tert-Butylated spirobifluorene derivative incorporating triphenylamine groups: A deep-blue emitter with high thermal stability and good hole transport ability for organic light emitting diode applications

Article abstract of DOI:10.1016/j.dyepig.2015.03.027

Abstract A tert-butylated spirobifluorene derivative incorporating triphenylamine groups was synthesized starting from the readily available reagent biphenyl. Without any hole-transport layer, an unoptimized double-layer device exhibits excellent electroluminescent performances with a radiance of 3013 cd/m² at 9.8 V, a maximum electroluminescent efficiency of 1.71 cd/A, a maximal external quantum efficiency of 2.58% at a brightness of 4.8 cd/m², narrow full width at half-maximum (<54 nm), and blue emission with Commission Internationale de l'Eclairage coordinates of (0.152, 0.103), which is close to the standard for blue. Compared to the reported organic light emitting diodes materials, the molecule displays very high thermal stability. In addition, the bilayer device showed a greatly improved performance as compared to a trilayer device with NPB as hole-transporting layer, which indicated that the molecule possesses good hole transport ability and can be good candidate for hole-transporting layer in organic light emitting diodes.

Full text of DOI:10.1016/j.dyepig.2015.03.027

Accepted Manuscript
tert-Butylated spirobifluorene derivative incorporating triphenylamine groups: a deep-
blue emitter with high thermal stability and good hole transport ability for organic light
emitting diode applications
Haibo Xiao, Lei Ding, Dong Ruan, Benyan Li, Ning Ding, Dongge Ma
PII:
S0143-7208(15)00102-3
10.1016/j.dyepig.2015.03.027
DYPI 4708
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 December 2014
Revised Date: 24 March 2015
Accepted Date: 25 March 2015
Please cite this article as: Xiao H, Ding L, Ruan D, Li B, Ding N, Ma D, tert-Butylated spirobifluorene
derivative incorporating triphenylamine groups: a deep-blue emitter with high thermal stability and
good hole transport ability for organic light emitting diode applications, Dyes and Pigments (2015), doi:
10.1016/j.dyepig.2015.03.027.
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ACCEPTED MANUSCRIPT
tert-Butylated spirobifluorene derivative incorporating
triphenylamine groups: a deep-blue emitter with high
thermal stability and good hole transport ability for organic
light emitting diode applications
Haibo Xiao,a,* Lei Ding,a Dong Ruan,a Benyan Li,a Ning Ding,a Dongge Mab
aDepartment of Chemistry, Shanghai Normal University, Shanghai 200234, P. R. China; E-mail:xiaohb@shnu.edu.cn
b
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun 130022, P. R. China
Abstract: A tert-butylated spirobifluorene derivative incorporating triphenylamine groups was
synthesized starting from the readily available reagent biphenyl. Without any hole-transport layer, an
unoptimized double-layer device exhibits excellent electroluminescent performances with a radiance
of 3013 cd/m2 at 9.8V, a maximum electroluminescent efficiency of 1.71 cd/A, a maximal external
quantum efficiency of 2.58% at a brightness of 4.8cd/m2, narrow full width at half-maximum (< 54 nm),
and blue emission with Commission Internationale de l’Eclairage coordinates of (0.152, 0.103), which is
close to the standard for blue. Compared to the reported organic light emitting diodes materials, the
molecule displays very high thermal stability. In addition, the bilayer device showed a greatly
improved performance as compared to a trilayer device with NPB as hole-transporting layer , which
indicated that the molecule possesses good hole transport ability and can be good candidate for
hole-transporting layer in organic light emitting diodes
.
Keywords
: spirobifluorene derivative; electroluminescent property; blue emitter; thermal stability; hole
transport
1. Introduction
Organic light emitting diodes (OLEDs) have attracted much attention because of their potential application in
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full-color flat-panel displays, solid state lighting sources, and large-area and flexible electronic devices [1-4]. To
compete with liquid crystal displays (LCDs) in the display market, OLEDs must have high device performances,
good thermal stabilities, excellent color purity, and low production costs [5]. Low molecular weight small
molecules are generally vacuum-deposited as thin films in device fabrication. Therefore, film-forming properties
and their thermal stability are prerequisite to the performance and longevity of devices. Amorphous materials
possessing high glass transition temperatures (Tg) should be propitious to realize high stability OLEDs [6]. A well
known class of amorphous materials is the spiro-linked compounds developed by Salbeck’s group [7].
Intramolecular bond rotation and vibration in the spiro-linked unit are not possible and an increase in Tg is
anticipated. In the past two decades, a lot of satisfactory red and green emitters for OLEDs have been developed;
however, efficient and stable non-doped deep blue OLEDs with good CIE coordinates are still rare [1, 5, 8-9], and
a host:dopant system is commonly adopted to use the energy transfer from the host to the guest to obtain efficient
blue OLEDs. The doping process would increase the complexity and thus the cost of the device fabrication.
Besides, potential phase-separation in the host:dopant system was also found to render energy-transfer ineffective
[1]. Therefore, to realize commercial full-color displays, high-performance blue emission is a pressing concern.
Moreover, developing non-doped deep blue emission, which is defined as having a Commission Internationale de
l’Eclairage (CIE) y coordinate value below 0.15 along with an (x + y) value < 0.30[8-12], is of special significance
because such an emitter 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 cascading to lower energy fluorescent or phosphorescent
dopants [13-19]. The 9,9’-spirobifluorene unit appears to be a very promising building block for the construction
of high Tg blue-emitting materials for high performance blue electroluminescent devices [20-24]. L. X. Xiao et al.
reported a pure blue emitter of 2,7-di(phenanthren-9-yl)- 9,9’-spirobifluorene (DPSF) with CIEyꢀ0.08 and Tg=
178°C [25]. However, the high ionization potential of fluorene-based compounds produces a relatively large
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hole-injection barrier from the indium tin oxide (ITO) (5.0 eV) or ITO/poly(3,4-ethylenedioxythiophene)
(PEDOT) layer (5.2 eV). To overcome this problem, arylamines, such as triphenylamine and carbazole, have been
incorporated in fluorene-based derivatives to improve the hole injection and transport abilities and their resulting
OLEDs show enhanced device efficiency and stability [5, 8, 26-28]. Such molecular modification can also simplify
device structures and reduce the overall cost [1,8].
In this paper, we designed a tert-butylated spirobifluorene derivative incorporating triphenylamine groups
(SPF-BP, Scheme 1). Most spirobifluorene derivatives are synthesized by the use of expensive reagent
2-bromobiphenyl as the starting material, whereas SPF-BP can be prepared starting from the readily available
reagent biphenyl [29]. The photophysical, electrochemical, thermal properties, and electroluminescent
characteristics of SPF-BP in unoptimized OLED devices are initially evaluated.
<Scheme1>
2. Experiments
2.1. Reagents and methods
As shown in Scheme 1, triphenylamine groups were directly introduced into the 2’, 7’-position of
spirobifluorene by the Suzuki coupling method. The intermediate 2,7-di-tert-butyl-9,9’-spirobifluorene was
prepared according to our previous method [29]. Melting points were determined with an XT-4A apparatus and are
1
uncorrected. Mass spectral studies were carried out using VG12-250 mass spectrometer. H NMR and 13C NMR
spectra were obtained on a Bruker DRX 400 MHz spectrometer. Chemical shifts were reported in ppm relative to a
Me4Si standard. Elemental analyses were performed by Atlantic Microlab. Steady-state emission and excitation
spectra were recorded on Perkin Elmer LS55 instrument. Visible absorption spectra were determined on Perkin
Elmer Lambda 35 spectrophotometer. TGA/DTA measurements were performed at heating rate of 10°C min-1 in
the temperature range of 25-800 °C, under nitrogen flow of 10 mL min-1 by instrument Shimadzu DT-40.
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Approximately 10 mg of sample were placed in standard aluminium crucible (40 ꢀL). The cyclic voltammograms
were recorded with a computer controlled Zahner IM6e electrochemical workstation (Germany) using polymer
films on glassy carbon disks as the working electrodes, Pt wire as the counter electrode, and Ag/Ag+ (0.01 M) as
the reference. Tetrabutylammonium perchlorate (Bu4NClO4) 0.1 M was used as supporting electrolyte in dry
CH2Cl2.
2.2. Preparation of 2,7-bis-(4-(N,N-diphenylamino)phenyl-2’,7’-di-tert-butyl-9,9’-spirobifluorene (SPF-BP)
A
mixture
of
2,7-dibromo-2’,7’-di-tert-butyl-9,9’-spirobifluorene
(0.53g,
0.9
mmol),
4-(diphenylamino)phenylboronic acid (0.59g, 2 mmol), Pd(OAc)2 (0.016g, 0.071 mmol), PPh3 (0.042g, 0.16 mmol)
and K2CO3 (0.6g, 4.3 mmol) in dry THF (20 ml) were reacted under nitrogen atmosphere. After the mixture was
refluxed for 18 h, the mixture was cooled to room temperature. The solvent was evaporated and the residue was
subjected to column chromatography on silica gel with CH2Cl2 and petroleum ether as eluent (1:30, V/V) to afford
1
white solid (0.45g, 55%). Mp > 250 °C. H NMR (400MHz, CDCl3) δ (ppm) : 1.17 (s, 18H), 6.75 (s,2H), 6.93 (s,
2H), 7.02 (m, 8H), 7.08 (d, 8H,J = 8.0Hz), 7.24 (t, 8H, J = 8.0Hz), 7.34 (d, 4H, J = 8.0Hz), 7.40 (d, 2H, J = 8.0Hz),
7.62 (d, 2H, J = 8.0Hz), 7.74 (d, 2H, J = 8.0Hz), 7.92 (d, 2H, J = 8.0Hz); 13C NMR (100MHz, CDCl3) δ (ppm) :
151.0, 149.2, 147.9,147.3, 140.5, 139.6, 136.4ꢁ129.5, 128.0, 126.5, 125.2, 124.6, 124.2, 123.1, 122.5, 121.0, 120.4,
119.4, 66.7, 35.1, 35.8. Elemental analysis (%) calcd for C69H58N2: C 90.55, H 6.39, N3.06. Found: C 91.00, H
6.37, N 3.05. MS(ESI+): 914.5.
2.3 OLED fabrication and characterization
The EL devices were fabricated by vacuum deposition of the materials at a base pressure of 5×10-6 Torr onto
glass precoated with a layer of indium tin oxide (ITO) with a sheet resistance of 25ꢁ / square. Before deposition of
an organic layer, the clear ITO substrates were treated with oxgen plasma for 5 min. The deposition rate of organic
compounds was 0.9-1.1Ås-1. Finally, a cathode composed of cesium pivalate (2nm) and aluminum (100nm) was
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sequentially deposited onto the substrate in the vacuum of 10-5 Torr. The current density (I)-voltage (V)-luminance
characteristics of the devices was measured with a Keithley 2400 Source meter and PR655. All measurements
were carried out at room temperature under ambient conditions.
3. Results and discussion
3.1 Optical properties
Key photophysical and thermal data are summarized in Table 1. The maximum peak of the UV/vis absorption
spectrum of SPF-BP was observed at 371 nm on account of the π-π* transition of the conjugated backbone.
From Fig. 1, it can be seen that the emission spectra of SPF-BP (λex=365nm) have large red-shift following the
increase of solvent polarity. For example, the maximum emission wavelength of SPF-BP is red-shifted by 40 nm
from toluene (416 nm) to DMSO (456 nm). The solvatochromic behavior is accompanied by a decrease of the
fluorescence quantum yield when the polarity of the solvent increases. The origin of this quenching phenomenon is
attributed to the lowered energies of the ICT states but still remains unclear and its amplitude rather unpredictable.
The fluorescence quantum yield of SPF-BP measured in CH2Cl2 solution is 0.76.
The photoluminescence spectrum of SPF-BP in thin-solid film presents a maximum peak at
442nm (Fig. 2), which showed obvious red shift than that in CH2Cl2 (422 nm) and that in toluene
(416nm). The quantum yield of the SPF-BP thin film is 0. 65, which was lower than the value
measured in solution. The thin film of SPF-BP was then gradually heated from rt to 200 °C. As no
obvious change in the emission spectra was observed (Fig. 2), the emission color of SPF-BP appears to
be stable and hence very promising for blue OLED applications [4] .
< Table 1>
<Fig. 1>
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< Fig. 2>
3.2 Thermal properties
The thermal properties of SPF-BP were investigated by DTA and TGA in a nitrogen atmosphere (Fig. 3). A
crystallization exothermal peak at 224°C (Tc) was detected. SPF-BP showed a small endothermic peak at 266°C
,
,
which revealed a high melting point (Tm) [8, 30]. The decomposition temperature of SPF-BP was around 466°C
much higher than that of DPSF [25]. Although crystallization is adverse for the morphological stability of the
SPF-BP thin film, the Tc of SPF-BP is higher than the Tgs of the reported blue light emitters and hole transport
materials and high enough to secure the stability and lifetime performance of SPF-BP in OLED
devices
during repetitive and long-term operations [ 3, 31]. As expected, incorporation of spirobifluorene is beneficial to
raising the thermally stability of the molecule [25].
<Fig. 3>
3.3 Electrochemical Characterization
The electrochemical properties of SPF-BP were studied by cyclic voltammetry (CV) experiments at ambient
temperature in CH2Cl2 (0.1 M n-Bu4NClO4 as a supporting electrolyte). Fig. 4 shows the cyclic voltammetry of
SPF-BP. The onset oxidation potential was 0.582 V(vs Ag/Ag+). The HOMO energy level of SPF-BP was
-5.29eV, estimated according to the following expression [32]:
HOMO (eV) =-[Eoxonset+ 4.71]
The hight-lying HOMO energy level and reversible electrochemical oxidation indicate that hole-injection and
transport are favored in SPF-BP.
<Fig.4>
3.4 Electroluminescence properties
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Two types of OLED devices were fabricated in this study (conditions had not been optimized):
ITO/MoO3(10nm)/NPB(60nm)/ SPF-BP (30nm)/TPBi(35nm)/LiF(1nm)/Al (device 1) and ITO/MoO3(10nm)/
SPF-BP (75nm)/TPBi(35nm)/LiF(1nm)/Al (device 2). The structures of OLED devices are very similar to those
previously reported by Müllen et al. [28]. In these devices, ITO is used as the anode; NPB
(1,4-bis[(1-naphthylphenyl)amino]biphenyl)
is
used
as
the
hole-transporting
layer
(HTL);
TPBi(1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene) is used as the electron-transporting layer ; and LiF/Al is used
as the cathode [31, 33]. In type 2, SPF-BP was used as a hole transporting and emitting layer. It is well known that
double- and multi-layer OLEDs often show better device performance than single-layer OLEDs, because of better
charge confinement [34]. To verify whether better charge carrier balance is needed, a hole-transport layer NPB (60
nm) with HOMO of -5.40eV was inserted in type 1 device. Adding NPB also helps in blocking electrons from
reaching the anode. Fig. 5 shows the current density (I)- voltage (V)-luminance characteristics of the devices, and
Fig.6 is the P-I (power efficiency vs current density) characteristics. The EL performances of two devices are
<Fig.5>
<Fig.6>
excellent and the performance characteristics are summarized in Table 2. In device 1, maximum luminance,
maximum current and power efficiencies of respectively 1732 cd/m2 ( at 11.4 V), 1.60cd/A, 1.19 lm/W were
obtained. The device has a onset voltage (defined as the voltage required to obtain a luminance of 1 cd/m2) of 5.7V.
Device 2 shows higher maximum efficiencies (maximum current efficiency = 1.71 cd/A; maximum power
efficiency = 1.79 lm/W), lower onset voltage of 3.2 V, and higher maximum luminance of 3013 cd/m2 (at 9.8V ).
In addition, the current densities of device 2 are higher than those of device 1. For instance, at a driving voltage of
5 V, the current density of device 2 is 33.5 mA/cm2, while that of device 1 is only 3.0 mA/cm2. Since the
luminescence and efficiency for type 1 device is not increased but decreased, the addition of a hole transporting
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layer seems to be unnecessary [28]. The results indicate that SPF-BP could function as a hole transporting material
used in organic LEDs [28, 34-36].
<Table2>
The EL spectra of device 1 showed a main emission peak at 424 nm with a small shoulder peak at about 447nm
(Fig.6) [4]. In device 2, dual band electroluminescent emissions at 429 and 447nm (Fig.7) were observed. It can
be found that no long-wavelength emission presented in the thin-film PL spectra (Fig. 2). So the long-wavelength
emissions (the shoulder peak in device 1 and the emission peak at 447nm in device 2 ) in the EL spectra probably
result from the excimer or exciplex emission [37-41], which has often been observed in the EL spectra of OLEDs
and is mainly attributed to the emission of photons resulting from the direct irradiative recombination of holes and
electrons residing in two adjacent molecules [42]. The intensity of the long-wavelength emission became relatively
strong in device 2. It may be due to the difference of light extraction came from the two devices with different
thickness. The chromatic coordinates calculated from the electroluminescence spectrum in the CIE1931
chromaticity diagram are (0.166, 0.119) for device 1 and (0.151, 0.103) for device 2, both of which are close to the
NTSC coordinates for blue of (0.15, 0.07) [8, 43]. Electroluminescence is also characterized by a narrow spectral
distribution ( spectral half width is 46 nm for device 1 and 54nm for device 2, respectively), which is lower
than the typical value [9]. Moreover, the EL spectra show no difference between their performance over a wide
range of operation conditions (Fig. 7 and Fig. 8), suggesting good EL stability. It is worthwhile to note that the
overall performances for the SPF-BP based devices are very promising in OLEDs application [2, 4, 23].
<Fig.7>
<Fig.8>
4. Conclusions
In summary, we have synthesized a tert-butylated spirobifluorene derivative incorporating triphenylamine
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groups by a convenient method and investigated its applications in non-doped OLEDs. The blue-emitting
compound was used to fabricate double-layer blue-emitting EL device with very promising performances
comparable to those of trilayer blue-emitting device and exhibit hole-transport properties. The concept presented
could be extended potentially to further generate a new class of advanced materials for full-color OLED displays
or white emitting OLEDs.
Acknowledgments
This work was financially supported by the Natural Science Foundation of Shanghai City of China
(No. 15ZR1431400) and the Innovation Program of Shanghai Municipal Education Commission (No. 13ZZ105).
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[40] Kotchapradist P, Prachumrak N, Sunonnam T, Tarsang R, Namuangruk S, et al. N-coumarin derivatives as
hole-transporting emitters for high efficiency solution-processed pure green electroluminescent devices. Dyes
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Tables:
1. Table 1. Optical, Thermal, and Electrical Properties of SPF-BP.
2. Table2. EL Performance of Compound SPF-BP.
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Table 1. Optical, Thermal, and Electrical Properties of SPF-BP.
opt
a
a
λmax Abs
,
λmax,PL
HOMO
(eV)
LUMOb ΔEg
Td
Tm
(nm)
(nm)
(eV)
(eV)
(°C)
(°C)
373
416
-5.29
-2.34
2.95
545
266
opt
a measured in toluene. bdeduced from HOMO and ΔEg
(ΔEgopt=1240/λonset, λonsetis the onset wavelength of absorption spectra) .
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Table2. EL Performance of Compound SPF-BP.
Device
ELmax
(nm)
Brightness Efficiency
CIE (x, y)
(cd/m2)
(cd/A)
1.60
1
2
424
1732
(0.166,0.119)
(0.152,0.103)
429, 447
3013
1.71
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Figures’ Captions
1. Scheme1. Synthesis of SPF-BP
.
2. Fig. 1.One-photon absorption and fluorescence spectra of SPF-BP in solvents of different polarity (10-5 M).
3. Fig. 2. Normalized thin-film emission spectra of SPF-BP after annealing, in air, at different temperatures .
4. Fig.3. TGA and DTA curve of SPF-BP.
5. Fig.4.CV of SPF-BP in CH2Cl2 (0.1 M n-Bu4NClO4 as a supporting electrolyte).
6. Fig.5.The current density (I)-voltage (V)-luminance characteristics of the devices.
7. Fig.6.The current density (I)-voltage (V)-luminance characteristics of the devices.
8. Fig.7. EL spectrum of device 1.
9. Fig.8. EL spectrum of device 2.
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Scheme1. Synthesis of SPF-BP.
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1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
4000
3500
3000
2500
2000
1500
1000
500
CH3CN
CH2Cl2
DMSO
THF
Toluene
0
350
400
450
500
550
Wavelength (nm)
Fig. 1.One-photon absorption and fluorescence spectra of SPF-BP in solvents of different polarity (10-5 M).
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1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
200ꢂ 1h
120ꢂ 1h
60ꢂ 1h
rt
400
450
500
550
600
650
Wavelength(nm)
Fig. 2. Normalized thin-film emission spectra of SPF-BP after annealing, in air, at different temperatures .
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110
0
100
90
-20
80
70
60
50
40
30
20
-40
-60
-80
-100
TGA
DTA
100
200
300
400
500
600
700
800
Temperature (
ꢂ
)
Fig. 3. TGA and DTA curve of SPF-BP.
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15
10
5
0
-5
-10
-15
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Potential/V
Fig.4.CV of SPF-BP in CH2Cl2 (0.1 M n-Bu4NClO4as a supporting electrolyte).
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600
Device 1
Device 2
500
103
400
300
200
100
0
102
101
100
2
4
6
8
10
12
14
Voltage (V)
Fig.5. The current density (I)-voltage (V)-luminance characteristics of the devices.
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2.0
Device 1
Device 2
1.5
1.0
0.5
0.0
0
100
200
300
400
500
600
Current Density (mA/cm2)
Fig.6. The current density (I)-voltage (V)-luminancecharacteristics of the devices.
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1.0
3.5V
4.5V
5.5V
6.5V
0.8
7.5V
8.5V
9.5V
0.6
10.5V
11.5V
0.4
0.2
0.0
400 450 500 550 600 650 700 750
Wavelength (nm)
Fig.7. EL spectrum of device 1.
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1.0
3.5V
4.5V
5.5V
6.5V
7.5V
0.8
8.5V
9.5V
0.6
0.4
0.2
0.0
400 450 500 550 600 650 700 750
Wavelength (nm)
Fig.8. EL spectrum of device 2.
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Highlight
►Quite high thermal stability for organic light emitting diode
applications.
►
Good hole-injection and -transporting ability.
The molecule can be prepared starting from the readily available
reagent biphenyl.
►
►
The CIE coordinates is close to the standard for blue.
The EL performances are among the best comparing to recently
reported blue emitting devices.
►