Dye-sensitized solar cells using supramolecular porphyrin arrays inspired by π-stacking structures of photosynthetic light-harvesting complexes

Article abstract of DOI:10.1246/cl.130880

A slipped cofacial π-stacks of bis(4-carboxyphenyl)bis(1- methylimidazol-2-yl)porphyrinatomagnesium complex can be self-assembled onto TiO₂ surface as a light-harvesting core. The porphyrin array allows a more favorable light-to-electricity conversion compared to the monomer analogue in dye-sensitized solar cells.

Full text of DOI:10.1246/cl.130880

Received: September 24, 2013 | Accepted: October 25, 2013 | Web Released: November 1, 2013
CL-130880
Dye-sensitized Solar Cells Using Supramolecular Porphyrin Arrays Inspired
by π-Stacking Structures of Photosynthetic Light-harvesting Complexes
Junko Fujimoto,1 Kazuhiro Manseki,2 and Hidekazu Miyaji*1
1Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University,
1-1 Yanagido, Gifu 501-1193
2Graduate School of Engineering, Environmental and Renewable Energy System Division,
Gifu University, 1-1 Yanagido, Gifu 501-1193
(E-mail: miyaji@gifu-u.ac.jp)
O
OH
OH
O
OH
O
A slipped cofacial π-stacks of bis(4-carboxyphenyl)bis(1-
methylimidazol-2-yl)porphyrinatomagnesium complex can be
self-assembled onto TiO2 surface as a light-harvesting core. The
porphyrin array allows a more favorable light-to-electricity
conversion compared to the monomer analogue in dye-sensitized
solar cells.
O
HO
O
N
N
N
N
N
N
HO
O
N
N
N
N
N
N
N
N
N
N
N
N
Mg
Mg
Mg
OH
1
2
3
HO
O
HO
O
HO
O
Dye-sensitized solar cells (DSSCs) have received much
attention as promising next-generation photovoltaics.1 Dyed-
semiconductor nanoparticle films, wherein electrolytes are
embedded in the nanopores of photoelectrodes, play crucial
roles in the efficient charge separation of electrons and holes.
Porphyrins are an important class of light-absorbing materials
owing to their high molar extinction coefficients (¾ > 104) from
UV to visible (or near-IR) region and easy substitution of
functional groups to the meso-position.2 As for DSSCs incor-
porating porphyrins, multiporphyrin arrays synthesized via
intermolecular covalent bonds have been reported to show
improved electron injection as well as light-harvesting efficiency
compared to the monomeric sturucture.3 On the other hand,
effects of the porphyrin π-stacks on the photovoltaic perform-
ance have been poorly understood, although photosynthetic
systems benefitting from the light-harvesting porphyrin subunits
have been clearly investigated.
Figure 1. A conceptual drawing of self-organization of 3 on
TiO2 electrode, in which the transition dipole interactions are
represented.
From the viewpoint of biomimetics, we focused on porphy-
rin supramolecules4 mimicking the antenna complexes of
photosynthetic system.5 We synthesized a porphyrin that has
1-methylimidazole and 4-carboxyphenyl groups6 for realizing
self-organization onto the semiconductor surface. It turned out
that the slipped cofacial π-stacking structure of porphyrin can be
controlled via the cooperative coordination of 1-methylimida-
zole to the central metal ion of the porphyrin, both in solutions
and in the solid state, i.e., dyed TiO2 films. We anticipated that
the exciton coupling, i.e., transition dipole interactions and the
overlapped π orbitals of the porphyrin units allow the efficient
electron injection resulting in a marked improvement in the
photovoltaic performance.
More specifically, we synthesized a novel magnesium
porphyrin 3 that has two 1-methylimidazoles and two carboxy-
phenyl groups as anchors for TiO2 (Figure 1). We prepared a
methoxycarbonylphenyl-substituted dipyrromethane part as the
building blocks for the targeted porphyrins, in which two 1-
methylimidazoles are introduced into the meso-positions and
two carboxyphenyl groups into the other facing meso-positions.7
Reactions with dipyrromethane and 1-methyl-2-imidazolecar-
baldehyde in propionic acid produced free-base porphyrin,
which was purified by column chromatography. The substituted
methoxycarbonylphenyl groups were then successfully convert-
ed to the carboxyphenyl groups by hydrolysis. The obtained
free-base porphyrin and magnesium perchlorate were refluxed
in pyridine to afford the Mg complex 3. For comparisons,
5,10,15-tris(4-carboxyphenyl)-20-(1-methylimidazolyl)porphyri-
natomagnesium6 complex (2) and 5,10,15,20-tetrakis(4-carboxy-
phenyl)porphyrinatomagnesium complex (1) were also synthe-
sized (see Supporting Information).8
Compounds 1, 2, and 3 showed low solubilities in organic
solvents such as dichloromethane and acetonitrile because of
their strong hydrogen bonds between the intermolecular carbox-
ylic acid groups. It should be noted that they were soluble in a
phosphate buffer solution (pH 7.4) because of the formation of
carboxylate salts. Figure 2 shows the UV-vis absorption spectra
of 1, 2, and 3 in the buffer solution. Judging from the sharp
Soret-band (-max = 421 nm), 1 was determined to be a monomer
(Figure 2 and Table 1). For 2, the broad Soret-band was
observed at around 428 nm, indicating that the interaction of
two transition dipoles, i.e., both “face-to-face” and “head-to-tail”
dipole interactions, plays a part in the marked peak broadening.9
The Soret-band of 3 was found to split into around 380 and
449 nm peaks. Similarly, this band splitting is due to the
interaction of transition dipoles by the stacking of 3. The
Chem. Lett. 2014, 43, 207–209 | doi:10.1246/cl.130880
© 2014 The Chemical Society of Japan | 207

0.8
0.6
0.4
0.2
0
1
2
3
550
600
Wavelength / nm
Wavelength / nm
Figure 3. Absorption spectra of 1, 2, and 3 on TiO2.
Figure 2. UV-vis absorption spectra of 1, 2, and 3 in
phosphate buffer solutions (pH 7.4).
Table 2. Photovoltaic properties of 1, 2, and 3
¹2
Compound
Jsc/mA cm
Voc/V
F.F.
©/%
Table 1. Absorption spectral data of 1, 2, and 3 in solutions
(phosphate buffer pH 7.4) and on TiO2
1
2
3
1.25
2.14
2.26
0.231
0.431
0.391
0.65
0.65
0.68
0.18
0.60
0.60
Solution
On TiO2
Compound
Soret band/nm Q-band/nm ¾α/¾ Q-band/nm ¾α/¾
β
β
1
2
3
421
428
564, 604
567, 613
0.58 566, 608
0.71 564, 607
0.66
0.41
380, 427, 449 568, 594, 611 0.50 565, 593, 611 0.30
splitting was found to be 69 nm (4040 cm¹1) for 3 and ca. 20 nm
(1150 cm¹1) for 2. Thus, it is postulated that the dimer formation
of 2 (22) and the porphyrin array of 3 (3n) are stable in a
phosphate buffer solution.10 In order to get more insight into the
stacking core, we estimated the ratio of ¾α/¾ of Q-bands. The
β
values significantly decreased from 0.71 (2) to 0.50 (3). In
addition, a new peak observed at 594 nm for 3 could be assigned
to the Mg site to be a six-coordination structure.11 From these
results, it is reasonable to conclude that porphyrin array 3n (J-
aggregate) is stabilized via the slipped cofacial π-stacking in a
phosphate buffer solution. Such a J-aggregate formation was
also supported by the red shift of fluorescence spectrum of 3,
which appeared at 621 nm (1 at 613 nm). This result also means
that the energy gap between HOMO-LUMO becomes smaller
for porphyrin array 3n.
Wavelength / nm
Figure 4. Photocurrent action spectra of DSSCs consisting of
light-harvesting porphyrins 1-3 on TiO2 electrodes.
In order to examine the self-organization on the TiO2
electrode, 1, 2, and 3 were adsorbed by immersing the TiO2
electrode in each methanol solution of the porphyrin.12 Figure 3
shows the absorption spectra of 1, 2, and 3 on the TiO2
electrode. The amount of dye adsorbed on TiO2 was controlled
by the adsorption time for 18-24 h, which can be estimated from
the absorption spectra of around 0.4 at 565 nm corresponding to
for porphyrin array 3n. It is likely that the π-stacking formation
of porphyrin array 3n (J-aggregate) contributed to the improved
photocurrent. The open circuit voltage (Voc) for both dimer 22
and porphyrin array 3n was significantly higher than that of 1.
This can be explained by the suppressed electron recombination
¹
reaction at the interface of TiO2 electrodes and I3 in electrolyte
the Q-bands. The ¾α/¾ value of Q-bands on TiO2 decreased
as being due to the densely packed structure on TiO2 for
π-stacking dimer and porphyrin arrays. As a consequence,
conversion efficiency (©) for 2 and 3 increased by a factor of 3.3
compared to that for 1 (from 0.18% to 0.60%). Higher (10-20%)
incident photon to current conversion efficiency (IPCE,
Figure 4) at the Soret-band of the dimer 22 and the porphyrin
array 3n, as well as the peak broadenings, was observed
compared to 1.13 It is likely that the broadening of IPCE at the
Soret-band is attributed to the splitting LUMO (S2) by transition
dipole interactions of “face-to-face” and “head-to-tail” modes
that are defined as allowed transitions. At the Q-band, the IPCE
β
from 0.66 (1) to 0.30 (3), which was similar to that observed in
solution. Furthermore, a new peak also appeared at 593 nm. We
found that the organized structure of porphyrin array 3n could be
controlled on the TiO2 surface. The red shift of Q-band from 608
to 611 nm could be explained by the splitting of LUMO (S1) due
to the intermolecular interactions of J-aggregates.
The photovoltaic performance of DSSCs fabricated with
porphyrins 1, 2, and 3 was evaluated (Table 2). The short-circuit
current density (Jsc) of dimer (22) was found to be 1.7 times
higher than that of 1, whereas 1.8 times higher Jsc was observed
208 | Chem. Lett. 2014, 43, 207–209 | doi:10.1246/cl.130880
© 2014 The Chemical Society of Japan

of the dimer 22 and porphyrin array 3n significantly increased
by a factor of 2.0-2.8, as compared to 1. In addition to the
broadening of the IPCE of 3n by transition dipole interactions,
the quantum yields of electron injection also seemed to improve.
It is postulated that the electron recombination from TiO2 to 3n
was effectively suppressed by hole trapping (i.e., hole migra-
tion14) due to the continuous π-stacking structure (via the
overlapped π orbitals) of the porphyrin array.
In summary, we successfully synthesized the Mg porphyrin
that has two 1-methylimidazoles and two carboxyphenyl groups.
The DSSC incorporating the supramoleclular porphyrin array
had positive effects on the increased photocurrent, which could
be explained by the strong exciton coupling, i.e., transition
dipole interactions and π-stacking of the oriented porphyrins,
leading to the favorable electron injection. We believe that this
result will provide a new strategy for the design and synthesis of
dyes for fabricating highly efficient DSSCs. Further applications
of DSSCs using exciton coupling and π-stacking system are in
progress.
Alessio, Springer, 2006, Vol. 121, p. 49, and references
therein. doi:10.1007/430_023.
5
A slipped cofacial π-stacking structure of a special pair, see:
a) J. Deisenhofer, O. Epp, K. Miki, R. Huber, H. Michel,
J. Mol. Biol. 1984, 180, 385. Slipped cofacial π-stacking
structures are also seen in light-harvesting complexes
(LH2 and LH1) of photosynthetic purple bacteria, see:
b) G. McDermott, S. M. Prince, A. A. Freer, A. M.
Hawthornthwaite-Lawless, M. Z. Papiz, R. J. Cogdell,
N. W. Isaacs, Nature 1994, 374, 517. c) A. W. Roszak,
T. D. Howard, J. Southall, A. T. Gardiner, C. J. Law, N. W.
Isaacs, R. J. Cogdell, Science 2003, 302, 1969.
H. Miyaji, J. Fujimoto, Tetrahedron Lett. 2010, 51, 2979.
a) T. Rohand, E. Dolusic, T. H. Ngo, W. Maes, W. Dehaen,
ARKIVOC 2007, 307. b) C.-H. Lee, J. S. Lindsey, Tetra-
hedron 1994, 50, 11427.
6
7
8
9
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
M. Kasha, H. R. Rawls, M. Ashraf El-Bayoumi, Pure Appl.
Chem. 1965, 11, 371.
We thank Prof. T. Yoshida (Yamagata University) and Prof.
M. Matsui (Gifu University) for useful discussions.
10 It seemed to exist as equilibrium mixtures of monomer,
dimer, trimer,+oligomer (3n) in a phosphate buffer solution
because of coordination of water to Mg. However, the
porphyrin array seemed to exist stable, considering from the
band splitting at 380 and 449 nm (¦E = 4040 cm¹1) and the
peak broadenings. The Co porphyrin which has two 1-
methylimidazoles and two alkyl chains was reported to form
90-500mer in an organic solvent, showing the band splitting
at 474 and 404 nm (¦E = 3650 cm¹1).4c Therefore, our
supramolecular porphyrin array (¦E = 4040 cm¹1) also
seemed to form a same level or higher levels of the
porphyrin array. It was considered that the formation of such
stable porphyrin array in water came from the effective
hydrophobic interaction and π-stacking, in addition to the
cooperative coordination from 1-methylimidazole to Mg.
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β
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Chem. Lett. 2014, 43, 207–209 | doi:10.1246/cl.130880
© 2014 The Chemical Society of Japan | 209