Oxadiazole based bipolar host materials employing planarized triarylamine donors for RGB PHOLEDs with low efficiency roll-off

Article abstract of DOI:10.1039/c3tc32338b

A series of 6 novel triarylamine-containing oxadiazole compounds (o-PCzPOXD, o-ICzPOXD, o-TPATOXD, o-PCzTOXD, o-ICzTOXD, o-CzTOXD) have been designed, synthesized and characterized concerning applications as host materials in PHOLED devices. To further improve the ortho-linkage concept, the impact of incorporating planarized electron-donating triarylamine (TAA) structures on intramolecular charge transfer was examined. The effect was evaluated for two series of electron-accepting oxadiazole scaffolds, realizing ortho-linkage on the benzene (POXD) and the thiophene (TOXD) cores. Thermal analysis shows increased glass-transition temperatures for planarized structures indicating an improved morphological stability. A higher degree of planarization also results in significantly increased singlet and triplet energy values, revealing the impact on the intramolecular charge transfer. Employing the developed materials, red (o-TPATOXD: CE_max: 28.8 cd A ^-1, EQE_max: 16.9%), green (o-PCzPOXD: CE_max: 62.9 cd A^-1, EQE_max: 17.1%) and blue (o-PCzPOXD: CE _max: 29.8 cd A^-1, EQE_max: 13.4%) devices were achieved showing remarkably low efficiency roll-off for planarized donors. Hence, this is the first report of efficient blue devices for this specific class of host materials. It is proposed that the results correlate with an increasing ortho-linkage effect and decreasing donor strength of the TAA moiety by planarization and, thus, tackling one of the major challenges in PHOLED research: improving both triplet energy and compound stability. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3tc32338b

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Fröhlich, J. Mater. Chem. C, 2014, DOI: 10.1039/C3TC32338B.
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DOI: 10.1039/C3TC32338B
Oxadiazole based bipolar host materials employing
planarized triarylamine donors for RGB PHOLEDs
with low efficiency roll-off†
Paul Kautny,a Daniel Lumpi,*a Yanping Wang,b Antoine Tissot,c
Johannes Bintinger,a Ernst Horkel,a Berthold Stöger,d Christian Hametner,a
Hans Hagemann,c Dongge Ma,b and Johannes Fröhlicha
a Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt
9/163, A-1060 Vienna, Austria
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun, 130022, China
c
Département de Chimie Physique, Université de Genève, 30, quai E. Ansermet, 1211
Geneva 4, Switzerland
d Institute of Chemical Technologies and Analytics, Vienna University of Technology,
Getreidemarkt 9/164, A-1060 Vienna, Austria
daniel.lumpi@tuwien.ac.at
† Electronic supplementary information (ESI) available: NMR spectra, TGA/DSC analyses,
cyclic voltammetry, phosphorescence as well as electroluminescence spectra, DFT
calculations and crystallographic information; See DOI:

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DOI: 10.1039/C3TC32338B
Abstract
A series of 6 novel triarylamine-containing oxadiazole compounds (o-PCzPOXD, o-
ICzPOXD, o-TPATOXD, o-PCzTOXD, o-ICzTOXD, o-CzTOXD) have been
designed, synthesized and characterized concerning applications as host materials in
PHOLED devices. To further improve the ortho-linkage concept, the impact of
incorporating planarized electron-donating triarylamine (TAA) structures on
intramolecular charge transfer was examined. The effect was evaluated for two
series of electron-accepting oxadiazole scaffolds, realizing ortho-linkage on the
benzene (POXD) and the thiophene (TOXD) core. Thermal analysis shows increased
glass-transition temperatures for planarized structures indicating an improved
morphological stability. A higher degree of planarization also results in significantly
increased singlet and triplet energy values, revealing the impact on the intramolecular
charge transfer. Employing the developed materials, red (o-TPATOXD: CEmax: 28.8
cd A-1, EQEmax: 16.9%), green (o-PCzPOXD: CEmax: 62.9 cd A-1, EQEmax: 17.1%)
and blue (o-PCzPOXD: CEmax: 29.8 cd A-1, EQEmax: 13.4%) devices were achieved
showing remarkably low efficiency roll-off for planarized donors. Hence, this is the
first report of efficient blue devices for this specific class of host materials. It is
proposed that the results correlate with an increasing ortho-linkage effect and
decreasing donor strength of the TAA moiety by planarization and, thus, tackling one
of the major challenges in PHOLED research: improving both triplet energy and
compound stability.

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DOI: 10.1039/C3TC32338B
Table of Content
Graphic:
Text:
The significant impact of the planarization of triarylamine donor structures on thermal,
photo-physical and electro-chemical properties of novel host materials is examined
with regard to PHOLED applications.

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DOI: 10.1039/C3TC32338B
Introduction
Organic Light Emitting Diodes (OLEDs) and their application in display technology
represent the most advanced technology among the rapidly growing field of organic
electronics.1-7 Since the groundbreaking work of Forrest et al. from 19988,9 great
efforts have been made in developing Phosphorescent Organic Light Emitting Diodes
(PHOLEDs) typically employing heavy transition metal complexes.10,11 In contrast to
fluorescent OLEDs phosphorescent emitters are capable of harvesting triplet and
singlet excitons simultaneously and, thus, can theoretically achieve 100% internal
quantum efficiency.12,13
However, high concentration of triplet excitons leads to triplet-triplet annihilation at
high current rates resulting in efficiency roll-off.14 Thus, phosphorescent emitters are
generally widely dispersed in an organic host material. Efficient host materials have
to fulfill some basic requirements:7 (i) higher triplet energy (Et) value than the dopant
to effectively confine triplet excitons on the phosphorescent emitter; (ii) suitable
HOMO / LUMO levels to facilitate charge injection; (iii) morphological stability for
durable and long lasting devices.
Bipolar host materials received great attention in recent years, due to their balanced
charge transport properties resulting in broad charge recombination zones. However,
combining donor and acceptor subunits in one molecule ultimately lowers the Et as a
result of the intramolecular charge transfer.7 Therefore, the molecular design of
bipolar host materials focuses on the interruption of the conjugated π-system in order
to reduce donor-acceptor interactions and thus to retain high Et values. Among the
most efficient ways to separate the molecular subunits is the introduction of specific
linkage modes.7 Particularly, ortho-linkage of donor and acceptor subunits, inducing

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twisted molecular conformations, proved to be a highly efficient strategyDtOoI: 1a0.c10h3i9e/Cv3eTC32338B
large triplet bandgap materials.15
In this contribution we reveal the enhancement of the effect of ortho-linkage by
applying increasingly planarized triarylamine donors (from N,N-diphenylbenzenamin
(TPA) to 9-phenyl-9H-carbazole (PCz) and indolo[3,2,1-jk]carbazole (ICz)) to an
established phenyl-oxadiazole-phenyl (POXD) acceptor. The corresponding
triphenylamine compound (o-TPAPOXD) has been previously demonstrated to be
highly efficient as host in red and green PHOLED devices.16,17 Utilizing the planarized
arylamines we were able to increase singlet as well as triplet energies resulting in
higher efficiencies and significantly reduced efficiency roll-off in non-optimized red,
green and blue PHOLED devices. We propose that these findings correlate with an
increasing ortho-linkage effect due to steric impact and decreasing donor strength of
the triarylamine moiety by planarization as a result of the contribution of the nitrogen
lone-pair to the aromaticity of the formed pyrrole subunit(s). To prove the developed
concept, TPA, PCz, ICz as well as carbazole (Cz) donors were also applied to a new
thiophene-oxadiazole-thiophene (TOXD) acceptor motive confirming the trends.
Employing the developed materials red (o-TPATOXD: CEmax: 28.7 cd A-1, EQEmax
:
16.9%), green (o-PCzPOXD: CEmax: 62.9 cd A-1, EQEmax: 17.1%) and blue (o-
PCzPOXD: CEmax: 29.8 cd A-1, EQEmax: 13.4%) devices have been obtained.
Hence, we report on an efficient approach to gain control of intramolecular charge
transfer, significantly affecting singlet as well as triplet energies, by the planarization
of electron donating triarylamine moieties. As a result, novel host materials have
been designed, synthesized and characterized exhibiting remarkably low efficiency
roll-off, which is of technological relevance for both single doped devices and multi-
color based white PHOLEDs.

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DOI: 10.1039/C3TC32338B
Results and Discussion
Synthesis
The synthesis of target host materials and precursors primarily relies on organo-
lithium assisted and transition-metal catalyzed (Ullman condensation, Suzuki cross-
coupling) reactions. The first part describes the synthetic pathways towards
planarized triarylamine (TAA) structures phenylcarbazole and indolocarbazole
particularly focusing on C-H activation as an efficient approach to acquire the desired
planarization. In the second part the realization of the ortho-linkage on the benzene
(5a-c) core and subsequently the thiophene core (8a-d), achieved via a Halogen
Dance (HD) reaction, is outlined.
Boronic esters of triphenylamine (3a) and phenylcarbazole (3b) were obtained in
analogy to literature.18-20 The synthesis of indolocarbazole boronic ester (3c) is based
on a recently developed methodology utilizing C-H activation.21 Compared to
previously reported procedures this synthetic approach towards indolo[3,2,1-
jk]carbazole 2 (Scheme 1) is shorter (less steps), more efficient and does not rely on
the application of special equipment (e.g. vacuum flash pyrolysis).22,23 Although good
yields of 83% (15 mol% Pd(OAc)2 catalyst) were accomplished the reduction of
catalyst loads drastically decreases the reaction conversion. The use of 5 mol%
Pd(OAc)2 even limits the product formation to trace amounts.21 Hence, we report on
an improved procedure at significantly reduced catalyst amounts (2 mol%) towards
indolo[3,2,1-jk]carbazole employing Pd(PPh3)4 and K2CO3 (71% yield). In fact, the
application of (NHC)Pd(allyl)Cl24,25 catalyst (2 mol%) further improves the yields to
94% (Scheme 1 and Table 1). Moreover, the substrate scope could be broadened to
chlorine derivatives (no conversion for Pd(PPh3)4, Table 1).

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DOI: 10.1039/C3TC32338B
Scheme 1 Synthetic pathways towards indolo[3,2,1-jk]carbazole 2; i: (NHC)Pd(allyl)Cl (2 mol%),
K2CO3 (2.0 eq.), DMA (0.2 M), 130 °C, 1-2h.
Table 1 Results of the C-H activation for substrates 1i-iii.
Substrate
Pd(PPh3)4
(NHC)Pd(allyl)Cl
71%a
n.c.b
n.c.b
94%a
94%a
80%a
1i
1ii
1iii
aIsolated yields. bNo conversion (GC-MS).
Particularly, the demonstrated two-fold C-H activation of chlorine derivatives allows
for a convenient and selective synthesis of a variety of novel target structures,
broadening the scope of accessible indolo[3,2,1-jk]carbazole motives.
Selective bromination (NBS, 54%)23 of 2 and organo-lithium assisted transformation
(n-BuLi, isopropyl pinacol borate, 69%) yielded the indolocarbazole pinacol boronic
ester 3c. The crystal structure (see ESI† for details) discloses the desired planarity of
this TAA type structure. 3c26 crystallizes with one crystallographically unique
molecule in the asymmetric unit located on a general position. The aromatic system
is close to flat: The N atom is located 0.1573(5) Å from the least squares (LS) plane
defined by the C atoms of the benzene rings.
The dibrominated POXD (benzene – oxadiazole – benzene structure) precursor 4
was synthesized according to literature.27 The conversion towards target compounds

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5a-c was accomplished utilizing TAA boronic esters 3a-c applying a standard Suzuki
DOI: 10.1039/C3TC32338B
protocol (Scheme 2).28 Good to excellent yields of 85% and 93% were achieved for
5a and 5b, respectively; the lower yield for 5c (39%) is attributed to a lower solubility
affecting both reaction conversion and work-up.
Scheme 2 Synthesis of POXD based host materials 5a-c; i: boronic acid ester 3a-c (2.5 eq.),
K2CO3 (5.0 eq., 2 M aq.), Pd(PPh3)4 (5 mol%), THF (~0.5 M), reflux.
In contrast to the POXD series the dibrominated TOXD (thiophene – oxadiazole –
thiophene structure), precursor 7i is unknown to literature. The ortho-substitution
pattern is realized by applying a double-sided Halogen Dance (HD)29-31 methodology
(Scheme 3) starting from 6 (synthesized according to literature). Due to coordinating
and activating effects of the oxadiazole moiety32 the lithium diisopropylamide (LDA)
induced metallation selectively takes place in the respective ortho-positions.
Subsequently, a rearrangement of the bromine atoms (“Halogen Dance”), resulting in
the thermodynamically most stable organo-lithium intermediate, yields the desired
ortho-substitution pattern.

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DOI: 10.1039/C3TC32338B
Scheme 3 Halogen Dance reaction towards ortho-substitution patterns (7i-ii); i: THF, -25 °C, LDA
(2.4 eq.), 30 min stirring prior to addition of electrophile (MeOH and TMS-Br).
As a consequence, this approach enables to directly introduce substituents R in the
5-positions by addition of suitable electrophiles (R), which has been demonstrated
utilizing TMS-Br to yield the silylated compound 7ii. Hence, specific alterations of
material properties e.g. solubility (for e.g. solution processing) can be achieved;
however, reaction conditions were not further optimized in this study.
The triarylamine and carbazole cap structures are attached to the TOXD core via
Suzuki cross-coupling reaction33-35 and Ullmann condensation,36-38 respectively
(Scheme 4).
Scheme 4 Synthesis of TOXD based host materials 8a-d; i: Suzuki Cross-Coupling: boronic acid
ester 3a-c (3.0 eq.), KOtBu (3.0 eq.), (NHC)Pd(allyl)Cl (2 mol%), iPrOH/H2O (3:1, 5 mM), rf; ii: Ullmann
.
condensation: carbazole (3.0 eq.), K2CO3 (3.0 eq.) and CuSO4 5H2O (6.4 mol%), 230 °C.

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The Suzuki reaction utilizing TAA precursors 3a-c was shown to proceedDiOnI: 1g0.o10o39d/Ct3oTC32338B
moderate yields using Pd(PPh3)4. However, improved yields and more reliable results
could be obtained by applying (NHC)Pd(allyl)Cl,24 (55% - 74%) which generally
turned out to be a highly efficient catalyst for thiophene-benzene cross-coupling.
Attempts of Buchwald-Hartwig reactions in order to introduce the carbazole did not
give satisfactory results, which led to a solvent-free Ullmann procedure20 using
.
CuSO4 5H2O as catalyst.
Thermal Properties
The thermal stability of the target host materials was investigated by TGA. High
decomposition temperatures (Td – determined from 5% mass loss) between 392 °C
and 426 °C were observed for all materials. Glass transition temperatures (Tg) were
analyzed by DSC. Whereas o-TPAPOXD exhibits glass transition at 94 °C16 the
values for o-PCzPOXD (125 °C) and o-ICzPOXD (152 °C) are significantly higher
indicating improved morphological stability of thin films in PHOLED devices. Within
the TOXD series o-PCzTOXD also features higher glass transition at 135 °C
compared to o-TPATOXD (101 °C) or o-CzTOXD (106 °C) while no glass transition
was detected for o-ICzTOXD. In general, the incorporation of the thiophene linker
(TOXD derivatives) resulted in slightly higher glass transition temperatures in relation
to the corresponding POXD compounds. Higher Tg values for PCz and ICz
compounds compared to TPA structure are attributed to an increased rigidity of the
TAA moieties. All observed values are distinctly higher compared to CBP (62 °C7).

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DOI: 10.1039/C3TC32338B
Photo-Physical Properties
Fig. 1 (left) displays the UV/VIS absorption and photoluminescence (PL) spectra of o-
PCzPOXD and o-ICzPOXD as well as “reference compound” o-TPAPOXD. Spectra
of the TOXD series are depicted in Fig. 1 (right). The optical bandgaps (opt. BG),
determined from absorption onsets, are summarized in Table 2.
Fig. 1 Absorption (full symbols) and PL (hollow symbols) characteristics of host materials 5a-c (POXD
series, left) and 8a-d (TOXD series, right); recorded in 5 μM solution in DCM at r.t.
The absorption bands of o-TPAPOXD and o-TPATOXD at 305 nm are assigned to
triphenylamine centered n-π* transition.39 All carbazole containing compounds exhibit
sharp absorption peaks around 290 nm, which are attributed to the n-π* transition of
the carbazole moiety.40 Indolocarbazole containing o-ICzTOXD and o-ICzPOXD
feature a related n-π* transition below 300 nm. Longer wavelength absorption bands
result from π-π* charge transfer transitions from electron-donating arylamines to
electron-accepting oxadiazole.41 Strikingly, the absorbance of charge transfer
transition is significantly lower for o-PCzPOXD and o-ICzPOXD compared to o-
PCzTOXD and o-ICzTOXD, indicating that the partial disruption of the conjugated π-
system by ortho-linkage is more effectively realized on the benzene spacer.
For the TOXD series also a significant blue shift of the absorption onset from o-
TPATOXD (~450 nm) to o-PCzTOXD and o-ICzTOXD (both located at ~400 nm) has

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been observed, indicating increased optical bands gap as a result of planarized TAA
DOI: 10.1039/C3TC32338B
donors.
Photoluminescence (PL) spectroscopy revealed a fluorescence emission maximum
of o-TPAPOXD located at 479 nm. A significant blue shift of the emission maxima is
observed for both o-PCzPOXD (428 nm) and o-ICzPOXD (379 nm). These results
impressively demonstrate the impact of planarizing TAA donors by shifting the singlet
emission by up to 100 nm (ΔE = 0.68 eV). Similar trends were observed for the
TOXD series (o-TPATOXD (502 nm), o-PCzTOXD (456 nm) and o-ICzTOXD (452
nm)). We propose that these findings correlate with an increasing ortho-linkage effect
(more twisted conformation) and decreasing donor strength of the TAA moiety (ICz <
PCz < TPA) by planarization. The alteration in donor strength is attributed to the fact
that the lone pair of the nitrogen is increasingly contributing to the aromaticity (PCz
and ICz). o-CzTOXD shows an emission maximum at 436 nm; the deep blue
emission characteristic is due to the less extended conjugated π-system compared to
the TAA structures. Clearly, all POXD compounds exhibit blue shifted emission with
respect to the thiophene analogs (TOXD) indicating a more effective donor-acceptor
separation for the benzene spacer.
Phosphorescence spectroscopy (77 K) revealed that planarization also impacts triplet
emission, although the effect is less pronounced compared to singlet emission. The
Et values, deduced from the highest vibronic sub-band of the phosphorescent
spectra, of o-TPAPOXD, o-PCzPOXD and o-ICzPOXD were determined to be 2.49
eV (2.46 eV16), 2.62 eV and 2.83 eV, respectively. Compared to the POXD series the
Et values of the TOXD materials o-TPATOXD (2.21 eV), o-PCzTOXD (2.29 eV), o-
ICzTOXD (2.37 eV) and o-CzTOXD (2.39 eV) are shifted to lower energies.

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Although the Et values significantly increase as a consequence of planarization for
DOI: 10.1039/C3TC32338B
both series, host materials of the TOXD series are restricted to low energy emitters.
In contrast, the triplet energies of the POXD series are suitable for the construction of
green devices.16,17 Et values of > 2.60 eV for the PCz and ICz donor structures
potentially broaden the scope even to blue phosphorescent emitters (e.g. FIrPic).
Particularly the unexpectedly high Et value (2.83 eV) for o-ICzPOXD reveals the
effect of incorporating indolo[3,2,1-jk]carbazole structures; a possible explanation for
these findings is given by DFT calculations (see below).
Additionally, lifetimes for the excited singlet and triplet states were determined. Most
striking are the significantly longer triplet lifetimes (τ) found in the POXD series (o-
TPAPOXD (~790 ms), o-PCzPOXD (~335 ms), o-ICzPOXD (~349 ms)) compared to
the TOXD compounds; triplet lifetimes for the TOXD structures range from 8.2 ms –
12.3 ms. The sulfur atoms in the TOXD compounds are subject to stronger spin-orbit
coupling which leads to somewhat relaxed selection rules for transitions between
singlet and triplet states (i.e. shorter lifetimes of the triplet). Further details on photo-
physical properties are given in the ESI†.
Electro-Chemical Properties
Electro-chemical properties (Table 2) of target materials were investigated by cyclic
voltammetry (CV). Predominately determined by electron-rich triarylamines, the
HOMO energy levels span in a narrow range of -5.73 eV to -5.64 eV with the
exception of o-TPAPOXD and o-TPATOXD at -5.25 eV16 and -5.30 eV, respectively.
Thus, the incorporation of planarized TAA donors significantly lowers the HOMO
energy levels. While o-TPATOXD shows reversible oxidation, all carbazole and

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indolocarbazole containing materials exhibit irreversible oxidation waves DdOuI:e10.1t0o39/tCh3eTC32338B
instability of the formed cations.23,42,43 Quasi reversible reductions were observed
during cathodic scans; however, no significant reduction process was observed for o-
ICzPOXD. The LUMO energy levels of the compounds are located between -2.68 eV
and -2.41 eV. These values indicate no significant injection barrier for charge carriers
from adjacent charge transporting layers.
Table 1 Physical data of synthesized materials.
Tg/Tc/Tm/Td
[°C]a
opt. BG
[eV]b,c
λPL,max
HOMO/LUMO
[eV]
ET (eV)
exp.f
[nm]c
exp.d
cal.e
cal.g
2.63
2.83
2.88
2.21
2.30
2.36
2.33
94/---/270/43216
125/n.o.h/252/395
152/231/308/404
101/n.o.h/184/414
135/n.o.h/261/426
n.o.h/302/341/402
106/n.o.h/211/392
3.24
3.51
3.26
2.88
3.19
3.19
3.11
479
428
379
502
456
452
436
-5.25/-2.4116 -5.14/-1.77
2.49
2.62
2.83
2.21
2.29
2.37
2.39
o-TPAPOXD
o-PCzPOXD
o-ICzPOXD
o-TPATOXD
o-PCzTOXD
o-ICzTOXD
o-CzTOXD
-5.64/-2.50
-5.73/-2.50i
-5.30/-2.57
-5.66/-2.68
-5.68/-2.56
-5.69/-2.66
-5.55/-2.04
-5.70/-1.84
-5.19/-2.11
-5.57/-2.37
-5.70/-2.03
-5.65/-2.39
aDetermined from TGA / DSC analysis; Tc: crystallization temperature. bEstimated from the absorption
onset. cMeasurement in DCM (5 μM) at r.t. dCalculated from onsets of oxidation and reduction peaks.
eCalculated applying density functional theory level (B3LYP/6-311+G*). fEstimated from the highest
energy vibronic transition in toluene at 77 K. gCalculated applying time-dependent density functional
theory level (B3LYP/6-311+G*). hNot observed. iCalculated from HOMO level and absorption onset.
Theoretical Calculations
To investigate the geometrical and electronic properties of the target compounds at
molecular level studies applying density functional theory (DFT) and time-dependent
DFT (TDDFT) calculations were conducted (for additional details see Experimental
Section). According to DFT calculations, absolute HOMO / LUMO values are in the
range of 5.14 eV – 5.70 eV / 1.77 eV – 2.39 eV, which correlates with the
experimental data; LUMO levels show a systematic shift of ~0.5 eV.

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The contour plots for 5a and 8a, comparing the POXD with the corresponding TOXD
DOI: 10.1039/C3TC32338B
compounds, are depicted in Fig. 2. As expected, HOMO levels are mainly located at
one of the TAA structures and the LUMOs at the oxadiazole containing cores (POXD
and TOXD). This separation, particularly crucial for high triplet energies, was found to
be more efficiently realized for the POXD series (benzene linker) for all substance
pairs, which is in agreement with photo-physical data (Table 2). Inspecting the TPA
(Fig. 2 (left)) and PCz (Fig. 3 (left)) scaffolds a similar trend is being observed: A
reduced HOMO / LUMO overlap is indicated for the more rigidified PCz versus the
TPA structures. The comparison of the average tilting angles of the of POXD and the
TOXD benzene to the TAA benzene core supports the aforementioned assumptions:
54.6° (56.2° / 53.0°) o-TPAPOXD < 58.7° (62.2° / 55.2°) o-PCzPOXD < 59.7° (61.9° /
57.5°) o-ICzPOXD and 50.2° (52.0° / 48.3°) o-TPATOXD < 54.3° (57.8° / 50.8°) o-
PCzTOXD < 54.9° (57.0° / 52.8°) o-ICzTOXD (the exact tilting angle values for each
side of the unsymmetric molecules are given in brackets).
A remarkable result of the calculation was acquired for o-ICzPOXD (Fig. 3 (right)).
Whereas the HOMO is positioned at the TAA (ICz) scaffold (as expected), the LUMO
is not located on the POXD core but on the opposed ICz moiety. Potentially, this fact
is an explanation for the aforementioned unexpectedly high triplet energy observed
as a result of the enhanced spatial separation and modified acceptor properties. This
finding outlines the significant deviation in donor strength for the applied TAA
structures and the effect on material properties. In contrast, the LUMO in the o-
ICzTOXD remains located at the TOXD core (as observed for all other molecules),
which corresponds to the detected triplet and singlet energy value relations; thus,
supports the hypothesis for the LUMO located on the ICz core in o-ICzPOXD.

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DOI: 10.1039/C3TC32338B
Fig. 2 HOMO (bottom) and LUMO (top) of
o-TPAPOXD (left) and o-TPATOXD (right).
Fig. 3 HOMO (bottom) and LUMO (top) of
o-PCzPOXD (left) and o-ICzPOXD (right).

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DOI: 10.1039/C3TC32338B
Crystallography
o-PCzPOXD (5b)44 (Fig. 4 (left)) crystallizes with one crystallographically unique
molecule in the asymmetric unit. The molecules are pseudo-symmetric by twofold
rotation around an axis passing through the O atom of the oxadiazole ring. The PCz
donor units are located on the side of the O atom of the oxadiazole ring. Due to steric
interaction of these groups, the core benzenes (POXD) are strongly inclined to each
other (angle between the LS planes 60.95(3)°) and to the central oxadiazole ring
(32.45(8)° and 33.64(8)°). The carbazole moieties are moderately inclined with
respect to the core benzenes (15.58(9)° and 16.25(9)°). On the other hand the PCz-
benzene rings are distinctly inclined to the POXD benzenes (53.49(6)° and 54.12(6)°)
and carbazoles (56.38(5)° and 57.68(5)°).
Fig. 4 Molecular structures of o-PCzPOXD (left) and o-PCzTOXD (right) – C, N, O and S atoms are
represented by white, blue, red and yellow ellipsoids drawn at 50% probability levels, H atoms by
spheres of arbitrary radius. For o-PCzTOXD only one out of two unique molecules is shown.
o-PCzTOXD (8b)45 (Fig. 4, (right)) crystallizes with two crystallographically different
molecules (Z' = 2) located on general positions. Both molecules are geometrically
virtually equivalent. One thiophene ring is in trans- and one in cis-conformation with
respect to the central oxadiazole ring. Thus, both PCz moieties face opposite
directions. This is in contrast to the crystal structures of o-PCzPOXD and the
solvates of the thiadiazole analog of o-PCzTOXD (as reported in earlier studies), in

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which the thiadiazole and thiophene rings are all in trans- and, therefore, the PCz
DOI: 10.1039/C3TC32338B
units in cis-conformation.46 The thiophene and oxadiazole rings in cis-conformation
are practically planar with tilt angles of 3.17(10)° and 3.53(10)°. In contrast, the
thiophenes in the trans-conformation feature distinct inclination to the oxadiazole ring
(12.03(11)° and 11.44(11)°). Whereas the carbazole connected via the PCz-benzene
to the cis-located thiophene is nearly coplanar with the latter (3.17(8)°, 2.33(8)°), the
trans-located thiophene is strongly inclined to the corresponding carbazole
(76.91(9)°, 76.86(9)°). The PCz-benzenes on the other hand are, like in o-PCzPOXD,
always strongly inclined to the neighboring aromatic moieties (tilt angles to thiophene:
45.82(10)° – 47.93(10)°; to carbazoles: 44.59(8)° – 61.06(8)°).
Charge Transport Properties
In order to study the influence of the four investigated arylamine-donors on charge
transport properties within the TOXD series, hole-only devices (HODs; structure: ITO
/ MoO3 (8 nm) / Host (80 nm) / MoO3 (8 nm) / Al) and electron-only devices (EODs;
structure: Al / Li2CO3 (1 nm) / Be:Li2CO3 (30 nm) / Host (80 nm) / Be:Li2CO3 (30 nm) /
Li2CO3 (1 nm) / Al) were fabricated. It is noted that the HOMO levels of o-CzTOXD,
o-PCzTOXD and o-ICzTOXD are nearly 0.4 eV lower compared to o-TPATOXD.
This fact may hamper hole injection to the HODs, however, displays the actual
situation in the PHOLED devices. Current-voltage curves are given in Fig. 5.

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DOI: 10.1039/C3TC32338B
Fig. 5 Current–voltage (I–V) curves of hole- (hollow symbols) and electron-only (full symbols) devices.
o-TPATOXD exhibits good hole transport properties due to the presence of the
triphenylamine moiety; significantly lower current density was observed in the EOD.
In contrast carbazole based o-CzTOXD and o-PCzTOXD feature better electron
transport properties. While o-PCzTOXD exhibits similarly low hole current density
compared to o-CzTOXD, the current density in the EOD applying o-PCzTOXD is
lower and therefore charge transport in o-PCzTOXD is more balanced than in o-
CzTOXD. For o-ICzTOXD the HOD shows higher current density than the EOD.
Furthermore, carrier properties in o-ICzTOXD are more balanced compared to the
other host materials. Thus, the incorporation of the indolocarbazole moiety resulted in
the most bipolar character of o-ICzTOXD among this series of materials. The same
trends were found for the POXD series with the exception of the EOD of o-ICzPOXD
featuring hardly any electron transport. This finding can be explained by the
significantly altered orbital distribution of o-ICzPOXD as discussed in the theoretical
part.

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DOI: 10.1039/C3TC32338B
Electroluminescent Properties
To evaluate the applicability of the POXD series as universal host materials for RGB
PHOLED devices with a standard architecture of ITO / MoO3 (8 nm) / NPB (50 nm) /
TCTA (5 nm) / EML (10 nm) / TPBI (25 nm) / LiF(1 nm) / Al, in which the EMLs
consist of coevaporated hosts o-TPAPOXD (I), o-PCzPOXD (II) or o-ICzPOXD (III)
and guest Ir(MDQ)2(acac) (R), Ir(ppy)3 (G) or FIrPic (B), were fabricated. NPB is
used as hole transporting layer while TPBI is applied as electron transporting and
hole blocking layer due to its low lying HOMO level.47-50 Additionally a thin TCTA
layer is inserted between the hole transporting and emitting layer in order to confine
triplet excitons more effectively in the EML as result of a larger triplet energy of TCTA
compared to NPB.48 The standard device architecture including HOMO and LUMO
levels of all employed materials is depicted in Fig. 6.
Fig. 6 Schematic energy level diagram of the device architecture employed in this work.
Current density-voltage-luminance and current efficiency-luminance-power efficiency
curves for all devices are depicted in Fig. 7 and the electroluminescent properties of
all fabricated PHOLED devices are summarized in Table 3. Among green devices the
best efficiency was achieved for device GII (host material: o-PCzPOXD), giving a
maximum current efficiency (CE) of 62.9 cd A-1, a maximum power efficiency (PE) of
53.5 lm W-1 and a maximum external quantum efficiency (EQE) of 17.1%, while lower
values were obtained for GI and GIII with CEmax of 35.1 cd A-1and 48.8 cd A-1, PEmax

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of 34.4 lm W-1 and 51.6 lm W-1 and EQEmax of 10.3% and 13.8%, respeDcOtiIv: 1e0.l1y0.39W/C3eTC32338B
emphasize that significantly higher efficiencies were obtained by applying o-
PCzPOXD and o-ICzPOXD in our standard device configuration compared to o-
TPAPOXD. In fact, using o-TPAPOXD Tao et al.17 reported highly efficient green
PHOLED devices reaching EQEmax of 22.0% (vs. EQEmax of 10.3%) after optimization
of device architecture, thus revealing the potential of the newly developed materials
within this paper. Notably, at illumination relevant brightness of 1000 cd m-2 devices
GII and GIII retained high CE of 61.2 cd A-1and 48.1 cd A-1. These values correspond
to remarkably low efficiency roll-offs of 2.7 and 1.5%. Even at a brightness of 5000 cd
m-2 GII showed a CE of 56.8 cd A-1 (9.7% roll-off), while GIII displayed a CE of 44.8
cd A-1 (8.2% roll-off). In contrast a significantly higher overall efficiency roll-off of
55.3% for device GI, featuring CE of 28.2 cd A-1and 15.7 cd A-1 at a brightness of
1000 cd m-2 and 5000 cd m-2, was observed. This efficiency roll-off is attributed to the
dominant hole transport properties of o-TPAPOXD, due to the TPA donor unit (more
balanced charge transport for the planarized donor structures; see chapter Charge
Transport Properties). The unbalanced charge transport properties lead to
accumulation of holes at the interface of the emissive and TPBI layer and a narrow
exciton recombination zone. Since triplet-triplet annihilation (TTA) is strongly
dependent on the triplet exciton concentration the thickness of the recombination
zone is of crucial importance and a major factor for efficiency roll-off in PHOLED
devices at high current densities.51 Thus, the incorporation of planarized arylamine
donors significantly decreases the efficiency roll-off as a result of more balanced
charge transport properties resulting in broader recombination zones.
View Article Online
Red devices RI, RII and RIII showed similar performance exhibiting CEmax of 14.0 cd
A-1, 16.1 cd A-1 and 15.0 cd A-1, PEmax of 13.7 lm W-1, 16.8 lm W-1 and 12.2 lm W-1
and EQEmax of 8.3%, 11.2% and 10.1%. In analogy to the green devices lower

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efficiency roll-off was observed for devices RII and RIII (incorporating planDaOrIi:z1e0.1d03h9/oC3sTtC32338B
materials) compared to RI.
Furthermore, blue device BII, utilizing o-PCzPOXD as host, was fabricated displaying
a CEmax of 29.8 cd A-1, a PEmax of 33.5 lm W-1 and an EQEmax of 13.4%. Thus, it was
demonstrated that o-PCzPOXD is applicable as universal host material in efficient
red, green and blue PHOLED devices. However, low device performance has been
observed for blue devices applying o-ICzPOXD. For these devices also the recorded
emission could not be assigned to the FIrPic emitter only. Systematic investigations
are necessary to understand these findings.
As a result of the lower triplet energies of the TOXD materials compared to the POXD
series, o-TPATOXD (IV), o-PCzTOXD (V), o-ICzTOXD (VI) and o-CzTOXD (VII)
were examined in red (R) devices only. Identical device architecture as for the POXD
series was applied. Among this series of devices RIV exhibited the highest CEmax of
28.8 cd A-1, PEmax of 28.4 lm W-1 and EQEmax of 16.9%, while RV, RVI, RVII
displayed CEmax of 14.4, 19.2 and 12.6 cd A-1, PEmax of 10.6, 16.7 and 10.0 lm W-1
and EQEmax of 11.2%, 13.2% and 9.4%, respectively, displaying similar device
performance as the corresponding POXD compounds. The same tendency for
decreased efficiency roll-off observed for the POXD series was found for devices
RIV-VII. While the CE of device RIV drops to 10.5 cd A-1 at a brightness of 5000 cd
m-2, corresponding to an efficiency roll-off of 64%, current efficiencies for devices RV,
RVI and RVII (incorporating planarized donors) decreased only by 24% to 27%.

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DOI: 10.1039/C3TC32338B
Fig. 7 Current density-voltage-luminance (full symbols: current density, hollow symbols: luminance)
and current efficiency-luminance-power efficiency (full symbols: power efficiency, hollow symbols:
current efficiency) curves of all devices investigated in this work.

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DOI: 10.1039/C3TC32338B
Table 3 Electroluminescent properties of devices GI-III, BII and RI-VII.
Von [V] Lmax [cd m-2]
CE [cd A-1]a
PE [lm W-1]a
EQE [%]a
2.8
3.0
2.6
2.8
2.8
3.0
3.0
2.6
3.2
2.8
3.0
15049
49386
48768
21710
16838
29615
27517
22160
27768
36975
25837
35.0/28.2/15.7/35.1
62.3/61.2/56.8/62.9
48.4/48.1/44.8/48.8
18.9/16.7/13.4/29.8
13.9/11.5/7.4/14.0
15.3/13.2/10.1/16.1
14.9/12.9/10.8/15.0
28.7/22.6/10.5/28.8
14.2/13.7/10.9/14.4
16.2/16.5/14.2/19.2
12.6/11.4/9.2/12.6
32.3/17.1/6.3/34.4
48.9/31.1/21.5/53.5
44.9/32.1/22.4/51.6
15.3/9.4/5.6/33.5
11.7/6.4/2.9/13.7
12.4/7.2/4.0/16.8
9.4/5.3/3.3/12.2
10.3/8.1/4.7/10.3
17.0/16.4/15.5/17.1
13.7/13.6/12.7/13.8
8.5/7.5/6.1/13.4
8.2/6.6/4.1/8.3
GI
GII
GIII
BII
RI
10.6/9.1/6.7/11.2
9.1/6.9/5.3/10.1
16.8/13.2/6.0/16.9
11.1/9.5/7.3/11.2
11.3/11.4/9.6/13.2
9.3/7.2/5.8/9.4
RII
RIII
RIV
RV
21.4/10.1/3.0/28.4
9.7/6.5/3.7/10.6
11.0/7.0/4.3/16.7
8.6/5.3/3.1/10.0
RVI
RVII
aMeasured at a brightness of 100 cd m-2/1000 cd m-2/5000 cd m-2/max

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DOI: 10.1039/C3TC32338B
Conclusion
In this study, the impact of planarizing TAA donors in bipolar host materials on the
ortho-linkage effect was investigated in detail. Hence, oxadiazole based structures
5a-c and 8a-d have been designed, synthesized and examined regarding thermal,
photo-physical and electro-chemical properties.
The straight-forward synthesis of target compounds relies on efficient cross-coupling
and organolithum based procedures. In particular, the developed C-H activation
protocol allows for a convenient and selective access to potential target scaffolds,
clearly broadening the scope of accessible indolo[3,2,1-jk]carbazole motives.
For increasingly planarized TAA donor structures the following conclusion can be
drawn: (i) improved morphological stability; (ii) elevated singlet and triplet energy
values, which are attributed to an enhanced ortho-linkage effect and reduced donor
strength of the planarized structures; (iii) significantly reduced efficiency roll-off for all
devices. Hence, the concept of planarization addresses major challenges in PHOLED
research: improving both triplet energy and compound stability. In fact, triplet
energies of >2.60 eV enable the construction of efficient blue (FIrPic) PHOLEDs,
which has not been reported for this specific substance class up to date.
Improved PHOLED performances in standard device configurations compared to
non-planarized reference compound o-TPAPOXD (EQEmax of 22.0% reported for
optimized architectures)17 reveal the great potential for the developed compounds as
highly efficient host materials.

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DOI: 10.1039/C3TC32338B
Experimental Section
General Information
All reagents and solvents have been purchased from commercial suppliers and used
without further purification. Anhydrous solvents were prepared by filtration through
drying columns; dry DMF was obtained from Acros. Column chromatography was
performed on silica 60 (Merck, 40-63 μm). NMR spectra were recorded on a Bruker
Avance DRX-400 Spectrometer or a Bruker Avance 200 Spectrometer. High
resolution mass spectra (HRMS) were obtained from a Thermo Scientific LTQ
Orbitrap XL hybrid FTMS (Fourier Transform Mass Spectrometer) and Thermo
Scientific MALDI LTQ Orbitrap interface; α-cyano-4-hydroxycinnamic acid was used
as matrix. Thermogravimetric (TG) and differential scanning calorimetry (DSC)
measurements were carried out with a heating rate of 5 K/min in a flowing argon
atmosphere (25 mL/min). For the TG measurements, a Netsch TG 209 F9 Tarsus
system, working with open aluminium oxide crucibles, was used. For the DSC
measurements, a Netsch DSC 200 F3 Maia, working with aluminium pans with
pierced lids, was employed. UV/VIS absorption and fluorescence emission spectra
were recorded in DCM solutions (5 μM) with a Perkin Elmer Lambda 750
spectrometer and an Edinburgh FLS920, respectively. Time resolved experiments
were obtained using a Quantel Brilliant tripled Nd-YAG laser (355 nm, 20 Hz
repetition rate, pulse width ~5 ns). Spectra were measured using a SPEX 270
monochromator equipped with both photomultiplier and CCD. This set-up is
controlled using a home-built Labview-based program which allows using different
instruments such as photon counting, oscilloscope, and additional mechanical
shutters. For the measurement of the triplet emission, a mechanical shutter was
triggered by the pulsed laser. A pretrigger period of 0.5 ms was followed by a 1 ms

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aperture and a rest time of 300 – 500 ms allowed obtaining the measurements shown
DOI: 10.1039/C3TC32338B
in the ESI. The slit of the monochromator was also opened further (up to 0.5 mm) to
measure the triplet emission. Triplet energy (ET) values were determined form the
highest energy vibronic subband (first significant peak/shoulder) of the time-resolved
low-temperature phosphorescence spectra. Cyclic voltammetry was performed using
a three electrode configuration consisting of a Pt working electrode, a Pt counter
electrode and an Ag/AgCl reference electrode and a PGSTAT128N, ADC164,
DAC164, External, DI048 potentiostat provided by Metrohm Autolab B.V.
Measurements were carried out in a 0.5 mM solution in anhydrous DCM (oxidation
scan) and THF or DMF (reduction scan) with Bu4NBF4 (0.1 M) as supporting
electrolyte. The solutions were purged with nitrogen for 15 minutes prior to
measurement. HOMO and LUMO energy levels were calculated from the onset of
oxidation and reduction, respectively. The onset potential was determined by the
intersection of two tangents drawn at the background and the rising of oxidation or
reduction peaks.
Synthetic Details
N,N-Diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzeneamine (3a),18,19
9-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-9H-carbazole (3b),19,20 2,5-
bis(5-bromo-2-thienyl)-1,3,4-oxadiazole (4),52 and 2,5-bis(2-bromophenyl)-1,3,4-
oxadiazole (6)27 have been prepared according to published procedures.
9-(2-Chlorophenyl)-9H-carbazole (1ii). 1ii was synthesized following procedures in
literature.20 Carbazole (2.51 g, 15.0 mmol, 1.00 eq.), 1-bromo-2-chlorobenzene (3.45
g, 18.0 mmol, 1.20 eq.), K2CO3 (2.07 g, 15.0 mmol, 1.00 eq.) and CuSO4∙5H2O (0.19
g, 0.75 mmol, 0.05 eq) were added to a 250 ml three-necked round-bottomed flask.

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The mixture was heated with a heating jacket to 230 °C for 72 h. After cooling to r.t.
DOI: 10.1039/C3TC32338B
H2O (100 ml) was added and the mixture was extracted with toluene. The brown
suspension was filtrated and the remaining solvent removed in vacuo. Purification by
crystallization from EtOH yielded 1ii as white solid (1.93 g, 6.9 mmol, 46%). 1H NMR
(400 MHz, CDCl3): δ = 8.21 (d, J=7.7 Hz, 2H), 7.73-7.71 (m, 1H), 7.56-7.44 (m, 5H),
13
7.35 (t, J=7.5 Hz, 2H), 7.15 (d, J=8.1 Hz, 2H) ppm. C NMR (100 MHz, CDCl3): δ =
140.8 (s), 135.0 (s), 133.7 (s), 131.0 (d), 130.8 (d), 129.8 (d), 128.1 (d), 125.9 (d),
123.3 (s), 120.3 (d), 120.0 (d), 109.9 (d) ppm. Calculated: m/z 277.07 [M]+. Found:
MS (EI): m/z 277.03 [M]+.
2,6-Dichloro-N,N-diphenylbenzenamine (1iii). Diphenylamine (1.02 g, 6.0 mmol,
1.00 eq.) was dissolved in dry DMF (18 ml). The solution was purged with argon,
subsequently heated to 50 °C and NaH (0.43 g, 18.0 mmol, 3.00 eq.) was added
quickly. The suspension was stirred for 5 min at 50 °C before 1,3-dichloro-2-
fluorobenzene (1.49, 9.0 mmol, 1.50 eq.) was added. Due to incomplete conversion
(TLC) after stirring overnight additional NaH (0.14 g, 6.0 mmol 1.00 eq.) was added.
After full conversion (TLC – 24 h) the reaction mixture was poured on H2O, extracted
with DCM repeatedly and the combined organic layers were dried over Na2SO4 and
concentrated under reduced pressure. Column chromatography (PE) yielded 1iii
1
(0.79 g, 2.5 mmol, 42%) as white solid. H NMR (400 MHz, CD2Cl2): δ = 7.46 (d,
J=8.1 Hz, 2H), 7.29-7.22 (m, 5H), 6.99-7.96 (m, 6H) ppm. 13C NMR (100 MHz,
CD2Cl2): δ = 145.7 (s), 140.7 (s), 137.6 (s), 130.2 (d), 129.6 (d), 129.3 (d), 122.6 (d),
121.0 (d) ppm. Calculated: m/z 313.04 [M]+. Found: MS (EI): m/z 313.01 [M]+.
Indolo[3,2,1-jk]carbazole (2). 1i/1ii/1iii (1.0 eq.), (NHC)Pd(allyl)Cl24,25 (2 mol%) and
K2CO3 (2.00 eq.) were dissolved in degassed N,N-dimethylacetamide (DMA, 0.2 M)
under argon atmosphere and heated to 130 °C until complete conversion (GCMS).

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The reaction mixture was poured on water and extracted with Et2O. The combined
DOI: 10.1039/C3TC32338B
organic layers were dried over anhydrous Na2SO4 and the solvent was removed
under reduced pressure. Purification was accomplished by column chromatography
(PE). Starting from 1i (9.66 g, 30.0 mmol), (NHC)Pd(allyl)Cl (0.34 g, 0.60 mmol) and
K2CO3 (8.29 g, 60.0 mmol) 2 was obtained as white powder (6.78 g, 28.1 mmol,
94%). Starting from 1ii (3.70 g, 13.3 mmol), (NHC)Pd(allyl)Cl (0.15 g, 0.27 mmol) and
K2CO3 (3.67 g, 26.6 mmol) 2 was obtained as white powder (3.02 g, 12.5 mmol 94%).
Starting from 1iii (0.47 g, 1.5 mmol), (NHC)Pd(allyl)Cl (0.02 g, 0.03 mmol) and
K2CO3 (0.43 g, 3.1 mmol) 2 was obtained as white powder (0.29 g, 1.2 mmol, 80%).
1H NMR (400 MHz, CD2Cl2): δ = 8.16 (d, J=7.8 Hz, 2H), 8.06 (d, J=7.4 Hz, 2H), 7.92
(d, J=8.1 Hz, 2H), 7.62-7.55 (m, 3H), 7.38 (t, J=7.7 Hz, 2H) ppm. 13C NMR (100 MHz,
CD2Cl2): δ = 144.2 (s), 139.2 (s), 130.5 (s), 127.3 (d), 123.6 (d), 123.4 (d), 122.3 (d),
119.9 (d), 118.9 (s), 112.7 (d) ppm. Calculated: m/z 241.09 [M]+. Found: MS (EI): m/z
241.11 [M]+.
2-Bromoindolo[3,2,1-jk]carbazole. 2 (14.26 g, 59.1 mmol, 1.0 eq.) was dissolved in
AcOH / CHCl3 = 1:1 (300 ml) and heated to 55 °C. NBS (10.51 g, 59.1 mmol, 1.0 eq.)
was added in small portions over a period of 1 h to the grey suspension. Since GC-
MS analysis indicated incomplete conversion, more NBS (1.05 g, 5.9 mmol, 0.10 eq.)
was added. After complete conversion the reaction was poured on aqueous NaOH
solution (1000 ml, 6M, 2 eq.) and extracted with DCM. The organic layer was dried
over Na2SO4 and the solvent was removed in vacuo. Crystallization from ACN
yielded 2-bromoindolo[3,2,1-jk]carbazole as beige powder (10.30 g, 32.2 mmol,
54%). Physical data in accordance to literature.23
2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)indolo[3,2,1-jk]carbazole
(3c).
The synthesis of 3c was accomplished analogously to published procedures.19,23 To