A twisting donor-acceptor molecule with an intercrossed excited state for highly efficient, deep-blue electroluminescence

Article abstract of DOI:10.1002/adfm.201200116

In an organic electroluminescent (EL) device, the recombination of injected holes and electrons produces what appears to be an ion-pair or charge-transfer (CT) exciton, and this CT exciton decays to produce one photon directly, or relaxes to a low-lying local exciton (LE). Thus the full utilization of both the energy of the CT exciton and the LE should be a pathway for obtaining high-efficiency EL. Here, a twisting donor-acceptor (D-A) triphenylamine- imidazol molecule, TPA-PPI, is reported: its synthesis, photophysics, and EL performance. Prepared by a manageable, one-pot cyclizing reaction, TPA-PPI exhibits deep-blue emission with high quantum yields (90%) both in solution and in the solid state. Fluorescent solvatochromic experiments for TPA-PPI solutions show a red-shift of 57 nm (3032 cm^-1) from low-polarity hexane (406 nm) to high-polarity acetonitrile (463 nm), accompanied by the gradual disappearance of the vibrational band in the spectra with increased solvent polarity. The photophysical investigation and DFT analysis suggest an intercrossed CT and LE excited state of the TPA-PPI, originating from its twisting D-A configuration. This is a rare instance that a CT-state material shows highly efficient deep-blue emission. EL characterization demonstrates that, as a deep-blue emitter with CIE coordinates of (0.15, 0.11), the performance of a TPA-PPI-based device is rather excellent, displaying a maximum current efficiency of >5.0 cd A^-1, and a maximum external quantum efficiency of >5.0%, corresponding to a maximum internal quantum efficiency of >25%. The effective utilization of the excitation energy arising from materials with intercrossed-excited-state (LE and CT) characters is thought to be beneficial for the improved efficiency of EL devices.

Full text of DOI:10.1002/adfm.201200116

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A Twisting Donor-Acceptor Molecule with an Intercrossed
Excited State for Highly Efficient, Deep-Blue
Electroluminescence
Weijun Li, Dandan Liu, Fangzhong Shen, Dongge Ma, Zhiming Wang,
Tao Feng, Yuanxiang Xu, Bing Yang,* and Yuguang Ma*
1. Introduction
In an organic electroluminescent (EL) device, the recombination of injected
Light emission from an organic molecule
holes and electrons produces what appears to be an ion-pair or charge-transfer
occurs when it relaxes to its ground state
(CT) exciton, and this CT exciton decays to produce one photon directly, or
by emitting a photon of light after being
relaxes to a low-lying local exciton (LE). Thus the full utilization of both the
energy of the CT exciton and the LE should be a pathway for obtaining high-
efficiency EL. Here, a twisting donor-acceptor (D-A) triphenylamine-imidazol
molecule, TPA-PPI, is reported: its synthesis, photophysics, and EL perform-
ance. Prepared by a manageable, one-pot cyclizing reaction, TPA-PPI exhibits
deep-blue emission with high quantum yields (90%) both in solution and in
the solid state. Fluorescent solvatochromic experiments for TPA-PPI solutions
show a red-shift of 57 nm (3032 cm−1 ) from low-polarity hexane (406 nm) to
high-polarity acetonitrile (463 nm), accompanied by the gradual disappear-
ance of the vibrational band in the spectra with increased solvent polarity.
The photophysical investigation and DFT analysis suggest an intercrossed
CT and LE excited state of the TPA-PPI, originating from its twisting D-A
configuration. This is a rare instance that a CT-state material shows highly
efficient deep-blue emission. EL characterization demonstrates that, as a
deep-blue emitter with CIE coordinates of (0.15, 0.11), the performance of
a TPA-PPI-based device is rather excellent, displaying a maximum current
efficiency of >5.0 cd A−1 , and a maximum external quantum efficiency of
>5.0%, corresponding to a maximum internal quantum efficiency of >25%.
The effective utilization of the excitation energy arising from materials with
intercrossed-excited-state (LE and CT) characters is thought to be beneficial
for the improved efficiency of EL devices.
excited to a higher quantum state, or
excited state, by absorption of some type
of energy. The kind of excited state of the
molecule determines its light-emitting
properties, with regard to efficiency and
energy. Organic light-emitting diodes
(OLEDs), as one of the important appli-
cations of these luminescent molecules,
have been widely studied.[1–7] In an OLED,
the recombination of injected holes and
electrons produces what appears to be an
ion-pair or charge-transfer (CT) exciton,
and this CT exciton decays to produce one
photon directly, or relaxes to a low-lying
local exciton (LE). Thus the full utilization
of both the energy of the CT exciton and
the LE may provide a pathway for obtaining
high-efficiency electroluminescence (EL).
Among OLED materials, the excited state
with the CT feature has been proved to
be very helpful in realizing the high effi-
ciency of devices. For example, the widely
used red dye 4-(dicyanomethylene)-2-me-
thyl-6-[4-(dimethylaminostyryl)-4H-pyran]
(DCM) and its derivatives possess typical
CT characteristics with a strong fluores-
cence solvatochromic effect.[3] Phospho-
rescent metal complexes such as tris[2-phenylpyridinato-C2,N]
iridium(III) (Ir(ppy)3) and bis[2-(4,6-difluorophenyl)pyridinato-
C2,N](picolinato)iridium(III) (FIrPic), which exhibit the highest
device efficiency, also have excited states of a CT nature, so-
called metal-to-ligand charge transfer (MLCT).[4] Great diffi-
culty has been met in expanding CT materials from current red
and green to blue and deep blue, because the CT excited-state
usually possesses a narrower band gap than the normal local
excited-state (LE) in the cases of both fluorescence and phos-
phorescence.[8,9] Considering the intrinsic wide-bandgap nature
of deep-blue emitters, it is a really great challenge to achieve
highly efficient deep-blue emission,[5] and it should be of sig-
nificance if a solution were a CT-state material.
Dr. W. J. Li, Dr. D. D. Liu, Dr. F. Z. Shen, Dr. Z. M. Wang,
T. Feng, Dr. Y. X. Xu, Prof. B. Yang, Prof. Y. G. Ma
State Key Lab of Supramolecular Structure and Materials
Jilin University
2699 Qianjin Avenue
Changchun, 130012, P.R. China
E-mail: yangbing@jlu.edu.cn; ygma@jlu.edu.cn
Prof. D. G. Ma
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022, P.R. China
DOI: 10.1002/adfm.201200116
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Scheme 1. The synthetic route to TPA-PPI (a: in CH3COOH and ammo-
nium acetate, refluxed for 2 h, yield: >80%; b: Pd(PPh3)4, sodium carbonate
in toluene and distilled water, refluxed for 48 h under nitrogen, yield: 75%.)
Herein, we report our newly synthesized D-A molecule, TPA-PPI,
based on a combination of triphenylamine (TPA) and 1,2-diphenyl-
1H-phenanthro[9,10-d]imidazole (PPI). PPI is a highly efficient
violet-blue chromophore with λmax at 369 nm, developed by our
group.[6] As PPI is modified by TPA to form a twisting, D-A mol-
ecule, TPA-PPI emits deep-blue light with λmax ≈ 438 nm, and an
efficiency up to 90%, both in solution and as a film. Solvatochromic
experiments indicate an intercrossed CT- and LE-state character for
TPA-PPI in the excited state, which is thought to contribute to the
observed very efficient fluorescence. By fabricating an EL device
with TPA-PPI as the emitter, we got an excellent deep-blue EL
performance with a maximum current efficiency of >5.0 cd A−1 , a
maximum external quantum efficiency of >5.0%, and Commission
Internationale de l’Eclairage (CIE) coordinates of (0.15, 0.11), which
are among the best results reported.[7] As follows, we present our
detailed experimental evidence and theoretical-calculation analysis.
Figure 1. Normalized UV and PL spectra of TPA-PPI in CHCl3 solution
(10−5 M) with PPI for comparison.
Firstly, in contrast to PPI, which shows violet-blue emission
at 369 nm with obvious vibronic structure features, TPA-PPI
in the same solution of chloroform shows a large red-shift of
69 nm, with deep-blue emission at 438 nm and a broad PL spectrum
without vibronic structure features. Secondly, as shown in Figure 2,
in different polar solvents (such as hexane, ethyl acetate, and
2. Results and Discussion
2.1. Synthesis
The molecular structure and synthetic route of TPA-PPI are
outlined in Scheme 1. TPA-PPI is composed of two main
components: triphenylamine (TPA) as the donor moiety and
1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) as the
acceptor moiety. The preparation was accomplished by two main
steps. Firstly, the important precursor, 2-(4-bromophenyl)-1-
phenyl-1H-phenanthro[9,10-d]imidazole (M1), was synthesized
by a one-pot cyclizing reaction via a mixture of aniline, phenan-
threnequinone, ammonium acetate, and 4-bromobenzaldehyde,
refluxed in acetic acid for 2 h. Then, through a Suzuki coupling
reaction, we synthesized our terminal compound, TPA-PPI.
2.2. Excitation-State Properties
The UV and photoluminescence (PL) spectra of TPA-PPI in CHCl3
solution are shown in Figure 1: the spectra of the compound PPI
without the TPA substituent are also shown for comparison. Com-
paredwiththePPI,theabsorptionpeakofTPA-PPIataround365nm
becomes very strong, which can be ascribed to a newly generated
CT transition from D (TPA) to A (PPI). This CT character of TPA-
PPI was observed more obviously in the fluorescent measurements.
Figure 2. PL spectra of TPA-PPI, measured in different solvents with
increasing polarity (the orientational polarizability of solvent, f, – hexane:
0.0012; triethylamine: 0.048; butyl ether: 0.096; ethyl ether: 0.167; ethyl
acetate: 0.200; acetone: 0.284; and acetonitrile: 0.305 (Table S1, Sup-
porting Information)).
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acetonitrile), the solvatochromic effect of the PL spectra of TPA-PPI
was observed. When the solvent polarity was increased gradually
from low-polarity hexane to high-polarity acetonitrile, the TPA-PPI
exhibited a large red-shift of 57 nm: 406 nm in hexane, 435 nm in
ethyl acetate, and 463 nm in acetonitrile. This large solvatochromic
shift, which is commonly consistent with the large dipole moment
of the CT state, indicates that the low-lying excited state, S1, of the
TPA-PPI possesses a strong-CT-state characteristic.[8]
The dipole moment of the S1 state can be estimated from
the slope of a plot of the Stokes shift (va – vf) against the solvent
parameters, or the orientation polarizability f(ε,n), according to
the Lippert–Mataga[8] equation:
2
2(µ −µ )
e
g
hc(νa − νf ) = hc(νa0 − νf0) +
f (ε, n)
3
a
ν0 − ν0
where ƒ is the orientational polarizability of the solvent,
a
f
corresponds to the Stokes shifts when ƒ is zero, μe is the excited-
state dipole moment, μg is the ground-state dipole moment, a
is the solvent Onsager cavity radius, and ε and n are the solvent
dielectric and the solvent refractive index, respectively
(Table S1, Supporting Information) (see the solvatochromic-
effect part in the supporting information).
Figure 4. The ground-state energy of TPA-PPI at different biphenyl twist
angles in the gas phase, using the cam-b3lyp method and a full geo-
metrical optimization at each twist angle.
Obviously, the experimental data do not obey the linear rela-
tionship predicted by the Lippert–Mataga equation well, in the
whole range of solvent polarity, as shown in Figure 3. A trans-
formation in the slope of the fitted line is observed between
the butyl ether (f = 0.10) and ethyl acetate (f = 0.20) solvents.
Through the analysis of the fitted line in high-polarity solvents, a
slope value of ≈14 272 (R = 0.96) was obtained. The ground-state
dipole, μg, of the TPA-PPI could be estimated from a long-range-
correction density-functional-theory (DFT) calculation using the
Gaussian 09 package[10a] (CAM-B3LYP/Coulomb-attenuating
method-B3LYP[10b,c] at the basis set level of 6-31G*), which gave
a μg of 4.09 D. Thus, the corresponding excited-state dipole, μe,
was calculated to be 18.5 D for TPA-PPI in high-polarity sol-
vents, according to the Lippert–Mataga equation. In the region
of low-polarity solvents, a linear relationship with a slope of near
zero could be drawn for TPA-PPI, indicating no obvious change
between μe and μg. However, we could not obtain the exact value
of μe, because the slope was so small that a tiny deviation from
the linear line would cause prodigious R values. Thus, the small
μe was estimated to be close to 4.09 D for TPA-PPI in low-polarity
solvents. This small μe of 4.09 D can be attributed to the usual
excited-state,[8,9] which is a LE state, while the large μe of 18.5 D
should not be treated as a small parameter in contrast to the usual
excited states (LE). It is actually smaller than, but very close to, the
μe value of 4-(N,N-dimethylamino)-benzonitrile (DMABN), a typ-
ical CT molecule, which has μe ≈ 23 D.[8] The fluorescent solva-
tochromic experiments and non-linear relationship of the Stokes
shift with solvent polarity indicate that our molecule, TPA-PPI,
possesses the intercrossed excited state of LE and CT: a bigger
contribution from the CT state in high-polarity solvents (f ≥ 0.2),
a bigger contribution from the LE state in low-polarity solvents
(f ≤ 0.1), and a totally intercrossed excited state of the LE and CT
in moderately polar environments between butyl ether and ethyl
acetate, or at the polarity level near chloroform.
DFT analysis of the molecular configuration and frontier
molecular orbital revealed that a twisting D-A linking with an
angle of 20–55° should be the origin of the CT and LE intercross.
The ground-state energy of TPA-PPI and the corresponding
electronic density distribution of the frontier molecular orbitals
(highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO)) at twist angles of 40° and 90°
are shown in Figure 4 and Figure 5, respectively. At a twist angle
of 90°, we can see the obviously separated HOMO and LUMO
on the TPA and PPI, respectively, which generates a pure-CT
transition of HOMO (donor) → LUMO (acceptor) in the TPA-
PPI. However, TPA-PPI in such a 90° twisting conformation is
unstable, because its energy is ≈0.1 eV higher than that of TPA-
PPI at a twist angle of ≈40°, based on DFT total-energy analysis.
In TPA-PPI with a D-A twisting of ≈40°, which is the lowest-
energy conformation, both the HOMO and the LUMO partially
delocalize to the whole TPA-PPI molecule, instead of there being
Figure 3. Linear correlation of orientation polarization (f) of solvent
media with the Stokes shift (va – vf) for TPA-PPI. (See Table S1 for data;
the lines in the high- and low-polarity regions were fitted to the points
with f ≥ 0.2 and f ≤ 0.1, respectively; the dashed parts of the fitted lines
are the extrapolated parts of them.)
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yield is the coexistence of the emissions from the LE and the
CT (or, to put it another way, an intercrossed CT and LE state).
As evidence, as shown in Table S3, the oscillator strengths for
absorption and emission were both enlarged in the moderately
polar solvent chloroform, in contrast to that in the gas phase,
which indicates the higher luminescence of the intercrossed
excited state in chloroform; further evidence is that the low-
lying CT state that dominates the luminescence results in a
greatly decreased efficiency in high-polarity environments, as
shown in Table S1.
2.3. Electroluminescence Properties
Figure 5. The frontier molecular orbitals (HOMO and LUMO) in the
ground state of TPA-PPI at different biphenyl twist angles (the isovalue
was set to be 0.02 a.u.).
TGA and DSC measurements have shown that TPA-PPI is a
highly thermally stable material. Its glass-transition tempera-
ture (Tg) is 130 °C, and its thermal-decomposition temperature
(Td) at 5% weight loss is 475 °C (Figure S4). Atomic-force-
microscopy (AFM) measurements showed that TPA-PPI film
exhibits a fairly smooth surface morphology with a roughness
of 0.79 nm, which was nearly unchanged after annealing at
90 °C for 2 h (Figure S5). These results indicate that TPA-PPI
can form a very stable and smooth film to support EL device
fabrication. Cyclic-voltammetry analysis was also carried out,
which gave HOMO and LUMO levels of –5.18 eV and –2.27 eV,
respectively, for TPA-PPI (Figure S3).
To investigate the EL properties of TPA-PPI, we con-
structed a device with a frequently used multilayered
structure: indium tin oxide (ITO)/MoO3 (10 nm)/N,N′-di-
1-naphthyl-N,N’-diphenylbenzidine (NPB) (80 nm)/4,4′,4″-
tri(N-carbazolyl)-triphenylamine (TCTA) (5 nm)/TPA-PPI
(20 nm)/1,3,5-tri(phenyl-2-benzimidazolyl)-benzene (TPBi)
(40 nm)/LiF (1 nm)/Al (100 nm), in which MoO3 was utilized
as a hole-injecting layer, NPB and TCTA as hole-transporting
and buffer layers, and TPBi as an electron-transporting
and hole-blocking layer. From the EL spectrum, shown in
Figure 6b, we could see a obvious deep-blue emission with
λmax at 434 nm and little vibronic feature. Such an EL spec-
trum is very similar to PL spectra observed from chloroform
solution and films, indicating the same source of EL and PL,
most likely the intercrossed LE and CT excited-state of the
TPA-PPI.
a fully separated (or localized) HOMO and LUMO. In this case,
HOMO → LUMO should not be treated as a pure-CT transition,
but an intercrossed transition of CT and π→π*. Naturally, the
intercrossed molecular orbitals bring the intercrossed character
of the LE and CT states. When in different polar solvents, the LE
and CT states show different properties because of their different
excited-state dipole moments. As the solvent polarity increases,
the CT energy level starts to decrease, due to strong interaction of
the solvent field with the CT excited state (large dipole moment),
while LE remains nearly unchanged.[8,9] In low-polarity solvents,
the LE, being the low-lying excited state, dominates the lumines-
cence with well-resolved LE emission features. With increased
solvent polarity, a depressed CT energy, which is close to energy
of the LE, results in both LE and CT emissions simultaneously,
like an intercrossing of the LE and CT states. In solvents with
very high polarity, the CT state may become the lowest excited
state, resulting in broad and red-shifted CT emissions.
With TPA-PPI as a film and as a bulk solid (the main form
for applications), its spectral character was very similar to that
of TPA-PPI in chloroform (Figure S2 and Table 1). This indi-
cates that the solid state (film) has a similar polar-environment
effect on the TPA-PPI molecule to that of chloroform, in which
TPA-PPI has been proved to possess the intercrossed excited
state. Thus, TPA-PPI in a film should also retain the char-
acter of the intercrossed excited state. The measurement of the
fluorescent quantum yield, by using quinine sulphate in 1 M
sulphuric acid as a standard, demonstrated a value as high as
90% for TPA-PPI in chloroform, and the fluorescence quantum
yield of TPA-PPI film, determined using an integrating-sphere
photometer, was also 90%. This is a rare instance that a CT state
material shows highly efficient deep-blue emission. It is highly
possible that the reason for the exceptionally high fluorescence
Thisdeviceexhibitedexcellentperformancewithamaximum
luminance of 13 675 cd m−2 , power efficiency > 6.0 lm W−1 ,
current efficiency > 5.0 cd A−1 , and maximum external
quantum efficiency (ηext) > 5.0%, as shown in Figure 6a,
Figure 7a and Table 2. Through calculation from the relation-
ship ηext = ηint· ηph, assuming a light out-coupling efficiency
of ηph ≈ 1/(2n2) ≈ 20% for a glass substrate with an index
T a b le 1 . The thermal, photophysical, and electrochemistry data of TPA-PPI.
fl (soln/film)b)
0.90/0.90
Tg
[°C]
Tda)
[°C]
UV (soln)
[nm]
UV (film)
[nm]
HOMOc)
[eV]
LUMOc)
[eV]
PL λmax (soln)
PL λmax (film) [nm]
Φ
[nm]
TPA-PPI
130
475
365
438
368
440
–5.22
–2.27
a)The temperature for 5% weight loss of the materials; b)The fluorescence quantum yield was measured in solution using a 0.5 M H2SO4 solution of quinine as a reference
(0.54); the solid - state quantum yield on the quartz plate was measured using an integrating - sphere apparatus; c)Estimated based on cyclic - voltammetry (CV) data of
each compound by comparing with ferrocene (Fc) (4.8 eV).
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Figure 6. a,b) The current-efficiency–luminance–power-efficiency curves
(a) and EL spectra at different operation voltages (b) of a device with
TPA-PPI as the emitter.
Figure 7. a) The external efficiency–luminance curve of a device with TPA-
PPI as the emitter. b) The simple scheme of recombination of electrons
and holes in our device with TPA-PPI as the emitter, which has the inter-
crossed LE and CT state.
of refraction n = 1.5, we tuned the ηext to the corresponding
maximum internal quantum efficiency ηint > 25%. Consid-
ering the TPA-PPI fluorescent quantum efficiency < 100%
(90%) and spin statistics (1/4), in the case of our EL device,
actually, the limit of the ηint was smaller than 25%.[11] This
indicates that TPA-PPI devices break through the limit of ηint
ruled by spin statistics, which is just found in triplet phos-
phorescence materials and special singlet materials.[12] This
breakthrough of ηint is tentatively proposed to be owing to the
fully effective utilization of both the energy of the LE and the
3
1
it is possible for the transition CT → CT to become allowed
1
3
due to the energetic closeness between CT and CT.[13] The
main reason for the high device efficiency is having both the
LE and CT emissions simultaneously in the device, arising
from the intercrossed excited-state (LE and CT) in the TPA-
PPI emitter.
1
3
CT exciton, including singlets, CT, and triplets, CT, because
T a b le 2 . The device performance of TPA-PPI.
Device performance at 100 cd m−2
Device
CIE coordinates
EQEmax
[%]
LEmax
PEmax
Lmax
[x, y]
[cd A−1
]
[lm W−1
]
[cd m−2
]
LE [cd A−1
]
PE [lm W−1
]
EQE [%]
3.76
TPA-PPI
0.15, 0.11
5.02
5.66
6.13
13 675
4.25
2.70
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Moreover, based on luminances of 100 cd m−2 (or 1000 cd m−2 ),
the TPA-PPI device also showed a high current efficiency
of 4.25 cd A−1 (or 3.50 cd A−1 ) and an external quantum effi-
ciency (EQE) of 3.76% (or 3.13%), which are among the best
results of nondoped deep-blue OLED structures that have been
reported.[7] The EL spectra showed the TPA-PPI device per-
formed with quite excellent color stability in the whole voltage
range at the same CIE coordinates of (0.15, 0.11), as shown in
Figure 6b.
(4.5 mL), and distilled water (3.0 mL), with tetrakis(triphenylphosphine)
palladium(0) (Pd(PPh3)4) (50 mg) acting as catalyst, was refluxed at
90 °C for 48 h under nitrogen. Then, some water was added to the
resulting solution and the mixture was extracted with chloroform several
times. The organic phase was dried over anhydrous magnesium sulfate.
After filtration and solvent evaporation, the liquid was purified by column
chromatography using petroleum ether/CH2Cl2 as the eluent to afford a
white solid. Yield: 75%; 1H NMR (500 MHz, DMSO, 25 °C, TMS, δ): 8.94
(d, J = 8.2 Hz, 1H, Ar H), 8.89 (d, J = 8.2 Hz, 1H, Ar H), 8.72 (d, J = 7.8
Hz, 1H, Ar H), 7.83–7.68 (m, 7H, Ar H), 7.64 (d, J = 6.2 Hz, 6H, Ar H),
7.57 (t, J = 7.4 Hz, 1H, Ar H), 7.34 (t, J = 7.9 Hz, 5H, Ar H), 7.11–7.06
(m, 7H, Ar H), 7.03 (d, J = 8.6 Hz, 2H, Ar H); 13C NMR (126 MHz,
CDCl3, 25 °C, TMS, δ): = 150.71 (C), 147.59 (C), 140.76 (C), 138.89 (C),
137.53 (C), 133.93 (C), 130.22 (CH), 129.83 (CH), 129.73 (CH), 129.33
(CH), 129.19 (CH), 128.87 (CH), 128.30 (C), 128.23 (C), 127.65 (CH),
127.23 (CH), 126.27 (CH), 126.24 (CH), 125.64 (CH), 124.88 (CH),
124.54 (CH), 124.14 (CH), 123.74 (CH), 123.12 (CH), 123.10 (CH),
122.84 (CH), 120.88 (CH); MALDI-TOF MS (mass m/z): 615.6 [M+ + H];
Anal. calcd for C45H31N3: C 88.06, H 5.09, N 6.85; found: C 88.46, H
5.35, N 6.37.
3. Conclusions
In summary, the intercrossed excited state has been demon-
strated to exist in a twisting D-A molecule, TPA-PPI, through
a combined photophysical and DFT investigation. This inter-
crossed excited state, with simultaneous LE and CT emission,
is responsible for the observed high quantum efficiency both
in solution and as a film. Such a special material is very suit-
able for OLED applications. A nondoped device with TPA-PPI
as the emitter reached an internal quantum efficiency (ηint) of
>25%, breaking through the limit of ηint for fluorescence-based
EL devices. The effective utilization of the excitation energy
arising from the intercrossed excited-state (LE and CT) is
thought to contribute to the improved efficiency. In this article,
new insight into the excited state of the twisting D-A molecules
provides us with an effective strategy to construct highly effi-
cient OLED materials that employ the intercrossed excited state
of the LE and CT.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
We are grateful for support from the National Science Foundation of
China (51073069, 20834006), the Ministry of Science and Technology of
China (2009CB623605), and PCSIRT (20921003).
Received: January 13, 2012
Revised: February 9, 2012
Published online:
4. Experimental Section
All of the reagents and solvents used for the syntheses were purchased
from Aldrich or Acros and were used as received. All of the reactions were
performed under a dry-nitrogen atmosphere. The 1H NMR spectra were
recorded on AVANCZ 500 spectrometers at 298 K by utilizing deuterated
dimethyl sulfoxide (DMSO) as solvents and tetramethylsilane (TMS)
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CHNS-O elemental analysis instrument. The MALDI-TOF-MS mass
spectra were recorded using an AXIMA-CFRTM plus instrument. UV-vis
absorption spectra were recorded on a UV-3100 spectrophotometer.
Fluorescence measurements were carried out with a RF-5301PC.
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Synthesis
of
2-(4-Bromophenyl)-1-phenyl-1H-phenanthro[9,10-d]-
imidazole (M1): A mixture of aniline (10.0 mmol), phenanthrenequinone
(2.0 mmol), 4-bromobenzaldehyde (2.0 mmol), ammonium acetate
(8.0 mmol), and acetic acid (15 mL) was refluxed 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 mixture (1:1, 30 mL) and water.
Then, it was dried in the vacuum and used directly for the next step
without further purification and characterization. 1H NMR (500 MHz,
DMSO, 25 °C, TMS, δ): 8.94 (d, J = 8.4 Hz, 1H, Ar H), 8.89 (d, J =
8.4 Hz, 1H, Ar H), 8.69 (d, J = 8.0 Hz, 1H, Ar H), 7.79 (t, J = 7.4 Hz, 1H,
Ar H), 7.77–7.68 (m, 6H, Ar H), 7.58 (m, 3H, Ar H), 7.52 (d, J = 8.4 Hz,
2H, Ar H), 7.35 (t, J = 7.7 Hz, 1H, Ar H), 7.09 (d, J = 8.3 Hz, 1H, Ar H);
MALDI-TOF MS (mass m/z): 449.0 [M+ + H].
Synthesis of N,N-Diphenyl-4′-(1-phenyl-1H-phenanthro[9,10-d]imidazol-
2-yl)biphenyl-4-amine (TPA-PPI): A mixture of N,N-diphenyl-4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (449.4 mg, 1.0 mmol)
(synthesized by: n-butyl-lithium added into 4-bromo-N,N-diphenylaniline
at –78 °C, then 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxabrorolane
added, and the mixture stirred for 24 h at room temperature), M1
(408.4 mg, 1.1 mmol), sodium carbonate (635.9 mg, 6.0 mmol), toluene
©
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Adv. Funct. Mater. 2012,
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www.MaterialsViews.com
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