Synthesis and luminescence properties of poly[2-(9,9-dihexylfluorene-2-yl)-1,4-phenylenevinylene] and its copolymers containing 2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene units

Article abstract of DOI:10.1021/ma011636t

A PPV derivative bearing the dihexylfluorenyl pendants, poly[2-(9,9-dihexylfluorene-2-yl)1,4-phenylenevinylene] (DHF-PPV) and its copolymers containing the dialkoxy-substituted phenylenevinylene units, i.e., poly[(2-(9,9-dihexylfluorene-2-yl)-1,4-phenylen

Full text of DOI:10.1021/ma011636t

2876
Macromolecules 2002, 35, 2876-2881
Articles
Synthesis and Luminescence Properties of
Poly[2-(9,9-dihexylfluorene-2-yl)-1,4-phenylenevinylene] and Its
Copolymers Containing
2-(2-Ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene Units
Byu n g-Hee Soh n ,†, Kyu n gk on Kim ,† Don Soo Ch oi,‡ You n g Kw a n Kim ,‡
Sa e Ch a e J eou n g,§ a n d J u n g-Il J in *,†
Division of Chemistry and Molecular Engineering and Center for Electro- and Photo-Responsive
Molecules, Korea University, Seoul 136-701, Korea; Department of Chemical Engineering,
Hong Ik University, Seoul 121-791, Korea; and Spectroscopy Laboratory, Korea Research Institute of
Standards and Science, Taejon 305-600, Korea
Received September 17, 2001; Revised Manuscript Received J anuary 9, 2002
ABSTRACT: A PPV derivative bearing the dihexylfluorenyl pendants, poly[2-(9,9-dihexylfluorene-2-yl)-
1,4-phenylenevinylene] (DHF-PPV) and its copolymers containing the dialkoxy-substituted phenylene-
vinylene units, i.e., poly[(2-(9,9-dihexylfluorene-2-yl)-1,4-phenylenevinylene)-co-(2-(2-ethylhexyloxy)-5-
methoxy-1,4-phenylenevinylene)s], were prepared, and luminescence properties of the light-emitting diodes
(LEDs) fabricated with them were studied. The copolymers 92.5 DHF/7.5 MEH-PPV and 50.5 DHF/49.5
MEH-PPV contained 7.5 and 49.5 mol % of the dialkoxy comonomer units, respectively. The structure
of LED devices was ITO/PEDOT (25 nm)/polymer (80 nm)/Ca (50 nm)/Al (50 nm). The wavelengths of
maximum emitted light of the devices were 519 nm (green), 560 nm (yellow), and 585 nm (orange-red)
for DHF-PPV and the two copolymers. Turn-on electric fields decreased in the order of DHF-PPV (0.54
MV/cm) > 92.5 DHF/7.5 MEH-PPV (0.43 MV/cm) > 50.5 DHF/49.5 MEH-PPV (0.29 MV/cm). Luminance
efficiencies of the three devices were 2.3, 1.0, and 0.9 cd/A for 92.5 DHF/7.5 MEH-PPV, 50.5 DHF/49.5
MEH-PPV, and DHF-PPV, respectively. The maximum luminance for the device of DHF-PPV was 1.6
× 104 cd/m2, and the value increased to 2.7 × 104 cd/m2 for 97.5 DHF/7.5 MEH-PPV. The maximum
luminance of the other copolymer was about 1.9 × 104 cd/m2. The study of time-resolved PL of the present
polymers strongly suggests that a correlation exists between their PL decay behavior and EL efficiency.
The mobilities of the charge carriers, i.e., hole and electron, are better balanced (υh/υe ∼ 10) in DHF-
PPV than in MEH-PPV (υh/υe ∼ 102).
In tr od u ction
and the last (MEH-PPV) polymers are homopolymers,
and the other two are random copolymers containing
the 2-ethylhexyloxy-5-methoxy-1,4-phenylenevinylene
(MEH-PV) comonomer units. It is well-known that
homo- and copolymers composed of only dialkyl-substi-
tuted fluorene repeating units without the vinylene
units in the backbone are capable of emitting blue light
in LED applications.4,5 And the comonomer unit of the
above copolymers is the building block of MEH-PPV
which has been thoroughly studied for EL properties
by Heeger’s group6 and others.7
Since the first report by the Cambridge group1 on the
electroluminescence (EL) of poly(p-phenylenevinylene)-
(PPV), EL properties of a number of PPV derivatives
have been studied by us2 and others.3 In our recent
papers, we discussed the EL properties of PPV deriva-
tives bearing either the carbazole2c or phenyloxadiazole2d
pendants. Dissymmetrical attachment of pendant sub-
stituents onto the PPV backbone alters not only the
wavelength of emitted light but also the external
efficiency of the light-emitting diodes (LEDs) based on
the polymers.
This article describes the synthesis of the new poly-
mers and EL properties of LEDs fabricated from them.
As a part of our continuing effort to establish the
structure-EL properties relationship of PPV deriva-
tives, we have synthesized polymers bearing dihexyl-
fluorenyl pendants (see Chart 1). The first (DHF-PPV)
Exp er im en ta l Section
The synthetic routes to the monomer and polymers are
shown in Schemes 1 and 2. The synthetic details of monomer
7 can be found in the Supporting Information.
Hom op olym er iza tion of Mon om er 7. Monomer 7 (0.50
g, 0.84 mmol) was dissolved in 50 mL of THF containing 2.0
mg of tert-butylbenzyl bromide.8 The solution was cooled to 0
°C, to which potassium tert-butoxide (2.5 mL of 1 M THF
solution) was added. The solution was stirred for 3 h at 0 °C.
The polymerization mixture was poured into 200 mL of
† Korea University.
‡ Hong Ik University.
§ Korea Research Institute of Standards and Science.
Permanent address: Polymer Lab, Samsung Advanced Insti-
tute of Technology, Taejon 305-380, Korea.
* To whom correspondence should be addressed. E-mail:
jijin@mail.korea.ac.kr.
10.1021/ma011636t CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/09/2002

Macromolecules, Vol. 35, No. 8, 2002
DHF-PPV and Its Copolymers 2877
Ch a r t 1
Sch em e 1. Syn th etic Rou te to th e 2-[1,4-Bis(bisbr om om eth yl)]p h en yl-9,9-d ih exylflu or en e Mon om er
Sch em e 2. Syn th esis of DHF -P P V, MEH-P P V, a n d Cop olym er s
methanol. The yellow precipitate was subjected to Soxhlet
extraction for 24 h using methanol. The final polymer was
dried at room temperature in a vacuum oven. The recovered
polymer yield was 0.28 g (78%).
wire (600 µm in diameter) electrodes were utilized as reference
and counter electrodes, respectively. Time-resolved PL spectra
of the polymer films were obtained as described in one of our
earlier papers.10
Cop olym er iza tion . A mixture of compound 7 and 1,4-bis-
(bromomethyl)-2-(2-ethylhexyloxy)-5-methoxybenzene was
polymerized in THF using potassium tert-butoxide initiator
under the same conditions as described in homopolymeriza-
tion. The two copolymers prepared were purified by Soxhlet
extraction with methanol. The two copolymers (refer to Scheme
2) were prepared using the monomer mixture of m:n ) 91:9.0
and 50:50 in mole ratio. The actual compositions of the
copolymers estimated from their H NMR are m:n ) 92.5:7.5
and 49.5:50.5, respectively. The recovered polymer yields were
73 and 68%, respectively.
F a br ica tion of LED Devices. The devices have a structure
of ITO/PEDOT (25 nm)/polymer (80 nm)/Ca (50 nm)/Al (50
nm). The ITO-coated glasses were cleaned as described
earlier.2a The conducting PEDOT (polyethylenedioxythiophene
doped with polystyrenesulfonate from Baeyer Co. with con-
ductivity of 10 S cm-1) layer (25 nm thick) was spin-coated
onto the ITO-coated glasses. The emitting polymer layer (80
nm) then was spin-coated onto the PEDOT layer using the
polymer solution in chlorobenzene. Finally, calcium (50 nm)
and aluminum (50 nm) electrodes were sequentially deposited
under a reduced pressure of 10-7 Torr on the polymer. The
device efficiency and electroluminescence spectra were ob-
tained as same method as described earlier.2a
Ch a r a cter iza tion of P olym er s. 1H NMR spectra were
collected on a Bruker Advance 300 spectrometer. UV-vis
absorption spectra were obtained on a J ASCO V-560 spectro-
photometer. Electroluminescence (EL) and photoluminescence
(PL) spectra were recorded with IS PC photon counting
spectrofluorometer. Molecular weights of polymers were de-
termined by gel permeation chromatography (GPC; Waters
410) using THF as eluent and polystyrene as standard. The
thermal stability of polymers was studied by thermo-
gravimetry, which was performed in air on an IBM 2050
instrument at a heating rate of 20 °C/min. The redox proper-
ties of polymers were examined by cyclic voltametry.9 The
polymer thin films were spin-coated on the ITO-coated glasses.
Chlorobenzene was used as the solvent. The electrolyte solu-
tion employed was 0.1 M tetrabutylammonium perchlorate,
Bu4NClO4, in N,N-dimethylformamide. The Ag/AgCl and Pt
Resu lts a n d Discu ssion
Gen er a l P r op er ties of P olym er s. All the polymers,
DHF-PPV and the two copolymers, were prepared by
the Gilch polymerization11,12 of monomer 7 or monomer
mixtures (refer to Scheme 1). They are soluble in
common organic solvents such as chloroform, THF,
dichloromethane, toluene, xylene, and N,N-dimethyl-
formamide. The number-average molar mass (Mh n) of the
polymers determined by GPC analysis was 420 × 103
for DHF-PPV, 290 × 103 for 92.5 DHF/7.5 MEH-PPV,
and 280 × 103 for 50.5 DHF/49.5 MEH-PPV. Sepa-

2878 Sohn et al.
Macromolecules, Vol. 35, No. 8, 2002
F igu r e 2. Photoluminescence spectra of the polymers ob-
tained at the excitation wavelength of 370 nm.
F igu r e 1. UV-vis absorption spectra of the polymers.
rately, we prepared poly(2-(2-ethylhexyloxy)-5-methoxy-
1,4-phenylenevinylene) (MEH-PPV), and its molar
mass was found to be Mh n ) 310 × 103. The polydisper-
sity indices of the four polymers were 2.7, 2.8, 3.2, and
4.2. Thermogravimetric analysis performed in air show
that thermal stability of the present polymers is fairly
good. The temperature revealing 5 wt % loss decreased
from 429 °C for DHF-PPV to 377 °C for MEH-PPV.
The corresponding temperatures of the two copolymers
lie in between. The presence of the alkoxy substituents
in MEH-PPV and copolymers appears to lower the
initial decomposition temperature. The glass transition
temperature of the polymers could not be identified on
their thermograms obtained by differential scanning
calorimetry (DSC). This can be ascribed to the rigidity
of their main chains.
Ta ble 1. P L Rise a n d Deca y Tim e a t Room Tem p er a tu r e
rise (ps) decay 1 (ps) decay 2 (ps)
DHF-PPV
28
32
26
350 (82.7%) 1080 (17.3%)
430 (76.7%) 1000 (23.3%)
230 (85.0%) 1190 (15.0%)
210 (84.6%) 870 (15.4%)
92.5 DHF/7.5 MEH-PPV
50.5 DHF/49.5 MEH-PPV
MEH-PPV
Ta ble 2. P L Efficien cies of P olym er s in Solu tion a n d F ilm
polymer
DHF-PPV
92.5 DHF/7.5 MEH
50.5 DHF/49.5 MEH
MEH-PPV
Φ(solution)a
relative efficiencyb
0.87
0.94
0.47
0.22
5.3
6.8
2.1
1
a
We used THF as a solvent and Coumarin 307 as a reference.14
The concentration was 5 × 10-6 mol/L of the repeating unit.
b
Relative value to the integrated PL intensity of MEH-PPV film.
UV-vis Absor p tion , P h otolu m in escen ce, a n d
Electr on ic Str u ctu r e. The UV-vis absorption and
photoluminescence (PL) spectra of the four polymers are
compared in Figures 1 and 2. According to the spectra
shown in Figure 1, all the polymers exhibit two major
absorptions: one below 350 nm and the other over a
longer wavelength region. The first absorptions in the
short wavelength region are originated from π electronic
transitions of aromatic structures and the second from
the π-π* transitions of main-chain π-systems. The
absorption maximum position (λmax) of the second peak
steadily increases from 432 nm for DHF-PPV to 435,
463, and 504 nm for the two copolymers and MEH-
PPV, respectively. In other words, as the content of the
MEH-PV comonomer unit increases, the λmax value
moves toward the longer wavelength. The bathochromic
shift13 by the alkoxy groups has been well established.
The optical band gaps (Eg’s) estimated from the UV-
vis absorption edges (503 nm for DHF-PPV, 517 and
582 nm for two copolymers, and 582 nm for MEH-PPV)
are 2.5 eV (DHF-PPV), 2.4 and 2.1 eV (two copolymers),
and 2.1 eV (MEH-PPV). It is evident that inclusion of
the MEH-PV unit lowers the band gap energy.
longer wavelength as the content of the MEH-PV unit
in the polymers is increased. A significant shift is
observed even when the copolymer contains only 7.5 mol
% of the MEH-PV unit; the maximum emission position
(λem) of the emission peak of DHF-PPV is 517 nm,
whereas that of 92.5 DHF/7.5 MEH-PPV is 554 nm.
Such a notable red shift of the emission peak by
including a very low level of the MEH-PV unit strongly
implies that excitons formed in higher energy structures
migrate to lower energy structures containing the
MEH-PV unit. The λem values increases to 597 and 595
nm respectively for 50.5 DHF/49.5 MEH-PPV and
MEH-PPV. The degree of red shift is greatly dimin-
ished, when the content of the MEH-PPV unit becomes
higher. All the emission peaks reveal a common feature
that a shoulder appears in the longer wavelength side.
Table 2 compares the PL efficiencies of different
polymers in tetrahydrofuran (THF) solution (5.0 × 10-6
mol/L of the repeating unit) and for thin (100 nm thick)
films. According to the data given in the table, surpris-
ingly enough, 92.5 DHF/7.5 MEH-PPV reveals the
highest PL quantum efficiency in solution as well as in
bulk film.
Figure 2 shows PL spectra of the polymers obtained
at the excitation wavelength 370 nm. The PL spectrum
of DHF-PPV reveals an emission peak whose maxi-
mum is located at 518 nm. The peak has one shoulder
at 548 nm. The position of emission peaks moves toward
From the cyclovoltametry,15 we could determine the
ionization potentials (IP), i.e., the HOMO levels, of the
polymers. The oxidative onset potentials were taken as
IP values. The LUMO levels were estimated from the

Macromolecules, Vol. 35, No. 8, 2002
DHF-PPV and Its Copolymers 2879
F igu r e 3. Electronic energy diagram of the polymers.
F igu r e 5. Dependence of current density and luminance on
electrical field in the LED devices.
and 592 nm (orange) with their shoulders locating at
588, 620, and 627 nm.
Figure 5 shows the I-V characteristics of the four
devices. The turn-on electric field increases from 0.28
MV/cm for the MEH-PPV device to 0.54 MV/cm for the
DHF-PPV device. The other two copolymers exhibit the
turn-on electric field of 0.43 and 0.29 MV/cm with the
value being lower for the copolymer containing higher
level of the MEH-PV unit. Comparison of the values
of turn-on electric field of the devices with the HOMO
levels (Figure 3) of the corresponding polymers leads
to the conclusion that the smaller the difference between
the work function of the ITO anode and the HOMO level
of a polymer, the lower the turn-on electric field
becomes. This, in turn, implies us that easier hole
injection from the anode controls the turn-on electric
field of the present devices, which should hold true only
when the major carrier is hole. Moreover, it is reminded
again that the LUMO levels (Figure 3) of the present
polymers are not so much dependent on the composi-
tions of the present polymers, and they all are lower
than the work function of calcium metal (2.8 eV).
Although the MEH-PPV device exhibits the lowest
turn-on electric field, Figure 5 tells us that the devices
based on DHF-PPV and copolymers are more efficient
in emitting light per current density. The most surpris-
ing phenomenon is that the copolymer (92.5 DHF/7.5
MEH-PPV) containing the low level of the MEH-PV
comonomer unit exhibits a remarkably higher light
output per current density when compared with DHF-
PPV, MEH-PPV, and another copolymer containing
about 50 mol % of the MEH-PPV unit.
F igu r e 4. Electroluminescence spectra of LEDs.
HOMO levels and optical band gaps (Eg’s) determined
from the absorption edges of the UV-vis absorption
spectra. The work function of ITO was taken as 4.8 eV.16
The HOMO and LUMO energy diagram thus obtained
of the present polymers is given in Figure 3. According
to the diagram, attachment of dihexylfluorene pendant
on the PPV backbone increases the HOMO level more
than the LUMO level; the HOMO and LUMO levels of
PPV17 are 5.5 and 3.2 eV, whereas they are 6.0 and 3.5
for DHF-PPV, respectively. In contrast, the alkoxy
pendant lowers the HOMO level while they affect
relatively little the LUMO level.
Electr ica l a n d Electr olu m in escen ce P r op er ties
of LED Devices. Electroluminescence of the LED
devices constructed with the four polymers was studied
in detail. The device configuration was ITO/PEDOT (25
nm)/polymer (80 nm)/Ca (50 nm)/Al (50 nm). Aluminum
was deposited onto the calcium electrode (cathode) in
order to protect the latter. The electroluminescence (EL)
spectra of the devices are presented in Figure 4. In
general, the shapes and positions of EL spectra are very
similar to those of the corresponding PL spectra. The
EL spectrum of the DHF-PPV device, however, reveals
a rather strong shoulder around 498 nm, in addition to
the major peak at 520 nm (green) and another shoulder
around 550 nm. A closer examination of the PL spec-
trum (Figure 3) of DHF-PPV indicates that a fairly
strong shoulder has been buried in the left-hand side
of the major emission peak at 517 nm. The other devices
show peak maxima at 561 nm (yellow), 585 nm (orange),
This is better compared in Figure 6, which shows the
dependence of luminance efficiency on the current
density for the four LED devices. One can see that the
92.5 DHF/7.5 MEH-PPV device’s maximum luminance
efficiency is about 2.4 cd/A, significantly higher than
those (0.65-1.0 cd/A) of other devices. The efficiencies
of the devices fabricated with DHF-PPV and 50.5 DHF/
49.5 MEH-PPV are not so much different from each
other, with the latter being only slightly better. One of
the plausible conjectures to explain the enhancement
of device efficiency by inclusion of a relatively low level

2880 Sohn et al.
Macromolecules, Vol. 35, No. 8, 2002
we clearly observe the rise time for DHF-PPV and
copolymers, indicating an efficient energy transfer from
the fluorene pendants to the backbone.10 In contrast,
with the exception of MEH-PPV, the slow time con-
stant did not vary as much with the content of the
MEH-PV unit. Since it is generally accepted that the
PL and EL mechanisms are basically the same, it is
expected that the excitons confined more effectively are
more likely to be involved in an emissive decay, result-
ing in a higher luminescence efficiency.18
In addition, balance in carriers mobilities is another
important factor controlling the LED device efficiency.
As is well-known, PPV transports the holes much faster
than the electrons.19,20 According to our own measure-
ments,18a by the time-of-flight method18b PPV’s hole
mobility is about 200 times its electron mobility. There-
fore, much of the holes injected from the anode into the
emitting layer are wasted on the cathode. That is one
of the reasons for the poor device performance based
on PPV. Although MEH-PPV is known to perform
better in LED,3a it is still far from satisfaction. The
presence of the alkoxy substituents hinders the inter-
chain contacts that is expected to reduce the chances
for the formation of interchain exciton pairs. Formation
of interchain exciton pairs was claimed to be another
factor diminishing device efficiency.7b The balance in the
mobility of the carriers, however, is not improved by the
two alkoxy substituents. In MEH-PPV the mobility of
holes is more than 100 times faster than the mobility
of electrons.21 In contrast, the imbalance in the mobility
of the carriers is significantly reduced in DHF-PPV,22
in which the hole mobility is only about 10 times faster
than the electron mobility. We believe that the presence
of 9,9-dihexylfluorenyl pendants stabilizes the positive
hole, which results in the reduction in the mobility of
the positive hole carrier. This, in turn, is expected to
reduce the imbalance in the mobilities of the positive
and negative carriers. As the content of the MEH-PV
units in the copolymers is increased, the imbalance in
the carrier mobility grows. In short, a combination of
reduced turn-on electric field, augmented excitons con-
finement, and improved balance in the mobilites of
carriers is judged to be the major reason for a higher
device efficiency of 92.5 DHF/7.5 MEH-PPV among the
polymers described in this report. It is added that one
of the recent publications by Lee et al.23 describes an
interesting approach in achieving a balance in the
mobilities of carriers. They attached the electron-
transporting oxadiazole pendants to the PPV backbone
through long dodecyloxy spacers so that the oxadiazole
groups2d can be uniformly dispersed in the hole-
transporting substituted PPV matrix. This is in contrast
with our earlier work where the oxadiazole groups were
attached directly to the PPV backbone through the
p-phenylene bridge.
F igu r e 6. Dependence of luminance efficiency of LEDs on
current density.
F igu r e 7. Time-resolved PL decay of the polymers.
of the MEH-PV unit is that inclusion of a small amount
of the comonomer unit along the main chain of the
polymers provides the sites where confinement of exci-
tons are promoted. As the level of the MEH-PV unit
in the backbone increases, confinement of excitons is
expected to diminish.
This assumption is strongly supported by the fact that
fluorescence lifetime is significantly lengthened when
the MEH-PV comonomer unit is included in a low level
into DHF-PPV as shown in the time-resolved photo-
luminescence spectra of Figure 7. Fluorescence decayed
fastest for MEH-PPV followed by 50 DHF/50 MEH-
PPV, DHF-PPV, and 92.5 DHF/7.5 MEH-PPV in that
order. When the PL decay curves were fit to the
biexponential function containing a slow and a fast
decay time constants, we obtained the results as shown
in Table 1. We clearly see that the fast decay time
constant governs the majority part of the PL decay,
which decreases in the order of 92.5 DHF/7.5 MEH-
PPV (430 ps) > DHF-PPV (350 ps) > 50.5 DHF/49.5
MEH-PPV (230 ps) > MEH-PPV (210 ps). Moreover,
Con clu sion
We have described the synthesis and luminescence
properties of a series of new PPV derivatives bearing
fluorenyl pendants. Attachment of the fluorenyl pen-
dants onto the phenylene rings of PPV not only renders
solubility but also improves the LED device efficiency.
Because of the presence of the bulky pendant, interchain
distance must have increased significantly as compared
with unsubstituted PPV, which would reduce the pos-
sibility for the formation of interchain exciton pairs.
This could be one of the major reasons for the improved

Macromolecules, Vol. 35, No. 8, 2002
DHF-PPV and Its Copolymers 2881
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