DOI: 10.1002/adma.200600049

Periodical Variation of Electronic Properties in PolyhydroxylatedMetallofullerene Materials**

By Jun Tang, Gengmei Xing, Yuliang Zhao,* Long Jing, Xingfa Gao, Yue Cheng, Hui Yuan, Feng Zhao,Zhen Chen, Huan Meng, Hui Zhang, Haijie Qian, Run Su, and Kurash Ibrahim

Endohedral metallofullerenes (fullerenes with metalatom(s) encapsulated), as a novel form of carbon-related ma-terials,[1] have attracted special attention for their potentialapplications[2–10] in electronic devices,[2,3] biomedical fieldssuch as therapeutic medicine,[4,5] and a new generation ofmagnetic resonance imaging (MRI) contrast agents,[6–8] etc.Recently, the polyhydroxylated Gd@C82 material was foundto possess a very high efficiency of inhibiting cancer growth invivo, and to have a strong capacity to improve immunity andinterfere with tumor invasion in normal muscle cells. Unlikeconventional anticancer chemicals that use a high toxicity tokill cells, this material shows non-toxicity in vivo and in vitroand does not kill normal cells directly.[9,10] This finding indi-cates that this type of material with proper surface modifica-tions may realize the human dream of cancer chemotherapeu-tics of high efficacy but low toxicity.

Although the structural and electronic properties of endo-hedral metallofullerenes have been well investigated,[1] thechemical functionalization of the metallofullerene and itsproperties are not yet well studied and understood.[11,12] Re-cently, a unique electronic structure of Gd@C82 caused by theencapsulation of the Gd atom was theoretically predicted.[13]

How do the electronic properties of the material vary if theC82 nanostructure is further modified with chemical groups?This is an intriguing topic for exploring the functions of a newnanomaterial because the modulation of the electronic prop-

erties of metallic atoms restricted to a nanospace is of signifi-cant and wide interest, though it is especially difficult. Re-cently, it was found that electronic configurations of atomsinside the fullerene cage can be tuned via chemical modifica-tions to the cage surface.[14] But, on the chemical modifica-tions of metallofullerenes it was found theoretically that i) thehighest occupied molecular orbital (HOMO) in M@C82 tendsto distribute locally,[13] ii) the addition locations prefer to initi-ate at the cage surface opposite to the metallic position,[15]

and iii) the addition pattern on the hollow fullerene cagetends to array as a cluster which shifts with the increasingnumber of added groups.[16] These raise many intriguing ques-tions, for example, how do the electronic properties of themodified material vary with the changing number of addedgroups to the outer surface? To this end, we synthesized andpurified the Gd@C82 and Gd@C82(OH)x materials with achanging number of hydroxyls. The electronic properties ofthe Gd@C82(OH)x film were then studied using synchrotronradiation photoemission spectroscopy (SRPES) and X-ray ab-sorption spectroscopy (SRXAS). Surprisingly, when the OHgroups reach a certain number in Gd@C82(OH)x, the electronemission of the innermost Gd shows a periodical emergenceor disappearance, depending on how many hydroxyls areadded to the outer surface of the fullerene cages. Such aunique phenomenon is observed for the first time. The resultssuggest that polyhydroxylation of metallofullerene may be anew way for designing materials with novel electronic, optical,or magnetic functions.

Figure 1(A1–D1) shows results from the Gd valence bandphotoemission spectroscopy (PES) of Gd@C82 andGd@C82(OH)x films obtained with an incident photon energyof 140.0 eV. In Gd@C82 the 31.4 eV energy level is not ob-served but it emerges in Gd@C82(OH)12. Surprisingly, thisphotoemission property vanishes in Gd@C82(OH)20 and ap-pears again in Gd@C82(OH)26. The energy level periodicallyappears or disappears with the changing number of OH groupsadded to the fullerene-cage surface. To identify the 31.4 eVpeak, we investigated the energy dependence of the valenceband PES for Gd@C82 and Gd@C82(OH)x, the results are giv-en in Figure 2. The PES spectra (measured and normalized bythe beam flux of the synchrotron radiation) of Gd@C82(OH)20

are similar to those of Gd@C82, while Gd@C82(OH)26 andGd@C82(OH)12 show similar spectra. So only the spectra forGd@C82 and Gd@C82(OH)12 are shown in Figure 2 as repre-sentative results. The spectra have less dependence on the inci-dent photon energy, excluding the possibility that the observed

CO

MM

UN

ICATI

ON

S

1458 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18, 1458–1462

–[*] Prof. Y. Zhao, Dr. J. Tang, Dr. G. Xing, L. Jing, Dr. X. Gao, Y. Cheng,

H. Yuan, F. Zhao, Z. Chen, H. Meng, Dr. H. ZhangLab for Bio-Environmental Health Sciences of Nanoscale MaterialsInstitute of High Energy Physics, The Chinese Academy of SciencesBeijing 100049 (P.R. China)E-mail: zhaoyuliang@ihep.ac.cnProf. Y. ZhaoNational Center for Nanoscience and NanotechnologyBeijing 100080 (P.R. China)Dr. J. Tang, Dr. X. Gao, Z. Chen, H. Meng, Dr. H. ZhangGraduate University of Chinese Academy of SciencesBeijing 100049 (P.R. China)Dr. H. Qian, R. Su, Prof. K. IbrahimBeijing Synchrotron Radiation Facility, Institute of High Energy PhysicsThe Chinese Academy of SciencesBeijing 100049 (P.R. China)

[**] The authors acknowledge funding from the Chinese NationalNatural Science Foundation (10490180 and 20571076), the Nation-al Science Fund for Distinguished Young Scholars (10525524), theMinistry of Science and Technology (2005CB724703), and the majordirection program of the Chinese Academy of Sciences (KJCX-SW-H12). Part of this work was carried out at the Beijing synchrotron ra-diation facility.

peak was from an Auger or resonance process. The Gd5p mul-tiplets around binding energy Eb = 31.4 eV were reported inthe valence band PES spectra of GdVO4,[17] GdCu,[18] andGdCu2.[19] This mulitplet shifts to a higher binding energywhen the environmental potential becomes higher due to thecharge transfer from Gd to a neighboring “unlike” atom. Thisis similar with the observations of the Eb = 31.4 eV peak in thepresent materials. Here, of the most interest is how theGd5p1/2 level appears or disappears periodically.

To understand how the outer chemical modifications domi-nate the periodical variation of electronic properties of theGd atom in Gd@C82(OH)x materials, we first need to knowthe electronic characters of the unmodified materials. InLa@C82, the 5p states are known to mix to a considerable ex-tent with the lowest r state of the C82 cage,[20,21] while inGd@C82, Gd bonds more tightly with the cage.[20] As the out-ermost levels beneath the valence level, the Gd5p level can besensitively influenced when it mixes with the r state of C82

due to their close energy levels. When OH is added to the C82

surface, the effects of interactions between the OH and thecage can directly feed back to the innermost metallic atomsvia the electronic interactions proposed in the literature.[14]

More importantly, the polyhydroxyl addition of the fullereneis an electrophilic process.[22,23] The unsaturated bonds of thefullerene cage first react with oxygen and form a transitionstate of C�O carboxyl groups (or epoxide) which are furtherhydrolyzed to yield C–OH groups.[24–27] In 4f elements, wherea strong exchange interaction between 4f and 5p levels occurs,it causes the 5p level to split into 5p1/2 and 5p3/2.[28] InGd@C82(OH)12 (Fig. 1, B2), the OH serves as the electrondonor due to the conjugation effect of the sp2 hybrid of thecage. The electronic donation from OH to C82 leads to a di-rect back-donation of electrons from C to Gd through themodel described in the literature.[14] This probably induces astrong exchange interaction between the 4f and 5p levels andcauses the level splitting with the emergence of the 5p1/2 ener-gy level. In Gd@C82(OH)20 (Fig. 1, C2), however, the conju-gation system of the C82 cage may be locally destroyed by thefurther addition of OH groups, whereby some “phenyl” struc-tures are converted into “alcohol” structures. Compared withGd@C82(OH)12, the consequence of the “phenyl”→“alcohol”structure shift in Gd@C82(OH)20 naturally counteracts theelectronic interaction between the cage and Gd and lowersthe interaction intensity between the 5p and 4f levels of Gd.Further, because nearly half of the total sp2 hybrid bonds havebeen occupied in Gd@C82(OH)26 (Fig. 1, D2), the conjuga-tion system of the fullerene cage can be more heavily or en-tirely destroyed by the additional OH groups. Thus, all theOH groups become electron acceptors relative to the carboncage. The enhanced intensity of electron donation from cageto oxygen in Gd@C82(OH)26 consequently enhances the elec-tronic interactions between the cage and the Gd atom, leadingto strong exchange interactions between the 4f and 5p levelsand the emergence of the 5p1/2 energy level again (Fig. 1, D1).

If the periodical variation observed in electronic properties isdue to the chemical-modification-induced variations in thestrong interactions between the Gd5p and 4f levels in theGd@C82(OH)x material, a similar effect should be observedfrom other levels of Gd such as the 4f electrons, or from otherelements such as oxygen or carbon. As indicated by the arrowsin Figure 1 (B1, D1), the 4f level (Eb ≈ 11 eV) emissions wereenhanced in Gd@C82(OH)12 and Gd@C82(OH)26 but not inGd@C82 and Gd@C82(OH)20. The peaks in the former two sam-ples are explicitly resolved although the Gd4f level is somewhatsubmerged by the broadened distribution of the C r band and

CO

MM

UN

ICATIO

NS

Adv. Mater. 2006, 18, 1458–1462 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 1459

(A2)

(B2)

(C2)

(D2)

G d @ C8 2

(O H )2 6

G d 5 p

-1 0 0 1 0 2 0 3 0 4 0 5 0 6 0

B in d in g E n e rg y (e V )

G d @ C8 2

G d @ C8 2

(O H )2 0

G d @ C8 2

(O H )1 2

(A1)

(B1)

(C1)

(D1)

Figure 1. The valence band PES of Gd@C82 (A1), Gd@C82(OH)12 (B1),Gd@C82(OH)20 (C1), Gd@C82(OH)26 (D1) with the incident photon en-ergy 140.0 eV. A2–D2 show schematic drawings for the hydroxyl addi-tions. The periodical emergence or disappearance of the energy level at31.4 eV is indicated by a dotted line.

-10 0 10 20 30 40 50 60B i n d i n g E n e r g y ( e V )

160

156.6on-resonance

150.0

146.6

145

143

140

G d @C 8 2(a)

-10 0 10 20 30 40 50 60

B i n d i n g E n e r g y ( e V )

140

143

145

on-resonance149.6

160

173

Gd @C 8 2

( OH )1 2

110

(b)

Figure 2. Dependence of the PES of the valence band of Gd@C82 (a) andGd@C82(OH)12 (b) on the incident photon energy.

the O2p peaks (indicated by asterisks in Fig. 1). The 4f emissionnature correlates with the periodical variation of the 5p elec-trons. Correspondingly, the widths of the peaks distributedaround 1–14 eV (Fig. 1) also exhibit an identically periodicalvariation with the changing hydroxyl number in Gd@C82(OH)x.

SRXAS is an efficient technique to investigate the electron-ic structure of materials since the fine structure of these ab-sorption-type spectra clearly reflects the unoccupied state ofselected elements. Thus the electronic interactions betweenthe encaged Gd and the C82 cage can be further confirmedfrom XAS for Gd4d, C1s, and O1s and the results are given inFigures 3–5, respectively (they are normalized by the base-

line). The periodical property exists in the atoms along theinteraction route: [outer modification group]→[nanosheaths]→[inner metallic atom]. The intensities in O1s(Fig. 5), C1s (Fig. 4), and Gd4d (Fig. 3) vary periodically,which is similar to the periodical variation observed from theGd5p and 4f electrons (Fig. 1). As the full width at half maxi-ma (FWHM) of the XAS peaks reflect the degeneracy of theenergy levels, the varying features of the O1s p* FWHM(Fig. 5D) and O1s r* FWHM (Fig. 5E) hence provide us withimportant information to testify the above processes. The data

CO

MM

UN

ICATI

ON

S

1460 www.advmat.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18, 1458–1462

1 .0

1 .1

1 .2 G d@ C82

(O H )26

(D)

1 .0

1 .1

1 .2

1 .3

1 .4

1 .5G d @ C

8 2(O H )

2 0(C)

Re

lative Inte

nsity

1 .0

1 .1

1 .2

1 .3

G d @ C8 2

(O H )1 2

(B)

1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0

1 .0

1 .2

1 .4

1 .6 G d @ C8 2

P h o to n E n e rg y (e V )

(A)

8D7/2

6D5/2

8PJ

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

Rela

tive I

nte

nsity

8D

7/2 at 144.2eV

6D

5/2 at 146.4eV

8P

J at ~150eV

Gd@

C82 (O

H)20

Gd@

C82 (O

H)20

Gd@

C82 (O

H)12

Gd@

C82

(E)

Figure 3. The XAS spectra of the Gd 4d→4f transitions for Gd@C82 (A),Gd@C82(OH)12 (B), Gd@C82(OH)20 (C), and Gd@C82(OH)26 (D). Thevariation of the relative intensities of the Gd 4d→4f transitions are givenin (E). They all show a periodical variation with the hydroxyl number inGd@C82(OH)x.

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Re

lative I

nte

nsity

π∗ at 284.4eV

π∗ at 288.4eV

σ∗at ~294.0eV

Gd@

C82

Gd@

C82 (O

H)12

Gd@

C82 (O

H)20

Gd@

C82 (O

H)26

(E)

2 6 0 2 7 0 2 8 0 2 9 0 3 0 0 3 1 0 3 2 0 3 3 0

1 .2

1 .6

2 .0

2 .4 G d @ C8 2

P h o to n E n e rg y (e V )

1s→π * 1s→σ*

(A)

1 .01 .52 .02 .53 .03 .54 .04 .55 .0 G d @ C

8 2(O H )

2 0(C)

1 .0

1 .2

1 .4

1 .6

1 .8 G d @ C8 2

(O H )1 2

(B)

1 .2

1 .6

2 .0

2 .4

2 .8 G d@ C82

(O H )2 6(D)

Re

lative Inte

nsity

Figure 4. The C1s absorption spectra for Gd@C82 (A), Gd@C82(OH)12

(B), Gd@C82(OH)20 (C), and Gd@C82(OH)26 (D). The periodical varia-tions of the relative intensities of C1s p* electronic absorptions at 284.4and 288.4 eV, and C1s r* electronic absorption at 293.0 eV in differentlymodified materials Gd@C82, Gd@C82(OH)12, Gd@C82(OH)20, andGd@C82(OH)26 are given in (E).

were analyzed using a Gaussian fitting process by searchingthe distributions that mostly fit the observed curve of the leftside without linear overlap for the peak until the baselines ofthe peaks were consistent. The FWHM of the O1s r* and p*absorptions show a similar variation to the Gd5p electrons. InGd@C82(OH)12, because of the lower coverage, the OHgroups prefer to loosely organize on the cage surface awayfrom the Gd vicinal region due to the HOMO distribution,leading to the electron donation of OH. With more hydroxyls,the OH groups are more densely added to the cage surface.Thus the conjugation between OH and the fullerene cage isgradually destroyed and some of the OH groups become elec-tron acceptors, e.g., in Gd@C82(OH)20. When even more hy-droxyls are added to the cage with increasing hydroxyl cover-age, more and more OH groups become electron acceptors asis the case in Gd@C82(OH)26. The narrower FWHMs of thep* and r* absorptions indicate a higher degeneracy of the en-ergy levels. This requires an intensive electronic interactionamong the OH groups whose energy levels should be as closeas possible. The hydroxyls in Gd@C82(OH)12 are electron do-nating groups, while in Gd@C82(OH)26 they become electronaccepting. Accordingly, the distribution of hydroxyls on thesurface of molecules in Gd@C82(OH)12 and Gd@C82(OH)26

leads to an easier reconstruction of the molecular orbits of theoxygen molecules, leading to a higher level degeneracy. Thisis why the O p* and r* transitions in Gd@C82(OH)12 andGd@C82(OH)26 show narrower FWHMs. However, as twokinds of OH groups, i.e., electron donating and electron ac-cepting groups, exist in Gd@C82(OH)20, the energy levels ofthe oxygen orbitals of the hydroxyls in Gd@C82(OH)20 have a

lower degeneracy and hence a widerFWHM as observed in Figure 5.

Metallofullerene is known as a strongelectron donor as well as an electron accep-tor compared to hollow fullerene.[29,30] In apristine Gd@C82 molecule, due to the elec-tron donation from the Gd atom, theHOMO orbitals distribute mainly on theopposite position of the C82 cage relative tothe Gd location,[13] which directly influ-ences the outer chemical group distribu-tions of electrophilic additions.[31] The pres-ent observations revealed that the directionand intensity of electronic interactionsalong outer chemical groups, the carboncage, and the Gd5p and 4f levels can becontrollably modulated, and their synergis-tic effects result in novel properties, i.e., aspringlike nature in the electronic proper-ties of these functionalized materials(Fig. 6).

In summary, polyhydroxylation of metal-lofullerenes was used to control the elec-tronic properties of the material. A Gd@C82

fullerene was chemically modified to obtainGd@C82(OH)x with OH numbers of 0,

12 ± 2, 20 ± 2, and 26 ± 2, and their electronic properties were in-vestigated using SRPES and SRXAS techniques. The periodi-cal variation in electronic properties of the Gd4f, 4d, and 5plevels were observed for the first time when different chemicalmodifications were applied to the C82 cage surface. The Gd5plevel splitting occurred in connection with an enhanced inten-sity of the 4f level photoemission. The results indicate that boththe direction and intensity of electron transfer between the in-nermost Gd and the outer carbon cage can be controlled by thepolyhydroxylation process. The synergistic effect efficientlymodulated the strong exchange interactions or coupling be-tween different electron levels of innermost metallic atoms,leading to a periodical occurrence of energy level splitting,and, consequently, novel electronic properties in the 4f atom of

CO

MM

UN

ICATIO

NS

Adv. Mater. 2006, 18, 1458–1462 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 1461

520 530 540 550 560

1.0

1.2

1.4

1.6

1.8

2.0

2.2Gd@C

82(OH)

12

Photon Energy(eV)

1.0

1.2

1.4

1.6

1.8 Gd@C82

(OH)26

1.0

1.1

1.2

1.3Gd@C

82(OH)

20

π* σ*

π* σ*

π*σ*

(C)

(B)

(A)

1.7

1.8π∗

Gd@

C82 (O

H)26

Gd@

C82 (O

H)20

Gd@

C82 (O

H)12

peak

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

σ∗peak

FW

HM

(eV

)

(D)

(E)

Figure 5. The O1s absorption spectra ofGd@C82(OH)12 (A), Gd@C82(OH)20 (B), andGd@C82(OH)26 (C), and the variation of thefull width at half maximum (FWHM) of p*(D) and r* (E) of O1s.

Figure 6. The electronic exchange pattern along the C82 cage, Gd5p, and4f levels. The electronic interactions between the Gd atom and the C82

cage are springlike and can be periodically modulated by polyhydroxyl ad-ditions.

the materials were found. The results indicate that polyhydrox-ylation of metallofullerenes can be a new way for creating de-sired materials with novel electronic, optical, or magnetic func-tions.

Experimental

The procedures for the preparation and characterization ofGd@C82 and Gd@C82(OH)x and for photoemission and XAS experi-ments have been described in our previous papers [14,32]. The metal-lofullerenes were synthesized using the arc discharge method [33].Separation and isolation of Gd@C82 were performed using high-per-formance liquid chromatography (HPLC, LC908-C60, Japan Analyti-cal Industry Co.) coupled with 5PBB and then Buckyprep columns(Nacalai Co. Japan, 20 mm × 250 mm). Matrix-assisted laser desorp-tion time-of-flight mass spectrometry (MADLI-TOF-MS, AutoFlex,Bruker Co., Germany) measurements show that the purity of the finalGd@C82 product was about 99.9 %. The synthesis method for water-soluble Gd-fullerenols was an alkaline reaction at room temperature[6,34]. The Gd@C82 toluene solution was first mixed with an aqueoussolution containing 50 % NaOH. Several drops of catalyst of 40 %TBAH (tetrabutylammonium hydroxide) were then added into the re-action system. The solution in the beaker changed from an originallydark yellow color into a colorless state, meanwhile a brown sludgeprecipitated to the bottom of the beaker. After adding more waterinto the brown sludge, it was stirred over night. The brown precipitatewas washed using MeOH to completely remove the remnant TBAHand NaOH and was then precipitated by vacuum evaporation. Thebrown precipitate was dissolved into deionized water by continuousstirring for 24 h until the color of the solution became a clear reddishbrown. It was then purified by column chromatography (SephadexG-25) (5 cm × 100 cm) with an eluent of neutralized water. The re-maining trace catalyst and Na+ ions were completely removed in thisprocess. To obtain a final Gd-metallofullerenol product with a narrowregion of distribution of the hydroxyl number, the fraction (eluate)was collected in a time interval of only several minutes. XPS measure-ment combined with elemental analysis was used to measure the num-ber of hydroxyl groups. The XPS experiments were performed at thephotoelectron station of the Beijing Synchrotron Radiation Facility,the Chinese Academy of Sciences. The high-purity platinum substratefor the XPS samples was prepared using magnetron and ion sputter-ing. Then Gd@C82 and Gd@C82(OH)x samples were deposited ontothe high-purity platinum substrates to obtain thin films for the XPSmeasurements, which were carried out in an ultravacuum chamberwith a background pressure of ca. 8 × 10–10 Torr (1 Torr ≈ 133 Pa). X-ray photoemission spectroscopy of metallofullerenol C1s was used todetermine the number of hydroxyl groups. PES of the valence band ofthe samples were acquired with a series of incident photon energies.The XAS of samples were obtained by collecting the partial electronyields. The energy resolution was ca. 0.5 eV. To inspect the contami-nation, XPS survey scans on the sample surface were performed be-fore and after each measurement.

Received: January 10, 2006Published online: April 26, 2006

–[1] H. Shinohara, Rep. Prog. Phys. 2000, 63, 843.[2] R. Klingeler, G. Kann, I. Wirth, S. Eisebitt, P. S. Bechthold, M. Neeb,

W. Eberhardt, J. Chem. Phys. 2001, 115, 7215.[3] J. Park, A. N. Pasupathy, J. I. Goldsmith, C. Chang, Y. Yaish, J. R.

Petta, M. Rinkoski, J. P. Sethna, H. D. Abruna, P. L. Mc-Euen, D. C.Ralph, Nature 2002, 417, 722.

[4] D. W. Cagle, T. P. Thrash, M. Alford, L. P. F. Chibante, G. J. Ehr-hardt, L. J. Wilson, J. Am. Chem. Soc. 1996, 118, 8043.

[5] D. W. Cagle, S. J. Kennel, S. Mirzadeh, J. M. Alford, L. J. Wilson,Proc. Natl. Acad. Sci. USA 1999, 96, 5182.

[6] M. Mikawa, H. Kato, M. Okumura, M. Narazaki, Y. Kanazawa,N. Miwa, H. Shinohara, Bioconjugate Chem. 2001, 12, 510.

[7] R. D. Bolskar, A. F. Benedetto, L. O. Husebo, R. E. Price, E. F.Jackson, S. Wallace, L. J. Wilson, J. M. Alford, J. Am. Chem. Soc.2003, 126, 5471.

[8] L. Qu, Y. L. Zhao, Z. L. Chen, X. F. Gao, Z. F. Chai, J. Zhang,G. M. Xing, Nucl. Phys. A 2004, 738, 459.

[9] C. Y. Chen, G. M. Xing, J. X. Wang, Y. L. Zhao, B. Li, J. Tang, G. Jia,T. C. Wang, J. Sun, L. Xing, H. Yuan, Y. X. Gao, H. Meng, Z. Chen,F. Zhao, C. Z. Fang, X. H. Fang, Nano Lett. 2005, 5, 2050.

[10] J. X. Wang, C. Y. Chen, B. Li, H. W. Yu, Y. L. Zhao, J. Sun, Y. F. Li,G. M. Xing, H. Yuan, J. Tang, Z. Chen, H. Meng, Y. X. Gao, C. Ye,Z. Chai, C. F. Zhu, B. C. Ma, X. H. Fang, L. J. Wan, Biochem. Phar-macol. 2006, 71, 872.

[11] T. Akasaka, T. Kato, K. Kobayashi, S. Nagase, K. Yamamoto, H. Fu-nasaka, T. Takahashi, Nature 1995, 374, 600.

[12] N. Tagmatarchis, A. Taninaka, H. Shinohara, Chem. Phys. Lett. 2002,355, 226.

[13] L. Senapati, J. Schrier, K. B. Whaley, Nano Lett. 2004, 4, 2073.[14] J. Tang, G. M. Xing, H. Yuan, W. B. Cao, L. Jing, X. F. Gao, L. Qu,

Y. Cheng, C. Ye, Y. L. Zhao, Z. F. Chai, K. Ibrahim, H. J. Qian,R. Su, J. Phys. Chem. B 2005, 109, 8779.

[15] I. E. Kareev, S. F. Lebedkin, V. P. Bubnov, E. B. Yagubskii, I. N.Ioffe, P. A. Khavrel, I. V. Kuvychko, S. H. Strauss, O. V. Boltalina,Angew. Chem. 2005, 117, 1880.

[16] J. G. Rodríguez-Zavala, R. A. Guirado-López, Phys. Rev. B 2004,69, 075 411.

[17] F. W. Kutzler, D. E. Ellis, D. J. Lam, B. W. Veal, A. P. Paulikas, A. T.Aldred, V. A. Gubanov, Phys. Rev. B 1984, 29, 1008.

[18] J. Szade, I. Karla, D. Gravel, M. Neumann, J. Alloys Compd. 1999,286, 153.

[19] J. Szade, G. Skorek, M. Neumann, B. Schneider, F. Fangmeyer,M. Matteucci, G. Paolucci, A. Goldni, Surf. Sci. 2002, 497, 29.

[20] E. Nishibori, K. Iwata, M. Sakata, Phys. Rev. B 2004, 69, 113 412.[21] D. M. Poirier, M. Knupfer, J. H. Weaver, W. Andreoni, K. Laasonen,

M. Parrinello, D. S. Bethune, K. Kikuchi, Y. Achiba, Phys. Rev. B1994, 49, 174 03.

[22] B. C. Wang, L. Chen, Y. M. Chou, J. Mol. Struct. (Theochem) 1998,422, 153.

[23] M. S. Meier, J. Kiegiel, Org. Lett. 2001, 3, 1717.[24] A. Naim, P. B. Shevlin, Tetrahedron Lett. 1992, 33, 7097.[25] D. Sun, H. Huang, S. Yang, Z. L. S. Liu, Chem. Mater. 1999, 11,

1003.[26] K. A. Gonzalez, L. J. Wilson, W. Wu, G. H. Nancollas, Bioorg. Med.

Chem. 2002, 10, 1991.[27] J. O. Escobedo, A. E. Frey, R. M. Strongin, Tetrahedron Lett. 2002,

43, 6117.[28] E. Vescovo, O. Rader, G. van der Laan, C. Carbone, Phys. Rev. B

1997, 56, 11 403.[29] Y. Maeda, J. Miyashita, T. Hasegawa, T. Wakahara, T. Tsuchiya,

L. Feng, Y. F. Lian, T. Akasaka, K. Kobayashi, S. Nagase, M. Kako,K. Yamamoto, K. M. Kadish, J. Am. Chem. Soc. 2005, 127, 2143.

[30] T. Suzuki, Y. Maruyama, T. Kato, K. Kikuchi, Y. Achiba, J. Am.Chem. Soc. 1993, 115, 11 006.

[31] A. Hirsch, Top. Curr. Chem. 1999, 199, 33.[32] G. M. Xing, J. Zhang, Y. L. Zhao, J. Tang, B. Zhang, X. F. Gao,

H. Yuan, L. Qu, W. B. Cao, Z. F. Chai, K. Ibrahim, R. Su, J. Phys.Chem. B 2004, 108, 11 473.

[33] S. H. Yang, H. J. Huang, Chem. Mater. 2000, 12, 2715.[34] D. T. Ros, M. Prato, Chem. Commun. 1999, 663.

CO

MM

UN

ICATI

ON

S

1462 www.advmat.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18, 1458–1462