4,5-Diazafluorene-incorporated ter(9,9-diarylfluorene): A novel molecular doping strategy for improving the electron injection property of a highly efficient OLED blue emitter

Article abstract of DOI:10.1021/ol050547o

(Chemical Equation Presented) A more efficient OLED device with blue emission characteristic of terfluorene has been achieved by using a novel molecular doping strategy, in which 4,5-diazafluorene was incorporated as the substitution group of terfluorene

Full text of DOI:10.1021/ol050547o

ORGANIC
LETTERS
2005
Vol. 7, No. 10
1979-1982
4,5-Diazafluorene-Incorporated
Ter(9,9-diarylfluorene): A Novel
Molecular Doping Strategy for Improving
the Electron Injection Property of a
Highly Efficient OLED Blue Emitter
Ken-Tsung Wong,*,† Ruei-Tang Chen,† Fu-Chuan Fang,† Chung-chih Wu,*,‡ and
Yu-Ting Lin‡
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, and
Department of Electrical Engineering, Graduate Institute of Electro-optical
Engineering and Graduate Institute of Electronics Engineering, National Taiwan
UniVersity, Taipei 106, Taiwan
kenwong@ntu.edu.tw
Received March 14, 2005
ABSTRACT
A more efficient OLED device with blue emission characteristic of terfluorene has been achieved by using a novel molecular doping strategy,
in which 4,5-diazafluorene was incorporated as the substitution group of terfluorene to facilitate electron injection from the metal cathode yet
without altering emission characteristics.
Oligomeric fluorenes with high thermal stability, high
solution- and solid-state fluorescence quantum yields,1 and
high bipolar charge carrier mobilities2 are promising for
highly efficient blue OLEDs. However, due to large carrier-
injection barriers between oligofluorene derivatives and
corresponding electrodes, efficient emitting devices using
fluorenes as emitters normally still require sophisticated
configurations incorporating both suitable hole-transporting
and electron-transporting layers. For improving the carrier
injection capability, structural modifications of the conjugated
fluorene backbone can be adopted by introducing heteroaro-
matic rings as functional components. However, molecular
design based on such a strategy would greatly perturb
structural features of the main chain and thus physical
properties of materials.3 By making use of flexibility in
functionalizing C-9 positions of fluorenes, such complexity
may be reduced through development of multifunctional
fluorene-based materials containing pendant groups of dif-
† Department of Chemistry, National Taiwan University.
‡ Graduate Institute of Electro-optical Engineering and Graduate Institute
of Electronics Engineering, National Taiwan University.
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10.1021/ol050547o CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/12/2005

ferent functionalities.4 With such a configuration, the tetra-
hedral C-9 carbon of fluorene serves as an insulating spacer,5
effectively hindering interactions between the central fluorene
chromophore and the C-9 substitution. In addition, materials
covalently combining different functional subunits also avoid
the problems of phase separation occasionally encountered
in using the doping or blending strategy.
Scheme 1
Among the oligofluorenes, ter(9,9-ditolylfluorene) (1) is
one of the most efficient emitters for blue OLEDs.6 Yet for
OLED applications, the electron affinity (Ea) of 1 is relatively
small, causing difficulty in electron injection from common
cathode electrodes and imposing the requirement of an
additional electron-transport layer with a more suitable
electron-injection capability. In this communication, 4,5-
diazafluorene7 has been facilely introduced as a functional
substituent spirally linked to the conjugated terfluorene main
chain. The resulting functionalized terfluorene (2) performs
a more balanced electron injection capability as compared
to the parent compound (1). The molecular design reported
in this communication possesses the advantage of permitting
the introduction of a higher electron affinity moiety without
altering the emission properties of the oligomeric fluorene
backbone.
The synthesis of the titled compound is depicted in Scheme
1. Starting from 4,5-diazafluoren-9-one,8 4,5-diaza-9,9′-
spirobifluorene9 was synthesized in moderate yield (70%)
with modified procedures (Supporting Information). The
selective bromination on the biphenyl branch of 4,5-diaza-
9,9′-spirobifluorene has been accomplished to afford 4,5-
diaza-2′,7′-dibromo-9,9′-spirobifluorene with a 66% isolated
yield in the presence of FeCl3 as a Lewis acid promoter in
CH2Cl2. The titled compound then was efficiently synthesized
with an isolated yield of 86% by Suzuki coupling reaction
of 4,5-diaza-2′,7′-dibromo-9,9′-spirobifluorene with the cor-
responding 9,9-ditolylfluorene pinacol boronic ester in the
presence of a catalytic amount of Pd(PPh3)4 and cocatalyst
PtBu3.
Cyclic voltammetry was conducted for probing electro-
chemical properties (Figure 1). To clearly differentiate
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Figure 1. Comparison of reduction cyclic voltammograms of 1
(blue) and 2 (red). The inset shows differential pluse voltammetry.
For CV experiments, which were performed in THF with 0.1 M of
nBu4ClO4 as a supporting electrolyte, a glass electrode was used
as the working electrode; scan rate ) 100 mV/s.
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electrochemical behaviors of different compounds, dif-
ferential pulse voltammetry (DPV) of terfluorenes 1 and 2
were also conducted under the same conditions (inset of
Figure 1). Terfluorene 2 exhibits a reduction onset at -1.76
V, whereas the parent terfluorene 1 has a higher reduction
onset at -1.87 V.
Three reduction potentials were detected for 2. The origin
of these reduction peaks was determined by comparing the
reduction potentials of 4,5-diaza-9,9′-spirobifluorene (with
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1980
Org. Lett., Vol. 7, No. 10, 2005

Table 1. Comparison of Photophyscal Properties of Terfluorenes 1 and 2
abs λmax solution
abs λmax film
PL λmax solution
PL λmax film
Qsol
Qfilm
Tg
compd
(nm)
(nm)
(nm)
(nm)
(%)
(%)
(°C)
1
2
353
353
353
353
393, 413, 440
393, 413, 440
405, 428, 450
405, 428, 450
85
78
80
60
190
220
a reversible reduction potential E1/2red at -2.00 V, Supporting
Information) and the parent terfluorene 1 (with two quasi-
reversible reduction potentials at -2.07 and -2.17 V). The
first reduction peak of 2 can be unambiguously ascribed to
the reduction occurring on the diazafluorene moiety, while
the second and third reduction peaks of 2 can be attributed
to the reductions occurring on the terfluorene backbone. The
assignments are even more obvious in comparing the
differential pulse voltammetry of terfluorenes 1 and 2 (Figure
1, inset). The slightly higher second and third reduction
potentials of 2, as compared to those of parent terfluorene
1, could be attributed to Coulombic interaction between the
reduced diazafluorene and the terfluorene backbone. On
oxidation, terfluorene 2 exhibits reversible oxidation peaks
at voltages similar to those of 1. Overall, introducing 4,5-
diazafluorene as the pendant substituent has led to a new
terfluorene exhibiting a lower reduction potential compared
to that of the parent terfluorene 1.
Table 1 and Figure 2 compare the photophysical properties
of terfluorenes 1 and 2. Both terfluorenes exhibit very similar
spectroscopic properties: nearly the same lowest-energy
absorption bands around 353 nm, similar PL spectra in either
solutions (with peaks around 393 and 413 nm) or in thin
films (with peaks around 405 and 428 nm), and similarly
high PL quantum yields (85% for 1 and 78% for 2 in
solutions and slightly lower values for both in thin films).
These transitions agree with characteristics of the lowest
π-π* transition of the central terfluorene chromophore and
appear to be consistent with the viewpoint that the tetrahedral
C9 carbon serves as an effective spacer blocking the
substitution from altering the electronic transition on the
backbone. On the other hand, bulky and rigid C9 substitutions
in both molecules are apparently beneficial to morphological
Figure 3. EL characteristics of devices using terfluorenes 1 and
2: (a) L-V and I-V characteristics (Inset: EL spectra of both
devices). (b) External EL quantum efficiency vs current.
Figure 2. Comparisons of photoluminescence of terfluorene 1 and
2 (a) in solution and (b) in solid thin films.
Org. Lett., Vol. 7, No. 10, 2005
1981

stability in thin films, as indicated by the high glass transition
temperatures (Tg) of both compounds (Table 1).
diazafluorene moiety, which due to its lower reduction
potential gives enhanced electron injection and device
performance. Nevertheless, the final emission originating
from the terfluorene backbone indicates that injected elec-
trons undergo efficient electron transfer from the reduced
diazafluorene to the terfluorene backbone, where they
eventually recombine with injected holes to generate emis-
sion characteristic of the terfluorene backbone. Thus, the
diazafluorene moiety molecularly spiro-linked to the ter-
fluorene chromophore effectively functions as a bridge for
electron injection from cathode to the terfluorene chro-
mophore.
Electroluminescent (EL) devices using these two terfluo-
renes were compared to investigate the influence of the 4,5-
diazafluorene substitution on EL properties. The device
structure used is ITO anode/PEDT:PSS (300 Å)/TCTA (500
Å)/terfluorene 1 or 2 (500 Å)/LiF (5 Å)/Al cathode, where
the conducting polymer polyethylene dioxythiophene:poly-
styrene sulfonate (PEDT:PSS) was used as the hole injection
layer,10 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA) as
the hole-transport layer,10 terfluorenes as the emitting/
electron-transport layer, and thin LiF as the electron-injection
layer.10 Both devices exhibit pure blue EL similar to PL
spectra of terfluorenes (inset of Figure 3a). The device using
2, however, exhibits a lower device voltage (Figure 3a) and
a much higher EL external quantum efficiency than the
device using 1 (1.6 vs 0.4% photon/electron, Figure 3b).
Performance enhancement with terfluorene 2 indicates that
poor performances of the device using 1 is due to unbalanced
electron injection from the cathode to terfluorene 1 and that
the new compound terfluorene 2 facilitates electron injection
and thus largely improves EL performance.
In summary, a novel molecular doping scheme to terfluo-
renes has been established by incorporating 4,5-diazafluorene
as the C9 substitution of terfluorenes. The original emission
characteristics of the terfluorene remains intact, whereas the
new terfluorene exhibits a lower reduction onset and
enhanced electron injection from the metal cathode. A more
efficient OLED device has been achieved by using this novel
terfluorene.
Acknowledgment. This work was financially supported
by the National Science Council of Taiwan.
On the basis of the EL, electrochemical, and photophysical
results, it is believed that electron injection onto terfluorene
2 in the EL device initially occurs at the spirally linked
Supporting Information Available: Detailed experi-
mental procedure and spectroscopic characterization of new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Org. Lett., Vol. 7, No. 10, 2005