C O MMU N I C A T I O N S
conversion of the coating from the monomeric complex to the
aggregate. Koutecky-Levich plots (Figure 3D) revealed that the
aggregate reduced O mainly with four electrons (napp ) 3.7), while
2
a two-electron reduction was predominant for the monomeric
complex (napp ) 2.0). Such a difference in the reactivity stems from
16
the presence of two parallel reactions, one involving the binding
of O to two proximate cobalt centers in the aggregate and giving
rise to the four-electron reduction of O to H O and the other in-
volving a single cobalt center and yielding the two-electron reduc-
2
2
2
17-20
2 2 2
tion of O to H O . Covalently linked cofacial metalloporphyrins
have been the typical platform for exploring such a cooperative
effect of metal centers in promoting the four-electron reduction of
O
2
. The short cobalt-cobalt distance in the CoTAPP aggregate
precludes O binding, suggesting that O is reduced at fracture or
2
2
defect points with a larger cobalt-cobalt separation.
In conclusion, the alcoholic solution of CoTAPP provided simple
systems developing the rodlike morphology that served as a
template for the cofacial orientation of porphyrins at the electrode
2
surface, allowing for both ends of O molecules to interact with
the cobalt centers in the transition state. Tuning the cobalt-cobalt
distance within the micelle and the microscopic analysis of the
nanorod are the topics of our continuous research.
Figure 3. (A) Cyclic voltammogram for the reduction of O2 at edge-plane
2
-10
-2
pyrolytic graphite electrodes (0.28 cm ) coated with 1.6 × 10 mol cm
of CoTAPP. The supporting electrolyte, 1 M HClO4, was saturated with
O2 (curves a and b) or argon (curve c). CoTAPP was deposited on the
electrode surface by transferring a pristine solution of CoTAPP in ethanol/
Supporting Information Available: Details of the preparation of
3
CoTAPP, the absorption spectrum, the DLS histogram, and the k -
weighted EXAFS function of the aggregate (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
1
-propanol 2/1 (v/v) to record curve b or a micellar solution of CoTAPP
prepared by ultrasonic irradiation of the pristine solution for 6 h to record
curves a and c. The electrodes were covered with 8 µL of 0.5 wt % Nafion
-
1
in 2-propanol and air-dried. Scan rate ) 0.1 V s . (B) Reduction of O2
with the electrode used to record curve a operated as a rotating disk electrode
under O2. Electrode rotation rates were 100, 200, 400, 600, 900, 1600, and
References
(
1) Uno, H.; Masumoto, A.; Ono, N. J. Am. Chem. Soc. 2003, 6, 12082-
2
500 rpm for curves 1-7, respectively. (C) Reduction of O2 with the
electrode used to record curve b in A. Electrode rotation rates were 100,
00, 400, 600, and 900 rpm for curves 1-5, respectively. (D) Koutecky-
12083.
(2) Shirakawa, M.; Kawano, S.; Fujita, N.; Sada, K.; Shinkai, S. J. Org. Chem.
2003, 68, 5037-5044.
2
-
1
-1/2
Levich plots of (plateau current) versus (rotation rate)
for the curves
(3) Kano, K.; Fukuda, K.; Wakami, H.; Nishiyabu, R.; Pasternack, R. F. J.
in B (b) and C (2). The broken lines were calculated for the diffusion-
convection-controlled reduction of O2 by two (n ) 2) or four (n ) 4)
electrons.
Am. Chem. Soc. 2000, 122, 7494-7502.
(4) Komatsu, T.; Yanagimoto, T.; Furubayashi, Y.; Wu, J.; Tsuchida, E.
Langmuir 1999, 15, 4427-4433.
(
5) Komatsu, T.; Yamada, K.; Tsuchida, E.; Siggel, U.; B o¨ ttcher, C.; Fuhrhop,
J.-H. Langmuir 1996, 12, 6242-6249.
of the stacked CoTAPP molecules. The dimension was in good
agreement with the cobalt-cobalt distance of l/n ) 0.25 nm based
on the SLS experiments.
(
6) Fuhrhop, J.-H.; Bindig, U.; Siggel, U. J. Am. Chem. Soc. 1993, 115,
11036-11037.
(7) Fuhrhop, J.-H.; Bindig, U.; Siggel, U. J. Chem. Soc., Chem. Commun.
1994, 1583-1584.
2
Another curious feature is the electrocatalysis of O reduction
(8) Fuhrhop, J.-H.; Demoulin, C.; B o¨ ttcher, C.; K o¨ ning, J.; Siggel, U. J. Am.
Chem. Soc. 1992, 114, 4159-4165.
by the CoTAPP aggregate adsorbed at the surface of a graphite
electrode immersed in an aqueous electrolyte solution. To determine
the effect of the cofacial orientation on the catalytic activity, one
(
9) Terech, P.; Scherer, C.; Lindner, P.; Ramasseul, R. Langmuir 2003, 19,
10648-10653.
(
10) Smith, K. M.; Kehres, L. A.; Fajer, J. J. Am. Chem. Soc. 1983, 105, 1387-
389.
2
can take advantage of the insolubility of CoTAPP in H O and the
1
very slow rate of aggregation in alcohols. Thus, a pristine alcoholic
solution of CoTAPP is presumed to contain the monomeric complex
alone, while the rodlike aggregate formed after ultrasonic irradiation
should survive during the electrode preparation and electrochemical
measurements. Indeed, when these solutions were used as mother
liquors to prepare the modified electrode by dip-coating, a quite
different electrochemical response was obtained. In Figure 3A,
curves a and b represent cyclic voltammograms for the reduction
(
11) Jesorka, A.; Balaban, T. S.; Holzwarth, A. R.; Schaffner, K. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 2861-2863.
(12) Tamiaki, H.; Kubota, T.; Tanikaga, R. Chem. Lett. 1996, 639-640.
(13) Aoudia, M.; Rodgers, M. A. J. J. Phys. Chem. B 2003, 107, 6194-6207.
2
2
2
-4
-1
(14) Optical constant K was given by 4π n
0
(dn/dc) λ
N
A
0
, with n and n
being the refractive index of the pure solvent and the solution, respectively,
dn/dc the refractive index increment, and NA Avogadro’s constant.
(15) Light Scattering from Polymer Solutions; Huglin, M. B., Ed.; Academic
Press: New York, 1972.
(
16) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. 1997, 36,
of O
2
at the modified electrode coated with the aggregate and the
4294-4295.
monomeric complex, respectively. A positive shift in the O
2
(17) Chang, C. J.; Deng, Y.; Shi, C.; Chang, C. K.; Anson, F. C.; Nocera, D.
G. Chem. Commun. 2000, 1355-1356.
reduction potential was accomplished by the aggregation of
CoTAPP, while the Co(III/II) couple was almost unaffected by the
aggregation and appeared at an even more positive potential (0.41
(
18) Guilard, R.; Brand e` s, S.; Tardieux, C.; Tabard, A.; L’Her, M.; Miry, C.;
Gouerec, P.; Knop, Y.; Collman, J. P. J. Am. Chem. Soc. 1995, 117,
11721-11729.
V) in the absence of O
Figures 3B and 3C show current-potential curves for the
reduction of O at a rotating disk electrode coated with the aggregate
2
(curve c).
(19) Ni, C.-L.; Abdalmuhdi, I.; Chang, C. K.; Anson, F. C. J. Phys. Chem.
987, 91, 1158-1166.
1
(
20) Durand, R. R., Jr.; Bencosme, C. S.; Collman, J. P.; Anson, F. C. J. Am.
2
Chem. Soc. 1983, 105, 2710-2718.
and the monomeric complex, respectively. While the reduction
occurred in a single step for both electrodes, the plateau current at
the slowest rotation rate (curve 1) was nearly doubled by the
(21) Mest, Y. L.; Inisan, C.; Laou e´ nan, A.; L’Her, M.; Talarmin, J.; Khalifa,
M. L.; Saillard, J.-Y. J. Am. Chem. Soc. 1997, 119, 6095-6106.
JA0486216
J. AM. CHEM. SOC.
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