Synthesis and characterization of phenanthroimidazole derivatives for applications in organic electroluminescent devices

Article abstract of DOI:10.1039/c1jm10326a

A series of phenanthroimidazole derivatives have been synthesized, characterized and applied as non-doped emitters in organic light-emitting devices. The compounds show high fluorescent quantum yields up to 0.75 as well as good thermal and film-forming ab

Full text of DOI:10.1039/c1jm10326a

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PAPER
Synthesis and characterization of phenanthroimidazole derivatives for
applications in organic electroluminescent devices†
b
ac
Ying Zhang,ab Shiu-Lun Lai, Qing-Xiao Tong,
Mei-Yee Chan,d Tsz-Wai Ng,b Zhi-Chun Wen,a
*
a
Guo-Qiang Zhang, Shuit-Tong Lee, Hoi-Lun Kwong and Chun-Sing Lee
b
e
b
*
Received 21st January 2011, Accepted 25th March 2011
DOI: 10.1039/c1jm10326a
A series of phenanthroimidazole derivatives have been synthesized, characterized and applied as non-
doped emitters in organic light-emitting devices. The compounds show high fluorescent quantum yields
up to 0.75 as well as good thermal and film-forming abilities. Crystal structures of the compounds were
determined by X-ray crystallography. Correlations between optoelectronic properties, energy levels
and molecular structures of the materials were investigated. It was found that introduction of
a thiophene ring on the C2-position of phenanthroimidazole can effectively decrease the ionization
potentials (Ip) of the compounds. Fluorescent properties of the materials were found to be related to the
dihedral angles in the compounds with different substituents. The low Ip (from 5.00 to 5.21 eV) of the
materials enables efficient hole-injection from the hole-transporting layer and results in low turn-on
(<2.7 V) and operation voltages (<5.5 V to give 1000 cd mꢀ2).
It is well-known that for organic semiconductors, conduc-
1. Introduction
tivities of emissive materials with electron-transport properties
are generally lagging behind those materials with hole-trans-
port properties. Thus, development of better emissive mate-
rials with electron-transport properties is an important
approach for performance enhancement. In previous works,
Tonzola et al. and Kuo et al. employed oligoquinoline and
imidazole respectively with electron-transport properties to
improve the power efficiency of their device.9,10 Here, we
Since the breakthrough in multilayered organic light-emitting
devices (OLEDs) by Tang and van Slyke,1 research on OLEDs
has been intensively pursued because of their applications in full-
color displays and solid-state lighting.2,3 In particular, much
research has been focused on material design and device struc-
ture optimization. It was found that the host/dopant emitter
approach4,5 is an effective strategy to tune emission color and
achieve high device efficiency. However, this approach also has
its drawbacks such as a more complicated fabrication process for
precisely controlling dopant concentration, and potential phase
separation which might affect device stability.6–8 So, there is still
much need to develop high efficiency non-doped emitters of
different colors.
developed
a series of new materials based on phenan-
throimidazole, for its good electron-transport properties.
Thiophene-based compounds have been reported to exhibit
either electron- and/or hole-transporting characteristics
depending on the aromatic groups attached to it.11–14 By
attaching different thiophene derivatives to the imidazole-C2-
position, five new compounds are synthesized and applied as
emitters in OLEDs. The devices show low onset voltages of
equal to or lower than 2.7 V with high brightness up to 15960
cd cmꢀ2 at a voltage of lower than 9.5 V. The relationship
between the molecular structure and the optoelectronic
properties of the materials was studied. Experimental results
indicated that the introduction of a thiophene ring to the
C2-position of the imidazole could effectively lead to shallow
HOMO energy levels (e.g. by introducing a thiophene ring to
1-(4-tert-butylphenyl)-2-(4-phenylbenzene-1-yl)-1H-phenanthro-
[9,10-d] imidazole to get Ph-TPI, the HOMO energy changes
from 5.41 eV to 5.19 eV) and this would decrease the hole-
injection barrier at the interface of the hole-transporting layer
(HTL) and the emissive layer.
aDepartment of Chemistry, Shantou University, Guangdong, 515063,
China. E-mail: qxtong@stu.edu.cn; Fax: +86-754-82902767
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; Fax:
+86-852-34420544
cJiangsu Key Lab. for Carbon-Based Funct. Mater. & Devices, Soochow
Univ., China
dDepartment of Chemistry, The University of Hong Kong, Hong Kong
SAR, China
eDept of Biology and Chemistry, City University of Hong Kong, Hong
Kong SAR, China
† Electronic supplementary information (ESI) available. CCDC
reference numbers 817159–817162. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1jm10326a
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8.70–8.81 (m, 3H). MS (ESI+): m/z 511.4 (MH+), Calc. for
2. Experimental
C29H23BrN2S: 510.08.
2.1. General experiment
Synthesis of 2a–2e
All solvents and materials were used as received from commercial
suppliers without further purification. Synthetic routes of the
phenanthroimidazole derivatives are outlined in Scheme 1. The
starting material 2-(5-bromothiophen-2-yl)-1-(4-tert-butyl-
phenyl)-1H-phenanthro [9,10-d] imidazole was prepared
according to the literature.15 Compounds 2a–2e were prepared
through Suzuki coupling reactions.16
1-(4-tert-butylphenyl)-2-(5-phenylthiophen-2-yl)-1H-phenanthro-
[9,10-d] imidazole (Ph-TPI) (2a). Ph-TPI was synthesized via
a classical Pd-catalyzed Suzuki coupling.16 A solution of
compound 1 (2.55 g, 5 mmol), phenylboronic acid (0.91 g, 7.5
mmol), Pd(PPh3)4 (0.29 g, 0.25 mmol) and aqueous Na2CO3 (2
M, 15 mL) in toluene (30 mL) and ethanol (10 mL) was heated to
reflux in an argon atmosphere for 24 h. The solution was cooled
to room temperature and extracted with dichloromethane. The
combined organic extracts were dried with anhydrous Na2SO4
and concentrated by rotary evaporation. The residue was puri-
fied by column chromatography (petroleum ether : CH2Cl2,
4 : 1) to obtain the pure product as light yellow powder. Yield:
2.13 g (83.8%). mp: 280 ꢁC. 1H NMR (400 MHz; CD2Cl2; Me4Si)
dH [ppm]: 1.54 (s, 9H), 6.69 (d, J ¼ 4.0 Hz, 1H), 7.16
(d, J ¼ 4.0 Hz, 1H), 7.25 (dd, J ¼ 8.3, 1.1 Hz, 1H), 7.28–7.37
(m, 2H), 7.41 (t, J ¼ 7.5 Hz, 2H), 7.49–7.85 (m, 9H), 8.74
(d, J ¼ 8.3 Hz, 1H), 8.77–8.85 (m, 2H); dC (50 MHz; CDCl3;
Me4Si) 31.53 (s), 35.22 (s), 120.67 (s), 123.01 (d), 123.99
(dd), 124.88 (s), 125.65 (s), 126.26 (d), 126.66 (s), 127.12
(d), 127.64 (d), 128.01 (d), 128.33 (d), 128.67 (d), 129.20 (s),
131.24 (s), 132.53 (s), 132.53 (s), 132.87 (s), 133.57 (s), 135.62 (s),
137.62 (s), 145.78 (s), 146.00 (s), 154.25 (s). MS (ESI+): m/z 509.5
(MH+), Calc. for C35H28N2S: 508.2. Anal. Found: C, 82.61; H,
5.56; N, 5.54%. Calc. for C35H28N2S: C, 82.64; H, 5.55; N,
5.51%.
2.2. Synthesis of 2-(5-bromothiophen-2-yl)-1-(4-tert-
butylphenyl)-1H-phenanthro[9,10-d] imidazole (1)
Compound
1
was prepared by refluxing 9,10-phenan-
threnequinone (4.16 g, 20 mmol), 5-bromothiophene-2-carbal-
dehyde (2.45 mL, 20 mmol), 4-tert-butylbenzenamine (4.8 mL,
30 mmol) and ammonium acetate (18.87 g, 244.7 mmol) in glacial
acetic acid (120 mL) for 24 h under an argon atmosphere. After
cooling to room temperature, the dark-yellow mixture was
poured into a methanol solution with stirring. The separated
solid was filtered off, washed with methanol and dried to give
a yellow solid. The solid was purified by column chromatography
(petroleum ether : CH2Cl2, 3 : 1) on silica gel. A yellow powder
was finally obtained after it was stirred in refluxing ethanol,
1
subsequently filtered and dried in vacuum. Yield: 8 g (78%). H
NMR (400 MHz; CD2Cl2; Me4Si) dH [ppm]: 1.52 (s, 9H), 6.50 (d,
J ¼ 4 Hz, 1H), 6.89 (d, J ¼ 4 Hz, 1H), 7.20 (dd, J ¼ 8.3, 0.8 Hz,
1H), 7.25–7.34 (m, 1H), 7.49–7.59 (m, 3H), 7.64–7.81 (m, 4H),
1-(4-tert-butylphenyl)-2-(5-(naphthalen-1-yl)thiophen-2-yl)-1H-
phenanthro[9,10-d]imidazole (1N-TPI) (2b). The synthesis
procedure is similar to that ꢁof Ph-TPI. Light yellow powder.
1
Yield: 2.3 g (85%). mp: 232 C. H NMR (400 MHz; CD2Cl2;
Me4Si) dH [ppm]: 1.52 (s, 9H), 6.82 (d, J ¼ 3.8 Hz, 1H), 7.10 (d,
J ¼ 3.8 Hz, 1H), 7.25 (d, J ¼ 7.4 Hz, 1H), 7.31 (dd, J ¼ 11.1, 4.1
Hz, 1H), 7.50–7.99 (m, 13H), 8.24–8.39 (m, 1H), 8.75 (d, J ¼ 8.2
Hz, 1H), 8.80 (d, J ¼ 8.1 Hz, 2H); dC (50 MHz; CDCl3; Me4Si)
31.51 (s), 35.19 (s), 120.68 (s), 122.87–123.21 (m), 124.11 (s),
124.90 (s), 125.30 (s), 125.68 (d), 126.13 (s), 126.38 (s), 126.59 (s),
126.97–127.34 (m), 127.54 (s), 127.83 (s), 128.07 (s), 128.22–
128.51 (m), 128.70 (d), 129.22 (s), 131.54 (s), 131.88 (s), 133.45
(s), 133.91 (s), 135.66 (s), 137.63 (s), 143.84 (s), 146.03 (s),
154.22 (s). MS (ESI+): m/z 559.5 (MH+), Calc. for C39H30N2S:
558.21.
1-(4-tert-butylphenyl)-2-(5-(naphthalen-2-yl)thiophen-2-yl)-1H-
phenanthro[9,10-d]imidazole (2N-TPI) (2c). The synthesis
procedure is similar to that of Ph-TPI. Yellow powder. Yield: 2.5
g (89.6%). mp: 242 ꢁC. 1H NMR (400 MHz; CD2Cl2; Me4Si) dH
[ppm]: 1.56 (s, 9H), 6.78 (d, J ¼ 3.9 Hz, 1H), 7.25–7.36 (m, 3H),
7.47–7.58 (m, 3H), 7.59–7.65 (m, 2H), 7.66–7.95 (m, 8H), 8.08 (d,
J ¼ 1.3 Hz, 1H), 8.75 (d, J ¼ 8.3 Hz, 1H), 8.81 (dd, J ¼ 12.7, 4.8
Hz, 2H); dC (50 MHz; CDCl3; Me4Si) 31.50 (s), 35.19 (s), 120.65
(s), 122.81–123.17 (m), 123.42 (s), 124.08 (s), 124.87 (s), 125.69
(d), 126.34 (s), 127.00 (s), 127.21 (s), 127.52 (s), 127.83 (d), 128.31
(d), 128.69 (s), 128.94 (s), 129.19 (s), 132.38 (s), 133.87 (s), 135.62
(s), 137.58 (s), 145.90 (d), 154.21 (s). MS (ESI+): m/z 559.5
(MH+), Calc. for C39H30N2S: 558.21.
Scheme 1 Synthetic routes for 1 and 2a–2e.
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ꢁ
N-(4-(5-(1-(4-tert-butylphenyl)-1H-phenanthro[9,10-d]imi-
dazol-2-yl)thiophen-2-yl)phenyl)-N-phenylbenzenamine (TPA-
TPI) (2d). The synthesis procedure is similarꢁto that of Ph-TPI.
quenched samples at a heating rate of 10 C minꢀ1. Ip of 2a–2e
were measured with the thin-films on ITO glass substrates by
ultra-violet photoelectron spectroscopy (UPS) in a VG ESCA-
LAB 220i-XL surface analysis system, while the values of EA
were estimated by subtracting from Ip with the optical band gap
(Egap) determined from the absorption spectra of their solid-state
films. It should be noted that as the optical band gap is typical
slightly smaller than the transfer band gap by about 0.5 eV for
most organic materials, the EA values estimated here are thus
slightly larger than the actual values.
1
Yellow powder. Yield: 2.3 g (99%). mp: 276 C. H NMR (400
MHz; CD2Cl2; Me4Si) dH [ppm]: 1.53 (s, 9H), 6.64 (d, J ¼ 4.0 Hz,
1H), 7.02–7.17 (m, 9H), 7.23 (d, J ¼ 7.3 Hz, 1H), 7.27–7.37 (m,
5H), 7.42–7.65 (m, 5H), 7.69 (dd, J ¼ 11.1, 4.2 Hz, 1H), 7.77 (t,
J ¼ 7.4 Hz, 3H), 8.61–8.93 (m, 3H); dC (50 MHz; CDCl3; Me4Si)
31.52 (s), 35.19 (s), 120.64 (s), 122.51 (s), 123.17 (dd), 124.08 (s),
124.74 (d), 125.60 (s), 126.41 (d), 127.09 (d), 127.80 (d), 127.50
(s), 128.29 (d), 128.70 (s), 129.15 (s), 129.38 (s), 131.41 (s), 135.68
(s), 137.59 (s), 145.78 (s), 146.12 (s), 147.40 (s), 147.61 (s), 154.15
(s). MS (ESI+): m/z 676.6 (MH+), Calc. for C47H37N3S: 675.27.
Anal. Found: C, 83.49; H, 5.47; N, 6.18%. Calc. for C47H37N3S:
C, 83.52; H, 5.52; N, 6.22%.
2.4. OLED fabrication
Patterned indium-tin oxide (ITO) coated glass substrates with
a sheet resistance of 30 U per square were sequentially cleaned
with isopropyl alcohol, Decon 90, rinsed in de-ionized water,
then dried in an oven, and finally treated in an ultraviolet-ozone
chamber. The ITO substrates were then transferred into a depo-
sition chamber with a base pressure of 10ꢀ6 mbar. Devices with
a configuration of ITO/NPB (70 nm)/one of 2a–2e (40 nm)/
BPhen (20 nm)/LiF (0.5 nm)/Al (80 nm) were prepared by
thermal vapor deposition. In the devices, NPB (4,40-bis[N-(1-
naphthyl)-N-phenyl amino] biphenyl) and BPhen (4,7-diphenyl-
1,10-phenanthroline) were used respectively as the HTL and the
electron-transporting layer (ETL). Deposition rates for both
organic and metal layers were monitored with an in situ quartz
1-(4-tert-butylphenyl)-2-(5-(pyren-1-yl)thiophen-2-yl)-1H-
phenanthro[9,10-d]imidazole (Py-TPI) (2e). The synthesis
procedure is similar to that of Ph-TPI. Bright yellow powder.
ꢁ
1
Yield: 2.6 g (82%). mp: 249 C. H NMR (400 MHz; CD2Cl2;
Me4Si) dH [ppm]: 1.54 (s, 9H), 6.90 (d, J ¼ 3.8 Hz, 1H), 7.16–7.30
(m, 2H), 7.33 (t, 1H), 7.51–7.60 (m, 1H), 7.69 (td, J ¼ 8.5, 4.0 Hz,
3H), 7.75–7.88 (m, 3H), 8.05–8.31 (m, 8H), 8.58 (d, J ¼ 9.3 Hz,
1H), 8.76 (d, J ¼ 8.2 Hz, 1H), 8.78–8.89 (m, 2H); dC (50 MHz;
CDCl3; Me4Si) 31.51 (s), 35.20 (s), 120.68 (s), 123.03 (d), 124.54–
125.23 (m), 125.44 (s), 125.67 (s), 126.29 (d), 127.04 (s), 127.31 (t),
127.57 (s), 127.83–128.52 (m), 128.81 (d), 129.19 (d), 130.92 (s),
131.14 (s), 131.44 (s), 133.98 (s), 135.66 (s), 137.65 (s), 144.52 (s),
154.27 (s). MS (ESI+): m/z 633.6 (MH+), Calc. for C45H32N2S:
632.23. Anal. Found: C, 85.37; H, 5.12; N, 4.40%. Calc. for
C45H32N2S: C, 85.41; H, 5.10; N, 4.43%.
oscillation crystal and controlled at 1–2 A sꢀ1. A shadow mask
ꢀ
was used to define the cathode to make four 0.1 cm2 devices on
each substrate. Electroluminescence (EL) spectra, CIE coordi-
nates, and current density–voltage–luminance (J–V–L) charac-
teristics of OLEDs were measured with a programmable
Keithley model 237 power source and Spectrascan PR 650
photometer under ambient air conditions without encapsulation.
2.3. Characterization
3. Results and discussion
Nuclear magnetic resonance (NMR) spectra were recorded using
CD2Cl2 (HNMR) and CDCl3 (CNMR) as solvent with a Varian
Gemin-400 Varian spectrometer. Mass spectra were recorded on
a PE SCIEX API-MS system. Single crystal structures of 2a–2e
were further studied by X-ray diffraction with a Bruker Smart
3.1. X-ray crystal structures
Single crystals of 2a–2e were obtained except for 2d (i.e. TPA-
TPI) by diffusing n-hexane into dichloromethane solution. Fig. 1
shows the single crystal structure of 2c (i.e. 2N-TPI), in which the
thiophene ring and the phenanthroimidazole ring adopt an
almost coplanar configuration with a dihedral angle (b) of ca.
ꢀ
Apex CCD diffractometer (Mo Ka, l ¼ 0.71073 A) using
SMART [Bruker AXS, Madison, WI, USA, 1997]. The crystal-
lographic calculations were conducted using the SHELXL-97
programs (experimental details and CIFs are available in the
ESI†). Elemental analyses were carried out on a Vario EL Ele-
mentar by the Flash EA 1112 method. Absorption and photo-
luminescence (PL) spectra of the materials were recorded on
a Perkin-Elmer Lambda 2S UV–visible spectrophotometer and
Perkin-Elmer LS50 fluorescence spectrometer, respectively.
Fluorescence quantum yields in dichloromethane solution (Ff)
were determined according to the previous literature17 by
a
comparative method, using 9,10-diphenylanthracene as
a standard reference with Ff ¼ 0.90 in cyclohexane. Thermog-
ravimetric (TGA) measurements were performed on a TA
Instrument TGAQ50 with a heating rate of 10 C minꢀ1 under
ꢁ
a nitrogen atmosphere. Differential scanning calorimetric (DSC)
measurements were performed on a TA Instrument DSC2910.
The samples were firstly heated at a rate of 10 C minꢀ1 to melt
ꢁ
Fig. 1 A (a): Intermolecular stacking of 2N-TPI viewed from a direc-
tion. A (b): offset p–p stacking of the intermolecular of 2N-TPI; B: single
molecular structure of 2N-TPI. Hydrogen atoms are removed for clarity.
and then quenched. Glass transition temperatures (Tg) and
crystallization temperatures (Tc) were recorded by heating the
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Table 1 Dihedral angles in Ph-TPI, 1N-TPI, 2N-TPI, and Py-TPI
which the material showed a 5% weight loss, were measured to be
351, 374, 393, 440, and 422 ꢁC for 2a–2e, resꢁpectively. DSC
measurements were performed, from 20 to 280 C for Ph-TPI,
Angle (a)
a[ꢁ]
Angle (b)
b[ꢁ]
Angle (c)
c[ꢁ]
ꢁ
1N-TPI, 2N-TPI, and from 20 to 330 C for TPA-TPI and Py-
Ph-TPI
1N-TPI
2N-TPI
Py-TPI
82
98
87
86.5
7.5
7.5
2.4
1.6
32
TPI, respectively. There appears to be a glass transition
temperature (Tg) in the range of 91–128 ꢁC for all of the materials
except for Ph-TPI in the second heating process. It is observed in
Fig. 2(a) that Ph-TPI has a melting temperature (Tm) of 280 ꢁC
but does not show any glass transition up to its Tm, while in the
second heating process there appears to be a crystallization
temperature (Tc) of 86 ꢁC. As seen in Fig. 2(d) and 2(e), TPA-TPI
and Py-TPI exhibit better thermal properties among the five
compounds, perhaps owing respectively to the non-planarity of
TPA and the rigidity of the pyrene moieties. Key thermal data of
the compounds are summarized in Table 2.
60.7
16.8
127.7
a
Dihedral angle between 4-tert-butylphenyl ring and imidazole ring.
Dihedral angle between thiophene ring and imidazole ring. Dihedral
angle between thiophene ring and R.
b
c
2.4ꢁ leading to an extended molecule. The 2-naphthalene ring is
twisted about the thiophene ring with a dihedral angle (c) of ca.
16.8ꢁ. In addition, the tert-butylbenzene is highly twisted about
the phenanthroimidazole ring with a dihedral angle (a) of ca. 87ꢁ.
Fig. 1b shows that the molecules are arranged in an offset face-
to-face p–p stacking with the thiophene moiety and the imid-
azole moiety facing each other giving an inter-planar distance of
3.3. Optical properties
Fig. 3 depicts absorption and PL spectra of the five materials
measured in both dichloromethane solutions and solid-films on
quartz substrates. Intensive blue to green emissions peaked at
wavelengths ranging from 433 to 521 nm were observed. Key
optical parameters are summarized in Table 2. It is apparent that
the five compounds have similar absorption and PL peaks in
solution while showing different amounts of bathochromic shifts
in the solid state. For example, in CH2Cl2 solutions, all
compounds show similar absorption peaks at ca. 260 nm, which
may originate from their common thiophene ring and benzene
ring.19 In addition, the absorption band between 380 to 400 nm is
assigned to the p–p* electronic transition in the phenan-
throimidazole. For compounds 1N-TPI, TPA-TPI, and Py-TPI,
the absorption and the PL spectra show relatively small differ-
ences for their corresponding states in solutions and thin-films.
ꢀ
3.9 A. The electron-donating thiophene and the electron-
accepting imidazole moieties would lead to intermolecular elec-
trostatic interaction.18 In addition, the dihedral angle (c) (Table
1) between the thiophene ring and R (R is naphthalene for the
present case) is an important parameter, as a large dihedral angle
can suppress intermolecular p–p stacking in solid state and thus
prevent self-quenching of fluorescence.
3.2. Thermal properties
We investigated thermal properties of the five phenan-
throimidazole derivatives using TGA and DSC under a nitrogen
atmosphere. All the compounds exhibited good thermal stability.
Decomposition temperatures (Td), defined as the temperature at
Fig. 2 DSC curves of (a) Ph-TPI, (b) 1N-TPI, (c) 2N-TPI, (d) TPA-TPI and (e) Py-TPI.
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Table 2 Photophysical properties and energy levels of the five phenanthroimidazole derivatives
Tg/Tm/Td
(ꢁC)
HOMOe
(eV)
LUMOf
(eV)
Abs Onset (nm)/
Egapg (eV)
Compound
lmax,absa (nm)
lmax,PLa (nm)
Ffb
lmax,absc (nm)
lmax,PLc (nm)
Ph-TPI
1N-TPI
2N-TPI
TPA-TPI
Py-TPI
239, 264, 381
229, 260, 377
230, 264, 381
232, 256, 400
243, 343, 381
433,450.5
461
446,464.5
468
484.5
0.75
0.18
0.51
0.65
0.23
243,268,389
223,261,384
226,401
262,409
243,406
482.5
474
487.5
469, 487
503.5, 521
nod, 280, 351
91, 232,374
100, 242, 393
121, 276, 440
128, 249, 422
ꢀ5.19
ꢀ5.21
ꢀ5.11
ꢀ5.00
ꢀ5.11
ꢀ2.45
ꢀ2.47
ꢀ2.47
ꢀ2.41
ꢀ2.60
452/2.74
452/2.74
470/2.64
479/2.59
493/2.51
a
b
Measured in CH2Cl2 solvent at room temperature. Measured in dichloromethane using 9,10-diphenylanthracene as a standard (Ff ¼ 0.90 in
c
d
e
cyclohexane). Solid state thin-film samples on quartz substrate. no ¼ not observed. By measuring ionization potential (Ip) with ultra-violet
f
g
photoelectron spectroscopy (UPS) in their thin-film state on ITO substrates. LUMO ¼ HOMO + Egap
.
Measured by the onset absorption
wavelength (Abs Onset) of the solid state.
This suggests that the bulky and non-coplanar end-capped
groups of these compounds have successfully restrained inter-
molecular aggregation. It can also be correlated to the dihedral
parameters of the crystal structure as shown in Table 1. For Ph-
TPI and 2N-TPI, the PL spectra of their solid films are red-
shifted by about 40–50 nm with respect to those in solution. The
larger red shifts in Ph-TPI and 2N-TPI can be explained by their
stronger p–p stacking interactions due to the smaller dihedral
angle (c) of the phenyl ring (32ꢁ) and the 1-naphthalene (16.8ꢁ)
from the thiophene ring. The larger dihedral angles in 1N-TPI
(60.7ꢁ) and Py-TPI (127.7ꢁ) would weaken the intermolecular
interaction and thus result in smaller PL peak shifts between the
solution and the thin film.
extent of conjugation in the molecule. That is, a smaller dihedral
angle (c) will give a more extended conjugation system and
a higher fluorescent quantum yield in solution. However, in the
solid state this will also give a stronger intermolecular interaction
and increases the chance of aggregation quenching. Therefore, it
is important to design a molecule with a suitable dihedral angle
(c) to achieve an ideal compromise between the fluorescent
quantum yield in solution and the aggregation induced quench-
ing in solid. On the other hand, it should be noted that the crystal
data and the quantum yield were measured in the solid state and
liquid state, respectively, the above correlations is subjected to
further confirmation.
Concerning the fluorescence quantum yields in solution (as
shown in Table 2), Ph-TPI, 2N-TPI, and TPA-TPI exhibit higher
values of 0.75, 0.51, and 0.65, respectively. In contrast, 1N-TPI
and Py-TPI show considerably lower fluorescent quantum yields
of only 0.18 and 0.23, respectively. One possible cause is that due
to the larger dihedral angle (c), the excited states of 1N-TPI and
Py-TPI might be easier to dissipate their energies via rotation of
the naphthalene and pyrene groups. It can also be seen that the
fluorescent quantum efficiency increases with the increasing
3.4. Electroluminescence (EL) performance using 2a–2e as
non-doped emitters
To investigate EL properties of the five phenanthroimidazole
derivatives, devices with a configuration of ITO/NPB/one of 2a-
2e/BPhen/LiF/Al were fabricated. Energy level diagrams20,26 of
the devices are shown in Fig. 4. Key device performance
parameters are summarized in Table 3.
Fig. 3 Absorption (circle) and PL (square) spectra of the five phenanthroimidazole derivatives in CH2Cl2 (empty symbols) and solid film state (solid
symbols).
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Fig. 4 Energy level diagram of the device based on the five phenan-
throimidazole derivatives, NPB and BPhen.
Fig. 5 EL spectra of the devices based on the five phenanthroimidazole
derivatives. Inset: CIE coordinates of the OLEDs.
All the fabricated devices emit brightly when positive bias was
applied to the ITO. Fig. 5 depicts normalized EL spectra of the
five devices. Peak wavelengths of the EL spectra for the devices
using Ph-TPI, 1N-TPI, 2N-TPI, TPA-TPI, and Py-TPI as the
emitting layer (EML) are 496, 494, 516, 471, 496, and 531 nm,
respectively. The EL spectra of the devices are almost the same as
the corresponding PL spectra of solid films of the five
compounds. This suggests that the EL is indeed derived from the
five compounds. Particularly, no exciplex emission can be
observed.21 The emission color ranged between blue to green,
corresponding to CIE coordinates (inset of Fig. 5) of (0.20, 0.39),
(0.19, 0.38), (0.26, 0.50), (0.19, 0.39), and (0.32, 0.59) for Ph-TPI,
1N-TPI, 2N-TPI, TPA-TPI, and Py-TPI, respectively. This
shows that by changing the conjugation length of the molecules,
the color of the emission is changed. For example, by joining the
thiophene to different positions of the naphthalene rings, the
dihedral angle c changes from 60.7ꢁ (1N-TPI) to 16.8ꢁ (2N-TPI).
This suggests a more extended conjugation in 2N-TPI and
explains why its emission color is considerably red-shifted
compared to that of 1N-TPI. These results show that changing
the moieties on the C5-position of the thiophene is an effective
way for tuning the optical properties of the present series of
phenanthroimidazole derivatives.
a voltage of lower than 9.5 V, respectively. Only 4.74, 5.42, 4.71,
4.58, and 4.48 V, respectively, are required to deliver a brightness
of 1000 cd mꢀ2 for the devices based on Ph-TPI, 1N-TPI, 2N-
TPI, TPA-TPI, and Py-TPI. Interestingly, the incorporation of
thiophene to the C2-position of the imidazole can render a much
shallower HOMO energy level (only 5.0–5.2 eV) for the present
phenanthroimidazole derivatives compared to previously
Fig. 6 shows J-V-L characteristics of the five devices. The
devices based on Py-TPI, 2N-TPI, TPA-TPI, Ph-TPI show high
brightness of 15 960, 15 780, 13 060, and 12 130 cd mꢀ2 at
Fig. 6 Current density–voltage and luminance–voltage characteristics of
the OLEDs based on the phenanthroimidazole derivatives.
Table 3 EL performances of devices based on the phenanthroimidazole derivatives
Onset
Voltage at
Voltage at
Max brightness Max CEb Max PEc
Max EQE EL
(%)
CIE
(x, y)
Emitter
voltagea (V) 1000 cd mꢀ2 (V) 20 mA cmꢀ2 (V) (cd mꢀ2
)
(cd Aꢀ1
)
(lm Wꢀ1
)
(nm)
Ph-TPI
1N-TPI
2N-TPI
TPA-TPI 2.6
Py-TPI
2.7
2.7
2.6
4.7
5.4
4.7
4.6
4.5
4.5
4.4
4.6
4.4
4.1
12130
7964
15780
13060
15960
3.67
1.48
4.24
4.60
2.93
3.61
1.53
4.11
4.68
3.02
1.50
0.62
1.45
1.91
0.88
496
494
516
(0.20, 0.39)
(0.19, 0.38)
(0.26, 0.50)
471, 496 (0.19, 0.39)
520, 531 (0.32, 0.59)
2.3
a
b
Obtained at 1 cd mꢀ2
.
Current efficiency of the devices. c Power efficiency of the devices.
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reported molecules11 without the thiophene ring. It also appears
that introduction of the thiophene ring to the C2-imidazole
mainly affects the HOMO energy level.
With these advantages, devices based on the five materials show
lower onset and operation voltages compared to devices with
recently reported emitters.
To further verify these, we synthesized another two
compounds (Fig. 7) by swapping the benzene ring and the thio-
phene ring (compound A) or replace the thiophene ring with
another benzene ring (compound B). Energy levels for the
compounds are determined via UPS and absorption spectros-
copy to be: HOMO: A: 5.41, B: 5.41, Ph-TPI: 5.19 eV and
LUMO: A: 2.49, B: 2.32 and Ph-TPI: 2.45 eV. It can be seen that
upon introduction of the thiophene ring, A and Ph-TPI show
slightly larger LUMO values. The isomeric compound A and Ph-
TPI show a large difference in their HOMO levels. It can be
explained by the decreased HOMO–LUMO energy gap which is
attributed to the more planar configuration between the thio-
phene ring and the imidazole ring in Ph-TPI. On the other hand,
the lone electron pair in the sulfur of the thiophene ring might
undergo an n-p* transition with lower energy absorption with
the more coplanar structure.22,23 This indicates that direct
connection of the thiophene to the imidazole plays a key role in
controlling the HOMO energy of the compound. With the
shallow HOMO, it can effectively lower the hole-injection barrier
from the NPB layer (i.e. HOMO ¼ 5.4 eV) into the phenan-
throimidazole derivatives, resulting in low turn-on voltages
(defined as the voltage at a brightness of 1 cd mꢀ2) below 2.7 V for
all five devices. Furthermore, the shallow HOMOs of the phe-
nanthroimidazole derivatives also imply that it is more difficult
for the holes to leak into the ETL. This better confines the
excitons within the phenanthroimidazole derivative layer and
results in high efficiency.
Current and power efficiencies of the devices are shown in
Fig. 8. The maximum efficiencies are 3.67 cd Aꢀ1 (3.61 lm Wꢀ1
and 1.50%), 1.48 cd Aꢀ1 (1.53 lm Wꢀ1 and 0.62%), 4.24 cd Aꢀ1
(4.11 lm Wꢀ1 and 1.45%), 4.60 cd Aꢀ1 (4.68 lm Wꢀ1 and 1.91%),
and 2.93 cd Aꢀ1 (3.02 lm Wꢀ1 and 0.88%), respectively, for the
devices based on Ph-TPI, 1N-TPI, 2N-TPI, TPA-TPI, and Py-
TPI. The maximum current/power efficiency as well as the
maximum external quantum efficiency (see in Table 3) is nearly
consistent with the trend in fluorescent quantum yields for the
five compounds. As shown in Fig. 8, the current efficiencies
decrease with the increase of the current density. However, for
the devices with 1N-TPI and Py-TPI the efficiency decreases are
mild. Among the performances of all the devices, the green-
emitting device based on 2N-TPI with CIE coordinates (0.26,
0.50) show high current efficiency (CE) and power efficiency (PE)
of 4.24 cd Aꢀ1 and 4.11 lm Wꢀ1, respectively, which are better
Table 4 Electroluminescent performance and/or values of the HOMO
energy of some typical hole-injection or -transport materials, typical
electron-transport or/and hole-blocking materials, and some recently
reported emitters
On-set
voltage
(V)
Voltage at
20 mA cmꢀ2
(V)
HOMO
(eV)
Emitters
Reference
Py-TPI
ꢀ5.11a
ꢀ5.00a
ꢀ5.11a
ꢀ5.19a
ꢀ5.21a
ꢀ5.7 a
ꢀ5.4a
2.3
2.6
2.6
2.7
4.1
4.4
4.6
4.5
4.4
5.38
–
5.2
3.3
–
this work
this work
this work
this work
this work
TPA-TPI
2N-TPI
Ph-TPI
1N-TPI
NPDPN
TDPFPA
DDPFTBC
TPyTPA
TDPFPC
DFDF
To more clearly show the advantage of the phenan-
throimidazole derivatives with low Ip, electroluminescent
performance of some recently reported non-doped emit-
ters4,9,21,24–37 are compared in Table 4. As we can see from the
table, all the present phenanthroimidazole derivatives have
HOMO levels similar or shallower than the classical hole trans-
port/injection materials,36–40 facilitating the hole injection/trans-
port from the HTL. Furthermore, the much shallower HOMO
level than those of the classical electron transport mate-
rials,41,43–45 effectively prevent the leakage of holes into the ETL.
2.7
3.12
2.65
2.8
2.6
2.53
3.7
24
9
ꢀ5.7a
25
21
26
27
28
ꢀ5.19a
ꢀ5.7a
ꢀ5.9a
5.2
–
o-/m-/p-TP-
EPY
ꢀ6.03–
ꢀ6.13b
ꢀ6.03b
ꢀ5.94b
ꢀ5.4b
8.2ꢂ8.7
TP-EPY
TP-EIF
TTPEPy
BLHPC
DSX-IF
DCDPF
POAn
9.2
11.2
3.6
–
10
<3
3
–
–
5.0
4.0
–
–
–
>20
–
–
–
8.8
6.2
5.6
9.5
29
29
30
31
32
33
34
35
4
ꢀ5.30b
ꢀ5.5b
ꢀ5.9a
ꢀ5.6b
ADN
ꢀ5.8b
MADNa
Cis-TSDTB
BDPQ
ꢀ5.7b
36
37
ꢀ5.7b
Typical hole-injection or -transport materials
CuPc
m-MTDATA
TPD
NBP, NPD
TCTA
ꢀ4.8b
ꢀ5.1b
ꢀ5.6b
ꢀ5.4b
ꢀ5.7c
38
39
40
41
42
Typical electron-transport or/and hole-blocking materials
Alq3
ꢀ5.8 a
ꢀ6.2b
ꢀ6.4b
ꢀ6.7c
43
41
44
45
TPBI
Bphen
BCP
a
obtained from ultra-violet photoelectron spectroscopy (UPS).
obtained from electrochemical method with cyclic voltammetry (CV).
methodology not mentioned.
b
c
Fig. 7 Structures of compounds A, B and Ph-TPI.
8212 | J. Mater. Chem., 2011, 21, 8206–8214
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direct connection of the thiophene ring with the imidazole ring
affects the HOMO energy level of the compounds.
Acknowledgements
This work was supported by the Research Grants Council of the
Hong Kong Special Administrative Region, China (Project No.
CityU 101508). We also thank the support from the National
Natural Science Foundation of China (No. 91027041) and the
Key Laboratory of Photochemical Conversion and Optoelec-
tronic Materials, TIPC, CAS. In addition, we also thank Dr Jun
1
He and Dr Shen-Mei Sun for H NMR measurement and Mass
Spectroscopy measurement.
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