High-efficiency violet-light-emitting materials based on phenanthro[9,10-d]imidazole

Article abstract of DOI:10.1002/chem.201203335

Novel violet: The material M2 (see picture), which is based on phenanthro[9,10-d]imidazole and carbazole, has a narrow band in the electroluminescence emission spectrum that results in saturated violet-light emission. The CIE of a non-doped device can reach (0.166, 0.056) with an external efficiency of 3.02 %. Copyright

Full text of DOI:10.1002/chem.201203335

DOI: 10.1002/chem.201203335
High-Efficiency Violet-Light-Emitting Materials Based on PhenanthroACTHNUTRGNE[NUG 9,10-
d]imidazole
Zhao Gao,[a] Yulong Liu,[a] Zhiming Wang,[a, b] Fangzhong Shen,[a] He Liu,[a]
Guannan Sun,[a] Liang Yao,[a] Ying Lv,[a] Ping Lu,*[a] and Yuguang Ma[a]
Highly efficient violet-light-emitting materials are of
great importance owing to their wide applications in biology
medical treatment,[1] sterilization,[2] and high-density infor-
mation storage.[3] They can also be utilized to generate light
of all colors, including blue, green, red, and white by energy-
transfer processes in devices.[4–8] Moreover, the power con-
sumption of a full-color display is highly dependent on the
saturation degree of the violet-light-emitting material; that
is, the higher saturation degree, the lower the power con-
sumption.[9] The violet emission region locates in the lowest
part of the CIE chromaticity diagram. Thus, material with a
lower y coordinate value would
semiconductors. The restriction in the p-conjugation length
often causes a decrease in carrier injection and trans-
port.[19,20] Herein, we report a new kind of violet-light-emit-
ting materials M1 and M2, composed of phenanthroACHTUNGTRENNUNG[9,10-
d]imidazole (PI) and carbazole moieties. PI itself is a typical
ultraviolet-light-emitting unit. PI also shows electron injec-
tion properties and good thermal stability based on our pre-
vious results,[21] which enables it to be an ideal unit for con-
structing stable, efficient violet-light-emitting material. After
connecting with different amount of carbazoles, we found
that M2 is a promising candidate for violet OLED with a
give higher saturation degree
because it would be closer to
the spectral locus. The pursuit
for efficient and stable violet-
light-emitting material has been
an ever-increasing issue in the
field of organic optoelectronics.
According to the European
Broadcasting Union (EBU)
standard blue CIE coordinate
of (0.15, 0.06), there have been
only a few reports on violet
OLEDs that can match the
emission
with
y coordinate
lower than 0.06 with high effi-
ciency.[10–18] One reason for the
scarcity of reports is that it is
difficult
to
simultaneously Scheme 1. Molecular structures of M1 and M2 and their synthesis from a and b, respectively.
inject electrons and holes into
such wide-band-gap organic
high external efficiency of 3.02%, negligible efficiency roll-
off, and good color stability. More importantly, the full
width at half maximum (FWHM) of the EL emission is very
narrow, which guarantees the violet emission color with CIE
coordinate of (0.166, 0.056).
[a] Dr. Z. Gao, Dr. Y. Liu, Dr. Z. Wang, Dr. F. Shen, Dr. H. Liu, G. Sun,
Dr. L. Yao, Dr. Y. Lv, Prof. P. Lu, Prof. Y. Ma
State Key Laboratory of Supramolecular Structure and Materials
Jilin University, 2699 Qianjin Avenue
Compounds M1 and M2 (Scheme 1) were synthesized in a
facile manner by a one-pot reaction. PI derivatives with var-
ious structures can be conveniently built by simply tuning
the aromatic aldehyde precursor involved. M1 and M2 were
isolated with good yields (over 70%). They were character-
ized spectroscopically, and the datacorresponded well to
their respective structures (Supporting Information).
Changchun, 130012 (P. R. China)
Fax : (+86)431-85193421
E-mail: lup@jlu.edu.cn
[b] Dr. Z. Wang
School of Petrochemical Engineering
Shenyang University of Technology
Liaoyang, 111003, (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203335.
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COMMUNICATION
High thermal stabilities with decomposition temperatures
(5% weight loss) of 4068C and 5158C for M1 and M2 were
observed, as expected (Supporting Information, Figure S3a).
The glass transition temperature (Tg) of M1 was measured
to be at 1328C, and is accompanied with two cold crystalli-
zation peaks, indicating its crystallization trend at higher
temperature. M2 showed a Tg of 2078C without a crystalliza-
tion peak, which was 758C higher than M1 because of the
attachment of one more PI group (Supporting Information,
Figure S3b). Such a high Tg value and glass state implies
that M2 could form morphologically stable films upon ther-
mal evaporation, which is highly important for application
in devices.[22,23] AFM characterization presented the similar
results. M2 film fabricated by vacuum deposition exhibited
a fairly smooth surface morphology with a roughness of
1.5 nm. After annealing at 1208C for 0.5 h, the morphology
became even better, with a roughness of only 0.59 nm (SI,
Figure S4). In contrast, the surface of the M1 film showed a
noticeable crystallization area, especially after annealing.
The thermal properties and the film morphology of com-
pounds have been significantly improved with the increased
number of PI unit.
shifts but were still located in the violet region. They exhibit
similar absorption spectra (Figure 1). The maximum absorp-
tion peak at 262 nm is attributed to the isolated benzene
ring connected with imidazole. The absorption band around
340 nm is attributed to the p–p* transition of phenanthro-
AHCTUNGTREG[NNNU 9,10-d]imidazole. The bandgap of M1 and M2 are calculat-
ed to be 3.24 and 3.18 eV according to their absorption edge
in the film state. The data were summarized in Table 1.
Table 1. The photophysical data of M1 and M2.
[b]
[c]
labs [nm][a] lPL [nm][a] labs [nm] lPL [nm] DEg
Ffl
max
max
max
max
in THF
in THF
in film
in film
[eV]
M1 266339
M2 262346
381399
390407
260347
268366
414
407
3.24
3.18
0.65
0.59
[a] The absorption and fluorescence spectra were measured in THF solu-
tion at a concentration of 1ꢁ10À5 molLÀ1. [b] Measured by the absorp-
tion edge of the thin film. [c] The solid-state quantum yield on the quartz
plate using an integrating sphere apparatus.
HOMO and LUMO levels were measured by cyclic vol-
tammetry using a glassy carbon disk (diameter 3 mm) as the
working electrode, a platinum wire as the auxiliary electrode
with a porous ceramic wick, and Ag/Ag+ as the reference
electrode. M1 exhibited one quasi-reversible oxidation wave
with an onset potential of 0.94 V, which gave a HOMO level
All of the compounds emit violet light in dilute THF solu-
tion (Figure 1). The emission peaks are at 381 nm and
399 nm for M1. As compared, M2 exhibits less than 10 nm
of À5.52 eV by comparison to ferrocene (EHOMO
=
À(eEox+4.58) eV). Calculated by the same method, M2 gave
a HOMO level of À5.35 eV, which was 0.17 eV higher than
M1, resulting in its easier injection of holes (Table 2; Sup-
porting Information, Figure S6).
Table 2. The thermal and electrochemistry data of M1 and M2.
[a]
Tg
Td
HOMO[b]
[eV]
LUMO[b]
[eV]
DEg
[eV]
[8C]
[8C]
M1
M2
132
207
405
515
À5.52
À5.35
À2.33
À2.22
3.19
3.13
[a] The temperature for 10% weight loss of the materials. [b] Calculated by
comparing with ferrocene (Fc) and calibrated using E1/2 (Fe/Fe+)=0.22 V.
For a better understanding of the effect of the structural
change on the energy levels and the carrier injection and
transport ability, the hole-only and the electron-only devices
of M1 and M2 were fabricated. The configuration of the
hole-only device was ITO/PEDOT/M1 or M2 (80 nm)/Au,
and the electron-only device had the configuration of ITO/
TPBi (20 nm)/M1 or M2 (100 nm)/LiF/Al. The results
(Figure 2) indicated that both M1 and M2 showed balanced
carrier injection properties. Furthermore, both the electron
and hole current values of M2 were higher than M1, indicat-
ing that more carriers could be transported to the emitting
layer of M2 in OLED, which is crucial to obtain high effi-
ciency owing to the dual-injection/transportation procedure
of OLEDs. After the comprehensive characterization of M1
and M2, M2 shows an overall enhanced performance com-
pared to M1, indicating that the introduction of one more
~
!
*
Figure 1. Normalized absorption (&, ) and emission spectra (
,
) of
~
!
*
,
M1 (&, ) and M2 (
) in THF. Concentration: 10À5 molLÀ1
.
red-shift, peaking at 390 nm and 407 nm. It is noteworthy
that both materials exhibit narrow emission with only 40 nm
FWHM in the spectra. This is helpful for obtaining saturat-
ed color with a low y CIE coordinate in OLEDs. We also
measured the fluorescence in different solvents with various
polarities, and they all showed the similar emission spectra,
implying that no intramolecular charge transfer existed in
these compounds. In the film state (Supporting Information,
Figure S5), the emission of these molecules showed red-
Chem. Eur. J. 2013, 19, 2602 – 2605
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2603

P. Lu et al.
only 40 nm, which is very helpful for obtaining saturated
color. Thus, the CIE coordinates of (0.165, 0.050) for M1
and (0.166, 0.056) for M2 are obtained. This result is a good
match with the requirement of EBU standard blue CIE co-
ordinate of (0.15, 0.06), and it is among the very few com-
pounds with a y coordinate lower than 0.06. Significantly, at
voltages from 5 V to 10 V the spectra of both devices are
kept unchanged and the CIE coordinates are almost identi-
cal, which show very good color stability for non-doped
violet light-emitting diodes.
The device performance of M1 and M2 is summarized in
Table 3. A maximum external quantum efficiency (EQE) of
1.94% was obtained for M1, with a luminance efficiency
Table 3. Summary of the device performance of M1 and M2.
LEmax
[cdAÀ1
PEmax
[lmWÀ1
Bmax
[cdmÀ12
EQE
[%][d]
CIE
ACHTUNGTRENNUNG
(x, y)[e]
[a]
[b]
[c]
G
]
G
]
N
]
M1
M2
0.65
1.53
0.48
0.86
3322
4329
1.94
3.02
(0.165, 0.050)
(0.166, 0.056)
~
!
*
Figure 2. The electron (&, ) and hole (
,
) current density versus elec-
~
)
tric field intensity curves of the single carrier devices based on M1 (&,
[a] The maximum values of luminance (LEmax). [b] The power efficiency
(PEmax). [c] The maximum brightness (Bmax). [d] External quantum effi-
ciency (EQE). [e] Taken at 8 V.
!
*
and M2 (
,
).
PI unit helps to give a higher Tg, better film stability, more
balanced carrier injection properties, and appropriate
HOMO and LUMO levels.
(LE) of 0.65 cdAÀ1 and a power efficiency (PE) of 0.48 Lm
WÀ1. The device utilizing M2 as emission layer shows en-
hanced maximum EQE of 3.02% at a current density of
2.03 mAcmÀ2 with a LE of 1.53 cdAÀ1 and a PE of 0.86 Lm
WÀ1 (Table 3). M1 and M2 show very low roll-off of the ex-
ternal efficiencies. Along with the increased number of PI
group, the EL performance has been significantly improved,
which is in correspondence with the comprehensive charac-
terization results obtained above.
In summary, we have designed a novel series of violet-
light-emitting materials, M1 and M2, consisting of covalently
bonded carbazole and phenanthroimidazole moieties. M2
exhibits a glass transition temperature Tg of 2078C, good
film stability, and balanced carrier injection properties. It is
a non-doped device with a violet CIE of (0.166, 0.056), and
an EQE of 3.02% was obtained. The FWHM of the EL
emission is only 40 nm, which guaranteed the low y coordi-
nate and saturated emission color. The color purity and the
efficiency are among the best results ever reported for non-
doped violet light-emitting diodes. This result gives us an in-
spiring basis for violet-light material design.
Non-doped devices with a typical three-layer structure of
ITO/NPB (80 nm)/M (30 nm)/TPBI (50 nm)/LiF (0.5 nm)/Al
(100 nm) were fabricated with respect to their appropriate
energy levels to investigate the potential applications of
compounds in violet OLEDs. The devices of M1 and M2
both exhibit violet emission with a peak centered at 420 nm
and 428 nm (Figure 3). The EL emission spectra of them
show very narrow distribution, and the FWHM for them is
Experimental Section
(2-(4-(9H-carbazol-9-yl)phenyl)-1-phenyl-1H-phenanthro
ACHTUNGTREN[NGNU 9,10-d]imida-
zole (M1): mixture of 4-(9H-carbazol-9-yl)benzaldehyde (1 g,
A
3.7 mmol), phenanthrene-9,10-dione (767.5 mg, 3.7 mmol), aniline (0.85 g,
18.5 mmol), ammonium acetate (1.14 g, 14.8 mmol), and acetic acid
(10 mL) were refluxed under nitrogen in an oil bath. After 2 h, the mix-
ture was cooled and filtered. The solid product was washed with an
acetic acid/water mixture (1:1, 150 mL) and water. It was then purified
by chromatography using CH2Cl2/petroleum ether (1:1) as an eluent to
obtain the product as white powder. Yield: 75%. 1H NMR (500 MHz,
Figure 3. The external quantum efficiency versus current density curves
!
of M1 (&) and M2 ( ). Inset: EL emission of M1 and M2 at different
voltages.
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Chem. Eur. J. 2013, 19, 2602 – 2605

Violet-Light-Emitting Materials
COMMUNICATION
DMSO, Hz): 8.96 (d, J=8.30 Hz, 1H), 8.91 (d, J=8.45 Hz, 1H), 8.74 (d,
J=7.36 Hz, 1H), 8.26 (d, J=7.83 Hz, 2H), 7.89 (d, J=8.22 Hz, 2H),
7.84–7.76 (m, 6H), 7.73 (t, J=7.50 Hz, 6.43 Hz, 1H), 7.66 (d, J=7.94 Hz,
2H), 7.59 (t, J=6.97 Hz, 7.68 Hz, 1H), 7.46 (t, J=7.50 Hz, 7.14 Hz, 2H),
7.40 (d, J=8.61 Hz, 2H), 7.38 (t, J=7.68 Hz, 7.68 Hz, 1H), 7.32 (t, J=
7.68 Hz, 7.68 Hz, 2H), 7.11 (d, J=8.47 Hz, 1H); 13C NMR (500 MHz,
CDCl3, Hz): 140.53, 130.80, 130.40, 130.14, 129.15, 128.39, 127.42, 126.61,
126.39, 126.02, 124.19, 123.57, 123.18, 120.91, 120.35, 120.20, 109.78;
FTIR (KBr, n, cmÀ1) 3058, 1625, 1532, 1475, 1451, 1429, 1378, 1358, 1241,
1225, 1174, 1147, 1111, 1018, 1004, 929, 844, 750, 724, 667, 620, 563, 535;
MALDI-TOF (m/z): [M+] calcd for C39H25N3: 535.64; Found: 535.90.
Anal. calcd for C39H25N3: C 87.45, H 4.70, N 7.84; Found: C 87.45, H
4.66, N 8.03.
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1-phenyl-2-(4-(3-(1-phenyl-1H-phenanthro
ACHTUNGTRNE[NUNG 9,10-d]imidazol-2-yl)-9H-car-
bazol-9-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (M2): A mixture of
AHCTUNGTRENNUNG
9-(4-formylphenyl)-9H-carbazole-3-carbaldehyde (957.3 mg, 3.198 mmol),
phenanthrene-9,10-dione (1.465 mg, 7.036 mmol), aniline (2.978 g), am-
monium acetate (1.92 g, 26.3 mmol), and acetic acid (10 mL) were re-
fluxed under nitrogen in an oil bath. After 2 h, the mixture was cooled
and filtered. The solid product was washed with an acetic acid/water mix-
ture (1:1, 150 mL) and water. It was then purified by chromatography
using CH2Cl2 as an eluent to obtain the product as white powder. Yield:
1
71%. H NMR (500 MHz, DMSO, Hz): 8.97–8.94 (m, 2H), 8.92–8.89 (m,
2H), 8.76–8.74 (m, 2H), 8.35 (s, 1H), 8.08 (d, J=8.22 Hz, 1H), 7.90 (d,
J=8.95, 2H), 7.85–7.77 (m, 10H), 7.73–7.70 (m, 5H), 7.67 (t, J=8.37 Hz,
2H), 7.60–7.55 (m, 2H), 7.49–7.46 (t, J=7.59 Hz, 7.59 Hz, 1H), 7.41–7.32
(m, 5H), 7.16 (t, J=8.37 Hz, 1H), 7.11 (d, J=7.59 Hz, 1H); FTIR (KBr,
n, cmÀ1) 3056, 1577, 1496, 1453, 1382, 1234, 1175, 1141, 1107, 1039, 886,
844, 817, 755, 725, 699, 617, 570, 534; MALDI-TOF (m/z): [M+] calcd for
C60H37N5: 827.97; Found: 828.0. Anal. calcd for C60H37N5: C 87.04, H
4.50, N 8.46; Found: C 87.30, H 4.46, N 8.28.
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Acknowledgements
This work is financially supported by the National Science Foundation of
China (Grant Nos. 21107050, 60906021, 51203091), the Ministry of Sci-
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Keywords: imidazoles · light-emitting materials · OLEDs ·
phenanthroquinone · violet light
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