Triphenylene-based polymers for blue polymerie light emitting diodes

Article abstract of DOI:10.1021/ma901912t

An efficient synthesis has been developed toward a novel series of conjugated blue emitting polymers containing triphenylene as repeating unit for PLEDs. Soluble 1,2,3,4-tetraphenyltriphenylene, 2,3-bis(4-octylphenyl)-l,4- diphenyltriphenylene, 2-heptyl-l

Full text of DOI:10.1021/ma901912t

Macromolecules 2010, 43, 137–143 137
DOI: 10.1021/ma901912t
Triphenylene-Based Polymers for Blue Polymeric Light Emitting Diodes
†
Moussa Saleh, Martin Baumgarten, Alexey Mavrinskiy, Thomas Schafer, and
†
†
‡
€
,†
€
Klaus Mullen*
†Contribution from the Max-Planck-Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany and ‡R&D, Ciba Inc., Schwarzwaldallee 215, CH-4002 Basel, Switzerland
Received August 28, 2009; Revised Manuscript Received November 11, 2009
ABSTRACT: An efficient synthesis has been developed toward a novel series of conjugated blue emitting
polymers containing triphenylene as repeating unit for PLEDs. Soluble 1,2,3,4-tetraphenyltriphenylene,
2,3-bis(4-octylphenyl)-1,4-diphenyltriphenylene, 2-heptyl-1,3,4-triphenyltriphenylene, 2-decyl-3-(4-octyl-
phenyl)-1,4-diphenyltriphenylene, and 2,3-diheptyl-1,4-diphenyltriphenylene-based co- and homopolymers
have been synthesized by the palladium-catalyzed Suzuki-Miyaura and the nickel-catalyzed Yamamoto
polycondensation reactions, respectively. The photophysical properties of the polymers were studied based
on the different main chains and substituents. Because of the twisted phenyl rings around the triphenylene
main core, π-π stacking in the polymers was prevented, resulting in almost identical photoluminescence (PL)
spectra in both solutions and films. All polymers exhibited narrow emission in the range of 430-450 nm,
where the human eye is most sensitive for the blue range.
Introduction
their various physical properties including one-dimensional
charge migration,29 one-dimensional energy migration,30 electro-
Since the discovery of electrical conductivity in π-conjugated
polymers three decades ago,1,2 semiconducting polymers have
been extensively studied and become of main interest in the
research activities around the globe.3-5 Because of the simple
processing, high flexibility, and ease of fabrication in large
area, conjugated polymers have been incorporated as active
materials into several kinds of electronic devices such as poly-
meric light emitting diodes (PLEDs),6-9 organic field effect
transistors (OFETs),10,11 polymer lasers,12 and organic photo-
voltaic cells.13-15
luminescence,31 ferroelectric switching,32 alignment, and self-as-
sembling behavior on surfaces.33,34 One example is the alkoxy-
substituted triphenylene which gained increased interest in the
fabrication of OLEDs.35
In this work, we present for the first time a series of tripheny-
lene-alt-arylene copolymers (P1-P5) and polytriphenylene
homopolymers (P6 and P7) with different alkyl or alkoxy chains
as solubilizing units. Concerning the design and the synthesis of
the presented polymers, several strongly twisted phenyl rings
have been introduced around the triphenylene core in order to
suppress the intermolecular interactions in the solid state. Such
behavior can result in self-quenching processes due to ground
state aggregation and results in a reduction in the efficiency and
luminescence of the PLED device.36
For the polymerization protocols, we decided first to use the
Pd-catalyzed Suzuki-Miyaura polycondensation reaction37,38
which is one of the most powerful methods for preparing
conjugated polymers. Furthermore, Suzuki synthesis gives the
ability to introduce additional arylene units between the repeat-
ing units in the polymer backbone. However, the preparation of
the arylenediboronic ester derivatives requires considerable syn-
thetic effort, especially when scale-up is needed. Moreover, such
AB type polymerizations require highly accurate stoichiometries,
where small deviations can lead to dramatically decreased mo-
lecular weights; this can be especially challenging for small scale
reactions. These two drawbacks of the Suzuki approach to
prepare copolymers motivated us to search for simpler but
comparatively powerful and robust methods for the preparation
of the functionalized triphenylene-based polymers.
During the past decade PLEDs have been intensively studied
because of their potential applications in a new generation of flat
display and lighting technologies.16,17 Phenylene based polymers
such as poly(fluorene),18 poly(indenofluorene),19 poly(ladder-
type pentaphenylene),20 and ladder-type poly(p-phenylene)s21
are some of the most important classes of conjugated wide band
gap polymers for blue emitting PLEDs. However, these polymers
suffer from problems of (i) poor charge injection and transport,
and (ii) a tendency to oxidize at the bridge-head positions. The
oxidation generates a ketone functionality, which leads to the
appearance of a long wavelength emission band and the dete-
rioration of device performance.22,23 A few years ago, our group
showed that replacement of the carbon with a nitrogen bridge
(carbazole)22 followed by replacement of the alkyl groups at C9-
head position with aryl groups24-27 has proved to be a successful
way to obtain stable blue emission from polyfluorenes. However,
one effect of aryl substituents on the physical properties of
polyfluorenes is to inhibit chain packing as demonstrated by
the lack of liquid crystalline phases, and the absence of any signs
of organization in the polymer films when studied by atomic force
microscopy (AFM).28 Therefore, new candidates for blue emis-
sion which do not suffer from the problems of keto-functionality
and low microscopic order was vital. Recently, triphenylene
containing small molecules and oligomers have received consi-
derable attention due to their discotic liquid crystalline nature and
A promising alternative was the Yamamoto polycondensa-
tion.39 Here, dibromo-functionalized starting materials are the
only monomers required and stoichiometric quantities of nickel
complexes take over the role of the organometallic counterpart.
Because of its simplicity, and because the Yamamoto protocol
has previously shown to be highly valuable for the synthesis
of constitutionally homogeneous, high molecular weight
*Corresponding E-mail: muellen@mpip-mainz.mpg.de.
r 2009 American Chemical Society
Published on Web 12/07/2009
pubs.acs.org/Macromolecules

138 Macromolecules, Vol. 43, No. 1, 2010
Scheme 1. Synthetic Routes of 6,11-Dibromo-1,2,3,4-Substituted-Triphenylene Monomers 5a-5e a
Saleh et al.
a Reagents and conditions: (i) Pd0, ethynyltrimethylsilane, CuI, piperidine, 80 °C, 91%; (ii) TBAF, THF, 61%; (iii) Pd0, 1-bromo-4-octylbenzene,
CuI, piperidine, 70 °C, 59%; (iv) Pd0, 1-dodecyne, CuI, piperidine, 50 °C, 70%; (v) KOH, MeOH, EtOH, 80 °C, 40%; (vi) Ph2O, reflux.
polyaromatics,39 we decided to utilize this method also in our
synthesis. Furthermore, a direct comparison between Suzuki and
Yamamoto polycondensation reactions can be available con-
cerning their practicability, constitutional homogeneity, degrees
of polycondensation, and finally the physical properties of the
resulting polymers. Microwave assisted organic synthesis
(MAOS) is a relatively new technique in the present-day organic
synthetic world and has gained increased interest in the past
decade.40 The main advantages of MAOS are the dramatic
acceleration of reactions, most often resulting in cleaner out-
comes and increased yields. Herein, we present the synthesis of
novel triphenylene-based polymers as promising blue light emit-
ters for PLEDs via the microwave assisted (MA) Suzuki-
Miyaura and Yamamoto polycondensation reactions.
different substituents. The synthesis of two new 1,2-di-substi-
tuted-ethynes 4b and 4c was accomplished by Sonogashira
coupling reactions.43
1,2-Di-substituted-ethynes 4 were then used for Diels-Alder
reactions to give 6,11-dibromo-1,2,3,4-substituted-triphenylenes
5a-5e (Scheme 1). Later, the triphenylene monomers were
subjected to the Suzuki-Miyaura and Yamamoto polymeriza-
tions (Schemes 2 and 3).
Suzuki-Miyaura Approach. With the triphenylene mono-
mers 5a-5e in hand, our attention focused on the subsequent
polymerization reactions. The MA-Suzuki-Miyaura po-
lymerization was first applied, therefore 1,4-bis(4,4,5,5-tet-
ramethyl-1,3,2-dioxaborolan-2-yl)benzene and its deri-
vatives were used (Scheme 2). It was challenging to find the
most suitable MA-Suzuki-Miyaura reaction conditions in
terms of the applied power (50 W), temperature (100 °C), and
reaction time (1 h). These complications arose from (i) the
relatively low yields of the Diels-Alder reactions (10-50%)
seen in Scheme 1, resulting from the steric hindrance between
the aryl groups on 3 and the aryl and/or alkyl groups on 4
and the debromination of the desired product 5 due to the
high temperature and the long time of the Diels-Alder
reactions, (ii) the need of ultra pure monomers, and (iii)
the fact that MAOS gives different results from one system
to the other. Fortunately, the MA-Suzuki-Miyaura
coupling reactions (Scheme 2) afforded moderate molecular
weight polymers (Mw = 15 Â 103 to 63 Â 103 g mol-1). The
Results and Discussion
The desired triphenylene monomers were synthesized in two
steps starting from the Knoevenagel condensation reaction41
between 2,7-dibromophenanthrene-9,10-dione (1) and 1,3-diphe-
nylpropan-2-one (2) to afford 5,10-dibromo-1,3-diphenyl-2H-
cyclopenta[l]phenanthren-2-one (3) (Scheme 1). Following that,
compound 3 was subjected to a series of Diels-Alder reactions42
with 1,2-di-substituted-ethynes 4. Utilizing different 1,2-di-sub-
stituted-ethynes 4 was crucial to afford the monomers with the
required solubility which is needed to achieve soluble high
molecular weight polymers. In addition the optical properties
of the resulting polymers can be studied depending upon those

Article
Macromolecules, Vol. 43, No. 1, 2010 139
Scheme 2. Synthetic Route of Polymers via Suzuki-Miyaura Coupling Reaction
Scheme 3. Synthetic Route of Polymers via Yamamoto Coupling Re-
action
Table 1. Molecular Weights and PDI of Polymers from GPC
Analysisa (Standard = PPP; Eluent = THF; Temperature = 25 °C)
polymer
yield (%)
Mw
PDI
P1
P2
P3
P4
P5
P6
P7
68
55
67
65
81.5
93
91
27 Â 103
36 Â 103
15 Â 103
18 Â 103
63 Â 103
224 Â 103
112 Â 103
2.7
1.4
1.9
1.6
3.6
2.7
2.7
a Refer to Synthesis and Characterization section.
Table 2. UV-Vis Absorption and PL Behavior of Monomers 5a-5e
(Solvent = DCM; Concentration ∼1Â 10-5 M;Temperature = 25 °C)
molecular weights and PDI of the resulting polymers are
summarized in Table 1. The GPC results for P5 revealed
unusual high PDI value due to the presence of two dominant
peaks in the GPC analysis corresponding to two high
molecular weight fractions which could not be separated.
The smaller molecular weight fraction under the GPC curve
could be attributed to the average molecular weight of the
polymer while the larger molecular weight fraction could be
ascribed to polymer aggregates due to the few number of the
phenyl groups around the triphenylene core.
Yamamoto Approach. Utilizing the MA-Yamamoto po-
lymerization (Scheme 3) the resulting homopolymers were
of higher molecular weights (Mw = 224 Â 103 and 112 Â
103 g mol-1 for P6 and P7, respectively) than the copolymers
(P1-P5) obtained by the MA-Suzuki- Miyaura polymeri-
zation (Table 1). Although polymers P5 and P6 have the same
side chains, P6 showed monomodal molecular weight dis-
tribution. The homopolymers were synthesized to study the
effect of the direct coupling between the triphenylene repeat-
ing units on the physical properties of the final polymers.
Synthesis and Characterization. The synthetic routes of
monomers 5a-5e and polymers P1-P7 are depicted in
Schemes 1-3. All of these polymers were easily dissolved
in common organic solvents such as chloroform (CHCl3),
tetrahydrofuran (THF), dichloromethane (DCM), and
toluene. In addition polymer solutions formed uniform,
transparent films on quartz substrates after spin coating.
a
b
a
b
λabs.sol
(nm)
λPL.sol
(nm)
λabs.sol
(nm)
λPL.sol
(nm)
monomer
monomer
5a
5b
5c
283
284
288
405
408
411
5d
5e
283
289
415
416
a Wavelength of the maximum absorbance. b Wavelength of the
maximum PL.
The weight-average molecular weights (Mw) were measured
by gel permeation chromatography (GPC), analysis was
6
4
˚
performed with SDV (PSS) columns (10 , 10 , and 500 A
porosity) connected to RI and UV (254 nm) detectors against
1,4-polyparaphenylene (PPP) and polystyrene standards
with THF as an eluting solvent. The polydispersity index
(PDI) was measured by dividing weight-average molecular
weight by the number-average molecular weight (Mw/Mn).
The data obtained from GPC are given in Table 1. The
weight-average molecular weights (Mw) of polymers were
between 15 Â 103 and 224 Â 103 g mol-1, while the poly-
dispersity indices (PDI) ranged from 1.4 to 3.6.
Absorption and PL in Solution. The photophysical char-
acteristics of the monomers as well as of the polymers were
studied by UV-vis absorption and PL spectroscopy in both
dilute solutions (DCM and THF, respectively) and thin
films. The wavelengths of the UV-vis absorptions and PL
maxima of the monomers and polymers in solutions as well

140 Macromolecules, Vol. 43, No. 1, 2010
Saleh et al.
Table 3. UV-Vis Absorption, PL Behavior, and Quantum Yields (ΦPL) of Polymers P1-P7 (Solvent = THF; Concentration ∼ 1 Â 10-5 M;
Temperature = 25 °C)
band gapb (eV)
λPL.sola (nm)
λPL.filma (nm)
ΦPLfilm
ΦPLsolution
c
d
polymer
λabs.sola (nm)
λabs.filma (nm)
P1
P2
P3
P4
P5
P6
P7
391
408
388
367
391
394
396
387
392
389
380
391
401
398
2.87
2.91
2.90
3.02
2.83
2.91
2.90
429
428
427
422
429
418
418
430
428
427
420
459
458
424
e
0.65
0.84
e
0.34
e
0.34
0.31
0.21
0.17
e
0.66
0.67
0.85
a Wavelength of the maximum absorbance or PL. b Band gaps were calculated from the onsets of UV-vis absorption spectra of polymer solutions.
c Relative to thin-film quantum efficiency of poly(9,9-dioctylfluorene) POF (sample polymer and reference POF were dissolved in toluene and spin
coated over quartz (1 min at 2000 rpm and 15 s at 5000 rpm). d Determined in THF relative to 9,10-diphenylanthracene in cyclohexane (ΦPL = 0.9) with
excitation at 365 nm. e Not measured.
Figure 1. UV-vis spectra of polymers in THF at 25 °C.
Figure 3. UV-vis spectra of polymer films.
Figure 2. PL spectra of polymers in THF at 25 °C.
Figure 4. PL spectra of polymer films.
as in films are summarized in Tables 2 and 3 and in Figures s1
and s2 in the Supporting Information, and Figures 1-4,
respectively.
polymers showed almost identical PL maxima in solution
at 429, 428 nm, and 427 nm, respectively (Figures 1 and 2).
The PL spectra were characterized by steep onsets and by
well-resolved vibrationally split maxima at higher wave-
lengths but with lower intensities (453 and 481 nm for P1,
453 and 480 nm for P2, and 452 and 479 nm for P3) which
resulted from the vibrational splitting. The UV-vis spec-
trum of the alkoxy substituted polymer P4 in solution was
characterized by featureless π-π* transition (one band) at
367 nm which was blue-shifted compared to the absorption
The UV-vis absorption spectra of the synthesized poly-
mers in solution (THF) are depicted in Figure 1. Polymers
P1, P2, and P3 exhibited two bands in the solution absorption,
with the absorption maxima at 391, 408, and 388 nm,
respectively, while the second bands were at 409, 392, 405
nm, respectively. These hypsochromically shifted absorption
maxima for P1 and P3 compared to P2 suggest a smaller
conjugation length for P1 and P3. However the three

Article
Macromolecules, Vol. 43, No. 1, 2010 141
maxima of the other polymers. This is most probably due to
the alkoxy groups on the poly(arylene)s backbone which
usually lead to an increase of the dihedral angle of consecu-
tive aromatic units, which reduces electronic conjugation
leading to an increased band gap.38
Table 4. HOMO and LUMO Energies and Electrochemical Prop-
erties of Polymers (Corrected to Ag/AgCl Reference Electrode)
polymer
Eox/onset (V)
Eox/peak (V)
EHOMO (eV)
ELUMO (eV)
P1
P2
P3
P4
P5
P6
P7
1.55
1.68
1.56
1.39
1.56
1.54
1.50
1.68
1.87
1.65
1.57
1.71
1.71
1.59
5.95
6.08
5.96
5.79
5.96
5.94
5.90
3.08
3.17
3.06
2.77
3.13
3.03
3.0
Unlike P1, P2, and P3, polymer P4 demonstrated one PL
maximum at 422 nm in solution with unresolved shoulder at
445 nm. Polymer P5 showed absorption maxima at 391 nm
with a second band at 408 nm, while the PL maximum was
observed at 429 nm. P5 exhibited also a second higher
wavelength but less intense PL maximum at 454 nm which
was homologous to the absorption and PL maxima of
polymer P3. Polymers P6 and P7 showed absorption maxima
at 394 and 396 nm, respectively with PL maxima at 418 nm in
solution. The PL spectra were similar to those of polymers
P1-P5 and characterized also by steep onsets with second
vibrationally split maxima at 442 and 441 nm for P6 and P7,
respectively. Furthermore, the observed PL spectra for P6
and P7 suggest that decreasing the number of the aryl units
around the triphenylene moiety has no significant effect in
solution. The observed PL maxima of ∼420 nm for P1-P7
compares very well with that of dialkylpolyfluorene (PL
maximum ∼ 415 nm)10 which has been widely studied as
an active material in blue PLED.
Absorption and PL in Thin Films. The thin films of the
above polymers were prepared by spin coating from toluene
solutions (∼1 Â 10-5 M) at 1000 rpm over quartz substrates.
As shown in Figures 3 and 4, polymers P1 and P2 showed
almost identical PL spectra in both solution and film. How-
ever the film absorption maxima were blue-shifted compared
to the solution maxima by 4 and 16 nm for P1 and P2,
respectively. Polymer P3 exhibited an almost identical ab-
sorption and PL spectra in both solution and film. In
addition a solid state related broadening of the spectra was
observed in the three polymers. These results suggest that
there is no significant change in the polymer conformations
in going from solution to film. From that point of view P1, P2,
and P3 can be considered as promising blue emitters
for PLED applications. While polymer P4 gave nearly
identical PL spectra in both solution and film, the film
absorption maximum was red-shifted by 13 nm from the
solution absorption maximum. In contrast to P1, P2, and P3,
polymer P4 showed insignificant broadening in the film
PL spectrum. Thus, polymer P4 displayed a clear advantage
over P1, P2, and P3 in terms of more deep and pure blue
emission. The absorption of polymer P5 was nearly the
same in both solution and film. However, the PL spectrum
revealed a broad bathochromically shifted emission band
of 30 nm in film compared to solution. This bathochromic
shift suggests that polymer P5 with fewer aryl units around
the triphenylene core undergoes aggregation in the solid
state.
maxima of polymer P7 were ∼9 nm red-shifted from those of
polymer P3 most probably due to the increased conjugation
length in case of P7. Moreover both polymers showed
roughly identical PL behavior aside from a broadening in
the PL spectra of polymer P7. These results suggest that the
direct coupling between the triphenylene repeating units in
the homopolymers did not hinder the conjugation in the
polymers backbones due to the probable increased twisting
of the different phenyl rings on the sequential triphenylene
moieties. Instead it led to analogous optical spectra for
polymers P7 and P3.
Quantum Yield. Polymers P1, P2, P5, P6, and P7 exhibited
high quantum efficiencies (ΦPL) in solution, which were
upon excitation at 365 nm in THF, using 9,10-diphenylan-
thracene (ΦPL = 0.9) as a standard44 found to be 0.65, 0.84,
0.66, 0.67, and 0.85, respectively. These values were close to
or even in case of P2 and P7 higher than polyfluorene
containing bis(2,2-diphenylvinyl)fluorene (ΦPL = 0.75).45
The values were also close and in case of P7 equal to the
quantum yield of POF (ΦPL = 0.85).46
We estimated the PL quantum yield of P2, P4, P5, P6, and
P7 in films to be 0.34, 0.34, 0.31, 0.21, and 0.17, respectively.
This was done by comparing their fluorescence intensity to
that of a thin film sample of POF polymer that was excited
at 380 nm (ΦPL = 0.55).45,46 The lower quantum efficiencies
of P2, P5, P6, and P7 in film compared to solution might
be due to the formation of intermolecular species, i.e.
46
excimer formation or aggregates as in case of P5 and P6
which suggests that decreasing the number of aryl units
around the triphenylene units in case of P5 and P6 was not
favored.
Cyclic Voltammetry. To investigate the redox properties,
cyclic voltammograms were recorded for the polymer thin
films against Ag/AgCl with a ferrocene standard (Figure s3,
Supporting Information). All polymers showed only anodic
peaks with irreversible oxidation which was in agreement
with the general case of conjugated polymers, where often
only reduction or oxidation is observed or where only one
process is found to be reversible.47-49 For that reason direct
estimation of the LUMO levels were not possible. Subse-
quently the HOMO levels, optical band gaps, and the
LUMO levels of the reduced polymers (were deduced from
the HOMO values and the polymers optical band gaps) were
calculated as described by Janietz et al.47 Polymers P1-P7
showed irreversible oxidation peaks at potentials ranging
from 1.57 to 1.87 V (Table 4) and onset potentials of
oxidation between 1.39 and 1.68 V in the anodic scan. This
could be attributed to the electron rich nature of the poly-
mers which may make it easier to oxidize rather than being
reduced.50,51 Compared with all polymers, the oxidation
onset potential of P4 showed the lowest value which could
be due to the electron-donating property of the alkoxy group
on the polymer backbone. All polymers exhibited HOMO
and LUMO values in the range of 6.0 and 3.0 eV, res-
pectively. These results were analogous to the obtained
results for POF which showed HOMO and LUMO values of
Similar to P5, polymer P6 exhibited approximately the
same absorption behavior in both solution and film. Never-
theless the PL spectrum showed a broader bathochromically
shifted maximum of about 40 nm in film compared to
solution. This resemblance to the spectra of polymer P5,
despite containing a different π-conjugated backbone (the
inclusion of a phenyl spacer), gave evidence that the solu-
blizing alkyl side chains found on monomer 5e result in the
broadening and the bathochromic shift by promoting poly-
mer aggregation. Accordingly polymers P5 and P6 with their
more pronounced green emission can not be considered as
good blue emitters.
Polymer P7 exhibited more or less the same absorption and
emission in both solution and film with a solid state related
broadening in the film emission spectrum. The absorption

142 Macromolecules, Vol. 43, No. 1, 2010
Saleh et al.
In contrast, the reflections for polymer P6 were almost
isotropic, indicating local order but poor macroscopic
orientation (Figure 5b).
Conclusions
In conclusion, we have synthesized a novel series of blue light
emitting triphenylene-based polymers in good yields, based on
Suzuki-Miyaura and Yamamoto polycondensation reactions.
All of the polymers showed deep blue emission in solution with
PL maxima centered around 420 nm. However the film UV-vis
absorption and PL spectra were diverse, that can be ascribed to
the number of aryl units around the triphenylene moiety. Since in
case of the less aryl substituted polymers P5 and P6 large bath-
ochromic shifts of 30 and 40 nm, respectively were observed in the
PL maxima (∼460 nm), while in the more aryl substituted
polymers P1, P2, P3, P4, and P7 the PL maxima were centered
around 430 nm to show deep blue emission. This suggests that
decreasing the number of aryl units around the triphenylene
moiety is not favored since it can lead to polymer aggregates.
However no distinct differences were found in the solution
UV-vis absorptions and PL spectra between the homopolymers
and the copolymers which may suggest similar conjugation
interactions along the polymers backbones. From the 2D-
WAX studies, we also found that the triphenylene-alt-arylene
copolymer P5 was related to a well-aligned macroscopic organi-
zation in the extruded sample, which was not obvious for the
polytriphenylene homopolymer P6. This difference in the macro-
scopic organization can be attributed to the present extra arylene
groups between the triphenylene repeating units in the copolymer
which reduced the twist in the polymer backbone and result in
well organized structure. Overall, the obtained results demon-
strated that triphenylene-based polymers are promising candi-
date for blue-PLEDs. Such devices have been investigated and
published elsewhere.56
Figure 5. 2D-WAXS of (a) P5 and (b) P6.
5.8 and 2.85 eV (estimated),47 respectively. The expected
electron injection barrier from LiF/Al cathode was ∼0.2 eV,
however the expected hole injection barrier from ITO/PED-
OT:PSS anode was ∼0.8 eV which should consequently lead
to limited charge injection into the polymers. In order to
overcome this problem and to obtain better interlayer
matching of the HOMO and LUMO properties in the PLED
device three additives were used, the electron transporting
layers t-PBD, the hole transporting material NPB, and on
the top of them a Bphen layer. These additives result in better
electron and hole injection into the polymer layers and thus
better device performance.53
Structure Investigation. The supramolecular structures of
polymers P5 and P6 have been investigated by two-dimen-
sional wide-angle X-ray scattering (2D-WAXS) on extruded
filaments.52
The samples were prepared as a thin filament of 0.7 mm
diameter by filament extrusion using a home-built mini
extruder. Polymers P5 and P6 were heated to 230 and
210 °C, respectively to reach a phase at which the polymer
became plastically deformable and could be extruded as a
thin filament.
Acknowledgment. Financial support from Ciba Inc., Basel,
Switzerland, and the IRTG graduate school (Mainz-Korea) is
gratefully acknowledged. The authors thank Dr. Wojciech Pisula
for fruitful discussions.
The structure investigation was performed using a 2D
wide-angle X-ray scattering (WAXS) setup, which is sche-
matically showed in Figure 5. The sample was positioned
perpendicular to the incident X-ray beam and vertical to the
2D detector.
In Figure 5 the 2D wide-angle X-ray scattering (2D-
WAXS) pattern of polymers P5 and P6 are presented. The
distinct and strong reflections for P5 were related to a well-
aligned macroscopic organization in the extruded sample,
which was not obvious for P6, confirming the influence of the
chemical structure difference on the ordering of the two
polymers. The presence of the arylene groups between the
triphenylene repeating units in P5 reduced the twist in the
copolymer backbone and result in more ordered structures
compared to P6 in which the triphenylene moieties were
directly coupled.
In case of polymer P5, the wide-angle equatorial scattering
intensity (marked in the pattern with arrows as “1”) in
Figure 5a was related to a stacking distance of 0.43 nm of
the adjacent molecules, confirming that the polymer chains
are aligned along the extrusion direction of the sample. This
value is close to the distance found for different polyfluorene
derivates which should result in identical charge carrier
transport for this material.53-55
Supporting Information Available: Text giving the experi-
mental information and figures showing UV-vis spectra, NMR
spectra, PL spectra, cyclic voltammograms of drop-casted polymer
films, and molar-mass distribution curves of polymers P1-P7
obtained by GPC. This material is available free of charge via
the Internet at http://pubs.acs.org.
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