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A Journal of
Accepted Article
Title: Structural Effect of Pendant Unit in Thiocyanate-Free Ru(II)
Sensitizers on Dye-Sensitized Solar Cell Performance
Authors: Mutsumi Kimura, Shogo Mori, Takahiro Kono, and Rei
Tamura
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700899
Link to VoR: http://dx.doi.org/10.1002/ejic.201700899
10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Structural Effect of Pendant Unit in Thiocyanate-Free Ru(II) Sensitizers on
Dye-Sensitized Solar Cell Performance
Rei Tamura, Takahiro Kono, Shogo Mori*, and Mutsumi Kimura*
Department of Chemistry and Materials, Faculty of Textile Science and Technology,
Shinshu University, Ueda 386-8567, Japan
E-mail: mkimura@shinshu-u.ac.jp & shogmori@shinshu-u.ac.jp
Corresponding Author: Prof. Dr. Mutsumi Kimura and Prof. Dr. Shogo Mori
Mail Address: Faculty of Textile Science and Technology, Shinshu University, Ueda
386-8567, Japan
TEL&FAX: +81-268-21-5499
E-mail: mkimura@shinshu-u.ac.jp & shogmori@shinshu-u.ac.jp
Graphical Abstract
Abstract: Combination of Ru complex sensitizers and Co complex redox couples for
dye-sensitized solar cells (DSSCs) generally results in low power conversion efficiency.
This has been interpreted with the facilitation of undesired electron transfer due to
strong intermolecular interactions between the dye and the redox couple. To retard the
interactions, two thiocyanate-free ruthenium (Ru) sensitizers are synthesized with a
terpyridine attached with a triphenylamine (TPA) unit having branched alkoxy chains to
the TPA unit. The main difference of the two dyes is the angle of the three phenyl rings
in the TPA unit. The DSSCs using both the new dyes show higher short-circuit currents
and open-circuit voltages in comparison to that using a Ru complex dye having
non-branched alkyl chains. The Ru sensitizer having a more twisted TPA unit displays
1
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
relatively high power conversion efficiency without coadsorption of chenodeoxycholic
acid, suggesting less intermolecular interaction among dyes and thus also with Co
complex redox couples.
Introduction
Dye-sensitized solar cells (DSSCs) have been attracted attention as an alternative
candidate for conventional solar cells.1 While thousands of sensitizers were synthesized,
various design strategies for sensitizers have been proposed.2 One of strategies is an
intramolecular arrangement of donor (D), light-harvesting chromophore as a p-linker,
and acceptor (A) units.2 The addition of donor and acceptor units to chromophore can
widen the absorption spectrum of the sensitizers. By the addition of a donor moiety to
porphyrin chromophore, a power conversion efficiency (PCE) of 11% was obtained.3
Further improvement of PCE for DSSCs was achieved by introducing sterically
protected structure to D-p-A porphyrin dye YD2-o-C8 for the use with cobalt2+/3+
tris(bipyridyl) complexes as a redox mediator.4 The cobalt complexes are able to
increase open-circuit voltage (Voc) due to their more positive redox potential in
comparison to that of I-/I3- redox couple.5 However, Co complexes generally cause
problems of fast charge recombination in DSSCs. Thus, large efforts have been made to
add a function to sensitizers to block the approach of Co complexes to the surface of
TiO2 electrode. While the attempts were successful for organic and porphyrin dyes, Ru
complex dyes had been suffering from fast recombination with Co complex redox
couples.6 Initially designed Ru dyes for DSSCs contain SCN ligands. Even the total
charge of Ru dyes is zero, such Ru complex dyes could even attract Co complex due to
2
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
electrostatic forces between localized negative charges at SCN ligands and positively
charged Co complex redox couples.7 The Ru complex has relatively small
HOMO-LUMO gap and thus large dispersion force, in other words, intermolecular
force, is expected. The dispersion force is proportional to molecule’s polarizability,
which scales with the HOMO-LUMO gap of the molecule. Thus, wide absorption
sensitizers have an intrinsic issue for DSSCs.8 One of remedies is to add moieties which
would not change the absorption spectrum but increase the distance between the
framework of the dye and redox species.9 The two properties, localized charge and large
polarizability, of the Ru complexs can thus increase the local concentration of Co
complex in the vicinity of TiO2 surface. Recently, Wu et al. reported SCN free
bipyridine based Ru complex giving 9.53 % energy conversion efficiency with Co
complex redox couple.10 We have recently reported that a Ru complex dye, T7, which
was designed by replacing thiocyanate ligands of a well-known Ru complex dye, called
Black dye, with a tridentate ligand and by adding a bulky TPA moiety to the terpyridine
ligand of the dye, increased charge separation efficiency and thus PCE values.11 Ru
complexes with a terpyridine ligand can give wider absorption spectrum than those with
a bipyridine ligand. Our result was interpreted with that the replacement of the SCNligands decreased the electrostatic interaction and the addition of bulky TPA moiety
decreased the effect of dispersion force between the dyes and Co complexes. The
addition of the TPA moiety did not widen the absorption spectrum, suggesting little
change in polarizability. Thus, the bulky TPA moiety probably did not increase the
intermolecular forces but worked to block the approach of Co complexes to the main
3
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
framework of the Ru complex dyes. The reduction of electrostatic and intermolecular
forces resulted in less influence on the concentration of Co3+ near the Ru complex dye,
retarding direct electron transfer from the excited dye to the Co complex and the charge
recombination between injected electrons and Co complexes. In this study, we explored
the structural modification of the SCN-free terpyridine based Ru sensitizers to reduce
further the molecular interactions between Ru and Co complexes to increase the
performance of DSSCs.
Results and Discussion
The chemical structures of new ligands (1 and 2) and dyes (T11 and T12) are shown
in Figure 1. We expected that the introduction of branched alkoxy chains and the steric
hindrance of free rotation in the TPA unit raise the blocking function of the Ru
sensitizers, retarding charge transfers from the TiO2 electrode to the Co complex redox
couples and from the dye to the redox couples. In addition, alkoxy was expected to
increase the electron donating ability to enhance the electron injection yield from the
dyes to TiO2.
Fig. 1
The nitrogen center in TPA is linked to three electron-rich phenyl groups in a
propeller-like geometry.12 The oxidation potential of TPA was negatively shifted when
electron-donating groups were attached at the para-phenyl positions.13 Two
terpyridine-based ligands 1 and 2 having different TPA structures were synthesized.
4
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Two TPA units were prepared by the Buchwald-Hartwig amination between
4-iodo-1-(2-ethylhexyloxy)benzene
and
4-bromoaniline
or
4-bromo-2,6-dimethylaniline.11 After the conversion of bromide to boronic acid, the
TPA units were coupled with the terpyridine ligand through the Suzuki coupling
reaction6d.
Fig. 2
Fig. 2a shows the absorption spectra of 1 and 2 in dimethylformamide (DMF).
While both ligands displayed a broad absorption band centered at 390 nm, the
molecular extinction coefficient (e) of 2 was lower than that of 1. The optimized
geometry of TPA in 1 calculated at the DFT/B3LYP level of theory using 6-31G* basis
set showed a propeller-like structure with D3 symmetry (Fig. 2b).14 The phenyl rings
were symmetrically twisted from the central plain made of NCCC atoms.The phenyl
rings attached with terpyridine ligand in 2 are more twisted from the central plain due to
the steric hindrance of the methyl groups attached to the one of the phenyl ring. Cyclic
voltammograms of 1 and 2 showed a reversible oxidation couple at +1.10 and +1.08 V
vs. NHE.
Fig. 3
Table 1
Two thiocyanate-free Ru dyes T11 and T12 were synthesized by the reaction of 1 or 2
with RuCl3·H2O and reacted with 2,6-bis(2’-(4’-trifluoromethyl)pyrazolyl) pyridine.15
The absorption spectra of T11 and T12 in DMF are shown in Fig. 3a, and the absorption
maxima (lmax) and absorption coefficients (e) are collected in Table 1. Heteroleptic
5
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
bis-tridentate RuII dyes T11 and T12 exhibited a broad absorption band in the whole
visible light region from 300 to 750 nm, and the spectral shapes of T11 and T12 were
similar to those of TF dyes, which were reported by Chou et. al. and have similar
structures,15 except for the absorption band at 420 nm corresponding to the TPA unit.
The absorption band centered at 505 nm can be assigned to the metal-to-ligand
charge-transfer (MLCT) transition to the dicarboxyterpyridine ligand.15 The Ru dyes
exhibit a board absorption band at longer wavelength, and the onsets of the absorption
spectra are close to 800 nm as shown in the inset of Fig. 3a. Differential pulse
voltammogram (DPV) of a DMF solution containing T11 and tetrabutylammonium
perchlorate as an electrolyte displayed bimodal oxidation waves at 0.90 and 0.98 V vs.
NHE. T7 dye exhibited two oxidation potentials at 0.92 and 1.17 V vs. NHE in solution.
The potential difference between the Ru2+/3+ center and the TPA in T11 was smaller than
that of T7 by changing alkyl chains to alkoxy chains at the peripheral positions of TPA
unit.11 T12 having the sterically crowding TPA unit showed a unimodal wave at 0.91 V
vs. NHE, suggesting the overlapping of two oxidation potentials of the Ru center and
the TPA. Figure 4 shows the optimized structure of T11 and T12 calculated at the
DFT/B3LYP level of theory using 3-21G* basis set.16 The effect of solvation by
acetonitrile was included by the conductor-like polarizable continuum mode.17 The
3-21G* basis set was chosen for the Ru complex dyes based on a paper by De Angelis
et al.18 Figure 4 shows that the angle between the planes of the terpyridine and the
dipyrazolylpyridine is 90 degree. The branched alkyl chains of T11 are close to on the
plane of the dipyrazolylpyridine ligand while those of T12 are on the plane, which has
6
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
almost 45 degree from both the planes of the terpyridine and the dipyrazolylpyridine
ligands. From the structure, the alkyl chains of T12 are expected to reduce the contact
between the terpyridine and the dipyrazolylpyridine ligands among dyes when they are
adsorbed on TiO2 surface. This is because the alkyl chains cover these ligands so that
mostly only the alkyl chains would be contacting each other.
Fig 4
Porous TiO2 films on the transparent conducting glass substrates were immersed
into 0.2 mM dye solutions containing 20 mM chenodeoxycholic acid (CDCA) in
toluene/ethanol (4:1 v/v).11 The co-adsorption of CDCA has been used to suppress
undesired dye aggregation on TiO2 surface. The onsets of absorption spectra of T11 and
T12 adsorbed onto TiO2 surface were blue-shifted by 30 nm relative to the spectra in
DMF (Fig. S1). The adsorption of dyes on TiO2 surface could change the absorption
spectral and electrochemical potential by the deprotonation of anchoring group and the
overlapping of molecular orbitals. The dissociation of COOH groups into carboxylate
anions affects the electron density for the coordination bonds between Ru ion and
pyridines in the terpyridine ligand by reducing the electron accepting nature.19 The TiO2
film stained with T12 and CDCA showed a peak at +0.94 V vs. NHE with a shoulder
peak at +0.85 V in a polarogram (Fig. S2). The oxidation potential for the Ru center in
T12 was slightly negatively shifted by the adsorption onto TiO2 surface. The HOMO
levels estimated from the first oxidation potentials were more positive than the redox
potentials of I-/I3- (+0.40 V vs. NHE) and CoII/III tris(1,10-phenantroline)
teteracyanoborate (Co(phen)3)5a (+0.61 V vs. NHE) (Fig. 3b). The LUMO energy levels
7
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
estimated from the absorption edge measured on TiO2 and the HOMO levels were more
negative than the conduction-band edge potential of TiO2. These HOMO and LUMO
levels for T11 and T12 fit the requirement for the operation of DSSCs.
Fig. 5
Table 2
The performance of these sensitizers in DSSCs was examined by using two
different redox electrolyte solutions. Fig. 5a and b show the photocurrent
density-voltage (J-V) curves and incident-photon to current conversion efficiency
(IPCE) spectra of DSSCs with I-/I3- redox couple using triple layered TiO2 electrodes
(13 µm thick mesoporous layer (particle size, 20 nm), 7 µm scattering layer (particle
size, 400 nm), and 7 µm reflection layer) stained with T11 and T12. The short-circuit
photocurrent density (Jsc), open-circuit voltage (Voc), fill factors (FF), and overall cell
efficiencies (PCE) for all the DSSCs are summarized in Table 2. The IPCE values of
400-550 nm for T11 and T12 cells reached above 85%. The PCEs of T11 and T12 cells
were higher than that of T7.11 Comparison between T7 and T11 shows that the addition
of branched alkoxy chains increased slightly the current density but resulted in little
effect on the Voc value. Between T11 and T12, T12 showed higher Voc but lower Jsc
value. These suggest that electron donating ability of alkoxy chain is effective for Jsc
when the molecule has a structure having better p conjugation, that is, the flat structure
of T11 has an advantage. The J-V characteristic of the DSSCs in the dark shows that the
DSSC with T12 showed higher voltage to flow current. The difference in the dark
current could be due to different TiO2 conduction band edge potential (Ecb) and/or
8
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
different charge recombination lifetimes and it is not obvious to discuss the
recombination based on the J-V curves. Our previous study with Ru complex dyes
showed that the Ecb did not depend on the structure of the dyes.11 Moreover we have
also observed similar Ecb from various dyes.20 We are aware that some papers have
shown different Ecb with different dyes. We suppose the apparent difference is due to the
effect of electrolytes.21 Since we use the same electrolyte solution, we assume the
values of Ecb for the DSSCs/T11 and T12 are the same. Then, the different in the dark
current is attributed to the different in the recombination lifetime, that is, T12 probably
retarded the charge recombination. Since the adsorbed dyes’ density in these DSSCs are
low (Table 2), the retardation of the recombination was not due to the blocking function
of the dye against the approach of Co3+ to TiO2 surface but probably due to the steric
hindrance to be close to the main framework of the dye molecules. Since dispersion
force is inversely proportional to the distance to the power of 6, the TPA unit and alkyl
chains can reduce to the effect of the dispersion force between Co3+ and the
dipyrazolylpyridine unit. Note that not only the length and number but also the location
of alkyl chains has been shown as an important parameter for the recombination
lifetime.6d
Between T7 and T11, they showed almost the same Jsc and Voc values,
suggesting they have similar charge recombination lifetime. Thus, to improve the
function reducing intermolecular interactions, non-flat structure seems effective but the
branched alkoxy chains did not add the blocking function for these dyes. T12 probably
had less intermolecular forces with I3- than T11, resulting in less I3- concentration near
TiO2 surface.
9
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Fig. 5
Solar cells were then fabricated with an electrolyte solution containing CoII/III(phen)3
redox couple and double-layered TiO2 electrode (8 µm thick mesoporous layer and
4 µm scattering layer) having a larger pore size than that for I-/I3-. Higher porosity and
thinner electrode were to avoid the mass transport limitation of larger-sized Co
complexes in the electrolyte solution. The DSSCs/T12 gave a higher PCE value of 6.7%
compared with T7 and T11 under the same electrode conditions (Figure 6a and Table 2).
The Voc of T12 cell with Co complex redox couple was 30 mV higher than those of T7
and T11. This is probably due to the same reason for the Voc improvement in T12 cells
with I-/I3- redox couple as described above. T12 also showed higher Jsc values than T7
and T11. This trend is different from the case when I-/I3- redox couple was employed.
When Co complex redox couple is used with simple structure Ru complex dyes, they
are attracted each other due to intermolecular forces and thus, electron transfer from
excited dyes to Co complex is facilitated, decreasing Jsc value.11 If the steric structure of
T12 reduces the intermolecular forces, it should result in less undesired electron transfer
from the dye to Co3+ and thus less loss in Jsc.
To check the effect of intermolecular interactions among the dyes, we also
fabricated solar cells without co-adsorption of CDCA. Table 3, Fig. S3 and Fig. S4
show the results. The amounts of the adsorbed dyes were about five times higher when
CDCA was not co-adsorbed. However, both Jsc and Voc values for the cells without
CDCA were lower than those with CDCA. The lower Jsc with higher dye density could
be due to that the electron/energy transfer among dyes becomes competitive with
10
This article is protected by copyright. All rights reserved.
10.1002/ejic.201700899
European Journal of Inorganic Chemistry
electron injection process. Difference in Jsc values for DSSCs/T12 with and without
CDCA was smaller than that for DSSCs/T11. This suggests that the structure of T12
was effective to reduce the intermolecular interactions not only with Co complex but
also with Ru complex dyes. The lower values of Voc for DSSCs/T11 and T12 without
CDCA are consistent with the interpretation that the dyes attract Co complexes, that is,
more adsorbed dyes result in more attracted Co complexes, facilitating charge
recombination between injected electrons in TiO2 and Co complexes in the electrolyte
solution. In other words, the blocking function of T12 is still not ideal and there is a
room to improve the structure further. For example, longer alkyl chains instead of
methyl groups would help increasing the block functions.
In conclusion, to retard intermolecular interactions between Ru complex dye and Co
complex redox couples, two thiocyanate-free ruthenium (Ru) sensitizers were
synthesized with a triphenylamine (TPA) unit having branched alkoxy chains (T11) and
additional methyl groups to make more twisted structure in the TPA unit (T12). DSSCs
using T11 showed higher short-circuit current in comparison to that using a Ru complex
dye having non-branched alkyl chains but the branched alkoxy chains did not retard
intermolecular interactions. DSSCs using T12 increased both open circuit voltage and
short circuit current. T12 also showed relatively high power conversion efficiency
without coadsorption of chenodeoxycholic acid, suggesting less intermolecular
interaction among dyes and thus with Co complex redox couples. Less interaction
resulted in lower local concentration of redox couples in the vicinity of TiO2 surface and
11
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
adsorbed dyes, reducing the electron transfer from TiO2 and excited dyes to Co
complexes.
Acknowledgements
This work was partially supported by the New Energy and Industrial Technology
Development Organization (NEDO) and JSPS KAKENHI Grant Number JP15H02172
and JP26288089. We thank Katsumi Kobayashi of Fujifilm Co. Ltd. for valuable
discussion and Shingo Takano of Sumitomo Osaka Cement Co. Ltd. for the supply of
TiO2 paste.
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European Journal of Inorganic Chemistry
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Figure Captions
Fig. 1. Structure of examined ligands and Ru complex dyes, and previously published
Ru complex dye.
Fig. 2 a) Absorption spectra of 1 (solid line) and 2 (dotted line) in DMF. b) Optimized
strcutre of TPA units in 1 and 2 by DFT calculations (TPy: terpyridine ligand). c)
Molecular orbitals of 1 and 2 calcurated by the B3LYP level of theory using 6-31G(d)
basis set.
Fig. 3 a) Absorption spectra of T11 (solid line) and T12 (dotted line) in DMF. b) Energy
level diagrams of TiO2, Ru complexes, and redox couple.
Fig. 4 Calculated structures for T11 (a, c) and T12 (b, d). Top row shows side view
(COOH are on left hand side and TPA on right hand side) and bottom row shows top
view (view from TPA to COOH direction).
Fig. 5 a) Photocurrent voltage curves obtained with DSSCs with I-/I3- redox couple
based on T11 (black line) and T12 (red line) under a standard global AM 1.5 solar
condition (solid line) and dark current (dotted line). b) Incident photon-to-current
conversion efficiency spectrum for DSSC based on T11 (black line) and T12 (red line).
Fig. 6 a) Photocurrent voltage curves obtained with DSSCs with Co(phen)3 redox
couple based on T11 (black line) and T12 (red line) under a standard global AM 1.5
solar condition (solid line) and dark current (dotted line). b) Incident photon-to-current
conversion efficiency spectrum for DSSC based on T11 (black line) and T12 (red line).
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Table 1: Photophysical and electrochemical data for T11 and T12
Dyes
lmax /nma (log e)
E0-0 /eVb
HOMO/Vc
LUMO/Vd
T11
715 (3.27),655 (3.28), 505
(4.11), 422 (5.50)
1.56
0.83
-0.73
T12
715 (3.27),655 (3.29), 505
(4.10), 420 (5.27)
1.56
0.84
-0.72
a
Absorption peaks were measured in DMF. b Optical energy gap E0-0 were estimated from the onset of
absorption spectra. c HOMO energy levels were estimated from the first peak of voltammogram measured
for the dyes adsorbed onto TiO2 in electrolyte solutions by DPV (vs. NHE). d LUMO energy levels were
estimated from LUMO = HOMO-E0-0 (vs. NHE).
17
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Table 2. IV characteristics of DSSCs using T11, T12, and T7 with I-/I3- a or Co(Phen)3b redox
couples under one sun condition.
Dye
Redox
shuttle
I-/I3-
T11
Co(Phen)3
I-/I3-
T12
Co(Phen)3
T7
a
-
e
-
I-/I3Co(Phen)3
Adsorption density
x 10-5 / mol cm-3
1.7c
8.5d
1.7c
1.6c
10.5d
1.6c
-
Jsc / mAcm-2
Voc / V
FF
PCE /%
17.1
7.7
11.0
16.7
9.8
12.0
16.7
10.1
0.76
0.74
0.80
0.79
0.80
0.83
0.76
0.80
0.69
0.55
0.71
0.71
0.69
0.66
0.70
0.70
9.0c
3.2d
6.3c
9.3c
5.4d
6.7c
8.9c
5.7c
I /I3 electrolyte system; [I2] = 0.05M, [LiI] = 0.10M, [DMPImI] = 0.60M, [tBP] = 0.50M in acetonitrile, b Co(phen)3 electrolyte
system; [CoII(phen)3(B(CN)4)2] = 0.22M, [CoIII(phen)3(B(CN)4)3] = 0.02M, [LiClO4] = 0.20M, [tBP] = 0.50M in acetonitrile.
DMPImI: dimethylpropylimidazolium iodide, tBP: 4-tert-butylpyridine. c Co-adsorbed with CDCA. d Without CDCA. e Data is from
ref 7.
18
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Fig. 1
Fig. 2
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Fig. 3
20
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Fig. 4.
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10.1002/ejic.201700899
European Journal of Inorganic Chemistry
Fig. 5
Fig. 6
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