Bipolar phenanthroimidazole derivatives containing bulky polyaromatic hydrocarbons for nondoped blue electroluminescence devices with high efficiency and low efficiency roll-off

Article abstract of DOI:10.1021/cm4030414

A series of bipolar phenanthroimidazole derivatives, TTP-TPI, DPT-TPI, and DPF-TPI, were designed and synthesized by incorporating polyaromatic hydrocarbon groups to phenanthroimidazole through a phenyl bridge. The bulky polyaromatic hydrocarbon units endow the molecules with high glass-transition temperatures, nonplanar twisty structures which reduce molecular aggregations. The bipolar transporting natures of these materials are demonstrated by single-carrier devices with supported of theoretical calculations. Nondoped organic light-emitting devices (OLEDs) using these phenanthroimidazole derivatives as emitters show blue-violet to sky-blue emissions with Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.05) for TTP-TPI, (0.16, 0.07) for DPT-TPI, and (0.17, 0.24) for DPF-TPI. The TTP-TPI-, DPT-TPI-, and DPF-TPI-based nondoped devices show impressive external quantum efficiencies (EQE) of 5.02, 5.25, 4.85%, respectively (corresponding current efficiencies: 2.10, 3.13, and 8.41 cd/A). These values are the best or among the best comparing to those of the reported nondoped OLEDs with the corresponding color gamuts. These devices also show small efficiency roll-off at high brightness (1000 cd/m²) with EQEs drop by 20.7, 12, and 0%. Moreover, with well-balanced carrier transport, DPF-TPI based device can achieve a higher brightness of 10000 cd/m² with EQE maintaining at 4.49% (only drop by 7.4%).

Full text of DOI:10.1021/cm4030414

Article
pubs.acs.org/cm
Bipolar Phenanthroimidazole Derivatives Containing Bulky
Polyaromatic Hydrocarbons for Nondoped Blue Electroluminescence
Devices with High Efficiency and Low Efficiency Roll-Off
Yi Yuan,†,‡ Jia-Xiong Chen,† Feng Lu,† Qing-Xiao Tong,*,† Qing-Dan Yang,‡ Hin-Wai Mo,‡ Tsz-Wai Ng,‡
Fu-Lung Wong,‡ Zheng-Qing Guo,§ Jun Ye,‡,⊥ Zhan Chen,⊥ Xiao-Hong Zhang,⊥ and Chun-Sing Lee*,‡
†Department of Chemistry, Shantou University, Guangdong 515063, China
§
‡Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Science and Department of
Biology and Chemistry, City University of Hong Kong, Hong Kong SAR, China
⊥Nano-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
S
* Supporting Information
ABSTRACT: A series of bipolar phenanthroimidazole derivatives, TTP-TPI, DPT-
TPI, and DPF-TPI, were designed and synthesized by incorporating polyaromatic
hydrocarbon groups to phenanthroimidazole through a phenyl bridge. The bulky
polyaromatic hydrocarbon units endow the molecules with high glass-transition
temperatures, nonplanar twisty structures which reduce molecular aggregations. The
bipolar transporting natures of these materials are demonstrated by single-carrier
devices with supported of theoretical calculations. Nondoped organic light-emitting
devices (OLEDs) using these phenanthroimidazole derivatives as emitters show blue-
violet to sky-blue emissions with Commission Internationale de l’Eclairage (CIE)
coordinates of (0.16, 0.05) for TTP-TPI, (0.16, 0.07) for DPT-TPI, and (0.17, 0.24)
for DPF-TPI. The TTP-TPI-, DPT-TPI-, and DPF-TPI-based nondoped devices show
impressive external quantum efficiencies (EQE) of 5.02, 5.25, 4.85%, respectively
(corresponding current efficiencies: 2.10, 3.13, and 8.41 cd/A). These values are the
best or among the best comparing to those of the reported nondoped OLEDs with the
corresponding color gamuts. These devices also show small efficiency roll-off at high brightness (1000 cd/m2) with EQEs drop
by 20.7, 12, and 0%. Moreover, with well-balanced carrier transport, DPF-TPI based device can achieve a higher brightness of
10000 cd/m2 with EQE maintaining at 4.49% (only drop by 7.4%).
KEYWORDS: organic light-emitting diodes, phenanthroimidazole, blue emitters, fluorescence
Therefore, fluorescent emitters are used for most high-efficiency
blue and deep-blue OLEDs.
INTRODUCTION
■
Blue organic light-emitting materials have attracted much
attention because of their significant applications in flat-panel
displays and solid-state lighting.1 Especially in full-color displays,
the blue emitter can not only effectively reduce the power
consumption of the devices but also be utilized to generate
emission of other colors by energy transfer to a suitable emissive
dopant.2 However, high-performance blue emitters are still
relatively rare.
To obtain high efficiency, phosphorescent organic light-
emitting devices (PhOLEDs) have attracted intense interest as
they can approach 100% internal quantum efficiency by utilizing
both singlet and triplet excitons.3,4 However, PhOLEDs typically
have shorter lifetime and sharper efficiency roll-off at high
brightness. In addition, it is difficult to synthesize efficient deep-
blue phosphors, because of the additional nonradiative pathway
via the metal d-orbitals when regulating the emissive metal−
ligand charge-transfer (MLCT) band into the deep blue.5
Charge transport and balance are two crucial factors for
obtaining high efficiency and stable OLEDs.6 One promising
strategy for this demand is to simultaneously incorporate
electron-donor and electron-acceptor groups into a single
molecule to obtain materials with bipolar transporting proper-
ties. This could facilitate better balanced transports of both holes
and electrons, and broaden the exciton formation zone,
consequently improving the device performance and reducing
efficiency roll-off.7 However, for application in blue OLEDs, this
strategy does have some disadvantages, such as a huge
bathochromic effect,8 low quantum yield due to the dipolar
quenching,9 as well as the unavoidable decrease in band gap of
the material due to intramolecular charge transfer.10 To address
Received: October 2, 2013
Revised: November 27, 2013
Published: November 27, 2013
© 2013 American Chemical Society
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(m, 3H), 7.46−7.40 (m, 4H), 7.26 (ddd, J = 18.2, 8.3, 7.2 Hz, 2H), 3.21
(s, 1H), 1.45 (s, 9H). MS (ESI+): m/z 450.14 (MH+). Calcd for
C33H26N2: 450.57.
these issues, one possible approach is to interrupt the π-
conjugation between the donor and the acceptor groups with a Si
or other sp3-hybridized atom. For example, Ma and Yang
developed a series of silicon-bridged host materials containing
both arylamine and heterocyclic groups for efficient PhOLED-
s.7a,11 However, this kind of material still cannot serve as an
efficient emitter for nondoped OLEDs because their emission
peaks would typically blue-shifted to the ultraviolet region. On
the other hand, Adachi et al. recently reported a series of
materials with thermally activated delayed fluorescence
(TADF).12 In these materials, reverse intersystem crossing
(RISC) from triplet to singlet states can be thermally activated
and thus enable a potentially 100% internal singlet yields. As
highly efficient TADF materials also need to be doped in
appropriate host materials to avoid concentration quenching of
the triplet excitons. Hence, high-performance bipolar hosts with
sufficiently wide energy gap are also desirable.
1-(4-(Tert-butyl)phenyl)-2-(3′,4′,5′-triphenyl-[1,1′:2′,1″-terphen-
yl]-4-yl)-1H-phenanthro[9,10-d]imidazole (TTP-TPI). Compound 1
(1.23 g, 2.73 mmol) and 2,3,4,5-tetraphenylcyclopenta-2,4-dienone
(0.95 g, 2.48 mmol) were dissolved in o-xylene (70 mL) under an argon
atmosphere, and the resultant mixture was heated for 24 h at 150 °C.
After the mixture was cooled to room temperature, ethanol (100 mL)
was added. The precipitate was filtered, washed with ethanol (100 mL),
and dried in a vacuum. Following column chromatography (petroleum
ether:CH2Cl2 = 1:1) on silica gel, TTP-TPI was obtained as a white
powder. Yield: 1.70 g (85.0%). 1 H NMR (400 MHz, CD2Cl2) δ 8.66
(dd, J = 20.7, 8.4 Hz, 3H), 7.66 (t, J = 7.3 Hz, 1H), 7.57 (t, J = 7.5 Hz,
1H), 7.55−7.51 (m, 2H), 7.43 (s, 2H), 7.34 − 7.30 (m, 4H), 7.19 (s,
1H), 7.11−7.07 (m, 6H), 7.02 (d, J = 8.4 Hz, 2H), 6.90 − 6.84 (m, 6H),
6.82 − 6.77 (m, 7H), 6.74 (dd, J = 6.6, 3.2 Hz, 2H), 1.36 (s, 9H). 13C
NMR (100 MHz, CDCl3) δ 146.35, 146.16, 141.85, 141.56, 140.82,
140.14, 139.83, 139.62, 139.16, 131.50, 131.41, 131.27, 129.90, 129.76,
128.65, 128.42, 127.60, 127.25, 126.97, 126.91, 126.62, 126.30, 125.73,
125.63, 125.34, 124.04, 123.05, 120.91, 35.00, 31.41 ppm. MS (ESI+):
m/z 807.51 (MH+). Calcd for C61H46N2: 807.03.
Phenanthroimidazole is a near-ultraviolet emitter with dual
nature due to different bonding structures of two nitrogen atoms
at the imidazole ring. It exhibits electron-donating properties
when a neutral or an electron-withdrawing moiety is attached to
its 2-position,13 but it shows electron withdrawing properties
when linked with an electron donor.14 Polycyclic aromatic
1-(4-(Tert-butyl)phenyl)-2-(4-(1,4-diphenyltriphenylen-2-yl)-
phenyl)-1H-phenanthro[9,10-d]imidazole (DPT-TPI). Using a similar
synthesis procedure for TTP-TPI by a Diels−Alder reaction, beige
1
powder was finally obtained. Yield: 1.32 g (80.2%). H NMR (400
hydrocarbon units (Mullen dendrons) have been shown to be
̈
MHz, CD2Cl2) δ 8.79 (dd, J = 20.5, 8.3 Hz, 3H), 8.49 (d, J = 8.2 Hz, 2H),
7.79 (s, 1H), 7.74 (dd, J = 8.4, 1.0 Hz, 1H), 7.72−7.64 (m, 4H), 7.57−
7.51 (m, 4H), 7.50−7.43 (m, 9H), 7.33 − 7.22 (m, 5H), 7.16−7.11 (m,
3H), 7.08−7.02 (m, 3H), 1.49 (s, 9H). 13 C NMR (100 MHz, CDCl3) δ
144.26, 141.73, 139.31, 138.14, 137.90, 137.25, 136.49, 132.17, 132.06,
131.91, 131.62, 131.38, 130.62, 130.06, 129.75, 129.63, 129.11, 128.71,
128.47, 127.25, 126.98, 126.78, 126.45, 126.24, 125.57, 125.27, 124.05,
123.21, 123.07, 120.92, 119.53, 112.75, 35.01, 31.42 ppm. MS (ESI+):
m/z 805.14 (MH+). Calcd for C61H44N2: 805.02.
1-(4-(Tert-butyl)phenyl)-2-(4-(7,10-diphenylfluoranthen-8-yl)-
phenyl)-1H-phenanthro[9,10-d]imidazole (DPF-TPI). Using a similar
synthesis procedure for TTP-TPI by a Diels−Alder reaction, yellow-
green powder was finally obtained. Yield: 1.57 g (84.9%). 1 H NMR (400
MHz, CD2Cl2) δ 8.76 (dd, J = 20.6, 8.4 Hz, 3H), 7.78 (t, J = 8.2 Hz, 3H),
7.73 − 7.66 (m, 3H), 7.64 (d, J = 2.0 Hz, 1H), 7.63 (d, J = 2.0 Hz, 1H),
7.59 (d, J = 1.6 Hz, 1H), 7.58 − 7.49 (m, 3H), 7.45 (dt, J = 4.5, 2.5 Hz,
6H), 7.42 (d, J = 2.8 Hz, 1H), 7.41 − 7.36 (m, 3H), 7.34 (d, J = 1.9 Hz,
1H), 7.33 − 7.25 (m, 3H), 7.20 (dd, J = 8.4, 2.2 Hz, 3H), 6.70 (d, J = 7.0
Hz, 1H), 1.47 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 139.86, 138.98,
138.29, 137.94, 136.46, 136.06, 135.76, 133.08, 131.00, 130.28, 129.79,
129.67, 129.08, 128.66, 128.53, 128.40, 127.87, 127.63, 127.44, 126.99,
126.74, 126.25, 124.04, 123.40, 123.04, 120.92, 35.02, 31.41 ppm. MS
(ESI+): m/z 779.04 (MH+). Calcd for C59H42N2: 778.98.
effective molecular building blocks for inhibiting molecular
aggregation or π−π stacking in the solid state.15 In addition, the
polyphenylated aromatic cores could serve as a platform for
tuning of donor−acceptor interactions.16 It can also act as a π-
acceptor and be incorporated to an electron donor to form
bipolar molecule.17
Herein, we designed and synthesized a series of new materials,
TTP-TPI, DPT-TPI and DPF-TPI, by combining phenanthroi-
midazole and bulky polyaromatic hydrocarbon groups. It is
anticipated that the electron-donating and -accepting groups
would impart the material with bipolar transporting nature;
meanwhile, emission spectra of the molecule are expected to red-
shift to the blue region due to the increase in conjugation length.
We also expect that the materials to possess good thermal and
morphological stability as a result of the bulky polyaromatic
hydrocarbon groups. Thermal, photophysical, and electro-
luminescent properties of the compounds were comprehensively
investigated. The new phenanthroimidazole derivatives have
been demonstrated as excellent blue fluorescent emitters for
nondoped OLEDs.
Characterization. Nuclear magnetic resonance (1 NMR) spectra
were recorded using CD2Cl2 (1 H NMR) and CDCl3 (13 C NMR) as
solvent with a Varian Gemin-400 Varian spectrometer. Mass spectra
were recorded on a PE SCIEX API-MS system. Absorption and
photoluminescence (PL) spectra of the materials were recorded with a
PerkinElmer Lambda 2S UV/vis spectrophotometer and a Perkin−
Elmer LS50B luminescence spectrophotometer, respectively. Thermog-
ravimetric analysis (TGA) measurements were performed on a TA
Instrument TGAQ50 at a heating rate of 10 °C/min under a nitrogen
atmosphere. Differential scanning calorimetric (DSC) measurements
were performed on a TA Instrument DSC2910. The samples were first
heated at a rate of 10 °C/min to melt and then quenched. Glass
transition temperature (Tg) and crystallization temperature (Tc) were
recorded by heating the quenched samples at a heating rate of 10 °C/
min. The HOMO energy of the materials were measured directly by
ultraviolet photoelectron spectroscopy (UPS), whereas the LUMO
energy was estimated by subtracting the HOMO energy with the optical
band gap determined from the lowest energy absorption edge of the
absorption spectrum.
EXPERIMENTAL SECTION
■
Starting materials 4-ethynylbenzaldehyde,18 1,3-diphenyl-2H-
cyclopenta[l] phenanthren-2-one,19 and 7,9-diphenyl-8H-cyclopenta-
[a]acenaphthylen-8-one20 were synthesized according to the literature
procedures. All other chemicals and solvents were used as received from
commercial suppliers without further purification.
Syntheses. 1-(4-(Tert-butyl)phenyl)-2-(4-ethynylphenyl)-1H-
phenanthro[9,10-d]imidazole (1). The product was prepared by
refluxing 9,10-phenanthrenequinone (2.08 g, 10.0 mmol), 4-ethynyl-
benzaldehyde (1.30 g, 10.0 mmol), 4-tert-butylbenzenamine (1.97 g,
12.0 mmol), and ammonium acetate (7.71 g, 100 mmol) in glacial acetic
acid (60 mL) for 24 h under an argon atmosphere. After cooling to room
temperature, a pale yellow mixture was obtained and poured into a
methanol solution with stirring. The separated solid was filtered off,
washed with methanol, and dried to give a pale 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, subsequently filtered and dried in vacuum. Yield:
3.71 g (82.3%). 1 H NMR (400 MHz, CD2Cl2) δ 8.77 (ddd, J = 25.0,
12.8, 4.7 Hz, 3H), 7.77−7.73 (m, 1H), 7.69−7.63 (m, 3H), 7.58−7.51
Device Fabrication and Measurement. Patterned indium tin
oxide (ITO) glass with a sheet resistance of 15 Ω per square was used as
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Scheme 1. Synthetic Routes for TTP-TPI, DPT-TPI, and DPF-TPI
the substrate. Before device fabrication, the ITO glass substrates were
cleaned with isopropanol, Decon 90 and deionized water, dried in an
oven at 120 °C, treated with UV-ozone, and finally transferred to a
vacuum deposition system with a base pressure better than 5 × 10−4 Pa
for organic and metal deposition. Devices were fabricated by thermal
deposition of organic layers at a deposition rate of 1−2 Å/s. The cathode
was completed through thermal deposition of LiF at a deposition rate of
0.1 Å/s, followed by Al deposition at a rate of 10 Å/s. Electro-
luminescence (EL) spectra and CIE color coordinates were measured
with a Spectrascan PR650 photometer and the current−voltage−
luminance characteristics were measured with a computer-controlled
Keitheley 2400 sourcemeter under ambient atmosphere.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Synthetic routes and
chemical structures of the compounds, 1-(4-(tert-butyl)phenyl)-
2-(3′,4′,5′-triphenyl-[1,1′:2′,1″-terphenyl]-4-yl)-1H-
phenanthro[9,10-d]imidazole (TTP-TPI), 1-(4-(tert-butyl)-
phenyl)-2-(4-(1,4-diphenyltriphenylen-2-yl)phenyl)-1H-
phenanthro[9,10-d]imidazole (DPT-TPI) and 1-(4-(tert-butyl)-
ph-enyl)-2-(4-(7,10-diphenylfluoranthen-8-yl)phenyl)-1H-
phenanthro[9,10-d]imidazole (DPF-TPI) are depicted in
Scheme 1. A key intermediate 1-(4-(tert-butyl)phenyl)-2-(4-
ethynylphenyl)-1H-phenanthro[9,10-d]imidazole (1) was pre-
pared according to the literature.21 The Diels−Alder reaction
between compound 1 and 2,3,4,5-tetraph-enylcyclopenta-2,4-
dienone, 1,3-diphenyl-2H-cyclopenta[l]phenanthren-2-one or
7,9-diphenyl-8H-cyclopenta[a]acenaphthylen-8-one give rise to
the final products TTP-TPI, DPT-TPI, and DPF-TPI,
respectively. All the compounds were purified with the silica
column method. Chemical structures of all of the compounds
were fully characterized with 1H NMR and 13C NMR
spectroscopy and mass spectrometry. Detailed synthetic
procedures and characterization of the new compounds have
been described in the Experimental Section.
Figure 1. DSC curves for (a) DPT-TPI and (b) DPF-TPI.
ravimetric analysis (TGA) and differential scanning calorimetry
(DSC) under a nitrogen atmosphere. All the compounds
exhibited good thermal stability. As shown in Figure S1 in the
Supporting Information, decomposition temperatures (Td,
Thermal Properties. Thermal properties of the three
phenanthroimidazole derivatives were investigated by thermog-
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Table 1. Key Physical Properties of the Three Compounds
λmaxabs (nm)
λmaxPL (nm)
a
compd
THF
film
THF
film
Φf
Tg /Tm/Td (°C)
HOMO (eV)
Eg (eV)
LUMO (eV)
TTP-TPI
DPT-TPI
DPF-TPI
262, 365
263, 365
262, 367
266, 370
270, 367
265, 372
406
432
458
424
439
472
∼1
∼1
∼1
n.o./n.o./421
197/335/458
176/351/439
−5.67
−5.68
−5.58
3.26
3.21
2.99
−2.41
−2.47
−2.59
a
n.o. = not observed.
Figure 2. UV−vis absorption (left panel) and PL spectra (right panel) of TTP-TPI, DPT-TPI, and DPF-TPI in THF solution (solid symbols) and in
thin film (open symbols).
Figure 3. Optimized molecular geometries and the calculated spatial distributions of the HOMOs and the LUMOs of TTP-TPI, DPT-TPI, and DPF-
TPI.
defined as temperature at which there is a 5% weight loss), were
measured to be 421, 458, and 439 °C for TTP-TPI, DPT-TPI,
and DPF-TPI, respectively. DSC measurements were performed,
from 20 to 355 °C for all of the materials. During the first heating
process, DPF-TPI only shows an endotherimic peak at a melting
point (Tm) of 351 °C. When the sample was heated again, a glass-
transition revealed at 176 °C. Thermal properties of DPT-TPI
are similar to those of DPF-TPI except for an additional phase
transformation at 316 °C (T1), and exhibited a Tg and a Tm of 197
and 335 °C, respectively. No obvious glass transition and
endothermic melting transition were observed for TTP-TPI and
it is most likely due to its smaller heat capacity and crystallinity.
Figure 1 shows the DSC curves of DPT-TPI and DPF-TPI. Key
thermal data of the phenanthroimidazole derivatives are
summarized in Table 1. As expected, the new materials have
much better thermal properties than their parent molecule 1,2-
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Table 2. EL Performance of the Phenanthroimidazole Derivatives
maximum
@ 1000 cd/m2
a
c
Von
λEL
b
b
b
b
b
b
emitter
(V)
ηC (cd/A)
ηP (lm/W)
ηext (%)
ηC (cd/A)
ηP (lm/W)
ηext (%)
(nm)
CIE (x, y)
TTP-TPI
3.1
2.10
1.88
5.02
(4.94 0.08)
1.47
0.81
(0.79 0.02)
3.98
(3.90 0.08)
424
0.16, 0.05
(2.06 0.04)
3.13
(3.09 0.05)
8.41
(8.33 0.11)
(1.85 0.03)
3.22
(2.96 0.26)
7.23
(6.91 0.32)
(1.44 0.03)
2.78
(2.75 0.03)
8.40
(8.32 0.08)
DPT-TPI
DPF-TPI
2.9
2.7
5.25
(5.19 0.07)
1.92
(1.84 0.08)
4.62
(4.55 0.07)
432
468
0.16, 0.07
0.17, 0.24
4.85
(4.83 0.04)
5.69
(5.55 0.14)
4.85
(4.82 0.03)
a
b
Von is the turn-on voltage at 1 cd/m2. ηC, ηP, and ηext are the current, power, and external quantum efficiency, respectively. Order of measured
c
efficiency values: maximum and average values in parentheses, averaged from four devices. The maximum electroluminescence wavelength.
Figure 5. Current density−voltage−luminance (J−V−L) characteristics
of the OLEDs based on the three phenanthroimidazole derivatives.
Figure 6. Energy level diagrams of the three phenanthroimidazole-
derivative-based devices.
improved their morphological stability, which is highly important
for applications in organic electronic devices.
Optical Properties. Figure 2 shows normalized UV−vis
absorption and PL spectra of TTP-TPI, DPT-TPI, and DPF-TPI
in diluted THF solutions and as thin film prepared by thermal
evaporation on quartz plates. Key photophysical data of the three
compounds are summarized in Table 1. These materials show
two similar absorption peaks in solution while showing slight
bathochromic shifts in the film state. A higher intensity
absorption band around 260 nm are probably originated from
Figure 4. EL spectra of the (a) TTP-TPI-, (b) DPT-TPI-, and (c) DPF-
TPI-based devices viewed in the normal direction at a luminance of 10,
100, and 1000 cd/m2.
the π−π* transition of their common benzene ring, and a lower
intensity band around 330−370 nm can be assigned to the
delocalized π−π* transition of the phenanthroimidazole.23 In
addition, DPT-TPI and DPF-TPI exhibit appreciable absorption
band in the range of 284−304 nm, which can be attributed to the
π−π* transition in the aryl core of the polyaromatic hydrocarbon
groups.24 As shown in Figure 2, all of the three compounds
diphenyl-1H-phenanthro[9,10-d]imidazole (PPI, Tg = 62 °C, Td
= 340 °C).22 The results confirm that introduction of highly
twisted and bulky polyaromatic hydrocarbons can greatly
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end-capping polyaromatic hydrocarbon groups. It should be
noted that all compounds show only small peak shifts in their
absorption and PL spectra from THF solutions to solid states.
This indicates that the highly twisted substituent on the 1-
imidazole position as well as the noncoplanar polyaromatic
hydrocarbon moieties can effectively suppress intermolecular
aggregation in the solid state. In addition, fluorescent quantum
yields of TTP-TPI, DPT-TPI and DPF-TPI were estimated to be
approaching unity in dichloromethane using 9,10-diphenylan-
thracene as a standard (Φf = 0.90 in cyclohexane).25 The optical
band gaps (Eg) determined from the threshold of the optical
absorption are 3.26, 3.21, and 2.99 eV for TTP-TPI, DPT-TPI
and DPF-TPI, respectively. The highest occupied molecular
orbital (HOMO) levels as measured by using ultraviolet
photoelectron spectroscopy (UPS) were −5.67, −5.68, and
−5.58 eV, respectively. The lowest unoccupied molecular orbital
(LUMO) levels of these compounds are estimated to be −2.41,
−2.47, and −2.59 eV respectively by adding the corresponding Eg
to the HOMO values.
Theoretical Properties. To better understand the struc-
ture−property relationship of the three phenanthroimidazole
derivatives at molecular level, we carried out density functional
theory (DFT) calculations with the geometry optimized at the
B3LYP/6-31G (d) theoretical level. The optimized geometry
and electron density distribution of the frontier molecular
orbitals are shown in Figure 3. All three molecules have highly
twisted conformation because of the large torsional angle
between t-Bu-benzene and phenanthroimidazole moieties, and
the steric hindrance of end-capping polyaromatic hydrocarbon
groups. Such highly twisted conformation can effectively reduce
intermolecular interaction and suppress problematic recrystalli-
zation, consequently leading to low fluorescence quenching and
stable amorphous thin films. According to DFT calculations, the
shapes of the HOMO orbitals are very similar, mainly located on
the phenanthroimidazole moiety and the phenyl group at the 2-
position of imidazole. However, their LUMO shapes are
different, and with increased conjugation length of Mullen
̈
dendron groups from TTP-TPI to DPF-TPI, the LUMO orbitals
are gradually localized on polyaromatic hydrocarbon groups. It
can be seen that all of the materials show clear separation of
HOMO and LUMO at the hole and the electron transporting
moieties respectively, especially for DPF-TPT, the HOMO and
LUMO orbitals are completely separated.
Electroluminescence Properties. To evaluate the bipolar
charge transporting properties of the compounds, we fabricated
single-carrier devices with structures of indium tin oxide (ITO)/
N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-dia-
mine (NPB, 20 nm)/TTP-TPI or DPT-TPI or DPF-TPI (80
nm)/NPB (20 nm)/Al (150 nm) for hole-only devices and ITO/
1,3,5-tri(phenyl-2-benzimidazolyl)-benzene (TPBI, 20 nm)/
TTP-TPI or DPT-TPI or DPF-TPI (80 nm)/TPBI (20 nm)/
LiF (1 nm)/Al (150 nm) for electron-only devices. NPB and
TPBI were used to prevent electron and hole injection from the
cathode and anode, respectively.26 As shown in Figure S2 in the
Supporting Information, the current density−voltage (J−V)
characteristics of the single-carrier devices illustrate that the three
phenanthroimidazole derivatives are capable of transporting
both holes and electrons. Among them, TTP-TPI based devices
exhibited a lower charge transporting properties, which may be
attributed to the conjugation of a relatively small polyaromatic
hydrocarbon group due to the large torsional angle between the
dendron core and all peripheral phenyl groups. In addition,
because the HOMO and LUMO orbitals are completely
Figure 7. Current efficiency and power efficiency versus current density
curves of the (a) TTP-TPI-, (b) DPT-TPI-, and (c) DPF-TPI-based
devices. (d) EQE of the three devices at different luminances.
exhibit blue emissions with an emission peak ranging from 406 to
472 nm. Compared to the PL spectra of TTP-TPI, the PL spectra
of DPT-TPI and DPF-TPI show further bathochromic shift
which could be attributed to the increase in π conjugation of the
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Table 3. Key Performance Parameters of the TTP-TPI-, DPT-TPI-, and DPF-TPI-Based Devices and Other High Performance
Blue-Violet, Deep-Blue, and Sky-Blue Light-Emitting Devices with CIEy < 0.06, 0.06 < CIEy < 0.10, and 0.15 < CIEy < 0.25,
Respectively
a
c
d
d
d
color
emitter
Von (V) λEL (nm)
ηC (cd/A)
ηP (lm/W)
ηext (%)
CIE (x, y)
ref
e
e
e
blue-violet
TTP-TPI
BPA-BPI
PATPA
TDAF1
TDAF2
CPhBzIm
TPA-(3)-F
M1
3.1
2.0
3.8
4.0
4.0
4.0
424
428
424
2.10/1.47
1.88/0.81
5.02/3.98
0.16, 0.05
0.15, 0.05
0.15, 0.06
0.158, 0.041
0.160, 0.044
0.16, 0.05
0.16, 0.06
0.165, 0.050
0.166, 0.056
0.155, 0.049
0.16, 0.07
0.156, 0.093
0.15, 0.09
0.15, 0.09
0.15, 0.09
0.15, 0.09
0.15, 0.09
0.15, 0.07
0.16, 0.07
0.15, 0.07
0.15, 0.09
0.151, 0.085
0.149, 0.086
0.147, 0.096
0.15, 0.07
0.17, 0.07
0.15, 0.07
0.17, 0.24
0.14, 0.21
0.16, 0.23
0.15, 0.16
0.17, 0.20
0.16, 0.23
0.166, 0.20
0.151, 0.161
0.14, 0.22
0.14, 0.20
0.14, 0.21
0.16, 0.20
this work
14a
14a
35
0.65/−
0.34/−
1.53/−
1.10/−
1.60/−
0.39/−
0.65/−
1.53/−
0.68/−
0.24/−
1.37/−
0.72/−
5.30/−
4.10/−
3.0/2.4
b
b
b
35
e
426
428
420
428
428
432
1.07/−
36
37
0.48/−
0.86/−
1.94/−
3.02/−
4.62/3.0
38
M2
38
be
,
TPAXAN
DPT-TPI
PhQ-CVz
CzPhB
3.4
2.9
3.5
6.5
2.8
3.9
3.2
3.2
3.0
3.3
3.1
3.8
3.2
3.0
3.2
7.5
3.5
4.0
2.7
2.7
3.8
3.0
3.7
3.7
39
e
e
e
e
deep-blue
3.13/2.78
3.22/1.92
1.77/0.51
1.3/−
5.25/4.62
This work
2c
b
be
,
be
,
2.06/−
3.3/2.6
2.45/2.0
be
be
,
,
449
448
445
4.3/3.9
8
e
e
TPA-BPI
TPIP
2.63/2.42
2.53/0.97
3.08/2.88
14a
21
b
4.69/−
2.71/1.6
5.43/−
e
e
e
e
e
e
Be(PPI)2
Zn(PPI)2
POAn
2.41/2.04
2.06/0.74
2.52/0.95
2.02/0.26
3.3/−
2.19/0.20
1.13/0.25
2.10/1.60
1.41/−
2.82/2.40
2.08/0.75
26b
26b
40
be
,
be
,
445
3.2/2.3
4.7/3.3
be
,
be
,
be
,
9TPAFSPO
9DPNAFSPO
NAF1
1.50/0.50
1.18/0.80
3.56/3.10
1.79/−
2.63/−
2.67/−
2.71/0.80
1.96/1.10
4.02/−
41
be
,
be
,
be
,
424
448
444
444
448
432
428
444
468
468
41
be
,
be
,
42
be
,
TBMFA
TBDNPA
TBMFPA
TFSDTC
1
3.58/2.10
43
be
,
1.96/−
1.92/−
5.17/4.10
43
be
,
4.76/4.0
2.7/−
2.9/−
43
b
be
,
1.91/1.70
44
be
,
2.2/1.1
1.6/−
45
be
,
BPPI
1.25/−
3.16/2.75
46
e
f
e
f
e
f
sky-blue
DPF-TPI
B3
8.41/8.40 / 7.69
7.23/5.69 / 3.23
4.85/4.85 / 4.49
this work
7b
b
3.63/3.40
2.60/−
4.10/3.2
2.41/−
3.60/3.4
e
e
e
BPPI
6.62/6.56
22
be
,
bf
,
b
bf
,
B2PPQ
Ph3TPE
DFDF
459
467
474
468
460
478
470
472
468
7.12/7.12 / 5.5
be
6.56/6.55 / 1.40
4.30/−/−
28
be
,
,
3.70/1.50
3.80/3.55
4.36/4.35
4.65/4.62
5.4/−
2.50/0.5
2.60/−
3.22/2.20
29
be
,
be
,
2.30/2.10
2.88/−
30
be
,
be
,
1-NaCPI
2-NaCPI
PhSPDPV
PhAA
2.73
2.71
31
be
,
be
,
3.18/2.20
3.61/−
3.4/3.0
31
be
be
,
,
5.7/3.0
4.0/−
3.5/−
32
be
,
3.2
2.9
4.0/3.6
33
be
,
NAA
5.5/5.3
33
be
,
be
,
Py-TPI
3.4
3.93/3.90
3.2/1.5
34
a
b
c
d
Von is turn-on voltage at 1 cd/m2. Values are estimates from the original figures. The maximum electroluminescence wavelength. ηc, ηp, and ηext
are current, power, and external quantum efficiencies, respectively. Efficiency values without superscript are maximum values; superscripts
e
f
Efficiencies measured at 1000 cd/m2. Efficiencies measured at 10000 cd/m2.
separated, DPF-TPI possess a more balanced charge transport
ability than TTP-TPI and DPT-TPI, which could support the
following explanation on the lower efficiency roll-off of the DPF-
TPI based device.
Key device performance parameters are summarized in Table 2
(Detailed parameters of replicate devices are summarized in the
Supporting Information, Figure S3 and Table S1).
Figure 4 depicts EL spectra of the three devices viewed in the
normal direction at a luminance of 10, 100, and 1000 cd/m2.
Peak wavelengths of the EL spectra for the devices using TTP-
TPI, DPT-TPI and DPF-TPI as the EML are 424, 432, and 468
nm, respectively. The EL spectra of the devices are almost the
same as the corresponding PL spectra in the film state, except for
small blue-shifts of DPT-TPI and DPF-TPI. This suggests that
the EL is indeed from the three phenanthroimidazole derivatives.
In addition, no excimer or exciplex emission can be observed. It is
attributed to the highly twisted structure due to the substituent
on the 1-imidazole position and the noncoplanar polyaromatic
hydrocarbon moieties, which results in large steric hindrance
To investigate electroluminescence (EL) properties of the
three phenanthroimidazole derivatives, we fabricated nondoped
devices with a configuration of ITO/NPB (70 nm)/4,4′,4″-
tri(N-carbazolyl)-triphenylamine (TCTA, 5 nm)/TTP-TPI or
DPT-TPI or DPF-TPI (30 nm)/TPBI (30 nm)/LiF (1 nm)/Al
(150 nm). In these devices, ITO and LiF/Al were used as the
anode and the cathode, respectively; NPB was used as the hole-
transporting layer (HTL); TCTA was used as a buffer layer;14b
TPBI was used as the electron-transporting layer (ETL) and
hole-blocking layer; and the three newly synthesized phenan-
throimidazole derivatives were used as the emitting layer (EML).
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Chemistry of Materials
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toward the phenanthroimidazole unit and the end group. As a
result, it prevents close-packing of the molecules. It is worth
noting that the EL spectra for the three devices are almost
independent of the driving voltages, with the spectrum remained
unchanged over a wide luminance range of 10−1000 cd/m2. It
suggests that the hole and electron recombination is well
confined within the EML, which might result from the wide band
gaps of the adjacent TCTA (3.4 eV) and TPBI (3.5 eV) layers.
The emission color ranged between blue-violet to sky-blue with
CIE 1931 chromaticity coordinates of (0.16, 0.05), (0.16, 0.07),
and (0.17, 0.24) for the devices based on TTP-TPI, DPT-TPI,
and DPF-TPI, respectively. In particular, the CIE coordinates of
the DPT-TPI-based device are close to the blue standards (0.14,
0.08) of the National Television Standards Committee
(NTSC).27
Current density−voltage−luminance (J−V−L) characteristics
of the three devices are shown in Figure 5. All devices show low
turn on voltage (Von, defined as voltage required to give a
luminance of 1 cd/m2) from 2.7 to 3.1 V, which can be attributed
to the small injection barriers in the devices. A schematic energy
level diagram of these OLEDs is illustrated in Figure 6. All three
compounds have similar HOMO at 5.7 eV, it mean that there are
negligible hole-injection barriers at the TCTA/EML junction.
The electron injection barriers at the TPBI/EML junction are 0.2
to 0.4 eV, and the hole injection barrier is also only 0.3 eV at the
NPB/TCTA junction. These small injection barriers enable
efficient injection of both holes and electrons into the emission
layer.
Figure 7 shows the efficiency of the three devices. Among
them, the DPF-TPI based sky blue device exhibits the best
performance, with a maximum current efficiency (ηC, max) of 8.41
cd/A, a maximum power efficiency (ηP, max) of 7.23 lm/W, and a
maximum external quantum efficiency (ηext, max) of 4.85%. It is
worth noting that the efficiency of DPF-TPI-based device
remains at 4.85% (8.40 cd/A) at a brightness of 1000 cd/m2.
Even at a much higher brightness of 10000 cd/m2, the external
quantum efficiency is still 4.49% with a small roll-off of 7.42%
(Figure 7d). To the best of our knowledge, this is the best result
reported in sky-blue nondoped OLEDs (Table 3).7b,22,28−34 The
low efficiency roll-off at high luminance for DPF-TPI-based
device can be attributed to the use of the bipolar emitter, which
may result in balanced charge fluxes and a broad distribution of
recombination regions within the emitting layer. Satisfactory EL
efficiencies are also obtained from blue-violet and deep blue
sky-blue nondoped OLEDs by using the phenanthroimidazole
derivatives as fluorescent emitters. The TTP-TPI, DPT-TPI, and
DPF-TPI-based nondoped devices can achieve excellent EQE
(current efficiency) of 5.02% (2.10 cd/A), 5.25% (3.13 cd/A),
and 4.85% (8.41 cd/A) with CIE coordinates of (0.16, 0.05),
(0.16, 0.07) and (0.17, 0.24), respectively. These values are the
best or among the best comparing to those of the reported
nondoped OLEDs with the corresponding color gamuts. The
DPF-TPI-based device exhibited lower efficiency roll-off than
TTP-TPI and DPT-TPI-based devices, which could be
attributed to its more balanced carrier transporting characteristic
of DPF-TPI. Our study presents a new molecular design strategy
of bipolar fluorescent emitters for efficient nondoped OLEDs.
ASSOCIATED CONTENT
* Supporting Information
■
S
TGA curves, J−V characteristics for single-carrier devices, and EL
performance of replicate devices are presented. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: qxtong@stu.edu.cn.
*E-mail: apcslee@cityu.edu.hk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work described in this paper was supported by the Research
Grants Council of the Hong Kong (Project T23-713/11), the
National Natural Science Foundation of China (Projects
51273108 and 91027041), and the National Basic Research
Program of China (973 Program 2013CB83 4803).
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■
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