1150
Chemistry Letters Vol.37, No.11 (2008)
Novel Green Small-molecule Host Materials for Solution-processed Organic Light-emitting Diodes
Dong-Ha Kim,2 Dae Hyuk Choi,2 Jung Joo Park,1 Seong Taek Lee,1 and Jang Hyuk Kwonꢀ1
1Department of Information Display, Kyung Hee University, Dongdaemoon-ku, Seoul 130-701, Korea
2Corporate R&D Center, LUDIS Co., Ltd., 404-1 Baekhyun-Dong, Bundang-Gu, Seongnam 463-420, Korea
(Received August 8, 2008; CL-080769; E-mail: jhkwon@khu.ac.kr)
The authors report novel small-molecule green-fluorescent
hosts for solution processed OLEDs. 9,10-Diarylanthracene
and fluorene moieties were introduced to the 9 and 10 positions
of an anthracene core to give the strong amorphous characteris-
tics. These novel hosts show sufficient optical, electrical, and
FADPA Ar =
Ar
thermal properties with very good solubility in organic solvents.
Utilizing these solution-processed hosts, a maximum current ef-
FADNA Ar =
Ar
ficiency of 7.8 cd/A is demonstrated with a general fluorescent
dopant.
Figure 1. Chemical structures of FADPA and FADNA.
core,
9-bromo-10-(9,9-dimethyl-9H-fluoren-2-yl)anthracene
was prepared from the Suzuki reaction of 9,10-dibromoanthra-
cene and 9,9-dimethyl-9H-fluoren-2-yl boronic acid. The Suzuki
coupling reaction of this 9-bromo-10-(9,9-dimethyl-9H-fluoren-
2-yl)anthracene and corresponding 9,10-diarylanthracene-2-yl
boronic acid which was prepared from 2-bromo-9,10-diarylan-
thracene produced FADPA and FADNA6 in 20% overall yield.
Optical and electrical properties of FADPA and FADNA are
summarized in Table 1. The UV–visible maximum absorption
wavelengths of FADPA and FADNA in THF solution are in
the range of 360–400 nm with structureless absorption bands
arising from ꢀ–ꢀꢀ transitions. And maximum wavelengths of
photoluminescence (PL) spectra in solution are in the blue re-
gion near 450 nm with a full width at half emission maximum
of 55 nm. In addition, the PL spectra of films on glass of these
compounds show about 10 nm red-shifts compared to those of
their solutions. This indicates that FADPA and FADNA do not
have planar stacking due to steric hindrance,7 and these small
red-shifts can be explained by the suppression of the intermolec-
ular ꢀ–ꢀꢀ stacking of FADPA and FADNA. The highest occu-
pied molecular orbital (HOMO) energy levels and the lowest un-
occupied molecular orbital (LUMO) energy levels of synthe-
sized materials were obtained according to our reported cyclic
voltammetry method.8a The HOMO energy levels of FADPA
and FADNA are 5.94 and 5.87 eV, respectively. The HOMO
and LUMO energy levels of our compounds with anthracene
core are consistent with previous results.4b,5c
Organic light-emitting diodes (OLEDs) have apparent ad-
vantages for the ultimate display because of its good self-emit-
ting nature and have been attracted intensive researches. Ex-
tremely high efficiency, low driving voltages, and long device
lifetime have been reported in devices to date.1 As far as device
fabrication, there are some concerns even with such good device
performance. The most common technology in the fabrication of
OLEDs is a fine-shadow-mask method utilizing thermal evapo-
ration of small molecular materials with which several manufac-
tures currently produce real products. However, there are still is-
sues regarding cost effectiveness, high resolution, and scalability
to large mother-glass formats. In contrast, solution processed
OLEDs have been considered to overcome these disadvantages
owing to simple printing and no vacuum technologies. Neverthe-
less, this method has serious limitations due to lack of good solu-
ble small molecules and polymer materials and relatively poor
device performance in efficiency and lifetime. Many efforts have
been carried out to improve device performance of solution-
processed OLEDs. Efforts have been limited to polymeric mate-
rials2 and soluble phosphorescent small molecular materials.3
In this letter, we report novel soluble small-molecular-host
materials and their application to green fluorescent OLEDs.
9,10-Bis(aryl)anthracenes are known to be blue emission dyes
with high fluorescent quantum yield and high chemical stabili-
ty.4 Their film-forming properties by solution process are very
poor. On the other hand, fluorene chemical units have been stud-
ied as OLED host materials for solution-processed OLEDs.5
They have excellent chemical and thermal stabilities, high fluo-
rescence quantum yields, good morphological properties, and
acceptable hole-transporting properties. Therefore, we designed
and synthesized two anthracene derivatives having 9,10-diaryl-
anthracene and fluorene moieties, namely, 100-(9,9-dimethyl-
9H-fluoren-2-yl)-9,10-diphenyl-2,90-bianthracene (FADPA) and
100-(9,9-dimethyl-9H-fluoren-2-yl)-9,10-di(naphthalen-2-yl)-2,90-
bianthracene (FADNA), as shown in Figure 1.
Table 1. Optical and electrical properties of FADPA and
FADNA
Solution
Film
(THF)
HOMO
/eV
LUMO
/eV
ꢁab
/nm
ꢁem
/nm
ꢁab
/nm
ꢁem
/nm
FADPA
FADNA
359
378
397
361
380
399
450
364
386
402
384
404
458
5.94
5.87
3.09
3.03
FADPA and FADNA were synthesized as follows. 2-Bro-
mo-9,10-diarylanthracene was prepared from aryl bromide and
2-bromoanthraquinone via arylmagnesium bromide. The aryl–
aryl bond formation was accomplished by the Suzuki coupling
reaction using Pd catalyst. As a fluorene moiety with anthracene
453
466
Copyright Ó 2008 The Chemical Society of Japan