The role of ruthenium and rhenium diimine complexes in conjugated polymers that exhibit interesting opto-electronic properties

Article abstract of DOI:10.1002/1521-3765(20011015)7:20<4358::AID-CHEM4358>3.0.CO;2-M

This paper reports the synthesis and opto-electronic properties of different conjugated polymers that contain the diimine complexes of ruthenium or rhenium. Conjugated poly(phenylene vinylene)s that contain aromatic 1,3,4-oxadiazole and 2,2′-bipyridine un

Full text of DOI:10.1002/1521-3765(20011015)7:20<4358::AID-CHEM4358>3.0.CO;2-M

FULL PAPER
The Role of Ruthenium and Rhenium Diimine Complexes in Conjugated
Polymers That Exhibit Interesting Opto-Electronic Properties
Po King Ng, Xiong Gong, Suk Hang Chan, Lillian Sze Man Lam, and Wai Kin Chan*[a]
Abstract: This paper reports the syn-
thesis and opto-electronic properties of
different conjugated polymers that con-
tain the diimine complexes of ruthenium
or rhenium. Conjugated poly(phenylene
vinylene)s that contain aromatic 1,3,4-
oxadiazole and 2,2'-bipyridine units on
the main chain were synthesized by the
palladium catalyzed olefinic coupling
reaction. Other types of polymers based
on 1,10-phenanthroline bis(2,2-bipyrid-
yl) ruthenium(ii) or chlorotricarbonyl
rhenium(i) complexes were also synthe-
sized by the same reaction. In general,
these polymers exhibit two absorption
bands due to the p ± p* transition of the
conjugated main chain and the d ± p*
metal-to-ligand charge-transfer transi-
tion of the metal complex. As a result,
the photosensitivity of the polymers
beyond 500 nm was enhanced. Charge-
carrier mobility measurements showed
that the presence of metal complexes
could facilitate the charge-transport
process, and the enhancement in carrier
mobility was dependent on the metal
content in the polymer. In addition, we
have also demonstrated that the ruthe-
nium complex could act as both photo-
sensitizer and light emitter. Photovoltaic
cells were constructed, and they were
subjected to irradiation with a xenon arc
lamp. Under illumination, the short
circuit current and the open circuit
voltage were measured to be
�
2
0.05 mAcm and 0.35 V, respectively.
The polymers were fabricated into sin-
gle-layer emitting devices, and light
emission was observed when the device
was subjected to forward bias. The
maximum luminance was determined
�
2
to be 300 cdm , and the external quan-
tum efficiency was approximately 0.05
to 0.2%. Although the efficiency was
relatively low when compared with oth-
er devices based on organic materials,
we have demonstrated the first exam-
ples of using transition metal complexes
for both photovoltaic and light-emitting
applications.
Keywords: luminescence ´ N ligands
´
rhenium
´
ruthenium
´
sensitizers
Introduction
mers also demonstrated various interesting electronic and
photonic properties, which are comparable to traditional
inorganic semiconductors. It has been reported that PPVs
Polymer metal complexes have remarkably specific struc-
tures, in which central metal ions are surrounded by an
enormous polymer chain. They show interesting and impor-
tant characteristics, especially catalytic activities different
from the corresponding ordinary metal complexes of low
molecular weight. Moreover, they also find potential appli-
cations in many important systems such as polymer-coated
electrodes, solar energy conversion, and chemical sensors and
[4]
[5]
have been used as laser materials, photodiodes, and
organic transistors.[ Most of the opto-electronic polymers
reported to date are based on pure organic conjugated
systems, in which different kinds of functional groups are
attached.[ Our group has been continuously investigating the
syntheses and properties of some conjugated polymers or
polymer composites that contain ruthenium or rhenium
6]
7]
[
1]
[8]
so on. Since the discovery of the first organic light-emitting
polypyridine complexes. The complexes were able to
[
2]
poly(p-phenylene vinylene) (PPV), there has been tremen-
dous advancement in this area in terms of material design,
device fabrication, emission wavelength, and performance
improvement in organic polymeric light-emitting diodes
enhance the charge-carrier mobilities of the polymers by
acting as extra charge carriers, and the mobilities were found
to be dependent on the metal complex content. In addition,
these transition metal complexes exhibit interesting electronic
and excited-state properties. As a result, by incorporating
metal complexes into the organic p-conjugated polymers, it is
possible to tune the photosensitivity, charge transport, and
electroluminescence (EL) activities of the resulting polymers.
[
3]
(
LED). Besides light-emitting properties, conjugated poly-
[
a] Dr. W. K. Chan, Dr. P. K. Ng, Dr. X. Gong, S. H. Chan, L. S. M. Lam
Department of Chemistry, University of Hong Kong
Pokfulam Road (Hong Kong)
2‡
The use of tris(2,2'-bipyridyl) ruthenium(ii) [Ru(bpy)3] or
,2'-bipyridyl rhenium(i) tricarbonyl complexes and their
derivatives as light absorption or emission sensitizers in
2
Fax : (‡852)2857-1586
E-mail: waichan@hkucc.hku.hk
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Chem. Eur. J. 2001, 7, No. 20

4358±4367
polymers constitutes one exam-
Br
CN
[
9]
6
ple. These d transition metal
complexes exhibit characteris-
tic long-lived metal-to-ligand
charge-transfer (MLCT) excit-
ed states, which can undergo
different photochemical or
photophysical processes. The
excited metal complexes may
serve as good energy donors,
electron donors, or electron
acceptors. One of the most
promising potentials of these
complexes is the conversion of
solar radiation into chemical
1
NaN , NH Cl
3
4
N
N
NH
N
Br
2
ClOC
COCl
Br
ClOC
Br
COCl
N
N
N
N
O
3
4
Br
N N
O
N N
O
Br
N
N
N
N
5
N
N
O
1
2
. [Ru(bpy)2(CF3SO3)2(acetone)2]
. KPF6
[
10]
N
N
energy. Conjugated polymers
incorporated with these com-
plexes have been reported in
6
N N
O
N N
O
Br
Br
[
11]
the literature.
It was also
N
N
N
proposed that these complexes
were able to participate in the
N
Ru
N
N
2 PF6-
[
8a, 12]
charge-transport processes.
In addition, they exhibit inter-
esting luminescence properties
7
: 5,5'-disubstituted bipyridine
: 4,4'-disubstituted bipyridine
8
originated from the MLCT
states. Light-emitting devices
Scheme 1. Synthesis of ruthenium complexes with oxadiazole-substituted bipyridine ligands.
based on ruthenium, rhenium,
or other transition metal complexes have also been con-
hexafluorophosphate as the counter anion because its salts
have higher solubility than halides in organic solvents, and
they are electrochemically inert such that they do not affect
the electronic or other charge-transport processes in the
polymer. The syntheses of the ruthenium and rhenium
complexes of 3,8-dibromo-1,10-phenanthroline (10 and 11)
are quite straightforward. Complex 11 was prepared
by refluxing the diimine ligand with rhenium penta-
carbonyl chloride in toluene in high yield (Scheme 2).
[
13]
structed.
It was suggested that by utilizing the emission
from the triplet excited states of transition metal complexes,
the theoretical maximum quantum efficiency of the resulting
light-emitting device could exceed 25% induced by light
[
14]
emission from singlet excitons.
In this paper, we describe the synthesis, photosensitization,
and light-emitting properties of a novel type of conjugated
polymers, which contain a tris(2,2'-bipyridyl)ruthenium(ii)
type complex as the photosensitizer and light emitter.
Aromatic oxadiazoles were proposed to be good candidates
for advanced materials and were used in many multilayered
light-emitting devices as electron-transport or hole-blocking
Br
Br
N
N
9
[
15]
layers.
They were used to enhance the electron-carrier
mobility and to facilitate the charge separation after the
photosensitization process. It has been shown that conjugate
polymers with aromatic oxadiazole moieties exhibit improved
light-emitting device performance because of the balanced
charge injections from both electrodes.[16] Our objective is to
investigate the roles played by these metal complexes when
they are incorporated into conjugated polymer systems. This
paper reports the results.
1
. [Ru(bpy)2(CF3SO3)2(acetone)2]
[
Re(CO)5Cl]
2. KPF6
Br
Br
N
Br
Br
N
N
N
OC Re Cl
N
Ru
N
OC CO
N
N
2 PF -
6
11
10
Scheme 2. Synthesis of ruthenium and rhenium complexes derived from
,8-dibromo-1,10-phenanthroline.
Results and Discussion
3
Syntheses of monomers and polymers: Ligands 5 and 6 were
synthesized by the reaction between the corresponding 2,2'-
bipyridinedicarbonyl chlorides 3 or 4 and 1-bromo-4-tetrazo-
yl-benzene 2 in excellent yield (Scheme 1). Reaction of the
ligands with the triflate salt of bis(2,2'-bipyridine)ruthe-
nium(ii) afforded monomers 7 or 8 in good yield. We chose
Polymerization was carried out by the palladium catalyzed
Heck coupling reaction by using 1,4-divinylbenzene and 1,4-
dibromo-2,5-dioctoxybenzene as the comonomers. We have
employed this reaction in the syntheses of different conju-
gated polymers functionalized with a variety of transition
[
17]
Chem. Eur. J. 2001, 7, No. 20
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4359

W. K. Chan et al.
FULL PAPER
metal complexes. By using this reaction, various oxophilic or
electrophilic functional groups were able to survive in the
polymerization reaction. Both the ruthenium and rhenium
complexes were robust in these reactions. No ligand exchange
reaction was observed in any one of these polymers. The metal
content in the polymer can be easily adjusted by changing the
proportion of the monomer metal complexes and other
monomers. Both the oxadiazole based (7 and 8) or phenan-
throline based monomers (10 and 11) yielded polymers in
good yield. For each series of polymers, polymers with
different metal content were synthesized, and the results are
summarized in Tables 1 and 2.
1
Spectroscopic properties: The H NMR spectra of ligand 5,
ruthenium complex 7, and polymer 15e are shown in Figure 1.
With reference to the NMR spectra of similar bis(2,2'-
[
8a]
bipyridyl)ruthenium complexes,
the NMR spectra of 5
and 7 (Figure 1a and b) clearly agree with their structures. In
Figure 1c, the spectra features of the polymer also agree with
the proposed structure. The signals at approximately d ˆ 8.2 ±
9
.4 correspond to the protons on the bipyridyl moieties, while
the peaks at d ˆ 7.5 ± 8.1 are due to the phenylene ± vinylene
units. The peaks at d ˆ 4.2, 1.9, 1.3 ± 1.4, and 0.9 are attributed
to the octoxy side chain. It can also be observed that there are
signals due to the vinyl end groups at d ˆ 5.3, 5.9, and 6.8. By
comparing the integration ratio of these peaks with those of
Figure 1. 1H NMR spectra of a) ligand 5 in CDCl ; b) ruthenium com-
plex 7 in CD CN; c) polymer 15e in [D ]DMF.
3 7
3
Table 1. Synthesis and physical properties of different ruthenium containing polymers with 1,3,4-oxadiazole-substituted bipyridine ligands.
yield [%][a]
T
[8C][b]
h
inh [dLg ][c]
� 1
m
[10 cm V
� 4
2
� 1 � 1
s
][d]
e
m [10 cm V s
][d]
� 4
2
� 1 � 1
polymer
metal complex
monomer
x
y
d
h
13a
13b
13c
13d
13e
13 f
14a
14b
14c
7
7
7
7
7
7
8
8
8
0.1
0.25
0.35
0.5
0.65
1
0.25
0.5
1
0.9
0.75
0.65
0.5
0.35
0
0.75
0.5
0
85
97
84
83
77
79
86
74
76
434
416
412
402
401
424
399
394
387
0.61
0.46
0.52
0.32
0.64
0.71
0.53
0.31
0.30
0.47
0.55
1.8
1.5
4.9
0.50
0.57
3.1
3.6
7.9
8.3
0.33
1.5
5.1
0.21
0.45
0.64
3.2
[
a] Yield after purification by washing with methanol for 2 days. [b] Onset temperature of decomposition determined by TGA under a nitrogen atmosphere.
�
1
[
c] Inherent viscosity measured in solution in NMP at 308C with concentration c ˆ 0.25 gdL . [d] Charge-carrier mobility measured at an electric field
� 1
strength of E ˆ 40 kVcm
.
Table 2. Synthesis and physical properties of different ruthenium or rhenium containing polymers based on 1,10-phenanthroline ligands.
yield [%][a]
T
[8C][b]
h
inh [dLg ][c]
� 1
m
h
[10 cm V
� 4
2
� 1 � 1
s
][d]
e
m (10 cm V s
][d]
� 4
2
� 1 � 1
polymer
metal complex
monomer
x
y
d
15a
15b
15c
15d
15e
15 f
15g
16a
16b
16c
16d
16e
10
10
10
10
10
10
10
11
11
11
11
11
0.1
0.2
0.3
0.4
0.5
0.6
1
0.1
0.2
0.3
0.5
0.7
0.9
0.8
0.7
0.6
0.5
0.4
0
0.9
0.8
0.7
0.5
0.3
85
87
80
76
75
70
72
86
83
88
87
78
392
383
372
371
383
382
392
408
403
410
415
407
0.46
0.44
0.41
0.33
0.40
0.34
0.32
0.38
0.36
0.32
0.40
0.32
0.8
1.3
1.3
1.5
1.8
2.4
3.1
0.4
1.0
1.1
1.4
2.1
2.1
2.8
3.0
3.3
3.8
4.6
6.5
1.2
1.9
2.4
2.7
4.0
[
[
a] Yield after purification by washing with methanol for 2 days. [b] Onset temperature of decomposition determined by TGA under a nitrogen atmosphere.
�
1
c] Inherent viscosities for polymers 15a ± g were measured in solution in NMP at 308C with concentration c ˆ 0.25 gdL . For polymers 16a ± e, they were
� 1
measured in solution in 1,1,2,2-tetrachloroethane. [d] Charge-carrier mobility measured at an electric field strength of E ˆ 40 kVcm
.
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Chem. Eur. J. 2001, 7, No. 20

Ruthenium and Rhenium Diimine Complexes in Conjugated Polymers
4358±4367
the methylene units of the alkyl pendant chain, the number
average degree of polymerization was estimated to be
approximately ten, which corresponds to the number average
molecular weight of approximately 12000. The metal content
in the polymers was estimated by NMR spectra by comparing
the ratio of the protons on the bipyridine units with those of
the methylene units. The monomer fed ratio also agrees well
with the metal content of the resulting polymers. In the FTIR
spectra, all polymers exhibit a characteristic pyridine CˆN
�
1
stretching band at 1600 cm . For polymers 13 ± 14
Scheme 3), the CˆN and C�O stretching bands for the
(
�
1
oxadiazole ring appear at 1576 and 1080 cm , respectively.
Another absorption band at 831 cm is ascribed to the typical
�
1
C�H out-of-plane bending of the 1,4-disubstituted phenylene
units. After the incorporation of the ruthenium complex, a
�
1
very strong band is found at 841 cm corresponding to the
Figure 2. UV/Vis absorption spectra of polymers 13b,d,f in solution
in DMF.
P�F stretching of the counter anion. In polymers 16a ± e,
three strong absorption bands are found at 2015, 1935, and
�
1
1
892 cm corresponding to the CO stretching of the rhenium
carbonyl with a fac configuration. This is strong evidence for
the presence of rhenium complexes on the polymer main
chain. These polymers exhibit modest thermal stabilities, and
�
1
their inherent viscosities are in the range of 0.3 ± 0.7 dLg . It
can be seen that the viscosities of polymers 15 and 16 are
slightly lower than those of polymers 13 and 14. This is due to
the fact that in monomers 10 and 11, the bromo groups are
quite close to the bulky metal complexes that lower the
reactivities of the corresponding monomers. In monomers 7
and 8, the reaction sites for the palladium coupling reaction
are far from the metal center. Nevertheless, optical quality
films were obtained by casting or spin coating techniques from
polymer solutions.
The electronic absorption spectra of some polymers are
shown in Figure 2, Figure 3, and Figure 4. All polymers with
the 2,2'-bipyridyl ruthenium moieties show an intense ab-
sorption band at 290 nm, which is assigned to the p ± p*
intraligand transition of the bipyridine ligand. The intensity of
these peaks increases with increasing metal content in the
polymers (Figures 2 and 3). Another strong and broad
absorption band is found in the region between 410 ±
Figure 3. UV/Vis absorption spectra of polymers 15a,d,g in solution
in DMF.
4
50 nm. This is attributed to the p ± p* electronic transition
of the conjugated polymer main chain. In general, it was
observed that the absorption maxima of these bands shifted to
Br
OC8H17
x
Br
M
Br
+
y
+
(x+y)
C8H17O
7,8,10,11
11a
Br
12
Pd(OAc)2, P(o-Tol)3 , Bu3N
C8H17O
Figure 4. UV/Vis absorption spectra of polymers 16b,d,e in solution
in 1,1,2,2-tetrachloroethane.
M
y
x
OC8H17
polymers 13-16
shorter wavelengths when the metal content was increased.
This is because the bulky metal complexes on the main chain
inhibit an efficient overlapping of the p-orbitals and thus
Scheme 3. Synthesis of polymers by palladium catalyzed olefinic
coupling reactions.
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W. K. Chan et al.
FULL PAPER
increase the transition energies. In addition, the characteristic
d ± p* MLCT transitions of the ruthenium or rhenium
complexes appear as shoulders at approximately 500 nm.
Although the intensity of these MLCT bands increases with
increasing amounts of metal complex, it is difficult to
correlate their intensities to the metal content in the polymers
as a result of the higher absorption coefficient exhibited by
the electronic transition of the p-systems.
For polymer 14b, the first reduction process is observed at
� 0.9 V, which is slightly lower than that in polymer 13 f. This
higher reduction potential is attributed to the 4,4'-disubsti-
tuted bipyridine main chain with less extended conjugation.
Other peaks observed in the cathodic scan originate from the
successive reduction of the ligands. In the reverse scan, a
similar anodic process is observed at approximately 0.8 V. The
phenanthroline based polymers 15g and 16e show similar
reduction processes in the cathodic scan, and their reduction
peaks are widely separated.
Electrochemical properties: The electrochemical properties
of the polymers were studied by cyclic voltammetry (CV).
Figure 5 shows the CV scans of some polymers in different
solvent systems; the systems used depend on the polymer
Charge-transport properties: Our previous work showed that
as a result of the presence of an electron-deficient polymer
main chain and other bipyridyl ligands, the electron-carrier
mobility of the polymers can be enhanced as they act as extra
charge carriers in the charge-transport process.[ We inves-
tigated the role of the metal complexes in charge-transport
processes by measuring their charge-carrier mobilities from
conventional time-of-flight experiments. The photocurrent
pulse shows a featureless decay, which is characteristic for
dispersive charge transport with non-Gaussian carrier distri-
bution. Drift mobility was calculated according to Equa-
tion (1).
12]
m ˆ L/t
T
E
(1)
In the equation, L is the film thickness, E is the applied field
strength, and t is the transit time for charge transport. The
T
electron- and hole-carrier mobilities of the polymers were
�
4
� 5
2
� 1 � 1
determined to be of the order of 10 to 10 cm V
s
(
Tables 1 and 2). In general, the charge-carrier mobilities
increase with the metal complex content in the polymers.
These values are significantly higher than those of the phenyl-
[
19]
substituted or unsubstituted PPVs
and polycarbonate
doped with oxadiazole derivatives.[ The enhancement in
electron-drift mobilities is possibly due to the presence of
oxadiazole moieties in the backbone, as most PPVs are p-type
polymers. The carrier mobilities were found to be temper-
ature dependent, and this indicates a thermally activated
charge-transport process. From the plot of temperature-
dependent charge mobility (Figure 6), the activation energies
20]
Figure 5. Cyclic voltammograms of polymers 13 f, 14b, 15g (in DMF/
CH
3 2 2
CN), and 16e (in CH Cl ). Tetra-n-butylammonium tetrafluoroborate
(
0.1m) was used as the supporting electrolyte. Glassy carbon was used as the
�
1
working electrode, and the scan rate was 50 mVs
.
solubility. As a result of the low solubility of polymers 13 ± 15
in acetonitrile, we carried out the CVexperiments by adding a
few drops of polymer solution in DMF to acetonitrile.
However, the scan range was reduced by this method due to
the presence of DMF. For polymer 16e, the CV experiments
were carried out in solution in dichloromethane. In the cyclic
voltammogram of polymer 13 f, two irreversible reduction
processes are observed at � 0.8 and � 0.95 V in the cathodic
scan. These are assigned to the reduction of the polymer main
chain, which is considered to involve the addition of elec-
trons to the polymer (n-doping). Such low reduction potential
may facilitate the transport of electrons in the polymers.
The reduction potentials are significantly lower than those
for other unsubstituted bipyridyl ruthenium complexes as
a result of the presence of the electron-withdrawing oxadia-
[
18]
zole moieties.
In the reverse scan, an anodic peak for
the undoping process corresponding to the removal of an
electron is observed at 0.7 V. In addition, another oxidation
process at 1.25 V is assigned to the metal centered RuIII/II
couple.
Figure 6. Plot of electron-carrier mobility for polymers 13d,f as a function
of temperature under different externally applied electric fields.
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Chem. Eur. J. 2001, 7, No. 20

Ruthenium and Rhenium Diimine Complexes in Conjugated Polymers
4358±4367
of charge transport were calculated to be approximately 0.1 ±
.2 eV. Figure 7 shows the plot of electron- and hole-carrier
mobilities of some polymers as a function of electric field
strength. The graph shows a negative slope, which is
voltaic cell consisting of a blend of ruthenium containing
polymer and C60 because the latter is incompatible with the
polymer matrix and is also insoluble in the polar solvent
medium (DMF) used for spin coating. When excited by light,
the charges generated by photoexcitation were separated at
the polymer/C60 interface. Due to the high electron affinity of
C , the electrons were collected at this layer, and photo-
0
60
current was generated as a result. Figure 8a shows the
current ± voltage characteristics of the photodiode in the dark
and under illumination by a xenon arc lamp that simulates
�
2
solar radiation with an intensity of 100 mWcm . Under
illumination, the short circuit current and the open circuit
�
2
voltage are approximately 0.05 mAcm and 0.35 V, respec-
tively. The conversion efficiency was estimated to be 0.5%.
Figure 8b shows the photocurrent density of the device under
different light intensity.
Figure 7. Plot of hole- and electron-carrier mobilities for polymers 13b,c,e
as a function of externally applied electric field at 298 K.
rationalized as the presence of off-diagonal disorder due to
the distribution of distances between molecules in an amor-
phous polymer system. The charge may hop against the field
direction in order to open a faster route for charge transport.
As the applied field is increased, ªbackward jumpsº and these
ªindirect routesº will be inhibited. This process is only
favorable under a low-field condition. As the electric field is
further increased, this effect will reach a maximum, and the
carrier mobilities are dependent on the distortion of the
density of hopping states by the applied field. A turning point
1
/2
may be observed in the log m versus E plot. However, when
we increased the electric field further, the polymer films were
subjected to dielectric breakdown due to the ionic species in
the polymer main chain. From these studies, we suggest that
the charge transport could involve inter- or intramolecular
hopping processes between the charge carrying species (i.e.
metal complexes). Because of the relatively high charge-
carrier mobilities, polymers with 5,5'-oxadiazole-substituted
bipyridine were selected for detail studies on opto-electronic
properties.
Photosensitizing properties: There have been several reports
on the use of molecular ruthenium complexes as photo-
sensitizing agents in solar cells or photorefractive materials.[
The photosensitivity of the polymers should be enhanced by
the metal complexes beyond 500 nm, at which the conjugated
systems do not absorb. A typical photosensitization process
consists of photoexcitation and charge separation. The
oxadiazole moieties may facilitate the initial hole ± electron
separation because of their affinity for electrons. In order to
study the photosensitivity of the polymer, a simple two-
layered photovoltaic cell with the structure ITO/polymer/C /
21]
Figure 8. a) Current ± voltage characteristics of the ITO/polymer 13 f/C60
/
�
2
Al device in the dark (solid circles) and under 100 mWcm illumination
open circles); b) A plot of photocurrent response of the device as a
(
function of irradiated light intensity under a forward bias of 1 V.
Light-emitting properties: Single-layer light-emitting devices
ITO/polymer/Al were constructed and they were subjected to
a dc voltage under a forward bias condition. The origin of light
emission is attributed to the radiative recombination of
exciton states, which are formed by the recombination of
oppositely charged polarons generated by the injection of
60
Al was constructed. Polymer 13 f was chosen for studies
because of its high metal content and charge-carrier mo-
bilities. We were not able to prepare a single-layered photo-
Chem. Eur. J. 2001, 7, No. 20
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4363

W. K. Chan et al.
FULL PAPER
electrons and holes. Table 3 summarizes the light-emitting
properties of some polymers. The PL and EL spectra of some
polymers are shown in Figure 9. It should be noted that the
emission from the polymer main chain was observed. This is
explained by an energy-transfer process between the con-
jugated system and the metal complex. After the conjugated
main chain has been excited, energy is transferred to the
adjacent low-lying MLCT states of the metal complexes.
When the metal complex content increases, this ªquenchingº
process is more efficient, and the luminescence spectra are
dominated by the MLCT emission. A similar energy-transfer
process was also observed in our previously reported con-
jugated polymers based on ruthenium dipyridophenazine
complexes and other systems.[ The turn-on voltages for the
devices were 6 to 8 V. For the polymer with the highest
ruthenium content (polymer 13 f), the maximum luminance
Table 3. Performance of some light-emitting devices.
polymer turn-on
maximum
external quantum l
max,em
voltage [V] luminance [cdm 2] efficiency [%]
�
[nm]
13d
13e
13 f
14a
14b
15b
15e
16b
16e
8
6
6
6
6
6.5
7
7.5
7
180
220
300
100
150
90
110
100
130
0.08
0.10
0.10
0.05
0.07
0.15
0.17
0.20
0.21
590, 690[a]
710
710
22]
[
[
[
a]
a]
a]
600, 700
690
570, 710
710
� 2
was determined to be 300 cdm at the driving voltage of 28 V.
570, 695
700
The external quantum efficiency of the devices was in the
range between 0.05 and 0.2%. A typical current ± voltage and
luminance ± voltage plot for polymer 13e is shown in Fig-
ure 10. Despite the relatively short lifetime for these devices
[
a] The emission band appears as a shoulder.
(
10 ± 20 min under ambient conditions), the design approach
of these polymers allows us to fine-tune the electronic and
emission properties of the polymers by simply adjusting the
amount of metal complex in the polymer.
Conclusion
Two series of conjugated polymers that were functionalized
with ruthenium or rhenium diimine complexes have been
synthesized. The first series of polymers contains the ruthe-
nium complexes of oxadiazole-substituted 2,2'-bipyridine.
The second type of polymers was derived from the ruthenium
or rhenium complexes of 1,10-phenanthroline. The use of
olefinic coupling reactions allowed the preparation of these
polymers directly from their corresponding monomer metal
complexes. The uniqueness of the electrochemical and
excited-state properties of the complexes was important for
the different opto-electronic processes. From the experimen-
tal studies, it was shown that these complexes were able to act
as photosensitizers, charge-transport species, as well as light
emitters. Their physical properties were dependent on the
Figure 9. Photoluminescence and electroluminescence spectra of poly-
mers 13d,f.
polymers consist of two light-emitting groups: the p-conju-
gated system and the metal complex. As a result, they show
very broad emission bands in the yellow-red region that
extend to very long wave-
lengths (>750 nm). For poly-
mer 13d, both PL and EL spec-
tra show major emission bands
centered at approximately
5
90 nm, which originates from
the p* ± p emission of the con-
jugated main chain. A small
shoulder due to the emission
from the MLCT states of the
ruthenium complex (p*-d) was
also found at approximately
6
50 nm. On the other hand,
polymer 13 f has a higher metal
complex content, and its PL
and EL spectra were dominated
by a relatively narrow emission
band centered at 690 nm. No
Figure 10. Current ± voltage and luminance ± voltage characteristics of the light-emitting device ITO/poly-
mer 13e/Al.
4364
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Chem. Eur. J. 2001, 7, No. 20

Ruthenium and Rhenium Diimine Complexes in Conjugated Polymers
4358±4367
metal complex content in the polymers. An energy-transfer
process between the conjugated main chain and the metal
complex moieties was proposed. The investigation of the
excited-state nature of the polymer metal complexes and
energy-transfer processes constitutes another very intriguing
project.
4,4'-Bis[2-(4-bromophenyl)-1,3,4-oxadiazoyl]-2,2'-bipyridine 6: This com-
pound was synthesized according to a similar procedure as for compound 5
except that 2,2'-bipyridine-4,4'-dicarboxylic acid was used instead. Yield
2
.5 g (93%).
1H NMR (300 MHz, CF
COOD-CDCl
): d ˆ 9.66 (d, J ˆ 1.0 Hz, 2H), 9.29
3
3
(
(
1
d, J ˆ 5.6 Hz, 2H), 8.72 (d, J ˆ 5.6 Hz, 2H), 8.15 (d, J ˆ 8.6 Hz, 4H), 7.89
13
d, J ˆ 8.6 Hz, 4H); C NMR (75 MHz, CF
3
COOD-CDCl
3
): d ˆ 170.1,
50.1, 149.4, 139.1, 135.6, 132.7, 131.1, 127.0, 123.1, 121.2; FTIR (KBr pellet):
�
1
nÄ ˆ 1602, 1574 (CˆN), 1075 (C�O stretching), 835 cm (1,4-disubstituted
‡
phenylene); MS (70 eV, EI): m/z (%): 602 [M] ; elemental analysis calcd
(
2
%) for C26
.48, N 13.55.
14 6 2 2
H N O Br (602.2): C 51.85, H 2.34, N 13.95; found: C 51.91, H
Experimental Section
Materials: All polymerizations were carried out under
atmosphere. N,N-dimethylformamide (DMF) was distilled over CaH
under reduced pressure. Pyridine was distilled over CaH . Other common
a
nitrogen
5,5'-Bis[2-(4-bromophenyl)-1,3,4-oxadiazoyl]-2,2'-bipyridine ruthenium(ii)
bis(2,2'-bipyridyl) hexafluorophosphate 7: Under a nitrogen atmosphere,
silver trifluoromethanesulfonate (0.99 g, 3.8 mmol) was added to a solution
of cis-dichlorobis(2,2'-bipyridine)ruthenium dihydrate (1 g, 1.9 mmol) in
acetone (200 mL). The solution was stirred at room temperature for 2 h,
and the silver chloride formed was filtered with a pad of Celite. The filtrate
was evaporated to dryness to give [Ru(bpy) (acetone) (SO CF ) ] (1.37 g,
2 2 3 3 2
90% yield). The triflate salt was added to a solution of 5 (2.1 g, 3.4 mmol) in
DMF (6 mL), and the solution was heated at 1208C for 12 h. The solution
2
2
organic solvents were analytical grade and used as received unless
otherwise stated. Silver trifluoromethanesulfonate, cis-dichlorobis(2,2'-
bipyridine) ruthenium dihydrate, and bromine were purchased from
Aldrich Chemical Co. and used as received. Sodium azide, 4-cyanobromo-
benzene, ammonium chloride, palladium(ii) acetate, tri-o-tolylphosphane,
tri-n-butylamine, and 1,10-phenanthroline monohydrochloride monohy-
drate were purchased from Lancaster Synthesis Ltd. and used as received.
Potassium hexafluorophosphate and rhenium(i) pentacarbonyl chloride
were purchased from Strem Chemical Co. The compounds 2,2'-bipyridine-
6
was filtered, and the filtrate was added to an aqueous solution of KPF . The
product was recrystallized by using a mixture of acetonitrile/diethyl ether
and was collected as a reddish brown solid (1.95 g, 77% yield).
[
23]
5
,5-dicarboxylic acid, 2,2'-bipyridine-4,4'-dicarboxylic acid, 3,8-dibromo-
1H NMR (300 MHz, CD
3
CN): d ˆ 8.74 (d, J ˆ 8.5 Hz, 2H), 8.68 (d, J ˆ
[24]
[25]
1,10-phenanthroline 9,
1,4-dibromo-2,5-dioctoxybenzene,
were synthesized according to the literature proce-
and 1,4-di-
8
.5 Hz, 2H), 8.60 (d, J ˆ 8.1 Hz, 2H), 8.53 (d, J ˆ 8.1 Hz, 2H), 8.25 ± 8.19
[26]
vinylbenzene
dures.
(
m, 4H), 8.05 (t, J ˆ 8.0 Hz, 2H), 7.90 ± 7.87 (m, 8H), 7.83 ± 7.80 (m, 4H),
13
7
.55 (t, J ˆ 6.1 Hz, 2H), 7.42 (t, J ˆ 6.1 Hz, 2H); C NMR (75 MHz,
Instruments: H and C NMR spectra ( H NMR 298 K, 13C NMR 298 K)
1
13
1
CD CN): d ˆ 165.9, 161.8, 159.3, 158.2, 157.9, 153.4, 153.2, 150.4, 139.4,
3
were collected on a Bruker 300DPXNMP spectrometer. FTIR spectra
139.3, 135.7, 133.8, 129.6, 128.8, 128.7, 127.7, 126.7, 125.6, 125.1, 123.4; FTIR
(KBr pellet): nÄ ˆ 1602, 1559 (CˆN), 1081 (C�O), 844 cm� 1 (P�F stretch-
(
KBr pellet) were collected on a Bio-RadFTS-7 FTIR spectrometer. UV/
Vis absorption spectra were collected on a HP8453 diode array spectro-
photometer. Mass spectrometry was performed on a high-resolution
FinniganMAT-95 mass spectrometer. Thermal analyses were performed
on Perkin ElmerDSC7 and TGA7 thermal analyzers. The photolumines-
cence (PL) and electroluminescence (EL) spectra were collected on an
ORIELMS-257 monochromator equipped with an ANDORDV420-BV
charge-coupled device (CCD) detector. Electrochemical experiments were
performed by using a Princeton Applied Research270 potentiostat with a
glassy carbon working electrode and silver/silver chloride reference
electrode. Distilled acetonitrile or dichloromethane were used as solvents,
and tetrabutylammonium tetrafluoroborate (0.1m) was used as the
supporting electrolyte. A small amount of ferrocene was added as an
internal standard.
ing); UV/Vis (CH CN): l (e) ˆ 290 (83000), 360 (62000), 440 (12000),
3
max
3
mol� 1 cm� 1); MS (FAB): m/z (%): 1161 [M � PF ]
‡
530 nm (5800 dm
6
, 1016
‡
[M � 2PF ]
6
46 30 10 2 2 2 12
; elemental analysis calcd (%) for C H N O Br RuP F
(1304.4): C 42.25, H 2.31, N 10.71; found: C 42.60, H 2.60, N 10.42.
4
,4'-Bis[2-(4-bromophenyl)-1,3,4-oxadiazoyl]-2,2'-bipyridine ruthenium(ii)
bis(2,2'-bipyridyl) hexafluorophosphate 8: The synthetic procedure was
similar to that for 7 except that ligand 6 (1.3 g, 4.7 mmol) was used instead.
Yield 1.7 g (66%).
1
H NMR (300 MHz, CD CN): d ˆ 8.88 (m, 8H), 8.26 (m, 6H), 8.05 (d, J ˆ
3
6.1 Hz, 2H), 7.97 (m, 6H), 7.89 (d, J ˆ 5.4 Hz, 2H), 7.89 (d, J ˆ 5.4 Hz, 2H),
7.76 (d, J ˆ 5.6 Hz, 2H), 7.59 (t, J ˆ 3.1 Hz, 2H), 7.53 (t, J ˆ 7.0 Hz, 2H);
13
C NMR (75 MHz, CD CN): d ˆ 166.9, 166.5, 163.1, 159.0, 157.9, 157.7,
3
1
1
8
(
PF
54.1, 153.0, 152.7, 149.2, 139.4, 133.8, 133.1, 129.9, 128.9, 128.8, 127.8, 125.5,
Synthesis. 1-bromo-4-tetrazoylbenzene 2: A mixture of 4-cyanobromoben-
zene (1 g, 5.5 mmol), sodium azide (5.7 g, 0.088 mol), ammonium chloride
25.1, 123.6, 122.4; FTIR (KBr pellet): nÄ ˆ 1600, 1562 (CˆN), 1079 (C�O),
�
1
41 cm (P�F stretching); UV/Vis (CH
CN): lmax (e) ˆ 292 (76000), 360
(
4.7 g, 0.088 mol), and dry DMF (40 mL) was stirred and heated under
reflux for 75 h. The cooled solution was added to dilute hydrochloric acid
150 mL). The precipitate was collected and washed with water and it was
3
3
� 1
� 1
14000), 480 nm (15000 dm mol cm ); MS (FAB): m/z (%): 1161 [M �
‡
‡
6
] , 1016 [M � 2PF
6
] ; elemental analysis calcd (%) for C46H N O Br2-
3
0
1
0
2
(
2
RuP F12 (1307.6): C 42.25, H 2.31, N 10.71; found: C 42.30, H 2.63, N 10.92.
recrystallized with methanol. Yield 1.0 g (81%).
1
H NMR (300 MHz, [D
6
]DMSO): d ˆ 7.99 (d, J ˆ 8.5 Hz, 2H), 7.84 (d, J ˆ
3,8-Dibromo-1,10-phenanthroline ruthenium(ii) bis(2,2'-bipyridyl) hexa-
fluorophosphate 10: Under a nitrogen atmosphere, silver trifluorometh-
anesulfonate (0.24 g, 0.96 mmol) was added to a solution of cis-dichloro-
bis(2,2'-bipyridine)ruthenium dihydrate (0.25 g, 0.48 mmol) in acetone
8
1
.3 Hz, 2H); 13C NMR (75 MHz, [D
6
]DMSO): d ˆ 132.4, 128.8, 128.6,
‡
24.6, 123.6; MS (70 eV, EI): m/z (%): 226 [M] .
5
,5'-Bis[2-(4-bromophenyl)-1,3,4-oxadiazoyl]-2,2'-bipyridine 5: Under a
(
50 mL). The solution was stirred at room temperature for 2 h, and the
silver chloride formed was filtered with a pad of Celite. After evaporation
of the solvent, the triflate salt [Ru(bpy) (acetone) (SO CF ] was added to
nitrogen atmosphere, a mixture of 2,2'-bipyridine-5,5'-dicarboxylic acid
1.14 g, 4.7 mmol) and thionyl chloride (20 mL) was heated under reflux for
(
2
1
2
2
3
3 2
)
4 h. The excess thionyl chloride was removed by distillation. Compound
-bromo-4-tetrazoylbenzene (2.3 g, 10 mmol) and dry pyridine (90 mL)
a solution of 3,8-dibromo-1,10-phenanthroline 9 (90 mg, 0.28 mmol) in
DMF (3 mL), and the solution was heated at 1408C for 24 h. The filtered
solution was added to an aqueous solution of KPF . The product was
6
recrystallized by using a mixture of acetone/diethyl ether and was collected
as a dark red solid (0.38 g, 75%).
were added to the acid chloride formed. The mixture was heated under
reflux for another 50 h. After cooling, the mixture was poured into water,
and the crude product was washed in hot methanol and then dried under
vacuum (2.45 g, 91% yield).
1H NMR (300 MHz, CF
COOD-CDCl
): d ˆ 9.85 (d, J ˆ 1.6 Hz, 2H), 9.31
1H NMR (300 MHz, CD
CN): d ˆ 8.84 (d, J ˆ 1.8 Hz, 2H), 8.50 (t, J ˆ
3
3
3
(
(
1
d, J ˆ 8.5 Hz, 2H), 8.97 (d, J ˆ 8.5 Hz, 2H), 8.14 (d, J ˆ 8.6 Hz, 4H), 7.91
7.5 Hz, 4H), 8.19 (d, J ˆ 1.2 Hz, 2H), 8.16 ± 7.98 (m, 6H), 7.75 (d, J ˆ 6.1 Hz,
2H), 7.65 (d, J ˆ 5.6 Hz, 2H), 7.42 (t, J ˆ 6.3 Hz, 2H), 7.27 (t, J ˆ 6.6 Hz,
13
d, J ˆ 8.6 Hz, 4H); C NMR (75 MHz, CF COOD-CDCl ): d ˆ 169.1,
3
3
49.8, 147.0, 143.6, 135.3, 132.4, 130.8, 126.4, 125.3, 122.5, 120.7, 110.0; FTIR
2H); 13C NMR (75 MHz, CD
3
CN): d ˆ 158.3, 157.9, 154.6, 153.4, 153.1,
�
1
(
KBr pellet): nÄ ˆ 1600, 1576 (CˆN), 833 cm (1,4-disubstituted phen-
147.1, 140.0, 139.2, 139.1, 132.5, 129.5, 128.7, 128.4, 125.4, 124.2, 122.1; FTIR
‡
� 1
ylene); MS (FAB): m/z (%): 602 [M] ; elemental analysis calcd (%) for
(KBr pellet): 1604 (CˆN), 841 cm (P�F stretching); UV/Vis (CH
3
CN):
3
� 1
� 1
C
26
H
14
N
6
O
2
Br
2
(602.2): C 51.85, H 2.34, N 13.95; found: C 51.90, H 2.42, N
l
max (e) ˆ 248 (36000), 292 (54000), 448 nm (11000 dm mol cm ); MS
‡
‡
13.95.
(FAB): m/z (%): 897 [M � PF
6
] , 752 [M � 2PF
6
] ; elemental analysis calcd
Chem. Eur. J. 2001, 7, No. 20
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0720-4365 $ 17.50+.50/0
4365

W. K. Chan et al.
FULL PAPER
(
3
%) for C32
H
22
N
6
Br
7.08, H 2.46, N 8.27.
,8-Dibromo-1,10-phenanthroline rhenium(i) tricarbonyl chloride 11: Com-
2
RuP
2
F
12 (1041.4): C 36.91, H 2.13, N 8.07; found: C
[5] a) G. Yu, C. Zhang, A. J. Heeger, Appl. Phys. Lett. 1994, 64, 1540;
b) J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H.
Friend, S. C. Moratti, A. B. Holmes, Nature 1995, 376, 498; c) G. Yu,
A. J. Heeger, J. Appl. Phys. 1995, 78, 4510; d) J. J. M. Halls, K. Pichler,
R. H. Friend, S. C. Moratti, A. B. Holmes, Appl. Phys. Lett. 1996, 68,
3
pound 3,8-dibromo-1,10-phenanthroline 9 (40 mg, 0.12 mmol) and rhe-
nium(i) pentacarbonyl chloride (40 mg, 0.12 mmol) were added to toluene
(
reflux for 2 h. After cooling, the orange-yellow solid was filtered, washed
with toluene and diethyl ether, and dried in vacuum (60 mg, 78% yield).
1H NMR (300 MHz, CDCl
1
1
1
(
(
3
120.
20 mL) under a nitrogen atmosphere. The solution was heated under
[
6] a) Y. Yang, A. J. Heeger, Nature 1994, 372, 344; b) Z. Bao, A.
Dodabalapur, A. J. Lovinger, Appl. Phys. Lett. 1996, 69, 4108; c) H.
Sirringhaus, N. Tessler, R. H. Friend, Science 1998, 280, 1741.
3
13
): d ˆ 9.42 (d, J ˆ 1.8 Hz, 2H), 8.71 (d, J ˆ
[
7] Semiconducting Polymers-Chemistry, Physics and Engineering (Eds.:
G. Hadziioannou, P. F. van Hutten), WILEY-VCH, Weinheim, 2000.
8] a) S. C. Yu, X. Gong, W. K. Chan, Macromolecules 1998, 31, 5639;
b) S. C. Yu, S. Hou, W. K. Chan, Macromolecules 1999, 32, 5251;
c) W. K. Chan, P. K. Ng, X. Gong, S. Hou, J. Mater. Chem. 1999, 9,
.9 Hz, 2H), 7.98 (s, 2H); C NMR (75 MHz, CDCl
3
): d ˆ 157.6, 154.4,
49.3, 146.7, 144.5, 139.9, 132.2, 127.8, 122.1; FTIR (KBr pellet): nÄ ˆ 2015,
[
�
1
935, 1892 (metal carbonyl CO stretching), 1591 cm (CˆN); UV/Vis
):
max (e) ˆ 248 (33000), 292 (30000), 335 (17000), 400 nm
CHCl
4,300 dm mol cm ); MS (FAB): m/z (%): 644 [M] , 613 [M � Cl] ;
ClBr Re (643.7): C 27.99, H
3
l
3
� 1
� 1
‡
‡
2
103; d) S. C. Yu, S. Hou, W. K. Chan, Macromolecules 2000, 33, 3529;
e) L. S. M. Lam, S. H. Chan, W. K. Chan, Macromol. Rapid Commun.
000, 21, 1081.
elemental analysis calcd (%) for C15
0
H
6
N
2
O
3
2
.94, N 4.35; found: C 28.18, H 1.02, N 4.28.
2
Palladium catalyzed polymerization: The synthesis of polymer 13d was
performed as in the general procedure. Divinylbenzene 12 (41 mg,
0
2
4
2
was added by means of a syringe, and the solution was stirred until all the
solid had dissolved. Tri-n-butylamine (0.22 mL) was added, and the
solution was stirred at 1008C for 24 h. The solution was poured into
methanol, and the polymer was washed with methanol in a Soxhlet
extractor for 2 days. The polymer was collected as a dark-brown solid.
[
9] W. E. Jones, Jr., L. Hermans, B. Jiang in Multimetallic and Macro-
molecular Inorganic Photochemistry (Eds.: V. Ramamurthy, K. S.
Schanze), Marcel Dekker, New York, 1999, p. 1.
.315 mmol), ruthenium complex 7 (0.206 g, 0.158 mmol), 1,4-dibromo-
,5-dioctoxybenzene 11 (78 mg, 0.158 mmol), palladium(ii) acetate (2.8 mg,
mol%), and tri-o-tolylphosphane (38 mg, 0.4 equiv) were added to a
5 mL round-bottomed flask under a nitrogen atmosphere. DMF (5 mL)
[
[
10] a) K. Kalyanasundaram, Coord. Chem. Rev. 1982, 46, 159; b) A. Juris,
V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. Von Zelewsky,
Coord. Chem. Rev. 1988, 84, 85.
11] For example: a) L. Trouillet, A. De Nicola, S. Guillerez, Chem. Mater.
2
000, 12, 1611; b) K. D. Ley, C. E. Whittle, M. D. Bartkerger, K. S.
Schanze, J. Am. Chem. Soc. 1997, 119, 3423; c) K. D. Ley, K. S.
Schanze, Coord. Chem. Rev. 1998, 171, 287; d) Q. Wang, L. M. Wang,
L. Yu, J. Am. Chem. Soc. 1998, 120, 12860; e) B. Wang, M. R.
Wasielewski, J. Am. Chem. Soc. 1997, 119, 12; f) C. G. Cameron, P. G.
Pickup, Chem. Commun. 1997, 303; g) S. C. Rasmussen, D. W.
Thompson, V. Singh, J. D. Petersen, Inorg. Chem. 1996, 35, 3449.
(
0.27 g, 83% yield).
Physical characterization: The polymer films for charge-carrier mobility
measurements were prepared by casting a polymer solution on an indium-
tin-oxide coated glass slide, and the solvent was evaporated slowly. The
typical thickness of the polymer film was approximately 0.5 ± 1.0 mm. A thin
layer of gold electrode (100 ± 120 Š) was coated on the polymer film
[12] a) W. K. Chan, X. Gong, W. Y. Ng, Appl. Phys. Lett. 1997, 71, 2919;
b) W. K. Chan, P. K. Ng, X. Gong, S. Hou, Appl. Phys. Lett. 1999, 75,
3920.
�
6
surface by thermal evaporation under high vacuum (10 mbar). The
charge-carrier mobility was determined by the time-of-flight method.
[13] a) V. W. W. Yam, C. L. Chan, S. W. K. Choi, K. M. C. Wong, E. C. C.
Cheng, S. C. Yu, P. K. Ng, W. K. Chan, K. K. Cheung, Chem.
Commun. 2000, 53; b) C. T. Wong, W. K. Chan, Adv. Mater. 1999,
11, 455; c) W. Y. Ng, X. Gong, W. K. Chan, Chem. Mater. 1999, 11,
1165; d) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson,
S. R. Forrest, Appl. Phys. Lett. 1999, 75, 4; e) E. S. Handy, A. J. Pai,
M. F. Rubner, J. Am. Chem. Soc. 1999, 121, 3525; f) X. Gong, P. K. Ng,
W. K. Chan, Adv. Mater. 1998, 10, 1337; g) C. H. Lyons, E. D. Abbas,
J.-K. Lee, M. F. Rubner, J. Am. Chem. Soc. 1998, 120, 12100; h) Y. Ma,
H.-Y. Chao, Y. Wu, S. T. Lee, W.-Y. Yu, C.-M. Che, Chem. Commun.
1998, 2491; i) Y. Ma, H. Zhang, J. Shen, C. M. Che, Synth. Met. 1998,
94, 245; j) J.-K. Lee, D. Yoo, M. F. Rubner, Chem. Mater. 1997, 9, 1710;
k) X. T. Tao, H. Suzuki, T. Watanabe, S. H. Lee, S. Miyata, H. Sasabe,
Appl. Phys. Lett. 1997, 70, 1503.
[
27]
A
Laser ScienceVSL-337 nitrogen laser was used to generate a pulsed laser
source [wavelength ˆ 337.1 nm, pulse energy ˆ 120 mJ, and pulse width full
width at half-maximum (fwhm) ˆ 3 ns]. The transient photocurrent was
monitored by an oscilloscope. The heterojunction photodiode was con-
structed by subliming a layer of C60 (50 nm) onto a polymer film (70 nm)
coated on an ITO glass slide. A layer of aluminum electrode (120 nm) was
coated on the surface by vacuum evaporation. The photocurrent was
measured with a Keithley238 sourcemeter. An ORIEL xenon arc lamp
(
150 W) that simulated solar radiation was used as the light source. The
�
2
irradiation light intensity was 100 mWcm
.
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4, 442.
This project was substantially supported by The Research Grants Council
of The Hong Kong Special Administrative Region, China (Project Nos.
HKU7093/97P, HKU7090/98P, and 7096/00P). Partial financial support
from The Committee on Research and Conference Grants (University of
Hong Kong) is also acknowledged. P.K.N, S.H.C, and L.S.M.L. would like
to thank The Graduate School, University of Hong Kong for the award of
their studentships. We also thank Prof. Wing Tak Wong for his assistance in
the CV experiments.
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