Photostability of a dyad of magnesium porphyrin and fullerene and its application to photocurrent conversion

Article abstract of DOI:10.1039/c2cc36988e

A new type of acceptor-donor₂ dyad system containing two magnesium porphyrin molecules and a fullerene molecule is much more stable than the magnesium porphyrin itself. When it is fabricated into a binary system using imidazole carboxylic acid as a linker to indium tin oxide, the molecule effects efficient photocurrent generation that surpasses the device using the corresponding zinc porphyrin. This journal is

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Photostability of a dyad of magnesium porphyrin and fullerene and its
application to photocurrent conversion
Takahiko Ichiki, Yutaka Matsuo,* and Eiichi Nakamura*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A new type of acceptor–donor2 dyad system containing two
magnesium porphyrin molecules and a fullerene molecule is
much more stable than magnesium porphyrin itself. When it
is fabricated into a binary system using imidazole carboxylic
10 acid as a linker to indium tin oxide, the molecule effects
efficient photocurrent generation that surpasses the device
using the corresponding zinc porphyrin.
Magnesium porphyrinoid (chlorophyll)1 plays the central role
2
in photosynthesis. Likewise, magnesium porphyrin has
15 attracted interest in a manꢀmade photo devices3 because it
offers several advantages over the widely studied zinc
porphyrins such as higher fluorescence quantum yields and
longer excitedꢀstate lifetime (5–10 ns for magnesium
porphyrins versus 2–2.5 ns for zinc porphyrins).4 In general,
20 however, magnesium porphyrins have a crucial disadvantage,
that is their instability. Because magnesium porphyrins have
highꢀlying HOMO levels due to their ionic bonding nature,
magnesium porphyrins tend to be easily oxidized by singlet
oxygen that generates through energy transfer from
25 photoexcited magnesium porphyrin to triplet oxygen. 5 We
conjectured the possibility of rapidly quenching the
photoexcited magnesium porphyrin via electron transfer to
[60]fullerene,6, 7 thereby both increasing the stability of the
magnesium porphyrin and achieving efficient photon–to–
30 electron conversion. We report here the synthesis and
properties of a acceptor–donor2 (A–D2) dyad system of
fullerene and two molecules of magnesium porphyrin (C60-
MgPor2)8, 9 and a binary photoelectric device composed of
C60-MgPor2 and imidazole carboxylic acid (ICA), which in
35 turn is layered on an indium tin oxide (ITO) surface (Scheme
1). The C60-MgPor2 was found to be quite stable, particularly
when imidazole or pyridine is coordinated to the magnesium
atom. Thus, the C60-MgPor2:ICA:ITO device shows much
higher performance than the corresponding device using zinc
40 porphyrin (C60-ZnPor2).
Scheme 1 Schematic drawing of a photo–current conversion system of a
fullerene–magnesium porphyrin A–D2 dyad on a selfꢀassembled
monolayer of imidazole carboxylic acid.
45
The new fullerene–fullerene A–D2 dyad (C60-Por2) was
synthesized first from fullerene in four linear steps (Scheme
2). After the Diels–Alder reaction of an in situꢀgenerated oꢀ
quinodimethane and deprotection, the fullerene diol was
esterified with two porphyrin carboxylic acids to obtain C60-
50 Por2. Introduction of magnesium(II) atoms into the two
porphyrin units was achieved by the known procedure using
magnesium iodide and triethylamine. 10 Interestingly, unlike
magnesium tetraphenylporphyrin (MgTPP), which is unstable,
C60-(MgPor)2 can be purified by silica gel chromatography
55 under air and ambient light without demetalation or
decomposition, indicating its stability under aerobic and
mildly acidic conditions.
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crossing processes in the presence of the fullerene unit.
30 Fig. 1 Fluorescence spectrum in ODCB at room temperature (left) and
phosphorescence spectrum in EtOH/toluene at 77 K (right) for C60-
(MgPor)2 (1.0 ꢀM, red line) and MgTPP as a reference (2.0 ꢀM, black
line).
To investigate the stabilizing effect of ligand coordination,
35 we titrated C60-(MgPor)2 with pyridine (Fig. S3). The
changes in the UV/vis spectrum were analyzed as a function
of ligand concentration according to the method of Jaffe and
Orchin.11 The slope of the fitted line indicated the formation
of a 1:1 coordination complex with a binding constant of 6.0 ×
40 104 M–1, which agrees with the data known for MgTPP
coordinated to pyridine. 12 In this concentration range, C60-
(MgPor)2 is fiveꢀcoordinated. The CV data for the pyridine
coordination to C60-(MgPor)2 showed a positive shift of the
oxidation wave of the porphyrin moiety, indicating that the
45 pyridine coordination lowers HOMO and hence stabilizes the
porphyrin against oxidation (Fig. S4).
Next, photooxidation of MgTPP (Fig. 2, left) and C60-
(MgPor)2 with and without pyridine upon irradiation of
incandescent light (60 W) under air was studied
50 spectroscopically (Fig. 2, left, and S5). Whereas MgTPP
immediately decomposed, C60-(MgPor)2 was much more
stable under the same conditions (Fig. S5), and its pyridine
complex was much more stable against photooxidation (Fig.
2, right).
Scheme 2 Synthesis of a fullerene–magnesium porphyrin A–D2 dyad and
a reference molecule.
The UV/vis absorption spectrum of C60-(MgPor)2 in oꢀ
dichlorobenzene (ODCB) showed a sharp Soret band at 429
nm and two Qꢀbands at 566 and 607 nm (Fig. S1). The
spectrum of C60-(MgPor)2 was similar to that of MgTPP,
suggesting that there is little interaction between the
porphyrin and fullerene in the ground state. In the cyclic
5
10 voltammetry (CV) measurements (Fig. S2), C60-(MgPor)2
showed twoꢀelectron oxidation and oneꢀelectron reduction for
each porphyrin moiety as well as two reduction waves for the
fullerene part. No electrochemical interaction between the two
magnesium porphyrin units was observed. The first oxidation
15 peak assigned to the magnesium porphyrin moieties of C60-
(MgPor)2 was slightly shifted to the positive side in
comparison with that of MgTPP. This result indicates that the
fullerene has an electronꢀwithdrawing effect on the
magnesium porphyrin moiety, thereby lowering the HOMO
20 level to stabilize against oxidation. Emission properties were
55
Fig. 2 Comparison of spectral changes during photooxidation of the stable
C60-(MgPor)2–pyridine complex (left) and MgTPP (right). ODCB
solutions (38 ꢀM) were irradiated under aerobic conditions.
With information on the photostability and the ability of
60 nitrogen atoms to coordinate to the two magnesium atoms in
C60-(MgPor)2, we next examined the C60-(MgPor)2/ICA/ITO
device. First, an ITO substrate was immersed in a DMF
solution of ICA (0.1 M) for 3 days; after washing with DMF
and ODCB, it was then immersed in an ODCB solution of
investigated
by
fluorescence
and
phosphorescence
spectroscopies (Fig. 1). These data suggest that the emission
from the porphyrin moieties is effectively suppressed by the
presence of the fullerene moiety, as judged by the comparison
25 with MgTPP, which is highly emissive under the same
conditions. We surmise that the formation of a chargeꢀ
separated state is favored over the emission and intersystem
65 C60-(MgPor)2 (0.01 M) for
3 h. UV/vis measurements
provided evidence of the presence of C60-(MgPor)2 on the
functionalized ITO substrate (Fig. 3a), and the surface
2
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coverage of C60-(MgPor)2 was 0.017 nmol/cm2, as estimated
from the peak height in the UV/vis spectrum (Fig. 3a). This
value is consistent with the formation of a monolayer of C60-
(MgPor)2.13
(MgPor)2 and ITO resulted in efficient photocurrent
generation. The results illustrate how one can utilize the
magnesium porphyrin for useful applications through
stabilization against oxidation by the use of an intramolecular
55 excitedꢀstate quencher and a nitrogen ligand that lowers the
HOMO level and creates a supramolecular array, which is
reminiscent of the photosynthesis system in nature.
We thank Dr. Xiaoyong Zhang (The University of Tokyo)
for contribution on early stage of this work. This study was
60 supported by the Funding Program for Next Generation
WorldꢀLeading Researchers (Y.M.) and the Strategic
Promotion of Innovative Research and Development from the
Japan Science and Technology Agency (E.N.).
5
Fig. 3 (a) UV/vis spectrum of the ITO substrate functionalized by ICA
(imidazole carboxylic acid linker) (blue line) and C60-(MgPor)2 (red line).
(b) Photocurrent generation of C60-(MgPor)2/ICA/ITO system in the
presence of MV2+ in aqueous solution containing Na2SO4 at –100 mV
Notes and references
65 Department of Chemistry, School of Science, The University of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: matsuo@chem.s.u-tokyo.ac.jp, nakamura@chem.s.u-tokyo.ac.jp;
Fax: +81 3-5800-6889
† Electronic Supplementary Information (ESI) available: Procedures for
70 Synthesis and selfꢀassembled monolayer formation as well as data for
photophysical, electrochemical, photoelectrochemical properties. See
DOI: 10.1039/b000000x/
10 applied bias voltage vs Ag/AgCl reference electrode.
Photocurrent generation experiments were carried out in a
0.1 M Na2SO4 aqueous solution containing 50 mM methyl
viologen (MV2+
)
as an electron acceptor. The C60-
(MgPor)2/ICA/ITO device was used as the working electrode
15 in a cell with a platinum counterꢀelectrode and an Ag/AgCl
(saturated KCl) reference electrode. In the presence of MV2+
,
1
2
(a) G. H. Krause, E. Weis, Annu. Rev. Plant Physiol. Plant Mol.
Biol., 1991, 42, 313; (b) W. Kühlbrandt, D. N. Wang, Y. Fujiyoshi,
Nature, 1994, 367, 614.
a stable cathodic photocurrent was observed upon light
irradiation (λex = 430 ± 10 nm, with a power of 207 ꢀW) at an
applied potential of –100 mV versus Ag/AgCl (Fig. 3b). A
20 photocurrent action spectrum at a bias of –100 mV versus
Ag/AgCl was measured (Fig. S6) and found to match well
with the corresponding absorption spectrum of C60-(MgPor)2
in solution, indicating that C60-(MgPor)2 fixed on the
substrate was responsible for the photocurrent.
(a) S. Izawa, R. L. Heath, G. Hind, Biochim. Biophys. Acta, 1969,
180, 388; (b) M. S. Tunuli, J. H. Fendler, J. Am. Chem. Soc., 1981,
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Photobiol., 1979, 31, 23.
3
4
5
6
25
The quantum yield of the photocurrent conversion was
7.8%, a respectable value for selfꢀassembled monolayer/ITO
systems.14 We compared this result with those obtained for the
(a) Q. Xie, F. Arias, L. Echegoyen, J. Am. Chem. Soc., 1993, 115,
9818; (b) L. Echegoyen, L. E. Echegoyen, Acc. Chem. Res., 1998, 31,
593.
corresponding zinc compound C60-(ZnPor)2 and
a noꢀ
7
8
(a) H. Imahori, Y. Sakata, Adv. Mater., 1997, 9, 537; (b) H. Imahori,
Y, Sakata, Eur. J. Org. Chem., 1999, 2445.
fullerene reference compound C6H4-(ZnPor)2 (Table 1; the
30 values were estimated from the data in Figs S7–S10). The
C60-(MgPor)2-based device was over 10 times more efficient
than the Znꢀbased and noꢀfullerene devices. We consider that
the longer lifetime of the singlet excited state and the higher
energy levels of HOMO and LUMO of the magnesium
35 porphyrin is responsible for the efficient charge separation
and hence better performance.
(a) J. Rochford, E. Galoppini, Langmuir 2008, 24, 5366; (b) W. M.
Campbell, A. K. Burrell, D. L. Officer, L. W. Jolley, Coord. Chem.
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Phys. Chem. B, 2004, 108, 12962; (d) H. Yamada, H. Imahori, Y.
Nishimura, I. Yamazaki, S. Fukuzumi, Chem. Commun., 2000, 1921.
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Yamada, Y. Nishimura, I. Yamazaki, Y. Sakata, J. Phy. Chem. B,
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Park, J. Park, W. Choi, YꢀK. Han, T. Joo, J. T. Park, Chem. Eur. J.,
2010, 16, 5586.
9
Table 1 Quantum yield and surface coverage of A–D2 dyad and reference
systems.
10 J. S. Lindsey, J. N. Woodford, Inorg. Chem., 1995, 34, 1063.
11 H. Jaffe, M. Orchin, Theory and Applications of Ultraviolet
Spectroscopy; Wiley: New York, 1962; pp 578–583.
12 K. M. Kadish, L. R. Shiue, Inorg. Chem., 1982, 21, 1112.
13 (a) D. A. Offord, S. B. Sachs, M. S. Ennis, T. A. Eberspacher, J. G.
Griffin, C. E. D. Chidey, J. P. Collman, J. Am. Chem. Soc., 1998,
120, 4478; (b) G. Ashkenasy, A. Ivanisevic, R. Cohen, C. E. Felder,
D. Cahen, A. B. Ellis, A. Shanzer, J. Am. Chem. Soc., 2000, 122,
1116.
14 (a) Y. Matsuo, K. Kanaizuka, K. Matsuo, Y.ꢀW. Zhong, T. Nakae, E.
Nakamura, J. Am. Chem. Soc., 2008, 130, 5016; (b) A. Sakamoto, Y.
Matsuo, K. Matsuo, E. Nakamura, Chem. Asian J., 2009, 4, 1208; (c)
Y. Matsuo, T. Ichiki, S. G. Radhakrishnan, D. M. Guldi, E.
Nakamura, J. Am. Chem. Soc., 2010, 132, 6342; (d) Y. Matsuo, T.
Ichiki, E. Nakamura, J. Am. Chem. Soc., 2011, 133, 9932.
Compound
40 C60-(MgPor)2 7.8 a
C60-(ZnPor)2
0.64 b
C6H4-(ZnPor)2 0.043 b
Quantum Yield (%)
Surface Coverage c (nmol/cm2)
0.017
0.016
0.032
a Measured with 430 nm irradiation. b Measured with 425 nm irradiation. c
Estimated from the peak area of UV/vis spectrum.
45
In conclusion, we found that covalent connection of a
fullerene molecule to magnesium porphyrin stabilized the
intrinsically photoꢀunstable magnesium porphyrin, probably
by the rapid quenching of the photoexcited state of the
magnesium porphyrin. Construction of a binary photocurrent
50 conversion system using ICA as a linker between C60-
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