Spiro-configured bipolar host materials for highly efficient electrophosphorescent devices

Article abstract of DOI:10.1002/asia.201100467

In this study, we synthesized and characterized a series of spirobifluorene-based bipolar compounds (D2 ACN, DNPACN, DNTACN, and DCzACN) in which a dicyano-substituted biphenyl branch, linked orthogonally to a donor biphenyl branch bearing various diarylamines, acted as an acceptor unit allowing fine-tuning of the morphological stability, triplet energy, bipolar transport behavior, and the HOMO and LUMO energy levels. The promising physical properties of these new compounds, together with their ability to transport electrons and holes with balanced mobilities, made them suitable for use as host materials in highly efficient phosphorescent organic light-emitting diodes (PhOLEDs) with green iridium-based- or red osmium-based phosphors as the emitting layer (EML). We adopted a multilayer structure to efficiently confine holes and electrons within the EML, thus preventing exciton diffusion and improving device efficiency. The device incorporating D2 ACN doped with the red emitter [Os(bpftz)₂(PPhMe₂)₂] (bpftz=3- (trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate) gave a saturated red electrophosphorescence with CIE coordinates of (0.65, 0.35) and remarkably high efficiencies of 20.3 % (21 cd A^-1) and 13.5 Lm W^-1 at a practical brightness of 1000 cd m^-2.

Full text of DOI:10.1002/asia.201100467

DOI: 10.1002/asia.201100467
Spiro-Configured Bipolar Host Materials for Highly Efficient
Electrophosphorescent Devices
Sung-Yu Ku,[a] Wen-Yi Hung,*[b] Chung-Wen Chen,[b] Shih-Wei Yang,[b] Ejabul Mondal,[a]
Yun Chi,[c] and Ken-Tsung Wong*[a]
Abstract: In this study, we synthesized
and characterized a series of spirobi-
fluorene-based bipolar compounds
(D2ACN, DNPACN, DNTACN, and
DCzACN) in which a dicyano-substi-
tuted biphenyl branch, linked orthogo-
nally to a donor biphenyl branch bear-
ing various diarylamines, acted as an
acceptor unit allowing fine-tuning of
the morphological stability, triplet
energy, bipolar transport behavior, and
the HOMO and LUMO energy levels.
The promising physical properties of
these new compounds, together with
their ability to transport electrons and
holes with balanced mobilities, made
them suitable for use as host materials
in highly efficient phosphorescent or-
ganic light-emitting diodes (PhOLEDs)
with green iridium-based- or red
osmium-based phosphors as the emit-
ting layer (EML). We adopted a multi-
layer structure to efficiently confine
holes and electrons within the EML,
thus preventing exciton diffusion and
improving device efficiency. The device
incorporating D2ACN doped with the
red emitter [OsACTHNUTRGENNUG(bpftz)2ACHTUNGTRNE(NUGN PPhMe2)2]
(bpftz=3-(trifluoromethyl)-5-(4-tert-
butylpyridyl)-1,2,4-triazolate) gave
a
saturated red electrophosphorescence
with CIE coordinates of (0.65, 0.35)
and remarkably high efficiencies of
20.3% (21 cdAꢀ1) and 13.5 LmWꢀ1 at
a practical brightness of 1000 cdmꢀ2.
Keywords: bipolar host · electro-
phosphorescence · oleds · osmium ·
spiro compounds
Introduction
tained through varying the structural features of the ligands
and the nature of the metal centers (e.g., Ir,[3] Os,[4] and
Pt[5]), thereby allowing the realization of full-color PhOLED
displays. To obtain high-performance PhOLEDs, the host
material must have a triplet energy higher than that of the
emitting phosphor, regardless of the emission colors from
transition-metal-centered phosphors, thereby preventing
detrimental reverse-energy transfer.[6] In addition, sophisti-
cated device configurations are typically required to facili-
tate electron/hole injection into the emitting layer (EML)
and to confine the emissive exciton efficiently onto the
phosphor. Apart from device engineering, the development
of tailor-made host compounds that combine various func-
tions into a multifunctional material with balanced hole-
and (bipolar) electron-transport properties is another poten-
tial alternative approach toward highly efficient PhOLEDs.
By hybridizing p- and n-type structural features into a bipo-
lar host material, more-balanced charge fluxes can be ach-
ieved with better carrier injection/transport properties in the
EML, thus possibly resulting in higher device performance.
Although effective bipolar host materials for red, green, and
blue phosphors have been reported in recent years,[7] re-
search directed toward the synthesis of bipolar host materi-
als for red phosphorescence has been rather limited. Chien
Organic light-emitting diodes (OLEDs) are being investigat-
ed extensively for their applications in high-efficiency, low-
drive voltage, large-area flat-panel displays.[1] Particular in-
terest has been focused recently toward phosphorescent
OLEDs (PhOLEDs) because of the higher efficiencies re-
sulting from strong spin-orbit coupling effects induced by
heavy-metal centers. PhOLEDs employing transition-metal-
containing complexes as emitting guests, which are normally
dispersed into appropriate host materials, can harvest both
singlet and triplet excitons for emission, thereby providing
the opportunity to realize internal quantum efficiencies
close to 100% (external quantum efficiency (hext)=ca. 20%
(ph/el)).[2] Transition-metal-centered phosphors with emis-
sion colors covering the full visible spectrum have been ob-
[a] Dr. S.-Y. Ku, E. Mondal, Prof. K.-T. Wong
Department of Chemistry
National Taiwan University
Taipei 106 (Taiwan)
E-mail: kenwong@ntu.edu.tw
[b] Prof. W.-Y. Hung, C.-W. Chen, S.-W. Yang
Institute of Optoelectronic Sciences
National Taiwan Ocean University
Keelung 202 (Taiwan)
et al. reported that doping the osmium phosphor [Os
(PPh2Me)2] into the host material 2,7-bis(diphenylphosphor-
yl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene
(POAPF) provided devices with maximum efficiencies as
high as 19.9% and 34.5 LmWꢀ1 [4a]
Ma and co-workers syn-
ACHTUNGTRENNUNG(fptz)2
AHCTUNGTRENNUNG
E-mail: wenhung@mail.ntou.edu.tw
[c] Prof. Y. Chi
Department of Chemistry
National Tsing Hua University
300 Hsinchu (Taiwan)
.
thesized an oxadiazole/carbazole-based bipolar compound
Chem. Asian J. 2012, 7, 133 – 142
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 133

FULL PAPERS
(CzOXD) as the host material; when combined with the iri-
within the emission layer, it would be necessary to fabricate
multilayer devices with the EML incorporating the spiro-
configured bipolar host. To optimize the device configura-
tion, it would be desirable to perform a systematic study of
the correlation between the physical properties (e.g., triplet
energies, energy levels, charge mobility) and the structural
features of 9,9’-spirobifluorene-based bipolar compounds.
Herein, we report that multilayer PhOLED devices employ-
ing those spiro-configured bipolar compounds as hosts
doped with green or red dopants can achieve high efficien-
cies. One of these devices, which adopts D2ACN as the host
dium complex [Ir
G
N
a
PhOLED device exhibiting
a
maximum luminance of
15232 cdmꢀ2 and a current efficiency of 20 cdAꢀ1.[8] Ma and
co-workers synthesized an ortho-linked oxadiazole/amine-
based bipolar compound as the host for the red dopant iridi-
um complex bis(1-phenylisoquinolinolato-C2,N)iridium-
(acetylacetonate) [(piq)2IrACTHNUTRGNEUGN(acac)], leading to a deep-red
electrophosphorescent device with a value of maximum hext
reaching 18.5%.[9] Shu and co-workers reported a sulfonyl/
triphenylamine-based bipolar host material that, when com-
bined with the dopant tris(1-phenylisoquinolinolato-
and [OsACHTNUGRTENN(UG bpftz)2ACHTUNGTRNE(NUGN PPhMe2)2] as the emitter, provided a satu-
C2,N)iridium
N
G
rated red electrophosphorescence with CIE coordinates (x,
y) of (0.65, 0.35) and remarkably high efficiencies of 20.3%
(21 cdAꢀ1) and 13.5 LmWꢀ1.
[(piq)2IrACHTUNGTRENNUNG(acac)] as the dopant to develop a PhOLED device
that exhibited a remarkable maximum external quantum ef-
ficiency (hext) of 21.6%.[11] From the point of view of molec-
ular design, the introduction of an electron-donating moiety
onto one of the biphenyl branches of a 9,9’-spirobifluorene
core and an electron-withdrawing moiety onto the other bi-
phenyl branch[12] would lead to a bipolar system.[13] The
energy levels of the highest occupied molecular orbital
(HOMO), and lowest unoccupied molecular orbital
(LUMO), and the carrier fluxes of the resulting spiro-con-
figured bipolar systems can be modulated by varying the
structural features of the donor (D) or acceptor (A) parts
independently, thereby making such systems potentially at-
tractive for use as active host materials in PhOLEDs.[14] In a
previous study, we applied a bipolar host—2,7-dicyano-2’,7’-
bisdiphenylamino-9,9’-spirobifluorene (D2ACN), configured
with appropriately positioned electron-deficient cyano- and
electron-rich diphenylamino moieties on the respective bi-
phenyl branches of 9,9’-spirobifluorene—in a simple config-
uration [indium tin oxide (ITO)/mt-DATA/D2ACN:-
Results and Discussion
The bipolar compounds comprise a cyano-substituted bi-
phenyl branch as an acceptor orthogonally bridged to a
donor branch bearing various diarylamino groups. Scheme 1
presents our synthetic routes toward these bipolar hosts,
starting from 2,7-dicyano-9,9’-spirobifluorene (1).[15] The di-
bromination of 1 was achieved through treatment with an
excess of Br2 and a catalytic amount of FeCl3 to afford the
dibromide 2 in 87% yield. Palladium-catalyzed C N bond
formation through cross-coupling of 2 with 1-naphthylphe-
nylamine gave DNPACN (62%). Unfortunately, the palladi-
ꢀ
ꢀ
um-catalyzed C N bond-formations of the dibromide 2 with
di(para-tolyl)amine and carbazole failed. Therefore, we in-
troduced more-reactive iodo groups onto compound 1 by re-
acting with I2 and HIO3 in AcOH and CHCl3 to give 2,2’-
diiodo-9,9’-spirobifluorene (3) in 80% yield. Subsequent
ꢀ
palladium-catalyzed C N bond-formation through cross-
coupling of the diiodospirofluorene 3 with di(para-tolyl)a-
mine afforded the bipolar host DNTACN in 60% yield. We
isolated the bipolar host DCzACN in a yield of 58% yield
through an Ullmann-type copper-mediated cross-coupling of
the diiodospirofluorene 3 and carbazole in 1,2-dichloroben-
zene.
A
ACHTUNGTRENNUNG
a
23,400 cdmꢀ2.[15] Although the bipolar character and suitable
frontier-orbital energies of D2ACN made it possible to ach-
ieve a cost-effective device configuration, this simple device
gives lower performance at higher voltages because the ab-
sence of neighboring functional layers results in the poor
confinement of excitons. To efficiently confine excitons
We used differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) to characterize the ther-
mal properties of the bipolar host compounds (Table 1).
Governed by their molecular weights, the bipolar hosts ex-
hibited glass-transition temperatures (Tg) in the order
DNPACN>DNTACN>D2ACN. The rigidity of the donor
branch had a great influence on the value of Tg—as exem-
plified by DCzACN, which exhibited a distinct glass transi-
tion at a temperature of 2008C. Nevertheless, all of these
materials formed homogeneous and stable amorphous films,
an essential property for OLED device applications. We as-
cribe the amorphous behavior and high values of Tg to the
rigidity of the spirobifluorene core and to the bulk of the di-
arylamine and carbazole substituents, which effectively sup-
pressed intermolecular interactions. The rigid spirobifluor-
ene skeleton and bulky substituent groups also imparted
Abstract in Chinese:
134
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 133 – 142

Spiro-Configured Bipolar Host Materials for Electrophosphorescent Devices
Scheme 1. Synthesis of the bipolar host compounds.
Table 1. Physical properties of D2ACN, DNPACN, DNTACN, and DCzACN.
Red
[c]
d
Tg/Td [8C]
aE1/2Ox/bE1/2 [V]
labs/lPL [nm] film
ET [eV]
HOMO/LUMO/Eg [eV]
mh/me [cm2/Vs]
D2ACN
116/350
184/395
156/360
200/420
0.79/ꢀ1.61
0.67/ꢀ1.62
0.63/ꢀ1.63
1.37/ꢀ1.54
330, 383/556
334, 382/550
329, 388/570
329, 347/469
2.40
2.37
2.38
2.61
5.4/2.4/3.0
5.3/2.3/3.0
5.3/2.4/2.9
5.7/2.4/3.3
2.4ꢁ10ꢀ5/2.2ꢁ10ꢀ5
3ꢁ10ꢀ5/7.2ꢁ10ꢀ5
2ꢁ10ꢀ5/2.3ꢁ10ꢀ5
1.9ꢁ10ꢀ5/1.9ꢁ10ꢀ6
DNPACN
DNTACN
DCzACN
[a] Oxidative CV was performed in 1.0 mm CH2Cl2 containing 0.1m TBAF6; [b] Reductive CV was performed in 1.0 mm THF containing 0.1m TBAP;
[c] Determined through AC2 measurements; LUMO=HOMO+ Eg; the value of Eg was calculated from the absorption onset of the solid film;
[d] Under an electric field (E) of 4.9ꢁ105 Vcmꢀ1. THF=tetrahydrofuran.
these materials with high tolerance toward thermal degrada-
tion, as indicated by their high decomposition temperatures
(Td).
We employed cyclic voltammetry (CV) to study the elec-
trochemical behavior of these spirobifluorene-based bipolar
compounds (Figure 1, Table 1). With the exception of
DCzACN, we observed reversible voltammograms in both
the oxidation and reduction scans, which indicated the for-
mation of stable radical cations and anions upon electro-
chemical redox processes, thus confirming the bipolar char-
ꢀ
acter of these spiro-configured D A compounds. The first
oxidation potentials (E1/2Ox) of D2ACN, DNPACN, and
DNTACN were 0.79, 0.67, and 0.63 V (vs. Ag/AgCl), respec-
tively. We ascribed the slight differences in oxidation poten-
tials to the structural variations in the diarylamino substitu-
ents, thereby providing a potential method for tuning the
HOMO energy levels. In contrast, DCzACN exhibited an
Figure 1. Cyclic voltammograms of D2ACN, DNPACN, DNTACN, and
initial oxidation peak at 1.37 V, a value that is much higher
than those of diarylamino-substituted derivatives. We as-
signed this peak to the oxidation of the carbazole moiety.
Notably, the radical cations of carbazole can easily undergo
rapid dimerization through coupling at their C3 and C6
sites. The carbazole dimer can be readily electrochemically
oxidized at 0.85 V.[16] Because the spiro-configured bipolar
compounds D2ACN, DNPACN, and DNTACN are all
equipped with a dicyano-substituted biphenyl branch as the
DCzACN.
A unit, they each provided a similar reversible reduction
peak near ꢀ1.6 V. The electronic nature of the D branch
had a slight coulombic repulsion effect on the reduction be-
havior of the A branch, as evidenced by the lower reduction
peak (ꢀ1.54 V) of DCzACN, which bears a less-electron-do-
nating carbazole unit on the D branch. Accordingly, modu-
Chem. Asian J. 2012, 7, 133 – 142
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135

W.-Y. Hung, K.-T. Wong et al.
FULL PAPERS
lation of the electronic character or energy levels of these
spiro-configured bipolar compounds can be achieved by
tuning the structure of the D and/or A branches. We used a
photoemission spectrometer (Riken AC-2) to determine the
HOMO energy levels of these bipolar compounds, and then
estimated their LUMO energy levels from the HOMO
energy level and the optical band gaps (Eg) by using the
equation LUMO=HOMO+Eg. Table 1 summarizes the
HOMO and LUMO energies.
Figure 2 presents the absorption and emission spectra of
D2ACN, DNPACN, DNTACN, and DCzACN in solid films;
Table 1 summarizes the photophysical data. The electronic
absorption spectra of D2ACN, DNPACN, and DNTACN
are very similar, giving values of lmax near 385 nm, which we
highest-energy 0–0 phosphorescence emissions recorded
from ethanol solutions at 77 K, we estimated the triplet en-
ergies (ET) of D2ACN, DNPACN, and DNTACN to be ap-
proximately 2.4 eV. Notably, the phosphorescence signals of
the bipolar compounds D2ACN, DNPACN, and DNTACN
were slightly blue-shifted relative to their fluorescence sig-
nals. To determine the origin of the phosphorescence of
D2ACN, we measured phosphorescence spectra of its indi-
vidual chromophore components, D2 and ACN (Figure 3).
The phosphorescence spectrum of A chromophore ACN
ꢀ
attribute to the p p* transitions of the diarylamino-capped
biphenyl branches;[17] in contrast, DCzACN, with its carba-
zole substituent, exhibited a lmax at 347 nm. The dicyano-
substituted biphenyl unit was responsible for an absorption
peak at 330 nm. Spiro-configured bipolar compounds typi-
cally provide weak emissions from the charge-separated ex-
cited state that originates from efficient photoinduced elec-
tron transfer.[15] Consequently, D2ACN, DNPACN, and
DNTACN gave very similar photoluminescence (PL) emis-
sion maxima, centered at lmax =560 nm. In contrast, the
spectrum of DCzACN displayed an emission maximum cen-
tered at 469 nm, a blue-shift of 90 nm, owing to the weakly
electron-donating character of the carbazole unit. From the
Figure 3. Phosphorescence (Phos) spectra of D2, ACN, and D2ACN re-
corded from their EtOH solutions at 77 K.
featured three emission peaks at 477, 512, and 544 nm, re-
spectively; the D chromophore had a lower-energy triplet
state, with phosphorescence signals appearing at 510 and
550 nm, similar to the spectrum of D2ACN. Thus, the phos-
phorescence of the bipolar compound D2ACN arose from
the D chromophore component, which features the lower
triplet energy level. The values of ET of D2ACN, DNPACN,
and DNTACN are sufficiently high to confine the triplet ex-
citons of red phosphors. On the other hand, DCzACN, with
its higher triplet energy (ET =2.61 eV), appeared to be a po-
tential host material for green and red triplet emitters.
We employed the time-of-flight (TOF) transient-photocur-
rent technique[18] at room temperature to investigate the car-
rier-transport properties of DNPACN, DNTACN, and
DCzACN; we have previously reported the carrier mobility
of D2ACN.[15] Figure 4 displays typical room-temperature
TOF transients for these materials under an applied electric
field; we observe a nondispersive hole transient photocur-
rent for DNTACN and dispersive transient photocurrents
for the other bipolar compounds. From double-logarithmic
representations (Figure 4a–f, insets), we extracted the carri-
er-transit times (tT), required to determine the carrier mobi-
lities, from the intersection points of the two asymptotes; we
then calculated the mobility using the formula m=d2/VtT,
where d is the sample thickness and V is the applied voltage.
Figure 5 reveals the dependence of the carrier mobilities
of these bipolar compounds on the electric field. The hole
Figure 2. Room-temperature absorption and emission (PL) spectra of
D2ACN, DNPACN, DNTACN, and DCzACN in the form of neat films
and corresponding phosphorescence (Phos) spectra recorded from their
EtOH solutions at 77 K.
136
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Chem. Asian J. 2012, 7, 133 – 142

Spiro-Configured Bipolar Host Materials for Electrophosphorescent Devices
the Poole–Frenkel factor.[19] By
extrapolating the data to the
electric field at 4.9ꢁ105 Vcmꢀ1,
these compounds all exhibited
a similar hole mobility of 2–3ꢁ
10ꢀ5 cm2 Vꢀ1 sꢀ1. Therefore, the
different D substituents (diaryl-
amine, carbazole) did not have
a pronounced effect on the hole
mobility. The films of the spiro-
configured bipolar compounds
with diarylamino substituents
(D2ACN,
DNPACN,
DNTACN) possessed electron
mobilities that followed the
same order of their hole mobili-
ties, giving ambipolar charge
transport characteristics that
should benefit the charge carri-
er balance in the EML. Howev-
er, the electron mobility of
DCzACN was one order of
magnitude lower than those of
its
diarylamino-substituted
counterparts. Presumably, the
attachment of different donors
(diarylamine or carbazole) onto
the spirobifluorene core results
in significant differences in the
Figure 4. Time-of-flight (TOF) transients of the spirobifluorene-based bipolar compounds: a) DNPACN, hole;
E=2.3ꢁ105 Vcmꢀ1
. . .
b) DNPACN, electron; E=4.8ꢁ105 Vcmꢀ1 c) DNTACN, hole; E=6ꢁ105 Vcmꢀ1
d) DNTACN, electron; E=4.8ꢁ105 Vcmꢀ1. e) DCzACN, hole; E=106 Vcmꢀ1. f) DCzACN, electron; E=6.2ꢁ
105 Vcmꢀ1
.
molecular packing. As a conse-
quence, the different internal
reorganization energies and positional disorder, which are
critical for carrier transport, will play crucial roles affecting
the electron mobility. Nevertheless, bipolar transporting be-
havior is necessary to maintain charge balance in the emis-
sive layer (EML)—in turn, generating broader recombina-
tion zones and steering them away from the interfaces with
the neighboring charge-transport layers to suppress emissive
exciton quenching.
Because of its ambipolar characteristics and suitable
energy levels, we have previously demonstrated the use of
D2ACN as a host material for an iridium-based red emitter
in a virtual single-layer PhOLED.[15] To achieve high-effi-
ciency OLEDs, here we adopted a multilayer device struc-
ture to facilitate carrier injection, decreased the energy bar-
riers between the interfacial layers, balanced the carrier
transport, and confined the excitons within the emissive
zone; this device structure was ITO/polyethylenedioxythio-
Figure 5. Hole- and electron mobilities for films of D2ACN, DNPACN,
DNTACN, and DCzACN plotted with respect to E1/2
.
phene:polystyrenesulfonate
bis[N-(1-naphthyl)-N-phenyl]biphenyldiamine
20 nm)/4,4ꢂ,4ꢂꢂ-tri(N-carbazolyl)triphenylamine
(PEDOT:PSS,
30 nm)/4,4ꢂ-
(a-NPD,
(TCTA,
mobilities fell within the range 1.2–4.7ꢁ10ꢀ5 cm2 Vꢀ1 sꢀ1 for
fields varying from 2.5–7.7ꢁ105 Vcmꢀ1; the electron mobili-
ties varied greatly: within the range from 2ꢁ10ꢀ6 to 8ꢁ
10ꢀ5 cm2 Vꢀ1 sꢀ1 for fields varying from 2.6–7.7ꢁ105 Vcmꢀ1.
The field-dependence of the carrier mobilities followed the
nearly universal Poole–Frenkel relationship m/exp (bE1/2)
typically observed for disordered organic systems, where b is
5 nm)/EML (25 nm)/1,3,5-tris(N-phenylbenzimidizol-2-yl)-
benzene (TPBI, 50 nm)/LiF (0.5 nm)/Al (100 nm). This
device architecture resembles those reported in the litera-
ture,[20] with PEDOT:PSS and a-NPD functioning as hole-
injection and hole-transport layers, respectively. To further
improve the efficiency and decrease the hole-injection barri-
Chem. Asian J. 2012, 7, 133 – 142
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137

W.-Y. Hung, K.-T. Wong et al.
FULL PAPERS
er, we inserted a thin layer of
TCTA between the NPB layer
and the EML to function as
both a hole-transporting materi-
al and an exciton blocker (ET =
2.76 eV)[21] preventing diffusion
of excitons to the a-NPD layer.
TPBI functioned as both
a
hole-blocking and electron-
transporting layer;[22] LiF was
the electron-injection material.
Finally, we completed the
device by depositing
a thin
layer of aluminum to serve as
the cathode. The EML com-
prised the bipolar hosts doped
with 10 wt% of the red dopants
osmium(II)
bis[3-(trifluoro-
methyl)-5-(4-tert-butylpyridyl)-
1,2,4-triazolate]dimethylphenyl-
phosphine
[OsACTHNGUTRENU(NG bpftz)2
ACHTUNGTRENNUNG
m(II) bis[3-(trifluoromethyl)-5-
(4-tert-butylpyridyl)-1,2,4-tria-
zolate]diphenylmethylphos-
Figure 6. HOMO and LUMO energy levels for the devices and chemical structures of the heavy-metal com-
plexes.
phine
[OsACHTUNGTRENNUNG(bpftz)2ACHTUNGRTNE(NUNG PPh2Me)2,
OS2].[24] We also inspected the
ability of DCzACN to function as a host in green phosphor-
escent devices; in this regard, we adopted bis(2-
work function of ITO and the HOMO energy levels of the
emitters (Figure 6). TPBI served as an effective hole-blocker
because of the large differences in the HOMO energies
(0.48–0.9 eV) of TPBI and the spirobifluorene derivatives.
In contrast, the relatively smaller differences in LUMO en-
ergies (0.27–0.37 eV) of TPBI and the spirobifluorene deriv-
atives facilitated injection of electrons from TPBI into the
emitter. In Figure 7a, we attribute the differences in the I–V
plots of the devices featuring the various hosts to slight dif-
ferences in the energy level alignments of the devices. The
devices that required higher operating voltages (with
DNPACN (B1) and DNTACN (C1) as hosts) had higher
LUMO energy levels than those of the other devices (using
D2ACN (A1) and DCzACN (D1)), thus resulting in imped-
ed flows of electrons. Among the devices, the D2ACN-
phenylpyridinato)iridium
(acac)][25] and tris(2-phenylpyridinato)iridium
(PPy)3][26] as dopants. Figure 6 displays the energy levels for
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
the devices and the chemical structures of the heavy-metal
complexes. Table 2 summarizes the electroluminescence
data obtained from all of the devices.
Figure 7 presents the current-density–voltage–luminance
(I–V–L) characteristics, device efficiencies, and EL spectra
for the devices incorporating D2ACN, DNPACN,
DNTACN, and DCzACN doped with OS1 (devices A1, B1,
C1, and D1, respectively). All of the devices in this study ex-
hibited relatively low turn-on voltages (2.5–3.0 V), presuma-
bly because of the small energy differences between the
Table 2. EL performance of devices incorporating various hosts and emitters.
[b]
Emitter[a]
Von [V]
L1000 nit [V,%]
Lmax [cdmꢀ2, V]
Imax [mAcmꢀ2
]
hext [%]
hc [cdAꢀ1
]
hp [lm/W]
A1
A2
B1
B2
C1
C2
D1
D2
D3
D4
D2ACN:OS1
D2ACN:OS2
DNPACN:OS1
DNPACN:Os2
DNTACN:OS1
DNTACN:OS2
DCzACN:OS1
DCzACN:OS2
DCzACN:Ppy2
DCzACN:Ppy3
2
2
2.5
2.5
2
5.2, 20.3
5.1, 19.5
6.1,16.4
6.1, 14.9
4.9, 15.6
4.9, 11.9
5.5, 16.7
5.6, 19.2
4.9, 12
134,700, 11
167,200, 12
86,300, 12
102,400, 12
69,200, 12
2420
2740
2110
1940
1610
1260
3080
2540
980
20.4
19.5
16.5
15
15.7
11.9
17.7
19.3
12.6
10.7
22.7
27.8
15.8
20.2
17
16.5
21.4
31.9
46
21.1
25.6
13
14.4
16.6
15.2
16.3
22.4
44.3
32
2
72,400, 11
2.5
2.5
2.5
2.5
84,400, 11.5
113,000, 12
66,800, 12.5
29,000, 12.5
5, 8.8
1100
32.4
[a] Device configuration: ITO/PEDOT:PSS/a-NPD (20 nm)/TCTAACTHNUGRTENUNG(5 m)/host:dopant 10 wt% (25 nm)/TPBI (50 nm)/LiF/Al. [b] Turn-on voltage at
which emission became detectable (10ꢀ2 cdmꢀ2).
138
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Chem. Asian J. 2012, 7, 133 – 142

Spiro-Configured Bipolar Host Materials for Electrophosphorescent Devices
indicating balanced injection and transport of holes and
electrons in the device, even at high current densities.
These results also reveal that a bipolar host material can
provide balanced charge fluxes within the EML, rendering
devices exhibiting only a limited external quantum efficien-
cy roll-off. The performance of device A1 is comparable
with the previously reported highest values for red PhO-
LEDs.[4a,27] In Figure 7c, we find that the features in the EL
spectra of devices resulted only from the red emission of
[OsACHTUNGRTENNUG(bpftz)2ACHTUNGTREN(NUGN PPhMe2)2], thus indicating that TCTA and
TPBI functioned as effective blockers to confine carriers
and excitons within the EML. Figure 8 displays the behavior
of the corresponding OS2-based devices. The best perfor-
mance was also that for the D2ACN-based device A2,
Figure 7. a) I–V–L characteristics, b) plots of EL efficiency with respect
to brightness, and c) EL spectra for devices incorporating hosts doped
with OS1.
based device A1 exhibited a relatively high brightness of
134,700 cdmꢀ2, driven at a current density of 2420 mAcmꢀ2
(11 V). Figure 7b displays the external quantum efficiencies
(hext) and power efficiencies (hp) of the devices, plotted with
respect to the luminance. The maximum values of hext and
hp for device A1 (20.4% and 21.1 LmWꢀ1, respectively)
were greater than those of devices B1 (16.5% and
13 LmWꢀ1, respectively), C1 (15.7% and 16.6 LmWꢀ1, re-
spectively), and D1 (17.7% and 16.3 Lm Wꢀ1, respectively).
The value of hext for device A1 of 20.4% is at the theoretical
limit of 20% for phosphorescent OLEDs. Notably, even
when the brightness reached 1000 cdmꢀ2, the external quan-
tum efficiency (hext) of device A1 remained at 20.3%, thus
Figure 8. a) I–V–L characteristics, b) plots of EL efficiency with respect
to brightness, and c) EL spectra for devices incorporating hosts doped
with OS2.
Chem. Asian J. 2012, 7, 133 – 142
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
139

W.-Y. Hung, K.-T. Wong et al.
FULL PAPERS
which exhibited saturated red emission CIE
(x,y)=0.63,0.37
present device structure, incorporating DCzACN as a bipo-
lar host, generally provides highly efficient OLEDs for both
red and green phosphors.
with maximum brightness, external quantum efficiency
(hext), and power efficiency (hp) of 167,200 cdmꢀ2, 19.5%,
and 25.6 LmWꢀ1, respectively.
To test the ability of DCzACN (ET =2.61 eV) to function
as a host material for green phosphorescent devices, we in-
corporated [(PPy)2IrACTHNUTRGNNEG(U acac)] and [IrHCATUNGTRNEN(GUN PPy)3] as dopants in de-
Conclusions
vices D3 and D4 having the same device configuration as
that described above. Figure 9 reveals that device D3 exhib-
ited a maximum brightness (Lmax) of 66,800 cdmꢀ2 at 12.5 V
and a maximum external quantum efficiency (hext) of
12.6%; device D4 provided a maximum brightness (Lmax) of
29,000 cdmꢀ2 at 12.5 V and a maximum external quantum
efficiency (hext) of 10.7%. These results suggest that our
We have synthesized and characterized a series of spirobi-
fluorene-based bipolar compounds (D2ACN, DNPACN,
DNTACN, DCzACN), and tailored the morphological sta-
bility, triplet energy (ET), bipolar charge-transport behavior,
and HOMO/LUMO energy levels by independently varying
the structural features of the D and A branches. Efficient
photoinduced electron transfer resulted in the bipolar com-
pounds D2ACN, DNPACN, and DNTACN, which have low-
lying excited states. Interestingly, the phosphorescence origi-
nating from the D branches was located at higher energy
relative to the charge-separated singlet emissions. The physi-
cal properties of these new compounds—most importantly,
their balanced electron and hole mobilities—make them
useful as host materials for high-efficiency phosphorescent
OLEDs. We fabricated multilayer devices employing these
new host materials, doped with iridium-based green or
osmium-based red phosphors, as the EML. This multilayer
structure efficiently confines holes and electrons within the
EML and prevents exciton diffusion, offering higher device
efficiencies. One of these devices, incorporating D2ACN
doped with the red emitter [OsACHTUNRGTENNUG(bpftz)2ACHTUNGTERN(NGUN PPhMe2)2], exhibited
saturated red electrophosphorescence with CIE coordinates
(x, y) of (0.65, 0.35) and remarkably high efficiencies of
20.3% (21 cdAꢀ1) and 13.5 LmWꢀ1 at a practical brightness
of 1000 cdmꢀ2.
Experimental Section
Cyclic Voltammetry
The oxidation potentials were determined in CH2Cl2 solutions (1.0 mm)
containing 0.1m tetra-n-butylammonium hexafluorophosphate (TBAPF6)
as the supporting electrolyte at a scan rate of 100 mVsꢀ1. The reduction
potentials were determined in THF solutions (1.0 mm) containing 0.1m
tetra-n-butylammonium perchlorate (TBAP). A glassy carbon electrode
and a platinum wire were used as the working and counter electrodes, re-
spectively. All potentials were recorded versus Ag/AgCl (saturated) as a
reference electrode. The ferrocenium/ferrocene redox couple in THF/
TBAP occurs at a value of Eo’=+0.61 V for oxidation and in CH2Cl2/
TBAPF6 at +0.45 V for reduction, with respect to Ag/AgCl (saturated).
Synthetic Procedures
Spiro-compound 3
A mixture of 1 (7.32 g, 20.00 mmol), I2 (6.88 g, 27.20 mmol), iodic acid
(1.65 g, 9.40 mmol), concentrated H2SO4 (2 mL), and CHCl3 (20 mL) in
AcOH (80 mL) was heated at 608C for 12 h. After cooling to room tem-
perature, the mixture was partitioned between CHCl3 and H2O and the
organic phase was washed with H2O and NaHCO3ACTHUNGRTENUNG(aq.). The organic
phase was collected, dried (MgSO4), and the crude product was purified
by column chromatography on silica gel to give a white powder (9.88 g,
80%). Rf =0.33 (EtOAc/hexanes); m.p.: 3738C (DSC); IR (KBr): n˜ =
Figure 9. a) I–V–L characteristics, b) plots of EL efficiency with respect
to brightness, and c) EL spectra for devices incorporating DCzACN
doped with green phosphors.
825, 932, 999, 1378, 1444, 1603, 2217 cmꢀ1 1H NMR (400 MHz, CDCl3,
;
258C): d=7.99 (dd, 3JH,H =8.0, 5JH,H =0.8 Hz, 2H), 7.78 (dd, 3JH,H =6.4,
140
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Chem. Asian J. 2012, 7, 133 – 142

Spiro-Configured Bipolar Host Materials for Electrophosphorescent Devices
4JH,H =1.2 Hz, 2H), 7.76 (dd, 3JH,H =6.0, 4JH,H =1.6 Hz, 2H), 7.61 (d,
Photophysical Measurements
4
3JH,H =8.0 Hz, 2H), 7.05 (d, 4JH,H =0.8 Hz, 2H), 6.94 ppm (d, JH,H
=
Steady-state spectra were recorded for both the solutions and solid films
prepared through vacuum (2ꢁ10ꢀ6 torr) deposition on a quartz plate
(1.6ꢁ1.0 cm). Absorption spectra were recorded using a U2800 A spec-
trophotometer (Hitachi). Fluorescence spectra were recorded at 300 K
and phosphorescence spectra at 77 K using a Hitachi F-4500 spectropho-
tometer, with excitation at the absorption maxima. Quantum efficiencies
were recorded using an integration sphere coupled with a photonic multi-
channel analyzer (Hamamatsu C9920), which gave anthracene a quantum
yield of 23%. The experimental HOMO energy levels were determined
using a Riken AC-2 photoemission spectrometer (PES); the LUMO
energy levels were estimated by subtracting the optical energy gap from
the measured HOMO energy level.
1.2 Hz, 2H); 13C NMR (100 MHz, CDCl3, 258C): d=148.6, 147.0, 143.7,
140.3, 138.1, 133.0, 132.9, 132.7, 128.0, 122.3, 122.0, 121.9, 118.1, 113.1,
93.9, 64.8, 53.4 ppm; MS (FAB+) m/z (%) 617.9 (100); HRMS calcd for
C27H12I2N2: 617.9090; found: 617.9066.
DNPACN
PdACHTUNGTRENNUNG(OAc)2 (0.02 g, 0.09 mmol), N-phenylnaphthalen-1-amine (1.32 g,
6.00 mmol), tBuONa (1.15 g, 12.00 mmol), and 1 (1.05 g, 2.00 mmol) were
placed in a 50 mL two-neck flask fitted with a septum. The flask was
evacuated and back-filled with argon. Toluene (15 mL) and PtBu3 (0.05m
in toluene, 4 mL, 0.20 mmol) were added via a syringe at room tempera-
ture. The reaction mixture was heated at 1108C for 2 days before being
cooled to room temperature and quenched with water (20 mL). The
aqueous phase was extracted with CH2Cl2 (3ꢁ15 mL). The combined or-
ganic phases were dried (MgSO4) and concentrated to obtain DNPACN
(980 mg, 64%), which was purified by column chromatography on silica
gel. Rf =0.50 (EtOAc/hexanes); m.p.>4008C (DSC); IR (KBr): n˜ =778,
TOF Mobility Measurements
The samples for the TOF measurements were prepared through vacuum
deposition in the configuration glass/Ag (30 nm)/organic (2–3 mm)/Al
(150 nm); they were then placed inside a cryostat and maintained under
vacuum. The thickness of the organic film was monitored in situ with a
quartz crystal sensor and calibrated through thin-film thickness measure-
ment (K-MAC ST2000). A pulsed nitrogen laser, used as the excitation
light source through the transparent electrode (ITO), induced photogen-
eration of a thin sheet of excess carriers. Under an applied dc bias, the
transient photocurrent was swept across the bulk of the organic film
toward the collection electrode (Al) and then recorded using a digital
storage oscilloscope. Depending on the polarity of the applied bias, se-
lected carriers (holes or electrons) were swept across the sample with a
transit time of tT. With an applied bias V, a sample thickness D, and an
applied electric field E equal to V/D, the carrier mobility was given using
the formula
1270, 1383, 1465, 1490, 1588, 2223 cmꢀ1 1H NMR (400 MHz, CDCl3,
;
258C): d=7.77–7.74 (m, 4H), 7.67–7.62 (m, 6H), 7.52–7.49 (m, 4H), 7.39
(t, 3JH,H =7.6 Hz, 2H), 7.35–7.27 (m, 4H), 7.15 (d, 3JH,H =7.6 Hz, 2H),
7.07–7.04 (m, 4H), 6.94 (dd, 3JH,H =8.4, 4JH,H =2.0 Hz, 2H), 6.87–6.82 (m,
6H), 6.22 ppm (d, 4JH,H =1.2 Hz, 2H);13C NMR (100 MHz, CDCl3,
258C): d=168.3, 150.3, 149.6, 147.8, 147.6, 146.9, 144.1, 143.0, 142.8,
135.3, 134.8, 133.5, 131.5, 130.2, 128.8, 128.0, 127.3,127.2, 126.4, 126.4,
126.0, 125.9, 123.9, 123.0, 122.0, 121.9, 121.7, 121.1, 121.0, 118.8, 116.8,
111.2, 110.4, 65.4 ppm; elemental analysis: calcd (%) for C59H36N4:
C 88.47, H 4.53, N 7.00; found: C 88.19, H 4.62, N 6.41.
DNTACN
Using the procedure described above, a mixture of PdACTHUNRGTNEUNG(OAc)2 (0.02 g,
m ¼ D=ðtT EÞ ¼ D2=ðVtT Þ
0.09 mmol), di-para-tolylamine (0.68 g, 3.50 mmol), tBuONa (0.38 g,
3.95 mmol), 1-biphenyldicyclohexylphosphine (0.050 g, 0.15 mmol), and 3
(0.93 g, 1.50 mmol) was heated at 1108C for 24 h to yield DNTACN
(680 mg, 56%), which was purified by column chromatography on silica
gel. Rf =0.33 (CH2Cl2/hexanes); m.p.: 3138C (DSC); IR (KBr): n˜ =809,
from which the carrier transit time, tT, was extracted from the intersection
point of two asymptotes to the plateau and the tail sections in double-
logarithmic plots.
1265, 1465, 1506, 1598, 2217 cmꢀ1 1H NMR (400 MHz, CDCl3, 258C):
;
d=7.80 (d, 3JH,H =8.0 Hz, 2H), 7.65 (dd, 3JH,H =8.0, 4JH,H =1.2 Hz, 2H),
7.54 (d, 3JH,H =8.4 Hz, 2H), 7.20 (d, 4JH,H =0.8 Hz, 2H), 7.96–7.93 (m,
10H), 6.83 (d, 3JH,H =8.4 Hz, 8H), 6.32 (d, 4JH,H =2.0 Hz, 2H), 2.24 ppm
(s, 12H); 13C NMR (100 MHz, CDCl3, 258C): d=150.6, 147.6, 146.3,
145.0, 143.8, 135.5, 132.4, 132.0, 129.7, 127.8, 124.0, 123.7, 121.8, 120.3,
118.6, 117.9, 112.3, 65.4, 20.7 ppm; MS (FAB+) m/z (%) 756.8 (100);
HRMS calcd for C59H36N4: 756.3253; found: 756.3245; elemental analysis:
calcd (%) for C55H40N4: C 87.27, H 5.33, N 7.40; found: C 87.33, H 5.36,
N 7.29.
OLED Device Fabrications
All chemicals were purified by vacuum sublimation prior to use. The
OLEDs were fabricated through vacuum deposition of the materials at
10ꢀ6 torr onto ITO-coated glass substrates that have a sheet resistance of
15 Wsqrꢀ1. The ITO surface was cleaned ultrasonically—sequentially with
acetone, methanol, and deionized water—and then it was treated with
UV-ozone. A hole-injection layer (PEDOT:PSS) was spin-coated onto
the substrates and dried at 1308C for 30 min to remove residual water.
Organic layers were then vacuum-deposited at a deposition rate of ca. 1–
2 ꢃsꢀ1. Subsequently, LiF was deposited at 0.1 ꢃsꢀ1 and then capped
with Al (ca. 5 ꢃsꢀ1) through shadow masking without breaking the
vacuum. The I-V-L characteristics of the devices were measured simulta-
neously using a Keithley 6430 source meter and a Keithley 6487 picoam-
meter equipped with a calibration silicon-photodiode in a glove box. EL
spectra were measured using a photodiode array (OTO SD1000) with a
spectral range from 300 to 1100 nm and a resolution of 1.5 nm.
DCzACN
A mixture of 3 (2.47 g, 4.00 mmol), carbazole (1.47 g, 8.80 mmol), Cu
(0.50 g, 8.00 mmol), K2CO3 (1.36 g, 12.00 mmol), and [18]crown-6 (0.21 g,
0.80 mmol) in 1,2-dichlorobenzene (60 mL) was heated under reflux at
1858C under argon for 2.5 days. The reaction mixture was filtered and
the filtrate concentrated under reduced pressure. The crude product was
purified by column chromatography on silica gel to give a white solid
(1.60 g, 58%). Rf =0.33 (CH2Cl2/hexanes); m.p.: 4208C (DSC); IR
(KBr): n˜ =819, 1229, 1326, 1439, 1495, 1598, 2217 cmꢀ1; NMR (400 MHz,
CDCl3, 258C): d=8.20 (d, 3JH,H =8.0 Hz, 2H), 8.10 (d, 3JH,H =7.2, Hz,
4
4H), 7.92 (dd, 3JH,H =8.0, 5JH,H =0.8 Hz, 2H), 7.76 (dd, 3JH,H =6.8, JH,H
=
4
Acknowledgements
1.6 Hz, 4H), 7.40–7.36 (m, 6H), 7.29–7.24 (m, 8H), 6.93 ppm (d, JH,H
=
1.6 Hz, 2H); 13C NMR (100 MHz, CDCl3, 258C): d=149.0, 147.5, 143.7,
140.2, 139.8, 137.8, 132.7, 127.8, 127.6, 125.9, 123.2, 122.3, 122.0, 121.9,
120.2, 120.1, 118.2, 112.9, 109.3, 65.5 ppm; MS (FAB+) m/z (%) 696.2
(100); HRMS calcd for C51H28N4: 696.2314; found: 696.2325; elemental
analysis: calcd (%) for C51H28N4: C 87.91, H 4.05, N 8.04; found: C 87.95,
H 4.13, N 8.02.
This study was supported financially by the National Science Council of
Taiwan.
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