New 4,4'-Bis(9-carbazolyl)-biphenyl derivatives with locked carbazole-biphenyl junctions: High-triplet state energy materials

Article abstract of DOI:10.1021/cm5045754

We synthesized a series of 4,4'-bis(9-carbazolyl)-biphenyl (CBP) derivatives, using methyl groups as spatially demanding groups, locking the angle between the carbazole subunit and the biphenyl backbone as potential matrix material for blue organic light-

Full text of DOI:10.1021/cm5045754

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Article
New CBP derivatives with locked carbazole-biphenyl
junctions – high triplet state energy materials
Markus Gantenbein, Manuel Hellstern, Loic Le Pleux, Markus Neuburger, and Marcel Mayor
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5045754 • Publication Date (Web): 08 Feb 2015
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New CBP derivatives with locked carbazole-
biphenyl junctions – high triplet state energy
materials
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Markus Gantenbein, Manuel Hellstern, Loïc Le Pleux, Markus Neuburger, Marcel Mayor
Abstract
We synthesized a series of CBP derivatives, using methyl groups as spatially demanding groups, locking
the angle between the carbazole subunit and the biphenyl backbone as potential matrix material for
blue organic light emitting diodes (OLEDs). The locked rotation was achieved by four methyl groups
either in the 1,8-positions of the carbazole subunit (1) or in the 3,5,3’,5’-positions of the biphenyl
subunit (2) and the fixed spatial arrangement was confirmed by x-ray analysis. The physical properties of
CBP derivatives based on the parent structure 2 were further tailored by electron withdrawing CF3-
groups in 3,6- or 2,7-positions of the carbazole subunits (3 and 4) or alternatively by electron donating
CH3O-groups in the 2,7-positions (5) of the same building blocks. Increased triplet energies (ET)
compared to the parent compound CBP were found for all synthesized CBP derivatives 1-5. Enhanced
glass transition temperatures ranging between 129°C to 205°C further corroborate the application
potential of these derivatives for matrix material in blue emitting OLEDs.
Introduction
The potential of cheap solar cells, organic field-effect transistors, and organic light-emitting diodes
(OLEDs) is fueling the interest in research on organic materials. The performance of such devices
critically depends on the efficiency in which charge carriers (holes and electrons) move within π-
conjugated materials. For the development of efficient π-conjugated materials, a profound
understanding of the interplay between the π-electron delocalization and geometrical structural
features is required.
Especially phosphorescent organic light emitting diodes (phOLEDs) have received considerable attention
because unlike standard fluorescent OLEDs, which feature singlet excitons, phOLEDs are based on triplet
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excitons in the emitter. 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 are
efficiently converted into the triplet state and hence, the theoretical limit of the internal quantum
efficiency is 100% with these phosphorescent emitters. In a typical phOLED, the emitters are usually
doped with suitable host materials for reducing concentration quenching and triplet–triplet annihilation.
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To maximize phOLED efficiency a high performing host molecule needs to fulfill the following criteria: (i)
the triplet energy (ET) of the host molecule should be higher than the corresponding one of the
phosphorescent guest emitter, to prevent the back energy transfer from guest to host and to promote
the exothermic energy transfer from host to guest, (ii) facile and balanced charge injection from
neighboring layers, and (iii) HOMO and LUMO levels of the host molecules should be appropriately
aligned with the hole transport layer (HTL) and electron transport layer (ETL) to reduce the driving
voltage for charge injection. The use of high-ET materials is imperative to confine the triplet excitons in
the emitter. In blue phosphorescent OLEDs in particular, a host material with a high ET of over 2.75 eV is
required. Due to its triplet energy ΔE (T1−S0) of 2.56 eV, 4,4′-Bis(9-carbazolyl)-biphenyl (CBP) is a suitable
matrix for green phosphorescent emitters like tris(2-phenylpyridine)iridium(III) (Ir(ppy)3). Blue emitting
materials such as the commonly used bis((4,6-difluorophenyl)-pyridinato-N,C2)picolinate-iridium(III)
(FIrpic) (ΔE (T1−S0) = 2.65 eV) require hosts with higher triplet energies. The key to obtain such high ET
materials is to confine the conjugated system within the host molecules.1
By variation of the torsion angle Φ between the planes of the two phenyl rings of biphenyl derivatives,
we have previously reported that the degree of π-overlap in the two phenyl rings and the resulting
extent of delocalization over both π-systems can be fine-tuned.2 In most cases this tuning is performed
by substituting biaryls in the 2 and 2′ position with different sterically demanding groups or by
interlinking the two positions with alkyl chains of different lengths. This approach, which aim to increase
the triplet energy of CBP based materials by reducing the extent of conjugation in the biphenyl scaffold
is currently thoroughly studied and was recently reviewed in details by Yang and co-workers.1,3 Tokito
and co-workers reported the new host 4,4′-bis(9-carbazolyl)-2,2′-dimethylbiphenyl (CDBP) with a high ET
of 2.79 – 3.0 eV (the reported values differ within this range) by introducing two methyl groups in 2- and
2′-positions of biphenyl in CBP structure.4 Strohriegl and co-workers modified the CDBP structure by
introducing CF3 groups instead of CH3 groups on the biphenyl core, while investigating the influence of
the substitution on the 3,6-positions of the carbazole. This leads not only to a loss of conjugation in the
central biphenyl core but also to high triplet energies.5,6
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Here we are less interested in the interphenyl angle of the biphenyl backbone. Instead we report here
compounds which were synthesized to investigate correlations between their physical properties and
the spatial arrangement between biphenyl and carbazole subunits. The molecular design is based on the
idea that reducing the conjugation between the subunits gives molecules which display optical features
of their individual subunits, with the whole molecule being able to form amorphous molecular materials.
In this context we have prepared a series of CBP derivatives with sterically restricted degrees of freedom
in their biphenyl-N-carbazole junctions, due to substituents either in the 1,8-positions of the carbazole
subunit (1) or in the 3,5,3’,5’-positions of the biphenyl subunit (2). In the latter case the electronic
features of the carbazole synthons were further varied by substituents either in the 3,6- or in the 2,7-
positions (Figure 1). We report here the synthesis of five amorphous CBP derivatives together with their
thermal and optical properties. The energy levels have been measured by electrochemical
measurements and optical spectroscopy techniques. Additional insights into geometrical features of the
CBP derivatives were obtained by analyzing their solid state structures by x-ray diffraction analysis and
with DFT calculations.
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Figure 1: Chemical structures of the CBP derivatives 1-5
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Results and discussion
Synthesis
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Scheme 1: Synthesis and molecular structure of rotational restricted CBP derivatives 1a and 1b.
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t
aReaction conditions: a) [(cinnamyl)2Pd])2 (2.5 mol%), BuBrettPhos (7 mol%), CsF, MePh, 110°C, 24h,
93%; b) Cs2CO3, DMF, 12h, 150°C, 11 (86%), 12 (87%), c) SnCl2 · 2H2O, EtOH, reflux, 4h, 13 (91%), 14
t
(94%); d) BF3 Et2O, BuONO, THF, KI, I2, MeCN, -10°C to rt, 12h, 15 (75%), 16 (79%); e) Turbo Grignard,
TEMPO, THF, -10°C to rt , 2h, 1 (96%), 2 (97%).
Carbazole 7 was synthesized according to literature7 in a one pot Pd - catalyzed Buchwald-Hartwig
amination followed by C-H activation procedure, whereas carbazole (6) was commercially available.
Fluoro-nitro compound 10 was synthesized starting from the commercially available 2,6-dimethyl-4-
nitro phenol. The phenolic alcohol was transformed into the triflate 8 using triflic anhydride and
pyridine. According to a procedure reported by Buchwald and co-workers, a Pd - catalyzed fluorine
insertion replacing the triflate moiety was used to provide the fluoro-nitro compound 10 in multigram
scale8. The 9-para-nitrophenylcarbazole compounds 11-12 were synthesized via an aromatic
nucleophilic substitution reaction (SNAr)9,10 between the carbazoles (6 or 7) and the para-
fluoronitrobenzene derivatives (9 or 10), using Cs2CO3 in DMF at reflux temperature. First attempts
based on Buchwald-Hartwig11 or Ullmann conditions12 did not lead to the desired conversion, what we
attributed to steric factors. Reduction of the nitro groups of 11-12 with tin chloride13 afforded the
anilines 13-14, which were transformed directly to the corresponding aryl iodide compounds 15-16 via
Sandmeyer reactions.14 The homo-coupling of the aryl iodide derivatives provided the desired CBP
derivatives 1 and 2. The reaction conditions for the homo-coupling were optimized by preliminary
screening experiments. Copper catalyzed reaction conditions like classical Ullmann conditions,12
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alternative Cu(I) sources,15 also including immobilized Cu(II) sources16 did not provide the desired CBP
derivatives in reasonable yields. To our delight, Knochel’s Turbo Grignard reaction17 conditions in
combination with TEMPO18 as co-oxidant provided the desired target CBP derivatives 1 and 2 in
excellent yields of 96% and 97%, respectively. Both target structures 1 and 2 were fully characterized by
1H- and 13C-NMR spectroscopy, mass spectrometry (MS), and elemental analysis. In addition optical and
electronic properties of these structures have been analyzed and will be discussed below. An important
finding of these studies is mentioned already here, as it steered the molecular design and thus also the
evolution of our experimental approach. Comparing 1 and 2 we don’t find big differences in their optical
properties and thus we conclude, that the methyl groups sterically locking the spatial arrangement of
the phenyl-carbazole junction are equally efficient whether they are mounted at the carbazole (1) or at
the biphenyl subunit (2). We thus decided to maintain with the methyl groups at the biphenyl backbone
and to profit from the not yet functionalized carbazole subunits to introduce substituents in order to
tune the physical properties of the CBP derivatives. The decision to favor analogs of 2 over the ones of 1
was also chemically encouraged by the relatively poor solubility and the challenging synthesis of 1.
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Scheme 2: Synthesis and molecular structure of CBP derivatives 3-5, as carbazole functionalized
analogs of 2.
Reaction conditions: a) SPhos Pd G2 (2 mol%), 3-(Trifluoromethyl)phenylboronic acid, K3PO4, THF : H2O
(20:1), 80°C, 12h, 93%; b) Pd(OAc)2 (2mol%), Cu(OAc)2 (20 mol%), MePh, 120°C, 24h, 98%; c) H2SO4 :
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MeOH (1:5), 80°C, 0.5h, quant; d) Pd2(dba)3, BINAP, NaOtBu, MePh, 90°C, 12h, 93%; e) NaOtBu,
HPtBu3][BF4], Pd(OAc)2, MePh, 160°C (MW), 3h, 81%; f) Cs2CO3, DMF, 12h, 150°C, 25 (82%), 26 (78%), 27
(86%); g) SnCl2 · 2H2O, EtOH, reflux, 4h, 28 (92%), 29 (93%), 30 (97)%; h) BF3 · Et2O, tBuONO, DCM, KI, I2, -
10°C - rt, 12h, 31 (82%), 32 (90%); i) PTSA, NaNO2, KI, MeCN, H2O, 10°C, 2h, 33 (83%); j) Turbo Grignard,
B(OiPr)3, neopentyl glycol, THF, -10°C to rt, 12h; k) SPhos Pd G2 (2 mol%), K3PO4, THF : H2O (20:1), 60°C,
12h, 3 (95%), 4 (93%), 5 (97%) over 2 steps.
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To apply the assembly strategy displayed above for 2, the substituted carbazoles 17-19 were required.
Carbazole 17 was synthesized by analogy to a literature procedure.19 Commercially available 2-chloro-4-
(trifluoromethyl)aniline was acetylated using acetyl chloride and pyridine yielding 20. The biphenyl 21
was obtained in very good 93% yield using a Suzuki-Miyaura cross-coupling protocol. The acetylated
carbazole 22 was formed via C-H-activation. Subsequent deprotection in a solvent mixture of sulfuric
acid in methanol gave the desired carbazole derivative 17 in excellent yield. Carbazole 18 was
synthesized in a two-step reaction. Initial attempts bases on a Buchwald-Hartwig reaction followed by
an in-situ C-H activation step7 gave mixtures of compounds with very similar polarities in all reaction
conditions investigated. The applied stepwise reaction protocol yielded carbazole 18 in very good yield
and in gram scale. First a Buchwald-Hartwig reaction between the aniline 23 and the commercially
available 1-bromo-3-(trifluoromethyl)benzene was performed to assemble the diarylamine 24. The
reaction turned out to be very sensitive to temperature with the competitive amination of the
chloroaryl 23 at temperatures above 80°C. For the stepwise reaction amine 24 was filtered through a
short silica plug and was used without further purification. Subsequently the carbazole was formed by
ring closing C-H activation yielding 18 in 81% over both steps. The 2,7-dimethoxy carbazole 19 was
synthesized following a reported procedure.20 With the carbazoles 17-19 in hands, we followed the
synthetic strategy described above for the synthesis of 2 as closely as possible. Namely the carbazoles
were coupled in a SNAr reaction with the fluoroaryl 10, providing the 9-para-nitrophenylcarbazole
derivatives 25-27 in good yields around 80%. Subsequently the nitro groups were reduced to the amines
28-30,13 which were exposed to a Sandmeyer14 reaction to provide the corresponding 9-para-
iodophenylcarbazole derivatives. While the conditions used above for 16 were successful for 31 and 32,
they failed in the case of 30. Most likely the BF3 etherate attacks the methoxy group resulting in the
liberation of phenolic substituents. Alternatively reaction conditions developed by Knochel and co-
workers21 were applied and provided the 9-para-iodophenylcarbazole compound 33 in good yields
(83%). The homo-coupling conditions developed above for 16 were applied to the iodoaryl precursors
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31-33. While in the case of 31 and 32 the target compounds 3 and 4 were only obtained in poor yields,
the method completely failed for 33. A more successful synthetic approach was to first transfer the 9-
para-iodophenylcarbazole derivatives 31-33 to the boronic ester derivatives 34-36,22 which were
exposed without purification to Suzuki-Miyaura cross-coupling reaction conditions together with
equimolar amounts of their iodoaryl precursors 31-33. The target CBP derivatives 3-5 were isolated in
excellent yields of 93%, 91%, and 94% respectively.
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The target CBP derivatives 3-5 and their precursors were fully characterized by H- and 13C-NMR
spectroscopy, mass spectrometry (MS), and elemental analysis. The identity of the CBP derivatives 2-4
was further corroborated by their solid state structures obtained by x-ray diffraction of suitable single
crystals. Optical and electronic properties of the target compounds will be discussed in the course of this
paper.
Thermal properties
To investigate the thermal properties of the presented CBP derivatives 1-5 differential scanning
calorimetry (DSC) measurements of all compounds were performed. Measurements were recorded on a
Perkin Elmer DSC Advanced Double-Furnace 8000 using scanning rates of 10 °C min-1, and the glass
transition temperature (Tg), crystallization temperature (Tc) and melting point (Tm) were obtained from
the second heating scan. The properties of the parent CBP were also recorded to calibrate the
experimental set-up and its recorded Tm, Tc and Tg values are in very good agreement with the
literature.6,23 Compound 2 remains in the amorphous state until 129°C, which is already twice as high as
for parent compound CBP, whereas no Tg value could be determined for compound 1. Furthermore, the
functionalized carbazole compounds 4 and 5 showed even higher Tg values of 158°C and 184°C,
respectively. Compound 3 reveals a Tg value of 202 °C, which is to the best of our knowledge, one of the
highest reported value for matrix compounds. Since there is no clear trend observed for electron
withdrawing or electron donating groups, we attribute these high glass transition temperatures to the
decreased crystallization probability of the compounds 1-5, due to the spatial demanding methyl groups
and the functional groups attached to the carbazole subunit. By analysis of the Tm values of target
compounds 1-5, a similar Tm 351°C and 348°C has been observed for compound 1 and 3, respectively. 4
also shows a high Tm of 314°C, whereas the compounds 2 and 5 revealed a slightly lower Tm value of
293°C and 299°C. Since the heating cycles were repeated three times starting from 20°C ending at
450°C, for all compounds 1-5 high thermal stability can be attributed. This improved thermal stability is
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important for their potential application in devices like e.g. phOLEDs. Melting point, crystallization and
glass transition temperature of all carbazoles 1-5 are summarized in Table 1. The traces of the second
heating and cooling scan are presented in the supporting information (Figure S81 to S85).
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Melting
pointb, Tm
(°C)
Crystallization
Glass transition
Compound
temperatureb, Tc temperatureb, Tg
(°C)
(°C)
CBPa
288; 283 lit.
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196; 205 lit.
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66; 62 lit.
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348
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184
Table 1: aCBP values are in good agreement with the literature6,23 bMelting point, crystallization temperature and glass
;
transition temperature were obtained from the second heating/cooling scan using a scan rate of 10 °C min-1.
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X-ray Crystallography
Single crystals, suitable for x-ray diffraction analysis, were obtained for 2, 3 and 4 using slow evaporation
techniques dissolving the compounds in a 1:1 mixture of DCM and cyclohexane. The obtained solid state
structures enable the investigation of the substituents’ effects on the spatial arrangement of the
subunits in the solid state. Experimental details have been summarized in the supporting information
(Figures S86 to S88) for all x-ray structures, which are discussed in this section. A particular focus is set
on the two inter-plane twist angles between the biphenyl and the carbazole subunit (α: between green
and orange plane; β: between blue and grey plane; Figure 2) and their influence on the physical
properties of the CBP derivative. In addition the molecular structures of compounds 1, 2 and 5 were
calculated using density functional theory (DFT) calculations using B3LYP hybrid functional theory with
6-31G* basis set in Spartan10. Even though the comparability of the obtained twist angles is to some
extent questionable as in the x-ray structures effects of intermolecular packing are considered as well,
the calculated molecular structures provide good ideas about the spatial arrangement of their subunits.
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Figure 2: Solid state structure of 2. The inter-plane angle between the central carbazole ring (green) and the connected
biphenyl ring (orange) is α and the one between the biphenyl (grey) and the carbazole ring (blue) is β.
Single crystal structure of compound 2 provides a non-standard monoclinic P21/n space group and thus,
the inter-plane angles α 85.05° and β 75.81° differ from each other. A similar inhomogeneity was
observed in the calculations with values of 82.84° and 80.12° obtained for α and β respectively. For the
CBP derivative 1 with the four methyl groups attached in the 1,8-positions of the carbazole subunits
even increased inter-plane twist angles were calculated. With 89.96° for α and 89.93° for β, the values
are very similar. Compound 3 crystallizes in the centro-symmetric orthorhombic space group Fddd, and
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thus the inter-plane angles α and β are the same both being 88.96°. CBP derivative 4 crystalizes in the
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ꢀ
triclinic space group P1, and its inter-plane twist angles of 79.67° for α and 76.66° for β are decreased
compared to 3. We attribute this effect to the attached CF3-groups which are further away from the CBP
axis in the 3,6-positions compared to the 2,7-positions. Intuitively one might thus argue that they have a
bigger lever to flatten the molecule during packing in the solid state. The calculated inter-plane angles
for 5 are almost perpendicular for both angles with α 89.95° and β 89.44°. For all derivatives inter-plane
angles larger than 75° have been found indicating a very limited interaction between the electron clouds
of neighboring aromatic subunits. In other words, the large angles break the π-electron conjugation
along the molecule’s axis.
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Photophysical Properties
The absorption and luminescence properties of the compounds were measured in dilute and aerated
dichloromethane (DCM) solution at room temperature. Figure 3 shows the corresponding UV−Vis and
Emission spectra. The optical band gap of all the studied molecules were also calculated from the onset
of optical absorption and are reported in Table 2.
250
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Wavelength (nm)
Figure 3: UV-visible and emission spectra of the compounds in DCM solution.
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Table 2: Structural, photophysical and electrochemical Data
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ε
Twist angles
α/β (°)
Eg
QY
(%)c
ET
Eox vs. Fc+/Fc EHOMO ELUMO
λmax uv
λonset uv
λmax emission
λmax phos.
(eV)d
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293
318
340
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335
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294
339
355
75380
45120
32700
23780
102530
98460
35920
17540
18160
76690
64920
44660
11430
13880
111570
107390
35300
12010
170210
83235
17050
23830
372
376
CPB 49.5/49.5b,24
352
348
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3.56
78.9
8.5
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0.87
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-5.90 -2.34
1
2
90.0/89.9a
414
0.80
0.92
82.8/80.1a
85.1/75.8b
343
359
347
3.57
36.9
432
2.87
-6.02 -2.45
339
355
3
4
89.0/89.0b
79.7/76.7b
342
364
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3.41
2.6
444
420
2.79
2.95
1.37
1.36
-6.47 -2.85
-6.46 -3.05
361
378
15.4
239
265
307
320
107650
61700
32500
27240
5
90.0/89.5a
334
3.71
444
1
431
2.88
0.56
-5.66 -1.95
a Data from geometry optimizations at the B3LYP/6-31G* level of theory. b From x-ray crystal structural data. c All data were collected in dilute dichloromethane solution at room temperature. d Eg =
1240/λonset uv.e Measured in 2-MeTHF glass matrix at 77K. f ET = 1240/λmax phos.
.
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In order to understand how the substitutions affect the excited states of these compounds, we first
consider the effect of the tetra-methyl substitution at the central biphenyl unit as well as at the 1,8-
positions of both carbazoles, by comparing 1 and 2 with CBP.
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The absorption bands at 291 nm, 326 nm and 339 nm recorded with 1 can be assigned to the absorption
of the 1,8-dimethylcarbazole units. In the case of CBP, the features associated with transitions localized
on the carbazole are present at 293 nm and 340 nm. In their fluorescence spectra CBP and 1 show very
similar structureless emission with a maximum at 374 nm and 414 nm for CPB and 1 respectively. The
bathochromic shift of approximately 40 nm observed in emission of 1 is caused by the additional methyl
groups at the carbazole units. In the absorption spectrum of CBP is an additional broad absorption
around 318 nm which is not observed in the case of 1. This feature is associated with transitions
between carbazole centered orbitals and orbitals localized on the biphenyl subunit and thus, the signal
can be considered as a measure of conjugation between the two subunits. This band assignment and its
role as conjugation indicator has been reported with comparable model compounds.5,25,26
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The flexibility of the carbazole-biphenyl junction can not only be limited by methyl groups in the 1,8-
positions of the carbazole, but alternatively by methyl groups in both ortho-positions of the attached
phenyl ring of the biphenyl subunit as realized in 2. Compound 2 displays almost the same carbazole
centered transitions as in CBP, with bands localized at 293 and 340 nm. The transition between the
carbazole moiety and the biphenyl core around 320 nm is weak, supporting the hypothesis of a frozen
flexibility and a perpendicular arrangement of both subunits at the carbazole-biphenyl junction. In
contrast to the broad emission spectrum of CBP, 2 exhibits a well resolved emission under identical
conditions. It is obvious that inserting methyl groups on the biphenyl scaffold causes a large blue shift of
the fluorescence (31 nm), caused by the decreased
strong twist.
π-conjugation with the biphenyl core, due to a
We now consider the effect of the di-trifluoromethane substitution at the carbazole, by comparing 2
with 3 (3,6-position) and 4 (2,7-position). The CBP derivative 3 displays similar carbazole centered
transitions as 2 (292 and 335 nm) as well as a similar emission profile (339 and 355 nm) which is
however marginally blue-shifted by 4 nm compared to 2. In the case of 4 some of the observed
transitions are red-shifted compared to 2. In particular the second of the carbazole centered absorptions
(294 and 355 nm) is bathochromically shifted by 15 nm. Even more pronounced is the 20 nm shift of the
emission bands (361 and 378 nm) which, apart from the shift, display a similar pattern as the parent
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structure 2. The difference between 3 and 4 might be rationalized by an increased conjugation over both
CF3 substituents in 4. The pioneering studies from Leclerc and co-workers comparing 3,6- with 2,7-
connected polycarbazoles displayed the extended conjugation in poly-2,7-carbazole comprising a rigid
poly-para-phenylene backbone compared to the poly-3,6-carbazole with a chain resembling benzidine
subunits interlinked by mutual nitrogen atoms.27 Replacing the electron withdrawing CF3-substituents of
3 with electron donating OCH3-groups in 5 results in blue-shifted absorption transitions (265, 320 nm).
In addition a broad emission band appears at 444 nm, pointing at excited state intramolecular charge
transfer (ICT) with the 3,6-dimethoxycarbazole moiety acting as the donor and the biphenyl as the
acceptor.28
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The fluorescence quantum yield (ΦF) of those carbazole derivatives in DCM at room temperature has
also been studied using a Hamamadzu Quantaurus-QY integrated sphere fluorimeter and the data are
reported in Table 2. In contrast to CBP which shows a strong emission in those conditions, the CF3-
substituted CBP derivatives 3 and 4 only display low ΦF values (0.1–0.03) suggesting an enhanced triplet
yield in these derivatives.
As a prerequisite for host materials, their triplet energy should be higher than that of the
phosphorescent emitter. The phosphorescence spectra of the molecules under investigation were
measured in 10-5 M aerated and frozen 2-methyltetrahydrofuran solutions at 77 K in gated steady state
conditions. The corresponding spectra are displayed in Figure 4 together with a spectrum of CBP for
comparison. The triplet energies were determined from the highest energy peak of the
phosphorescence spectra (λmax phos) and are also listed in Table 2. The triplet energies levels are
calculated according to ET = 1240/λmax phos
.
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CBP
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350
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450
500
550
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Wavelength (nm)
Figure 4: Phosphorescence spectra of the carbazole derivatives in 2-MeTHF glass matrix at 77K
All the synthetized CBP derivatives 1-5 show phosphorescence emission at 77 K. In spite of the broad
signals the triplet energies were detectable for all five compounds and were with values between 2.75
and 2.95 eV higher than the 2.7 eV recorded for CBP (Figure 4). Apparently the carbazole and biphenyl
subunits are efficiently decoupled by the induced twist between both in these model compounds and
consequently, we can estimate that the triplet excited state in those CBP derivatives is mainly located on
the biphenyl unit of the molecules, as substitution on the carbazole units does not affect their triplet
state energy. Triplet energies of 3.1 eV for N-ethylcarbazole29 and 2.9 eV for biphenyl (67. ± 0.6 kcal·mol-
1 30
)
corroborates this hypothesis as the triplet state will mainly be localized on the fragment with the
lowest triplet energy. This hypsochromic shift of their phosphorescence makes these CBP derivatives
with locked carbazole-biphenyl junctions interesting host materials for blue phosphorescent emitters.
In most cases exclusively fluorescence is observed in steady state conditions and phosphorescence being
usually orders of magnitude weaker is difficult to observe. Here phosphorescence emission is clearly
observed coexisting with fluorescence at low temperatures (see SI, figure S89 to S94). This and the
decreased emission quantum yield observed at room temperature clearly points at an increase in the
triplet yield of those CF3-substituted compounds.
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Of particular interest were the electrochemical features of these CBP derivatives with sterically locked
carbazole-biphenyl junctions. As displayed in Figure 5 with the orbital plots of 2 the highest occupied
molecular orbital (HOMO) is almost exclusively localized on the carbazole subunit while the lowest
unoccupied molecular orbital (LUMO) is mainly located on the biphenyl subunit. The localization of both
frontier orbitals on both decoupled subunits raised the hope that their energy levels might be tuned
independently in these model compounds.
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HOMO
LUMO
Figure 5: Contour plots of the frontier orbitals of 2.
The oxidation potential of the CBP derivatives 1-5 were determined by DPV experiments and are
summarized in Table 2. For none of the compounds a reduction process was observed within the
detectable window (recorded to -2.0 V vs. SCE). Thus the HOMO energy levels of the compounds were
estimated by their oxidation potentials after correction of the vacuum energy level (5.1 eV),31 while the
LUMO energy levels were obtained by adding the optical energy gap (Eg) obtained by absorption
spectroscopy to the HOMO level (ELUMO = EHOMO + Eg). The recorded oxidation potentials are summarized
in Table 2 and the determined energy levels of the frontier orbitals are depicted in Figure 6.
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-2.3
-2.4
-2.4
-2.8
-3.1
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Figure 6: Energy diagram showing the position of the frontier orbital of the CBP derivatives. The dark grey line displays the
position of the triplet energies.
As the HOMO of these CBP derivatives is mainly localized on the carbazole subunit (Error! Reference
source not found.), the oxidation potentials of the series 2-5 with various carbazole substituents were
particular interesting. The energy levels of the frontier orbitals of the parent structure 2 with four
methyl groups at the biphenyl core are very comparable with the ones of CBP, while the four methyl
substituents in the 1,8-postions of both carbazoles in 1 result in a slight lifting of the HOMO level due to
the inductive electron donation (+I-effect) by the methyl substituents. Interestingly the LUMO of 1 is
lifted by the same value in spite of the decoupling of the carbazole and the biphenyl subunits. On the
other hand functionalizing the carbazole subunits with electron withdrawing CF3-substituents (-I-effect)
in 3 and 4 lower the energy level of the HOMO considerably and increase the oxidation potential by 450
mV and 440 mV respectively compared to 2. But also in the case of 3 and 4 a similar shift of the LUMO
energy level is involved, falsifying the design proposition of individually addressable energy levels of the
frontier orbitals. That the HOMO level can also be lifted is displayed with 5, in which the pronounced +I-
effect of the methoxy substituents lower the oxidation potential by 360 mV compared to 2. Also here a
concomitant lifting of the LUMO is observed. In their cyclic voltammograms the CBP derivatives 1, 2, 4
and 5 displayed an irreversible oxidation behavior which is exemplified with 2 in Figure 7.
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2nd cycle
3rd cycle
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3rd cycle
0.6
0.9
1.2
1.5
1.8
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Potential applied (V)
Potential applied (V)
Figure 7: Cyclic voltagramm (three cycles, scan rate 100 mV/s, 10-3 M in DCM) of 2 (left) and 3 (right) recorded against SCE.
In the anodic scan an oxidation peak at about 1.5 V vs. SCE is observed which is assigned to the oxidation
of the carbazole units. In the back scan not only the reduction peak between 1.2 and 1.3 V is observed,
but also an additional well defined peak at about 0.9 to 1 V. It is known that after oxidation the
generated radical cation is localized in the 3,6-positions of the carbazole subunit and that the proximity
of radical cations at the electrode surface results in an intermolecular coupling between carbazole
subunits. This dimerization is electrochemically observed as irreversible oxidation. In the subsequent
scan a new oxidation peak in the region of 1.05 to 1.15 V is recorded attributed to the oxidation of the
dimerization product accumulated at the Pt electrode. Similar behavior has been reported for
structurally related CBP derivatives5 and also for N-aryl carbazoles.32 On the other hand, compound 3
reveals a fully reversible oxidation behavior which can be attributed to the blocking of the reactive 3,6-
position of the carbazole units (Figure 7).
In summary the electrochemical studies show that the energy levels of these CBP derivatives can be
fine-tuned to some extent by varying the substituents, and in a lesser extent by altering the substitution
pattern of the pendant carbazoles. These design option is particular appealing to fine tune the energy
levels of the materials involved in multilayer OLED architectures. In addition, the functionalization of the
3,6-position of the carbazole units gives electrochemically stable compounds, which would be a certain
advantage as radical ions are involved in charge transport within the device.
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A series of CBP derivatives with sterically locked conformation at the carbazole-biphenyl junction 1-5
has been synthesized and fully characterized. All CBP derivatives were assembled by first synthesizing
the corresponding N-para-nitrophenyl-carbazole precursors, which were converted into N-para-
iodophenyl derivatives, and then subsequently dimerized either by Turbo Grignard reactions or by
Suzuki-Miyaura type couplings, requiring in addition the corresponding boronic acid derivatives. The key
step was the nucleophilic aromatic substitution reaction (SNAr) of the carbazole nucleophile profiting
from the perfectly activated para-nitrofluorobenzenes, allowing to overcome the limitations of the
sterically restricted reaction space at the carbazole-phenyl junctions. The rigid and almost perpendicular
arrangements of the carbazole-phenyl junctions were corroborated by solid state structures of the
derivatives 2, 3 and 4. A particular focus was set on the CBP derivatives with four methyl groups in the
3,5,3’,5’-positions of the central biphenyl core 2-5, enabling to fine tune the physical properties of the
CBP derivatives by additional carbazole substituents. And indeed, optical and electrochemical analysis
displayed that electron withdrawing CF3-substituents (either in the 3,6-position in 3 or in the 2,7-
position in 4) stabilized the frontier orbitals, while electron donating CH3O-substituents (in 2,7 position
in 5) increased the energy levels of the frontier orbitals. All new CBP derivatives displayed enhanced
triplet energies compared to the parent CBP and DSC studies showed a long lasting amorphous states,
emphasizing the application potential of these CBP derivatives as matrix material in blue emitting
OLED’s.
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We are currently looking at alternative central motives of potential matrix materials as well as variations
in the substitution patterns.
Supporting Information Available
Synthetic procedures and full characterization for all synthesized compounds, DSC traces, as well as
gated steady state phosphorescence spectra for compounds 1-5 and x-ray structures for compounds 2, 3
and 4 are provided as Supporting Information. This information is available free of charge via the
Internet at http://pubs.acs.org/.
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Acknowledgement
We thank the Swiss National Science Foundation and the University of Basel for financial support. This
work has received funding from the European Union’s Seventh Framework Program for research,
technological development and demonstration under the ROSFEN project (grant agreement no.
312829).
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