Investigation of spacer influences in phosphorescent-emitting nonconjugated PLED systems

Article abstract of DOI:10.1002/pola.23796

An assay was introduced to clarify influences on electroluminescent behavior for RGB-colored phosphorescent terpolymers with N,N-Di-p-tolyl-aniline as hole-transporting unit, 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4- oxadiazole (tert- BuPBD) as electron-transporting unit, and different iridium complexes in RGB-colors as triplet emitting materials. All monomers were attached with spacer moieties to the "para" position of a polystyrene. Polymer light emitting diodes (PLEDs) were built to study the electro-optical behavior of these materials. The gist was a remarkable influence of hexyl-spacer units to the PLED performance. For all three colors only very restricted PLED performances were found. In comparison RGB-terpolymers were synthesized with directly attached charge transport materials to the polymer backbone. For this directly linked systems efficiencies were 28 cd A^-1 @ 6 V (green), 4.9 cd A^-1 @ 5 V (red) and 4.3 cd A^-1 @ 6 V (bluish). In summary we assume that an improved charge percolation pathways regarding to the higher content of semiconducting molecules and an improved charge transfer to the phosphorescent dopand in the case of the copolymers without spacers are responsible for the better device performance comparing the copolymers with hexyl spacers. The approach of the directly connected charge transport materials at the nonconjugated styrene polymer backbone should be favored for further investigations, therefore.

Full text of DOI:10.1002/pola.23796

Investigation of Spacer Influences in Phosphorescent-Emitting
Nonconjugated PLED Systems
MANUEL W. THESEN,1 HARTMUT KRUEGER,1 SILVIA JANIETZ,1 ARMIN WEDEL,1 MARION GRAF2
1Fraunhofer-Institut fuer Angewandte Polymerforschung, Wissenschaftspark Golm, Geiselbergstr. 69, Potsdam D-14476, Germany
2Department Chemie der Ludwig-Maximilians-Universitaet Muenchen, Butenandtstraße 5-13, Muenchen D-81377, Germany
Received 19 August 2009; accepted 23 October 2009
DOI: 10.1002/pola.23796
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: An assay was introduced to clarify influences on
electroluminescent behavior for RGB-colored phosphorescent
terpolymers with N,N-Di-p-tolyl-aniline as hole-transporting
unit, 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tert-
BuPBD) as electron-transporting unit, and different iridium com-
plexes in RGB-colors as triplet emitting materials. All monomers
were attached with spacer moieties to the ‘‘para’’ position of a
polystyrene. Polymer light emitting diodes (PLEDs) were built to
study the electro-optical behavior of these materials. The gist
was a remarkable influence of hexyl-spacer units to the PLED
performance. For all three colors only very restricted PLED per-
formances were found. In comparison RGB-terpolymers were
synthesized with directly attached charge transport materials to
the polymer backbone. For this directly linked systems efficien-
cies were 28 cd Aꢀ1 @ 6 V (green), 4.9 cd Aꢀ1 @ 5 V (red) and
4.3 cd Aꢀ1 @ 6 V (bluish). In summary we assume that an
improved charge percolation pathways regarding to the higher
content of semiconducting molecules and an improved charge
transfer to the phosphorescent dopand in the case of the
copolymers without spacers are responsible for the better device
performance comparing the copolymers with hexyl spacers. The
approach of the directly connected charge transport materials at
the nonconjugated styrene polymer backbone should be favored
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for further investigations, therefore.
2009 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 48: 389–402, 2010
KEYWORDS: copolymerization; charge transport; electrolumines-
cence; organic light-emitting diode; semiconducting polymers
INTRODUCTION Since Burroughes et al.1 reported the first
discovery of a PLED in 1990, these materials became more
and more attractive because of cost efficiency and ease pro-
cessing by printing techniques from solution. Large area dis-
plays or plane lighting with light emitting polymers printed
via ink-jet2 or roll-to-roll3 processes are now conceivable. The
use of phosphorescent organometallic complexes of transition
metals and their chromophoric ligands show higher quantum
efficiencies compared to singlet emitting materials.4 Light
emissions of conjugated polymers like polyfluorene or polyvi-
nylenephenylene, generated by fluorescence, are restricted by
spin statistics to efficiencies of 25%. Some transition metal
complexes, first to name cyclometalated iridium-(III) com-
plexes, provide very short triplet lifetimes of excited states
that up to 100% efficiencies of these systems in adapted
matrices are possible.5 Several attempts have been applied to
establish triplets in polymeric, respectively soluble matrices
for example, poly(N-vinylcarbazole) (PVK) as a nonconjugated,
redox-active matrix.6,7 Yang et al.8,9 achieved high efficient and
bright PLEDs by blending the PVK-matrix with iridium-(III)
complexes, electron-, and additional hole-transporting mole-
cules. The wide bandgap of PVK (ca. 3.7 eV) allows different
emission colors without any interaction between the active
components and the polymer matrix. Furthermore, conjugated
polymers, especially polyfluorenes, were applied as host sys-
tems for triplet emitters.10–13 Efficient PLEDs were obtained
for the colors green and red, but not for blue because of inter-
actions with the polymer matrix emission.14 The bandgap of
these conjugated polymers is lower than 3 eV. To prevent
phase separation, Evans et al.15 reported the introduction of
spacer groups into a polyfluorene host matrix to the phos-
phorescent dopand. They used a red emitting iridium-(III)
complex with 2-(20benzo[b]thienyl)pyridinato-ligands (btp)
and tethered it at the 9-position of 9-octylfluorene with an
octyl-spacer moiety. With this approach they avoided a triplet
energy transfer from the red phosphorescent iridium complex
to the polyfluorene backbone and, thereby, achieved an aug-
mentation of the photoluminescence intensity. Graf et al.16
established an iridium-(III) complex with 2-phenylpyridinato
(ppy) chromophoric ligands and the saturation ligand acetyla-
cetonate connected to one hexyl moiety of 1,4-dibromo-2,5-
bis(hexyloxy)benzene. This is another example of a nonconju-
gated connection of phosphorescent materials into a conju-
gated polymer matrix system using spacer moieties. Den-
drimers were introduced to realize charge transport materials
with adequate film building properties.17–19 According to the
wide bandgap of dendronic hosts (>4 eV), it was possible to
cover the full visible range.
Correspondence to: H. Krueger (E-mail: hartmut.krueger@iap.fraunhofer.de)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 389–402 (2010) 2009 Wiley Periodicals, Inc.
PHOSPHORESCENT-EMITTING NONCONJUGATED PLED, THESEN ET AL.
389

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
An alternative strategy to eliminate phase separation in poly-
mer systems is to introduce a nonconjugated covalent link-
age between phosphorescent dopands and nonconjugated
polymer hosts. In such matrix systems the polymer backbone
does not take place in charge transport or emission because
of its extremely high bandgap of more than 4 eV. The usage
of this kind of linkage realizes an unlimited mixing of active
side groups and causes a stable morphology, since migration
and aggregation is suppressed very hard by fixing the active
molecules as side groups. Nonconjugated polymer matrices
are on principle much better soluble in common solvents,
compared to conjugated polymers, what alleviate their appli-
cation to solution processed PLEDs. Polystyrene is known to
be a relatively chemically inert and optical stable polymer
material. Therefore, Suzuki et al.20 reported a polymer ma-
trix system based on polystyrene with covalently fixed elec-
tron- and hole-transporting side functionalities. To avoid
phase separation and degradation the iridium complex was
also attached to the polystyrene backbone and for these opti-
mized systems external quantum efficiencies up to 12%
were observed.
scanning calorimetry (DSC) using a Netzsch DSC 204 with
10 K minꢀ1 scanning rate.
UV/vis spectra were recorded with a Perkin–Elmer Lambda
950 spectrometer at room temperature as dropcasted films
on silica glass substrates. Therefore, the polymer solution
from THF was dropcasted on silica glass substrates, to per-
form layer thicknesses between 400 and 500 nm after the
evaporation of the solvent at room temperature. Photolumi-
nescence spectra were measured using a Perkin–Elmer
LS50B spectrometer and the mentioned dropcasted films.
CV Measurements
Voltammograms were obtained using an EG&G Parc model
273 potentiostat, controlled by an IBM P70 computer. A
three electrode configuration was applied, contained in an
undivided cell consisting of a glassy carbon electrode (area
0.5 cm2) where the polymer film was deposited, a platinum
mesh as the counter electrode, and an Ag/AgCl (3 M NaCl
and sat. AgCl) reference electrode. 0.1 M Bu4NBF4 in acetoni-
trile was used as electrolyte and prior each measurement
the electochemical cell was deoxygenated with nitrogen. Fur-
thermore, the electrochemical cell was calibrated by the use
of a ferrocene standard and the ferrocene half-wave potential
was estimated to 435 mV for this assembly. 1 wt % polymer
solutions in chloroform were prepared and 5 lL were depos-
ited at a glassy carbon electrode. The prepared electrodes
were kept under vacuum and dried at 60 ꢁC for 2 h. For
measurements in solution a platinum disc was used as work-
ing electrode and the concentrations of organic compounds
EXPERIMENTAL
Materials and Methods
All materials were obtained from Aldrich Chemical Company
or Acros Organics and used without any further purification
unless otherwise stated. THF for polymerization reactions
was freshly distilled over sodium and stored under nitrogen.
Silica gel 60 (Merck) and C18 reversed phased silica gel
(Merck) was used in the separation and purification of com-
pounds by column chromatography. Solvents for column
chromatography, recrystallization and purification were
received from Th. Geyer GmbH and J. T. Baker. High-resolu-
tion (500 MHz) 1H NMR spectra were recorded using a
UNITY INOVA 500 spectrometer from Varian. Elemental ana-
lyzes were obtained using a Thermo Scientific FlashEA 1112
CHNS/O Automatic Elemental Analyser. For the iridium ana-
lyzes, 10 mg of each sample were solved into 1 mL of nitric
acid 65% (Merck) and 1 mL of perchloric acid 70%, and
subsequently radiated in a microwave oven PAAR Physica
Multiwave for 30 min. The sample was diluted with water to
15 mL and analyzed with a ICP-OES Optima 2100 DV using
iridium standards (Merck). Gel-permeation chromatography
in the electrolyte system were close to 10ꢀ3 mol Lꢀ1
.
Device Fabrication
The transparent indium-tin-oxide (ITO) covered glass slides
were received from OPTREX Europe GmbH (sheet resistance
20 Ohm/h) and were chemical wet cleaned in an ultra sonic
bath before use. The hole injecting layer was polyethylene-
dioxythiophene doped with polystyrenesulphonic acid
(CH8000, H.C. Starck) and dried at temperatures above 130
ꢁC for 5 min to remove the solvent residues, followed by a
spin-coating process of the emitting polymer solution from
chlorobenzene to receive polymer layer thicknesses of about
60 to 80 nm after another annealing step for 10 min at 110
ꢁC. In advance, the polymer solutions were solved before for
24 h at room temperature in chlorobenzene, using a shaker.
Subsequently, they were filtered through PTFE-syringe filters
with pore sizes of 0.2 lm. The cathodes (CsF 4 nm; Ca 15
nm; Ag 50 nm) were assembled by a thermal evaporation
process at pressures below 10ꢀ5 Pa. Four devices of each
polymer were build to guarantee uniformity. The complete
sample preparation and measurements were carried out in a
clean room under nitrogen atmosphere. Device characterisa-
tion: The current-voltage and the luminance-voltage charac-
teristics of the devices were measured simultaneously with a
computer controlled Keithley 236 Source-Measure-Unit in
combination with an Optometer Model GO352, equipped
with a calibrated sensor head for the luminance measure-
ments. The electroluminescence spectra were recorded in a
ꢁ
(GPC) at 25 C in THF was used to determine the molecular
weight. For this purpose a combination of a Waters HPLC-
Pump 515, Autosampler 717 plus, Dual l Absorbance Detec-
tor 2487 and a Refrative Index Detector 2414 was used. A
precolumn and three columns from Waters (7.8 mm ꢂ 300
mm; Styragel HR3, HR4, HR5), filled with a copolymer of sty-
rene and divinylbenzene 5 lm and PS-Standards from Poly-
mer Laboratories (Varian), were used. The polymer solution
(2 mg Lꢀ1 in THF) stirred for 24 h at room temperature and
filtered through a syringe PTFE filter 1 lm, before 2 ꢂ 100
lL of each polymer-solution were injected. Molecular
weights were calculated with the Empower software from
Waters. Thermal analysis was performed with differential
390
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ARTICLE
glove box system under nitrogen atmosphere with a diode-
array Spectrometer EPP 2000 from Stella Net.
organic phase was washed several times with water, dried
over Na2SO4, filtrated and concentrated. Column chromatog-
raphy with hexane/ethyl acetate 1:1 gave 5 as a slight yel-
low oil 4.94 g (81%).
Synthesis
4-Methyl-N-p-tolyl-N-(4-vinylphenyl)aniline (2)
1H NMR (500 MHz, CDCl3, d): 7.02 (m, 6H, ArH), 6.96 (m, 6H,
ArH), 3.64 (t, J ¼ 6.6 Hz, 2H, CH2OH), 2.54 (t, J ¼ 7.8 Hz, 2H,
ArCH2), 2.30 (s, 6H, CH3), 1.58 (m, 4H, alkyl chain), 1.39 (m,
4H, alkyl chain). Anal. calcd. for C26H31NO (Mw 373.53 g
molꢀ1): C 83.60, H 8.37, N 3.75; found: C 83.42, H 8.33, N 3.69.
Methyl triphenyl phosphonium bromide (1.48 g; 4.14 mmol)
was weighed into a round-bottom flask with septum. The
apparatus was evacuated and flushed with Ar three times.
15 mL of dry THF were added and the mixture was cooled
to 0 ꢁC. n-BuLi (1.6 M in hexane; 2.6 mL; 4.14 mmol)ꢁ was
injected slowly and the mixture stirred for 10 min at 0 C to
perform a slightly yellow liquid. A solution of N,N-bis(4-
methylphenyl)-aminobenzaldehyde 1 (0.50 g; 1.66 mmol) in
10 mL THF was injected drop wise and the reaction mixture
stirred at room temperature for 2 h. CH2Cl2 was added and
the solution washed several times with water, dried with
Na2SO4, filtered and concentrated. Column chromatography
with 10:1 hexane/ethyl acetate as eluant gave 0.42 g (85%)
2 as slight yellow solid.
Di-p-tolyl-{4-[6-(4-vinyl-benzyloxy)-hexyl]-phenyl}-amin
(6)
Small portions of Na (0.16 g; 6.68 mmol) were weighed into
a round-bottom flask with septum. The apparatus was evac-
uated and flushed with Ar three times. 5 (2.50 g; 6.68
mmol) in 20 mL THF was added and the mixture was stirred
for 16 h at reflux. 1-chloromethyl-4-vinyl-benzene (1.15 g;
7.53 mmol, technical, purified before with silica gel 60) was
injected and the mixture stirred another 16 h at reflux tem-
perature. The mixture was diluted with diethyl ether, washed
several times with water, dried with Na2SO4, filtered and
concentrated. Column chromatography over RP-18 KG with
acetone/water 10:1 gave 6 as slight yellow oil 2.0 g (61%).
1H NMR (500 MHz, CDCl3, d): 7.24 (d, J ¼ 8.6 Hz, 2H, ArH),
7.05 (d, J ¼ 8.5 Hz, 4H, ArH), 6.98 (d, 4H, ArH), 6.96 (d, 2H,
ArH), 6.63 (dd, J ¼ 10.8 Hz, 17.6 Hz, 1H, ArCHCH2), 5.60 (d,
1H, CHCH2), 5.11 (d, 1H, CHCH2), 2.30 (s, 6H, CH3). Anal.
calcd. for C22H21N (Mw 299.41 g molꢀ1): C 88.25, H 7.07, N
4.68; found: C 88.46, H 6.99, N 4.55.
1H NMR (500 MHz, CDCl3, d): 7.41 (d, J ¼ 8.0 Hz, 2H, ArH),
7.32 (d, 2H, ArH), 7.04 (m, 6H, ArH), 6.98 (m, 6H, ArH), 6.73
(dd, J ¼ 10.8 Hz, 17.6 Hz 1H, ArCHCH2), 5.76 (d, 1H,
CHCH2), 5.25 (d, 1H, CHCH2), 4.51 (s, 2H, ArCH2O), 3.48 (t, J
¼ 6.6 Hz, 2H, CH2O), 2.56 (t, J ¼ 7.7 Hz, 2H, ArCH2), 2.32 (s,
6H, CH3), 1.63 (m, 4H, alkyl chain), 1.40 (m, 4H, alkyl chain).
Anal. calcd. for C35H39NO (Mw 489.69 g molꢀ1): C 85.84, H
8.03, N 2.86; found: C 85.52, H 7.98, N 2.77.
2-(6-Bromohexyloxy)tetrahydro-2H-pyran (4)
6-bromohexane-1-ol (35 g; 0.19 mol) and some p-toluenesul-
phonic acid crystals were dissolved in 60 mL of dry toluene.
3,4-Dihydropyrane (30 mL, 0.33 mol), solved in 40 mL of
dry toluene were injected drop wise. The mixture was
heated to 100 ꢁC and stirred for 4 h. K2CO3 (4.7 g; 34.0
mmol) was added and the mixture allowed to stir at room
temperature over night. Afterwards, the liquid phase was
separated and concentrated. Column chromatography with
hexane/ethyl acetate 10:1 gave the pure product 4 35.62 g
(71%) as slight yellow oil.
2-[4-(40-Acetylbiphenylyl)-5-(4-tert-butylphenyl)]-
1,3,4-oxadiazole (8)
tert-BuPBD 7 (11.5 g; 32.4 mmol) was dissolved in 300 mL
of dry CH2Cl2. Freshly distilled acetyl chloride (5.8 mL; 82.0
mmol) and AlCl3 (32.0 g; 0.24 mol) were added. The mixture
was heated to reflux for 5 h. The cooled mixture was slowly
poured into 300 mL ice/water and 50 mL concentrated HCl.
The separated organic phase was washed three times with
water and dried with Na2SO4. After removal of the solvent,
the raw product was crystallized in EtOH/CHCl3 to obtain
10.0 g (78%) 8 as slight yellow crystals.
1H NMR (500 MHz, CDCl3, d): 8.25 (d, J ¼ 8.6 Hz, 2H,
biphenyl), 8.09 (d, J ¼ 8.6 Hz, 2H, ArH), 8.08 (d, 2H,
biphenyl), 7.80 (d, J ¼ 8.6 Hz, 2H, biphenyl), 7.76 (d, 2H,
ArH), 7.57 (d, 2H, biphenyl), 2.67 (s, 3H, COCH3), 1.38 (s, 9H,
CH3). Anal. calcd. for C26H24N2O2 (Mw 396.48 g molꢀ1): C
78.76, H 6.10, N 7.07; found: C 78.29, H 6.39, N 6.54.
1H NMR (500 MHz, CDCl3, d): 4.54 (m, 1H, OCHO), 3.83 (m,
1H, OCH2-pyrane), 3.70 (m, 1H, OCH2-alkyl), 3.47 (m, 1H,
OCH2-pyrane), 3.36 (t, 2H, CH2OH), 3.34 (m, 1H, OCH2-alkyl),
1.84 (m, 2H, CH2CH2OH), 1.80 (m, 1H, CHCH2), 1.67 (m, 1H,
CHCH2), 1.6...1.35 (m, alkyl chain). Anal. calcd. for
C11H21BrO2 (Mw 265.19 g molꢀ1): C 49.82, H 7.98; found: C
49.69, H 7.84.
6-(4-(Di-p-tolylamino)phenyl)hexan-1-ol (5)
3 (5.76 g; 16.4 mmol) was loaded into a round-bottom flask
with septum and the apparatus was evacuated and flushed
with Ar three times. 80 mL of dry THF and 80 mL of dry
hexane were added and the mixture was cooled to ꢀ78 ꢁC.
n-BuLi (1.6 M in Hexane; 10.25 mL, 16.4 mmol) wꢁas injected
slowly and the mixture stirred for 45 min at ꢀ78 C. 4 (4.77
g; 18.0 mmol) was added and the reaction mixture was
allowed to stir at room temperature for 16 h. Afterwards,
the solution was diluted with diethyl ether, washed several
times with water, dried over Na2SO4, filtrated and concen-
trated. The raw product was solubilized in 20 mL THF and
20 mL 2 N HCl and stirred at 50 ꢁC for 2 h. The separated
2-[4-(40-(1-Hydroxyethyl)biphenylyl)]-5-(4-tert-butyl-
phenyl)-1,3,4-oxadiazole (9)
8 (4.52 g; 11.4 mmol) and NaBH4 (1.02 g; 27.0 mmol) were
dissolved in 100 mL EtOH and stirred at room temperature
for 5 h. 2 N HCl was added until no more gasification could
be observed. The mixture was diluted with CHCl3, washed
three times with water and dried over Na2SO4. After removal
PHOSPHORESCENT-EMITTING NONCONJUGATED PLED, THESEN ET AL.
391

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
of the solvent, 4.40 g (97%) of 9 were obtained as slight yel-
3H, CH3). Anal. calcd. for C21H26O4 (Mw 342.43 g molꢀ1): C
low crystals.
73.66, H 7.65; found: C 73.98, H 7.41.
1H NMR (500 MHz, CDCl3, d): 8.19 (d, J ¼ 8.6 Hz, 2H,
biphenyl), 8.08 (d, J ¼ 8.6 Hz, 2H, ArH), 7.74 (d, 2H,
biphenyl), 7.64 (d, J ¼ 8.2 Hz, 2H, biphenyl), 7.56 (d, 2H,
ArH), 7.50 (d, 2H, biphenyl), 4.98 (q, J ¼ 6.5 Hz, 1H, CHOH),
1.55 (d, 3H, CH3), 1.38 (s, 9H, CH3). Anal. calcd. for
C26H26N2O2 (Mw 398.50 g molꢀ1): C 78.36, H 6.58, N 7.03, O
8.03; found: C 78.51, H 6.65, N 7.06, O 8.03.
40-(6-Hydroxyhexyloxy)biphenyl-4-carbohydrazide (14)
13 (2.00 g; 5.84 mmol) was dissolved into 50 mL of isopro-
ꢁ
panol and heated to 60 C. Hydrazine hydrate (8.5 mL; 0.17
mol) was added drop wise and the mixture was stirred at
reflux for 48 h. The precipitation was filtered and washed
with water. After crystallization from MeOH 1.59 g (83%) 14
were obtained.
1H NMR (500 MHz, Aceton-d6, d): 7.94 (d, J ¼ 8.3 Hz, 2H,
biphenyl), 7.70 (d, 2H, biphenyl), 7.66 (d, J ¼ 8.8 Hz, 2H,
biphenyl), 7.04 (d, 2H, biphenyl), 4.06 (t, J ¼ 6.5 Hz, 2H,
ArOCH2), 3.54 (t, J ¼6.5 Hz, 2H, CH2OH), 1.81 (m, 2H, alkyl
chain), 1.54 (m, 2H, CH2OH), 1.52 (m, 2H, alkyl chain), 1.46
(m, 2H, alkyl chain). Anal. calcd. for C19H24N2O3 (Mw 328.41
g molꢀ1): C 69.49, H 7.37, N 8.53; found: C 69.49, H 7.37, N
8.53.
2-[4-(40-Vinylbiphenylyl)]-5-(4-tert-butylphenyl)-
1,3,4-oxadiazole (10)
9 (4.38 g; 11.0 mmol) and p-toluenesulfonic acid mono
hydrate (0.22 g; 1.15 mmol) were dissolved in 150 mL tolu-
ene and the mixture was heated to reflux in a Dean-Stark ap-
paratus for 5 h to remove azeotropic toluene/water. The
mixture was concentrated and final column chromatography
with hexane/ethyl acetate 4:1 gave 2.18 g (52%) 10 as
white pellet crystals.
4-tert-Butylbenzoyl Chloride (15)
1H NMR (500 MHz, CD2Cl2, d): 8.20 (d, J ¼ 8.8 Hz, 2H,
biphenyl), 8.07 (d, J ¼ 8.8 Hz, 2H, ArH), 7.80 (d, 2H,
biphenyl), 7.66 (d, J ¼ 8.3 Hz, 2H, biphenyl), 7.59 (d, 2H,
ArH), 7.54 (d, 2H, biphenyl), 6.78 (dd, J ¼ 10.7 Hz, 17.6 Hz,
1H, CHCH2), 5.84 (d, 1H, CHCH2), 5.31 (d, 1H, CHCH2), 1.37
(s, 9H, CH3). Anal. calcd. for C26H24N2O (Mw 380.48 g
molꢀ1): C 82.07, H 6.36, N 7.36, O 4.21; found: C 82.32, H
6.38, N 7.34, O 4.40.
4-tert-Butylbenzoic acid (5.00 g; 28.1 mmol) was placed into
a round-bottom flask with septum, evacuated and flushed
three times with Ar before. Thionyl chloride (14.2 mL; 0.20
mol) was added and the mixture was stirred at 85 ꢁC over
night. Vacuum distillation leads to 3.96 g (72%) of the pure
product 15. Anal. calcd. for C11H13ClO (Mw 196.67 g molꢀ1):
C 67.18, H 6.66; found: C 67.11, H 6.69.
N0-(4-tert-Butylbenzoyl)-40-(6-hydroxyhexyloxy)biphenyl-4-
carbohydrazide (16)
Ethyl 40-hydroxybiphenyl-4-carboxylate (12)
11 (10.00 g; 46.7 mmol) was diluted in 170 mL EtOH and 5
mL H2SO4. The mixture was stirred for 5 h at 100 ꢁC. The
reaction mixture was dropped into saturated NaHCO3 (aq.)
and the precipitation was filtered and washed with water.
After recrystallization from hexane, 9.66 g (85%) of 12
were obtained.
14 (1.10 g; 3.35 mmol) was dissolved into 17 mL of pyridine
and a solution of 15 (0.66 g; 3.35 mmol) in 7 mL of THF
was added drop wise under stirring. The reaction mixture
was stirred for 54 h at room temperature, poured into 130
mL of 5 N HCl and the white precipitate was filtered and
washed with water. After recrystallization from EtOH, 1.25 g
(76%) of 16 were obtained.
1H NMR (500 MHz, CDCl3, d): 8.09 (d, J ¼ 8.4 Hz, 2H,
biphenyl), 7.61 (d, 2H, biphenyl), 7.53 (d, J ¼ 8.6 Hz, 2H,
biphenyl), 6.94 (d, 2H, biphenyl), 5.14 (s, 1H, OH), 4.40 (q, J
¼ 7.1 Hz, 2H, OCH2), 1.42 (t, 3H, CH3). Anal. calcd. for
1H NMR (500 MHz, DMSO-d6, d): 10.52 (s, 1H, NH), 10.45 (s,
1H, OH), 7.99 (d, J ¼ 8.3 Hz, 2H, biphenyl), 7.88 (d, J ¼ 8.6
Hz, 2H, ArH), 7.78 (d, 2H, biphenyl), 7.70 (d, J ¼ 8.8 Hz, 2H,
biphenyl), 7.54 (d, 2H, ArH), 7.05 (d, 2H, biphenyl), 4.02 (t,
2H, CH2OAr), 3.40 (m, 2H, CH2OH), 1.74 (m, 2H, alkyl chain),
1.45 (m, 4H, alkyl chain), 1.36 (m, 2H, alkyl chain), 1.32 (s,
9H, CH3). Anal. calcd. for C30H36N2O4 (Mw 488.62 g molꢀ1):
C 73.74, H 7.43, N 5.73; found: C 73.40, H 7.68, N 5.78.
C
15H14O3 (Mw 242.27 g molꢀ1): C 74.36, H 5.82; found: C
74.21, H 5.88.
Ethyl 40-(6-hydroxyhexyloxy)biphenyl-4-carboxylate (13)
12 (9.42 g; 38.9 mmol) was dissolved into 150 mL of dry
DMF. K2CO3 (7.24 g; 52.4 mmol) and some crystals of KJ
were added and the mixture was stirred under nitrogen
atmosphere at reflux. After the injection of 6-Bromo-hexa-
nole (11.83 g; 65.3 mmol) the mixture was stirred for 3 h at
reflux. The reaction mixture was poured into water and the
precipitation filtered. After recrystallization from EtOH, 8.55
g (64%) of 13 were obtained.
1H NMR (500 MHz, CDCl3, d): 8.07 (d, J ¼ 8.5, 2H, biphenyl),
7.61 (d, 2H, biphenyl), 7.56 (d, J ¼ 8.8 Hz, 2H, biphenyl),
6.98 (d, 2H, biphenyl), 4.39 (q, J ¼ 7.1 Hz, 2H, OCH2CH3),
4.01 (t, J ¼ 6.6 Hz, 2H, ArOCH2), 3.67 (t, J ¼ 6.6 Hz, 2H,
CH2OAr), 1.83 (m, 2H, alkyl chain), 1.62 (m, 2H, CH2OH),
1.54 (m, 2H, alkyl chain), 1.45 (m, 2H, alkyl chain), 1.41 (t,
6-(40-(5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl)biphenyl-
4-yloxy)hexyl Acetate (17)
16 (1.60 g; 3.27 mmol), acetic acid anhydride (12.7 mL;
0.13 mol) and 3 drops of H2SO4 conc. were stirred for 2 h at
100 ꢁC. The mixture was precipitated into ice/water and
extracted with CHCl3. The organic phase was separated,
washed several times with water, dried over Na2SO4, filtrated
and concentrated. Column chromatography with hexane/
ethyl acetate 1:1 gave the pure product (0.43 g; 25%) as
white crystals.
1H NMR (500 MHz, CDCl3, d): 8.18 (d, J ¼ 8.3 Hz, 2H,
biphenyl), 8.08 (d, J ¼ 8.4 Hz, 2H, ArH), 7.72 (d, 2H,
392
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ARTICLE
biphenyl), 7.60 (d, J ¼ 8.7 Hz, 2H, biphenyl), 7.56 (d, 2H,
ArH), 7.01 (d, 2H, biphenyl), 4.09 (t, J ¼ 6.6 Hz, 2H,
CH2OCO), 4.02 (t, J ¼ 6.4 Hz, 2H, CH2OAr), 2.06 (s, 3H, CH3),
1.84 (m, 2H, alkyl chain), 1.68 (m, 2H, alkyl chain), 1.54 (m,
2H, alkyl chain), 1.47 (m, 2H, alkyl chain), 1.37 (s, 9H, CH3).
Anal. calcd. for C32H36N2O4 (Mw 512.64 g molꢀ1): C 74.97, H
7.08, N 5.46; found: C 74.85, H 7.06, N 5.42.
ture was stirred at 60 ꢁC over night. The reaction mixture
was allowed to cool to room temperature and poured into
200 mL of water. The precipitate was filtered, solved in
CHCl3, washed twice with water, dried over Na2SO4, filtrated
and concentrated. Column chromatography with CHCl3 gave
the pure product (0.92 g, 78%) as white crystals.
1H NMR (500 MHz, CDCl3, d): 8.64 (m, 1H, pyridyl), 7.88 (m,
1H, benzthienyl), 7.84 (s, 1H, benzthienyl), 7.81 (m, 2H, pyri-
dyl), 7.73 (m, 1H, benzthienyl), 7.36 (m, 2H, benzthienyl),
7.21 (m, 1H, pyridyl). Anal. calcd. for C13H9NS (Mw 211.28 g
molꢀ1): C 73.90, H 4.29, N 6.63, S 15.18; found: C 74.06, H
4.83, N 6.40, S 14.46.
6-(40-(5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl)biphenyl-
4-yloxy)hexan-1-ol (18)
17 (0.50 g; 0.97 mmol) was dissolved into 8 mL of THF and
5.0 mL of 1 N NaOH (aq.) and stirred for 4 h at reflux. The
mixture was diluted with toluene and washed several times
with water, dried over Na2SO4, filtered and concentrated.
0.29 g (63%) of 18 were obtained.
2-(2,4-difluorophenyl)pyridine (F2ppy)
Using the same procedure like btp, 2-bromopyridine (1.00 g;
6.34 mmol) and 2,4-difluorophenyl boronic acid (1.10 g;
6.97 mmol) lead to the pure product F2ppy (2.93 g, 80%)
as white crystals.
1H NMR (500 MHz, CDCl3, d): 8.18 (d, J ¼ 8.5 Hz, 2H,
biphenyl), 8.08 (d, J ¼ 8.5 Hz, 2H, ArH), 7.72 (d, 2H,
biphenyl), 7.59 (d, J ¼ 8.8 Hz, 2H, biphenyl), 7.55 (d, 2H,
ArH), 7.00 (d, 2H, biphenyl), 4.03 (t, J ¼ 6.5 Hz, 2H, CH2OAr),
3.68 (t, J ¼ 6.5 Hz, 2H, CH2OH), 1.84 (m, 2H, alkyl chain),
1.63 (m, 2H, alkyl chain), 1.53 (m, 2H, alkyl chain), 1.47 (m,
2H, alkyl chain), 1.38 (s, 9H, CH3). Anal. calcd. for
1H NMR (500 MHz, CDCl3, d): 8.71 (m, 1H, pyridyl), 8.00 (m,
1H, Ar), 7.74 (m, 2H, pyridyl), 7.24 (m, 1H, pyridyl), 7.00 (m,
1H, Ar), 6.91 (m, 1H, Ar). Anal. calcd. for C11H7F2N (Mw
191.18 g molꢀ1): C 69.11, H 3.69, N 7.33; found: C 69.00, H
3.66, N 7.31.
C
30H34N2O3 (Mw 470.60 g molꢀ1): C 76.57, H 7.28, N 5.95;
found: C 76.72, H 7.22, N 5.98.
6-(4-Vinylphenyl)-hexan-2,4-dion (ket)
2-(4-tert-Butylphenyl)-5-(40-(6-(4-vinylbenzyloxy)hexyloxy)
biphenyl-4-yl)-1,3,4-oxadiazole (19)
In a round-bottom flask with septum sodium hydride (1.97
g; 82.1 mmol) was weighed and the apparatus was evac-
uated and flushed with Ar three times. 100 mL THF abs. and
hexamethylphosphamide (1.6 mL; 9.17 mmol) were added
and cooled to 0 ꢁC. Acetyl acetonate (4.00 g; 40.0 mmol)
was injected while foaming and a white precipitation could
18 (1.10 g; 2.34 mmol) and Na (54 mg; 2.34 mmol) were
placed into a round-bottom flask with septum. The appara-
tus was evacuated and flushed with Ar several times. 3 mL
of THF abs. were added and the mixture was stirred at 60
ꢁC for 18 h until the Na was solved and a white precipitation
was formed. A solution of 1-(Chloromethyl)-4-vinylbenzene
(1.16 g; 7.60 mmol, technical, purified before with silica gel
60) in 3 mL of THF was injected drop wise. The solution
was stirred for 2 h at 60 ꢁC, diluted with diethyl ether,
washed with water, dried over Na2SO4, filtered and concen-
trated. After crystallization from EtOH 0.34 g (25%) of 19
were obtained.
ꢁ
be observed. The mixture stirred for 20 min at 0 C. n-BuLi
(1.6 M in hexane, 28 mL; 44.8 mmol) was slowly added and
the mixture stirred for another 20 min. 1-(Chloromethyl)-4-
vinylbenzene (6.41 g; 42.0 mmol, technical, purified before
with silica gel 60) was injected drop wise and the mixture
was allowed to stir 20 min at RT. To get rid of spare sodium
hydride, 2 N HCl was added slowly until no further gasifica-
tion could be observed. The reaction mixture was diluted
with diethyl ether and washed several times with water,
dried with Na2SO4, filtered and concentrated. Column chro-
matography with hexane/CH2Cl2 1:1 lead to the pure prod-
uct (6.80 g; 79%) as colorless oil.
1H NMR (500 MHz, CDCl3, d): 8.18 (d, J ¼ 8.5 Hz, 2H,
biphenyl), 8.09 (d, J ¼ 8.8 Hz, 2H, ArH), 7.72 (d, 2H,
biphenyl), 7.59 (d, J ¼ 8.8 Hz, 2H, biphenyl), 7.57 (d, 2H,
ArH), 7.39 (d, J ¼ 8.2 Hz, 2H, ArH), 7.30 (d, 2H, ArH), 7.00
(d, 2H, biphenyl), 6.71 (dd, J ¼ 11.0 Hz, 17.6 Hz, 1H,
CHCH2), 5.74 (d, 1H, CHCH2), 5.23 (d, 1H, CHCH2), 4.50 (s,
2H, ArCH2O), 4.02 (t, J ¼ 6.6 Hz, 2H, CH2OAr), 3.49 (t, J ¼
6.6 Hz, 2H, ArCH2OCH2), 1.83 (m, 2H, alkyl chain), 1.67 (m,
2H, alkyl chain), 1.49 (m, 4H, alkyl chain), 1.38 (s, 9H, CH3).
Anal. calcd. for C39H42N2O3 (Mw 586.76 g molꢀ1): C 79.83, H
7.21, N 4.77; found: C 79.69, H 7.20, N 4.71.
1H NMR (500 MHz, CDCl3, d): keto (15%): 7.32 (d, J ¼ 8.1,
2H, ArH), 7.14 (d, 2H, ArH), 6.68 (dd, J ¼ 10.7 Hz, 17.6 Hz,
1H, CHCH2), 5.71 (d, 1H, CHCH2), 5.20 (d, 1H, CHCH2), 3.54
(s, 2H, COCH2CO), 2.89 (t, 2H, ArCH2), 2.82 (t, 2H, COCH2),
2.19 (s, 3H, CH3). enol (85%): 7.33 (d, J ¼ 8.1 Hz, 2H, ArH),
7.15 (d, 2H, ArH), 6.68 (dd, J ¼ 10.7 Hz, 17.6 Hz, 1H,
CHCH2), 5.71 (d, 1H, CHCH2), 5.47 (s, 1H, COHCHCO), 5.20
(d, 1H, CHCH2), 2.92 (t, 2H, CH2Ar), 2.58 (t, 2H, COCH2), 2.03
(s, 3H, CH3). Anal. calcd. for C14H16O2 (Mw 216.28 g molꢀ1):
requires C 77.75, H 7.46; found: C 78.04, H 7.63.
2-(benzo[b]thiophen-2-yl)pyridine (btp)
In a round-bottom flask with septum 2-bromopyridine (1.00
g; 5.60 mmol) and Tetrakis(triphenylphosphine)palladium
(0) (1.50 ꢂ 10ꢀ3 g; 0.013 mmol) were weighed and the ap-
paratus was evacuated and flushed with Ar three times. 30
mL of dry THF, thianaphthen-2-boronic acid (0.98 g; 6.18
mmol) and 20 mL 1 N Na2CO3 (aq.) were added. The mix-
[{Ir(l-Cl)(L)2}2]
L ¼ 2-phenylpyridine (ppy); 2-benzo[b]thiophen-2-yl-pyri-
dine (btp); 2-(2,4-difluorophenyl)-pyridine (F2ppy). [{Ir(l-
Cl)(coe)2}2] (0.55 mmol) and the corresponding ligand L
PHOSPHORESCENT-EMITTING NONCONJUGATED PLED, THESEN ET AL.
393

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
(2.20 mmol) were dissolved in 30 mL of toluene under
nitrogen atmosphere and refluxed with stirring for 1h. After
cooling to room temperature, the precipitate was filtered off,
washed twice with 10 mL portions of toluene and dried in
vacuum.
Polymerization Conditions
All polymers described were synthesized by means of a free
radical process. The monomer concentration of each reaction
was 100 g Lꢀ1 in freshly distilled THF and 2 mol % N,N-azo-
bisisobutyronitrile (AIBN) was chosen as initiator. ꢁAll reac-
tions were carried out in a glove box system at 50 C for 72
h. The resulting solutions were allowed to cool to room tem-
perature and purified by repeated precipitation into 120 mL
of MeOH/diethyl ether 2:1. The polymers were solved again
in THF and filtered through a syringe PTFE filter 0.2 lm.
The solvent was removed in vacuum and the solutions pre-
cipitated again into 120 mL MeOH/diethyl ether 2:1. The
residues were filtered by the use of PTFE filters pore size
1[{Ir(l-Cl)(ppy)2}2]. Yield
92%,
Anal.
calcd.
for
C44H32Ir2N4Cl2 (Mw 1072.11 g molꢀ1): C 49.29, H 3.01, N
5.23; found: C 49.31, H 3.05, N 5.19.
2[{Ir(l-Cl)(btp)2}2]. Yield 84%, Anal. calcd. for C52H36Ir2N4
S4Cl2 (Mw 1300.47 g molꢀ1): C 48.03, H 2.79, N 4.31; found:
C 48.08, H 2.73, N 4.29.
3[{Ir(l-Cl)(F2ppy)2}2]. Yield 74%, Anal. calcd. for C44H28
F8Ir2N4Cl2 (Mw 1220.07 g molꢀ1): C 43.32, H 2.31, N 4.59;
found: C 43.35, H 2.30, N 4.54.
ꢁ
0.45 lm and dried for 30 h at 80 C in vacuum.
RESULTS AND DISCUSSION
In this article, we report on influences of alkyl spacer
lengths between a polystyrene backbone and electron-trans-
porting respectively hole-transporting units attached with
and without a hexyl-spacer at the "para" position of each sty-
rene monomer. The kind of linkage of active side groups to
the inert polystyrene backbone, especially the spacer length
between the side group and the polystyrene main chain,
seems to be important for the material’s electronic proper-
ties. To verify this influence, the active side groups were con-
nected via hexyl spacers on one hand, and without these
spacers directly to the polymeric backbone on the other
hand. In the following, the syntheses of functionalized sty-
rene monomers and of the resulting different copolymers
are presented. One layer PLEDs with very simple hole- and
electron-transporting molecules were utilized to study the
emission properties of these materials. As hole-transporting
material N,N-Di-p-tolyl-aniline and as electron-transporting
[Ir(L)2(ket)]
L ¼ 2-phenylpyridine (ppy); 2-benzo[b]thiophen-2-yl-pyri-
dine (btp); 2-(2,4-difluorphenyl)-pyridine (F2ppy); ketone ¼
6-(4-vinyl-phenyl)-hexane-2,4-dione. [{Ir(l-Cl)(L)2}2] (0.47
mmol) and silvertrifluoroacetate (208 mg, 0.94 mmol) were
dissolved under nitrogen atmosphere in 40 mL of acetone
and refluxed with stirring for 1 h. After cooling to room tem-
perature, the precipitated AgCl was filtered off. The ketone
(1.00 mmol) and 0.5 mL of NEt3 were added and the result-
ing solution was stirred overnight at room temperature
whereupon the solvent was evaporated under reduced pres-
sure. The remaining residue was dissolved in 10 mL of
CH2Cl2 and purified by column chromatography on alumina
with CH2Cl2 as the eluant. The solvent was reduced to about
3 mL and the compound precipitated by adding hexane (20
mL). The precipitate was filtered off, washed twice with 10
mL portions of hexane, and dried in vacuum.
unit
2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
TEG (Yield: 78%). 1H NMR (400 MHz, CDCl3, d): 8.47 (d, J
¼ 1Hz, 6 Hz, 1H), 8.21 (d, J ¼ 1Hz, 6 Hz, 1H), 7.80 (t, J ¼ 6
Hz, 2H), 7.66 (m, 2H), 7.53 (t, J ¼ 8 Hz, 2H), 7.17 (d, J ¼ 8
Hz, 2H), 7.09 (m, 1H), 6.94 (m, 1H), 6.89 (d, J ¼ 8 Hz, 2H),
6.80 (m, 2H), 6.69 (m, 3H), 6.28 (dd, J ¼ 8 Hz, 12 Hz, 2H),
5.67, (dd, J ¼ 1 Hz, 18 Hz, 1H), 5.19 (d, J ¼ 10 Hz, 1H), 5.17
(s, 1 Hz). Anal. calcd. for C36H30IrN2O2 (Mw 714.87 g molꢀ1):
C 60.49, H 4.23, N 3.92; found: C 60.25, H 4.31, N 3.89.
(tert-BuPBD) were established.
Synthesis
Monomer Syntheses
Hole-Transporting Monomers. A convenient way to intro-
duce a vinyl group into very basic triarylamines is to utilize
N,N-Bis(4-methylphenyl)-aminobenzaldehyde
1
as educt.
Wittig-reaction with in situ prepared phosphonium ylide of
methyl triphenyl phosphonium bromide and butyl lithium
lead directly to the vinyl functionalized hole-transporting
monomer 2 (Scheme 1). For the introduction of a hexyl-
spacer group, 4-Bromo-N,N-di-p-tolylaniline 3 was selected
TER (Yield: 58%). 1H NMR (270 MHz, Aceton-d6, d): 8.50 (m,
1H), 8.16 (m, 1H), 7.98 (m, 2H), 7.72 (m, 4H), 7.24 (m, 1H),
7.08 (m, 5H), 6.83 (m, 4H), 6.68 (dd, J ¼ 10 Hz, 18 Hz, 1H),
6.25 (m, 2H), 5.71 (dd, J ¼ 1 Hz, 18 Hz, 1H), 5.36 (s, 1H), 5.17
(dd, J ¼ 1 Hz, 11 Hz, 1H), 2.58 (m, 2H), 2.38 (m, 2H), 1.70 (s,
3H). Anal. calcd. for C40H30IrN2O2 (Mw 827.03 g molꢀ1): C
58.09, H 3.66, N 3.39; found: C 57.98, H 3.55, N 3.28.
TEB (Yield: 68%). 1H NMR (400 MHz, Aceton-d6, d): 8.50
(m, 1H), 8.23 (m, 3H), 8.01 (m, 2H), 7.42 (m, 1H), 7.22 (d, J
¼ 8 Hz, 3H), 6.88 (d, J ¼ 8 Hz, 2H), 6.68 (dd, J ¼ 6.7 Hz, 18
Hz, 1H), 6.46 (m, 2H), 5.72 (dd, J ¼ 1 Hz, 18 Hz, 1H), 5.65
(dd, J ¼ 4Hz, 8 Hz, 2H), 5.32 (s, 1H), 5.16 (dd, J ¼ 1 Hz, 11
Hz, 1H), 2.62 (m, 2H), 2.45 (m, 1H), 2.32 (m, 1H), 1.69 (s,
3H). Anal. calcd. for C36H26IrF4N2O2 (Mw 786.83 g molꢀ1): C
54.95, H 3.33, N 3.56; found: C 54.79, H 3.37, N 3.60.
SCHEME 1 Synthesis of the hole-transporting monomer 2 with-
out spacer.
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ARTICLE
SCHEME 2 Reaction scheme for the hole-transporting monomer 6 with hexyl spacer.
as starting material (Scheme 2). Dehalogenation with n-Butyl
lithium, followed by the addition of pyrane protected 6-Bro-
mohexanol 4, lead to an intermediate, unprotected easy by
the use of 2 N hydrochloric acid to generate the free alcohol
5. The last step could be done by a Williamson ether synthe-
sis of 1-chloromethyl-4-vinyl-benzene with the in situ pre-
pared sodium-alcoholate of 5 to obtain the vinyl functional-
ized monomer 6.
Emitting Materials. To introduce polymerisable functional
groups into iridium-(III) complexes with chromophoric
ligands, it was necessary to vinylise the saturation ligand,
which reacts with the dichloro diiridium intermediate in the
last step of each reaction sequence. An acetylacetonate deriv-
ative with a styrene moiety was used. The triplet emitting
complex was achieved for green (TEG), red-orange (TER),
and bluish-green (TEB) with color endowing ligands ppy for
TEG, btp for TER and 2-(2,4-difluorophenyl)pyridine
(F2ppy) for TEB (Fig. 1).
Electron-Transporting Monomers. The synthesis of vinyl
functionalized tert-BuPBD is shown in Scheme 3, and was
done in accordance to Boiteau et al.21 The introduction of the
hexyl-spacer group into tert-BuPBD turned out to be a little
more difficult compared to the hole-transporting material.
Direct insertion of the spacer moiety into tert-BuPBD was dis-
covered during the first attempts to be unpractical. Therefore,
the final seven step reaction scheme, shown in Scheme 4, was
established. Starting material is 40-Hydroxybiphenyl-4-carbox-
ylic acid 11 and the first step was a sulphuric acid catalyzed
esterification with ethanol. The protected ethyl ester com-
pound 12 was functionalized with the C6-spacer group, using
6-Bromohexanol as reagent. In a further step, hydrazine
hydrate leads to the carbonyl hydrazide derivative 14 in a
good yield. By a following amidation with 4-tert-Butylbenzoyl
chloride 15 the dicarbonyl hydrazide derivative 16 was
obtained. Final dehydration cyclization leads to the 1,3,4-oxa-
diazole derivative 17. After an alkaline saponification of the
ethyl ester and final Williamson ether synthesis of the alcohol
with sodium and 1-(Chloromethyl)-4-vinylbenzene, the mono-
mer 19 with a styrene-functionality was obtained.
Polymerization
Combining the synthesized styrene monomers seven copoly-
mers 20–26 were obtained with hexyl spacer, without
spacer, and in different emitting colors (Fig. 2). Polymer 20
consists of both spacer functionalized transporting mono-
mers 6 and 19. Polymer 21, 22, and 23 consisted of the un-
tethered electron-transporting material 9 and the hole-trans-
porting material 6 with spacer unit, whereas polymers 24,
25, and 26 only exhibited the two un-tethered monomers 2
and 9. Free radical polymerisations of the styrene monomer
mixtures were carried out using azobisisobutyronitrile
(AIBN) as initiator and tetrahydrofuran as solvent. All syn-
thesized polymers were obtained in acceptable yields of
ꢃ70%. The ratio of transporting monomers was selected
with 2:1 electron to hole transporting monomer in each po-
lymerization reaction. This ratio was discovered to perform
the best results in PLEDs by preliminary experiments using
blend systems of active molecules in a polystyrene matrix.
The ratio of the hole- respectively electron-transporting
SCHEME 3 Synthesis of the electron-transporting monomer 9 without spacer.
PHOSPHORESCENT-EMITTING NONCONJUGATED PLED, THESEN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 4 Reaction scheme for the electron-transporting monomer 19 with a hexyl spacer.
component was estimated with 1H NMR spectroscopy. As
listed in Table 1, the selected monomer ratio could only be
obtained in copolymers consisting of structural equal mono-
mer types. For polymer 20, with two of the monomers teth-
ered to the polystyrene backbone via hexyl spacers, the ratio
could be achieved as selected before. For polymers 22 and
23, with hexyl-spacer connected hole-transporting mono-
mers, it was shown that the monomer with spacer function-
ality is enriched. Finally, polymers 25 and 26 featured again
the selected ratio of the un-tethered monomers. Hence it
could be concluded that monomers carrying a hexyl spacer
exhibited higher polymerization rates compared to those
without. This may be caused by different solubilities gener-
ated mainly by the hexyl spacer moieties.
mers 20–23). With two un-tethered monomers, see copoly-
mers 24–26, the content of the phosphorescent dopand
decreased to ꢃ70% of the preselected amount. The number-
average molecular weights of all polymers of ꢃ25 kg molꢀ1
determined by size exclusion chromatography were sufficient
to perform good film-building properties. Polydispersities of
about 2, in the case when hexyl spacers were present for
copolymers 20–23, are quite normal for free radical poly-
merisations. Copolymers 24–26, without any spacer, exhib-
ited a little higher value of about 2.5, what may be caused
by a lower solubility of these materials. Furthermore, it
could be expected that glass transition temperatures depend
on the introduced spacer length. In Table 1 the limitation of
Tg to 78 ꢁC is listed for polymer 20 with the two tethered
monomers. In the case when only one charge transport
The triplet emitting monomer ratio was selected with 7.4 wt
% with respect to the transporting monomer mixture. The
molar ratio of the iridium complex in all synthesized copoly-
mers could not be identified by NMR spectroscopy because
of its small amount and because of very broad polymer sig-
nals overlapping the emitter signals. Therefore, an acidic
pulping of the polymer in concentrated nitric acid and con-
centrated perchloric acid under microwave irradiation had to
be done, before the Ir-content could be measured on ICP-
OES. In Table 1 it is noticed that the iridium content varies
of about 30% in the obtained polymers to the preselected
amount. It could be concluded that the content of the irid-
ium complex is about 90% of the selected monomer ratio in
copolymers, whenever a tethered monomer is present (poly-
FIGURE 1 Structure of the different styrene-functionalized Ir-
complexes TEG, TER, and TEB used as emitting materials.
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ARTICLE
FIGURE 2 Chemical structure of the investigated copolymers 20–26.
monomer carries a hexyl spacer, an increasing of glass tran-
sition temperatures to about 125–140 C in polymers 21–23
around 307 nm could be mainly related to the transporting
polymer matrix, as well. 1MLCT and 3MLCT bands were, in
accordance with the literature, observed in the range of
400–500 nm.25 For photoluminescence spectra (full symbols)
plotted in Figure 3(b) an excitation wavelength of 495 nm
was selected. The emission maxima of the red emitting poly-
mer systems were ascertained at about 620 nm which could
be assigned to electronic origin transitions broadened inho-
mogeneously and electron-phonon coupling. Furthermore
they exhibited an additional shoulder at around 665 nm and
above which depicts overlapping vibrational satellites, while
the Ir(btp)2acac emitter itself shows an emission maximum
at 612 nm in 2-methyltetrahydrofuran solution.26
ꢁ
could be observed. Polymers 24–26 showed enhanced glass
transition temperatures of nearly 200 ꢁC. This drastically
increase of the glass transition temperature can be explained
by the bulky side groups, directly linked to the polystyrene
backbone. They involved an extension of the polystyrene main
chain and thereby a decrease of theirꢁmovability, compared to
un-substituted polystyrene (Tg ¼ 105 C).
UV/Vis- and Photoluminescence Spectroscopy
UV/Vis spectra of the three green emitting polymer films of
20, 21, and 24 are plotted in Figure 3(a). It was shown that
absorbance maxima of all polymers (open symbols) were in
1
the range of about 307 nm, which indicated spin-allowed p-
UV/Vis and photoluminescence behavior of polymers 23 and
26 are shown in Figure 3(c) and summarized in Table 2.
Maxima of absorbance (open symbols) were with 307 nm in
the same range as the absorbance maxima of the green
respectively red emitting polymers and could be mainly
regarded to the polymeric matrix system. The excitation
wavelength for photoluminescence spectra (full symbols)
was 307 nm for both polymer films. The un-tethered blue
emitting polymer 26 showed an emission maximum at 493
nm. Especially remarkable is the 10 nm bathochromic shift
between the maxima of the tethered polymer 23 and the un-
tethered polymer 26 but the origin of this phenomenon is
unknown recently. A slight shoulder at 457 nm was observed
for both polymers and showed an increased emission in the
un-tethered case. Compared with literature,27 the maximum
of absorbance of the Ir(F2ppy)2acac triplet emitter itself
p* interligand transitions of the emitter between 200 and
340 nm on the one hand and the absorbance maximum of
the polymer matrix with tert-BuPBD and N,N-Di-p-tolyl-ani-
line at 307 nm22 on the other hand. Whereas, the spin-
allowed metal to ligand charge transfer (1MLCT) could be
3
ascribed between 350 and 450 nm. The spin-forbidden p-p*
and 3MLCT could be anticipated from 450 to 550 nm.23 An
excitation wavelength of 465 nm was selected for photolumi-
nescence spectra, shown in Figure 3(a) (full symbols). The
emission maxima were ascertained at about 530 nm for all
green emitting polymers, while the Ir(ppy)2acac triplet emit-
ter itself shows an emission maximum at 516 nm in 2-meth-
yltetrahydrofuran solution.24
Figure 3(b) shows the UV/vis behavior of the red triplet
emitting polymer films of 22 and 25. Absorbance maxima at
PHOSPHORESCENT-EMITTING NONCONJUGATED PLED, THESEN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Overview on Selected Monomer Ratios, Obtained Copolymer Composition, Molecular Weight and Glass Transition
Temperatures (Tg) of the Terpolymers 20–26
Copolymer
20
66
21
69
22
67
23
74
24
78
25
73
26
71
Approximate
yield in %
Elemental
C
79.04 (80.03) 80.59 (81.83) 80.63 (81.68) 80.32 (81.44) 81.42 (82.23) 81.58 (81.94) 81.79 (81.75)
analysis in %
(expected from
monomer ratio)
H
N
Ir
7.08 (7.23)
4.18 (4.19)
1.69 (1.99)
6.68 (6.82)
5.68 (5.47)
1.76 (1.99)
6.78 (6.73)
5.51 (5.54)
1.60 (1.73)
6.73 (6.70)
5.44 (5.45)
1.63 (1.82)
6.32 (6.41)
6.50 (6.42)
1.22 (1.98)
6.18 (6.32)
6.48 (6.37)
1.20 (1.72)
6.20 (6.28)
6.49 (6.39)
1.24 (1.78)
Ir analysis in %
(expected from
monomer ratio)
Selected molar
monomer ratio
x
y
z
0.628
0.314
0.058
0.63
0.637
0.319
0.044
0.62
0.640
0.321
0.039
0.56
0.639
0.320
0.041
0.54
0.642
0.321
0.037
0.72
0.645
0.322
0.033
0.67
0.644
0.322
0.034
0.67
Molar copolymer
composition
x
y
z
0.37
0.38
0.44
0.46
0.28
0.33
0.33
(x, y calcd. from
1H NMR integrals)
(z calcd. from
Ir analysis)
0.049
0.043
0.036
0.036
0.023
0.023
0.021
GPC in 103 g molꢀ1
Mn 22.9
Mw 40.9
PDI 1.79
27.2
57.6
2.12
140
29.6
60.7
2.05
137
26.8
55.5
2.07
125
22.3
61.3
2.75
194
33.9
74.7
2.50
196
28.2
67.8
2.41
197
DSC in ꢁC
Tg
78
should be at 476 nm, measured in chloroform solution.
There is an overall adjustment of more than 25 nm observ-
able for both polymer systems, while the adjustment is maxi-
mized when a tethered hole transporting monomer is used.
side moieties are the main charge transport mechanism, in
contrast to conjugated polymers, where transport along the
polymer backbone is a basic mechanism. In our case, organic
p-conjugation of the attached charge transport materials pro-
vides redox-isolated sites and/or charge hopping sites for
electrons and permits intermolecular electron transfer pro-
cesses, therefore. Cyclic voltammetry (CV) is one suitable pos-
sibility to investigate electronic properties in the solid state
Cyclovoltammetric Investigations
For nonconjugated polymer backbones carrying the active
molecules as side-chains, hopping processes between these
FIGURE 3 UV/Vis- and photoluminescence spectra (a) of the green emitting polymers 20, 21, and 24 (with TEG) as solid films
(open symbols absorbance, full symbols photoluminescence). (b) Of the red emitting polymers 22 and 25 (with TER) as solid
films (open symbols absorbance, full symbols photoluminescence). (c) Of the blue emitting polymers 23 and 26 (with TEB) as
solid films (open symbols absorbance, full symbols photoluminescence).
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ARTICLE
TABLE 2 phPLED, UV/vis and Photoluminescence Data of the Terpolymers 20–26
Copolymer
20
21
22
23
24
25
26
Onset voltage in V
Brightness in cd mꢀ2@ 8 V
Luminous efficiency in cd Aꢀ1 @ 6 V
Electroluminescence maxima in nm
Absorbtion maxima as film in nm
Photoluminescence maxima as film in nm
4.4
40
2.9
3.2
3.5
2.4
2.5
2.6
2,250
15.7
523
309
526
450
2.6
160
1.9
3,200
27.7
521
306
529
700
4.9
400
4.3
1.8
523
310
532
615
308
621
507
307
493
615
307
616
498
308
483
and to get an idea of the electrochemical redox behavior.
Additionally, it is possible to estimate HOMO- and LUMO-
energy levels from CV measurements. Consequently, we used
this method to extract information about the electron transfer
activities of our synthesized polymer materials as electro-
active films, deposited on a glassy carbon working electrode.
0.1 M tetrabutylammonium tetrafluoroborate (Bu4NBF4) in
acetonitrile was used as electrolyte, besides a platinum mesh
as counter electrode and an Ag/AgCl-electrode as reference.
step at around þ0.90 V.29 The oxidation processes of the
polymers were more or less not of complete reversibility,
since additional hole transfers from the hole transporting
material to TEG are imaginable. Measured in solution, TEG
exhibited a reversible and a second quasi-reversible oxida-
tion step [Fig. 4(b)]. The ascertained half wave potential of
the first oxidation step was at E1/2: þ0.87 V, whereby the
electrochemical activity of TEG was in the same range as
that one of the hole transport material. The assumption, that
the possible hole transfer from the N,N-Di-p-tolyl-aniline side
group to TEG is reasonable for noncomplete reversible oxi-
dation processes of the terpolymers, is supported by the oxi-
dation behavior of the corresponding copolymers without
TEG. Only copolymers without an attached emitter showed
good reversibility.30 The electrochemical behavior seemed to
be finally independent from the spacer length between the
polymer backbone and the attached active molecules. The
electrochemical redox performance was defined by intro-
duced p-conjugated hole- and electron-transporting mole-
cules and Ir-complexes connected to the polymer backbone.
It was shown that in all cases intermolecular charge transfer
is possible through hopping processes between the attached
charge transport materials. This is an important requirement
for the electronic application for these polymers in PLEDs.
Reversible reduction processes were detected in the cathodic
sweep direction at very negative values for polymers 20, 21
and 24 [Fig. 4(a)]. The identified reduction peak potentials
were in the range from ꢀ2.10 V to ꢀ2.19 V and the corre-
sponding re-reduction peaks at ꢀ1.95 V. This electrochemical
redox behavior could be attributed to the tert-BuPBD units
attached as side groups to the polystyrene backbone. In solu-
tion tert-BuPBD 7 exhibits a reversible reduction step with a
peak potential at ꢀ1.96 V,28 whereas the polymer films
exhibited a 150 mV shift to more negative values, allegeable
by the diffusion of counter ions into the polymer film to
maintain charge neutrality. In the oxidation direction only
quasi-reversible oxidation steps were observed, assigned
mainly to the hole-transporting units in the polymer struc-
ture [Fig. 4(a)]. The oxidation peaks of the polymer films
were located at þ0.98 V 6 20 mV and the corresponding ca-
thodic peaks could be found at þ0.90 V. In solution the N,N-
Di-p-tolyl-aniline molecule shows also a reversible oxidation
Device Characterization
For all obtained terpolymers 20–26, phosphorescent PLEDs
were built in similar configuration (glass/ITO/PEDOT:PSS/
FIGURE 4 Cyclic voltammograms (a) of green emitting polymers 20, 21, and 24 as film on a glassy carbon electrode (0.1 M
Bu4NBF4 in acetonitrile, scan rates 20 mV sꢀ1). (b) Of TEG in solution (0.1 M Bu4NBF4 in acetonitrile, scan rate 100 mV sꢀ1).
PHOSPHORESCENT-EMITTING NONCONJUGATED PLED, THESEN ET AL.
399

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Device characteristics (a) of the green emitting polymers 20, 21 and 24 (with TEG; full symbols brightness; open sym-
bols luminous efficiency; layer thicknesses about 80 nm). (b) Of the red emitting polymers 22 and 25 (with TER; full symbols
brightness; open symbols luminous efficiency; layer thicknesses about 60 nm). (c) Of the blue emitting polymers 23 and 26 (with
TEB; full symbols brightness; open symbols luminous efficiency; layer thicknesses about 75 nm).
terpolymer/CsF/Ca/Ag) to compare their electronic proper-
ties with respect to the spacer length and the emission col-
ors. For the prepared green emitting polymers the results
are shown in Figure 5(a) and Table 2 for polymer layer
thicknesses of about 80 nm. The onset voltage of polymer
20 was with 4.4 V considerably higher compared to that of
polymers with just one 21 (2.9 V) or respectively none 24
(2.4 V) spacer attached transport monomer. The best overall
performance of the green emitting polymers was presented
by polymer 24 with nontethered units, showing a brightness
of 4500 cd mꢀ2 at 8.5 V, a luminous efficiency of 28 cd Aꢀ1
at 6 V and the low mentioned onset voltage of 2.4 V. Polymer
20, where both transporting monomers were connected via
hexyl spacer groups to the polymer backbone, presented the
worst performance. In Figure 6 the electroluminescence
spectra of the green emitting PLEDs of polymer 21 and 24
are plotted (open and filled triangles). For these two green
emitting materials, the electroluminescence emission maxima
were identical at 523 nm and independent from spacer
length, therefore. There was also a good accordance to the
photoluminescence spectra [Fig. 3(a)]. A similar influence on
the emission behavior could be noted for the red-emitting
polymers as well, illustrated in Figure 5(b) and Table 2. For
the red-emitting polymer 22, consisting of the tethered hole-
transporting monomer, a brightness of 500 cd mꢀ2 at 8 V
with a maximum luminous efficiency of 2.5 cd Aꢀ1 at 5 V
was observed. The onset voltage of polymer 22 with 3.2 V
was, compared to the un-tethered polymer 25 with 2.5 V,
substantially higher and showed a similar behavior—com-
paring the green emitting case. Polymer 25, contained of
curves showed emission maxima at 615 nm and local max-
ima at 665 nm and a good accordance to the photolumines-
cence behavior measured in solid films. With respect to the
blue emitting polymers 23 and 26 an analog behavior con-
cerning the green and red-emitting polymers was noticed
[Fig. 5(c) and Table 2]. The luminous efficiency for polymer
system 23, containing hexyl-spacer units to fix the hole-
transporters, was limited to 1.9 cd Aꢀ1 at 6 V with a bright-
ness of 300 cd mꢀ2 at 10 V and an onset voltage of about
3.5 V. With a maximum luminous efficiency of 4.3 cd Aꢀ1 at
6 V, a brightness of 800 cd mꢀ2 at 10 V, and an onset volt-
age of about 2.7 V, the performance of polymer 26 with un-
tethered transporting units was significantly increased as
well as for the other colors. The electroluminescence plot of
directly connected transporting units, exhibited, with
a
brightness of 700 cd mꢀ2 at 8 V and a luminous efficiency
of 4.9 cd Aꢀ1 at 5 V, a remarkable increase in contrast to the
performance of the tethered polymer 22. Therefore, the best
overall performance of the investigated red-emitting poly-
mers is presented by polymer 25 without the spacer groups.
The electroluminescence spectra of the red-emitting materi-
als are plotted in Figure 6 (open and filled circles). Both
FIGURE 6 Electroluminescence spectra of the investigated
PLEDs (full symbols tethered hole-transporting component,
open symbols nontethered components; triangles polymers
with TEG, circles polymers with TER and squares polymers
with TEB).
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ARTICLE
polymer 23 and 26 is shown in Figure 6 (open and filled
squares). Polymer 26, with the un-tethered monomers,
showed with a maximum at 496 nm the behavior expected
from photoluminescence experiments. For polymer 23, con-
sisting of the tethered hole-transporting component, two
maxima could be observed: one at the expected 503 nm and
an additional unexpected local maximum at 583 nm. The ori-
gin of this additional emission peak in the electrolumines-
cence spectra is recently unknown and will be investigated
in further studies. Furthermore, it has to be noticed, that the
choice of device characteristics was done exclusively with
respect to comparable layer thicknesses, that is not the best
gainable performance for the selected device adjustment was
listed finally. It shows only a comparison of polymers, with
and without spacer units between the polymer main chain
and the connected charge transport materials, and does not
illustrate the maximum accessible luminous efficiencies or
brightnesses of these materials.
a lesser extend with no spacer at both charge carrier moi-
eties. That polymer (monomers 10 and 2) exhibits a mini-
mized nonconductive ratio of about 8% and should be the
best material for efficient charge carrier transportation of
that array. This statement is further based on the current-
voltage-characteristics of these materials. Polymer 20 with
the two tethered charge carrier moieties shows a current
density of only 0.65 mA cmꢀ2 at 8 V, whereas polymer 24
with no spacer groups shows at 8 V a current density of
11.6 mA cmꢀ2. This drastic increase of current density
means that much more charge carriers can be injected into
the solid film to form excitons for efficient light emission.
Overall it can be summarized, that this established approach
of a direct linkage of active components as side functionalities
is favorable for polymers with a nonconjugated backbone and
their potential for efficient charge transport processes. It is
shown that spacer moieties may affect the efficient charge car-
rier transport from the transporting materials to the phos-
phorescent dopand in two ways. The first is the maximization
of the nonconductive ratio in the nonconjugated polymer and,
the second is the possibility of a formation of a nonconductive
shield around the phosphorescent dopand, caused by the non-
conductive spacer moieties. In further experiments it was
established that a white light emitting phosphorescent PLED
should be possible with these polymers because of their sup-
pressing undesired energy transfer from the blue to the red
emitter in multi component copolymers.31
In summary, the experiment has proven that a hexyl spacer
unit exhibits a disadvantageous influence regarding all the
determined PLED characteristics for the investigated colors.
The onset voltage of the phPLED increases on one hand,
while the luminous efficiency and the brightness decrease in
all cases on the other hand with an application of spacer
groups. A possible explanation for this phenomenon could be
that the introduction of hexyl-spacers between the transport-
ing units and the polymer main chain, resulting in a drastic
decrease of the glass transition temperature, and therefore,
allows an unfavorable steric order.
CONCLUSION
It was possible to synthesize copolymers with hole- and elec-
tron-transporting components and additional phosphorescent
triplet emitters with and without spacer moieties. The investi-
gation of the resulting polymerization products leads to the
prediction that spacer-functionalized monomers exhibit higher
polymerization rates. Further, the introduction of spacer moi-
eties has no influence on the electrochemical behavior of the
polymers, as shown by CV measurements. Additionally, it was
found that an introduction of hexyl-spacer tethered transport-
ing monomers into polymers has a crucial influence on their
PLED properties. The glass transition temperatures of poly-
mers containing tethered components are much lower, in con-
trast to those without the latter. Luminous efficiencies, turn-
on voltages, and brightnesses were also influenced and re-
stricted by the introduction of spacer units. It was empha-
sized that transporting materials which are directly connected
to the polymer main chain, exhibited the best PLED perform-
ance. An explanation for this phenomenon was given by a
more efficient charge transport because of a higher content of
semiconducting molecules in the copolymer, by a more effi-
cient charge transfer to the phosphorescent dopand and a
more efficient exciton formation in the case of the spacerless
copolymers. Therefore, the preferred approach for further
investigations, based on this study, is the direct linkage of the
active molecules without the use of spacer moieties onto the
polymer backbone of polystyrene.
It is imaginable, that the nonconjugated hexyl spacer units of
the charge carrier molecules linked to the polymer main
chain forming a tube-like nonconductive shield around the
emitter, because the emitter itself is connected in a shorter
distance to the nonconjugated polymer backbone. In polymer
20 for example, the distance between the electron- and hole-
transporting units to the polymer backbone is about 7 Å
higher than that one of the phosphorescent dopand to the
polymer backbone. As a result of such an emitter-shielding
the transfer of the injected charges by hopping processes to
the phosphorescent dopand becomes more inefficient. So the
formation of excitons at the phosphorescent dopand will be
reduced and a nonemissive charge recombination becomes
more important. This leads to a more inefficient device in
the case of the thetered copolymers.
Another possible declaration causing the inefficient PLED
performances of the tethered polymers can be that the
amount of nonactive parts of the polymer, showing neither
emission behavior nor adopting any charge transport tasks,
are increased by the use of spacer groups. The semiconduc-
tive ratio of the un-tethered polymers is maximized without
the use of spacers, therefore. For example, if an imagined co-
polymer of the tethered electron- and hole-transporting sty-
rene monomers in the ratio 2:1 is at hand, the nonconduc-
tive ratio of the polymer with spacers (monomers 19 and 6)
is with 40% very high. Whereas, in the case of just one
spacer at the hole-transporting monomer the nonconductive
ratio of the copolymer is calculated to 20% and it comes to
The authors thank the Federal Ministry of Education and
Research (BMBF) of Germany for financial support within the
PHOSPHORESCENT-EMITTING NONCONJUGATED PLED, THESEN ET AL.
401

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
15 Evans, N. R.; Devi, L. S.; Mak, C. S. K.; Warkins, S. E.; Pascu, S. I.;
Ko¨hler, A.; Friend, R. H.; Williams, C. K.; Holmes, A. B. J Am Chem Soc
2006, 128, 6647.
CARO project (01 BD 0685). They also thank Steffi Kreissl for
the device preparation and Anna Koehler, University of
Bayreuth, for the helpful discussion.
16 Graf, M.; Gancheva, V.; Thesen, M.; Kru¨ger, H.; Mayer, P.; Su¨nkel, K.
Inorg Chem Comm 2008, 11, 231.
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