π-conjugated aromatic enynes as a single-emitting component for white electroluminescence

Article abstract of DOI:10.1021/ja058188f

By use of the organolanthanide catalysts Me₂Si(C₅Me₄)(NAr)Lu(CH₂SiMe₃)(THF) and (C₅Me₅)₂LaCH(SiMe₃)₂, carbazole-substituted phenyl enynes (Z)-CPE

Full text of DOI:10.1021/ja058188f

Published on Web 04/07/2006
π-Conjugated Aromatic Enynes as a Single-Emitting Component for White
Electroluminescence
†
†
‡
,†
Yu Liu, Masayoshi Nishiura, Yue Wang, and Zhaomin Hou*
Organometallic Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research),
Hirosawa 2-1, Wako, Saitama 351-0198, Japan, and Key Laboratory for Supramolecular Structure and
Materials of Ministry of Education, Jilin UniVersity, Changchun 130023, People’s Republic of China
Received December 2, 2005; E-mail: Houz@riken.jp
White organic light-emitting devices (WOLEDs) have attracted
much current interest because of their good potential for various
lighting applications, such as low-cost, large-area flat-panel displays.
An ideal white emission should be composed of the three primary
colors (blue, green, and red) and cover the whole visible range from
400 to 700 nm. Most of the WOLEDs reported so far have relied
on the use of a combination of several organic components that
emit different colors of light to fully span the entire visible
1
spectrum. Compared to the multi-emitting-component WOLEDs,
a single-emitting-component WOLED could show many advan-
tages, such as better stability, better reproducibility, and a simpler
fabrication process. However, few materials are known to show
white-light emission as a single-emitting component, and none have
2
been reported to emit “pure” white light. Therefore, the search
for new organic light-emitting materials with new structures for
use in single-emitting-component WOLEDs is of obvious interest
and importance.
We report here the synthesis and photo- and electroluminescence
properties of carbazole-substituted aromatic (E)- and (Z)-enynes.3
These new π-conjugated compounds, in particular, the (E)-enyne
isomer, can serve as an excellent single-emitting component for
WOLEDs, emitting almost “pure” white light with stable CIE
Figure 1. Photoluminescence (PL) spectra of (E)-CPEY (red dotted line,
in CH2Cl2; red solid line, in a vacuum-deposited film), (Z)-CPEY (blue
dotted line, in CH2Cl2; blue solid line, in a vacuum-deposited film), NPB
in a vacuum-deposited film (black solid line), and (E)-CPEY doped in NPB
(ca. 1:1 w/w) in a vacuum-deposited film (black dotted line). Inset:
Excitation spectra of (E)-CPEY monitored at 455 and 550 nm in a vacuum-
deposited film.
Scheme 1. Regio- and Stereoselective Synthesis of Conjugated
Enynes
(Commission Internationale de l’Eclairage) coordinates under
different driving voltages.
Catalytic dimerization of terminal alkynes by organolanthanide
complexes to give conjugated enynes has been reported previously
4
by our group and others. By use of the organolanthanide catalysts
1
and 2, the carbazole-substituted phenyl enynes (Z)-CPEY and
(
E)-CPEY could be easily prepared, respectively, with excellent
5
regio- and stereoselectivity, as shown in Scheme 1. On irradiation
at 365 nm in CH Cl , both (E)-CPEY and (Z)-CPEY showed intense
2
2
blue fluorescence, with an emission peak centered at λmax ) 455
nm and a quantum yield of ca. 0.75 for (E)-CPEY and 0.70 for
(Z)-CPEY (Figure 1). In the thin films, however, the photolumi-
nescence (PL) spectra showed extra broad emissions at 510-520
nm and tailed to 600 nm in addition to the strong emission at 455
nm, probably because of formation of excimers (vide infra).
To investigate the electroluminescence (EL) properties of these
compounds, a single-layer device with a structure of ITO/
nm) and orange-red (590 nm) emissions. The analogous (Z)-CPEY-
based device ITO/(Z)-CPEY(60 nm)/LiF/Al (device 2) showed a
similar EL spectrum, although its CIE coordinates (0.23, 0.26) were
not as close to those of pure white light (0.33, 0.33) under the same
conditions (Figure 2). To facilitate the hole injection from the anode
and increase the efficiency of the EL device 1, a thin layer of 4,4′-
bis(1-naphthylphenylamino)biphenyl (NPB) (HOMO ) -5.5 eV,
LUMO ) -2.4 eV)1 was added by vacuum deposition between
the (E)-CPEY layer (HOMO ) -5.96 eV, LUMO ) -3.01 eV)
and ITO (work function, -4.7 eV). As shown in Figure 2, such a
double-layer device ITO/NPB(30 nm)/(E)-CPEY(40 nm)/LiF/Al
(device 3) showed much better performance as a WOLED which
(E)-CPEY(60 nm)/LiF/Al (ITO ) indium-tin-oxide) (device 1)
was fabricated by vacuum deposition of (E)-CPEY. Compared to
the PL spectrum of the (E)-CPEY film, the EL spectrum of the
(
E)-CPEY-based device became significantly broader, with the long-
wavelength emissions around 510 and 590 nm being much stronger
Figure 2). The whole device emitted white light with CIE coor-
(
k,l
dinates of (0.29, 0.30), as a result of the combination of the blue
emission around 455 nm with the longer-wavelength green (510
†
RIKEN.
Jilin University.
‡
5592
9
J. AM. CHEM. SOC. 2006, 128, 5592-5593
10.1021/ja058188f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
and orange-red) from eximers. The (E)-CPEY-based double-layer
device (device 3) emitted almost pure white light with CIE coor-
-
2
dinates of (0.32, 0.33), maximum brightness of 1395 cd m , and
-
1
maximum current efficiency of 2.07 cd A . To the best of our
knowledge, this is the purest white emission with high lumi-
nescence and high efficiency ever reported for a single-emitting-
component WOLED. The quality of the white emission remained
almost unchanged under varying driving voltages, demonstrating
an advantageous potential of single-emitting-component WOLEDs.
Acknowledgment. This work was partly supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No. 14078224,
“
Reaction Control of Dynamic Complexes”) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan and
by NSFC (50225313, and 50520130316), 973 and ITDU Programs
(
2002CB613401 and B06009). Dr. Yuichi Ito of Toppan Printing
Figure 2. Electroluminescence (EL) spectra of single-emitting-component
E)-CPEY- or (Z)-CPEY-based devices: device 1, ITO/(E)-CPEY(60 nm)/
LiF/Al; device 2, ITO/(Z)-CPEY(60 nm)/LiF/Al; device 3, ITO/NPB(30
nm)/(E)-CPEY(40 nm)/LiF/Al; device 4, ITO/NPB(30 nm)/(Z)-CPEY(40
nm)/LiF/Al. NPB ) 4,4′-bis(1-naphthylphenylamino)biphenyl. Inset: Pho-
tograph of device 3 at a driving voltage of 15 V.
Co. is gratefully acknowledged for measurement of the HOMO
and LUMO band energies of (E)-CPEY. We are also grateful to
the Japan Society for the Promotion of Science (JSPS) for a
Postdoctoral Fellowship for Y.L.
(
Supporting Information Available: Experimental details and
spectral data for all new compounds and OLED structure, characteriza-
tion, and performance data for all devices. This material is available
free of charge via the Internet at http://pubs.acs.org.
emitted white EL with the spectrum covering the whole visible
range from 400 to 700 nm. Under a driving voltage of 16 V, an
almost pure white emission was achieved with CIE coordinates of
-
2
(
0.32, 0.33), maximum brightness of 1395 cd m , and maximum
References
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by the little variations in the CIE coordinates (0.30, 0.32)-(0.32,
(
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5
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film.7
a,b
Accordingly, the long-wavelength emissions in the PL
spectra of the (E)-CPEY film should result from excimers. Since
the long-wavelength broad peaks at 510 and 590 nm in the EL
spectra of devices 1 and 3 (Figure 2) are very close to the loca-
tion of the longer-wavelength shoulders in the PL spectra of the
(3) Carbazolyl was chosen as a subsituent, because it is well known to show
7a,b
high charge mobility and interesting optical and electronic properties.
(
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(E)-CPEY film (Figure 1), the longer-wavelength emissions in the
(
5) For mechanistic aspects of such alkyne dimerization, see ref 4.
EL spectra should also result from excimers. The difference in line
shape (or emission intensity) between the EL spectra and the PL
spectra might be because electron injection can sometimes generate
excitons that cannot be induced by optical excitation.7
(6) In fact, the PL spectrum of (E)-CPEY doped in NPB (ca. 1:1 w/w) in a
vacuum-deposited film showed a sharp emission around 455 nm with-
out tailing to long wavelength (Figure 1). This strongly suggests that
(E)-CPEY excimer formation was hindered even by NPB in the doped
film and thus excludes again the possibility of exciplex formation between
c
(E)-CPEY and NPB.
In summary, carbazole-substituted aromatic enynes, such as
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(E)-CPEY, which are easily prepared by catalytic dimerization of
a terminal alkyne, can act as an excellent single-emitting component
for WOLEDs, as a result of combination of the blue emission from
an isolated molecule with the longer-wavelength emissions (green
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