Doubly ortho-linked quinoxaline/diphenylfluorene hybrids as bipolar, fluorescent chameleons for optoelectronic applications

Article abstract of DOI:10.1021/ja062660v

The titled dipolar hybrids bearing a central quinoxaline-fused dibenzosuberene optoelectronic unit with functional C5 and C8 appendages and spiro-fluorene junction act as fluorescent bipolar OLED chameleons. The emission colors can be tuned from blue, gre

Full text of DOI:10.1021/ja062660v

Published on Web 08/08/2006
Doubly Ortho-Linked Quinoxaline/Diphenylfluorene Hybrids as Bipolar,
Fluorescent Chameleons for Optoelectronic Applications
Chien-Tien Chen,*,† Yi Wei,† Jin-Sheng Lin,† Murthy V. R. K. Moturu,† Wei-San Chao,†
Yu-Tai Tao,‡ and Chin-Hsiung Chien‡
Department of Chemistry, National Taiwan Normal UniVersity, and Institute of Chemistry,
Academia Sinica, Taipei, Taiwan
Received April 17, 2006; E-mail: chefv043@ntnu.edu.tw
There has been great interest in the development of new
electroluminescent molecular materials for organic light emitting
diodes (OLEDs) in flat-panel display.1a.b In the aspect of advanced
designs, integrating electron-transporting (ET) and hole-transporting
(HT) segments into molecular materials may effectively improve
the OLED device performance because of more balanced charge
transports of both carriers.1c Several linearly D-A conjugated
templates based on this concept2-5 and attempts on single-layered
devices6 have thus been examined with significant success.
Figure 1. ORTEP diagram for X-ray crystal structure (ellipsoids are shown
at 20% probability level) of (a) 5b (left) and (b) 5d (right).
Recently, we unraveled the first attempt of using doubly ortho-
linked quinoxaline/triarylamine 1:1 hybrid as a bifunctional, dipolar
electroluminescent clip for bluish green and orange OLED applica-
tions.7 As part of our ongoing program on the use of dibenzo-
suberene (DBE)-based triarylcarbenium ions and over-crowded
triarylethenes in catalysis8 and LC optical switches,9 we thought
to merge the DBE template with fused quinoxaline backbones of
varying electronic attributes for optoelectronic applications. In
addition, to improve the rigidity, redox stability, and morphology
of the DBE template, spiro-fluorene10 is incorporated at the C5-
position of the DBE template. Conceptually, the new molecular
design connects the individual ortho-position of two phenyl rings
in a HT-type 9,9-diphenylfluorene framework to the C2-C3 edge
of a functionalized ET-type quinoxaline. Therefore, the resultant
1:1 hybrid is potentially bipolar and may act as a central fluorescent
chameleon unit with tunable emission colors by judicious choices
of the C5- and C8-appendages. Herein, we report the preliminary
results toward this end with good to excellent optoelectronic
performance in OLED applications (Scheme 1).
The starting 2, derived from DBE-1, was oxidized by benzene-
seleninic anhydride (BSA) to give diketone-3 in 87% yield.
Condensation of 3 and 3,6-dibromo-1,2-diaminobenzene with
catalytic p-TSA in refluxed CHCl3 provided quinoxaline-fused
DBE-4 in 90% yield. Attachments of respective aryl and arylamino
components to both the C5 and C8 positions of the quinoxaline
backbone in 4 were achieved by Suzuki and Hartwig coupling
reactions, leading to 5a-d, respectively in 70-95% yields.
The structural features for 5a-d were resolved by NMR
spectroscopy and ultimately proven by X-ray crystallographic
analyses. For the aryl-conjugated systems 5a-c, they share common
structural attributes. They exist as a dimmer in unit cell. The
flanking aryl appendages at both the C5 and C8 positions in the
quinoxaline backbone are turned by 45-56° to avoid epi-interaction
with the quinoxaline nitrogen lone pairs and steric interaction with
the spirofluorene unit in another molecule (Figure 1a). The
spirofluorene prevents facile conformational flipping of the central
seven-memberd ring in the DBE core. Conversely, only one flanking
nitrogen lone pair of the methylphenylamino groups in 5d is
Scheme 1. Assembly Concept of the Quinoxaline/
Diphenylfluorene Optoelectronic System
Table 1. Optical, Morphological, and CV Data for 5a-d and 6
a,b
a
absorbance
Em λmax
nm
Φfa
%
Tg/Td
Ered/Eox
V
compd
λmax
(
ꢀ a
)
°C
5a
276 (63.8),
367 (17.2)
288 (60.6),
374 (15.7)
323 (17.4),
441 (3.6)
332 (23.0),
482 (2.7)
284 (58.9),
365 (17.6)
482 (74)
521 (89)
599 (95)
650 (110)
497 (97)
41
78
34
42
57
181/395
160/372
186/412
153/333
74 /321
-2.07/- - -
-2.07/- - -
-2.05/+0.46
5b
5c
5d
6
-2.11/+0.08,
+0.27
-2.07/- - -
a λmax, in nm; (ꢀ) × 10-3; measured in CH2Cl2. b The data in parentheses
correspond to full-width at half-maximum (fwhm).
somewhat coplanar (i.e., conjugated) with the quinoxaline backbone
(Figure 1b). Nevertheless, resonance conjugations between the
pendant aryl (or phenylamino) group and the quinoxaline backbone
are prominent in solution, exerting progressive, bathochromic shifts
in their UV and photoluminescence spectra, Table 1.
Upon UV excitation, these materials show different emission
colors in solution varying from bluish, to green, to orange, to red.
The full-width at half-maximum (fwhm) for each individual
emission also increases progressively from 74, to 89, to 95, to 110
nm (Table 1), with quantum yields ranging from 34 to 78%. The
increased molecular size and rigidity arising from the spiral
framework of these materials are manifested in their increased
thermal stability (Td, 333-412 °C) and glassy transition temperature
† National Taiwan Normal University.
‡ Academia Sinica.
9
10992
J. AM. CHEM. SOC. 2006, 128, 10992-10993
10.1021/ja062660v CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Electroluminescence Data for 5a-d and 6
of 1.1, 2.9, and 3.2%. Their respective luminance and power
efficiencies (ηc/ηp) are 2.9/1.2, 7.8/3.7, and 2.3/1.2 cd A-1/lm W-1
.
c
Emb
λmax
Von
V
Lc
c
confa
ηext
ηcc
/
ηpc
cd/m2
Overall, the optoelectronic performance is 2 times better when
5b is utilized as an ET and emitting layer. On the contrary, the
optoelectronic performances are 2-3 times better when 5c or 5d
is employed as HT and emitting layers. Nevertheless, both the 5b
and 5c can function as bipolar electroluminescent materials with
more balanced charge transporting properties in view of the
satisfactory OLED performances in complementary device con-
figurations A and B. To further optimize the device efficiency, we
have also fabricated a four-layered device with a configuration of
R-NPB/5c (40 nm)/BCP/Alq3 (configuration C). For this combina-
tion of using 5c as the sole emitting layer, the OLED device exhibits
Lmax and L20 of 69700 and 2503 cd/m2 with ηext of 4.9%. The
luminance and power efficiencies (ηc/ηp) are 13.1/6.5 cd A-1/lm
W-1. To our knowledge, green emitting 5b, yellow-emitting 5c,
and red emitting 5d represent one of the best bipolar fluorescent
materials to date.12
5a/A
5b/A
5b/B
6/A
Alq3/A
5c/A
5c/B
5c/C
5d/A
5d/B
490 (70)
520 (82)
524 (88)
502 (90)
516(104)
576 (95)
580 (97)
578 (96)
642(102)
644(101)
4.5 (4.9)
2.5 (4.0)
4.7 (7.4)
2.7 (4.0)
3.9 (5.6)
2.8 (3.9)
3.9 (6.5)
4.4 (6.3)
3.7 (5.7)
4.1 (5.8)
0.45
1.92
1.06
1.75
1.11
0.98
2.90
4.92
0.81
3.17
0.82/0.52
6.35/5.02
2.90/1.19
5.07/3.95
3.29/1.84
2.54/2.05
7.75/3.73
13.10/6.54
0.85/0.47
2.31/1.21
5995 (205)
35372 (1268)
11733 (414)
20640 (1055)
28911 (638)
23716 (524)
20683 (1542)
69700 (2503)
3558 (167)
11484 (1102)
a Configuration (conf) A: ITO/NPB (40 nm)/5a-d (40 nm)/LiF (1 nm)/
Al; B: ITO/5b-d (40 nm)/BCP (10 nm)/Alq3 (40 nm)/LiF (1 nm)/Al; C:
ITO/NPB (40 nm)/5c (40 nm)/BCP (10 nm)/Alq3 (40 nm)/LiF (1 nm)/Al.
b The data in parentheses correspond to full-width at half-maximum (fwhm).
c The data in parentheses for Von and L and the data for ηext (%), ηc (cd/A),
and ηp (lm/W) were measured at 20 mA/cm2.
(Tg, 153-186 °C). For example, the Td and Tg (372 °C and 160
°C) for 5b (C47H32N2O2, fw: 656.8) are 51 and 86 °C higher than
those (321 °C and 74 °C) for the corresponding open-form system-6
[i.e., 5,8-bis-(4-methoxyphenyl)-2,3-diphenyl-quinoxaline (C34H26-
N2O2, fw: 494.6)].
The redox behaviors of 5a-d were evaluated by cyclic volta-
mmetry (CV) experiments at ambient temperature. They exhibit
similar reduction potentials with reversible redox couples at -2.08
( 0.03 V. No discernible oxidations were found in less than +1.5
V for 5a and 5b. Nevertheless, reversible two-electron oxidation
redox couples can be identified for triarylamino-containing 5c and
5d. They show oxidation potential at +0.46 V for 5c and a pair of
+0.08 and +0.27 eV for 5d owing to sequential oxidations of the
two amino appendages. When compared with Ar3N/quinoxaline4a,b
and spiro-fluorene-based dipolar hybrids,5d the current new designs
show slightly lower redox potentials, which may facilitate better
charge injection from both carriers.
Acknowledgment. We thank the National Science Council of
Taiwan for generous financial support of this research.
Supporting Information Available: Experimental, device, and
spectra details of 2-6 and cif files for 5b and 5d. This material is
available free of charge via the Internet at http://pubs.acs.org.
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m2, respectively, with individual external quantum efficiency (ηext
)
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J. AM. CHEM. SOC. VOL. 128, NO. 34, 2006 10993