Synthesis of diimidazolylstilbenes as n-type blue fluorophores: Alternative dopant materials for highly efficient electroluminescent devices

Article abstract of DOI:10.1002/adma.201202222

Two new n-type diimidazolylstilbenes as blue-fluorescent dopant materials are synthesized and characterized. Blue-fluorescent devices based on these two compounds as the dopants reveal outstanding external quantum efficiencies (EQEs) (current efficiencies) of 7.8% (10.4 cd A^-1) and 7.7% (7.9 cd A ^-1) with Commission internationale de l'Eclairage (CIE) co-ordinates of (0.14, 0.15) and (0.15, 0.11). Copyright

Full text of DOI:10.1002/adma.201202222

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Synthesis of Diimidazolylstilbenes as n-Type Blue
Fluorophores: Alternative Dopant Materials for Highly
Efficient Electroluminescent Devices
Ho-Hsiu Chou, Yi-Hsiang Chen, He-Pei Hsu, Wen-Hsin Chang, Yu-Han Chen,
and Chien-Hong Cheng*
Organic light-emitting diodes (OLEDs) have attracted great
attention due to their application in high-resolution, full-color,
flat-panel displays and lighting sources.[1] The search for stable
and highly efficient emitters for the three primary colors is
crucial for OLEDs to be useful commercially.[2] Recently, the
development of red and green electrophosphorescent devices
with high efficiencies, long lifetimes, and pure color Commis-
sion International de l’Eclairage (CIE) co-ordination has been
achieved. However, it is much more difficult to find a blue phos-
phorescent emission with long lifetime and pure color CIE, due
to the inherent wide bandgap. Thus, although many blue phos-
phorescent devices can achieve high external quantum efficien-
cies (EQEs),[3,4] high CIE co-ordinates (y co-ordinate ≥ 0.25)
and/or short device lifetimes are not suitable for them to be
commercially used. Therefore, in order to achieve marketable
OLEDs, the hunt for highly efficient blue-fluorescent materials
and devices is still a subject of current interest.
For a blue-fluorescent device, it is well known that the device
efficiency, color co-ordinates, and operational stability can be
significantly improved by using a doped emitter.[5,6] To date,
many blue-fluorescent dopant materials containing p-type moi-
eties such as arylamino or carbazolyl have been reported. The
distyrylarylene (DSA) amines, 4,4′-bis((E)-2-(9-ethyl-9H-car-
bazol-3-yl)vinyl)-1,1′-biphenyl (BCzVBi)[7] and 4,4′- 4,4′-bis[2-(4-
(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi),[8] were
first reported by Hosokawa et al. and were widely used as the
blue dopants for electroluminescent devices.[7–9] A mono(styryl)
CIE co-ordinates of (0.14, 0.16). We recently also used BCzVBi
as the dopant and phenanthrene derivatives as hosts to achieve
a current efficiency as high as 9.8 cd A−1 at CIE co-ordinates of
(0.14, 0.14).[12] In addition, several reports using p-type mole-
cules containing the styrylamine moiety as the blue fluorescent
dopant are also known.[1c,6–8,13]
Unlike p-type blue-fluorescent dopant materials, blue-
fluorescent dopant materials with n-type moieties are relatively
rare. The n-type imidazole moiety, has been widely employed as an
electron-transporting material[14] and as the electron-withdrawing
group of bipolar host materials.[15] We have shown that [1,4-bis(2-
(4-benzoylphenyl)-5-phenyl-1H-4-imidazolyl) benzene] is an n-type
blue dopant emitter for electroluminescent devices, achieving
CIE co-ordinates of (0.15, 0.12), although the device efficien-
cies are low.[16] The n-type spirobifluorene-based pyrazoloquino-
line dopant materials developed by Shu and coworkers showed
a current efficiency of 4.5 cd A−1 with CIE co-ordinates of (0.14,
0.17) for one of the corresponding devices.[17] Recently, the syn-
thesis of phenanthroimidazolyl derivatives and the application of
these materials as the host emitters for nondoped devices were
reported by our group.[18a] In addition, Ma and coworkers further
demonstrated that the phenanthroimidazolyl unit is an excellent
electron-injection structure.[14c] In this paper, we report the syn-
thesis of two new imidazole derivatives (E)-1,2-bis(4-(1-phenyl-1H-
phenanthro[9,10-d]imidazol-2-yl)phenyl) ethene (PPIE) and (E)-
1,2-bis(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)ethene (TPIE)
and the application of these two molecules as extremely high effi-
ciency blue-fluorescent dopants for electroluminescent devices.
Both PPIE and TPIE adopt a stilbene group as the core structure
end-capped with two 1-phenyl-1H-phenanthro[9,10-d]imidazol-
2-yl and 1,4,5-triphenyl-1H-imidazol-2-yl groups, respectively.
They were synthesized in one pot from the corresponding dial-
dehyde and dione derivatives in the presence of aniline, ammo-
nium acetate, and acetic acid (Scheme 1).[18] The crystal structures
of PPIE and TPIE were determined using single-crystal X-ray dif-
fraction (Supporting Information, Figure S1 and S2).[19] In both
crystal structures, the central stilbene group adopts a planar struc-
ture, but the stilbene and imidazole planes are no longer coplanar,
showing dihedral angles of 41° and 37°, respectively. Similarly,
in both compounds, the N-phenyl and the imidazole planes
intersect with dihedral angles of ca. 68°. On the other hand, the
1H-phenanthro[9,10-d]imidazole group in PPIE is planar, but in
TPIE the imidazole moiety and the two phenyl moieties attached
reveal dihedral angles of 28° and 46°, respectively. This structural
difference between PPIE and TPIE is expected to alter the highest
occupied molecular orbital (HOMO) and lowest unoccupied
amine
(E)-4-(2-([1,1′:4′,1′′-terphenyl]-4-yl)vinyl)-N,N-dipheny-
laniline (BD1)-based device showing a current efficiency of
5.4 cd A−1 and an external quantum efficiency of 5.1%, with
CIE co-ordinates of (0.14, 0.13), was demonstrated by Chen
and coworkers.[1c] Jin and coworkers. reported blue-fluorescent
devices using BCzVBi as the dopant, with a current efficiency
of 6.9 cd A−1 at CIE co-ordinates of (0.155, 0.157).[10] A series of
spiro styrylamines were synthesized and used as blue dopants
by Lee et al. and Yoon and coworkers.[11] One of the doped
devices showed a high current efficiency of 9.11 cd A−1 with
Dr. H.-H. Chou, Y.-H. Chen, H.-P. Hsu,
W.-H. Chang, Y.-H. Chen, Prof. C.-H. Cheng
Department of Chemistry
National Tsing Hua University
Hsinchu 30013, Taiwan
E-mail: chcheng@mx.nthu.edu.tw
DOI: 10.1002/adma.201202222
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DOI: 10.1002/adma.201202222
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Scheme 1. The synthesis route of PPIE and TPIE.
molecular orbital (LUMO) energy levels, as well as the thermal
spectra are summarized in Table 1. The absorption spectra of
these two compounds appear simple, showing only one band at
387 and 373 nm for PPIE and TPIE, respectively. There are two
emission maxima in the blue region for each of these two com-
pounds. As expected, the emission maxima of PPIE in tetrahydro-
furan (THF) solution at 444 and 469 nm are slightly red-shifted
compared with those of TPIE at 432 and 452 nm. The emission
spectra of PPIE and TPIE doped in 1-(2,5-dimethyl-4-(1-pyrenyl)
phenyl)pyrene (DMPPP)[20] thin films via vacuum deposition
with a concentration of 5% and thickness of 25 nm on a quartz
plate were also measured. These emission peaks in DMPPP are
red-shifted by ca. 10 nm relative to the corresponding peaks in
THF. In the neat thin film, the PL spectra are further red-shifted
by 41 nm for PPIE and by 26 nm for TPIE (Figure S3). The large
red shift of the PPIE neat film is probably due to the presence
of the planar phenanthroimidazolyl ring that facilitates stronger
π–π stacking. Both PPIE and TPIE show very high PL quantum
yields (QYs) of 0.83 and 0.80, respectively, in dilute cyclohexane
solution. The values were measured by using 9,10-diphenylan-
threcene (QY = 0.90 in cyclohexane) as in work by Li et al.[21] In
addition, we also measured the solid-state photoluminescence
QY of PPIE and TPIE doped in DMPPP thin films (50 nm) with
a concentration of 2%. Both PPIE and TPIE exhibited excellent
QYs of 0.92 and 0.83, respectively, and were higher than the QY
of DPAVBi (QY = 0.80) and BCzVBi (QY = 0.83) measured also
in the DMPPP film under the same conditions.
The molecular orbital levels of these two compounds were
determined by photoelectron spectrometry and UV–vis absorp-
tion measurements. The HOMO levels of PPIE and TPIE
appeared at 5.35 and 5.47 eV, respectively. The LUMO levels,
both appearing at 2.66 eV, were calculated by subtracting the
energy gap from their HOMO levels. The HOMO level of PPIE
was higher than that of TPIE due to the extended π-conjugation
of the fused phenanthroimidazolyl structure. To gain insight
into the electronic states of these two compounds, density func-
tionalized theory (DFT) calculations were performed (Figure S4).
The results show that the LUMO of PPIE and TPIE are localized
on the stilbene and imidazole moieties. However, the HOMO
of PPIE is further extended to the phenanthrene moiety, while
the HOMO of TPIE is extended to the two phenyl groups at the
4,5-position of imidazole. The observations are in a good agree-
ment with the fact that PPIE and TPIE have the same LUMO
levels but different HOMO levels. Moreover, the involvement
of the imidazole group in the LUMO’s of both PPIE and TPIE
suggests that they can be considered as n-type materials.[14,15]
1
and photoluminance properties. The H and 13C NMR spectra,
mass spectral data, and elemental analysis are in agreement with
the structures determined by X-ray-diffraction analysis.
The absorption and photoluminescence (PL) spectra of PPIE
and TPIE are shown in Figure 1, while the peak maxima of these
1.2
a)
Abs-PPIE-THF(10-5M)
Abs-TPIE-THF(10-5M)
PL-PPIE-THF(10-5M)
PL-TPIE-THF(10-5M)
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
1.2
PL-PPIE(5%):DMPPP(25 nm)
PL-TPIE(5%):DMPPP(25 nm)
b)
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
Figure 1. a) Normalized absorption and PL spectra of PPIE and TPIE in
THF (1 × 10−5 M). b) Normalized PL spectra of PPIE- and TPIE-doped
DMPPP films (25 nm) at a dopant concentration of 5%.
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T a b le 1 . The photophysical and thermal properties of PPIE and TPIE.
c)
a)
a)
b)
Compound
Eg
HOMOd)
[eV]
LUMOe)
[eV]
Tg/Tm/Td
[°C]
QYf)
λabs
λfl
λfl
[eV]
2.69
2.81
[nm]
387
373
[nm]
[nm]
PPIE
TPIE
444, 469
432, 452
454, 478
442, 464
5.35
5.47
2.66
2.66
190/401/497
151/358/459
0.83 (0.92)
0.80 (0.84)
a)The absorption and fluorescence spectra were measured in THF solution at concentration = 1 × 10−5 M; b)The fluorescence spectra were measured in the PPIE- or TPIE-
doped DMPPP film (25 nm); c)Estimated from the thin-film optical-absorption threshold; d)Estimated by photoelectron spectrometry (AC-II); e)LUMO = HOMO – Eg; f)The
quantum yield was measured in cyclohexane using 9,10-diphenylanthrecene (QY = 0.90) as the standard; the data presented in parentheses are the solid-state photolumi-
nescent quantum yields of the dopants in DMPPP thin film.
1.2
To find further support for the electron-transporting char-
Device A (PPIE)
Device B (TPIE)
Device C (PPIE)
acter of diimidazolylstilbenes, two electron-only devices were
fabricated. The devices consist of the following structure:
1.0
indium tin oxide (ITO)/BCP (15 nm)/dopant (30 nm)/BCP
(15 nm)/LiF (1 nm)/Al (100 nm), where the dopant is either
PPIE or TPIE and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phen-
anthroline. We also fabricated a similar electron-only device
using DPAVBi as the dopant for comparison. In these devices,
BCP layers were used to prevent hole injection from the anode.
The current density versus voltage curves of these three devices
(Figure S5) show that the TPIE- and PPIE-based electron-only
devices gave a higher current density than the DPAVBi-based
one. The observations strongly support that both PPIE and
TPIE are n-type materials capable of transporting electrons.
The thermal properties of PPIE and TPIE were determined by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). The resultant data including melting point (Tm),
glass-transition temperature (Tg), and decomposition tempera-
ture (Td) are shown in Table 1 and Figure S6. Among these two
diimidazolylstilbenes, the decomposition temperatures are in
the order PPIE > TPIE. In addition, the DSC results show that
PPIE with two fused phenanthroimidazolyl structures possesses
a higher thermal stability. After rapid cooling from the melt
state followed by a slow warm up, PPIE shows two endothermic
transitions at 190 and 401 °C, corresponding to the Tg and Tm,
and one exothermic transition at 255 °C, cor-
Device D (TPIE)
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
Figure 2. EL spectra for devices A–D at 8 V.
DMPPP as the host (Figure S8). The EL spectra, luminance and
current density vs. applied voltage, and EQE and current effi-
ciency vs. luminance of these devices are displayed in Figure 2–4.
Devices A and B consist of the layers: NPNPB (50 nm)/NPB
(10 nm)/DMPPP: PPIE or TPIE (5%) (25 nm)/BAlq (20 nm)/
responding to the crystallization temperature
Device A (PPIE)
(T ). For TPIE, the Tg appears at 151 °C and
c
Device B (TPIE)
Device C (PPIE)
Device D (TPIE)
800
600
400
200
0
Tm at 376 °C, whilst no T was observed. To
c
10000
examine the thermal morphological stability
further, we used atomic force microscopy
(AFM) to measure the topography of PPIE
and TPIE thin films at room temperature
1000
and at 90 °C for 18 h. The root-mean-square
roughness (Rrms) of these thin-film surfaces
showed no substantial changes before and
after annealing (90 °C) (Figure S7). The
results of the thermal-property studies reveal
that PPIE and TPIE have very high glass-
transition temperatures and melting points,
and that the morphology of their thin films
show high thermal stability.
To know the suitability of these n-type diim-
idazolylstilbenes as the electroluminescent
(EL) materials, we fabricated devices A–D by
utilizing PPIE and TPIE as the dopants and
100
10
1
0
5
10
15
20
Voltage (V)
Figure 3. Luminance and current density versus voltage for devices A–D.
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20
15
10
5
C showed a low turn-on voltage of 2.8 eV with
an excellent power efficiency of 6.4 lm W−1 .
As shown in Figure 4 and Figure S9, the EQE,
current efficiency and power efficiency vs.
luminance curves showed very low efficiency
roll-off for all of the devices. Interestingly, the
EL spectra of devices A–D and the PL spectra
of PPIE and TPIE doped in DMPPP thin
films are nearly the same. We also used the
common p-type deep-blue dopant BCzVBi to
fabricate device E with the same configuration
(see Table 2 and Figure S10). The perform-
ance of device E showed an EQE of 5.7%, a
current efficiency of 7.1 cd A−1 , and CIE co-
ordinates of (0.14, 0.14). The result demon-
strates that our n-type dopants have a higher
device efficiency than the p-type one. In
addition, compared with the performance of
previously reported blue-fluorescent devices
with true blue CIE co-ordinates, the extremely
high efficiencies of these PPIE- and TPIE-
based devices appear to be among the highest
reported to the best of our knowledge.
8
6
4
2
0
Device A (PPIE)
Device B (TPIE)
Device C (PPIE)
Device D (TPIE)
0
20000
40000
60000
Luminance (cd m-2)
Figure 4. EQE and current efficiency versus luminance for devices A–D.
LiF (1 nm)/Al (100 nm), where NPNPB = N,N′-diphenyl-N,N′-
di-[4-(N,N-diphenyl-amino)phenyl]benzidine, NPB = N,N′-di(1-
naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine, and BAlq =
bis(2-methyl-8-quinolinolate)-4-(phenylphenolato) aluminum.
Devices A and B exhibited high EQEs of 8.1 and 6.9%, current
efficiencies of 10.0 and 6.8 cd A−1 , and CIE co-ordinates of (0.14,
0.14) and (0.15, 0.10), respectively. The performance of PPIE and
TPIE-based devices can be improved by inserting a thin layer of
4,4′,4′′-tri(9-carbazoyl)triphenylamine (TCTA) (10 nm) between
NPB and the emitting layer (see Table 2). Devices C and D thus
fabricated emit deep-blue light with CIE co-ordinates of (0.14,
0.15) and (0.15, 0.11) very efficiently. Extremely high EQEs of
7.8 and 7.7% and current efficiencies of 10.4 and 7.9 cd A−1 ,
respectively, were observed. In particular, the PPIE-based device
In summary, we have demonstrated in this work the use of
blue-fluorescent dopants PPIE and TPIE containing the n-type
imidazole moiety for EL devices. These two dopants exhibited
very high fluorescence quantum yields and thermal stability. The
PPIE- and TPIE-based EL devices can achieve excellent EQE (cur-
rent efficiency) of 7.8% (10.4 cd A−1 ) and 7.7% (7.9 cd A−1 ) with
true blue CIE co-ordinates of (0.14, 0.15) and (0.15, 0.11), respec-
tively. These n-type materials provide an alternative for dopant
emitters in OLEDs other than the well-known p-type ones.
Experimental Section
Procedure
for
the
Synthesis
of
(E)-1,2-bis(4-(1-phenyl-1H-
mixture of
phenanthro[9,10-d]imidazol-2-yl)phenyl)ethene (PPIE):
A
9,10-phenanthrenequinone (1.06 g, 5.08 mmol), (E)-4,4′-(ethene-1,2-
diyl)dibenzaldehyde (500 mg, 2.12 mmol), aniline (0.46 mL, 5.08
mmol) and ammonium acetate (3.26 g, 42.3 mmol) in glacial acetic
acid (28 ml) was heated at 140 °C under a nitrogen atmosphere for 48
h. After cooling to room temperature, the reaction mixture was poured
into a methanol solution with stirring. The separated solid was filtered
off, washed with methanol, and dried to give the expected product.
The product was sublimed at 370 °C (1 × 10−5 Torr) to give the desired
final product in a 43% yield. 1H NMR (500 MHz, CDCl3, δ): 8.86 (d, J =
6.5 Hz, 2H), 8.76 (d, J = 8.0 Hz, 2H), 8.70 (d, J = 8 Hz, 2H), 7.73 (s,
2H), 7.63–7.49 (m, 16H), 7.45 (s, 2H), 7.40 (d, J = 9 Hz, 4H), 7.18 (s,
4H), 7.05 (s, 2H); HRMS (M+): calcd for C56H36N4, 764.2940; found,
764.2947. Anal. calcd. for C56H36N4: C 87.93, H 4.74, N 7.32; found, C
87.44, H 4.83, N 7.29.
T a b le 2 . Performances of devices A, B, C, and D.
Devicea)
Dopant
PPIE
Von
3.0
3.1
2.8
3.2
3.5
Lmax
CIEf)
b)
c)
d)
e)
ηext
ηc
ηp
[cd m−2
]
[x, y ]
[%]
[cd A−1
]
[lm W−1
]
A
B
C
D
E
62 956
8.1
10.0
4.8
(0.14, 0.14)
(0.15, 0.10)
(0.14, 0.15)
(0.15, 0.11)
(0.14, 0.14)
(20.0 V)
TPIE
34 846
6.9
7.8
7.7
5.7
6.8
10.4
7.9
2.8
6.4
3.1
3.4
(19.5 V)
PPIE
50 560
(20.0 V)
TPIE
37 484
Similar procedures were used to prepare TPIE from the
corresponding aldehyde derivatives. The yields and spectral data are
shown below.
(20.0 V)
BCzVBi
40 894
7.1
(E)-1,2-bis(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)ethene
(TPIE):
(18.0 V)
The product was sublimed at 330 °C (1 × 10−5 Torr) to give the desired
1
final product in a 65% yield. H NMR (400 MHz, CD2Cl2, δ): 7.58–7.55
a)The cathode of the general device is LiF (1 nm)/Al (100 nm); the structure of
devices A and B: ITO/NPNPB (50)/NPB (10)/DMPPP: Dopant (5%) (25)/BAlq
(20); devices C, D and E: ITO/NPNPB (50)/NPB (10)/TCTA (10)/DMPPP: Dopant
(5%) (25)/BAlq (20); the unit of thickness is nm; b)The maximum values of lumi-
nance (Lmax); c)The external quantum efficiency (ηext); d)The current efficiency (ηc);
e)The power efficiency (ηp); f)Taken at 8 V.
(m, 4H), 7.44–7.38 (m, 10H), 7.34–7.16 (m, 20H), 7.13–7.11 (m, 4H),
7.07 (s, 2H); 13C NMR (125 MHz, CDCl3, δ): 146.2, 137.5, 131.1, 131.0,
129.3, 128.7, 128.4, 128.3, 128.3, 127.6, 127.1, 126.4. HRMS (M+): calcd
for C56H40N4, 768.3253; found, 768.3251. Anal. calcd. for C56H40N4: C
87.47, H 5.24, N 7.29; found: C 87.43, H 5.28, N 7.27.
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CCDC-872027 and CCDC-872028 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Supporting Information
Supporting Information is available from the Wiley Online Library or
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Acknowledgements
We thank the Ministry of Economic Affairs (100-EC-17-A-08-S1-042) and
the National Science Council (NSC-100-2119-M-007-010-MY3) for support
of this research and the National Center for High-Performance Computing
(Account number: u32chc04) of Taiwan for providing the computing time.
Received: June 2, 2012
Revised: July 23, 2012
Published online:
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Adv. Mater. 2012,
DOI: 10.1002/adma.201202222
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