Novel blue luminescent materials for organic light-emitting diodes based on C9-fluorenyl anthracenes

Article abstract of DOI:10.1016/j.dyepig.2012.10.013

A series of twisted fluorenyl anthracenes linked by a rigid sp ³-hybridized carbon atom, containing an end-capping aryl substituent at either the C2-position of fluorene or at the C10-position of anthracene, have been synthesized and characterized. These materials exhibited good thermal stabilities and amorphous properties. Theoretical calculations and X-ray diffraction studies revealed that the six blue emitters possess non-coplanar structures to suppress intermolecular interactions. The relationship between the chemical structures with different end-capping groups and the electroluminescent properties has been investigated. Organic light-emitting diodes utilizing 9-(2-phenyl-9H-fluoren-9-yl) anthracene as the emitter exhibited deep-blue emissions CIE x, y (0.166, 0.153) with a current efficiency of 1.47 cd/A, external quantum efficiency of 1.84% at 20 mA/cm² and a maximum brightness of 3013 cd/m² at 11 V. It was found that the introduction of the aryl substituents to the C2 position of fluorene could improve the performance of OLEDs based on C9-fluorenyl anthracenes.

Full text of DOI:10.1016/j.dyepig.2012.10.013

Dyes and Pigments 96 (2013) 642e652
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel blue luminescent materials for organic light-emitting diodes based
on C9-fluorenyl anthracenes
,
a
b
a
a
a
c
Wen Wan a , Heliang Du , Jing Wang , Yuping Le , Haizhen Jiang , Hua Chen , Shizheng Zhu ,
**
*
,c,
Jian Hao a
a Department of Chemistry, Shanghai University, Shanghai 200444, China
b School of Materials Science and Engineering, Shanghai University, China
c Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, China
a r t i c l e i n f o
a b s t r a c t
A series of twisted fluorenyl anthracenes linked by a rigid sp3-hybridized carbon atom, containing an
end-capping aryl substituent at either the C2-position of fluorene or at the C10-position of anthracene,
have been synthesized and characterized. These materials exhibited good thermal stabilities and
amorphous properties. Theoretical calculations and X-ray diffraction studies revealed that the six blue
emitters possess non-coplanar structures to suppress intermolecular interactions. The relationship
between the chemical structures with different end-capping groups and the electroluminescent prop-
erties has been investigated. Organic light-emitting diodes utilizing 9-(2-phenyl-9H-fluoren-9-yl)
anthracene as the emitter exhibited deep-blue emissions CIE x, y (0.166, 0.153) with a current efficiency
of 1.47 cd/A, external quantum efficiency of 1.84% at 20 mA/cm2 and a maximum brightness of 3013 cd/
Article history:
Received 6 September 2012
Received in revised form
23 October 2012
Accepted 27 October 2012
Available online 12 November 2012
Keywords:
Blue OLED
Electroluminescence
Fluorescence spectra
Fluorenyl anthracenes
Synthesis
m
2 at 11 V. It was found that the introduction of the aryl substituents to the C2 position of fluorene could
improve the performance of OLEDs based on C9-fluorenyl anthracenes.
Ó 2012 Elsevier Ltd. All rights reserved.
Twisted structure
1. Introduction
materials have to be developed for OLEDs in the past decade, there
is still a clear need for further improvements of blue emission. At
present, it becomes a prime issue of concern in the development of
a satisfactory full-color display based on OLED devices.
The applications of organic light-emitting devices (OLEDs) to flat
panel display have been widely studied [1,2]. Though materials and
devices have developed rapidly, the search for new emitting
materials with high efficiency, thermal stability and excellent CIE
color coordinate for a full-color organic electroluminescent display
remains one of the most active areas of the studies. Compared with
red or green emitting materials, the electroluminescene (EL)
characteristics of blue-emitting material have to be improved,
particularly in terms of efficiency, color purity and lifetime [3e8].
However, it is much more difficult to produce a blue emission due
to its intrinsic characteristic of having a wide band gap, irrespective
of the type of the materials. This results in the observation that the
performance of blue electroluminescent emitters is usually inferior
to that of green or red emitters. Although many blue-light emitting
Because of their outstanding solution and solid-state photo-
luminescence (PL) quantum yields, research into blue-emitting
materials has been intensively focused on fluorene and anthra-
cene derivatives in recent years [9e15]. Spiro-type derivatives
containing fluorene or anthracene as OLED fluorescent materials
have received a great deal of attention due to their inherent char-
acteristics of morphological stability, high glass transition
temperature and amorphous properties [16e20]. The anthracene
and fluorene moiety, were often directly connected via arylearyl
(sp2esp2 bond) coupling to construct the desired highly twisted
blue emitters [21e23]. In our previous work, a series of one fold C-9
fluorene functionalized anthracene derivatives, in which the
anthracene and fluorene moieties were linked through a tetrahe-
dral sp3-hybridized carbon atom, was first presented [24]. Similar to
a spiro-annulated molecule, the tetrahedral sp3-hybridized atom
can also maintain an almost orthogonal linkage between the con-
nected conjugated moieties via a sp2esp3 bonded network. Due to
its structural steric configuration and non-planarity, the C-9
* Corresponding author. Tel.: þ86 81 21 66135060; fax: þ86 81 21 66133380.
** Corresponding author. Department of Chemistry, Shanghai University, Shanghai
200444, China. Tel.: þ86 81 21 66135060; fax: þ86 81 21 66133380.
E-mail addresses: wanwen@staff.shu.edu.cn (W. Wan), jhao@shu.edu.cn (J. Hao).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.10.013

W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
643
introduction of a bulky anthracene moiety to fluorene could
effectively minimize its degree of intermolecular e stacking in the
solid state and these compounds have been proved to be a prom-
ising host materials for blue OLED.
In this work, we report the synthesis and luminescent proper-
ties of a new series of aryl-substituted fluorenyl-anthracene
derivatives. The anthracene and fluorene, which behave as impor-
tant building blocks to construct blue luminescent emitters due to
their high fluorescent quantum yield, wide band gap and ready
modification, were connected through a tetrahedral sp3-hybridized
carbon atom. Also, various end-capping groups, such as a phenyl
yellowish crystal 5.72 g (74%), m.p. 147e148 ꢁC (ref. 140e141 ꢁC)
[25]; IR (KBr, cmꢀ1): 3051, 1708, 1515, 1462, 1440, 886, 827, 782.
1H NMR (500 Hz, CDCl3):
d
7.90 (d, J ¼ 1.5 Hz, 1H), 7.72 (dd, J ¼ 1.5,
8.0 Hz, 1H), 7.68 (d, J ¼ 7.5 Hz, 1H), 7.64 (d, J ¼ 10.5 Hz, 2H), 7.57 (d,
J ¼ 13.0 Hz, 1H), 7.54 (d, J ¼ 7.5 Hz, 1H), 7.51 (d, J ¼ 6.5 Hz, 1H), 7.48
(d, J ¼ 8.5, 2H), 7.38 (t, J ¼ 7.5 Hz, 1H), 7.29 (m, 1H). 13C NMR
(125 MHz, CDCl3):
d 194.0 (eCOe), 144.4, 144.2, 143.3, 142.4, 139.9,
135.0, 134.9, 134.8, 134.6, 133.3, 129.1, 128.0, 126.9, 124.4, 123.1,
120.8, 120.5, 120.4.
Compounds 4b and 4c were synthesized according to the
procedure described for 4a.
group (1a, 2a),
a 4-fluorophenyl group (1b, 2b) and a 2-
naphthalenyl group (1c, 2c), were incorporated into the fluorene
or anthracene backbone, respectively, to investigate the structuree
property relationship. These highly twisted molecules designed in
this way have non-planar structures, which are thus expected to
effectively suppress self-aggregation and facilitate the formation of
stable amorphous films.
2.3. Synthesis 2-(4-fluorophenyl)-9H-fluoren-9-one (4b)
Yield 67%, m.p 155e156 ꢁC. IR (KBr, cmꢀ1): 3062, 1710, 1514,
1466, 1440, 890, 830, 786. 1H NMR (500 Hz, CDCl3):
d 7.84 (d,
J ¼ 1.5 Hz,1H), 7.67 (td, J1 ¼8.0 Hz, J1 ¼6.5 Hz, J2 ¼ 1.5 Hz, 2H), 7.59e
7.49 (m, 5H), 7.30 (td, J1 ¼7.0 Hz, J1 ¼7.5 Hz, J2 ¼ 1.0 Hz, 1H), 7.15 (t,
J ¼ 8.5 Hz, 2H). 13C NMR (125 MHz, CDCl3):
d
193.9, 162.9 (d, 1JC-
¼ 245 Hz), 144.4, 143.3, 141.4, 136.2 (d, 4JC-F ¼ 3.0 Hz), 135.1, 135.0,
2. Experimental
F
134.6, 133.1, 129.2, 128.6 (d, 3JC-F ¼ 8.0 Hz), 124.6, 122.9, 120.9, 120.5,
2
116.0 (d, JC-F ¼ 23 Hz). 19F NMR (470 MHz, CDCl3):
d
ꢀ114.46
2.1. Materials and measurements
to ꢀ114.50 (m). MS (EI): m/z 274.6, Anal. Calcd for C19H11FO: C
83.20, H 4.04%. Found: C 83.17, H 4.13%.
The reagents and chemicals were used as received unless noted
otherwise. The IR spectra were determined on a PE-1700 IR spec-
trophotometer by dispersing samples in KBr disks. 1H, 13C, 19F NMR
spectra were recorded on Bruker AV-500 spectrometer with CDCl3
as deuterium solvent. Mass spectra were recorded on Agilent
5975N GCeMS. Elemental analyses were performed with
Elemental Vario EL III instrument. UVevis absorption spectra were
recorded on CRAY-100 spectrophotometer. Fluorescence measure-
ments were carried out with PerkineElmer LS55 in a solution of
10ꢀ6 mol/L and solid state, respectively. OxidationeReduction
Potentials were measured using a Solartron SI 1287 electro-
chemical workstation (vs SCE). The melting points were measured
by DSC on Netzstch STA409PC using a scanning rate of 10 ꢁC/min
with nitrogen flushing. The redox potentials of the compounds
were determined with cyclic voltammetry (CV) using a Base 2000
CV system. HOMO values were recalculated from reported litera-
ture values as HOMO ¼ [Eonset]red/ox (vs SCE) þ 4.4, Eg ¼ 1241/
2.4. Synthesis of 2-(naphthalen-2-yl)-9H-fluoren-9-one (4c)
Yield 81%, m.p. 170e171 ꢁC. IR (KBr, cmꢀ1): 3045, 1718, 1608,
1513, 1456, 878, 841, 779, 746. 1H NMR (500 Hz, CDCl3):
d 8.09 (s,
1H), 8.04 (s, 1H), 7.92 (dd, J ¼ 5.0, 10.0 Hz, 2H), 7.87 (dd, J ¼ 5.0,
10.0 Hz, 2H), 7.76 (dd, J ¼ 3.5, 5.0 Hz,1H), 7.76 (d, J ¼ 5.0 Hz,1H), 7.76
(d, J ¼ 3.5 Hz, 1H), 7.57e7.49 (m, 4H), 7.32 (t, J ¼ 10.0 Hz, 1H). 13C
NMR (125 MHz, CDCl3):
d 194.0 (eCOe), 144.4, 143.4, 142.3, 137.2,
135.1, 135.0, 134.6, 133.7, 133.5, 133.0, 129.2, 128.8, 128.4, 127.8,
126.6, 126.4, 125.8, 125.0, 124.6, 123.3, 120.9, 120.6. MS (EI): m/z
306.3, Anal. Calcd for C23H14O: C 90.17, H 4.61%. Found: C 90.24, H
4.56%.
2.5. Synthesis of 2-phenyl-9H-fluoren-9-ol (5a)
2-Phenyl-9H-fluoren-one (2.36 g, 9.22 mmol) was dissolved in
anhydrous alcohol (30 mL) in a 100 mL round-bottomed flask and
cooled in an ice base, sodium borohydride (0.7 g, 18.52 mmol) was
added in several times. After the mixture reacted for one hour at
room temperature, hydrochloric acid (10%) was added until pH ¼ 7.
Methanol was removed by distillation and the residue was
extracted with ethyl acetate (3 ꢂ 50 mL). The organic layer was
washed with brine for three times and dried over anhydrous
magnesium sulfate (MgSO4), filtered, and concentrated to provide
a crude product which was purified by recrystallization from
petroleum ether to afford compound 5a as white crystal 1.87 g
(77%), m.p. 167e169 ꢁC. IR (KBr, cmꢀ1): 3448, 3036,1610,1507,1476,
UV(onset). Single-crystal XRD was performed with graphite-
ꢀ
monochromatic MoKa radiation (
l
¼ 0.71073A) on a Bruker
Smart ApexⅡ CCD diffractometer at T ¼ 273(2) K. The structures
were solved by direct method with SHELXS-97 program and
refined by full matrix least-squares on F2 with SHELXL-97 program.
All non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were located and included at their calculated
position. The luminanceeCurrentevoltage characteristics were
measured simultaneously by the programmable Keithley model
2400 and PR 650 spectrometer. All measurements were performed
under an ambient atmosphere at room temperature.
2.2. Synthesis of 2-phenyl-9-H-fluoren-9-one (4a)
878, 854, 776, 750. 1H NMR (500 Hz, CDCl3):
d 7.87 (s,1H), 7.70e7.63
(m, 6H), 7.48e7.45 (t, J ¼ 7.5 Hz, 2H), 7.41e7.38 (m, 2H), 7.34e7.31 (t,
2-Bromo-9H-fluoren-9-one (7.74 g, 30 mmol), phenylboronic
acid (5.4 g, 44.3 mmol), K2CO3 (11.7 g, 84.8 mmol), TBAB (0.5 g,
1.5 mmol) and Pd(OAc)2 (0.34 g, 1.5 mmol) were placed in a 250 mL
round-bottomed flask with deionized water (100 mL) and ethanol
(10 mL). The mixture was degassed and the reaction was refluxed
for 24 h under nitrogen. After the solution was cooled, the solvent
was evaporated under vacuum and the product was extracted with
ethyl acetate (3 ꢂ 50 mL). The ethyl acetate solution was washed
with water and dried with anhydrous MgSO4. Evaporation of the
solvent, followed by column chromatography on silica gel (petro-
J ¼ 7.0 Hz, 1H), 5.60 (s, 1H), 2.03 (s, 1H). 13C NMR (125 MHz, CDCl3):
d
146.5, 146.0, 141.0, 139.8, 139.2, 129.3, 129.0, 128.4, 128.1, 127.9,
127.5, 127.2, 125.3, 124.0, 120.4, 120.2, 75.3. MS (EI): m/z 258.2, Anal.
Calcd for C19H14O: C 88.34, H 5.46%. Found: C 88.29, H 5.36%.
Compounds 5b and 5c was synthesized according to the
procedure described for 5a.
2.6. Synthesis of 2-(4-fluorophenyl)-9H-fluoren-9-ol (5b)
Yield 86%, m.p 133e135 ꢁC. IR (KBr, cmꢀ1): 3480, 3053, 1599,
leum ether/ethyl acetate
¼
8:1), afforded compound 4a as
1516, 1473, 890, 858, 779, 751. 1H NMR (500 Hz, CDCl3):
d 7.82

644
W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
(s, 1H), 7.70e7.64 (m, 3H), 7.61e7.56 (m, 3H), 7.41 (t, J ¼ 7.6 Hz,
J ¼ 6.9 Hz, 1H), 7.34 (td, J1 ¼7.4 Hz, J2 ¼ 1.0 Hz, 1H) 7.15 (t, J ¼ 8.6 Hz,
1H), 7.79 (s, 2H), 7.71e7.68 (m, 2H), 7.56e7.50 (m, 2H), 7.43 (t,
J ¼ 7.0 Hz, 1H), 7.39e7.35 (m, 1H), 6.07 (s, 1H). 13C NMR (125 MHz,
2H), 5.61 (s, 1H), 1.97 (s, 1H). 13C NMR (125 MHz, CDCl3):
d
162.5 (d,
CDCl3): d 145.1, 144.5, 141.3, 139.6, 139.1, 138.1, 133.8, 132.9, 129.4,
4
1JC-F ¼ 245 Hz), 146.4, 145.8, 139.9, 139.6, 139.1, 137.1 (d, JC-
128.7, 128.4, 128.2, 127.8, 126.4, 126.5, 126.2, 125.9, 125.6, 125.4,
120.8, 120.5, 46.3. MS (EI): m/z 370.3, Anal. Calcd for C23H15Br: C
74.41, H 4.07%. Found: C 74.56, H 4.16%.
¼ 3.6 Hz), 129.2, 128.6 (d, 3JC-F ¼ 7.0 Hz), 127.9, 127.8, 125.1, 123.8,
F
120.3, 120.1,115.7 (d, 2JC-F ¼ 22 Hz), 75.2. 19F NMR (470 MHz, CDCl3):
d
ꢀ115.43 to ꢀ114.49 (m). MS (EI): m/z 276.0, Anal. Calcd for
C19H13FO: C 82.59, H 4.74%. Found: C 82.73, H 4.84%.
2.7. Synthesis of 2-naphthalene-2-yl-9H-fluoren-9-ol (5c)
Yield 92%, m.p. 170e171 ꢁC; IR (KBr, cmꢀ1): 3453, 3030, 1617,
2.11. Synthesis of 9-(2-phenyl-9H-fluoren-9-yl)-anthracene (1a)
Anthracene (0.89 g, 5.00 mmol), zinc powder (0.5 g, 8.0 mmol),
and the 9-bromo-2-phenyl-9H-fluorene (1.6 g, 4.98 mmol) were
stirred in CS2 (50 mL) and refluxed for 24 h. After the reaction was
cooled to room temperature and the solvent was removed by
distillation, the residue was extracted with methylene chloride
(3 ꢂ 50 mL) and filtered to remove excess Zn powder. The combined
organic layer was washed with brine for three times and dried over
anhydrous magnesium sulfate (MgSO4), filtered, and concentrated
to provide a crude product which was purified by recrystallization
from petroleum ether to afford compound 1a as white crystal 0.86 g
(41%). IR (KBr, cmꢀ1): 3050, 3027, 1623, 1477, 884, 834, 770, 755,
1518, 1473, 886, 856, 776, 756. 1H NMR (500 Hz, CDCl3):
d 8.11 (s,
1H), 8.02 (s, 1H), 7.96e7.86 (m, 4H), 7.87 (dd, J ¼ 2.0, 8.5 Hz, 1H),
7.76e7.74 (m, 2H), 7.73e7.66 (m, 2H), 7.54e7.49 (m, 3H), 7.42e7.40
(m, 1H), 7.34e7.32 (m, 1H) 5.66 (s, 1H), 2.04 (s, 1H); 13C NMR
(125 MHz, CDCl3):
d 144.6, 146.1, 140.9, 139.8, 139.3, 138.3, 133.8,
132.8, 129.3, 128.6, 128.4, 128.3, 127.9, 127.8, 126.5, 126.1, 125.8,
125.5, 125.3, 124.2, 120.5, 120.2, 75.4. MS (EI): m/z 308.2, Anal. Calcd
for C23H16O: C 89.58, H 5.23%. Found: C 89.72, H 5.07%.
728. 1H NMR (500 Hz, CDCl3):
d
8.73 (d, J ¼ 9.0 Hz, 1H), 8.51 (s, 1H),
2.8. Synthesis of 9-bromo-2-phenyl-9H-fluorene (6a)
8.16 (d, J ¼ 8.0 Hz, 1H), 8.08 (d, J ¼ 8.0 Hz, 1H), 8.04 (d, J ¼ 8.0 Hz,
1H), 7.94 (d, J ¼ 8.5 Hz, 1H), 7.73 (d, J ¼ 1.0 Hz, 1H), 7.71 (t, J ¼ 1.0 Hz,
1H), 7.67 (dd, J ¼ 6.5, 8.0 Hz, 1H), 7.49e7.46 (m, 2H), 7.45 (s, 1H),
7.32e7.28 (m, 3H), 7.26e7.17 (m, 3H), 7.09 (d, J ¼ 7.5 Hz, 1H), 6.90e
6.87 (m,1H), 6.78 (d, J ¼ 9.0 Hz,1H), 6.60 (s,1H). 13C NMR (125 MHz,
2-Phenyl-9H-fluoren-9-ol (5.50 g, 21.32 mmol) in dichloro-
methane (90 mL) were placed in a 250 mL three-necked round
bottomed flask equipped with a reflux condenser, a constant press
drop funnel, an off gas absorption equipment. The flask was cooled
to 0 ꢁC and phosphorus tribromide (4.3 mL, 25.8 mmol) was slowly
added over 15 min. The mixture was stayed at 0 ꢁC for two hours
and then saturated sodium bicarbonate was slowly added under
stirring until no bubble generated. Then the water was extracted
with dichloromethane (3 ꢂ 50 mL). The combined organic layer
was washed with brine for three times and dried over anhydrous
magnesium sulfate (MgSO4), filtered, and concentrated to provide
a crude product which was purified by recrystalization from
petroleum ether to afford compound 6a as pale yellow crystal
5.57 g (82%), m.p: 129e131 ꢁC. IR (KBr, cmꢀ1): 3033, 1617, 1515,
CDCl3):
d 149.9, 149.5, 141.2, 140.6, 140.0, 139.5, 132.4, 132.2, 131.9,
131.6, 129.8, 129.7, 129.0, 128.7, 128.0, 127.6, 127.3, 127.2, 126.7,
126.4, 125.9, 125.3, 125.0, 124.9, 124.6, 124.0, 123.2, 120.9, 120.6,
49.1. MS (EI): m/z 418.3, Anal. Calcd for C33H22: C 94.70, H 5.30%.
Found: C 94.57, H 5.49%.
Compounds 1b and 1c was synthesized according to the
procedure described for 1a.
2.12. Synthesis of 9-(2-(4-fluorophenyl)-9H-fluoren-9-yl)-
anthracene (1b)
1456, 877, 852, 770, 746. 1H NMR (500 Hz, CDCl3):
(d, J ¼ 7.0 Hz, 1H), 7.69e7.63 (m, 5H), 7.49e7.48 (m, 2H), 7.46e7.34
(m, 3H), 6.06 (s, 1H). 13C NMR (125 MHz, CDCl3):
144.9, 144.5,
d
7.89 (s, 1H), 7.73
Yield 41%. IR (KBr, cmꢀ1): 3047, 2924, 1621, 1597, 1510, 1478,
1448, 1217, 1158, 824. 1H NMR (500 Hz, CDCl3):
d
8.71 (d, J ¼ 9 Hz,
d
1H), 8.49 (s,1H), 8.16 (d, J ¼ 8.5 Hz,1H), 8.05 (d, J ¼ 8, 2H), 7.93e7.92
(d, J ¼ 8 Hz, 1H), 7.67e7.63 (m, 2H), 7.59e7.55 (m, 1H), 7.48e7.45
(m, 1H), 7.39e7.37 (m, 2H), 7.24e7.21 (m, 2H), 7.19e7.16 (m, 1H),
7.07 (d, J ¼ 7.5 Hz, 1H), 6.99e6.95 (m, 2H), 6.88e6.85 (m, 1H), 6.73
141.4, 140.8, 139.6, 139.0, 129.4, 129.0, 128.4, 128.2, 127.6, 127.3,
126.5, 125.2, 120.7, 120.4, 46.1. MS (EI): m/z 320.3, Anal. Calcd for
C19H13Br: C 71.04, H 4.08%. Found: C 71.11, H 4.20%.
Compounds 6b and 6c was synthesized according to the
procedure described for 6a.
(d, J ¼ 9 Hz,1H), 6.57 (s, 1H). 13C NMR (125 MHz, CDCl3):
d
160.1 (1JC-
¼ 240 Hz), 149.8, 149.3, 139.8, 139.5, 139.4, 139.4, 137.2, 132.3,
F
132.0, 131.8, 131.4, 129.8, 129.6, 128.9, 128.7, 128.6, 127.9, 127.5,
127.2, 126.6, 126.1, 125.7, 124.9, 124.5, 123.9, 122.9, 120.8, 115.5,
2.9. Synthesis of 9-bromo-2-(4-fluorophenyl)-9H-fluorene (6b)
Yield 75%, m.p. 201e203 ꢁC. IR (KBr, cmꢀ1): 3047, 1512, 1464,
115.3, 48.9. 19F NMR (470 MHz, CDCl3):
d
ꢀ115.93 (m). MS (EI): m/z
436.5, Anal. Calcd for C33H21F: C 90.80, H 4.85%. Found: C 90.98, H
5.12%.
1432, 874, 846, 776. 1H NMR (500 Hz, CDCl3):
d 7.83 (s, 1H), 7.69 (t,
J ¼ 8.5 Hz, 3H), 7.62e7.59 (m, 2H), 7.56 (dd, J1 ¼ 7.5 Hz, J2 ¼ 1.5 Hz,
1H), 7.42 (t, J1 ¼ 7.5 Hz, J2 ¼ 0.5 Hz, 1H), 7.37 (td, J1 ¼ 7.5 Hz,
2.13. Synthesis of 9-(2-(naphthalene-2-yl)-9H-fluoren-9-yl)-
anthracene (1c)
J2 ¼ 1.0 Hz, 1H), 7.16 (td, J1 ¼ 8.5 Hz, J2 ¼ 2.0 Hz, 2H), 6.03 (s, 1H). 13
C
1
NMR (125 MHz, CDCl3):
d
162.5 (d, JC-F ¼ 246 Hz), 144.9, 144.3,
140.2, 139.3, 138.8, 136.8 (d, 4JC-F ¼ 2.6 Hz), 129.3, 128.7, 128.6, 128.1
Yield 57%. IR (KBr, cmꢀ1): 3050, 3025, 1621, 1595, 1521, 1462,
1447, 1409, 1324, 1157, 885, 818, 775, 761, 729.1H NMR (500 Hz,
3
2
(d, JC-F ¼ 3.0 Hz), 126.3, 124.9, 120.5, 120.3, 115.87 (d, JC-F
F ¼ 22 Hz), 45.8. 19F NMR (470 MHz, CDCl3):
d
ꢀ115.09 to ꢀ115.13
CDCl3):
d
8.74 (d, J ¼ 9.0 Hz, 1H), 8.50 (s, 1H), 8.16 (d, J ¼ 8.5 Hz, 1H),
(m). MS (EI): m/z 338.1, Anal. Calcd for C19H12BrF: C 67.28, H 3.57%.
8.10 (d, J ¼ 8.0 Hz, 1H), 8.04 (d, J ¼ 8.0 Hz, 1H), 7.93 (d, J ¼ 8.5 Hz,
1H), 7.88 (d, J ¼ 8.0 Hz, 1H), 7.83 (dd, J ¼ 0.5, 7.0 Hz, 1H), 7.75 (dd,
J ¼ 3.0, 6.0 Hz, 3H), 7.68e7.64 (m, 1H), 7.61e7.57 (m, 2H), 7.48e7.39
(m, 4H), 7.24e7.17 (m, 2H), 7.07 (dd, J ¼ 0.5, 7.5 Hz, 1H), 6.89e6.86
(m, 1H), 6.76 (dd, J ¼ 0.5, 9.0 Hz, 1H), 6.61 (s, 1H). 13C NMR
Found: C 67.38, H 3.69%.
2.10. Synthesis of 9-bromo-2-(naphthalene-2-yl)-9H-fluorene (6c)
Yiled 66%, m.p: 153e155 ꢁC. IR (KBr, cmꢀ1): 3055, 1608, 1514,
(125 MHz, CDCl3):
d 149.8, 149.4, 140.3, 139.9, 139.5, 138.4, 133.5,
1470, 1428, 884, 773. 1H NMR (500 Hz, CDCl3):
d
8.11 (s, 1H), 8.02 (s,
132.5, 132.3, 132.1, 131.8, 131.4, 129.7, 129.6, 128.8, 128.2, 128.1, 127.9,
127.5, 127.4, 127.2, 126.6, 126.2, 125.8, 125.7, 125.6, 125.5, 125.3,
1H), 7.94e7.92 (m, 2H), 7.91e7.88 (m, 1H), 7.87 (dd, J ¼ 1.5, 8.5 Hz,

W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
645
124.9, 124.8, 124.5, 123.9, 123.2, 120.9, 120.6, 49.0. MS (EI): m/z
468.8, Anal. Calcd for C37H24: C 94.84, H 5.16%. Found: C 94.82, H
5.09%.
2.17. Synthesis of 9-(9H-fluoren-9-yl)-10-(naphthalen-2-yl)
anthracene (2c)
Yield 70%. IR (KBr, cmꢀ1): 3059, 3010, 811, 761, 749. 1H NMR
(500 Hz, CDCl3):
2.14. Synthesis of 9-bromo-10-(9H-fluoren-9-yl)anthracene (8)
d
8.80 (d, J ¼ 9 Hz, 1H), 8.08e8.02 (m, 4H), 7.97 (s,
1H), 7.92e7.91 (t, J ¼ 1.5, 7.0 Hz, 1H), 7.84e7.82 (d, J ¼ 9 Hz, 1H),
7.63e7.59 (m, 5H), 7.49e7.46 (t, J ¼ 7.5 Hz, 2H), 7.43e7.40 (m, 1H),
7.24e7.17 (m, 4H), 7.08e7.05 (t, J ¼ 8 Hz, 1H), 6.85e6.82 (t, J ¼ 9 Hz,
1H), 6.76e6.74 (d, J ¼ 9 Hz, 1H), 6.63 (s, 1H). 13C NMR (125 MHz,
9-Bromoanthracene (0.33 g, 1.28 mmol), ZnCl2 (0.8 g,
5.88 mmol), and the 9-bromo-9H-fluorene (0.5 g, 2.04 mmol) were
stirred in CS2 (12 mL) and refluxed for 48 h. After the reaction was
cooled to room temperature and the solvent was removed by
distillation, the residue was extracted with methylene chloride
(3 ꢂ 50 mL) and filtered to remove excess ZnCl2. The combined
organic layer was washed with brine for three times and dried over
anhydrous magnesium sulfate (MgSO4), filtered, and concentrated
to provide a crude product which was purified by recrystallization
from petroleum ether to afford compound 8 as light yellow crystal
0.32 g (60%), m.p: 211.5e212.7 ꢁC. IR (KBr, cmꢀ1): 3080, 3038, 1622,
CDCl3):
d 149.3, 140.3, 137.8, 136.9, 133.5, 132.8, 132.1, 131.8, 130.9,
130.6, 130.3, 129.6, 129.3, 128.5, 128.2, 127.7, 127.2, 126.6, 126.4,
125.9, 124.9, 124.6, 124.1, 120.7, 49.2. MS (EI): m/z 468.6, Anal. Calcd
for C37H24: C 94.84, H 5.16%. Found: C 94.71, H 5.22%.
3. Results and discussion
3.1. Synthesis of aryl-containing fluorenyl-anthrancene derivatives
1442, 1341, 932, 788, 743. 1H NMR (500 Hz, CDCl3):
d 8.74e8.72 (m,
2H), 8.51e8.49 (m, 1H), 8.01e8.00 (d, J ¼ 7.5, 2H), 7.71e7.69 (m, 2H),
7.46e7.43 (t, J ¼ 7.5 Hz, 2H), 7.37e7.34 (m, 1H), 7.19e7.16 (m, 2H),
7.06e7.04 (dd, J ¼ 1 Hz, 7 Hz, 2H), 6.86e6.89 (m, 1H), 6.70e6.68 (m,
The main synthetic routes to the aryl containing fluorene-
functionalized anthracene derivatives are shown in Scheme 1.
Compound 4 was obtained by bromination of 9H-fluoren-9-one in
the presence of bromine in H2O, and subsequent Pd-catalyzed
Suzuki coupling reaction in aqueous potassium carbonate solu-
tion [26]. Reduction of the obtained fluorenone 4 with sodium
borohydride in ethanol or tetrahydrofuran provided the 9-hydroxyl
fluorene derivative 5, which were further treated with phosphorus
tribromide in methylene dichloride to give the corresponding 9-
bromo fluorene derivatives 6 with aryl substituents at the C2
position in good yield. Radical reaction of compounds 6ae6c with
anthracene in the presence of zinc powder in CS2 provided the
blue-emitting materials 1ae1c in moderate yields. Compound 8
was prepared by FriedeleCrafts reaction of 9-bromo-9H-fluorene
and 9-bromoanthracene using ZnCl2 as catalyst. Finally, Pd cata-
lyzed Suzuki coupling reaction of the arylboronic acid with
compound 8 was accomplished by using a catalytic amount of
Pd(Ph3)2(Cl)2 to afford corresponding compounds 2ae2c in good
yields.
Thus, novel fluorene-functionalized anthracene derivatives 1ae
1c and 2ae2c with a linkage of a rigid sp3-hybridized carbon atom
between fluorene and anthracene moieties (at the C-9 position of
the fluorene) were prepared. The aromatic substituents, such as
phenyl, 4-fluorophenyl and 2-naphthalenyl substituents, were
introduced to the fluorene or anthracene moieties, respectively. The
chemical structures of 1ae1c and 2ae2c were confirmed by 1H and
13C NMR and FT-IR spectroscopy. The results of the mass spec-
trometry and elemental analysis also supported the formation of
the six blue emitting materials and matched well with the calcu-
lated data. Compounds 1ae1c and 2ae2c had good solubility in
common organic solvents so that they could be easily purified by
column chromatography and recrystallization to high purity for
spectroscopic characterization and OLEDs application. Various
properties such as UVevis absorption, photoluminescene, and
electroluminescence including EL efficiency and color purity were
also evaluated.
1H), 6.50 (s, 1H). 13C NMR (125 MHz, CDCl3):
d 148.8, 140.1, 132.7,
131.5,129.9,129.7,129.1,128.9,127.5,127.2,126.8,126.3,126.3,126.1,
126.0,125.4,125.3,124.4,124.3,120.6, 48.9. MS (EI): m/z 420.4. Anal.
Calcd for C27H17Br: C 76.97, H 4.07%. Found: C 76.82, H 3.88%.
2.15. Synthesis of 9-(9H-fluoren-9-yl)-10-phenylanthracene (2a)
A mixture of phenylboronic acid (1.0 g, 8.2 mmol), bis(-
triphenylphosphine) palladium(II) chloride (0.24 g, 0.38 mmol), 9-
bromo-10-(9H-fluoren-9-yl) anthracene (1.6 g, 3.8 mmol), toluene
(120 mL), ethanol (20 mL) and saturated solution of K2CO3 (20 mL)
were heated at 100 ꢁC with magnetic stirring under nitrogen
atmosphere. Some solid began to appear after 2 h and heating was
continued for another 48 h. The reaction mixture was cooled to
room temperature and the precipitated solid was isolated by
filtration, washed with water, and concentrated to provide a crude
product which was purified by recrystalization from
dichloromethane-anhydrous ethanol to afford compound 2a as
white crystal 1.0 g (63%). IR (KBr, cmꢀ1): 3062, 3031, 1597, 763, 734,
700. 1H NMR (500 Hz, CDCl3):
d
8.81 (d, J ¼ 8.9 Hz, 1H), 8.06e8.05
(d, J ¼ 7.4 Hz, 2H), 7.86 (d, J ¼ 8.8 Hz, 1H), 7.64e7.58 (m, 5H), 7.52 (d,
J ¼ 7.6, 5H), 7.23e7.19 (m, 4H), 7.14e7.11 (t, J ¼ 7.6 Hz, 1H), 6.87 (t,
J ¼ 9 Hz, 1H), 6.79e6.77 (d, J ¼ 9 Hz, 1H), 6.64 (s, 1H). 13C NMR
(125 MHz, CDCl3):
d 149.3, 140.3, 139.4, 138.0, 132.1, 131.7, 130.7,
129.3, 128.5, 127.6, 127.2, 126.3, 125.9, 124.9, 124.8, 124.6, 124.0,
120.6, 49.21. MS (EI): m/z 418.6, Anal. Calcd for C33H22: C 94.70, H
5.30%. Found: C 94.87, H 5.19%.
Compounds 2b and 2c was synthesized according to the
procedure described for 2a.
2.16. Synthesis of 9-(9H-fluoren-9-yl)-10-(4-fluorophenyl)
anthracene (2b)
Yield 75%. IR (KBr, cmꢀ1): 3065, 3011, 2920, 1600, 1508, 1487,
833, 762, 733. 1H NMR (500 Hz, CDCl3):
d
8.79 (d, J ¼ 9.0 Hz, 1H),
3.2. Crystallographic analyses
8.04 (d, J ¼ 7.6 Hz, 2H), 7.80 (d, J ¼ 8.7 Hz, 1H), 7.66e7.62 (m, 1H),
7.57 (d, J ¼ 8.7 Hz, 1H), 7.49e7.43 (m, 5H), 7.32 (t, J ¼ 8.7 Hz, 2H),
7.21 (td, J1 ¼7.5 Hz, J2 ¼ 1.0 Hz, 2H), 7.17 (d, J ¼ 7.5 Hz, 2H), 7.14e7.11
A colorless single crystal of 2b (CCDC 85994) was obtained by
diffusion of hexane into its dichloromethane solution and was
analyzed by X-ray diffraction in order to elucidate its molecular
structure and bulk-packing characteristics (Fig. 1(a) and (b)). The
angle between the plane of anthracenyl moiety and that of fluo-
renyl moiety is 74.7ꢁ, which indicates that the bulky fluorene ring is
almost orthogonally linked with the anthracene ring through the
tetrahedral sp3-hybridized carbon atom. In addition, the dihedral
(m, 1H), 6.87e6.83 (m, 1H), 6.77 (s, 1H), 6.62 (s, 1H). 13C NMR
1
(125 MHz, CDCl3):
d
162.5 (d, JC-F ¼ 245 Hz), 149.2, 140.3, 136.8,
135.2, 133.0, 132.1, 130.9, 130.6, 129.3, 128.1, 127.60, 127.3, 126.3,
125.9,125.0,124.6,124.1,120.7,115.6 (d, 2JC-F ¼ 21 Hz), 49.2. 19F NMR
(470 MHz, CDCl3):
d
ꢀ114.86 to ꢀ114.87 (m); MS (EI): m/z 436.3,
Anal. Calcd for C33H21F: C 90.80, H 4.85% Found: C 90.68, H. 5.08%.

646
W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
Scheme 1. Synthetic route toward aryl substituted C-9 fluorenyl anthracene derivatives.
angle of the anthracene and benzene was analyzed to be 84.9ꢁ
(Fig. 1(a)). This conformation of 9-fluorenyl,10-phenyl anthracene
indicates that the steric interactions among anthracene, fluorene
and benzene cores are effectively released. As shown in the crystal
packing diagram of 2b presented in Fig. 1(b), the fluorene and
benzene rings lie on the each side of the anthracene plane, which
would result in the efficient suppression of the interchromophore
interactions. The plane-to-plane distance between two neigh-
3.3. Thermal properties
Thermal properties were examined using thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC). The
melting transition (Tm), decomposition temperature (Td) and
glass transition (Tg) are summarized in Table 1. The Td is defined by
a 5%-weight-loss temperature under a nitrogen atmosphere.
All of the synthesized aryl-substituted fluorenyl-anthracene
derivatives 1ae1c, 2ae2c were found to exhibit stable thermal
properties upon several heating and cooling cycles, as revealed by
the DSC studies. A clear improvement was shown in the thermal
stability as a naphthalenyl group was incorporated to the C2 posi-
tion on fluorene (1c) or the C10 position on anthracene (2c). In
addition, distinct glass transition temperatures around 104e143 ꢁC
were observed for 1ae1c, 2ae2c on the second heating process.
Upon further heating beyond Tg, endothermic peaks, which were
ascribed to the melting, were observed only for 1a, 2b, 2c. However,
completely different behavior was observed in the case of 1b, 1c
ꢀ
boring anthracene moieties is calculated to be 11.23 A, indicating
that there are no aromaticearomatic interactions between the
anthracene cores. In the crystal structure, intermolecular hydrogen
bonding of F.H was found, which could be effective in the stabi-
lization of the structure. The orthogonal structural features con-
taining
a
tetrahedral sp3-hybridized carbon atom center are
believed to reduce the excimer formation that causes the concen-
tration quenching of fluorescence in the solid state [27]. In addition,
characteristics of morphological stability, high glass transition
temperature and amorphous properties could also be expected.
Fig. 1. Molecular view of the crystal structure (a) and crystal packing diagram (b) of 2b.

W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
647
Table 1
UV absorption, PL, energy levels and thermal properties of the six blue materials.
Compound
lmax Absa
(nm)
lmax PLa
(nm)
lmax PLb
(nm)
QEc
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
Td
(ꢁC)
Tm
(ꢁC)
Tg
(ꢁC)
(
f)
1a
1b
1c
2a
2b
2c
358, 373, 390
352, 370, 390
370, 390
358, 379, 398
357, 377, 397
358, 378, 399
395, 418, 443
395, 419, 444
395, 419, 442
413, 432
412, 433
420, 436
432, 446
452
435, 446, 450
438, 457
437, 456
446, 455
0.64
0.65
0.35
0.69
0.62
0.80
ꢀ5.69
ꢀ5.69
ꢀ5.70
ꢀ5.65
ꢀ5.78
ꢀ5.79
ꢀ2.58
ꢀ2.58
ꢀ2.58
ꢀ2.63
ꢀ2.76
ꢀ2.79
3.11
3.11
3.12
3.02
3.02
3.00
325
329
382
315
317
352
231
189
290
223
231
270
118
107
105
110
104
143
a
Measured in CH2Cl2 solution.
Film samples.
b
c
Fluorescence quantum efficiency, relative to quinine sulfate in cyclohexane (ff z 0.55).
and 2a. Fig. 2 displays the representative DCS curves of compound
1c and the insert shows its TG curve. It can be seen that compound
1c melted at 290 ꢁC on the first heating run and then changed into
a glass state upon cooling from the melt. A glass transition occurred
at 105 ꢁC when the amorphous glass sample was heated again. No
clear endothermic peak due to melting was found when the
temperature was raised until decomposition occurred around
382 ꢁC. This phenomenon was also observed for the DSC studies on
compounds 1b, 2a. Such thermal behavior has already been found
for similar bulky structures such as spiro fluorene-anthracene
derivatives [28,29]. The prominent stability of the amorphous
glass states of 1b, 1c and 2a in this work is due presumably to the
presence of the sp3-hybridized carbon atom linkage between flu-
orene and anthracene. Thus, based on the thermal and amorphous
stability of C9-fluorenyl anthracene derivatives, homogeneous and
stable amorphous thin films of these compounds are expected to be
achieved in the fabrication of OLEDs.
TG studies show that all the compounds exhibited good thermal
stabilities with decomposition temperatures in the range of 315e
382 ꢁC, demonstrating the good thermal stability of the materials.
The introduction of fluorine to the aryl substituent resulted in
a slight increase of Td, when compared within the compound 1a, 2a
and their fluorinated analogs 1b, 2b. Compounds 1ae1c containing
C2 aryl substituents on fluorene showed higher Td temperatures
than corresponding analogs 2ae2c which have C10 aryl substitu-
ents on antrhacene. These results indicate that the introduction of
the aryl substituents at the C2 positions is beneficial for improving
the morphological stability and resistance to the thermal
decomposition.
3.4. Optical and photoluminescent characteristics
The photophysical properties of 1a, 1b, 1c and 2a, 2b, 2c were
investigated by UVevis and photoluminescence spectra for dilute
solutions and solid states as shown in Fig. 3. All compounds
exhibited the characteristic vibrational patterns of an isolated
anthracene group with three bands in the region of 340e420 nm in
the UVevis spectra in Fig. 3(a). It was interesting to find that
compounds 1ae1c containing the extended aryl group on the flu-
orenyl moiety, which had similar steric configurations, showed
similar vibronic patterns at 350, 370 and 390 nm with a broad band
centered within the region of 260e360 nm, which are assigned to
the
pep* absorption band of conjugated aromatic rings. While
compounds 2ae2c containing extended aryl group on the anthra-
cenyl moiety gave the vibronic pattern at 360, 380 and 400 nm. In
comparison with compound 1ae1c, a red shift of 10 nm was pre-
sented in the absorption spectra of compounds 2ae2c. The intro-
duction of a fluorine atom to 1b and 2b gave a blue shift when
studied on UVevis spectra within the series 1ae1c or the series 2ae
2c. The PL spectra of the compounds 1ae1c in solution showed an
almost identical fluorescent emission pattern with three emission
peaks 395, 418 and 443 nm (Fig. 3(b)). Compared with compounds
2ae2c, the emission spectra of 1ae1c showed blue shifts of 20 nm
due to the shortened p-conjugation by the distorted structure in the
end-capping groups. As seen from Fig. 3(c), the PL spectra in the
solid states of all of the synthesized compounds exhibited a maxima
emission peak in the blue region of 432e450 nm, which had red
shift about 5e15 nm in comparison with those obtained in dilute
solution. The bathochromic shift on the fluorescent emissions is
probably due to the difference in dielectric constant of the envi-
ronment. The small spectral shifts in the solid state suggest that the
intermolecular interactions are weak, i.e., the molecules are
restrained effectively by the twisted bulky fluorenyl anthracene
units. The PLmax in the solid state of 2a, which processes a phenyl
substituent at the C10 position of the anthracene, presented a red
shift of 6 nm in comparison with that of 1a, which contains a phenyl
substituent at the C2 position of the fluorene. These phenomena of
PL were also observed by careful studies between compounds 2b, 2c
and the corresponding 1b, 1c. The introduction of aryl group into
the C10 of the anthracene (2ae2c) would result in the stronger
bathochromic effect in comparison with the introduction of aryl
group into the C2 position of the fluorene (1ae1c). When the effect
of substituent was studied within the compounds 1ae1c, the
introduction of a phenyl group to the twisted fluorene functional-
ized anthracenes (1a) was found to exhibit a distinct blue-shift in
the solid state in comparison with the introduction of 4-F phenyl
(1b) or 2-naphthalenyl group (1c). However, compounds 2a, 2b and
2c exhibited almost identical PL spectra in the solid state, indicating
that the substituents on anthracene caused little effect on the
emission process. The fluorescence quantum yield was measured by
calibrating against quinine sulfate (ff z 0.55) [30]. The emission
Fig. 2. DSC traces of 1c recorded at a heating rate of 10 ꢁC/min. Insert: TGA thermo-
gram of 1c recorded at a heating rate of 10 ꢁC/min.

648
W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
a three-electrode cell setup with 0.1 M tetrabutyl-ammonium
hexafluorophosphate (Bu4NPF6) as a supporting electrolyte in
anhydrous CH2Cl2 to probe the electrochemical behavior of the
materials. The electrochemical properties are shown in Fig. 4 and
Table 2. HOMO values were calculated from as HOMO ¼ [Eonset red/
]
ox (vs SCE) þ 4.4, Eg ¼ 1240/UV(onset) [31]. All compounds showed
a reversible oxidation process, indicating their electrochemical
stabilities. As seen from Table 1, all HOMO energies of the
aryl substituted fluorenyl-anthracene derivatives were at about
5.65e5.80 eV. Such a low HOMO energy level greatly reduced the
energy barrier for hole injection from indium tin oxide (ITO) to the
emissive anthracene-fluorene derivatives. The HOMOeLUMO
energy gaps (Eg) of about 3.0e3.1 eV estimated from the onset of
the absorption spectra were nearly identical for compound 1ae1c
and 2ae2c, respectively, which suggested that our newly
prepared compounds were potentially suitable blue emitting
material. The LUMO energy levels for compounds 1ae1c and 2ae2c
were determined to be around 2.6e2.8 eV by combining the HOMO
energy levels together with the band gaps. It was found that
compounds 1ae1c showed relatively higher HOMOs energy with
larger band gaps. Introducing the substituent into the C2 position of
fluorene for the compounds 1ae1c decreased the
effectively and improved the HOMO and LUMO energy level. The
distorted -conjugation on the fluorene molecule could also lead to
p-conjugation
p
a larger energy gap.
Density functional theory (DFT) calculations have also been
applied to characterize the three-dimensional geometries and the
frontier molecular orbital energy levels of compounds 1ae1c and
2ae2c at the B3LYP/6-31G* level by using the Gaussian 03 program
[32e34]. The orbital plots of the HOMO-LOMOs are illustrated in
Fig. 5. The calculated geometries and further X-ray diffraction
analysis of compound 2b clearly demonstrated that the fluorene
and the anthracene moieties were significantly twisted through the
linkage of the sp3-hybridized carbon atom, resulting in a non-
coplanar twisted conformation for each molecule. The fluorene
and benzene rings were found to be almost orthogonally linked
with the anthracene. This dramatically twisted conformation can
effectively prevent intermolecular interactions between p-systems
of aromatic rings. It leads to the information that crystallization of
the materials can be suppressed and finally the morphological
stability of the molecules will be improved, which were supported
by the observation of high Tg (above 100 ꢁC) on the DSC studies of
the target compounds 1ae1c and 2ae2c. Such a unique structural
feature is beneficial to form the uniform amorphous morphology in
Fig. 3. Absorption spectra (a) PL spectra (b) in CH2Cl2 solution and PL spectra (c) in
solid state.
quantum yield was high in the region of 0.35e0.80. Therefore, these
compounds were good candidates for use as efficient blue-emitting
materials in OLED devices.
3.5. Electrochemical properties
The redox potentials of the compounds were determined by
cyclic voltammetry (CV) measurements which were carried out in
Fig. 4. CV curves for compounds 1ae1c and 2ae2c.

W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
649
Table 2
EL performances of aryl-substituted fluorenyl-anthracene compounds.
LEb
(cd/A)
PEc
(lm/W)
EQEd
(%)
CIE (x, y)e
EL
a
Compound
lmax
Lmax
(nm)
(cd/m2)
1a
1b
1c
2a
2b
2c
464
492
464
468
492
462
3013
860
2131
775
626
967
1.47
0.93
1.20
0.85
0.65
0.71
1.27
0.76
1.06
0.64
0.55
0.57
1.84
1.17
1.48
1.21
1.09
1.16
(0.166, 0.153)
(0.186, 0.237)
(0.172, 0.162)
(0.154, 0.147)
(0.168, 0.230)
(0.156, 0.131)
a
Maximum luminance achieved.
b
c
Luminous efficiency at 20 mA/cm2.
Power efficiency at 20 mA/cm2.
d
e
External quantum efficiency at 20 mA/cm2.
1931 CIE chromaticity coordinates.
the film state of the OLED device. The electron densities of the
HOMOs and LUMOs are localized mainly on the anthracene unit,
indicating that the absorption and emission process are only
attributed to the
pep* transition centered at the anthracene
moiety. Thus, the excellent luminescence efficiency of the anthra-
cene moiety can be maintained.
3.6. OLED fabrication
To investigate the potential applications in an OLED, the aryl
substituted fluorenyl-anthracene compounds, 1aec and 2aec, as
emitting materials were processed into films by a thermal evapo-
ration technique to produce simple non-doped OLED devices with
the constitution of ITO/MoOx (4 nm)/NPB (20 nm)/emitting mate-
rial (30 nm)/Bphen (20 nm)/LiF (0.7 nm)/Al. The device structure is
presented in Fig. 6(a). ITO (indium tin oxide) and LiF:Al were used
as anode and composite cathode, respectively. 4,7-Dipheny1-l,10-
phcnanthroline (Bphen) was as an electron transporting layer and
N,N0-bis(naphthalen-1-yl)-N,N0-bis(phenyl) benzidine (NPB) as
a hole transporting layer. The ITO glass substrate was cleaned with
detergent and deionized water and dried in an oven. It was then
treated with UV-ozone for 30 min before loading into a deposition
chamber. The organic films of NPB, emitting material, Bphen and
LiF/Al were sequentially deposited onto the ITO substrate at
a pressure of around 3 ꢂ 10ꢀ4 Pa.
As shown in the schematic energy level diagram of these OLEDs
in Fig. 6(a), the hole-injection barrier at the NPB/emitter was about
0.4 V, while the electron-injection barrier at the emitter was about
0.3 V. These small injection barriers could enhance the efficient
injection of both holes and electrons into the emission layer.
Fig. 6(b) shows the EL spectra of OLEDs based on 1ae1c and 2ae2c
as the emitting layers. All devices exhibited blue emission with
peak maxima at 450e490 nm. A stable emission in EL spectra and
CIE coordinates from all diodes 1ae1c and 2ae2c were observed to
be insensitive to the driving voltage, which indicated that
compounds 1ae1c and 2ae2c revealed excellent operating stabil-
ities for blue emission. The similarity of the PL and EL spectra for
each compound suggests that the EL is attributed to an emission
from the corresponding emitting layer. The observed tails in the
longer wavelength region could be due to charge transfer complex
formation at the interface between hole-transporting layer and
emitting layer [35,36].
Fig. 5. HOMO and LUMO electronic density distributions of 1ae1c and 2ae2c.
Compounds 2a, 2c showed emission peaks at 468, 462 nm with
a shoulder emission at around 444 nm and a width at half
maximum (FWHM) about 90 nm, while compounds 1a, 1c gave the
emission peaks at around 464 nm with a width at half maximum
(FWHM) around 70 nm. The narrower FWHM for 1a, 1c indicates
that the introduction of phenyl or 2-nathracenyl unit into the C2-
position of fluorene would reduce the aggregate formation emit-
ting at longer wavelength [37,38]. The EL spectra for compounds 1a,
1c, 2a, 2c were found to show a red shift about 10 nm, compared
with corresponding PL of solid states. This is a frequently observed
phenomenon caused by the influence of the electrical field on the
excited states in OLEDs [39]. While, compounds 1b and 2b
exhibited a larger red shift of about 40 nm of EL spectra to their PL
spectra in the solid states.

650
W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
Fig. 7. Luminanceevoltage (a) and current densityevoltage (b) for the devices using
1ae1c and 2ae2c.
and a maximum brightness of 860 cd/m2 at 11 V with a turn-on
voltage of 4.3 V. In the case of the other three compounds 2a, 2b,
2c which have C10 aryl substituents on antrhacene, however,
a much lower device performances in terms of efficiency of 0.85,
Fig. 6. Schematic energy level of different layer (a) and EL spectra (b) at 8 V.
The key performance parameters of the devices are summarized
in Table 2.
The devices based on compounds 1a, 1c and 2a, 2c showed
a deep-blue emission with CIE coordinates at the driving voltage of
8 V for 1a (0.166, 0.153), 1c (0.172, 0.162), 2a (0.154, 0.147), 2c (0.156,
0.131), while a sky-blue emission for devices from 1b and 2b with
CIE coordinates (0.186, 0.273) and (0.168, 0.230) were presented.
Compared with the analogs of 1a and 2a, the fluorine effect on the
EL spectra, such as obvious red shifts of the emission peaks, wider
FWHM and poorer CIE coordinates with y > 0.23, was observed for
compounds 1b and 2b.
As shown by the voltageecurrent densityeluminance charac-
teristics (Fig. 7(a) and (b)) and the current efficiency curves in Fig. 8,
the devices based on the blue emitters 1a and 1c exhibited much
higher EL efficiency with a current efficiency of 1.47 cd/A, an
external quantum efficiency of 1.84% at 20 mA/cm2 and a maximum
brightness of 3013 cd/m2 at 11 V with a turn-on voltage of 5.2 V for
1a, and a maximum current efficiency of 1.20 cd/A, an external
quantum efficiency of 1.48% at 20 mA/cm2 and a maximum
brightness of 2131 cd/m2 at 11 V with a lower turn-on voltage of
4.8 V for 1c, respectively. Due to the fluorine effect on EL, a litter
lower performance for 1b was observed with a current efficiency of
0.93 cd/A, an external quantum efficiency of 1.17% at 20 mA/cm2
Fig. 8. Luminous efficiency e current density for the devices using 1ae1c and 2ae2c.

W. Wan et al. / Dyes and Pigments 96 (2013) 642e652
651
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0.65, 0.71 cd/A, and the maximum brightness of 775, 626, 967 cd/
m2, respectively, were obtained with the same device configura-
tion. It was found that all devices have turn-on voltages of 4.3e
5.5 V at a brightness of 1 cd/m2. One important reason for such
similar turn-on voltages is that the injection barriers in the devices
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The current density at given voltage in device based on 1ae1c
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The best EL characteristics of compound 1a of all the synthe-
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results in efficient injection of holes from the hole-transporting
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anthracene compounds, which contain
a twisted structural
configuration due to the rigid linkage of a sp3-hybridized carbon
atom, were first prepared and described here. All compounds
exhibited glass transition temperatures around 104e143 ꢁC. Our
initial results from the unoptimized devices indicated that 1a with
phenyl and 1c with naphthalenyl at C2 position of fluorene module
exhibited better electroluminescent performances for blue OLEDs,
in comparison with the corresponding 2a and 2c, which possess the
aryl substituents at the C10 position of anthracene module. Intro-
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anthracene backbone should be a reasonable method to obtain
novel, promising blue-light-emitting OLED host emitter materials
with better luminous performances.
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Acknowledgments
This work is financially supported by National Natural Science
Foundation of China (21072127, 21032006, and 21172141), Science
and Technology Commission of Shanghai Municipality
(08JC1409900, 10JC1405600) and Shanghai Municipal Education
Commission. The authors are grateful to Professors Xueyin Jiang, Dr.
Hongmei Deng and Dr. Huaping Lin for their help in fabrication of
OLED devices.
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