Nondoped deep-blue organic light-emitting diodes with color stability and very low efficiency roll-off: Solution-processable small-molecule fluorophores by phosphine oxide linkage

Article abstract of DOI:10.1002/chem.201201512

A series of solution-processable small molecules PO1-PO4 were designed and synthesized by linking N-phenylnaphthalen-1-amine groups to a phenyl phosphine oxide core through a π-conjugated bridge, and their thermal, photophysical, and electrochemical properties were investigated. The phosphine oxide linkage can disrupt the conjugation and allows the molecular system to be extended to enable solution processability and high glass transition temperatures (159-181 °C) while preserving the deep-blue emission. The noncoplanar molecular structures resulting from the trigonal-pyramidal configuration of the phosphine oxide can suppress intermolecular interactions, and thus these compounds exhibit strong deep-blue emission both in solution and the solid state with high photoluminescent quantum yield (PLQY) of 0.88-0.99 in dilute toluene solution. Solution-processed nondoped organic light-emitting diodes featuring PO4 as emitter achieve a maximum current efficiency of 2.36 cd A^-1 with CIE coordinates of (0.15, 0.11) that are very close to the NTSC blue standard. Noticeably, all devices based on these small-molecular fluorescent emitters show striking deep-blue electroluminescent color stability and extremely low efficiency roll-off. Copyright

Full text of DOI:10.1002/chem.201201512

FULL PAPER
DOI: 10.1002/chem.201201512
Nondoped Deep-Blue Organic Light-Emitting Diodes with Color Stability
and Very Low Efficiency Roll-Off: Solution-Processable Small-Molecule
Fluorophores by Phosphine Oxide Linkage
Cui Liu,[a] Yu Gu,[a] Qiang Fu,[b] Ning Sun,[b] Cheng Zhong,[a] Dongge Ma,*[b]
Jingui Qin,[a] and Chuluo Yang*[a]
Abstract: A series of solution-processa-
ble small molecules PO1–PO4 were
designed and synthesized by linking N-
phenylnaphthalen-1-amine groups to a
phenyl phosphine oxide core through a
p-conjugated bridge, and their thermal,
photophysical, and electrochemical
properties were investigated. The phos-
phine oxide linkage can disrupt the
conjugation and allows the molecular
system to be extended to enable solu-
tion processability and high glass tran-
sition temperatures (159–1818C) while
preserving the deep-blue emission. The
noncoplanar molecular structures re-
sulting from the trigonal-pyramidal
configuration of the phosphine oxide
can suppress intermolecular interac-
tions, and thus these compounds exhib-
it strong deep-blue emission both in
solution and the solid state with high
(PLQY) of 0.88–0.99 in dilute toluene
solution. Solution-processed nondoped
organic light-emitting diodes featuring
PO4 as emitter achieve a maximum
current efficiency of 2.36 cdAÀ1 with
CIE coordinates of (0.15, 0.11) that are
very close to the NTSC blue standard.
Noticeably, all devices based on these
small-molecular fluorescent emitters
show striking deep-blue electrolumi-
nescent color stability and extremely
low efficiency roll-off.
photoluminescent
quantum
yield
Keywords: electrochemistry · elec-
troluminescence fluorescence
·
·
light-emitting diodes · phosphine
oxides
Introduction
often inferior to that of green and red counterparts because
of the intrinsic wide bandgap of blue-emitting materials,
which makes it hard to inject charges into the emitting
layer.
Since the pioneering work on organic light-emitting diodes
(OLEDs) by the Kodak[1] and Cambridge groups,[2] OLEDs
have attracted considerable scientific and industrial atten-
tion because of their applications in flat-panel displays and
solid-state lighting.[3–12] Full-color displays require primary
RGB emission of relatively equal stability, efficiency, and
color purity. It is critical to develop highly efficient deep-
blue emission, which is defined as having blue electrolumi-
nescent (EL) emission with a CIE y coordinate of less than
0.15, because such emitters can not only effectively reduce
the power consumption of a full-color OLED, but also be
utilized to generate light of other colors by energy cascade
To obtain high efficiency, phosphorescent emitters have
aroused intensive interest, as they have the ability to boost
the internal quantum efficiency to 100%.[16] However, the
difficulties of synthesizing applicable large-bandgap dopants
and host materials make blue phosphorescent systems less
desirable than their fluorescent counterparts, and only a few
deep-blue phosphorescent emitters with a CIE coordinate of
y<0.15 have been reported.[17–20] Therefore, deep-blue
OLEDs mainly focus on fluorescent emitters, and extensive
research has been carried out to develop high-performance
deep-blue fluorophores.
Commonly, thermal high-vacuum evaporation technology
is used for fabrication of small molecule based OLEDs
(SMOLEDs), and solution processing technology for poly-
mer-based devices (PLEDs). The vapor deposition techni-
ques enable complicated multilayer device architectures and
give excellent devices with high efficiencies.[21–22] However,
thermal evaporation deposition has critical drawbacks in-
cluding high manufacturing costs and low utilization of the
expensive materials.[23] Although solution processing limits
fabrication of composite device structures because the sol-
vent used for one layer can redissolve or damage the previ-
ous layers,[24] it is advantageous over thermal evaporation
processing in terms of its low-cost and large-area manufac-
to
a
lower-energy fluorescent or phosphorescent
dopant.[13–15] However, the performance of blue LEDs is
[a] C. Liu, Y. Gu, C. Zhong, Prof. J. Qin, Prof. C. Yang
Department of Chemistry
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials
Wuhan University, Wuhan, 430072 (P.R. China)
Fax : (+86)27-68756757
E-mail: clyang@whu.edu.cn
[b] Q. Fu, N. Sun, Prof. D. Ma
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun, 130022 (P.R. China)
Fax : (+86)431-85262357
E-mail: mdg1014@ciac.jl.cn
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

turing.[25] Compared with light-emitting polymers, small mol-
ecules have advantages such as precise molecular structure
and excellent chemical purity, which may greatly impact effi-
ciency and stability of the OLEDs. Therefore, it would be a
good strategy to develop SMOLEDs via a solution process.
A number of solution-processed small molecules have
been reported as blue electroluminescent materials.[26–37]
However, deep-blue fluorescent SMOLEDs fabricated by
spin-coating with stable color coordinates and high efficien-
cy remain rare. It is not easy to design a small molecule that
has both deep-blue emission and solution processability. Sol-
ution processability usually requires large enough molecule
size, while the molecular conjugation system should not be
too extended to ensure the deep-blue emission of the mate-
rials.
Figure 1. TGA thermograms of PO1–PO4 recorded at a heating rate of
108CminÀ1. Inset: DSC traces recorded at a heating rate of 108CminÀ1
.
In this work, we designed and synthesized a series of solu-
tion-processable small molecules with phenyl phosphine
oxide core and N-phenylnaphthalen-1-amine groups as end
caps. The deep-blue emission of these emitters can be well-
preserved due to disruption of conjugation by the phosphine
oxide linkage, even though the molecular size is enlarged.
Introduction of the electron-withdrawing phosphine oxide
can also improve the electron-transporting ability of the ma-
terials. Moreover, the existence of the phosphine oxide
group results in a trigonal-pyramidal structure, which could
effectively hinder close molecular packing in the solid state
and prevent excimer formation and fluorescence quenching.
All of these compounds show high fluorescence quantum
yields in solution when using 9,10-diphenylanthracene
(DPA) as standard. We fabricated simple double-layer devi-
ces that achieve efficient deep-blue performance of
2.36 cdAÀ1 (2.06%) with CIE coordinate of (0.15, 0.11).
Moreover, all devices based on these materials show excel-
lent color stability and extremely low efficiency roll-off.
tures (Td, corresponding to 5% weight loss) in the range of
478–5558C in the thermogravimetric analysis (Figure 1).
Though PO3 and PO4 have rigid fluorene structures and
higher molecular weights than PO1 and PO2, the former
pair has lower Td values than the latter. This can be attribut-
ed to decomposition of the ethyl substituents at the 9-posi-
tion of fluorene rings of PO3 and PO4. Their glass transi-
tion temperatures Tg, determined by differential scanning
calorimetry, are between 159 and 1818C (inset in Figure 1),
which are much higher than that of 1,4-bis[(1-naphthylphe-
nyl)amino]biphenyl (NPB, 988C).[42] The enhanced thermal
stability may be due to the presence of the phosphine oxide
group. The trigonal-pyramidal structure can cause a more
crowded arrangement of the aryl groups around the P atom,
prevent intermolecular packing, and improve the stability of
the amorphous morphology against crystallization. The Tg
order of PO1<PO2, PO3<PO4 could be attributable to
the increased molecular size and molecular weight. The ex-
cellent thermal and morphological stability of these materi-
als enables preparation of homogeneous and stable amor-
phous thin films through solution processing, which is cru-
cial for operation of OLEDs.
Results and Discussion
Synthesis and characterization: The synthetic routes and
chemical structures of PO1–PO4 are depicted in Scheme 1.
Key intermediates 1–3 were prepared from 1,4-dibromoben-
zene, 4,4’-dibromobiphenyl, and 2,7-dibromo-9,9-diethyl-9H-
fluorene, respectively, by sequential lithium–halogen ex-
change with nBuLi, coupling with dichlorophenylphosphine,
and oxidation with hydrogen peroxide. Compounds 4, 5 and
2,7-dibromo-9,9-diethyl-9H-fluorene were prepared by fol-
lowing reported procedures.[38–41] Lithiation of 5 with an
excess of nBuLi followed by treatment with trimethyl
borate and hydrolysis in aqueous HCl gave 9,9-diethyl-7-[1-
Photophysical properties: Figure 2 shows the absorption and
fluorescence spectra of PO1–PO4 in the film state. The pho-
tophysical data of the compounds are presented in Table 1.
The absorption band in the region from 270 to 310 nm can
naphthylACHTUNGTRENNUNG(phenyl)amino]-9H-fluoren-2-ylboronic acid (6).
Target compounds PO1–PO4 were prepared by Suzuki
cross-coupling reactions in yields of about 70%. All com-
pounds were fully characterized by 1H NMR, 13C NMR,
MALDI-TOF MS, and elemental analysis.
Thermal properties: The good thermal stability of the com-
pounds is indicated by their high decomposition tempera-
Figure 2. UV/Vis absorption and PL spectra of PO1–PO4 in film state.
&
2
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Nondoped Deep-Blue Organic Light-Emitting Diodes
FULL PAPER
Scheme 1. Synthesis of PO1–PO4. a) nBuLi, THF, dichlorophenylphosphine, À788C; b) 30% H2O2, CH2Cl2; c) [Pd
ACHTUNGTREN(NNUG PPh3)4], toluene, ethanol, 2m
Na2CO3, reflux; d) i) nBuLi, THF, trimethyl borate, À788C, ii) 2m HCl, 08C.
be attributed to the p–p* transition of the terminal function-
al aryl groups,[43] while the absorption peaks around 355 nm
for PO1 and PO2 and 375 nm for PO3 and PO4 are as-
signed to the p–p* transition of the conjugated bridges be-
tween the electron-donating N-phenylnaphthalen-1-amine
(NPA) moiety and the electron-accepting phenyl phosphine
oxide (PPO) moiety. The emission maxima of the com-
pounds in the film state, which range from 453 to 457 nm,
exhibit only a small blueshift compared with those in di-
chloromethane solution, that is, intermolecular interactions
do not occur in the solid state. The noncoplanar structure of
these compounds effectively prevents close packing of con-
stituent molecules and shields the intermolecular interac-
tions. All of the blue emitters show relatively high photolu-
minescent quantum yield (PLQY) of 0.88–0.99 in dilute tol-
uene solution with DPA (F=0.90) as calibration standard.
Electrochemical properties and DFT calculations: The elec-
trochemical properties of the compounds were probed by
cyclic voltammetry (CV, Figure 3). PO1 and PO2 exhibit ir-
reversible oxidation waves, while PO3 and PO4 show rever-
sible oxidation process. The oxidation wave around 0.5 eV
could be attributed to oxidation of the terminal aryl groups,
and the presence of fluorene units in PO3 and PO4 makes
the oxidation process reversible. The HOMO energy levels
estimated from the onsets of the oxidation potentials are in
the range of 5.10–5.30 eV with respect to the energy level of
ferrocene (4.8 eV below vacuum), which matches well with
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
3
&
ÞÞ
These are not the final page numbers!

D. Ma, C. Yang et al.
Table 1. Thermal, photophysical, and electrochemical data of PO1–PO4.
located on terminal functional aryl groups, while LUMO
and LUMO+1 are mainly distributed on the p-conjugated
bridges. Considering the interchange between HOMO and
HOMOÀ1 and between LUMO and LUMO+1 when the
molecules vibrate, the HOMO and LUMO distributions for
PO1 and PO2 and for PO3 and PO4 are similar. The termi-
nal functional aryl groups have twisted configurations that
result in a noncoplanar structure in each molecule, and all
four molecules show trigonal-pyramidal structure.
PO1
PO2
PO3
PO4
Td/Tg [8C]
552/163
279, 355
349
555/181
296, 354
347
484/159
284, 375
376
478/170
299, 382
375
labs[a] [nm]
labs[b] [nm]
(e [mÀ1 cmÀ1])
lem,max[a] [nm]
lem,max[b] [nm]
HOMO[c]/LU-
MO[d] [eV]
(52000)
453
466
(63300)
457
471
(72700)
455
464
(84700)
455
466
5.26/2.30
5.24/2.28
5.14/2.21
5.11/2.23
[e]
FPL
0.92 (0.35) 0.99 (0.49) 0.88 (0.46) 0.95 (0.49)
[a] Measured in film. [b] Measured in CH2Cl2. [c] Determined from the
onset of oxidation potential. [d] Deduced from HOMO and Eg. [e] Fluo-
rescence quantum yields in toluene solution were measured with 9,10-di-
phenylanthracene (F=0.90) as standard, and fluorescence quantum
yields in solid-state films on a quartz plate by using an integrating sphere
(in parentheses).
Electroluminescence: To evaluate the EL performance of
PO1–PO4 as nondoped blue emitters, double-layer devices
were fabricated with the configurations of indium tin oxide
(ITO)/PEDOT:PSS (40 nm)/emission layer (EML, 40 nm)/
1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene
(TPBI,
30 nm)/LiF (1 nm)/Al (100 nm), in which the emitter is PO1
in device A, PO2 in device B, PO3 in device C, and PO4 in
device D, the emission layer was spin-coated from chloro-
benzene solution on top of PEDOT:PSS, PEDOT:PSS and
LiF served as hole- and electron-injecting layers, respective-
ly, and TPBI was inserted between the EML and LiF as
electron-transporting layer.
The devices based on these emitters exhibit deep-blue
emission in their EL spectra with CIE coordinates of (0.15,
0.09) for device A, (0.15, 0.12) for device B, (0.15, 0.10) for
device C, and (0.15, 0.11) for device D (Figure 5). The EL
spectra of all materials show only slight blueshifts relative to
the PL spectra of thin-film samples, which indicates that ex-
cimers or exciplexes are effectively suppressed during the
EL process.
Figure 3. Cyclic voltammograms of PO1–PO4 in CH2Cl2 for oxidation.
that of 5.2 eV for poly(3,4-ethylenedioxythiopene):poly(styr-
ene sulfonate) (PEDOT:PSS) and may result in efficient
hole injection from PEDOT:PSS to the emissive layer. The
band gaps Eg of PO1–PO4, estimated on the basis of the
red edge of the longest absorption wavelength for the solid-
film sample, are 2.96–2.88 eV. The LUMO levels range from
2.21 to 2.30 eV, as deduced from the HOMO level and Eg
(Table 1).
Density functional calculations [B3LYP; 6-31G(d)] were
carried out to obtain a better understanding of the geometri-
cal and electronic properties of the compounds. As depicted
in Figure 4, HOMO and HOMOÀ1 of PO1–PO4 are mainly
More importantly, these devices exhibit striking blue EL
color stability at various driving voltages, as shown in
Figure 5. With increasing driving voltage from 4 to 9 V, the
EL spectra of all these devices remain nearly unchanged,
and the CIE coordinates show only slight changes, and this
suggests a remarkable voltage-independent and stable EL
spectrum. The excellent color stability can be attributed to
the noncoplanar conformation, which may suppress close
packing of molecules in the solid state and improve the mor-
phological stability by resisting crystallization and morpho-
logical transition induced deterioration during device opera-
tion.
Figure 6 shows the current density–voltage–brightness
AHCTUNGERTG(NNUN J–V–L) characteristics, and efficiency versus current density
curves of the devices, and the device data are summarized in
Table 2. All of these devices display a low turn-on voltage
around 4 V, which can be attributed to the small injection
barriers in the devices. A schematic energy-level diagram of
these devices is presented in Figure 7. The electron-injection
barriers at the EML/TPBI junction are about 0.4–0.5 eV,
while the hole injection barriers at the PEDOT:PSS/EML
junction are smaller than 0.1 eV. These small injection barri-
ers enhance injection efficiency of both holes and electrons
into the emission layer.
The best EL performance is achieved by PO4 with a max-
imum current efficiency hc,max of 2.36 cdAÀ1, a maximum
power efficiency hp,max of 1.86 LmWÀ1, and a maximum ex-
Figure 4. Calculated spatial distributions of the HOMO and LUMO
levels of PO1–PO4.
&
4
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Nondoped Deep-Blue Organic Light-Emitting Diodes
FULL PAPER
Figure 6. a) Current density–voltage–brightness (J–V–L) characteristics
for devices A–D. b) Current efficiency and power efficiency. c) EQE
versus current density curves for devices A–D.
Figure 5. Normalized EL spectra of devices a) A, b) B, c) C, and d) D re-
corded at various driving voltages.
ternal quantum efficiency hext,max of 2.06%, which is compa-
rable with the best device performances in the literature for
solution-processed small-molecular nondoped deep-blue
OLEDs.[26–27,30,32–37] For example, Ma et al. reported fluo-
rene-based small molecule TCPA-6, and fabricated a deep-
blue device with hc,max =1.35 cdAÀ1 and CIE coordinates of
(0.16, 0.05);[26] Huang et al. reported pyrene-functioned fluo-
rene 2PPPF and obtained a deep-blue emitting OLED with
hc,max =1.13 cdAÀ1 and CIE coordinates of (0.16, 0.10);[30]
Park et al. obtained a deep-blue emitting OLED with
Figure 7. Energy-level diagrams for devices A–D.
hc,max =1.15 cdAÀ1 and CIE coordinates of (0.159, 0.105) by
utilizing anthracene-derived compound PCAN as emitter.[34]
Notably, the efficiency of device D remains at 2.01%
(2.32 cdAÀ1) on increasing to the practical brightness of
100 cdmÀ2. Even at a much higher brightness of 1000 cdmÀ2,
the external quantum efficiency is still 1.87% with a roll-off
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!

D. Ma, C. Yang et al.
Table 2. Electroluminescence characteristics of the devices.
[c,f]
[d,f]
Device
Von [V][a]
Lmax [cdmÀ2
]
[b], voltage [V]
hc [cdAÀ1
]
hp [lm WÀ1
]
hext [%][e,f]
CIE (x, y)[g]
A
B
C
D
4.1
4.1
4.1
3.9
5913, 9.1
5892, 9.3
4575, 9.1
5650, 8.9
1.78, 1.69, 1.76
1.84, 1.79, 1.79
1.72, 1.72, 1.67
2.36, 2.32, 2.15
1.11, 1.08, 0.91
1.11, 1.06, 0.86
1.19, 1.10, 0.86
1.86, 1.55, 1.18
1.79, 1.70, 1.77
1.54, 1.50, 1.50
1.66, 1.66, 1.61
2.06, 2.01, 1.87
0.15, 0.09
0.15, 0.12
0.15, 0.10
0.15, 0.11
[a] Turn-on voltage, recorded at a brightness of 1 cdmÀ2. [b] Maximum luminance. [c] Current efficiency. [d] Power efficiency. [e] External quantum effi-
ciency. [f] Order of measured values: maximum, then values at 100 and 1000 cdmÀ2. [g] Measured at 7 V.
thermal stability of the samples under a nitrogen atmosphere was deter-
mined by measuring their weight loss while heating at a rate of 108C
value of 9.2%. Although the other three devices showed in-
ferior EL performance to the PO4-based device, they exhib-
minÀ1 from 20 to 8008C. Cyclic voltammetry (CV) was carried out in ni-
trogen-purged dichloromethane (oxidation scan) at room temperature
it much lower efficiency roll-off. At
a brightness of
with a CHI voltammetric analyzer. Tetrabutylammonium hexafluoro-
phosphate (TBAPF6) (0.1m) was used as supporting electrolyte. The con-
ventional three-electrode configuration consisted of a platinum working
electrode, a platinum wire auxiliary electrode, and an Ag wire pseudore-
ference electrode with ferrocenium/ferrocene (Fc+/Fc) as internal stand-
ard. Cyclic voltammograms were obtained at scan rate of 100 mVsÀ1. The
onset potential was determined from the intersection of two tangents
drawn at the rising and background current of the cyclic voltammogram.
1000 cdmÀ2, the roll-off values of devices A, B, and C for
hext are 1.1, 2.6, and 3.0%, respectively. The high efficiencies
and low efficiency roll-off at high luminance for these devi-
ces can be attributed to the bipolar conformation of the
emitters, which may result in balanced charge fluxes and a
broadly distributed recombination region within the emit-
ting layer.
Device fabrication and measurement: The hole-injection material PE-
DOT:PSS and electron-transporting material TPBI were commercially
available. Commercial ITO-coated glass with sheet resistance of
10 WsquareÀ1 was used as starting substrates. Before device fabrication,
the ITO/glass substrates were precleaned carefully and treated with
oxygen plasma for 2 min. PEDOT:PSS was spin-coated to smooth the
ITO surface and promote hole injection, and then the emissive layer was
spin-coated from chlorobenzene solution. The samples were annealed at
1208C for 30 min to remove residual solvent. The thickness of the EML
was about 40 nm. Finally, an electron-transporting layer of TPBI (30 nm)
and a cathode composed of lithium fluoride (LiF, 1 nm) and aluminum
(Al, 100 nm) were sequentially deposited onto the substrate by vacuum
deposition at 10À6 Torr. The J–V–L characteristics of the devices were
measured with a Keithley 2400 Source meter and a Keithley 2000 Source
multimeter equipped with a calibrated silicon photodiode. The electrolu-
minescence (EL) spectra were measured on a JY SPEX CCD3000 spec-
trometer. The EQE values were calculated according to previously re-
ported methods.[44] All measurements were carried out at room tempera-
ture under ambient conditions.
Conclusion
We have designed and synthesized four deep-blue small-
molecule fluorescent emitters with solution processability.
Theoretical calculations reveal that the blue emitters have
noncoplanar structures resulting from the phosphine oxide
core and peripheral functional aryl groups. The steric hin-
drance effectively suppresses close molecular packing in the
solid state and prevents excimer formation and fluorescence
quenching. Simple nondoped OLEDs based on spin coating
of the new materials exhibit deep-blue electroluminescence.
Moreover, all devices show excellent color stability, high ef-
ficiency, and very low roll-off. The PO4-based device ach-
ieves a maximum current efficiency of 2.36 cdAÀ1 with CIE
coordinates of (0.15, 0.11). Our findings provide a valuable
strategy for the rational design and development of solu-
tion-processable deep-blue fluorescent materials for use in
OLED displays.
Synthesis:
4’-[1-naphthyl
(phenyl)amino]biphenyl-4-ylboronic
acid
Bis(4-bromophenyl)ACTHUNRGTNEUNG(phenyl)phosphine oxide (1): n-Butyllithium (2.4m in
hexane, 7.9 mL, 19 mmol) was added dropwise to a solution of 1,4-dibro-
mobenzene (4.72 g, 20 mmol) in anhydrous THF (160 mL) at À788C. The
reaction mixture was kept at this temperature for 3 h, and then dichloro-
phenylphosphine (1.26 mL, 9.3 mmol) was added. The resulting mixture
was stirred for a further 3 h at À788C before quenching with 5 mL of
methanol. Water was added, and the mixture was extracted with CH2Cl2,
washed with water, and dried over anhydrous Na2SO4. After the solvent
had been completely removed, 30% hydrogen peroxide (30 mL) and
CH2Cl2 (60 mL) were added to the obtained residue and the mixture stir-
red overnight at room temperature. The organic layer was separated and
washed with water and then brine. The extract was evaporated to dry-
ness, and the residue was purified by column chromatography on silica
gel with dichloromethane/methanol (30/1 v/v) as eluent to give a white
solid in 70% yield (%). 1H NMR (300 MHz, CDCl3): d=7.68–7.58 (m,
6H), 7.58–7.45 ppm (m, 7H); 13C NMR (75 MHz, CDCl3): d=133.38,
133.31, 132.35, 131.80, 131.77, 131.47, 131.12, 130.77, 130.43, 128.69,
128.60, 127.38, 127.36 ppm; MS (EI): m/z calcd for C18H13Br2OP: 435.91,
found: 435.83.
Experimental Section
General information: 1H NMR and 13C NMR spectra were measured on
a Mercury-VX300 spectrometer. Elemental analyses of carbon, hydrogen,
and nitrogen were performed on a Vario EL III microanalyzer. EI-MS
spectra were recorded on VJ-ZAB-3F Mass spectrometer. MALDI-TOF
mass spectra were recorded on a Bruker BIFLEX III TOF mass spec-
trometer. UV/Vis absorption spectra were recorded on a Shimadzu UV-
2500 recording spectrophotometer. Photoluminescence spectra were re-
corded on a Hitachi F-4500 fluorescence spectrophotometer. The PL
quantum yields of solid-state films were measured by an absolute method
using the Edinburgh Instruments (FLS920) integrating sphere excited
with an Xe lamp. Differential scanning calorimetry (DSC) was performed
on a NETZSCH DSC 200 PC unit at a heating rate of 108CminÀ1 from
room temperature to 4008C under argon. The glass transition tempera-
ture was determined from the second heating scan. Thermogravimetric
analysis was performed with a NETZSCH STA 449C instrument. The
Bis(4’-bromobiphenyl-4-yl)ACTHNUGRTENUNG(phenyl)phosphine oxide (2): Compound 2 was
prepared in 67% yield according to a similar procedure to 1 by using
4,4’-dibromobiphenyl instead of 1,4-dibromobenzene. 1H NMR
&
6
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Nondoped Deep-Blue Organic Light-Emitting Diodes
FULL PAPER
(300 MHz, CDCl3): d=7.82–7.71 (m, 6H), 7.68 (d, J=8.1 Hz 4H), 7.61
(d, J=8.7 Hz 4H), 7.54–7.45 ppm (m, 7H); 13C NMR (75 MHz, CDCl3):
d=143.34, 138.52, 132.59, 132.52, 131.93, 131.70, 131.58, 130.88, 128.64,
128.50, 128.43, 126.82, 122.44 ppm; MS (EI): m/z calcd for C30H21Br2OP:
587.97, found: 587.81.
as eluent to give PO3 as a yellow powder in 69% yield (%); 1H NMR
(300 MHz, CDCl3): d=7.95–7.87 (m, 4H), 7.83–7.71 (m, 10H), 7.68–7.40
(m, 16H), 7.38–7.30 (m, 4H), 7.24–7.18 (m, 5H), 7.10–7.04 (m, 6H),
6.99–6.91 (m, 4H), 1.79–2.00 (m, 8H), 0.355 ppm (t, J=7.2 Hz 12H);
13C NMR (75 MHz, CDCl3): d=151.56, 150.58, 148.68, 148.35, 145.23,
143.80, 141.71, 137.56, 135.27, 134.88, 132.65, 132.20, 131.92, 130.92,
129.08, 128.55, 128.35, 127.07, 126.73, 126.23, 126.04, 124.36, 121.96,
121.68, 121.35, 120.51, 119.29, 116.80, 56.11, 32.62, 8.50 ppm; elemental
analysis calcd (%) for C84H69N2OP: C 87.67, H 5.90, N 2.42; found: C
87.47, H 6.03, N 2.43; MALDI-TOF MS: m/z 1153.0.
Bis(7-bromo-9,9-diethyl-9H-fluoren-2-yl)ACTHNUTRGNE(NUG phenyl)phosphine oxide (3):
Compound 3 was prepared in 81% yield according to a similar procedure
to 1 by using 2,7-dibromo-9,9-diethyl-9H-fluorene instead of 1,4-dibromo-
benzene. 1H NMR (300 MHz, CDCl3): d=7.80–7.64 (m, 6H), 7.64–7.44
(m, 11H), 1.99 (q, J=6.9 Hz 8H), 0.27 ppm (t, J=6.9 Hz 12H);
13C NMR (75 MHz, CDCl3): d=152.54, 149.58, 149.50, 144.05, 138.95,
133.08, 132.39, 131.80, 131.21, 131.15, 131.08, 130.52, 130.12, 128.36,
128.28, 126.35, 126.28, 126.20, 122.36, 121.62, 119.38, 56.60, 32.28, 32.23,
8.37, 8.09 ppm; MS (EI): m/z calcd for C40H37Br2OP: 724.09, found:
724.02.
7’,7’’-(Phenylphosphoryl)bis(9,9,9’,9’-tetraethyl-N-1-naphthyl-N-phenyl-
9H,9’H-2,2’-bifluoren-7-amine) (PO4): PO4 was prepared in 64% yield
according to a similar procedure to PO3 by using 3 instead of 1.
1H NMR (300 MHz, CDCl3): d=7.96 (d, J=8.1 Hz 2H), 7.91 (d, J=
8.1 Hz 2H), 7.85–7.72 (m, 10H), 7.72–7.41 (m, 20H), 7.39–7.30 (m, 4H),
7.25–7.18 (m, 5H), 7.12–7.03 (m, 6H), 7.00–6.91 (m, 4H), 2.16–1.80 (m,
16H), 0.45–0.30 ppm (m, 24H); 13C NMR (75 MHz, CDCl3): d=151.70,
151.64, 150.78, 149.03, 148.38, 145.40, 144.12, 142.28, 141.23, 139.45,
139.34, 135.54, 131.19, 129.36, 128.65, 127.02, 126.82, 126.58, 126.43,
126.33, 124.67, 122.10, 121.81, 121.64, 121.48, 121.00, 120.64, 119.45,
117.24, 56.89, 56.43, 32.99, 8.88 ppm; elemental analysis calcd (%) for
C106H93N2OP: C 88.61, H 6.42, N 1.87; found: C 88.30, H 6.50, N 1.94;
MALDI-TOF: m/z 1441.1.
4’’,4’’’-(Phenylphosphoryl)bis(N-1-naphthyl-N-phenyl-1,1’:4’,1’’-terphenyl-
4-amine) (PO1): Degassed toluene (18 mL) and ethanol (4 mL) were
added to a mixture of compound 1 (0.305 g, 0.7 mmol), 4 (0.872 g,
2.1 mmol), Na2CO3 (2 m, 3.5 mL, 7 mmol), and [PdACHTNUGTRNEGNU(PPh3)4] (0.048 g,
0.042 mmol). The resulting mixture was stirred and heated to reflux at
1008C for 48 h under argon atmosphere. After cooling to room tempera-
ture, the solution was extracted with CH2Cl2 and the organic layer was
washed with brine and water and then dried over anhydrous Na2SO4.
After the solvent had been removed, the residue was purified by column
chromatography on silica gel with dichloromethane/methanol (30/1 v/v)
as eluent to give a yellow powder in 67% yield. 1H NMR (300 MHz,
CDCl3): d=7.97 (d, J=7.8 Hz 2H), 7.92 (d, J=6.9 Hz 2H), 7.83–7.69 (m,
12H), 7.68–7.59 (m, 8H), 7.54–7.42 (m, 9H), 7.42–7.33 (m, 4H), 7.27–
7.18 (m, 6H), 7.09 ppm (t, J=8.4 Hz 8H), 7.01–6.93 (m, 2H); 13C NMR
(75 MHz, CDCl3): d=148.36, 148.29, 144.54, 143.50, 140.78, 138.16,
135.56, 133.22, 132.94, 132.42, 131.48, 129.46, 128.90, 128.72, 127.83,
127.61, 127.25, 126.95, 126.79, 126.67, 126.49, 124.45, 122.60, 122.42,
121.72 ppm; elemental analysis calcd (%) for C74H53N2OP: C 87.04, H
5.02, N 2.68; found: C 87.38, H 5.25, N 2.75; MALDI-TOF MS: m/z
1017.1.
Acknowledgements
We thank the National Science Fund for Distinguished Young Scholars
of China (No. 51125013), the National Basic Research Program of China
(973 Program 2009CB623602 and 2009CB930603), the National Natural
Science Foundation of China (No. 90922020), and the Open Research
Fund of State Key Laboratory of Polymer Physics and Chemistry, Chang-
chun Institute of Applied Chemistry, Chinese Academy of Sciences for fi-
nancial support.
4’’’,4’’’’-(Phenylphosphoryl)bis(N-1-naphthyl-N-phenyl-1,1’:4’,1’’:4’’,1’’’-qua-
terphenyl-4-amine) (PO2): PO2 was prepared in 71% yield according to
a similar procedure to PO1 by using 2 instead of 1. 1H NMR (300 MHz,
CDCl3): d=7.98 (d, J=8.7 Hz, 2H), 7.92 (d, J=8.7 Hz, 2H), 7.83–7.61
(m, 25H), 7.61–7.42 (m, 12H), 7.42–7.34 (m, 4H), 7.28–7.18 (m, 6H),
7.18–7.02 (m, 8H), 7.01–6.95 ppm (m, 2H); 13C NMR (75 MHz, CDCl3):
d=148.38, 148.26, 144.51, 143.59, 140.83, 140.20, 138.86, 138.74, 135.59,
133.55, 133.02, 132.90, 132.46, 132.34, 131.54, 129.44, 128.96, 128.71,
127.93, 127.80, 127.70, 127.57, 127.40, 127.20, 126.91, 126.75, 126.65,
126.47, 124.49, 122.56, 122.36, 121.83 ppm; elemental analysis calcd (%)
for C86H61N2OP: C 88.27, H 5.18, N 2.29; found: C 88.33, H 5.26, N 2.40;
MALDI-TOF MS: m/z 1169.0.
[1] C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 1987, 51, 913–915.
[2] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.
Mackay, R. H. Friend, P. L. Burn, A. B. Holmes, Nature 1990, 347,
539–541.
[3] M. A. Baldo, M. E. Thompson, S. R. Forrest, Nature 2000, 403, 750–
753.
[4] B. W. DꢁAndrade, S. R. Forrest, Adv. Mater. 2004, 16, 1585–1595.
[5] C. C. Wu, Y. T. Lin, K. T. Wong, R. T. Chen, Y. Y. Chien, Adv.
Mater. 2004, 16, 61–65.
[6] Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, S. R.
Forrest, Nature 2006, 440, 908–912.
[7] J. Luo, X. Li, Q. Hou, J. B. Peng, W. Yang, Y. Cao, Adv. Mater. 2007,
19, 1113–1117.
[8] S.-J. Su, E. Gonmori, H. Sasabe, J. Kido, Adv. Mater. 2008, 20, 4189–
4194.
[9] S. Beauprꢂ, P.-L. T. Boudreault, M. Leclerc, Adv. Mater. 2010, 22,
E6-E27.
[10] K. T. Kamtekar, A. P. Monkman, M. R. Bryce, Adv. Mater. 2010, 22,
572–582.
N,N’-{(Phenylphosphoryl)bis[(9,9-diethyl-9H-fluorene-7,2-diyl)-4,1-phe-
nylene]}bis(N-phenylnaphthalen-1-amine) (PO3): nBuLi (2.4m in hexane,
2.9 mL, 6.96 mmol) was added dropwise to a solution of 7-bromo-9,9-di-
ethyl-N-1-naphthyl-N-phenyl-9H-fluoren-2-amine (5, 3.0 g, 5.8 mmol) in
anhydrous THF (60 mL), at À788C. At this temperature, the reaction
mixture was stirred for 1 h before adding trimethyl borate (0.97 mL,
8.7 mmol). After the mixture was stirred for 1 h at À788C, it was allowed
to warm to room temperature, stirred overnight, quenched with HCl
(50 mL, 2.0m), and poured into a large amount of water. After extraction
with diethyl ether three times, the organic portions were washed with
brine before drying over anhydrous Na2SO4. The solvents were removed
under reduced pressure, and boronic acid 6 was obtained (2.38 g 85%
yield). Degassed toluene (15 mL) and ethanol (4 mL) were added to a
[11] C. H. Chien, C. K. Chen, F. M. Hsu, C. F. Shu, P. T. Chou, C. H. Lai,
Adv. Funct. Mater. 2009, 19, 560–566.
[12] M. Zhu, Q. Wang, Y. Gu, X. Cao, C. Zhong, D. Ma, J. Qin, C. Yang,
J. Mater. Chem. 2011, 21, 6409–6415.
mixture of compound 1 (0.262 g, 0.6 mmol), boronic acid
6 (0.87 g,
[13] M. Lee, Appl. Phys. Lett. 2004, 85, 3301–3303.
[14] M. T. Lee, C. H. Liao, C. H. Tsai, C. H. Chen, Adv. Mater. 2005, 17,
2493–2497.
[15] S. J. Lee, J. S. Park, K.-J. Yoon, Y.-I. Kim, S.-H. Jin, S. K. Kang, Y.-S.
Gal, S. Kang, J. Y. Lee, J.-W. Kang, S.-H. Lee, H.-D. Park, J.-J. Kim,
Adv. Funct. Mater. 2008, 18, 3922–3930.
1.8 mmol), Na2CO3 (2 m, 3 mL, 6 mmol), and [Pd(PPh3)4] (0.042 g,
AHCTUNGTRENNUNG
0.036 mmol). The resulting mixture was stirred and heated to reflux at
1008C for 48 h under argon atmosphere. After cooling to room tempera-
ture, the solution was extracted with CH2Cl2 and the organic layer was
washed with brine and water and then dried over anhydrous Na2SO4.
After the solvent had been removed, the residue was purified by column
chromatography on silica gel with dichloromethane/methanol (30/1 v/v)
[16] M. A. Baldo, D. F. OꢁBrien, Y. You, A. Shoustikov, S. Sibley, M. E.
Thompson, S. R. Forrest, Nature 1998, 395, 151–154.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!

D. Ma, C. Yang et al.
[17] C.-F. Chang, Y.-M. Cheng, Y. Chi, Y.-C. Chiu, C.-C. Lin, G.-H. Lee,
P.-T. Chou, C.-C. Chen, C.-H. Chang, C.-C. Wu, Angew. Chem. 2008,
120, 4618–4621; Angew. Chem. Int. Ed. 2008, 47, 4542–4545.
[18] Y.-C. Chiu, Y. Chi, J.-Y. Hung, Y.-M. Cheng, Y.-C. Yu, M.-W.
Chung, G.-H. Lee, P.-T. Chou, C.-C. Chen, C.-C. Wu, H.-Y. Hsieh,
ACS Appl. Mater. Interfaces 2009, 1, 433–442.
[31] L. Duan, L. Hou, T.-W. Lee, J. Qiao, D. Zhang, G. Dong, L. Wang,
Y. Qiu, J. Mater. Chem. 2010, 20, 6392–6407.
[32] A. L. Fisher, K. E. Linton, K. T. Kamtekar, C. Pearson, M. R. Bryce,
M. C. Petty, Chem. Mater. 2011, 23, 1640–1642.
[33] M. Zhu, T. Ye, C.-G. Li, X. Cao, C. Zhong, D. Ma, J. Qin, C. Yang,
J. Phys. Chem. C 2011, 115, 17965–17972.
[19] Y.-C. Chiu, J.-Y. Hung, Y. Chi, C.-C. Chen, C.-H. Chang, C.-C. Wu,
Y.-M. Cheng, Y.-C. Yu, G.-H. Lee, P.-T. Chou, Adv. Mater. 2009, 21,
2221–2225.
[20] C.-H. Hsieh, F.-I. Wu, C.-H. Fan, M.-J. Huang, K.-Y. Lu, P.-Y. Chou,
Y.-H. O. Yang, S.-H. Wu, I. C. Chen, S.-H. Chou, K.-T. Wong, C.-H.
Cheng, Chem. Eur. J. Chem.–Eur. J. 2011, 17, 9180–9187.
[21] C. Adachi, M. A. Baldo, S. R. Forrest, M. E. Thompson, Appl. Phys.
Lett. 2000, 77, 904–906.
[34] I. Cho, S. H. Kim, J. H. Kim, S. Park, S. Y. Park, J. Mater. Chem.
2012, 22, 123–129.
[35] L. Yao, S. Xue, Q. Wang, W. Dong, W. Yang, H. Wu, M. Zhang, B.
Yang, Y. Ma, Chem. Eur. J. Chem.–Eur. J. 2012, 18, 2707–2714.
[36] K. L. Chan, J. P. F. Lim, X. Yang, A. Dodabalapur, G. E. Jabbour, A.
Sellinger, Chem. Commun. 2012, 48, 5106–5108.
[37] C. Liu, Y. Li, Y. Zhang, C. Yang, H. Wu, J. Qin, Y. Cao, Chem. Eur.
J. 2012, 18, 6928–6934.
[22] J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, S. Liu, Appl.
Phys. Lett. 2002, 80, 139–141.
[38] B. E. Koene, D. E. Loy, M. E. Thompson, Chem. Mater. 1998, 10,
2235–2250.
[23] H. Kim, Y. Byun, R. Ragini Das, B.-K. Choi, P.-S. Ahn, Appl. Phys.
Lett. 2007, 91, 093512–093513.
[39] W. L. Jia, M. J. Moran, Y.-Y. Yuan, Z. H. Lu, S. Wang, J. Mater.
Chem. 2005, 15, 3326–3333.
[24] S. R. Forrest, Nature 2004, 428, 911–918.
[25] M. C. Gather, A. Kçhnen, K. Meerholz, Adv. Mater. 2011, 23, 233–
248.
[26] S. Tang, M. R. Liu, P. Lu, H. Xia, M. Li, Z. Q. Xie, F. Z. Shen, C.
Gu, H. P. Wang, B. Yang, Y. G. Ma, Adv. Funct. Mater. 2007, 17,
2869–2877.
[27] Y.-J. Pu, M. Higashidate, K.-i. Nakayama, J. Kido, J. Mater. Chem.
2008, 18, 4183–4188.
[28] B. Walker, A. Tamayo, J. Yang, J. Z. Brzezinski, T.-Q. Nguyen, Appl.
Phys. Lett. 2008, 93, 063302–063303.
[29] T.-W. Lee, T. Noh, H.-W. Shin, O. Kwon, J.-J. Park, B.-K. Choi, M.-
S. Kim, D. W. Shin, Y.-R. Kim, Adv. Funct. Mater. 2009, 19, 1625–
1630.
[40] A. Baheti, P. Tyagi, K. R. J. Thomas, Y.-C. Hsu, J. T. s. Lin, J. Phys.
Chem. C 2009, 113, 8541–8547.
[41] R. Kannan, G. S. He, L. Yuan, F. Xu, P. N. Prasad, A. G. Dombrosk-
ie, B. A. Reinhardt, J. W. Baur, R. A. Vaia, L.-S. Tan, Chem. Mater.
2001, 13, 1896–1904.
[42] J. N. Moorthy, P. Venkatakrishnan, D.-F. Huang, T. J. Chow, Chem.
Commun. 2008, 2146–2148.
[43] Y. Tao, Q. Wang, Y. Shang, C. Yang, L. Ao, J. Qin, D. Ma, Z. Shuai,
Chem. Commun. 2009, 77–79.
[44] S. R. Forrest, D. D. C. Bradley, M. E. Thompson, Adv. Mater. 2003,
15, 1043–1048.
Received: May 1, 2012
Revised: June 26, 2012
Published online: && &&, 0000
[30] F. Liu, C. Tang, Q.-Q. Chen, F.-F. Shi, H.-B. Wu, L.-H. Xie, B. Peng,
W. Wei, Y. Cao, W. Huang, J. Phys. Chem. C 2009, 113, 4641–4647.
&
8
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Nondoped Deep-Blue Organic Light-Emitting Diodes
FULL PAPER
Deep-blue small molecule fluorescent
emitters with solution processACTHNUTRGNEsNUG ability
Electroluminescence
were designed and synthesized by link-
ing N-phenylnaphthalen-1-amine
groups to a phenyl phosphine oxide
core through a p-conjugated bridge.
Solution-processed small molecule
based organic light-emitting diodes
featuring PO4 as emitter (see figure)
achieve a maximum current efficiency
of 2.36 cdAÀ1 with CIE coordinates of
(0.15, 0.11).
C. Liu, Y. Gu, Q. Fu, N. Sun,
C. Zhong, D. Ma,* J. Qin,
C. Yang* ....................... &&&& — &&&&
Nondoped Deep-Blue Organic Light-
Emitting Diodes with Color Stability
and Very Low Efficiency Roll-Off:
Solution-Processable Small-Molecule
Fluorophores by Phosphine Oxide
Linkage
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
9
&
ÞÞ
These are not the final page numbers!