Thiazolocatechol: Electron-Withdrawing Catechol Anchoring Group for Dye-Sensitized Solar Cells

Article abstract of DOI:10.1002/cphc.201900342

Anchoring groups adopting a five-membered bidentate chelating are attractive to realize high power conversion efficiency (η) and long-term durability in dye-sensitized solar cells (DSSCs). In this regard, we chose catechol as an anchoring group that can a

Full text of DOI:10.1002/cphc.201900342

Accepted Article
Title: Thiazolocatechol: Electron-withdrawing Catechol Anchoring
Group for Dye-Sensitized Solar Cells
Authors: Tomohiro Higashino, Hitomi Iiyama, Yuma Kurumisawa, and
Hiroshi Imahori
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10.1002/cphc.201900342
ChemPhysChem
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Thiazolocatechol: Electron-withdrawing Catechol Anchoring
Group for Dye-Sensitized Solar Cells
Tomohiro Higashino,[a] Hitomi Iiyama,[a] Yuma Kurumisawa,[a] and Hiroshi Imahori*[a,b]
Abstract: Anchoring groups adopting a five-membered bidentate
chelating are attractive to realize a high power conversion efficiency
(h) and long-term durability in dye-sensitized solar cells (DSSCs). In
this regard, we chose catechol as an anchoring group that can adopt
the chelating. However, the DSSCs with catechol-based sensitizers
have never exceeded an h-value of 2%. These poor photovoltaic
performances may be associated with the electron-donating ability of
the hydroxy groups in catechol. Considering these, we envisioned
that fusing an electron-withdrawing thiazole moiety with a catechol
group for DSSCs owing to its simple synthesis and sufficient
binding ability to TiO2 in organic solvents. Nevertheless,
carboxylic acid tends to detach from TiO2 during the cell
operation under illumination and in the presence of water. To
attain the long-term durability of the DSSCs toward their
practical application, stable anchoring groups have been
explored.[7] In this connection, we previously reported that push-
pull porphyrin sensitizers with tropolone[8a] and hydroxamic
acid[8b] anchoring groups attained both high h-values and long-
anchoring group would improve its photovoltaic performance. Herein, term durability of the DSSCs. In contrast to the possible binding
we report
a
push-pull porphyrin sensitizer ZnPTC with
a
mode of four-membered bidentate chelating for carboxylic acid,
tropolone and hydroxamic acid anchoring groups can adopt a
five-membered bidentate chelating as possible binding modes
(Figure 1a). Thus, utilization of anchoring groups possessing a
five-membered bidentate chelating would be an effective means
to realize both high h-value and long-term durability. Along this
line, we focused on catechol as an anchoring group that is
known to adopt a five-membered bidentate chelating.[9]
thiazolocatechol anchoring group. The DSSC with ZnPTC exhibited
h = 4.87%. This value is the highest ever reported for catechol-
anchor based DSSCs. Meanwhile, the long-term cell durability was
not improved, although the robust anchoring properties were
attained under harsh conditions.
Introduction
a)
carboxylic acid
tropolone
hydroxamic acid
NH
catechol
In recent years, the development of anchoring groups to bind
organic molecules and metal oxides have been actively explored
for heterogeneous catalysts and photoelectrochemical cells,
which are promising technologies to realize sustainable energy
systems. In this regard, photoelectrochemical cells, which
convert solar energy into electricity or useful chemicals, have
attracted much attention.[1,2] In particular, dye-sensitized solar
cells (DSSCs) are a fascinating candidate for an alternative to
conventional silicon-based solar cells because of their potential
low-cost production and a high power conversion efficiency (h).[1]
Since the epoch-making paper by Grätzel et al. in 1991,[3a]
diverse ruthenium[3] and organic sensitizers[4] have been
assessed for their applications to DSSCs. Among various kinds
of sensitizers, porphyrins are attractive candidates because of
their intense absorption in the visible region and easily
modulated structures.[5] Push-pull porphyrin sensitizers have
achieved an excellent light-harvesting ability and demonstrated
high h-values more than 10%.[6]
O
O
O
O
O
O
O
O
Ti
Ti
Ti
Ti
b)
C6H13
C6H13
C6H13
C6H13
N
N
C8H17
O
O
OC8H17 C8H17
O
O
OC8H17
OC8H17
N
N
N
N
N
N
Zn
Zn
N
N
C8H17
OC8H17 C8H17
S
N
O
OH
To date, carboxylic acid is the most widely used anchoring
HO
OH
YD2-o-C8
ZnPTC
[a]
[b]
Dr. T. Higashino, H. Iiyama, Y. Kurumisawa, Prof. Dr. H. Imahori
Department of Molecular Engineering
Graduate School of Engineering, Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: imahori@scl.kyoto-u.ac.jp
Prof. Dr. H. Imahori
Institute for Integrated Cell-Material Sciences (WPI-iCeMS)
Kyoto University
Figure 1. a) Possible binding modes for anchoring groups on TiO2. b)
Molecular structures of porphyrin sensitizers in this study.
Although several organic sensitizers with catechol anchoring
groups have been tested,[9b,10] their photovoltaic performances
have never exceeded an h-value of 2%.[10a] Porphyrin sensitizers
with a catechol anchoring group have appeared, but their
photovoltaic performances are also limited to less than an h-
value of 1%.[10e] These poor photovoltaic performances may be
Sakyo-ku, Kyoto 606-8501 (Japan)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cphc.201900342
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NH2
N
Br
associated with the electron-donating ability of the hydroxy
groups in catechol. In terms of molecular design for excellent
sensitizers in DSSCs, the electron-donating nature of the
catechol group is inappropriate for a fast electron injection
process from the excited sensitizer to TiO2 as well as efficient
intramolecular charge-transfer interaction in push-pull structures.
Bearing these in mind, we expected that fusing an electron-
withdrawing thiazole moiety with a catechol anchoring group
would improve photovoltaic performance.
S
S
N
NO2
OH
NO2
OAc
NH2
OAc
(CH3CO)2
DMAP
O
KSCN
Br2
1) t-BuNO2
2) CuBr
H2, Pd/C
CHCl3
THF
CH3CN
CH3CN
HO
AcO
AcO
AcO
OAc
AcO
OAc
1
2
3
4
C6H13
C6H13
C6H13
C6H13
N
N
C8H17
O
O
OC8H17
OC8H17
C8H17
O
O
OC8H17
OC8H17
N
N
N
N
N
N
N
Zn
Zn
Pd2(dba)3
AsPh3
N
NaOH
THF/CH3OH/H2O
C8H17
C8H17
+
4
ZnPTC
THF/Et3
N
Herein, we report a thiazolocatechol as a new catechol
anchoring group with an electron-withdrawing thiazole moiety.
We anticipated the improvement of the light-harvesting ability,
photovoltaic performances, and robust anchoring properties
owing to its π-expansion and enhanced electron-withdrawing
nature arising from the thiazole-fused structure. To examine the
effect of the thiazolocatechol anchoring group, selected as the
basic motif was YD2-o-C8, which is one of the best sensitizers
in DSSCs.[6b] The carboxyphenyl group of YD2-o-C8 was
replaced with the thiazolocatechol anchoring group to give a
push-pull porphyrin sensitizer ZnPTC (Figure 1b). Their optical,
electrochemical, and photovoltaic properties were described in
this paper.
H
S
N
5
AcO
OAc
6
Scheme 1. Synthesis of ZnPTC.
Absorption, Fluorescence, and Electrochemistry
The UV-visible absorption spectra of ZnPTC and YD2-o-C8 in
THF are depicted in Figure 2a. The Soret and Q-bands of
ZnPTC are shifted toward longer wavelengths compared to
those of YD2-o-C8. This improved light-harvesting ability of
ZnPTC can be rationalized by higher electron-withdrawing
nature of the thiazolocatechol group than that of the
carboxyphenyl group. The emission spectrum of ZnPTC in THF
reveals a red-shifted peak at 677 nm relative to that of YD2-o-
C8 (669 nm, Figure 2b). By using the crossing of the normalized
absorption and fluorescence spectra, the optical HOMO–LUMO
gaps were estimated to be 1.86 eV for ZnPTC and 1.89 eV for
YD2-o-C8 (Figure S6). The fluorescence lifetimes (tF) were
determined by using time-correlated single photon counting
(TCSPC) method. ZnPTC (tF = 1.3 ns) shows a slightly shorter
Results and Discussion
Synthesis and characterization
The synthetic scheme of ZnPTC is shown in Scheme 1. The
reaction of 4-nitrocatechol with acetic acid anhydride gave
acetylated catechol 1. After reduction of the nitro group of 1, the
reaction of
2
with potassium thiocyanate provided 2-
aminothiazolocatechol 3. Sandmeyer reaction of 3 afforded 2-
bromothiazolocatechol 4. In the next step, we synthesized the
porphyrin core 5 according to the literature.[6b] Sonogashira
coupling of 5 with 4 gave the porphyrin intermediate 6. Finally,
hydrolysis of 6 with an excess amount of NaOH furnished the
push-pull porphyrin sensitizer ZnPTC with the thiazolocatechol
anchoring group. All the compounds were fully characterized by
value than YD2-o-C8 (tF
= 1.6 ns), indicating that the
thiazolocatechol anchoring group has no significant impact on
the excited-state dynamics. Since the decaying processes are
much slower than the typical electron injection processes from
the excited singlet state to TiO2,[11] they would not affect the h-
values (vide infra).
1
high-resolution mass spectrometry and H and 13C NMR and IR
spectroscopies (Figures S1–S5). Although the 1H NMR
spectrum of ZnPTC in CDCl3 was broad, the signals became
sharp by adding [D5]-pyridine into the CDCl3 solution (Figure S5).
These results indicate the aggregation tendency of ZnPTC,
which may be ascribed to the intermolecular coordination of
hydroxy moieties on the thiazolocatechol group to the zinc metal
of the porphyrin core. YD2-o-C8 was also prepared as a control
compound according to the literature.[6b]
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C8.
[a]
[a]
opt
[b]
[c]
[d]
Eox1
Ered1
Eg
E*ox
DGinj
DGreg
[V]
[V]
[eV]
[V]
[eV]
[eV]
ZnPTC
0.76
0.79
–1.21
−1.21
1.86
1.89
–1.10
–1.10
–0.60
–0.60
–0.19
–0.22
YD2-o-C8[e]
[a] Obtained by using DPV (vs. NHE). [b] Obtained by adding Egopt to Eox1. [c]
Free energy change for electron transfer from the singlet excited state of the
porphyrin to the conduction band (CB) of TiO2 (–0.5 V vs. NHE). [d] Free
energy change for electron transfer from tris(bipyridyl)CoII/III redox couple
(+0.57 V vs. NHE) to the oxidized porphyrin. [e] From ref 12.
We carried out calculations for the simplified porphyrins
using density functional theory (DFT) at the B3LYP/6-31G(d)
level to obtain information on the structural and electronic
properties of the porphyrins (Figure S8). Both the porphyrins
show similar ground-state geometry. The calculated HOMO–
LUMO gaps are 2.30 eV for ZnPTC and 2.32 eV for YD2-o-C8,
which agree with the trends on the HOMO–LUMO gaps obtained
optically and electrochemically. The orbital distribution on the
thiazolocatechol group in the LUMO of ZnPTC is smaller than
that on the carboxyphenyl group in the LUMO of YD2-o-C8.
Since the orbital distributions of LUMO around an anchoring
group are known to affect the electronic coupling between 1ZnP*
and the 3d orbital of TiO2,[13] the electron injection of ZnPTC on
TiO2 may be slower than that of YD2-o-C8 on TiO2 (vide infra).
In contrast, the orbital distribution on the thiazolocatechol group
in the HOMO of ZnPTC is larger than that on the carboxyphenyl
group in the HOMO of YD2-o-C8, accelerating the charge
recombination (CR) from electron in the CB of TiO2 to ZnP+• for
ZnPTC (vide infra).
Figure 2. a) UV-visible absorption and b) normalized emission spectra of
ZnPTC (red) and YD2-o-C8 (black) in THF. For the emission measurement,
wavelengths of Soret band maxima were used for the excitation.
The electrochemical properties of ZnPTC (versus NHE) in
THF were studied by using cyclic voltammetry (CV) and
differential pulse voltammetry (DPV). As an electrolyte
tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) was
employed (Figure S7 and Table 1). The first oxidation potential
(Eox) of ZnPTC is shifted slightly to a negative direction with
respect to YD2-o-C8, whereas their first reduction potentials
(Ered) are identical. These results are consistent with the higher
electron-withdrawing nature of the thiazolocatechol group than
that of the carboxyphenyl group. The electrochemical HOMO-
LUMO gaps were calculated from the Eox and Ered values. The
HOMO–LUMO gap of ZnPTC (1.97 eV) is slightly smaller
compared to that of YD2-o-C8 (2.00 eV), which matches with
the tendency on the HOMO–LUMO gaps determined optically.
On a basis of the measurements, free energy changes for the
electron transfer (ET) processes, i.e., electron injection (DGinj)
and dye regeneration (DGreg), were calculated.[8b] Considering
sufficient free energy changes for the ET processes (<–0.19 eV),
efficient ET is possible for their processes.
Binding structure
We examined the binding property of the thiazolocatechol
anchoring group on TiO2. A TiO2 electrode was immersed in a
1:4 mixture of THF and ethanol containing porphyrin (0.2 mM) to
give
a porphyrin-sensitized TiO2 electrode. The porphyrin
surface coverage (G) on the TiO2 surface was estimated by
measuring the porphyrin absorbance at 640 nm on the TiO2
electrode in comparison with that of YD2-o-C8 (Figure 3). During
the immersion, ZnPTC and YD2-o-C8 reach the saturated
surface coverages on TiO2 in 3 and 2 h, respectively. It is
noteworthy that the saturated G value of ZnPTC (1.3´10–10 mol
cm–2) exceeds that of YD2-o-C8 (8.2´10–11 mol cm–2). Although
the molecular structures of the two porphyrins are almost
identical, their anchoring groups affect the G values significantly.
The high G value of ZnPTC may be related with the high
aggregation tendency of ZnPTC.
Table 1. Redox potentials and free energy changes of ZnPTC and YD2-o-
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equivalents of CDCA (Figure S12). The use of the large excess
amount of CDCA for improving the cell performance also
supports the strong aggregation behavior of ZnPTC on TiO2.
The highest value (h = 4.87%) of the DSSC with ZnPTC under
our optimized conditions is significantly low compared to that
with YD2-o-C8 (h = 9.93%) under the optimized conditions
without the addition of CDCA (Figure
4 and Table 2).
Notwithstanding, as far as we know, the DSSC with ZnPTC
attains the record efficiency of the DSSCs based on porphyrin
sensitizers with a catechol anchoring group (0.8%)[10e] and even
organic sensitizers with a catechol anchoring group (1.6%).[10a]
Figure 3. Adsorption profiles of porphyrins YD2-o-C8 (black) and ZnPTC (red)
versus immersion time. The porphyrin surface coverages (G) were determined
for the TiO2 electrodes with no light-scattering layers.
In the next step, we examined the binding structure of the
thiazolocatechol anchoring group on TiO2. X-ray photoelectron
spectroscopic measurements were performed for catechol and
ZnPTC (Figures S9 and S10). The O1s photoelectron spectrum
of the catechol powder displays a peak at 532.1 eV, which is
originated from the OH moieties. After adsorption of catechol on
TiO2, the spectrum displays two peaks at 531.4 and 530.2 eV,
which are assigned to the O atoms of the C–O–Ti moiety and
TiO2. The shift of the corresponding binding energies from 532.1
eV to 531.4 eV together with appearance of the single peak at
531.4 eV suggests the bidentate adsorption mode of catechol to
TiO2.[14] The O1s photoelectron spectrum of the ZnPTC powder
illustrates two peaks at 532.4 and 531.5 eV, arising from the
oxygen atoms of the OH moieties in the thiazolocatechol
anchoring group and of the octyloxy moieties. After adsorption of
ZnPTC on TiO2, the spectrum shows three peaks at 531.3,
530.3, and 529.4 eV, which are assigned to the oxygen atoms of
the octyloxy moieties, of TiO2, and of the C–O–Ti moiety. From
the X-ray photoelectron spectroscopic measurements, we can
conclude that the binding mode of thiazolocatechol moiety on
TiO2 is symmetrical bidentate coordination, i.e., bidentate
chelating and/or bidentate bridging modes.
Figure 4. Photocurrent-voltage curves of the DSSCs with YD2-o-C8 (black)
and ZnPTC (red) under the optimized conditions.
Table 2. Photovoltaic performances of YD2-o-C8 and ZnPTC.[a]
JSC [mA cm–2
]
VOC [V]
ff[b]
h [%]
ZnPTC[c]
8.46
(8.23 ± 0.12)
0.757
(0.763
0.006)
0.760
(0.754
0.009)
4.87
(4.73
0.07)
±
±
±
±
±
±
YD2-o-
C8[d]
14.6
(14.2 ± 0.3)
0.912
(0.897
0.010)
0.746
(0.743
0.007)
9.93
(9.49
0.23)
Solar cell properties
The device performances of DSSCs were evaluated in standard
AM1.5 conditions. Tris(bipyridyl)cobaltII/III complexes were used
as the redox couple. To optimize the cell performance, we first
examined the effect of immersion time on the photovoltaic
properties without co-adsorbent (Figure S11). The DSSC with
ZnPTC exhibits a highest h-value of 3.7% in an immersion time
of 0.5 h. The decreasing trend after reaching the maximum
suggests the aggregation behavior of ZnPTC on TiO2, which
agrees with the higher surface coverage of ZnPTC than YD2-o-
C8 (vide supra). By fixing the immersion time at 0.5 h, we also
evaluated the effect of chenodeoxycholic acid (CDCA), a co-
adsorbent for the suppression of porphyrin aggregation on TiO2.
Indeed, the co-adsorption of CDCA improved the cell
performance. A maximal h-value of 4.87% was attained with 20
[a] Values represent solar cell parameters exhibiting the best h-value.
Values in bracket show average values obtained from eight different
experiments. [b] Fill factor. [c] Immersion time: 0.5 h, chenodeoxycholic acid
(CDCA): 20 equiv. [d] Immersion time: 3 h, CDCA: 0 equiv.
The absorption spectra of the porphyrin-adsorbed TiO2
electrodes and the action spectra of the DSSCs are shown in
Figure 5. Although the ZnPTC-sensitized TiO2 electrode displays
red-shifted absorption, the photocurrent generation efficiency
(incident photon-to-current efficiency (IPCE)) of the DSSC based
on ZnPTC are lower than those with YD2-o-C8 in all the
wavelength regions. The IPCE value is calculated from the
following equation: IPCE = LHE ´ finj ´ hcol, where LHE (light-
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harvesting efficiency) is the number of adsorbed photons per
incident photon, finj is the efficiency of electron injection, and hcol
is the efficiency of charge collection. Because the LHE of
ZnPTC and YD2-o-C8 are comparable (Figure 5a), the finj
and/or hcol of ZnPTC should be lower than those of YD2-o-C8.
Although the aggregation of ZnPTC on TiO2 is suppressed by
co-adsorption of the large excess amount of CDCA to some
extent, the finj value would be low compared to that of the DSSC
based on YD2-o-C8, considering the smaller orbital distributions
of the thiazolocatechol anchoring group in the LUMO in addition
1
to the aggregation-induced fast quenching of ZnP* on TiO2. To
examine the ET process from the redox shuttle to the oxidized
porphyrin, microsecond time-resolved transient absorption (TA)
measurements were performed for the porphyrin-sensitized TiO2
films using neat acetonitrile and cobalt electrolyte solutions
(Figure S13). We monitored the decay profiles of characteristic
absorption at 800 nm corresponding to ZnP•+, which was
confirmed by the UV/Vis absorption measurements of the
chemically oxidized porphyrins (Figure S14). The lifetime of
ZnP•+ in the absence of the cobalt redox shuttle for ZnPTC (15.3
µs) was significantly shorter than that for YD2-o-C8 (40.7 µs),
which shows the faster CR process between ZnP•+ and electron
in the CB of TiO2 for ZnPTC. From the lifetimes of ZnP•+ in the
absence and presence of the cobalt redox shuttle, the dye
regeneration efficiencies (jreg) can be estimated to be 72% for
ZnPTC and 73% for YD2-o-C8. The similar jreg values suggest
that the difference in the IPCE values at least to some extent
results from fast CR processes from the electron in the CB of
TiO2 to ZnP•+, which may be caused by the larger orbital
distribution on the thiazolocatechol group than the
carboxyphenyl group in HOMOs (Figure S7), and/or to the
oxidized redox shuttle in the case of ZnPTC. In addition, we
evaluated current-voltage curves in dark conditions (Figure S15).
The onset voltage of the DSSC with ZnPTC is shifted to a
negative direction by ca. 0.15 V relative to that with YD2-o-C8.
This suggests the faster CR from the electrons in the CB of TiO2
to the oxidized redox couple in the electrolyte solution for the
DSSC with ZnPTC than that with YD2-o-C8. We have
demonstrated that with an increase in the length of linkers
between a porphyrin core and TiO2, the porphyrin tends to be
inclined to TiO2, leading to fast CR on the time region of pico-
and nanosecond and in turn low photovoltaic performances.[15]
More tilted geometry of ZnPTC than YD2-o-C8 as the result of
the longer linker of ZnPTC than YD2-o-C8 together with the
unfavorable orbital distribution of ZnPTC in HOMO would result
in the plausible occurrence of the fast CR even on the time
region of pico- and nanosecond, in addition to the microsecond
region, and the eventual decrease in the VOC and hcol values.
Consequently, these fast CR processes together with the slow
electron injection lower the finj and hcol values, resulting in the
moderate JSC and h of the DSSC with ZnPTC.
Figure 5. a) UV-Visible absorption spectra of the porphyrin-adsorbed TiO2
electrodes with YD2-o-C8 (black) and ZnPTC (red). Light-scattering TiO2
layers were not used to obtain an accurate absorption profile. b) Plots of IPCE
versus wavelength: DSSCs with YD2-o-C8 (black) and ZnPTC (red) in the
best conditions for achieving the highest h-values.
Long-term durability
We investigated the binding stability of YD2-o-C8 and ZnPTC on
TiO2. The porphyrin-sensitized TiO2 electrodes were immersed
in a THF-H2O mixture (v/v = 1:1) containing 28 mM acetic acid or
0.1 mM NaOH. The amount of dye retained on TiO2 was
determined versus the immersion time from the absorbance of
the TiO2 electrode (Figure S16). After immersion in 8h, the YD2-
o-C8 molecules were dissociated from TiO2 by 70% and 100%,
under acidic and alkaline conditions, respectively. In contrast,
the ZnPTC molecules were detached from TiO2 by 10% and
29% under the respective conditions. We also investigated the
binding ability in organic and aqueous solutions without acetic
acid or NaOH (Figure S17). ZnPTC showed no desorption under
both conditions, whereas the YD2-o-C8 molecules were
desorbed from TiO2 under the aqueous conditions. These results
demonstrate the superior binding ability of the thiazolocatechol
anchoring group to the carboxylic acid anchoring group. Then,
we evaluated the long-term durability of DSSCs with ZnPTC and
YD2-o-C8 under continuous white light illumination (100 mW
cm–2) and dark conditions at 25 °C (Figure S18). Unfortunately,
both cells exhibited similar decreasing profiles in the h-values
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4-Nitro-1,2-phenylene diacetate (1): 4-Nitrocatechol (1.0 g, 6.4 mmol)
with increasing the illumination time despite the superior binding
ability of ZnPTC. On the other hand, the ZnPTC-based DSSC
showed almost no decrease in the h-value over 500 h, whereas
the YD2-o-C8-based DSSC revealed a decrease of 16% in 500
h. Although the long-term durability under white light illumination
is not improved for ZnPTC, the superior binding ability and cell
durability under dark conditions signify the potential utility of the
thiazolocatechol anchoring group for sensitizers in DSSCs.
and 4-dimethylaminopyridine (40 mg, 0.32 mmol,
5 mol%) were
dissolved in dry THF (32 mL) and the mixed solution was cooled to 0 °C.
Acetic anhydride was added to this solution and the solution was stirred
at 0 °C. After 1 h, the reaction mixture was added to water and extracted
with CH2Cl2, dried over anhydrous Na2SO4 and the solvent was
evaporated under reduced pressure. The reaction mixture was purified
by silica gel column chromatography using CH2Cl2 as eluent to give 1 as
a white solid (1.5 g, 6.3 mmol, 98%). 1H NMR[17] (CDCl3, 400 MHz): d =
8.16 (dd, J = 8.8, 2.0 Hz, 1H), 8.12 (d, J = 7.6 Hz, 1H), 7.40 (d, J = 8.8
Hz, 1H) and 2.34 (s, 6H) ppm.
Conclusions
4-Amino-1,2-phenylene diacetate (2): A solution of 1 (1.5 g, 6.3 mmol)
in CHCl3 (115 mL) was poured into a hydrogenator bomb and treated
with 10 wt% Pd/C (401 mg). After 5 h, the reaction mixture was filtered
on celite and the solvent was evaporated under reduced pressure. The
crude product was purified by silica gel column chromatography using
CH2Cl2/MeOH (v/v=50/1) as eluent to give 2 as a white solid (1.1 g, 5.3
mmol, 84%). 1H NMR[17] (CDCl3, 400 MHz): d = 6.93 (d, J = 8.4 Hz, 1H),
6.52 (dd, J = 8.6, 2.8 Hz, 1H), 6.49 (d, J = 2.8 Hz, 1H), 3.68 (s, 2H), 2.26
(s, 3H) and 2.25 (s, 3H) ppm.
We designed and synthesized a push-pull porphyrin sensitizer
ZnPTC with a thiazolocatechol anchoring group to evaluate the
effect of the thiazolocatechol group on the photovoltaic
properties and long-term durability. The DSSC with ZnPTC
exhibited the moderate photovoltaic performance (h = 4.87%)
compared to that with YD2-o-C8 (h = 9.93%). The moderate h-
value for ZnPTC can be ascribed to the low finj and hcol values
from the unfavorable orbital distribution of the thiazolocatechol
anchoring group on LUMO and HOMO as well as the fast CR
from the electrons in the CB of TiO2 to the oxidized redox couple
in the electrolyte solution or to the oxidized porphyrin on TiO2.
Nevertheless, the photovoltaic performance of the DSSC with
ZnPTC (h = 4.87%) has overcome the barrier of h = 4% for the
first time in catechol-based sensitizers. Therefore, fusion of the
electron-withdrawing thiazole moiety with catechol was found to
be effective to improve the photovoltaic performance of DSSCs.
Moreover, we corroborated the superior binding ability of the
thiazolocatechol anchoring group to the conventional carboxylic
acid anchoring group under harsh conditions. We believe that
further rational molecular design of catechol anchoring groups
with a suitable electron-withdrawing moiety would make a
breakthrough in DSSC sensitizers for achieving both high
photovoltaic performances and long-term cell durability.
2-Amino-5,6-diacetoxybenzothiazole (3): To a solution of 2 (500 mg,
2.4 mmol) and KSCN (465 mg, 4.8 mmol, 2 eq.) in AcOH (12 mL), a
solution of Br2 (0.25 mL, 4.8 mmol, 2 eq.) in AcOH (12 mL) was added
dropwise at room temperature. The mixture was stirred for 5.5 h at room
temperature and then the reaction mixture was added to a saturated
Na2S2O3 aqueous solution and extracted with CH2Cl2, dried over
anhydrous Na2SO4. After the solvent was evaporated under reduced
pressure, the reaction mixture was purified by silica gel column
chromatography using CH2Cl2/MeOH (v/v=100/1) as eluent to give 3 as a
white solid (281 mg, 1.1 mmol, 46%). 1H NMR (CDCl3, 400 MHz): d =
7.41 (s, 1H), 7.33 (s, 1H), 5.24 (s, 1H), 2.31 (s, 3H) and 2.30 (s, 3H) ppm.
13C NMR (CDCl3, 100 MHz): d = 168.8, 168.7, 166.8, 150.5, 141.1, 137.7,
129.4, 115.3, 113.8 and 20.8 ppm. FT-IR (ATR): n = 2369, 2229, 2196,
2028, 1967, 1914, 1763, 1558, 1476, 1436, 1369, 1278, 1197, 1168,
1129, 1011, 967, 910 and 871 cm–1
. MALDI-MS: m/z calcd for
C11H10N2O4SNa: [M+Na]+ 289.0253; found 289.0251. m.p.: 162–164 °C.
2-Bromo-5,6-diacetoxybenzothiazole (4): To a solution of 3 (25 mg,
0.094 mmol) and CuBr (20 mg, 0.14 mmol, 1.5 eq.) in MeCN (1.3 mL) a
solution of tert-butyl nitrite (90%, 17 µL, 1.50 mmol) in MeCN (0.85 mL)
was added dropwise at 0 °C. The mixture was stirred for 1 h at room
temperature and then 3 h at 65 °C. The reaction mixture was added to
water and extracted with EtOAc, washed with 1 M HCl aqueous solution,
saturated NaHCO3 aqueous solution, brine and water, dried over
anhydrous Na2SO4. After the solvent was evaporated under reduced
pressure, the reaction mixture was purified by silica gel column
chromatography using CH2Cl2/MeOH (v/v=100/1) as eluent to give 4 as a
white solid (25 mg, 0.076 mmol, 81%). 1H NMR (CDCl3, 400 MHz): d =
7.81 (s, 1H), 7.67 (s, 1H), 2.35 (s, 3H) and 2.33 (s, 3H) ppm. 13C NMR
(CDCl3, 100 MHz): d = 168.4, 150.3, 141.7, 140.9, 140.2, 134.9, 117.3,
115.3, 20.8 and 20.7 ppm. FT-IR (ATR): n = 3439, 3039, 2531, 2422,
2156, 1760, 1646, 1535, 1459, 1372, 1209, 1190, 1137, 1016, 904 and
867 cm–1. MALDI-MS: m/z calcd for C11H979BrNO4S: [M+H]+ 329.9430;
found 329.9443. m.p.: 85–86 °C.
Experimental Section
Materials and Instruments: Commercially available reagents and
solvents were employed without additional purification unless otherwise
noted. Silica-gel column chromatography was carried out using UltraPure
Silica Gel (230-400 mesh, SiliCycle) unless otherwise described. Thin-
layer chromatography (TLC) was performed with Silica gel 60 F254
(Merck). Size exclusion gel permeation chromatography (GPC) was
implemented using Bio-beads S-X1 (Bio-rad). UV-visible-near infrared
absorption and steady-state fluorescence spectra, 1H and 13C NMR
spectra, high-resolution mass spectra (HR-MS), and attenuated total
reflectance-Fourier transform infrared (ATR-FTIR) spectra were recorded
following the previously reported methods.[8b] Electrochemical and X-ray
photoelectron spectroscopy measurements were conducted according to
our previous paper.[8b] DFT calculations were carried out using the
Gaussian 09 program.[16] All the structures of the porphyrins were fully
optimized without any symmetry restriction at the B3LYP/6-31G(d) level
for C, H, O, N, S, and Zn.
Porphyrin 6: Porphryin 5 (0.060 mmol) was dissolved in a mixture of dry
THF (11 mL) and NEt3 (4.1 mL). To the mixture, 4 (100 mg, 0.30 mmol,
5.0 eq.), Pd2(dba)3 (69 mg, 0.075 mmol, 25 mol%) and AsPh3 (150 mg,
0.48 mmol, 1.6 eq.) were added. The mixture was refluxed for 3 h under
argon. Then, the reaction mixture was cooled to room temperature,
washed with water, extracted with CH2Cl2, and dried over anhydrous
Na2SO4. The solvent was evaporated under reduce pressure and the
Synthesis: Porphyrin 5 was prepared according to literature.[6b]
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10.1002/cphc.201900342
ChemPhysChem
FULL PAPER
reaction mixture was purified by silica gel column chromatography using
CH2Cl2 as eluent and GPC using toluene as eluent to give 6 as a green
solid (34 mg, 0.021 mmol, 35%). 1H NMR (CDCl3, 400 MHz): d = 9.66 (d,
J = 4.8 Hz, 2H), 9.18 (d, J = 4.4 Hz, 2H), 8.90 (d, J = 4.4 Hz, 2H), 8.68 (d,
J = 4.4 Hz, 2H), 8.02 (s, 1H), 7.83 (s, 1H), 7.66 (t, J = 8.4 Hz, 2H), 7.20
(d, J = 8.8 Hz, 4H), 6.95 (t, J = 9.2 Hz, 8H), 3.84 (t, J = 6.4 Hz, 8H), 2.46
(t, J = 7.8 Hz, 4H), 2.40 (s, 3H), 2.38 (s, 3H), 1.55–1.51 (m, 4H), 1.31–
1.26 (m, 12H), 1.02–0.96 (m, 8H), 0.85–0.78 (m, 14H) and 0.65–0.45 (m,
44H) ppm. 13C NMR (CDCl3, 100 MHz): d = 168.5, 159.9, 152.4, 152.0,
151.7, 151.5, 150.8, 150.6, 141.9, 141.0, 134.9, 133.3, 133.0, 132.2,
131.0, 130.2, 130.1, 128.9, 124.6, 122.2, 120.5, 117.5, 115.4, 105.2,
102.8, 95.1, 88.6, 68.7, 35.4, 31.9, 31.7, 31.5, 29.3, 28.9, 28.7, 28.6,
25.3, 22.8, 22.4, 20.9, 20.8, 14.2 and 13.9 ppm. MALDI-MS: m/z calcd
for C101H124N6O8SZn: [M]•+ 1644.8487; found 1644.8504. FT-IR (ATR): n
= 3864, 3676, 3630, 3556, 2949, 2922, 2853, 2344, 2179, 2162, 1988,
1777, 1587, 1504, 1449, 1367, 1338, 1297, 1243, 1199, 1094, 996, 933,
910, 793 and 712 cm–1. m.p.: 63 °C.
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porphyrin 6 (33 mg, 0.020 mmol). The solution was heated at 40 °C for 1
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the mixture was diluted with CH2Cl2, washed with brine, dried over
anhydrous Na2SO4. After the solvent was removed to give ZnPTC as a
green solid (31 mg, 0.020 mmol, quant.). 1H NMR (CDCl3/pyridine-d5,
400 MHz): d = 9.57 (d, J = 4.4 Hz, 2H), 9.00 (d, J = 4.4 Hz, 2H), 8.78 (d,
J = 4.8 Hz, H), 8.57 (d, J = 4.4 Hz, 2H), 7.66 (s, 1H), 7.63 (t, J = 8.2 Hz,
2H), 7.39 (s, 1H), 7.00 (d, J = 8.0 Hz, 4H), 6.94 (d, J = 8.0 Hz, 4H), 6.80
(d, J = 8.4 Hz, 4H), 3.81 (t, J = 6.6 Hz, 8H), 2.40 (t, J = 7.6 Hz, 4H), 1.49–
1.46 (m, 4H) and 0.93–0.42 (m, 78H) ppm. 13C NMR (CDCl3/pyridine-d5,
100 MHz): d = 160.0, 152.3, 151.5, 150.6, 150.4, 149.5, 149.1, 148.9,
135.8, 135.6, 135.4, 134.3, 132.2, 131.7, 130.5, 130.0, 129.7, 128.6,
123.5, 123.2, 123.0, 121.7, 121.4, 114.4, 107.8, 105.2, 105.1, 95.1, 88.7,
77.5, 77.4, 77.2, 76.8, 76.5, 68.6, 35.3, 31.8, 31.6, 31.1, 29.8, 29.3, 28.8,
28.72, 28.66, 25.2, 22.7, 22.4, 14.2 and 14.0 ppm. MALDI-MS: m/z calcd
for C97H120N6O6SZn: [M]•+ 1560.8276; found 1560.8247. FT-IR (ATR): n =
3899, 3805, 3751, 3725, 3687, 3462, 2924, 2851, 2234, 2173, 2146,
2028, 1964, 1588, 1504, 1451, 1243, 1093, 995, 792 and 711 cm–1. m.p.:
96–97 °C.
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Photovoltaic Measurements: The TiO2 electrodes and the sealed cells
for photovoltaic measurements were prepared according to
literatures.[8b,12,18] The TiO2 electrode was soaked into a THF/ethanol
solution (v/v = 1/4) containing the porphyrins (0.20 mM) at 25 °C. The
electrolyte solution consisted of 0.25
[Co(bpy)3](TFSI)3, 0.1 lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI), and 0.5 4-tert-butylpyridine in acetonitrile. Photovoltaic
M [Co(bpy)3](TFSI)2, 0.05 M
M
M
measurements were conducted according to our previous papers.[8b,11]
Acknowledgements
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This work was supported by the JSPS (KAKENHI Grant
Numbers JP18H03898 (H.I.) and JP18K14198(T.H.)). In this
work, DFT calculations were performed by the supercomputer of
ACCMS, Kyoto University.
Keywords: porphyrin • thiazole • catechol • solar cell •
anchoring group
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We designed and synthesized a
push-pull porphyrin sensitizer ZnPTC
with a thiazolocatechol as a catechol
anchoring group with an electron-
withdrawing thiazole moiety. The dye-
sensitized solar cell with ZnPTC
revealed an energy conversion
efficiency of 4.87%, exceeding the
barrier of 4% for the first time in
catechol-based sensitizers.
T. Higashino, H. Iiyama, Y.
Kurumisawa, H. Imahori*
Page No. – Page No.
Thiazolocatechol: Electron-
withdrawing Catechol Anchoring
Group for Dye-Sensitized Solar Cells
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Author(s), Corresponding Author(s)*
((Insert TOC Graphic here; max. width: 11.5 cm; max. height: 2.5 cm))
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Title
Text for Table of Contents
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