A series of CBP-derivatives as host materials for blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c0jm03321a

We report a series of CBP-derivatives with superior thermal and electronic properties for the use as host materials for blue electrophosphorescent organic light emitting diodes. We applied a systematic variation of the substitution pattern in the 2- and 2′-position of the biphenyl unit and the 3- and 6-position of the carbazole moieties. In contrast to the crystalline parent compound CBP, all methyl and trifluoromethyl substituted derivatives show amorphous behaviour. Substitution in the 2- and 2′-position of the biphenyl causes a twisting of the phenyl rings. Hence, the degree of conjugation of the molecules is limited which leads to enlarged triplet energies of approximately 2.95 eV compared to 2.58 eV for CBP. The methyl substitution at the active 3- and 6-position of the pendant carbazole units yields materials with an electrochemically stable behaviour against oxidation. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0jm03321a

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PAPER
A series of CBP-derivatives as host materials for blue phosphorescent organic
light-emitting diodes
a
Pamela Schrogel, Ausra Tomkeviciene,† Peter Strohriegl, Sebastian T. Hoffmann, Anna Kohler
a
a
b
b
*
€
ꢀ
ꢀ
_
€
and Christian Lennartzc
Received 2nd October 2010, Accepted 15th November 2010
DOI: 10.1039/c0jm03321a
We report a series of CBP-derivatives with superior thermal and electronic properties for the use as host
materials for blue electrophosphorescent organic light emitting diodes. We applied a systematic
variation of the substitution pattern in the 2- and 20-position of the biphenyl unit and the 3- and
6-position of the carbazole moieties. In contrast to the crystalline parent compound CBP, all methyl
and trifluoromethyl substituted derivatives show amorphous behaviour. Substitution in the 2- and
20-position of the biphenyl causes a twisting of the phenyl rings. Hence, the degree of conjugation of the
molecules is limited which leads to enlarged triplet energies of approximately 2.95 eV compared to
2.58 eV for CBP. The methyl substitution at the active 3- and 6-position of the pendant carbazole units
yields materials with an electrochemically stable behaviour against oxidation.
triplet energies. The key to such materials is to confine the
conjugated system in the host molecules. In N,N0-dicarbazolyl-
3,5-benzene (mCP) this is accomplished by exchanging the
biphenyl group by a single benzene unit in combination with
meta conjugation instead of para which leads to a triplet energy
of approximately 2.90 eV.7,8 Another approach to enlarge the
triplet energy of CBP based materials comprises the attachment
of two methyl groups in the 2- and 20-position of the central
biphenyl which leads to 4,40-bis(9-carbazolyl)-2,20-dimethylbi-
phenyl (CDBP) with a triplet energy of 2.79 eV.4,9,10 Over the
years many different carbazole based host materials have been
described.11–13 Ma et al. recently described a series of non-
conjugated carbazole host materials where the linkage groups
between two carbazole moieties were varied. This leads to a loss
of conjugation in the molecules and high triplet energies.14
Another crucial requirement for the successful operation of
OLEDs is the ability of the materials to form stable amorphous
films.15 This property guarantees that the emitter is uniformly
diluted in the host to minimize the effect of concentration
quenching. In addition, the absence of grain boundaries, which
may act as trap states, makes the use of organic glasses as OLED
materials advantageous.16,17 The glass transition temperature of
materials for OLED applications is ideally above 100 ꢁC. In
general, the introduction of bulky substituents hinders packing
of the molecules and leads to an amorphous behaviour of the
material. On the other hand, detrimental effects on the charge
carrier transport properties are observed. 9-(4-tert-Butylphenyl)-
3,6-bis(triphenylsilyl)-carbazole (CzSi), for example, combines
appropriate thermal properties with a confined conjugated
system; however, the hole mobility of 5 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 is
rather low.18 By exchanging the triphenylsilyl groups by trityl
Introduction
Recent developments of efficient emitters for organic light
emitting diodes (OLEDs) are often focused on phosphorescent
transition metal complexes. Due to elementary spin statistics
75% triplet excitons and 25% singlet excitons are formed on
initial charge recombination. By fast intersystem crossing all
singlet excitons will be efficiently converted to the triplet state.
Hence, with these phosphorescent emitters the theoretical limit
of the internal quantum efficiency is 100%.1–3 Due to concen-
tration quenching effects phosphorescent materials show a loss in
efficiency if the neat material is used in OLEDs. To avoid self-
quenching it is necessary to dope the emitters into an appropriate
host. It is essential that the triplet energy DE (T1 ꢀ S0) of the host
is higher than that of the emitter in order to prevent energy back
transfer from the phosphorescent guest to the host. 4,40-Bis-
(9-carbazolyl)-biphenyl (CBP) is a widely used matrix material
for phosphorescent emitters. Due to its triplet energy DE
(T1 ꢀ S0) of 2.55 eV,4 CBP is a suitable matrix for green phos-
phorescent emitters like tris(2-phenylpyridine)iridium(III)
(Ir(ppy)3).3,5 Blue emitting materials such as the commonly used
bis((4,6-difluorophenyl)-pyridinato-N,C2)picolinate-iridium(III)
(FIrpic) (DE (T1 ꢀ S0) ¼ 2.62 eV6) require hosts with higher
aLehrstuhl Makromolekulare Chemie I, Universita€t Bayreuth, 95440
Bayreuth, Germany. E-mail: peter.strohriegl@uni-bayreuth.de; Fax:
+49-921-553206; Tel: +49-921-553296
bLehrstuhl Experimentalphysik II, Universita€t Bayreuth, 95440 Bayreuth,
Germany
cBASF SE, 67056 Ludwigshafen, Germany
† Current address: Kaunas University of Technology, Department of
Organic Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania
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groups the hole mobility rises by one order of magnitude to 5 ꢂ
10ꢀ4 cm2 Vꢀ1 sꢀ1. However, this material shows a stronger effi-
ciency roll off at higher current densities than CzSi.19
electrochemical stability of the CBP derivatives. In addition,
DFT calculations were carried out to obtain the ground state
geometries of the CBP derivatives.
We have prepared a series of CBP derivatives in which the
substitution pattern in both the 2- and 20-position of the biphenyl
unit and in the 3- and 6-position of the carbazole unit has been
systematically varied (Scheme 1). The attachment of the methyl
and trifluoromethyl groups in these positions leads to amor-
phous materials with large triplet energies making them suitable
as hosts for blue phosphorescent emitters.
Results and discussion
Synthesis
In a two step synthesis 3-methylcarbazole 3 and 3,6-di-
methylcarbazole
4 were prepared from 4-methyl-phenyl-
hydrazine and cyclohexanone or 4-methyl-cyclohexanone as the
starting materials in a Borsche reaction (Scheme 2a). The first
step yields the 1,2,3,4-tetrahydrocarbazoles 1 and 2 which are
subsequently dehydrogenated with palladium on activated
charcoal.20,21
In this work we report the synthesis of five amorphous deri-
vatives of CBP together with their thermal and optical proper-
ties. The energy levels have been measured by cyclic voltammetry
and absorption measurements. Furthermore, a detailed cyclic
voltammetry study of the materials gives insight into the
The synthesis of the diiodobiphenyls is shown in Scheme 2b.
4,40-Diiodo-2,20-dimethylbiphenyl 5 and 4,40-diiodo-2,20-bis(tri-
fluoromethyl)biphenyl 6 were prepared by diazotization of the
corresponding diamine and subsequent reaction with sodium
iodide.22 The carbazole containing host materials 7–11 were
prepared via the Ullmann coupling reaction of diiodo compound
5 or 6 with carbazole or the methylcarbazoles 3 and 4, respec-
tively (Scheme 2c).
Mass spectrometry, 1H-NMR spectroscopy and 13C-NMR
spectroscopy were used to identify the materials and the data are
given in the Experimental section. The purity of the materials
was monitored by SEC measurements.
Thermal properties
Scheme 1 Chemical structures of the substituted 4,40-bis(9-carbazolyl)-
The thermal properties of the newly synthesized compounds were
examined by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) in a nitrogen atmosphere at a scan-
ning rate of 10 K minꢀ1. All materials reveal high thermal
biphenyls.
stabilities, with an onset of weight loss at temperatures (TID
)
exceeding 310 ꢁC, as determined by TGA measurements
(Table 1).
In Fig. 1, the DSC thermograms of CBP, the methyl
substituted compound 8 and the trifluoromethyl derivative 11 are
presented. The parent compound CBP shows a crystalline
ꢁ
behaviour in the DSC. The melting peak is observed at 285 C
and upon cooling the material crystallises at 183 ꢁC. In contrast,
Table 1 Thermal properties of the carbazole-substituted biphenyls
CBP, CDBP, and 7–11d
a
Entry
Tg/ꢁC
Tm/ꢁC
Tcr/ꢁC
TID /ꢁC
CBP
CDBP
7
8
9
—
94
283
—
—
277
232
210b
233b
205
—
365
310
310
312
310
337
333
106
121
100
105
119
—
200c
—
10
11
—
—
a
Scheme 2 Synthetic routes to (a) methyl substituted carbazoles, (b) til-
ted biphenyls and (c) methyl substituted CBP derivatives. Reagents and
conditions: (i) acetic acid, 80 ꢁC, 30 min; (ii) Pd(C), 1,2,4-trime-
thylbenzene, 170 ꢁC, 6 h; (iii) H2O, HCl, NaNO2, 0–5 ꢁC; (iv) I2, NaI,
dichloromethane, rt, 24 h; (v) Cu, K2CO3, 18-crown-6, o-dichloroben-
zene, reflux, 24 h.
TID is the temperature at which an initial loss of mass was observed in
a thermogravimetric experiment with a heating rate of 10 K minꢀ1 in
a nitrogen atmosphere. Observed only in the first heating scan.
b
c
d
Observed during the heating scan. Tg: glass-transition temperatures,
Tm: melting temperatures, Tcr: crystallisation temperatures and TID
initial decomposition temperatures.
:
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Optical properties
The compounds CBP, CDBP and 7–11 were analyzed by UV/
Vis, fluorescence and phosphorescence spectroscopy. In addi-
tion, the molecular structures of all compounds were simulated
via DFT-calculations to facilitate the interpretation of the
experimental results. The geometry optimized structures of CBP,
8, and 11 are visualized in Fig. 2. Due to the steric demand of the
substituents at the 2- and 20-position at the biphenyl unit, 8 and
11 reveal twisted molecular structures with torsion angles
between the two phenyl rings of the biphenyl unit of 82ꢁ and 74ꢁ
which are significantly higher compared to the more planar CBP
(33ꢁ).
Fig. 1 DSC traces of carbazole based compounds CBP, 8, and 11 at a
scan rate of 10 K minꢀ1, N2 atmosphere. Shown are from top to
bottom: the first heating, second heating, first cooling and second
cooling traces.
Fig. 3 displays the room temperature absorption and fluores-
cence spectra taken from 10ꢀ5 M cyclohexane solutions together
with the phosphorescence spectra obtained from 10 wt% solid
solutions of CBP, 8, and 11 in PMMA at 10 K. In order to
understand how the substitutions affect the excited states of these
compounds, we first consider the effect of the methyl substitution
at the central biphenyl unit by comparing CBP and 8. For 8, the
first and second absorption are at 351 nm and 300 nm which can
both be assigned to the absorption of the 3,6-dimethylcarbazole
units. In the case of CBP, the features associated with transitions
localized on the carbazole are still present at 339 nm and 290 nm.
The observed bathochromic shift of approximately 10 nm in
compound 8 is caused by the additional methyl groups at the
compound 8 exhibits a melting peak at 277 ꢁC. In the cooling
cycle no crystallisation is observed as the material solidifies in an
amorphous phase. In the second heating curve the glass transi-
tionꢁis observed at 121 ꢁC followed by a recrystallisation at
200 C. The CF3-substituted derivative 11 remains in the amor-
phous phase after the first melting atꢁ233 ꢁC. In the second
heating only the glass transition at 119 C is observed.
These results show that by the introduction of additional
methyl- and trifluoromethyl groups into the basic CBP structure
the thermal properties are improved. While the parent
compound CBP is highly crystalline, all CBP derivatives 7–11
reveal high glass transition temperatures ranging from 94 to 121
ꢁC and all materials remain amorphous upon cooling (Table 1).
For OLED applications materials with high glass transition
temperatures (Tg z 100 ꢁC) are advantageous for the opera-
tional stability of the device. In a morphologically stable amor-
phous host material the emitter molecules are homogenously
diluted which prevents concentration quenching. The melting,
crystallisation and glass transition temperatures of all derivatives
are summarised in Table 1.
Fig. 3 Comparison of the absorption (dark grey with circles), fluores-
cence (light grey with triangles) and phosphorescence (black) spectra of
CBP, 8, and 11. Absorption and fluorescence were taken in 10ꢀ5
M
Fig. 2 Geometry optimized structures of CBP, 8, and 11 (top to bottom)
cyclohexane solutions at 300 K, phosphorescence was measured in
10 wt% solid solutions in PMMA at 10 K.
with different torsion angles.
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carbazole units. In CBP, however, there is an additional broad
absorption centred at about 320 nm. This feature is likely to be
associated with transitions between orbitals that involve the
central biphenyl unit of the molecule. In the fluorescence spectra,
in contrast, CBP and 8 show very similar vibrational structures
with a strong maximum at about 355 nm for both compounds.
We now consider how changing the substituents on the central
biphenyl unit from methyl to trifluoromethyl affects the optical
transitions. While 8 shows the signature of the carbazole-based
transitions in the absorption and fluorescence spectra, 11 has an
additional contribution to the absorption, centred at about
325 nm. As for CBP, we consider the additional absorption to
involve the central biphenyl unit. The main difference of
compound 11 compared to CBP and 8 is the observed featureless
fluorescence centred at 410 nm. The broad bathochromically
shifted fluorescence is clearly not due to transitions localized on
the carbazole. The absence of vibrational fine structure rather
points to a charge-transfer type transition, for example from the
carbazole moiety to the central, trifluoromethyl substituted
biphenyl rings.
phosphorescence spectroscopy was carried out in 10 wt% solid
solutions of PMMA.
Cyclic voltammetry
The electrochemical behaviour of the CBP-derivatives was
studied by cyclic voltammetry in a conventional three-electrode
cell using a platinum working electrode, a platinum wire counter
electrode, and a Ag/AgNO3 reference electrode. In particular, the
oxidation processes were investigated in dichloromethane solu-
tions. The HOMO levels of the compounds were estimated from
the half-wave potential of the first oxidation relative to ferrocene.
The LUMO levels were calculated by adding the optical band
gap to the HOMO levels. Table 3 lists the values for the HOMO
and LUMO levels. In the CV-experiments CBP and CDBP show
very similar HOMO levels of 5.63 eV and 5.64 eV. This obser-
vation is not surprising as in both molecules the HOMO is
mainly located on the electron rich carbazole units. In the case of
the methyl substitution in the 3- and 6-position of the carbazole
units the additional +I-effect on the carbazole shifts the HOMO
levels to slightly higher values. The HOMO level is 5.56 eV for
compound 7 and 5.52 eV for compound 8. In the case of the
compounds 9–11 the strong ꢀI-effect of the trifluoromethyl
substituents causes a decrease of the HOMO level of approxi-
mately 0.1 eV (5.74 eV for 9) in comparison with CDBP
(5.64 eV). By the subsequent introduction of methyl groups at the
carbazole moieties in compounds 10 and 11 the HOMO level
rises again to 5.68 eV and 5.65 eV, respectively. In contrast, the
LUMO levels of all compounds are less affected by the intro-
duction of the substituents on the carbazole units. However, in
For applications of compounds 7–11 as host materials for blue
phosphorescent emitters the triplet energies are of major interest.
In the phosphorescence spectra of compounds 8 and 11, we
observe two sharp peaks at 420 nm and 450 nm as well as a broad
peak centred at 480 nm. On the contrary, the emission of CBP is
bathochromically shifted by 60 nm. As a result, 8 and 11 reveal
a triplet energy of approximately 2.95 eV (420 nm), significantly
higher than that of CBP of 2.58 eV (480 nm). Apparently, the
conjugation between the two phenyl rings is interrupted by
introducing CH3 and CF3 substituents at the 2- and 20-position
of the central biphenyl leading to higher triplet energies.
All results of the photophysical investigations of compounds
CBP, CDBP, and 7–11 are summarised in Table 2. With
increasing CH3-content on the pendant carbazole units in the
series CDBP, 7, and 8 as well as in the series of 9, 10, and 11
a small bathochromic shift is noticeable in both the absorption
and fluorescence spectra. We attribute this to the electron
donating effect of the CH3-units. All compounds which are
twisted by substituents at the central biphenyl show triplet
energies of approximately 2.95 eV. These high triplet energies of
the materials CDBP and 7–11 make them suitable host materials
for deep blue phosphorescent emitters. It should be noted that
the triplet energies of CBP and CDBP are slightly higher than in
neat films reported by Tokito et al.4,9,10 as in our case the
Table 3 Experimentally determined energy levels of the CBP derivatives
Entry
HOMOa/eV
LUMOb/eV
CBP
CDBP
7
8
9
5.63
5.64
5.56
5.52
5.74
5.68
5.65
2.16
2.13
2.13
2.13
2.20
2.22
2.21
10
11
a
Estimated from the half-wave potential of the first oxidation in the
cyclic voltammetry measurements. Estimated from the HOMO values
and the optical band gap.
b
Table 2 Optical properties of CBP, CDBP and the CBP-derivatives 7–11
Entry
lEAa/nm, solution
lEAb/nm, film
lRT c/nm
l10K d/nm
DE (S0 ꢀ S1)e/eV
DE (T1 ꢀ S0)/eV
em
em
CBP
CDBP
7
8
9
350
346
353
357
343
350
353
357
353
361
365
350
358
360
356, 374
342, 358
349, 366
355, 372
368
477
419
417
421
419
419
420
3.47
3.51
3.43
3.39
3.54
3.46
3.44
2.58
2.95
2.97
2.95
2.95
2.95
2.95
10
11
373
379
a
Edge of absorption measured in 10ꢀ5 M cyclohexane solutions at room temperature. b Edge of absorption measured on neat films at room temperature.
Wavelengths of the intensity maxima of the fluorescence at 300 nm excitation of 10ꢀ5 M cyclohexane solutions at room temperature. d Wavelength of
c
the highest energy maximum measured on film samples of 10 wt% compound in PMMA at 10 K. e The optical band gap was determined from the UV/
Vis absorption onset of neat films.
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compounds 9–11 the trifluoromethyl substitution at the biphenyl
lowers the LUMO level slightly by approximately 8 meV
compared to compounds CDBP, 7, and 8.
These considerations show that the energy levels of the CBP
derivatives can be fine tuned to some extent by the variation of
the substitution pattern at the connecting biphenyl moiety as well
as at the pendant carbazoles. Especially, the HOMO levels can be
varied as can be seen in Fig. 4. Thus, with these slight variations
in the molecular structure the energy levels of the different layers
in an OLED can be adjusted to each other in order to minimize
energy barriers within the device.
Fig. 6 Formation of the radical cation of N-phenylcarbazole upon
oxidation and subsequent dimerisation at the active 3-position.
oligomeric species. The electrochemical behaviour of 7 is very
similar to CBP. However, the signal between 0.4 and 0.6 V
indicating the coupling of two carbazole units is growing more
slowly compared with CBP, as in compound 7 oligomerisation
can occur only at the unblocked 6-position. In contrast,
compound 8 reveals a fully reversible oxidation behavior which
can be attributed to the complete blocking of the active 3- and 6-
position of the carbazole units.
Cyclic voltammetry experiments with repeated cycles give an
insight into the electrochemical stability of the CBP derivatives.
As radical cations and anions are involved in charge transport
processes the electrochemical stability of the materials used in
electroluminescent devices contributes to the overall stability of
the device. Fig. 5 shows the cyclic voltammograms with five
repeated oxidation cycles of CBP, 7, and 8 in 2 ꢂ 10ꢀ3
M
dichloromethane solutions.
CBP reveals an irreversible oxidation behaviour as here oli-
gomerisation reactions of the oxidised species can take place at
the active 3- and 6-position of the carbazole units. This kind of
dimerisation is known from triphenylamine and N-phenyl-
carbazole. The mechanism is shown in Fig. 6.23,24 In the first step
the molecule is oxidised at the electron lone pair of the nitrogen
atom and a radical cation is formed. The radical stabilises into
the 3-position of the carbazole where recombination of two
radical molecules takes place under elimination of two protons.
The oligomeric species are oxidised more easily, i.e. at lower
voltages. In the cyclic voltammogram the emerging signal at 0.6
V to 0.7 V is assigned to the oxidation of the newly formed
Experimental section
Materials
All chemicals and reagents were used as received from
commercial sources without further purification. 2,20-Bis(tri-
fluoromethyl)benzidine was synthesised according to a proce-
dure described by Rogers et al.25 The solvents for reactions and
purification were all distilled before use.
Characterization
1H- and 13C-NMR spectra were recorded with a Bruker AC 300
(300 MHz, 75 MHz) and CDCl3 as a solvent. All data are given
as chemical shifts d (ppm) downfield from Si(CH3)4. For optical
measurements, 10ꢀ5 M cyclohexane solutions of the materials as
well as thin films on quartz substrates were prepared. Both neat
films and films with 10 wt% of the compound in poly-
(methylmethacrylate) were prepared by spincoating. The UV/Vis
spectra were measured in solution and on neat films with
a Hitachi U-3000 spectrometer. Fluorescence spectra in solution
were obtained from a Shimadzu spectrofluorophotometer RF-
5301PC using excitation at 300 nm. The phosphorescence spectra
were taken with the thin film samples mounted in a continuous
flow helium cryostat. The temperature was controlled with an
Oxford Intelligent temperature controller-4 (ITC-502). Excita-
tion was provided by a pulsed, frequency-tripled Nd:YAG laser
Fig. 4 Energy diagram showing the location of the HOMO and LUMO
levels of the different CBP-derivatives. The solid line displays the position
of the triplet energies DE (T1 ꢀ S0).
Fig. 5 Cyclic voltammograms of CBP, 7, and 8 (five scans, scan rate 50 mV sꢀ1, 2 ꢂ 10ꢀ3 M in CH2Cl2).
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at 355 nm (3.49 eV) (Spectron SL401). This wavelength corre-
sponds to the red tail of the first absorption band in our
compounds. The duration of the laser pulses was 6 ns and the
laser was operated at a repetition rate of 10 Hz by a self-made
electronic delay generator. The light emitted by the sample was
dispersed and subsequently detected by a time gated intensified
CCD camera (Andor iStar DH734-18F-9AM). The measure-
ments were taken with a delay time of 500 ns and a gate width of
60 ms. The measurements were carried out at an excitation
density of about 250 mJ cmꢀ2 pulseꢀ1 on films of about 150 nm
thickness as determined by a Dektak profilometer. To increase
the signal-to-noise-ratio, all spectra were obtained by averaging
over 2000 laser shots. For differential scanning calorimetry
(DSC) measurements a Diamond DSC apparatus from Perkin
Elmer was used (heating/cooling rate 10 K minꢀ1). Thermogra-
vimetric analysis (TGA) was performed on a Mettler Toledo
TGA/SDTA815e machine at a heating rate of 10 K minꢀ1 in
a nitrogen atmosphere. Cyclic voltammetry measurements were
carried out in absolute solvents measuring at a platinum working
dichloromethane and filtered over neutral aluminium oxide.
After the removal of dichloromethane, hexane was added and the
product was obtained as white precipitate. Yield: 88%. EI-MS m/
z: 181 (100, M+). 1H-NMR (300 MHz, CDCl3), d (ppm): 8.06 (dd,
1H), 7.94 (s, 1H, NH), 7.90 (ds, 1H), 7.41 (m, 2H), 7.35 (d, 1H),
7.27–7.21 (m, 2H), 2.55 (s, 3H). 13C-NMR (75 MHz, CDCl3),
d (ppm): 139.82, 137.71, 137.26, 128.75, 127.18, 125.65, 123.14,
120.26, 120.24, 119.22, 110.55, 110.24, 21.44.
3,6-Dimethylcarbazole (4). Compound 4 was prepared
according to the procedure given for 3. Yield: 86%. EI-MS m/z:
1
195 (100, M+). H-NMR (300 MHz, CDCl3), d (ppm): 7.85 (m,
2H), 7.84 (s, 1H, NH), 7.31 (d, 2H), 7.24 (dd, 2H), 2.55 (s, 6H).
13C-NMR (75 MHz, CDCl3), d (ppm): 138.07, 128.48, 126.99,
123.41, 120.18, 110.22, 21.44.
General procedure for the preparation of substituted biphenyls
4,40-Diiodo-2,20-dimethylbiphenyl (5). 2,20-Dimethylbenzidine
dihydrochloride (14.3 g, 50 mmol) was suspended in 125 mL of
water and 15 mL of conc. hydrochloric acid and the suspension
was cooled to 0–5 ꢁC. A solution of sodium nitrite (7.18 g,
100 mmol) in 20 mL of water was added dropwise. The resulting
cold tetrazonium salt solution was added slowly to a well-stirred
solution of iodine (30.5 g, 120 mmol) and sodium iodide (30.0 g,
200 mmol) in 50 mL of water and 100 mL of dichloromethane at
electrode versus
a Ag/AgNO3 reference electrode. Each
measurement was calibrated against an internal standard
(ferrocene/ferrocenium redox system). The purity of the target
compounds was checked with a Waters size exclusion chroma-
tography system (SEC) for oligomers (analytical columns:
crosslinked polystyrene gel (Polymer Laboratories), length: 2 ꢂ
ꢁ
ꢁ
60 cm, width: 0.8 cm, particle size: 5 mm, pore size: 100 A, eluent:
a temperature below 5 C. The mixture was stirred for 24 h at
THF (0.5 mL minꢀ1, 80 bar), polystyrene standard).
room temperature and the excess of iodine was removed by the
addition of a sodium thiosulfate solution. The product was
extracted with dichloromethane and washed several times with
water. After drying the organic layer over anhydrous sodium
sulfate, the solvent was evaporated and the residue was purified
by column chromatography on silica gel with hexane as eluent to
afford compound 5. Yield: 16.6 g (76%). EI-MS m/z: 434 (100,
Calculations
Geometries were optimized using the BP86-functional26,27 in
combination with a split-valence basis set (SV(P)) including
polarization functions on all heavy atoms.28 All calculations were
carried out with the turbomole program package.29
1
M+). H-NMR (300 MHz, CDCl3), d (ppm): 7.63 (d, 2H), 7.55
(dd, 2H), 6.79 (d, 2H), 1.98 (s, 6H). 13C-NMR (75 MHz, CDCl3),
d (ppm): 139.99, 138.75, 138.22, 134.81, 130.87, 93.13, 19.47.
Synthetic procedures
General procedure for the preparation of methyl substituted
carbazoles
4,40-Diiodo-2,20-bis(trifluoromethyl)biphenyl (6). Compound
6 was prepared according to the procedure given for 5. Yield:
1.91 g (75%). EI-MS m/z: 542 (100, M+). H-NMR (300 MHz,
1
3-Methyl-6,7,8,9-tetrahydro-5H-carbazole (1). 3-Methyl-
phenylhydrazine hydrochloride (10.0 g, 63 mmol) was added to
a solution of cyclohexanone (6.2 g, 63 mmol) in acetic acid
(30 mL) under an argon atmosphere over a period of one hour.
After stirring at 80 ꢁC for half an hour the reaction mixture was
extracted with dichloromethane and washed several times with
5% aqueous sodium hydrogen carbonate solution to neutralise
the acetic acid. After drying the organic layer over anhydrous
sodium sulfate the solvent was evaporated. Yield: 9.25 g (79%).
EI-MS m/z: 185 (95, M+).
CDCl3), d (ppm): 8.07 (d, 2H), 7.89 (dd, 2H), 6.99 (d, 2H). 13C-
NMR (75 MHz, CDCl3), d (ppm): 139.97, 135.88, 135.06, 132.86,
1
2
130.32 (q, J(C-F) ¼ 30.8 Hz), 122.56 (q, J(C-F) ¼ 273.0 Hz),
93.55.
General procedure for the Ullmann condensation
4,40-Bis(3-methylcarbazolyl)-2,20-dimethylbiphenyl (7). 4,40-
Diiodo-2,20-dimethylbiphenyl (5) (1 g, 2.3 mmol), 3-methyl-
carbazole (3) (1.0 g, 5.52 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) were refluxed in 15 mL of o-dichloroben-
zene 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 hexane/tetrahydrofuran (20 : 1) as
eluent yielded 0.72 g (58%) of 7 as white solid. EI-MS m/z: 540
3,6-Dimethyl-6,7,8,9-tetrahydro-5H-carbazole (2). Compound
2 was prepared according to the procedure described for 1. Yield:
93%.
3-Methylcarbazole (3). To 9.25 g (50 mmol) of 3-methyl-
6,7,8,9-tetrahydro-5H-carbazole (1) in 30 mL of 1,2,4-trime-
thylbenzene was added 10% palladium on activated charcoal
(2.66 g, 25 mmol). The mixture was refluxed for 6 h. In order to
remove the catalyst the mixture was diluted with
1
(100, M+). H-NMR (300 MHz, CDCl3), d (ppm): 8.14 (d, 4H),
7.97 (m, 4H), 7.55–7.43 (m, 12H), 7.32–7.27 (m, 4H), 2.58 (s, 6H),
2.29 (s, 6H). 13C-NMR (75 MHz, CDCl3), d (ppm): 141.11,
139.83, 139.23, 137.88, 137.11, 130.78, 129.31, 128.19, 127.24,
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J. Mater. Chem., 2011, 21, 2266–2273 | 2271

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125.76, 124.16, 123.57, 123.30, ꢁ120.28, 119.72, 109.88, 109.65,
7–11 which have a much lower tendency to crystallise. Their glass
transition temperatures range from 94–121 ꢁC. The highest glass
transition temperatures were determined for 3,6-dimethylcarba-
zole bearing derivatives 8 and 11 with 121 ꢁC and 119 ꢁC,
respectively.
21.45, 20.17. Tm: —ꢁC; Tg: 106 C.
4,40-Bis(9-carbazolyl)-2,20-dimethylbiphenyl
(CDBP),
4,40-bis(3,6-dimethylcarbazolyl)-2,20-dimethylbiphenyl (8), 4,40-
bis(9-carbazolyl)-2,20-bis(trifluoromethyl)biphenyl (9),30 4,40-
bis(3-methylcarbazolyl)-2,20-bis(trifluoromethyl)biphenyl (10),
and 4,40-bis(3,6-dimethylcarbazolyl)-2,20-bis(trifluoromethyl)-
biphenyl (11) were prepared according to the procedure given
for 7.
The main effect of the methyl substitution in 2- and 20-position
of the biphenyl unit is a twisting of the two central phenyl rings.
Due to this electronic decoupling, the conjugation length in the
molecule is limited which causes an increase of the triplet energy
DE (T1 ꢀ S0) from 2.58 eV for CBP to 2.95–2.97 eV for 7–11.
Fine-tuning of the energy levels, especially the HOMO levels,
can be achieved by a suitable choice of substitution pattern of the
CBP derivatives. Cyclic voltammetry with repeated cycles shows
that by introducing substituents at the 3- and 6-position of the
pendant carbazole units the oxidation to the radical cation
becomes fully reversible and thus electrochemical stable host
materials are accessible.
4,40-Bis(9-carbazolyl)-2,20-dimethylbiphenyl (CDBP). Yield:
1
0.59 g (50%). EI-MS m/z: 512 (100, M+). H-NMR (300 MHz,
CDCl3), d (ppm): 8.20 (d, 4H), 7.58–7.45 (m, 14H), 7.36–7.31 (m,
4H), 2.31 (s, 6H). 13C-NMR (75 MHz, CDCl3), d (ppm): 140.96,
140.01, 137.95, 136.92, 130.81, 128.36, 125.94,ꢁ 124.34, 123.42,
120.36, 119.94, 109.94, 20.17. Tm: —ꢁC; Tg: 94 C.
4,40-Bis(3,6-dimethylcarbazolyl)-2,20-dimethylbiphenyl
(8).
Yield: 0.7 g (50%). EI-MS m/z: 568 (100, M+). H-NMR (300
MHz, CDCl3), d (ppm): 7.93 (ds, 4H), 7.52–7.40 (m, 10H), 7.26
(dd, 4H), 2.57 (s, 12H), 2.28 (s, 6H). 13C-NMR (75 MHz, CDCl3),
d (ppm): 139.64, 139.40, 137.82, 137.32, 130.75, 129.06, 128.01,
127.07, 123.98, 123.46, 120.22, 109.60, 21.45, 20.19. Tm: 277 ꢁC;
1
Acknowledgements
The authors thank Irene Bauer and Dr Michael Rothmann for
the help during synthesis and characterisation of the novel CBP
€
derivatives. We also thank Dr Ingo Munster, Dr Evelyn Fuchs
ꢁ
Tg: 121 C.
and Dr Nicolle Langer for fruitful discussions. Financial support
from the BMBF project TOPAS 2012 (FKZ 13N 10447) is
4,40-Bis(9-carbazolyl)-2,20-bis(trifluoromethyl)biphenyl (9).
Yield: 0.71 g (62%). EI-MS m/z: 620 (100, M+). H-NMR (300
€
gratefully acknowledged. P.S. thanks the Universitat Bayern e.V.
1
for a grant.
MHz, CDCl3), d (ppm): 8.20 (d, 4H), 8.09 (d, 2H), 7.93 (dd, 2H),
7.71 (d, 2H), 7.55–7.48 (m, 8H), 7.40–7.35 (m, 4H). 13C-NMR (75
MHz, CDCl3), d (ppm): 140.41, 138.33, 135.32, 133.49, 133.41,
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Conclusions
We have described a number of CBP derivatives in which the
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