Synthesis and Light-Emitting Properties of Bipolar Oligofluorenes Containing Triarylamine and 1,2,4-Triazole Moieties

Full text of DOI:10.1021/ol0615477

ORGANIC
LETTERS
2006
Vol. 8, No. 19
4271-4274
Synthesis and Light-Emitting Properties
of Bipolar Oligofluorenes Containing
Triarylamine and 1,2,4-Triazole Moieties
Zhong Hui Li,†,§ Man Shing Wong,*,† Hiroshi Fukutani,‡ and Ye Tao*,‡
Department of Chemistry and Centre for AdVanced Luminescence Materials, Hong
Kong Baptist UniVersity, Hong Kong SAR, China, and Institute for Microstructural
Sciences, National Research Council of Canada, M-50 Montreal Road,
Ottawa, Ontario, Canada K1A 0R6
mswong@hkbu.edu.hk; ye.tao@nrc.ca
Received June 22, 2006
ABSTRACT
A facile approach for synthesis of bipolar oligofluorenes, TAZ-OF(n)-NPh, n
) 2 or 3 end-capped with hole-transporting diphenylamino and
electron-transporting triazole moieties by Suzuki cross-coupling as the key reaction has been developed. This novel bipolar oligofluorenes
exhibited blue-emission, high thermal and morphological stabilities. The single-layer OLED based on TAZ-OF(2)-NPh exhibited superior device
-
2
-1
performance with a maximum luminance of 1128 cd m and luminance efficiency of up to 0.83 cd A
.
Maintaining the charge balance in an organic light-emitting
diode (OLED) is considered to be an important issue for
achieving high device performance1,2 as it is a double-charge
injection device, which requires the simultaneous supply of
both electrons and holes to the electroluminescent (EL)
material sandwiched between two electrodes. Two strategies
have been developed to meet this demand which include the
fabrication of multilayer configuration devices2c and an
introduction of hole- and electron-transporting moieties into
one single light-emitting molecular material.3 The latter has
been thought of as a promising approach to improve device
performance because of the simplified device structure and
fabrication procedure as well as the possibility of reducing
(2) (a) Adachi, C.; Nagai, K.; Tamoto, N. Appl. Phys. Lett. 1995, 66,
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Mater. Chem. 2000, 10, 1. (d) Loy, D. E.; Koene, B. E.; Thompson, M. E.
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† Hong Kong Baptist University.
‡ National Research Council of Canada.
§ Present address: Department of Chemistry, Sichuan College of
Education, Chengdu, 610041, Sichuan Province, China.
(1) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(b) Adachi, C.; Tokito, S.; Tsutsui, T.; Saito, S. Jpn. J. Appl. Phys. 1988,
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10.1021/ol0615477 CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/15/2006

or eliminating exciplex formation between layers.4 Highly
electron-rich triphenylamines known for their high hole
mobility have been extensively used as hole-transporting
materials in OLEDs.5 It has been shown that light-emitting
polymers grafted or end-capped with triphenylamine moieties
exhibit improved OLED device performance.6 Furthermore,
highly electron-deficient triazole (TAZ) derivatives have been
demonstrated to have more efficient electron-transport and
hole-blocking characteristics and have a higher stability to
high current density than the oxadiazole derivatives (e.g.,
PBD) in OLEDs.7 Monodisperse oligofluorenes (OF) have
recently attracted much attention because of their chemical
and thermal stabilities and potential applications for opto-
electronic molecular materials, especially for blue-emitting
OLEDs devices.8 As part of our efforts to investigate the
structural factors of functional materials9 that can enhance
OLED performance and stability and toward the goal of
integrating three active components including hole-transport-
ing, electron-transporting, and light-emitting moieties into a
single molecule to simplify device fabrication and to improve
performance of resulting devices, we report herein the first
synthesis of bipolar oligofluorenes, TAZ-OF(n)-NPhs, n )
2 or 3, in which the blue emitting bi- or terfluorene core is
asymmetrically disubstituted with diphenylamino hole-
transporting moiety and electron-transporting triazole deriva-
tive forming a multifunctional molecule and the investigation
of their use in single-layer OLEDs. Meanwhile, to obtain
blue light-emitting materials, the strongly electron-withdraw-
ing triazole moiety must be decoupled with π-conjugated
system of the oligofluorene core which was thus linked at
the nonconjugated 3-position of the phenyl ring of the triazole
moiety. Because of the bipolar character, good luminescence
properties, high thermal stability, and good amorphous
morphological stability, TAZ-OF(n)-NPhs show great po-
tential for use as double charge carrier transport emitters in
OLEDs
The synthetic strategy for asymmetrically disubstituted
diphenylamine and triazole based bipolar oligofluorenes
TAZ-OF(n)-NPhs, n ) 2 or 3, is outlined in Scheme 1.
Scheme 1. Synthesis of Asymmetric Bipolar Oligofluorenes
TAZ-OF(n)-NPh
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Transformation of the dibromide 19b into the corresponding
2-(trimethylsilyl)-7-fluorenylboronic acid 2 was carried out
by a one-pot reaction affording a 52% overall yield. After
first lithium bromide exchange at -78 °C followed by
reaction with trimethylsilyl chloride at room temperature,
second lithium bromide exchange at -78 °C was subse-
quently proceeded and followed by reaction with trimethyl
borate and then acid hydrolysis. Reaction of 4-tert-butyl-
4272
Org. Lett., Vol. 8, No. 19, 2006

Table 1. Summaries of Measurements of TAZ-OF(n)-NPhs
λabs a/nm
max
f
(ꢀ × 104/M-1 cm-1
)
λem a,b/nm
Φa,c
E1/2oxdd/V
HOMOe/eV
E1/2 redd/V
LUMOe/eV
Tg /°C
Tdecg/°C
max
TAZ-OF(2)-NPh
TAZ-OF(3)-NPh
375 (4.38)
374 (4.76)
435
435
0.52
0.52
0.37, 0.91
0.35, 0.82
-5.17
-5.15
-1.38
-1.38
-2.99
-2.99
127
126
448
452
d
a Measured in CHCl3. b Excited at the absorption maxima. c Using quinine sulfate monohydrate (Φ313 ) 0.48) in 1.0 M H2SO4 as a standard. E1/2 vs
Fc+/Fc estimated by CV method using platinum disk electrode as a working electrode, platinum wire as a counter electrode, and SCE as a reference electrode
with an agar salt bridge connecting to the oligomer solution and all the potentials were calibrated with ferrocene, E1/2 (Fc/Fc+) ) 0.43 V vs SCE. e Estimated
from CV measurements. f Determined by differential scanning calorimeter from remelt after cooling with a heating rate of 10 °C/min under N2. g Determined
by thermal gravimetric analyzer with a heating rate of 10 °C/min under N2.
benzoyl hydrazide and 3-bromobenzoyl chloride in the
presence of triethylamine at room temperature afforded the
disubstituted hydrazine 4 in a quantitative yield. Condensa-
tion of 4 and aniline in the presence of POCl3 at 195 °C
afforded 1,2,4-triazole derivative 5 in 67% yield. Palladium-
catalyzed Suzuki cross-coupling of the bromide 5 and boronic
acid 2 using Pd(OAc)2/2P(o-tolyl)3 as a catalyst afforded the
desired trimethylsilylfluorene derivative 6 in an excellent
yield. Iododesilylation of 6 was carried out in the presence
of silver trifluoroacetate at 80 °C affording the corresponding
iodide 7 in quantitative yield.10 Palladium-catalyzed Suzuki
cross-coupling of iodide 7 and 7-diphenylamino-2-fluore-
nylboronic acid, 3, which was prepared according to the
literature procedure,9b afforded TAZ-OF(2)-NPh in 87%
yield. On the other hand, cross coupling of 7 and boronic
acid 2 afforded the trimethylsilylbifluorene derivative 8 in
an excellent isolated yield of 96%. Iododesilylation of 8 gave
the corresponding bifluorene iodide 9. Palladium-catalyzed
Suzuki cross-coupling of 9 and boronic acid 3 afforded the
target compound TAZ-OF(3)-NPh in a good yield. All the
newly synthesized asymmetrically disubstituted bipolar oli-
Although the λabs
constant, their molar absorptivities (ꢀmax) increase slightly
as the oligofluorene chain length increases (Figure 1a and
of TAZ-OF(n)-NPhs remain fairly
max
Figure 1. (a) Absorption and emission spectra measured in CHCl3
and (b) cyclic voltammogram measured in CH2Cl2 of TAZ-OF-
(n)-NPh, n ) 2 and 3.
1
gofluorenes were fully characterized with H NMR, 13C
Table 1). On the other hand, their emission spectra are
structureless, indicating noncoplanar excited state with
NMR, MS, and elemental analysis and found to be in good
agreement with their structures.
slightly red-shifted (∆ 3-10 nm) emission maxima (λem
These bipolar oligofluorenesTAZ-OF(n)-NPhs show dis-
tinct glass transition temperatures (Tg’s ∼ 126 °C) as
determined by differential scanning calorimetry (DSC),
which remain fairly constant despite an increase in fluorenyl
unit, suggesting that these oligofluorenes could form mor-
phologically stable amorphous thin film. Furthermore, TAZ-
OF(n)-NPhs show high thermal stabilities with decomposi-
tion temperatures (Tdec’s) of ∼450 °C which slightly increase
as the oligofluorene chain length increases as determined
by thermal gravimetric analyzer (TGA) (Table 1)
The absorption maxima (λabsmax) of these bipolar oligof-
luorenes TAZ-OF(n)-NPhs peaked at 375 nm and are
relatively blue shifted (∼10 nm) as compared to the
symmetrically diphenylamino-disubstituted analogues, indi-
cating the nonconjugated nature of the oligofluorene core
and triazole moiety.9b In addition, the incorporation of triazole
moiety onto the nonconjugated position of the oligofluorene
backbone may cause the resulting molecule to be less
max
) 435 nm) when compared to those of the corresponding
symmetrically diphenylamino-disubstituted oligofluorenes.
Upon excitation either at 269 nm corresponding to the
λabsmax of triazole moiety or at 314 nm attributed to the nfπ*
transition of triarylamine or at 375 nm corresponding to the
πfπ* transition of oligofluorene backbone, the emission
spectra obtained are almost identical, suggesting that energy
or exciton can efficiently transfer from the end-substituent
to the oligofluorene core. The fluorescence quantum yields
(ΦPL) of TAZ-OF(n)-NPhs measured in chloroform using
quinine sulfate monohydrate in 1.0 M H2SO4 as a standard12
are 0.52 which does not increase with an extension of chain
length, indicating the longer homologue does not improve
coplanarity of the π-conjugated system. These ΦPL values
(11) (a) Berlman, I. B. J. Phys. Chem. 1970, 74, 3085. (b) Nijegorodov,
N. I.; Downey, W. S. J. Phys. Chem. 1994, 98, 5639. As revealed from the
MM2-optimized geometries, TAZ-OF(2)-NPh showed a sligthly greater
twisting between the two fluorene units than the symmetrically dipheny-
lamino end-capped analogoues (∼1°). The twisting between bifluorene
skeleton and phenyl ring of the traizole moiety is ∼35.8°. These twistings
may be the cause of blue shift.
coplanar than the analogues end-capped with diphenylamino
11
groups in their ground state leading to a blue shift in λabs
.
max
(10) Wilson, S. R.; Jacob, L. A. J. Org. Chem. 1986, 51, 4833.
Org. Lett., Vol. 8, No. 19, 2006
(12) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
4273

are slightly smaller than those of the corresponding sym-
metrically diphenylamino-disubstituted analogous,9c consis-
tently indicating that introduction of triazole moiety onto the
oligofluorene skeleton might perturb the coplanarity altering
the fluorescent nature of oligofluorene core.
subsequently investigated for their electroluminescent (EL)
properties. The EL spectra of the TAZ-OF(2)-NPh-based
devices compare very well with the solution and thin film
photoluminescence (PL) spectra indicating the blue emission
originate from the oligofluorenes and showed some red-shifts
relative to the solution and thin film PL spectra. The EL
emission maxima (λELmax) of these devices also showed a
red-shift with an increase in the thickness of the emitting
layer. For TAZ-OF(2)-NPh-based devices, λELmax shift from
436 to 464 nm and the 1976 CIE coordinates shift from (u′,
V′ ) 0.165, 0.262) to (u′, V′ ) 0.149, 0.296) with a layer
thickness increasing from 40 to 80 nm. Such a small red
shift might be caused by the microcavity effect in these
devices.13 However, TAZ-OF(3)-NPh-based devices showed
Cyclic voltammetry was carried out in a three-electrode
cell setup with 0.1 M of Bu4NPF6 as a supporting electrolyte
in CH2Cl2 to examine the electrochemical properties of the
oligofluorenes. The results are tabulated in Table 1. The
asymmetrically substituted oligofluorenes TAZ-OF(n)-NPhs
exhibited two reversible one-electron anodic redox couples
which correspond to the sequential removal of electrons from
the arylamino group and oligofluorene core forming radical
cations and dications, respectively. (Figure 1b) Both the
arylamine oxidation and the oligofluorene core oxidation
proceed more easily with an increase in conjugated length
as this electrochemically formed radical cation can be
efficiently delocalized and hence stabilized. Importantly,
TAZ-OF(n)-NPhs also exhibited a quasi-reversible reduction
at -1.38 V under the same CV conditions corresponding to
the one-electron reduction of the triazole moiety which does
not affect by the conjugated length of oligofluorene due to
the nonconjugated linking. The advantages of incorporation
of diphenylamino endcap leading to the HOMO level of 5.15
eV to reduce the energy barrier for the hole injection from
ITO to the emissive oligofluorene core9 and incorporation
of triazole moiety resulting in the LUMO level of 2.99 eV
to facilitate electron injection and electron transport (Table
1) of this oligofluorene are apparent. As a result, TAZ-OF-
(n)-NPhs can also be used as double charge carrier transport/
injection materials (Figure 2).
much larger thickness-dependent red shift of λEL
(from
max
436 to 500 nm and the 1976 CIE coordinates from (u′, V′ )
0.153, 0.313) to (u′, V′ ) 0.193, 0.502)) with spectral
broadening. In addition to the possible microcavity effect,
the presence of molecular aggregation as inferred from the
red-shift and band broadening of the thin-film PL spectrum
of TAZ-OF(3)-NPh would further aggravate the adverse
situation. Importantly, the single-layer TAZ-OF(2)-NPh-
based device exhibited superior device performance with a
maximum luminance of 1128 cd m-2 and a luminance
efficiency of up to 0.83 cd A-1, whereas the TAZ-OF(3)-
NPh-based devices exhibited a maximum luminance of 370
cd m-2 and a luminance efficiency of up to 0.20 cd A-1.
In summary, a facile approach for the synthesis of the
asymmetrically disubstituted bipolar oligofluorenes contain-
ing hole-transporting and electron-transporting moieties using
Suzuki cross-coupling as the key reaction has been presented.
This approach could also be applicable for the construction
of other structurally uniform and well-defined π-conjugated
functional bipolar oligomers. The newly synthesized bipolar
oligofluorenes, TAZ-OF(n)-NPhs exhibited desirable lumi-
nescence and material properties. The single-layer TAZ-OF-
(2)-NPh-based OLED exhibited a blue emission with a
maximum luminance of 1128 cd m-2 and a luminance
efficiency of up to 0.83 cd A-1.
Acknowledgment. We are grateful to the Hong Kong
Research Grants Council (HKBU 2018/05P) for financial
support of this work.
Supporting Information Available: The synthetic pro-
cedures and physical data of intermediates and TAZ-OF-
(n)-NPh, n ) 1-2 and their device data. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 2. (a) Electroluminescence spectra and (b) luminance-
voltage and efficiency-voltage plots of TAZ-OF(n)-NPh-based
OLED devices.
OL0615477
The undoped single-layer OLEDs with the structure of
ITO/TAZ-OF(n)-NPh (40-80 nm)/LiF (1 nm)/Al (150 nm)
were fabricated by the conventional vacuum deposition and
(13) Bulovic´, V.; Khalfin, V. B.; Gu, G.; Burrows, P. E.Garbuzov, D.
Z.; Forrest, S. R. Phys. ReV. B 1998, 58, 3730.
4274
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