Sterically hindered fluorenyl-substituted poly(p-phenylenevinylenes) for light-emitting diodes

Article abstract of DOI:10.1021/ma010643e

A new series of processable dihexylfluorenyl-substituted poly(p-phenylenevinylene) derivatives (DHF-PPVs) were synthesized by dehydrohalogenation-condensation polymerization (GILCH polymerization). The polymers were characterized by NMR, FT-IR, and elemen

Full text of DOI:10.1021/ma010643e

1356
Macromolecules 2002, 35, 1356-1364
Sterically Hindered Fluorenyl-Substituted Poly(p-phenylenevinylenes)
for Light-Emitting Diodes
Sa n g Ho Lee,† Bo-Bin J a n g,† a n d Tetsu o Tsu tsu i*,†,‡
CREST, J apan Science and Technology Corporation (J ST), J apan, and Department of Applied Science
for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University,
Kasuga, Fukuoka, 816-8580, J apan
Received April 12, 2001; Revised Manuscript Received November 14, 2001
ABSTRACT: A new series of processable dihexylfluorenyl-substituted poly(p-phenylenevinylene) deriva-
tives (DHF-PPVs) were synthesized by dehydrohalogenation-condensation polymerization (GILCH
polymerization). The polymers were characterized by NMR, FT-IR, and elemental analysis and were
completely soluble in common organic solvents. All of the polymers demonstrated bright blue-green
emission with high photoluminescence (PL) efficiencies of 68-71% in chloroform, good thermal stability
with decomposition temperatures above 322 °C, and high glass transition temperatures in the range of
113-148 °C. Cyclic voltammetry studies revealed that these polymers undergo both irreversible oxidation
and reduction onsets around 0.9 and -1.5 V, which are attributed to the conjugated backbone. Polymer
light-emitting diodes (PLEDs), fabricated with DHF-PPVs, as the emitting layer, poly(3,4-ethylenediox-
ythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS) (PEDOT:PSS), as the hole injection/
transporting layer, Ca cathodes, and indium-tin oxide (ITO) anodes, (ITO/PEDOT:PSS/DHF-PPVs/
Ca), emitted blue-green emission with a maximum peak around 500 nm and a shoulder peak around 532
nm. For the poly[2-(9,9-dihexylfluorenyl)-1,4-phenylenevinylene] (DHF-PPV) device, a low turn-on voltage
of 2.8 V (0.13 MV/cm), a maximum luminance of 12 000 cd/m2 at 0.4 MV/cm, and a maximum external
quantum efficiency of 0.53% were obtained. The latter was 33-37 times brighter and 1.6-2.8 times more
efficient than those devices with poly[2-(7-methoxy-9,9-dihexylfluorenyl)-1,4-phenylenevinylene] (MDHF-
PPV) and poly[2-(7-cyano-9,9-dihexylfluorenyl)-1,4-phenylenevinylene] (CNDHF-PPV).
In tr od u ction
years, some of conjugated polymers with phenyl,12,13
biphenyl,14 or 2,3-dialkoxy15 side chains appended to the
polymer backbone have exhibited excellent PL efficien-
cies in the solid state. Fluorene derivatives show
interesting and unique chemical and physical properties
because they contain a rigid planar biphenyl unit and
are easily substituted at the remote C-9 position, which
improves the polymer processability. Incorporating the
9,9-dialkylfluorenyl unit, where alkyl chains are out of
plane to the fluorene residue, into the parent PPV
backbone forms a new type of conjugated polymer, poly-
[2-(9,9-dihexylfluorenyl)-1,4-phenylenevinylene] (DHF-
PPV). Its geometry is nonplanar due to both a strong
steric hindrance at inter-ring linking positions between
the fluorene residue and the PPV backbone and inter-
molecular interactions due to the long side chains of the
fluorene unit. In addition, the sterically hindered alkyl
chains of the fluorene unit may result in an amorphous
solid state of PPV, giving excellent device performance.16
This also suppresses ordered regions within emissive
layer of PLED and thus enhances the internal efficiency
of the device.17
We have reported that sterically hindered fluorenyl-
substituted PPV have excellent solubility and exhibit a
high quantum efficiency from PLEDs with the config-
uration of ITO/PEDOT:PSS/DHF-PPV/MgAg.18 In this
study we use functionalized fluorene units (e.g., cyano-
or methoxy-substituted fluorene) as pendant groups to
the PPV backbone. Synthesis and characterization of
the conjugated polymers, DHF-PPVs, are described,
and their thermal, optical, electrochemical, and light-
emitting properties are also investigated.
Since the first report on electroluminescence (EL)
from a conjugated polymer, poly(p-phenylenevinylene)
(PPV),1 many studies of polymer light-emitting diodes
(PLEDs) have been reported due to their potential
application in full color flat panel displays.2-4 PLEDs
present many advantages such as easy fabrication with
low cost, low operating voltage, color tunability, flex-
ibility, etc.5
For developing practical PLEDs with a high quantum
efficiency, low operating voltage, and long lifetime, the
design of processable conjugated polymers with high
purity and photoluminescence (PL) efficiency, good
thermal and oxidation stability, and balance of charge
carrier mobility is required.4 This can be satisfied by
introducing long aliphatic chains6,7 or charge transport-
ing materials8-10 as pendant groups or as part of the
polymer backbone or by controlling either the highest
occupied molecular orbital (HOMO) or the lowest unoc-
cupied molecular orbital (LUMO) energy level of poly-
mer through chemical modification. For example, the
functionalization of PPV through the introduction of
alkoxy,6 silyl,7 or oxadiazole8 units as side-chain groups
on the polymer backbone or the interruption of π-con-
jugation along the polymer backbone in copolymers11
provide solubility and permit tuning of the emission
color as well as control of HOMO or LUMO energy
levels.
Few sterically hindered polymers have been reported
due to the difficulty of their synthesis. In recent
† CREST/J ST.
Exp er im en ta l Section
Mea su r em en ts. 1H and 13C NMR spectra were recorded
on a J EOL J NM-LA 400 spectrometer (400 MHz) or J EOL
‡ Kyushu University.
* To whom correspondence should be addressed. e-mail: tsuigz@
mbox.nc.kyushu-u.ac.jp.
10.1021/ma010643e CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/17/2002

Macromolecules, Vol. 35, No. 4, 2002
Fluorenyl-Substituted Poly(p-phenylenevinylenes) 1357
J NM-LA 600 spectrometer (600 MHz) with chloroform-d,
dichloromethane-d2, or DMSO-d6 as solvents and tetrameth-
ylsilane (TMS) as the internal standard. Mass spectra were
obtained with a J EOL J MS-70 spectrometer with FAB (fast-
atom bombardment) system. FT-IR spectra were recorded on
a BIO-RAD FTS 6000 spectrometer. UV-vis and fluorescence
spectra were obtained on a Hitachi 330 spectrophotometer and
on an Edinburgh Instruments FL/FS900 spectrophotometer,
respectively. Thermogravimetric analysis (TGA) was conducted
on a Seiko Instruments SSC 5200 TG/DTA 320 system under
a heating rate of 20 °C/min and nitrogen flow rate of 80 mL/
min. Differential scanning calorimetry (DSC) was run on a
Seiko Instruments SII DSC 22 system. Elemental analyses
were performed by Analytical Center, Kyushu University for
C, H, and N determination. Cyclic voltammetry was performed
on an ALS electrochemical analyzer model 660 with a three-
electrode cell in a solution of Bu4NClO4 (0.1 M) in acetonitrile
at a scan rate of 10 mV/s. Polymer films were prepared from
glass coated with indium-tin oxide (ITO) by spin-coating the
corresponding toluene solutions and then dried under reduced
pressure. A Pt mesh was used as the counter electrode, and a
Ag/AgCl (0.1 M in acetonitrile) electrode was used as the
reference electrode. Prior to each series of measurements, the
cell was deoxygenated with argon. Gel permeation chroma-
tography (GPC) analysis was conducted on a Waters 515 HPLC
system equipped with Waters 2487 dual λ absorbance detector
and a Shodex GPC K-804 column, using polystyrene as
standard and chloroform as eluent.
LED Device Fabr ication an d Ch ar acter ization . Double-
layer PLED devices were fabricated on glass substrates coated
with ITO. The substrate was cleaned by a general procedure,
which included sonication in detergent followed by repeated
rinsing in deionized water, acetone, and ethanol, and, prior
to use, placed in hot ethanol for 10 min and treated with an
oxygen plasma. Conducting polymer dispersion of poly(3,4-
ethylenedioxythiophene) (PEDOT) doped with poly(styrene-
sulfonic acid) (PSS) (PEDOT:PSS, Baytron P) was obtained
from Bayer Corp. The hole injection layer of PEDOT:PSS was
prepared from a water dispersion and baked at 170 °C for 20
min under argon atmosphere with the thickness of 50 nm. The
emitting layers of the polymers (DHF-PPV, MDHF-PPV, and
CNDHF-PPV) were spin-coated from a toluene solution onto
the hole injection layer. Finally, a Ca cathode was vacuum-
deposited onto the polymer layers at a pressure below 1 × 10-6
Torr. The emitting areas of the EL devices were 2 × 2 mm2.
EL spectra of the devices were measured by connecting a
Hamamatsu PMA-11 spectrophotometer to a programmable
Keithley 238 electrometer. Luminance-current density-volt-
age (L-I-V) curves were recorded with a programmable
Keithley 238 electrometer and MINOLTA LS-110 colorimeter
in a nitrogen atmosphere.
The reaction mixture was refluxed with vigorous stirring under
nitrogen for 24 h. After the reaction was complete, the mixture
was diluted with ethyl acetate and water. The organic phase
was separated and washed with brine and dried over MgSO4.
The crude product was purified by column chromatography
(silica gel, hexane) to give 8.1 g (56.3%) of 4 as a colorless oil.
1H NMR (CDCl3): δ 0.50-0.70 (m, 10H), 0.90-1.10 (m, 15H),
1.30 (t, J ) 8 Hz, 3H), 1.90 (t, J ) 7.5 Hz, 4H), 4.00 (q, J ) 7.2
Hz, 2H), 4.32 (q, J ) 7.3 Hz, 2H), 7.25 (m, 4H), 7.65 (d, J )
8.2 Hz, 2H), 7.77 (d, J ) 8.2 Hz, 1H), 8.00 (m, 3H).
2-(9,9-Dih e xylflu or e n yl)-1,4-d ih yd r oxym e t h ylb e n -
zen e (5). 4 (5.6 g, 0.01 mol) in 70 mL of dry THF was added
dropwise with stirring to a suspension of LiAlH4 (1 g, 26.3
mmol) in 100 mL of dry THF over 30 min at 0 °C. The mixture
was stirred at room temperature for 30 min and further
refluxed for 5 h. It was then cooled in an ice bath, and 50 mL
of water was added dropwise to the suspension over 20 min.
An additional 50 mL of 2 M HCl was added to destroy the
remaining LiAlH4. The organic layer was extracted with ethyl
acetate, washed with brine, and dried over MgSO4. The residue
was purified by column chromatography (silica gel, hexane/
ethyl acetate ) 2/1) to afford 4.54 g (95.6%) as 5 of a colorless
oil. 1H NMR (CDCl3): δ 0.60-0.70 (m, 4H), 0.75 (t, J ) 7.3
Hz, 6H), 1.00-1.15 (m, 12H), 1.78 (s, 2H), 1.97 (t, J ) 8.5 Hz,
4H), 4.64 (s, 2H), 4.75 (s, 2H), 7.30 (m, 7H), 7.56 (d, J ) 7.7
Hz, 1H), 7.72 (m, 2H).
2-(9,9-Dih exylflu or en yl)-1,4-dich lor om eth ylben zen e (6).
To 5 (2.68 g, 5.69 mmol) in 50 mL of dry toluene was added
thionyl chloride (3 mL, 34.6 mmol) containing one drop of
pyridine. The reaction mixture was then heated to 100 °C for
12 h. Upon cooling to room temperature, the mixture was
concentrated under reduced pressure and diluted with ethyl
acetate and brine. The organic layer was separated and
washed with brine and then dried over MgSO4. The residue
was purified by column chromatography (silica gel, hexane)
to afford 2.7 g (93.5%) as 6 of a colorless oil. 1H NMR (CDCl3):
δ 0.62-0.80 (m, 10H), 1.02-1.15 (m, 12H), 2.00 (t, J ) 6.5
Hz, 4H), 4.50 (s, 2H), 4.55 (s, 2H), 7.26-7.40 (m, 6H), 7.50
(m, 2H), 7.72 (m, 2H).
2-Br om o-1,4-d ih yd r oxym eth ylben zen e (7). 3 (20.9 g,
0.069 mol) in 200 mL of dry THF was added dropwise with
stirring to a suspension of LiAlH4 (6.5 g, 0.17 mol) in 300 mL
of dry THF over 30 min at 0 °C. The mixture was stirred at
room temperature for 30 min and further refluxed for 6 h. It
was then cooled in an ice bath, and 15 mL of water was added
dropwise to the suspension over 20 min. An additional 30 mL
of 2 M HCl was added to destroy the remaining LiAlH4. The
organic layer was extracted with ethyl acetate, washed with
brine, and dried over MgSO4. The crude product was recrystal-
lized from hexane/ethyl acetate to give 11.7 g (78%) of 7 as a
white solid. 1H NMR (DMSO-d6): δ 4.48 (dd, J ) 5.0 Hz, 4H),
5.28 (t, J ) 5.8 Hz, 1H), 5.39 (t, J ) 5.6 Hz, 1H), 7.30 (d, J )
7.8 Hz, 1H), 7.46-7.50 (m, 2H).
Ma ter ia ls. Tetrahydrofuran was distilled over sodium/
benzophenone, 1,4-dioxane was distilled over sodium, N,N-
dimethylformamide was vacuum-distilled over CaH2, and
toluene was distilled over P2O5. All other reagents were
analytical grade quality, purchased commercially, and used
without any further purification. 2-Bromo-9,9-dihexylfluorene
(1), 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexy-
lfluorene (2), and 2,7-dibromo-9,9-dihexylfluorene (10) were
prepared according to literature procedures.19
Dieth yl 2-Br om oter ep h th a la te (3). 2-Bromoterephthalic
acid (20 g, 0.08 mol) was dissolved in 10 mL of thionyl chloride
with catalytic amounts of DMF and warmed at 60 °C for 7 h.
The excess thionyl chloride was removed under reduced
pressure. The resulting pale yellow oil was added to 300 mL
of absolute ethanol in an ice bath and stirred for 2 h at room
temperature. The solvent was removed by rotary evaporation,
and the crude 2-bromodiethyl terephthalate was purified
by column chromatography (silica gel, hexane/ethyl acetate )
10/1) to afford a pale yellow oil (24 g) quantitatively.
2-Br om o-1,4-d i(2-t et r a h yd r op yr a n yloxym et h yl)b en -
zen e (8). To 7 (4.77 g, 0.02 mol) in DMF (3 mL) and
dichloromethane (50 mL) was added 2,3-dihydropyran (5.9 g,
0.07 mol) and a catalytic amount (0.79 g) of p-toluenesulfonic
acid. The reaction mixture was stirred for 12 h at room
temperature. The organic layer was washed with brine and
dried over MgSO4. The residue was purified by column
chromatography (silica gel, hexane/ethyl acetate ) 15/1) to
1
afford 7.25 g (86.4%) of 8 as a colorless oil. H NMR (CDCl3):
δ 1.50-1.90 (m, 12H), 3.50 (s, 2H), 3.86 (s, 2H), 4.43 (dd, J )
6.6 Hz, 1H), 4.53 (dd, J ) 6.3 Hz, 1H), 4.65-4.81 (m, 4H), 7.30
(s, 1H), 7.45 (s, 1H), 7.54 (s, 1H).
2-(4,4,5,5-Tet r a m et h yl-1,3,2-d ioxa bor ola n -2-yl)-1,4-d i-
(2-tetr a h yd r op yr a n yloxym eth yl)ben zen e (9). To a solu-
tion of 8 (9.64 g, 0.025 mol) in dry THF (200 mL) at -78 °C
was added, by syringe, 18.74 mL (0.033 mol) of n-butyllithium
(1.6 M in hexane). The mixture was stirred at -78 °C, warmed
to 0 °C for 20 min, and cooled again at -78 °C for 10 min.
2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6.98 g,
0.037 mol) was added rapidly to the solution, and the resulting
mixture was slowly warmed to room temperature and stirred
for 12 h. The reaction mixture was poured into water and
2-(9,9-Dih exylflu or en yl)dieth yl Ter ep h th a la te (4). To
2 (11.92 g, 26 mmol) in toluene (260 mL) and aqueous
Na2CO3 (120 mL, 2 M) was added 3 (10.13 g, 34 mmol). After
the solution was degassed with nitrogen for 20 min, tetrakis-
(triphenylphosphine)palladium(0) (0.6 g, 0.5 mmol) was added.

1358 Lee et al.
Macromolecules, Vol. 35, No. 4, 2002
extracted with ethyl acetate. The organic layer was washed
with brine and dried over MgSO4. The residue was purified
by column chromatography (silica gel, hexane/ethyl acetate )
tion of 6. Colorless oil was obtained in 92% yield. 1H NMR
(CDCl3): δ 0.50-0.70 (m, 10H), 0.92-1.05 (m, 12H), 1.80-
1.95 (m, 4H), 3.75 (s, 3H), 4.40 (s, 2H), 4.47 (s, 2H), 6.78 (m,
2H), 7.20 (m, 1H), 7.30 (m, 3H), 7.42 (m, 1H), 7.54 (m, 2H).
Anal. Calcd for C34H42Cl2O: C, 75.96; H, 7.87. Found: C, 75.27;
H, 7.89.
2-(7-Cya n o-9,9-d ih exylflu or en yl)-1,4-d ich lor om et h yl-
ben zen e (18). 18 was synthesized under the same conditions
as 6. Colorless oil was obtained in 93% yield.1H NMR
(CDCl3): δ 0.55-0.70 (m, 4H), 0.75 (t, J ) 7.2 Hz, 6H), 1.00-
1.20 (m, 12H), 2.03 (m, 4H), 4.5 (s, 2H), 4.65 (s, 2H), 7.45 (m,
3H), 7.50 (s, 1H), 7.56 (d, J ) 8 Hz, 1H), 7.66 (m, 2H), 7.82 (t,
J ) 6.8 Hz, 2H). Anal. Calcd for C34H39Cl2N: C, 76.68; H, 7.38;
N, 2.63. Found: C, 76.67; H, 7.40; N, 2.54.
1
30/1 to 15/1) to afford 5.1 g (47.2%) of 9 as a colorless oil. H
NMR (CDCl3): δ 1.30 (s, 12H), 1.50-1.90 (m, 12H), 3.53 (m,
2H), 3.93 (m, 2H), 4.50 (d, J ) 12 Hz, 1H), 4.69 (t, J ) 3.6 Hz,
1H), 7.47 (s, 2H), 7.76 (s, 1H).
2-Br om o-7-m et h oxy-9,9-d ih exylflu or en e (11). Sodium
ethoxide was prepared by adding sodium (3.9 g, 0.17 mol) into
50 mL of anhydrous ethanol under a nitrogen atmosphere.
After all the sodium disappeared 10 (16.7 g, 0.034 mol) in 150
mL of dry DMF and copper iodide (7.1 g, 0.037 mol) was added
to the above solution and refluxed for 7 h. The reaction mixture
was poured into ice water and then extracted with ethyl
acetate. The organic layer was washed with dilute HCl and
brine and dried over MgSO4. The resulting oil was purified by
column chromatography (silica gel, hexane) to give 7.84 g
(52.1%) of 11 as a colorless oil. 1H NMR (CDCl3): δ 0.50-0.65
(m, 4H), 0.75 (t, J ) 7.3 Hz, 6H), 1.00-1.15 (m, 12H), 1.90 (t,
J ) 7.6 Hz, 4H), 3.87 (s, 3H), 6.85 (m, 2H), 7.40 (m, 3H), 7.55
(d, J ) 8 Hz, 1H).
2-Br om o-7-cya n o-9,9-d ih exylflu or en e (12). A mixture of
10 (20 g, 0.04 mol) and copper cyanide (2.7 g, 0.03 mol) in 300
mL of dry DMF was refluxed for 10 h. The solvent was
concentrated under the reduced pressure. To the resulting
dark brown mixture was added ethyl acetate and brine. The
organic phase was separated and extracted with brine and
dried over MgSO4. The crude product was purified by column
chromatography (silica gel, hexane/ethyl acetate ) 50/1) to give
6.36 g (35.7%) of 12 as a yellow solid. 1H NMR (CDCl3): δ
0.50-0.65 (m, 4H), 0.75 (t, J ) 7 Hz, 6H), 0.95-1.15 (m, 12H),
1.97 (t, J ) 7 Hz, 4H), 7.50 (t, J ) 8 Hz, 2H), 7.62 (t, J ) 11.5
Hz, 3H), 7.73 (d, J ) 7 Hz, 1H).
P oly[2-(9,9-d ih e xylflu or e n yl)-1,4-p h e n yle n e vin yl-
en e](DHF -P P V). 6 (3.17 g, 6.25 mmol) was dissolved in 475
mL of dry dioxane and pursed with nitrogen for 30 min at 73
°C. To the above solution was added dropwise 15.6 mL (15.6
mmol) of potassium tert-butoxide (1 M in hexane) for 30 min.
The additional potassium tert-butoxide (12.5 mL, 12.5 mmol)
was added, refluxed for 3 h, and cooled slowly to 45 °C. To the
reaction mixture, 5 mL of mixture solvent (dioxane/acetic acid
) 1/1) was added and stirred for 30 min. The reaction mixture
was then poured into 500 mL of the stirred water. The
resulting yellow precipitate was collected by filtration and was
washed with methanol. The polymer was purified through
reprecipitation of THF and methanol (three times) and dried
under vacuum oven at 35 °C to afford 2.05 g (75%) of a bright
1
yellow fiber. H NMR (CD2Cl2): δ 0.70 (br s, 10H), 0.98 (br s,
12H), 1.92 (br s, 4H), 7.00-7.80 (br m, 12H). Anal. Calcd for
(C33H38)n: C, 91.24; H, 8.76. Found: C, 90.24; H, 8.76.
P oly[2-(7-m et h oxy-9,9-d ih exylflu or en yl)-1,4-p h en yl-
en evin ylen e] (MDHF -P P V). MDHF-PPV was synthesized
through the polymerization condition of DHF-PPV. Bright
2-(7-Met h oxy-9,9-d ih exylflu or en yl)-1,4-d i(2-t et r a h y-
d r op yr a n yloxym eth yl)ben zen e (13). 13 was synthesized
through the same condition of 4. Colorless oil was obtained in
1
yellow fiber was obtained in 70% yield. H NMR (CD2Cl2): δ
0.71 (br s, 10H), 0.99 (br s, 12H), 1.90 (br s, 4H), 3.86 (s, 3H),
6.80-7.60 (br m, 11H). Anal. Calcd for (C34H40O)n: C, 87.93;
H, 8.62. Found: C, 86.15; H, 8.69.
1
70% yield. H NMR (CDCl3): δ 0.60-0.70 (m, 4H), 0.75 (t, J
) 7.3 Hz, 6H), 1.00-1.15 (m, 12H), 1.50-1.80 (m, 12H), 1.90
(m, 4H), 3.46 (t, J ) 6.1 Hz, 1H), 3.55 (t, J ) 6.1 Hz, 1H), 3.88
(s, 3H), 3.84-3.94 (m, 2H), 4.40 (d, J ) 11.5 Hz, 1H), 4.56 (d,
J ) 12 Hz, 1H), 4.62 (t, J ) 3.4 Hz, 1H), 4.71 (s, 1H), 4.75 (dd,
J ) 2.7 Hz, 1H), 4.87 (d, J ) 10.7 Hz, 1H), 6.90 (m, 2H), 7.35
(m, 4H), 7.60 (m, 3H).
P oly[2-(7-cya n o-9,9-d ih exylflu or en yl)-1,4-p h en ylen evi-
n ylen e] (CNDHF -P P V). 18 (1.5 g, 2.82 mmol) was dissolved
in 150 mL of dry THF and pursed with nitrogen gas for 30
min at room temperature. Potassium tert-butoxide (10 mL, 10
mmol) was added dropwise for 30 min and stirred for 48 h at
room temperature. The reaction mixture was then poured into
250 mL of the stirred water. The resulting yellow precipitate
was collected by filtration and was washed with methanol. The
polymer was purified through reprecipitation of THF and
methanol (three times) and dried under vacuum oven at 35
°C to afford 0.66 g (51%) of a bright yellow fiber. 1H NMR
(CD2Cl2): δ 0.70 (br. s, 10H), 0.97 (br. s, 12H), 1.94 (br. s, 4H),
7.00-7.80 (br. m, 11H). Anal. Calcd for (C34H37N)n: C, 88.89;
H, 8.06; N, 3.05. Found: C, 87.61; H, 8.02; N, 2.93.
2-(7-Cya n o-9,9-d ih exylflu or en yl)-1,4-d i(2-t et r a h yd r o-
pyr an yloxym eth yl)ben zen e (14). 14 was synthesized through
the same condition of 4. Colorless oil was obtained in 75%
1
yield. H NMR (CDCl3): δ 0.50-0.65 (m, 4H), 0.78 (t, J ) 6.8
Hz, 6H), 1.00-1.15 (m, 12H), 1.50-1.90 (m, 12H), 2.00 (t, J )
8 Hz, 4H), 3.46 (t, J ) 6.1 Hz, 1H), 3.55 (t, J ) 6.1 Hz, 1H),
3.85 (t, J ) 8.8 Hz, 1H), 3.95 (t, J ) 8.8 Hz, 1H), 4.40 (d, J )
11.5 Hz, 1H), 4.56 (d, J ) 12 Hz, 1H), 4.63 (t, J ) 3.2 Hz, 1H),
4.71 (d, J ) 11.5 Hz, 1H), 4.76 (t, J ) 3.7 Hz, 1H), 4.87 (d, J
) 12.2 Hz, 1H), 7.36-7.47 (m, 4H), 7.60-7.69 (m, 3H), 7.78
(m, 2H).
Resu lts a n d Discu ssion
2-(7-Met h oxy-9,9-d ih exylflu or en yl)-1,4-d ih yd r oxym e-
th ylben zen e (15). A mixture of 13 (3.3 g, 4.99 mmol) and
p-toluenesulfonic acid (0.19 g, 1.0 mmol) in 70 mL of ethanol
was stirred for 5 h at room temperature. The organic layer
was washed with brine, dried over MgSO4, and then concen-
trated under reduced pressure. The crude product was purified
by column chromatography (silica gel, hexane/ethyl acetate )
6/1) to give 2.3 g (92%) of 15 as a colorless oil. 1H NMR
(CDCl3): δ 0.60-0.70 (m, 4H), 0.75 (t, J ) 7.3 Hz, 6H), 1.00-
1.15 (m, 12H), 1.95 (m, 4H), 3.00 (br. s, 2H), 3.85 (s, 3H), 4.60
(s, 2H), 4.65 (s, 2H), 6.88 (m, 2H), 7.25 (m, 4H), 7.50 (d, J )
7.6 Hz, 1H), 7.6 (m, 2H).
2-(7-Cya n o-9,9-d ih exylflu or en yl)-1,4-d ih yd r oxym eth yl-
ben zen e (16). 16 was synthesized through the same condition
of 15. Colorless oil was obtained in 87% yield. 1H NMR
(CDCl3): δ 0.55-0.70 (m, 4H), 0.75 (t, J ) 7.3 Hz, 6H), 1.00-
1.15 (m, 12H), 1.75 (br. s, 2H), 2.00 (t, J ) 7.6 Hz, 4H), 4.6
(s, 2H), 4.75 (s, 2H), 7.40 (m, 4H), 7.62 (m, 3H), 7.80 (d, J ) 5
Hz, 2H).
Syn th esis a n d Ch a r a cter iza tion . The monomer
syntheses are depicted in Scheme 1. The DHF-PPV
monomer, 2-(9,9-dihexylfluorenyl)-1,4-dichloromethyl-
benzene (6), was synthesized in four steps from the
starting material, 2-bromo-9,9-dihexylfluorene (1), which
was converted to 2-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)-9,9-dihexylfluorene (2). The arylboronic ester
2 underwent Suzuki cross-coupling with diethyl 2-bro-
moterephthalate (3) using Pd(PPh3)4 catalyst to yield
fluorenyl-containing diethyl terephthalate (4). Reduc-
tion with lithium aluminum hydride (LiAlH4) and
subsequent reaction with thionyl chloride finally af-
forded the key monomer 6. Another synthetic route was
selected to synthesize methoxy- or cyanofluorenyl-
containing 1,4-dichloromethylbenzene. Diethyl 2-bro-
moterephthalate (3) was reduced with LiAlH4, protected
with 2,3-dihydropyran, and then treated with 2-isopro-
poxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane to give aryl-
2-(7-Meth oxy-9,9-d ih exylflu or en yl)-1,4-d ich lor om eth -
ylben zen e (17). 17 was synthesized through the same condi-

Macromolecules, Vol. 35, No. 4, 2002
Fluorenyl-Substituted Poly(p-phenylenevinylenes) 1359
Sch em e 1. Syn th etic Rou tes to th e Mon om er s. Rea gen ts a n d Con d ition sa
a
(i) n-BuLi, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF, -78 °C; (ii) 3, Pd(PPh3)4, toluene/aqueous Na2CO3, reflux;
(iii) LiAlH4, THF, 0 °C to reflux; (iv) SOCl2, toluene, 100 °C; (v) SOCl2, 60 °C, EtOH, then LiAlH4, THF, 0 °C to reflux; (vi)
2,3-dihydropyran, p-toluenesulfonic acid, DMF/CH2Cl2, room temperature; (vii) NaOEt, CuI, DHF, or CuCN, DMF, reflux; (viii)
Pd(PPh3)4, toluene/aqueous Na2CO3, reflux; (ix) p-toluenesulfonic acid, EtOH, room temperature.
boronic ester 9. Suzuki cross-coupling of 9 with 2-bromo-
7-methoxy- or 2-bromo-7-cyano-9,9-dihexylfluorene using
Pd(PPh3)4 catalyst yielded 13 or 14, respectively. Depro-
tection of the tetrahydropyranyl group in 13 or 14 with
p-toluenesulfonic acid in ethanol and subsequent treat-
ment with thionyl chloride afforded monomers 17 or 18,
respectively. Base-induced polymerization (GILCH
polymerization) of monomers 6, 17, or 18 with potas-
sium tert-butoxide in refluxing dioxane or in THF at
room temperature yielded the polymers, DHF-PPV,
MDHF-PPV, and CNDHF-PPV, respectively (Scheme
2). The polymers were obtained as bright yellow fibers

1360 Lee et al.
Macromolecules, Vol. 35, No. 4, 2002
Sch em e 2. Syn th etic Rou tes to P olym er s, DHF -P P V, MDHF -P P V, a n d CNDHF -P P V
Ta ble 1. Aver a ge Molecu la r Weigh ts a n d Th er m a l
polymerization, while a new band with strong intensity
occurred at 962 cm-1, which is due to the vinylic out-
of-plane deformation. This result suggests that the
vinylene group formed through GILCH route is in the
trans configuration. The insoluble polymer, CNDHF-
PPV, was obtained, when the monomer 18 was polym-
erized with potassium tert-butoxide in refluxing dioxane.
This indicates that the side-reaction (e.g., the formation
of cross-linked polymer) between the CN group of
monomer 18 and the quinodimethane derivative formed
in the first step of polymerization occurred. No observa-
tion of the sharp stretching peak of CN group at 2224
cm-1 in insoluble CNDHF-PPV suggests that cross-
linked polymer was formed. The 1H NMR spectra of the
polymers and the monomer 17 exhibit the chemical
shifts of alkyl side chains between 0.55 and 2.00 ppm
(Figure 2). As an example, the methoxy protons (-OCH3)
of the monomer 17 appear at 3.75 ppm, and the
methylene protons adjacent to the phenyl rings and the
aromatic phenyl protons are found at 4.40 and 4.47 ppm
and in the range of 6.80-7.60 ppm, respectively. As the
polymerization proceeded, the methylene protons of
monomer 17 around 4.40 ppm disappeared completely,
and new vinylic protons appeared around 7.10 ppm.
P r op er ties of DHF -P P Vs
yield
Mn
Mw
polymer
(%) (×10-4
)
(×10-4
) Mw/Mn Tg (°C) Td (°C)
DHF-PPV
75
70
31.3
23.8
4.2
56.3
51.9
13.2
1.80
2.18
3.13
113
118
148
322
331
345
MDHF-PPV
CNDHF-PPV 51
Th er m a l P r op er ties. The thermal stability of the
polymers was evaluated by thermogravimetric analysis
(TGA) under a nitrogen atmosphere. Figure 3 shows
that the polymers exhibit good thermal stability. The
polymers have onset degradation temperatures above
322 °C, and no weight loss was observed at lower
temperatures. The glass transition temperatures (Tg)
of the polymers were determined by differential scan-
ning calorimetry (DSC) in a nitrogen atmosphere at a
heating rate of 20 °C/min. As shown in Figure 4, all
three polymers have high Tg’s of 113-148 °C, which are
50-80 °C higher than that of the soluble polymer
MEH-PPV (∼65 °C).20 The Tg of CNDHF-PPV is 15
°C higher than that of DHF-PPV. This can be explained
by the intermolecular electrostatic interaction between
the cyano groups in the fluorene units of one polymer
backbone and those on adjacent chains in the bulk
materials. The relatively high glass transition temper-
F igu r e 1. FT-IR spectra of (a) DHF-PPV, (b) MDHF-PPV,
(c) CNDHF-PPV, and (d) monomer 18.
after repeated precipitation with THF and methanol
and readily dissolved in common organic solvents, such
as chloroform, THF, toluene, and xylene.
The number-average molecular weights (Mn) and the
weight-average molecular weights (Mw) of the polymers
were determined to be 42 000-310 000 and 130 000-
560 000 with the polydispersity index of 1.8-3.1, re-
spectively, by gel permeation chromatography (GPC)
using polystyrene as a standard (Table 1). The chemical
1
structure of the polymers was confirmed by FT-IR, H
NMR, 13C NMR, and elemental analysis. Figure 1 shows
the FT-IR spectra of the polymers and the monomer 18.
The strong stretching absorption bands for C-Cl in the
monomer between 660 and 800 cm-1 disappeared after

Macromolecules, Vol. 35, No. 4, 2002
Fluorenyl-Substituted Poly(p-phenylenevinylenes) 1361
Ta ble 2. Op tica l a n d Electr och em ica l P r op er ties of DHF -P P Vs
Eogp (eV)c
Eegl (eV)d
HOMO eV)e
LUMO (eV)f
b
polymer
UV λmax (nm)a
PL λmax (nm)a
ΦPL
DHF-PPV
MDHF-PPV
CNDHF-PPV
428
428
423
504
510
502
0.68
0.71
0.68
2.51
2.52
2.53
2.40
2.35
2.32
-5.61
-5.55
-5.57
-3.21
-3.20
-3.26
a
b
Determined from thin films on the quartz plate. PL efficiencies in chloroform determined relative to quinine sulfate. c Calculated
from the absorption spectra edges. Calculated from the equation Eegl ) LUMO - HOMO. e Determined from the onset of oxidative
d
curve. f Determined from the onset of reductive curve.
F igu r e 4. DSC thermograms of (a) DHF-PPV, (b) MDHF-
F igu r e 2. 1H NMR spectra of (a) DHF-PPV, (b) monomer
PPV, and (c) CNDHF-PPV.
17, (c) MDHF-PPV, and (d) CNDHF-PPV.
F igu r e 3. TGA thermograms of (a) DHF-PPV, (b) MDHF-
PPV, and (c) CNDHF-PPV.
F igu r e 5. UV-vis spectra of (a) DHF-PPV, (b) MDHF-PPV,
and (c) CNDHF-PPV films spin-coated on a quartz plate.
atures of three polymers designed as emissive materials
in this study may present an advantage (e.g., improve-
ment of the operating lifetime) for the PLED device
fabrication.
fluorene units which are confirmed by the same two
absorption peaks of the compound 1. The absorption
onset wavelengths of the polymers were the 490-494
nm, which correspond to band gaps of 2.51-2.53 eV.
The PL spectra of DHF-PPVs as thin film are shown
in Figure 6. The polymer films with the broad band blue-
green PL emission exhibit similar vibronic features with
maximum emission peaks around 503 nm and the
shoulder peaks at 530 nm, which are significantly blue-
shifted compared with the two emission peaks (520 and
551 nm) of the parent polymer, poly(p-phenylenevi-
nylene) (PPV). The result demonstrates that the com-
bination of the strong steric effect21 due to bulky groups
in PPV backbone and the poor electronic effect22 of
electron-donating or -withdrawing groups in nonconju-
gated positions of PPV backbone gives rise to disruption
Op tica l a n d P h otolu m in escen ce P r op er ties. The
UV-vis absorption of DHF-PPV, MDHF-PPV, and
CNDHF-PPV as thin films is shown in Figures 5. The
peak wavelengths of the spectra and the band gaps (Eg)
of the polymers estimated from the absorption spectra
edges are summarized in Table 2. The thin films were
prepared by spin-coating on quartz plates from the
polymer solutions in toluene. All three polymers exhibit
very similar absorption spectra with a maximum peak
of 428 nm attributed to π-π* transition of the conju-
gated backbones and two additional absorption peaks
at 279-295 and 311-323 nm due to π-π* transition of

1362 Lee et al.
Macromolecules, Vol. 35, No. 4, 2002
F igu r e 6. Photoluminescence (PL) spectra of (a) DHF-PPV,
(b) MDHF-PPV, and (c) CNDHF-PPV films spin-coated on
a quartz plate.
F igu r e 7. Cyclic voltammograms of DHF-PPVs films coated
on ITO plate electrodes in acetonitrile containing 0.1 M Bu4-
NClO4. Counter electrode: platinum mesh. Reference elec-
trode: Ag/AgCl (0.1 M in acetonitrile). Scan rate: 10 mV/s.
of the effective conjugating length of the polymer
backbone. The PL efficiencies (ΦPL) of the polymers in
chloroform were measured by comparing to quinine
sulfate (ca. 2 × 10-5 M, assuming ΦPL of 0.546 at 365
nm excitation in 1 N H2SO4)23 as standard. All three
polymers exhibited very high PL efficiencies in chloro-
form in the range of 68-71%, as shown in Table 1.
These PL efficiencies are comparable with those of
biphenyl-substituted PPVs (73-77%), which are typical
sterically hindered phenyl-substituted PPVs with maxi-
mum emission peaks of 504-520 nm in the films. The
high PL efficiency of the polymers may result from the
steric hindrance between fluorene units and the PPV
backbone which forces the pendant fluorene unit to be
twisted away from the plane of the conjugated backbone,
which prevents close packing in solid state. In addition,
the asymmetric bulky side group-containing DHF-
PPVs will be less ordered than the symmetric side
chain-containing PPV, poly(2,5-bis(2-ethylhexyloxy)-1,4-
phenylenevinylene (BEH-PPV)24 in the film state.
Therefore, it is expected that PL efficiencies of DHF-
PPVs thin films are less reduced.
tion onsets of the polymers can be used for calculating
LUMO energy levels and are determined to be -1.49,
-1.50, and -1.45 V for DHF-PPV, MDHF-PPV, and
CNDHF-PPV, respectively. As in the oxidation scan,
the reduction scans of the polymers are less sensitive
to the variation of substitution at the 7-position of the
fluorene unit and show irreversible processes. The result
demonstrates that these reduction onsets must also
originate from the PPV backbone. The calculated LUMO
energy levels of the polymers are about 3.2 eV, which
is consistent with the results (about 3.1 eV) estimated
from the HOMO and optical band gap. These values are
comparable with that of cyano-substituted poly(p-phe-
nylenevinylene) (CN-PPV) (3.1 eV),26 which shows good
electron transport in PLEDs.
Because of low-lying LUMO energy levels of DHF-
PPVs, we believe that the operation of PLEDs fabricated
using DHF-PPVs as an emitting layer and Ca (IP )
2.8 eV) as cathode would be limited by the electron
mobility within the emitting layer rather than electron
injection into the polymer film.
Electr och em ica l P r op er ties. Cyclic voltammetry
(CV) was employed to investigate the redox behaviors
of the polymers, DHF-PPVs, and to estimate the
HOMO and LUMO energy levels of the polymers. The
cyclic voltammograms of the polymers coated on a ITO
plate from chloroform solution were performed in a
three-electrode cell with a Pt counter electrode and a
Ag/AgCl reference electrode. The electrolyte (0.1 M
tetrabutylammonium perchlorate in acetonitrile) was
purged with argon. All measurements were calibrated
against an internal standard, ferrocene (Fc), which has
the IP value (-4.8 eV) of the Fc/Fc+ redox system.25 The
potential of Fc/Fc+ was measured as 0.1 V vs Ag/AgCl
in this work.
The CV curves of DHF-PPVs are shown for compari-
son in Figure 7. All of the polymers exhibit irreversible
processes in an oxidation scan. The oxidation onsets of
the polymers are 0.91, 0.85, and 0.87 V for DHF-PPV,
MDHF-PPV, and CNDHF-PPV, respectively, which
correspond to HOMO energy level of about 5.6 eV. The
small difference of oxidation onsets for the polymers is
due to the PPV backbone rather than the electronic
effect of functional groups in fluorene unit. The reduc-
Device Ch a r a cter istics. The double-layered poly-
mer light-emitting diodes (PLEDs) with the configura-
tion of ITO/PEDOT:PSS/DHF-PPVs/Ca were fabricated
to investigate the electroluminescent properties of the
new polymers, DHF-PPVs. As shown in Figure 8, all
EL spectra of three DHF-PPVs show a maximum peak
around 500 nm and a shoulder around 532 nm, which
are very similar to the PL spectra of the corresponding
polymer films. The blue-green light emission from the
devices originates from the main chain of the emitting
polymers. PLEDs were characterized by measuring the
current density as a function of the electric field and
the dependence of luminance on the electric field and
by calculating the maximum external quantum efficien-
cies (ηmax, %) and the turn-on voltage (MV/cm) (defined
as the electric field required for a luminance of 0.1 cd/
m2). Figure 9 compares the I-V curves for the double-
layered diodes of ITO/PEDOT:PSS/DHF-PPVs/Ca for
the three polymers. With an increasing forward bias
voltage, the current in all of the devices increases, which
is a typical rectifying characteristic. The EL perfor-
mance of the device fabricated using a DHF-PPV
emitting layer has the lowest turn-on voltage, the

Macromolecules, Vol. 35, No. 4, 2002
Fluorenyl-Substituted Poly(p-phenylenevinylenes) 1363
F igu r e 8. Electroluminescence (EL) spectra of PLEDs of (a)
DHF-PPV, (b) MDHF-PPV, and (c) CNDHF-PPV with a
configuration of ITO/PEDOT:PSS/DHF-PPVs/Ca.
F igu r e 10. Luminance-electric field characteristics of PLEDs
of (a) DHF-PPV, (b) MDHF-PPV, and (c) CNDHF-PPV with
a configuration of ITO/PEDOT:PSS/DHF-PPVs/Ca.
CN groups in CNDHF-PPV act as deep traps of hole
carriers. Furthermore, the functional groups of the side
fluorene units may give rise to unbalanced charge
transport in addition to the carrier trapping of injected
charged carriers. Comparison of the performance of the
device of DHF-PPV with those of MDHF-PPV and
CNDHF-PPV shows that maximum external quantum
efficiency (0.53%) of DHF-PPV is higher by a factor of
1.6 (MDHF-PPV, 0.33%) and 2.8 (CNDHF-PPV, 0.19%).
Con clu sion s
A new series of processable polymers with fluorene
groups appended to a PPV backbone were synthesized
using the dehydrohalogenation-condensation polymer-
ization (GILCH polymerization). These polymers have
quite good thermal stability with decomposition starting
above 322 °C together with high glass transition tem-
peratures in the range of 113-148 °C. The double-layer
polymer light-emitting diodes (PLEDs) of dihexylfluo-
renyl-substituted poly(p-phenylenevinylene) derivatives
(DHF-PPVs) emit bright blue-green light with a maxi-
mum peak around 500 nm and a shoulder peak around
532 nm. DHF-PPV devices reveal both a lower turn-
on voltage (2.8 V, 0.13 MV/cm) and a greatly enhanced
external quantum efficiency (0.53%) when compared to
that of MDHF-PPV and CNDHF-PPV devices.
F igu r e 9. Current-electric field characteristics of PLEDs of
(a) DHF-PPV, (b) MDHF-PPV, and (c) CNDHF-PPV with
a configuration of ITO/PEDOT:PSS/DHF-PPVs/Ca.
highest maximum luminance, and the highest maxi-
mum external quantum efficiency, compared to the
other two polymers. As shown by the L-V curve in
Figure 10, the maximum luminance of a DHF-PPV
device as the emitting layer is 12 000 cd/m2 at 0.4 MV/
cm, which is 37 times higher than that of MDHF-PPV
and 33 times higher than that of CNDHF-PPV at the
same electric field. The turn-on voltages of DHF-PPV,
MDHF-PPV, and CNDHF-PPV are 0.13, 0.21, and
0.21 MV/cm, respectively. The increase of the turn-on
voltages for MDHF-PPV and CNDHF-PPV may be
closely related to the carrier trapping ability of func-
tional groups in the fluorene units. For the structure of
the DHF-PPVs designed in this study, it is reasonable
that the functional groups of the side fluorene units lie
outside the rigid-rod polymer backbone and thus act as
electron or hole carrier traps to injected charge carriers.
It was found that DHF-PPV has a hole mobility of 4.3
× 10-4 cm2/(V s) at an applied field of 0.25 MV/cm,
which is 4 times faster than that of MDHF-PPV (1.0
× 10-4 cm2/(V s)) and 3 orders faster than that of
CNDHF-PPV (4.0 × 10-7 cm2/(V s)) at the same applied
field.27 It means that the strong electron-withdrawing
Ack n ow led gm en t. This work has been supported
by the Core Research for Evolutional Science and
Technology, J apan Science and Technology Corporation
(CREST/J ST). S.H.L. thanks Mr. Toshikazu Nakamura
for his help in the CV measurement.
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