synthetic route. Unlike poly(2,7-silafluorene), poly(3,6-
silafluorene) homopolymers showed an extremely wide band-
gap (ca. 4 eV) with a PL emission peak in the violet region
(
40.0 g, 714 mmol) and ethanol (1000 mL) was heated to
reflux, aqueous HCl (36% w/w, 120 mL) was added dropwise
over 30 min. After refluxing for a further 2 h, the excess iron
powder was filtered off and the filtrate was evaporated to
dryness. The residue was added to a mixture of citric acid
1
3
15
(
360–370 nm). As was reported by Chan et al., both the
poly(2,7-silafluorene) homopolymer and the poly(2,7-silafluor-
ene-alt-2,7-fluorene) copolymer showed EL and PL properties
similar to those of poly(9,9-dioctyl-2,7-fluorene) (PFO). It will
be interesting to see how the incorporation of a wide bandgap
(
110.0 g, 0.52 mol) and potassium sodium tartrate (130.0 g,
.46 mol) in 500 mL of water. Then saturated aqueous
Na CO solution was added until the pH reached 8. The
precipitate was filtered and then washed with water to give
,29-diaminobiphenyl (2), which was added to a Na CO
solution (30 g in 200 mL of water) with acetic anhydride
50 mL). After being cooled to 0 uC, 3 was obtained by
filtration. Recrystallization from ethanol–water (3 : 1) afforded
as white crystals (13.0 g, 59%). Mp 168–169 uC. GC-MS (EI
0
2
3
3
,6-silafluorene unit into the polyfluorene main chain would
affect the emitting properties of the copolymers. In this paper,
we report the synthesis and characterization of a new series
of copolymers (PSiFF) based on 3,6-silafluorene (SiF) and
2
2
3
(
2
,7-fluorene. The influences of the 3,6-silafluorene content
in the copolymers on the thermal, photophysical, EL and
electrochemical properties were investigated. Both fluorene
and 3,6-silafluorene are expected to take part in p-conjugation
of the main chain and thereby the emission spectra will be
significantly blue-shifted with increasing the wide bandgap 3,6-
silafluorene content. We found out that the incorporation of
3
+
m/z): 268 (M ). Anal. Calcd for C16
.01; N, 10.44. Found: C, 71.58; H, 6.69; N, 10.70. IR (solid
state): 3291, 3206, 3173, 3013, 2821, 1660, 1578, 1532, 1470,
16 2 2
H N O (%): C, 71.62; H,
6
1
7
450, 1433, 1369, 1312, 1298, 1032, 1007, 972, 944, 768,
21 1
46 cm . H NMR (300 MHz, CDCl ) d(ppm) 8.07 (2H, d,
3
3
,6-silafluorene into the polyfluorene main chain suppresses
J = 8.1, ArH), 7.42 (2H, t, J = 9.1, ArH), 7.24 (4H, m,
the long-wavelength emission and improves the spectral stabi-
lity of the copolymers at annealing and the device efficiencies.
1
3
ArH), 7.06 (2H, b, NH), 1.96 (6H, s, CH
75 MHz, CDCl ) d(ppm) 169.2, 135.4, 130.5, 129.3, 125.3,
23.6, 77.1, 24.1.
3
). C NMR
(
3
1
Experimental
N,N9-(5,59-Dibromobiphenyl-2,29-diyl)biacetamide (4).
mixture of 3 (10.0 g, 37.3 mmol), N-bromosuccinimide
14.2 g, 79.8 mmol) and N,N-dimethylformamide (100 mL)
was stirred for 12 h at room temperature, and then poured into
00 mL of water. The precipitate was filtered and washed with
A
Materials
All chemicals and reagents were used as received from Aldrich,
ABCR, and Acros Chemical Co. unless otherwise specified. All
solvents were carefully dried and purified under nitrogen. All
manipulations involving air-sensitive reagents were performed
under a dry argon atmosphere.
(
2
water. Recrystallization from ethanol afforded 4 as a yellow–
white powder (14.0 g, 88%). Mp 280 uC. Anal. Calcd for
C
16
H
14Br
2
N
2
O
2
(%): C, 45.10; H, 3.31; N, 6.57. Found: C,
4.93; H, 3.51; N, 6.67. IR (solid state): 3356, 3057, 2931, 1675,
1575, 1515, 1450, 1413, 1392, 1374, 1298, 1085, 1025, 888, 828,
Measurement and characterization
4
1
13
H and C NMR spectra were recorded on a Bruker AV-300
300 MHz and 75 MHz respectively) in deuterated chloroform
2
1 1
(
695, 664, 597 cm . H NMR (300 MHz, DMSO-d
6
) d(ppm)
or dimethyl sulfoxide solution. The Fourier transform
infrared (FTIR) spectra of all complexes were recorded on a
Bruker FTIR spectrometer using KBr pellets at room
temperature. Melting points were determined using a polarized
optical microscope of Zeiss Axiophot with a Linkam hot stage.
GC-MS experiments were recorded on a Finnigan Trace 2000.
8.92 (2H, d, J = 11.6, NH), 7.56 (4H, b, ArH), 7.39 (2H, t,
3
1
ArH), 1.80 (6H, d, J = 2.8, CH
3
). C NMR (75 MHz, DMSO-
6
d ) d(ppm) 169.1, 135.6, 133.7, 133.6, 131.5, 127.8, 117.5, 23.5.
5,59-Dibromo-2,29-diiodobiphenyl (6). To a refluxing solution
of 4 (10.0 g, 23.5 mmol) in ethanol (200 mL) and water
(200 mL) was added dropwise concentrated HCl (100 mL).
After refluxing for a further 3 h, the ethanol was evaporated
off. NaOH was added to the mixture until the pH reached 10.
The precipitate was filtered to yield 5,59-dibromobiphenyl-
2,29-diamine (5), which was added to a mixture of sodium
nitrite (3.4 g, 49.3 mmol in 30 mL of water) and HCl (w/w
36%, 24 mL in 50 mL water) at 0 uC in 10 min. After stirring
for a further 20 min, the mixture was added within 30 min to a
vigorously stirred solution of KI (83.0 g, 0.5 mol) in 200 mL of
water at room temperature, and then stirred overnight. NaOH
n w
Number-average (M ) and weight-average (M ) molecular
weights were determined by a Waters GPC 2410 in tetra-
hydrofuran (THF) using a calibration curve with standard
polystyrene as a reference. The elemental analyses were
performed on a Vario EL elemental analysis instrument
(
ELEMENTAR Co.). UV-vis absorption spectra were
recorded on a Hitachi UV-3010 spectrophotometer. The
PL quantum yields were determined in an integrating sphere
IS080 (Labsphere) with 325 nm excitation of a HeCd laser
(
Mells Griot).
4
134 | J. Mater. Chem., 2006, 16, 4133–4140
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solution was added until the pH reached 7. The solution was
extracted several times with diethyl ether. The organic layer
Copolymer PSiFF70. Monomer 7 (0.30 equiv.), monomer 8
(0.20 equiv.), and monomer 9 (0.50 equiv.) were used in this
polymerization. Anal. Calcd for polymer (%): C, 87.61; H,
10.25. Found: C, 86.34; H, 10.36.
was washed with water, dried over anhydrous MgSO , and
4
evaporated. The residue was purified by column chromato-
graphy on silica gel using hexane as the eluent to yield 6 as an
off-white solid (4.3 g, 32%). Mp 207–210 uC. GC-MS (EI m/z):
Copolymer PSiFF90. Monomer 7 (0.10 equiv.), monomer 8
(0.40 equiv.), and monomer 9 (0.50 equiv.) were used in this
polymerization. Anal. Calcd for polymer (%): C, 88.95; H,
10.33. Found: C, 88.54; H, 10.32.
+
64 (M ). Anal. Calcd for C12
5
6
H Br
2
I
2
(%): C, 25.56; H, 1.07.
Found: C, 25.77; H, 0.77. IR (solid state): 3072, 3054, 2920,
2
1
850, 1629, 1562, 1535, 1437, 1385, 1354, 1086, 1058, 1040,
21 1
016, 998, 879, 822, 806, 722, 710, 698, 665, 624, 455 cm . H
NMR (300 MHz, CDCl ) d(ppm) 7.80 (2H, d, J = 8.4, ArH),
3
Copolymer PSiFF95. Monomer 7 (0.05 equiv.), monomer 8
(0.45 equiv.), and monomer 9 (0.50 equiv.) were used in this
polymerization. Anal. Calcd for polymer (%): C, 89.29; H,
10.35. Found: C, 88.73; H, 10.41.
7
.34 (2H, d, J = 2.3, ArH), 7.24 (2H, dd, J = 8.5, 2.4, ArH).
1
3
C NMR (75 MHz, CDCl
22.3, 97.3.
3
) d(ppm) 149.3, 140.3, 132.9, 132.6,
1
3
,6-Dibromo-9,9-di-n-octylsilafluorene (7). To a solution of 6
3 g, 5.32 mmol) in THF (50 mL) at 2100 uC was added
Device fabrication and characterization
(
2
1
The LED was fabricated on prepatterned indium–tin oxide
21
ITO) with a sheet resistance of 10–20 ohm square . The
dropwise 1.6 mol L n-BuLi in hexane (11.6 mL, 18.6 mmol)
over 1 h. The resulting mixture was stirred for a further 2 h
at 2100 uC. After that, di-n-octyldichlorosilane (3.44 g,
(
substrate was ultrasonically cleaned with acetone, detergent,
deionized water, and 2-propanol subsequently. Onto the
ITO glass a layer of polyethylenedioxythiophene–polystyrene
sulfonic acid (PEDOT : PSS) film with a thickness of 40 nm
was spin-coated from its aqueous dispersion (Baytron P 4083,
Bayer AG). The PEDOT–PSS film was dried at 80 uC for 8 h
in a vacuum oven. Then polyvinylcarbazole (PVK) was spin-
coated onto PEDOT–PSS and dried at 80 uC for 2 h under
nitrogen. The solution of the copolymers in toluene was spin-
coated onto the ITO/PEDOT : PSS/PVK surface. After that, a
thin layer of barium as an electron injection cathode and the
subsequent 150 nm thick aluminium capping layer were
thermally deposited by vacuum evaporation through a mask
1
0.6 mmol) was added and the mixture was allowed to warm
to room temperature and stirred overnight. The reaction was
quenched with distilled water, and then the product was
extracted with diethyl ether. The organic layer was washed
with brine and water, dried over anhydrous MgSO4 and
evaporated. The residue was chromatographed on silica gel
using hexane as the eluent to yield 7 as a colorless oil (2.1 g,
+
0%). GC-MS (EI m/z): 564 (M ). Anal. Calcd for
7
28 2
C H40Br Si (%): C, 59.57; H, 7.14. Found: C, 59.67; H,
7
1
8
.18. IR (solid state): 3066, 3037, 2956, 2923, 2854, 2670, 1626,
578, 1538, 1457, 1393, 1373, 1260, 1244, 1086, 1063, 1013,
2
1
1
. H NMR (300 MHz,
71, 819, 752, 726, 529, 416 cm
2
4
at a base pressure below 2 6 10 Pa. The deposition speed
and thickness of the barium and aluminium layers were
CDCl ) d(ppm) 7.91 (2H, d, J = 1.4, ArH), 7.46 (2H, d, J = 7.6,
3
ArH), 7.42 (2H, dd, J = 7.6, 1.5, ArH), 1.19–1.28 (24H, m,
1
3
monitored by a thickness/rate meter (model STM-100,
CH
2
), 0.91 (4H, m, CH
75 MHz, CDCl ) d(ppm) 149.1, 136.8, 134.5, 130.6, 125.2,
24.7, 33.2, 31.8, 29.3, 29.1, 29.0, 23.8, 22.6, 14.3.
2 3
), 0.87 (6H, t, J = 7.1, CH ), C NMR
Sycon Instrument, Inc.). The spin coating of the EL layer
and the device performance tests were carried out within a
glove box (Vacuum Atmosphere Co.) with nitrogen circula-
tion. Current–voltage (I–V) characteristics were measured
with a computerized Keithley 236 Source Measure Unit. The
luminance of the device was measured with a photodiode
calibrated by using a PR-705 SpectraScan Spectrophotometer
(
3
1
1
General procedure of polymerization . Carefully purified
5
monomer 7, 8, 9, tricyclohexylphosphine (5–7 mol%), and
palladium(II) acetate (1.5–2 mol%) were dissolved in a mixture
2
of toluene ([monomer]
1
=
0.1 mol
L
) and aqueous
(
Photo Research). External quantum efficiency was verified
tetraethylammonium hydroxide (20% w/w, 3:2 in volume)
under argon. The resulting mixture was refluxed by vigorously
stirring for 48 h, and then cooled to room temperature and
poured into methanol. The precipitated material was collected
by filtration through a funnel. The polymer was purified by
washing with acetone for 24 h to remove catalyst residues.
by measurement in the integrating sphere (IS080, Labsphere)
after encapsulation of the devices with UV-cured epoxy and
thin cover glass.
Results and discussion
Synthesis and characterization
Copolymer PSiFF50. Monomer 7 (0.50 equiv.) and mono-
mer 9 (0.50 equiv.) were used in this polymerization. Anal.
The chemical structures and synthetic routes of 3,6-(9,9-di-
n-octylsilafluorene) monomer and copolymers are outlined in
Schemes 1 and 2. 1 was reduced with iron powder to the
corresponding diamine 2. Acetic anhydride added to the
Calcd for polymer (%): C, 86.30; H, 10.16. Found: C, 85.83; H,
1
0.31. H NMR (300 MHz, CDCl
1
3
) d(ppm) 7.44–7.84 (brm,
1
3
ArH), 2.07 (b, CH
2
2 3
), 0.68–1.46 (brm, CH +CH ); C NMR
(
75 MHz, CDCl ) d(ppm) 151.8, 149.1, 144.7, 142.1, 140.4,
3
mixture of 2 without further purification in Na CO aqueous
2 3
1
3
35.4, 135.0, 128.6, 128.0, 126.1, 121.44, 120.1, 55.4, 40.5, 33.5,
1.9, 31.7, 30.0, 29.7, 29.3, 29.2, 24.9, 24.1, 23.8, 23.2, 23.0,
2.7, 22.6, 16.5, 14.1, 12.5. Other copolymers showed similar
solution gave stable 3 in 59% yield. The bromination of 3 took
place selectively at the 5,59-position to give 4 in 88% yield. The
5 obtained by hydrolysis of 4 was diazotized with sodium
nitrite in dilute hydrochloric acid to give 6 in a low yield (32%)
2
1
H NMR spectra to that of PSiFF50.
This journal is ß The Royal Society of Chemistry 2006
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Scheme 1 Synthesis of 3,6-dibromo-9,9-di-n-octylsilafluorene.
owing to the formation of a byproduct dibenzoiodonium
than the corresponding PFO homopolymer (see Table 1), so it
would be expected that incorporation of the SiF unit into the
PFO backbone would lead to lower Tg for the copolymer.
Surprisingly, with an initial increase of SiF molar ratio in the
copolymers, Tg increases to 80 uC for PSiFF90 and then
decreases with further increase in SiF molar ratio, reaching the
level of the PSiF homopolymer for PSiFF50. As we will show
below PSiFF90 gives the best device performance and the
narrowest EL emission peak. At this stage of experiment, it is
1
5,17
iodide salt.
n-BuLi was added dropwise to the THF
solution of 6 followed by addition of di-n-octyldichlorosilane
1
to afford 7 in 70% yield. The PSiFF copolymers based on
8
1
monomers 7, 8 and 9 were synthesized via a modified Suzuki
6
1
5
polycondensation (Scheme 2). In order to obtain high
molecular weights, the monomers were purified carefully by
column chromatography on silica gel and their purities were
determined by elemental analysis and NMR before use. The
feed compositions of monomer 7 in the polycondensation
were 50, 30, 10, and 5 mol%, and the corresponding
copolymers are respectively named PSiFF50, PSiFF70,
PSiFF90, and PSiFF95. The copolymers are readily soluble
in common organic solvents, such as toluene, xylene, THF and
chloroform. The chemical structures of the polymers were
not clear why T increases with the initial incorporation of SiF
g
unit in the copolymers. Further morphological and structural
studies are in progress and the results will be reported in a
forthcoming paper. The molecular weights and thermal data of
the polymers are summarized in Table 1.
1
13
verified by H NMR, C NMR, and elemental analyses. The
number-average molecular weights (M ) of the polymers
Electrochemical properties
n
The electrochemical behaviors of the polymers were investi-
gated by cyclic voltammetry (CV); the results are summarized
in Table 2. CV was performed on a Potentiostat Galvanostat
Model 283 (Princeton Applied Research) electrochemical
workstation with platinum working electrodes at a scan
determined by gel permeation chromatography (GPC) ranged
from 19000 to 39000 with polydispersity indices of 1.4 to 2.5
using polystyrene as the standard. To investigate the thermal
stability of the copolymers, differential scanning calorimetry
(
DSC) and thermal gravimetric analysis (TGA) were per-
formed. All the copolymers exhibited good thermal stability
with degradation temperatures (T ) of 395–418 uC (5% weight
2
1
rate of 50 mV s against a calomel reference electrode with
1
2
a nitrogen-saturated anhydrous solution of 0.1 mol L
6
tetrabutylammonium hexafluorophosphate (Bu NPF ) in
d
4
loss). DSC results indicate that Tg is very sensitive to the
acetonitrile (CH CN). The evaporation-deposition of poly-
3
content of 3,6-silafluorene unit. As we reported previously, the
mers onto the working electrode was conducted from a dilute
chloroform solution. All the polymers displayed reversible
p-doping and the onset potentials for oxidation were 1.55,
poly(3,6-silafluorene) homopolymer showed much lower T
g
1
.44, 1.40, 1.38, and 1.40 V for PSiFF50, PSiFF70, PSiFF90,
PSiFF95, and PFO, respectively. However, the reduction
Table 1 Molecular weights and thermal data of the polymers
Yield
Polymers (%)
4
4
M
n
(610 )
M
w
(610 )
M
w
/M
n
T
d
/uC
g
T /uC
a
PSiF
—
1.0
1.9
2.4
3.2
3.9
4.5
1.6
2.6
5.5
8.0
9.4
10.1
1.6
442
395
418
408
408
385
58
58
60
80
72
72
PSiFF50 50
PSiFF70 60
PSiFF90 80
PSiFF95 85
1.39
2.25
2.49
2.41
2.24
PFO
a
85
Data of poly(3,6-(9,9-dihexyl)silafluorene) taken from reference 13.
Scheme 2 Synthesis of copolymers.
4
136 | J. Mater. Chem., 2006, 16, 4133–4140
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Table 2 Electrochemical and optical properties of the polymers
Solution in THF lmax/nm
Film lmax/nm
Polymer
HOMO/eV
LUMO/eV
E
g
/eV
Abs
PL
Abs
PL
FWHM
W
PL
a
PSiF
26.1
22.1
4.0
283
333
356
384
386
384
365
283
332
357
378
380
381
363
46
59
47
46
46
48
14%
38%
77%
84%
71%
67%
PSiFF50
PSiFF70
PSiFF90
PSiFF95
PFO
25.95
25.84
25.80
25.78
25.80
22.62
22.87
22.86
22.86
22.87
3.33
2.97
2.94
2.92
2.93
376, 414
415, 436
415, 439
416, 438
416, 438
398, 418
418, 442
422, 443
422, 445
431, 446
a
Data of poly(3,6-(9,9-dihexyl)silafluorene) taken from reference 13.
processes were hardly recorded after many attempts. The
HOMO levels of the polymers were calculated according to the
peaks responsible for wide and narrow bandgap units, the
relative intensities of which change with the relative ratios of
20
1
9
empirical formula EHOMO = 2(Eox + 4.40) (eV), the LUMO
the comonomer composition in the copolymers, poly(3,6-
levels of the copolymers estimated by subtraction of the optical
silafluorene-co-2,7-fluorene) showed only one absorption peak
blue-shifting from 381 nm (for neat PFO) to 283 nm (for neat
PSiF) with increasing 3,6-silafluorene content in the copoly-
mers (Fig. 1b). There are two possible mechanisms for the
significant blue shift in the copolymers. One is that the blue
shift might originate from both fluorene and 3,6-silafluorene
comonomers taking part in the p-conjugation along the
polymer backbone and forming a uniform band structure by
bandgap E (eV) from the HOMO values.
g
Optical and photoluminescence properties
The absorption spectra of the polymers in THF solution and
films spin-coated from toluene solution are shown in Fig. 1a
and b, respectively. The absorption spectra of the polymers in
the solid state are similar to (a little broader than) those of the
corresponding copolymers in THF solution. The optical
bandgaps deduced from the absorption edge and the optical
properties are summarized in Table 2. The absorption spectra
of the poly(3,6-silafluorene) homopolymers synthesized in our
21
mixing the electronic states of the two monomer units. As
13
reported previously, the poly(3,6-silafluorene) homopolymer
has a wider bandgap than PFO. As a result, the bandgaps of
the copolymers increase with increasing 3,6-silafluorene
content. The other is that the blue shift might result from
the interruption of the delocalization of the p-electrons along
the polymer backbone by the isolated 3,6-silafluorene linkages.
In this case, the 3,6-silafluorene unit does not join in the
p-conjugation but functions only as a defect which breaks the
1
3
previous work are also included in Fig. 1 for comparison. As
we reported previously, poly(3,6-silafluorene) had a very wide
1
bandgap and showed the absorption maximum at 283 nm,
3
while that of PFO homopolymer was observed at 381 nm.
Unlike most polyfluorene-based copolymers containing
narrow bandgap units showing two distinguished absorption
22
p-conjugation of the PFO main chain. However, judging
23
from the absorption peaks of the fluorene oligomers and the
PSiFF copolymers, the latter mechanism is less probable, for
the peak position of the PSiFF copolymers with statistically
distributed PFO units: 50/50 (corresponding an isolated
monomer unit): 332 nm; 70/30 (corresponding to statistically
2
.3 units): 357 nm is much more bathochromic than that of the
corresponding fluorene oligomers: 329 nm for the dimer,
2
3
3
49 nm for the trimer and 363 nm for the tetramer.
The PL spectra of the polymers in THF solution and in films
are shown in Fig. 2a and b, respectively. Except for PSiFF50,
the PL spectra of the other copolymers in films were only
slightly red shifted with respect to their corresponding spectra
in solution, indicating that there were no noticeable molecular
conformation changes from solution to the solid state. As
shown in Fig. 2b the PL spectra of the PSiFF copolymers in
films show lower intensity of the shoulder at 460–490 nm
compared to those of the polyfluorene homopolymer (PFO)
which demonstrates that the incorporation of 3,6-silafluorene
could effectively suppress the p-stacking/aggregation of poly-
fluorenes in the solid state. As a result, the color purity of
the copolymer-based devices is significantly improved.
Similar to the absorption spectra, the PL spectra of the
PSiFF copolymers in films are significantly blue-shifted from
4
31 nm for the PFO homopolymer to 420, 416 and 400 nm for
Fig. 1 UV-vis spectra of the polymers: (a) in THF solution and (b) in
PSiFF90, PSiFF70 and PSiFF50 respectively with increasing
3,6-silafluorene content in the copolymers. All the PSiFF
the solid state.
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Fig. 3 EL spectra of the devices fabricated from the PSiFF
copolymers.
emission decreased with increasing 3,6-silafluorene content
except for the alternating copolymer, which showed a broad-
ening of both the EL peak and the PL peak. As a result, the
full widths at half maximum (FWHM) of the EL spectra for
PSiFF90 (19 nm) and PSiFF70 (15 nm) (Table 3) decreased
dramatically in comparison with those of the PL spectra for
the corresponding copolymers in the solid state. Band
narrowing is very important for the improvement of the color
purity of polymer LEDs, for the FWHM of p-conjugated
emitting polymers are typically more than 50 nm due to the
broad distribution of the conjugation length and the formation
2
5
21
Fig. 2 PL spectra of the polymers: (a) in THF solution (10 mol L
)
and (b) in the solid state.
2
4
25
of interchain emissive species. Following Ariu et al. the
decrease in the intensity of the lower energy vibronic emissive
states with increasing 3,6-silafluorene content in the copoly-
mers suggests that the chain rigidity of the PSiFF copolymers
has been significantly enhanced due to the incorporation of
copolymers show similar PL profiles in solid state except the
alternating copolymer PSiFF50. Absolute PL efficiencies of
the copolymer films measured in the integrating sphere
increase initially reach the maximum of 84% for PSiFF90
with increasing 3,6-silafluorene content, and then decrease
very quickly to 38% for the alternating copolymer PSiFF50
with further increase in 3,6-silafluorene content (Table 2). The
fact that the PL emission profile for PSiFF50 is much broader
than those of the other copolymers (Fig. 2b) is consistent
with the fact of the reduced PL efficiency for the alternating
copolymer PSiFF50.
3
,6-silafluorene into the polyfluorene main chain. Similarly,
that EL emission peaks are narrower than their PL emission
peaks has been reported for oligoarylfluorenes bearing
2
6
oligothiophene as a central core by the Tao group, which
is attributed to the different contributions of vibronic states
2
6
in the EL and PL processes. Further experiments are in
progress to get a full understanding of the origin behind the
phenomenon. As a result of band narrowing, the CIE
coordinates (0.18, 0.09), (0.18, 0.15), and (0.16, 0.07) have
been reached (Fig. 4) for PSiFF50, PSiFF70, and PSiFF90,
respectively. These values are among the CIE coordinates of
the best deep-blue polymer emitters. It is important to note
that not only does the incorporation of 3,6-silafluorene into
the polyfluorene main chain efficiently suppress the long-
wavelength emission (compared to the EL emission of the PFO
homopolymer in Fig. 3), the spectral stability after high
temperature annealing is also significantly improved. Fig. 5
compares the changes of the EL spectra of PSiFF90 and PFO
homopolymer after annealing for 1 h at 150 uC under nitrogen.
In contrast to the pronounced increase in the long-wavelength
Electroluminescence properties
To investigate the EL properties of the copolymers, double-
layer devices were fabricated with a configuration of ITO/
PEDOT : PSS(40 nm)/PVK(40 nm)/polymer(80 nm)/Ba(4 nm)/
Al(150 nm), in which PEDOT–PSS and PVK respectively
represent poly(3,4-ethylenedioxythiophene)–poly(styrenesulfo-
nate) and polyvinylcarbazole as hole injection/transport layers.
Fig. 3 shows the EL spectra of the devices from the copoly-
mers. The peak position of EL emission of the copolymers was
close to that of the PL emission (Fig. 2b). In contrast with the
distinct fine vibronic structure of the PL emission in the solid
state, the relative intensity of the vibronic shoulders of the EL
22
Table 3 EL performance of the polymers in devices with the configuration of ITO/PEDOT/PVK/polymer/Ba/Al at 20 mA cm
2
1
22
L/cd m
Polymer
lEL,max/nm
FWHM/nm
CIE 1931
LE/cd A
EQE (%)
Voltage/V
PSiFF50
PSiFF70
PSiFF90
PSiFF95
PFO
400
416
420
418
438
57
15
19
53
35
0.18, 0.09
0.18, 0.15
0.16, 0.07
0.16, 0.12
0.19, 0.19
0.96
0.43
2.02
0.47
0.36
0.75
1.36
3.34
1.42
0.51
189
96
326
90
11.2
10.1
6.7
8.3
10.3
66
4
138 | J. Mater. Chem., 2006, 16, 4133–4140
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Fig. 6 LE–J–L characteristics of the device based on PSiFF90.
Fig. 4 CIE coordinates of the devices fabricated from the PSiFF
copolymers.
emission for the PFO homopolymer, almost no change
was observed for PSiFF90 after annealing. The improved
thermal stability of the EL emission may be related partially
to the higher T of PSiFF90 copolymer. It is reported that
g
the higher color purity of electroluminescence devices
based on conjugated oligomers is usually associated with
2
7
their higher glass transition temperatures. We note that
Chan et al. also reported that poly(2,7-dibenzosilole) showed
significantly improved color purity after thermal annealing
Fig. 7 J–V–L characteristics of the device based on PSiFF90.
1
5
over polyfluorenes.
2
2
The EL performance of the polymers at 20 mA cm is
summarized in Table 3. Of all the copolymers, the device from
the PSiFF90 copolymer showed the best performance with an
external quantum efficiency of 3.34% and a luminous effici-
3
,6-silafluorene unit not only suppressed the long-wavelength
emission but the efficiency and color purity of PSiFF
copolymer-based devices are significantly improved. The
device based on copolymer PSiFF90 exhibited high perfor-
2
1
22
ency of 2.02 cd A at a luminance of 326 cd m . Luminous
efficiency–current density–luminance (LE–J–L) and current
density–voltage–luminance (J–V–L) characteristics of the
PSiFF90-based device are plotted in Fig. 6 and 7, respectively.
As shown in Fig. 6, the device efficiency remains as high as
mance with an external quantum efficiency of 3.34% and
21
a luminous efficiency of 2.02 cd A
22
at a brightness of
3
26 cd m with a surprisingly narrow FWHM (19 nm) and
CIE coordinates of (0.16, 0.07), which almost match the NTSC
standard blue pixel coordinates of (0.14, 0.08) (see Fig. 4). The
results are among the best reported for pure blue-emitting
polymers in the literature so far. Moreover, the incorporation
of 3,6-silafluorene into the polyfluorene main chain signifi-
cantly enhances the spectral stability of copolymers at
annealing. The fascinating properties of poly(3,6-silafluorene-
co-2,7-fluorene) copolymers show that the incorporation of a
wide bandgap 3,6-silafluorene into the p-conjugated poly-
fluorenesmain chain is a promising route to high-efficiency
spectrum-stable color-pure blue-emitting polymers.
2
1
22
1
.1 cd A at a current density of 500 mA cm with the
2
2
luminance over 5000 cd m . The results indicate poly(3,6-
silafluorene-co-2,7-fluorene) could be a promising candidate
for blue-emitting polymers of high efficiency and good
color purity.
Conclusions
In conclusion, 3,6-silafluorene was successfully incorporated
into the polyfluorene main chain. The incorporation of the
Acknowledgements
This work was supported by research grants from the Ministry
of Science and Technology, China (MOST) Project
(
#2002CB613402) and the National Science Foundation of
China (NSFC) Project (#50433030).
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