Spirobifluorene derivative: A pure blue emitter (CIEy ≈ 0.08) with high efficiency and thermal stability

Article abstract of DOI:10.1039/c2jm32512h

A spirobifluorene derivative containing phenanthrene moiety, 2,7-di(phenanthren-9-yl)-9,9′-spirobifluorene (DPSF), has been synthesized. It shows absorption peaks at 254 nm, 310 nm, and 327 nm and a fluorescence peak at 383 nm in CHCl₃ that shifts to 398 nm in the film state. The quantum yield is 0.79 calibrated with a standard of coumarin 102 (0.93). A pure blue emission at Commission Internationale de l′Eclairage (CIE) (0.15, 0.08), has been achieved using DPSF as the emitter, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) as the hole injecting layer, 4,4′-bis[N-(1-naphthyl)-N-phenyl- amino] biphenyl (NPB) as the hole transporting layer, and 1,3,5-tris(N- phenylbenzimidazol-2-yl)-benzene (TPBI) mixing with 2-tert-butylphenyl-5- biphenyl-1,3,4-oxadiazole (PBD) (2:1) as the electron transporting material. The maximum current efficiency (CE) and power efficiency (PE) of the DPSF device are 3.24 cd A^-1 and 2.54 lm W^-1, corresponding to 5.41% of maximum external quantum efficiency (EQE). The spirobifluorene derivative show high thermal stabilities, 178 °C for the glass transition temperature (T_g) and 503 °C for the decomposition temperature (T _d). The synthesized spirobifluorene derivative shows potential application as a highly efficient pure blue emitter for organic light emitting devices (OLED). The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm32512h

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PAPER
Spirobifluorene derivative: a pure blue emitter (CIEy z 0.08) with high
efficiency and thermal stability
*
Xing Xing, Lixin Xiao, Lingling Zheng, Shuangyuan Hu, Zhijian Chen, Bo Qu and Qihuang Gong
*
Received 21st April 2012, Accepted 6th June 2012
DOI: 10.1039/c2jm32512h
A spirobifluorene derivative containing phenanthrene moiety, 2,7-di(phenanthren-9-yl)-9,90-
spirobifluorene (DPSF), has been synthesized. It shows absorption peaks at 254 nm, 310 nm, and 327
nm and a fluorescence peak at 383 nm in CHCl3 that shifts to 398 nm in the film state. The quantum
yield is 0.79 calibrated with a standard of coumarin 102 (0.93). A pure blue emission at Commission
0 ꢀ
Internationale de l Eclairage (CIE) (0.15, 0.08), has been achieved using DPSF as the emitter, poly(3,4-
ethylene dioxythiophene) : poly(styrene sulfonic acid) (PEDOT : PSS) as the hole injecting layer, 4,40-
bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl (NPB) as the hole transporting layer, and 1,3,5-tris(N-
phenylbenzimidazol-2-yl)-benzene (TPBI) mixing with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole
(PBD) (2 : 1) as the electron transporting material. The maximum current efficiency (CE) and power
efficiency (PE) of the DPSF device are 3.24 cd Aꢀ1 and 2.54 lm Wꢀ1, corresponding to 5.41% of
maximum external quantum efficiency (EQE). The spirobifluorene derivative show high thermal
ꢁ
stabilities, 178 C for the glass transition temperature (Tg) and 503 ꢁC for the decomposition
temperature (Td). The synthesized spirobifluorene derivative shows potential application as a highly
efficient pure blue emitter for organic light emitting devices (OLED).
anthracene (POAn) at a saturated deep-blue CIE (0.15, 0.07)
although the fluorescence quantum yield (F) of POAn in dilute
cyclohexane reached as high as 0.98, the F in the solid state
reduced to 0.71. The glass transition temperature (Tg) of it was
146 ꢁC.33 Recently, T. Peng et al. reported a fluorescent beryllium
complex of bis(2-(2-hydroxyphenyl)-4-methyl-pyridine)beryl-
lium (Be(4-mpp)2) with a saturated deep blue emission by CIE
(0.14, 0.09) and EQE of 5.4%, however the quantum yield of
Be(4-mpp)2 was as low as 0.37, moreover beryllium is
poisonous.34 Kwon et al. reported a pure blue emission with a
National Television Standards Committee (NTSC) standard
blue at CIE (0.14, 0.08) from 9, 10-bis[(200,700-di-t-butyl)-90,900-
spirobifluorenyl] anthracene (TBSA), however the power
efficiency (PE) needed to be improved (PE, 0.54 lm Wꢀ1).35
Therefore, it is more desirable to develop a highly efficient pure
blue emitter with CIEy z 0.08 and higher thermal stability.
In order to achieve this goal, we designed and synthesized a
spirobifluorene derivative through considering the high thermal
stability of the spirobifluorene moiety. Although anthracene
derivatives are highly efficient emitters, it is still a challenge to get
a highly efficient saturated deep-blue emission with them.
Considering anthracene is more conjugated than phenanthrene,
we introduced highly efficient emitting phenanthrene moieties to
improve its colour purity and obtained 2,7-di(phenanthren-9-yl)-
9,90-spirobifluorene (DPSF) with a higher Tg of 178 ꢁC than that
1. Introduction
Although organic light-emitting devices (OLED) have been
commercialized since 1997, a pure blue emitter with high effi-
ciency is still a challenge for full colour display. In addition,
100% internal quantum efficiency of sky-blue electro-
phosphorescence has been obtained,1,2 however, highly efficient,
stable deep-blue phosphorescence is still under developed.3,4
Therefore, many efforts have been paid on highly efficient stable
blue fluorescent emitters which can be listed as single-cyclic
aromatics;5,6 e.g. 4,40-bis(2,2-diphenylvinyl)biphenyl (DPVBi),5
anthracene derivatives;7–11 e.g. 9,10-di-(2-naphthyl) anthracene
(ADN),8 fluorene and spirobifluorenes;12–21 multi-cyclic
aromatic;22–24 aromatic amines;25–28 nitrogen-containing hetero-
cyclic blue emitters;11,29,30 other hetero-atom compounds.31
Among them, there have been reported a few emissions with a
high external quantum efficiency (EQE) and Commission
0 ꢀ
Internationale de l Eclairage (CIE) coordinates in the range of
(CIEx <0.15, CIEy <0.15) which was usually defined as a deep-
blue colour.7,11,12,16,22,24,28,32 However, a saturated deep-blue
(CIEx <0.15, CIEy <0.10) emitter with high efficiency is a
pressing concern for the development of full-colour saturated
displays.33–38 C. H. Chien et al. reported a saturated deep blue
emitter of 2-tert-butyl-9,10-bis[40-(diphenylphosphoryl) phenyl]
of POAn33 and a higher F of 0.79 than that of Be(4-mpp)2.34
A
State Key Laboratory for Mesoscopic Physics and Department of Physics,
Peking University, Beijing, 100871, P. R. China. E-mail: lxxiao@pku.edu.
cn; qhgong@pku.edu.cn
pure blue emission at CIE (0.15, 0.08) with a maximum EQE of
5.41% has been achieved by using DPSF as the emitter.
15136 | J. Mater. Chem., 2012, 22, 15136–15140
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2. Experimental sections
3. Results and discussions
General
DPSF was synthesized through a Suzuki–Miyaura cross-
coupling reaction40 by 2,7-dibromo-spirobifluorene with 9-
boronic acid phenanthrene as shown in Scheme 1.
The 1H NMR spectra were recorded on a JEOL 270 (270 MHz)
spectrometer. Mass spectra were obtained using a JEOL JMS-K9
mass spectrometer. Ultraviolet-visible (UV-vis) absorption and
photoluminescence spectra were obtained on an Agilent UV-vis
Spectroscopy System (8453E). The electrochemical properties of
DPSF was studied through cyclic voltammetry (CV) with a three-
electrode configuration with a Pt disk as the working electrode,
Pt wire as the counter electrode, and a saturated calomel elec-
trode (SCE) as the reference electrode in an acetonitrile solution
containing 0.1 M of tetrabutylammonium perchlorate as the
supporting electrolyte. Differential scanning calorimetry (DSC)
was performed on a Perkin-Elmer Diamond DSC Pyris instru-
The absorption spectra of DPSF in CHCl3 and as a film which
was deposited by high-vacuum (10ꢀ6 Torr) thermal evaporation
on quartz are shown in Fig. 1. The absorption peaks at around
254 nm, 310 nm, and 327 nm for DPSF in solution, shift to
257 nm, 313 nm, and 336 nm for DPSF in its film state. The 254
nm absorption peak can be assigned to the p–p* transitions of
the molecule. The peaks at 310 nm and 327 nm can be attributed
to the absorption nature of the phenanthrene moiety.41 The
emission peak of DPSF at around 400 nm is more red shifted
than phenanthrene at around 350 nm42 in solution due to the
larger conjugated system of DPSF. According to the absorption
edges, the highest occupied molecular orbital–lowest unoccupied
molecular orbital (HOMO–LUMO) energy gap (Eg) of DPSF
can be calculated as 3.5 eV. The energy gap of DPSF is even
wider than those of ADN (3.2 eV), DPVBi (3.1 eV), Be(4-mpp)2
(3.1 eV), POAn (3.1 eV), and TBSA (2.8 eV) which are listed in
Table 1. A fluorescence peak at 383 nm in CHCl3 shifts to 398 nm
in the film state. The quantum yield of DPSF in CHCl3 is 0.79
calibrated with a standard of coumarin 102 (F, 0.93).
ment under nitrogen atmosphere at a heating rate of 20 ꢁC minꢀ1
.
Thermogravimetric analysis (TGA) was undertaken using a
SEIKO EXSTAR 6000 TG/DTA 6200 unit under nitrogen
atmosphere at a heating rate of 10 ꢁC minꢀ1. HOMO levels were
determined by atmospheric ultraviolet photoelectron spectros-
copy (Rikken Keiki AC-2).
Synthesis of DPSF
The electrochemical properties of DPSF are shown in Fig. 2.
Two reduction peaks were observed, the first peak located at
0.70 V (vs. SCE) was referred to monoanion formation (X/Xꢀ1),
and the second peak located at 0.98 V (vs. SCE) was the dianion
(Xꢀ1/Xꢀ2) as reported.43 The formation of dianion indicates that
the monoanion is quite stable, which should be a benefit for a
long time operation under charge transporting. The unsymmet-
rical redox processes may be due to the protonation reaction
induced by the reactivity of the supporting electrolyte.43
A standard Suzuki–Miyaura cross-coupling reaction procedure
was followed to synthesize DPSF. To a solution of 2,7-dibromo-
9,9-spirobifluorene (1.314 g, 2.773 mmol) and phenanthrene-9-
boronic acid (1.353 g, 6.097 mmol) in toluene (200 mL) was
added ethanol (100 mL) and a Na2CO3 solution (2 M, 100 mL)
under nitrogen and agitation, followed by the addition of
Pd(PPh3)4 (710 mg, 0.614 mmol). The resultant solution was
refluxed for 24 h. Water was added to quench the reaction, then
extracted with CH2Cl2, finally purified by column chromato-
graphy eluting with: CH2Cl2 : hexane (1 : 4), a solid was
The thermal properties of the spirobifluorene derivative were
measured by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) (Table 1). The Tg of DPSF is 178 ꢁC,
1
obtained with a yield of 46.7%. H NMR (270 MHz, CDCl3) d:
ꢁ
8.70 (d, J ¼ 8.31 Hz, 2H), 8.65 (d, J ¼ 8.19 Hz, 2H), 8.05 (d, J ¼
7.82 Hz, 2H), 7.81 (d, J ¼ 8.05 Hz, 4H), 7.75 (d, J ¼ 7.60 Hz, 2H),
7.54–7.63 (m, 10H), 7.44 (t, J ¼ 7.92 Hz, 2H), 7.34 (t, J ¼ 7.72
Hz, 2H), 7.19 (t, J ¼ 7.33 Hz, 2H), 7.00 (d, J ¼ 3.77 Hz, 2H), 6.96
(s, 2H). MS (EI, m/z): calcd for C53H32, 668.8; found, 669.3.
Anal. calcd for C53H32: C 95.17%, H, 4.83%; found: C 95.10%, H
5.07%.
much higher than 64 C for that of DPVBi. The decomposition
temperature (Td) of DPSF is 503 ꢁC. The highly thermal stability
of DPSF can be attributed to the existence of high thermal
stability of the spirobifluorene moiety.44
To study the emitting properties of the spirobifluorene deri-
vative, devices were fabricated by using a 30 nm layer of 10 : 1
(wt) of poly(3,4-ethylene dioxythiophene) : polystyrene sulfonic
acid (PEDOT : PSS) as the hole injecting material, a 30 nm layer
of 4,40-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPB) as
the hole transporting material, a 30 nm layer of DPSF or
DPSF : N,N0-dicarbazolyl-3,5-benzene (mCP) (4 : 1 by weight)
as the emitter and a 30 nm layer of 1,3,5-tris(N-phenyl-
benzimidazol-2-yl)-benzene (TPBI) or TPBI : 2-tert-butyl-
phenyl-5-biphenyl-1,3,4-oxadiazole (PBD) (2 : 1 by weight) as
the electron transporting (ET) material. Finally, 0.5 nm of LiF
together with 100 nm of Al was deposited as the cathode. The
Fabrication of organic electrophosphorescent devices and the
measurements
Organic layers were deposited by high-vacuum (10ꢀ6 Torr)
thermal evaporation as previously reported.39 First, a 30 nm
layer of PEDOT : PSS (10 : 1 by weight) was coated via a
dichloroethane solution as the hole injecting layer, before it was
loaded into the evaporation system. After this the hole trans-
porting, emitting, and electron transporting layers were sequen-
tially deposited on the surface via thermal evaporation in a
vacuum of 10ꢀ6 Torr. Finally, 0.5 nm of LiF together with
100 nm of Al were deposited as the cathode. The emitting area
was defined by using a cyclic shadow mask. EQE was determined
based on luminance, electroluminescence spectra, and current
densities.
Scheme 1 Synthesis of the spirobifluorene derivative.
J. Mater. Chem., 2012, 22, 15136–15140 | 15137
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Fig. 1 Absorption and PL spectra of DPSF.
Table 1 Physical properties of the blue emitters
Materials
Tg (ꢁC)
Ea (eV)
Ip (eV)
Eg (eV)
Ref.
DPSF
ADN
DPVBi
POAn
Be(4-mpp)2
TBSA
178
2.3
2.6
2.8
2.5
2.6
3.0
5.8
5.8
5.9
5.6
5.7
5.8
3.5
3.2
3.1
3.1
3.1
2.8
This work
13
5, 45
33
34
64
146
207
35
Fig. 3 Chemical structures and energy levels of the organic materials
used in the OLEDs.
Fig. 2 CV curves of DPSF at a scan-rate of 10 mV$sꢀ1
.
emitting area was defined by using a shadow mask with 2 mm
diameter opening. The device architecture and their energy levels
are shown in Fig. 3.
Device A: ITO/PEDOT : PSS 30 nm/NPB 30 nm/DPSF
30 nm/TPBI 30 nm/LiF 0.5 nm/Al 100 nm.
Device B: ITO/PEDOT : PSS 30 nm/NPB 30 nm/DPSF 30 nm/
TPBI : PBD (2 : 1) 30 nm/LiF 0.5 nm/Al 100 nm.
Device C: ITO/PEDOT : PSS 30 nm/NPB 30 nm/DPSF : mCP
(4 : 1) 30 nm/TPBI 30 nm/LiF 0.5 nm/Al 100 nm.
Device D: ITO/PEDOT : PSS 30 nm/NPB 30 nm/
DPSF : mCP (4 : 1) 30 nm/TPBI : PBD (2 : 1) 30 nm/LiF 0.5 nm/
Al 100 nm.
Fig. 4 Current density and luminance–voltage curves of device A: ITO/
PEDOT : PSS 30 nm/NPB 30 nm/DPSF 30 nm/TPBI 30 nm/LiF/Al (O);
device B: ITO/PEDOT : PSS 30 nm/NPB 30 nm/DPSF 30 nm/
TPBI : PBD (2 : 1) 30 nm/LiF/Al (:); device C: ITO/PEDOT : PSS
30 nm/NPB 30 nm/DPSF : mCP (4 : 1) 30 nm/TPBI 30 nm/LiF/Al (B);
device D: ITO/PEDOT : PSS 30 nm/NPB 30 nm/DPSF : mCP (4 : 1) 30
nm/TPBI : PBD (2 : 1) 30 nm/LiF/Al (C).
Current density–voltage curves of the devices are shown in
Fig. 4, in which the current density of device B with TPBI : PBD
as the electron transporting material is higher than that of device
A where only TPBI functions as the electron transporting
material. Accordingly, the turn-on voltage of device B (4.1 V) is
lower than that of device A (4.5 V). This is due to the slightly
46
lower electron mobility of TPBI (10ꢀ6 to 10ꢀ5 cm2 V sꢀ1
) than
1.9 ꢂ 10ꢀ5 cm2 V sꢀ1 for PBD.47 The deeper HOMO of PBD
(6.5 eV)48 might also be the reason for the higher efficiency for
device B. The holes should be more effectively blocked by PBD
than TPBI. Accordingly, the current efficiency (CE) and PE of
device B, 3.26 cd Aꢀ1 and 2.27 lm Wꢀ1, respectively, are higher
than those of device A as shown in Fig. 5. Device B achieved a
15138 | J. Mater. Chem., 2012, 22, 15136–15140
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Fig. 6 EQE-current density of device A: ITO/PEDOT : PSS 30 nm/NPB
30 nm/DPSF 30 nm/TPBI 30 nm/LiF/Al (O); device B: ITO/
PEDOT : PSS 30 nm/NPB 30 nm/DPSF 30 nm/TPBI : PBD (2 : 1)
30 nm/LiF/Al (:); device C: ITO/PEDOT : PSS 30 nm/NPB 30 nm/
DPSF : mCP (4 : 1) 30 nm/TPBI 30 nm/LiF/Al (B); device D: ITO/
PEDOT : PSS 30 nm/NPB 30 nm/DPSF : mCP (4 : 1) 30 nm/
TPBI : PBD (2 : 1) 30 nm/LiF/Al (C).
Fig.
5 CE and PE-current density curves of device A: ITO/
PEDOT : PSS 30 nm/NPB 30 nm/DPSF 30 nm/TPBI 30 nm/LiF/Al (O);
device B: ITO/PEDOT : PSS 30 nm/NPB 30 nm/DPSF 30 nm/
TPBI : PBD (2 : 1) 30 nm/LiF/Al (:); device C: ITO/PEDOT : PSS
30 nm/NPB 30 nm/DPSF : mCP (4 : 1) 30 nm/TPBI 30 nm/LiF/Al (B);
device D: ITO/PEDOT : PSS 30 nm/NPB 30 nm/DPSF : mCP (4 : 1)
30 nm/TPBI : PBD (2 : 1) 30 nm/LiF/Al (C).
Fig. 7 EL spectra of the devices: ITO/PEDOT : PSS 30 nm/NPB 30 nm/
DPSF 30 nm/TPBI 30 nm/LiF/Al (O); ITO/PEDOT : PSS 30 nm/NPB
30 nm/DPSF 30 nm/TPBI : PBD (2 : 1) 30 nm/LiF/Al (:); ITO/
PEDOT : PSS 30 nm/NPB 30 nm/DPSF : mCP (4 : 1) 30 nm/TPBI
30 nm/LiF/Al (B); ITO/PEDOT : PSS 30 nm/NPB 30 nm/DPSF : mCP
(4 : 1) 30 nm/TPBI : PBD (2 : 1) 30 nm/LiF/Al (C).
higher EQE of 4.99%. The device performances are listed in
Table 2.
To reduce exciton quenching in the emitting layer, doping is an
effective way. In device C, we introduced mCP as the diluter
because of its large gap which may prevent the energy transfer
from DPSF to the dopant and its deep HOMO to confine exci-
tons in the emitting layer. The doping ratio is 4 : 1 by weight of
DPSF : mCP, which is quite high for the content of DPSF, thus it
is quite easy to fabricate. This results in the reduced exciton
quenching effect in the emitter of DPSF. The maximum lumi-
nance of device C is 4430 cd mꢀ2, 64% higher than that of device
A. The efficiencies of device C are also accordingly increased to
2.75 cd Aꢀ1 and 1.73 lm Wꢀ1 from 1.97 cd Aꢀ1 and 1.23 lm Wꢀ1
for those of device A. The current density of device C is higher
than that of device A. This might be because the LUMO of mCP
is 2.6 eV49 lower than that of 2.3 eV for the emitter of DPSF and
much closer to the LUMO level of TPBI, therefore the electrons
should transfer more easily from the electron transporting layer
to the emitting layer.
By combining the advantages of device B and device C, we
prepared device D by using a mixture of TPBI and PBD as the
electron transporting and hole blocking layer, and DPSF doped
with mCP as the emitting layer. Thus we get not only a high
luminance of 4745 cd mꢀ2, but also the highest EQE of 5.41%
(Fig. 6), corresponding to nearly 100% of internal quantum
efficiency obtained by using doped DPSF as the emitter. All the
devices of A, B, C, and D show an electroluminescence at around
Table 2 The performance of devices with different electron transporting layers
Turn-on voltage
(V@1 cd mꢀ2
Max luminance
(cd mꢀ2
Device
)
)
Max PE (lm Wꢀ1
)
Max CE (cd Aꢀ1
)
Max EQE (%)
CIE (x, y)
A
B
C
D
4.5
4.1
4.5
3.5
2711
2632
4430
4745
1.23
2.27
1.73
2.54
1.97
3.26
2.75
3.24
3.41
4.99
4.82
5.41
0.15, 0.08
0.15, 0.08
0.15, 0.08
0.15, 0.08
This journal is ª The Royal Society of Chemistry 2012
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ꢁ
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435 nm (Fig. 7), and pure blue emission at CIE (0.15, 0.08) has
been achieved using DPSF as the emitter. The synthesized
spirobifluorene derivative shows a potential application as a
highly efficient pure blue emitter for OLEDs.
ꢁ
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properties in deep blue devices were investigated. The maximum
CE and PE of the DPSF device are 3.24 cd Aꢀ1 and 2.54 lm Wꢀ1
,
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Acknowledgements
This work was financially supported by the National Basic
Research Program of China (2009CB930504), NSFC (grants
61177020, 10934001, 60907015, and 11121091), and the Beijing
Municipal Science and Technology Project (Z101103050410002).
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