Phenanthroimidazole-derivative semiconductors as functional layer in high performance OLEDs

Article abstract of DOI:10.1039/c1nj20072k

Four phenanthroimidazole derivatives (a: 1,2-diphenyl-1H-phenanthro[9,10-d] imidazole; b: 2-phenyl-1-p-tolyl-1H-phenanthro[9,10-d]imidazole; c: 1-phenyl-2-p-tolyl-1H-phenanthro[9,10-d]imidazole; d: 1,2-di-p-tolyl-1H- phenanthro[9,10-d]imidazole) were synthesized and their single crystal structures, photophysical, electrochemical and mobility properties were carefully studied. Taking advantage of the thermal stability and the hole transporting (HT) ability, the highly efficient Alq₃-based organic light-emitting diodes (OLEDs) have been achieved by employing the compounds a-d as a functional layer between NPB (4,4-bis(N-(1-naphthyl)-N-phenylamino) biphenyl) and Alq₃ (tris(8-hydroxyquinoline)aluminium) layers. For the device of [ITO/NPB/d/Alq₃/LiF/Al], a maximum luminous efficiency (LE) of 8.1 cd A^-1 was obtained with a maximum brightness of 65 130 cd m^-2, which exhibited much higher efficiency compared to the device with structure of [ITO/NPB/Alq₃/LiF/Al]. The results demonstrated not only an alternative idea to design novel HT materials, but also a convenient way to improve the performance of the NPB/Alq₃-based devices by introduction of a suitable organic buffer layer. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/c1nj20072k

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PAPER
Phenanthroimidazole-derivative semiconductors as functional layer
in high performance OLEDsw
Yang Yuan, Di Li, Xueqiang Zhang, Xingjia Zhao, Yu Liu,* Jingying Zhang and
Yue Wang*
Received (in Montpellier, France) 27th January 2011, Accepted 7th April 2011
DOI: 10.1039/c1nj20072k
Four phenanthroimidazole derivatives (a: 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole;
b: 2-phenyl-1-p-tolyl-1H-phenanthro[9,10-d]imidazole; c: 1-phenyl-2-p-tolyl-1H-phenanthro-
[9,10-d]imidazole; d: 1,2-di-p-tolyl-1H-phenanthro[9,10-d]imidazole) were synthesized and their
single crystal structures, photophysical, electrochemical and mobility properties were carefully
studied. Taking advantage of the thermal stability and the hole transporting (HT) ability, the
highly efficient Alq3-based organic light-emitting diodes (OLEDs) have been achieved by
employing the compounds a–d as a functional layer between NPB (4,4-bis(N-(1-naphthyl)-N-
phenylamino)biphenyl) and Alq3 (tris(8-hydroxyquinoline)aluminium) layers. For the device of
[ITO/NPB/d/Alq3/LiF/Al], a maximum luminous efficiency (LE) of 8.1 cd AÀ1 was obtained with
a maximum brightness of 65 130 cd mÀ2, which exhibited much higher efficiency compared to the
device with structure of [ITO/NPB/Alq3/LiF/Al]. The results demonstrated not only an alternative
idea to design novel HT materials, but also a convenient way to improve the performance of the
NPB/Alq3-based devices by introduction of a suitable organic buffer layer.
luminous efficiency (LE) and device duration. Enhancement
Introduction
and optimization of charge injection and transport and carrier
Since the observation of highly efficient electroluminescence
balance are very important issues in achieving efficient
devices.8 In this paper, we investigate the significant enhance-
(EL) from a bilayer device composed of triphenylamine deri-
vative and tris(8-hydroxyquinoline)aluminium (Alq3),1 NPB
ment of the EL performance by inserting an organic
(4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl) and Alq3
buffer layer composed of a series of phenanthroimidazole
derivatives9 with the thickness of tens of nanometres between
gradually became the most widely used hole-transporting,
electron-transporting as well as host emitting materials in
the NPB and Alq3 layer. Devices with structure of [ITO/NPB/
organic light-emitting diodes (OLEDs) due to their own many
buffer layer/Alq3/LiF/Al] were fabricated, which presented
high brighness (>60000 cd mÀ2) and efficiency (>8 cd AÀ1).
advantages such as thermal and morphological stability, and
easy synthesis and purification.2–3 However, the electron and
In addition, the characteristics of single crystal, thermal,
hole mobilities in Alq3 and NPB are at the level of 10À4–10À5
photophysical, electrochemical, EL and hole mobility properties
and 10À3–10À4 cm2 VÀ1 sÀ1, respectively, in an electric field of
will be reported.
2 Â 106 V cmÀ1 4,5
,
which gives rise to an accumulation of
excess holes at the NPB/Alq3 boundary. Great efforts have
been invested to improve and balance carrier injection into the
emitting layer to achieve high-efficiency devices. Among them,
insertion of an anodic buffer layer including organic6 and
inorganic7 materials between indium-tin oxide (ITO) and the
hole-transporting layer (HTL) as a hole injection layer (HIL)
could control the hole injection and have resulted in enhanced
Results and discussion
X-Ray structures and molecular packing properties
Four phenanthroimidazole derivatives (a: 1,2-diphenyl-1H-
phenanthro[9,10-d]imidazole; b: 2-phenyl-1-p-tolyl-1H-phen-
anthro[9,10-d]imidazole; c: 1-phenyl-2-p-tolyl-1H-phenanthro-
[9,10-d]imidazole; d: 1,2-di-p-tolyl-1H-phenanthro[9,10-d]-
imidazole) have been synthesized. The single crystals of
compounds b–d were obtained during sublimation. The mole-
cular structures of compounds b–d were determined by single
crystal X-ray crystallography. Fig. 1 shows the Oak Ridge
thermal ellipsoid plot (ORTEP) drawing of b–d. Fig. 2
illustrates the molecular packing features of compounds b, c
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun 130012,
P. R. China. E-mail: yuewang@jlu.edu.cn, yuliu@jlu.edu.cn;
Fax: +86 431-85193421; Tel: +86 431-85168484
w Crystallographic data for b–d, CCDC reference numbers
806932–806934. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1nj20072k
c
1534 New J. Chem., 2011, 35, 1534–1540
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Fig. 1 ORTEP drawing of b–d with 30% thermal ellipsoids.
and d in crystals. In the asymmetric units for compounds b and
c each have two independent molecules. In crystal b, there
are two orientations of intermolecular interactions. In one
orientation, intermolecular C–HÁ Á Áp interactions (2.67 A) exist
between the aniline and the phenanthrene plane. In another
orientation, intermolecular C–HÁ Á Áp interactions (2.81 A)
between the aniline and the phenanthrene plane are observed.
There are intermolecular C–HÁ Á ÁN interactions (2.71 A)
between two orientations. In the crystal of c, there are also
two orientations of intermolecular interactions with the same
C–HÁ Á ÁN interaction distance of 2.74 A between phenyl and N
atom of five-membered ring. Two kinds of intermolecular
C–HÁ Á Áp interactions have been observed: (i) between phenyl
and benzyl plane (2.66 A), and (ii) between phenyl and
phenanthrene plane (2.63 A) in two orientations. In solid d,
there are just intermolecular hydrogen bonding interactions
between benzyl and benzyl (2.89 A), and between phen-
anthrene and phenanthrene (2.89 A). There is no interfacial
pÁ Á Áp interaction in the packing structure of b–d.
Fig. 2 Stacking diagram showing intermolecular interactions in solid
b, c and d.
these complexes are used in EL devices since the high Tm and
Tg values could improve the lifetime of the devices.10 The Tg
(glass transition temperature) values are 82, 74, 74 and 64 1C
for a, b, c and d (Table 1), respectively.
Thermal analysis
The thermal properties of a–d were investigated by differential
scanning calorimetry (DSC) and thermal gravimetric analyses
(TGA) under a nitrogen atmosphere (Fig. 3). The melting
points of a–d measured by DSC examinations were 206, 211,
199 and 194 1C, respectively. Compound b has the highest
melting point among these compounds, which is probably due
to the intermolecular C–HÁ Á Áp interactions of two orientations
that can induce more condensed molecular packing. The
decomposition temperatures with a 5% weight loss (Td5) for
a–d are 343, 342, 337, 336 1C, respectively. The high Tm and
Td5 values indicate that the compounds a–d are thermally
stable and would be capable of enduring the vacuum thermal
sublimation process, which might serve as an advantage when
Photophysical and electrochemical properties
Absorption and photoluminescence studies were performed on
compounds a–d in chloroform (10À4 M) to investigate the
relationship between methyl substituents and the physical
properties (Fig. 4). For all the compounds, their absorption
spectra do not show remarkable differences and have a
maxima at 262 nm and a shoulder at around 311 nm suggesting
the very similar optical energy gap. The photoluminescence
spectra of compounds a and b have a same sharp emission
peak at 384 nm accompanied by a shoulder around 370 nm,
and a slight red shift of the emission bands for compounds c
and d with the peak of 387 nm and the shoulder of 373 nm
c
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were observed. No major difference has been shown between
the photophysical properties of all the compounds a–d
indicating that the substitution of methyl has little effect on
the photophysical properties of a–d. The electrochemical
properties of compounds a–d were studied in solution through
cyclic voltammetry (CV) measurements using ferrocene as the
internal standard. Their cyclic voltammograms are shown in
Fig. 5 and the respective electrochemical data are summarized
in Table 1. All the compounds a–d displayed one reversible
oxidation wave and no reduction wave was detected within the
electrochemical window of dichloromethane. The introduction
and the increased amount of methyl groups only lead a slight
negative shift respect to oxidation potentials from compound
a to d (0.84 V, 0.82 V, 0.80 V and 0.78 V, respectively). The
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels of
compounds were calculated from the onset potentials for
oxidation together with their absorption spectra. The HOMO
energy levels varied from À5.64 eV for compound a to
À5.62 eV, À5.60 eV, À5.58 eV for compound b, c and d,
and the corresponding LUMO levels were À2.29 eV,
À2.27 eV, À2.27 eV and À2.24 eV, respectively.
Electroluminescence and mobility properties
To investigate the EL properties of compounds a–d as a buffer
layer, a series of devices (I, II, III and IV) with the device
configuration of [ITO/NPB (10 nm)/buffer layer (30 nm)/Alq3
(60 nm)/LiF (1 nm)/Al] have been fabricated, where NPB was
the hole-transporting layer (HTL), Alq3 was employed as the
electron-transporting layer (ETL) and the emitting layer
(EML). The compound a–d film was the buffer layer in device
I, II III and IV, respectively. For comparison, the reference
device (device V) with the structure of [ITO/NPB (40 nm)/Alq3
(60 nm)/LiF/Al] and device VI with the thiner layer (10 nm) of
compound d by maintaining the total thickness (40 nm) of
HTL and buffer layer were fabricated also. Fig. 6 and 7 show
the current density-brightness-voltage and the luminous
efficiency (LE)-current density characteristics, respectively.
The devices exhibited bright green emission (Fig. 8) from
Alq3, independent of the applied voltages in the range of 3 V
to 16 V. At the same driven voltage, the current density and
the EL intensity of devices I–IV with a buffer layer was much
Fig. 3 DSC (top) and TGA (bottom) curves of compounds a–d at a
heating rate of 10 1C minÀ1
.
Table 1 Thermal, electrochemical and photophysical data of a–d
Tm/Tg/Td5/1C lmax,abs/nma em/nma Eox/V HOMO/LUMO/eVb
l
a 206/82/343
b 211/74/342
c 199/74/337
d 194/64/336
262,311
262,311
262,311
263,311
384,370 0.84 À5.64/À2.29
384,370 0.82 À5.62/À2.27
387,373 0.80 À5.60/À2.27
387,374 0.78 À5.58/À2.24
a
b
In CHCl3. The HOMO and LUMO energies were determined from
cyclic voltammetry and absorption data.
Fig. 4 UV-Vis absorption and PL spectra (normalized) in CHCl3
(10À4 M) compounds a–d.
Fig. 5 Cyclic voltammograms of a–d.
c
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Therefore, the decreased amount of holes should be attrib-
uted to the relatively low HT ability of compounds a–d, which
is supported by the current density–voltage characteristic of
device IV and VI with different thickness of buffer layer based
on compound d (Fig. 6).13 The current density of device IV
with thicker compound d layer was much lower than that of
device V with the traditional device structure. For the device
VI with thinner compound d layer, it showed a higher current
density than that of device V. The dependence of the current
density of device on the thickness of buffer layer indicates
compound d and the series of phenanthroimidazole derivatives
can prevent excessive holes to enter the EML when the
thickness of functional layer is as thick as tens nanometres.
The higher efficiency was realized by improving the balance of
the hole and electron.
Fig. 6 Current density-brightness-voltage characteristics of devices I,
II, III, IV and V. Current density-voltage characteristics of VI.
Furthermore, the hole mobility of compound d was
measured using the conventional time of flight (TOF) technique
14
based on the device configuration of [ITO/d (1.36 mm)/Al].
Fig. 9 shows typical TOF transients of holes for complex d
under an applied field, and the transient photocurrent signals
were dispersive, suggesting the presence of charge carrier
traps. The hole mobility of compound d was determined to
be 3.4 Â 10À4 cm2 VÀ1
s
À1, while that of the commonly used
HT material NPB is 7.4 Â 10À4 cm2 VÀ1 sÀ1 based on the same
measurement condition. Clearly, the hole-mobility alignment
of compound d and NPB supported partly also the result that
compound d can decrease the amount of holes transported into
the emitting layer effectively due to its relatively low hole mobility.
The compounds a–d with almost alike molecular structures
exhibit similar thermal, electrochemical and photophysical
properties. However, the devices I–IV displayed different turn
on voltage, highest brightness and maxima LE (Table 2). We
tentatively attribute the different EL property to the difference
of molecular packing and nanostructures of a–d in thin film
systems. The charge transport of organic semiconductors is
strongly molecular packing and morphology dependent. It has
been demonstrated that organic molecules with similar
molecular structures exhibited remarkably different mobility.15
On the other hand, in EL devices the carrier mobility and
balance is often dependent on the interfacial characteristics
between the buffer layer and NPB or Alq3 layer, which have
effect on the EL performance. At this stage, we can not
provide rational explanation for this phenomenon.
Fig. 7 Luminous efficiency (LE)-current density characteristics of
devices I–V.
lower than those of device V. However, the devices I–IV
exhibited higher LE than that of device V. For instance, at
an injection current density of 100 mA cmÀ2, the LE of of
device V without buffer layer is 3.1 cd AÀ1, while device I–IV is
5.4, 4.1, 3.8, 6.4 cd AÀ1, respectively. Furthermore, all the
devices containing buffer layer showed very high peak bright-
ness and color stability under different driving voltage. Among
them, device IV based on compound d had the highest
efficiency among all the devices, the peak LE of 8.1 cd AÀ1
was obtained at the peak brightness of 65 130 cd mÀ2 at 15 V.
This is rather high EL efficiency under such high-level bright-
ness for the OLED based on fluorescent materials, which has
been significantly enhanced also compared to that of device V
(3.22 cd AÀ1 at 27 390 cd mÀ2 and 10.5 V). Obviously, the
presence of the buffer layer between the NPB and Alq3 layers
could increase the LE value. We believed that the improved
efficiency of the devices I–IV is due to the decrease of the
amount of holes through the buffer layer, which results in
balancing the electrons and holes in the emitting layer and thus
eliminating the nonproductive hole current. From the electro-
chemical data of the materials used in the devices listed in
Table 1, the HOMO energy level of compounds a–d is about
À5.6 eV, which is in between that of NPB (À5.4 eV) and Alq3
(À6.0 eV)11 and could regulate the hole injection by acted as a
ladder between the energy levels of NPB and Alq3.6a,b,12
Fig. 8 The EL spectra of devices I–VI at applied voltages of 9 V.
c
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spectrometer with tetramethylsilane as the internal standard.
Mass spectra were recorded on a Shimadzu AXIMA-CFR
MALDITOF mass spectrometer. Elemental analyses were
performed on a flash EA 1112 spectrometer. Differential
Scanning Calorimetric (DSC) measurements were performed
on a NETZSCH DSC204 instrument. Thermogravimetric
analyses (TGA) were performed on a TAQ500 thermo-
gravimeter. UV-vis absorption spectra were recorded using a
PE UV-vis lambdazo spectrometer. The emission spectra were
recorded using a Shimadzu RF-5301 PC spectrometer. The IR
spectra were obtained on a FT-IR Vertex 80 V spectrometer.
Cyclic voltammetry was performed on a BAS 100 W instru-
ment with a scan rate of 100 mV sÀ1. A three-electrode
configuration was used for the measurement: a platinum
electrode as the working electrode, a platinum wire as the
counter electrode, and an Ag/Ag+ electrode as the reference
electrode. A 0.1 M solution of tetrabutylammonium perchlorate
(TBAPF6) in CH2Cl2 was used as the supporting electrolyte.
Prior to device fabrication, all the compounds were purified by
gradient sublimation.
Fig. 9 TOF transient of complex d for hole at room temperature
(E = 4.4 Â 105 V cmÀ1 and Tt = 0.9 ms). Insert: log of the
photocurrent vs. the log of time in a time-of-flight measurement.
Table 2 Performance of OLEDsa
Device
Von (V)b
L (cd mÀ2
)
Zc (cd AÀ1
)
CIE(x,y)@9 V
I
II
III
IV
V
5.0
4.9
4.1
3.8
2.7
2.7
35970
24220
33680
65130
37360
38910
6.10
4.07
4.16
8.03
3.22
4.17
(0.33,0.55)
(0.32,0.55)
(0.33,0.55)
(0.35,0.56)
(0.32,0.52)
(0.32,0.52)
Preparation of phenanthroimidazole derivatives
The synthetic route of phenanthroimidazole derivatives is
shown in Scheme 1. The preparation of 1,2-diphenyl-1H-
phenanthro[9,10-d]imidazole (compound a) is employed as
an example to describe the synthesis procedure. A mixture of
phenanthrene-9,10-dione (2.0 g, 9.6 mmol), benzaldehyde
(1.02 g, 9.6 mmol), aniline (4.46 g, 48 mmol), ammonium
acetate (2.96 g, 38.4 mmol) and glacial acetic acid (60 mL) was
heated at 123 1C for 3 h under nitrogen atmosphere. After
cooling to room temperature, the reaction mixture was poured
into distilled water with stirring. The separated solid was
filtered off, washed with water and dried to give the expected
product in good yields. The yields and characterization data
are given below.
VI
a
The brightness (L) and current efficiency (Zc) are the maximum values
of the devices. Von is defined as the voltage required for 1 cd mÀ12
.
b
Conclusions
Four phenanthroimidazole derivatives (a, b, c and d) have
been synthesized and the detailed studies of their single crystal
structures, photophysical and electrochemical properties are
presented. The high Tm and Td5 values indicate that these
compounds are thermally stable and can be used to fabricate
efficient EL devices. The highly efficient Alq3-based EL devices
have been achieved by employing compound a–d as an organic
buffer layer inserted between NPB and Alq3 layers. A
maximum LE of 8.1 cd AÀ1 was obtained with a maximum
brightness of 65130 cd mÀ2 at 15 V, which exhibited much
higher efficiencies compared with the device of [ITO/NPB/
Alq3/LiF/Al] without a–d. This improvement is due to the
relatively weak HT ability of the phenanthroimidazole deri-
vatives, which can depress excessive holes to transport into the
EML, thus improve the balance of the hole and electron and
realize the higher efficiency. The results of this study not
only demonstrated an alternative idea to design novel HT
materials, but also provided a convenient way to improve the
performance of the NPB/Alq3-based devices by inserting a
suitable organic buffer layer.
1,2-Diphenyl-1H-phenanthro[9,10-d]imidazole (a). Yield: 65%.
MS: m/z 370.5 (M+). 1H NMR (500 MHz, d6-DMSO)
d (ppm): 7.09 (d, J = 7.5 Hz, 1H), 7.33–7.40 (m, 4H),
7.55–7.60 (m, 3H), 7.68–7.72 (m, 6H), 7.78 (t, J = 7 Hz, 1H),
8.70 (d, J = 7 Hz, 1H), 8.88 (d, J = 8.5 Hz, 1H), 8.93 (d, J =
8.5 Hz, 1H). Anal. calcd (%) for C27H18N2: C, 87.54; H, 4.90;
N, 7.56. Found: C, 87.43; H, 4.95; N, 7.62. IR (cmÀ1): 428 (w),
517 (w), 534 (w), 613 (w), 667 (w), 700 (s), 710 (m), 720 (s),
753 (s), 776 (s), 806 (w), 1035 (w), 1070 (w), 1106 (w), 1121 (w),
Experimental
General information
All reactants and solvents, unless otherwise stated, were purchased
from commercial sources and used as received. 1H NMR
spectra were measured on a Bruker AVANCE 500 MHz
Scheme 1 Synthetic route for compounds a, b, c and d.
c
1538 New J. Chem., 2011, 35, 1534–1540
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1147 (w), 1167 (w), 1233 (w), 1344 (w), 1383 (s), 1452 (s), 1496
(s), 1523 (w), 1596 (w), 3056 (m).
c = 20.461(4) A, a = 901, b = 901, g = 901, V = 4058.6(14) A3,
Z = 8, Dc = 1.258 g cmÀ3. The refinement converged to R1 =
0.0732, wR2 = 0.1100 (I > 2s(I)), GOF = 0.984.
2-Phenyl-1-p-tolyl-1H-phenanthro [9,10-d]imidazole (b). Yield:
55%. MS: m/z 384.6 (M+). 1H NMR (500 MHz,
d6-DMSO) d (ppm): 2.35 (s, 3H), 7.14 (d, J = 8.5 Hz, 1H),
7.34–7.37 (m, 4H), 7.47 (d, J = 8 Hz, 2H), 7.538–7.599
(m, 5H), 7.68 (t, J = 7.5 Hz, 1H), 7.77 (t, J = 7.5 Hz, 1H),
8.68 (d, J = 8 Hz, 1H), 8.87 (d, J = 8.5 Hz, 1H), 8.92 (d, J =
8.5 Hz, 1H). Anal. calcd (%) for C28H20N2: C, 87.47; H, 5.24;
N, 7.29. Found: C, 87.30; H, 5.33; N, 7.37. IR (cmÀ1): 428 (w),
520 (w), 533 (w), 615 (w), 662 (m), 697 (s), 725 (s), 755 (s),
775 (m), 802 (w), 833 (m), 917 (w), 954 (w), 977 (w), 1020 (w),
1075 (w), 1102 (w), 1145 (m), 1324 (m), 1345 (m), 1384 (s),
1450 (s), 1470 (s), 1512 (s), 1573 (m), 1611 (m), 1907 (w),
2954 (w), 3063 (w).
Crystal data for d: CCDC 806934; C29H22N2; FW = 398.5,
monoclinic, P21/c, a = 13.726(3) A, b = 8.4643(17) A, c =
19.367(4) A, a = 901, b = 109.14(3)1, g = 901, V = 2125.6(7) A3,
Z = 4, Dc = 1.245 g cmÀ3. The refinement converged to R1 =
0.0740, wR2 = 0.1596 (I > 2s(I)), GOF = 1.025.
Device fabrication and measurements
ITO coated glass was used as the substrate. It was cleaned by
sonication successively in a detergent solution, acetone,
methanol, and deionized water before use. The devices were
prepared in vacuum at a pressure of 5 Â 10À6 torr. Organic
layers were deposited onto the substrate at a rate of 0.1 nm sÀ1
.
After the organic film deposition, LiF and aluminum were
thermally evaporated onto the surface of organic layer. The
thicknesses of the organic materials and the cathode layers
were controlled using a quartz crystal thickness monitor. The
electrical characteristics of the devices were measured with a
Keithley 2400 sourcemeter. The EL spectra and luminance of
the devices were obtained on a PR650 spectrometer. All
measurements of the devices were carried out in ambient
atmosphere without further encapsulations.
1-Phenyl-2-p-tolyl-1H-phenanthro [9,10-d] imidazole (c). Yield:
1
50%. MS: m/z 384.6 (M+). H NMR (500 MHz, d6-DMSO)
d (ppm): 2.29 (s, 3H), 7.07 (d, J = 8 Hz, 1H), 7.15 (d, J =
8 Hz, 2H), 7.33 (t, J = 8 Hz, 1H), 7.46 (d, J =
8.5 Hz, 2H), 7.55 (t, J = 7 Hz, 1H), 7.67–7.72 (m, 6H), 7.77
(t, J = 7.5 Hz, 1H), 8.68 (d, J = 8 Hz, 1H), 8.87 (d, J = 8.5 Hz,
1H), 8.92 (d, J = 8.5 Hz, 1H). Anal. calcd (%) for C28H20N2:
C, 87.47; H, 5.24; N, 7.29. Found: C, 87.38; H, 5.32; N, 7.30.
IR (cmÀ1): 428 (w), 511 (w), 520 (w), 534 (w), 614 (w), 659 (w),
671 (w), 698 (s), 709 (m), 726 (s), 755 (s), 771 (w), 822 (m), 921
(w), 948 (w), 1036 (w), 1143 (w), 1188 (w), 1237 (w), 1284 (w),
1235 (w), 1379 (s), 1426 (w), 1450 (s), 1470 (s), 1495 (s), 1513
(w), 1577 (w), 1594 (w), 1610 (w), 3059 (m).
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20921003, 50733002 and 50903036),
the Major State Basic Research Development Program
(2009CB623600).
1,2-Di-p-tolyl-1H-phenanthro [9,10-d]imidazole (d). Yield:
1
60%. MS: m/z 398.6 (M+). H NMR (500 MHz, d6-DMSO)
d (ppm): 2.30 (s, 3H), 2.35 (s, 3H), 7.127–7.173 (m, 3H), 7.35
(t, J = 7.5 Hz, 1H), 7.47 (d, J = 8 Hz, 4H), 7.55 (d, J = 8.5
Hz, 3H), 7.68 (t, J = 7.5 Hz, 1H), 7.76 (t, J = 8 Hz, 1H), 8.67
(d, J = 8 Hz, 1H), 8.87 (d, J = 8.5 Hz, 1H), 8.92 (d, J = 8 Hz,
1H). Anal. calcd (%) for C29H22N2: C, 87.41; H, 5.56; N, 7.03.
Found: C, 87.23; H, 5.68; N, 7.09. IR (cmÀ1): 430 (w), 506 (w),
529 (w), 671 (m), 724 (s), 755 (s), 773 (w), 816 (m), 837 (m), 855
(w), 870 (w), 950 (w), 975 (w), 992 (w), 1017 (w), 1035 (w),
1106 (w), 1142 (w), 1184 (w), 1235 (w), 1280 (w), 1327 (w),
1379 (s), 1428 (m), 1450 (s), 1470 (s), 1513 (s), 1578 (w), 1610
(w), 2918 (m), 3033 (m).
References
1 (a) C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett., 1987, 51,
913; (b) C. W. Tang, S. A. Vanskyke and C. H. Chen, J. Appl.
Phys., 1989, 65, 3610.
2 (a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151;
(b) X. H. Zhang, B. J. Chen, X. Q. Lin, O. Y. Wong, C. S. Lee,
H. L. Kwong, S. T. Lee and S. K. Wu, Chem. Mater., 2001, 13,
1565; (c) K. Q. Ye, J. Wang, H. Sun, Y. Liu, Z. C. Mu, F. Li,
S. M. Jiang, J. Y. Zhang, H. X. Zhang, Y. Wang and C.-M. Che,
J. Phys. Chem. B, 2005, 109, 8008; (d) H. Bi, K. Q. Ye, Y. F. Zhao,
Y. Yang, Y. Liu and Y. Wang, Org. Electron., 2010, 11, 1180.
3 L. S. Hung and C. H. Chen, Mater. Sci. Eng., R, 2002, 39, 143.
4 (a) G. G. Malliaras, Y. Shen, D. H. Dunlap, H. Murata and
Z. H. Kafafi, Appl. Phys. Lett., 2001, 79, 2582; (b) S. C. Tse,
H. H. Fong and S. K. So, J. Appl. Phys., 2003, 94, 2033;
(c) S. C. Tse, S. K. So, M. Y. Yeung, C. F. Lo, S. W. Wen and
C. H. Chen, Jpn. J. Appl. Phys., 2006, 45, 555.
Single crystal X-ray diffraction
The single crystals suitable for X-ray structural analysis were
obtained by vacuum sublimation. Diffraction data were
collected on a Rigaku R-Axis Rapid diffractometer (Mo-Ka
radiation, graphite monochromator) in the c rotation scan
mode. The structure determination was performed by direct
methods using SHELXTL 5.01v and refinements with full-
matrix least squares on F2.
5 (a) S. C. Tse, K. C. Kwok and S. K. So, Appl. Phys. Lett., 2006, 89,
262102; (b) K. K. Tsung and S. K. So, Appl. Phys. Lett., 2008, 92,
103315.
6 (a) Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada,
Y. Yonemoto, S. Kawami and K. Imai, Appl. Phys. Lett., 1994, 65,
807; (b) S. A. VanSlyke, C. H. Chen and C. W. Tang, Appl. Phys.
Lett., 1996, 69, 2160; (c) S. A. Carter, M. Angelopoulos, S. Karg,
P. J. Brock and J. C. Scott, Appl. Phys. Lett., 1997, 70, 2067.
7 (a) Z. B. Deng, X. M. Ding, S. T. Lee and W. A. Gambling, Appl.
Phys. Lett., 1999, 74, 2227; (b) H. J. Jiang, Y. Zhou, B. S. Ooi,
Y. W. Chen, T. Wee, Y. L. Lam, J. S. Huang and S. Y. Liu, Thin
Solid Films, 2000, 363, 25; (c) C. F. Qiu, H. Y. Chen, Z. L. Xie,
M. Wong and H. S. Kwok, Appl. Phys. Lett., 2002, 80, 3485;
(d) Y. Kurosaka, N. Tada, Y. Ohmori and K. Yoshino, Synth.
Met., 1999, 102, 1101; (e) I. M. Chan, T. Y. Hsu and E. C. Hong,
Crystal data for b: CCDC 806932; C28H20N2; FW = 384.5,
triclinic, P-1, a = 9.776(2) A, b = 12.364(3) A, c = 18.480(4) A,
a = 73.75(3)1, b = 80.53(3)1, g = 76.89(3)1, V = 2076.3(7) A3,
Z = 4, Dc = 1.230 g cmÀ3. The refinement converged to R1 =
0.0465, wR2 = 0.1234 (I > 2s(I)), GOF = 1.05.
Crystal data for c: CCDC 806933; C28H20N2; FW = 384.5,
orthorhombic, P212121, a = 10.527(2) A, b = 18.843(4) A,
c
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Appl. Phys. Lett., 2002, 81, 1899; (f) W. P. Hu, M. Matsumura,
K. Furukawa and K. Torimitsu, J. Phys. Chem. B, 2004, 108,
13116.
8 (a) J. Kwon, T. H. Kwon, H. S. Cho, M. K. Kim, I. S. Shin,
D. Y. Shin, S. J. Park and J. I. Hong, New J. Chem., 2008, 32,
1368–1372; (b) L. Aubouy, N. Huby, L. Hirsch, A. van der Lee and
P. Gerbier, New J. Chem., 2009, 33, 1290–1300.
9 I. Yoshikatsu and M. Teruo, J. Org. Chem., 1979, 44, 41.
10 (a) S. Tokito, H. Tanaka, K. Noda, A. Okada and Y. Taga, Appl.
Phys. Lett., 1997, 70, 1929; (b) Z. K. Chen, H. Meng, Y. H. Lai and
W. Huang, Macromolecules, 1999, 32, 4351; (c) T.-R. Chen,
R.-H. Chien, M.-S. Jan, A. Yeh and J.-D. Chien, J. Organomet.
Chem., 2006, 691, 799.
11 (a) T. Tsuzuki and S. Tokito, Adv. Mater., 2007, 19, 276;
(b) C. Adachi, M. E. Thompson and S. R. Forrest, IEEE J. Sel.
Top. Quantum Electron., 2002, 8, 372.
12 C. Adachi, K. Nagai and N. Tamoto, Appl. Phys. Lett., 1995, 66, 2679.
13 L. X. Xiao, S.-J. Su, Y. Agata. H. Lan and J. Kido, Adv. Mater.,
2009, 21, 1271.
14 H. H. Fong, K. C. Lun and S. K. So, Chem. Phys. Lett., 2002, 353, 407.
15 Z. X. Xu, H. F. Xiang, V. A. Roy, S. S. Y. Chui, Y. Wang,
P. T. Lai and C. M. Che, Appl. Phys. Lett., 2009, 95, 123305.
c
1540 New J. Chem., 2011, 35, 1534–1540
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011