Synthesis, characterization and photovoltaic properties of a low-bandgap platinum(II) polyyne functionalized with a 3,4-ethylenedioxythiophene-benzothiadiazole hybrid spacer

Article abstract of DOI:10.1016/j.jorganchem.2008.08.025

The synthesis, characterization and optical spectroscopy of a deep blue platinum(II) polyyne functionalized with the (3,4-ethylenedioxythiophene)-benzothiadiazole hybrid spacer (P1) and its dinuclear platinum molecular model complex (M1) are described. This metalated polymer P1 exhibits good thermal stability and possesses a narrow bandgap of 1.76 eV. Optical spectroscopic measurements of these materials reveal a substantial donor-acceptor interaction along the rigid backbone of the organometallic polyynes through the interaction of metal center and the conjugated tricyclic ligand. Preliminary study shows that P1: methanofullerene acceptor blend can be used as an active layer of bulk-heterojunction polymer solar cells. Photoexcitation of this blend layer in some yet-to-be optimized cells results in a photo-induced electron transfer from the π-conjugated metallopolyyne electron donor to [6,6]-phenyl C₆₁-butyric acid methyl ester with a power conversion efficiency (PCE) close to 0.30% under air mass (AM1.5) simulated solar illumination. The power dependencies of the solar cell parameters (including the short-circuit current density, open-circuit voltage, fill-factor and PCE) were also studied.

Full text of DOI:10.1016/j.jorganchem.2008.08.025

Journal of Organometallic Chemistry 693 (2008) 3603–3612
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization and photovoltaic properties of a low-bandgap
platinum(II) polyyne functionalized with a 3,4-ethylenedioxythiophene-benzothia-
diazole hybrid spacer
Wai-Yeung Wong a, , Xingzhu Wang , Hai-Liang Zhang , Kai-Yin Cheung , Man-Kin Fung ,
a,b
b
c
c
*
Aleksandra B. Djuriši c´ , Wai-Kin Chan d
Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, PR China
c,
*
a
b
College of Chemistry and Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105,
Hunan Province, PR China
c
Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2008
Received in revised form 19 August 2008
Accepted 20 August 2008
Available online 26 August 2008
The synthesis, characterization and optical spectroscopy of a deep blue platinum(II) polyyne functional-
ized with the (3,4-ethylenedioxythiophene)-benzothiadiazole hybrid spacer (P1) and its dinuclear plati-
num molecular model complex (M1) are described. This metalated polymer P1 exhibits good thermal
stability and possesses a narrow bandgap of 1.76 eV. Optical spectroscopic measurements of these mate-
rials reveal a substantial donor–acceptor interaction along the rigid backbone of the organometallic
polyynes through the interaction of metal center and the conjugated tricyclic ligand. Preliminary study
shows that P1: methanofullerene acceptor blend can be used as an active layer of bulk-heterojunction
polymer solar cells. Photoexcitation of this blend layer in some yet-to-be optimized cells results in a
Keywords:
Polyynes
Metallopolymers
Photovoltaics
Ethylenedioxythiophene
Platinum
photo-induced electron transfer from the
p-conjugated metallopolyyne electron donor to [6,6]-phenyl
C61-butyric acid methyl ester with a power conversion efficiency (PCE) close to 0.30% under air mass
(AM1.5) simulated solar illumination. The power dependencies of the solar cell parameters (including
the short-circuit current density, open-circuit voltage, fill-factor and PCE) were also studied.
Ó 2008 Elsevier B.V. All rights reserved.
1
. Introduction
lar cells are a major driving force for research in bandgap-
g
engineering. However, polymers with E > 2.0 eV only absorb radi-
The discovery of the metal-like conductivity in oxidized
poly(acetylene) and polyaromatic systems (e.g. polypyrrole, poly-
thiophene or polyaniline) in the late seventies and early eighties
ation in the UV and green part of the visible range. Therefore,
chemists have been making efforts to synthesize new low-bandgap
polymers (i.e. polymers with E < 1.9–2.0 eV) to harvest more pho-
g
[
1–4] generated a considerable interest in functional
p
-conjugated
ton energies at long wavelengths to increase the photocurrent.
With the emergence of the donor–acceptor (D–A) concept for
bandgap reduction, a large series of purely organic polymers have
been developed [38,39]. The basic idea is that conjugated systems
with a regular alternation of electron-rich donor and electron-defi-
cient acceptor groups will exhibit a broadening of the valence and
conduction bands and thus bandgap reduction [38–42]. 2,1,3-Ben-
zothiadiazole [38,43,44] and thiophene [45–48] derivatives have
systems. These systems have a potential use as organic semicon-
ductors in a variety of applications, such as polymer light-emitting
diodes [5–10], polymer solar cells (PSCs) [11–19], photodetectors
[
17], biosensors [20,21] and field-effect transistors [22,23]. Among
these, much attention is currently paid to the development of no-
vel conjugated materials [24–26] and polymers [27–34] as low-
cost renewable energy resources to be exploited in photovoltaic
technology [24–26].
been widely used as
p-conjugating spacers in push–pull chro-
At present, the optical bandgaps (E
g
) of the substituted poly[p-
mophores designed for the production of low-bandgap conjugated
polymers. Recently, photovoltaic devices based on a blend of [6,6]-
phenyl C61-butyric acid methyl ester (PCBM) and some of these
low-bandgap conjugated organic polymers have been reported
phenylenevinylene] and the polythiophenes commonly used in the
PSCs are typically 2.0–2.2 eV [12,35,36] and not optimized with re-
spect to the solar spectrum (ca. 350–1500 nm) [19,37]. Organic so-
[
49–52]. Apart from the thiophene ring, 3,4-ethylenedioxythio-
phene (EDOT) also occupies a prominent position as the functional
component for conjugated polymers due, among other things, to
the strong electron donor effect, the self-rigidification of the
*
Corresponding authors. Tel.: +852 34117074; fax: +852 34117348.
E-mail addresses: rwywong@hkbu.edu.hk (W.-Y. Wong), dalek@hkucc.hku.hk
(
A.B. Djuriši c´ ).
0
022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.08.025

3
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W.-Y. Wong et al. / Journal of Organometallic Chemistry 693 (2008) 3603–3612
structure by non-covalent intramolecular interactions as well as
quinoidization of the conjugated system [53].
and M1 are air and thermally stable. Thermogravimetric analysis
(TGA) traces (at a heating rate of 20 °C/min) showed the onset of
the polymer decomposition at around 365 °C. Decomposition on-
set was defined by a 39 wt.-% loss for P1, corresponding to the re-
Although PSCs using organic polymers in a bulk-heterojunction
(
BHJ) architecture have been extensively studied, their metalated
analogues remain under-explored [54,55]. Metallopolymers de-
rived from platinum alkynyl structural units have shown promis-
ing results in this area recently [56–63]. While the field is still in
3
moval of two PBu ligands from P1. In differential scanning
calorimetry (DSC) studies, no phase transition signals are de-
tected during repeated heating/cooling DSC cycles for P1 over
the temperature range from 20 to 300 °C. This observation prob-
its infancy, low-bandgap soluble
p-conjugated organometallic
0
polyynes have gained growing interest [56,60,61,63]. We have re-
cently demonstrated efficient solar cells based on poly-
platinyne:PCBM BHJ with high and tunable PCEs [56,60,61,63]. It
is generally admitted that a conjugated system serving as donor
in a BHJ solar cell should have a bandgap lower than 1.80 eV to
achieve a better harvesting of the energy of solar photons. In fact,
it has been shown that an optimal band gap value for a single junc-
tion cell is 1.5 eV, while the two absorbers in tandem cells should
have gaps of 1.7 eV and 1.1 eV, respectively, to maximize efficiency
ably results from the stiffness of the polymer s chains.
0
Although 4,7-bis[2 -(3,4-ethylenedioxythiophene)]-2,1,3-ben-
zothiadiazole has been reported before [44], its solid-state struc-
ture is not yet known. The X-ray crystal structure of this
compound as well as that for L1-2TMS were determined and per-
spective views of the molecules are depicted in Fig. 1. From
Fig. 1a, the molecule adopts a fully planar geometry with the
dihedral angle between the S(1)- and S(2)-planes of only 3.1°.
Furthermore, we note a short-contact hydrogen bonding interac-
tion between H(9A) and O(2) [H(9A)ꢀꢀꢀO(2) = 2.193 Å]. Close
examination of the non-bonded distance between sulfur and
nitrogen atoms of the adjacent cycles shows that the sulfur-
nitrogen distance [S(1)ꢀꢀꢀN(1) = 2.868 Å] is markedly smaller than
the sum of the van der Waals radii of the individual atoms, thus
demonstrating the existence of non-covalent intramolecular
interactions which contribute to planarize the system [53]. For
L1-2TMS, the two trimethylsilylacetylide groups are bonded to
the central tricyclic ring which also reveals the involvement of
non-bonded short contacts [S(1)ꢀꢀꢀN(1) = 2.882, S(3)ꢀꢀꢀN(2) = 2.848,
H(16A)ꢀꢀꢀO(3) = 2.201, H(17A)ꢀꢀꢀO(2) = 2.218 Å] to ensure the pla-
narity and rigid skeleton of the molecule. The S(2)-plane makes
dihedral angles of 1.0° and 3.4° with the S(1)- and S(3)-planes,
respectively. Thus, in addition to the electronic effects already
observed in the tricyclic systems [53], EDOT introduces a self-
rigidification effect which contributes to a further reduction of
the bandgap.
[
34].
Although this E
g
value has been demonstrated for purely organ-
ic polymers, the problem is complicated by the lack of examples of
narrow-gap organometallic polyynes. Most metal polyyne poly-
mers characterized so far have large E (>2.0 eV) [64–67], which
g
compare unfavorably with those of their organic counterparts.
We report here the synthesis, characterization and photovoltaic
properties of a new low-bandgap metallopolyyne P1 with 4,7-
0
bis[2 -(3,4-ethylenedioxythiophene)]-2,1,3-benzothiadiazole tricy-
clic chromophore as a versatile building block that use a donor–
acceptor approach to achieve absorption in the almost red spectral
region.
2
. Results and discussion
2.1. Synthesis and chemical characterization
The synthesis and chemical structures of platinum-containing
metallopolyyne (P1) and its well-defined model complex (M1)
2.2. Photophysical and electrochemical characterization
are shown in Scheme 1. 4,7-Dibromo-2,1,3-benzothiadiazole
[
44] was first coupled with a monosubstituted tributylstannyl
The photophysical properties of the polymer and model com-
derivative of EDOT utilizing the standard Stille chemistry [68]
to give the tri-block 4,7-bis[2 -(3,4-ethylenedioxythiophene)]-
plex were measured in CH
1). Fig. 2a shows the absorption spectra of L1-2TMS, P1 and M1 in
2 2
Cl solutions at 293 K (Fig. 2 and Table
0
5
3
2
,1,3-benzothiadiazole system, which upon iodination with N-
CH
2
Cl
2
(conc. ꢁ5 ꢂ 10 mol/dm ), as well as P1 in solid thin film.
iodosuccinimide (NIS) can form L1-I. Conversion of the diiodo
precursor to its corresponding trimethylsilyl-substituted congener
L1-2TMS is accomplished via the typical organic synthetic routes
for alkynylation of aromatic halides [69]. The platinum(II) com-
pounds P1 and M1 were prepared by the Sonogashira dehydroha-
logenative reactions between the diethynyl derivative (which
should be prepared in situ under an inert atmosphere from the
The spectra displayed two major absorption bands in each case.
The lowest-energy absorption features are lowered significantly
(by ca. 0.31 eV) when the metal is introduced in the polymer chain,
i.e., on going from L1-2TMS to P1, which is expected for a donor–
acceptor (D–A)
p-conjugated framework [57,71,72]. The absorp-
tion properties of M1 (kmax = 385, 586 nm) are only slightly blue-
shifted from those of P1 (kmax = 387, 590 nm). From the similar
reaction of L1-2TMS with Bu
4
NF in THF to avoid its rapid decom-
pattern of the absorption bands of ligand and polymer (Fig. 2a),
*
position) and the platinum chloride or dichloride precursors in an
amine solvent in the presence of CuI [56–67]. The phenyl end-
capped complex M1 represents a good molecular model com-
pound for the elucidation of the spectroscopic and photophysical
properties of P1. The polymer is purified by silica column chro-
matography and repeated precipitation. The structures of both
materials were characterized using elemental analyses, mass
spectrometry, IR and NMR spectroscopies. The NMR data are con-
sistent with the proposed structure of P1 and M1. The dichloro-
methane-soluble fraction of P1 was isolated by Soxhlet
extraction. The synthetic yield is not high for P1 since a certain
portion of the polymer formed under our experimental conditions
was organic insoluble. As a consequence of the poorer solubility
of P1, the molecular weight of the soluble fraction is believed
to be relatively low (see Section 4). Th narrow polydispersity
we infer that the absorption is mainly due to the
p
ꢃ
p
transition
of the conjugated ligand from the highest occupied molecular orbi-
tal (HOMO) to the lowest unoccupied molecular orbital (LUMO),
although the energy levels of the orbitals involved are perturbed
by strong interactions with the metal center. Moreover, the
absorption spectrum of the polymer in solid thin films is batho-
2 2
chromically shifted compared to that recorded in CH Cl solution,
presumably due to the interchain interactions in the film state.
For many organic systems, it could be the case that aggregates con-
sisting of many polymer chains are formed in the solid state, and
the electronic communication of the
p-systems via tight packing
and a stacking effect leads to the observed bathochromic shift.
p–p
Alternatively, the occurrence of the red-shifted feature is a single-
molecule effect, similarly to that for polythiophenes, in which
planarization of the
forced planarization then results in increased conjugation and thus
leads to a lower E . The optical bandgap of 1.76 eV was estimated
p-system is induced by aggregation [70]. The
(
PDI < 2) in molecular weights is consistent with the proposed
linear structure from the condensation polymerization. Both P1
g

W.-Y. Wong et al. / Journal of Organometallic Chemistry 693 (2008) 3603–3612
3605
O
O
O
O
O
O
Br
Br
+
(i)
N
N
Bu Sn
3
S
S
S
S
N
N
S
(
ii)
O
O
O
O
O
O
O
O
(
iii)
TMS
TMS
I
I
S
S
S
S
N
N
N
N
S
S
L1-2TM
S
L1-2I
(iv)
PBu3
Pt
O
O
O
O
(
v)
S
S
n
N
N
PBu3
PEt3
O
O
O
O
PEt3
S
P1
Pt
Pt
S
S
PEt3
N
N
PEt3
S
M1
Scheme 1. Synthesis of metallopolymer P1 and the model complex M1. Conditions and reagents: (i) Pd(PPh
3
)
4
, PhCH
3
, reflux; (ii) CHCl
NH/CH Cl
3
/AcOH, NIS; (iii) Me
3
SiC„CH,
i
i
Pd(OAc)
2
, PPh
3
, CuI, Et
3
N; (iv) Bu
4
NF, trans-[Pt(PEt
3
)
2
PhCl], CuI, Pr
2
2
NH/CH Cl
2
4
and (v) Bu NF, trans-[Pt(PBu
3
2
) Cl
2
], CuI, Pr
2
2
2
.
0
Fig. 1. X-ray crystal structures (with thermal ellipsoids drawn at 25% probability levels) of (a) 4,7-bis[2 -(3,4-ethylenedioxythiophene)]-2,1,3-benzothiadiazole. Selected
bond lengths (Å) and angles [°]: S(1)–C(1), 1.704(5); S(1)–C(4), 1.741(4); C(4)–C(8), 1.445(6); S(2)ꢃN(1), 1.608(4); C(7)ꢃN(1), 1.354(5); N(1)–S(2)–N(1A), 101.1(3). (b) L1-
2
TMS. The labels on the methyl carbon atoms are omitted for clarity. Selected bond lengths (Å) and angles [°]: Si(1)ꢃC(4), 1.858(5); C(4)ꢃC(5), 1.158(5); S(1)–C(6), 1.729(4);
S(1)–C(9), 1.743(4); C(9)–C(12), 1.469(5); S(2)ꢃN(1), 1.620(3); S(2)ꢃN(2), 1.610(3); C(15)ꢃC(18), 1.465(5); S(3)ꢃC(18), 1.742(4); S(3)ꢃC(21), 1.734(4); Si(2)ꢃC(25), 1.858(5);
C(24)ꢃC(25), 1.168(5); Si(1)–C(4)–C(5), 173.9(4); C(4)–C(5)–C(6), 177.4(5); N(1)–S(2)–N(2), 100.6(2); Si(2)–C(25)–C(24), 175.3(4); C(21)–C(24)–C(25), 176.3(4).

3
606
W.-Y. Wong et al. / Journal of Organometallic Chemistry 693 (2008) 3603–3612
able acceptor groups tends to reduce E
of kmax to the red. Due to the presence of a more extended
tron delocalized system throughout the chain and the creation of
an alternate D–A chromophore based on an electron-accepting
benzothiadiazole group in combination with the electron-donating
g
accompanied by a shift
-elec-
a
p
L1-2TMS
P1
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
film P1
M1
g
EDOT rings, the E value of P1 is significantly reduced by ca.
0
.79 eV relative to the purely electron-rich bithienyl (2.55 eV) con-
gener [59]. A reduction of the bandgap by 0.44 eV is achieved in P1
upon adding one EDOT ring on each side of benzothiadiazole,
which results in onset of optical absorption at 590 nm. An increase
of the donor strength in P1 with the electron-releasing ethylenedi-
oxy bridge leads to a bathochromic shift of kmax accompanied by a
0
decrease in E
g
relative to that with a 4,7-di-2 -thienyl-2,1,3-benzo-
thiadiazole spacer. Comparison of the electronic absorption spec-
trum of P1 to those of terthienyl counterpart also reveals a
considerable red shift of the absorption maximum indicative of
the reduction of the HOMO-LUMO gap. The triblock heteroaro-
matic spacer in P1 with a functional median group inserted
between two lateral EDOT groups already has a strong D–A inter-
3
00
400
500
600
700
800
Wavelength (nm)
b
L1-2TMS
P1
action which can be enhanced by the extensive
p-delocalization
1
.0
in the conjugated polymer backbone containing the electron-rich
platinum (Fig. 3). This reflects an important role of strong
D–A interaction in P1, which is created by a regular alternation
of D- and A-like moieties, possibly separated by neutral parts. If
the electron donor and acceptor regions are extended through
the Pt center and alkynyl units we get a system analogous to the
inorganic n-i-p-i superlattice quantum well structure that exhibit
M1
0
0
0
0
0
.8
.6
.4
.2
.0
g
a much lower E [71,72].
The photoluminescence (PL) spectra of P1 and M1 are depicted
in Fig. 2b. They displayed one strong structureless peak in the
near-infrared region at 720 and 732 nm, respectively. Compared
to the Pt polyyne with bis(thienyl)-benzothiadiazole which fluo-
resces at 680 nm in CH
sion wavelength for P1 (
2
Cl
2
, we note a clear red shift in the emis-
k = 40 nm). The measured emission
500
600
700
800
90
D
Wavelength (nm)
lifetime in the nanosecond range for P1 and M1 (2.51 and
2.27 ns, respectively) suggests a radiative decay from the singlet
excited state (i.e. fluorescence). There is no emission from the
long-lived triplet excited state over the measured spectral win-
dow between 1.2 and 3.1 eV for P1 and M1. This can be explained
in terms of the energy gap law for non-radiative decay for conju-
gated Pt polyynes and their molecular diynes, whereby the non-
radiative decay rate increases exponentially with decreasing
triplet–singlet energy gap according to the expression
Fig. 2. (a) UV–Vis absorption and (b) PL spectra for L1-2TMS, M1 and P1.
Table 1
Photophysical data of L1-2TMS, M1 and P1
(nm)b
(eV)c
E
g
Samples
k
abs in
k
em in CH
2
Cl
2
2 2
CH Cl (nm)
L1-2TMS
M1
357 (3.9), 513 (2.8)
332sh (1.7), 385 (4.4),
630 (0.74, 10.5)
732 (0.026, 2.27)
2.12
1.84
(
T
k
nr
)
ꢃS
P
1 exp(ꢃA
DESꢃT), where DESꢃT is the energy gap for the
transition and A is a term controlled by the molecular
401sh (4.0), 586 (3.1)
1
0
P1
332sh, 387, 398sh, 590
720 (0.021, 2.51)
1.83
(
parameters and vibrational modes [74]. This phenomenon also in-
volves the bandgap/phosphorescence decay rate trade-off com-
1.76)d
(
333sh, 368sh, 389, 407sh, 619)a
g
monly observed in such systems. Hence, a high-E polymer will
a
k
abs values obtained from thin film.
Emission quantum yields ( ) and lifetimes (
). kex = 500 nm.
favor phosphorescence whereas a low E value will be detrimen-
g
b
U
s) in ns are shown in parentheses
tal for the triplet emission. The fully extended heteroaryl rings in
the ligand chromophore greatly reduces the influence of heavy Pt
ion which is mainly responsible for the intersystem crossing and
the phosphorescence. Hence, it is not the triplet state but a
(
U
,
s
c
Optical bandgaps obtained from absorption spectra in CH
Optical bandgap from the solid-state thin film spectrum.
2 2
Cl .
d
from the onset of solid-state absorption spectrum. There is only a
small difference in E between P1 and M1. To date, the value of
.76 eV is one of the lowest among the reported polyplatinynes
and clearly represents a significant step forward in the develop-
ment of low-bandgap metallopolyyne polymers [64–67]. The sig-
g
1
PBu3
Pt
O
O
O
O
S
S
nificant D–
p
ꢃA structural motif constitutes a major tool for the
n
N
N
modulation of the electronic properties of extended systems and
PBu3
S
has an immediate effect on the resulting optical properties. Table
P1
2
compares the solid-state colors, E
P1 with some related platinum(II) polyynes in the literature in
terms of their -conjugating ability in which the addition of suit-
g
and thermal stability data of
Fig. 3. The expected direction of electron transfer after excitation in metallopoly-
mer P1.
p

W.-Y. Wong et al. / Journal of Organometallic Chemistry 693 (2008) 3603–3612
3607
Table 2
Structural effects on the optical and thermal stability data of various low-bandgap platinum(II)-based metallopolyynes with different central spacer Ar
PBu3
Pt
Ar
n
PBu3
Ar
Color
Peak kmax in solid film (nm)
E
g
(eV)
Tdec (°C)
Ref.
Yellow
446
2.55
278
[59]
S
S
Orange-yellow
Orange-red
484
528
2.40
290
[59]
[73]
S
S
S
2.20
1.85
Not reported
N
N
S
Purple
548
616
284
[60]
S
S
N
N
S
Deep blue
1.76
365
This work
O
O
O
O
S
S
N
N
S
ligand-based
p
ꢃ
p* excited state that contributes to the photoin-
2.3. Polymer photovoltaic behavior
duced charge separation in the energy conversion for P1 without
the participation of the triplet state [57].
To examine if the low-energy absorption band of P1 contributes
to the photovoltaic energy conversion in PSCs, BHJ solar cells were
fabricated by using P1 as an electron donor and PCBM as an elec-
tron acceptor (Table 3). The hole collection electrode consisted of
indium tin oxide (ITO) with a spin-coated poly(3,4-ethylenedioxy-
thiophene):poly(styrene sulfonate) (PEDOT:PSS), while Al served
The HOMO and LUMO energy levels of P1 were calculated using
the onset oxidation and reduction potentials as determined from
cyclic voltammetric method. The experiments were performed by
casting the polymer films on the glassy-carbon working electrode
with a Ag/AgCl wire as the reference electrode, at a scan rate of
ꢃ1
5
0 mV s . The solvent in all measurements was deoxygenated
n
MeCN, and the supporting electrolyte was 0.1 M [ Bu
From the onset values of oxidation (Eonset ox) and reduction
red), the HOMO and LUMO levels of P1 were calculated
according to the following equations EHOMO = ꢃ(Eonset ox + 4.72) eV
and ELUMO = ꢃ(Eonset red + 4.72) eV (where the unit of potential is
V vs. Ag/AgCl) [75]. P1 shows a quasi-reversible oxidation wave
with the Eonset ox at 0.62 V, giving rise to a HOMO level of
ꢃ5.34 eV. Oxidation arises from the -conjugated heteroaromatic
4 4
N][BF ].
,
Table 3
onset
(E ,
Solar cell performance of best devices with P1
,
PSC
Voc (V)
J
sc (mA
FF
Max. PCE
(%)
Max. EQE
(%)
,
ꢃ2
c
cm
)
P1:PCBM
0.55 (0.54) 1.68 (1.68)
0.53 (0.53) 1.93 (1.83)
0.32 (0.31) 0.29 (0.28)
8.5 (567)
,
a
(
1:5)
p
P1:PCBM
0.28 (0.28) 0.28 (0.27) 10.2 (567)
(1:5)b
segment of P1. The elevated HOMO level of P1 relative to the
one with dithienyl-benzothiadiazole unit (ꢃ5.37 eV) corroborates
with the stronger donor property of the EDOT groups in the former
case, which is also consistent with the absorption data shown
above. The LUMO level of P1 was then estimated to be ꢃ3.74 eV
for measurements obtained from a polymer thin film.
The numbers in parentheses denote average from 6 devices.
a
Obtained from toluene.
Obtained from chlorobenzene.
For EQE maxima, the numbers in brackets denote the wavelength in nm at
b
c
which maximum occurs.

3
608
W.-Y. Wong et al. / Journal of Organometallic Chemistry 693 (2008) 3603–3612
0
0
0
.2
.1
.0
affected by the blend film morphology. Comprehensive study of
phase separation in the blend, film morphology and charge trans-
port is necessary to further improve FF and overall device
performance.
The external quantum efficiency (EQE) for the solar cells has
been measured, as shown in Fig. 6. All EQE curves of the polymer
photodiodes show onset near 750 nm, and there is a photoinduced
charge transfer from the polymer to PCBM. The EQE photoaction
spectrum shows a major peak at around 567 nm that corresponds
to the absorption spectral profile of P1. The similarity of the
absorption spectrum and EQE response demonstrates that the exci-
tons produced by absorption in the polymer are dissociated into
charge carriers at the contact between P1 and PCBM in the active
layer, and are subsequently collected at the electrodes. The broad
EQE curves for P1 cover the visible-light region from 400 up to
4
00
500
600
700
800
ꢁ
800 nm with maxima values of 8.5–10.2%. In addition to other
Wavelength (nm)
prerequisites of the donor materials such as their electronic prop-
erties and absorption ability, they must combine solution process-
ability and appropriate compatibility with PCBM to provide a
composite material with optimal nanoscale morphology. Never-
theless, these devices are still not fully optimized and similar
materials of different molecular weight ranges are anticipated to
show better results. Further studies are currently underway to
obtain devices of higher performance.
Fig. 4. The absorption spectrum of the P1:PCBM (1:5) active blend layer.
2
1
0
1
2
toluene
chlorobenzene
toluene (dark current)
chlorobenzene (dark current)
20
15
10
5
0
chlorobenzene (1.7 mA cm−2)
-
-
−2
toluene (1.4 mA cm
)
0
.0
0.2
0. 4
0.6
0.8
V (V)
Fig. 5. J–V curves of the polymer solar cells with P1:PCBM active layers (at the
blend ratio of 1:5 w/w) in the dark and under simulated AM1.5 solar irradiation.
4
00
500
600
700
800
as the electron collecting electrode. The absorption spectrum of P1
blended with PCBM (1:5 by weight) is shown in Fig. 4, which
shows a broad band peaking at around 600 nm. The higher energy
peak in the blend film is less pronounced because of the dramati-
cally enhanced absorption in the UV region due to PCBM. Fig. 5 dis-
plays the current density–voltage (JꢃV) characteristics of the PSCs
recorded under air mass (AM 1.5) simulated solar illumination and
the corresponding dark current curves. The BHJ devices (1:5 blend
ratio) prepared from toluene solution exhibited an open-circuit
V (V)
Fig. 6. EQE wavelength dependencies of solar cells with P1:PCBM (1:5) active
layers.
0.5
0
.0
voltage VOC of 0.55 V, a short-circuit current density JSC of
2
1
.68 mA/cm , and a fill factor (FF) of 0.32, corresponding to a PCE
1
0
of ꢁ0.29%. Similar values were obtained from the chlorobenzene
solvent. These values are comparable to those for some low-band-
gap near-infrared-absorbing organic polymer diodes reported in
the literature [51,76]. It should be noted that in spite of low band-
gap, the obtained power conversion efficiency of this polymer is
not as high as that obtained for other low bandgap metal-contain-
ing polymers [60,61]. There are several possible reasons for that,
such as lower absorption, lower molecular-weight as well as solu-
bility yielding unfavorable phase separation and film quality and
morphology. The JSC and FF values are relatively low, which in
addition to factors mentioned above can also be affected by pro-
cessing and measurement in ambient atmosphere, which can re-
sult in the presence of traps. In addition, JSC and FF would be
affected by the charge collection efficiency, which is in turn
-0.5
20
30
40
-
1.0
50
60
-1.5
70
80
90
-
2.0
100
DC
0
.0
0.2
0. 4
0.6
V (V)
Fig. 7. JꢃV curves of the P1:PCBM (1:5) device for different illumination power (in
ꢃ2
units of mW cm ). The dark current (DC) curve is also included.

W.-Y. Wong et al. / Journal of Organometallic Chemistry 693 (2008) 3603–3612
3609
2
1
1
0
.0
.5
.0
.5
will result in further improvements in the efficiency. The depen-
dence of solar cell performance on molecular weight should not
be overlooked, which can cause significant changes in the blend
morphology, and polymers of higher molecular weights should
benefit to the overall performance.
0.5
0.4
0.3
0.2
4
. Experimental
4.1. General information
Voc (V)
Solvents were carefully dried and distilled from appropriate
drying agents prior to use. Commercially available reagents were
used without further purification unless otherwise stated. trans-
FF
PCE (%)
–
2
0.0
Jsc (mA cm )
[
2 3 2 3 2
PtCl (PBu ) ] [80] and trans-[PtPh(Cl)(PEt ) ] [81] were prepared
0
20
40
60
80
100
as described in the literature. All reagents for chemical syntheses
were purchased from Aldrich or Acros Organics. PCBM was pur-
chased from American Dyes. PEDOT:PSS (Baytron VPAI 4083) was
purchased from H.C. Starck. Reactions and manipulations were car-
ried out under an atmosphere of prepurified nitrogen using
Schlenk techniques. All reactions were monitored by thin-layer
chromatography (TLC) on silica with Merck pre-coated glass plates.
Flash column chromatography was carried out using silica gel from
Merck (230ꢃ400 mesh).
–
2
Power (mW cm )
Fig. 8. Power dependencies of open-circuit voltage (VOC), short-circuit current
density (JSC), fill factor (FF) and power conversion efficiency (PCE) for the P1:PCBM
(
1:5) cells.
To study the performance of PSCs, it is important to understand
which mechanisms control the JꢃV characteristics of a given device
and the fate of the photogenerated electrons and holes. So, the
influence of light intensity on the solar cell parameters is very
informative for analyzing internal recombination losses [77]. The
JꢃV curves for P1 under different excitation powers and the power
dependence of the solar cell parameters are shown in Figs. 7 and 8,
respectively. The JSC exhibits a linear dependence on the optical
power, while Voc shows some increase and then saturates at higher
intensity, as expected [78]. The almost linear photocurrent depen-
4.2. Physical measurements
2 2
Infrared spectra were measured as CH Cl solutions using a
Perkin–Elmer Paragon 1000 PC FTIR spectrometer. Fast atom bom-
bardment (FAB) mass spectra were recorded on a Finnigan MAT
SSQ710 system. NMR spectra were measured in CDCl
3
on a Varian
Inova 400 MHz FT-NMR spectrometer and chemical shifts are
1
13
dence on the light intensity for voltage approaching VOC does not
quoted relative to tetramethylsilane for H and C nuclei and
3
31
give a slope of ꢁ
4
, which indicates the absence of space-charge-
H
8
3
PO
453 diode array spectrophotometer. The emission spectra of the
polymers were measured on a Photon Technology International
PTI) Fluorescence QuantaMaster Series QM1 spectrophotometer.
The emission quantum yields ( ) were determined in CH Cl solu-
4
for P nucleus. UV–Vis spectra were obtained on an HP-
limited device performance and that the charge-carrier losses in
the absorber bulk are dominated by monomolecular recombina-
tion under short-circuit conditions. The PCE only decreases slightly
with the illumination intensity, reaching a peak of 0.32% at
0 mW cm . The FF shows a maximum of 0.35 at a low power le-
vel of 10 mW cm but drops gradually at higher light intensities,
in agreement with previously reported power dependence of FF
in a BHJ cell [79]. In another trial on another polymer sample of
comparable molecular weights, we even witnessed a PCE = 0.42%
at 20 mW cm (see Figs. S1 and S2 in Supporting Information)
and similar trends in the device performance parameters were
observed.
(
U
2
2
ꢃ2
3
tions at 293 K against the Rhodamine B in EtOH standard [82]. The
CV measurements were carried out at a scan rate of 50 mV/s using
a eDAQ EA161 potentiostat electrochemical interface equipped
with a thin film coated ITO covered glass working electrode, a plat-
inum counter electrode and a Ag/AgCl (in 3 M KCl) reference elec-
ꢃ2
ꢃ2
trode. The solvent in all measurements was deoxygenated MeCN,
n
and the Supporting electrolyte was 0.1 M [ Bu
4
4
N][BF ]. Thin poly-
mer films were deposited on the working electrode by dip coating
3
in chlorobenzene solution (6 mg/cm ). The onset oxidation and
3
. Conclusion
reduction potentials were used to determine the HOMO and LUMO
energy levels using the equations EHOMO = [ꢃ(Eonset,ox (vs. Ag/AgCl)
–
–
In summary, a metalated push–pull conjugated polymer using
,4-ethylenedioxythiophene as the donor group and 2,1,3-benzo-
E
onset (N.H.E. vs. Ag/AgCl)] ꢃ 4.50 eV and ELUMO = [ꢃ(Eonset,red (vs. Ag/AgCl)
3
Eonset (N.H.E. vs. Ag/AgCl)] ꢃ 4.50 eV, where the potentials for N.H.E. vs.
thiadiazole as the acceptor block was prepared and characterized.
Optical spectroscopy suggests a very narrow bandgap system for
this polymer. Preliminary results on unoptimized BHJ solar cells
indicate a PCE of 0.30% under solar-light illumination in AM 1.5
conditions. The present work emphasizes good opportunities of-
fered by the EDOT building block for the design and synthesis of
vacuum and N.H.E. vs. Ag/AgCl are 4.50 and ꢃ0.22 V, respectively
[75]. The molecular weights of the polymer were determined by
GPC (HP 1050 series HPLC with visible wavelength and fluorescent
detectors) using polystyrene standards and THF as eluent. Thermal
analyses were performed with the Perkin–Elmer Pyris Diamond
DSC and the Perkin–Elmer TGA6 thermal analyzers.
new classes of functional
p-conjugated metallopolymers for ad-
vanced electronic and photonic applications. To further improve
the solubility and solution-processability of this type of highly con-
jugated polyaromatic structure we should aim at a parent organic
precursor in which EDOT is substituted by a long alkoxy chain
grafted at the ethylenedioxy bridge. Also, device optimization re-
quires further studies of the blend morphology and charge trans-
port in the blends with different compositions. It is expected that
optimization at all levels of device construction and composition
4.3. Preparation of compounds
4.3.1. Preparation of 2-tributylstannyl-3,4-ethylenedioxythiophene
To a solution of 3,4-ethylenedioxythiophene (2.30 g, 160 mmol)
0
0
and N,N,N ,N -tetramethylethylenediamine (TMEDA) (2.10 g,
185 mmol) in anhydrous diethyl ether (60 mL), a solution of n-
butyllithium (10.0 mL, 160 mmol, 1.6 M in hexane) was slowly
added using a syringe under an argon atmosphere at room temper-

3
610
W.-Y. Wong et al. / Journal of Organometallic Chemistry 693 (2008) 3603–3612
ꢃ1
ature. The mixture was stirred at room temperature for 10 min,
and then refluxed for 30 min. A pink, milk-like mixture was
formed. The mixture was cooled to ꢃ78 °C, and tributyltin chloride
(KBr):
m
(C„C) 2135 cm . 1H NMR (CDCl
3
, d): 8.41 (s, 2H, benzo),
1
3
4.39 (m, 8H, ethylene), 0.27 ppm (s, 18H, TMS). C NMR (CDCl
d): 152.09, 144.26, 139.40, 127.00, 123.40, 114.92, 104.15,
100.95, 94.95 (Ar), 64.76, 64.72 (CH ), 0.00 ppm (TMS). FAB-MS:
Si : C, 55.23; H, 4.64;
3
,
(
5.18 g, 160 mmol) was added slowly to it over 1.5 h. The reaction
2
+
mixture was warmed up to room temperature and stirred for 3 h.
After it was quenched with saturated sodium chloride solution
m/z 608 (M ). Anal. Calc. for C28
H
28
N
2
O
4
S
3
2
N, 4.60. Found: C, 55.38; H, 4.55; N, 4.80%.
(
(
(
50 mL), its organic layer was extracted with diethyl ether
3 ꢂ 50 mL), dried over MgSO and filtered. Triethylamine
10 mL) was added to the filtrate, and the solvents were evapo-
4
,
4.3.5. Synthesis of platinum metallopolyyne P1
Polymerization was carried out by mixing L1-2TMS (25 mg,
0.04 mmol), trans-[Pt(PBu Cl (27 mg, 0.04 mmol) and CuI
(40 mL, 1:1, v/v). Bu NF (0.08 mL of a
1 M solution in THF) was subsequently added and the solution
quickly turned deep blue. After stirring at room temperature for
20 h under nitrogen, the solution mixture was evaporated to dry-
ness. The residue was redissolved in CH Cl and filtered through
2 2
a short silica column using the same eluent to remove the ionic
impurities and catalyst residues. After removal of the solvent, the
rated. The residue was purified using column chromatography
with hexane on a pre-treated silica gel (silica gel was washed with
neat triethylamine, and then with hexane). The solvent was re-
3
)
2
2
]
i
(2.0 mg) in Pr
2
NH/CH
2
Cl
2
4
moved under vacuum and the residue was further purified through
1
vacuum distillation to produce a colorless liquid (yield: 75%);
NMR (CDCl
m, 6H, SnCH
J = 7.2 Hz, 9H, CH
H
3
, d): 6.59 (s, 1H, thienyl), 4.17 (m, 4H, ethylene), 1.58
), 1.35 (m, 6H, CH ), 1.11 (m, 6H, CH ), 0.91 ppm (t,
).
(
2
2
2
3
2 2
crude product was purified twice by precipitation in CH Cl from
MeOH. Subsequent washing with hexane and drying in vacuo gave
0
4
.3.2. Preparation of 4,7-bis[2 -(3,4-ethylenedioxythiophene)]-2,1,3-
benzothiadiazole
Pd(PPh (20.0 mg) was added to a solution of 4,7-dibromo-
ꢃ1
a deep blue solid of P1 (23 mg, 53%). IR (KBr):
NMR (CDCl
4H, ethylene), 2.22ꢃ2.07 (m, 12 , CH
m
(C„C) 2099 cm
.
3
)
4
3
, d): 8.39 (s, 2H, benzo), 4.35 (m, 4H, ethylene), 4.28 (m,
), 1.59ꢃ1.46 (m, 24H, CH ),
); P NMR (CDCl , d): 3.40 ppm
( JPꢃPt = 2390 Hz). GPC (THF): = 5400, = 8800,
= 1.63. Anal. Calc. for (C46 PtS
N, 2.64. Found: C, 52.43; H, 6.18; N, 2.98%.
2
3
(
,1,3-benzothiadiazole (1.47 g, 5.00 mmol) and 2-tributylstannyl-
,4-ethylenedioxythiophene (5.40 g, 12.5 mmol) in toluene
50 mL). The mixture was refluxed in a nitrogen atmosphere for
2 h, and the mixture was then cooled to room temperature, after
2
2
3
1
0.99ꢃ0.94 ppm (m, 18H, CH
3
3
1
M
w
M
n
w
M /
1
M
n
H
64
N
2
O
P
4 2
)
3 n
: C, 52.01; H, 6.07;
which the solvent was removed under reduced pressure. The prod-
uct was purified by column chromatography eluting with hexane/
1
CH
CDCl
H, ethylene), 4.32–4.30 ppm (m, 4H, ethylene). C NMR (CDCl
d): 152.36, 141.67, 140.27, 126.65, 123.7, 113.74, 101.98 (Ar),
2
Cl
2
(1:2, v/v) to give a red solid in 65% yield (2.70 g). H NMR
4.3.6. Synthesis of platinum model complex M
i
(
4
3
, d): 8.39 (s, 2H, benzo), 6.56 (s, 2H, thienyl), 4.41–4.39 (m,
To a stirred mixture of L1-2TMS (13.0 mg, 0.02 mmol) in Pr
2
NH
PhCl]
NF (0.04 mL of a 1 M
13
3
,
(10 mL) and CH
(23 mg, 0.04 mmol) and CuI (1.0 mg). Bu
2 2 3 2
Cl (10 mL) was added trans-[Pt(PEt )
4
+
6
C
3
5.01, 64.37 ppm (CH
: C, 51.91; H, 2.90; N, 6.73. Found: C, 51.78; H,
.02; N, 6.83%.
2
). FAB-MS: m/z 416 (M ). Anal. Calc. for
solution in THF) was added. The solution was stirred at room tem-
perature under nitrogen over a period of 24 h, after which all vol-
atile components were removed under vacuum. The crude product
18
12 2 4 3
H N O S
was taken up in CH
with hexane/CH Cl
tained as a deep blue solid (18 mg, 62%). IR (KBr): m
2
Cl
(2:5, v/v) as eluent. The product M was ob-
(C„C)
3
H NMR (CDCl , d): 8.34 (s, 2H, benzo), 7.32 (d,
2
and purified on preparative silica TLC plates
4
.3.3. Preparation of L1-2I
2
2
0
4
,7-Bis[2 -(3,4-ethylenedioxythiophene)]-2,1,3-benzothiadiaz-
ꢃ1
1
ole (417 mg, 1.0 mmol) was dissolved in a mixture of chloroform
30 mL) and acetic acid (30 mL) under a nitrogen atmosphere,
2080 cm
.
(
J = 7.2 Hz, 4H, Hortho of Ph), 6.97 (t, J = 7.4 Hz, 2H, Hmeta of Ph),
6.80 (t, J = 7.2 Hz, 4H, Hpara of Ph), 4.37–4.35 (m, 4H, ethylene),
and N-iodosuccinimide (490 mg, 2.20 mmol) was immediately
added. After the reaction mixture had been stirred at room temper-
ature for 6 h, it was then poured into cold water and the mixture
was extracted with chloroform. The organic layer was washed
4.31–4.30 (m, 4H, ethylene), 1.82–1.75 (m, 24H, CH
2
),
1
3
1.14ꢃ1.06 ppm (m, 36H, CH
3
).
C NMR (CDCl , d): 156.32,
3
152.49, 141.23, 140.02, 139.20, 127.26, 127.02, 126.05, 123.10,
twice with water and dried over MgSO
4
. The solution was then
121.19, 109.61, 108.19, 99.40 (Ar + C„C), 64.83, 64.29 (ethylene),
3
1
concentrated under vacuum and methanol was added. The precip-
itate formed was filtered off, washed with methanol and dried in
an oven to give the crude product, which was recrystallized from
15.09, 8.10 ppm (Et);
P
NMR (CDCl
3
,
d): 9.85 ppm
( JPꢃPt = 2628 Hz). FAB-MS: m/z: 1478 (M ). Anal. Calc. for
Pt : C, 47.09; H, 5.45; N, 1.89. Found: C, 46.82; H,
5.23; N, 2.02%.
1
+
C H
58 80
N
2
O
P
4 2
2 3
S
a mixture of CH
in 87% yield (582 mg). H NMR (CDCl
.39 ppm (m, 8 H, ethylene). FAB-MS: m/z 668 (M ). Anal. Calc.
for C18 : C, 32.35; H, 1.51; N, 4.19. Found: C, 32.45; H,
.56; N, 4.34%.
2 2
Cl and hexane (1:3, v/v) to afford a dark red solid
1
3
, d): 8.35 (s, 2H, benzo),
+
4
4.4. Solar cell fabrication and characterization
10 2 2 4 3
H N I O S
1
The device structure was ITO/poly(3,4-ethylenedioxythio-
phene):poly(styrene (PEDOT:PSS)/polymer:PCBM
sulfonate)
4.3.4. Preparation of L1-2TMS
blend/Al. ITO glass substrates (10 X per square) were cleaned by
To an ice-cooled mixture of L1-2I (669 mg, 1.00 mmol) in
sonication in toluene, acetone, ethanol and deionized water, dried
in an oven, and then cleaned with UV ozone for 300 s. As-received
freshly distilled triethylamine (20 mL) and CH
added CuI (20.0 mg) and Pd(OAc) (60.0 mg). After the solution
was stirred for 30 min, trimethylsilylacetylene (0.15 mL,
2 2
Cl (20 mL) was
2
PEDOT:PSS solution was passed through the 0.45
spin-coated on patterned ITO substrates at 5000 r.p.m. for 3 min,
followed by baking in N at 150 °C for 15 min. The metal-
lm filter and
1
3
.00 mmol) was then added and the suspension was stirred for
0 min in an ice-bath before being warmed to room temperature.
After reacting for 30 min at room temperature, the mixture was
heated to 50 °C for 24 h. The solution was then allowed to cool
to room temperature and the solvent mixture was evaporated in
vacuo. The crude product was purified by column chromatography
2
lopolyyne:PCBM active layer (1:5 w/w) was prepared by spin-coat-
ing each of the chlorobenzene or toluene solutions at 1000 r.p.m.
for 2 min. The substrates were dried at room temperature in low
vacuum (vacuum oven) for 1 h, and then stored in high vacuum
ꢃ5
ꢃ6
(10 ꢃ10 Torr) overnight. Al electrode (100 nm) was evaporated
through a shadow mask to define the active area of the devices
(2 mm circle). All the fabrication procedures (except drying, PED-
2 2
on silica gel with a solvent combination of hexane/CH Cl (2:5, v/v)
as eluent to provide L1-2TMS as a dark red solid (430 mg, 71%). IR

W.-Y. Wong et al. / Journal of Organometallic Chemistry 693 (2008) 3603–3612
3611
Table 4
Crystal data for 4,7-bis[2 -(3,4-ethylenedioxythiophene)]-2,1,3-benzothiadiazole
EDOT-BTD-EDOT) and L1-2TMS
Appendix A. Supplementary material
0
(
CCDC 694929 and 694930 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2008.08.025.
EDOT-BTD-EDOT
L1-2TMS
Formula
C
18
H
12
N
2
O
S
4 3
28 28 2 4 3 2
C H N O S Si
M
r
416.48
608.88
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
0.30 ꢂ 0.28 ꢂ 0.12
Orthorhombic
Pbcn
4.3945(6)
15.655(2)
23.950(3)
90
90
90
1647.7(4)
4
1.679
0.30 ꢂ 0.24 ꢂ 0.15
Monoclinic
C2/c
41.462(5)
8.258(1)
17.814(2)
90
105.450(2)
90
5878(1)
8
References
a
(°)
b (°)
(°)
[
1] T.A. Skotheim, R.L. Elesenbaumer, J.R. Reynolds (Eds.), Handbook of
Conducting Polymers, second ed., Marcel Dekker, New York, 1998.
c
[2] H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers,
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3
V (Å )
Z
D
l
ꢃ3
calc (mg m
)
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0.371
ꢃ1
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0.481
[
[
[
[
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F(000)
856
2544
2
h Range (°)
2.60ꢃ25.00
2.32ꢃ25.00
Number of reflections collected
Number of unique reflections
6693
13502
1400
5142
R
int
0.0295
0.0458
[
9] A.J. Heeger, Solid State Commun. 49 (1998) 319.
Number of reflections with I > 2.0
r
(I)
1253
3428
[
10] J.S. Wilson, A.S. Dhoot, A.J.A.B. Seeley, M.S. Khan, A. Kohler, R.H. Friend, Nature
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Number of parameters
123
352
0.0541, 0.1372
0.0901, 0.1538
(
a
R
R
1
, wR
, wR
2
[I > 2.0
(all data)
r
(I)]
0.0649, 0.1383
0.0713, 0.1425
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