Doubly ortho-linked cis-4,4′-bis(diarylamino)stilbene/fluorene hybrids as efficient nondoped, sky-blue fluorescent materials for optoelectronic applications

Article abstract of DOI:10.1021/ja070822x

The titled hybrids bearing a central dibenzosuberene optoelectronic unit with functional C3 and C7, N,N-diarylamino appendages, and spiro-fluorene junction act as blue fluorescent OLED materials. Sharp blue fluorescent (464 nm, Δλ_fwhm = 47-60 n

Full text of DOI:10.1021/ja070822x

Published on Web 05/25/2007
Doubly Ortho-Linked cis-4,4′-Bis(diarylamino)stilbene/Fluorene Hybrids as
Efficient Nondoped, Sky-Blue Fluorescent Materials for Optoelectronic
Applications
Yi Wei and Chien-Tien Chen*
Department of Chemistry, National Taiwan Normal UniVersity, Taipei, Taiwan 11650
Received February 5, 2007; E-mail: chefv043@ntnu.edu.tw
There has been great interest in developing electroluminescent
molecular materials with sharp and bright blue emissions for organic
light emitting diodes (OLEDs) in flat-panel display in recent years.1
Several efficient UV molecular host materials integrated with
suitable blue-emitting dopants have been developed with pending
stability issues during prolonged operation.2 Conversely, many
researchers have tried to come up with ultimate, nondoped hosts
by utilizing the derivatives of bistriphenylenyl,3 diarylfluorene/
spirobifluorene,4 and diarylanthracene.5
trans-4,4′-Bis(diarylamino)stilbene derivatives are generally
Figure 1. Chem-3D presentation for the X-ray crystal structure of 5c.
adopted as blue fluorescent materials.6 However, their ready trans
to cis photoisomerization profiles and close interchromophoric
contact significantly reduce their emission efficiency both in
solution and in the solid state, limiting their extensive applications
in OLED.7 To prevent the radiationless photoisomerization through
the confinement of the cis-stilbene configuration in a cyclic
framework, we sought a new concept of this type by affixing the
two individual ortho-positions of the phenyl rings in cis-stilbene
onto the C-9 position of fluorene (Scheme 1). The resultant spirally
connected, butterfly-shaped framework8 can avoid intimate inter-
molecular chromophoric π-π stacking responsible for emission
quenching or shifting due to eximer formation. By appending two
hole-transporting diarylamino segments onto both para positions
of the cis-stilbene template, sky-blue fluorescent OLED materials
can be furnished with high EL brightness and working efficiencies.
Herein, we report the preliminary result toward this end.
Scheme 1. The Assembly Concept of the
cis-4,4′-Bis(N,N-diarylamino)stilbene/Fluorene Optoelectronic
System
Table 1. Optical, Morphological, and Electrochemical Data for
5a-c, 6, and 8
a
a
absolute
λ
(
max ꢀ a
)
λmax
nm
Φfa
%
Tg/Td
oC
Eox
V
compd
nm
The target molecules 5a-c can be synthesized in three steps
from 3,7-dibromo-dibenzosuberenone-3 by treatment with 2-lithio-
biphenyl and subsequent intramolecular Friedel-Crafts alkylation
of the resulting 3° alcohol and final coupling with diarylamines.9
The structural features of 5a-c were determined by NMR
spectroscopic and X-ray crystallographic analysis. As represented
in 5c, it exists as a dimer in unit cell with planar nitrogen units
(Σ ) 359 ( 0.6°),9 Figure 1. Both the flanking diarylamino units
are tilted by 28-38° relative to the central dibenzosuberene (DBE)
template. The 4-methoxyphenyl units are tilted by 27.4 ( 0.9° and
58.4 ( 2.8°, respectively. The rigid butterfly-shaped structure with
a spiral tail not only prevents facile conformational flipping of the
central 7-membered ring in the DBE core but also avoids
intermolecular parallel π-π stacking.
Upon UV excitation, 5a-c show similar blue emission color
with emission maxima in the range of 454-483 nm with quantum
yields up to 90%. The full-width at half-maximum (fwhm) for each
individual emission ranges from 53 to 60 nm, which may act as
ideal sky-blue emitters particularly for 5a and 5b (Table 1).
Conversely, the rigid spiral framework also helps to increase their
thermal stability (Td, 441-473 °C) and glassy transition temperature
(Tg, 117-137 °C). In comparison, the Td (443 °C) for 5a is 34 and
46 °C higher than those of the corresponding open-form and
methylene-linked systems 6 and 8, respectively (Table 1). Con-
5a
306 (22.2)
349 (12.3)
406 (23.6)
265 (44.5)
347 (26.4)
403 (33.0)
302 (30.5)
370 (16.6)
414 (27.5)
307 (15.9)
370 (10.4)
306 (22.9)
370 (23.0)
461 (57b)
454 (53)
483 (60)
90
58
90
123/443
137/473
117/441
+0.15
+0.32
+0.15
5b
5c
+0.36
+0.19
+0.34
+0.26
6
8
441 (58)
440 (62)
99
99
50/409
101/397
+0.34
+0.71
a ꢀ × 10-3; measured in CH2Cl2. b The data in parentheses correspond
to full-width at half-maximum (fwhm).
versely, the Tg (123 °C) for 5a is 73 and 22 °C higher than those
of 6 and 8, respectively.
The redox behaviors of 5a-c were evaluated by cyclic volta-
mmetry (CV) experiments at ambient temperature with Ag/AgCl
as a reference electrode. All of these show similar, paired oxidation
potentials with reversible redox couples at +0.17 ( 0.02 and +0.34
( 0.02 V owing to sequential oxidations of the two diarylamino
appendages. In marked contrast, the corresponding open-form and
9
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J. AM. CHEM. SOC. 2007, 129, 7478-7479
10.1021/ja070822x CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Electroluminescence Data for 5a-c, 6, and 8
ficiency in configuration-B were improved by about 4 times for
5a and by 1.7-1.9 times for 5b and 5c, albeit with slightly higher
operational voltages by 0.6-0.9 V. Conversely, when 2,9-dimethyl-
4,7-diphenyl-1,10-phenanthroline (BCP10c) was used to replace
TPBI in device-B, the resulting device-C operated 75% as efficient.9
Notably, the external quantum efficiencies (ηext) in device
configuration-B for 5a,b are approaching or greater than the
theoretical maximum of about 6.5% for common fluorescent-type
molecular materials. A recent report by Fujihira suggested that extra
singlet excited-state formation may be facilitated through triplet-
triplet annihilation of a given triplet excited state, which attributes
to an additional fluorescent emission by 60% of the original
maximum (i.e., corrected max for ηext is 10.4%).11 The un-
precedented high external quantum efficiency (ηext, 7.9%) exhibited
by 5a in device configuration-B indicates potential facile triplet-
triplet annihilation of 5a with enhanced singlet-state formation in
the cyclic DBE skeleton by placing the triplet biradical at both
pseudoaxial positions of C10 and C11. The pseudoaxially disposed
triplet biradical can thus be readily transformed back to the
corresponding singlet biradical through conjugation with both the
diarylamino moieties. To our knowledge, blue-emitting 5a repre-
sents one of the best blue fluorescent materials to date, and its
optoelectronic performance is also somewhat comparable to some
doped phosphorescent devices.12 Particularly, saturated sky-blue
OLED device-B [CIE (x,y):(0.15,0.25)] resulted well from 5a augur
for its further applications in full-color display technology.
Acknowledgment. We thank the National Science Council of
Taiwan for generous financial support of this research.
c
configurationa
Em. λmax
Von, V
ηext
ηc
/
ηpc
Lmax,
d cd/m2
5a/A
5a/B
5b/A
5b/B
5c/A
5c/B
6/A
462 (47b) 2.9 (4.3c) 1.94 2.9/2.1
11313 (595c)
7.87 13.6/8.2 17609 (2689)
3.44 5.4/4.2 14690 (1071)
6.35 10.2/6.9 17599 (2034)
3.26 7.4/6.5
13615e (1454)
5.19 12.1/8.6 13806 (2423)
466 (60)
458 (49)
462 (52)
480 (59)
484 (77)
464 (47)
468 (70)
2.8 (5.2)
2.8 (4.0)
2.7 (4.6)
2.5 (3.6)
2.5 (4.4)
4.6 (8.6)
3.2 (4.3)
1.12 1.6/0.6
2.42 3.0/1.9
1223 (319)
8939 (621)
8/A
a Configuration A: ITO/5a-c, 6, or 8 (40 nm)/TPBI(40 nm)/LiF(1 nm)/
Al; configuration B: ITO/PEDOT:PSS(20 nm)/5a-c(40 nm)/TPBI (40 nm)/
LiF(1 nm)/Al. b The data in parentheses correspond to full-width at half-
c
maximum (fwhm). Von, ηext (%), ηc (cd/A), ηp (lm/W), and L20 measured
at 20 mA/cm2. d At 10.5 V. e At 8.5 V.
Figure 2. The stacked plots of the I-V-L and I-ηext characteristics of
devices-B for 5a-c.
methylene-linked systems 6 and 8 exhibit only one reversible, two-
electron oxidation, redox couple at +0.26 and +0.34 V, respec-
tively. Therefore, the current spiral design shows lower first
oxidation potential, which may facilitate better hole injection from
ITO to these materials in OLED devices. The second oxidation
potential at +0.71 V for 8 is due to facile oxidation at the C(5)-
position of the central DBE template.
Their optoelectronic performances were assessed by fabricating
5a-c as an individual hole transport-type emitting layer (40 nm
thickness) and making it into a bilayer OLED device configura-
tion-A by using 1,3,5-tris-(N-phenyl-benzimidazole-2-yl)benzene
(TBPI; 40 nm thickness)10a as the electron-transport (ET) layer
Supporting Information Available: Experimental, device, and
spectra details of 2-5, 6, and 8 and cif file for 5c. This material is
available free of charge via the Internet at http://pubs.acs.org.
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respectively, with individual external quantum efficiencies (ηext
)
of 7.9, 6.4, and 5.2% (Figure 2). Their ηc/ηpvalues are 13.6/8.2,
10.2/6.9, 12.1/8.6 cdA-1/lmW-1, respectively. Overall, their external
quantum efficiency, operational brightness, and operational ef-
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