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Element-Specific Magnetic Properties of Di-Erbium Er2@C82 and Er2C2@C82 Metallofullerenes: A Synchrotron Soft X-ray Magnetic Circular Dichroism Study Haruya Okimoto,† Ryo Kitaura,† Tetsuya Nakamura,‡ Yasuhiro Ito,† Yutaka Kitamura,† Takao Akachi,† Daisuke Ogawa,† Naoki Imazu,† Yuko Kato,† Yuki Asada,† Toshiki Sugai,† Hitoshi Osawa,‡ Tomohiro Matsushita,‡ Takayuki Muro,‡ and Hisanori Shinohara*,†,§ Department of Chemistry and Institute for AdVanced Research, Nagoya UniVersity, Nagoya 464-8602, Japan, Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan, and CREST, Japan Science and Technology Corporation, c/o Department of Chemistry, Nagoya UniVersity, Nagoya 464-8602, Japan ReceiVed: December 15, 2007; In Final Form: February 7, 2008
The magnetic properties of mono- and dierbium(Er) metallofullerenes have been studied by means of a synchrotron soft X-ray magnetic circular dichroism (SXMCD) technique. The effective magnetic moment of the Er ions has been estimated to be 8.48 µB and is independent of the number of Er ions encapsulated inside the fullerene cage. These Er metallofullerenes exhibit an antiferromagnetic-like behavior at low temperatures that is stronger in Er@C82(C2V isomer) monometallofullerene than in Er2@C82(C2V) and Er2C2@C82(C2V) dimetallofullerenes. The result suggests that the unpaired spin on the monometallofullerene cages causes an indirect magnetic interaction between Er 4f spins of neighboring metallofullerenes. Furthermore, we have found that the Weiss temperatures of the metallofullerene remain the same even when one of the Er ions in Er2C2@C82(Cs) is replaced by a nonmagnetic Y ion (i.e., ErYC2@C82). The results indicate that interaction between the two Er ions in the same fullerene cage does not contribute significantly to the overall magnetic properties.
1. Introduction Encapsulations of metal atom(s) into hollow fullerene cages, the so-called endohedral metallofullerenes,1 can provide a substantial change of chemical and physical properties of the fullerene molecules. This has attracted widespread interests not only in their fundamental properties but also in various applications such as a field effect transistor (FET)2-4 and a contrast agent for magnetic resonance imaging (MRI).5-7 So far, various endohedral metallofullerenes, especially monometallofullerenes (M@C2n), have been synthesized, characterized, and their specific chemical reactivity,8-12 redox activity,13-18 and optical properties19-24 have been investigated. Because of fundamental interests on the magnetism of metal atoms confined in a nanometer scale cage and a possible application as high-performance spintronic devices25 and MRI contrast agents,5-7 magnetic properties arising from encapsulated metal atoms are one of the most important properties of metallofullerenes. Until now, magnetic properties of monometallofullerenes (M@C82, M ) Sc, La, Ce, Ho, Tb, Dy, Er, etc.) have been investigated by electron spin resonance (ESR)26-30 and superconducting quantum interference devices (SQUID) magnetometry,31-34 which reveals the existence of intermolecular antiferromagnetic-like interactions and the magnetic moments reduction of encapsulated metal atoms compared to free metal ions. In addition, Sc@C82 and La@C82 metallofullerenes have shown a characteristic magnetic behavior that originates from * Corresponding author. Telephone: +81-52-789-2482. Fax: +81-52789-1169. E-mail:
[email protected]. † Department of Chemistry and Institute for Advanced Research, Nagoya University. ‡ Japan Synchrotron Radiation Research Institute. § CREST, Japan Science and Technology Corporation.
a structure phase transition correlated with molecular and metal rotational motion.35-37 In contrast, dimetallofullerenes (M2@C2n) and dimetal carbide fullerenes (M2C2@C2n)24,38-44 are other series of interesting metallofullerenes to investigate their unique magnetic properties because of the presence of two metal atoms confined in the hollow space. During the past several years, Sc, Y, Dy, and Er dimetal carbide fullerenes have been characterized by nuclear magnetic resonance (NMR)42,45-47 and X-ray diffraction (XRD) studies40,43,44,47-49 in reference to the corresponding pure dimetallofullerenes. Unfortunately, very few attempts to investigate magnetic properties of dimetallofullerenes have so far been made successfully primarily because of much lower synthetic yields of such dimetallofullerenes. More importantly, the magnetic properties of dimetallofullerenes are complicated because they have two kinds of magnetic interactions, that is, inter- and intramolecular magnetic interactions. Therefore, detailed investigations of magnetic properties of dimetallofullerenes require extremely highly sensitive and element-specific measurements to distinguish interand intramolecular magnetic interaction. To obtain detailed information on such magnetic interactions, we have investigated the temperature dependence on the magnetic susceptibility of hetero-dimetallofullerenes (containing Er and Y atoms) by using synchrotron soft X-ray magnetic circular dichroism (SXMCD) spectroscopy. Recently, syntheses of hetero-metallofullerenes of M2@C2n type50,51 and M3N@C2n type52-57 were reported, and their physical properties were discussed. By replacing one of the encapsulated magnetic ions with a nonmagnetic ion and selectively observing magnetization of remaining encapsulated magnetic ions by SXMCD, one may evaluate the intermolecular magnetic interaction. SXMCD spectroscopy has been developed
10.1021/jp711776j CCC: $40.75 © 2008 American Chemical Society Published on Web 03/26/2008
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Figure 1. HPLC profiles for isolation of ErYC2@C82(Cs). (a) First stage HPLC chromatogram of the extract containing ErY metallofullerenes on a 5PYE column; (b) second stage recycling HPLC chromatogram of subfraction A on a 5PYE column; (c) third stage recycling HPLC chromatogram of subfraction B on Buckyclutcher-I column. (d) Fourth stage recycling HPLC chromatogram of subfraction C on Buckyoprep and the inset is extended figure from 600 min to 645 min. Toluene was used as eluent.
as a powerful technique for studying the magnetic property of a variety of materials.58-62 One of the main advantages of SXMCD is the orbital specificity of an inner-shell electron excitation, which enables one to measure element-specific magnetization. SXMCD employing high-brilliance synchrotron soft X-ray source requires only a very small quantity of the sample (a few micrograms) compared to that required for SQUID magnetometry.63-67 In fact, we have reported recently that SXMCD is a very powerful technique to obtain information on the element-specific magnetization of metallofullerenes and the corresponding metallofullerene-nanopeapods, that is, carbon nanotubes encapsulating metallofullerenes.68-70 The effective magnetic moment of the metal atom inside fullerene cage was reduced considerably by the crystal field effect from the fullerene cage and orbital hybridization. Here, we report the element specific magnetic property of Er@C82(isomer C2V), Er2@C82(C2V), Er2C2@C82(C2V) by SXMCD together with hetero-dimetallofullerenes such as ErYC2@C82(Cs) to clarify the magnetic interaction between Er atoms inside dimetallofullerenes. 2. Experimental Section Er metallofullerenes were produced by the DC arc-discharge method. The details of the synthesis and separation of Er metallofullerenes have been reported elsewhere.24,39 ErY metallofullerenes were produced by DC arc-discharge using composite rods containing Er 0.6 atom % and Y 0.6 atom % (Toyo Tanso Co. Ltd.). The soot generated was collected under ambient conditions and extracted with CS2. The soot extract contained empty fullerenes, a series of Y-metallofullerenes (Y@C2n, Y2@C2n, and Y2C2@C2n) and Er-metallofullerenes (Er@C2n, Er2@C2n, and Er2C2@C2n) together with ErY-metallofullerenes (ErY@C2n, ErYC2@C2n). ErYC2@C82 (isomer Cs) was isolated by multistage HPLC incorporating three different types of columns: 5PYE (φ 20 × 250 mm2; flow rate: 21 mL min-1; Nacalai Tesque), Buckyclutcher-I (φ 21.1 × 500 mm2, flow rate: 10 mL min-1; Regis Chemical), and Buckyprep (φ 20 ×
250 mm2; flow rate: 21 mL min-1; Nacalai Tesque) for the first, second, and third stage, respectively. Toluene was used as the mobile phase. The identification of isolation of the metallofullerenes was performed by LD-TOF mass spectrometry (MALDI AXIMACFR, Shimadzu). The UV-vis-NIR absorption spectra of the metallofullerenes in CS2 were obtained on a Jasco V-570 spectrophotometer. SXMCD measurements at 1.9 T were performed at a twin helical undulators beam line BL25SU of SPring-8. The solutions of the various metallofullerenes were dropped to coat on a Cu sample plate and baked at 400 K under a pressure of 1 × 10-5 Pa to remove the CS2 solvent molecules present in metallofullerene powders. The sample plate was cooled down to 10 K and the cooling rate was -20 K min-1 in the absence of a magnetic field. X-ray absorption and the MCD spectra were measured by means of the total electron yield method. The intensity of the MCD spectra is proportional to magnetization of the target element, which is the Er atom in the present case.60 On the basis of this principle, we determined the absolute value of magnetization by simulations of MCD spectra. The simulations were performed by the Cowan code, which solves the Schro¨dinger equation for a multielectron atom in a multiconfiguration expansion approximation with relativistic corrections treated perturbatively.71 The solutions were used to evaluate the radiative transition probabilities of the electric dipole, electric quadrupole, and magnetic dipole type. 3. Results and Discussion 3.1. Preparation and Isolation of ErY Hetero-dimetallofullerenes. Figure 1a shows the first-stage HPLC profile for the CS2 extract of the ErY metallofullerenes with 5PYE column for a crude separation. Subfraction A contains some dimetallfullerenes such as Y2C2n, Er2C2n, and ErYC2n. Figure 1b shows an HPLC recycling profile for separation of subfraction A with the 5PYE column. Subfraction B contains ErYC84 with higher empty fullerenes, Y2C2@C82(Cs), Er2@C82(Cs), and Er2C2@C82(Cs). Figure 1c shows the HPLC profile of subfraction B using
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Figure 3. UV-vis-NIR absorption spectra of ErYC84(I), Y2C2@C82(Cs), Er2C2@C82(Cs) in CS2 solvent.
Figure 2. (a). LD-TOF mass spectra of subfraction C. Insets are the corresponding high-resolution spectra. (b). LD-TOF mass spectrum of isolated ErYC84(I).
Buckyclutcher-I column for elimination of empty fullerenes. Subfraction C contains Y2C2@C82(Cs), Er2C2@C82(Cs), and ErYC84(I) as shown in Figure 2a. Figure 1d shows the final-stage HPLC profile using the Buckyprep column for the isolation of ErYC84(I), where three fractions due to Y2C2@C82(Cs), ErYC84(I), and Er2C2@C82(Cs) are seen for the first, second, and third fractions, respectively. After several recycling HPLC procedures, ErYC84(I) metallofullerene was isolated. The relative yields of Y2C2@C82(Cs), Er2C2@C82(Cs), and ErYC84(I) were 1:2:1 judging from the mass spectral peak intensity and absorption integral intensity of the corresponding HPLC profile. As shown in Figure 2b, we confirmed that the isolated ErYC84(I) contained neither empty fullerenes nor other metallofullerenes based on LD-TOF mass spectroscopic measurements. Furthermore, we have found that fraction D contains only Er2@C82(Cs) metallofullerene. The interesting observation is that fraction D does not contain any ErY@C82(Cs) metallofullerene. Hence, we suggest that the cage symmetry of ErY metallofullerenes is similar to that of Y metallofullerene series. It has been generally understood that the retention time of the Buckyprep column depends on the size of the fullerene cage, valence on the fullerene cage, and the number of metal atoms inside, and not on the kind of the metal atoms encapsulated.1 For example, the retention time of Y2C2@C82(Cs) on the Buckyprep column is close to that of Er2C2@C82(Cs). In this case, the retention time of ErYC84(I) is similar to those of Y2C2@C82(Cs) and Er2C2@C82(Cs). On the basis of these similarities, we suggest that ErY@C84 possesses a C82(Cs) fullerene cage. Their retention times are, however, different: 584, 590, and 594 min for Y2C2@C82(Cs), ErYC84(I), and Er2C2@C82(Cs), respectively. The probable reason for this difference in the retention times is the difference in the number of electrons available for charge transfer, although this difference might be very small. 3.2. UV-vis-NIR Absorption Spectra of ErYC2@C82(I). It is well known that the UV-vis-NIR absorption spectra of dimetallofullerenes reflect the cage size, symmetry, and valence
of the fullerenes.42 We have compared the UV-vis-NIR absorption spectra of ErY@C84(I) with those of Y2C2@C82(Cs) and Er2C2@C82(Cs). Figure 3 shows the UV-vis-NIR absorption spectra of ErYC84(I), Er2C2@C82(Cs), and Y2C2@C82(Cs). The spectrum of ErYC84(I) shows characteristic peaks at 628, 709, 789, 1026, and 1191 nm. The absorption spectra of ErYC84(I) is almost identical to those of Er2C2@C82(Cs) and Y2C2@C82(Cs), suggesting that ErYC84(I) is not ErY@C84 but ErYC2@C82(Cs) and that the formal electronic state can be expressed as (ErYC2)6+@C826-. 3.3. Temperature-Dependent Magnetization of Er@C82(C2W), Er2@C82(C2W) and Er2C2@C82(C2W) by SXMCD. We have measured the temperature dependence on X-ray absorption (XAS) and SXMCD spectra of Er@C82(C2V), Er2@C82(C2V), and Er2C2@C82(C2V) at 1.9 T from 10 to 40 K. All of the metallofullerenes possess the same C82(C2V) fullerene cage with different encapsulated species. Figure 4 shows the observed and calculated XAS and SXMCD spectra at the M5 edge corresponding to 3d-4f transition of the Er ion in Er@C82(C2V), Er2@C82(C2V), and Er2C2@C82(C2V) at 10 K and 1.9 T. The M5 edge spectra of Er metallofullerenes are almost identical to the calculated spectra of free Er3+ ion, which ensures that the Er ion inside the fullerene cage is in a trivalent state. This means that the electronic structure of the Er ion inside the fullerene cage is not substantially affected by the number of metal atoms and encapsulated C2 residing between the Er ions. Similar results on Sc and Y metallofullerenes have been observed by X-ray diffraction.49,72 To characterize the magnetic behavior of the Er metallofullerenes, the temperature-dependent magnetization at 1.9 T was measured in a cooling process. Figure 5 shows the inverse magnetic susceptibility-temperature plot for Er@C82(C2V), Er2@C82(C2V), and Er2C2@C82(C2V). The data pertaining to Er@C82(C2V) are consistent with the previous data.34 The susceptibility of the encapsulated Er ions in dimetallofullerenes is 1.7 times as high as that of mono-Er metallofullerene, that is, Er@C82(C2V). Almost 6 electrons are transferring from the two Er metal atoms to the carbon cages in Er2@C82(C2V) and Er2C2@C82(C2V), indicating that di-Er-metallofullerenes have no unpaired spin on their fullerene cages. Because the presence of spin-orbital overlap integrals (arising from unpaired spin on carbon cages of Er@C82) usually result in intermolecular antiferromagnetic-like interactions,37,70 the closed shell nature of carbon cage of Er dimetallofullerenes may reduce such intermolecular antiferromagnetic-like interactions between the neighboring fullerenes compared to the Er@C82 case. The inverse susceptibility-temperature plot of each Er metallofullerene in a temperature range from 10 to 40 K is reproduced well by the Curie-Weiss law χ ) C/(T - θ) using the Curie constant C and Weiss temperature θ listed in Table
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Figure 5. Inverse susceptibility of Er@C82, Er2@C82, and Er2C2@C82 on 1.9 T. Dashed line is a fitting by the least-square method.
TABLE 1: Curie Constant, Weiss Temperature, and Effective Magnetic Moment of Er@C82(C2W), Er2@C82(C2W), and Er2C2@C82(C2W)
Er@C82(C2V) Er2@C82(C2V) Er2C2@C82(C2V)
Figure 4. Er M5 edge (a) XAS and (b) SXMCD spectra of Er@C82(I), Er2@C82(II), and Er2C2@C82(II). Circle plot is experimental data, and solid line is calculation by Cowan code. The spectra are normalized to a constant edge jump. The atomic calculations for the respective free ion are shown as a solid line.
1. The Weiss temperature obtained for Er@C82(C2V) is -19 K, which implies that a substantial antiferromagnetic-like interaction is exerted between the neighboring metallofullerenes. In contrast, the Weiss temperature of dimetallofullerenes is close to zero, indicating that the magnetic interactions in Er2@C82 and Er2C2@C82 are much smaller and are paramagnetic characteristics. As discussed above, the origin of these magnetic behaviors can be attributable to the existence or nonexistence of unpaired spin on the fullerene cage. We have calculated the effective magnetic moments of these Er metallofullerenes from the corresponding Curie constants. As shown in Table 1, the effective magnetic moments of Er@C82, Er2@C82 and Er2C2@C82 are 8.48, 8.47, and 8.39 µB, respectively, which is much lower than that of the free Er3+ ion (9.59 µB, 4I15/2). One of the main reasons for the observed reduction of the magnetic moment is most probably due to the crystal field and orbital hybridization effect as discussed previously.68 The observed effective magnetic moment is consistent with the result of a density functional
C (µBKT-1ion-1)
θ (K)
effective magnetic moment (µB)
16.1 ( 1.0 16.1 ( 1.1 15.8 ( 1.1
-18.8 -3.3 -7.7
8.48 ( 0.27 8.47 ( 0.27 8.39 ( 0.26
calculation on Er2@C82(C2V).73 Because virtually no difference on the effective magnetic moment for encapsulated Er atoms is observed between the di- and monometallofullerenes, the effective magnetic moment of the Er atoms inside the fullerene cage is not substantially affected by the valence on the fullerene cage. 3.4. Element-Specific Magnetization Behavior between Er2C2@C82(Cs) and ErYC2@C82(Cs). In the previous section, we have discussed that the magnetic susceptibility of the diEr-metallofullerenes is higher than that of the mono-Ermetallofullerene. In order to further investigate the nature of the magnetic interaction exerted between the two Er atoms in Er2@C82 and Er2C2@C82, Er-Y hetero-dimetallofullerenes (ErYC84) have been synthesized, purified, and subjected to SXMCD measurements. To evaluate inter- and intramolecular magnetic interactions separately, we have measured the temperature dependence on magnetization of ErYC2@C82 (Cs) and compared it to that of Er2C2@C82(Cs). ErYC2@C82(Cs) is one of the ideal hetero-dimetallofullerenes for studying both the intra- and intermolecular magnetic interactions because it has an Er3+ (paramagnetic ion) and an Y3+ (diamagnetic ion) inside the fullerene cage. The ErYC2@C82 metallofullerene should not have any intramolecular magnetic interaction because the encapsulated Y3+ ion has no unpaired spins and an unpaired spin only resides in the Er3+ ion. In this sense, the formal electronic structure of this metallofullerene is similar to monometallofullerenes without any unpaired spin on the fullerene cage. The difference in the magnetic behavior between Er2C2@C82(Cs) and ErYC2@C82(Cs) should, therefore, stem from the intramolecular magnetic interactions. Figure 6 shows the XAS and SXMCD spectra for the M5 edge of the Er ion in Er2C2@C82(Cs) and ErYC2@C82(Cs) at 10 K and 1.9 T. The peak position and spectral shape of ErYC2@C82(Cs) are almost the same as those of Er2C2@C82(Cs). This supports an idea that the replacement of Er3+ with Y3+ does not significantly alter the electronic state of the other Er3+ in carbon cages.
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Figure 7. Inverse susceptibility of Er2C2@C82(Cs) and ErYC2@C82(Cs) on 1.9 T. Dashed line is a fitting by the least-square method.
Figure 6. Er M5 (a)XAS and (b)SXMCD spectra of and Er2C2@C82(Cs) and ErYC2@C82(Cs). Circle plot is experimental data and solid line is calculation by Cowan code. The spectra are normalized to a constant edge jump. The atomic calculations for the respective free ion are shown as solid line.
Figure 7 shows inverse magnetic susceptibility-temperature plots of Er2C2@C82(Cs) and ErYC2@C82(Cs) at 1.9 T. As seen clearly in the figure, the magnetization process of Er3+ ions in ErYC2@C82(Cs) is almost exactly the same as that of Er2C2@C82(Cs). The Weiss temperature and the effective magnetic moment determined by a fitting with the Curie-Weiss law is -4 K and 7.75 µB, respectively (cf. Table 2), which are relatively close to those of Er2C2@C82(C2V). In contrast to the similarity discussed above, the difference of the cage symmetry between Er2C2@C82(C2V) and Er2C2@C82(Cs) has a substantial effect on the effective magnetic moments of these metallofullerenes (cf. Tables 1 and 2). This clearly shows that the intramolecular magnetic interaction due to encapsulated Er ions is fairly weak: both the direct exchange and super-exchange interactions mediated by the presence of encapsulated C2 molecules and of fullerene cages are weak in the present dimetallofullerenes. This is shown schematically in
Figure 8. Schematic representation on the magnetic interaction of Er metallofullerenes. The endohedral structures of Er2C2 and ErYC2 clusters inside C82 cage are schematically drawn based on the reported X-ray diffraction studies on other dimetallofullerenes such as Y2C2@C82 and Sc2C2@C82.
TABLE 2: Curie Constant, Weiss Temperature, and Effective Magnetic Moment of Er2C2@C82(Cs) and ErYC2@C82(Cs)
Er2C2@C82(Cs) ErYC2@C82(Cs)
C (µBKT-1ion-1)
θ (K)
effective magnetic moment (µB)
13.5 ( 0.8 13.5 ( 0.8
-4.53 -4.33
7.75 ( 0.25 7.75 ( 0.39
Figure 8. Encapsulated Er ions in Er2C2@C82 and ErYC2@C82 can be considered as a kind of isolated ions, which is in strong contrast to the case of other di-Er complexes such as a tripledecker-type phthalocyanine complex, which shows an intracomplex antiferromagnetic interaction below 40 K,74 and other metallofullerenes such as Ho3N@C80 and Tb3N@C80, which show ferromagnetic interaction between M3N clusters.75,76
6108 J. Phys. Chem. C, Vol. 112, No. 15, 2008 Conclusions We have reported element-specific magnetization of Er@C82(C2V), Er2@C82(C2V), Er2C2@C82(C2V and Cs isomers), and ErYC2@C82(Cs) by SXMCD measurements. The Er atoms inside the monometallofullerenes have an antiferromagneticlike interaction due to the presence of unpaired spin on the fullerene cages, whereas the Er atoms inside the dimetallofullerenes show paramagnetic behaviors because Er2@C82(C2V) and Er2C2@C82(C2V). On the basis of the analysis of the SXMCD results, we suggest that the magnetic interaction of the Er3+ ions encapsulated in the dimetallofullerenes is fairly weak, although their effective magnetic moments are reduced by the crystal field effect arising from the fullerene cage. The present element-specific magnetic measurements via SXMCD is one of the powerful methods for evaluating both the intra- and intermolecular magnetic interactions of the metallofullerenes having magnetic atoms inside the fullerene cage. Acknowledgment. This work has been supported by the JST CREST Program for Novel Carbon Nanotube Materials and partially supported by the Grant-in-Aid for Scientific Research A (no. 19205003) of MEXT, Japan. The SXMCD experiments were performed at the twin helical undulators beam line BL25SU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal nos. 2006B1565 and 2007A2012). References and Notes (1) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843. (2) Kanbara, T.; Shibata, K.; Fujiki, S.; Kubozono, Y.; Kashino, S.; Urisu, T.; Sakai, M.; Fujiwara, A.; Kumashiro, R.; Tanigaki, K. Chem. Phys. Lett. 2003, 379, 223. (3) Kobayashi, S.; Mori, S.; Iida, S.; Ando, H.; Takenobu, T.; Taguchi, Y.; Fujiwara, A.; Taninaka, A.; Shinohara, H.; Iwasa, Y. J. Am. Chem. Soc. 2003, 125, 8116. (4) Nagano, T.; Kuwahara, E.; Takayanagi, T.; Kubozono, Y.; Fujiwara, A. Chem. Phys. Lett. 2005, 409, 187. (5) Mikawa, M.; Kato, H.; Okumura, M.; Narazaki, M.; Kanazawa, Y.; Miwa, N.; Shinohara, H. Bioconjugate Chem. 2001, 12, 510. (6) Kato, H.; Kanazawa, Y.; Okumura, M.; Taninaka, A.; Yokawa, T.; Shinohara, H. J. Am. Chem. Soc. 2003, 125, 4391. (7) Bolskar, R. D.; Benedetto, A. F.; Husebo, L. O.; Price, R. E.; Jackson, E. F.; Wallace, S.; Wilson, L. J.; Alford, J. M. J. Am. Chem. Soc. 2003, 125, 5471. (8) Maeda, Y.; Miyashita, J.; Hasegawa, T.; Wakahara, T.; Tsuchiya, T.; Nakahodo, T.; Akasaka, T.; Mizorogi, N.; Kobayashi, K.; Nagase, S.; Kato, T.; Ban, N.; Nakajima, H.; Watanabe, Y. J. Am. Chem. Soc. 2005, 127, 12190. (9) Feng, L.; Nakahodo, T.; Wakahara, T.; Tsuchiya, T.; Maeda, Y.; Akasaka, T.; Kato, T.; Horn, E.; Yoza, K.; Mizorogi, N.; Nagase, S. J. Am. Chem. Soc. 2005, 127, 17136. (10) Maeda, Y.; Matsunaga, Y.; Wakahara, T.; Takahashi, S.; Tsuchiya, T.; Ishitsuka, M. O.; Hasegawa, T.; Akasaka, T.; Liu, M. T. H.; Kokura, K.; Horn, E.; Yoza, K.; Kato, T.; Okubo, S.; Kobayashi, K.; Nagase, S.; Yamamoto, K. J. Am. Chem. Soc. 2004, 126, 6858. (11) Maeda, Y.; Miyashita, J.; Hasegawa, T.; Wakahara, T.; Tsuchiya, T.; Feng, L.; Lian, Y. F.; Akasaka, T.; Kobayashi, K.; Nagase, S.; Kako, M.; Yamamoto, K.; Kadish, K. M. J. Am. Chem. Soc. 2005, 127, 2143. (12) Iiduka, Y.; Ikenaga, O.; Sakuraba, A.; Wakahara, T.; Tsuchiya, T.; Maeda, Y.; Nakahodo, T.; Akasaka, T.; Kako, M.; Mizorogi, N.; Nagase, S. J. Am. Chem. Soc. 2005, 127, 9956. (13) Wakahara, T.; Kobayashi, J.; Yamada, M.; Maeda, Y.; Tsuchiya, T.; Okamura, M.; Akasaka, T.; Waelchli, M.; Kobayashi, K.; Nagase, S.; Kato, T.; Kako, M.; Yamamoto, K.; Kadish, K. M. J. Am. Chem. Soc. 2004, 126, 4883. (14) Fan, L.; Yang, S.; Yang, S. H. Chem.sEur. J. 2003, 9, 5610. (15) Tsuchiya, T.; Sato, K.; Kurihara, H.; Wakahara, T.; Maeda, Y.; Akasaka, T.; Ohkubo, K.; Fukuzumi, S.; Kato, T.; Nagase, S. J. Am. Chem. Soc. 2006, 128, 14418. (16) Tsuchiya, T.; Wakahara, T.; Lian, Y. F.; Maeda, Y.; Akasaka, T.; Kato, T.; Mizorogi, N.; Nagase, S. J. Phys. Chem B 2006, 110, 22517.
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