Novel cyclohexylsilyl- or phenylsilyl-substituted poly(p-phenylene vinylene)s via the halogen precursor route and Gilch polymerization

Article abstract of DOI:10.1021/ma011618b

Novel cyclohexylsilyl- or phenylsilyl-substituted poly(1,4-phenylene vinylene) (PPV) derivatives, poly[2,5-bis(dimethylcyclohexylsilyl)-1,4-phenylene vinylene] (BDMCyS-PPV), poly[2,5-bis(dimethylphenylsilyl)-1,4-phenylene vinylene] (BDMPS-PPV), poly[2-dim

Full text of DOI:10.1021/ma011618b

Macromolecules 2002, 35, 3495-3505
3495
Novel Cyclohexylsilyl- or Phenylsilyl-Substituted Poly(p-phenylene
vinylene)s via the Halogen Precursor Route and Gilch Polymerization
Ta ek Ah n , Seu n g-Won Ko, J a em in Lee, a n d Hon g-Ku Sh im *
Center for Advanced Functional Polymers, Department of Chemistry and School of Molecular Science
(BK 21), Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea
Received September 11, 2001; Revised Manuscript Received February 7, 2002
ABSTRACT: Novel cyclohexylsilyl- or phenylsilyl-substituted poly(1,4-phenylene vinylene) (PPV) deriva-
tives, poly[2,5-bis(dimethylcyclohexylsilyl)-1,4-phenylene vinylene] (BDMCyS-PPV), poly[2,5-bis(di-
methylphenylsilyl)-1,4-phenylene vinylene] (BDMPS-PPV), poly[2-dimethylcyclohexylsilyl-1,4-phenylene
vinylene] (DMCyS-PPV), and poly[2-dimethylphenylsilyl-1,4-phenylene vinylene] (DMPS-PPV), were
synthesized via the bromine precursor route (BPR) and Gilch dehydrohalogenation polyaddition. Thin
films of the insoluble BDMCyS-PPV and BDMPS-PPV were fabricated from soluble polymer precursor
materials by thermal conversion, and the electronic properties of these films were investigated. Monosilyl-
substituted DMCyS-PPV and DMPS-PPV exhibited good solubility in the conjugated state, good film-
forming properties, and high molecular weights. Moreover, they showed better thermal stability and
higher values of Tg (DMCyS-PPV, 125 °C; DMPS-PPV, 127 °C) than other PPV derivatives including
alkylsilyl-substituted PPVs; this improved mechanical stability led to good electroluminescence perfor-
mance. Monocyclohexylsilyl- or phenylsilyl-substituted DMCyS-PPV and DMPS-PPV exhibited sharp
PL emissions at about 511 and 513 nm, respectively, along with extremely high photoluminescence (PL)
efficiencies in both solution and film (DMCyS-PPV, Φfilm ) 0.83; DMCPS-PPV, Φfilm ) 0.82). LED devices
fabricated from DMCyS-PPV and DMPS-PPV using the configuration ITO/polymer/Al showed elec-
troluminescence (EL) maxima at 510 and 515 nm, respectively, with external EL quantum efficiencies of
0.02% and 0.03%. Incorporation of PVK as a hole-transporting layer between the ITO and polymer with
an air-stable aluminum cathode caused a substantial improvement in the EL quantum efficiencies,
increasing the values to 0.07% for DMCyS-PPV and 0.08% for DMPS-PPV.
In tr od u ction
cathode) efficiencies of a PPV derivative with one
octylsilyl substituent.17,18 Most recently, Huang et al.
reported the synthesis of a family of bis-silyl-substituted
PPVs with side-chain lengths ranging from C1 to C18.19
They found that the chemical/electrical stability and
charge injection and transporting ability of the alkyl-
silyl-substituted polymers decreased with increasing
chain length. In addition, they revealed that the intro-
duction of silyl groups into PPVs retards the hole
injection ability and that modification of the anode is
the key factor in LED device performance. However, all
the silyl-containing polymers synthesized to date have
been composed of simple alkylsilyl-type side chains.
Although such long alkylsilyl- or alkoxy-substituted
PPV derivatives have good solubility in common organic
solvents, they tend to have lower glass transition
temperatures (Tg) than PPV.3 The low Tg in the PPV
derivatives represent a drawback of the PPV precursor
route. Tokito et al. found the operating lifetime of an
EL device to be directly related to the Tg and thermal
stability of the polymers used.20 It is therefore important
to synthesize silyl-substituted PPVs characterized by
good mechanical properties (high Tg) and high solubility
in order to obtain high PL efficiency and enhanced EL
device performance.
The discovery by the Cambridge group in 19901 that
conjugated polymers can be used as active light-emitting
layers in light-emitting devices triggered extensive
research on this class of polymers.2-4 Polymeric light-
emitting diodes (PLEDs) have many properties that are
advantageous for the development of a large-area light-
emitting displays: good processability, low operating
voltages, fast response times, and color tunability over
the full visible range by the control of the HOMO-
LUMO band gap of the emissive layer.5-7 A number of
conjugated backbone structures such as poly(p-phen-
ylene vinylene) (PPV), poly(p-phenylene) (PPP),8,9 poly-
thiophene (PT),10 and polyfluorene (PF)11-13 have been
shown to be of importance in realizing a range of
emissive colors from polymeric light-emitting diodes
(PLEDs). The most promising and extensively studied
group of polymers is poly(p-phenylene vinylene) (PPV)
and its derivatives on account of their good device
performance.14
There has been increasing interest in the silyl-
substituted PPV derivatives as luminescent polymers
since the first silicon-containing derivative, poly[2-(3-
epi-cholestanol)-5-dimethylthexylsilyl-1,4-phenylene vi-
nylene] (CS-PPV), was reported by Wudl et al.15,16 Silyl-
substituted PPVs have many valuable properties such
as high photoluminescence (PL) quantum efficiency,
good solubility, and uniform film morphology. Holmes
et al. demonstrated the high PL (ca. 60% in the film
state) and electroluminescence (EL) (ca. 0.05% with Al
Here we report the synthesis of bis (or mono)-
cyclohexylsilyl- and phenylsilyl-substituted PPVs via the
halogen precursor route21-23 and Gilch dehydrohaloge-
nation14,24,25 method. Our results show that the cyclo-
hexylsilyl- and phenylsilyl-substituted PPVs have en-
hanced thermal stabilities, mechanical properties, and
PL and EL quantum efficiencies in comparison to the
PPV derivatives prepared in the past.
* 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/ma011618b CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/27/2002

3496 Ahn et al.
Macromolecules, Vol. 35, No. 9, 2002
renesulfonic acid) (PSS) (PEDOT:PSS) (BAYTRON P 4083)
was prepared from water dispersion and baked at 100 °C in
vacuo for 1 h with a thickness of about 30 nm. PVK and
DMPS-PPV layers were fabricated successively onto the
PEDOT:PSS layer. The alkaline metal compound, LiF (2 nm),
as an electron injection material (EIM) and Al (100 nm)
cathode were vacuum-deposited onto the DMPS-PPV film at
a pressure below 2 × 10-6 Torr, yielding an active area of 0.2
cm2. EL spectra of the LED device were measured with a
Minolta CS-1000 spectroradiometer. Luminance-current-
voltage (L-I-V) characteristics were recorded using a pro-
grammable current/voltage source (Keithley 238) and a lumi-
nance meter (Minolta LS-100).
Exp er im en ta l Section
Ma ter ia ls. 2,5-Dibromo-p-xylene, 2-bromo-p-xylene, chlo-
rodimethylcyclohexylsilane, chlorodimethylphenylsilane, N-
bromosuccinimide (NBS), potassium tert-butoxide (1.0 M
solution in THF), poly(9-vinylcarbazole) (PVK), and magne-
sium turnings from Aldrich Chemical Co. were used without
further purification. Tetrahydrofuran (THF) was dried and
fractionally distilled from sodium. All other solvents and
reagents were analytical-grade quality, purchased commer-
cially, and used without further purification.
Mea su r em en ts. 1H NMR and 13C NMR spectra were
recorded with the use of Bruker AVANCE 300 and 400
spectrometers, and chemical shifts were reported in ppm units
with tetramethylsilane as an internal standard. Chloroform
(CDCl3) was mainly used as the solvent for recording NMR
spectra. Melting points (mp) were determined using an Elec-
trothermal model 1307 digital analyzer. Elemental analyses
were performed by the Analytical Laboratory of Korea Ad-
vanced Institute of Science and Technology using an elemental
analyzer (EA1110-FISONS). The FT-IR spectra were obtained
by a Bruker EQUINOX 55 spectrometer by dispersing samples
in KBr. Differential scanning calorimetry (DSC) and thermo-
gravimetric analysis (TGA) were performed under nitrogen at
a heating rate of 10 °C/min with a DuPont 9900 analyzer.
Cyclic voltammetry was performed on an AUTOLAB/PG-
STAT12 model system with a three-electrode cell in a solution
of Bu4NBF4 (0.10 M) in acetonitrile at a scan rate of 50 mV/s.
Polymer films were coated on a square Pt electrode (0.50 cm2)
by dipping the electrode into the corresponding solutions and
then drying in argon. A Pt wire was used as the counter
electrode and a Ag/AgNO3 (0.10 M in acetonitrile) electrode
as the reference electrode. Gel permeation chromatography
(GPC) analysis was conducted on a Waters GPC-150C model
system using polystyrene as standard and THF as eluent. UV-
vis spectra were measured on a Shimadzu UV-3100S spec-
trometer, and PL spectra of the polymers were obtained using
a Spex Fluorolog-3 (model FL3-11) spectrofluorometer. Ab-
sorption and PL spectra were measured on polymer films spin-
coated onto quartz substrates. The film thicknesses of the
polymers were found by a Tencor Alpha-Step 500 surface
profiler.
EL Device F a br ica tion . BDMCyS-P P V a n d BDMP S-
P P V. Single-layer EL devices were fabricated with the polymer
film sandwiched between an indium tin oxide (ITO) anode and
an aluminum cathode. A glass substrate coated with transpar-
ent ITO was cleaned by successive ultrasonic treatment in
isopropyl alcohol and acetone, then dried with nitrogen gas,
and heated for further drying. Precursor polymer solutions (15
mg/mL in anhydrous chloroform) were spin-coated onto pre-
cleaned ITO/glass substrates at a spin speed of 1500 rpm for
30 s. Thermal conversion of the precursor films was achieved
by heating to 250 °C in vacuo for 6 h. Aluminum electrodes
(100 nm) were then evaporated on top of the polymer film
below 2 × 10-6 Torr, yielding a circular active layer with a
diameter of 2 mm.
DMCyS-P P V a n d DMP S-P P V. Single-layer and multi-
layer LED devices were fabricated on glass substrates coated
with indium tin oxide (ITO). The substrate was cleaned by the
same method mentioned above prior to use. The hole-
transporting layer of poly(9-vinylcarbazole) (PVK) (Mw 69 000
and Mn 27 200) was fabricated. PVK polymer solution (5 mg/
mL in cyclohexanone) was spin-coated onto ITO/glass sub-
strates at a spin speed of 3000 rpm for 30 s and baked at 100
°C in vacuo for 1 h with the thickness of about 60 nm. The
light-emitting layer (80 nm thick) of DMCyS-PPV or DMPS-
PPV was spin-coated from chlorobenzene solutions (10 mg/mL)
onto the ITO surface or the hole-transporting PVK layer.
Light-emitting polymers coated on top of the PVK layer did
not cause swelling or dissolution of the PVK layer, so good
quality film interfaces were obtained. Finally, the Al cathode
was vacuum-evaporated onto the light-emitting polymer layer
as with the BDMCyS-PPV or BDMPS-PPV. For the en-
hanced LED device performance, a hole injection layer of poly-
(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(sty-
2,5-Bis(d im eth ylcycloh exylsilyl)-p-xylen e (1). To a solu-
tion of 10 g (38.0 mmol) of 2,5-dibromo-p-xylene diluted in
anhydrous THF was added 3.7 g (152 mmol) of clean magne-
sium turnings after initiation by 5 mol % of dibromoethane,
and then this solution was refluxed for 3 h under a nitrogen
atmosphere. When
a gray, soupy Grignard reagent was
formed, 21.1 mL (114 mmol) of chlorodimethylcyclohexylsilane
was slowly added to this solution, and the mixture was
refluxed for 9 h. The reaction was quenched with dilute HCl
solution. The organic layer was separated and washed with
water several times. The solution was dried with magnesium
sulfate, and then THF was removed by evaporation. A white,
solid product was obtained by recrystallization from methanol.
1
The product yield was 5.7 g (39%); mp 112-114 °C. H NMR
(CDCl3, ppm): δ 7.14 (s, 2H, Ar H), 2.38 (s, 6H, CH3), 1.70-
1.51 (m, 10H, CH and CH2 of cyclohexyl), 1.25-0.93 (m, 12H,
CH2 of cyclohexyl), 0.24 (s, 12H, SiCH3). 13C NMR (CDCl3,
ppm): δ 139.33, 137.36, 136.41, 28.22, 27.74, 26.99, 25.74,
23.01, -3.77. Anal. Calcd for C24H42Si2: C, 74.53; H, 10.95.
Found: C, 73.95; H, 11.26.
2,5-Bis(d im et h ylcycloh exylsilyl)-1,4-b is(b r om om et h -
yl)ben zen e (2). Compound 2 was prepared by reacting 3.0 g
(7.76 mmol) of 1 with 2.9 g (16.3 mmol) of N-bromosuccinimide
(NBS) in 80 mL of carbon tetrachloride (CCl4). A small amount
of benzoyl peroxide was added as an initiator. The reaction
mixture was refluxed at 80 °C for 3 h under nitrogen
atmosphere. The completion of the reaction was indicated by
the appearance of succinimide on the surface of the reaction
solution. The organic layer was washed with water and brine
and then dried over anhydrous magnesium sulfate. The
solution was concentrated and poured into methanol. The pure
white solid was obtained by recrystallizing the resulting
precipitate from methanol. The product yield was 2.8 g (66%);
mp 94-96 °C. 1H NMR (CDCl3, ppm): δ 7.45 (s, 2H, Ar H),
4.60 (s, 4H, CH2Br), 1.70-1.52 (m, 10H, CH and CH2 of
cyclohexyl), 1.24-1.06 (m, 12H, CH2 of cyclohexyl), 0.32 (s,
12H, SiCH3). 13C NMR (CDCl3, ppm): δ 142.01, 139.06, 138.11,
34.48, 27.99, 27.57, 26.83, 26.02, -3.58. Anal. Calcd for C24H40
Br2Si2: C, 52.93; H, 7.40. Found: C, 52.13; H, 8.05.
-
2,5-Bis(d im eth ylp h en ylsilyl)-p-xylen e (3). To a solution
of 10 g (38.0 mmol) of 2,5-dibromo-p-xylene diluted in anhy-
drous THF was added 3.7 g (152 mmol) of clean magnesium
turnings after initiation by 5 mol % of dibromoethane, and
then this solution was refluxed for 3 h under a nitrogen
atmosphere. When
a gray, soupy Grignard reagent was
formed, 19.1 mL (114 mmol) of chlorodimethylphenylsilane
was slowly added to this solution, and the mixture was
refluxed for 9 h. The following synthetic procedures were
performed in analogy to the procedures given for compound
1. The product yield was 6.8 g (48%); mp 121-123 °C. 1H NMR
(CDCl3, ppm): δ 7.52 (m, 4H, Ar H), 7.35 (m, 6H, Ar H), 7.25
(m, 2H, Ar H), 2.46 (s, 6H, CH3), 0.59 (s, 12H, SiCH3). 13C NMR
(CDCl3, ppm): δ 140.03, 138.97, 137.38, 136.72, 134.04, 128.90,
127.80, 22.77, -1.38. Anal. Calcd for C24H30Si2: C, 76.94; H,
8.07. Found: C, 76.66; H, 8.01.
2,5-Bis(dim eth ylph en ylsilyl)-1,4-bis(br om om eth yl)ben -
zen e (4). Compound 4 was prepared by reacting 5.0 g (13.4
mmol) of 3 with 4.9 g (28 mmol) of N-bromosuccinimide (NBS)
in 100 mL of carbon tetrachloride (CCl4). The following
synthetic procedures were performed in analogy to the proce-
dures given for compound 2. The product yield was 5.1 g (72%);
1
mp 199-121 °C. H NMR (CDCl3, ppm): δ 7.57 (s, 2H, Ar H),

Macromolecules, Vol. 35, No. 9, 2002
Poly(p-phenylenevinylene)s 3497
7.52 (m, 4H, Ar H), 7.36 (m, 6H, Ar H), 4.41 (s, 4H, CH2Br),
0.66 (s, 12H, SiCH3). 13C NMR (CDCl3, ppm): δ 142.54, 139.21,
138.40, 137.86, 133.98, 129.44, 128.07, 34.00, -1.10. Anal.
Calcd for C24H28Br2Si2: C, 54.14; H, 5.30. Found: C, 53.99;
H, 5.89.
bis(dimethylcyclohexylsilyl)-1,4-bis(bromomethyl)benzene (2)
(0.5 g, 0.92 mmol) in dry tetrahydrofuran (3 mL), which was
cooled with an acetone/ice bath under nitrogen. A viscous sky-
blue solution was formed. The reaction mixture was allowed
to warm to room temperature after 10 min and stirred for 2
h. Polymer 9 was precipitated by adding the reaction mixture
dropwise to ice-cooled methanol (30 mL). The mixture was
filtered, and the residue, polymer 9, was briefly dried under
vacuum. The residue was dissolved in dry chloroform, and the
polymer was precipitated again by addition to an excess
methanol. The mixture was filtered, and the residue was
collected. The procedure was repeated twice more to yield a
sky-blue solid of 9. The polymer yield was 49% (0.21 g). 1H
NMR (CDCl3, ppm): δ 8.10-7.81 (br m, vinyl H), 7.45-6.61
(br m, Ar H and vinyl H), 5.65-5.15 (br m, CHBr), 3.97-2.96
(br m, ArCH2), 2.75-0.80 (br m, CH and CH2 of cyclohexyl),
0.79 to -0.60 (br m, -Si(CH3)2).
P oly[{2,5-b is(d im et h ylp h en ylsilyl)-1,4-p h en ylen e}(1-
br om oeth ylen e)}-co-{2,5-bis(d im eth ylp h en yl- silyl)-1,4-
p h en ylen evin ylen e}] (10). A solution of potassium tert-
butoxide (89 mg, 0.75 mmol) dissolved in dry tetrahydrofuran
(3 mL) was added to a stirred solution of 2,5-bis(dimethylphe-
nylsilyl)-1,4-bis(bromomethyl)benzene (4) (0.5 g, 0.94 mmol)
in dry tetrahydrofuran (3 mL), which was cooled with an
acetone/ice bath under nitrogen. A viscous sky-blue solution
was formed. The following synthetic procedures were per-
formed in analogy to the procedures given for precursor
polymer 9. The polymer yield was 55% (0.23 g). 1H NMR
(CDCl3, ppm): δ 7.77-7.59 (br m, vinyl H), 7.39-6.63 (br m,
Ar H and vinyl H), 5.25-5.01 (br m, CHBr), 3.47-2.66 (br m,
ArCH2), 1.02 to -0.15 (br m -Si(CH3)2).
2-Dim eth ylcycloh exylsilyl-p-xylen e (5). To a solution of
2-bromo-p-xylene (10 g, 54.0 mmol) in anhydrous THF was
slowly added clean magnesium turnings (2.27 g, 93.5 mmol)
after initiation by 5 mol % of 1,2-dibromoethane at 80 °C.
When the magnesium turnings were completely consumed,
chlorodimethylcyclohexylsilane (13.4 mL, 71.5 mmol) was
added. The mixture was heated to reflux for 6 h, and the
reaction was quenched with dilute aqueous HCl solution. The
THF layer was separated and washed with water several
times, and the solvent was removed on the rotary evaporator.
The residue was vacuum-distilled to give a colorless liquid
1
product. The yield was 5.2 g (38%). H NMR (CDCl3, ppm): δ
7.23 (m, 1H, Ar H), 7.06 (m, 2H, Ar H), 2.4 (s, 3H, CH3), 2.3
(s, 3H, CH3), 1.72-1.61 (m, 5H, CH and CH2 of cyclohexyl),
1.28-1.08 (m, 6H, CH2 of cyclohexyl), 0.28 (s, 6H, SiCH3). 13
C
NMR (CDCl3, ppm): δ 140.43, 136.60, 135.79, 133.72, 129.66,
128.89, 28.19, 27.71, 26.95, 25.72, 22.84, 21.10, -3.75. Anal.
Calcd for C16H26Si: C, 77.97; H, 10.63. Found: C, 77.23; H,
9.89.
2-Dim et h ylcycloh exylsilyl-1,4-b is(br om om et h yl)ben -
zen e (6). To a solution of 2-dimethylcyclohexylsilyl-p-xylene
(7.0 g, 28.4 mmol) in carbon tetrachloride (80 mL) was added
N-bromosuccinimide (10.7 g, 59.6 mmol) and benzoyl peroxide
as an initiator. The reaction mixture was heated to reflux at
80 °C for 4 h under a nitrogen atmosphere. The completion of
the reaction was indicated by the appearance of succinimide
on the surface of the reaction solution. The organic layer was
washed with water and brine and then dried over anhydrous
magnesium sulfate. After evaporation of the solvent, a yel-
lowish oil was obtained. Chromatography on a silica column
using hexane as eluent gave the brominated product as a
colorless oil. The product yield was 5.4 g (47%). 1H NMR
(CDCl3, ppm): δ 7.42-7.24 (m, 2H, Ar H), 7.26-7.22 (m, 1H,
Ar H), 4.58 (s, 2H, CH2Br), 4.47 (s, 2H, CH2Br), 1.71-1.59 (m,
5H, CH and CH2 of cyclohexyl), 1.27-0.90 (m, 6H, CH2 of
cyclohexyl), 0.34 (s, 6H, SiCH3). 13C NMR (CDCl3, ppm): δ
143.41, 138.36, 136.70, 135.84, 132.07, 130.22, 41.44, 40.41,
28.11, 27.50, 27.31, 14.11, -4.11. Anal. Calcd for C16H24Br2Si:
C, 47.54; H, 5.98. Found: C, 47.88; H, 5.78.
P oly[{2-(d im et h ylcycoh exylsilyl)-1,4-p h en ylen e }(1-
b r om oe t h yle n e )}-co-{2-(d im e t h ylcycloh e xylsilyl)-1,4-
p h en ylen e vin ylen e}] (11). A solution of potassium tert-
butoxide (131 mg, 1.10 mmol) dissolved in dry tetrahydrofuran
(3 mL) was added to a stirred solution of 2-dimethylcyclohexy-
lsilyl-1,4-bis(bromomethyl)benzene (6) (0.5 g, 1.23 mmol) in
dry tetrahydrofuran (3 mL) cooled with an acetone/ice bath
under nitrogen. A viscous sky-blue solution was formed. The
following synthetic procedures were performed in analogy to
the procedures given for precursor polymer 9. The polymer
1
yield was 45% (0.18 g). H NMR (CDCl3, ppm): δ 7.79-6.65
(br m, vinyl H and Ar H), 5.75-4.95 (br m, CHBr), 3.71-2.89
(br m, ArCH2), 2.35-0.80 (br m, CH and CH2 of cyclohexyl),
0.75 to -0.17 (br m, -Si(CH3)2).
2-Dim eth ylp h en ylsilyl-p-xylen e (7). To a solution of
2-bromo-p-xylene (10.0 g, 54.0 mmol) in anhydrous THF was
slowly added clean magnesium turnings (2.23 g, 92.0 mmol)
after initiation by 5 mol % of 1,2-dibromoethane at 80 °C.
When the magnesium turnings were completely consumed,
chlorodimethylphenylsilane (15.4 mL, 92.0 mmol) was added.
The following synthetic procedures were performed in analogy
to the procedures given for compound 5. The yield was 6.2 g
P o ly [{2-(d im e t h y lp h e n y ls ily l)-1,4-p h e n y le n e }(1-
br om oeth ylen e)}-co-{2-d im eth ylp h en ylsilyl)-1,4-p h en yl-
en e vin ylen e}] (12). A solution of potassium tert-butoxide
(134 mg, 1.13 mmol) dissolved in dry tetrahydrofuran (3 mL)
was added to a stirred solution of 2-dimethylphenylsilyl-1,4-
bis(bromomethyl)benzene (8) (0.5 g, 1.26 mmol) in dry tet-
rahydrofuran (3 mL) cooled with an acetone/ice bath under
nitrogen. A viscous sky-blue solution was formed. The follow-
ing synthetic procedures were performed in analogy to the
procedures given for precursor polymer 9. The polymer yield
1
(48%). H NMR (CDCl3, ppm): δ 7.52 (m, 2H, Ar H), 7.36 (m,
4H, Ar H), 7.14 (d, 1H, Ar H), 7.08 (d, 1H, Ar H), 2.35 (s, 3H,
CH3), 2.24 (s, 3H, CH3), 0.60 (s, 6H, SiCH3). 13C NMR
(CDCl3): δ 140.90, 139.05, 135.99, 135.80, 133.94, 133.53,
130.28, 129.83, 128.83, 127.76, 22.60, 21.08, -1.36. Anal. Calcd
for C16H20Si: C, 79.93; H, 8.38. Found: C, 78.80; H, 8.09.
2-Dim eth ylp h en ylsilyl-1,4-bis(br om om eth yl)ben zen e
(8). To a solution of 2-dimethylphenylsilyl-p-xylene (7.0 g, 29.0
mmol) in carbon tetrachloride (100 mL) was added N-bromo-
succinimide (11.5 g, 64.0 mmol) and benzoyl peroxide as an
initiator. The following synthetic procedures were performed
in analogy to the procedures given for compound 6. The
product yield was 6.0 g (52%). 1H NMR (CDCl3, ppm): δ 7.51-
7.42 (m, 5H, Ar H), 7.38-7.24 (m, 3H, Ar H), 4.46 (s, 2H, CH2-
Br), 4.36 (s, 2H, CH2Br), 0.64 (s, 6H, SiCH3). 13C NMR (CDCl3,
ppm): δ 143.82, 137.76, 136.95, 133.93, 131.68, 130.87, 129.79,
129.45, 128.85, 128.16, 33.62, 33.14, -1.05. Anal. Calcd for
1
was 50% (0.20 g). H NMR (CDCl3, ppm): δ 7.67-6.69 (br m,
ArH and vinyl H), 6.12-5.75, 5.65-5.32, 5.25-4.85, 4.84-4.61
(br m, CHBr), 3.67-2.86 (br m, ArCH2), 0.89-0.25 (br m -Si-
(CH3)2).
P oly[2-dim eth ylcycloh exylsilyl-1,4-ph en ylen e vin ylen e]
(DMCyS-P P V). The monomer 6 (0.5 g 1.23 mmol) was
dissolved in 30 mL of THF with 29 mg (0.123 mmol) of tert-
butylbenzyl bromide as an end-capper, and then the solution
was cooled to 0 °C. The initiator, 3.69 mL (3.69 mmol) of
potassium tert-butoxide (1.0 M in THF), was added to the
solution. The reaction mixture became progressively green and
viscous during the addition. The highly viscous solution was
stirred at 0 °C for 3 h. The polymerization reaction was poured
into 300 mL of methanol with vigorous stirring. The resulting
yellow precipitate was collected, dissolved in chloroform, and
reprecipitated in methanol/acetone (1/1) cosolvent twice more.
The polymer was extracted with a Soxhlet extractor from
methanol for 24 h and dried under vacuum. The polymer yield
C
16H18Br2Si: C, 48.26; H, 4.56. Found: C, 47.88; H, 4.78.
P olym er ization . P oly[{2,5-bis(dim eth ylcycoh exylsilyl)-
1,4-p h en ylen e}(1-br om oeth ylen e)}-co-{2,5-bis(d im eth yl-
cycloh exylsilyl)-1,4-p h en ylen e vin ylen e}] (9). A solution
of potassium tert-butoxide (87 mg, 0.73 mmol) dissolved in dry
tetrahydrofuran (3 mL) was added to a stirred solution of 2,5-
was 57% (0.17 g) of a yellow fiber type. FT-IR (KBr) νmax/cm-1
3059, 2924, 2845, 1489, 1449, 1252, 1142, 1102, 1070, 1000,
:

3498 Ahn et al.
Macromolecules, Vol. 35, No. 9, 2002
Sch em e 1. Syn th etic Rou te for BDMCyS-P P V a n d BDMP S-P P V
1
960, 889, 842, 802, 763, 645. H NMR (CDCl3, ppm): δ 7.88-
6.80 (br m, 5H, Ar H and vinylic H), 1.98-1.33 (br s, 5H, CH
and CH2 of cyclohexyl), 1.31-0.78 (br s, 6H, CH2 of cyclohexyl),
0.71-0.40 (br s, 6H, -Si(CH3)2). Anal. Calcd for (C16H22Si)n:
C, 79.25; H, 9.16. Found: C, 79.01; H, 9.78.
method yielded polymer particles that were insoluble
in common organic solvents. This result is in contrast
to the case of bis(alkylsilyl)-substituted PPVs, which
have been shown to have soluble final conjugated
forms.19 The use of inflexible side chains causes bis-
(silyl)-substituted BDMCyS-PPV and BDMPS-PPV to
have symmetric structures and therefore to show lower
solubility. To circumvent the problem of the polymer
solubility, we carried out the 1,6-polymerization and the
1,2-elimination separately. Using this method, a precur-
sor polymer is first obtained which is then converted to
the corresponding bis(silyl)-containing polymers. This
precursor approach has been used extensively for the
fabrication of insoluble PPV thin films from a soluble
polymer precursor material.3,21 The bis(bromomethyl)
monomers 2 and 4 were polymerized under the standard
conditions to give the precursor polymers 9 and 10,
respectively. In each case, a solution of 0.9 equiv of
potassium tert-butoxide in tetrahydrofuran was added
to a solution of 2 or 4 in tetrahydrofuran cooled in an
acetone/ice bath. The reaction mixtures were then
allowed to warm to room temperature and stirred for 2
h. The precursor polymers 9 and 10 were purified by
reprecipitation from chloroform using methanol. The
monosilyl-containing monomers 6 and 8 were also
synthesized via the BPR and Gilch polymerization
routes. Monocyclohexylsilyl- or monophenylsilyl-substi-
tuted precursor polymers were obtained using polym-
erization conditions similar to those described above for
the synthesis of the bis(silyl)-containing precursor poly-
mers. In the case of the polymers containing monosilyl,
most attention was focused on the direct polymerization
into the final conjugated forms DMCyS-PPV and
DMPS-PPV. The monomers containing monosilyl (6
and 8) were easily polymerized via the Gilch dehydro-
halogenation route under ambient conditions using
excess potassium tert-butoxide as the base in dry THF.
Chloroform solutions of the polymers were precipitated
P oly[2-d im et h ylp h en ylsilyl-1,4-p h en ylen e vin ylen e]
(DMP S-P P V). The monomer 8 (0.5 g 1.26 mmol) was
dissolved in 30 mL of THF with 30 mg (0.126 mmol) of tert-
butylbenzyl bromide as an end-capper, and then the solution
was cooled to 0 °C. The initiator, 3.78 mL (3.78 mmol) of
potassium tert-butoxide (1.0 M in THF), was added to the
solution. The reaction mixture became progressively green and
viscous during the addition. The following synthetic procedures
were performed in analogy to the procedures given for
DMCyS-PPV. The polymer yield was 60% (0.18 g) of a yellow
fiber type. FT-IR (KBr) νmax/cm-1: 3074, 3043, 3011, 2956,
1481, 1426, 1252, 1110, 1063, 961, 905, 810, 771, 731, 692,
645, 582, 463. 1H NMR (CDCl3, ppm): δ 7.98-6.82 (br m, 10H,
Ar H and vinylic H), 0.98-0.10 (br s, 6H, -Si(CH3)2). Anal.
Calcd for (C16H16Si)n: C, 81.28; H, 6.84. Found: C, 80.88; H,
6.78.
P oly[2-m eth oxy-5-(2-eth ylh exyloxy)-1,4-p h en ylen e vi-
n ylen e] (ME H-P P V). The well-known MEH-PPV was
synthesized by the method of dehydrohalogenation route (the
Gilch procedure).26,27 The final polymer was soluble in common
organic solvents.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . The synthetic
routes used to prepare the mono (or bis)cyclohexylsilyl-
or phenylsilyl-containing bis(bromomethyl)-type mono-
mers, the resulting precursor polymers (halogen pre-
cursor route),21-23 and the soluble polymers (Gilch
route)14,24,25 are presented in Schemes 1 and 2. We
incorporated a dimethylcyclohexylsilyl or dimethylphe-
nylsilyl group as a polymer side chain using the Grig-
nard reaction. Each bis(bromomethyl) monomer was
polymerized using two synthetic methods: the bromine
precursor route (BPR) and Gilch polymerization. First,
bis(silicon)-substituted 2 and 4 monomers were polym-
erized using excess base (Gilch route). However, this

Macromolecules, Vol. 35, No. 9, 2002
Poly(p-phenylenevinylene)s 3499
Sch em e 2. Syn th etic Rou te for DMCyS-P P V a n d DMP S-P P V
Ta ble 1. P olym er iza tion Resu lts a n d Th er m a l P r op er ties
of P olym er s
twice or more in methanol/acetone (1/1) cosolvent,
yielding yellow solid products. The collected polymers
were further purified by Soxhlet extraction through
methanol. The two polymers synthesized by the Gilch
route, DMCyS-PPV and DMPS-PPV, were completely
soluble in common organic solvents such as chloroform,
chlorobenzene, THF, 1,2-dichloroethane, and toluene.
Huang et al. reported that silyl-substituted PPVs have
few structural defects (bis(benzyl) and diphenylaceth-
ylene moieties, etc.) in their polymer chains and that
the silyl groups contribute to the satisfactory solubility
of these polymers because of the larger atomic size of
Si compared to C and O as well as the longer bond
length of Si-C compared to C-C and C-O.19
b
yield
(%)
Tg
TID
polymer
Mn
Mw
PDIa (°C) (°C)
a
a
9
49
55
45
50
57
60
9.8 × 104 2.5 × 105 2.56
1.1 × 105 2.8 × 105 2.59
7.3 × 104 5.9 × 105 8.10
7.2 × 104 6.3 × 105 8.70
168c
163c
179c
164c
10
11
12
DMCyS-PPV
DMPS-PPV
3.5 × 105 1.4 × 106 4.10 125 428
2.9 × 105 1.1 × 106 3.88 127 435
a
Mn, Mw, and PDI of the polymers were determined by gel
b
permeation chromatography using polystyrene standards. Tem-
perature at which initial loss of mass (5%) was observed. c Mainly
due to the loss of hydrogen bromide.
cyclohexylsilyl exhibit peaks in the region of 2-1 ppm,
corresponding to the CH and CH2 of the cyclohexyl. FT-
IR spectra of the precursor polymers 11 and 12,
DMCyS-PPV and DMPS-PPV, are measured. The
spectra of these polymers are very similar except for the
additional peak at about 960 cm-1 in the spectra of
DMCyS-PPV and DMPS-PPV, which is attributed to
the out-of-plane deformation of the trans C-H of the
alkene moiety.7,23 This result suggests that the vinylene
group formed through the Gilch route is in the trans
configuration. In addition, all polymers showed the
The molecular weights of the polymers were mea-
sured by gel permeation chromatography using THF as
eluent and polystyrene as standard. The number-
average molecular weights, Mn, of the monosilyl-
substituted DMCyS-PPV and DMPS-PPV, obtained by
the Gilch route, were 351 000 (Mw/Mn ) 4.1) and
292 000 (Mw/Mn ) 3.9), respectively, which are very high
compared to those of the simple alkylsilyl-substituted
PPVs prepared in the past. The four precursor polymers
(9, 10, 11, and 12) synthesized by the BPR method
showed relatively low molecular weights compared with
the polymers prepared by the Gilch route. The number-
average molecular weight (Mn), weight-average molec-
ular weight (Mw), and polydispersity index (PDI) results
absorption band characteristic of Si-CH3 at 1250 cm-1
,
with a very sharp peak of strong intensity.19
Thermogravimetric analysis of the precursor polymers
9-12 showed a significant weight loss at approximately
160 °C due to the loss of hydrogen bromide.23 DMCyS-
PPV and DMPS-PPV from the Gilch route showed less
than 5% weight loss on heating to about 400 °C. The
5% weight loss temperatures for DMCyS-PPV and
DMPS-PPV were 428 and 435 °C, respectively. These
polymers therefore show much higher thermal stability
than those of the simple alkylsilyl-PPVs (the 5% weight
loss temperatures for butylsilyl-PPV and decylsilyl-PPV
are reported as 392 and 371 °C, respectively)19 (Figure
1). In addition to higher thermal stability, the cyclo-
hexylsilyl- or phenylsilyl-substituted PPVs (DMCyS-
1
are summarized in Table 1. The H NMR spectra of the
synthesized polymers showed signal broadening, but the
chemical shifts are consistent with the proposed polymer
structures. The assignments for these spectra are
documented in the Experimental Section. The signal at
7.24 ppm originates from the chloroform-d used as a
solvent, and the peaks in the ranges 6-5 and 4-2.5 ppm
observed for all precursor polymers are assigned to the
protons in CHBr and ArCH2, respectively.22 However,
the conjugated DMCyS-PPV and DMPS-PPV poly-
mers synthesized by the Gilch method show no peaks
between 2.5 and 6 ppm. The polymers containing

3500 Ahn et al.
Macromolecules, Vol. 35, No. 9, 2002
F igu r e 1. TGA thermograms of 9-12, DMCyS-PPV, and
DMPS-PPV.
PPV and DMPS-PPV) also have improved mechanical
properties, as measured by differential scanning calo-
rimetry (DSC) measurements under a nitrogen atmo-
sphere. The DSC curves show glass transition temper-
atures (Tg) of around 125 °C for DMCyS-PPV and 127
°C for DMPS-PPV. These values are high compared to
those of the well-known alkoxy-substituted MEH-PPV
(65 °C)28 and are even comparable to those of polyfluo-
rene derivatives (75-125 °C).29,30 This indicates that the
incorporation of cyclohexylsilyl or phenylsilyl groups
into the polymer side chain considerably suppresses the
flexibility of the polymer and thus increases the Tg of
the polymer. Recently, it was reported that the lifetime
of EL devices is directly related to the Tg value and
thermal stability of the polymers used.20 Consequently,
the good thermal stability and high Tg values of
DMCyS-PPV and DMPS-PPV make them more suit-
able as EL polymers in terms of mechanical properties
than simple alkylsilyl- or alkoxy-substituted PPVs. The
polymerization results and thermal properties of all the
polymers are summarized in Table 1.
F igu r e 2. UV-vis absorption spectra of (a) BDMCyS-PPV
and (b) BDMPS-PPV thin films thermally converted at the
indicated temperatures for 6 h.
Op tica l a n d P h otolu m in escen ce P r op er ties. BD-
MCyS-P P V a n d BDMP S-P P V. The solid-state UV-
vis absorption spectra of (a) BDMCyS-PPV and (b)
BDMPS-PPV converted at various temperatures for 6
h in a vacuum are shown in Figure 2. Precursor
polymers that have not been thermally treated do not
show any absorption because they lack conjugation in
the polymer main chain. On conversion at 100 °C, the
absorption changes only a small amount, which suggests
that only a small fraction of the precursor polymers was
converted to the conjugated form at this temperature.
However, polymer films converted at 150 °C in a
vacuum show obvious changes in their absorption. This
indicates that considerable H-Br elimination occurs at
150 °C, which is in accordance with the TGA analysis
of the initial decomposition temperature of H-Br. Thin
films containing BDMCyS-PPV and BDMPS-PPV
with higher levels of conjugation were obtained from
their respective precursor polymers by conversion at 200
and 250 °C than by treatment at lower temperatures.
These polymers showed similar absorption profiles in
terms of both the absorption maximum (413 nm for
BDMCyS-PPV and 370 nm for BDMPS-PPV) and edge
(493 nm for BDMCyS-PPV and 491 nm for BDMPS-
PPV). The films treated at 300 °C for 6 h exhibited a
diminished optical density and a slight blue shift in the
absorption spectra (not shown here). These changes are
due to the degradation of the conjugated polymer main
chain. From these results we conclude that the onset of
F igu r e 3. Photoluminescence (PL) spectra of (a) BDMCyS-
PPV and (b) BDMPS-PPV thin films thermally converted at
the indicated temperatures for 6 h.
conversion occurs at about 150 °C and fully conjugated
polymers form at 250 °C. Figure 3 shows the solid-state
photoluminescence (PL) spectra of (a) BDMCyS-PPV

Macromolecules, Vol. 35, No. 9, 2002
Poly(p-phenylenevinylene)s 3501
F igu r e 4. UV-vis absorption spectra of DMCyS-PPV and
DMPS-PPV in solution and in thin films coated on a quartz
plate.
F igu r e 5. Photoluminescence (PL) spectra of DMCyS-PPV
and DMPS-PPV in solution and in thin films coated on a
quartz plate.
and (b) BDMPS-PPV converted for 6 h in a vacuum at
temperatures ranging from 100 to 250 °C. Only a small
amount of emission is observed from the thin films
maintained at 100 °C, indicating that the precursors are
not converted at this temperature (as discussed above
for the UV-vis absorption spectra). Above the onset
conversion temperature (150 °C), however, greenish-
yellow emissions appear. BDMCyS-PPVs converted at
200 and 250 °C show emission maxima at 519 nm with
a shoulder at 555 nm, which are slightly red-shifted (4
nm) compared with those of the film converted at 150
°C. As with the absorption spectra, conversion above 300
°C leads to a blue shift in the emission and a significant
decrease in emission intensity. Phenylsilyl-containing
BDMPS-PPV converted at 250 °C shows a peak at 525
nm and a shoulder at 560 nm, corresponding to green-
ish-yellow emission.
DMCyS-P P V a n d DMP S-P P V. The final conju-
gated forms of the monosilyl-containing DMCyS-PPV
and DMPS-PPV obtained by the Gilch method were
completely soluble in common organic solvents. The
UV-vis absorption spectra of these polymers both in
dilute chloroform solutions and as thin films were
measured and are shown in Figure 4. The maximum
UV-vis absorptions of DMCyS-PPV and DMPS-PPV
in solution appear at approximately 414 and 418 nm,
respectively. The absorption maxima of the film samples
were bathochromically red-shifted compared to the
corresponding solution samples. The DMPS-PPV film
shows a slight red shift of the absorption peak to 422
nm, whereas that of DMCyS-PPV shifts to 420 nm.
This indicates that the π-electron delocalization of the
polymer containing cyclohexylsilyl is interrupted to a
greater extent than that in the polymer containing
phenylsilyl.
Figure 5 shows the PL spectra recorded using excita-
tion wavelengths corresponding to the absorption maxi-
mum of each polymer in solution and film. The PL
spectra of DMCyS-PPV and DMPS-PPV in solution
exhibit maximum emission peaks at 483 and 485 nm,
respectively. The spectra of the film samples resemble
those of the corresponding solutions but are shifted to
longer wavelength. The DMCyS-PPV film shows a
maximum emission peak at about 510 nm, with a
shoulder at 545 nm. The phenylsilyl-substituted DMPS-
PPV exhibits an almost the same emission profile
similar to that of DMCyS-PPV, but with slightly red-
shifted maximum emission and shoulder peaks at 513
and 547 nm, respectively. The reason for the red shift
in the emission of DMPS-PPV compared with DMCyS-
PPV is the same as that outlined above for the UV-vis
absorption maxima of the two polymers. For comparison
purposes, the PL spectra of the well-known PPV and
alkoxy-substituted MEH-PPV are included in Figure
5. It can be seen that all the emission spectra of the
silyl-substituted polymers are blue-shifted with respect
to the PPV and MEH-PPV spectra. This blue shift
results from the electronic properties of the silicon
atoms, which have a slight electron-donating effect, and
the greater steric hindrance of the bulky cyclohexylsilyl
and phenylsilyl groups compared to other alkyl or
alkoxy groups. These factors lead to a breaking of the
coplanarity within the polymer main chain. Conse-
quently, the cyclohexylsilyl or phenylsilyl substituents
enlarge the semiconductor band through their blue-
shifted emission compared with other PPV derivatives.
Bis(silyl)-substituted BDMCyS-PPV and BDMPS-PPV
show emissions that are slightly red-shifted with respect
to DMCyS-PPV and DMPS-PPV; these results cor-
relate with the symmetric structures of the bis(silyl)-
substituted polymers ascertained from their low solu-
bilities. Interestingly, the DMCyS-PPV and DMPS-
PPV polymers are highly fluorescent in both solution
and film. The quantum efficiency of each polymer in
chloroform solution was determined twice more using
a dilute quinine sulfate as a standard (ca. 1 × 10-5
M
solution in 0.10 M H2SO4).31,32 The measured values
were then averaged. The PL efficiency of quinine sulfate
in 0.10 M H2SO4 solution was taken to be 0.546 at 365
nm excitation, and the refractive indices of the dilute
polymer solutions in chloroform and 0.10 M H2SO4 were
taken to be 1.446 and 1.333, respectively.19 Using these
values, the quantum yields of DMCyS-PPV and DMPS-
PPV were determined as Φsol ) 0.88 and 0.86, which
are much higher than those of other PPVs containing
alkoxy or alkyl side chains and are even comparable to
those of polyfluorenes. In considering the utility of these
compounds for applications in thin film displays, the
film PL quantum efficiency is more meaningful than the
solution value. The film PL efficiencies were measured
using an optically dense configuration and diphenyl-
anthracene (dispersed in a PMMA film at a concentra-
tion of less than 10-3 M, assuming a PL efficiency of
0.83) as the standard.29,32-34 DMCyS-PPV and DMPS-
PPV showed extremely high film PL quantum efficien-
cies of Φfilm ) 0.82 and 0.83, respectively. These values
are among the highest reported for the film PL efficiency
of PPV derivatives. In our previous work in this area,

3502 Ahn et al.
Macromolecules, Vol. 35, No. 9, 2002
Ta ble 2. UV-vis Sp ectr a , P h otolu m in escen ce (P L)
Sp ectr a , a n d Qu a n tu m Yield in Solu tion a n d F ilm of
P olym er s
λmax
λmax
PL efficiency
(UV, nm)a
(PL, nm)a,b
(Φ)a
polymer
soln film
soln
film
soln
film
BDMCyS-PPV
BDMPS-PPV
DMCyS-PPV
DMPS-PPV
413
370
519 (555)
525 (560)
0.53
0.56
0.82
0.83
414 420 483 (515) 510 (545) 0.88
418 422 485 (517) 513 (547) 0.86
a
These values listed were measured in chloroform solution and
b
thin film state onto the quartz plate. These values listed in
parentheses represent shoulder peak.
we reported a solid PL quantum efficiency of Φfilm
)
0.81 from the copolymer composed of alkyl-substituted
fluorene and trimethylsilyl-substituted phenyl moi-
eties.7 The polymers containing bis(silyl) obtained by
thermal elimination at 250 °C, BDCyS-PPV (Φfilm
)
0.53) and BDMPS-PPV (Φfilm ) 0.56), exhibited lower
film PL efficiencies than the mono-Si-substituted
DMCyS-PPV and DMPS-PPV. It seems that the better
packing of the polymers with symmetric cyclohexylsilyl
or phenylsilyl side chains reduces the quantum ef-
ficiency. In other words, the chain packing and conjuga-
tion are more effectively interrupted in the monosilyl-
substituted systems (DMCyS-PPV and DMPS-
PPV) than the bis(silyl)-substituted symmetric ones
(BDMCyS-PPV and BDMPS-PPV), which results in
the formation of amorphous polymer thin films for an
increased luminescence. The absorption and photo-
luminescence results for the cyclohexylsilyl- and phen-
ylsilyl-substituted PPVs in solution and film states are
summarized in Table 2.
Electr och em ica l P r op er ties. Cyclic voltammetry
(CV) is an easy and effective approach for measuring
redox reversibility, reproducibility, and stability of
polymer films on electrodes. In addition, it can be
utilized to simultaneously evaluate both the HOMO and
LUMO energy levels and the band gap of a polymer.
CV was performed on a solution of 0.1 M tetrabutylam-
monium tetrafluoroborate (Bu4NBF4) in acetonitrile
with a scan rate of 50 mV/s at room temperature under
the protection of argon. A platinum electrode (∼0.5 cm2)
was coated with a thin polymer film (DMCyS-PPV or
DMPS-PPV) and used as the working electrode. Plati-
num wire was used as the counter electrode, and a Ag/
0.10 M AgNO3 electrode was used as the reference
electrode. The cyclic voltammograms from DMCyS-
PPV, DMPS-PPV, and MEH-PPV are displayed in
Figure 6. During the cathodic scan, DMPS-PPV exhib-
its reversible n-doping processes. Epc/pa were found at
-1.89/-1.68 V vs SCE, and the onset of the n-doping
process occurs at about -1.75 V. DMCyS-PPV showed
similar peaks and onset potentials in the n-doping
process, but with a slightly lower onset potential (-1.81
V) than DMPS-PPV for reduction. This indicates that
DMCyS-PPV has a lower electron affinity than DMPS-
PPV for accepting electrons from the cathode in an EL
device. In the anodic scan, DMCyS-PPV and DMPS-
PPV show similar irreversible p-doping processes.
DMCyS-PPV and DMPS-PPV exhibit peak potentials
at 1.47 and 1.27 V, respectively. The onset potential of
oxidation for DMCyS-PPV (1.25 V) is somewhat higher
than the value for DMPS-PPV (1.16 V). For compari-
son, we also measured the electrochemical properties
of the well-known polymer MEH-PPV. The cyclic
voltammogram of MEH-PPV measured in the present
F igu r e 6. Cyclic voltammograms of MEH-PPV, DMCyS-
PPV, and DMPS-PPV films coated on platinum plate elec-
trode in acetonitrile containing 0.1 M Bu4NBF4. Counter
electrode: platinum wire. Reference electrode: Ag/AgNO3 (0.1
M in acetonitrile). Scan rate: 50 mV/s.
study coincides well with previous results.35 Compared
to MEH-PPV, DMCyS-PPV and DMPS-PPV show
much lower peak currents in the cathodic reaction. This
result indicates that polymers with cyclohexylsilyl or
phenylsilyl in PPV backbone have a greater potential
for accepting or transporting electrons than alkoxy-
substituted PPV derivatives (e.g., MEH-PPV). The
onset potentials (Eox and Ered) of p- and n-doping are
used to determine the HOMO and LUMO energy levels
of the conjugated polymers by means of the empirical
relationship proposed by Leeuw et al.36 This relationship
connects the solid-state IP (HOMO) and EA (LUMO)
to the Eox and Ered using the relations IP (EHOMO) )
-(Eox + 4.39) (eV) and EA (ELUMO) ) -(Ered + 4.39) (eV),
where EHOMO is the HOMO energy level and ELUMO is
the LUMO energy level below the vacuum. These data
are useful for the selection of suitable electrode materi-
als and hole- or electron-transporting materials in the
fabrication of effectively balanced polymer EL devices.
All the data, including the IP (EHOMO) and EA (ELUMO
)
values, electrochemical band gaps, oxidation and reduc-
tion onset potentials, and n-doping and p-doping peak
potentials, are listed in Table 3. As shown in Table 3,
DMCyS-PPV has a more negative value of IP (HOMO)
than DMPS-PPV. The cyclohexylsilyl- and phenylsilyl-
substituted polymers have much more negative [IP
(HOMO)] values than other PPV derivatives. This
property means that it is difficult to inject holes from
indium tin oxide (ITO) into the polymers. Similar results
are reported elsewhere for other simple alkylsilyl-
substituted polymers.19 Alkyl- or alkoxy-substituted
PPV derivatives usually have hole major characteristics
in EL devices, imbalance of injected hole and electron,
with an air-stable metal cathode.37 However, DMCyS-
PPV and DMPS-PPV with retarded hole injection show

Macromolecules, Vol. 35, No. 9, 2002
Poly(p-phenylenevinylene)s 3503
Ta ble 3. Electr och em ica l P r op er ties a n d En er gy Levels of MEH-P P V, DMCyS-P P V, a n d DMP S-P P V
n-doping (V)a
Epa
p-doping (V)a
energy levels (eV)
EA
polymer
Eonset
Epc
Eonset
Epa
Epc
IP
band gap
DMCyS-PPV
DMPS-PPV
MEH-PPV
-1.81
-1.75
-1.57
-1.77
-1.68
-1.66
-2.01
-1.89
-1.70
1.25
1.16
0.55
1.47
1.27
0.82
-5.64
-5.55
-4.94
-2.58
-2.64
-2.82
3.06
2.91
2.12
0.77
a
Eonset, Epa, and Epc stand for onset potential, anodic peak potential, and cathodic peak potential, respectively.
F igu r e 7. Electroluminescence (EL) spectra of single-layer
LEDs of BDMCyS-PPV, BDMPS-PPV, DMCyS-PPV, and
DMPS-PPV with a configuration of ITO/polymer/Al.
more balanced hole and electron injection than PPV
derivatives, allowing the construction of an improved
EL device with an air-stable cathode.
E lect r olu m in escen t Device Ch a r a ct er ist ics.
BDMCyS-P P V a n d BDMP S-P P V. Single-layer EL
devices were fabricated from polymer films sandwiched
between ITO anodes and aluminum cathodes. Precursor
polymer solutions were spin-coated onto ITO and then
thermally converted to give the conjugated polymers.
Aluminum electrodes were then evaporated onto the top
of the polymer film to complete the devices. Electro-
luminescence (EL) was observed from both polymer
devices; the spectra are shown in Figure 7. The EL
emission maxima from the ITO/BDMCyS-PPV/Al and
ITO/BDMPS-PPV/Al devices are at 518 and 525 nm,
respectively. As a consequence, emission from these
devices appears greenish-yellow to the eye. In addition,
the devices show similar vibronic emissions at about 542
nm (BDMCyS-PPV) and 556 nm (BDMPS-PPV), as
in the PL spectra. The luminance of the ITO/BDMCyS-
PPV/Al device was rather low, reaching only 6 cd/m2 at
15 V, 90 mA/cm2. BDMPS-PPV showed a higher
maximum luminance (11 cd/m2 at 14 V, 165 mA/cm2)
than BDMCyS-PPV in the device of structure ITO/
polymer/Al. The threshold voltages of BDMCyS-PPV
and BDMPS-PPV polymers were about 8 and 9 V (film
thickness approximately 100 nm), respectively. Conse-
quently, EL devices consisting of BDMCyS-PPV or
BDMPS-PPV films fabricated by thermal conversion
exhibited rather low performance compared with other
silyl-substituted PPVs. We believe that these results
have the same origin as the PL efficiency results
outlined above; namely, the polymer chain packing of
symmetric structures decreases the efficiency of PL and
EL. Nevertheless, the thermally converted BDMCyS-
PPV and BDMPS-PPV films do not dissolve or swell
in common organic solvents. This solubility behavior can
be applied advantageously in the design of multilayer
polymer LED devices.3
F igu r e 8. (a) Current-voltage or (b) luminance-voltage
characteristics of single- and double-layer LEDs of MEH-PPV,
DMCyS-PPV, and DMPS-PPV with a configuration of ITO/
polymer/Al and ITO/PVK/polymer/Al.
DMCyS-P P V a n d DMP S-P P V. The EL spectra of
DMCyS-PPV and DMPS-PPV from the device config-
uration ITO/polymer/Al are shown in Figure 7.
DMCyS-PPV and DMPS-PPV exhibit EL emissive
bands at 510 and 515 nm, respectively. These emissions
correspond to green light, which represents a slight blue
shift with respect to the bis(silyl) systems BDMCyS-
PPV and BDMPS-PPV. Figure 8 shows (a) current-
voltage and (b) luminance density-voltage character-
istics of the ITO/polymer/Al and ITO/PVK/polymer/Al
devices. The characteristics of the well-known dialkoxy-
substituted MEH-PPV were also determined to com-
pare device performance. All polymer films were about
80 nm in thickness. DMCyS-PPV and DMPS-PPV
showed good device performance in single-layer EL
devices with air-stable Al electrodes compared with
BDMCyS-PPV, BDMPS-PPV, and MEH-PPV.
DMCyS-PPV and DMPS-PPV showed maximum lu-
minance values of about 54 and 91 cd/m2 at 11 and 13
V, respectively. The external quantum efficiency and
power efficiency of DMPS-PPV reached 0.025% and
0.047 lm/W, which are much higher than those of
MEH-PPV (measured at about 2 × 10-3% external
efficiency).38 Moreover, DMCyS-PPV and DMPS-PPV

3504 Ahn et al.
Macromolecules, Vol. 35, No. 9, 2002
F igu r e 10. Current-luminance-voltage (I-V-L) character-
istics for an ITO/PEDOT:PSS (30 nm)/PVK (60 nm)/DMPS-
PPV (80 nm)/LiF (2 nm)/Al.
polymer LUMO.39-41 Figure 10 shows that maximum
brightness of this multilayer device is 2450 cd/m2 at 370
mA/cm2 (12 V), with a turn-on voltage of 5 V. This value
is much higher than the currently reported value (1384
cd/m2 at 12 V) of alkoxy- and bis(silyl)-substituted
BDMPS-co-DB-PPV in a similar EL device configura-
tion (ITO/PEDOT/polymer/Ca), which differs only in the
lack of the PVK layer used in our device.42
Con clu sion s
We have reported the synthesis of novel cyclohexyl-
silyl- or phenylsilyl-substituted PPVs using the bromine
precursor route (BPR) to prepare bis(silyl)-substituted
polymers and Gilch polymerization to prepare mono-
silyl-substituted polymers. We successfully fabricated
insoluble BDMCyS-PPV and BDMPS-PPV thin films
from precursor polymers by thermal conversion. These
insoluble films are suitable for applications in multi-
layer polymer EL devices. The monosilyl systems con-
taining cyclohexyl or phenyl were shown to be superior
to the bis(silyl) systems in solubility (processability),
film PL efficiency, and EL efficiency. The monosilyl-
substituted polymers DMCyS-PPV and DMPS-PPV
showed higher Tg, molecular weight, and thermal
stability than the alkoxy or simple alkylsilyl-PPVs. In
addition, DMCyS-PPV and DMPS-PPV exhibited very
high solution and film PL efficiencies combined with
good processability, amorphousness, and good film-
forming ability. Overall, cyclohexylsilyl- or phenylsilyl
substituents have much better properties than other
substituents such as alkoxy or alkyl groups. The single
and double-layer EL devices of DMCyS-PPV and
DMPS-PPV using air-stable aluminum cathodes emit-
ted green light with good brightness and efficiency.
Addition of a thin PEDOT:PSS layer and a dielectric
LiF layer led to an improved maximum luminance of
2450 cd/m2 at 370 mA/cm2 (12 V) and a turn-on voltage
of 5 V. The characteristics of DMCyS-PPV and DMPS-
PPV such as good thermal stability and luminance
properties make them potential candidates for applica-
tion in polymer LEDs.
F igu r e 9. (a) External quantum efficiency and (b) power
efficiency of single- or double-layer light-emitting diodes of
DMCyS-PPV and DMPS-PPV.
showed lower turn-on voltages (7 and 6 V, respectively)
than octylsilyl-substituted DMOS-PPV (>10 V)17 and
comparable turn-on voltages to those measured for
simple alkylsilyl-substituted PPVs in a single-layer EL
device with a Mg:Ag cathode.19 The bilayer devices with
an ITO/PVK/polymer/Al structure showed drastically
improved luminance values. Compared with alkyl- or
alkoxy-PPVs, DMCyS-PPV and DMPS-PPV have
higher energy barriers (from CV data) between ITO and
the HOMO of the polymers. To reduce the effect of this
barrier, we inserted a layer of PVK (5.4 eV) between
the ITO (4.8 eV) and polymer layers (DMCyS-PPV
(5.64 eV) and DMPS-PPV (5.55 eV)) to facilitate hole
transportation. Figure 8 shows that the turn-on voltages
of the double-layer devices are about 2-3 V higher than
those of the single-layer devices, but the DMCyS-PPV
and DMPS-PPV double-layer devices containing PVK
exhibit much higher values of the maximum brightness,
external quantum efficiency, and power efficiency. The
ITO/PVK/DMPS-PPV/Al device reached a maximum
brightness of about 220 cd/m2 at 14 V and showed
0.075% and 0.187 lm/W for the external quantum
efficiency and power efficiency, respectively. The EL
spectra of these devices are the same as those of the
single-layer devices, presented in Figure 7. The external
quantum efficiency and power efficiency curves of the
single- and double-layer EL devices of DMCyS-PPV
and DMPS-PPV are shown in Figure 9. To improve the
performance of the EL device, a PEDOT:PSS layer of
thickness 30 nm was inserted between the ITO and the
PVK layer, and an LiF layer of thickness 2 nm was
inserted between the DMPS-PPV and Al. PEDOT:PSS
is widely used as a hole injection layer, and the thin
LiF layer acts as an insulating layer that decreases the
barrier to electron injection from the cathode (Al) to the
Ack n ow led gm en t. This work was supported by the
Center for Advanced Functional Polymers through the
Korea Science and Engineering Foundation (KOSEF)
and ILJ IN. The authors are grateful to Dr. J eong-Ik Lee
at ETRI for helpful experiments and discussions on the
EL device fabrication and measuring the I-V-L char-
acteristics. This paper is dedicated to Professor J ung-Il
J in on the occasion of his 60th birthday.

Macromolecules, Vol. 35, No. 9, 2002
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