Toward functional π-conjugated organophosphorus materials: Design of phosphole-based oligomers for electroluminescent devices

Article abstract of DOI:10.1021/ja0567182

The photophysical, electrochemical, and optoelectronic properties of conjugated systems incorporating dibenzophosphole or phosphole moieties are described. Dibenzophosphole derivatives are not suitable materials for OLEDs due to their weak photoluminescen

Full text of DOI:10.1021/ja0567182

Published on Web 12/24/2005
Toward Functional π-Conjugated Organophosphorus
Materials: Design of Phosphole-Based Oligomers for
Electroluminescent Devices
Hai-Ching Su,† Omrane Fadhel,‡ Chih-Jen Yang,† Ting-Yi Cho,† Claire Fave,‡
Muriel Hissler,‡ Chung-Chih Wu,*,† and Re´gis Re´au*,‡
Contribution from the Department of Electrical Engineering, Graduate Institute of
Electro-optical Engineering and Graduate Institute of Electronics Engineering, National Taiwan
UniVersity, Taipei, Taiwan 106, and Institut de Chimie, UMR 6509, UniVersite´ de Rennes 1,
Campus de Beaulieu, 35042 Rennes Cedex, France
Received October 10, 2005; E-mail: regis.reau@univ-rennes1.fr; chungwu@cc.ee.ntu.edu.tw.
Abstract: The photophysical, electrochemical, and optoelectronic properties of conjugated systems
incorporating dibenzophosphole or phosphole moieties are described. Dibenzophosphole derivatives are
not suitable materials for OLEDs due to their weak photoluminescence (PL) in the solid state and the
instability of the devices. Variation of the substitution pattern of phospholes and chemical modification of
their P atoms afford thermally stable derivatives, which are photo- and electroluminescent. Comparison of
the optical properties of solution and thin film of thioxophospholes shows that these compounds do not
form aggregates in the solid state. This property, which is also supported by an X-ray diffraction study of
three novel derivatives, results in an enhancement of the fluorescence quantum yields in the solid state.
In contrast, (phosphole)gold(I) complexes exhibit a broad emission in thin film, which is due to the formation
of aggregates. Single- and multilayer OLEDs using these P derivatives as the emissive layer have been
fabricated. The emission color of these devices and their performances vary with the nature of the P material.
Interestingly, di(2-thienyl)thiooxophosphole is an efficient host for the red dopant DCJTB, and devices using
the gold complexes have broad emission spectra.
organic materials2 following the seminal reports of efficient
organic light emitting diodes (OLEDs) based on small molecules
Introduction
In the past decade, the synthesis of organic π-conjugated
materials has been the focus of much work because of their
potential applications in electronic and optoelectronic devices.1,2
This growing interest is mainly due to the possibility of tailoring
their physical properties and supramolecular organization via
structural variations.1,2 Predictive structure-property relation-
ships can be established, and the optimization of characteristic
properties to suit a desired function is possible via chemical
engineering at the molecular level. This thought process is nicely
illustrated by the development of advanced electroluminescent
and conjugated polymers.3
Varying the chemical composition of conjugated systems is
thus a major concern to further optimize organic materials. Due
to the high flexibility of organic synthesis, many strategies can
be envisaged to diversify the structure of π-conjugated molecular
materials. The most usual ones imply the synthesis of all-carbon
macromolecules incorporating different C-sp2- and C-sp-based
building blocks (aryle, alkyne, alkene...), and the grafting of
substituents with specific properties on these skeletons.1,2
Another fruitful approach involves the incorporation of hetero-
atomic moieties into the conjugated frameworks. Nitrogen and
sulfur have been intensively used for such a purpose; poly-
(aniline)s, poly(pyrrole)s, and poly(thiophene)s are among the
most widely investigated π-conjugated systems.1,2 Organosilicon
derivatives such as siloles have also been successfully employed
for the construction of efficient molecular materials for opto-
electronic applications.4 In marked contrast, organophosphorus
moieties have only recently been considered as potential building
blocks for the tailoring of π-conjugated systems.5 To date, the
most widely used phosphorus building block is phosphole A
† National Taiwan University.
‡ Universite´ de Rennes 1.
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R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.;
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Materials and Polymers; John Wiley and Sons: New York, 1997. (e)
Roncali, J. Chem. ReV. 1997, 97, 173-206.
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9
10.1021/ja0567182 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 983-995
983

A R T I C L E S
Su et al.
Table 1. Photophysical Data for Benzophosphole Derivatives 2, 3,
and 4
Figure 1. Organophosphorus moieties used to construct π-conjugated
systems.
(Figure 1). However, before 2000 few oligomers6a and polymers6b
featuring this P unit were known. In recent years, the family of
phosphole-containing conjugated systems has been considerably
extended with the synthesis of various mixed oligomers and
polymers.7 Moieties B,8a-c C,8d,e and D8f,g leading to P analogues
of poly(aniline) and poly(para-phenylene-vinylene), respectively,
have also been recently investigated.
a
b
b
c
Td10
C)
λmax
λem
φfb
Ep1
/2
(
°
(nm)
log
ꢀ
(nm)
(%)
(eV)
2
3
4
213
220
220
332
330
333
2.9
2.9
3.0
366
366
366
4.2
0.2
13.4
-1.93
-1.92
-1.80d
a TGA, 10% weight loss. b Measured in CH2Cl2, fluorescence quantum
yield relative to quinine sulfate. c In CH2Cl2, referenced to SCE. Epc.
d
The structural diversity of these building blocks illustrates
the impact of phosphorus chemistry for the construction of
conjugated macromolecules. Furthermore, one appealing prop-
erty of these P building blocks is that they possess reactive
phosphorus centers potentially allowing direct access to a range
of novel π-conjugated systems. This possibility has been fully
exploited for phosphole-based derivatives. We have shown that
chemical modifications of the phosphorus atoms allow a fine-
tuning of the optical and electrochemical properties of phos-
phole-based π-conjugated systems.7a-f In other words, starting
from one phosphole-containing π-conjugated system, it is
possible to readily access a family of derivatives with different
physical characteristics. The next key challenge was to show
that this unique possibility could be used to tailor conjugated
materials for optoelectronic functions. For this purpose, we have
selected OLEDs because this type of device demands multi-
functional properties from a single organic material (thermal
stability, luminescence efficiency, hole and electron injection,
and transport capabilities...), which are difficult to optimize. In
this paper, we report a full detailed study on the synthesis, use,
and tailoring of phosphole-based oligomers and of their gold
complexes as advanced materials for single- and multilayer
OLEDs.9 The conceptual design and specific properties that
directly result from the presence of the P atom will be illustrated.
We will show that exploiting the reactivity of the P atom is a
key to tuning and to optimizing the material performances and
also to avoiding or to inducing the aggregation of these
P-chomophores in the solid state.
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Results and Discussion
Physical Properties of Dibenzophosphole Derivatives. We
started this study using the long-standing and readily available
dibenzophosphole derivatives 1-4 (Table 1),10 although these
derivatives do not display the usual properties of phospholes.
In fact, due to their benzo-annulated structure, no significant
interaction of the endocyclic π-system with the P moiety occurs
and they behave as nonflexible arylphosphines.10 For example,
while pentasubstituted σ3,λ3-diarylphospholes are air-stable
derivatives (vide infra),7 σ3,λ3-dibenzophosphole 1 gives the
corresponding oxide 2 upon exposure to air in the solid state.
Its oxygen sensitivity precludes 1 from being used as a material
for optoelectronic applications. Quite surprisingly, to the best
of our knowledge, no physical data for compounds 2-4 are
reported in the literature, although the optical properties of
dibenzophosphole-aryl copolymers were recently described.11
Derivatives 2-4 exhibit good thermal stabilities as estimated
by thermogravimetric analysis (TGA) performed under nitrogen
(Table 1). This parameter is important because low-molecular-
weight species are deposited as thin films by vacuum evapora-
tion for the fabrication of OLEDs. Furthermore, the thermal
(9) For a preliminary communication, see: Fave, C.; Cho, T.-Y.; Hissler, M.;
Chen, C.-W.; Luh, T.-Y.; Wu, C.-C.; Re´au, R. J. Am. Chem. Soc. 2003,
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Chemistry: The Rise of a New Domain; Mathey, F., Ed.; Elsevier Science
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Attar, S.; Bearden, W. H.; Alcock, N. W.; Alyea, E. C.; Nelson, J. H. Inorg.
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984 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Design of Oligomers for Electroluminescent Devices
A R T I C L E S
Figure 2. Structures of multilayer OLED constituent materials: TCTA (HTM), TPBI (ETM), R-NPD (HTM), Alq3 (ETM), and DCJTB (dopant).
instability of organic materials is considered to be a major cause
of the poor high-temperature operating stability of OLEDs.2,12
Cyclic voltammetry (CV) performed at 200 mV s-1 in CH2Cl2
solution revealed reduction processes occurring at similar
potentials for the three compounds (Table 1). The reduction
processes are reversible for compounds 2 and 3 and irreversible
for the gold complex 4. No oxidation peak was observed under
these experimental conditions. The UV-vis spectra of diben-
zophospholes 2-4 are similar; they show weak absorptions
around 330 nm (Table 1). These absorptions in the UV region
reveal that these derivatives possess large “optical” HOMO-
LUMO gaps, as observed for structurally related fluorenes13 and
fused dithienophospholes.7h,i This feature is in accordance with
the view that dibenzophospholes have to be considered as
P-bridged biphenyl derivatives. The three dibenzophosphole
derivatives 2-4 are photoluminescent and emit in the UV region
at similar wavelengths (Table 1). Note that the Stokes shifts
are relatively small (ca. 32-36 nm), suggesting minimal
rearrangement of these rigid molecules upon photoexcitation.
The quantum yields depend on the nature of the P-modification,
the gold(I) complex 4 exhibiting the highest value in solution
(Table 1). In marked contrast, in the solid state only photo-
luminescence (PL) from the oxodibenzophosphole 2 can be
detected (λem ) 370 nm). This result prompted us to investigate
only this compound as a material for the fabrication of OLEDs.
OLEDs Incorporating Dibenzophosphole Derivatives. The
devices employed in this study have the typical structure of
organic layers sandwiched between the bilayer anode [indium-
tin oxide/poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate,
ITO/PEDOT-PSS] and the cathode. The low-molecular-weight
organic layers were deposited by thermal evaporation under high
vacuum.14 The configuration of the single-layer OLEDs is ITO/
PEDOT:PSS(25 nm)/organic layer(100 nm)/Mg:Ag(80 nm)/
Ag(150 nm). Using 2 as the organic layer, light emission was
observed for a turn-on voltage of 4 V. However, the device is
unstable and its electroluminescence characteristics (emission
wavelength, brightness...) vary rapidly with applied driving
current. Derivative 2 was also tested in two-layer OLEDs having
the configuration of ITO/PEDOT/TCTA/2/Mg:Ag/Ag and ITO/
PEDOT/2/TPBI/Mg:Ag/Ag, respectively. In the former hetero-
structure device, 2 is used in combination with the known large-
gap hole-transport material (HTM) TCTA (Figure 2), to check
whether 2 can serve as an electron-transport and emitting layer,
while in the latter device, 2 is used in combination with the
known large-gap electron-transport material (ETM) TPBI
(Figure 2), to test whether 2 can serve as a hole-transport and
emitting layer. Here also, the device characteristics changed
rapidly with applied driving current. These results show that
dibenzophospholes are not suitable derivatives for the construc-
tion of OLEDs and prompted us to study another series of
P-containing oligomers incorporating phosphole units.
Synthesis and Physical Properties of Phosphole Deriva-
tives. Phospholes are interesting building blocks for the tailoring
of π-conjugated oligomers because they possess properties that
are markedly different from those of thiophenes and pyrroles.5,10
The tricoordinate phosphorus atom of phospholes has a pyra-
midal geometry with a lone pair having pronounced s-character.
These geometric and electronic features prevent an efficient
endocyclic conjugation of the electron sextet, and, as a
consequence, the phosphole ring exhibits a low aromatic
character and a reactive heteroatom.10a,b,15 These properties are
of particular interest for the construction of π-conjugated
systems with low HOMO-LUMO separation7 because conjuga-
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811. (h) Wu, F.-I.; Reddy, D. S.; Shu, C.-F.; Liu, M. S.; Jen, A. K.-Y.
Chem. Mater. 2003, 15, 269-274. (i) Shu, C.-F.; Dodda, R.; Wu, F.-I.;
Liu, M. S.; Jen, A. K.-Y. Macromolecules 2003, 36, 6698-6703.
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Appl. Phys. Lett. 2001, 79, 3023-3025.
(15) (a) Nyulaszi, L. Chem. ReV. 2001, 101, 1229-1246. (b) Mattmann, E.;
Mathey, F.; Sevin, A.; Frisson, G. J. Org. Chem. 2002, 67, 1208-1213.
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Org. Chem. 2000, 65, 2631-2636. (d) Delaere, D.; Nguyen, M. T.;
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A R T I C L E S
Su et al.
Scheme 1. General Synthetic Route to 2,5-Diarylphospholes, Their Thiooxo Derivatives, and Gold Complexes, and Structures of the
Studied P Compounds
Table 2. Photophysical and Electrochemical Data for Derivatives 6, 7a,b, 8a, 9a, and 10a,b
a
b
b
d
d
f
f
Td10
C)
λmax
λem
φfc
τb
(ns)
λmax
λem
φfe
τd
(ns)
Epc
Epa
(
°
(nm)
log
ꢀ
(nm)
(%)
(nm)
(nm)
(%)
(eV)
(eV)
6
7a
7b
8a
9a
306
210
253
287
252
251
218
395
380
432
396
412
358
428
4.36
4.13
3.98
3.90
3.44
3.80
4.10
497
516
548
527
542
506
544
15.2
0.92
0.025
0.41
0.024
0.16
0.52
1.83
410
345
435
397
410
360
440
520
517
553
526
545
505/630
550/690
3.6
0.5
6.6
0.8
4.2
0.30
0.38
1.33
0.38
0.52
0.94/5.20
0.25/1.54
-1.95
-1.74
-1.55
-1.70
-1.68
-1.75
-1.35
+1.04
+1.52
+1.08
+1.45
+1.31
+1.85
+1.22
0.05g
4.6
0.37
2.0
7.3
10a
10b
11.2
0.6
12.9
a TGA, 10% weight loss. b In CH2Cl2. c Absolute fluorescence quantum yield in CH2Cl2. d In neat film. e Absolute fluorescence quantum yield in neat
film. f In CH2Cl2, referenced to SCE. g The QY is too low to be measured upon excitation at 442 nm with HeCd Laser; this value was obtained in CH2Cl2
solution relative to quinine sulfate.
tion is enhanced for macromolecules composed of units exhibit-
ing low resonance energies.1e,15c-f However, the most appealing
property of the phosphole ring for tailoring molecular materials
is the possibility of fine-tuning the physical characteristics (redox
potentials, absorption and emission wavelengths...) of phosphole-
containing conjugated systems by simple chemical modification
(oxidation, complexation...) of the P centers.5,7 Note that the
tuning of the electron affinity (EA) and ionization potential (IP)
is of great interest to optimize the energy differences between
the LUMO and HOMO levels and work functions of cathodes
and anodes, respectively, to favor hole and electron injection
into OLEDs.
The first property to be considered is the thermal stability of
phosphole-based oligomers, because the OLED fabrication
process involves thermal evaporation of these low-molecular-
weight oligomers. Derivatives 5a,b (Scheme 1) featuring σ3,λ3-
phosphole rings possess rather low decomposition temperatures,7c
which preclude their deposition by vacuum sublimation. It is
likely that the thermal instability of derivatives 5a,b is due to
the isomerization of these 1H-phospholes into 2H-phospholes,
which are extremely reactive species.16 Two strategies were
investigated to overcome the thermal instability of σ3,λ3-
phospholes. The first approach, which is a “trial and error” one,
consists of varying the ring substituents at the 2,5-positions. A
systematic study involving the synthesis of compounds bearing
substituted phenyl, thienyl, and pyridyl moieties revealed that
the mixed σ3-derivative 6 (Scheme 1) has a high decomposition
temperature (306 °C). The second approach involves the
chemical modification of the P atom, because it is known that
the 1H-phosphole f 2H-phosphole isomerization process is less
favored for σ4-phospholes.16b Indeed, the decomposition tem-
peratures of σ4-thiooxophospholes 7a and 7b (Scheme 1) are
increased by 86 and 48 °C, respectively, in comparison to their
σ3-phosphole precursors 5a and 5b. These results prompted us
to varying the substitution pattern of σ4-thiooxophospholes to
extend the family of these thermally stable compounds. The
molecular design for the new σ4-thiooxo(2-phenyl)phospholes
relies on the introduction of 4-biphenyl or 4-stilbenyl substit-
uents at the 5-position of the P-ring with the view to vary the
conjugated path. Indeed, these compounds 8a and 9a (Scheme
1) also have rather high decomposition temperatures as estimated
by TGA (Table 2). The synthetic route to the newly synthesized
derivatives employs the one-pot “Fagan-Nugent method”17
involving the oxidative coupling of functionalized octa-1,7-
diynes with zirconocene followed by treatment with PhPBr2
(17) (a) Fagan, P. J.; Nugent, W. A. J. Am. Chem. Soc. 1988, 110, 2310-2312.
(b) Fagan, P. J.; Nugent, W. A.; Calabrese, J. C. J. Am. Chem. Soc. 1994,
116, 1880-1889. (c) Sava, X.; Me´zaille, N.; Maigrot, N.; Nief, F.; Ricard,
L.; Mathey, F.; Le Floch, P. Organometallics 1999, 18, 4205-4215. (d)
Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124-130.
(16) (a) Mathey, F. Acc. Chem. Res. 2004, 37, 954-960. (b) Charrier, C.;
Bonnard, H.; De Lauzon, G.; Mathey, F. J. Am. Chem. Soc. 1983, 105,
6871-6877.
9
986 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Design of Oligomers for Electroluminescent Devices
A R T I C L E S
Table 3. Selected Bond Lengths [Å] and Twist Angles between the Phosphole Ring and Its 2- and 5-Substituents [deg] for
Thiooxophospholes 7a, 8a, and 9a (Y ) S) and Complexes 10a,b (Y ) Au-Cl)
7a (Y
)
S)
8a (Y
)
S)
9a (Y
)
S)
10a (Y
)
AuCl)
10b (Y ) AuCl)
P-C(1)
1.816(2)
1.346(3)
1.493(3)
1.345(3)
1.806(2)
1.809(2)
1.364(3)
1.489(3)
1.354(3)
1.809(2)
1.812(2)
1.351(3)
1.494(3)
1.353(3)
1.8109(18)
1.801(6)
1.350(9)
1.484(9)
1.346(9)
1.802(6)
2.229(1)
2.284(1)
93.3(3)
110.4(3)
114.4(2)
105.8(3)
116.7(2)
114.2(2)
177.64(7)
47.3/54.7
1.817(6)
1.358(9)
1.483(9)
1.359(6)
1.806(3)
2.2300(16)
2.2895(19)
93.2(3)
105.0(3)
121.30(18)
107.3(3)
C(1)-C(2)
C(2)-C(7)
C(7)-C(8)
C(8)-P
P-Au
Au-Cl
C(1)-P-C(8)
C(1)-P-C(27)
C(1)-P-Y
C(8)-P-C(27)
C(8)-P-Y
C(27)-P-Y
P-Au-Cl
twist angles
93.38(8)
105.09(8)
119.54(6)
107.95(8)
115.27(6)
113.40(6)
93.67(11)
102.71(10)
119.13(8)
110.21(10)
116.31(8)
112.70(8)
92.97(8)
106.17(8)
118.24(7)
111.87(9)
112.78(6)
113.12(7)
116.48(19)
111.48(19)
174.63(8)
3.8/13.0
50.3/50.5
36.3/35.9
47.2/44.6
Table 4. Crystallographic Data and Structural Refinement Details for 7a, 8a, 9a, 10a, and 10b
7a
8a
9a
10a
10b
molecular formula
molecular weight, g/mol
C26H23PS
398.47
C32H27PS
474.57
C34H29PS
500.60
C26H23AuClP
641.29
C22H19AuClPS2
610.88
a, Å
b, Å
c, Å
9.634(5)
15.725(5)
14.360(5)
90
93.621(5)
90
2171.1(15)
4
1.219
9.907(5)
10.282(5)
12.655(5)
90
105.568(5)
90
1241.8(10)
2
1.269
9.256(5)
12.399(5)
13.325(5)
95.791(5)
109.093(5)
108.521(5)
1334.0(10)
2
10.377(5)
17.347(5)
13.471(5)
90
91.769(5)
90
2423.8(16)
4
1.757
9.0350(10)
9.542(3)
14.598(2)
71.62(2)
78.250(10)
63.25(2)
1063.7(4)
2
R, deg
â, deg
γ, deg
V, Å3
Z
D
calc, g cm-3
1.246
1.907
crystal system
space group
monoclinic
P21/n
monoclinic
P21
triclinic
P-1
monoclinic
P21/n
triclinic
P-1
T, K
293(2)
293(2)
293(2)
293(2)
293(2)
wavelength Mo KR, Å
0.71069
0.231
840
0.71069
0.214
500
0.71069
0.203
528
0.71069
6.368
1244
0.71069
0.732
588
µ, mm-1
F(000)
θ limit, deg
2.93-27.54
2.33-27.12
3.27-27.49
2.44-30.03
1.47-26.94
no. reflns collected
no. ind reflns
reflections [I > 2σ(I)]
data/restraints/parameters
GOF on F2
9750
5273
10 446
12 868
4887
4985
3877
4985/0/254
1.052
5273
4735
5273/1/308
1.077
6083
4163
60 837/0/326
1.025
7066
5271
7066/0/285
1.032
4593
3785
4593/0/245
1.075
final R indices
[I > 2σ(I)]
R indices (all data)
R1 ) 0.0456
wR2 ) 0.1231
R1 ) 0.0616
wR2 ) 0.1361
0.241 and -0.298
R1 ) 0.0424
wR2 ) 0.1092
R1 ) 0.0495
wR2 ) 0.1165
0.311 and -0.316
R1 ) 0.0467
wR2 ) 0.1158
R1 ) 0.0783
wR2 ) 0.1334
0.381 and -0.267
R1 ) 0.0461
wR2 ) 0.1074
R1 ) 0.0701
wR2 ) 0.1189
1.483 and -1.428
R1 ) 0.0356
wR2 ) 0.0914
R1 ) 0.0541
wR2 ) 0.0972
1.187 and -1.148
largest diff peak
and hole (e Å-3
)
(Scheme 1). It is worth noting that this synthetic pathway is
rather simple and efficient, allowing gram-scale preparations,
because the functionalized octa-1,7-diynes are obtained in good
yields by Sonogashira couplings. The thiooxophospholes are
readily prepared by treatment of the corresponding phosphole
with elemental sulfur (Scheme 1). Derivatives 6 and 7a-9a are
air-stable compounds that can be easily purified by column
chromatography. They are readily soluble in common organic
solvents such as CH2Cl2, THF, or CH3CN. They exhibit classical
NMR data, and their structures were supported by high-
resolution mass spectrometry and elemental analysis. σ4-
Thiooxophospholes 8a and 9a, as well as the related 2,5-
diphenylphosphole 7a, were subjected to X-ray diffraction study
to elucidate their molecular structures and bulk molecular-
packing characteristics (Tables 3 and 4, Figures 3-5). The bond
lengths and valence angles of the P-ring of 8a and 9a are
typical7c and compare with those of 7a (Table 3). In these three
compounds, the P atom exhibits a distorted tetrahedral geometry
(Figures 3-5) due to the small endocyclic C(1)-P-C(8) angles
[7a, 93.38°(8); 8a, 93.67°(11); 9a, 92.97°(8)]. Of particular
interest, these crystal structures show that the diarylphospholyl
moieties do not have a planar conformation. The rotational
disorder depends on the substitution pattern of the σ4-thiooxo-
phospholes. The twist angles between the phosphole ring and
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 987

A R T I C L E S
Su et al.
Figure 3. Perspective view of the packing of 2,5-diphenylthiooxophosphole 7a in crystalline phase. H atoms are omitted for clarity.
Figure 4. Perspective view of the packing of 2-phenyl-5-biphenyllthiooxophosphole 8a in crystalline phase. H atoms are omitted for clarity.
the aromatic substituents are similar for 2,5-diphenylphosphole
7a (50.3° and 50.5°) and 2-phenyl-stylbenylphosphole 9a (44.6°
and 47.2°), but much smaller for 2-biphenyl-5-phenylphosphole
8a (35.9° and 36.3°). The lower rotational disorder observed
for derivative 8a should favor the delocalization of the π-system
because the orbital overlap varies approximately with the cosine
of the twist angle. This is supported by the fact that the inter-
ring phosphole-phenyl distances are slightly smaller for deriva-
tive 8a [1.470(3)-1.473(3) Å] than for the highly rotationally
disordered compounds 7a [1.479(2)-1.480(2) Å] and 9a
[1.479(2)-1.481(2) Å]. Note that the twist angle (23.0°) and
the inter-ring distance [1.484(3) Å] of the biphenyl moiety of
8a are classical and that the stilbenyl fragment of 9a is almost
planar. The above results indicate that σ4-thiooxophospholes
have a three-dimensional nonplanar molecular structure. These
structural features are of particular interest because they could
reduce the intermolecular interaction and the likelihood of
excimer formation in solid films (vide infra).
The photophysical and electrochemical properties of these
derivatives are presented in Table 2. The published optical data
for 7a,b7b have been completed by lifetime measurements of
excited states and studies of thin films deposited by vacuum
sublimation. It is noteworthy that 2,5-diphenylthiooxophosphole
7a exhibits properties that are significantly different from those
of the corresponding dibenzophosphole 3. The Stokes shift of
the less rigid compound 7a is much higher than that of 3 (7a,
167 nm; 3, 36 nm), and thiooxophosphole 7a is more easily
reduced and oxidized than 3 (Tables 1 and 2). These physical
9
988 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Design of Oligomers for Electroluminescent Devices
A R T I C L E S
Figure 5. Perspective view of the packing of 2-phenyl-5-stilbenylthiooxophosphole 9a in crystalline phase. H atoms are omitted for clarity.
data nicely illustrate the profound difference in electronic
structures between 2,5-diarylphospholes and dibenzophospholes.
All phosphole derivatives exhibit an intense band in the UV-
visible region attributed to the π-π* transitions of the conju-
gated system. Upon increasing the length of the conjugated path
of σ4-thiooxophospholes (7a/8a/9a, Scheme 1), the λmax is red-
shifted, indicating a decrease in the optical HOMO-LUMO
energy separations (Table 2). Only a single emission peak was
observed for the three compounds with Stokes shifts ranging
from 75 to 165 nm (Table 2). The lifetimes (τ) of excited σ3-
and σ4-phosphole derivatives are in the nanosecond range,
suggesting fluorescence processes involving emission from
Figure 6. UV-vis and fluorescence spectra of 7b in CH2Cl2 solution and
singlet excited states (Table 2). The absolute quantum yields
in thin film.
in solution measured with a calibrated integrating sphere
and the nonplanarity of the diarylphosphole moiety, which
system18 are rather modest and depend on the structure of the
preclude a close cofacial organization of these P chromophores
phospholes (Table 2), the most efficient fluorophore being the
in the solid state. This hypothesis is supported by two facts.
σ3-phosphole 6 (φf ) 15.2%). This behavior is remarkable
First, the UV-vis and fluorescence spectra of σ4-thiooxophos-
pholes in solution and in thin film are similar (Table 2); no
red-shifted emission revealing excimer or aggregate formation
is observed (Figure 6). Second, although care must be taken
when comparing different states of matter (amorphous vs
crystalline, vide infra), the packing of σ4-thiooxophospholes 7a,
8a, and 9a in monocrystals clearly shows the site isolation of
these P chromophores (Figures 3-5). The steric protection
provided by the substituents of the tetrahedral P atoms can be
seen plainly in these figures. Such rare enhancement of
fluorescence in the solid state was also reported for phenyl-
substituted siloles20a-d and is attributed to a restriction in the
solid state of the rotation of the 2,5-phenyl substituents, which
because chromophores featuring σ3-P centers usually exhibit
almost no fluorescence as a result of quenching by the P lone
pair.19 This result nicely illustrates the unique properties of
phospholes as compared to usual phosphines. It is interesting
to note that, while the absolute quantum yield of 6 decreases in
the solid state (φf ) 3.6%), the quantum yields18 for all σ4-
thiooxophospholes are higher in the solid state than in dilute
solution (Table 2). This behavior is surprising because photo-
luminescent organic chromophores are usually less emissive in
the solid state than in solution due to molecular aggregation
forming less emissive species such as excimers.20 It is likely
that this rather unusual behavior is due to the steric protection
provided by the substituents (Ph, S) of the tetrahedral P atom
(20) (a) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qui, C.; Kwok,
H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740-
1741. (b) Chen, J.; Xie, Z.; Lam, J. W. Y.; Tang, B. Z. Macromolecules
2003, 36, 1108-1117. (c) Ren, Y.; Lam, J. W. Y.; Dong, Y.; Tang, B. Z.;
Wong, K. S. J. Phys. Chem. 2005, 109, 1135-1140. (d) Chen, J.; Xu, B.;
Cao, Y.; Sung, H. H. Y.; Williams, I. D.; Tang, B. Z. J. Phys. Chem. 2005,
109, 17086-17093. (e) Levitus, M.; Schmieder, K.; Ricks, H.; Shimizu,
K. D.; Bunz, U. H. F.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2001,
123, 4259-4265. (f) McQuade, D. T.; Kim, J.; Swager, T. M. J. Am. Chem.
Soc. 2000, 122, 5885-5886. (g) Levitsky, I. A.; Kishikawa, K.; Eichhorn,
S. H.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 2474-2479. (h) Yeh,
H.-C.; Yeh, S.-H.; Chen, C.-T. Chem. Commun. 2003, 2632-2633.
(18) (a) Wu, C.-C.; Hung, W.-Y.; Liu, T.-L.; Zhang, L.-Z.; Luh, T.-Y. J. Appl.
Phys. 2003, 93, 5465-5471. (b) Chao, T.-C.; Lin, Y.-T.; Chang, C.-Y.;
Hung, T.-S.; Chou, H.-C.; Wu, C.-C.; Wong, K.-T. AdV. Mater. 2005, 17,
992-996.
(19) (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Organomet. Chem. 2002,
646, 277-281. (b) Wang, Y.; Ranasinghe, M. I.; Goodson, T. J. Am. Chem.
Soc. 2003, 125, 9562-9563. (c) Smith, R. C.; Protasiewicz, J. D. J. Chem.
Soc., Dalton Trans. 2000, 4738-4741. (d) Smith, R. C.; Earl, M. J.;
Protasiewicz, J. D. Inorg. Chem. Acta 2004, 357, 4139-4143. (e) Kang,
Y.; Song, D.; Schmider, H.; Wang, S. Organometallics 2002, 21, 2413-
2421.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 989

A R T I C L E S
Su et al.
Figure 7. Perspective view of the packing of gold complex 10a in crystalline phase. H atoms are omitted for clarity.
makes a large contribution to the nonradiative transition process
in solution. It is likely that a similar explanation can be invoked
for phosphole-based fluorophores. Overall, these results nicely
illustrate that the use of novel heterocyclic building blocks
(siloles, phospholes...) can induce rather unusual photophysical
properties.
It is interesting to note that the P chromophores depicted in
Scheme 1 have different physical properties depending on their
molecular structure. For example, their maximum emission
wavelengths (λem) in the solid state range from 517 to 553 nm,
and both their oxidation and their reduction potentials vary by
0.4-0.5 eV (Table 2). This tuning is promising for OLED
development because it would allow the emission color of the
devices to be variant and also to reach a good match between
the LUMO and HOMO levels and the work functions of
electrodes. Unfortunately, the oxidation and reduction potentials
were irreversible, so we cannot estimate their HOMO and
LUMO energies reliably.
The results presented above show that the tuning of the
electronic properties of phosphole-based oligomers by chemical
modifications of the P center is a key point toward the
development of these novel π-conjugated materials for OLEDs.
To further exploit this unique capability, metal complexes were
investigated, because phospholes act as 2-electron donors toward
a wide range of transition metals.21 In a first approach, Au(I)
complexes were selected for two main reasons. First, despite
the numerous studies on electroluminescent metal complexes,22
OLEDs based on luminescent Au(I) complexes have few
precedents in the literature.23 Second, Au(I) complexes have a
strong tendency to form metal-metal interactions (aurophilicity)
arising from correlation effects that are enhanced by a relativistic
effect.24 Au(I) complexes can thus self-associate in the solid
state leading to luminescent aggregates.25 Note that lumino-
phores that can form aggregates with a broad spectral emission
are of great interest for the development of white organic light-
emitting diodes (WOLEDs),2e,25 a promising device for lighting
applications.2e
Phospholes 5a and 5b reacting in CH2Cl2 solution at room
temperature with (tetrahydrothiophene)AuCl gave rise to the
corresponding complexes 10a and 10b (Scheme 1), which were
isolated as air-stable powders in 77% and 90% yield, respec-
tively. Coordination of the P atom to the gold center is evidenced
by the downfield 31P NMR coordination shift (∆δ ≈ 26 ppm).
1
For both complexes, the simplicity of the H and 13C NMR
spectra favors a symmetric structure. These NMR data discount
a chelating P,S-coordination mode for ligand 5b. The proposed
structures were confirmed by X-ray diffraction studies (Tables
3 and 4, Figures 7 and 8). The bond lengths within the phosphole
moiety are similar for both complexes and compared to those
of the thiooxophosphole derivatives 7a and 9a (Table 3). As
expected, the P atoms are tetrahedral [∑(CPC angles): 10a,
309.5°; 10b, 305.5°], and the two-coordinate Au(I) atoms have
an almost linear geometry [P-Au-Cl angles: 10a, 171.64°;
10b, 174.63°]. This slight deviation from ideal linearity is typical
(21) (a) Leca, F.; Sauthier, M.; Deborde, V.; Toupet, L.; Re´au, R. Chem.-Eur.
J. 2003, 9, 3785-3795. (b) Leca, F.; Lescop, C.; Rodriguez, E.; Costuas,
K.; Halet, J.-F.; Re´au, R. Angew. Chem., Int. Ed. 2005, 44, 4362-4365.
(c) Leca, F.; Sauthier, M.; Le Guennic, B.; Lescop, C.; Toupet, L.; Halet,
J.-F.; Re´au, R. Chem. Commun. 2003, 1774-1775. (d) Leca, F.; Lescop,
C.; Re´au, R. Organometallics 2004, 23, 6191-6201. (e) Sauthier, M.; Leca,
F.; Toupet, L.; Re´au, R. Organometallics 2002, 21, 1591-1602. (f) Hay,
C.; Sauthier, M.; Deborde, V.; Hissler, M.; Toupet, L.; Re´au, R. J.
Organomet. Chem. 2002, 643-644, 494-497. (g) Caballero, A.; Jalon, F.
A.; Manzano, B. R.; Sauthier, M.; Toupet, L.; Re´au, R. J. Organomet.
Chem. 2002, 663, 118-126. (h) Sauthier, M.; Le Guennic, B.; Deborde,
V.; Toupet, L.; Halet, J.-F.; Re´au, R. J. Organomet. Chem. 2001, 40, 228-
231.
(22) For recent examples, see: (a) Lu, W.; Mi, B. X.; Chan, M. C. W.; Hui, Z.;
Che, M.; Zhu, N.; Lee, S. T. J. Am. Chem. Soc. 2004, 126, 4958-4971.
(b) Lin, W.; Jen, A. K. Y.; Niu, U. H.; Zhang, L. Chem. Commun. 2005,
1002-1004. (c) Liu, C. Y.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 4190-
4191. (d) Welter, S.; Krunner, K.; Hofstraat, J. W.; De Cola, L. Nature
2003, 421, 54-57. (e) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov,
A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-
154.
(23) For Au(I) complex-based OLEDs, see: (a) Ma, Y.; Zhou, X.; Shen, J.;
Chao, H.-Y.; Che, C.-M. Appl. Phys. Lett. 1999, 74, 1361-1363. For Au-
(III) complex-based OLEDs, see: (b) Wong, K. M.-C.; Zhu, X.; Hung,
L.-L.; Zhu, N.; Yam, V. W. W.; Kwok, H.-S. Chem. Commun. 2005, 2906-
2908.
9
990 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Design of Oligomers for Electroluminescent Devices
A R T I C L E S
Figure 8. Perspective view of the packing of gold complex 10b in crystalline phase. H atoms are omitted for clarity.
of the sp hybridization of gold(I).24m The Au-P [2.2290(16),
2.2300(16) Å] and Au-Cl [2.2638(18), 2.2895(19) Å] distances
are usual for two-coordinated gold(I) complexes.23m Two
structural features are noteworthy. First, the diphenylphosphole
moiety of complex 10a exhibits an important rotational disorder
(twist angles, 43.3-54.5°), whereas the three heterocycles of
the di(2-thienyl)phosphole moiety of complex 10b are almost
coplanar (twist angles, 3.8-13.0°). Basically, these properties
can increase the degree of conjugation in the thienyl-capped
derivative 10b as compared to 10a. Second, the shortest distance
between two Au(I) centers is 8.7 Å for 10a and 9.0 Å for 10b,
and the interchromophore spacing is rather large (>5.2 Å).
Figure 9. UV-vis and emission spectra of 10a and 10b in solution (CH2-
These large separation values clearly indicate the absence of
any aurophilic or π-π interactions in the crystal lattice (Figures
7 and 8).
Cl2) and in thin film.
In CH2Cl2 solution, complexes 10a and 10b have a maximum
absorption in the visible region, and, as observed in the
thiooxophosphole series, the value for the thienyl-capped
complex 10b is 70 nm red shifted as compared to that of its
phenyl analogue 10a (Table 2). On excitation at their λmax, these
gold complexes display one broad emission band in the greenish
region (10a, 506 nm; 10b, 544 nm, Figure 9) with absolute
quantum yields varying from 7.3% (10a) to 12.9% (10b). The
Stokes shifts (126-146 nm) lie within the range of those of
thiooxophospholes (116-167 nm), and the nanosecond lifetimes
of the excited state indicate that the emission is due to
fluorescence (Table 2). These data strongly suggest that the
absorption and emission of gold complexes 10a,b in solution
are associated with metal-perturbed intraligand (IL) π-π*
transitions of the 2,5-diarylphosphole ligands. It is noteworthy
that the quantum yields of complexes 10a,b are much higher
than those of the corresponding thiooxophospholes 7a,b (Table
2), illustrating that the nature of the P modification has a
considerable impact on the optical properties of these fluoro-
phores.
(24) (a) Schmidbaur, H. Gold Bull. 1990, 23, 11-21 (b) Scherbaum, F.;
Grohmann, A.; Huber, B.; Kru¨ger, C.; Schmidbaur, H. Angew. Chem., Int.
Ed. Engl. 1988, 27, 1544-1546. (c) Pyykko¨, P. Chem. ReV. 1997, 97, 597-
636. (d) Jiang, Y.; Alvarez, S.; Hoffmann, R. Inorg. Chem. 1985, 24, 749-
757. (e) Mingos, D. M. P. J. Chem. Soc., Dalton Trans. 1976, 1163-
1169. (f) Harwell, D. E.; Mortimer, M. D.; Knobler, C. B.; Anet, F. A. L.;
Hawthorne, M. F. J. Am. Chem. Soc. 1996, 118, 2679-2685. (g)
Narayanaswany, R.; Young, M. A.; Parkhurst, E.; Ouellette, M.; Kerr, M.
E.; Ho, D. M.; Elder, R. C.; Bruce, A. E.; Bruce, M. R. M. Inorg. Chem.
1993, 32, 2506-2517. (h) Zhang, H. X.; Che, C. M. Chem.-Eur. J. 2001,
7, 4887-4893. (i) Assefa, Z.; McBurnett, B. G.; Staples, R. J.; Fackler, J.
P., Jr.; Assmann, B.; Angermaier, K.; Schmidbaur, H. Inorg. Chem. 1995,
34, 75-83. (j) Jones, W. B.; Yuan, J.; Narayanaswamy, R.; Young, M.
A.; Elder, R. C.; Bruce, A. E.; Bruce, M. R. M. Inorg. Chem. 1995, 34,
1996-2001. (k) Forward, J. M.; Bohmann, D.; Fackler, J. P., Jr.; Staples,
R. J. Inorg. Chem. 1995, 34, 6330-6336. (l) Hanna, S. D.; Zink, J. I. Inorg.
Chem. 1996, 35, 297-302. (m) Li, C.-K.; Lu, X.-X.; Wong, K. M.-C.;
Chan, C.-L.; Zhu, N.; Yam, V. W. W. Inorg. Chem. 2004, 43, 7421-
7430. (n) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412-4456. (o)
Yam, V. W. W.; Lo, K. K. W.; Fung, W. K. M.; Wang, C. R. Coord.
Chem. ReV. 1998, 171, 17-41.
(25) (a) D’Andrade, B. W.; Brooks, J.; Adamovitch, V.; Thompson, M. E.;
Forrest, S. R. AdV. Mater. 2002, 14, 1032-1036. (b) Adamovitch, V.;
Brooks, J.; Tamayo, A.; Alexander, A. M.; Dlurovitch, P. I.; D’Andrade,
B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. New J. Chem. 2002,
26, 1171-1178. (c) Feng, J.; Li, F.; Gao, W.; Liu, S.; Liu, Y.; Wang, Y.
Appl. Phys. Lett. 2001, 78, 3947-3949. (d) D’Andrade, B. W.; Thompson,
M. E.; Forrest, S. R. AdV. Mater. 2001, 14, 147-151. (e) Wang, L.; Lei,
G.; Qiu, Y. J. Appl. Phys. 2005, 97, 114503-114508. (f) Yan, S.; Yang,
Y. Appl. Phys. Lett. 2005, 86, 7351-7353.
Gold complexes 10a,b are thermally stable enough (Td10
g
218 °C, Table 2) to give thin films upon vacuum sublimation.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 991

A R T I C L E S
Su et al.
aurophilic nor π-π interaction is observed in the solid-state
crystal structures of complexes 10a,b (Figures 7 and 8).
However, the films obtained by vacuum deposition do not
exhibit long-range ordering, as revealed by the broad powder
X-ray diffraction signal of complex 10b. These data indicate
that the bulk organization of the gold-coordinated phospholes
depends on the conditions of solid formation and that crystalline
structures are not always good models for amorphous thin films
obtained by vacuum sublimation.20d,26 Whatever the driving
force for aggregate formation (aurophilic or/and π-π inter-
actions), thin films of complexes 10a,b cover a broad range of
the visible spectrum due to dual monomeric and aggregate
emissions, a feature which may have some interesting implica-
tions for WOLED fabrication.
Hence, fine chemical tailoring based on the variation of the
phosphole substituents and the nature of the P-modification
afforded phosphole fluorophores with appealing properties. They
are thermally stable enough to be sublimed, they form uniform
thin films by vacuum deposition, and they exhibit different
optical and electrochemical properties. The next step was
therefore to evaluate these P-based oligomers as materials for
OLEDs fabrication.
Figure 10. Absorption and excitation spectra of 10b in thin film recorded
for emission.
The absorption spectra in solution or in thin film of these
complexes are similar (Table 2, Figure 9). In marked contrast,
the solution and thin-film emission spectra of the gold complexes
are different. Two broad emission bands are observed for the
thin films, one at a wavelength similar to that of the solution
spectrum (10a, 505 nm; 10b, 550 nm), and a second that is
considerably red-shifted (10a, 630 nm; 10b, 690 nm) (Figure
9). Note that the relative intensity of these two bands depends
on the nature of the phosphole ligand. They have a comparable
intensity for the thienyl-capped derivative 10b, whereas for its
phenyl analogue 10a the long-wavelength band is far more
intense (Figure 9). The lifetimes of the red-emitting species are
longer than those of the higher-energy emitting species (Table
2), but they are still in the nanosecond range, suggesting that
both emissions are due to fluorescence. The emission bands at
shorter wavelength (10a, 505 nm; 10b, 550 nm) can reasonably
be assigned to IL π-π* transitions of site-isolated gold
complexes because their positions closely match those recorded
in dilute solution (Figure 9). The second emission band at longer
wavelengths is not due to the formation of a new emitting
species arising from chemical transformation of complexes
10a,b, since we have checked that they do not decompose or
transform upon sublimation under high vacuum (4 × 10-5 Torr).
Moreover, the excitation spectra of 10b in thin film recorded
for emission at 550 or 700 nm are similar to its absorption
spectrum (Figure 10), clearly indicating that both emissions are
due to the (phosphole)gold complexes. The second emission
can also not be attributed to vibronic couplings or aggregation-
induced planarization4i because the energy difference between
the two emissions is large (>4000 cm-1). Hence, the low-energy
luminescence bands obtained in thin film with phosphole Au-
(I) complexes 10a,b probably arise from the formation of
aggregates. This hypothesis is supported by the fact that the
low-energy emission bands are no longer observed when
complexes 10a,b are dispersed in a polymethylmetacrylate
matrix. In this discussion, the term “aggregate” is used to
describe both excited-state (excimer) and ground-state (oligo-
mers) aggregated species arising from π-π stacking and metal-
metal interactions, respectively. The high-wavelength emissions
are not due to metal-centered excited states (MC) because MCs
involving Au centers are triplet in nature with microsecond
lifetimes.24 However, it is clear that the gold centers play a key
role in the formation of these aggregates because no low-energy
luminescence bands are observed with the thiooxophosphole
derivatives 7a,b (Figure 6). It is striking to note that neither
OLEDs Incorporating Phosphole Derivatives. Derivatives
6, 7a,b, 8a, and 9a (Scheme 1) were investigated as organic
materials in single-layer OLEDs A-E (Table 5), having a ITO/
PEDOT:PSS(25/nm)/organic layer/Mg:Ag(80/nm)/Ag(150/nm)
configuration. Not surprisingly, the performances of each device
are significantly dependent on the phosphole materials em-
EL
ployed. The OLED device A displays greenish emission (λmax
) 510 nm) for a turn-on voltage of 6.0 V (Table 5). The
electroluminescence spectrum (EL) of the device resembles the
thin-film PL of 6 (Figure 11), showing that the EL emission
band is from the σ3-phosphole derivative 6. The maximum
brightness attains 1000 cd m-2, but the EL quantum yield is
low (Table 5). With relatively low turn-on voltages (2.0-3.6
V), devices B and C emit green to yellow light (λmaxEL: B, 531
nm; C, 557 nm) with a broad emission band (full width at half-
maximum of ca. 100 nm) (Table 5). The EL spectra of these
devices B and C match those of the solid-state PL spectra of
the σ4-thiooxophospholes 7a and 7b (Figure 12), respectively,
indicating similar mechanisms for both types of emission. As
expected from the thin-film study, no low-energy emission
arising from exciplex is observed. The maximum brightness
(Bmax) and external EL quantum efficiency (EEQE) are by far
higher with the thienyl-capped phosphole 7b than for its phenyl
analogue 7a (Table 5, C versus B). Devices D and E also exhibit
a single broad PL emission, which matches those of the σ4-
thiooxophosphole materials 8a and 9a, respectively (Table 5).
Here also, the turn-on voltages are rather low (2.2-3.2 V), the
B
max’s are satisfactory, and yet the EEQE’s are low (Table 5).
Overall, these results show that π-conjugated oligomers contain-
ing σ3- or σ4-phosphole moieties can be employed as multi-
functional materials in single-layer OLEDs.
One appealing property of these P-materials is that, as
expected from the data recorded in thin film (Table 2), the λem
values of the OLEDs vary with the nature of the phosphole
derivative (Table 5). Among devices A-E, the best perfor-
mances in all aspects are obtained with C including the bis(2-
(26) Van Hutten, P. F.; Krasnikov, V. V.; Hadziioannou, G. Acc. Chem. Res.
1999, 32, 257-265.
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992 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Design of Oligomers for Electroluminescent Devices
A R T I C L E S
Table 5. Electroluminescence Characteristics of Devices A-Q
B20e [cd
Bmaxf [cd
EL
b
c
d
organic layer
[nm]a
λmax
Von
[V]
V20
[V]
Vmax
[V]
2
2
g
h
i
j
device
[nm]
m-
]
m-
]
η20
0.07
ηmax
0.08
lm/W20
lm/Wmax
A
B
C
D
E
F
G
H
I
6(80)
510
531
557
528
524
510
567
550
544
549
617
6.0 14.0 16.1 150
1000
0.056
0.056
7a(60)
7b(75)
8a(50)
9a(18)
3.6
2.0
3.2
2.2
4.0
3.2
2.2
3.2
2.6
2.4
2.2
2.8 10.0
4.5 8.0
7.6 10
3.7 7.2
8.5 12.2 500
9.2 10.0
8.5 15.0 450
9.4 10
7.9 10
8.0 13.5 530
8.6 15.0 780
3.4 × 10-4
5.7 6.9 × 10-5 0.048
1.8 × 10-6 4.8 × 10-3
50
7.2
6.5
3613
0.09
76.5 0.01
0.16
0.014
0.017
0.29
0.21
0.80
0.14
0.32
1.83
3.70
0.025
0.175
0.015
0.024
0.33
0.047
0.827
0.13
0.37
1.12
1.40
0.222
0.016
0.035
0.37
0.20
1.14
0.29
0.59
1.61
6.0
692
5600
57
37 830
152
2110
0.009
0.27
0.05
0.74
0.14
0.32
1.83
2.20
R-NPD(40)/6(40)/Alq3(20)
R-NPD(40)/7a(20)/Alq3(40)
R-NPD(40)/7b(20)/Alq3(40)
R-NPD(40)/8a(20)/Alq3(40)
R-NPD(40)/9a(20)/Alq3(40)
R-NPD(40)/7b:DCJTB(20)/Alq3(40)k
R-NPD(40)/Alq3:DCJTB(20)/Alq3(40) 613
10a(71)
10b(56)
26.6
84
188
J
K
L
M
N
O
P
36 538
50 719
640 14
16.2 20.0
7.8 10.0
9.0 12.0 22.0
0.20
0.15
0.50
1.2 0.021
4.3 × 10-3 6.5 × 10-3
7.5 × 10-3 8.5 × 10-3
565
637
540
4.0
2.6 3.7 × 10-4 2.7 × 10-4 3 × 10-4
1.7 × 10-3
R-NPD(40)/10a(40)/Alq3(20)
60
2786
36 538
0.045
0.35
0.67
0.05
0.36
1.22
R-NPD(40)/10b(20)/Alq3(40)
2.2 10.0 16.0 180
2.2 10.5 15.5 200
0.28
0.30
0.39
1.02
Q
R-NPD(40)/10b:DCJTB(20)/Alq3(40)k 623
a Device configurations (thickness): ITO/PEDOT:PSS(25/nm)/organic layer/Mg:Ag(80/nm)/Ag(150/nm). b Turn-on voltage at which emission becomes
f
detectable (∼10-4 cd m-2). c Voltage for a current density of 20 mA cm-2
.
d Voltage at the maximum brightness (Bmax). e Brightness at V20. Bmax
.
g External
EL quantum efficiency (EELQ) at 20 mA cm-2 h Maximal EELQ. i Luminous efficiency (LE) at 20 mA cm-2 j Maximal LE. k DCJTB 1.4 wt %.
. .
Figure 11. PL spectrum of 6 in thin film and EL spectra of devices A and
F.
Figure 13. Relative energy level diagram for device F.
IP(eV) ) Eonsetox(vs SCE) + 4.4.2c If one considers work
functions φc ) 3.70 eV for the Mg:Ag cathode and φa ) 5.10
eV for the ITO:PEDOT anode,27 these data show rather large
barriers for electron injection for phosphole-based materials (φc
- EA ≈ 0.7-1.1 eV). The barriers for hole injection are lower
(φa - IP ≈ 0.2-0.7), indicating an unbalanced carrier injection.
To further improve device characteristics, multilayered
devices F-J in which the phosphole layer is sandwiched
between a hole-transport layer (R-NPD, Figure 2) and an
electron-transport layer (Alq3, Figure 2) were prepared. Figure
13 shows schematically the energy-level diagram4c for one of
the multilayer devices, from which one may expect reduced
barriers for carrier injection and thus improved device charac-
teristics. The EL spectra of devices F-J are similar to those of
the corresponding single-layer OLEDs A-E (Figures 11 and
12), suggesting that the EL emission arises purely from the
phosphole materials. The turn-on voltages required to drive
multilayer devices are lower (A/F, B/G, Table 5) or comparable
to those of the corresponding single-layer OLEDs. The external
EL quantum efficiency (EELQ) and brightness of the multilayer
devices are dramatically superior to those of their single-layer
Figure 12. PL spectrum of 7b in thin film and EL spectra of devices C,
H, and K.
thienyl)thiooxophosphole 7b as the multifunctional organic
layer. The EL is obtained with a low turn-on voltage (2.0 V),
and the maximum brightness reaches 3613 cd m-2. However,
the EL quantum yield is modest (0.16%), presumably due to
unbalanced carrier injection and/or transport. Single-layer
OLEDs usually give poor efficiency and brightness because EL
organic materials tend to have either p-type (hole-transport) or
n-type (electron-transport) charge-transport characteristics.2,3 The
experimental determination of IP and EA values for organic
materials is difficult. It has been suggested by Jenekhe et al.
that a fair estimation of IP and EA can be derived from cyclic
voltammetry by taking EA(eV) ) Eonsetred(vs SCE) + 4.4 and
(27) (a) Gu, G.; Bulovic, V.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E.
Appl. Phys. Lett. 1996, 68, 2606-2608. (b) Burrows, P. E.; Gu, G., Forrest,
S. R.; Vincenzi, E. P.; Zhou, T. J. Appl. Phys. 2000, 87, 3080-3085.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 993

A R T I C L E S
Su et al.
Figure 14. Comparison of external EL quantum efficiency versus current
density for devices C and F.
Figure 16. External EL quantum efficiency versus current density for
devices K and L.
sponding “nondoped” device H (Table 5). Upon doping with
DCJTB, the turn-on voltage and maximal brightness remain
similar (Table 5, devices H/K), but the EL efficiency increases
from 0.74% to 1.83% photon/electron and the luminous ef-
ficiency from 0.83 to 1.12 lm/W. One of the major concerns
for practical applications of OLEDs is the current-induced
luminescence quenching, particularly in red OLEDs.2 We
therefore studied the current-density dependence of the device
efficiency. The external EL quantum efficiency of device K is
unaffected by the driving current density in the range of 0-90
mA/cm2 (Figure 16). This behavior is promising, because the
efficiency of the conventional DCJTB-doped Alq3 devices
usually decays rapidly upon increasing the driving current due
to quenching effects of charged excited states of Alq3 on red
dopants.2,28 This behavior is illustrated by the DCJTB(1.4%)-
doped Alq3 device L consisting of ITO/PEDOT:PSS(25/nm)/
R-NPD(40 nm)/Alq3:DCJTB(20 nm)/Alq3(40 nm)/Mg:Ag(80
nm)/Ag(150 nm) (Figure 16). To circumvent this problem,
which affects reliability and lifetime of a DCJTB-doped device,
an assisting dopant (rubrene, quinacridone...) was added in the
fabrication of some reported device.2d,27a,d The role of these
assisting dopants is to reduce the formation of [Alq3].+, which
is a well-known quenching species for red-emitting dopants.28e,f
Thus, 2,5-di(2-thienyl)thiooxophosphole 7b appears to be an
attractive host material for DCJTB, because it solves the problem
of luminescence quenching at high drive current without using
the more complicated assisting-dopant technique. This is an
important feature because the stability in EL quantum efficiency
and color over a wide range of current density is crucial for
OLED applications that require high current density (i.e., passive
matrix displays, solid-state lasers...).
Figure 15. Current density and brightness versus voltage for device F.
counterparts (Table 5). For example, with 7b, the maximum
brightness is enhanced by 1 order of magnitude (C/H, 3613/
37830 cd m-2) and the maximum EELQ is multiplied by 5 (C/
H, 0.16/0.80). The higher efficiency of multilayer over single-
layer OLEDs is further illustrated in Figure 14, which compares
the external EL quantum efficiency of devices C and H versus
current density. The current density-voltage-brightness char-
acteristics of device H are presented in Figure 15.
This first report on the use of EL organophosphorus deriva-
tives proves that there is no inherent limit to the development
of P-containing conjugated systems for optoelectronic functions.
Furthermore, these results demonstrate that exploiting the
reactivity of the P center of phosphole-based oligomers is a clue
to optimizing these materials. Hence, it is clear that there is
plenty of room for further improvement of the OLED perfor-
mances through structural modifications of these phosphole-
based materials.
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.2 This guest-host doped
emitter system is widely used for solving the problem of red-
emissive materials being prone to aggregation in the solid state.2d
2,5-(2-Dithienyl)thiooxophosphole 7b was thus evaluated as the
host material for DCJTB (Figure 2), one of the best red-emitting
dopants for OLEDs.2d The testing device K has the same
configuration of multilayer device H except for 1.4 wt % of
DCJTB doped in the organic layer of 7b (Table 5). The dopant
concentration was not optimized, although the effective dopant
range can be narrow. Device K showed red emission from
DCJTB (Figure 12), indicating effective resonant energy transfer
(Fo¨rster resonance) or carrier trapping from 7b to DCJTB.
Moreover, this new device is more efficient than the corre-
Similarly, gold complexes 10a and 10b were subjected to
EL studies using also the single-layer structure (devices M and
N, Table 5) and the multilayer structure (devices O and P, Table
5). Figures 17 and 18 compare the thin-film PL and EL spectra
of 10a (devices M and O) and 10b (devices N and P),
respectively. As observed for thin-film PL of 10a and 10b, their
(28) (a) Liu, T.-H.; Iou, C.-Y.; Chen, C. H. Appl. Phys. Lett. 2003, 83, 5241-
5243. (b) Young, R. H.; Tang, C. W. Appl. Phys. Lett. 2002, 80, 874-
876. (c) Feng, J.; Li, F.; Gao, W.; Cheng, G.; Xie, W.; Liu, S. Appl. Phys.
Lett. 2002, 81, 2935-2937. (d) Xie, Z. Y.; Hung, L. S.; Lee, S. T. Appl.
Phys. Lett. 2001, 79, 1048-1050. (e) Young, R. H.; Tang, C. W.; Marchetti,
A. Appl. Phys. Lett. 2002, 80, 1465-1467. (f) Young, R. G.; Tang, C. W.;
Marchetti, A. P. Appl. Phys. Lett. 2002, 80, 874-876.
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994 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Design of Oligomers for Electroluminescent Devices
A R T I C L E S
energies. Such results indicate that with DCJTB doping, the
transition from the high-energy emission states of 10b to the
lower-energy emission states of 10b is rather suppressed by the
effective transfer to DCJTB excited states. The emission
efficiency of this device Q is markedly enhanced as compared
to device P (Table 5); its external EL quantum efficiency and
maximum brightness reach 1.22% and 36 538 cd/m2, respec-
tively.
Conclusion
The phosphole-based EL materials presented in this work
possess appealing properties such as ease of synthesis and
purification, thermal stability, and enhancement of PL efficiency
in the solid state. Moreover, they are attractive because of the
flexibility available for fine-tuning their electronic properties
through the manipulation of their chemical structures. This is
nicely illustrated by the fact that the nature of the P moiety
affects the bulk molecular environment and hence the photo-
physical behavior of these chromophores in the solid state;
thiooxophosphole derivatives are isolated luminophores, whereas
gold(I) phosphole complexes form aggregates leading to broad
emission spectra from both monomer and aggregate states. This
dichotomy shows that the presence of reactive P atoms allows
for a unique molecular design to vary and to control the
electronic structures of π-conjugated materials. Moreover, we
have shown that σ3- and σ4-phosphole-based conjugated oligo-
mers can act as multifunctional materials for OLEDs. Single-
and multilayer OLEDs have been prepared, and a color tuning
was achieved by the proper selection of the phosphole substit-
uents. Di(2-thienyl)thiooxophosphole 7b appeared to be the most
efficient P material in many respects. Of particular interest, it
is a host for the fabrication of red OLEDs stable over a wide
range of current densities, which is important for display and
solid-state lighting applications. OLEDs using phosphole com-
plexes 10a,b, which are among the rare gold(I) complexes used
for this type of application, cover broad emission spectra due
to EL from monomer and aggregate states. This property can
be of interest for future development of WOLEDs based on a
single emissive material. The results presented in this paper are
the basis for further development of P materials for optoelec-
tronic applications because they show that there is no inherent
impossibility in using organophosphorus moieties to construct
functional materials. Furthermore, they demonstrate the versatil-
ity provided by the exploitation of P chemistry in this field. It
can be expected that, besides the classic applications of
organophosphorus derivatives (ligands design, medicinal chem-
istry...), more and more P materials featuring different moieties
(Figure 1) and optoelectronic functions will emerge soon.
Figure 17. PL spectrum of 10a in thin film and EL spectra of devices M
and O.
Figure 18. PL spectrum of 10b in thin film and EL spectra of devices N,
P, and Q.
EL spectra also show two broad emission bands, suggesting
similar emission mechanisms for both PL and EL. With 10a as
the emitting material, the long-wavelength band (around 630
nm) is dominant, and the devices gave rather saturated red
emission (Table 5). The CIE (Commission Internationale de
l’Eclairage) 1931 color chromatography coordinates of device
M are x ) 0.61 and y ) 0.39, close to those of the standard red
CRT Phosphors of the Society of Motion Picture and Television
Engineers (x ) 0.64, y ) 0.34). Note that nondoped red OLEDs
are relatively rare.2d,20d Using 10b, the relative intensity of the
long-wavelength band varies with the device structures, which
may be due to optical effects (e.g., reflection, interference, etc.)
of the device structure on emitting species at different locations.
It is interesting to note that in the single-layer device of 10b,
the EL spectrum covers uniformly a broad spectral range of
500-800 nm due to the well-balanced dual monomer and
aggregate emissions (Figure 18). Once again, these results
underline that the properties of phosphole-based materials can
be tuned through specific chemical modifications of the P atom.
As in previous cases, incorporating the hole-transport layer and
the electron-transport layer to form the multilayer devices O
and P also significantly enhances the efficiencies and brightness
of devices (Table 5). Here also, the best performances are
obtained with the thienyl-substituted phosphole derivative
(device P). The OLED using the di(2-thienyl)phosphole-gold
complex 10b shows a low turn-on voltage (2.2 V); the external
EL quantum efficiency and the maximum brightness reach
0.36% and 2786 cd/m2, respectively. Complex 10b was also
evaluated as the host material for DCJTB in multilayer device
Q with a 1.4 wt % DCJTB doped in the organic layer of 10b
(Table 5). Device Q showed red emission from DCJTB (Figure
18), even though 10b itself has emission states of even lower
Acknowledgment. We thank the Ministe`re de l’Education
Nationale, de la Recherche et de la Technologie, the Institut
Universitaire de France, the Centre National de la Recherche
Scientifique, and the National Science Council of the Republic
of China for financial support. Thanks are due to Christophe
Lescop for X-ray diffraction studies.
Supporting Information Available: Experimental, spectro-
scopic, and other experimental details of new compounds, table
of crystallographic data, structure refinement details, atomic
coordinates, interatomic distances and angles, and anisotropic
displacement parameters (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
JA0567182
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J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 995