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
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.
References
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Publishers: Amsterdam, 1997.
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The terfluorene derivatives with highest thin-film PL quantum
yields have been subject to electroluminescent (EL) studies. The
device structure used is ITO/PEDT:PSS (300 Å)/terfluorene 4aa
or 4cc (500 Å)/TPBI (370 Å)/LiF (5 Å)/Al, where the conducting
(8) Wong, K.-T.; Wang, Z.-J.; Chien, Y.-Y.; Wang, C.-L. Org. Lett. 2001, 3,
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