Low-Symmetry Ω-Shaped Zinc Phthalocyanine Sensitizers with Panchromatic Light-Harvesting Properties for Dye-Sensitized Solar Cells

Article abstract of DOI:10.1002/chem.201603705

Two low-symmetry phthalocyanines (Pcs) substituted with thiophene units at the non-peripheral (α) and peripheral (β) positions were synthesized and their optical, electronic-structure, and electrochemical properties were investigated. The substitution of thiophene units at the α positions of the phthalocyanine skeleton resulted in a red shift of the Q band and significantly modified the molecular-orbital electronic distributions just below the HOMO and just above the LUMO, with distortion of the typical Gouterman four-orbital arrangement of MOs. Two amphiphilic Ω-shaped ZnPcs (αPcS1 and αPcS2) bearing a π-conjugated side chain with an adsorption site at an α position of the Pc macrocycle were synthesized as sensitizers for dye-sensitized solar cells (DSSCs). The absorption spectra of αPcS1 and αPcS2 showed red shifted Q bands and a broad band from 350 to 550 nm assignable to the intramolecular charge-transfer transition from the ZnPc core to the side chains. Time-dependent DFT calculations provided a clear interpretation of the effect of the thiophene conjugation on the typical phthalocyanine core π MOs. Compound αPcS1 was used as a light-harvesting dye on a TiO₂electrode for a DSSC, which showed a panchromatic response in the range 400–800 nm with a power conversion efficiency of 5.5 % under one-sun conditions.

Full text of DOI:10.1002/chem.201603705

DOI: 10.1002/chem.201603705
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&
Phthalocyanines
Low-Symmetry W-Shaped Zinc Phthalocyanine Sensitizers having
Panchromatic Light-Harvesting Property for Dye-Sensitized Solar
Cells
Satoshi Yamamoto,[a] Angel Zhang,[b] Martin J. Stillman,[b] Nagao Kobayashi,[a] and
Mutsumi Kimura*[a]
Chem. Eur. J. 2016, 22, 1 – 10
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Abstract: Two low-symmetry phthalocyanines (Pcs) substi-
tuted with thiophene units at the non-peripheral (a) and pe-
ripheral (b) positions were synthesized and their optical,
electronic-structure, and electrochemical properties were in-
vestigated. The substitution of thiophene units at the a posi-
tions of the phthalocyanine skeleton resulted in a red shift
of the Q band and significantly modified the molecular-orbi-
tal electronic distributions just below the HOMO and just
above the LUMO, with distortion of the typical Gouterman
four-orbital arrangement of MOs. Two amphiphilic W-shaped
ZnPcs (aPcS1 and aPcS2) bearing a p-conjugated side chain
with an adsorption site at an a position of the Pc macrocy-
cle were synthesized as sensitizers for dye-sensitized solar
cells (DSSCs). The absorption spectra of aPcS1 and aPcS2
showed red shifted Q bands and a broad band from 350 to
550 nm assignable to the intramolecular charge-transfer
transition from the ZnPc core to the side chains. Time-de-
pendent DFT calculations provided a clear interpretation of
the effect of the thiophene conjugation on the typical
phthalocyanine core p MOs. Compound aPcS1 was used as
a light-harvesting dye on a TiO2 electrode for a DSSC, which
showed a panchromatic response in the range 400–800 nm
with a power conversion efficiency of 5.5% under one-sun
conditions.
Introduction
two dyes on the TiO2 surface. Ince et al. reported the panchro-
matic response of hybrid MPc-based sensitizers having periph-
eral p-conjugated bis-thiophene substituents.[6] However, the
reported PCEs of DSSCs sensitized with the hybrid sensitizers
were less than 3% due to their strong tendency for intermo-
lecular aggregation. In this work, we investigated the optical
effects of substitution of thiophene at the a or b position of
ZnPcs 1 and 2. Since a-substituted ZnPc 1 showed a larger red
shift of the Q band, we chose it for the design of two new
hybrid chromophoric systems, aPcS1 and aPcS2, in which a p-
conjugated side chain with an adsorption site was introduced
at an a position of the MPc to act as the photosensitizer for
DSSCs. A photoresponsive cell sensitized with aPcS1 provided
a PCE of 5.5% under a simulated air mass with AM1.5 global
sunlight; aPcS1 could convert photon energy to electronic
energy over a wide wavelength range from the visible to the
NIR regions of the solar spectrum to electronic energy.
Phthalocyanines (Pcs) and their metal complexes (MPcs) are
synthetic analogues of porphyrins and have attracted special
attention as sensitizers in solar-energy conversion applications
because of their intense absorption bands in the red/near-in-
frared (NIR) region (Q bands), high molar extinction coeffi-
cients, and excellent stabilities.[1] The molecular engineering of
MPc-based sensitizers has been widely explored to improve
photon-to-electron power conversion efficiency (PCE) in organ-
ic-based solar cells.[2] The PCE values for MPc-based dye-sensi-
tized solar cells (DSSCs) have been enhanced significantly by
applying molecular-design rules such as a low-symmetry struc-
ture, steric isolation, intramolecular push–pull structure, and
optimization of the adsorption site.[3] The expansion of the
light-harvesting wavelength range in the NIR region is one of
the key issues to be solved in order to enhance the solar-cell
performance. To date, several attempts have been made to
expand the light-harvesting wavelength range of MPc-based
DSSCs by extension of the p-conjugated system and substitu- Results and Discussion
tion of peripheral positions.[4] We recently reported the light-
Effect of thiophene substitution positions on the optical and
harvesting property in the NIR region of ring-expanded sensi-
tizer NcS1, and a DSSC sensitized with NcS1 showed a photo-
response at wavelengths in the 600–850 nm region.[4c] Howev-
er, these MPc-based solar cells exhibited little sensitization in
the region between the B and the Q bands due to the absence
of transitions in this area. Co-sensitization with different dyes
that have complementary absorption properties in the visible
region has been applied to cover whole region from visible to
NIR.[3d,5] The fabrication of co-sensitized DSSCs requires careful
control of the adsorption process to optimize the ratio of the
electrochemical properties
The effect of substituents on the position and intensity of the
Q bands has been systematically investigated.[7] Peripheral sub-
stitution with electron-donating groups such as alkoxyl and al-
kylthio groups at the a positions of the MPc skeleton shifts the
Q band to longer wavelength compared to b-substituted
counterparts, and these effects can be reasonably explained by
considering the magnitude of the atomic orbital coefficients of
the carbon atoms derived from molecular orbital calcula-
tions.[7b] However, the effect of the introduction of thiophene
units at the a and b positions of the MPc skeleton on the opti-
cal and electrochemical properties has not been examined. We
synthesized two structural isomers 1 and 2 having two thio-
phene units at the a or b positions on one side of the macro-
cycle (Figure 1) and investigated their optical, electronic struc-
ture and electrochemical differences.
[a] S. Yamamoto, Prof. Dr. N. Kobayashi, Prof. Dr. M. Kimura
Division of Chemistry and Materials
Faculty of Textile Science and Technology
Shinshu University, Ueda 386-8567 (Japan)
E-mail: mkimura@shinshu-u.ac.jp
[b] A. Zhang, Prof. Dr. M. J. Stillman
Department of Chemistry and Department of Biology
The University of Western Ontario, London, Ontario, N6A 5B7 (Canada)
Phthalonitriles 3 and 4 bearing two thiophene rings at differ-
ent positions were synthesized by palladium-catalyzed cou-
pling reactions of 3,6-dihydroxyphthalonitrile and 4,5-dichlo-
Supporting information and ORCIDs of the authors for this article are avail-
able on the WWW under http://dx.doi.org/10.1002/chem.201603705.
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ZnPc(tBu)4. Furthermore, the molar extinction e at the Q band
for 1 was significantly lower than that of 2 (Table 1). Splitting
of the Q band was not observed in the previously reported
zinc a-octaphenylphthalocyanine complexes.[13] Thus, the intro-
duction of the thiophene units at both the a and b positions
of the MPc skeleton strongly affects the electronic properties
of the MPc core p system. The emission maximum of 1 is red
shifted by 14 nm relative to that of 2, and this suggests that
1 has a narrower bandgap than 2 (Figure 2a). The magnetic
circular dichroism (MCD) spectrum of 1 showed a Faraday A-
term-like curve with a peak at 672 nm and trough at 697 nm
(Figure 2b). This is assignable as a pseudo-Faraday A term pro-
duced by superimposition of close lying Faraday B terms of op-
posite sign because of the approximate C2v symmetry.[14] This
suggests splitting of the LUMO and LUMO+1 energy levels of
the low-symmetry complexes. The HOMO energy levels of
1 and 2 were also determined by using a differential pulse vol-
tammetric (DPV) technique (Supporting Information, Fig-
ure S3). The first oxidation potentials of 1 (+0.02 V vs. Fc+/Fc)
and 2 (+0.04 V vs. Fc+/Fc) are negatively shifted compared to
that of ZnPc(tBu)4 (+0.09 V vs. Fc+/Fc). The increased energy
of the HOMO level of 1 compared with that of 2 indicates the
effective destabilization of the HOMO levels by substitution at
a positions.
Figure 1. Substitution patterns in the ZnPc(tBu)4 core of compounds 1 and
2.
rophthalonitrile, respectively (Scheme 1). The phthalonitrile
having thiophene rings at the 3- and 6-positions was synthe-
sized by Migita–Kosugi–Stille cross-coupling between 3,6-bis-
(trifluoromethanesulfonyloxy)phthalonitrile and 2-(tributylstan-
nyl)thiophene in 81% yield.[8,9] The 4,5-dithiophenephthaloni-
trile was synthesized by Suzuki–Miyaura cross-coupling of 4,5-
dichlorophthalonitrile with 2-thiopheneboronic acid by using
2-dicyclohexylphosphino-2’,6’-methoxybiphenyl (SPhos) as
ligand in 88% yield.[10] After treatment with N-bromosuccini-
mide (NBS), the resulting bromides were treated with 4-tert-bu-
tylphenylboronic acid in the presence of [Pd(PPh3)4] to produce
3 and 4 in 90 and 86% yield, respectively. The peripherally sub-
stituted, low-symmetry ZnPcs 1 and 2 were prepared by mixed
condensation reaction between 4-tert-butylphthalonitrile and 3
or 4 in 3:1 molar ratio in N,N-dimethylaminoethanol (DMAE) in
the presence of Zn(AcO)2, and were isolated by column chro-
matography and preparative HPLC.[11]
Figure 2a shows absorption spectra of a-substituted 1 and
b-substituted 2 in THF. While the absorption spectrum of
ZnPc(tBu)4 lacking thiophene rings showed the expected sharp
and strong
Q band at 672 nm (Supporting Information,
Figure S2), the low-symmetry 1 and 2 displayed broader, split
Q bands red shifted by about 20 nm relative to that of
Figure 2. a) Absorption and emission spectra of 1 (solid and dotted lines)
and 2 (dashed and dashed-dotted line) in THF. b) MCD spectra of 1 (solid
line) and 2 (dashed line) in THF.
DFT calculations at the CAM-B3LYP/6-31G* level of theory (6-
31G* is also termed 6-31G(d))[15] (Gaussian 09) were carried out
for ZnPc, 1, and 2 to enhance our understanding of the spec-
troscopic and electrochemical data. Figure 3 shows energy-
level diagrams of 1, 2, and the core ZnPc, and calculated elec-
tronic absorption spectra of 1 and 2. Whereas the estimated
LUMO and LUMO+1 levels of unsubstituted ZnPc are degen-
erate, the degeneracy of LUMO and LUMO+1 levels for 1 and
2 is broken by substitution with thiophene units. The appear-
ance of thiophene-related orbitals between the HOMO and the
Scheme 1. a) 2-(Tributylstannyl)thiophene, [Pd(PPh3)4], LiCl, dioxane; b) NBS,
DMF; c) [Pd(PPh3)4], 1m K2CO3 aq., DME; d) 2-Thiopheneboronic acid,
Pd(AcO)2, SPhos, K3PO4, toluene.
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stepwise coupling reactions, as shown in Scheme 2
(Supporting Information, Scheme S1). Low-symmetry
AB3-type ZnPcs were synthesized by statistical tetra-
merization of the bulky 4,5-bis(2,6-diisopropyl)phe-
noxyphthalonitrile 8 and 7 or 12. After deprotection
of the pinacol-protected aldehyde group, the alde-
hyde terminal group was allowed to react with an
excess of cyanoacetic acid in the presence of piperi-
dine to afford aPcS1 and aPcS2. We also synthesized
symmetrical ZnPc 5 as a reference compound using
phthalonitrile 8.[19]
Table 1. Optical and electrochemical data for 1 and 2.
[d]
[e]
[f]
Dye Absorption[a]
[nm] (10ꢀ5 e [mꢀ1 cmꢀ1
Emission[b] EHOMO
E0–0 ELUMO
]
[nm] (f)[c] [V vs. Fc+/Fc] [eV] [V vs. Fc+/Fc]
f
1
2
694 (0.68), 678 (0.69), 347 (0.72) 713 (0.1)
690 (1.56), 675 (1.1), 350 (0.82) 698 (0.2)
+0.02
+0.04
1.76 ꢀ1.74
1.79 ꢀ1.75
[a] In THF. [b] In THF by exciting at the peak position of the B band. [c] Fluorescence
quantum yield f was determined with ZnPc(tBu)4 as standard in THF (f =0.3[12]).
f
f
[d] EHOMO was determined from the first oxidation potential of the dye in o-DCB solu-
tion; [e] Optical bandgap E0–0 was determined from the intercept of the normalized
absorption and emission spectra. [f] ELUMO was calculated from the HOMO and the
band gap.
typical Gouterman HOMOꢀ1,[16] which are identified by the
nodal patterns for the +/ꢀ4 angular momentum of the core
porphyrazine ring, is an important aspect of the use of the
thiophene units. Whereas HOMOꢀ1 for Pcs is always lower in
energy by a significant amount (here >2 eV) for peripheral
substituents that do not interact with the conjugation, no orbi-
tals are usually introduced between the HOMO and its paired
four-node orbitals HOMOꢀ1 nominally the 2nd of the eu pair
in symmetry. The energy splitting correlates well with the Q-
band intensity.[17] Time-dependent (TD) DFT calculations repro-
duced the spectral differences between 1 and 2 (Figure 3b
and Table S1 of the Supporting Information) very well. The
split Q bands can be assigned to the HOMO!LUMO and
HOMO!LUMO+1 transitions of the Pc ligand. The lowest-
lying electronic transition of 1 was red shifted by 13 nm com-
pared with that of 2 due to the narrowing of the HOMO–
LUMO bandgap through destabilization of the HOMO level
and stabilization of the LUMO level. For isolated MOs like
those in ZnPc, the HOMO–LUMO gap can closely predict the
Q-band energy.[17] These results reveal that substitution of thio-
phene units at the a positions of the Pc skeleton leads to ex-
pansion of the light-harvesting region in the NIR region by red
shifting of Q bands as well as tuning of the HOMO and LUMO
energy levels to optimize the photoinduced electron-transfer
Figure 3. a) Calculated energy levels of 1 (left), ZnPc (middle), and 2 (right);
b) Calculated absorption spectra of 1 (red) and 2 (blue) derived from CAM-
B3LYP/6-31G(d) DFT calculations. LUMO and LUMO+1 at about ꢀ2 eV are
degenerate for ZnPc, and slightly split for 1 and 2.
reaction for the DSSC system. Thus, the a-substituted ZnPc
sensitizers were used to prepare aPcS1 and aPcS2 by adding
an adsorption site that was designed and synthesized to im-
prove the enhancement of DSSC performance.
Low-symmetry ZnPc sensitizers for DSSCs
Optical data for aPcS1 and aPcS2
We designed and synthesized two ZnPc-based dyes based on
1, aPcS1 and aPcS2, in which a cyanocarboxyl group was at-
tached at one terminal of the conjugated thiophene side chain
on the Pc ring (Figure 4). Since the ring expansion of Pc leads
to more red shifting of the Q band,[18] the ring-expanded dye
aPcS2 was synthesized from 1,4-dihydroxy-2,3-dicyanonaph-
thalene, with the expectation of further enhancement of the
light-harvesting region for DSSCs. For both dyes, six bulky 2,6-
diisopropylphenoxyl substituents were added to the peripheral
positions of the ZnPc core to prevent the formation of inter-
molecular aggregates.[3f,19] Phthalonitriles 7 and 12 bearing
a protected terminal aldehyde group were synthesized by
Figure 5 shows the absorption and MCD spectra of aPcS1 and
aPcS2 in THF. The attachment of thiophene side chains at the
a positions on one side of the Pc macrocycle resulted in a red
shift and splitting of the Q band compared to symmetrical
ZnPc 5. The Q MCD bands of aPcS1 and aPcS2 appeared at
longer wavelengths and their bandwidths were larger than
that of 5 (particularly the Q band for aPcS1), and hence the Q-
band MCD can be interpreted as a pseudo-Faraday A terms.
The ring-expanded aPcS2 shows a sharp Q band at 708 nm,
which is slightly red shifted compared with that of aPcS1. The
splitting width of the Q band for aPcS2 predicted by the
TDDFT calculation (15.0 nm) was narrower than that of aPcS1
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Figure 4. Structures of aPcS1, aPcS2, and 5.[19]
Figure 5. a) Absorption and b) MCD spectra of aPcS1 (solid line), aPcS2
(dashed line) and 5 (dashed-dotted line) in THF solution.
Identification of a thiophene-specific charge-transfer band
from the MCD spectra and TDDFT calculations
Scheme 2. a) 4,4,5,5-Tetramethyl-2-[5-(4,4,5,5-tetrametyl-1,3-dioxolan-2-yl)-
thien-2-yl]-1,3,2-dioxaborolane, [Pd(PPh3)4], DME, 1m K2CO3 aq.; b) 4-tert-Bu-
tylphenylboronic acid, DME, 1m K2CO3 aq.; c) Zn(AcO)2, DMEA, o-DCB;
d) 10% CHl aq., THF; e) Piperidine, cyanoacetic acid, CHCl3.
A new broad band appeared in the 380–480 nm region in the
absorption spectrum of aPcS1 and aPcS2 that is not seen for
3, 4, or 5. The MCD spectra in the 380–480 nm regions are sur-
prisingly very similar for aPcS1, aPcS2, and 5. The lack of sig-
nificant MCD intensity under the broad absorption band in the
380-480 nm region implies that negligible angular momentum
is involved in these electronic transitions.[20] This is consistent
with the TDDFT calculations for aPcS1 and aPcS2, that is, the
transition identified between the B and Q bands for aPcS1 at
467 nm (f=0.69) is assigned to the HOMO!LUMO+2 transi-
tion (Figure 6, Table 3, Supporting Information, Figure S6),
where the LUMO+2 (#549) MO is not a core p ring MO. The
transition for aPcS1 showed negative solvatochromism with
increasing solvent polarity, which indicates a change of the
energy level in this excited state following the interaction be-
tween the dye and solvent molecules (Supporting Information,
(29.2 nm). This narrow splitting width of aPcS2 results in in-
creased sharpness of the Q MCD pseudo-Faraday A term and
larger absorption coefficient of the Q band. The first oxidation
potential of aPcS2,measured by DPV in ortho-dichlorobenzene
(o-DCB), was +0.02 V versus Fc+/Fc, which is more negative
than that of aPcS1 by 50 mV (Supporting Information, Fig-
ure S5). The TDDFT calculation also suggested destabilization
of the HOMO energy level for aPcS2 by annulation with a ben-
zene unit. Therefore, we can conclude that the HOMO–LUMO
gap of aPcS2 decreased because of destabilization of the
HOMO level through annulation with a benzene ring.
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Table 2. Optical and electrochemical data for aPcS1 and aPcS2.
0[f]
[g]
Dye
Absorption[a]
[nm] (10ꢀ5 e [mꢀ1 cmꢀ1
Emission
Absorption[c]
[nm]
EHOMO
[V vs. NHE]
E0–
ELUMO
]
[nm] (f)[b]
[eV]
[V vs. NHE]
f
aPcS1
aPcS2
702 (1.0), 687 (1.1), 359 (0.83)
708 (2.2), 357(0.97)
713 (0.1)
715 (0.02)
692
708
+0.70[d] (+0.88)[e]
+0.65[d] (+0.80)[e]
1.76
1.74
ꢀ1.06 (ꢀ0.88)
ꢀ1.09 (ꢀ0.94)
[a] In THF. [b] Fluorescence quantum yields f were determined with ZnPc(tBu)4 as standard in THF (Supporting Information, Figure S6). [c] On TiO2 elec-
f
trode. [d] EHOMO was determined from the first oxidation potential of the dye in o-DCB solution. [e] EHOMO was determined from the first oxidation potential
of dye-stained TiO2 electrode. [f] Optical bandgap E0ꢀ0 was estimated from the crossing point of the normalized absorption and emission spectra. [g] ELUMO
was calculated as EHOMOꢀE0–0
.
Figure S7).[16b,21] The introduction of the electron-accepting cya-
nocarboxyl group on one end of the conjugated side chain is
considered to create this intramolecular charge-transfer (ICT)
transition from the electron-rich Pc core to the side chain. Be-
cause of the close connection to the Pc core, this broad ab-
sorption band will contribute to improving the light-harvesting
efficiency in the wavelength region between the Q and the B
bands in DSSCs.
TiO2 surface. In general, the adsorption of dyes on the TiO2 sur-
face results in absorption-spectral and electrochemical changes
because of deprotonation of the anchoring group[22] or elec-
tronic interaction between the dye LUMO and the conduction
band of TiO2.[23] Whereas the Q-band positions of aPcS1 and
aPcS2 were almost the same as those in solution, red shifts of
the Q-band onsets and positive shifts of the HOMO levels from
the electrochemical measurements were observed (Supporting
Information, Figure S8, Table 2). The adsorption densities of
aPcS1 and aPcS2 on the TiO2 surface were determined from
the absorbance of the ZnPcs released from the dye-stained
TiO2 electrodes. The adsorption densities of aPcS1 and aPcS2
are lower than that previously reported for PcS18 (1.4ꢁ
10ꢀ4 molcmꢀ3).[4c] The occupied areas of aPcS1 and aPcS2 on
the TiO2 surface were estimated to be 3.0 nm2 by using their
Performance of aPcS1- and aPcS2-based DSSCs
Mesoporous TiO2 electrodes were immersed in THF solutions
of aPcS1 and aPcS2 (5ꢁ10ꢀ5 m) at room temperature to give
dye-stained TiO2 electrodes by formation of an ester linkage
between the anchoring carboxyl groups of the dyes and the
Figure 6. DFT-calculated energy level diagrams and molecular orbital surfaces of aPcS1 (left) and aPcS2 (right) (The HOMO is #546 in aPcS1 and #571 in
aPcS2).
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Table 3. Calculated transition energies, oscillator strengths f, and configurations.
Dye Energy [eV (nm)] Configurations (main contributions)
f
aPcS1 1.87 (661.50)
1.96 (632.22)
2.65 (467.27)
3.04 (408.40)
3.14 (395.40)
3.25 (382.10)
3.27 (379.10)
3.29 (376.47)
3.34 (371.17)
3.44 (360.45)
aPcS2 1.84 (672.50)
1.89 (657.54)
2.59 (477.96)
3.07 (403.38)
3.12 (397.76)
3.23 (383.90)
3.30 (376.28)
3.36 (368.73)
3.37 (367.42)
3.46 (358.47)
0.69 546 (H)!547 (L) (90%)
0.62 546!547 (3%), 546!548 (89%)
0.69 545!549 (9%), 546!549 (62%), 546!550 (8%)
0.19 545!547 (73%)
0.08 545!548 (37%), 545!549 (20%)
0.52 542!548 (21%), 545!548 (26%), 545!549 (17%)
0.19 541!547 (73%)
0.04 542!548 (50%), 545!548 (19%)
0.08 543!548 (66%)
0.11 546!550 (65%)
0.66 571 (H)!572 (L) (82%), 571!573 (4%), 571!574 (8%)
0.67 571!573 (84%), 571!574 (9%)
0.18 571!572 (9%), 571!574 (66%), 571!575 (12%)
0.82 565!574 (2%), 569!572 (11%), 569!574 (15%), 570!572 (19%), 570!574 (9%), 571!574 (11%), 571!575 (13%)
0.27 569!574 (7%), 571!575 (65%)
0.01 557!572 (62%)
0.12 568!572 (60%)
0.05 566!572 (11%), 566!573 (47%), 566!574 (14%)
0.06 566!573 (14%), 570!573 (40%)
0.04 570!572 (41%), 570!574 (15%)
optimized molecular models (Supporting Information, Fig-
ure S9). These estimated areas are about 1.6 times of that of
PcS18. The enlargement of the molecular size by the attach-
ment of the conjugated side chain to the ZnPc core reduced
the packing density of the dyes in the self-assembled monolay-
er on the TiO2 surface.
lumination (100 mWcm2) for cells based on the two ZnPcs. The
aPcS1 cell showed a short-current density Jsc of 10.1 mAcmꢀ2,
an open-circuit voltage Voc of 0.61 V, and a fill factor (FF) of
0.73, giving an overall PCE of 4.5% (Supporting Information,
Figure S10 and Table 4). In contrast, the aPcS2-based cell
showed a lower Jsc, leading to a lower PCE value of 3.2%. Che-
nodeoxycholic acid (CDCA) was used as a co-adsorbent in the
DSSCs to prevent dye aggregation on the TiO2 surface.[24] The
half-width of the Q band of aPcS1 decreased from 63 to
56 nm following the addition of CDCA (Supporting Informa-
tion, Figure S8), which suggests that the aggregates on the
TiO2 surface were dissociated by co-adsorption with CDCA. The
aPcS1-based cell with co-adsorbed CDCA provided a higher
PCE of 5.5% compared with the cell without CDCA (Support-
ing Information, Figure S11). The onset voltage in dark cur-
rent–voltage measurements was improved by co-adsorption
The photovoltaic performances of the aPcS1 and aPcS2
cells were evaluated with a liquid electrolyte containing 0.6m
1,2-dimethyl-3-propylimidazolium iodide, 0.1m LiI, 0.05m I2,
and 0.5m tert-butylpyridine in acetonitrile. Figure 7a shows the
J–V curves measured under AM 1.5G simulated full-sunlight il-
with CDCA, and this implies an enhanced blocking effect of
ꢀ [24b]
I3
.
Since the enlarged dyes make space within the dye
layer on the TiO2 surface, electrons in the TiO2 are lost by con-
ꢀ
tact with I3 in the electrolyte passing through the loose pack-
ing layer of dye molecules. The dissociation of aggregates and
ꢀ
the enhancement of the blocking of I3 by co-adsorption with
CDCA could improve the performance of aPcS1 and aPcS2
cells. The incident photon-to-current (IPCE) spectrum of the
aPcS1-based cell followed the absorption feature of aPcS1 ad-
sorbed on the TiO2 electrode, and showed a panchromatic re-
sponse from the visible to the NIR regions. The aPcS1 cell
showed a photoresponse in the wavelength range of 400–
600 nm corresponding to the broad ICT band. The substitution
with oligothiophene units bearing an adsorption site at the
a positions of the Pc skeleton results in absorption in the typi-
cal ZnPc window at 500 nm and expansion of the light-har-
vesting range by 50 nm compared to that of PcS18. However,
the maximum IPCE value at the Q band of aPcS1 was 47%,
which is lower than that of PcS18 (ca. 80%). This implies
*
Figure 7. a) J–V curves for DSSCs based on aPcS1+CDCA ( ) and
~
aPcS2+CDCA ( ) under AM 1.5G simulated solar light. b) IPCE spectra of
~
*
DSSCs based on aPcS1+CDCA ( ) and aPcS2+CDCA ( ).
Chem. Eur. J. 2016, 22, 1 – 10 www.chemeurj.org
7
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to internal TMS. UV/Vis spectra and fluorescence spectra were mea-
sured with a JASCO V-650 and a JASCO FP-750. MALDI-TOF mass
spectra were obtained with a Bruker autoflex spectrometer with di-
thranol as matrix. High-resolution mass spectra with electrospray
ionization were obtained with a Bruker Daltonics micrOTOF II. DPV
data were recorded with an ALS 720C potentiostat, and electro-
chemical experiments were performed under purified nitrogen gas.
Nanoporous TiO2 working electrodes (thickness: 6 mm) were pre-
pared by applying pastes of TiO2 nanoparticles having 15–20 nm
diameter onto transparent conducting glass substrates (SnO2:F on
1.8 mm-thick glass substrate, Asahi Glass). The TiO2 electrodes
were immersed in 0.05 mm THF solutions of aPcS1 and aPcS2 for
3 h, and the dye-stained electrodes were used as working electro-
des. The reference electrode was Ag/AgCl, corrected for junction
potentials by referencing to the ferrocenium/ferrocene (Fc+/Fc)
couple. DFT and TDDFT calculations were performed with the Cou-
lomb-attenuating B3LYP (CAM-B3LYP) functional and 6-31G* basis
set as implemented in the Gaussian 09 software suite.[25] The fabri-
cation of DSSCs and the evaluation of solar-cell performance was
conducted according to the methods reported by us.[3d]
a lower electron-injection efficiency from the excited ZnPc to
the conduction band of the TiO2 electrode compared to
PcS18. This is probably caused by poor electronic coupling be-
tween the ZnPc core and the adsorption site at the LUMO and
LUMO+1 levels, as shown in Figure 6. Further improvement of
the IPCE values can be achieved by structural optimization of
the p-conjugated linker between the ZnPc core and the ad-
sorption site.
Table 4. Performance of ZnPc-sensitized DSSCs.
Dye[a]
[CDCA] Voc
Jsc
FF
PCE Absorption density
[%]
[mm]
[V]
[mAcmꢀ2
]
[105 molcmꢀ3
]
0
0.5
0
0.61 10.1
0.62 11.9
0.60 7.1
0.63 7.8
0.73 4.5
0.75 5.5
0.74 3.2
0.77 3.8
6.8
4.1
5.2
2.8
aPcS1
aPcS2
0.5
[a] [Dye]=0.05 mm in THF.
All chemicals were purchased from commercial suppliers and used
without further purification. Column chromatography was per-
formed with activated alumina (Wako, 200 mesh). Recycling prepa-
rative gel permeation chromatography was carried out with a JAI
recycling preparative HPLC with CHCl3 as eluent. Analytical TLC
was performed with commercial Merck plates coated with alumi-
Conclusions
We have demonstrated expansion of the light-harvesting
wavelength range of ZnPc-based DSSCs by hybridization with
p-conjugated side chains. The effect of substitution at the
a position of low-symmetry ZnPcs on the optical and electro-
chemical properties was investigated. We found that substitu-
tion with thiophene units at the a positions resulted in red
shifting of the Q band into the NIR region as well as tuning of
the HOMO and LUMO energy levels. Two ZnPc-based photo-
sensitizers aPcS1 and aPcS2, in which the p-conjugated side
chain bearing an adsorption site was attached at the a posi-
tions of the Pc ring, were synthesized by stepwise coupling re-
actions. The presence of the nonperipheral thiophene groups
resulted in insertion of thiophene-based MOs between the
HOMO and the typical four-node a2u HOMOꢀ1 of the Gouter-
man model. Similarly, there were new thiophene-related MOs
just above the LUMO and LUMO+1. The absorption spectra of
aPcS1 and aPcS2 showed a broad band from about 380 to
480 nm, which was assigned to an ICT transition from the
ZnPc core to the side chain on the basis of the lack of signifi-
cant MCD signal intensity and the predicted spectral bands
from TDDFT calculations. Whereas the previously reported
ZnPc-based sensitizers for DSSCs did not harvest in the green
region, the aPcS1 showed a panchromatic response in the
range 400–800 nm with a PCE of 5.5% when used as a light-
harvesting dye on a TiO2 electrode for DSSCs under one-sun
conditions. The hybridization of chromophores is one way to
design panchromatic ZnPc-based sensitizers with high energy-
conversion efficiencies.
num oxide 60 F254
.
Synthesis of aPcS1
A mixture of 7 (34 mg, 0.54 mmol), 8 (78 mg, 0.16 mmol), and
Zn(CH3COO)2 (9.8 mg, 0.54 mmol) in 6 mL of DMAE and 3 mL of o-
DCB was heated at 1608C with stirring overnight. After the reac-
tion mixture was cooled, it was diluted with MeOH. A precipitate
formed, which was collected by filtration and washed with MeOH
several times to remove excess Zn ion. The residue was purified by
column chromatography on activated alumina by eluting with
CH2Cl2, followed by recycling preparative HPLC to give 9 (27 mg,
1
24%). H NMR (400.13 MHz, CDCl3): d=8.93 (d, J=4.0 Hz, 1H, thio-
phene H), 8.78 (d, J=4.0 Hz, 1H, thiophene H), 8.34 (s, 1H, PcH),
8.32 (s, 1H, PcH), 8.21 (s, 1H, PcH), 8.12 (s, 1H, PcH), 8.15 (s, 2H,
PcH), 8.14 (s, 2H, PcH), 7.64–7.54 (m, 10H, ArH), 7.49–7.42 (m, 12H,
ArH), 7.19 (d, J=3.6 Hz, 1H, thiophene H), 7.15 (d, J=3.6 Hz, 1H,
thiophene H), 6.41 (d, J=4.0 Hz, 1H, thiophene H), 6.34 (d, J=
4.0 Hz, 1H, thiophene H), 6.28 (s, 1H, CH), 3.49–3.34 (m, 12H, CH),
1.47 (s, 9H, C(CH3)3), 1.44 (s, 6H, CH3), 1.36 (s, 6H, CH3), 1.22 ppm
(brs, 72H, CH3); MALDI-TOF MS (dithranol): m/z 2140.35 [M]
(100%); calcd for C133H142N8O8S3Zn: m/z 2139.94.
Complex 9 (27 mg, 0.13 mmol) was dissolved in a 1.2 molLꢀ1 aque-
ous solution of HCl (3 mL) and THF (6 mL). The mixture was heated
at 508C for 3 h and then poured into water. Ethyl acetate was
added, and the organic layer was washed with water until neural
pH, dried over anhydrous Na2SO4, and the solvent was evaporated.
The residue was purified by column chromatography on activated
alumina with CH2Cl2 to give 10 (20 mg, yield 78%). 1H NMR
(400.13 MHz, CDCl3): d=9.71 (s, 1H, CHO), 8.84 (d, J=4.0 Hz, 2H,
thiophene H), 8.34 (s, 1H, PcH), 8.27 (s, 1H, PcH), 8.21 (s, 2H, PcH),
8.15 (s, 4H, PcH), 7.74 (d, J=4.0 Hz, 1H, thiophene H), 7.64–7.54
(m, 10H, ArH), 7.49–7.44 (m, 8H, ArH), 7.38 (s, 4H, ArH), 7.33 (d, J=
4.0 Hz, 1H, thiophene H), 6.54 (d, J=4.0 Hz, 1H, thiophene H), 6.42
(d, J=4.0 Hz, 1H, thiophene H), 3.48–3.34 (m, 12H, CH), 1.47 (s,
9H, C(CH3)3), 1.26 ppm (s, 72H, CH3); MALDI-TOF MS (dithranol):
Experimental Section
General
NMR spectra were recorded for H and 13C in CDCl3 solution with
a Bruker AVANCE 400 FT NMR. Chemical shifts are reported relative
1
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(ATR): n˜ =1666.50 cmꢀ1 (CHO).
Cyanoacetic acid (8.3 mg, 9.8 mmol) and a catalytic amount of pi-
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90%). H NMR (400.13 MHz, CDCl3): d=8.82 (brs, 1H, CH=), 8.35–
8.30 (m, 4H, thiophene H), 8.23–8.05 (m, 8H, PcH), 7.60–7.54 (m,
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CH3); MALDI-TOF MS (dithranol): m/z 2105.2 [M+H] (100%); calcd
for C130H131N9O8S3Zn: m/z 2105.9; UV/Vis (THF): lmax (loge)=702
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M.J.S. (37-2015).
Keywords: density functional calculations · dyes/pigments ·
phthalocyanines · sensitizers · solar cells
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Published online on && &&, 0000
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FULL PAPER
Omega sensitizers: Two amphiphilic W-
shaped zinc phthalocyanines, denoted
aPcS1 and aPcS2, bearing a p-conju-
gated side chain with an adsorption site
(see figure), were synthesized as sensi-
tizers for dye-sensitized solar cells
(DSSCs). Compound aPcS1 was used as
a light-harvesting dye on a TiO2 elec-
trode for DSSCs, which showed a pan-
chromatic response in the range 400–
800 nm with a power conversion effi-
ciency of 5.5% under one-sun condi-
tions.
&
Phthalocyanines
S. Yamamoto, A. Zhang, M. J. Stillman,
N. Kobayashi, M. Kimura*
&& – &&
Low-Symmetry W-Shaped Zinc
Phthalocyanine Sensitizers having
Panchromatic Light-Harvesting
Property for Dye-Sensitized Solar Cells
Panchromatic Phthalocyanine Sensitizer
A p-conjugation side chain with an adsorption site was
introduced at an a position of a zinc phthalocyanine (ZnPc)
macrocycle. The compound (aPcS1) shows a red shifted Q
band and a broad absorption band from 350 to 550 nm,
which is assigned to the intramolecular charge–transfer
transition from the ZnPc core to the side chains. TD-DFT
calculations provide a clear interpretation of the effect of
the thiophene conjugation on the typical p molecular
orbitals of the Pc core. The aPcS1 cell showed
a panchromatic response in the range 400–800 nm with
a power conversion efficiency of 5.5% when used as a light-
harvesting dye on a TiO2 electrode for the dye-sensitized
solar cell under one-sun conditions. For more information,
see the Full Paper by M. Kimura et al. on page&& ff.
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!