Solution-processable polyphenylphenyl dendron bearing molecules for highly efficient blue light-emitting diodes

Article abstract of DOI:10.1021/ol0478180

(Chemical Equation Presented) Novel solution-processable blue light-emitting materials Blue A-D, bearing a polyphenylphenyl dendron, have been synthesized and characterized. The energy levels and band gaps can be facilely tuned by changing the central aro

Full text of DOI:10.1021/ol0478180

ORGANIC
LETTERS
2005
Vol. 7, No. 3
391-394
Solution-Processable Polyphenylphenyl
Dendron Bearing Molecules for Highly
Efficient Blue Light-Emitting Diodes
Chun Huang,†,‡ Chang-Gua Zhen,† Siew Ping Su,† Kian Ping Loh,‡ and
Zhi-Kuan Chen*,†
Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602,
Republic of Singapore, Department of Chemistry, National UniVersity of Singapore,
Singapore 119260, Republic of Singapore
zk-chen@imre.a-star.edu.sg
Received October 25, 2004
ABSTRACT
Novel solution-processable blue light-emitting materials Blue A−D, bearing a polyphenylphenyl dendron, have been synthesized and characterized.
The energy levels and band gaps can be facilely tuned by changing the central aromatic ring of the molecules. A highly efficient deep blue
light-emitting OLED device based on Blue C with a maximum current efficiency of 2.2 cd/A has been achieved using PVK as the host material
through a solution process.
Electroluminescent materials have attracted significant at-
tention ever since the first contribution of small molecular
organic light-emitting diodes (SMOLED) by Tang1 et al. and
polymeric light-emitting diodes (PLED) by Friend2 et al. due
to their great potential application in flat panel displays. In
the past decade, great efforts have been paid to develop novel
electroluminescent materials with intense luminescent ef-
ficiency, good thermal/electrical/optical stability, and desir-
able film morphology.3-7 Most of the organic light-emitting
materials developed so far have high quantum efficiency in
solution but few exhibit high quantum yield in films due to
the existence of more nonradiative decay channels, which
are associated with aggregates, excimers, exciplexes, and
impurities in the solid state. Complete amorphous morphol-
ogy of films can effectively prevent the formation of
aggregates, excimers, and exciplexes, which can result in
higher quantum efficiency and a stable emission spectrum.
From the structural aspect, molecules with asymmetric and
nonplanar structure can be readily processed into amorphous
films through either spin coating or vacuum deposition.
Starburst and dendrimer-type conjugated molecules or con-
jugated molecules bearing such types of hyperbranched side
groups have been developed for this purpose.8-10 However,
the application of these hyperbranched structural materials
as an emissive layer for OLED application is very limited
due to the tedious synthesis and purification procedure.
Recently, a new type of fully conjugated polyphenylphenyl
† Institute of Materials Research & Engineering.
‡ National University of Singapore.
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Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature (London)
1990, 347, 539.
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(London) 1999, 397, 414.
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Burn, P. L.; Holmes, A. B. Nature (London) 1993, 365, 628.
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(7) Jin, S. H.; Kim, M. Y.; Kim, J. Y.; Lee, K.; Gal, Y. S. J. Am. Chem.
Soc. 2004, 126, 2474.
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E.; Frechet, J. M. J. J. Am. Chem. Soc. 2004, 122, 12385.
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P. L.; Salata, O. V.; Samuel, I. D. W. AdV. Mater. 2002, 14, 975.
(10) Lupton, J. M.; Samuel, I. D. W.; Beavington, R.; Burn, P. L.;
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10.1021/ol0478180 CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/07/2005

(tetraphenylphenyl and pentaphenylphenyl) dendron has been
successfully incorporated into both small molecular weight
light-emitting materials11 and polymeric light-emitting ma-
terials12,13 through simple synthetic procedure.14 Such den-
drons have been proven to be effective molecular construc-
tion blocks to inhibit the formation of molecular aggregation
or π-π stacking in the solid state.12,13 In this letter, we report
our new research effort in developing highly efficient light
emitting materials, which are composed of three segments
with well-defined conjugated length to achieve blue light
emission. The molecules are designed with different central
aromatic rings, which are coupled with tetraphenylphenyl
group at one side and an N,N-dimethylaminophenyl attached
at the other. Obviously, incorporation of nonplanar poly-
phenylphenyl dendron into the molecules is to suppress the
chance of formation of molecular aggregates in films. This
dendron structure also renders the molecules good thermal
stability and solubility for spin coating. Solution process
could simplify the device fabrication procedure and can
reduce the cost. Attachment of a strong electron-donating
group of N,N-dimethylaminophenyl ring at another end of
the molecule can further enhance the device efficiency by
elevating the HOMO energy levels of the molecules for ease
of hole injection/transporting in the film. The central group
is varied from an electron-donating thienyl ring, 2,5-
dimethoxyphenyl ring, and phenyl ring to an electron-
withdrawing pyridyl ring for the aim of tuning the energy
levels and charge-transporting properties of the resulting
materials. Deep blue light-emitting OLED devices with good
device performance have readily been fabricated from the
newly developed materials through solution process.
molar numbers of the starting materials were used, good to
excellent yields of monosubstituted products can be obtained
(yields ranged from 42% to 90%). The extremely high yield
of compound 2A is due to the good selectivity of oxidative
addition of 2-bromo of 2,5-dibromopyridine to the palladium
catalyst over the 5-bromo position. Thereafter, tetraphenyl-
phenyl groups were introduced into the molecular backbones
through the simple Diels-Alder cycloaddition reaction
between the arylethynyl and tetraphenylcyclopentadienone.
Pure products of 3A, 3B, and 3C can be easily obtained
through recrystallization or flash column purification. How-
ever, purification of 3D is rather difficult because of the
similar solubility and polarity between the product and the
starting materials. Therefore, the crude product 3D was
directly used for the next reaction to prepare compound Blue
D. p-Dimethylaminophenyl groups were introduced to the
molecular backbones through the Suzuki coupling reaction
to improve the hole charge transport property of the
molecules. The Suzuki coupling reactions achieved very high
yields (>90%) except for Blue C. Pure Blue D was also
obtained after silica gel column purification. The impurity
in crude compound 3D did not affect this coupling reaction.
Thermal properties of the materials Blue A-D were
investigated with thermal gravimetric analyses (TGA) and
differential scanning calorimetry (DSC). All the materials
show very good thermal stabilities. The onset decomposition
temperatures are 332, 296, 306, and 326 °C for Blue A-D,
respectively. Obvious glass transmission temperatures (Tg)
were observed for Blue B-D at 124, 100, and 118 °C,
respectively, in the second heating run. High Tg is believed
to be good for long lifespan device operation. No Tg for Blue
A was observed probably because of the existence of strong
dipole-dipole interaction between Blue A molecules, which
makes the molecules too rigid in solid state.
Ultraviolet absorption (UV), photoluminescence (PL)
spectra, and the relative fluorescence quantum efficiencies
(ηPL) of the four compounds in chloroform solution with the
concentration of about 1 × 10-5 M were measured at room
temperature. The quantum efficiencies were determined to
be 33%, 14%, 35%, and 7.0% for Blue A-D, respectively,
using quinine sulfate (1 × 10-5 M dissolved in 0.1 M H2SO4)
as a reference. As shown in Table 1, the π-π* energy band
gaps of these materials were estimated from the UV spectra
absorption edges to be 3.10, 3.25, 2.84, and 3.25 eV for Blue
A-D, respectively. It can be seen that the energy band gaps
of these compounds can be easily tuned by changing the
central aromatic ring in the compounds, and the lower band
gap of Blue C is ascribed to the electron-rich nature as well
as the better conjugation of the thienyl ring with the two
adjacent phenyl rings linked at 2 and 5 positions. Theoretical
calculation indicated the dihedral angle of the most stable
These four blue light-emitting polyphenylphenyl com-
pounds, Blue A-D, were synthesized following the synthetic
routes outlined in Scheme 1. Palladium-catalyzed Sonogash-
ira coupling reactions between the dibromo compounds and
(trimethylsilyl)acetylene were employed to introduce the
ethynyl group into the central aromatic ring. When equal
Scheme 1
(11) Chen, C. T.; Chiang, C. L.; Lin, Y. C.; Chan, L. H.; Huang, C. H.;
Tsai, Z. W.; Chen, C. T. Org. Lett. 2003, 5, 1261.
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K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001, 123,
946.
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A. C.; Mu¨llen, K. AdV. Mater. 2002, 14, 1061.
(14) Watson, M. D.; Fechtenkotter, A.; Mu¨llen, K. Chem. ReV. 2001,
101, 1267.
392
Org. Lett., Vol. 7, No. 3, 2005

Table 1. Physical Properties of the Materials
p-doping (V)
energy levels (eV)
Td
(°C)
Tg
(°C)
UV λmax in
CHCl3 (nm)
band gap
(eV)
PL λmax in
CHCl3 (nm)
ηpl in
CHCl3
material
Eonset
Ea
Ec
HOMO
LUMO
Blue A
Blue B
Blue C
Blue D
332
296
306
326
/
124
100
118
332
323
362
329
3.10
3.25
2.84
3.25
382 (429)
373
419
33%
14%
35%
7.0%
0.70
0.66
0.50
0.56
0.95
0.91
0.72
0.82
0.77
0.62
0.52
0.60
-5.10
-5.06
-4.90
-4.96
-2.00
-1.81
-2.06
-1.71
396
conformation between thienyl ring and phenyl ring is about
40°, while that in biphenyl is 45-46°.15,16 The energy band
gap of Blue D will not be affected by the alkoxy substituents
on the central phenylene ring compared to Blue B. However,
the maximum absorption of Blue D is red shifted by 6 nm
compared to Blue B in solution. It is noteworthy that the
PL spectrum of Blue A is much broader than the others and
also shows a large Stockes shift. The full width at half-
maximum (fwhm) of PL spectra of Blue B-D are 59, 73,
and 55 nm and the Stockes shifts are 50, 57, and 67 nm,
respectively; the fwhm and Stockes shift of Blue A are 108
and 97 nm, respectively. Similar broader emission was also
observed in solutions using different solvent, e.g., hexane,
toluene, and THF. The much broader PL spectrum and large
Stockes shift are characteristic of intermolecular excimer
emission, which have been observed in a few conjugated
polymers.17-19 The formation of excimers of Blue A in
solution may be attributed to the strong dipole-dipole
interaction between two molecules due to existence of
strong electron-donating phenylamino group and electron-
withdrawing pyridine ring in the same molecule.
band gaps obtained from the edge absorption of the solution
samples. All data are summarized in Table 1, from which
we can find that Blue C possesses the highest HOMO energy
level, lowest LUMO energy level, and lowest energy band
gap among the four materials.
Multilayer devices were fabricated to investigate the
performance of the four blue light-emitting materials with
the device configuration of ITO/PEDOT:PSS (50 nm)/blue
material:PVK (2%, w:w) (50 nm)/BCP (10 nm)/Alq3 (20
nm)/LiF (0.5 nm)/Al (200 nm). The first layer of poly(3,4-
ethylenedioxythiophene) doped with poly(styrenesulfonic
acid) (PEDOT:PSS) was spin-coated on a glass substrate with
patterned ITO to form a hole injection layer. After this, dried
in an oven at 120 °C for 5 min. A solution containing 4 mL
of toluene, 40 mg of PVK, and 0.8 mg of blue light-emitting
material (2% dispersed in PVK) was spin-coated onto the
first layer to form an emitting layer with a thickness of about
50 nm. On the emissive layer, 10 nm of 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline (BCP), 20 nm of aluminum
tris-8-hydroxyquinoline (Alq3), 0.5 nm of LiF, and 200 nm
of Al were thermally deposited sequentially under vacuum
of 3 × 10-4 Pa. The organic electroluminescent devices
obtained were examined in a drybox with a Keithley 2420
source meter and with a calibrated photodiode. EL spectra
were recorded with an Ocean Optics USB2000 miniature
fiber optic spectrometer. The current density-voltage and
luminance-voltage characteristics for device of Blue C are
Cyclic voltammetry (CV) was employed to investigate the
electrochemical behaviors of these materials. The CV graphs
were obtained with positively scan from 0 to 1.2 V with the
materials in 0.10 M of tetrabutylammonium perfluorate
phosphate (Bu4NPF6) in anhydrous CH2Cl2 solution. The
onset potentials for oxidation of the materials were observed
to be 0.70, 0.66, 0.50, and 0.56 V (vs SCE) for Blue A-D,
respectively. The HOMO energy levels were estimated to
be -5.10, -5.06, -4.90, and -4.96 eV for the four materials
according to the equation HOMO ) -([Eonset]
ox + 4.4) eV.20
The HOMO energy level for Blue C is the highest and higher
than that of Blue A by 0.20 eV. It is obvious that the
electron-donating nature of the central aromatic ring can help
to raise the HOMO energy levels and electron-withdrawing
groups will lower the HOMO levels of the materials.
Therefore, the HOMO energy levels of the compounds can
be easily controlled by changing the central aryl group or
introducing substituents with different electronic properties.
The respective LUMO energy levels of the compounds were
estimated to be -2.00, -1.81, -2.06, and -1.71 eV, by
combining the HOMO energy levels together with the energy
(15) Pan, J. F.; Chua, S. J.; Huang, W. Chem. Phys. Lett. 2002, 363, 18.
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(18) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903.
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E. F. Synth. Met. 1997, 87, 53.
Figure 1. Current density-voltage and luminance-voltage char-
acteristics of OLED device of Blue C with a configuration of ITO/
PEDOT: PSS (50 nm)/Blue C:PVK (2%, w:w) (50 nm)/BCP (10
nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al (200 nm). Insert is the EL spectra
of the four materials Blue A-D.
Org. Lett., Vol. 7, No. 3, 2005
393

plotted in Figure 1, and the EL spectra of the four materials
are shown as an insert. The turn on voltage (at a brightness
of 1.0 cd/m2) of Blue C is 8 V and the device reaches the
maximum luminance of 1120 cd/m2 at 14 V. The EL
spectrum of Blue C shows two peak emission at 451 and
493 nm, with the CIE coordinates of (0.158, 0.201),
corresponding to deep blue emission. Blue A, B, and D emit
sky blue light with the maximum emission at about 406 nm
and a clear side peak at 496 nm. The peak wavelength of
emission, the CIE coordinates and the current efficiency/
quantum efficiency of the devices are summarized in Table
2. It is interesting to note that device based on Blue C
layer and electron transport layer, respectively, could be
easily transferred to the guest material of Blue C. Therefore,
the charge recombination and light emission will occur at
the guest materials rather than PVK. On the contrary, in
devices of Blue A, B, and D, the holes are able to be
transferred from PVK to the guest materials but the electrons
will mostly be trapped in the host matrix. Thus, most excitons
will be generated from the host materials in the three cases.
The EL spectra of Blue A, B and D indicated that the
emission is mainly from the host materials as expected, which
demonstrated characteristic PVK emission peaked at 406 to
409 nm.23 The side peak emission at 496 nm with low
intensity is from the guest materials. As we know that PVK
is a good host and hole transport material but not a good
light emitter for OLEDs, the much lower efficiency for
devices of Blue A, B, and D is thus rational.
Table 2. Device Performances of the Four Materials
material EL λmax (nm)
CIE (x, y)
ηCurr (cd/A) ηex (%)
In summary, a series of novel blue light-emitting materials
comprising a polyphenylphenyl dendron, phenylamino group,
and central aromatic ring that varied from pyridylene,
phenylene, thienylene, and 2,5-dimethoxyphenylene has been
synthesized through a simple synthetic route. All the materi-
als show good thermal stability and high glass transition
temperature (>100 °C). The four materials demonstrate very
good solubility in common organic solvents, which makes
them promising candidates for OLED device fabrication
through solution process. The electronic properties of the
resulting materials can be easily tuned through changing the
central aromatic ring. A highly efficient and deep blue light-
emitting OLED device has been demonstrated by using
compound Blue C as the emitter and PVK as the host
material through spin coating. The current efficiency and
external quantum efficiency of Blue C are 2.2 cd/A and
1.6%, respectively, which is the highest efficiency reported
so far for solution-processed small molecules blue OLEDs.
The higher EL efficiency of Blue C compared to the others
is ascribed to the higher PL efficiency and matched energy
levels between Blue C and the host material (PVK), in which
the charge carriers injected into the PVK matrix can be fully
transferred to the guest materials to achieve highly efficient
light emission.
Blue A
Blue B
Blue C
Blue D
409 (496)
406 (496)
451 (493)
406 (496)
0.188, 0.179
0.200, 0.186
0.158, 0.201
0.197, 0.201
0.16
0.15
2.2
0.16
0.15
1.6
0.15
0.14
demonstrated much higher EL efficiency with a maximum
current efficiency of 2.2 cd/A (400 cd/m2 at 18 mA/cm2)
and external quantum efficiency of 1.6%, whereas the device
efficiencies of other three materials are of only about 0.15
cd/A and 0.15% with the same device configuration. To the
best of our knowledge, 2.2 cd/A is the highest efficiency
reported so far for small molecules blue light-emitting
devices fabricated through solution process. Solution process
is cost-effective compared to vacuum deposition and thus is
more desirable. This efficiency is also comparable to most
of the reported small molecules blue OLEDs fabricated
through vacuum deposition.21,22
Much lower electroluminescent efficiencies of the other
three materials compared to Blue C could be ascribed to
two factors: (i) PL efficiency lower than that of Blue C,
especially for Blue B and D, and (ii) the mismatching energy
levels of the three light-emitting materials with that of the
host material of PVK, as shown in Figure 2. By comparing
the energy levels of Blue C with PVK, it can be seen that
both the HOMO and LUMO of Blue C are lying between
the energy levels of PVK. Thus both the hole and electron
charge carriers injected into PVK from the hole injection
Acknowledgment. We thank Institute of Materials Re-
search & Engineering for the financial support for this
research. Mr. Patrick W. S. Lau, Ms. Tang Shih Ju, Ms.
Huang Cailing, Ms. Shwe Zin Ma, and Ms. Teo Tang Lin
should also be acknowledged for their assistance in OLED
device fabrication and other characterization.
Supporting Information Available: Synthetic proce-
dures, UV-vis and PL spectra, and cyclic voltammetric
graphs for the compounds of Blue A-D. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL0478180
(21) Kim, Y.-H.; Shin, D.-C.; Kim, S.-H.; Ko, C.-H.; Yu, H.-S.; Chae,
Y.-S.; Kwon, S.-K. AdV. Mater. 2001, 13, 1690.
(22) Li, Y.; Fung, M. K.; Xie, Z.; Lee, S. T.; Hung, L. S.; Shi, J. AdV.
Mater. 2002, 14, 1317.
(23) Kido, J.; Hongkawa, K.; Okuyama, K.; Nagai, K. Appl. Phys. Lett.
1993, 63, 2627.
Figure 2. Energy levels of PVK and the four blue light-emitting
materials. The energy levels of PVK were cited from ref 24. The
values are in good agreement with the data obtained from
electrochemical measurement performed in our lab.
(24) He, G.; Chang, S. C.; Chen, F. C.; Li, Y.; Yang, Y. Appl. Phys.
Lett. 2002, 81, 1509.
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