Multifunctional electron-transporting indolizine derivatives for highly efficient blue fluorescence, orange phosphorescence host and two-color based white OLEDs

Article abstract of DOI:10.1039/c2jm14904d

In this work, derivatives of indolizine are first used as electron-transporting host materials for hybrid fluorescence/phosphorescence white organic light-emitting devices (F/P-WOLED). Of the indolizine derivatives, a blue fluorescent material BPPI (3-(4,4′-biphenyl)-2-diphenylindolizine) was found to have: (1) blue emission with high quantum yields, (2) good morphological and thermal stabilities, (3) electron-transporting properties, and (4) a sufficiently high triplet energy level to act as a host for red or yellow-orange phosphorescent dopants. The multifunctional BPPI enables adaptation of several simplified device configurations. For example, a non-doped blue fluorescent device exhibits good performance with an external quantum efficiency of 3.16% and Commission Internationale de l'Eclairage coordinates of (0.15, 0.07). A high-performance orange phosphorescent device was found to have a high current efficiency of 23.9 cd A^-1. Using BPPI, we also demonstrate a F/P-WOLED with a simplified structure, stable emissions and respectable performance (current and external quantum efficiencies of 17.8 cd A^-1 and 10.7%, respectively).

Full text of DOI:10.1039/c2jm14904d

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PAPER
Multifunctional electron-transporting indolizine derivatives for highly
efficient blue fluorescence, orange phosphorescence host and two-color based
white OLEDs
b
a
Jing Wan,†ac Cai-Jun Zheng,†ab Man-Keung Fung,b Xiao-Ke Liu,ac Chun-Sing Lee and Xiao-Hong Zhang
*
*
Received 30th September 2011, Accepted 22nd November 2011
DOI: 10.1039/c2jm14904d
In this work, derivatives of indolizine are first used as electron-transporting host materials for hybrid
fluorescence/phosphorescence white organic light-emitting devices (F/P-WOLED). Of the indolizine
derivatives, a blue fluorescent material BPPI (3-(4,40-biphenyl)-2-diphenylindolizine) was found to have:
(1) blue emission with high quantum yields, (2) good morphological and thermal stabilities, (3)
electron-transporting properties, and (4) a sufficiently high triplet energy level to act as a host for red or
yellow-orange phosphorescent dopants. The multifunctional BPPI enables adaptation of several
simplified device configurations. For example, a non-doped blue fluorescent device exhibits good
performance with an external quantum efficiency of 3.16% and Commission Internationale de
l’Eclairage coordinates of (0.15, 0.07). A high-performance orange phosphorescent device was found to
have a high current efficiency of 23.9 cd Aꢀ1. Using BPPI, we also demonstrate a F/P-WOLED with
a simplified structure, stable emissions and respectable performance (current and external quantum
efficiencies of 17.8 cd Aꢀ1 and 10.7%, respectively).
layer should not only show highly efficient blue fluorescence, but
also sensitize the green, red or yellow-orange phosphors.
However, as the structure of OLED devices has become
increasingly complicated, the material requirements have also
become more critical. For these new F/P-WOLEDs, the host
materials play a significant role and generally should have high
blue luminous efficiencies and suitable triplet state levels. So far,
there are limited numbers of molecular systems fulfilling these
requirements.2–11
Introduction
Lighting takes up a significant part of the energy resources of the
world, with a large share still consumed by inefficient incandes-
cent lamps. White organic light-emitting diodes (WOLEDs)
show promise in future ambient lighting applications due to their
favorable properties, including homogenous large-area emission,
good color rendering and potential realization on flexible
substrates, which also opens new areas in lighting design, such as
light ceilings or luminous objects of almost any shape.1 At
present, WOLEDs already find application in color flat panel
displays; however, in the illuminant and lighting domains, the
performance characteristics (e.g., efficiency and lifetime) of
WOLEDs are still not adequate.
Recently, we have found that indolizine derivatives, which
have never been applied to OLEDs, possess the following novel
properties:12,13 (1) blue emissions with high quantum yields, (2)
electron-transporting properties, and (3) sufficiently high triplet
energy (ET) levels. These demonstrate that indolizine derivatives
have the potential to be good host materials in F/P-WOLEDs.
Therefore, in this work, we designed, synthesized and charac-
terized the following indolizine derivatives: 1,4-di-30-(20-phenyl-
indolizyl)-benzene (PPI), 1,4-di-30-(20-naphthalenylindolizyl)-
benzene (PNI), 3-(4,40-biphenyl)-2-diphenylindolizine (BPPI),
3-(4,40-biphenyl)-2-dinaphthalenylindolizine (BPNI), 1,3,5-tri[4-
30-(20-naphthalenylindolizyl)phenyl]benzene (p-TPPNI) and 1,3,5-
tri[3-30-(20-phenylindolizyl)phenyl]benzene (m-TPPPI), with the
aim of clarifying the relationships between their properties and
molecular structures. Each of the above compounds consists of
two to three indolizine cores linked together with different
phenyl bridging groups (Scheme 1). The phenyl linking groups
are expected to enhance the light-emitting efficiency by providing
an extended conjugated structure and increasing the number of
Materials are vital components of OLEDs. At present,
searching for high performance materials has gained increasing
attention, driving the further development of OLEDs. In 2006,
Forrest et al. proposed a hybrid fluorescent/phosphorescent
route to realize efficient WOLEDs.2 In this design, the emitting
aNano-organic Photoelectronic Laboratory and Key Laboratory of
Photochemical Conversion and Optoelectronic Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing, 100190, China. E-mail: xhzhang@mail.ipc.ac.cn
bCenter of Super-Diamond and Advanced Films (COSDAF) and
Department of Physics and Materials Science, City University of Hong
Kong, Hong Kong SAR, China. E-mail: apcslee@cityu.edu.hk
cGraduate University of Chinese Academy of Sciences, Beijing, 100039,
China
† These authors contributed equally to this work
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Scheme 2 Synthesis of the indolizine derivatives. Reagent: Pd(PPh3)4,
NaOH (aq. 2M), DME, 40 h, 85 ꢁC.
2-phenylindolizine and 2-naphthalenylindolizine with excess
iodine.13 In addition, the dibromo-aromatic compounds and
tribromo-aromatic compounds reacted readily with n-butyl
lithium in dry tetrahydrofuran at ꢀ78 ꢁC to form the corre-
sponding difunctional or trifunctional lithium reagents, result-
ing in satisfactory yields (35%–60%) of the corresponding
boronic acids (3–6) when treated with ethereal trimethyl borate
at ꢀ78 ꢁC, with subsequent acidification.18 Then, the Suzuki-
cross coupling of 3-iodo-2-phenylindolizine (1) and 3-iodo-2-
naphthalenylindolizine (2) with the corresponding boronic
acids resulted in all 6 blue fluorescent materials producing
moderate to ideal yields.13,19
Scheme 1 Molecular structures of the indolizine derivatives.
indolizine units. In addition, bulky phenyl groups can further
increase steric hindrance and cause molecular non-planarity and
result in better thermal stability. Due to the electron-deficient
nitrogen-containing heterocyclic moieties in the indolizine cores,
the materials have natural electron-transporting properties.14,15,16
Meanwhile, the attachment of indolizine groups at the 2-position
also leads to amorphous materials.12a Of these materials, we
found that BPPI has high blue fluorescent efficiency, reasonable
electron-transport properties, good thermal stability, and suit-
able triplet state energy levels. Based on this multifunctional
material, blue fluorescent, orange phosphorescent devices as well
as F/P-WOLEDs with simplified structures were fabricated and
shown to have remarkable performances.
All the blue emitters were fully characterized by 1H NMR, fast
atom bombardment or electron impact-mass spectrometry
(FAB-MS or EI-MS), and were consistent with the proposed
structures. All the data are given in the Experimental section.
Thermal properties
Thermal properties of the newly synthesized compounds were
examined by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) in a nitrogen atmosphere at a scan-
ning rate of 10 K minꢀ1. All the materials revealed good thermal
stabilities, with a 5% weight loss at temperatures (TD) exceeding
385 ꢁC, as determined by TGA measurements. Key thermal
properties including TD, melting temperatures, Tm, and glass-
transition temperatures, Tg, of the compounds are summarized
in Table 1. It can be seen that TD can be increased by replacing
the phenyl group with a naphthyl group at the 2-position of the
indolizine unit or by increasing the size of the linking group
between the indolizine units. The compounds (except BPPI and
BPNI) also show Tg values greater than 170 ꢁC. Probably due to
their bulky structures, Tg in BPPI and BPNI were not observed.
The good thermal properties of the compounds are advanta-
geous for application as host materials in OLEDs. This is
because in a morphologically stable amorphous host material,
the emitter molecules are homogenously diluted, thus minimizing
concentration quenching.20
Results and discussion
Synthesis and characterization
As shown in Scheme 2, the 6 blue fluorescent materials were
synthesized from iodo-substituted species, 3-iodo-2-phenyl-
indolizine and 3-iodo-2-naphthalenylindolizine, which were
obtained as reported in literature.14,17 Firstly, 2-bromoaceto-
phenone and 2-bromoacetonaphthone were successfully
obtained by the bromination of acetophenone and acetonaph-
thone with NBS.17 Secondly, 2-phenylindolizine and 2-naph-
thalenylindolizine were readily prepared in good yields
(67% and 87%) from 2-bromoacetophenone and 2-amino-
pyrididine and the corresponding 2-bromoacetophenones.13
Thirdly, 3-iodo-2-phenylindolizine (1) and 3-iodo-2-naph-
thalenylindolizine (2) were easily obtained by the iodination of
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Table 1 Thermal, photophysical and electrochemical properties as well as energy levels of the six indolizine derivatives
labs
,
lem (Ff),
lf,
lp (ET),
Tm/TD/Tg [ꢁC]a CH2Cl2 [nm] b CH2Cl2 [nm (%)] c film [nm]c 77 K [nm (eV)]d DEg [eV]e EOX [eV]f HOMO/LUMO [eV]g
PPI
PNI
BPPI
BPNI
280/389/175
292/408/184
348/414/—
326/463/—
334.3
337.8
338.3
343.4
330.2
314.8
411.2 (53.02%)
425.4 (43.08%)
414.4 (83.00%)
423.6 (66.15%)
410.7 (41.70%)
399.4 (37.93%)
435
444
430
437
398
404
475 (2.62)
511 (2.42)
478 (2.59)
506 (2.45)
503 (2.47)
507 (2.45)
3.20
3.17
3.19
3.17
3.23
3.21
1.263
1.285
1.285
1.293
1.293
1.349
5.52/2.32
5.54/2.37
5.54/2.35
5.55/2.38
5.55/2.32
5.61/2.40
p-TPPNI 294/507/187
m-TPPPI 279/492/186
a
TD: Decomposition temperatures, observed in a thermogravimetric experiment with a heating rate of 10 K minꢀ1 in a nitrogen atmosphere. Tm:
b
c
Melting temperatures, Tg: Glass transition temperatures. labs: Absorption maximum. lem: Emission maxima were measured both in solution and
in thin films. The quantum yields (Ff) were measured in dichloromethane solution, see ref. [21]. d lp: Phosphorescence emission peak and ET: Triplet
energy measured in 2-methyltetrahydrofuran at 77 K. DEg: The band-gap energies were estimated from the optical absorption edges of solid films
e
f
on quartz substrates. EOX: Electrochemical first oxidation potentials with respect to the ferrocene/ferrocenium redox potential, which is 0.4825 eV
g
vs. SCE, determined by differential pulse voltammetry in a 0.1 M solution of TBAPF6 in dichloromethane. HOMO ¼ ꢀEOX (vs SCE) ꢀ ESCE and
LUMO ¼ HOMO + DEg, the energy level of SCE (ESCE) is 4.74 eV vs. vacuum level.22
Photophysical properties
groups between the indolizine units are star-shaped four benzene
ring systems, both Ff and lem decrease. This is due to the fact that
the star-shaped bridging group would hinder the conjugation
between each indolizine unit.
Absorption and emission spectra of these blue emitters were
studied in dichloromethane and solid film. Table 1 indicates that
the absorptions are only mildly sensitive to the molecular
structure. In the first four compounds, slight red shifts in the
absorption (labs value) were observed when the extent of
molecular conjugation was increased by replacing the phenyl
groups with naphthyl groups at the 2-position of the indolizine
units (PNI > PPI and BPNI > BPPI) or by using a diphenyl
instead of a phenyl group for bridging the indolizine units (BPPI
> PPI and BPNI > PNI). On the other hand, using a star-shaped
four benzene bridging group would hinder conjugation between
indolizine units, resulting in blue-shifted absorption in p-TPPNI
and m-TPPPI.
Phosphorescence spectra of these compounds were also
studied (measured at 77 K) (Fig. 1c). The ET levels of the
compounds were estimated by taking the highest energy peak of
the phosphorescence as the vibronic 0–0 transition energy of
T1 / S0. As shown in Table 1, the ET levels of all 6 compounds
are greater than 2.4 eV. These suggest that the materials should
have good potential for application as host materials for red or
orange phosphorescent dopants.
Electrochemical properties
All six compounds show blue or near ultraviolet fluorescence
in both solution and solid states. Emission peaks of the
compounds show similar trends to the absorption. Key photo-
physical parameters of the compounds are summarized in
Table 1. By using quinine bisulfate as the calibration standard,
fluorescence quantum yields of PPI, PNI, BPPI, BPNI, p-TPPNI
and m-TPPPI in dichloromethane were determined to be 53.02%,
43.08%, 83.00%, 66.15%, 41.70% and 37.93% respectively. Due
to the fact that the electron delocalization ability of the naphthyl
moiety is stronger than that of the phenyl moiety, PPI and BPPI
have slightly higher Ff values than PNI and BPNI, respectively.
Although some of these values are not high enough to enable
application as high performance blue emitters for OLEDs, BPPI,
whose fluorescence quantum yield reaches a high level of 83.00%,
can obviously take this role.
Electrochemical properties of the indolizine derivatives were
studied by cyclic voltammetry (CV) in a conventional three-
electrode cell, using a glassy carbon working electrode, a plat-
inum wire counter electrode, and a saturated calomel electrode
(SCE) reference electrode. The oxidation processes were inves-
tigated in dichloromethane solution. The highest occupied
molecular orbital (HOMO) levels of the compounds were esti-
mated from the first oxidation peak relative to ferrocene. The
lowest unoccupied molecular orbital (LUMO) levels were
calculated by adding the band gaps (Eg), determined from the
optical absorption edges of solid films on quartz substrates, to
the HOMO levels (Table 1). It can be seen that PPI, PNI, BPPI
and BPNI have similar HOMO levels of 5.52, 5.54, 5.54 and
5.55 eV, and LUMO levels of 2.32, 2.37, 2.35 and 2.38 eV,
respectively.
Due to the greater electron delocalization ability and larger p-
conjugation plane, naphthyl moieties on the 2-position of the
indolizine units are relatively more delocalized than the phenyl
moiety and thus increase the extent of p-orbital conjugation
within each indolizine unit. In turn, this decreases the p-orbital
conjugation between different indolizine units in the whole
OLED characterization
Based on the above, BPPI was identified to have the highest
potential for application in OLEDs. Using BPPI, a blue fluo-
rescent device (device A) with a configuration of indium tin oxide
(ITO)/NPB (30 nm)/TCTA (10 nm)/BPPI (30 nm)/LiF (1.5 nm)/
Al was fabricated to check whether BPPI can be used as a blue
emitter. In this device, ITO and LiF/Al served as the anode and
the cathodes, respectively; 4,40-bis[N-(1-naphthyl)-N-phenyl-
amino]biphenyl (NPB) was used as the hole-transporting layer
(HTL); and 4,40,400-tris(N-carbazolyl)triphenylamine (TCTA)
molecule. This would decrease both Ff and the red shift of lem
.
On the other hand, the p-orbital conjugation between different
indolizine units in the whole molecule increases when there are
more bridging phenyl groups between the indolizine units (e.g.,
biphenyl vs. phenyl). This leads to an increase in Ff (e.g. BPPI vs.
PPI, BPNI vs. PNI). In comparison, if the bridging phenyl
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was the electron blocking layer (EBL). Considering that the
multifunctional compound BPPI has both electron-transporting
properties and efficient blue emission, we simplified the device
structure by using BPPI as the electron-transporting layer (ETL)
and emitting layer (EML), simultaneously.
As shown in Fig. 2(c), the fluorescent device exhibits stable
deep-blue electroluminescence (EL) at different luminances. The
Commission Internationale de l’Eclairage (CIE) coordinates of
the blue device are (0.15, 0.07), which is very close to the NTSC
standard blue color (0.14, 0.08). Fig. 2(b) shows EQE–current
density characteristics of the device. The maximum EQE of the
blue device is 3.16%, which is a respectable result among blue
fluorescent devices.23,24,25
To confirm whether BPPI can be used as a phosphorescent
host, tris(2-phenylquinoline)iridium (Ir(2-phq)3) doped orange
phosphorescent device (device B) with a configuration of ITO/
NPB (30 nm)/TCTA (10 nm)/BPPI: 2%Ir(2-phq)3 (30 nm)/BPPI
(30 nm)/LiF (1.5 nm)/Al, was fabricated. A 60 nm BPPI layer
was also used as both EML and ETL in this device. From
Fig. 2(d), the turn-on voltage of the device is about 4.0 V, and the
maximum current density at a driving voltage of 10 V is around
60 mA cmꢀ2. The relatively low current density is probably due to
the thick BPPI layer, and also the possibly lower electron
transporting mobility of BPPI, compared to common electron
transporting materials, such as TPBI, BCP, etc. From Fig. 2(e),
the maximum current efficiency of the device is 23.9 cd Aꢀ1 and
shows a mild efficiency roll-off at high current densities. Fig. 2(f)
shows the EL spectra of the device at different luminances; the
EL spectra only show the Ir(2-phq)3 emission from 10 to
10 000 cd mꢀ2. These confirm that BPPI efficiently sensitizes the
triplet emission of the orange phosphor.26,27,28
The results of the blue fluorescent and the orange phospho-
rescent devices suggest that BPPI can be used as a host for F/P-
WOLEDs. We thus fabricated a white device (device C) with
a simplified configuration of ITO/NPB (30 nm)/TCTA (10 nm)/
BPPI (10 nm)/BPPI:2%Ir(2-phq)3 (20 nm)/BPPI (30 nm)/LiF
(1.5 nm)/Al. Since TCTA and BPPI mostly conduct holes and
electrons respectively, the main exciton generation zones would
be located at the TCTA/BPPI interfaces (Fig. 3). Considering
the short diffusion lengths (3 nm) of the singlet excitons, 10 nm
undoped BPPI spacers were placed to capture the singlet exci-
tons for blue emission. The triplet excitons were then trans-
ferred to Ir(2-phq)3 doped in the middle layers for their long
diffusion lengths of 100 nm. The high singlet energy level
(3.4 eV) and ET level (2.8 eV) of TCTA confine all excitons,
such that they can be used in the EML. The maximum EQE
(Fig. 4(b)) of the BPPI-based F/P-WOLED is 10.7%.2,6 This is
a respectable result for WOLEDs considering the simplified
device structure. The EL spectra at different luminances of the
white device are shown in Fig. 4(c), which reveals that the
amount of blue emission slightly decreases with increasing
current density. The reason for the decrease in blue emission
might be that the concentration annihilation between BPPI
molecules is higher than the triplet–triplet annihilation at high
exciton concentrations. At different luminances, the
device exhibits white emission. From luminances of 100 to
10 000 cd mꢀ2, the color coordinates are only slightly shifted by
(+0.03, +0.03). Electroluminescent data of the devices A–C are
shown in Table 2.
Fig. 1 a) Absorption spectra in dichloromethane solution. b) Emission
spectra in dichloromethane solution. c) Phosphorescence emission
spectra in 2-methyltetrahydrofuran at 77 K.
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Fig. 2 a), d) Current density and luminance plotted against applied electric field, characteristic for devices A and B. b), e) Current efficiency plotted
against current density for devices A and B. c), f) EL spectra for devices A and B at different luminances.
structure delivers a respectable performance with high efficiency
Conclusions
and stable emissions.
In summary, we synthesized and characterized a series of indo-
lizine derivatives and demonstrated their applications as elec-
tron-transporting hosts and deep-blue emitters. To the best of
our knowledge, the current work is the first attempt to apply
indolizine derivatives in OLEDs. With a systematic variation of
the molecular structures and studies of their properties, BPPI
exhibits the following properties: (1) blue emission with high
quantum yields, (2) high morphological and thermal stabilities,
(3) electron transporting properties, and (4) a sufficiently high ET
level to act as red or yellow-orange phosphorescent host. Based
on this multifunctional compound, the non-doped blue fluores-
cent device exhibits good performance with an EQE of 3.16%
and CIE coordinates of (0.15, 0.07); moreover, the orange
phosphorescent device achieves a high current efficiency of
23.9 cd Aꢀ1. Finally, the proposed F/P-WOLED with a simplified
Experimental
General methods
The starting materials, 4,40-dibromobiphenyl, 2,20-dibromo-
phenyl, 3-bromoacetophenone and 4-bromoacetophenone were
purchased from Alfa Asia and used without further purification.
All of the arylboronic acids were prepared in two steps from
corresponding aryl halides and the final compounds were
prepared by using Suzuki-type cross-coupling reactions accord-
ing to published procedures.14,15 All of the reactions
and manipulations were carried out under N2 with the use of
a
standard inert atmosphere and Schlenk techniques.
The solvents were dried by standard procedures. All of the
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Fig. 3 Energy diagram and device structure of the F/P-WOLED. The
numbers in parentheses indicate the energy gaps of materials. T1: Triplet
energy. The red dotted line square zone indicates the exciton formation
zone.
column-chromatography measurements were performed using
silica gel (100–200 mesh or 200–300 mesh, Sinopharm Chemical
Reagent Co.) as the stationary phase in a column 15–20 cm in
length and 2.0 cm–6.0 cm in diameter. The NMR spectra were
recorded on Varian-Gemini 300 and Bruker Avance P-400
spectrometer at room temperature. The electronic absorption
spectra were measured using a Hitachi UV-Vis spectrophotom-
eter U-3010. The fluorescence spectra were recorded using
a Hitachi fluorescence spectrometer F-4500. The emission
quantum yields were measured with reference to quinine bisul-
fate in 0.1 N sulfuric acid solution.18 The fluorescence lifetimes of
the compounds in solution were determined using a single-
photon counting fluorescence lifetime apparatus F900 (Edin-
burgh Instruments Company). The phosphorescence spectra of
the compounds (in the 2-methyltetrahydrofuran solution) were
measured using a Hitachi fluorescence spectrometer F-4500 at
77 K. The cyclic voltammetry (CV) experiments were performed
using a Potentiostat/Galvanostat Model 283 electrochemical
analyzer. All of the measurements were carried out at room
temperature with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum wire
counter electrode, and a SCE (saturated calomel electrode)
reference electrode. The EOX values were determined as electro-
chemical first oxidation potentials with respect to the ferrocene/
ferrocenium redox potential. The solvent in all of the experi-
ments was CH2Cl2 and the supporting electrolyte was 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6). The DSC
measurements were carried out using a TA Instruments DSC
2910 thermal analyzer at a heating rate of 10 ꢁC minꢀ1. The TGA
measurements were performed on a TA Instruments TGA 2050
thermal analyzer. Low- and high-resolution mass spectra were
recorded using a BIFLEXIII MALDI-TOF mass spectrometer
in MALDI mode or in EI mode.
Fig. 4 a) Current density and luminance plotted against applied electric
field, characteristic for device C. b) Current efficiency plotted against
current density for device C. c) EL spectra for device C at different
luminances.
filtered, washed with cold water, ether and ethanol, and dried
under reduced pressure to give the intermediate compound.
Synthesis of 1,3,5-trialkylbenzenes: general procedure. Thionyl
chloride (0.8 mL, 11 mmol) was added slowly by a syringe to
a stirred solution of bromoacetophenone (1 g, 5 mmol) in
anhydrous ethanol (2 mL, 35 mmol). The mixture was stirred for
1 h at reflux temperature and then neutralised with a saturated
sodium carbonate solution. The solid obtained on cooling was
1,3,5-Tri(4-bromophenyl)benzene. Light yellowish solid. M.P.
ꢁ
1
ꢁ
262–263 C. H NMR (400 MHz, CDCl3, 25 C, TMS) d 7.69
(s, 3H), 7.59–7.62 (m, 6H), 7.52–7.55 (m, 6H). TOF EI-MS:
543.8333 [M]+.
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Table 2 Electroluminescent data of the devices A–C
Lmax (Voltage)
hL,max
[cd Aꢀ1
]
f
Von [V]a
[cd mꢀ2 (V)]b
lem [nm]c
CIE @100 cd mꢀ2 [x, y]d
hext,max [%]e
Device A
Device B
Device C
4.00
4.00
4.20
2204 (10.0)
13010 (10.0)
14830 (11.0)
444
582
446
(0.15, 0.07)
(0.55, 0.44)
(0.41, 0.33)
3.16
12.4
10.7
1.25
23.9
17.8
a
b
c
Von (turn-on voltage) was obtained from the x-intercept of plot of log(luminance) against applied voltage. L: Luminance; max: maximum. lem
:
Emission wavelength. CIE (x,y): Commission Internationale de l’Eclairage coordinates. hext: External quantum efficiency; max: maximum. hL:
Current efficiency; max: maximum.
d
e
f
1
1,3,5-Tri(3-bromophenyl)benzene. Light yellowish solid. M.P.
167–168 ꢁC. 1H NMR (400 MHz, CDCl3, 25 ꢁC, TMS) d 7.73 (s,
3H), 7.63–7.67 (m, 3H), 7.37–7.43 (m, 6H), 7.25–7.28 (m, 3H).
TOF EI-MS: 543.8421 [M]+.
2-Phenylindolizine. white bꢁeige solid, M.P. 135–136 ꢁC; H
NMR (400 MHz, CDCl3, 25 C, TMS) d 8.13 (d, J ¼ 6.76 Hz,
1H), 7.97 (d, J ¼ 7.4 Hz, 2H), 7.88 (s, 1H), 7.66 (d, J ¼ 9.12 Hz,
1H), 7.44 (t, J ¼ 7.44 Hz, 2H), 7.34 (t, J ¼ 7.36 Hz, 1H),
7.18 (t, J ¼ 7.68 Hz, 1H), 6.79 (t, J ¼ 6.72 Hz, 1H); TOF EI-MS:
194.0582 [M]+.
Synthesis of arylboronic acids 3–6: general procedure
Biphenyl-4,40-diboronic acid (4).8,9 n-Butyl lithium (1.6 mL,
2.72 M in hexanes; 4.35 mmol) was added dropwise to
a solution of 4,40-dibromo-1,10-biphenyl (0.5 g, 1.6 mmol) in
THF at ꢀ78 ꢁC. The solution was stirred for 10 min at
ꢀ78 ꢁC, and 1 h at room temperature. The mixture was re-
cooled to ꢀ78 ꢁC and trimethylborate (1.0 mL, 8 mmol) was
added rapidly. The mixture was allowed to warm to room
temperature and stirred for 16 h before being quenched with
aqueous hydrochloric acid (8 mL, 2 M). The solvent was
removed in vacuum to give a white residue, which was dis-
solved in a sodium hydroxide solution (500 mL, 2 M) and
filtered. Acidification of the filtrate to pH ¼ 1 with hydro-
chloric acid (35%) gave a fine, white precipitate, which was
collected by centrifugation. The crude diboronic acid 4 was
dried and used unpurified in the next step.
2-Naphthalenylindolizine. yield 67%, white beige solid, M.P.
155–162 ꢁC; 1H NMR (400 MHz, CDCl3, 25 ꢁC, TMS) d 8.53 (s,
1H), 8.17 (d, J ¼ 6.9 Hz, 1H), 7.83–8.04 (m, 5H), 7.69 (d, J ¼
9.0 Hz, 1H), 7.46–7.50 (m, 2H), 7.18–7.26 (m, 1H), 6.81 (t, J ¼
7.1 Hz, 1H); TOF EI-MS: 244.0681 [M+].
Synthesis of iodo compounds 1, 2: general procedure for reaction
with iodine. To a solution of 2-phenylindolizine (0.50 g,
2.6 mmol) in pyridine (5 mL) was added iodine (1 g, 4 mmol).
The reaction mixture was heated at 50 ꢁC for 5 h and then poured
into water (50 mL). The aqueous solution was extracted with
dichloromethane (30 mL ꢂ 3). The combined organic extracts
were washed with water (50 mL), Na2S2O3 (aq) (50 mL), and
brine (50 mL). The dichloromethane solution was dried over
sodium sulfate, and the filtrate was evaporated in vacuo. The
crude product was subjected to column chromatography on
alumina (solvent: dichloromethane) to obtain 1 (2.2 mmol; 84%)
Synthesis of 2-bromoacetophenone and 2-bromoacetonaph-
thone: general procedure. To a mixture of acetophenone 7 (1.17
mL, 10 mmol) and NBS (1.869 g, 10.5 mmol) in dry Et2O (10 mL)
was added NH4OAc (77.8mg, 1 mmol). After stirring at 25 ꢁC for
0.5 h, the mixture was filtered and the filtrate was washed with
water, dried and evaporated. The residue was chromatographed
(hexane–acetone ¼ 10 : 1) on silica gel to give the product (92%).
2-bromoacetonaphthone was also prepared from acetonaph-
thone and NBS by a similar procedure (87%).
3-Iodo-2-phenylindolizine (1). pale yellow solid, M.P. 169–
171 ꢁC; 1H NMR (400 MHz, CDCl3, 25 ꢁC, TMS) d 8.27 (d, J ¼
6.84 Hz, 1H), 8.08 (d, J ¼ 7.4 Hz, 2H), 7.63 (d, J ¼ 8.7 Hz, 1H),
7.50 (t, J ¼ 7.24 Hz, 2H), 7.37–7.44 (m, 1H), 7.20–7.32 (m, 1H),
6.94 (t, J ¼ 6.9 Hz, 1H); TOF EI-MS: 320.3632 [M]+.
3-Iodo-2-naphthalenylindolizine (2). pale yellow solid, M.P.
150–152 ꢁC; 1H NMR (400 MHz, CDCl3, 25 ꢁC, TMS) d 8.24 (d,
J ¼ 6.9 Hz, 1H), 8.07 (d, J ¼ 6.9 Hz, 2H), 7.63 (d, J ¼ 8.7 Hz,
1H), 7.49 (t, J ¼ 7.4 Hz, 2H), 7.37–7.44 (m, 1H), 7.20–7.32 (m,
1H), 6.94 (t, J ¼ 6.9 Hz, 1H); TOF EI-MS: 369.9746 [M]+.
Synthesis of indolizines: general procedure. A mixture of 2-
aminopyridine (0.58 g, 6.2 mmol), 2-bromoacetophenone (1.0 g,
5.0 mmol), and sodium hydrogen carbonate (0.65 g, 7.8 mmol) in
ethanol (10 mL) was stirred for 12 h at room temperature. The
solution was evaporated in vacuo, and water (50 mL) was added
to the residue. This was extracted with chloroform twice (50 mL
ꢂ 2), and the chloroform solution was washed with water (50
mL) and brine (50 mL). The chloroform solution was dried over
sodium sulfate, and the filtrate was evaporated in vacuum. The
crude product was subjected to column chromatography on
alumina (solvent: dichloromethane) to obtain 2-phenylindolizine
(4.3 mmol; 86%). 2-naphthalenylindolizine was also prepared
from 2-aminopyridine and 2-bromoacetonaphthone by a similar
procedure except for the requirement of higher reaction
temperature (80 ꢁC).
General procedure for the Suzuki reaction. To a mixture of
3-iodo-2-phenylindolizine 1 (0.96 g, 3 mmol) and Pd(PPh3)4
(120 mg, 0.1 mmol) in 1,2-dimethoxyethane (DME, 16 mL) was
added 1,4-benzenediboronic acid (0.2 g, 1.27 mmol) followed by
the addition of sodium hydroxide (0.16 g, 4 mmol) in water
(8 mL). The reaction mixture was refluxed at 85 ꢁC for 40 h under
argon. Then the mixture was poured into water (40 mL). The
aqueous solution was extracted with dichloromethane (30 mL ꢂ
3). The combined organic extracts were washed with water
(50 mL) and brine (50 mL). The dichloromethane solution was
4508 | J. Mater. Chem., 2012, 22, 4502–4510
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View Online
dried over sodium sulfate, and the filtrate was evaporated in
vacuo. The residue was subjected to column chromatography on
alumina (solvent: dichloromethane) to obtain PPI (0.87 mmol;
57%).
MALDI-TOF MS: calcd. for C63H42N6 882.35, found
882.322 [M]+.
Device fabrication. OLEDs were fabricated on patterned ITO-
coated glass substrates with a sheet resistance of 30 U sqꢀ1. The
substrates were cleaned with Decon 90, rinsed in de-ionized
water, dried in an oven, and finally exposed to UV-ozone for
about 25 min. The ITO substrates were then immediately
transferred into a deposition chamber with a base pressure of 1 ꢂ
10ꢀ6 mbar. Organic layers were sequentially deposited at a rate of
0.1–0.2 nm sꢀ1 by conventional vapor vacuum deposition. The Al
cathode was then prepared by evaporation of Al after LiF of
1.5 nm was deposited.
1,4-Di-30-(20-phenylindolizyl)-benzene (PPI). pale yellow
1
ꢁ
solid; H NMR (400 MHz, CDCl3, 25 C, TMS) d 8.15 (d, J ¼
6.88 Hz, 4H), 7.84 (d, J ¼ 7.44 Hz, 4H), 7.72 (q, J1 ¼ 8.24 Hz,
J2 ¼ 1.68 Hz, 8H), 7.65 (s, 8H), 7.35 (m, 16H), 6.90 (t, J ¼ 6.52
Hz, 4H); 13C NMR (CDCl3, 100 MHz): d 148.5, 138.2, 137.7,
133.0, 129.3, 129.2, 128.7, 128.0, 127.5, 126.2, 116.8, 114.6; TOF
EI-MS: calcd. for C32H22N4 462.18, found 462.1678 [M]+.
1,4-Di-30-(20-naphthalenylindolizyl)-benzene (PNI). pale
yellow solid; 1H NMR (400 MHz, CDCl3, 25 ꢁC, TMS) d 8.30 (s,
2H), 8.18 (d, J ¼ 6.88, 2H), 7.80 (m, 10H) 7.68 (s, 4H), 7.47 (m,
4H), 7.30 (t, J ¼ 7.32, 2H), 6.87 (t, J ¼ 6.32, 2H); 13C NMR
(CDCl3, 100 MHz): d 148.7, 138.7, 133.8, 133.2, 132.5, 129.5,
128.5, 128.1, 127.8, 127.7, 127.4, 126.8, 117.8, 115.6; TOF
EI-MS: calcd. for C40H26N4 562.22, found 562.2174 [M]+.
Acknowledgements
This work was supported by National High-tech R&D Program
of China (863 Program) (Grant No. 2011AA03A110), National
Natural Science Foundation of China (Grant No. 50825304,
51033007, 51103169, 51128301) and Beijing Natural Science
Foundation (Grant No. 2111002), P. R. China.
3-(4,40-Biphenyl)-2-diphenylindolizine (BPPI). pale yellow
1
ꢁ
solid; H NMR (400 MHz, CDCl3, 25 C, TMS) d 8.08 (d, J ¼
6.92 Hz, 2H), 7.87 (d, J ¼ 8.24 Hz, 4H), 7.74 (q, J1 ¼ 6.76 Hz,
J2 ¼ 1.6 Hz, 6H), 7.60 (d, J ¼ 8.28 Hz, 4H), 7.31 (m, 8H), 6.81 (t,
J ¼ 8.17 Hz, 2H); 13C NMR (CDCl3, 100 MHz): d 145.1, 142.9,
140.6, 134.2, 131.4, 129.5, 128.5, 128.4, 128.2, 127.8, 125.0, 123.4,
120.7, 117.8, 112.6; TOF EI-MS: calcd. for C38H26N4 538.22,
found 538.1967 [M]+.
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