Ter(9,9-diarylfluorene)s: Highly efficient blue emitter with promising electrochemical and thermal stability

Article abstract of DOI:10.1021/ja0269587

Novel ter(9,9-diarylfluorene)s were synthesized by a Suzuki-coupling reaction of 2-bromofluorene (1) and 2,7-fluorenediboronic ester derivatives (3) with high isolated yields (63-86%). The X-ray structure analysis of ter(9,9′-spirobifluorene) (4aa) revealed that the conjugated chromophore adopts a helical conformation. This conformation effectively releases the steric interaction between the fluorene moieties and prevents inter-chromophore interactions. The introduction of aryl groups at the C9 position of fluorene was highly beneficial to the thermal and morphological stability of these oligomers. These terfluorenes exhibit intense blue fluorescence with excellent quantum yields both in solution (～100%) and in solid state (66-90%), and possess interesting reversible redox properties. Highly efficient blue light-emitting OLED devices were fabricated using 4aa and 4cc as emitters as well as hole transporters. The devices exhibit low turn-on voltage (～3 V) and high EL external quantum efficiency (2.5-3%). Copyright

Full text of DOI:10.1021/ja0269587

Published on Web 09/10/2002
Ter(9,9-diarylfluorene)s: Highly Efficient Blue Emitter with Promising
Electrochemical and Thermal Stability
Ken-Tsung Wong,* Yuh-Yih Chien, Ruei-Tang Chen, Chung-Feng Wang, Yu-Ting Lin,
Huo-Hsien Chiang, Ping-Yuan Hsieh, Chung-Chih Wu,* Chung Hsien Chou, Yuhlong Oliver Su,
Gene-Hsiang Lee, and Shie-Ming Peng
Department of Chemistry, Department of Electrical Engineering, Graduate Institute of Electro-optical Engineering,
and Graduate Institute of Electronics Engineering, National Taiwan UniVersity, Taipei 106, Taiwan
Received May 18, 2002
Scheme 1. Synthetic Pathways of Ter(9,9-diaryfluorene)s
Advances made in the molecular design of modern optoelectronic
materials have made significant contributions toward the develop-
ment of organic electronics. The organic light-emitting devices
(OLED) employing monodisperse1 or polymeric conjugated materi-
als2 possess the most promising prospects. However, materials
suitable for long-term use as blue light emitters are still far from
optimization in terms of stability. Terfluorenes,3 fully spiro-
configured terfluorene,4 oligomeric fluorenes,3a,5 and polyfluorenes6
are generally regarded as the most promising candidates for blue
OLEDs. However, the long-wavelength emission due to the photo-
or electrooxidized cleavage of C9-substituted alkyl pendant group(s),7
results in poor color purity and has inhibited their prospective
utilization. In this communication, we report a convergent synthesis,
the single-crystal structure, and intriguing properties of novel
ter(9,9-diaryfluorene)s. These ter(9,9-diarylfluorene)s which contain
intrinsically stronger Csp3-Csp2 bonds between the pendant aryl
groups and the C9 carbon of fluorene exhibit excellent morphologi-
cal and thermal stability. Together with the high quantum yields
in thin films, these terfluorenes are promising for applications in
OLEDs as efficient blue emitters.
The desired 9,9-diaryl substituted 2-bromofluorene 1a-d and
2,7-dibromofluorene 2a-d were synthesized in good yields (75-
92%) by aryl Grignard addition onto the corresponding 2-bromo-
fluorenone and 2,7-dibromofluorenone respectively,8 followed by
a sulfuric acid-promoted Friedel-Crafts reaction (Scheme 1). The
transformation of the dibromide 2 into the corresponding dipina-
colato boronic esters 3 was achieved in modest yields (54-70%)
by lithiating 2 with an excess amount of n-BuLi at -78 °C and by
successively treating the dilithiated complex with 2-isopropoxy-
4,4,5,5-tetramethly-[1,3,2]dioxaborolane. Finally, Pd-catalyzed Su-
zuki coupling reactions of diboronic esters 3 and 1 were accom-
plished by adding a catalytic amount of bulky tri-tert-butylposphine
as a promoter to afford the target ter(9,9-diaryfluorene)s in good
yields (Table 1).
All ter(9,9-diarylfluorene)s in this study are highly fluorescent
with an emission maximum centered at ca. 394 nm along with
vibronic features at ca. 413 and ca. 441 nm (Table 1, also see
Supporting Information). The photoluminescent (PL) features reveal
typical characteristics of conjugated fluorene derivatives.3,4 The
quantum yields of terfluorenes 4 in ethyl acetate were measured to
be close to 100% by using Coumarin 1 and 9,10-diphenylanthracene
as standards. More importantly, the vacuum-deposited thin films
of terfluorenes 4 exhibit excellent quantum yields (Table 1), which
were determined by using a calibrated integrating sphere system.
These results indicate that the C9 aryl substituents play a less
relevant role in the absorption and emission maxima but have a
pronounced effect on the solid-state PL efficiencies. The introduc-
tion of aryl substituents is significantly beneficial in improving the
morphological stability and the resistance to thermal decomposition.
Except for crystalline 4aa, terfluorenes 4 exhibit distinct glass
transition temperatures (Tg) ranging from 189 to 231 °C. The highly
crystalline feature of ter(spirobifluorene) 4aa can be attributed to
the rigidity of its peripheral 2,2′-biphenyl substituents. Thus,
combining the rigid-relaxed fluorene moieties 1b and 1c with 3a
generates terfluorene 4ab and 4ac with distinct Tg’s, respectively.
It is noteworthy that the C9 carbon of 1d is a chiral center.
Therefore, combining 1d with 3a affords 4ad as a diastereomeric
mixture, which exhibited a high Tg up to 231 °C. More importantly,
no further phase transition was observed upon heating 4ad over its
Tg to 350 °C. This result represents a new approach for the
molecular design of amorphous materials by introducing chiral
centers in close proximity of the conjugated chromophore backbone.
By TGA analysis, these terfluorenes were found to be thermally
Due to the rigidity of the peripheral aryl substituents, colorless
single crystals of 4aa suitable for X-ray analysis were obtained by
carefully layering the THF solution of 4aa with hexane. Inspection
of the molecular structure (Figure 1a) indicates the two outside
fluorene moieties of the conjugated chromophore backbone are
highly twisted relative to the central one by 134.6° and -128.3°
respectively, rendering the conjugated chromophore a helical
conformation as viewed along the molecular axis (Figure 1b). This
conformation effectively releases the steric interaction between the
fluorene moieties and prevents inter-chromophore interactions.
* Corresponding author. E-mail: kenwong@ccms.ntu.edu.tw.
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J. AM. CHEM. SOC. 2002, 124, 11576-11577
10.1021/ja0269587 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Isolated Yields and Physical Properties of Ter(9,9-diarylfluorene)s 4
1
2
1
2
cmpd
yield (%)
λmax (nm), log ꢀ
PL λmax (nm)
Qsolution (%)
Qfilm (%)
T (°C)
g
TGA (°C)
oxid. E , E (V)
red. E , E (V)
4aa
4bb
4cc
4ab
4ac
4ad
83
81
75
68
63
86
353, 4.54
355, 4.70
354, 4.98
353, 4.73
353, 4.77
356, 4.83
393, 412, 441
395, 413, 441
393, 414, 441
393, 412, 438
394, 414, 440
395, 415, 441
99
99
99
99
99
99
90
78
90
82
87
66
no
450
400
410
425
435
430
1.32, 1.56
1.36, 1.60
1.32, 1.57
1.31, 1.56
1.31, 1.56
1.30, 1.55
-2.01, -2.21
-1.97, -2.15
-2.02, -2.18
-2.11, -2.23
-2.10, -2.22
-2.03, -2.17
189
204
201
210
231
polymer polyethylene dioxythiophene:polystyrene sulfonate (PEDT:
PSS) was used as the hole-injection layer, 1,3,5-tris(N-phenylbenz-
imidazol-2-yl)benzene (TPBI) as the electron-transport layer, and
5 Å LiF as the electron-injection layer. These devices exhibit pure
blue EL similar to PL spectra of terfluorenes (see Supporting
Information), indicating emission purely from terfluorenes. The
rather low turn-on voltage of ∼3 V, defined as the voltage where
EL emission becomes detectable, and low operation voltage (100
cd/m2 at ∼6 V, 1000 cd/m2 at ∼8 V) of these devices suggest these
terfluorenes also function well as hole transporters. High EL external
quantum efficiency of 2.5-3% photon/electron and high brightness
over 5000 cd/m2, were observed for these devices, consistent with
the high thin-film PL quantum yields. High PL/EL quantum
efficiencies in conjunction with carrier transport properties render
these terfluorenes a very interesting and promising class of
optoelectronic materials.
Figure 1. (a) Molecular view of X-ray structure of 4aa. (b) Top view
from the molecular axis.
Acknowledgment. We thank Professor C.-T. Chen (Academia
Sinica) for providing TPBI and Mr. S. C. Kao for DSC and TGA
analysis. This work was supported by the National Science Council
and Ministry of Education of Taiwan.
Supporting Information Available: Complete experimental details,
X-ray data of 4aa (CIF), a comparison of UV-vis and PL spectra of
4aa and 4cc, and OLED devices characteristics (I-V, brightness-V,
EL/PL) (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 2. Cyclic voltammogram of terfluorene 4aa. (Inset) Differential
pulse voltammetry (DPV) of reduction region.
stable with temperatures corresponding to a 5% weight loss upon
heating (20 °C/min) under nitrogen ranging from 400 to 450 °C.
Cyclic voltammetry experiments were conducted on terfluorenes
4 at room temperature to probe their electrochemical properties.
Two discrete reversible oxidation potentials were observed in
CH2Cl2 (0.1 M nBu4NPF6 as a supporting electrolyte), and two
reversible reduction potentials were detected in THF (0.1 M nBu4-
NClO4 as a supporting electrolyte). A representative example of
the cyclic voltammogram of 4aa is depicted in Figure 2. Those of
the two reductions are relatively poorly resolved compared to that
of the oxidations. However, differential pulse voltammetry (DPV)
clearly showed the appearance of two distinguishable reductions.
Terfluorenes 4aa, 4ab, and 4ac, which have the same structural
features of the central subunit, that is, spirobifluorene, exhibited
very similar redox behaviors. The pronounced potential differences
in their oxidation regions (240-250 mV) and reduction regions
(120-200 mV) indicate that the monocationic and anionic species
can efficiently delocalize the charge over the entire conjugated
chromophore.
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