Isomeric and Structural Impacts on Electron Acceptability of Carbon

Predicting the changes of the fullerene dimers during processes of multiple electron additions is of significant interest. We found that the fullerene...
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J. Phys. Chem. C 2008, 112, 741-746

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Isomeric and Structural Impacts on Electron Acceptability of Carbon Cages in Atom-Bridged Fullerene Dimers Baoyun Sun,*,† Tongxiang Ren,† Xiaopei Miao,‡,§ Fucai Dai,†,| Long Jin,† Hui Yuan,† Gengmei Xing,† Meixian Li,*,‡ Jinquan Dong,† Fei Chang,| Jingbo Hu,§ Hao Chen,† Feng Zhao,† Xueyun Gao,† and Yuliang Zhao*,† Laboratory for Biological Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049, National Center for Nanosciences & Technology of China, Beijing 100080, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, College of Chemistry and Chemical Engineering, Inner Mongolia UniVersity, Huhhot 010021, and Department of Chemistry, Beijing Normal UniVersity, Beijing 100875, China ReceiVed: September 5, 2007; In Final Form: October 23, 2007

The electrochemical properties of the carbon-bridged fullerene dimers C121(I), C121(II), C121(III), C131, and C141 were characterized systematically for the first time in this study. Cyclic voltammogram and differential pulse voltammogram analyses revealed that they first underwent three reversible fullerene-unit-based reduction processes where each of the two carbon cages accepted one electron in each step and then possessed a different deep reduction sequence from the fourth to sixth reduction potentials of the fullerene cages. The electronic interactions between cages in the atom-bridged dimers (e.g., C60-C-C60) were found to be different from those of dimers in which two cages were connected directly. Comparison studies of the redox properties of the five dimers revealed that the C60 dimerization via [5.6]-[6.6] connection influenced the cage electron acceptability much more than that of [5.6]-[5.6] or [6.6]-[6.6] connections and the dimerization with C70 cages influenced the reduction potentials of dimerized products more potently than that with C60 cages. Further results from controlled potential electrolysis, high-performance liquid chromatography, matrix-assisted laser desorption and ionization time-of-flight mass spectrometry, ultraviolet absorption spectral analyses demonstrated the reduction processes and a dissociation of the dimers based on reductions. The theoretical understanding of the experiments was investigated by using time-dependent density functional calculations for the ionic states of C121(I, II, III)n- with n ) 0, 1, 2, 3, or 4.

Introduction Electrochemical characterization, as one of the most useful tools in the study of electronic properties, has been successfully performed on a lot of fullerenes and their derivatives. The properties, stability, mechanism, and reactivity of fullerene derivatives could be deduced from the experiment.1-6 Many kinds of fullerene-cage-based derivatives have been synthesized for the purpose of developing electrochemically or optically active systems with different applications, whose basic constructing units are dimer, carbon-bridged dimer, or trimer structures. Their electronic properties are mostly dominated by the electronic interactions between the neighboring fullerene cages of the dimerized units. How the dimerization influences the electronically functional properties is of particular interest for developing as well as understanding the ultimate materials of fullerene derivatives with the desired functions and expected applications such as photovoltaic conversion materials and alloptical switch or optical limiting materials.7,8 In this work, electrochemical analyses were performed on a series of fullerene dimers, in which the two fullerene cages were * To whom correspondence should be addressed. Phone: 86-10-88233191. Fax: 86-10-8823-3186. E-mail: [email protected] (Y.Z.); [email protected] (B.S.). † Chinese Academy of Science and National Center for Nanosciences & Technology of China. ‡ Peking University. § Beijing Normal University. | Inner Mongolia University.

separated by a carbon atom bridge. The study was designed to explore the relationship between the redox properties and the structures of fullerene dimers by using electrochemical analyses. The electrochemical probes were performed to a deep reduction sequence: from the first up to the sixth reduction potentials, associated with controlled potential electrolysis, high-performance liquid chromatography (HPLC), matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF-MS), ultraviolet (UV) absorption spectroscopy, and theoretical calculations. Experimental Methods The five fullerene dimers C121(I, II, III), C131, and C141 were synthesized by the neutron irradiation method. The details are described in previous papers.9-11 In brief, C60, C70, or a mixture of C60 and C70 was irradiated for 2 h by neutron beams with a composition of fast neutrons of ∼5.4 × 1012 cm-2 s-1 and thermal neutrons of ∼5.4 × 1013 cm-2 s-1 and a fast-to-thermal neutron flux ratio of about 10%. Under this condition, the energetic neutrons transferred their energy to fullerene cages, which perhaps caused a secondary ionization and induced some fullerene cages’ dissociation. The dissociation of fullerene cages produced fragmental carbon atoms and finally resulted in fullerene dimers.12 A high-purity germanium detector (HPGe) was used to ensure no radioactivity remained with the irradiated fullerene samples.

10.1021/jp0771217 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/28/2007

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Figure 1. Cyclic (a) and differential pulse (b) voltammograms of C121(I, II, III) in MeCN/C6H5CH3 (1:4, v/v) containing 0.1 M (n-Bu)4NClO4 (the dashed lines are the CVs of dimers in a small scan range).

Isolation of the five isomers was performed by HPLC separation (LC908-C60, Japan Analytical Industry Co.) with 5PBB (Nacalai Co., Japan, 20 × 250 mm) and Buckyprep (Nacalai Co., Japan, 10 × 250 mm) columns. The mass spectra were measured via MALDI-TOF-MS (AutoFlex, Bruker Co., Germany) with 9-nitroanthracene as the matrix. The purity of C121(I, II) and C131 used in this study was 99%, and that of C121(III) and C141 was about 90%. A CHI 660A electrochemical workstation (CH Instruments) with a conventional three-electrode cell was used to perform electrochemical measurements. All the experiments were conducted in MeCN/C6H5CH3 (1:4, v/v) containing 0.1 M (nBu)4NClO4 at ambient temperature (20 ( 2 °C). The working electrode was a glassy carbon electrode with a diameter of 4 mm. A Ag wire coated with AgCl was used as the reference electrode and a platinum electrode as the auxiliary electrode. Cyclic voltammograms (CVs) were recorded at 100 mV/s, and differential pulse voltammograms (DPVs) were obtained at 20 mV/s by using a pulse amplitude of 50 mV, a pulse width of 60 ms, and a pulse period of 200 ms. The interference of oxygen was carefully avoided. All the potentials in this paper are with respect to the ferrocene/ferrocenium redox couple. The concentrations of C121(I, II, III), C131, and C141 were 10-4 M. C60, C70, and a mixture of C60 and C70 (at a molar ratio of 1:1) were measured under the same electrochemical conditions for comparison studies. The absorption spectra in the UV-vis regions were measured with a TU-1901 UV-vis spectrophotometer (Beijing Perkinje General Instrument Co., Ltd., China). The structures of the anions of C121(I, II, III)n- (n ) 0, 1, 2, 3, 4) were calculated by using the Gaussian 98W package, and their molecular orbitals were optimized by B3LYP/3-21G*.13,14 Results and Discussion Electrochemical Analysis of C121(I, II, III), C131, and C141. The CVs and DPVs of the five fullerene dimers and the 1:1 mixture of C60 and C70 are shown in Figures 1 and 2. The CV

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Figure 2. Cyclic (a) and differential pulse (b) voltammograms of C131, C141, and the 1:1 mixture of C60 + C70 in MeCN/C6H5CH3 (1:4, v/v) containing 0.1 M (n-Bu)4NClO4 (the dashed lines are the CVs of dimers in a small scan range).

TABLE 1: DPV Peak Potentialsa (V) of the Five Dimers, C60, and C70 versus Fc/Fc+ red

E1

C121(I) C121(II) C121(III) C131 C141 C60 C70

-0.98 -0.97 -0.98 -0.96 -0.94 -0.98 -0.97

red

E2

-1.35 -1.32 -1.27 -1.35 -1.43 -1.40 -1.38

red

E3b

-2.00 -1.80 -1.85 -1.86 -1.90 -1.83

red

E4

-2.30 -2.26 -2.33 -2.36 -2.36 -2.39 -2.25

red

E5

-2.80 -2.72 -2.80 -2.85 -2.84 -2.89 -2.73

red

E6

-2.99 -2.92 -2.99 -3.03 -3.00 -3.33 -2.98

a The electrochemical experiments were conducted in MeCN/ C6H5CH3 (1:4, v/v) containing 0.1 M (n-Bu)4NClO4, pulse amplitude 50 mV, pulse width 60 ms, pulse period 200 ms, scan rate 20 mV/s. b Ill-defined.

and DPV curves of C60 and C70 were recorded under the same conditions of measuring the five dimers and the 1:1 mixture of C60 + C70. Six one-electron reversible reductions could be observed for C60 and C70 in the potential range from 0 to -3.5 V, which agrees well with those reported before.1 The CVs and DPVs of the mixture of C60 and C70 (Figure 2) show that the first and second reduction potentials of C60 and C70 locate at almost the same position and cannot be well resolved from each other. The third reduction potential of C70 has a little bit positive shift to that of C60 and behaves as a slight shoulder. The obvious big shifts are observed from the fourth reduction. Detailed analysis of the CV and DPV results in different scan ranges of redox potential indicates that the electrochemical properties of C121(I), C121(II), C121(III), C131, and C141 display similar redox characteristics. The first three (e.g., the first, second, and third) reductions are all fullerene-unit-based reversible reductions whose potentials are similar to those of parent fullerene C60 or C70, but the subsequent (e.g., the fourth, fifth, and sixth) reductions are obviously different from the first three (e.g., the first, second, and third). The oxidation process of the five isomers was not observed in the range from 0 to 0.9 V. The DPV potentials for the five dimers, C60, and C70 are summarized in Table 1. All values reported here are relative to the Fc/Fc+ couple. The first reductions of all the fullerene dimers show a negative shift in the sequence of C141, C131, and C121.

Carbon Cage Electron Acceptability in Fullerenes Taking into account their compositions, C70dCdC70 (C141), C60dCdC70 (C131), and C60dCdC60 (C121),3,4,6,9 and the known electrochemical properties of the C70 or C60 monomer,1 we can hence understand the observed results that the electron-accepting ability of C131 locates between that of C141 and C121. Namely, the electron acceptability changed in the sequence C141 > C131 > C121, which acted in accord with their different compositions. This result provides the new insight that the carbon-bridged fullerene dimers can maintain the electrochemical characteristics of their parent cages. The first reduction potentials of the three C121 isomers are a little different from each other (Figure 1 and Table 1), though the only difference among them is the structural conjugation pattern of bridgehead carbon atoms between two cages.15,16 The first reduction potentials for C121(I) and C121(III) locate at similar positions for C60, but C121(II) shifts to a more positive potential. This indicates that isomer II is easier to reduce than isomers I and III. Three different connections have been identified for the C121 isomers: [6.6]-[6.6] for isomer I, [5.6]-[6.6] for isomer II, and [5.6]-[5.6] for isomer III.15 Comparing their redox properties, one can find an important property that the dimerization of fullerene cages through the [5.6]-[6.6]-type bridge connection has more impact on the electron-accepting ability of cages than the other types of bridge connections. Undoubtedly, the first three (e.g., the first, second, and third) reduction potentials of the solutions of the five dimers are all fullerene-unit-based ones (Figures 1 and 2). The similar intensity indicates that the number of electrons transferred in each of three reduction steps is the same. The first and second reduction peaks are slightly broadened because each peak corresponds to a two-electron-transfer process involving reduction of both fullerene units in the dimers. These two electrons were assigned to each of the two cages, but splitting of the peaks was not observed, indicating that each of the two C60 monomers in C60d CdC60 could be reduced at the same potential, similar to the first reduction of the mixture of C60 and C70. These suggest that the electronic interactions between the cage units of the atom-bridged C60dCdC60 dimers are quite weak. As for fullerene dimers such as C120, C120O, or C120C2H4,17-19 they exhibit a step-by-step reduction process of each unit, i.e., strong interactions between cage units in this type of dimer. This may be understood from the geometry where the cage units are connected directly and their electronic communication easily proceeds. In other similar structures in which the two fullerene units are not directly connected but separated by a great distance by atom chains, such as butadiyne-1,4-diylbisfullerenes, the reductions of each cage are simultaneous.20-22 In this study, two cages in the atom-bridged dimers are separated by the chemical bond of the carbon bridge. The presence of the sp3 center carbon atom prevents the interaction between the cage units. Such poor electronic interactions have been proved upon comparing their electronic absorption spectra.6,23 It is noteworthy that the potential difference between the second and third reductions is bigger than that between the first and second reductions. This increase in separation is likely to result from the effect of increasing Coulombic repulsion. In the third reduction, the wave was split into two peaks in all of the dimers, indicating that as more and more electrons are added to the cages, their mutual interactions become intensive and cannot be neglected anymore. In the same way, we compared the first reduction potential of C131 with that of the 1:1 mixture of C60 + C70 and the potential of C141 with that of C70 (Figure 2 and Table 1). There are 16 possible isomers for C131 and 64 possible isomers for

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Figure 3. Cyclic voltammograms of C121(I) recorded at a scan rate of 100 mV/s in different potential ranges.

C141.3,4 Under the severe neutron irradiation condition, the yield of isomers should be totally dependent on their thermodynamic stabilities.11 The theoretical calculations concluded that the most possible structures of C131 and C141 were both [6,6]-[6,6] connections, in which C70 opened the [6,6] ring that located at the equator area to create new chemical bonds with the bridged carbon atom.3,4 We speculated that the structure of C131 and C141 in this study should be a [6,6]-[6,6] connection, which was similar to that of C121(I). However, compared with those of C121 and C60, the first reduction potentials of C131 and C141 show a very positive shift versus that of the C60 + C70 mixture and C70, respectively. In other words, the dimerization with C70 cages influenced the reduction potentials of the dimers more potently than that with C60 cages. In previous studies, the electrochemical scans were limited to the first three reductions of fullerene and no behavior beyond more negative potentials was observed.6,17-22 To our knowledge, this is the first time the electrochemical study has been extended to a very deep reduction (from the first to sixth reductions). The electrochemistry of the five dimers in the accessible potential window is somewhat complicated. The sudden increase of the intensity of the fourth peak and the irreversible sequent reduction peaks are clearly remarkable features. Figure 3 shows the CVs of C121(I) recorded at a scan rate of 100 mV/s in different potential ranges. The CVs of C121(II), C121(III), C131, and C141 show electrochemical behaviors similar to that of C121(I). The first and second reductions of all the species are reversible, the third reduction is quasi-reversible, and the fourth is irreversible. In the experiment, we found that the working electrode surface was covered with a black precipitate if the scan was extended over the fourth reduction. This reduction was not ascribed to the carbon-bridged fullerene dimers but to intermediates arising from rearrangement or dissociation of the C2mdCdC2n dimers; i.e., different species formed by an irreversible chemical process occurred with the reduction reactions, as those reported for other fullerene derivatives.24-27 For instance, the homofullerene is able to rearrange into the methanofullerene under the conditions of the electrochemical reduction.28,29 Controlled Potential Electrolysis (CPE) of C121(I). To understand this observation, we performed CPE with C121(I) at the fourth reduction potential for 12 h, in association with the CV and DPV measurements every 2 h. Finally, the electrolytic solution was oxidized at 0 mV. Figure 4 shows the DPV results for C121(I) recorded after the CPE for 2 and 8 h and reoxidation of the electrolytic solution. After 2 h of electrolysis, the intensities of all the peaks of the original C121(I) decreased evidently. With the electrolytic time, the height of all reduction waves became lower and lower

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Figure 4. Differential pulse voltammograms of C121(I) at four different conditions: before controlled potential electrolysis, after controlled potential electrolysis (for 2 and 8 h, respectively), and reoxidation. Figure 6. MALDI-TOF-MS of the HPLC fractions at retention times of 15 min (a) and 75 min (b) and the precipitate after electrolysis (c).

Figure 5. HPLC chromatograms of C121(I) solutions before and after electrolysis.

till the first three (e.g., the first, second, and third) reduction peaks almost disappeared after 8 h of electrolysis. Furthermore, the fifth and sixth reductions shifted to more negative potentials. This observation suggests that different fullerene-containing species might be formed during the electrolysis process. After 8 h of electrolysis, the CV and DPV results of later electrolysis were similar to the results of 8 h of electrolysis. No obvious change occurred by prolonging the electrolysis time, which suggests that 8 h is the time limit for electrolysis processes of the C2mdCdC2n (m ) n or m * n) type fullerene dimers. After the reoxidation, the first two (e.g., the first and second) reduction peaks reappeared but could not be separated from each other. Meanwhile, the fifth and sixth potentials shifted in the more negative direction. It seems that the dissociated species were oxidized but the original dimers could not be recovered by reoxidation. During the electrolysis processes, we found that some precipitate dispersed in the electrolytic solution of C121. To analyze these phenomena, the precipitate was collected carefully and we performed further HPLC separation of the C121 solutions before and after electrolysis (Figure 5). The acetonitrile and supporting electrolyte were removed before the solution was analyzed by HPLC. After electrolysis, HPLC shows fractions at retention times of 15, 30, 50, and 75 min, indicating that the categories of species existing in the electrolytic solution are much more complicated than those of the original C121 solution. The fraction at the retention time (tR) of 15 min is C60, which was confirmed by both HPLC retention time and MALDI-TOFMS analyses (Figure 6a). This means that a part of the C121 has dissociated into C60 during the electrolysis process. The fraction at tR ) 75 min was identified as the unchanged C121 (Figure 6b). This agrees well with the fourth reduction peak observed after electrolysis, though its intensity decreased significantly. The HPLC fractions observed at tR ) 30 and 50 min could not be identified. Nevertheless, the MALDI-TOF-MS analysis of

Figure 7. UV-vis spectra of C121(I) before and after electrolysis and enlarged spectra from 400 to 800 nm (inset).

the precipitate showed the presence of C60, C121, etc. (Figure 6c), which implies that the precipitate on the electrode and solution should be a complex containing at least C60 and C121 species. The UV-vis spectrum of the C121(I) electrolytic solution (Figure 7) becomes featureless, compared with that of C121(I) before electrolysis. As we have discussed above, the electrolytic solution has become a mixture rather than a pure C121 solution, which should be the reason for the absence of the characteristic absorption peaks. The broadened reduction peaks in the DPV and CV of the reoxidized solution might have the same cause. However, characteristic absorptions around 300 and 500-600 nm were also detected, indicating that the compounds in the solution still contain C60 cages. Actually, similar dissociation and rearrangement of other fullerene derivatives were observed previously: A “retrocyclopropanation reaction” occurred for the substituted methanofullerene.24 The derivatives with phosphonate or sulfone groups are able to lose their addend from the fullerene cage, to react with another molecule or to form a dimer when electrochemistry is performed.26 C120 undergoes dissociation into a C60 anion under electrochemical reduction.19 The removal of addend groups on the C78 cage enables the synthesis of a C78 isomer which is not accessible by direct synthesis.30 In the present compounds studied, there exists not only a C60 cage but also a C61 unit. Thus, the reaction should be more complicated than that of the compounds studied previously.19,24,26,28 Predicting the changes of the fullerene dimers during processes of multiple electron additions is of significant interest. We found that the fullerene dimers remained stable after the first, second, and third reductions. No dissociation evidence was observed. However, since the bridge atom is tetrahedrally

Carbon Cage Electron Acceptability in Fullerenes

Figure 8. Geometry of the three isomers C121(I) (a), C121(II) (b), and C121(III) (c).

coordinated, the addition of more electrons to the dimers may alter the electrostatic interactions between the cages and open the cyclopropane ring more or less to release the strain of the bonds. This can lead to a rapid dissociation or reaction with other species in the system of electrolysis. Quantum Chemical Computations. The relative stabilities and the structures of C121(I, II, III)n- (n ) 0, 1, 2, 3, 4) ions were calculated on the basis of the time-dependent density functional theory (TD-DFT).13,14 The geometries and electronic structures were optimized using B3LYP/3-21G* in the Gaussian 98W program. Within the framework of the van der Lugt and Oosterhoff model, the geometrical features of the excited-state path are assumed to be similar to those of the ground-state path.31,32 Though this prevents the exact localization of stationary points on an excited-state potential energy surface, an approximate excited-state potential energy surface can hence be generated. In this study, the structures of dimer anions C121(I, II, III)n(n ) 0, 1, 2, 3, 4) were calculated according to this approximation (Figure 8). However, the structures of anions with higher charge states could not be accessed by this calculation because the excited-state features could not be neglected anymore. The center carbon atoms, the bridgehead atoms, and the bond angles are marked with C1-C5, R, and β (Figure 8). The variety of the energies and distances of the mass center of C121(I, II, III)n(n ) 0, 1, 2, 3, 4) ions are given in Figure 9. The corresponding structural parameters are summarized in Tables S1 and S2 (Supporting Information). The theoretical structures are consistent with the observations of the electrochemical experiment. As one electron was added onto the fullerene cages, the energies of all the dimer anions exhibited a regular decreasing feature. When the fullerene units accepted two electrons, the energy of the dimer became the lowest, suggesting the dimers have strong electron acceptability. However, as more electrons were further added onto the cage, the energy of the C121 dimer showed an increasing tendency; hence, the dimer anions with a higher charge state became more and more unstable. To release the relatively higher energy, dissociation or rearrangement of the dimer structures became possible.

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Figure 9. Corresponding energies (a) and distances of the mass center (b) of the negative ions of the three isomers C121(I, II, III)n- with n ) 0, 1, 2, 3, or 4.

It was further found that both the bond lengths between the center carbon and the bridgehead carbons (e.g., C1-C5, C2C5, C3-C5, and C4-C5) and the bond angles (R, β) varied with the number of accepted electrons. When the bond lengths became longer and longer, the bond angles became smaller and smaller as the charge state of the C121 dimer increased. Measurement of the distances between two mass centers of the fullerene cages of the dimers showed that the two cages pulled away from each other when more electrons were added onto the cages. This is in agreement with the energy tendency and should be responsible for the dissociation or rearrangement of the dimers observed in the above-mentioned electrochemical experiment. Conclusions To analyze the electrochemical properties of atom-bridged fullerene dimers, C121(I, II, III), C131, and C141 were prepared and investigated by cyclic voltammetry and differential pulse voltammetry, associated with HPLC and MALDI-TOF-MS analyses, as well as UV-vis spectroscopy. The results indicated that the above dimers have different electron-accepting abilities. In a comparison study of the first reduction potential of the three C121 isomers, C131, and C141, we revealed that their reduction potentials are closely related to the type of conjugation of the bridge atom between cage units. All five studied compounds underwent three reversible fullerene-unit-based reduction processes: one cage accepted one electron in each step of reduction. Furthermore, three irreversible reductions were observed, which should be ascribed to other fullerene-based derivatives instantly formed during the reduction processes. Controlled potential electrolysis showed that the reduction waves varied with the electrolysis time. Further HPLC, MALDI-TOFMS, and UV absorption spectral analyses of the reoxidized C121(I) electrolytic solution demonstrated that the C121 dimer dissociated into C60 and other fullerene-based derivatives. The theoretical calculations of C121(I, II, III)n- (n ) 0, 1, 2, 3, 4) ions showed that the energies of the dimers increased and the two cages pulled away from each other when more electrons were added to the cages of the C121 dimer, which agrees well with the electrochemical results observed.

746 J. Phys. Chem. C, Vol. 112, No. 3, 2008 Acknowledgment. We acknowledge funding support from the National Natural Science Foundation of China (Grants 20571076, 10525524, and 20575004), the MOST 973 Programs (Grants 2006CB705601 and 2005CB724703), and the Directionary Project of the Chinese Academy of Sciences (Grant KJCSW-H12). We acknowledge the generous support of Prof. Dr. Xiaoyi Li at the Graduate University of the Chinese Academy of Sciences in the theoretical calculation. Supporting Information Available: Structure parameters of C121(I, II, III)n- (n ) 0, 1, 2, 3, 4) calculated on the basis of TD-DFT. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Reed, C. A.; Bolskar, R. D. Chem. ReV. 2000, 100, 1075. (2) Segura, J. L.; Martin, N. Chem. Soc. ReV. 2000, 29, 13. (3) Gao, X. F.; Zhao, Y. L.; Yuan, H.; Chen, Z. L.; Chai, Z. F. Chem. Phys. Lett. 2006, 418, 24. (4) Gao, X. F.; Yuan, H.; Chen, Z. L.; Zhao, Y. L. J. Comput. Chem. 2004, 25, 2023. (5) Forman, G. S.; Tagmatarchis N.; Shinohara H. J. Am. Chem. Soc. 2002, 124(2), 178. (6) Dragoe, N.; Shimotani, H.; Wang, J.; Iwaya, M.; Bettencourt-Dias, A.; Balch, A. L.; Kitazawa, K. J. Am. Chem. Soc. 2001, 123, 1294. (7) Ishihara, S.; Ikemoto, I.; Suzuki, S.; Kikuchi, K.; Achiba, Y.; Kobayashi, T. AdV. Funct. Mater. 2001, 11, 15. (8) Stepanov, A. G.; Portella-Oberli, M. T.; Sassara, A.; Chergui, M. Chem. Phys. Lett. 2002, 358, 516. (9) Zhao, Y. L.; Chen, Z. L.; Yuan, H.; Gao, X. F.; Qu, L.; Chai, Z. F.; Xing, G. M.; Yoshimoto, S.; Tsutsumi, E.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 11134. (10) Zhao, Y. L.; Nishinaka. I.; Nagame. Y.; Tsukada. K.; Sueki. K.; Goto. S.; Tanikawa. M.; Nakahara. H. J. Radioanal. Nucl. Chem. 2003, 255 (1), 67. (11) Ren, T. X.; Sun, B. Y.; Chen, Z. L.; Qu, L.; Yuan, H.; Gao, X. F.; Wang, S. K.; He, R.; Zhao, F.; Zhao, Y. L.; Liu, Z. S.; Jing, X. P. J. Phys. Chem. B 2007, 111, 6344. (12) Chen, Z. L.; Zhao, Y. L.; Qu, L.; Gao, X. F.; Zhang, J.; Yuan, H.; Chai, Z. F.; Xing, G. M.; Cheng, Y. Chin. Sci. Bull. 2004, 49, 793. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,

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