Diarylmethylene-bridged triphenylamine derivatives encapsulated with fluorene: Very high Tg host materials for efficient blue and green phosphorescent OLEDs

Article abstract of DOI:10.1039/b927576b

Two bridged triphenylamine/fluorene hybrids, BTPAF1 and BTPAF2, were designed and synthesized through Friedel-Crafts reaction. Their thermal, electrochemical, electronic absorption and photoluminescent properties were fully investigated. Very high glass t

Full text of DOI:10.1039/b927576b

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www.rsc.org/materials | Journal of Materials Chemistry
Diarylmethylene-bridged triphenylamine derivatives encapsulated with
fluorene: very high Tg host materials for efficient blue and green
phosphorescent OLEDs†
a
Cong Fan, Yonghua Chen, Zuoquan Jiang, Chuluo Yang, Cheng Zhong, Jingui Qin and Dongge Ma
b
a
a
a
a
b
*
*
Received 5th January 2010, Accepted 3rd February 2010
First published as an Advance Article on the web 4th March 2010
DOI: 10.1039/b927576b
Two bridged triphenylamine/fluorene hybrids, BTPAF1 and BTPAF2, were designed and
synthesized through Friedel–Crafts reaction. Their thermal, electrochemical, electronic absorption and
photoluminescent properties were fully investigated. Very high glass transition temperatures (Tg) were
ꢀ
observed at 204 ꢀC for BTPAF1 and 211 C for BTPAF2, owing to the introduction of rigid
fluorene and bridged triphenylamine unit. The encapsulation of a fluorene unit at the para positions of
bridged triphenylamine greatly enhances their electrochemical stability. The linkage by the
quaternary carbon atom of the fluorene moiety (C-9) effectively prevents the extension of
p-conjugation of the bridged triphenylamine core, and consequently means that the compounds have a
high triplet energy of 2.86 eV. Phosphorescent organic light-emitting devices (PHOLEDs) fabricated by
using the two compounds as the hosts and the blue emitter bis[2-(40,60-difluorophenyl)pyridinato-
0
N,C2 ]iridium(III) picolate (FIrpic) as the guest exhibit good EL performances with a maximum current
efficiency of 20 cd Aꢁ1, a maximum power efficiency of 14 lm Wꢁ1, and a maximum external
quantum efficiency of 9.4%. Green electrophosphorescent devices by using green-emitter iridium(III)
fac-tris(2-phenylpyridine) [Ir(ppy)3] as guest and the two new compounds as the hosts display excellent
EL performances with a maximum current efficiency of 75 cd Aꢁ1, a maximum power efficiency of 60 lm
Wꢁ1, and a maximum external quantum efficiency of 19.5%. The device figures of merit, together
with the excellent morphological and electrochemical stabilities, make the new compounds ideal host
materials for PHOLEDs, especially for high-temperature applications of devices.
feedback from the guest to the host; (ii) larger energy gap (Eg)
between the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) than the phos-
phorescent guest, inducing deep charge trap on dopant emitter;3
(iii) high glass transition temperature (Tg) to acquire good film
morphology, lowering the risk of phase separation upon heating
and/or at high loads. However, such requirements become
challenged in designing host materials for blue phosphorescent
emitter with high triplet energy (ET > 2.62 eV).4 Up to now, the
major host materials suitable for blue PHOLEDs are based on
carbazole derivatives, such as 4,40-bis(9-carbazolyl)-2,20-
biphenyl (CBP),1b 1,3-bis(9-carbazolyl)benzene (mCP),4b
3,5-bis(9-carbazolyl)tetraphenylsilane (SimCP),5a and 9-(4-tert-
butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), etc.5b
Triphenylamine (TPA) is well known for its high triplet energy
of 3.04 eV and good hole-transporting ability.6,7 However, TPA
derivatives usually exhibit undesired morphological stabilities
due to the nonrigid TPA molecular configuration. In addition,
TPA derivatives may polymerize during the device operation due
to the existence of an electrochemical active position.8 The
above-mentioned factors hamper the application of triphenyl-
amine derivatives in OLEDs.9 In the search for new materials
capable of hosting blue-emitting phosphors, we have designed
two bridged triphenylamine/triphenylsilane hybrids with high
triplet energy (2.95 eV).10 The introduction of triphenylsilane
moieties can block the electrochemically active sites and
enhance their electrochemical stability. However the bulky
Introduction
Organic light-emitting diodes (OLEDs) have shown promising
potential applications in next-generation flat-panel displays and
light illumination. Among these devices, tremendous efforts have
been made in the development of highly efficient phosphorescent
organic light-emitting diodes (PHOLEDs) because they can
approach 100% internal quantum efficiency in theory by har-
vesting both electro-generated singlet and triplet excitons for
emission.1 To fabricate good-performance PHOLEDs, a phos-
phorescent heavy metal complex as triplet emitter is usually
doped into a host matrix to reduce aggregation quenching and
triplet–triplet annihilation etc.,2 and thus the choice of host
material is of equal importance to triplet dopant. Generally, host
material should fulfill the following requirements: (i) higher
triplet energy than phosphorescent guest, forbidding energy
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan, 430072, People’s
Republic of China. E-mail: clyang@whu.edu.cn; Fax: +86 27-68756757;
Tel: +86 27-68756101
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, People’s Republic of China. E-mail: mdg1014@ciac.
jl.cn; Fax: +86 431-85262357; Tel: +86 431-85262357
† Electronic supplementary information (ESI) available: UV-vis
absorption and PL spectra of BTPAFs in film, and power
efficiency-external quantum efficiency versus current density curve for
devices A–D. See DOI: 10.1039/b927576b
3232 | J. Mater. Chem., 2010, 20, 3232–3237
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non-conjugated triphenylsilane groups could be disadvantageous
to the charge transporting ability of the compounds. In this
article, we report two new diarylmethylene-bridged triphenyl-
amine/fluorene hybrids, BTPAF1 and BTPAF2, by connecting
the active para positions of the diarylmethylene-bridged triphe-
nylamine with the sp3-hybridized carbon atom at the C-9 posi-
tion of the fluorene unit. We choose fluorene to replace
triphenylsilane as building block because of its ambipolar carrier
transporting properties, high triplet energy (2.95 eV), and high
photoluminescent efficiency.11 Accordingly, we anticipate that
the combination of bridged triphenylamine and rigid fluorene
unit could impart them with good charge transporting ability,
high triplet energy, and high thermal and morphological
stabilities, and therefore the hybrids could be efficient hosts for
phosphorescent triplet emitters.
Fig. 1 TGA and DSC (inset) thermograms of BTPAF1 and BTPAF2.
can be attributed to a p–p* transition of the bridged triphenyl-
amine core, while the absorption around 268 nm can be assigned
to a p–p* transition of the fluorene moiety. The characteristic
peaks indicate that the linkage by the quaternary carbon atom of
the fluorene moiety (C-9) effectively prevents the extension of
p-conjugation of the bridged triphenylamine core. The optical
band gaps (Eg) determined from the onset of the film absorption
spectra are 3.10 eV.
Results and discussion
Synthesis and characterization
The diarylmethylene-bridged triphenylamine/fluorene hybrids,
namely BTPAF1 and BTPAF2, were readily prepared through
Friedel–Crafts reaction of diarylmethylene-bridged triphenyl-
amine with 9-aryl-9H-fluorenol under the mediation of
BF3$OEt2 in good yields (Scheme 1). The two compounds were
The new compounds exhibit ultraviolet emission at ca. 373 nm
in the CH2Cl2 solution, and 376 nm in film (see ESI, Fig. S1†).
The tiny bathochromic shift of film emission with respect to
solution reveals that the rigid fluorene moiety as well as the
peripheral aryl group on the bridgehead carbon atom can act as
effective spacers to suppress the intermolecular aggregation. The
inset of Fig. 2 depicts the phosphorescence spectra of the two
compounds measured from a frozen toluene matrix at 77 K.
Their triplet energies (ET) are determined to be ca. 2.86 eV by the
highest energy vibronic sub-band of the phosphorescence
spectra. This value is high enough to host red, green and blue
phosphorescent emitters, such as the common blue emitter
1
fully characterized by H NMR, 13C NMR, mass spectrometry
and elemental analysis (see Experimental).
Thermal properties
The good thermal stability of the compounds is indicated by the
high decomposition temperatures (Td, corresponding to 5%
weight loss) of 491 ꢀC for BTPAF1 and 496 ꢀC for BTPAF2
through thermogravimetric analysis (Fig. 1). Their glass transi-
tion temperatures (Tg) appear at 204 ꢀC for BTPAF1 and 211 ꢀC
for BTPAF2, in the differential scanning calorimetry (DSC)
thermograms (inset of Fig. 1). The Tg values are significantly
higher than previous reported triphenylamine/triphenylsilane
hybrids (168–173 ꢀC),10 which should be attributed to the
introduction of rigid fluorene unit. As a consequence, the new
compounds can form highly thermally durable and morpholog-
ically stable thin-film, an essential property for OLEDs upon
thermal evaporation.
0
bis[2-(40,60-difluorophenyl)pyridinato-N,C2 ]iridium(III) picolate
(FIrpic, 2.62 eV) and green emitter iridium(III) fac-tris(2-phe-
nylpyridine) (Ir(ppy)3, 2.42 eV). All data are summarized in
Table 1.
Electrochemical properties and DFT calculations
The electrochemical behaviors of the new compounds were
examined by cyclic voltammetry (CV). They all exhibit a revers-
ible oxidation process. Noticeably, upon repeated scanning, both
show reproducible and reversible oxidation behavior (Fig. 3).
Photophysical properties
Fig. 2 shows the absorption and photoluminescence spectra of
the new compounds. The absorptions in the range of 300–350 nm
Fig. 2 UV-vis absorption spectra, PL spectra of BTPAF1 and BTPAF2
in CH2Cl2 solution, and their phosphorescence spectra in toluene at 77 K
(inset).
Scheme 1 Synthesis of BTPAF1 and BTPAF2.
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Table 1 Optical and thermal properties of BTPAF1 and BTPAF2
BTPAF1
BTPAF2
Tg/Tm/Td(ꢀC)
labs
204/374/491
325,268/326,269
374/376
5.19/1.89
2.86
211/380/496
325,268/328,270
373/376
5.15/1.85
2.86
,
max (nm) sola/filmb
lPL, max/nm sola/filmb
HOMOc/LUMO (eV)d
ET (eV)e
a
b
Measured in CH2Cl2 solution. Measured in film. Determined from
c
d
the onset of oxidation potentials. Deduced from HOMO and Eg.
Triplet energy measured in toluene in 77 K.
e
This contrasts with the irreversible oxidation process of the
pristine triphenylamine.8 Such electrochemical stability can be
attributed to the blocking of three active sites of para-phenyl
positions by t-butyl and fluorene moieties, and should benefit the
stability of their electroluminescence devices. The HOMO energy
levels determined from the onsets of the oxidation potentials are
ꢁ5.19 eV for BTPAF1 and ꢁ5.15 eV for BTPAF2 (relative to
vacuum level), respectively, which are higher than that of most
widely used NPB (5.4 eV),12 suggesting no hole-injection barriers
from NPB to the compounds.
Fig. 4 Spatial distributions of the calculated HOMO and LUMO energy
levels of BTPAF1 and BTPAF2.
(TAZ) was utilized as electron transporting as well as hole
blocking material; FIrpic or Ir(ppy)3 doped in host was used as
the emitting layer, with optimized doping levels of FIrpic and
Ir(ppy)3 at 6% and 9%, respectively; MoO3 and LiF served as
hole- and electron-injecting layer, respectively. The EL spectra of
the devices are displayed in Fig. 6. The luminance-current density-
current efficiency (L-J-hc) characteristics of the devices are shown
in Fig. 7, and the device data are summarized in Table 2.
The geometrical and electronic properties of the compounds
were studied by density functional theory (DFT) calculations
using the B3LYP hybrid functional. As shown in Fig. 4, the
HOMO orbitals of the two molecules are mainly located on the
bridged triphenylamine moiety, while their LUMO orbitals are
exclusively distributed on the fluorene moiety. The complete
separation between HOMO and LUMO levels is favourable to
the efficient hole- and electron-transporting properties and the
prevention of reverse energy transfer, and thus to benefit to the
EL performance of the PHOLEDs.13
The EL spectra show the typical emissions from the blue
ꢀ
emitter FIrpic with the Commission Internationale de l’Eclairage
(CIE) coordinates of (0.16, 0.32) for devices A and B, and the
green emitter Ir(ppy)3 with CIE coordinates of (0.35, 0.62)
for devices C and D. The four devices turn on at voltages of
3.5–4.3 V, which are not very low. This may relate to the large
electron injection barrier from the electron transporting TAZ
layer to the emissive layer. Device A with BTPAF1 as host
exhibits a maximum current efficiency of 20 cd Aꢁ1, a maximum
power efficiency of 14 lm Wꢁ1, and a maximum external quantum
efficiency of 9.4%, and those data are 19 cd Aꢁ1, 14 lm Wꢁ1, and
8.9% for device B with BTPAF2 as host. These efficiencies are
comparable with or even better than the blue devices using FIrpic
as blue emitter under the similar device structures.14 Both devices
C and D reveal highly efficient green electrophosphorescence,
with maximum current efficiency, power efficiency and external
quantum efficiency of 75 cd Aꢁ1, 60 lm Wꢁ1 and 19.5% for device
C, and 78 cd Aꢁ1, 54 lm Wꢁ1 and 18.8% for device D, respectively.
These efficiencies are comparable with the best green phospho-
rescent OLEDs we recently reported.2c,10 It is noteworthy that all
Phosphorescent OLEDs
To evaluate the performance of the two compounds as host
materials for blue and green phosphorescent emitters, we fabri-
cated the devices with the following configurations (Fig. 5):
device A–B: ITO/NPB (40 nm)/TCTA (5 nm)/host:FIrpic
(20 nm)/TAZ (40 nm)/LiF (1 nm)/Al (100 nm); device C–D:
ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/host:Ir(ppy)3
(20 nm)/TAZ (40 nm)/LiF (1 nm)/Al (100 nm). In these
devices, 1,4-bis(1-naphthylphenylamino)-biphenyl (NPB) was
used as the hole-transporting material; 4,40,400-tri(N-carbazolyl)
triphenylamine (TCTA) was used as exciton blocking material;
(3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
Fig. 5 Device structures and energy level diagram of materials used in
Fig. 3 Cyclic voltammogram of the compounds vs. Ag/AgCl.
3234 | J. Mater. Chem., 2010, 20, 3232–3237
the devices.
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Table 2 Summary of device performance
Host
BTPAF1
BTPAF2
Dopant
Device
FIrpic
A
4.1
6988
9.4
20
Ir(ppy)3
C
3.7
27 528
19.5
75
FIrpic
B
4.3
10 223
8.9
19
Ir(ppy)3
D
3.5
39 411
18.8
78
Vturn-on/V
L max/cd mꢁ2
a
E.Q.E max (%)b
c
L.E max/cd Aꢁ1
d
P.E max/lm Wꢁ1 14
60
(0.16, 0.32) (0.35. 0.62) (0.17, 0.33) (0.34, 0.62)
14
54
CIE (x, y)
a
b
Maximum luminance.
Maximum external quantum efficiency.
c
Maximum current efficiency. d Maximum power efficiency.
Fig. 6 EL spectra of devices A–D.
the devices display significantly higher luminance than the
similar devices using triphenylamine/triphenylsilane hybrids as
host materials in our previous report.10 For example, device B
shows a maximum luminance of 10 223 cd mꢁ2 at 14 V, which is
remarkably enhanced compared to the device hosted by its
analogue of triphenylamine/triphenylsilane (1865 cd mꢁ2 at
16 V). This could be attributed to the improvement of carrier
transporting properties of hosts after the replacement of fluorene
for triphenylsilane.
the hybrids. Devices with the new compounds as host materials
show maximum external quantum efficiency as high as 9.4% for
blue and 19.5% for green electrophosphorescence. The high
device efficiencies could be attributed to the high triplet energy of
the host, and complete spatial separation of HOMO and LUMO
energy levels. The good device performance, together with the
excellent morphological and electrochemical stability, makes the
new compounds practical host materials for PHOLEDs.
Experimental
Conclusions
General Information
In conclusion, we have designed and synthesized two new host
materials for PHOLEDs by the combination of bridged-triphe-
nylamine and fluorene units. The introduction of a fluorene
moiety at the para positions of bridged triphenylamine greatly
improves their thermal and morphological stability, as well as
their electrochemical stability. The non-conjugated linkage
between the two moieties also ensures the high triplet energies of
1H NMR and 13C NMR spectra were measured on a Varian
Unity 300 MHz spectrometer using CDCl3 as the solvent.
Elemental analyses of carbon, hydrogen, and nitrogen were
performed on a Vario EL-III microanalyzer. MALDI-TOF mass
spectrometric measurement was performed on Bruker Biflex III
MALDI TOF instrument. Differential scanning calorimetry
(DSC) was performed on a NETZSCH DSC 200 PC unit at
a heating rate of 10 ꢀC minꢁ1 from 30 to 600 ꢀC under a nitrogen
atmosphere. Thermogravimetric analysis (TGA) was undertaken
with a NETZSCH STA 449C instrument. The thermal stability
of the samples under nitrogen atmosphere was determined by
measuring their weight loss, heated at a rate of 10 ꢀC minꢁ1 from
25 to 600 ꢀC. UV-Vis absorption spectra were recorded on
Shimadzu UV-2550 spectrophotometer. PL spectra were recor-
ded on Hitachi F-4500 fluorescence spectrophotometer. Cyclic
voltammetric measurements were carried out on a computer-
controlled EG&G Potentiostat/Galvanostat model 283 using
tetrabutylammonium hexafluorophosphate (0.1 M) as support-
ing electrolyte in freshly distilled CH2Cl2 solution and conven-
tional three-electrode cell with a Pt work electrode of 2 mm
diameter, a platinum-wire counter electrode, and a Ag/AgCl
reference electrode.
Preparation of the compounds
7-t-Butyl-5,5,9,9-tetraphenyl-13b-aza-naphtho[3,2,1-de]anthracene
(BTPA1) and 7-t-butyl-5,5,9,9-tetra-p-tolyl-13b-aza-naphtho-
[3,2,1-de]anthracene (BTPA2) were prepared according to our
previous report.10 All reagents commercially available were used
as received unless otherwise stated. The solvents (THF, diethyl
ether, dichloromethane) were purified by conventional procedures
and distilled under dry argon before using. All reactions were
carried out using Schlenk techniques in an argon atmosphere.
Fig. 7 L-J-hc characteristics (a) for the FIrpic-based devices A and B; (b)
for the Ir(ppy)3-based devices C and D.
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Synthesis of BTPAF1. A solution of BF3$OEt2 (0.15 ml,
1.2 mmol) in 2 ml of dichloromethane was added dropwise to
a solution of 9-phenyl-9H-fluoren-9-ol (0.26 g, 1 mmol) and
BTPA1 (0.31 g, 0.5 mmol) in 10 ml dichloromethane. The
mixture was stirred at room temperature under argon atmo-
sphere for 3 h. Ethanol (5 ml) and water (15 ml) was successively
added to quench the reaction. The mixture was extracted with
dichloromethane, and then the combined dichloromethane
solution was washed with brine and dried over with anhydrous
sodium sulfate. After removal of the solvent, the crude product
was purified by silicon gel chromatography using 2 : 1 (v : v)
petroleum–chloroform as the eluent to afford the product as
Computational details
The geometrical and electronic properties of the compounds
were performed with the Gaussian 03 program package. The
calculation was optimized by means of the B3LYP (Becke three
parameters hybrid functional with Lee-Yang-Perdew correlation
functionals) with the 6-31G(d) atomic basis set 32. Molecular
orbitals were visualized using Gaussview.
Acknowledgements
We thank the National Natural Science Foundation of China
(Project Nos. 90922020, 50773057 and 20621401), and the
National Basic Research Program of China (973 Program-
2009CB623602, 2009CB930603) for financial support.
1
white solid. Yield: 86%. H NMR (300 MHz, CDCl3, d): 7.67
(d, J ¼ 7.2 Hz, 4H), 7.27–7.25 (m, 6H), 7.19–7.02 (m, 24H),
6.99–6.81 (m, 16H), 6.74–6.62 (m, 4H), 1.04 (s, 9H); 13C NMR
(75 MHz, CDCl3, d): 151.66, 151.46, 146.86, 146.22, 146.06,
140.32, 140.11, 138.92, 137.51, 134.92, 131.87, 131.54, 131.16,
129.31, 128.11, 127.87, 127.71, 127.64, 127.42, 126.45, 126.23,
125.53, 125.35, 120.26, 120.11, 116.51, 65.04, 57.64, 34.48, 31.54.
Anal. Calcd. for C86H63N (%): C, 93.02; H, 5.72; N, 1.26. Found:
C, 93.00; H, 5.92; N, 1.15. MS (MALDI-TOF) m/z 1109.8 [M+].
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Device fabrication and measurement
The hole-injection material MoO3, hole-transporting material
NPB
(1,4-bis(1-naphthylphenylamino)-biphenyl),
exciton
blocking material TCTA (4,40,400-tri(N-carbazolyl)triphenyl-
amine), hole-blocking and electron-transporting material TAZ
(3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole)
were commercially available. Commercial indium tin oxide
(ITO) coated glass with sheet resistance of 10U/square was used
as the starting substrates. Before device fabrication, the ITO
glass substrates were pre-cleaned carefully and treated by UV/O3
for 2 min. Then the sample was transferred to the deposition
system. 10 nm of MoO3 was firstly deposited to ITO substrates,
followed by 80 nm NPB, 5 nm TCTA, emissive layer, 40 nm
TAZ. Finally, a cathode composed of 1 nm of lithium fluoride
and 100 nm of aluminium was sequentially deposited onto the
substrates in the vacuum of 10ꢁ6 Torr to construct the device.
The I–V-B of EL devices was measured with a Keithey 2400
Source meter and a Keithey 2000 Source multimeter equipped
with a calibrated silicon photodiode. The EL spectra were
measured by JY SPEX CCD3000 spectrometer. All measure-
ments were carried out at room temperature under ambient
conditions.
3236 | J. Mater. Chem., 2010, 20, 3232–3237
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