4,4',5,5'-tetracarboxy-2,2'-bipyridine Ru(II) sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1021/ic501178f

Two Ru(II) sensitizers TCR-1 and TCR-2 bearing four carboxy anchoring groups were prepared using 4,4',5,5'-tetraethoxycarbonyl-2,2'-bipyridine chelate and 4-(5-hexylthien-2-yl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine and 6-t-butyl-1-(3-trifluoromethyl-1H-pyrazol-5-yl)isoquinoline, respectively. Dissolution of these sensitizers in DMF solution afforded a light green solution up to 10^-5 M, for which their color gradually turned red upon further dilution and deposition on the surface of a TiO₂ photoanode due to the spontaneous deprotonation of carboxylic acid groups. These sensitizers were characterized using electrochemical means and structural analysis time-dependent density functional theory (TDDFT) simulation and were also subjected to actual device fabrication. The as-fabricated DSC devices showed overall efficiencies η = 6.16% and 6.23% versus their 4,4'-dicarboxy counterparts TFRS-2 and TFRS-52 with higher efficiencies of 7.57% and 8.09%, using electrolyte with 0.2 M LiI additive. Their inferior efficiencies are possibly caused by the combination of blue-shifted absorption on TiO ₂, inadequate dye loading, and the perpendicularly oriented central carboxy groups.

Full text of DOI:10.1021/ic501178f

Article
pubs.acs.org/IC
4,4′,5,5′-Tetracarboxy-2,2′-bipyridine Ru(II) Sensitizers for Dye-
Sensitized Solar Cells
Chun-Cheng Chou,† Fa-Chun Hu,† Kuan-Lin Wu,† Tainan Duan,† Yun Chi,*,† Shih-Hung Liu,‡
Gene-Hsiang Lee,‡ and Pi-Tai Chou*,‡
†Department of Chemistry and Low Carbon Energy Research Center, National Tsing Hua University, Hsinchu 30013, Taiwan
‡Department of Chemistry and Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei 10617,
Taiwan
S
* Supporting Information
ABSTRACT: Two Ru(II) sensitizers TCR-1 and TCR-2 bearing four
carboxy anchoring groups were prepared using 4,4′,5,5′-tetraethoxycarbonyl-
2,2′-bipyridine chelate and 4-(5-hexylthien-2-yl)-2-(3-trifluoromethyl-1H-
pyrazol-5-yl)pyridine and 6-t-butyl-1-(3-trifluoromethyl-1H-pyrazol-5-yl)-
isoquinoline, respectively. Dissolution of these sensitizers in DMF solution
afforded a light green solution up to 10−5 M, for which their color gradually
turned red upon further dilution and deposition on the surface of a TiO2
photoanode due to the spontaneous deprotonation of carboxylic acid groups.
These sensitizers were characterized using electrochemical means and
structural analysis time-dependent density functional theory (TDDFT)
simulation and were also subjected to actual device fabrication. The as-
fabricated DSC devices showed overall efficiencies η = 6.16% and 6.23%
versus their 4,4′-dicarboxy counterparts TFRS-2 and TFRS-52 with higher
efficiencies of 7.57% and 8.09%, using electrolyte with 0.2 M LiI additive.
Their inferior efficiencies are possibly caused by the combination of blue-shifted absorption on TiO2, inadequate dye loading, and
the perpendicularly oriented central carboxy groups.
carboxy anchor from 4,4′- to either 3,3′- or 5,5′-positions was
evaluated, and their performances were then compared with
that of the benchmark Ru sensitizer, N719,11 or their respective
parent metal complexes.12 With an aim to further investigate
the functional behaviors of these 2,2′-bipyridine anchors, we
proceed to design and synthesize the Ru(II)-based complexes
with unprecedented four carboxylic acid groups on the single
2,2′-bipyridine anchor, e.g., the 4,4′,5,5′-tetracarboxy-2,2′-
bipyridine, with the hope to probe the photophysical properties
and cell characteristics, for which the extra carboxy groups may
hopefully induce stronger binding to TiO2, increase the
absorption extinction coefficient, and cause the red-shifting of
absorption profile for better response to solar irradiation.
INTRODUCTION
■
Dye-sensitized solar cells (DSCs) are considered to be a leading
contender to the emerging photovoltaics,1 complementary to
other modern competing technologies such as organic
photovoltaics2 and Pervoskite cells.3 These DSC devices were
typically fabricated by depositing a sensitizer on a mesoporous
TiO2 electrode, together with incorporation of an electrolytic
solution with an I−/I3− redox couple for rapid dye regeneration,
−
and a Pt-based counter electrode for reducing the oxidized I3
ions. Despite rapid progression, DSC dye design remains one of
the most challenging areas and has been attracting extensive
research activities into the Ru(II) metal,4 organic push−pull
dyes,5 squaraine dyes for transparent solar cells,6 and zinc
porphyrin relevant sensitizers,7 among which superior DSC
efficiencies have been reported. In most cases, they were
strategically designed by incorporation of bulky substituents at
the basal skeleton of dyes to suppress intermolecular
aggregation,8 or by addition of one or more carboxy groups
that can tightly bind the TiO2 photoanode for increasing device
stability. Notably, the Ru(II) sensitizers with a single carboxy
group always showed inferior efficiency,9 whereas those with
multiple carboxy groups, such as 4,4′-dicarboxy-2,2′-bipyridine,
4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine, and analogues,10 have
exhibited the highest conversion efficiency reported among
sensitizers with optimal device stability. As for the 2,2′-
bipyridine chelate, the effect on reshuffling the substituents or
RESULTS AND DISCUSSION
■
As shown in Scheme 1, the 4,4′,5,5′-tetraethoxycarbonyl-2,2′-
bipyridine anchor was prepared using a multistep protocol
starting from ethyl cinchomeronate, followed by m-chloroper-
oxybenzoic acid (mCPBA) oxidation and chlorination with
POCl3, chloride-to-bromide metathesis using bromotrimethyl-
silane, and coupling of the resulting 6-bromo-3,4-diethox-
ycarbonylpyridine in the presence of Sn2Bun and PdCl2(PPh3)2
6
by employing Stille coupling.13
Received: May 21, 2014
© XXXX American Chemical Society
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a
Scheme 1
a
Synthetic protocols: (a) H2SO4, EtOH, reflux; (b) mCPBA, CH2Cl2,
rt; (c) POCl3, reflux; (d) SiBrMe3, propionitrile, reflux; (e) Sn2Bun ,
6
PdCl2(PPh3)2, toluene, reflux.
This anchoring chelate was then reacted with [RuCl2(p-
cymene)]2 in dimethylformamide (DMF) and with each of two
tailor-made ancillaries, namely, 4-(5-hexylthien-2-yl)-2-(3-
trifluoromethyl-1H-pyrazol-5-yl)pyridine and 6-t-butyl-1-(3-
trifluoromethyl-1H-pyrazol-5-yl)isoquinoline, in refluxing xy-
lenes. Flash column chromatography and subsequent hydrolysis
in basic media yield the demanded tetracarboxy Ru(II)
complexes, coded TCR-1 and TCR-2, in ∼30% yields (Scheme
2).
Figure 1. Perspective view of TCR-1-Et. Only one carbon for hexyl
groups is shown, the hydrogen atoms were omitted for clarity, and the
thermal ellipsoids were drawn at the 30% probability level. Selected
bond lengths: Ru1−N1 = 2.023(1) Å, Ru1−N2 = 2.030(7) Å, Ru1−
N3 = 2.084(9) Å, Ru1−N4 = 2.043(2) Å, Ru1−N6 = 2.066(3) Å, and
Ru1−N7 = 2.030(6) Å. Selected bond angles: N1−Ru1−N6 =
171.8(7)°, N2−Ru1−N3 = 173.4(5)°, N4−Ru1−N7 = 165.3(2)°.
Selected dihedral angles: plane (C2, C3, C4)−plane (C11, O1, O2) =
21.8°, plane (C2, C3, C4)−plane (C14, O3, O4) = 76.7°, plane (C7,
C8, C9)−plane (C20, O7, O8) = 9.8°, plane (C7, C8, C9)−plane
(C17, O5, O6) = 81.1°, plane (C24, C25, C26)−plane (S1, C32, C33)
= 22.8°, plane (C43, C44, C45)−plane (S2, C51, C52) = 20.2°.
Scheme 2. Structural Drawings of the Studied Ru(II)
Sensitizers
a result, the simulated 4,4′-dicarboxy-2,2′-bipyridine dihedral
angles are (21.11°, 25.66°) for TCR-1 and (25.39°, 26.76°) for
TCR-2, which are notably deviated from 0°. In comparison, the
4,4′-dicarboxy-2,2′-bipyridine dihedral angles of their analogues
bearing 4,4′-dicarboxy-2,2′-bipyridine chelate, i.e., TFRS-2 and
TERS-52, are calculated to be (0.79°, 0.30°) and (0.43°, 0.33°)
for TFRS-2 and TFRS-52, respectively, giving a nearly planar
configuration. For short, these calculated molecular structures
seem to suggest the existence of reduced steric congestion
between the carboxylic acid groups versus the ethoxycarbonyl
groups of the ester counterparts. In fact, a recent study of
Ru(II) sensitizer bearing an ortho-dicarboxyphenylterpyridine
anchoring unit also showed similar distortion of the carboxylic
acid groups by computer simulation.14
To examine whether the above-mentioned molecular
geometry is also applicable to the dye adsorption onto the
TiO2 nanocrystals, we then further simulated the interacting
mode with the anatase (101) (TiO2)38 surface.15 The calculated
4,4′-dicarboxy-2,2′-bipyridine dihedral angles of TFRS-2/
(TiO2)38, TFRS-52/(TiO2)38, TCR-1/(TiO2)38, and TCR-2/
(TiO2)38 are recorded to be (0.16° and 0.31°), (0.85° and
3.97°), (25.66° and 25.04°), and (28.47° and 29.75°),
respectively. As a result, when TFRS-2 and TFRS-52 are
anchored onto the TiO2 surface, it is reasonable to expect that
their 4,4′-dicarboxy-2,2′-bipyridine dihedral angles are nearly
unchanged, similar to those of the free dye molecule. For
TFRS-2/(TiO2)38 and TFRS-52/(TiO2)38, the approaching
coplanar dihedral 4,4′-dicarboxy-2,2′-bipyridine makes a perfect
π-conjugation pathway that facilitates the interfacial charge
For confirming their structural features, single-crystal X-ray
analysis of ester substituted derivative TCR-1-Et was carried
out, for which the perspective view and the associated metric
parameters are shown in Figure 1. As can be seen, the molecule
consists of a distorted octahedral framework with Ru−N
distances spanning the narrow range of 2.023(1)−2.084(9) Å.
All chelates seem to be planar, and the azolate fragments adopt
the expected trans-orientations. The thienyl appendages on the
azolate chelate also exhibit small dihedral angles of 20.2° and
22.8°, showing excellent π-conjugation extended over these
ancillaries. As for the bipyridine anchor, the outer ethox-
ycarbonyl groups at both the 5- and 5′-positions adopt the
essentially parallel orientation with dihedral angles of 10−22°
relative to the basal plane of bipyridine. In sharp contrast, the
central ethoxycarbonyl groups at the 4- and 4′-sites are rotated
by 77−81°, for which this perpendicular orientation indicates a
severe disruption of π-conjugation.
We are then very curious about whether the carboxy groups
for the hydrolyzed TCR-1 and TCR-2 are similar to
ethoxycarboxy groups of TCR-1-Et at the 4- and 4′-sites,
being subject to significant deviation from the planarity versus
bipyridine. To gain insight into this issue, we then executed the
ground-state geometry optimization of the studied sensitizers
obtained from the density functional theory (DFT) method. As
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injection. Conversely, due to the perpendicularly arranged 4,4′-
dicarboxy-2,2′-bipyridine fragments and hence a disrupted π-
conjugation, the charge injection may be hampered in the cases
of TCR-1/(TiO2)38 and TCR-2/(TiO2)38 (vide infra).
The absorption spectra of TCR-1 and TCR-2 in DMF are
measured and are shown in Figure 2. In addition to the higher
amount of ligand-to-ligand charge transfer (LLCT) involving
pyrazolate ancillaries. Furthermore, the electron distributions of
LUMO, LUMO+1, and LUMO+2 molecular orbitals of TCR-1
and TCR-2 are localized at the 4,4′,5,5′-tetracarboxy-2,2′-
bipyridine anchor (see Figures S2 and S3, Supporting
Information), which are beneficial to the excited-state electron
injection. Note that the peaks at 452.9 nm for TCR-1 and 503.6
nm for TCR-2 are accompanied by a minor part of LLCT from
the thienyl (TCR-1) or quinolinyl fragment (TCR-2) to the
4,4′,5,5′-tetracarboxy-2,2′-bipyridine anchor.
Upon depositing these samples onto TiO2, the higher energy
ππ* absorption is blue-shifted toward ∼413 nm; likewise, the
lower energy MLCT band also undergoes a significant blue-
shift toward ∼560 nm. We speculate that such an abrupt shift in
absorption could be associated with the spontaneous
deprotonation upon depositing TCR dyes on the TiO2 surface,
giving formation of a negatively charged carboxylate that, in
turn, could destabilize the π*-orbital of the bipyridine anchor.
In fact, the pKa of cinchomeronic acid (2.63)16 was reported to
be higher than that of isonicotinic acid (4.9),17 implying more
facile deprotonation for 4,4′,5,5′-tetracarboxy than that for 4,4′-
dicarboxy-2,2′-bipyridine. Further evidence of deprotonation is
given by the gradual variation of UV/vis absorption spectra by
addition of water into the DMF solution (see Figures S4 and
S5, Supporting Information). In fact, this experiment would be
visualized by the changing of solution color from green to red,
which also occurred upon deposition onto TiO2 and
basification mentioned earlier. Moreover, at the DMF/H2O
ratio of 6:4, the recorded spectral patterns are essentially
identical to those observed on the TiO2 thin film, confirming
our speculation. In fact, several organic push−pull sensitizers
were also reported to exhibit similar blue-shifting of absorption
spectra on the TiO2 surface due to the in situ deprotonation.18
We then made a further attempt to deprotonate both
sensitizers using tetra-n-butyl ammonia hydroxide [TBA]OH
(1.0 × 10−2 M) in DMF solution. However, the visual color
remained the same even after addition of ≥6 equiv of
[TBA]OH. This result is different from the titration experiment
using a methanol solution of [TBA]OH at the same
concentration, for which a gradual change of color was detected
up to an addition of 4−5 equiv of [TBA]OH. Then, an abrupt
change to red color was identified after addition of 6 equiv of
[TBA]OH. This change of color seems to be not directly
proportional to the amount of base added, implying that the
proposed deprotonation also critically depends on the protic
media (i.e., water or methanol) in the solution. Figure S6 of the
Supporting Information depicts the UV−vis absorption spectra
of sensitizers at various amounts of added [TBA]OH in
methanol.
Figure 2. UV−vis absorption spectra of TCR-1 and TCR-2 in DMF
(solid line, left axis) and those deposited on TiO2 thin film (dashed
line, right axis).
energy ππ* transition at ∼436 nm, both complexes exhibit
strong absorption with a peak wavelength at ∼604 nm. The
valley of the absorption spectrum at ∼510 nm gave them a
distinctive green color in solution (10−4 M). On the basis of the
time-dependent DFT (TDDFT) simulations in DMF (see the
Supporting Information for details), the simulated absorption
peaks of TCR-1 (TCR-2) at 759.7 (756.8), 606.8 (619.7),
458.4 (460.1), and 416.7 (413.6) nm (see Figure 3 for TCR-1
Figure 3. Experimental (black solid line) and TDDFT calculated (blue
dashed line) absorption spectra of TCR-1 in DMF (a Gaussian
convolution σ = 0.2 eV for spectral fitting). For clarity, the calculated
absorption wavelengths (red vertical lines) and the relative transition
probabilities (magnitude of vertical lines) are shown. Also displayed
are frontier orbitals (pink color: occupied orbital; yellow color:
unoccupied orbital) that relate to the major transitions.
Cyclic voltammetry was then performed to verify if their
HOMOs (the ground-state oxidation potential or Eox°′)
matched the redox potential of electrolytic solution for rapid
dye regeneration. As shown in Table 1, the onset potentials for
oxidation of TCR-1 and TCR-2 on TiO2 appeared at 0.90 and
0.88 V (vs NHE), respectively, which are attributed to the
Ru(II) metal oxidation. These data are more positive than that
of the I−/I3 redox couple (ca. 0.4 V vs NHE), confirming the
−
and Figure S1, Supporting Information, for TCR-2) are
attributed to the metal-to-ligand charge-transfer (MLCT)
excitation. The lowest singlet optical transition (S0 → S1) of
TCR-1 and TCR-2 located at 759.7 and 756.8 nm, respectively,
is assigned to the HOMO → LUMO transition (see Tables S1
and S2, Supporting Information), which is ascribed to the
metal-to-bipyridine MLCT transition, together with a small
existence of sufficient driving force for the dye regeneration.
Alternatively, the excited-state oxidation potentials (E°′*),
e.g., −0.84 and −0.83 V, estimated from the difference of
ground-state oxidation potential and the energy gap (E0−0), are
more negative than the conduction band potential of the TiO2
electrode (ca. −0.5 V vs NHE). Furthermore, there are two
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a
Table 1. Electrochemical and Photovoltaic Data of the Studied Ru(II) Sensitizers
dye
Eox°′
E0−0
E°′*
JSC
VOC
FF
η
dye loading
TFRS-2
TFRS-52
TCR-1
0.85
0.83
0.90
0.88
1.86
1.80
1.74
1.71
−1.01
−0.97
−0.84
−0.83
16.55
16.90
13.98
13.73
0.64
0.67
0.61
0.63
0.715
0.716
0.722
0.720
7.57
8.09
6.16
6.23
2.05 × 10−7
1.90 × 10−7
1.08 × 10−7
0.89 × 10−7
TCR-2
a
Eox°′ is the redox potential of sensitizer deposited on TiO2 film. All measurements were in reference to the Fc/Fc+ standard, and the data were
converted to a value relative to NHE (+0.63 V). E0−0 was estimated using the onset of the MLCT absorption on the TiO2 electrode, and E°′* was
calculated using the equation E°′* = Eox°′ − E0−0. Open-circuit potential (VOC) of DSC devices is reported in units of V, while short-circuit current
density (JSC) and dye loading are in units of mA·cm−2 and mol·cm−2.
Figure 4. Frontier molecular orbitals HOMO (pink mesh) and LUMO (green mesh) of TFRS-52/(TiO2)38 (left) and TCR-2/(TiO2)38 (right).
Figure 5. (a) J−V characteristics. (b) IPCE action diagrams of as-fabricated DSCs. (c) TiO2 electron density versus voltage deduced from charge
extraction measurement. (d) Electron lifetime versus chemical capacitance obtained by IMVS measurement.
viable methods for E0−0 measurement, namely, positioning the
interaction between absorption and emission bands.20 In this
study, we use the 10% maxima of the lowest energy absorption
MLCT shoulder at the 10% maxima,19 and measuring the
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to assess the E0−0, as the UV/vis spectra were measured on
samples deposited on TiO2, while the respective emission on
TiO2 cannot be observed due to the rapid photoinduced
electron injection. Moreover, both Eox°′ and E°′* of TCR-1
and TCR-2 are found to be more positive and less negative
compared with those of dicarboxy counterparts TFRS-2 and
TFRS-52. These observations are consistent with the greater
electron deficiency at the Ru(II) metal center and the lowering
of the empty π*-orbital energy level of the bipyridine anchor;
both are induced by the two extra carboxy groups.
Furthermore, the observed E0−0 data are also consistent with
those deduced from the simulated TFRS-52/(TiO2)38 and
TCR-2/(TiO2)38 models. As shown in Figure 4, the calculated
lowest-lying transition involves mainly HOMO → LUMO
character, for which HOMO and LUMO are localized at the
TFRS-52 (or TCR-2) and the (TiO2)38 surface, respectively,
displaying an interfacial electron transfer or the electron
injection process from the dye to the (TiO2)38 surface. Similar
results are obtained for the optimized TFRS-2/(TiO2)38 and
TCR-1/(TiO2)38 models shown in Figure S7 (Supporting
Information).
Comparative studies were next executed to reveal their
device performances. All DSCs were fabricated using 15 + 7 μm
of 20 and 400 nm in diameter of TiO2 layers and with a
projected area of 4 × 4 mm2 defined by a metal mask. The
anodes were stained with 0.3 mM of TCR-1 (or TCR-2) and
0.6 mM deoxycholic acid in a 4:1 volume ratio of mixed ethanol
and DMSO solvent over a period of 16 h. The electrolyte
consists of 0.6 M PMII, 0.03 M I2, 0.1 M guanidinium
thiocyanate (GuNCS), 0.1 M t-BP, and 0.2 M LiI in mixed
acetonitrile and valeronitrile (v/v, 85/15). Performances of
DSC were then evaluated under 1 sun irradiation (AM 1.5G,
100 mW cm−2), for which the numeric data are summarized in
Table 1, while J−V characteristics and IPCE diagrams are
depicted in Figure 5a,b. As can be seen, the efficiency of TCR-1
and TCR-2 was recorded to be 6.16% and 6.23%, respectively.
Under identical conditions, the TFRS-2 and TFRS-52
references showed higher efficiencies of 7.57% and 8.09%,
respectively. We also like to point out that, upon adoption of an
electrolyte with a reduced concentration of LiI, both reference
sensitizers have been capable of achieving a DSC efficiency of
over 9.5%.21 In comparison, without this excessive LiI in the
electrolyte, DSCs fabricated with TCR-1 and TCR-2 have still
afforded a much reduced JSC, showing an insufficient driving
force for electron injection. Also, comparing that of TFRS-2
and TFRS-52 references, a slightly lower VOC for TCR-1 and
TCR-2 is also noticed (see Figure 5a and Table 1). This can
generally be explained by variation of TiO2 conduction band
potential (investigated via charge extraction) and recombina-
tion lifetimes (investigated via intensity-modulated photo-
voltage spectroscopy, IMVS). As showed in Figure 5c, both
the TCR-1 and TCR-2 cells feature the higher extracted charge
at the same VOC compared with the TFRS-2 and TFRS-52
reference cells, suggesting the downward movement of the
TiO2 conduction band potential.
Alternatively, the lower dye loading for the TCR compounds
may be consistent with binding through one set of adjacent
carboxylate groups at either 4,5- or 4′,5′-positions, which is
expected to give a large footprint on the TiO2 surface. Note
that the IMVS measurements at various light intensities (cf.
Figure 5d) also showed a decreased electron lifetimes for TCR-
1 and TCR-2 cells. Again, this could be attributed to the
reduction of dye loading, leading to the increased electron
recombination. However, the TiO2 surface uncovered by
sensitizers should effectively interact with the t-BP additive in
the electrolyte and afford an upward movement of conductive
band potentials accordingly.22 Failure to observe this behavior
in the charge extraction experiment suggests the dominance by
other factors such as the increased interaction to the dissociated
proton, or even Li+ ions added in the electrolyte.
CONCLUSION
■
In summary, we have synthesized two Ru(II) sensitizers bearing
the 4,4′,5,5′-tetracarboxy-2,2′-bipyridine anchor. Structural data
and subsequent spectroscopic studies suggest the increased
tendency for deprotonation in solution as well as on the TiO2
surface. This, together with the simultaneous twisting of the
4,4′-substituted carboxy fragments against the bipyridine
chelate, is detrimental to the fabrication of very high efficiency
DSC devices. Thus, our works have shown that the excessive
introduction of carboxy groups to the sensitizers in the hope of
improving cell stability and dye loading may just lead to the
opposite consequences, which must be executed with caution.
EXPERIMENTAL SECTION
■
General Procedures. All purchased commercial chemicals were
used without purification. Solvents were dried using a VAC solvent
purifier prior to use. All reactions were conducted under an inert N2
atmosphere. All reactions were monitored by precoated TLC plates
(Merck, 0.20 mm with fluorescent indicator UV254). Compounds
were visualized using a UVGL-25 Compact UV Lamp. Flash column
chromatography was carried out using silica gel with a particle size of
230−400 mesh. Mass spectra were obtained on a JEOL SX-102A
instrument operating in electron impact (EI) or fast atom bombard-
1
ment (FAB) mode. H and 19F NMR spectra were measured on a
Bruker-400 or INOVA-500 instrument; chemical shifts are quoted
with reference to the internal standard Me4Si. Elemental analysis was
carried out with a Heraeus CHN-O Rapid Elementary Analyzer. UV/
vis absorption spectra were recorded on a Hitachi U3900
spectrophotometer.
Synthesis of TCR-1-Et. [RuCl2(p-cymene)]2 (145 mg, 0.23
mmol) and 4,4′,5,5′-tetraethoxycarbonyl-2,2′-bipyridine (200 mg,
0.45 mmol) were added in 30 mL of DMF, and the mixture was
heated to 60 °C for 6 h according to literature procedures.23 After
then, DMF was removed under vacuum, and 4-(5-hexylthien-2-yl)-2-
(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine (171 mg, 0.45 mmol),
KOAc (221 mg, 2.3 mmol), and xylene (30 mL) were added to the
flask. After refluxing for 8 h, the solvent was removed and the crude
product was purified by silica gel column chromatography (ethyl
acetate:CH2Cl2 = 1:10). Yield: 235 mg, 40%. Another Ru(II)
derivative, TCR-2-Et, was synthesized from [RuCl2(p-cymene)]2 and
a stoichiometric amount of the respective 6-(tert-butyl)-1-(3-(trifluoro-
methyl)-1H-pyrazol-5-yl)isoquinoline using identical procedures.
Selected Spectral Data of TCR-1-Et. 1H NMR (400 MHz,
CDCl3, 298 K): δ 8.42 (s, 2H), 8.34 (d, JHH = 3.6 Hz, 2H), 8.31 (s,
2H), 7.73 (d, JHH = 2.0 Hz, 2H), 7.24 (d, JHH = 6.0 Hz, 2H), 7.12 (dd,
JHH = 6.0, 2.0 Hz, 2H), 6.99 (s, 2H), 6.79 (d, JHH = 3.6 Hz, 2H), 4.41
(q, JHH = 7.2 Hz, 4H), 4.20 (q, JHH = 7.2 Hz, 4H), 2.82 (t, JHH = 7.2
Hz, 4H), 1.71−1.64 (m, 4H), 1.41−1.27 (m, 18H), 1.15−1.11 (m,
6H), 0.88−0.85 (m, 6H). 19F NMR (376 MHz, CDCl3, 298 K): δ
−59.91 (s, 6F).
The inferior DSC performance for TCR-1 and TCR-2, in
part, should be due to the fact that the dye loading for both
TCR-1 and TCR-2 is significantly less than that of the
dicarboxy counterparts TFRS-2 and TFRS-52 (see Table 1). In
other words, the above-enhanced deprotonation and the
perpendicularly oriented central carboxy groups for TCR-1
(TCR-2) may retard dye absorption, resulting in a smaller dye−
TiO2 association constant (cf. TFRS-2 and TFRS-52).
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Selected Spectral Data of TCR-2-Et. 1H NMR (400 MHz,
CDCl3, 298 K): δ 8.65 (d, JHH = 9.2 Hz, 2H), 8.30 (s, 2H), 8.24 (s,
2H), 7.76 (dd, JHH = 8.8, 1.6 Hz, 2H), 7.64 (s, 2H), 7.45 (s, 2H), 7.25
(d, JHH = 6.4 Hz, 2H), 7.19 (d, JHH = 6.4 Hz, 2H), 4.39 (q, JHH = 7.2
Hz, 4H), 4.06 (q, JHH = 7.2 Hz, 4H), 1.38 (s, 18H), 1.36−1.34 (m,
6H), 1.26−1.22 (m, 6H). 19F NMR (376 MHz, CDCl3, 298 K): δ
−59.89 (s, 6F).
AUTHOR INFORMATION
Corresponding Authors
*E-mail: ychi@mx.nthu.edu.tw (Y.C.).
*E-mail: chop@ntu.edu.tw (P.-T.C.).
Notes
■
The authors declare no competing financial interest.
Synthesis of TCR-1. TCR-1-Et (100 mg, 0.08 mmol) was
dissolved in a mixed acetone (30 mL) and 2 M NaOH(aq) solution
(2 mL). After stirring for 8 h, the solvent was evaporated under
vacuum and the residue was dissolved in 10 mL of H2O and titrated
with 2 N HCl to pH 3 to induce a black precipitation. This black
product was washed with a small amount of deionized water and
acetone in sequence, to yield TCR-1 (69 mg, 76%). Another Ru(II)
derivative, TCR-2, was synthesized by hydrolysis of the obtained TCR-
2-Et using identical procedures.
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Science and
Technology, Taiwan, and the computation was executed using
the facility at the National Center for High-Performance
Computing (NCHC).
REFERENCES
Selected Spectral Data of TCR-1. MS (FAB, 102Ru): m/z 1189.1
■
1
(M + 1)+. H NMR (400 MHz, d6-DMSO, 298 K): δ 9.07 (s, 2H),
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Selected Spectral Data of TCR-2. MS (FAB, 102Ru): m/z 1069.9
1
(M + 1)+. H NMR (400 MHz, d6-DMSO, 298 K): δ 8.97 (s, 2H),
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of 0.1 M TBAOH. The solar cells were covered with a black metal
mask with an aperture (0.4 × 0.4 cm2) to define the active area during
measurement.
X-ray Crystallography. All single-crystal X-ray diffraction data
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ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray structural data of ester derivative TCR-1-Et in CIF
format. Synthetic procedures for 4,4′,5,5′-tetraethoxycarbonyl-
2,2′-bipyridine, experimental details for TDDFT and DFT
computation, electrochemistry and photovoltaic character-
ization, and computational results and UV/vis spectral analyses
of both TCR-1 and TCR-2. This material is available free of
charge via the Internet at http://pubs.acs.org.
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