Synthesis and color tuning of new fluorene-based copolymers

Article abstract of DOI:10.1021/ma011155+

A series of copolymers, poly{9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl-co-2,5-bis(2-thienyl-1-cyanov inyl)-1-(2′-ethylhexyloxy)-4-methoxybenzene-5″, 5?-diyl} (PFTCVB), were synthesized from the monomers 2,7-dibrome-9,9-bis(2′-ethylhexyl)fluorene and 2,5-bis

Full text of DOI:10.1021/ma011155+

1224
Macromolecules 2002, 35, 1224-1228
Synthesis and Color Tuning of New Fluorene-Based Copolymers
Na m Su n g Ch o,† Do-Hoon Hw a n g,‡ J eon g-Ik Lee,§ Byu n g-J u n J u n g,† a n d
Hon g-Ku Sh im *,†
Center for Advanced Functional Polymers, Department of Chemistry and School of Molecular Science
(BK21), Korea Advanced Instituted of Science and Technology, Tae-jon 305-701, Korea;
Department of Applied Chemistry, Kumoh National University of Technology, Kumi, 730-701, Korea;
and Basic Lab., ETRI, Taejon 305-350, Korea
Received J uly 5, 2001; Revised Manuscript Received October 19, 2001
ABSTRACT: A series of copolymers, poly{9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl-co-2,5-bis(2-thienyl-1-
cyanovinyl)-1-(2′-ethylhexyloxy)-4-methoxybenzene-5′′,5′′′-diyl} (PFTCVB), were synthesized from the
monomers 2,7-dibromo-9,9-bis(2′-ethylhexyl)fluorene and 2,5-bis(2-(5′-bromothienyl)-1-cyanovinyl)-1-(2′′-
ethylhexyloxy)-4-methoxybenzene (BTCVB) through the Ni(0)-mediated polymerization. The copolymers
were characterized using FT-IR spectroscopy, UV-vis spectroscopy, TGA, photoluminescence (PL) and
electroluminescence (EL) spectroscopy, elemental analysis, and molecular weight studies. The synthesized
copolymers showed an absorption maximum at about 380 nm, and between 425 and 600 nm the absorption
increased with increasing fraction of thiophene-containing monomer 4 (BTCVB). In PL, the emission
maxima of the copolymers were red-shifted as the fraction of BTCVB increases, despite the copolymers
showing little variation in UV-vis absorption characteristics. Light-emitting devices were fabricated in
an ITO (indium-tin oxide)/PEDOT/polymer/LiF/Al configuration. The EL spectra showed similar emissions
to the PL results, and the copolymer containing 15 mol % of BTCVB showed bright-red emission.
In tr od u ction
porting layer.24,25 However, appropriate blue-emitting
materials that meet the requirements for display ap-
plications have yet to be obtained, and further improve-
ments are necessary.
Since poly(p-phenylenevinylene) (PPV) was first re-
ported as an electroluminescene material by Burroughes
et al.,1 enormous effort has been devoted to the synthesis
of light-emitting polymers because of their potential
applications as active materials for organic electrolu-
minescent displays.2-5 PPV and its derivatives have
attracted much attention because of their optical and
physical properties, and many efforts have been made
to improve the performance of electroluminescent
devices.6-11 Although all three primary colors (red,
green, and blue) have been demonstrated in LEDs, at
present only green and orange LEDs meet the require-
ments of commercial use.
Meanwhile, several efforts have been made to tune
color using fluorene-based polymers. One successful
method for obtaining color tuning was the doping of
green- or red-emitting materials into PFs. The other
method is copolymerization with low band gap comono-
mers. Inbasekaran et al. at the Dow Chemical Company
have reported several fluorene-based copolymers syn-
thesized by alternating copolymerization using 5,5′-
dibromo-2,2′-bithiophene and 4,7-dibromo-2,1,3-ben-
zothiadiazole, which showed yellow and green emissions,
respectively.26 In addition, Lee et al. at IBM have
reported fluorene-based copolymers synthesized by ran-
dom copolymerization using 3,9(10)-dibromoperylene,
4,4′-dibromo-R-cyanostilbene, and 1,4-bis(2-(4′-bromo-
phenyl)-1-cyanovinyl)-2-(2′-ethylhexyl)-5-methoxyben-
zene.27 A small amount (5 mol %) of the comonomers in
the copolymers was sufficient to change the blue emis-
sion of PF to yellow and green emission. Basically, the
introduction into PF of comonomers with a lower band
gap than fluorene monomer leads to successful color
tuning based on PF.
A number of polyfluorene (PF) polymers and their
derivatives have been studied since poly(9,9-di-n-hexyl-
fluorene) (PDHF) was first reported as a blue-light-
emitting polymer. The interest in these polymers arose
because they show highly efficient photoluminescence
(PL) and electroluminescence (EL), excellent thermal
and oxdative stability, and good solubility in common
organic solvents.12-18 Major drawbacks of PFs, however,
are that they show excimer and/or aggregate formation
upon thermal annealing or the passage of current.
Moreover, the efficiency of devices based on PFs are not
sufficient for applications. Continuing efforts have been
made to suppress excimer formation and improve ef-
ficiency in PFs, including copolymerization with an-
thracene,17 end-capping with a sterically hindered
groups19,20/hole-trapping moieties,21 introducing steri-
cally hindered substituents at the 9-position of fluo-
rene,22,23 and combining with cross-linked hole-trans-
In contrast to the body of work on blue-emitting
copolymers, only a few reports investigate red emission
from copolymers based on PF. To address this issue, in
the present work we report on the preparation and
characterization of copolymers containing a low band
gap comonomer, 2,5-bis(2-(5′-bromothienyl)-1-cyanovi-
nyl)-1-(2′′-ethylhexyl)-4-methoxybenzene (BTCVB), which
not only has extended π-conjugation but also contains
the thiophene moiety. We prepared copolymers covering
the full color range from blue to red by changing the
feed ratio of the low band gap comonomer, BTCVB.
Using this method, we succeeded in synthesizing new
fluorene-based copolymers, poly{9,9-bis(2′-ethylhexyl)-
fluorene-2,7-diyl-co-2,5-bis(2-thienyl-1-cyanovinyl)-1-(2′-
† Korea Advanced Instituted of Science and Technology.
‡ Kumoh National University of Technology.
§ ETRI.
* To whom correspondence should be addressed. Tel +82-42-
869-2827; Fax +82-42-869-2810; e-mail hkshim@sorak.kaist.
ac.kr.
10.1021/ma011155+ CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/18/2002

Macromolecules, Vol. 35, No. 4, 2002
Fluorene-Based Copolymers 1225
Sch em e 1. Syn th etic Sch em e of 2-Br om oth iop h en e-(2-cya n o)vin ylen e-2-m eth oxy-5-(2-eth ylh exyl)-1,4-p h en ylen e-
(1-cya n o)vin ylen e-5-br om oth iop h en e (BTCVB)
Sch em e 2. Syn th etic Sch em e of P oly{2,7-(9,9′-bis(2-eth ylh exyl)flu or en e)-co-2,5-th iop h en e-(2-cya n o)vin ylen e-
2-m eth oxy-5-(2-eth ylh exyloxy)-1,4-p h en ylen e-(1-cya n o)vin ylen e-2,5-th iop h en e} (P F TCVB)
on glass substrates coated with indium-tin oxide (ITO). The
device configuration was ITO/poly(3,4-ethylenedioxythiophene)-
(PEDOT)/polymer/LiF/Al. PEDOT was used as the hole-
injection layer. The LED structure used in this study con-
sists of an aluminum contact on the copolymer surface that
is spin-casted onto an PEDOT-coated ITO glass substrate
from a solution of PFTCVBs in p-xylene. The spin-casting
yielded uniform films with thickness of approximately 100 nm.
Aluninum was deposited onto the polymer films using the
vacuum evaporation method at a pressure of 10-6 Torr. EL
spectra of the devices were measured using a Minolta CS-1000.
Current-voltage-luminance (I-V-L) characteristics were
recorded simultaneously with the measurement of the EL
intensity by attaching the photospectrometer to a Keithley 238
and Minolta LS-100 as the luminance detector. All measure-
ments were carried out at room temperature under ambient
atmosphere.
ethylhexyloxy)-4-methoxybenzene-5′′,5′′′-diyl} (PFTCVB),
which show full color emissions. The synthetic routes
and details are given in Schemes 1 and 2.
Exp er im en ta l Section
Mea su r em en ts. NMR spectra were recorded on a Bruker
AM 300 MHz spectrometer with tetramethylsilane as internal
reference. Elemental analysis was performed by an EA 1110-
Fisons. FT-IR spectra were recorded using an EQUINOX 55
spectrometer. UV-vis and PL spectra were recorded on J asco
V-530 and Spex Fluorolog-3 spectrofluorometers. Thermo-
gravimetric analysis (TGA) was performed on a DuPont
9900 analyzer with a heating rate of 10 °C/min under a
nitrogen atmosphere. The number- and weight-average mo-
lecular weights of polymers were determined by gel permea-
tion chromatography (GPC) on a Waters GPC-150C instru-
ment, using tetrahydrofuran (THF) as eluent and polysty-
rene as standard. Single-layer LED devices were fabricated
Ma ter ia ls. 4-Methoxyphenol, 2-ethylhexyl bromide, 1,5-
cyclooctadiene, 2,7-dibromofluorene, 2,2′-dipyridyltoluene (99.8%,

1226 Cho et al.
Macromolecules, Vol. 35, No. 4, 2002
Ta ble 1. P olym er iza tion Resu lts
anhydrous), N,N-dimethylformamide (99.8%, anhydrous), and
5-bromo-2-thiophenecarboxaldehyde were purchased from Al-
drich. Bis(1,5-cyclooctadiene)nickel(0) was purchased from
Strem. All chemicals were used without further purification.
1,4-Bis(chloromethyl)-2-(2-ethylhexyloxy)-5-methoxybenzene
(2) and 2,7-dibromo-9,9-bis(2′-ethylhexyl)fluorene (5) were
synthesized according to procedures outlined in the litera-
ture.11,28,29
copolymers
Mw
PFTCVB1 PFTCVB3 PFTCVB5 PFTCVB15
47000
20000
2.3
23000
15000
1.5
33000
13000
2.5
61000
22000
2.7
Mn
PDI (Mw/Mn)
polymer
yield (%)
y ratioa (%)
72
66
75
61
1.4
3.1
7.0
17.5
Syn t h esis of 1,4-Bis(cya n om et h yl)-2-m et h oxy-5-(2′-
eth ylh exyloxy)ben zen e (3). A mixture of 10.0 g (30 mmol)
of compound 2 and 4.4 g (90 mmol) of sodium cyanate in N,N-
dimethylformamide was stirred at 45 °C for 72 h. The resulting
mixture was extracted with methylene chloride and brine and
then dried with MgSO4. The extract was filtered and evapo-
rated in vacuo. The resulting liquid was poured into water,
yielding a pale yellow precipitate that was filtered. The pre-
cipitate was recrystallized in methylene chloride and hexane
two times. The product yield was 60% (5.7 g). 1H NMR (CDCl3,
ppm): δ 6.90 (d, 2H), 3.85 (d, 2H), 3.83 (s, 3H), 3.68 (s, 4H),
1.72 (m, 1H), 1.55-1.29 (m, 8H), 0.91 (q, 6H). 13C NMR (CDCl3,
ppm): δ 150.43, 150.30, 119.10, 119.00, 117,73, 112.48, 111.82,
71.13, 56.12, 39.48, 30.59, 29.07, 23.98, 22.99, 18.63, 18.58,
14.03, 11.16 Anal. Calcd for C19H26N2O2; C, 72.58; H, 8.33; N,
8.91. Found: C, 72.52; H, 8.11; N, 8.67.
a
y ratios are BTCVB ratios in copolymers, and they measured
by elemental analysis through calculation of the amount of
nitrogen contained in copolymers.
Syn th esis of 2,5-Bis-{2-(4′-br om oth ien yl)-1-cya n ovi-
n yl}-2-(2′-eth ylh exyloxy)-5-m eth oxyben zen e (BTCVB) (4).
A mixture of 10.0 g (32 mmol) of compound 3 and 18.5 g (97
mmol) of 5-bromothiophene-2-carbaldehyde was stirred in 100
mL of methanol at room temperature. A catalytic amount of
potassium tert-butoxide in methanol was added to this mix-
ture. After 2 days, the bright yellow solid was filtered and
dried. The resulting solid was recrystallized in methylene
chloride and methanol, and then the solid was dried in vacuo.
The resulting product yield was 62% (13.0 g). 1H NMR (CDCl3,
ppm): δ 7.95 (s, 1H), 7.79 (s, 1H), 7.31 (d, 1H), 7.28 (d, 1H),
7.10-7.06 (m, 4H) 3.82 (d, 2H), 3.76 (s, 3H), 3.68 (s, 4H), 1.72
(m, 1H), 1.55-1.29 (m, 8H), 0.91 (q, 6H). 13C NMR (CDCl3,
ppm): δ 150.94, 239.64, 138.20, 133.22, 132.84, 130.55, 123.84,
118.55, 118.21, 113.85, 113.32, 104.84, 71.79, 56.51, 39.66,
F igu r e 1. UV-vis absorption and photoluminescence emis-
sion spectra of 2,7-dibromo-9,9-bis(2′-ethylhexylfluorene) and
BTCVB in CHCl3.
parent and homogeneous thin films. The number-
average molecular weight (Mn) and the weight-average
molecular weight (Mw) of the copolymers, as determined
by gel permeation chromatography using polystyrene
standard, ranged from 13 000 to 22 000 and 23 000 to
61 000, respectively, with a polydispersity index ranging
from 1.5 to 2.7. The resulting polymer yields were
60-75%. All of the PFTCVB and the homopolymer poly-
(9,9-bis(2′-ethylhexyl)fluorene) (PBEHF) were end-
capped with 9-bromoanthracene in an effort to suppress
excimer emission.30 The nature of the end groups
apparently affects the π-stacking tendency of the fluo-
rene backbone which is determined by the rigidly
planarized biphenyl units within the polymer backbone.
PBEHF and PFTCVBs terminated with a more steri-
cally hindered end-capper such as 9-bromoanthracene
are less prone to excimer formation. The polymerization
results of the synthesized copolymers are summarized
in Table 1. In addition, the actual fraction of BTCVB in
the PFTCVBs, as determined by elemental analysis
through calculation of the amount of nitrogen contained
in the copolymers, is somewhat higher than the feed
ratio. These results indicate that BTCVB is more active
than fluorene monomer (5) in polymerization reactions.
The infrared absorption of the cyano subsituent at 2210
cm-1 increased with increasing amount of the cyano-
stilbene monomer in the comonomer feed.
30.81, 29.16, 24.14, 23.01, 14.03, 11.24. Anal. Calcd for C29H28
-
Br2N2O2S2: C, 52.74; H, 4.27; N, 4.84; S, 9.71. Found: C, 52.72;
H, 4.10; N, 4.53; S, 9.84.
P olym er iza tion . Homopolymer and statistical copolymers
were synthesized by nickel(0)-mediated polymerization.30 The
feed ratios of BTCVB (4) were 1, 3, 5, and 15 mol % of the
total amount of monomer, and the total amount of reactant
was 1.8 mmol. Each Schlenk tube containing 5 mL of DMF,
bis(1,5-cyclooctadienyl)nickel(0), 2,2′-dipyridyl, and 1,5-cy-
clooctadiene (the latter three in a molar ratio of 1:1:1) was
kept under argon at 80 °C for 30 min, and then 5 mL of
anhydrous toluene was added to the mixture. The polymeri-
zation was maintained at 80 °C for 72 h. After this time, 0.1
g of 9-bromoanthracene was dissolved in toluene and added
to the reaction mixture for end-capping. After the reaction had
finished, each polymer was precipitated from an equivolume
mixture of concentrated HCl, methanol, and acetone. The
isolated polymers were dissolved in chloroform and precipi-
tated in methanol. Finally, the resulting polymers were
purified by Soxhlet extaction and dried in vacuo. The resulting
polymer yields ranged from 60 to 75% (poly(9,9-bis(2-ethyl-
hexyl)fluorene-2,7-diyl): 60%, copolymer containing 1 mol %
of BTCVB: 72%, copolymer containing 3 mol % of BTCVB:
66%, copolymer containing 5 mol % of BTCVB: 75%, copolymer
containing 15 mol % of BTCVB: 61%). Elemental analysis for
All of the PFTCVBs exhibited very good thermal
stabilities, losing less than 5% of their weight on heating
to approximately 300 °C evaluated by means of TGA
under a nitrogen atmosphere.
PFTCVBs-PFTCVB1: C, 86.71; H, 10.52; N, 9.91 × 10-2
.
PFTCVB3: C, 84.97; H, 10.23; N, 0.22. PFTCVB5: C, 86.52;
H, 10.33; N, 0.50. PFTCVB15: C, 82.59; H, 9.47; N, 1.20.
Op tica l a n d P h otolu m in escen ce P r op er ties. Fig-
ure 1 shows the absorption and emission spectra of 2,7-
dibromo-9,9-bis(2′-ethylhexyl)fluorene and BTCVB co-
monomer in CHCl3. BTCVB exhibits a large red shift
in both absorption and emissions compared with 2,7-
dibromo-9,9-bis(2′-ethylhexyl)fluorene. The emission
maxima of BTCVB are 518 and 628 nm, respectively.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of P olym er s. All
of the synthesized copolymers were soluble in common
organic solvents. However, this solubility decreased
slightly with increasing ratio of BTCVB. The copolymers
were spin-coated onto an ITO substrate, giving trans-

Macromolecules, Vol. 35, No. 4, 2002
Fluorene-Based Copolymers 1227
Ta ble 2. Sp ectr u m Da ta
λ
max (nm)
UV
PL
EL
emission
b
copolymers
absorptiona
emissiona
ΦPL
PBEHF
380
380
380
380
379
420
536
544
583
620
419
532
535
580
630
0.80
0.69
0.57
0.51
0.34
PFTCVB1
PFTCVB3
PFTCVB5
PFTCVB15
a
b
Measured in the thin film onto fused quartz plates. Photo-
luminescence quantum yield in chloroform determined relative to
quinine sulfate in 0.1 M H2SO4 solution; see refs 32-34.
F igu r e 2. UV-vis absorption spectra of the thin films of
PFTCVBs coated onto fused quartz plates.
F igu r e 4. Electroluminescence spectra of PFTCVBs and
PBEHF which have ITO/PEDOT/polymer/LiF/Al configuration.
full colors from blue to red with these new fluorene-
based copolymers by making small changes in the
amount of red-emissive BTCVB. In Table 2 we present
a summary of all of the optical absorptions, UV-vis
maximum absorptions, PL and EL maximum emissions
in film, and PL quantum efficiencies in solution of the
synthesized copolymers. As shown in Table 2, the PL
quantum efficiencies decreases with increasing fraction
of BTCVB.32-34
Electr olu m in escen ce P r op er ties a n d Cu r r en t-
Volta ge-Lu m in a n ce (I-V-L) Ch a r a cter istics. The
EL spectra of the PFTCVBs are similar to the PL
spectra of the copolyemrs with the same components
(Figure 4), which indicates that the same energy
transfer is involved in EL and PL. It is reasonable to
suppose that these results are due to energy transfer
from the high-energy-state fluorene moiety to the low-
energy-state BTCVB moiety. As a result, the transferred
excitons decay through radiative recombination in
BTCVB and then exhibit a red-shifted emission in
comparison to the emission of the homopolymer.35 In
particular, PFTCVB15 (which contains 17.5 mol % of
BTCVB) showed almost pure red emission at 630 nm.
Figure 5 shows the current-voltage-luminance charac-
teristics of the LED of ITO/PEDOT/PFTCVB/LiF/Al.
The threshold voltages of PFTCVBs ranged from about
5 to 13 V and decreased stepwise as the fraction of
BTCVB increased. The EL intensity of the PFTCVBs
increased with increasing BTCVB fraction at lower
voltages as shown in Figure 5b. A detailed study of these
LED devices is currently underway.
F igu r e 3. Photoluminescence spectra of the thin films of
PFTCVBs and PBEHF coated onto fused quartz plates.
Figure 2 shows the UV-vis absorption spectra of the
thin films of PFTCVBs coated onto fused quartz plates.
All of the PFTCVBs exhibit absorption maxima close
to 380 nm, regardless of copolymer composition. The
absorption maximum of PFTCVB15 is slightly blue-
shifted because of its high molecular weight, contrary
to the copolymer having low molecular weight.30,31 As
the fraction of BTCVB increases, however, the absorp-
tion between 425 and 600 nm increases. These absorp-
tion bands could be due to the BTCVB units incorpo-
rated into the polyfluorene main chain, and the increase
of this unit affects the absorption intensities because
of the low band gap of BTCVB. The film of PBEHF
homopolymer showed PL emission maxima at 420 and
440 nm. Interestingly, these two strong and sharp PL
peaks reduce in size dramatically in the PL spectra of
the PFTCVBs, as shown in Figure 3. In contrast, the
emission peaks from the BTCVB part increase greatly,
such that even PFTCVB1 (which contains only 1.4
mol % of BTCVB) clearly exhibits this phenomenon.
PFTCVB1 greatly shifted its PL maximum to 540 nm
with a large and broad shoulder at 575 nm. These large
shifts in PL maxima are probably due to intramolecular
energy transfer from the BEHF part to BTCVB chro-
mophores. Therefore, the emission from the latter is
major in PL spectrum. The emission maxima of the
copolymers gradually shift to the long wavelength as
BTCVB content increases. This shift in the emission
maxima might be explained that the number of BTCVB
unit in copolymer increases when the fraction of BTCVB
in polymerization increases. As a result, PFTCVB15
showed a PL emission maximum at 620 nm, yielding
red emission. Conclusively, we succeeded in embodying
Su m m a r y
We have successfully prepared novel fluorene-based
copolymers that have luminescent properties from blue
to red. The UV-vis absorption spectra of these copoly-
mers showed absorption maxima at almost the same
wavelength (380 nm). However, the PL and EL emission

1228 Cho et al.
Macromolecules, Vol. 35, No. 4, 2002
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spectra showed quite different trends. Compared to
poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) (BEHF), PL
and EL maxima for PFTCVBs shifted to longer wave-
lengths as the ratio of BTCVB was increased. Dramati-
cally, the wavelengths of the PL and EL maxima are
red-shifted about 115 nm when only a small fraction
(1.4%) of BTCVB is used. In particular, PFTCVB15
showed an EL emission maximum at 630 nm, yielding
an almost pure red color. The threshold voltages step-
wise decreased as the fraction of BTCVB increased.
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Ack n ow led gm en t. We thank M. S. Lee, Y. S. Suh,
and T. Ahn for their lots of help and gratefully acknowl-
edge financial support from the Center for Advanced
Functional Polymers through Korea Science and Engi-
neering Foundation (KOSEF) and ILJ IN. We also thank
Dr. Y. W. Kim of K.R.I.C.T. for carrying out the ele-
mental analysis and K. S. Pyun for assistance with GPC
measurements.
Refer en ces a n d Notes
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R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B.
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