Triazine based bipolar host materials for blue phosphorescent OLEDs

Article abstract of DOI:10.1021/cm4023216

Two novel bipolar host materials BPTRZ and MBPTRZ were synthesized, in which the hole transporting carbazole is separated from the electron transporting triazine moiety by a fully aromatic, but nonconjugated meta-linked biphenyl unit. The additional twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2- and 2′-position of the biphenyl leads to a higher triplet energy of 2.81 eV compared to 2.70 eV for BPTRZ. Both materials possess high thermal stabilities and good glass forming properties. An organic light emitting diode with MBPTRZ as host for the blue phosphorescence emitter FIrpic shows a maximum luminance of 30600 cd/m² and a maximum external quantum efficiency of 7.0%.

Full text of DOI:10.1021/cm4023216

Article
pubs.acs.org/cm
Triazine Based Bipolar Host Materials for Blue Phosphorescent
OLEDs
Daniel Wagner,† Sebastian T. Hoffmann,‡ Ute Heinemeyer,§ Ingo Munster,§ Anna Kohler,‡
̈
̈
and Peter Strohriegl†,*
†Makromolekulare Chemie I and Bayreuther Institut fur Makromolekulforschung, University of Bayreuth, 95440 Bayreuth, Germany
̈
̈
‡Experimentalphysik II, University of Bayreuth, 95440 Bayreuth, Germany
§BASF SE, 67056 Ludwigshafen, Germany
ABSTRACT: Two novel bipolar host materials BPTRZ and MBPTRZ were synthesized,
in which the hole transporting carbazole is separated from the electron transporting
triazine moiety by a fully aromatic, but nonconjugated meta-linked biphenyl unit. The
additional twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2-
and 2′-position of the biphenyl leads to a higher triplet energy of 2.81 eV compared to 2.70
eV for BPTRZ. Both materials possess high thermal stabilities and good glass forming
properties. An organic light emitting diode with MBPTRZ as host for the blue
phosphorescence emitter FIrpic shows a maximum luminance of 30600 cd/m2 and a
maximum external quantum efficiency of 7.0%.
KEYWORDS: host material, bipolar, triazine, carbazole, phosphorescence, blue OLED
N,N′-dicarbazolyl-3,5-benzene (mCP) with a high triplet energy
INTRODUCTION
■
of 2.90 eV has been reported as host material for saturated blue
OLEDs.4,7 The introduction of torsion in the biphenyl unit of the
host material 4,4′-bis(9-carbazolyl-2,2′-dimethylbiphenyl
(CDBP) caused by steric hindrance of two methyl substituents
in 2- and 2′-position leads to a triplet energy of 2.79 eV.8,9 By
tuning the linking between the carbazole and the central biphenyl
unit from the para to the meta position, the triplet energy in 3,3′-
bis(carbazolyl)biphenyl (m-CBP)10,11 is enlarged to 2.98 eV.11
To achieve efficient OLEDs a balanced charge carrier transport
and a broad recombination zone are required. By the design of
bipolar host materials, this task can be accomplished.12 In bipolar
host materials, holes and electrons are transported through
different parts of the molecule. The hole transport in many cases
occurs through carbazole units due to sufficiently high triplet
energy and good hole transporting properties,13,14 whereas
electron transport is often realized by the use of electron
accepting N-heterocycles such as triazines15 or oxadiazoles.16 In
recent years, a number of bipolar host materials such as 2,6-bis(3-
(carbazol-9-yl)phenyl)pyridine (26DCzPPy)17 and bis-4-(N-
carbazolyl)phenyl)phenylphosphine oxide (BCPO)18 have been
described. The literature on bipolar host materials has been
summarized by Chaskar et al.19
The discovery of phosphorescent emitters was one of the major
breakthroughs toward efficient OLEDs. According to spin
statistics, 75% triplet excitons and 25% singlet excitons are
formed by the recombination of an electron and a hole. By
harvesting both singlet and triplet excitons through intersystem
crossing (ISC), phosphorescent organic light emitting diodes
(PhOLEDs) approach 100% internal quantum efficiency.1,2 In a
PhOLED, the phosphorescent emitter is usually embedded in a
suitable host to avoid self-quenching and triplet−triplet-
annihilation. For an efficient energy transfer, it is essential that
the triplet energy of the host is higher compared to the emitter to
prevent reverse energy transfer from the emitter back to the host
and to confine triplet excitons effectively on the emitter
molecules.3,4 Effective host−guest systems have been commer-
cialized for green and red emitters, whereas blue phosphors and
suitable matrix materials are still a challenge. The best known
blue emitter is the light blue bis[(4′,6′-difluorophenyl)-
pyridinato-N,C2′]iridium(III) picolinate (FIrpic) with a triplet
energy of 2.65 eV (470 nm).4 Other blue phosphors with a more
saturated blue emission are iridium(III) bis(4′,6′-
difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6)
and mer-tris(N-dibenzofuranyl-N′-methylimidazole)iridium(III)
(Ir(dbfmi)) showing triplet levels of 2.72 eV (458 nm)5 and 2.79
eV (445 nm).6
1,3,5-triazine derivatives such as 3-(4,6-diphenyl-1,3,5-triazin-
2-yl)-9-phenyl-9H-carbazole (DPTPCz),20 2-(3-(3-(N,N-bis(4-
(1,1-dimethylehtyl)phenyl)-amino)phenyl)-phenyl)-4,6-di-
phenyl-1,3,5-triazine (tBu-TPA-m-TRZ),21 9-(4,6-diphenyl-
1,3,5-triazine-2-yl)-9′-phenyl-3,3′-bicarbazole (CzT),22 and 2-
The confinement of the conjugated system is the key to host
materials with high triplet energies. A widely used host material
for green and red emitters is 4,4′-bis(9-carbazolyl)-biphenyl
(CBP) with a triplet level of 2.56 eV.3 By replacing the central
biphenyl group in CBP by a single, meta-linked benzene unit,
Published: August 13, 2013
© 2013
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3-(Carbazol-9-yl)-3′-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan)biphenyl (6). Yield: 87%. 1H NMR (300 MHz,
CDCl3), δ (ppm): 8.16 (d, 2H), 8.08 (s, 1H), 7.90−7.80 (m, 2H),
7.74 (dt, 2H), 7.66 (t, 1H), 7.53 (dt, 1H), 7.51−7.37 (m, 5H), 7.32−
7.26 (m, 2H), 1.35 (s, 12H). 13C NMR (75 MHz, CDCl3), δ (ppm):
143.3, 141.0, 139.6, 138.2, 134.3, 133.7, 130.3, 130.1, 128.5, 126.5, 126.1,
126.0, 125.9, 123.5, 120.4, 120.0, 110.0, 84.1, 25.1, 25.0. EI-MS m/z: 445
(100, M+), 345 (36).
3-(Carbazol-9-yl)-6,6′-dimethyl-3′-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan)biphenyl (7). Yield: 85%. 1H NMR (300 MHz,
CDCl3), δ (ppm): 8.15 (d, 2H), 7.74 (d, 1H), 7.65 (s, 1H), 7.52−7.38
(m, 6H), 7.35−7.25 (m, 4H), 2.23 (s, 3H), 2.19 (s, 3H), 1.35 (s, 12H).
13C NMR (75 MHz, CDCl3), δ (ppm): 143.2, 141.1, 140.2, 139.3, 135.7,
(4-(3,6-dimethylcarbazol-9-yl)-phenoxy)-bis-4,6-biscarbazolyl-
1,3,5-triazine (PCTrz)23 have been reported as bipolar host
materials owing to the high triplet energies and the excellent
electron transport ability of the triazine moiety.
In a previous paper, we have presented the bipolar matrix
material PCTrz with a hole transporting carbazole and an
electron transporting triazine moiety.23 By using the concepts of
twisted24 or meta-linked11 biphenyls to break the conjugation,
we have achieved CBP-like materials with large triplet energies.
Here, we present a combination of both concepts. In the two
novel bipolar matrix materials BPTRZ and MBPTRZ, a hole
transporting carbazole and an electron transporting triazine
moiety are connected by a fully aromatic but nonconjugated
biphenyl bridging group.
135.6, 135.1, 134.1, 131.3, 129.6, 128.0, 126.0, 125.8, 123.4, 120.4, 119.8,
110.0, 83.9, 25.1, 25.0, 20.4, 19.8. EI-MS m/z: 473 (100, M+), 415 (22),
373 (88), 229 (23), 166 (52).
General Procedure for Suzuki-Miyaura Cross-Coupling. 2,4-
Bis(carbazol-9-yl)-6-chloro-1,3,5-triazine 1 (0.71 g, 1.6 mmol) and 3-
(carbazol-9-yl)-3′-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan)biphenyl 6
or 7 (1.9 mmol) were dissolved in dioxane (65 mL) and toluene (22
mL). After the addition of potassium phosphate (0.57 g, 2.7 mmol) in 18
mL water, the mixture was degassed by three freeze−pump−thaw cycles
and the flask was backfilled with argon. Pd2(dba)3 (45 mg, 0.05 mmol)
and PCy3 (30 mg, 0.11 mmol) were added, and the mixture was
degassed again by three freeze−thaw cycles and the flask backfilled with
argon. The mixture was stirred at 90 °C for 20 h. After cooling to room
temperature, the mixture was poured into water and then extracted with
DCM. The combined organic phase was washed with water, filtered, and
dried over sodium sulfate. The solvent was evaporated and the crude
product was boiled in ethyl acetate and hot filtered to yield BPTRZ and
MBPTRZ as white solids after cooling. The product was further purified
by train sublimation.
3-(Carbazol-9-yl)-3′-(4,6-(dicarbazol-9-yl)-1,3,5-triazin-2-yl)-
1,1′-biphenyl (BPTRZ). Yield: 52%. 1H NMR (300 MHz, CDCl3), δ
(ppm): 9.11−9.00 (m, 5H), 8.73 (d, 1H), 8.20 (d, 2H), 8.12−8.03 (m,
4H), 7.98 (d, 2H), 7.90 (d, 1H), 7.82−7.71 (m, 2H), 7.66 (d, 1H), 7.52
(d, 2H), 7.43−7.27 (m, 12H). 13C NMR (75 MHz, CDCl3), δ (ppm):
172.6, 164.7, 142.3, 140.9, 140.6, 138.8, 136.8, 131.3, 130.6, 129.7, 128.4,
127.9, 127.0, 126.5, 126.1, 125.8, 123.4, 120.4, 120.0, 119.7, 117.5, 109.8.
EI-MS m/z: 728 (100, M+), 344 (22), 166 (22).
EXPERIMENTAL SECTION
Materials. All chemicals and reagents were used as received from
commercial sources without further purification. The solvents for
reactions and purification were distilled before use.
2,4-Bis(carbazol-9-yl)-6-chloro-1,3,5-triazine (1). Compound 1
was synthesized according to a procedure reported by Rothmann et al.25
Yield: 70%.
■
3,3′-Dibromobiphenyl (2). 3-Bromophenylboronic acid (5.0 g,
24.9 mmol) was dissolved in methanol (65 mL). After addition of
copper(I) chloride (0.05 g, 0.51 mmol), the mixture was stirred at room
temperature for 4 h in air. The reaction mixture was filtered and washed
with ethyl acetate. The solvent was evaporated and the crude product
was purified by column chromatography on silica gel with hexane as
eluent to afford 2.32 g (7.4 mmol, 60%) of 2 as white solid. 1H NMR
(300 MHz, CDCl3), δ (ppm): 7.70 (t, 2H), 7.52−7.45 (m, 4H), 7.31 (t,
2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 141.9, 130.9, 130.5, 130.3,
125.9, 132.1. EI-MS m/z: 312 (100, M+), 152 (100), 76 (51).
3,3′-Diiodo-6,6′-dimethylbiphenyl (3). Compound 3 was
synthesized according to the procedure reported by Schrogel et al.11
̈
Yield: 32%.
General Procedure for Ullmann-type Reaction. The 3,3′-
dihalogenobiphenyls 2 or 3 (9.2 mmol), carbazole (1.54 g, 9.2 mmol),
copper iodide (0.18 g, 0.9 mmol), trans-1,2-diaminocyclohexane (0.11
g, 0.9 mmol), and potassium phosphate (4.11 g, 19.4 mmol) were
dissolved in dioxane (60 mL) under argon atmosphere. The mixture was
refluxed for 20 h. After cooling to room temperature, the mixture was
diluted with THF. The copper catalyst and inorganic salts were removed
by filtration over neutral aluminum oxide. The solvent was evaporated
and the residue was purified by column chromatography on silica gel
with hexane/toluene (4:1) as eluent to yield 4 and 5 as light yellow
solids.
3-(Carbazol-9-yl)-6,6′-dimethyl-3′-(4,6-(dicarbazol-9-yl)-
1
1,3,5-triazin-2-yl)-1,1′-biphenyl (MBPTRZ). Yield: 92%. H NMR
(300 MHz, CDCl3), δ (ppm): 9.03 (d, 4H), 8.62 (m, 2H), 8.16 (d, 2H),
8.09 (d, 4H), 7.62−7.28 (m, 18H), 2.37 (s, 3H), 2.35 (s, 3H). 13C NMR
(75 MHz, CDCl3), δ (ppm): 172.8, 164.6, 142.4, 141.4, 141.1, 138.8,
135.4, 135.2, 133.8, 131.6, 130.9, 130.2, 128.3, 128.0, 126.9, 126.4, 126.3,
125.9, 123.3, 120.3, 119.8, 119.7, 117.4, 109.7, 20.4, 19.7. EI-MS m/z:
756 (100, M+), 378 (26), 166 (32).
1
Characterization. H and 13C NMR spectra were recorded with a
1
3-(Carbazol-9-yl)-3′-bromobiphenyl (4). Yield: 47%. H NMR
Bruker AC-300 (300 MHz, 75 MHz) and CDCl3 as solvent. All data
were given as chemical shifts δ (ppm) downfield from Si(CH3)4. MS
spectra were received on a Finnigan Mat 8500, MAT 112 S Varian
machine using EI-ionization. For differential scanning calorimetry
measurements (DSC), a Perkin-Elmer Diamond DSC apparatus with
heating and cooling rates of 10 K/min under nitrogen atmosphere was
used. Thermogravimetric analysis (TGA) was performed on a Mettler
Toledo TGA/SDTA851e machine at a heating rate of 10 K/min under a
nitrogen atmosphere. Cyclic voltammetry measurements (CV) were
performed with an EG&G Princeton Applied Research Potentiostat/
Galvanostat Model 263 A. The measurements were carried out in
absolute solvents measuring at a platinum working electrode vs a Ag/
AgNO3 reference electrode. Each measurement was calibrated against
an internal standard (ferrocene/ferrocenium redox system). The target
molecules were finally purified by sublimation at a Carbolite HZS 12/
450 three zone tube furnace equipped with a Leybold PT 151/361 KIT
turbomolecular pump. 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, 2 × 60 cm; width, 0.8 m; particle size, 5 μm; pore
size, 100 Å; eluent, THF (0.5 mL/min, 80 bar), polystyrene standard).
(300 MHz, CDCl3), δ (ppm): 8.17 (dt, 2H), 7.78 (dt, 2H), 7.73−7.63
(m, 2H), 7.61−7.39 (m, 7H), 7.37−7.27 (m, 3H). 13C NMR (75 MHz,
CDCl3), δ (ppm): 142.3, 141.8, 140.9, 138.5, 130.9, 130.6, 130.4, 126.6,
126.3, 126.2, 125.9, 125.8, 123.6, 123.2, 120.5, 120.2, 109.8. EI-MS m/z:
398 (100, M+), 317 (24), 159 (21).
3-(Carbazol-9-yl)-6,6-dimethyl-3′-iodobiphenyl (5). Yield:
1
48%. H NMR (300 MHz, CDCl3), δ (ppm): 8.14 (d, 2H), 7.62−
7.54 (m, 2H), 7.48 (s, 2H), 7.42 (d, 4H), 7.31−7.15 (m, 3H), 7.04 (d,
1H), 2.18 (s, 3H), 2.12 (s, 3H). 13C NMR (75 MHz, CDCl3), δ (ppm):
142.9, 141.7, 141.0, 137.8, 136.7, 135.6, 135.4, 135.3, 132.1, 131.6, 127.6,
126.3, 126.0, 123.4, 120.4, 120.0, 109.8, 90.6, 19.7. EI-MS m/z: 473
(100, M+), 165 (35), 91 (20).
General Procedure for Miyaura-Borylation Reaction. The 3-
(carbazol-9-yl)-3′-halogenobiphenyls 4 or 5 (3.8 mmol), bis-
(pinacolato)diboron (1.05 g, 4.1 mmol), PdCl2(dppf) (0.09 g, 0.1
mmol) and anhydrous potassium acetate (1.11 g, 11.3 mmol) were
dissolved in dry DMSO (50 mL) under argon atmosphere. The mixture
was stirred at 80 °C for 3 h. After cooling to room temperature, the
mixture was extracted with diethyl ether and washed several times with
water. The organic phase was filtrated and the solvent was evaporated to
yield 6 and 7 as light yellow solids.
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a
Scheme 1. Synthesis of BPTRZ and MBPTRZ
a
Reagents and conditions: (i) 2 equiv. carbazole, n-BuLi, THF, reflux, 6 h. (ii) CuCl, methanol, RT, 4 h. (iii) I2/HIO3, CHCl3, H2SO4, AcOH, H2O,
80 °C, 6.5 h. (iv) Carbazole, CuI, trans-1,2-diaminocyclohexane, K3PO4, dioxane, reflux, 20 h. (v) Bis(pinacolato)diboron, PdCl2(dppf), KOAc,
DMSO, 80 °C, 3 h. (vi) Pd2dba3, PCy3, K3PO4, H2O, dioxane, toluene, 90 °C, 20 h.
Spectroscopic Measurements. All optical measurements were
carried out on thin films. The films of about 150 nm thickness as
determined by a Dektak profilometer were prepared by dissolving
BPTRZ and MBPTRZ in toluene at a concentration of 20 mg/mL and
casting films from this solution. Absorption spectra were measured with
a Cary5000 spectrometer from Varian. Fluorescence spectra were
obtained in a nitrogen-purged atmosphere from a JASCO FP-8600
luminescence spectrometer using excitation at 330 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 (ITC-
502). Excitation was provided by a pulsed nitrogen laser (LTB
MNL106PD) at 337.1 nm. The light emitted by the sample was
dispersed and subsequently detected by a time gated intensified CCD
camera (Andor iStar DH734-18F-9AM) with a delay time of 1 μs and a
gate width of 10 ms. The measurements were carried out at an excitation
density of about 10 μJ cm−2 pulse−1.
OLED Fabrication. The ITO substrate (10 ohm/square) used as the
anode is first cleaned with an acetone/isopropanol mixture in an
ultrasonic bath. For eliminating any possible organic residues and for
improving the hole injection, the substrate is thereafter exposed to
ozone for 30 min. Then, Plexcore OC AJ20-1000 (commercially
available from Plextronics Inc.) is spin-coated and dried to form a hole
injection layer (∼40 nm). Thereafter, the organic materials are applied
by vapor deposition in ultrahigh vacuum (<10−6 mbar).
Computational Calculations. Density Functional Theory (DFT)
calculations were carried out using the B3LYP hybrid functional
together with the basis set 6-311G*.26,27 The excited states were
calculated by using Time-Dependent-DFT with the optimized ground-
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state geometries. All DFT calculations were carried out with the
Gaussian 09 program.
RESULTS AND DISCUSSION
Synthesis. We have prepared the two novel matrix materials
BPTRZ and MBPTRZ. In both materials, a triazine unit is
■
Table 1. Thermal Properties of Matrix Materials BPTRZ and
a
MBPTRZ
Tg (°C)
Tm (°C)
Tc (°C)
Td (°C)
b
BPTRZ
134
154
286, 290
228, 236
445
475
c
c
MBPTRZ
284
225
a
Tg: glass transition temperature; Tm: melting temperature; Tc:
crystallization temperature; Td: 1% weight loss in N2 atmosphere.
b
c
Observed only upon heating. Observed only in the first heating scan.
connected to a carbazole by a fully aromatic but nonconjugated
linker. In BPTRZ, a meta-substituted biphenyl unit acts as
connecting group whereas in MBPTRZ the biphenyl unit is
additionally twisted by two methyl groups in 2- and 2′-position.
BPTRZ and MBPTRZ are prepared using a triazine substituted
with two carbazoles that we have described before.25 The
synthesis of the carbazole substituted unsymmetrical biphenyls 6
and 7 has been newly developed. Both materials represent
versatile building blocks for the synthesis of unsymetrically
substituted matrix materials. The synthesis of the two host
materials BPTRZ and MBPTRZ is outlined in Scheme 1. The
electron deficient biscarbazolyl-triazine 1 is synthesized by a
nucleophilic substitution of carbazole at the triazine core. The
number of carbazole units is controlled by the ratio of carbazole
and triazine and the reaction temperature. Due to the stepwise
deactivation of the chlorines at the triazine core after substitution
with an electron donating carbazole unit,25 the reaction of two
equivalents of carbazole with cyanuric chloride leads to 1 in 70%
yield. The biphenyl building blocks for BPTRZ and MBPTRZ
are synthesized by two different routes. For BPTRZ, 3,3′-
dibromobiphenyl 2 is obtained by copper mediated homocou-
pling of 3-bromophenylboronic acid.28 Via Goldberg-type
reaction, one carbazole unit is attached to the biphenyl moiety
to receive 3-(carbazol-9-yl)-3′-bromobiphenyl 4.29 The boryla-
tion of 4 is achieved by Miyaura-type cross coupling reaction.30
Finally, BPTRZ is prepared by Suzuki-Miyaura cross-coupling
reaction between the biscarbazolyl-triazine 1 and the borylated
carbazole substituted biphenyl 6.31 MBPTRZ was prepared in a
similar way except that 3,3′-diiodo-2,2′-dimethylbiphenyl 3 was
Figure 2. Absorption (solid line), fluorescence (dash-dotted line), and
phosphorescence spectra (dotted line) of (a) BPTRZ, (b) MBPTRZ,
and (c) N-phenylcarbazole (NPC) in neat film. Absorption and
fluorescence (Fl) were taken from thin films at room temperature,
phosphorescence (Ph) (film) was taken at 5 K. In each subfigure, the
bottom axis refers to wavelength and the top axis denotes energy.
used, which was synthesized by iodination of 2,2′-dimethylbi-
phenyl with iodine/iodic acid.11 The following steps are
analogous to BPTRZ, that is, Goldberg-coupling of carbazole,
subsequent borylation, and Suzuki cross-coupling with 1 to
MBPTRZ.
Thermal Characterization. The thermal properties of the
triazines were investigated by differential scanning calorimetry
(DSC), and their thermal stability was checked by thermogravi-
Figure 1. DSC-curves of BPTRZ (left) and MBPTRZ (right). Shown are the first and second heating scans, an additional heating scan after tempering
the sample for 18 h at 100 °C as well as the first and second cooling scans at a scan speed of 10 K/min in N2-atmosphere.
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Figure 3. (a) DFT optimized ground state geometries for BPTRZ (left side) and MBPTRZ (right side), along with the orbitals having major
contributions to the first and second singlet excited state. Also indicated are significant torsion angles between adjacent units. (b) Scheme indicating the
order and dominant character of the first and second singlet and triplet excited states (see text).
Figure 4. Energy diagram showing the location of the HOMO and LUMO levels of BPTRZ, MBPTRZ, and two CBP-derivatives 8 and 9 for
comparison. The HOMO levels were determined from the half-wave potential of the first oxidation in the cyclic voltammetry experiment. The LUMO
levels were estimated from the HOMO values and the optical bandgaps. The solid lines display the triplet energies ΔE (T1−S0).
Table 2. Optical Properties of BPTRZ and MBPTRZ
than 400 °C. A weight loss of 1% (Td) is observed at 445 °C for
BPTRZ and 475 °C for MBPTRZ. These values are more than
100 °C above the sublimation temperatures of BPTRZ and
MBPTRZ and therefore prove their high thermal stability. In
Figure 1, the DSC curves are shown. BPTRZ exhibits a double
melting peak at 286 and 290 °C. During cooling no phase
transition can be seen. The Tg of BPTRZ is observed at 134 °C in
the second DSC heating scan followed by two exothermic peaks
at 228 and 236 °C corresponding to the recrystallization of the
material. The melting of BPTRZ occurs again at 286 and 290 °C.
To check the stability of the amorphous phase of BPTRZ formed
during cooling we annealed the sample for 18 h at 100 °C. The
heating curve shows a recrystallization at 226 °C and a melting at
286 °C.
a
λEA
c
d
5K
RT
[nm]
film
ΔE(S0−
λem
λem
ΔE(T1−
e
b
S1) [eV] [nm] film [nm] film
S0) [eV]
BPTRZ
405
385
3.06
3.22
431
440
459
441
2.70
MBPTRZ
2.81
b
a
Absorption edge measured in neat films at room temperature. The
optical bandgap was determined form the UV/vis absorption onset of
c
neat films. Fluorescence maxima (excitation: 330 nm, 10−5
M
d
cyclohexane solution, room temperature). Wavelength of 0−0
phosphorescence transition measured on film samples (2 wt % matrix
e
material in PMMA at 5 K). The triplet energy was determined from
the 0−0 transition of the phosphorescence spectra of neat films at 5 K.
metric analysis (TGA). In Table 1 the thermal data are
summarized. TGA measurements show that both triazines
BPTRZ and MBPTRZ possess high thermal stabilities of more
MBPTRZ (Figure 1 right) exhibits a recrystallization peak in
the range from 210 to 240 °C and an endothermic peak at 283
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crystallization, which can be seen from the melting peak at 284
°C. The melting enthalpy is about 16% of the enthalpy during the
first heating run, which demonstrates that only a small amount of
the material has crystallized during tempering at 100 °C. The
high recrystallization temperature in the range from 215 to 243
°C for BPTRZ and the small amount of recrystallization after
tempering MBPTRZ at 100 °C demonstrate the stability of the
glassy state in pure BPTRZ and MBPTRZ. In OLEDs the
stability is usually further enhanced by mixing the matrix material
with an emitter.
Spectroscopic Measurements and DFT Calculations.
The thin film absorption and emission spectra for BPTRZ and
MBPTRZ are shown in Figure 2, along with the corresponding
data for N-phenylcarbazole (NPC) for ease of comparison. The
absorption spectra of BPTRZ and MBPTRZ are nearly identical.
With increasing photon energy, they feature three bands of
increasing oscillator strengths as well as a tail from about 3.44−
3.10 eV (360−400 nm). This tail is also present in solution of
BPTRZ and MBPTRZ, in contrast to the reference compound
NPC. The lowest energy band shows a shoulder at 3.60 eV (344
nm) and peaks at 3.76 and 3.92 eV (329 nm, 316 nm). These
features have an equidistant spacing of 160 meV (about 1290
cm−1), which is a typical mean vibrational energy.32 Furthermore,
the peaks coincide with the energies of the 0−0, 0−1 and 0−2
vibrational peak of the first absorption band in NPC. Similarly,
the second absorption band displays features at 4.19, 4.34, and
4.49 eV that concur with the vibrational peaks in the second
absorption band of NPC. Evidently, the excited states in BPTRZ
and MBPTRZ have significant contributions from transitions
that are localized on the carbazole moieties.
Figure 5. Top: Chemical structures of materials used in the OLEDs.
Bottom: Energy level diagram of device 2 with MBPTRZ as host
material; ionization potentials and electron affinity levels of the different
materials are indicated. The dotted lines represent the levels of the
emitter FIrpic. * For device 1, BPTRZ with a HOMO level of 5.60 eV
and a LUMO level of 2.36 eV was used as host material.
The emission spectra were taken from thin films in two
different ways such as to distinguish the short-lived intense
fluorescence from the weak long-lived phosphorescence. To
measure the fluorescence, we used a JASCO luminescence
spectrometer at room temperature and excited at 3.75 eV (330
nm). In order to detect phosphorescence, the samples were
°C, which is assigned to the melting of the material and is only
observed in the first heating. Upon cooling, the material solidifies
in an amorphous state without recrystallization. Tempering
MBPTRZ (18 h at 100 °C) leads to a small degree of
Figure 6. Top left: Electroluminescence spectra of the devices 1 (BPTRZ) and 2 (MBPTRZ). Top right: Quantum efficiency-luminance characteristics
for the blue phosphorescent OLEDs with BPTRZ (device 1) and MBPTRZ (device 2) as host materials. Bottom: Luminance−voltage-current density
characteristics for the blue phosphorescent OLEDs with BPTRZ (left) and MBPTRZ (right) as host materials.
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a
Table 3. Performance of OLEDs using BPTRZ and MBPTRZ as Host Material for the Blue Emitter FIrpic
at 100 cd/m2
at 1000 cd/m2
voltage [V]
ηC [cd/A]
ηP [lm/W]
ηext [%]
voltage [V]
ηC [cd/A]
ηP [lm/W]
ηext [%]
BPTRZ
10.5
9.4
14.4
15.6
4.3
5.3
6.1
6.7
12.1
11.1
11.8
12.9
3.1
3.7
5.0
5.5
MBPTRZ
a
ηC: current efficiency; ηP: power efficiency; ηext: external quantum efficiency.
cooled to 5 K, excited with a laser pulse at 3.68 eV (337 nm) and
measured using a ICCD camera with gated detection. After a
delay of 1 μs with respect to the excitation pulse the gate was
opened for 10 ms. The phosphorescence spectra for BPTRZ
show peaks spaced about 160 meV apart at 2.70, 2.54, and 2.39
eV, with relative intensities that resemble the vibrational peaks in
the NPC phosphorescence spectrum. MBPTRZ has essentially
the same phosphorescence spectrum than BPTRZ, albeit shifted
to higher energies by 110 meV and less structured. With these
vibrational spacings and relative intensities, the phosphorescence
spectra of MBPTRZ and BPTRZ look like the phosphorescence
spectrum of N-phenylcarbazole, albeit somewhat broadened and
shifted to the red by 140 meV for MBPTRZ and 250 meV for
BPTRZ. In contrast to this resemblance in the phosphorescence
spectra, the fluorescence of NPC and BPTRZ or MBPTRZ show
marked differences. For the reference N-phenylcarbazole, the
fluorescence is well-structured with narrow vibrational peaks and,
in particular, without any noticeable Stokes’ shift between the 0−
0 peaks of fluorescence and absorption. In contrast to this, the
fluorescence spectra of MBPTRZ and BPTRZ are broader and
display a pronounced energy shift between the maximum of the
fluorescence spectra and either shoulder or peak in the first
absorption bands. Furthermore, the energy difference between
the fluorescence spectra of NPC and MBPTRZ or BPTRZ is
much larger than that found for the phosphorescence spectra.
In order to interpret these spectra, it is helpful to consider DFT
calculations of the ground state geometry and the excited states,
calculated using Gaussian with the B3LYP hybrid functional. In
both compounds, BPTRZ and MBPTRZ, the central triazine
ring forms an approximately planar system with the two adjacent
carbazole moieties and the adjacent phenyl ring (Figure 3). The
torsion angles between the triazine ring and the adjacent
carbazoles is about 20°, and that with the adjacent phenyl ring is
15° for BPTRZ and 11° for MBPTRZ. This roughly planar half of
the molecule is separated from the third carbazole unit by strong
torsions, as indicated in the figure. Consistent with these strong
torsions is a strong localization of the frontier orbitals. While the
HOMO is localized onto the carbazole moiety, the LUMO is
confined to the central triazine and the adjacent phenyl ring.
Since hole transport takes place between HOMOs and electron
transport between LUMOs, this implies that the transport
pathways for both carriers are spatially separated even when they
happen to meet on one molecule. We shall now consider the
dominant transitions of the excited states. The first calculated
excited state, S1, involves mainly the transition from the
carbazole-based HOMO to a triazine-based LUMO. In agree-
ment with the strong charge-transfer (CT) character of this state,
it carries no oscillator strength. The next higher excited state, S2,
is calculated to be 0.25 eV above S1 and has oscillator strength
(0.08). The dominant transition occurs from the HOMO-2 to
the LUMO+1. It has a strong ππ* character involving the π-
system that extends over two carbazoles connected by the central
triazine ring.
red-shifted fluorescence results from this S1 state after geometric
relaxation in the excited state and thus concomitant planarization
and somewhat improved wave function overlap. By virtue of their
small wave function overlap, charge transfer states are
characterized by a small exchange energy, so that the associated
triplet excited state can be expected to be energetically close. In
contrast, the exchange energy associated with ππ* transitions is
significantly larger, typically in the range 0.7−1.0 eV.33 We
therefore attribute the phosphorescence of BPTRZ and
MBPTRZ to a triplet state based on ππ* transitions such as
the HOMO-2 to LUMO+1 transition involved in S2. Such an
involvement of carbazole-based transitions between a π-systems
extending over two connected carbazoles is in good agreement
with the observed vibrational structure of the phosphorescence
spectrum. A qualitative scheme of the excited state order is
indicated in Figure 3. A similar relative order of states has also
been observed for related carbazole-derivatives with charge
transfer character.34
Electrochemical Properties. The electrochemical behavior
was studied by cyclic voltammetry (CV). The HOMO levels
were determined from the half-wave potential of the first
oxidation relative to ferrocene and the LUMO levels were
calculated by adding the optical band gap to the HOMO levels.
The first oxidation events of BPTRZ and MBPTRZ are observed
at 0.80 and 0.78 V (vs Fc/Fc+). The potentials can be translated
to HOMO levels of 5.60 and 5.58 eV. In Figure 4, the HOMO
and LUMO values of BPTRZ and MBPTRZ are compared to
CBP-derivatives 8 and 9 from a previous paper.24 In these
materials, the meta-substituted biphenyl linkers are identical to
BPTRZ and MBPTRZ, but the biphenyl units are substituted
with carbazoles on both sides. The HOMO levels of all four
matrix materials are between 5.58 and 5.65 eV and almost
identical. The well-known host material CBP also has a HOMO
of 5.63 eV. This shows that the HOMOs of all four materials are
mainly located at the electron rich carbazole adjacent to the
biphenyl linkers (Figure 3). The LUMOs of the triazine
containing BPTRZ and MBPTRZ are at 2.36 and 2.28 eV, and
therewith somewhat lower compared to the carbazole based
compounds 8 and 9. The triplet energies of 8 and 9 are both at
2.98 eV. In BPTRZ, where the meta-substituted biphenyl is
linked to an electron withdrawing triazine unit, the triplet energy
is at 2.70 eV. In MBPTRZ, where the two phenyl rings in the
biphenyl unit are twisted by 90°, the triplet energy goes up to
2.81 eV (Table 2). This shows that an efficient decoupling of the
electron donating and the electron accepting unit in bipolar host
materials is important to achieve a high triplet energy. The triplet
energy of 2.81 eV for MBPTRZ is high enough for blue emitters.
Phosphorescent Organic Light-Emitting Diode. To
demonstrate the potential of BPTRZ and MBPTRZ as host
materials for blue phosphorescent emitters, OLEDs were
fabricated. The materials and the device setup are presented in
Figure 5. On top of an indium−tin−oxide (ITO) glass substrate
poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PE-
DOT:PSS) was spin coated as hole injection layer followed by 10
nm DPBIC p-doped with molybdenum(VI) oxide as hole
We associate the CT-type S1 state with the weak tail observed
in the absorption spectra. We further consider that the broad and
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Chemistry of Materials
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transporting layer. An additional 10 nm thick layer of tris[(3-
phenyl-1H-benzimidazol-1-yl-2(3H)-ylidene)-1,2-phenylene]-
iridium (DPBIC) followed as exciton and electron blocking layer.
The 40 nm thick emission layer consisted of 5% FIrpic doped
into the triazine host materials BPTRZ (device 1) or MBPTRZ
(device 2). As hole and exciton blocking layer 5 nm of 2,8-
bis(triphenylsilyl)-dibenzofuran (DBFSi) were deposited fol-
lowed by 20 nm cesium carbonate doped 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline (BCP) as electron transporting
layer. The devices were finalized by deposition of 100 nm
aluminum as cathode.
Figure 6 (top left) displays the electroluminescence spectra of
both devices at a current density of 5 mA/cm2. The two spectra
are almost identical. Both devices show the pure emission of
FIrpic with a maximum at 473 nm.
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Yang, C. J. Mater. Chem 2012, 22, 2894.
The luminance−voltage−current density characteristics and
the efficiencies of the OLED devices 1 and 2 are shown in Figure
6 and summarized in Table 3. For OLEDs with BPTRZ as host
100 cd/m2 are reached at 10.5 V and 1000 cd/m2 at 12.1 V. For
MBPTRZ, the corresponding voltages are lower at 9.4 V (100
cd/m2) and 11.1 V (1000 cd/m2). The maximum brightness is
24400 cd/m2 (at 15.7 V) for BPTRZ and 30600 cd/m2 (at 15.0
V) for MBPTRZ. Device 1 with BPTRZ as host material exhibit a
maximum external quantum efficiency of 6.2% and a power
efficiency of 4.6 lm/W, whereas device 2 with MBPTRZ reaches
7.0% and 6.3 lm/W, respectively.
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CONCLUSION
■
We have synthesized two new bipolar host materials BPTRZ and
MBPTRZ with a hole transporting carbazole moiety that is
separated from the electron transporting biscarbazolyl-triazine
by a nonconjugated meta-linked biphenyl unit. Both host
materials possess high thermal stabilities and good glass forming
properties. The additional twist at the biphenyl unit in MBPTRZ,
which is achieved by two additional methyl groups in the 2- and
2′-position of the biphenyl reduces the conjugation in MBPTRZ,
leading to higher triplet energy of 2.81 eV compared to 2.70 eV
for BPTRZ. A phosphorescent OLED with MBPTRZ as host and
FIrpic as blue emitter has an external quantum efficiency of 7.0%,
a current efficiency of 16.3 cd/A and a power efficiency of 6.3 lm/
W. These results demonstrate the potential of donor-substituted
triazines as bipolar host materials for blue phosphorescent
OLEDs.
(23) Rothmann, M. M.; Fuchs, E.; Schildknecht, C.; Langer, N.;
Lennartz, C.; Munster, I.; Strohriegl, P. Org. Electron. 2011, 12, 1192.
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Kohler, A.; Lennartz, C. J. Mater. Chem. 2011, 21, 2266.
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Schildknecht, C.; Strohriegl, P. Chem. Mater. 2010, 22, 2403.
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Soc. 2001, 123, 7727.
AUTHOR INFORMATION
Corresponding Author
*E-mail: peter.strohriegl@uni-bayreuth.de.
■
(30) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60,
7508.
(31) Su, S.-J.; Cai, C.; Kido, J. Chem. Mater. 2011, 23, 274.
Notes
The authors declare no competing financial interest.
(32) Kohler, A.; Khan, A. L. T.; Wilson, J. S.; Dosche, C.; Al-Suti, M. K.;
Shah, H. H.; Khan, M. S. J. Chem. Phys. 2012, 136, 94905.
̈
(33) Kohler, A.; Wilson, J. S.; Friend, R. H.; Al-Suti, M. K.; Khan, M. S.;
̈
ACKNOWLEDGMENTS
■
Gerhard, A.; Bassler, H. J. Chem. Phys. 2002, 116, 9457.
̈
We thank I. Bauer for the help with the synthesis of the host
materials and C. Lennartz, S. Watanabe, P. Schrogel, and M.
Rothmann for helpful discussions. We are grateful to the
Graduiertenkolleg GRK 1640 (German Science Foundation)
and the BMBF (TOPAS 2012) for financial support.
(34) Hoffmann, S. T.; Schrogel, P.; Rothmann, M.; Albuquerque, R.
̈
̈
Q.; Strohriegl, P.; Kohler, A. J. Phys. Chem. B 2010, 115, 414.
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