First examples of organophosphorus-containing materials for light-emitting diodes

Article abstract of DOI:10.1021/ja035155w

Exploiting the reactivity of the P-atom of phosphole-based oligomers, we have achieved access to the first organophosphorus-containing organic light-emitting diode (OLED) materials. The versatility of these P-materials is demonstrated with the synthesis of a corresponding gold complex that has also been used as an OLED material. Optimization of the OLED devices by doping the phosphole layer with a red fluorescent dye is described. Copyright

Full text of DOI:10.1021/ja035155w

Published on Web 07/12/2003
First Examples of Organophosphorus-Containing Materials for Light-Emitting
Diodes
Claire Fave,† Ting-Yi Cho,‡ Muriel Hissler,† Chieh-Wei Chen,‡ Tien-Yau Luh,§ Chung-Chih Wu,*,‡ and
Re´gis Re´au*,†
Institut de Chimie de Rennes, CNRS UMR 6509, UniVersite´ de Rennes 1, Campus de Beaulieu,
35042 Rennes Cedex, France, Department of Electrical Engineering, Graduate Institute of Electrooptical
Engineering and Graduate Institute of Electronics Engineering, National Taiwan UniVersity, Taipei, Taiwan 106,
and Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115
Received March 14, 2003; E-mail: regis.reau@univ-rennes1.fr
Table 1. Photophysical Data for Derivatives 1, 2,2b and 3
Over the past decade, considerable effort has focused on the
synthesis of organic π-conjugated systems for optoelectronic
applications.1 One remarkable example is the development of small
molecules or polymers for the preparation of organic light-emitting
diodes (OLEDs), which represent a realistic alternative to the well-
established liquid crystal devices used in flat-panel display tech-
nologies.1 Although monochromatic OLEDs are already used
commercially, considerable advances are still to be made, especially
toward full-color displays.1d Optimization of device performance
is often tied to the use of new materials with readily tunable
properties (thermal stability, redox potentials, emission wavelength,
etc.).
a
b
b
c
Td10
λmax
λem
φfb
×102
Epa
Epcc
[V]
[°C]
[nm]
log ꢀ
[nm]
[V]
1
2
3
205
253
218
412
432
428
3.93
3.98
4.18
501
548
544
5.0
4.6
14.0
+0.40
+0.68
+0.82
<-2.10
-1.95
-1.75
a TGA, 10% weight loss. b Measured in THF; fluorescence quantum yield
relative to fluorescein. c In CH2Cl2, referenced to Fc/Fc+ half cell.
We have recently shown that phospholes are versatile building
blocks for the engineering of mixed π-conjugated oligomers such
as 1 (Table 1).2 Chemoselective reactions involving the nucleophilic
phosphorus center allow direct access to a range of new π-conju-
gated systems, without the need for additional multistep syntheses.
Furthermore, these simple chemical modifications permit fine-tuning
of the physical properties of phosphole-based π-conjugated
systems.2a,b We now report that, by exploiting the reactivity of the
phosphole core of derivative 1, access to the first organophosphorus-
containing OLED materials can be achieved.
The devices employed in this study have the typical structure of
organic layers sandwiched between a bilayer anode and cathode
(Table 2).3 The low-molecular-weight organic layers were deposited
by thermal evaporation in high vacuum. This procedure led to the
decomposition of phosphole 1;2a however, its thioxo-derivative 22a
(Table 1), which possesses a higher decomposition temperature,
gave homogeneous thin films. The photoluminescence spectra (PL)
of thin films of 2 showed a single maximum at 542 nm (Figure 1),
a value close to that recorded in THF solution (Table 1). The OLED
device A, containing a single layer of 2 (Table 2), exhibited yellow
emission for a relatively low turn-on voltage of 2 V. The electro-
luminescence spectrum (EL) of device A resembles the thin-film
PL of 2, indicating similar mechanisms for both types of emission.
The EL quantum efficiency is approximately constant up to high
current (e.g., 600 mA cm-2), indicating a good operating endur-
ability.4a However, the maximum brightness and EL quantum yields
(Table 2) are low, presumably due to unbalanced carrier injection
or transport. Therefore, a more advanced device B, in which 2 was
sandwiched between the hole-transport layer R-NPD and the
electron-transport layer Alq3,1a,c was prepared. Comparison of the
EL of device B with that of a reference device G (Table 2) suggests
that its emission arises purely from 2 (Figure 1). Very interestingly,
as compared to device A, the turn-on voltage required to drive
device B is almost unchanged, while the EL quantum efficiency
and brightness are increased by nearly 1 order of magnitude.
Another effective way to further improve OLED performance,
and also to tune their color, is to dope highly fluorescent dyes as
guests into an emissive host matrix.1a,5 Thioxophosphole 2 was thus
evaluated as a host material for DCJTB, a popular pyran-containing
red-emitting dopant for OLEDs.5b Device C, which has a config-
uration related to that of device B except for a 1.4 wt % doping
concentration in the organic layer of 2 (Table 2), showed red
emission from DCJTB (Figure 1). This indicates effective resonant
energy transfer or carrier trapping from 2 to DCJTB. Upon doping
with DCJTB, the EL efficiency is further enhanced up to 1.83%
photon/electron with a maximal brightness of ca. 37 000 cd/m2
(devices B and C, Table 2). Of particular interest for practical appli-
cations, the EL quantum efficiency of phosphole-containing device
C does not decrease with the driving current.4a This is an important
improvement of the conventional DCJTB-doped Alq3 device H
(Table 2), of which EL efficiency drops rapidly with the driving
current, a common occurrence for this type of device due to the
quenching effect of charged excited states of Alq3 on red dopants.5d
To the best of our knowledge, these results give the first examples
of OLEDs based on π-organophosphorus materials. This study
raises interesting opportunities because the performance of these
phosphole-based devices may well be further enhanced by tailoring
the P-ring substituents. In this respect, because phospholes act as
ligands toward metals,6 a less classical approach was envisaged
whereby metal complexes are investigated as materials for OLE-
Ds.7,8 In a preliminary study, d10 Au(I) complexes were explored
because they generally exhibit interesting luminescent properties
in the solid state.8 Derivative 1 reacted with ClAu(tetrahy-
drothiophene) to afford complex 3 (Table 1), which was isolated
as an air-stable orange powder in 84% yield. The downfield 31P
NMR coordination shift (∆δ, 26.3 ppm) clearly establishes
coordination of the P-atom to the gold center.9 The simplicity of
† Universite´ de Rennes 1.
‡ National Taiwan University.
§ Academia Sinica.
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J. AM. CHEM. SOC. 2003, 125, 9254-9255
10.1021/ja035155w CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Electroluminescence Characteristics of Devices A-H
-2
h
i
j
k
device
organic layer (nm)a
λmaxEL [nm]
V
on
c [V]
V
20
d [V] Vmaxe [V]
B
20
f cd m-2 Bmaxg cd m
η20
ηmax
lm/W
lm/W
max
20
A
B
C
D
E
F
compound 2(75)
557
550
617
565
540
623
530
613
2.0
2.2
2.4
4.0
2.2
2.2
2.2
2.2
4.5
8.0
15.0
13.5
50
450
530
3613
37830
36538
0.09
0.74
1.83
0.16
0.80
1.83
0.175
0.827
1.12
0.222
1.14
1.61
R-NPD(40)/2(20)/Alq3(40)
R-NPD(40)/2:DCJTB(20)/Alq3(40)b
compound 3(56)
8.5
8.0
7.8
0.15
2.6 3.7 × 10-4 2.7 × 10-3 3 × 10-4 1.7 × 10-3
R-NPD(40)/3(20)/Alq3(40)
R-NPD(40)/3:DCJTB(20)/Alq3(40)b
R-NPD(40)/Alq3(60)
10.0
10.5
8.8
16.0
15.5
15.5
15.0
180
200
900
780
6575
2786
68880
50719
0.35
0.67
1.46
2.20
0.36
1.22
1.52
3.70
0.28
0.30
1.62
1.40
0.39
1.02
2.07
6.0
G
H
R-NPD(40)/Alq:DCJTB(20)/Alq3(40)b
8.6
a Device configurations (thickness): ITO/PEDOT:PSS(25 nm)/organic layer/Mg:Ag(80 nm)/Ag(150 nm). b DCJTB 1.4 wt %. c Turn-on voltage at which
emission becomes detectable (∼10-4 cd/m2). d Voltage for a current density of 20 mA cm-2
.
e Voltage at the maximum brightness (MB). f Brightness at V20.
g MB. h External EL quantum efficiency (EELQ) at 20 mA cm-2 i Maximal EELQ. j Luminous efficiency (LE) at 20 mA cm-2 k Maximal LE.
.
.
organophosphorus π-conjugated oligomers for optoelectronic ap-
plications. π-Conjugated systems including organophosphorus
moieties are still very rare;2,12 yet this work shows that the presence
of reactive P-centers in these derivatives can offer new perspectives
in the field of π-conjugated systems.
Acknowledgment. We thank the CNRS, the MENRT, the
Conseil Re´gional de Bretagne (PRIR n° 99CC10), and the National
Science Council and Ministry of Education of R.O.C. for financial
support.
Supporting Information Available: X-ray crystallographic data,
analytical, spectroscopic data for 3, OLED device characteristics (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 1. PL spectra of thin films of 2 and 3 and EL spectra of devices
B-H.
References
(1) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed.
1998, 37, 402. (b) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed.
1999, 38, 8, 1350. (c) Hung, L. S.; Chen, C. H. Mater. Sci. Eng., R 2002,
39, 143. (d) Tullo, A. H. Chem. Eng. News 2001, 79, 49.
(2) (a) Hay, C.; Fischmeister, C.; Hissler, M.; Toupet, L.; Re´au, R. Angew.
Chem., Int. Ed. 2000, 10, 1812. (b) Hay, C.; Hissler, M.; Fischmeister,
C.; Rault-Berthelot, J.; Toupet, L.; Nyulaszi, L.; Re´au, R. Chem.-Eur. J.
2001, 7, 4222. (c) Fave, C.; Hissler, M.; Se´ne´chal, K.; Ledoux, I.; Zyss,
J.; Re´au, R. Chem. Commun. 2002, 1674.
(3) Wu, C.-C.; Chen, C.-W.; Lin, Y.-T.; Yu, H.-L.; Hsu, J.-H.; Luh, T.-Y.
Appl. Phys. Lett. 2001, 79, 3023.
(4) See the Supporting Information for (a) external EL quantum efficiency
versus current for devices A, C, and H; (b) spectroscopic, analytical, and
crystallographic data of 3.
(5) (a) Tang, C. W.; Vanslyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65,
3610. (b) Chen, C. H.; Tang, C. W.; Shi, J.; Klubek, C. Macromol. Symp.
1997, 125, 49. (c) Roundhill, D. M.; Fackler, J. P. Optoelectronic
Properties of Inorganic Compounds; Plenum Press: New York, 1999.
(d) Young, R. H.; Tang, C. W.; Marchetti, A. P. Appl. Phys. Lett. 2002,
80, 874.
(6) (a) Mathey, F. Chem. ReV. 1988, 88, 429. (b) Quin, L. D.; Quin, G. S. In
Phosphorus-Carbon Heterocyclic Chemistry: The Rise of a New Domain;
Mathey, F., Ed.; Elsevier Science Ltd.: Oxford, 2001.
the 13C NMR spectrum favors a symmetric structure, discounting
a chelating coordination mode of 1 involving a gold-sulfur
interaction.4b These spectroscopic data fit perfectly with the solid-
state structure of 3 established by an X-ray diffraction study.4b As
expected, the σ4-phosphorus atom adopts a tetrahedral shape, with
the metal center in an almost linear geometry [P-Au-Cl, 174.6-
(1)°]. It is noteworthy that the shortest Au(I)-Au(I) distance in
the solid state is 7.3 Å, indicating the absence of any aurophilic
interactions.8 The optical data of 3 are very similar to those of 2
(Table 1), strongly suggesting that the absorption and emission of
complex 3 in solution are associated with intraligand π-π*
transitions.10 Interestingly, the quantum yield of 3 is 3 times as
high as that of 2 (Table 1). The Epa and Epc of derivatives 2 and
3 differ noticeably (Table 1), a feature that may have an important
impact on their electron-transporting properties.11
Complex 3 is thermally stable enough to give homogeneous thin
films upon sublimation in high vacuum. The PL spectra of these
thin films showed several maxima (Figure 1). The strong emission
centered at 565 nm probably involves an intraligand π-π*
transition, because this value is close to that recorded for 3 in THF
solution (Table 1). The apparition of low energy emissions in the
solid state is clearly associated with the presence of the Au(I) metal
center,8 nicely illustrating the contribution of P-chemistry to the
tuning of the optical properties of phosphole-based materials (Figure
1, derivatives 2/3). Device D containing a single layer of complex
3 (Table 2) exhibited an EL emission resembling the PL of the
thin film. The fact that the EL emission covers the 480-800 nm
domain is encouraging for the development of white-emitting
OLEDs based on phosphole-Au complexes. It is noteworthy that
the EL performances can be considerably enhanced by adding
charge-transporting layers or doping with DCJTB as well (Table
2, Figure 2, devices E, F). The fact that phosphole complexes can
be used as materials for OLEDs expands considerably the potential
of phosphole-based derivatives for further OLED developments.
In conclusion, we have given the first example of the use of
(7) (a) Buda, M.; Kalyuzhny, G.; Bard, A. J. J. Am. Chem. Soc. 2002, 124,
6090. (b) Rudmann, H.; Shimada, S.; Rubner, M. F. J. Am. Chem. Soc.
2002, 124, 4918 and references therein.
(8) (a) Lee, Y.-A.; McGarrah, J. E.; Lachicotte, R. J.; Eisenberg, R. J. Am.
Chem. Soc. 2002, 124, 10663. (b) Yam, V. W. W.; Chang, C.-L.; Li,
C.-K.; Wong, K. M.-C. Coord. Chem. ReV. 2001, 216-217, 173.
(9) (a) Attar, S.; Bearden, W. H.; Allcock, N. W.; Alyea, E. C.; Nelson, J. H.
Inorg. Chem. 1990, 29, 425. (b) Gimeno, M. C.; Laguna, A. Chem. ReV.
1997, 97, 511.
(10) Large Stokes shifts (>300 nm) are generally observed for the triplet Au-
(I) center state: Ma, Y.; Zhou, X.; Shen, J.; Chao, H.-Y.; Che, C.-M Appl.
Phys. Lett. 1999, 74, 1361 and ref 7.
(11) The redox processes likely involve the π-conjugated ligand and not the
metal center because the Epa and Epc of (phosphane)AuCl complexes
are generally much higher and lower than those observed with 3,
respectively. For example, (Ph3P)AuCl, Epc ) -2.4 V, no oxidation peak
appeared prior to CH2Cl2 discharge: Koelle, U.; Laguna, A. Inorg. Chim.
Acta 1999, 290, 44.
(12) (a) Bevierre, M. O.; Mercier, F.; Ricard, L.; Mathey F. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 655. (b) Deschamps, E.; Ricard, L.; Mathey, F.
Angew. Chem., Int. Ed. Engl. 1994, 11, 1158. (c) Mao, S. S. H.; Tilley,
T. D. Macromolecules 1997, 30, 5566. (d) Wright, V. A.; Gates, D. P.
Angew. Chem., Int. Ed. 2002, 41, 2389. (e) Lucht, B. L.; St. Onge, N. O.
Chem. Commun. 2000, 2097. (f) Morisaki, Y.; Aiki, Y.; Chujo, Y.
Macromolecules 2003, 36, 2594. (g) Hissler, M.; Dyer, P.; Re´au, R. Coord.
Chem. ReV. 2003, in press.
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