Surface bottom-up fabrication of porphyrin-terminated metal complex molecular wires with photo-electron conversion properties on ITO

Article abstract of DOI:10.1246/cl.2008.404

We fabricated photoelectron conversion system with porphyrin-terminated "molecular wires" on an ITO surface synthesized using stepwise metal-terpyridine complexation reactions. The efficiency and the electrode potential singniflcantly depended on the meta

Full text of DOI:10.1246/cl.2008.404

404
Chemistry Letters Vol.37, No.4 (2008)
Surface Bottom-up Fabrication of Porphyrin-terminated Metal Complex Molecular Wires
with Photo-electron Conversion Properties on ITO
Mariko Miyachi, Makiko Ohta, Misaki Nakai, Yoshihiro Kubota, Yoshinori Yamanoi,
Tetsu Yonezawa, and Hiroshi Nishiharaꢀ
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received January 11, 2008; CL-080037; E-mail: nisihara@chem.s.u-tokyo.ac.jp)
We fabricated photoelectron conversion system with por-
N
N
N
phyrin-terminated ‘‘molecular wires’’ on an ITO surface synthe-
sized using stepwise metal–terpyridine complexation reactions.
The efficiency and the electrode potential singnificantly depend-
ed on the metal center of the bis(terpyridine) complex unit in the
molecular wire.
N
N
N
N
N
N
N
NH
O
Zn
O
O
H
R
2
1
N
N
N
N
Zn
N
N
N
N
R
Zn
R
Molecular assembling has attracted attention in the forma-
tion of functional devices and materials. Among the technolo-
gies of molecular assembling, ‘‘self-assembled monolayer
(SAM)’’ provides effective alternatives to obtain target functions
in a simple way using solutions. SAM is especially used for pho-
tochemistry as a means to arrange functional molecules, since
the arrangement of photoreceptors, donors, and acceptors signif-
icantly controls the efficiency of photoelectric conversion. For
example, Imahori et al. have reported achieving high quantum
efficiency and long life with a photoelectrode with donor–photo-
sensitizer–acceptor triad molecules.1
HN
O
R = O(CH2)10COOH
4
R
N
3
N
M
N
N
N
N
N
M
N
N
N
N
N
(ii)
(i)
(iii)
M = Fe2+, Co2+, Zn2+
O
O
O
O
O
O
ITO
ITO
ITO
ITO
Chart 1. Chemical structure of the ligands used in this study,
and stepwise coordination methods for the preparation of modi-
fied ITO electrodes: (i) immobilization at carboxylic acid 1, (ii)
complexation with metal ion, and (iii) complexation with 2.
Recently, preparation of layer-by-layer assembled multilay-
ers by stepwise chemical bond formation on the surface is beeing
developed extensively.2 Especially, coordination reactions are
useful to synthesize oligomeric molecular wires with the desired
number of complex units and the desired hetero-layered struc-
tures. When we use a linear bis(terpyridine) complex as a ‘‘mo-
lecular wire,’’ it shows a one-dimensional ordered structure and
exhibits redox conduction through the molecular wire using its
redox-active d orbital electrons.3 This type of electron transfer
is also expected to provide efficient photo-electron transport us-
ing the d orbitals in the wire. In the present study, we designed
ITO electrodes modified with M(tpy)2 (tpy = 2,20:60,200-terpyri-
dine, M = Co, Fe, and Zn) complex wires with a terminal por-
phyrin moiety as a photosensitizer for studying photo-electron-
transfer behavior from porphyrin to ITO through the ‘‘molecular
wire’’ by changing the metal element in the M(tpy)2 moieties.
We fabricated the modified ITO electrodes4 by a combina-
tion of SAM formation with a terpyridine derivative and
stepwise metal–terpyridine coordination reactions in a manner
similar to that described in our previous reports (Figure 1).3
4-[(2,20:60,200-Terpyridin)-400-yl]benzoic acid (1) was immobi-
lized on cleaned ITO5 by immersing them in a 0.1 mmol dmꢁ3
solution of 1 in chloroform for 12 h in order to anchor the
carboxyl group to ITO.6,7 The modified ITO was immersed in
the aqueous solution of 0.1 mol dmꢁ3 CoCl2, Fe(BF4)2, or
Zn(BF4)2 for 2–3 h to form metal–terpyridine coordination
reactions. Finally, the metal-coordinated ITO was immersed in
a 0.1 mmol dmꢁ3 acetonitrile solution of a terpyridine-function-
alized porphyrin, 2, to give the target molecular wires, 4, on
electrodes (Chart 1). In addition, a carboxylate-functionalized
porphyrin, 3, was immobilized on cleaned ITO to afford a
modified ITO, 5, by immersing them in a 0.1 mmol dmꢁ3 ethanol
solution of the porphyrin as a reference.8
Cyclic voltammetry (CV) of 4 and 5 was carried out in
Bu4NClO4–CH2Cl2 in order to estimate the surface coverage.
The adsorbed amount of terpyridine-functionalized porphyrin
was found to be 1:8 ꢂ 10ꢁ12 mol cmꢁ2 for the Co complex
(4-Co), 1:1 ꢂ 10ꢁ11 mol cmꢁ2 for the Fe complex (4-Fe), 1:2 ꢂ
10ꢁ11 mol cmꢁ2 for the Zn complex (4-Zn), and 2:8 ꢂ 10ꢁ11
mol cmꢁ2 for 5 by calculation from the peak area of the
porphyrin and M(tpy)2.
The absorption spectra of 4-Co and 2 in acetonitrile show
the Soret band of porphyrin, while that of the former was broader
than that of the latter. The ꢀmax value of the Soret band of 4-Co
was nearly identical to that of 2 in methanol, but 12-nm red shift
was observed. These spectral changes could be due to partial ag-
gregation of porphyrin moieties, according to a previous study
which shows similar broadening and a red shift for porphyrin
SAMs on ITO and gold.9 The adsorbed amount of 4-Co calculat-
ed from the absorption spectra was 1:9 ꢂ 10ꢁ12 mol cmꢁ2. This
value is consistent with that derived from CV.
Photoelectrochemical measurements using the modified
ITO as working electrodes were carried out in an argon-
saturated 0.1 mol dmꢁ3 Na2SO4 aqueous solution containing
50 mmol dmꢁ3 triethanolamine (TEA) as an electron sacrificer.10
A stable photocurrent appeared immediately upon irradiation of
monochromatic light with a 28-nm width between 400 and
600 nm for 4 and 5. The magnitude of the anodic photocurrent
showed a dependence on the applied potential and the excitation
wavelength (Figure 1). The dependency of the wavelength is
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.4 (2008)
405
nature of the molecular wire is critical to the photo-electron
transfer from the porphyrin to ITO. These results indicate a
new facile fabrication method of molecular assemblies effective
for construction of photo-electron-transfer systems. This system
will be upgraded by increasing the wire length, making the redox
potential step in the wire, and/or incorporating donors and
acceptors. Experiments along this line are in progress.
(a)
(b)
Zn(P)0/-
-1.9 V
*
π
*
π
-
25
20
15
10
5
-1.9 V
-1.7 V
4-Co
4-Fe
4-Zn
5
CoI/II
-1.1 V
*
π
-1.1 V
CoII/III
0.0 V
0
FeII/III
0.8 V
-5
a bc d
Zn(P)+/0
0.5 V
-10
-15
This work was partially supported by Grants-in-Aid for
Scientific Research (Nos. 16047204 (area 434; Chemistry of
Coordination Space) and 17205007). MM thanks the JSPS
fellowship for Japanese Junior Scientists.
+
Fe complex
porphyrin
Co complex
-0.3
0.2
0.7
Applied potential / V vs. Ag/AgCl
Figure 1. Potential dependence of the anodic photocurrent of
modified ITO electrodes excited at 410 nm (a) and the energy
diagram (b); a–d refers to EF (4-Co), EF (4-Zn), EF (4-Fe),
and EF (5), respectively.
References and Notes
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similar for all the modified electrodes. However, the potential at
which the photocurrent intersected at zero (EF) was different for
each electrode (Figure 1a). The potential became more positive
as follows: 4-Co < 4-Zn ꢃ 4-Fe < 5. These results indicate
that photo-electron transfer is related to the molecular orbital
levels of the molecular wires. The molecular wires with bis-
(terpyridine)metal complexes have two possible photo-electron
pathways. One is through the d orbital of the central metal ion,
and the other is through the ꢁꢀ orbital of the terpyridine moieties
(Figure 1b). In the case of 4-Co, the level of the CoI/II orbital
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0.57%, while those of 4-Zn and 5 were 0.20% and 0.15%, re-
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more effectively than the ꢁꢀ orbital in the molcular wire.
In conclusion, we fabricated ITO electrodes modified with
porphyrin-terminated M(tpy)2 complex wires by the stepwise
coordination method, and demonstrated that the electronic
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Published on the web (Advance View) March 1, 2008; doi:10.1246/cl.2008.404