Highly efficient red electrophosphorescent device incorporating a bipolar triphenylamine/bisphenylsulfonyl-substituted fluorene hybrid as the host

Article abstract of DOI:10.1039/b910292b

We have fabricated highly efficient red phosphorescent organic light-emitting diodes (PHOLEDs) incorporating a bipolar host material, 2,7-bis(phenylsulfonyl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (SAF), doped with 7 wt% tris(1-phenylisoquinolin

Full text of DOI:10.1039/b910292b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Highly efficient red electrophosphorescent device incorporating a bipolar
triphenylamine/bisphenylsulfonyl-substituted fluorene hybrid as the host
a
a
a
a
a
Fang-Ming Hsu, Chen-Han Chien, Ya-Jou Hsieh, Chen-Hao Wu, Ching-Fong Shu, Shun-Wei Liub
*
b
*
and Chin-Ti Chen
Received 27th May 2009, Accepted 19th August 2009
First published as an Advance Article on the web 15th September 2009
DOI: 10.1039/b910292b
We have fabricated highly efficient red phosphorescent organic light-emitting diodes (PHOLEDs)
incorporating a bipolar host material, 2,7-bis(phenylsulfonyl)-9-[4-(N,N-diphenylamino)phenyl]-9-
phenylfluorene (SAF), doped with 7 wt% tris(1-phenylisoquinolinolato-C2,N)iridium(III) [Ir(piq)3].
Attaching the electron-donating (p-type) triphenylamine group onto the electron-accepting (n-type)
2,7-bis(phenylsulfonyl)fluorene segment (through the C9 position of the fluorene unit) imparts SAF
with good morphological stability, high triplet energy gap (ET), bipolar transporting ability, and
matching energy levels with adjacent carrier-transporting layers. Consequently, the SAF-based red-
PHOLED exhibited a very low turn-on voltage (2.4 V) and high electroluminescence efficiencies of
15.8% and 22.0 lm Wꢀ1, superior to those of the corresponding device incorporating a conventional
host material, 4,40-N,N0-dicarbazolbiphenyl (CBP; 3.2 V, 8.5%, and 8.4 lm Wꢀ1, respectively). At
a practical brightness of 1000 cd mꢀ2, the efficiencies of the SAF-based red-PHOLED remained high
(13.1%, 14.4 lm Wꢀ1).
In this study, we synthesized a host material, SAF, comprising
Introduction
electron-rich triphenylamine and electron-deficient 2,7-bis(phe-
nylsulfonyl)fluorene units linked through the C9 position of the
fluorene moiety. This molecular design endows SAF with several
advantageous properties: (i) bipolarity, arising from the presence
of the electron-accepting phenylsulfonyl groups appended to the
triphenylamine/fluorene hybrid,23–25 thereby facilitating both
electron and hole injection and transport; (ii) a high triplet energy
gap (ET ¼ 2.71 eV), resulting from the individual building blocks
being connected through a nonconjugated linkage (the sp3-
hybridized C9 atom of the fluorene unit), to effectively suppress
reverse energy transfer from the guest emitter to the SAF host;
and (iii) a sterically bulky and hindered structure imparting good
thermal and morphological stabilities. As a result, we expected
SAF to be an appropriate host for red-PHOLEDs. Indeed, an
SAF-based red-PHOLED doped with 7 wt% tris(1-phenyl-
isoquinolinolato-C2,N)iridium(III) [Ir(piq)3] exhibited a very low
turn-on voltage (2.4 V) and high electroluminescence (EL) effi-
ciencies of 15.8% and 22.0 lm Wꢀ1, superior to those of the
reference device based on the conventional material CBP (3.2 V,
8.5%, and 8.4 lm Wꢀ1, respectively).
Phosphorescent organic light-emitting diodes (PHOLEDs) have
attracted much attention because they can, in theory, approach
100% internal quantum efficiency by harvesting both singlet and
triplet excitons.1–10 In these PHOLEDs, the triplet emitters are
usually doped into a host material to reduce the self-quenching
associated with the relatively long excited state lifetimes of triplet
emitters and triplet–triplet annihilation.11 Thus, the development
of effective hosts and phosphors is equally important for the
fabrication of highly efficient PHOLEDs. Being one of the
primary colors, the efficient emission of red light from OLEDs is
critical for applications in full-color displays and solid state
lighting.12–16 Realizing efficient red-PHOLEDs, however,
remains a great challenge because of the lack of suitable host
materials. Although the carbazole derivative 4,40-N,N0-dicarba-
zolbiphenyl (CBP) has been used widely as a host for red-
PHOLEDs, they usually require high driving voltages because
the wide energy gap of CBP leads to large energy barriers for
charge injection from adjacent carrier-transporting layers. In
addition, CBP is inherently a p-type material; its hole mobility
is at least one order of magnitude higher than its electron
mobility.17,18 Moreover, CBP exhibits a relatively low glass-
Recently, we reported an efficient bipolar host 2,7-
bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-
phenylfluorene (POAPF) for a blue-PHOLED.26 However, the
relatively weak electron-withdrawing nature of diphenylphos-
phoryl segments in the POAPF molecule leads to an elevated
lowest unoccupied molecular orbital (LUMO) level (ꢀ2.40 eV),
which may impede electron injection from the electron-trans-
porting layer. In addition, the mismatch of the LUMO energy
level of POAPF with that of Ir(piq)3 (ꢀ3.10 eV) would cause
serious electron trapping at the dopant sites. Both of these
influences will significantly increase the operating voltage of the
red-PHOLED based on POAPF.
transition temperature (Tg ¼ 62 C)19 and is prone to crystalli-
ꢁ
zation.20–22 Its poor thermal and morphological stabilities
may reduce CBP’s ability to bear joule heat during device
operation.
aDepartment of Applied Chemistry, National Chiao Tung University,
Hsinchu, 300 Taiwan. E-mail: shu@cc.nctu.edu.tw
bInstitute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, Republic
of China
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Results and discussion
Synthesis and characterization
Scheme 1 illustrates the synthetic route followed for the prepa-
ration of the bipolar host SAF. The starting material, 2,7-
dibromofluorenone, was treated with thiophenol in the presence
of potassium carbonate in N,N-dimethylformamide (DMF) at
130 ꢁC to afford the disulfide 1, which was then reacted with the
Grignard reagent of bromobenzene to give the corresponding
alcohol 2. Subsequent oxidation with 30% aqueous hydrogen
peroxide yielded the disulfonyl intermediate 3. The target
compound SAF was obtained from the acid-promoted Friedel–
Craft-type substitution of 3 with excess triphenylamine. We
characterized the structure of SAF using 1H and 13C NMR
spectroscopy, high-resolution mass spectrometry, and elemental
analysis.
Thermal properties
Fig. 1 AFM images of 7 wt% Ir(piq)3-doped films based on the hosts (a)
SAF without annealing, (b) SAF with annealing, (c) CBP without
annealing, and (d) CBP with annealing.
SAF exhibits very high thermal stability; thermogravimetric
analysis (TGA) performed under a nitrogen atmosphere revealed
that the decomposition temperature (Td, corresponding to 5%
weight loss) was 430 ꢁC. The high value of Td of SAF implies that
it is capable of enduring vacuum thermal sublimation, which
meets one of the basic requirements of a host to be used in
PHOLEDs. From differential scanning calorimetry (DSC)
measurements, we observed a distinct glass transition tempera-
annealing the CBP-based film induced a degradation of the sur-
face morphology, with large crystals appearing on the annealed
film. The RMS roughness before and after the annealing of the
CBP-based film were 0.57 and 18.6 nm, respectively. SAF
maintained its morphological stability in the glassy state for long
periods of heat exposure.
ꢁ
ture (Tg) at 127 C—much higher than that of the conventional
host material CBP (Tg ¼ 62 ꢁC)19—and no exothermic peak
ꢁ
resulting from crystallization at temperatures up to 280 C. We
attributed these superior thermal properties to the nonplanar,
rigid structure of SAF, featuring a bulky triphenylamine group
and two phenylsulfonyl units appended at the 9- and 2,7-posi-
tions of the fluorene ring, respectively. It is important for OLEDs
to be constructed from materials having relatively high values of
Tg if they are to avoid degradation through morphological
changes of the amorphous organic layer.27–29 To further investi-
gate the morphological stability of the emitting layer (EML)
within the red-PHOLEDs, we used vacuum deposition to
prepare SAF and CBP thin films (30 nm) doped with 7 wt%
Ir(piq)3 on silicon wafers and then measured their surface
morphologies using atomic force microscopy (AFM) before and
Photophysical properties
Fig. 2a displays the UV-Vis absorption and photoluminescence
(PL) spectra of SAF measured under different conditions; Table 1
summarizes the key spectral data. The absorption profiles of
SAF in cyclohexane, toluene, and THF are nearly identical—i.e.,
independent on solvent polarity—indicating that the Franck–
Condon excited state is subject to a rather small dipolar change
with respect to the ground state. In sharp contrast, the fluores-
cent maxima of SAF revealed a salient bathochromic shift: from
ꢁ
after annealing at 90 C for 12 h under a nitrogen atmosphere.
Fig. 1 reveals that the SAF-based thin film had a fairly stable
surface morphology, which did not change upon annealing. The
root mean square (RMS) roughness before and after annealing
were 0.42 and 0.45 nm, respectively. On the other hand,
Fig. 2 (a) Room-temperature absorption and PL spectra (excited at
335 nm) of SAF in cyclohexane (-), toluene (C), and THF (:) solu-
tions and as a solid film (B). The absorption spectrum of the Ir(piq)3 film
is also depicted (O). All films were formed on quartz substrates through
thermal evaporation (30 nm). (b) Emission peak frequencies plotted as
a function of the solvent polarities for SAF in various solutions. The
solvent polarity parameter function Df is expressed using the formula
Df ¼ [(3 ꢀ 1)/(23 + 1)] ꢀ [(n2 ꢀ 1)/(2n2 + 1)], where 3 is the static dielectric
constant and n is the optical refractivity index of the solvent.
Scheme 1 Synthetic route for SAF.
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Table 1 Optical and thermal properties of SAF
a
labs [nm]
lem, max [nm]
Host
cHex
Tol
THF
Film
cHex
Tol
506
THF
560
Film
528
ET [eV]
2.71
Tg/Td [ꢁC]
SAF
299, 321
299, 323
299, 323
301, 325
460
127/430
a
Absorption wavelength, measured in cyclohexane (cHex), toluene (Tol), and tetrahydrofuran (THF) solutions and in the thin film.
460 nm in nonpolar cyclohexane to 560 nm in highly polar THF.
Furthermore, the excitation spectra of SAF, within experimental
error, were effectively identical to the absorption spectra, indi-
cating that the emissions result from a common Franck–Condon
excited state. In addition, when we varied the concentration of
SAF in each solution over the range 10ꢀ4 to 10ꢀ6 M, the
absorption and emission profiles all exhibited concentration-
independence. Thus, we can eliminate the possibility of the
emission being caused by aggregates and/or excimers. We
tentatively attribute the distinct solvatochromism in PL spectra
of SAF to a mechanism involving rapid photoinduced electron
transfer between the electron donor (triphenylamine) and the
electron acceptor [2,7-bis(phenylsulfonyl)fluorene], resulting in
Fig. 3 Cyclic voltammograms of SAF and CBP.
a large change in the dipole moment in the excited state;
a subsequent solvent relaxation process leads to the solvent
polarity-dependent emission. When we plotted the emission peak
frequencies of SAF in various organic solvents (from cyclo-
hexane to ethanol) as a function of the solvent polarity (Fig. 2b),
we obtained a linear relationship together with a slope as steep
as ꢀ14 240 cmꢀ1, consistent with our assignment of a charge-
transfer emission. Similar results were reported and reviewed for
other bipolar-type molecules.30–32
onset potentials at ꢀ1.92 and ꢀ2.70 V, respectively. The end-
capping of the two electron-withdrawing phenylsulfonyl units at
the fluorene group provided SAF with a less negative reduction
potential relative to that of CBP. When performing anodic scans
in CH2Cl2, SAF and CBP underwent oxidation processes with
onset potentials of 0.49 and 0.72 V, originating from their elec-
tron-rich triphenylamine and carbazole segments, respectively.
The oxidation of SAF occurred at a lower potential, indicating
that the oxidation process was facilitated upon replacing the
carbazole building block with a triphenylamine unit.25 On the
basis of the onset potentials for oxidation and reduction, we
estimated the highest occupied/lowest unoccupied molecular
orbital (HOMO/LUMO) levels of SAF and CBP with regard to
the energy level of ferrocene (4.8 eV below vacuum). Fig. 4
displays the relative HOMO/LUMO energy levels of the mate-
rials used to prepare the devices. SAF possesses lower LUMO
(ꢀ2.88 eV) and shallower HOMO (ꢀ5.29 eV) levels than those of
CBP (ꢀ2.10 and ꢀ5.52 eV, respectively), revealing its superior
ability to inject electrons and holes.
Moreover, there is an effective overlap between the PL spec-
trum of SAF and the absorption spectrum of the red-emitting
phosphor Ir(piq)3 (Fig. 2a). Thus, we anticipated that efficient
€
Forster energy transfer of singlet excitons would occur from SAF
to Ir(piq)3. In the EL spectrum of the 7 wt% Ir(piq)3–doped device
(vide infra), we observe only the red emission from the triplet
dopant, indicating the complete energy transfer from SAF to
Ir(piq)3. The absorption spectra (not shown) of the blue-emitting
0
bis[(4,6-difluorophenyl)pyridinato-N,C2 ](picolinato)iridium(III)
(FIrpic) and green-emitting fac-tris(2-phenylpyridinato)-
iridium(III) [Ir(ppy)3] overlap poorly with the emission spectrum
of SAF, thereby prohibiting efficient energy transfer between the
host and these phosphors. In addition, the nonconjugated linkage
between the n- and p-type groups endows SAF with a high value of
ET of 2.71 eV, estimated from the highest-energy 0–0 phospho-
rescent emission measured in frozen 2-methyltetrahydrofuran at
77 K. This value of ET is sufficiently high for SAF to undergo
efficient exothermic energy transfer of triplet excitons to the red
triplet emitter Ir(piq)3 (ET ¼ 2.0 eV)33 without reversing.
Electroluminescence properties
To evaluate the bipolar characteristics of SAF, we prepared
hole-only devices having the architecture indium tin oxide
(ITO)/N,N0-diphenyl-N,N0-bis(3-methylphenyl)-(1,10-biphenyl)-
4,40-diamine (TPD, 30 nm)/host (30 nm)/TPD (30 nm)/Al
(100 nm) and electron-only devices having the configuration
Electrochemical properties
Table 2 Electrochemical data for the hosts
Fig. 3 displays the cyclic voltammograms of SAF and CBP in
solutions containing ferrocene/ferrocenium (Fc/Fc+) as the
internal standard. Table 2 summarizes the redox onset potentials
and calculated energy levels. During cathodic sweeping in DMF,
both SAF and CBP exhibited reversible reduction waves, with
onset
ox
onset
[V]
red
Host
E
[V]
E
HOMO [eV]
LUMO [eV]
SAF
CBP
0.49
0.72
ꢀ1.92
ꢀ2.70
ꢀ5.29
ꢀ5.52
ꢀ2.88
ꢀ2.10
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field of 4.1 ꢂ 10ꢀ5 V cmꢀ1. In contrast, the electron and hole
mobilities of CBP were on the orders of 10ꢀ4 and 10ꢀ3 cm2 Vꢀ1 sꢀ1,
respectively,17,18 indicating relatively unbalanced charge fluxes in
the CBP-based EML. Although the electron and hole mobilities
of SAF were more than one order of magnitude lower than those
of CBP, the measured current densities of the SAF-based carrier-
only devices (Fig. 5) were higher than those of its CBP counter-
parts at the same driving voltages. This behavior may suggest that
the current fluxes in CBP carrier-only devices are significantly
limited by the potential barriers presented at the carrier-
transporting layer–CBP interfaces, with the device currents being
dictated primarily by the efficiencies of carrier injection (i.e.,
energy barriers), rather than the carrier mobility of the host
material.34,35
Fig. 4 Relative HOMO/LUMO energy levels of the materials used for
the EL devices.
Subsequently, we doped the bipolar host material SAF with
a commercially available triplet phosphor, Ir(piq)3, to realize
a highly efficient red-PHOLED. The device was fabricated in the
configuration ITO/TPD (30 nm)/EML (30 nm)/BPhen (30 nm)/
ITO/4,7-diphenyl-1,10-phenanthroline (BPhen, 30 nm)/host
˚
(30 nm)/BPhen (30 nm)/LiF (15 A)/Al (100 nm). Here, the host
materials were SAF and the conventional host CBP. In the case
of the hole-only devices, because of the large energy injection
barrier between TPD (LUMO ¼ ꢀ2.20 eV) and Al (ꢀ4.30 eV),
most electrons were restrained from injection into the organic
layer. Only holes could be injected from the anode to the organic
layers; therefore, the measured current density–voltage (I–V)
characteristics were dominated by holes. On the other hand, in
the electron-only devices, the BPhen layer possessed a large
HOMO energy level (–6.40 eV) and, therefore, it behaved as
a hole-blocker to inhibit the injection of holes from the ITO
anode (ꢀ4.80 eV) to the organic layer. Thus, the measured I–V
curves mostly reflect the injected electrons. Fig. 5 reveals that
both the SAF-based hole- and electron-only devices exhibited
lower turn-on voltages and higher current densities than did the
corresponding CBP-based devices under the same bias. We
attribute the reduced driving voltages in the SAF-based devices
to the decreased barrier heights for hole and electron injection
from the charge transport layers to SAF. From these studies of
carrier-only devices, we anticipated that the use of SAF as a host
in PHOLEDs would provide suitable energy levels to facilitate
carrier injection.
˚
LiF (15A)/Al (100 nm). Here, ‘‘EML’’ refers to a layer of SAF
doped with 7 wt% Ir(piq)3; TPD and BPhen functioned not only
as hole- and electron-transporting layers but also as the carrier-
blockers for effective emission.36,37 For comparison, we also
fabricated the reference device incorporating the conventional
host material CBP. Fig. 6 displays the current density–voltage–
luminance (I–V–L) characteristics of these red-PHOLEDs. The
SAF-based device exhibited significantly lower operating volt-
ages than its CBP counterpart under the same current density,
presumably because of the smaller energy barriers when using
SAF as the host material, as demonstrated in the carrier-only
devices (vide supra). As a result, the SAF device had a relatively
low turn-on voltage of 2.4 V (corresponding to 1 cd m2). In
addition, the SAF-based device exhibited a much better peak
external quantum efficiency (EQE, 15.8% at 21.9 cd mꢀ2) than its
CBP counterpart (8.5% at 93.0 cd mꢀ2). We attribute this
behavior to the relatively balanced bipolar transport of SAF
(vide supra), thereby increasing the opportunity for carrier
recombination, and the higher value of ET of SAF (2.71 eV)
relative to that of CBP (2.56 eV), thereby effectively suppressing
reverse energy transfer from the triplet dopant to the host.20 The
combination of a low operating voltage and a high EQE afforded
our SAF-based device with an extremely high power efficiency
(PE) of 22.0 lm Wꢀ1. Even at a practical brightness of
1000 cd mꢀ2, the operating voltage was merely 3.6 V and the
Furthermore, we measured the carrier mobility in the SAF film
using time-of-flight (TOF) transient photocurrent techniques.
The bipolar charge-transporting capability of SAF was evidenced
by the similar values for the electron (3.4 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1
)
and hole mobilities (3.5 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1) under an electric
Fig. 5 I–V characteristics of (a) hole- and (b) electron-only devices.
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Fig. 6 I–V–L characteristics of Ir(piq)3–doped devices.
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device exhibited almost the same profiles, with the maximum
signal intensity at 622 nm, over the entire range of operating
voltages (Fig. 8), with Commission Internationale de L’Eclairage
(CIE) coordinates at the standard red (0.67, 0.33) demanded by
the NTSC. The spectra were assigned to the emission from
Ir(piq)3, without any other emission originating from the adja-
cent organic layer, indicating the excellent confinement of
carriers within the EML.
Conclusion
We have synthesized a novel bipolar host (SAF) for the fabri-
cation of highly efficient Ir(piq)3-based red-PHOLEDs. SAF was
constructed by linking an electron-rich triphenylamine group to
an electron-deficient bis(phenylsulfonyl)-substituted fluorene
through the C9 atom of its fluorene unit. This geometry afforded
SAF with a rather bulky and nonplanar structure and, hence,
high thermal and morphological stabilities. In addition, the
introduction of a nonconjugated linkage resulted in a high value
of ET (2.71 eV), which effectively prevented reverse energy
transfer from the triplet emitter to the SAF host. More impor-
tantly, the favorable bipolar characteristics of SAF facilitated
both hole and electron injection from adjacent carrier-trans-
porting layers and balanced charge fluxes within the EML.
Consequently, the 7 wt% Ir(piq)3-doped SAF device exhibited
Fig. 7 (a) EQE and (b) PE of the Ir(piq)3-doped devices plotted with
significantly improved EL efficiencies (15.8% and 22.0 lm Wꢀ1
)
respect to the current density.
relative to those of the reference device incorporating the
conventional host CBP (8.5%, 8.4 lm Wꢀ1).
Table 3 Electroluminescence data for the devices
Host
SAF
CBP
Experimental section
V a [V]
2.4/3.6
3.2/4.9
8.5/7.3
8.4/5.7
10.6/8.8
0.67,0.33
Characterization
EQE b [%]
PE b [lm Wꢀ1
15.8/13.1
22.0/14.4
19.6/16.2
0.67,0.33
1H and 13C NMR spectra were recorded using Varian UNITY
INOVA 500 MHz and Varian UNITY 300 MHz spectrometers.
Mass spectra and high-resolution mass spectra (HRMS) were
obtained using JEOL JMS-HX 110 and Finnigan Thermo Quest
MAT 95XL mass spectrometers. Differential scanning calorim-
etry (DSC) was performed using a SEIKO EXSTAR 6000DSC
]
LE b [cd Aꢀ1
]
CIE c [x, y]
a
Measured operating voltages, presented in the order of the values at 1
Measured values, presented in the order of the
c
b
and 1000 cd mꢀ2
maximum value and the value at 1000 cd mꢀ2
.
.
Measured at 7 V.
unit operated at heating and cooling rates of 10 and 50 ꢁC minꢀ1
,
respectively. The glass transition temperature (Tg) was deter-
mined from the second heating scan. Thermogravimetric analysis
(TGA) was undertaken using a DuPont TGA 2950 instrument.
The thermal stability of the samples under a nitrogen atmosphere
was determined by measuring their weight losses while heating at
a rate of 20 ꢁC minꢀ1. Atomic force microscopy (AFM)
measurements were performed in the tapping mode under
ambient conditions using a Digital Nanoscope IIIa instrument.
UV-Vis spectra were measured using an HP 8453 diode array
spectrophotometer. PL spectra were obtained using a Hitachi
F4500 luminescence spectrometer. The low-temperature phos-
phorescence spectra were obtained using a composite spec-
trometer containing a monochromator (Jobin Yvon, Triax 190)
coupled with a liquid nitrogen-cooled charge-coupled device
(CCD) detector (Jobin Yvon, CCD-1024 ꢂ 256-open-1LS).
Cyclic voltammetry (CV) measurements were performed using
a BAS 100 B/W electrochemical analyzer operated at a scan rate
of 100 mV sꢀ1. The potentials were measured against an Ag/Ag+
(0.01 M AgNO3) reference electrode using ferrocene/ferrocenium
(Fc/Fc+) as the internal standard.
Fig. 8 EL spectra of the SAF-based device recorded under various
operating voltages.
efficiencies remained high (13.1%, 14.4 lm Wꢀ1). Fig. 7 presents
plots of the EQE and PE with respect to the current density for
these red-electrophosphorescent devices; Table 3 lists their key
characteristics. Moreover, the EL spectra of the SAF-based
8006 | J. Mater. Chem., 2009, 19, 8002–8008
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Carrier mobility
2,7-Bis(phenylthio)-9-phenyl-9-fluorenol (2)
The carrier mobility was measured using time-of-flight transient
photocurrent techniques at ambient temperature. The sample
was fabricated in the configuration glass/ITO (100 nm)/SAF
(1 mm)/Ag (20 nm). The SAF layer was deposited at a constant
rate of 0.15 nm sꢀ1. During the film deposition of the devices, the
pressure of the chamber was maintained below 2 ꢂ 10ꢀ6 torr to
minimize defects and the evaporation rates of the organic
materials were controlled using respective quartz-crystal moni-
tors. The thickness of the organic layer was measured using
a surface profiler (Dektak 150; Veeco). The frequency-doubled
output of a 5 ns-pulsed dye laser at 295 nm was chosen to
maximize the optical absorption of the organic layer. The laser
beam was directed through the samples from the Ag side to
create photoexcited carriers. The excitation power density was
set at ca. 1.6 W cmꢀ2 to avoid space-charge effects. The instru-
mental response time was maintained much shorter than the
transit time. The transient photocurrent was recorded by
measuring the voltage across a 50 U resistor using a 1 GHz digital
oscilloscope (LC574 AM, LeCroy). The transit time, tT, was
taken from the intersection of two clear straight lines in the
double-logarithmic plot of the transient photocurrent profile.
The carrier mobility m was given by m ¼ D/tTE, where D is the
thickness of the organic layer and E is the applied electric field.
The detection of the transient photocurrent signal was performed
using a process similar to one described previously.38
The Grignard reagent was prepared from Mg powder (0.37 g,
15.2 mmol) and bromobenzene (1.71 mL, 16.2 mmol) in ether
(150 mL). 1 (5.08 g, 12.8 mmol) was added into the cooled
Grignard solution. The reaction mixture was stirred for 8 h at
room temperature and then it was quenched with saturated
NH4Cl aqueous solution (150 mL) and extracted with ethyl
acetate (3 ꢂ 100 mL). The collected organic phase was dried
(MgSO4) and concentrated under reduced pressure. The residue
was washed with hot n-hexane (150 mL) to give a pale-orange
solid (5.50 g, 90%). 1H NMR (300 MHz, CDCl3, d): 7.58 (d, J ¼
7.8 Hz, 2H), 7.38–7.24 (m, 19H), 2.50 (s, 1H); 13C NMR
(75 MHz, CDCl3, d): 151.62, 142.64, 138.28, 136.71, 135.84,
132.17, 131.30, 129.58, 128.69, 127.87, 127.76, 127.53, 125.62,
121.10, 83.74; MS (EI, m/z): 474 [M+].
2,7-Bis(phenylsulfonyl)-9-phenyl-9-fluorenol (3)
30% Aqueous hydrogen peroxide (30 mL) was added to a solu-
tion of 2 (2.58 g, 5.43 mmol) in acetic acid (70 mL). The reaction
ꢁ
mixture was stirred at 80 C for 24 h and then it was cooled to
room temperature, saturated with NaCl (ca. 15 g), and extracted
with CH2Cl2 (4 ꢂ 70 mL). The combined organic phases were
dried (MgSO4) and the solvents evaporated under reduced
pressure to provide a pale-yellow residue, which was recrystal-
lized from a mixture of CHCl3 and n-hexane to give a white solid
(2.76 g, 95%). 1H NMR (300 MHz, CDCl3, d): 7.94–7.90 (m, 5H),
7.87–7.84 (m, 5H), 7.77–7.73 (m, 3H), 7.56–7.42 (m, 8H); 13C
NMR (75 MHz, CDCl3, d): 152.56, 142.98, 142.55, 141.38,
140.72, 133.61, 129.61, 128.97, 128.47, 127.86, 125.36, 124.64,
122.06, 83.60; MS (EI, m/z): 538 [M+].
Fabrication of PHOLEDs
The EL devices were fabricated through vacuum deposition of
the materials at 10ꢀ6 torr onto ITO glass having a sheet resistance
of 25 U squareꢀ1. All of the organic layers were deposited at a rate
2,7-Bis(phenylsulfonyl)-9-(4-N,N-diphenylamino)phenyl-9-
phenylfluorene (SAF)
of 1.0 A sꢀ1. The cathode was completed through thermal
˚
CF3SO3H (1.10 mL, 12.5 mmol) was added dropwise to a solu-
tion of triphenylamine (4.85 g, 19.8 mmol) and 3 (5.26 g,
9.78 mmol) in CH2Cl2 (200 mL). The mixture was stirred for
30 min at room temperature and then quenched with saturated
aqueous NaHCO3 solution (100 mL). The organic phase was
washed twice with water (100 mL) and then it was dried
(MgSO4). After evaporation of the volatiles, the residue was
purified through column chromatography (SiO2; CH2Cl2/
deposition of LiF at a deposition rate of 0.1 A sꢀ1; it was then
˚
capped with Al metal through thermal evaporation at a rate of
4.0 A sꢀ1. The current density–voltage and brightness–voltage
˚
relationships of the devices were measured using a Keithley 2400
source meter and a Newport 1835C optical meter equipped with
an 818ST silicon photodiode. The EL spectrum was obtained
using a Hitachi F4500 luminescence spectrometer.
1
hexane, 1:5) to yield SAF as a yellow solid (6.14 g, 82%). H
NMR (300 MHz, [D8]THF, d): 8.11 (s, 2H), 8.07 (d, J ¼ 8.1 Hz,
2H), 7.98 (dd, J ¼ 8.1, 1.5 Hz, 2H), 7.89 (d, J ¼ 6.9 Hz, 4H),
7.55–7.43 (m, 6H), 7.25–7.14 (m, 9H), 7.04–6.98 (m, 8H), 6.88 (d,
J ¼ 8.7 Hz, 2H); 13C NMR (75 MHz, [D8]THF, d): 153.48,
147.79, 147.33, 144.12, 143.05, 142.99, 142.69, 137.26, 133.08,
129.34, 128.72, 128.69, 127.89, 127.86, 127.69, 127.39, 125.89,
124.62, 123.18, 123.11, 122.35, 65.67; HRMS (FAB, m/z): calcd
for C49H36NO4S2 [M + H]+, 766.2086; found, 766.2073; Anal.
Calcd for C49H35NO4S2: C 76.84, H 4.61, N, 1.83; found:
C 76.46, H 4.70, N 1.72.
2,7-Bis(phenylthio)fluorenone (1)
2,7-Dibromofluorenone (3.06 g, 9.05 mmol) was added to
a mixture of K2CO3 (2.76 g, 20.0 mmol) and N,N-dime-
thylformamide (150 mL). Thiophenol (1.95 mL, 19.0 mmol) was
ꢁ
added with stirring and then the solution was heated at 130 C
for 6 h. The reaction mixture was cooled to room temperature
and poured into water (600 mL). The orange precipitate was
collected by filtration, washed with water, and dried under
vacuum. The solid was recrystallized (ethyl acetate/n-hexane)
Acknowledgements
1
to afford orange needles (2.94 g, 82%). H NMR (300 MHz,
CDCl3, d): 7.51 (s, 2H), 7.43–7.28 (m, 14H); 13C NMR (75 MHz,
CDCl3, d): 192.83, 142.51, 138.85, 136.19, 135.19, 134.32, 132.44,
129.85, 129.31, 126.20, 121.12; MS (EI, m/z): 396 [M+].
We thank the National Science Council for funding and
Professor C.-H. Cheng for his support during the preparation
and characterization of the PHOLEDs.
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 8002–8008 | 8007

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8008 | J. Mater. Chem., 2009, 19, 8002–8008
This journal is ª The Royal Society of Chemistry 2009