Meta-linked CBP-derivatives as host materials for a blue iridium carbene complex

Article abstract of DOI:10.1016/j.orgel.2011.08.012

We present four derivatives of 4,4′-bis(9-carbazolyl)biphenyl (CBP) for the use as host materials in blue phosphorescent organic light emitting diodes. By replacing the para-linkage by a meta-linkage of the carbazole substituents at the central biphenyl unit materials with improved thermal and optical properties are obtained. The triplet energy of the meta-linked host materials is significantly increased to more than 2.90 eV compared to 2.58 eV for the para-linked CBP. Moreover, selective methyl substitution of the basic meta-CBP structure leads to materials with high glass transition temperatures up to 120 °C and electrochemical stability of the oxidised species against dimerisation. The high triplet energy allows the use of the meta-CBP derivatives as host materials for the carbene emitter mer-tris(N-dibenzofuranyl-N′- methylimidazole)iridium (III) (Ir(dbfmi)) with a pure blue emission at 450 nm.

Full text of DOI:10.1016/j.orgel.2011.08.012

Organic Electronics 12 (2011) 2047–2055
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Meta-linked CBP-derivatives as host materials for a blue iridium
carbene complex
a
b
b
b
Pamela Schrögel , Nicolle Langer , Christian Schildknecht , Gerhard Wagenblast ,
b
a,
⇑
Christian Lennartz , Peter Strohriegl
Lehrstuhl Makromolekulare Chemie 1, Universität Bayreuth, D-95440 Bayreuth, Germany
BASF SE, D-67056 Ludwigshafen, Germany
a
b
a r t i c l e i n f o
a b s t r a c t
0
Article history:
We present four derivatives of 4,4 -bis(9-carbazolyl)biphenyl (CBP) for the use as host
materials in blue phosphorescent organic light emitting diodes. By replacing the para-link-
age by a meta-linkage of the carbazole substituents at the central biphenyl unit materials
with improved thermal and optical properties are obtained. The triplet energy of the meta-
linked host materials is significantly increased to more than 2.90 eV compared to 2.58 eV
for the para-linked CBP. Moreover, selective methyl substitution of the basic meta-CBP
structure leads to materials with high glass transition temperatures up to 120 °C and elec-
trochemical stability of the oxidised species against dimerisation. The high triplet energy
allows the use of the meta-CBP derivatives as host materials for the carbene emitter
Received 22 June 2011
Received in revised form 19 August 2011
Accepted 20 August 2011
Available online 10 September 2011
Keywords:
Host material
Electroluminescence
Blue phosphorescent OLED
Carbazole
0
mer-tris(N-dibenzofuranyl-N -methylimidazole)iridium (III) (Ir(dbfmi)) with a pure blue
emission at 450 nm.
Ó 2011 Elsevier B.V. All rights reserved.
1
. Introduction
The presentation of OLEDs based on the electrophospho-
To avoid self-quenching the emitter has to be doped
into an appropriate host material [6]. For an efficient
exothermic energy transfer from the host to the guest
molecules, the triplet energy of the host must be higher
compared to the emitter. In the case of blue emitters the
rescence was a breakthrough in device performance. With
the introduction of phosphorescent emitters both the elec-
trogenerated singlet and triplet excitons contribute to the
emission of light. The fast intersystem crossing efficiently
converts singlet excitons to the triplet state leading to a the-
oretical internal quantum efficiency of 100% [1–3]. The
most frequently used blue phosphorescent emitter is the
greenish blue phosphor FIrpic with an emission maximum
host’s energy gap between the singlet ground state (S
0
)
and the first excited triplet state (T ) has to be at least
1
2.8 eV. To obtain high triplet energies for the host materi-
als the conjugation of the materials must be limited. This
requirement makes the development of host material for
blue triplet emitters particularly challenging because low-
ering the conjugation, i.e. increasing the singlet and triplet
energies may negatively affect the charge transport prop-
erties [7].
(k
em) at ꢀ470 nm. For white OLEDs, however, a saturated
blue emission with a kem at 450 nm would be beneficial. A
number of blue phosphors has been reported in the last
years and recently been reviewed by Sasabe and Kido [4].
Carbene type emitters such as mer-tris(N-dibenzofuranyl-
N -methylimidazole)iridium (III) (Ir(dbfmi)) [5] are attrac-
tive candidates for a saturated blue emission.
Many organic compounds used as host materials for
phosphorescent emitters are based on carbazole [8–11].
One reason is the high intrinsic triplet energy of carbazole
of 3.0 eV [12]. The most prominent example for a carbazole
0
0
based host material is 4,4 -bis(9-carbazolyl)-biphenyl (CBP)
⇑
Corresponding author. Tel.: +49 921 553296; fax: +49 921 553206.
E-mail address: peter.strohriegl@uni-bayreuth.de (P. Strohriegl).
where the N-atoms of the carbazole units are connected
with a biphenyl unit at the 4,4 -positions. The molecular
0
1
566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.08.012

2
048
P. Schrögel et al. / Organic Electronics 12 (2011) 2047–2055
structure allows for an extended conjugation of the carba-
zole units over the biphenyl structure leading to a decrease
in triplet energy to around 2.56 eV [13]. If the conjugation
between the constituting units in carbazole derivatives is
minimised the triplet energy can be maintained at a high le-
vel. Many approaches to confine the conjugated system in
carbazole derivatives have been described in literature.
One way to minimise the conjugation in carbazole based
host materials introduced by Tokito et al. is to attach two
units such as a single phenyl ring instead of biphenyl in
N,N -dicarbazolyl-3,5-benzene (mCP) [18,13] or non-conju-
0
gated units as, for example, an aliphatic cyclohexyl ring, an
oxygen-bridge [19] or a diphenylsilane linker [20]. In mCP
another effect reduces the conjugation: instead of the
para-connection, the two carbazole moieties are meta-
linked over the phenyl ring [21,22]. A major drawback of
mCP however is its high crystallinity. Matrix materials with
0
carbazole units linked to a biphenyl in the 3,3 -positions
0
methyl groups in the 2,2 -positions of the central biphenyl
have been described in a patent of Adachi and Forrest
[23] and a number of following patents. In the scientific lit-
erature meta-linked CBP derivatives are to the best of our
knowledge only reported in a theoretical study about het-
erofluorene linker groups between the two carbazoles as
alternatives to biphenyl [24]. Besides the high triplet en-
ergy a host material has to reveal a stable amorphous phase
at device operating temperatures [25]. This ensures homo-
geneous mixing of the emitter within the matrix material
and minimises the effects of concentration quenching. In
addition the use of organic glasses in OLEDs helps to avoid
0
in CBP leading to the host material 4,4 -bis(9-carbazolyl)-
0
2
,2 -dimethylbiphenyl (CDBP) [14,15]. Due to the steric
hindrance the biphenyl is forced into a tilted conformation
which reduces the conjugation in the molecule and con-
comitantly enlarges the triplet energy to 2.79 eV. We re-
cently reported a series of CBP-based materials with high
triplet energies using a similar concept by introducing
0
methyl and trifluoromethyl substituents in the 2,2 -posi-
tions of the central biphenyl unit [16,17]. Another design
concept is to choose different linkers between the carbazole
(
a)
(b)
Fig. 1. Chemical structures and synthetic routes to (a) meta-linked CBP derivatives 1 and 2 and (b) twisted meta-linked CBP derivatives 3 and 4. Reagents
and conditions: (i) Cu(OAc)
reflux, 24 h.
2 2 3 2 4 2 4 2 3
, DMF, 100 °C, 90 min; (ii) I , HIO , H SO , H O, CCl , acetic acid, 80 °C, 18 h; (iii) Cu, K CO , 18-crown-6, o-dichlorobenzene,

P. Schrögel et al. / Organic Electronics 12 (2011) 2047–2055
2049
grain boundaries which may act as trap states [26,27]. Or-
ganic glasses can be obtained by avoiding strong intermo-
voltammetry measurements were carried out in absolute
solvents measuring at a platinum working electrode versus
lecular forces like hydrogen bonding or
p–p
stacking
3
a Ag/AgNO reference electrode. Each measurement was
between the molecules. A very common design concept
for molecular glasses is the space-filling star-shaped topol-
ogy, as in the well-known hole transporting organic glass
calibrated against an internal standard (ferrocene/ferroce-
nium redox system). The purity of the target compounds
was checked with a Waters size exclusion chromatography
system (SEC) for oligomers (analytical columns: cross-
linked polystyrene gel (Polymer Laboratories), length:
0
00
4
,4 ,4 -tris(carbazol-9-yl)triphenylamine (TCTA) [28]. Fur-
thermore, the introduction of bulky substituents leads to
a larger intermolecular distance and a hindrance in packing
and therefore to amorphous behaviour. For example, in
2 ꢂ 60 cm, width: 0.8 cm, particle size: 5
lm, pore size
100 Å, eluent THF (0.5 mL/min, 80 bar), polystyrene
9
-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-carbazole
standard).
(
(
CzSi) [29] and 3,5-di(N-carbazolyl)-tetra-phenylsilane
SimCP) [30] packing of the molecules is avoided by intro-
2
.3. Computational methods
ducing bulky triphenylsily groups. However, the separation
of the conducting units usually leads to a decrease in charge
carrier mobility.
The transport levels of the materials used were deter-
mined via density functional calculations. For the ionisa-
tion potential and the electron affinity first the geometry
of the neutral as well as the charged states were opti-
mised using the BP86-functional [31,32] in combination
with a split-valence basis set (SV(P)) including polarisa-
tion functions on all heavy atoms [33]. For iridium an
effective core potential was employed [34]. For the ener-
getics we performed single point calculations at the
optimised geometries using the same functional in combi-
nation with a TZVP-basis set [35]. To account for dielectric
solid state effects a UPS/IEPS-calibrated version of the
conductor like screening model (COSMO) [36] was used
in conjunction with these single point calculations. All cal-
culations were carried out with the turbomole program
package [37].
In this work we describe the preparation, the thermal
and optical characterisation of four meta-linked CBP-deriv-
atives. Materials revealing high triplet energies and amor-
phous behaviour with high glass transition temperatures
are achieved by attaching the carbazole units at the
meta-position of the phenyl rings of the biphenyl moiety
in combination with a selective methyl substitution at the
meta-CBP structure. The high triplet energy allows the use
as host materials for the pure blue carbene emitter mer-
0
tris(N-dibenzofuranyl-N -methylimidazole)iridium (III) (Ir
(
dbfmi)). The molecular structures of the host materials
1–4 are depicted in Fig. 1.
2
. Experimental
2.1. Materials
2.4. Synthetic procedures
All chemicals and reagents were used as received from
The synthesis of tris[(3-phenyl-1H-benzimidazol-1-yl-2
3H)-yliden-1,2-phenylene]-irdidium (DPBIC) [38], 2,8-bis
commercial sources without further purification. The sol-
vents for reactions and purification were all distilled before
use.
(
(
triphenylsilyl)-dibenzofuran (DBFSi) [39], mer-tris(N-dib-
0
enzofuranyl-N -methylimidazole)-iridium (III) (Ir(dbfmi))
[
5,39] and 3,6-dimethyl-carbazole [16] is described in
2
.2. Characterisation
literature.
0
3,3 -Dibromobiphenyl was synthesized according to a
1H and 13C NMR spectra were recorded with a Bruker
procedure reported by Demir et al. [40] Yield: 43%. EI-MS
+
1
AC 300 (300, 75 MHz) and CDCl
ven as chemical shifts d (ppm) downfield from Si(CH
3
as solvent. All data are gi-
. For
m/z: 312 (100, M ), 232 (5), 152 (95). H NMR (300 MHz,
CDCl ), d (ppm): 7.70 (dd, 2H), 7.52–7.46 (m, 4H), 7.31
(dd, 2H). C NMR (75 MHz, CDCl
131.16, 130.74, 130.50, 126.09, 123.33.
3
)
4
3
ꢁ5
Ò
13
optical measurements, 10 M cyclohexane (Ultrasolv )
solutions of the materials as well as thin films on quartz
substrates were prepared by spincoating. The UV/Vis spec-
tra were measured in solution and on neat films with a Hit-
achi U-3000 spectrometer. Fluorescence spectra in solution
were obtained from a Shimadzu spectrofluorophotometer
RF-5301PC using excitation at 300 nm. Phosphorescence
spectra of thin films have been measured at 5 K (Helium
cryostat Optistat CF from Oxford Instruments) applying
the technique of time-gated spectroscopy (with fluores-
cence and phosphorescence lifetime spectrometer FLSP
3
), d (ppm): 142.11,
0
0
0
5,5 -Diiodo-2,2 -dimethylbiphenyl: 2,2 -Dimethylbiphen-
yl (9.9 g, 54 mmol) in 120 mL of acetic acid and 5.5 mL of
water were heated to 90 °C to get a clear solution. After
the addition of iodine (10.9 g, 43 mmol), iodic acid (5.7 g,
32 mmol), 5.5 mL of sulphuric acid and 5.5 mL of tetrachlo-
romethane the solution was stirred at 80 °C for 18 h. The
excess of iodine was removed by adding a solution of so-
dium hydrogen sulphide. The product was extracted with
dichloromethane and washed several times with water.
After drying the organic layer over anhydrous sodium sul-
phate, the solvent was evaporated. The resulting yellowish
oily product was recrystallised from ethyl acetate to yield
9
20 from Edinburgh Instruments). For differential scanning
calorimetry measurements (DSC) a Diamond DSC appara-
tus from Perkin Elmer was used (heating/cooling rate
1
was performed on a Mettler Toledo TGA/SDTA815e ma-
chine at a heating rate of 10 K/min under nitrogen. Cyclic
ꢁ1
0 K min , nitrogen). Thermogravimetric analysis (TGA)
6.9 g (16 mmol, 30%) of a white solid. EI-MS m/z: 434
(100, M+), 180 (50), 165 (47). H NMR (300 MHz, CDCl
d (ppm): 7.58 (dd, 2H), 7.42 (ds, 2H), 7.00 (d, 2H). 1.99 (s,
1
3
),

2
050
P. Schrögel et al. / Organic Electronics 12 (2011) 2047–2055
6
1
H). 13C NMR (75 MHz, CDCl
36.56, 135.48, 131.83, 90.41, 19.38.
3
), d (ppm): 142.30, 137.65,
3. Results and discussion
.1. Synthesis
3
2
.5. General procedure for the Ullmann- type reaction
In Fig. 1a the synthetic route to the meta-linked CBP
derivatives 1 and 2 is depicted. In a copper-mediated
homocoupling reaction 3-bromophenylboronic acid reacts
to 3,3 -dibromobiphenyl [40]. An Ullmann-type reaction
of dibromobiphenyl with carbazole or 3,6-dimethylcarba-
zole [16] yields the host materials.
Dihalogenobiphenyl (2.3 mmol), carbazole (5.06 mmol),
potassium carbonate (2.5 g, 18.4 mmol), copper powder
0.58 g, 9.2 mmol) and 18-crown-6 (0.12 mg, 0.46 mmol)
0
(
were refluxed in 15 mL of o-dichlorobenzene in an argon
atmosphere for 24 h. Copper and inorganic salts were
filtered off and the solvent was evaporated. Column chro-
matography on silica gel with mixtures of hexane/tetrahy-
drofurane as eluent yielded the products 1–4 as white
To obtain the twisted meta-linked CBP-derivatives 3
0
0
and 4, first, 5,5 -diiodo-2,2 -dimethyl-biphenyl was pre-
0
pared by direct iodination [41] of 2,2 -dimethylbiphenyl
with iodine and iodic acid in acetic acid (Fig. 1b). Via 2D
solids.
1
H NMR experiments (see Supporting Information) the
0
3
,3 -Bis(carbazolyl)biphenyl (1): Yield: 47%. EI-MS m/z:
0
1
selective iodination at the 5- and 5 -positions of the biphe-
4
84 (100, M+), 316 (10), 234 (39). H NMR (300 MHz,
nyl could be confirmed. Here, the inductive effect of the
CDCl3), d (ppm): 8.17 (d, 4 H), 7.88 (m, 2H), 7.75 (dt, 2H),
0
methyl groups in the 2- and 2 -positions of the biphenyl in-
7
7
1
1
.70 (dd, 2 H), 7.60 (dt, 2H), 7.51 (d, 4H), 7.43 (dt, 4H),
0
1
3
creases the electron density in the 5- and 5 -positions and
.31 (dt, 4H).
( C NMR (75 MHz, CDCl3), d (ppm):
+
0
favours the electrophilic attack of I . We consider this 5,5 -
42.42, 141.19, 138.82, 130.84, 126.80, 126.52, 126.39,
0
diiodo-2,2 -dimethylbiphenyl unit as a versatile building
26.10, 123.79, 120.71, 120.40, 110.08.
block for the synthesis of materials with confined conjuga-
tion which, to the best of our knowledge, has not been de-
scribed in the literature before. The tilted meta-linked CBP
derivatives 3 and 4 were prepared via the Ullmann-type
0
3
,3 -Bis(3,6-dimethylcarbazolyl)biphenyl (2): Yield: 55%.
1
EI-MS m/z: 540 (100, M+), 345 (5), 270 (24). H NMR
300 MHz, CDCl ), d (ppm): 7.92 (m, 4H), 7.86 (m, 2H),
.70 (dt, 2H), 7.66 (dd, 2H), 7.57 (dt, 2H), 7.38 (d, 4H),
(
3
7
7
d
1
2
0
0
.24 (dd, 4H), 2.56 (s, 3H). 1 C NMR (75 MHz, CDCl
3
reaction of 5,5 -diiodo-2,2 -dimethylbiphenyl with carba-
zole or 3,6-dimethylcarbazole [16].
3
),
(ppm): 142.38, 139.63, 139.24, 130.70, 129.55,
27.52, 126.41, 126.05, 125.76, 123.84, 120.56, 109.75,
1.74.
Mass spectrometry, H and 13C NMR spectroscopy were
used to identify the materials. The data are given in the
experimental part. All materials were purified by repeated
zone sublimation.
1
0
0
3
,3 -Bis(carbazolyl)-6,6 -dimethylbiphenyl (3): Yield: 50%.
+
1
EI-MS m/z: 512 (100, M ), 329 (8), 257 (17), 166 (12).
H
NMR (300 MHz, CDCl ), d (ppm): 8.15 (d, 4H), 7.55–7.39
3
13
3.2. Thermal properties
(
(
1
1
m, 14H), 7.32–7.26 (m, 4H), 2.33 (s, 6H).
75 MHz, CDCl ), d (ppm): 142.51 141.25 135.65, 135.46,
31.85, 127.96 126.45, 126.26, 123.67, 120.65, 120.20,
C NMR
3
We examined the thermal properties of the meta-linked
CBP derivatives 1–4 by thermal gravimetric analysis (TGA)
and differential scanning calorimetry (DSC) in nitrogen
atmosphere at a scanning rate of 10 K min . All results
are summarised in Table 1.
Both the para-linked CBP and the meta-linked 1 show
crystalline behaviour in the DSC experiment. For the
meta-derivative 1 the melting and the crystallisation are
observed at around 10 °C lower temperatures than for
CBP. Methyl substitution at the pendant carbazoles in 2
leads to a material with a much lower tendency to crystal-
10.09, 20.11.
0
0
+
3,3 -Bis(3,6-dimethylcarbazolyl)-6,6 -dimethylbiphenyl (4):
ꢁ1
Yield: 55%. EI-MS m/z: 568 (100, M ), 358 (15), 284 (29),
1
1
4
7
92 (24). H NMR (300 MHz, CDCl
3
), d (ppm): 7.90 (s,
.21(d, 4H), 2.55 (s, 12H), 2.30 (s, 6H). 1 C NMR (75 MHz,
), d (ppm): 142.47, 139.72, 136.06, 134.98, 131.70,
H), 7.52–7.45 (m, 4H), 7.41–7.38 (m, 4H), 7.33 (d, 4H),
3
CDCl
3
1
2
29.30, 127.69, 127.39, 126.10, 123.70, 120.49, 109.74,
1.73, 20.06.
2.6. OLED fabrication
Table 1
Thermal properties of CBP and the meta-linked host materials 1–4.
The organic layers were deposited by thermal evapora-
tion in high vacuum (<10 mbar) onto indium-tin-oxide
ITO, 10 ohm/square) precoated glass substrates. Prior to
ꢁ6
a
T
g
(°C)
T
m
(°C)
Tcr (°C)
T
id (°C)
(
205b
CBP
1
2
3
4
–
–
107
108
120
283
271
270
237
212
365
315
349
319
319
b
use the ITO glass was degreased using organic solvents
and cleaned using an UV-ozon oven for 30 min. The organic
layers and the metal cathode were evaporated without
breaking the vacuum. The current density–luminance–
voltage (J–L–V) characteristics of the OLEDs were
measured by a Keithley source meter 2400 and a Konica
Minolta CS-200, respectively. EL spectra were taken by a
CCD spectral analyser by Zeiss. EQEs were calculated from
the luminance, current density, and EL spectrum, assuming
a Lambertian distribution.
191
184
176
c
c
–
T
g
m
: glass-transition temperature, T : melting temperature, Tcr: crystalli-
sation temperature and Tid: initial decomposition temperature.
a
T
id is the temperature at which an initial mass loss was observed in a
ꢁ1
thermogravimetric experiment with a heating rate of 10 K min in a
nitrogen atmosphere.
b
Observed during cooling scan.
Observed only during the heating scan.
c

P. Schrögel et al. / Organic Electronics 12 (2011) 2047–2055
2051
lise. The glass transition of 2 is observed at 107 °C and the
recrystallisation is observed at a high temperature of
and 3 and in compounds 2 and 4 are identical it is not sur-
prising that these two pairs of materials show nearly the
same absorption and fluorescence spectra. For compound
2 and 4 the additional methyl substituents on the chromo-
phore 3,6-dimethylcarbazole cause a bathochromic shift of
13 nm in the absorption and emission spectra compared to
compounds 1 and 3.
1
84 °C. In compound 3 the introduction of methyl groups
only at the central biphenyl unit leads to nearly the same
glass transition temperature of 109 °C and nearly the same
recrystallisation temperature of 176 °C. The biphenyl sub-
stitution in combination with the methyl substitution at
the carbazoles increases the T
g
even to 120 °C for com-
As a prerequisite for the application of 1–4 as host
materials, their triplet energy has to be higher compared
to that of the blue phosphorescent emitter. By measuring
phosphorescence at low temperatures the triplet energies
can be experimentally determined as the highest energy
peak of luminescence.
In order to understand how the meta-linkage of the car-
bazole units affects the triplet energy we compare the
phosphorescence spectra of the para-linked CBP and the
meta-linked derivative 1–4. The most striking difference
in the normalised spectra (see Fig. 2, bottom) is the
remarkable blue shift in phosphorescence of 1–4 compared
with CPB. The emission of 1, for example, is shifted to low-
er wavelengths by 65 nm compared with CBP, correspond-
pound 4. Moreover, for compound 4 crystallisation is ob-
served neither in the cooling nor in the heating cycle
which renders 4 the derivative of the series with the high-
est tendency to form a molecular glass. This thermal inves-
tigation shows that the introduction of additional methyl
groups at the central biphenyl and/or the adjacent carba-
zole moieties of the meta-linked molecular structure has
beneficial effects on the thermal properties, i.e. amorphous
g
materials with high T s in the range of 107–120 °C are
obtained.
3.3. Optical properties
1 0
ing to triplet energies (DE(T –S )) of 2.98 eV for 1 and
The room temperature UV/Vis absorption and fluores-
ꢁ5
2.58 eV for CBP. This is a proof of the design concept of
meta-connection to achieve high triplet energies. In fact,
the methyl substitution at the central biphenyl moiety is
expected to lead to even higher triplet energies by intro-
ducing torsion between the phenyl rings of the biphenyl
unit as is well known from CDBP. However, in the case of
the meta-linked host materials the additional twisted
molecular structure has only minor effects on the emission
characteristics. Similar to the emission at room tempera-
ture, compounds 1 and 3 and compounds 2 and 4 reveal
the highest energy peaks at the same wavelength, while
the shape of the phosphorescence spectra is different. In
the case of the twisted methyl substituted biphenyl con-
taining compounds 3 and 4 the 0–0 transition is more pro-
nounced and the width of the spectra is smaller than for 1
and 2. As an effect of the methyl substitution at the 3- and
6-positon of the carbazole units we observe again a small
bathochromic shift of ca. 10 nm for compounds 2 and 4
compared to 1 and 3.
cence spectra of 10 M cyclohexane solutions of 1–4 are
depicted on top of Fig. 2. Compounds 1 and 3 as well as
compounds 2 and 4 show very similar spectra. In each
compound the absorption and photoluminescence behav-
iour is dominated by optical transitions localised on the
carbazole units. Since the chromophore in compounds 1
2
1
1
0
0
.0
.5
.0
.5
.0
RT
1
2
3
4
1.0
0.8
0.6
0.4
0.2
0.0
450
All results of the photo physical investigations of CBP
and the meta-linked matrix materials 1–4 are summarised
in Table 2.
2
50
300
350
400
wavelength / nm
1
2
3
4
1
0
0
0
0
0
.0 5 K
3
.4. Cyclic voltammetry
.8
.6
.4
.2
.0
The electrochemical behaviour of 1–4 was investigated
CBP
by cyclic voltammetry in a conventional three-electrode
setup using a platinum working electrode, a platinum wire
as counter electrode, and a Ag/AgNO reference electrode.
3
Particularly, the oxidation behaviour of 1–4 in dichloro-
methane solutions was studied. From the half-wave poten-
tial of the first oxidation relative to ferrocene the HOMO
value of the compounds can be deduced. By adding the
optical band gap to the HOMO value the LUMO value can
be estimated. As the cyclic voltammetry is measured in
solution and not in the solid state the values of HOMO
and LUMO may differ from the solid state values in an
OLED. Both the experimentally received and the calculated
values for the energy levels of CBP and of 1–4 are listed in
400
450
500
550
600
wavelength / nm
ꢁ
5
Fig. 2. Top: absorption and fluorescence spectra of 1–4 taken in 10
M
solutions in cyclohexane at 300 K. Bottom: normalised phosphorescence
spectra of CBP and 1–4 measured in 2 wt.% solid solution of the
compounds in PMMA at 5 K.

2
052
P. Schrögel et al. / Organic Electronics 12 (2011) 2047–2055
Table 2
Optical properties and energy levels of CBP and the meta-linked host materials 1–4.
Solution
Film
Solution
Film
Film
Calculated
Experimental
HOMOg (eV)
a
b
kRTem (nm)
c
k5Kem (nm)
d
k5Kem (nm)
e
LUMOh (eV)
k
EA (nm)
k
EA (nm)
HOMO (eV)
LUMO (eV)
CBP
1
2
3
4
350
344
357
346
358
357
354
365
354
369
356, 374
340, 357
353, 370
342, 359
355, 373
484
421
452
423
431
477
415
421
415
421
ꢁ5.58
ꢁ5.71
ꢁ5.48
ꢁ5.64
ꢁ5.44
ꢁ1.78
ꢁ1.77
ꢁ1.71
ꢁ1.51
ꢁ1.47
ꢁ5.63
ꢁ5.65
ꢁ5.56
ꢁ5.65
ꢁ5.53
ꢁ2.16
ꢁ2.15
ꢁ2.17
ꢁ2.16
ꢁ2.17
f
a
b
c
ꢁ5
Absorption edge measured in 10 M cyclohexane solutions at room temperature.
Absorption edge measured on neat films at room temperature.
ꢁ5
Wavelengths of the intensity maxima of the fluorescence at 300 nm excitation of 10 M cyclohexane solutions at room temperature.
Measured on film samples (100%) at 5 K.
Measured in 2 wt.% PMMA at 5 K.
d
e
f
No 0–0 transition is observed.
g
h
Estimated from the half-wave potential of the first oxidation in the cyclic voltammetry experiment.
Estimated from the HOMO values and the optical band gap (determined from the UV/Vis absorption onset of neat films).
Table 2. The meta-linked compounds 1 and 3 with unsub-
stituted carbazole units reveal almost the same energy lev-
els as for CBP. In contrast, 2 and 4 show ca. 10 mV higher
HOMO levels which can be attributed to the inductive ef-
fect of the additional methyl groups in the 3- and 6-posi-
tons of the carbazole units. The cyclic voltammetry
experiments show that the behaviour upon oxidation is
clearly dominated by the molecules’ site with the highest
electron density, i.e. the carbazole moieties. Contrary to
the theoretical expectations, the substitution on the cen-
tral biphenyl unit does not affect the HOMO levels and
fairly no effects of the different substitution pattern on
the LUMO levels are observed.
y = 0.34) [14] and 458 nm (CIE colour coordinates x = 0.16,
y = 0.26) [44], respectively. For solid state lighting, phos-
phors with a more saturated blue emission would be bene-
ficial. Due to the high triplet energy of more than 2.9 eV the
host materials 1–4 should be suitable for saturated blue
emitters. Thus, we have tested the host materials with the
pure blue carbene emitter Ir(dbfmi) with a kem = 450 nm
and CIE colour coordinates of x = 0.16, y = 0.18.
Charge transport processes in organic semiconductors
involve the formation of radical anions and cations. Thus,
investigations of the electrochemical stability can give
information about the stability of the materials in OLED
devices. To evaluate the electrochemical stability of the
materials we carried out multicycle experiments. The cyc-
lic voltammograms showing five repeated oxidation cycles
ꢁ3
of 1 and 2 in 2 ꢂ 10 M dichloromethane solutions are gi-
ven as Supporting Information. In the case of 1, the oxida-
tion cycles are not reproducible. After the first reduction
into the uncharged molecule at 0.88 V an additional
reduction at around 0.59 V is observed. The latter can be
attributed to new oligomeric species formed during dimer-
isation reactions at the active 3- and 6-positions of the car-
bazole units after the first oxidation of the molecule
4
1
1
1
1
1
0
3
1
1200
000
800
0
1
2
0
[
42,43]. As the oligomeric species is oxidised more easily
1
0
the corresponding oxidation process takes place at a lower
voltage of around 0.70 V. In contrast, 2 reveals a fully
reversible oxidation behaviour. The methyl substitution
at the adjacent carbazole units blocks the electroactive
600
400
200
0
0
-
1
10
3
- and 6-positions and renders the oxidised species stable
-2
10
against dimerisation in solution. The same stability of the
radical cations was also found for compound 4 in contrast
to 3.
2
3
4
5
6
7
8
9
voltage / V
Fig. 3. Top: energy level diagram of the OLED device with the configu-
ration ITO/PEDOT:PSS/MoO -doped DPBIC (10 nm)/DPBIC (10 nm)/1:
Ir(dbfmi) (5 %, 20 nm)/1 (10 nm)/Cs CO -doped BCP (20 nm)/Cs CO
1 nm)/Al (100 nm) employing 1 as host for Ir(dbfmi); ionisation poten-
3
3.5. Blue phosphorescent organic light-emitting diodes
2
3
2
3
(
tials and electron affinities obtained from calculations are indicated. The
grey dotted lines represent the levels of the emitter Ir(dbfmi). Bottom:
luminance-voltage plot (filled symbols) and current density–voltage
characteristic (open symbols).
Common blue phosphors as for example FIrpic and FIr6
show blue emission tailing to the green region with emis-
sion maxima at 472 nm (CIE colour coordinates x = 0.17,

P. Schrögel et al. / Organic Electronics 12 (2011) 2047–2055
2053
The first OLED device setup is shown in Fig. 3. On top of
the ITO glass substrate poly(3,4-ethylene-dioxythioph-
ene):poly(styrenesulphonate) (PEDOT:PSS) was applied
by spin coating as hole injection layer followed by 10 nm
DPBIC p-doped with molybdenum(VI) oxide as hole trans-
porting layer. An additional 10 nm thick layer of DPBIC fol-
lowed as exciton and electron blocker. The 20 nm emission
layer consisted of 5% Ir(dbfmi) doped into the meta-linked
CBP host materials. Ten nanometer of the host material
were deposited as hole and exciton blocking layer followed
by 20 nm of caesium carbonate doped 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline (BCP) as electron transport-
observed a small contribution of the hole transporting
material DPBIC to the emission. This indicates that the
exciton formation is shifted towards the hole conducting
layer. We attribute this to the high energy barrier for hole
injection from the DPBIC into the emission layer causing as
well the high turn-on voltage. In contrast, electrons do not
have to overcome major energy barriers.
To improve the hole injection into the emission layer
we coevaporated the hole transporting DPBIC into the
emission layer providing a barrier free path for holes. The
optimised device setup and the molecular structures of
the materials used are shown on top of Fig. 4. Ten nanome-
ter molybdenum(VI) oxide doped DPBIC as hole transport-
ing layer and an additional 10 nm thick layer of DPBIC
were used as exciton and electron blocker. The 20 nm thick
emission layer comprises a mixed matrix system of the
carbazole containing host materials 1–4 with DPBIC and
the blue emitter Ir(dbfmi) in the ratio 75:20:5. 2,8-Bis
2 3
ing layer. As cathode 1 nm Cs CO and 100 nm aluminium
were deposited.
The turn-on voltage at 1 cd m 2 of 4.5 V is quite high
ꢁ
ꢁ2
and luminances of 100 and 1000 cd m are achieved at
voltages of 5.6 and 6.8 V, respectively. In some cases we
(
triphenylsilyl)-dibenzofuran (DBFSi) (5 nm) was used as
exciton and hole blocking material followed by 5 nm BCP
and 40 nm BCP n-doped with Cs CO . As cathode 1 nm
Cs CO and 100 nm aluminium were deposited.
In the electroluminescence spectrum of the optimised
2
3
2
3
device setup the pure emission of the phosphor Ir(dbfmi)
can be observed with CIE-coordinates of x = 0.16 and
y = 0.18 (Fig. 4, bottom). The CIE-coordinates do not change
within the voltage range between 4.2 and 8.2 V. By
decreasing the hole injection barrier in the mixed matrix
4
3
2
1
0
4000
1
2
3
4
1
1
1
1
1
0
0
0
0
0
3
2
1
0
000
000
000
-
1
1
0
2
3
4
5
6
7
8
1
0
0
0
0
0
.0
.8
.6
.4
.2
1
Voltage / V
1
2
3
4
1
0
8
6
4
2
0
.0
3
50 400 450 500 550 600 650
wavelength/ nm
Fig. 4. Top: energy level diagram of the optimised OLED device with the
configuration ITO/PEDOT:PSS/MoO -doped DPBIC (10 nm)/DPBIC
10 nm)/1: DPBIC:Ir(dbfmi) (75:20:5, 20 nm)/DBFSi (5 nm)/BCP (5 nm)/
Cs CO -doped BCP (40 nm)/Cs CO (1 nm)/Al (100 nm) employing 1 as
host for Ir(dbfmi) with chemical structures of the materials used;
Ionisation potentials and electron affinities obtained from calculations
are indicated. The grey dotted lines represent the levels of the emitter
Ir(dbfmi). Bottom: electroluminescence spectrum of the devices with 1 as
host material for Ir(dbfmi) at 4.25 V. CIE coordinates: x = 0.16, y = 0.18.
3
0
1
2
3
4
1
0
10
10
10
10
(
2
2
3
2
3
Luminance / cd/m
Fig. 5. Top: luminance–voltage curves (black filled symbols) and current
density–voltage characteristics (grey open symbols) of the optimised
device setup. Bottom: external quantum efficiency–luminance curves of
the devices with 1–4 as host material for Ir(dbfmi).

2
054
P. Schrögel et al. / Organic Electronics 12 (2011) 2047–2055
Table 3
Performance data at 100 cd m and 1000 cd m using 1–4 as host materials for the blue emitter Ir(dbfmi).
ꢁ
2
ꢁ2
At 100 cd m 2
ꢁ
At 1000 cd m
Voltage (V)
ꢁ2
Voltage (V)
g
C
(cd Aꢁ1)
g
P
(lm W
ꢁ1
)
g
ext (%)
g
C
(cd A
ꢁ1
)
g
P
(lm W
ꢁ1
)
gext (%)
1
2
3
4
3.5
4.9
3.7
4.3
11.5
0.9
12.2
4.5
10.2
0.6
10.2
3.3
8.2
0.5
8.7
2.6
4.4
6.6
4.4
5.3
5.7
0.6
8.5
3.1
4.1
0.3
6.0
1.9
4.1
0.4
6.1
1.8
C P
g : Current efficiency; g : power efficiency; gext: external quantum efficiency.
system the exciton formation zone could be successfully
separated from the hole transporting layer. Moreover, the
the biphenyl unit, the tendency to crystallise could be
lowered. Glass transition temperatures in the range of
107–120 °C were achieved. The meta-linkage reduces the
conjugation of the molecules effectively, leading to high
triplet energies in the range of 2.93–2.98 eV compared
with 2.58 eV for CBP. Here, the additional twist by the
methyl substitution at the biphenyl unit does not increase
the triplet energy any further, but narrows the phospho-
rescence spectra of 3 and 4 compared with 1 and 2. Intro-
ducing methyl groups at the 3- and 6-position of the
pendant carbazole units in the matrix materials 2 and 4
leads to electrochemical stability of the oxidised species
in cyclic voltammetry. First results from OLED tests show
that meta-linked CPB-derivatives are suited host materials
for blue emitters, e.g., Ir(dbfmi), with pure blue emission at
450 nm. The highest efficiencies were achieved with the
twisted meta-linked derivative 3 giving external quantum
efficiencies of 8.7% and 6.1% and power efficiencies of
ꢁ
2
applied voltages at 100 and 1000 cd m could be signifi-
cantly lowered, to 3.5 and 4.4 V, respectively, in the case
of host 1 (see Fig. 5).
ꢁ
2
ꢁ2
Peak luminances are 5300 cd m for 1, 5000 cd m for
ꢁ2
ꢁ2
2
, 10,800 cd m for 3 and 9300 cd m for 4. For hosts 1
and 3 higher luminances are reached at lower voltages
than for 2 and 4. Comparing the efficiencies of the four
meta-linked CBP-derivatives 1–4, the compounds 2 and 4
with methyl substituents at the 3- and 6-positions of the
carbazole units show significantly lower efficiencies than
1
and 3. As evident from the current density–voltage-plot
in Fig. 5, higher operation voltages are needed in the de-
vices employing the host materials 2 and 4. One possibility
might be that the transport properties of 2 and 4 with
methyl substituents at the carbazole units are worse com-
pared to 1 and 3 with unsubstituted carbazole units. In
contrast, 1 and 3 give similar performances with slightly
better values for 3 at higher luminances. The highest
efficiency was achieved for 3 with an external quantum
ꢁ1
ꢁ2
10.2 and 6.0 lm W at 100 and 1000 cd m , respectively.
Acknowledgements
ꢁ
ꢁ2
1
efficiency of 8.7% and a power efficiency of 10.2 lm W
at 100 cd m and 6.1% and 6.0 lm W at 1000 cd m
ꢁ
2
ꢁ1
.
We thank Irene Bauer for the help during synthesis and
characterisation of the host materials and Christian Bonsi-
gnore and Mustapha Al-Helwi for their support during the
device fabrication and characterisation. We also thank Dr.
Ingo Münster, Dr. Evelyn Fuchs and Dr. Soichi Watanabe
for scientific discussions. Financial support from the BMBF
project TOPAS 2012 (FKZ 13N 10447) is gratefully acknowl-
edged. P.S. thanks the Universität Bayern e.V. for a grant.
Supporting Information is available online.
These efficiencies are comparable to those of Ir(dbfmi) in
the phosphine oxide based host material PO9 recently re-
ꢁ2
ported by Sasabe et al. [5] At a brightness of 1000 cd m
they reported an external quantum efficiency of 6.2%
which compares well with the external quantum efficiency
of 6.1% we found for the host material 3. All performance
data are summarised in Table 3.
The large differences in device performance as evident
from Table 3 are quite surprising as the chemical struc-
tures of all hosts 1–4 are very similar. These findings show
that each combination of emitter and host needs to be cho-
sen very carefully as the host-emitter-system is very sensi-
tive to even small structural variations which may change
the charge-carrier balance in the device or the chemical
compatibility of host and emitter. Both effects in conjunc-
tion with the relatively long triplet lifetime of Ir(dbfmi) of
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.orgel.2011.
0
8.012.
References
ꢀ
20
l
s
[5] could negatively influence the device
performance.
[
[
[
[
1] M.A. Baldo, D.F. O’Brian, Y. You, A. Shoustikov, S. Sibley, M.E.
Thompson, S.R. Forrest, Nature 395 (1998) 151–154 (London, U. K.).
2] D.F. O’Brian, M.A. Baldo, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett.
74 (4) (1999) 442–444.
3] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest,
Appl. Phys. Lett. 75 (1999) 4–6.
4
. Conclusions
We have described a series of CBP derivatives with a
4] H. Sasabe, J. Kido, Chem. Mater. 23 (2011) 621–630.
meta-linkage of the carbazole moieties to the central
biphenyl unit. By selective methyl substitution at two sites
of the basic meta-CBP structure, the 3- and 6-position of
[5] H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe, G. Wagenblast,
N. Langer, O. Molt, E. Fuchs, C. Lennartz, J. Kido, Adv. Mater. 22
(
2010) 5003–5007.
[
6] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90
(2001) 5048–5051.
0
the pendant carbazoles and at the 6- and 6 -position of

P. Schrögel et al. / Organic Electronics 12 (2011) 2047–2055
2055
[
[
[
7] A.B. Padmaperuma, L.S. Sapochak, P.E. Burrows, Chem. Mater. 18
2006) 2389–2396.
[26] Y. Shirota, J. Mater. Chem. 10 (2000) 1–25.
[27] R. Schmechel, H. Von Seggern, Phys. Status Solidi A 201 (2004)
1215–1235.
[28] Y. Kuwabara, H. Ogawa, H. Inada, N. Noma, Y. Shirota, Adv. Mater. 6
(2004) 677–679.
(
8] P.-I. Shih, C.-L. Chiang, A.K. Dixit, C.-K. Chen, M.-C. Yuan, R.-Y. Lee, C.-
T. Chen, E.W.-G. Diau, C.-F. Shu, Org. Lett. 8 (2006) 2799–2802.
9] M.-H. Tsai, Y.-H. Hong, C.-H. Chang, H.-C. Su, C.-C. Wu, A.
Matoliukstyte, J. Simokaitiene, S. Grigalevicius, J.V. Grazulevicius,
C.-P. Hsu, Adv. Mater. 19 (2007) 862–866.
[29] M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C.-C. Wu, F.-C. Fang, Y.-L.
Liao, K.-T. Wong, C.-I. Wu, Adv. Mater. 18 (2006) 1216–1220.
[30] M.-F. Wu, S.-J. Yeh, C.-T. Chen, H. Murayama, T. Tsuboi, W.-S. Li, I.
Chao, S.-W. Liu, J.-K. Wang, Adv. Funct. Mater. 17 (2007) 1887–1895.
[31] J.P. Perdew, W. Yue, Phys. Rev. B: Condens. Matter Mater. Phys. 33
(1986) 8800–8802.
[32] A.D. Becke, Phys. Rev. A: At., Mol. Opt. Phys. 36 (1988) 3098–3100.
[33] A. Schäfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 97 (1992) 2571–2577.
[34] D. Andrae, U. Häußermann, M. Dolg, H. Stoll, H. Preuss, Theor. Chim.
Acta 77 (1990) 123–141.
[35] A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829–
5835.
[36] A. Klamt, J. Phys. Chem. 99 (1995) 2224–2235.
[37] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett.
162 (1989) 165–169.
[
[
10] H. Fukagawa, K. Watanabe, T. Tsuzuki, S. Tokito, Appl. Phys. Lett. 93
2008) 133312–133314.
11] J.-H. Jou, W.-B. Wang, S.-M. Shen, S. Kumar, I.-M. Lai, J.-J. Shyue, S.
Lengvinaite, R. Zostautiene, J.V. Grazulevicius, S. Grigalevicius, S.-Z.
Chen, C.-C. Wu, J. Mater. Chem. 21 (2011) 9546–9552.
(
[
[
[
12] J.E. Adams, W.W. Mantulin, J.R. Huber, J. Am. Chem. Soc. 95 (1973)
5
477–5481.
13] R.J. Holmes, S.R. Forrest, Y.J. Tung, R.C. Kwong, J.J. Brown, S. Garon,
M.E. Thompson, Appl. Phys. Lett. 82 (2003) 2422–2424.
14] S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, F. Sato, Appl. Phys.
Lett. 82 (2003) 569–571.
15] I. Tanaka, Y. Tabata, S. Tokito, Chem. Phys. Lett. 400 (2004) 86–89.
16] P. Schrögel, A. Tomkevi cˇ ien e˙ , P. Strohriegl, S.T. Hoffmann, A. Köhler,
C. Lennartz, J. Mater. Chem. 21 (2011) 2266–2273.
17] S.T. Hoffmann, P. Schrögel, M. Rothmann, R.Q. Albuquerque, P.
Strohriegl, A. Köhler, J. Phys. Chem. B 115 (3) (2011) 414–421.
18] V. Adamovich, J. Brooks, A. Tamayo, A.M. Alexander, P. Djurovich,
B.W. D’Andrade, C. Adachi, S.R. Forrest, M.E. Thompson, New J. Chem.
[
[
[38] M. Bold, C. Lennartz, M. Prinz, H.-W. Schmidt, M. Thelakkat, M. Bäte,
C. Neuber, W. Kowalsky, C. Schildknecht, H.-H. Johannes, WO Patent
(2005) 019373.
[39] N. Langer, K. Kahle, C. Lennartz, O. Molt, E. Fuchs, J. Rudolph, C.
Schildknecht, S. Watanabe, G. Wagenblast, WO Patent (2009)
003898.
[
[
2
6 (2002) 1171–1178.
[
[
19] J. He, H. Lui, X. Dai, X. Ou, J. Wang, S. Tao, X. Zhang, P. Wang, D. Ma, J.
Phys. Chem. C 113 (2009) 6761–6767.
20] R. Zostautiene, J.V. Grazulevicius, Y.M. Lai, W.B. Wang, J.H. Jou, S.
Grigalevicius, Synthetic Metals 161 (2011) 92–95.
[40] A.S. Demir, Ö. Reis, M. Emrullahoglu, J. Org. Chem. 68 (2003) 10130–
10134.
[41] H.O. Wirth, O. Königstein, W. Kern, Liebigs Ann. Chem. 634 (1960)
84–104.
[
[
[
[
21] S.-J. Su, T. Chiba, T. Takeda, J. Kido, Adv. Mater. 20 (2008) 2125–2130.
22] S.-J. Su, C. Cai, J. Kido, Chem. Mater. 23 (2) (2011) 274–284.
23] C. Adachi, S.R. Forrest, US Patent 6 (2002) 458–475.
24] J. Yin, S.-L. Zhang, R.-F. Chen, Q.-D. Ling, W. Huang, Phys. Chem.
Chem. Phys. 12 (2010) 15448–15458.
[42] J.F. Ambrose, R.F. Nelson, J. Electrochem. Soc. 115 (11) (1968) 1159–
1164.
[43] J.F. Ambrose, L.L. Carpenter, R.F. Nelson, J. Electrochem. Soc. 122 (7)
(1975) 876–894.
[44] R.J. Holmes, B.W. D’Andrade, S.R. Forrest, X. Ren, J. Li, M.E.
Thompson, Appl. Phys. Lett. 83 (2003) 3818–3820.
[
25] P. Strohriegl, J.V. Grazulevicius, Adv. Mater. 14 (2002) 1439–1452.