Efficient solution-processed deep-blue organic light-emitting diodes based on multibranched oligofluorenes with a phosphine oxide center

Article abstract of DOI:10.1021/cm401640v

A series of multibranched oligofluorenes with a phosphine oxide center were designed and synthesized through Suzuki cross-coupling reaction. Their thermal, photophysical, and electrochemical properties were investigated. The phosphine oxide linkage can di

Full text of DOI:10.1021/cm401640v

Article
pubs.acs.org/cm
Efficient Solution-Processed Deep-Blue Organic Light-Emitting
Diodes Based on Multibranched Oligofluorenes with a Phosphine
Oxide Center
†
‡
†
,†
,‡
†
‡
Cui Liu, Yanhu Li, Yifan Li, Chuluo Yang,* Hongbin Wu,* Jingui Qin, and Yong Cao
†Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
‡
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South
China University of Technology, Guangzhou 510640, People’s Republic of China
ABSTRACT: A series of multibranched oligofluorenes with a
phosphine oxide center were designed and synthesized through
Suzuki cross-coupling reaction. Their thermal, photophysical,
and electrochemical properties were investigated. The phosphine
oxide linkage can disrupt the conjugation, and then allow the
molecule system to extend to improve the solution processability
and photoluminescent quantum yields without depreciating the
deep blue emission. The noncoplanar molecular structures
resulting from the phosphine oxide triangular pyramidal
configuration can suppress the intermolecular interaction. All
compounds display strong deep-blue emission both in solution
and the solid state. Solution-processed devices based on these oligofluorenes exhibit highly efficient deep-blue
electroluminescence, and the device performances are significantly enhanced with the extension of the oligofluorene branches.
The double-layered device featuring PPO-TF3 as emitter shows a maximum current efficiency of 1.88 cd A− and a maximum
external quantum efficiency of 3.39% with Commission Internationale de l′Eclairage (CIE) coordinates of (0.16, 0.09) that are
very close to the National Television Standards Committee’s blue standard.
1
KEYWORDS: deep-blue fluorescent OLEDs, solution process, oligofluorene, phosphine oxide
INTRODUCTION
wavelength emission in the process of photoirradiation, heat
treatment, or device operation, which has been attributed to
■
Organic light-emitting diodes (OLEDs) have attracted consid-
erable scientific and industrial attention because of their
applications in flat-panel displays and solid-state lighting
27−29
physical (excimers or aggregate formation)
or chemical
degradation processes. Recently, monodis-
perse oligofluorenes that can be deposited via solution process
30,31
(
keto defects)
1
−16
resources.
Full-color displays require primary RGB emission
32−41
of relatively equal stability, efficiency, and color purity. The
development of deep-blue emission, which is defined as having
blue electroluminescence (EL) emission with a Commission
Internationale de l′Eclairage (CIE) y coordinate value of less
than 0.15, is of special significance 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 a lower-energy fluorescent or phosphorescent
have been of great interest for use in OLEDs.
In comparison
with polymers, monodisperse oligofluorenes are characterized by
well-defined and uniform molecular structures as well as high
chemical purity. From a practical standpoint, high chemical
purity and structural uniformity are crucial factors that could
influence the performance of OLEDs. It has been demonstrated
that the performance and stability of monodisperse oligofluor-
42−44
enes in OLEDs are higher than that of polyfluorenes.
1
7−19
dopant.
However, the performance of blue light emitting
Monodisperse star-shaped oligofluorenes with different cores,
devices is often inferior to that of green and red counterparts for
the intrinsic wide band gap of blue-emitting materials which
makes it hard to inject charges into emitting layer.
4
0
32,34
35
such as benzene, triazatruxene,
pyrene, 4,4′,4″-tris-
33
(
1
carbazol-9-yl)-triphenylamine (TCTA), 1,3,5-tri(anthracen-
0-yl)benzene, fully bridged triphenylamine, truxene, and
spirofluorene, have been reported. However, there is still no
multibranched oligofluorene reported so far.
39
41
45
Polyfluorene and its derivatives are considered to be the most
promising blue light emitting materials due to their high
photoluminescence (PL) efficiencies, wide band-gaps, good
thermal stabilities, interesting morphological properties, and easy
tunability of properties by substitution and/or copolymeriza-
46
41
Received: May 21, 2013
Revised: July 28, 2013
2
0−26
tion.
However, the application of polyfluorenes as blue
emitters in OLEDs is hampered by their undesirable long-
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/cm401640v | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
In this context, we designed and synthesized a series of
multibranched oligofluorenes with a phosphine oxide center. We
wish that the deep-blue emission of these materials could be well-
preserved with the extension of molecules due to the disrupted
conjugation via phosphine oxide center. The introduction of the
electron-withdrawing phosphine oxide can also improve the
electron transporting ability of the materials. Moreover, the
existence of phosphine oxide group results in a triangular
pyramidal structure, which could effectively hinder the close
molecular packing in the solid state and prevent excimer
formation and fluorescence quenching. The thermal, photo-
physical, and electrochemical properties of oligofluorenes as well
as the characteristics of devices incorporating these molecules
were investigated. All these compounds show deep-blue
emission with high fluorescence quantum yields in film.
Solution-processed devices based on these oligomers exhibit
efficient deep-blue electroluminescence. Furthermore, the device
performance is significantly enhanced with the extension of the
oligofluorene branches. The double-layered device with PPO-
TF3 as emitter shows a maximum current efficiency of 1.88 cd
nm) and Al (120 nm) layer were evaporated with a shadow mask at a
−4
base pressure of 3 × 10 Pa. The thickness of the evaporated cathode
was monitored by using a quartz crystal thickness/ratio monitor (model
STM-100/MF, Sycon), and the thickness of spin-coated PEDOT:PSS
and EML was measured by using the terrace detector. The overlapping
2
area between the cathode and anode defined a pixel size of 19 mm .
Except for the deposition of the PEDOT layers, all the fabrication
processes were carried out inside a controlled atmosphere of nitrogen
drybox (Vacuum Atmosphere Co.) containing less than 10 ppm oxygen
and moisture. The current density−luminance−voltage characteristic
was measured using a Keithley 236 source measurement unit and a
calibrated silicon photodiode. The forward-viewing luminance was
calibrated by using a spectrophotometer (SpectraScan PR-705, Photo
Research), and the forward-viewing LE was calculated accordingly.
Throughout the whole paper, reported values of luminance and LE are
for forward-viewing direction only. The external quantum efficiency of
EL was collected by measuring the total light output in all directions in
an integrating sphere (IS-080, Labsphere). The EL spectra were
collected via a PR-705 photometer.
Synthesis. Oligofluorene boronic acids of different chain lengths
4
5,46
(F1−F3) were prepared according to a reported procedure.
The
Suzuki coupling reaction was conducted under a nitrogen atmosphere
−1
and by avoiding light exposure.
A
and a maximum external quantum efficiency of 3.39% with
Bis(4-bromophenyl)(phenyl)phosphine Oxide. To a solution of 1,4-
dibromobenzene (4.72 g, 20 mmol) in anhydrous THF (160 mL), n-
butyllithium (2.26 M in hexane, 8.4 mL, 19 mmol) was added dropwise
at −78 °C. The reaction was kept at this temperature for 2 h, and then
1.26 mL (9.3 mmol) of dichloro(phenyl)phosphine was added. After the
mixture was stirred for 1 h at −78 °C, it was allowed to warm to room
temperature, stirred overnight followed by quenching with 5 mL of
CIE coordinates of (0.16, 0.09).
EXPERIMENTAL SECTION
■
General Information. 1H NMR and 13C NMR spectra were
measured on a MECUYR-VX300 spectrometer. Elemental analyses of
carbon, hydrogen, and nitrogen were performed on a Vario EL III
microanalyzer. Mass spectra were measured on a ZAB 3F-HF mass
spectrometer. MALDI-TOF (matrix-assisted laser-desorption/ioniza-
tion time-of-flight) mass spectra were performed on a Bruker BIFLEX
III TOF mass spectrometer. UV−vis absorption spectra were recorded
on a Shimadzu UV-2500 recording spectrophotometer. Photo-
luminescence spectra were recorded 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
methanol. Water was added, and the mixture was extracted with CH
washed with water, and dried over anhydrous Na SO . After the solvent
had been completely removed, 30% hydrogen peroxide (30 mL) and
CH Cl (60 mL) were added to the obtained residue and they were
Cl ,
2
2
2
4
2
2
stirred overnight at room temperature. The organic layer was separated
and washed with water and then brine. The extract was evaporated to
dryness, and the residue was purified by column chromatography on
silica gel using dichloromethane/methanol (30:1 by vol) as the eluent to
1
(
FLS920) integrating sphere excited with Xe lamp. Differential scanning
give a white solid (3.77 g). Yield: 93%. H NMR (300 MHz, CDCl3, δ):
13
calorimetry (DSC) was performed on a NETZSCH DSC 200 PC unit at
7.68−7.58 (m, 6H), 7.58−7.45 (m, 7H). C NMR (75 MHz, CDCl3,
−1
a heating rate of 10 °C min from room temperature to 300 °C under
argon. The glass transition temperature was determined from the
second heating scan. Thermogravimetric analysis was undertaken with a
NETZSCH STA 449C instrument. The thermal stability of the samples
under a nitrogen atmosphere was determined by measuring their weight
δ): 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. Anal. Calcd for C18
(%): C 49.58, H 3.00; found: C 49.50, H 3.33. MS (EI) m/z calcd for
OP: 435.91; found: 435.83.
Tris(4-bromophenyl)phosphine Oxide. Tris(4-bromophenyl)-
H13Br OP
2
C H13Br
18 2
−1
loss while heating at a rate of 15 °C min from 25 to 800 °C. Cyclic
voltammetry was carried out in nitrogen-purged THF (reduction scan)
and dichloromethane (oxidation scan), respectively, with a CHI
voltammetric analyzer. Tetrabutylammonium hexafluorophosphate
phosphine oxide was prepared according to the similar procedure to
bis(4-bromophenyl)(phenyl)phosphine oxide by using phosphorus
1
trichloride to replace dichloro(phenyl)phosphine. Yield: 30%. H NMR
1
3
(300 MHz, CDCl3, δ): 7.68−7.60 (m, 6H), 7.54−7.44 (m, 6H).
C
(
TBAPF6) (0.1 M) was used as the supporting electrolyte. The
NMR (75 MHz, CDCl3, δ): 133.60, 133.51, 132.64, 132.33, 132.20,
conventional three-electrode configuration consists of a platinum
131.14, 130.09, 129.00, 127.97. Anal. Calcd for C H Br OP (%): C
18
12
3
working electrode, a platinum wire auxiliary electrode, and an Ag wire
pseudoreference electrode with ferrocenium-ferrocene (Fc /Fc) as the
41.98, H 2.35; found: C 42.16, H 2.54. MS (EI) m/z calcd for
+
C H Br OP: 513.82; found: 514.54.
18
12
3
internal standard. Cyclic voltammograms were obtained at scan rate of
Bis(4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl)(phenyl)phosphine
−1
1
00 mV s . Formal potentials are calculated as the average of cyclic
voltammetric anodic and cathodic peaks. The onset potential was
determined from half-wave potential of the oxidation.
Oxide (PPO-BF1). To a mixture of bis(4-bromophenyl)(phenyl)-
phosphine oxide (0.44 g, 1.00 mmol), fluorene boronic acid (F1)
(0.98 g, 2.60 mmol), Pd(PPh ) (0.069 g, 0.06 mmol), and 2 M K CO
3
4
2
3
Device Fabrication and Measurement. Patterned ITO coated
(5.00 mL, 10.00 mmol) was added degassed toluene (20 mL) and
ethanol (5 mL). The solution was heated to reflux for 48 h under argon.
After cooling to room temperature, the solution was extracted with
−1
glass with a sheet resistance of 15−20 Ω square were cleaned by a
surfactant scrub, then underwent a wet-cleaning process inside an
ultrasonic bath, beginning with deionized water, followed by acetone
and isopropanol. After oxygen plasma cleaning for 4 min, 40 nm of
poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PE-
DOT:PSS) (Bayer Baytron P 4083) used as a hole-injection layer at
the anode interface was spin-coated on the ITO substrate and then dried
in a vacuum oven at 80 °C overnight. The emissive layer (EML) was
coated onto the anode by spin-coating from chlorobenzene solution,
and then annealed at 80 °C for 10 min to remove the solvent residue.
The thickness of the EML was about 50 nm. Finally, an electron-
transporting layer of TPBI (30 nm) and a cathode composed of CsF (1.5
CH
Cl
and the organic layer was washed with brine and H
O, and then
2
2
2
dried over anhydrous Na
SO . After the solvent had been removed, the
2
4
residue was purified by column chromatography on silica gel using
petroleum/ethyl acetate (2:1 by vol) as the eluent to give a white
1
powder (0.75g). Yield: 79%. H NMR (300 MHz, CDCl
, δ): 7.88−7.70
3
(m, 14H), 7.63−7.55 (m, 5H), 7.55−7.47 (m, 2H), 7.38−7.30 (m, 6H),
2.00 (t, J = 7.2 8H), 1.15−0.95 (m, 24H), 0.75 (t, J = 6.6 12H), 0.71−
0.60 (m, 8H). 13C NMR (75 MHz, CDCl , δ): 151.92, 151.35, 145.55,
3
141.71, 140.75, 138.90, 133.02, 132.88, 132.52, 132.39, 131.93, 130.54,
128.96, 128.80, 127.58, 127.42, 127.18, 126.48, 123.23, 121.85, 120.41,
B
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Chemistry of Materials
Article
Scheme 1. Synthesis of the Multibranched Oligofluorenes
1
20.22, 55.52, 40.69, 31.76, 29.97, 24.06, 22.84, 14.26. Anal. Calcd for
CDCl , δ): 7.91−7.70 (m, 21H), 7.62−7.57 (m, 6H), 7.38−7.30 (m,
3
C H OP (%): C 86.58, H 8.44; found: C 86.70, H 8.53. MS (MALDI-
TOF) m/z calcd for C H OP: 942.59; found: 943.12.
6H), 2.00 (m, 12H), 1.11−0.96 (m, 36H), 0.74 (t, J = 6.6 18H), 0.70−
68
79
0.59 (m, 12H). 13C NMR (75 MHz, CDCl , δ): 151.58, 151.02, 145.25,
68
79
3
Bis(4-(9,9,9′,9′-tetrahexyl-9H,9′H-[2,2′-bifluoren]-7-yl)phenyl)-
141.38, 140.41, 138.58, 132.58, 130.27, 127.29, 126.84, 126.14, 122.88,
(
phenyl)phosphine Oxide (PPO-BF2). PPO-BF2 was prepared
1
21.52, 119.89, 55.17, 40.35, 31.42, 29.64, 23.71, 22.51, 13.94. Anal.
according to the same procedure as PPO-BF1 but using bifluorene
Calcd for C H111OP (%): C 87.55, H 8.77; found: C 87.88, H 8.95. MS
93
boronic acid (F2) and bis(4-bromophenyl)(phenyl)phosphine oxide to
(
MALDI-TOF) m/z calcd for C H111OP: 1275.84; found: 1276.45.
93
1
give a white power. Yield: 80%. H NMR (300 MHz, CDCl , δ): 7.89−
3
Tris(4-(9,9,9′,9′-tetrahexyl-9H,9′H-[2,2′-bifluoren]-7-yl)phenyl)-
7
1
.73 (m, 17H), 7.72−7.49 (m, 16H), 7.42−7.31 (m, 6H), 2.15−1.98 (m,
phosphine Oxide (PPO-TF2). PPO-TF2 was prepared according to the
same procedure as PPO-BF1 but using bifluorene boronic acid (F2) and
13
6H), 1.20−1.01 (m, 48H), 0.95−0.65 (m, 40H). C NMR (75 MHz,
CDCl , δ): 152.23, 152.12, 151.79, 151.30, 145.55, 141.41, 141.27,
3
tris(4-bromophenyl)phosphine oxide to give a white power. Yield: 57%.
1
1
5
41.05, 140.68, 139.91, 138.89, 133.04, 132.89, 132.53, 128.96, 127.58,
27.42, 127.09, 126.58, 126.35, 123.23, 121.81, 120.46, 120.18, 120.02,
5.67, 55.47, 40.65, 31.73, 29.96, 24.08, 22.82, 14.25. Anal. Calcd for
C118H143OP (%): C 88.12, H 8.96; found: C 88.45, H 9.01. MS
MALDI-TOF) m/z calcd for C118H143OP: 1608.09; found: 1607.43.
1
H NMR (300 MHz, CDCl , δ): 7.95−7.71 (m, 22H), 7.71−7.59 (m,
3
2
0
1
1
1
2
3H), 7.38−7.29 (m, 6H), 2.17−1.97 (m, 24H), 1.20−0.99 (m, 72H),
13
.82−0.62 (m, 60H). C NMR (75 MHz, CDCl , δ): 151.92, 151.80,
3
51.46, 150.97, 145.25, 141.10, 140.94, 140.72, 140.34, 139.58, 138.59,
32.75, 132.62, 127.31, 127.15, 126.78, 126.24, 126.03, 122.90, 121.62,
21.41, 120.15, 119.86, 119.71, 55.35, 55.15, 40.35, 31.43, 29.66, 23.77,
2.52, 13.96. Anal. Calcd for C168H207OP (%): C 88.76, H 9.18; found:
(
Bis(4-(9,9,9′,9′,9″,9″-hexahexyl-9H,9′H,9″H[2,2′:7′,2″- terfluo-
ren]-7-yl)phenyl)(phenyl)phosphine Oxide (PPO-BF3). PPO-BF3 was
prepared according to the same procedure as PPO-BF1 but using
terfluorene boronic acid (F3) and bis(4-bromophenyl)(phenyl)-
C 88.59, H 8.91. MS (MALDI-TOF) m/z calcd for C168H207OP:
2272.59; found: 2272.81.
Tris(4-(9,9,9′,9′,9″,9″-hexahexyl-9H,9′H,9″H-[2,2′:7′,2″-terfluo-
ren]-7-yl)phenyl)phosphine Oxide (PPO-TF3). PPO-TF3 was prepared
according to the same procedure as PPO-BF1 but using terfluorene
1
phosphine oxide to give a pale yellow power. Yield: 87%. H NMR
(
300 MHz, CDCl , δ): 7.89−7.73 (m, 22H), 7.73−7.49 (m, 23H),
3
7
0
1
1
1
1
8
.39−7.26 (m, 6H), 2.17−1.97 (m, 24H), 1.20−1.00 (m, 72H), 0.90−
.65 (m, 60H). 13C NMR (75 MHz, CDCl , δ): 151.48, 151.12, 150.66,
3
boronic acid (F3) and tris(4-bromophenyl)phosphine oxide to give a
44.88, 140.60, 140.44, 140.02, 139.58, 139.28, 138.26, 132.39, 132.25,
31.89, 128.30, 128.14, 126.92, 126.76, 126.44, 125.82, 122.57, 121.17,
19.82, 119.61, 119.35, 54.98, 54.81, 40.00, 31.09, 29.30, 23.47, 22.17,
3.62. Anal. Calcd for C168H207OP (%): C 88.76, H 9.18; found: C
8.74, H 9.15. MS (MALDI-TOF) m/z calcd for C168H207OP: 2272.59;
1
pale yellow power. Yield: 69%. H NMR (300 MHz, CDCl , δ): 7.98−
3
7
3
.75 (m, 27H), 7.75−7.59 (m, 36H), 7.42−7.30 (m, 6H), 2.19−1.98 (m,
13
6H), 1.20−1.00 (m, 108H), 0.89−0.63 (m, 90H). C NMR (75 MHz,
CDCl , δ): 152.10, 151.74, 151.27, 145.57, 141.39, 141.22, 141.04,
3
1
1
40.63, 140.18, 139.90, 138.90, 133.04, 132.91, 127.60, 127.44, 127.06,
26.44, 123.18, 121.77, 120.47, 120.24, 119.98, 55.60, 55.43, 40.64,
found: 2271.97.
Tris(4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl)phosphine Oxide (PPO-
TF1). PPO-TF1 was prepared according to the same procedure as PPO-
BF1 but using fluorene boronic acid (F1) and tris(4-bromophenyl)-
31.72, 29.93, 24.11, 22.80, 14.26. Anal. Calcd for C243H303OP (%): C
89.23, H 9.34; found: C 88.95, H 9.03. MS (MALDI-TOF) m/z calcd
for C243H303OP: 3270.35; found: 3270.15.
1
phosphine oxide to give a white power. Yield: 51%. H NMR (300 MHz,
C
dx.doi.org/10.1021/cm401640v | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
−1
Figure 1. TGA thermograms (a, c) and DSC traces (b, d) of the materials recorded at a heating rate of 10 °C min .
Figure 2. UV−vis absorption and PL spectra of the compounds in toluene solution at 5 × 10−6 M (a, c) and film state (b, d).
4
5,46
RESULTS AND DISCUSSION
were prepared by reported procedure.
The key intermediates
■
bis(4-bromophenyl)(phenyl)phosphine oxide and tris(4-
bromophenyl)phosphine oxide were prepared from 1,4-
dibromobenzene, sequentially through a lithium-halogen ex-
Synthesis and Characterization. The synthetic routes and
chemical structures of the materials are depicted in Scheme 1.
Oligofluorene boronic acids of different chain lengths (F1−F3)
D
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Chemistry of Materials
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Table 1. Thermal, Photophysical, and Electrochemical Data of the Materials
a
b
a
b
cd
e
T /T (°C)
λabs (nm)
λabs (nm)
λem, max (nm)
λem, max (nm)
HOMO/LUMO (eV)
ΦPL (%)
d
g
PPO-BF1
PPO-BF2
PPO-BF3
PPO-TF1
PPO-TF2
PPO-TF3
417/66
420/85
420/94
415/77
420/92
321
347
361
322
349
321
352
367
326
355
367
355 (sh), 371
394, 413 (sh)
408, 429 (sh)
357 (sh), 372
395, 414 (sh)
409, 430 (sh)
379
−5.53/−2.10
−5.52/−2.40
−5.45/−2.47
−5.58/−2.18
−5.56/−2.46
−5.48/−2.50
31
58
90
33
71
99
404 (sh), 421
415, 437 (sh)
401
408 (sh), 419
423/103
b
363
416, 437 (sh)
a
c
d
Measured in toluene. Measured in film. Determined from the onset of oxidation potentials. Deduced from HOMO and E estimated from the
red edge of the longest absorption wavelength for the solid-film sample. Fluorescence quantum yields in solid state films, measured on the quartz
g
e
plate using an integrating sphere.
change reaction with n-butyllithium (n-BuLi), a coupling
reaction with dichlorophenylphosphine and phosphorus tri-
chloride, respectively, and an oxidation reaction with hydrogen
peroxide. The target compounds were successfully prepared
through Suzuki cross-coupling reactions of the phosphine oxide
intermediates and the oligofluorene boronic acids. All the
branches increase from two to three. Fluorescence quantum
yields of three-branched oligofluorenes are 0.33, 0.71 and 0.99,
respectively.
Electrochemical Properties. The electrochemical proper-
ties of the compounds were probed by cyclic voltammetry (CV,
Figure 3). All compounds exhibit overlapped multireversible
1
13
compounds were fully characterized by H NMR, C NMR,
MALDI-TOF, and elemental analysis.
Thermal Properties. The good thermal stability of the
compounds is indicated by their high decomposition temper-
atures (T , corresponding to 5% weight loss) in the range of
d
4
15−423 °C in the thermogravimetric analysis (Figure 1). The
reason for the first weight loss to around 65% of the original
weight is the cleavage of alkyl side chains. Their glass transition
temperatures (T ) determined through differential scanning
g
calorimetry are between 66 and 103 °C, and enhanced with
increaing oligofluorene branch length. Moreover, when the
oligofluorene branches of these molecules extended from two to
three, T of the corresponding compound rises by about 10 °C,
g
which can be attributed to the increased molecular size and
molecular weight. The good thermal and morphological stability
of these materials enables the preparation of homogeneous and
stable amorphous thin films through solution processing, which
is crucial for the operation of OLEDs.
Photophysical Properties. Figure 2 shows the absorption
and fluorescence spectra of these compounds in both toluene
solution and solid state films. The photophysical data of the
compounds are presented in Table 1. For two-branched
molecules, the absorption spectra of three compounds in toluene
solution show intense π−π* absorption bands, which pro-
gressively red-shift from 321 nm for PPO-BF1 to 361 nm for
PPO-BF3 with increasing fluorene units. The emission spectrum
of PPO-BF1 locates in near-ultraviolet region, while PPO-BF2
and PPO-BF3 display deep-blue emission. The emission maxima
of three compounds are 371, 394, and 408 nm in solution,
respectively. The absorption and emission spectra of the films are
similar to those of the solution, and only exhibit a small red-shift
in the emission maximum (8−10 nm), indicating the absence of
intermolecular interactions in thin films. This can be attributed to
the noncoplanar configuration resulted by phosphine oxide
group that have effectively prevented close-packing of con-
stituent molecules and restrained intermolecular aggregation.
Figure 3. Cyclic voltammograms of the materials in CH Cl for
oxidation.
2
2
oxidation waves, attributed to the oxidation of oligofluorene
branches. With the extension of fluorene units in each branch, the
onset of the oxidation potentials decreases from 0.73 (PPO-BF1)
to 0.65 eV (PPO-BF3) for two-branched oligofluorenes, and 0.78
(PPO-TF1) to 0.68 eV (PPO-TF3) for three-branched
oligofluorenes. The highest occupied molecular orbital
(HOMO) energy levels estimated from the onsets of the
oxidation potentials are around −5.50 eV with regard to the
energy level of ferrocene (4.8 eV below vacuum). The band gaps
Fluorescence quantum yields (Φ ) of two-branched materials in
PL
solid state measured on the quartz plate using an integrating
sphere are 0.31 for PPO-BF1, 0.58 for PPO-BF2, and 0.90 for
PPO-BF3, significantly enhanced with the extension from one
fluorene unit to two and three. The three-branched oligofluor-
enes show similar absorption and emission spectra to their two-
branched analogues with slight red-shift, indicating that the
conjugation has not been expanded when the oligofluorene
E
dx.doi.org/10.1021/cm401640v | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Eg) of these compounds, on the basis of the red edge of the
Article
(
longest absorption wavelength for the solid-film sample, were
estimated to be 2.98−3.43 eV. The lowest unoccupied molecular
orbital (LUMO) levels range from 2.10 to 2.50 eV, which are
deduced from HOMO and E (Table 1).
g
Electroluminescence. Compounds PPO-BF2, PPO-BF3,
PPO-TF2, and PPO-TF3 exhibit deep-blue emission, high
fluorescence quantum yield and good solubility in common
solvents. To evaluate EL properties of these materials, simple
double-layer devices, featuring PPO-BF2, PPO-BF3, PPO-TF2,
and PPO-TF3 as nondoped blue emitters, were fabricated with
the configurations of indium tin oxide (ITO)/PEDOT:PSS (40
nm)/emission layer (EML, 50 nm)/1,3,5-tris(N-phenylbenzi-
midazol-2-yl)benzene (TPBI) (30 nm)/CsF (1.5 nm)/Al (120
nm) (emitter: PPO-BF2, device A; PPO-BF3, device B; PPO-
TF2, device C; PPO-TF3, device D), in which the emission layer
was spin-coated from chlorobenzene solution. PEDOT:PSS and
CsF served as hole- and electron-injecting layers, respectively.
TPBI was inserted between the EML and LiF as the electron-
transporting layer.
The devices based on these emitters exhibit deep-blue
emission in their EL spectra with emission maxima of 424 nm
for devices A and C, and 438 nm for devices B and D (Figure 4).
Figure 5. (a) Current efficiency and external quantum efficiency versus
current density curves and (b) current density−voltage−brightness (J−
V−L) characteristics for devices A−D.
performances are comparable with those of the star-shaped
41
oligofluorenes we have reported before and among the highest
for nondoped deep-blue OLEDs based on solution-processable
32−41
starburst oligofluorenes until now.
We noted that the EL performance is significantly enhanced
with the extension of fluorene units from two to three in each
branch. The improvements can be mainly elucidated from the
substantially higher photoluminescence quantum yields of PPO-
BF3 over PPO-BF2, PPO-TF3 over PPO-TF2. In addition, the
HOMO and LUMO energy levels of PPO-BF3 and PPO-TF3
are more matched with adjacent PEDOT and TPBI layers than
those of their analogues PPO-BF2 and PPO-TF2, respectively.
Therefore, more balanced charge flux can be anticipated in device
B and D.
Figure 4. Normalized EL spectra of devices A−D.
The EL spectras of all materials are similar to their PL spectra
from the thin film samples, indicating that the excimer or exciplex
have been effectively suppressed during the EL process.
Figure 5 shows the current density−voltage−brightness (J−
V−L) characteristics, and efficiency versus current density curves
of the devices. All the device data are summarized in Table 2. The
turn-on voltage of these devices is in the following order: A > B,
C > D, which could be attributed to the matched HOMO and
LUMO energy level for device B and D (Figure 5), resulting in
lower hole and electron injection barrier than device A and C.
Device A exhibit a turn-on voltage of 5.4 V, a maximum current
The operating stability test under continuous stress was also
carried out with an initial luminance of 100 cd m− for device D.
The lifetime (defined as the time it takes for the display to reach
50% of the original luminance) of the device is found to be ∼10 h,
indicating that the lifetime of the devices still remains as the
major hindrance to practical applications. We note that the
operating lifetime of the real device is closely related to some of
extrinsic factors (such as the interface properties of the electrode,
the reliability of encapsulation, etc.); the observed short lifetime
of the devices should not be solely attributed to the intrinsic
properties of the compound reported here. Since the lifetime of
the device can be significantly improved through rational
materials design by improving the charge transporting properties
and thermal properties, our forthcoming work will pay more
attention to this issue.
2
−1
efficiency (ηc, max) of 1.08 cd A , and a maximum external
quantum efficiency (ηext, max) of 1.74%, with CIE coordinates of
(
0.17, 0.11). The performances of device C are comparable to
those of device A. The device based on PPO-BF3 shows
significantly improved performances with a low turn-on voltage
−1
of 4.4 V, ηc, max of 1.76 cd A , η
of 3.17%, and CIE
ext, max
coordinates of (0.16, 0.08), which is very close to the National
Television Standards Committee’s blue standard (0.14, 0.08).
PPO-TF3-based device D exhibits better performance compared
to device B, with turn-on voltage of 4.2 V, ηc, max of 1.88 cd A ,
ηext, max of 3.39%, and CIE coordinates of (0.16, 0.09). These
−1
F
dx.doi.org/10.1021/cm401640v | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Table 2. Electroluminescence Characteristics of the Devices
a
Lmax (cd m−2) , V (V)
b
ηc, max (cd A−1
c
d
e
device
A
EML
Von (V)
)
ηext, max (%)
CIE (x, y)
PPO-BF2
5.4
564, 10.4
1.08
1.74
(0.17, 0.11)
(0.16, 0.08)
(0.17, 0.10)
(0.16, 0.09)
1
.06 ± 0.02
1.72 ± 0.04
3.17
B
PPO-BF3
PPO-TF2
PPO-TF3
4.4
5.0
4.2
672, 9.2
1.76
1
.71 ± 0.07
3.07 ± 0.14
1.82
C
D
257, 16.0
1.13
1
.12 ± 0.01
1.80 ± 0.03
3.39
944, 9.8
1.88
1
.90 ± 0.02
3.41 ± 0.03
a
−2
b
c
Turn-on voltage, recorded at the brightness of 1 cd m . Maximum luminance. Maximum current efficiency; statistics (italic) based on 10 cells of
d
e
−2
each type. Maximum external quantum efficiency; statistics (italic) based on 10 cells of each type. Measured at 10 mA m .
CONCLUSION
(12) Zhu, M.; Ye, T.; Li, C.-G.; Cao, X.; Zhong, C.; Ma, D.; Qin, J.;
Yang, C. J. Phys. Chem. C 2011, 115, 17965.
13) Gong, S.; Zhao, Y.; Wang, M.; Yang, C.; Zhong, C.; Qin, J.; Ma, D.
Chem.Asian J. 2010, 5, 2093.
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■
In summary, we have designed and synthesized a series of
multibranched oligofluorenes with a phosphine oxide center. All
of the compounds show excellent thermal stabilities, pronounced
PL efficiencies, and good solution processability. Double-layered
nondoped OLEDs based on these materials by solution process
exhibit highly efficient deep-blue electroluminescence. The PPO-
TF3-based device achieves a maximum current efficiency of 1.88
(
(
−1
cd A and maximum external quantum efficiency of 3.39% with
CIE coordinates of (0.16, 0.09). Our findings provide a valuable
strategy to the rational design and development of efficient blue
EL oligofluorenes and other related materials.
(
18) Lee, M. T.; Liao, C. H.; Tsai, C. H.; Chen, C. H. Adv. Mater. 2005,
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AUTHOR INFORMATION
■
*
(
Corresponding Author
E-mail: clyang@whu.edu.cn (C.Y.); hbwu@scut.edu.cn
H.W.).
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(
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(
Notes
The authors declare no competing financial interest.
2
(
ACKNOWLEDGMENTS
■
(
C.Y. thanks the National Basic Research Program of China (973
Program 2013CB834805 and 2009CB623602) and the National
Science Fund for Distinguished Young Scholars of China (No.
1125013); H.W. thanks the National Natural Science
Foundation of China (61177022).
(
1
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dx.doi.org/10.1021/cm401640v | Chem. Mater. XXXX, XXX, XXX−XXX