Electronic structure of Eu@C60 studied by XANES and UV-VIS absorption spectra

Article abstract of DOI:10.1016/S0009-2614(99)01309-3

Eu endohedral C₆₀, Eu@C₆₀, has been extracted with aniline from soot prepared by arc-heating of a graphite/Eu₂O₃ composite rod and obtained at high concentration by combining sublimation and repeated high-performance liquid chromatography. The laser desorption time-of-flight mass spectrum showed a pronounced peak of Eu@C₆₀⁺. The UV-VIS absorption spectrum of this sample has a red-shift in the onset (>900 nm) in comparison with those for C₆₀ and C₇₀, as expected from electron transfer from the Eu atom to the C₆₀ cage. The valence state of the Eu atom in Eu@C₆₀ has been determined to be +2 by Eu L_III-edge XANES.

Full text of DOI:10.1016/S0009-2614(99)01309-3

21 January 2000
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Chemical Physics Letters 316 2000 381–386
www.elsevier.nlrlocatercplett
Electronic structure of Eu@C60 studied by XANES and UV–VIS
absorption spectra
a
a
a
b
T. Inoue , Y. Kubozono a,), S. Kashino , Y. Takabayashi , K. Fujitaka ,
b
a
a
c
d
M. Hida , M. Inoue , T. Kanbara , S. Emura , T. Uruga
a
Department of Chemistry, Okayama UniÕersity, Okayama 700-8530, Japan
Department of Mechanical Engineering, Okayama UniÕersity, Okayama 700-8530, Japan
b
c
ISIR, Osaka UniÕersity, Osaka 567-0047, Japan
d
JASRI, Sayo 679-5198, Japan
Received 8 September 1999; in final form 3 November 1999
Abstract
Eu endohedral C60, Eu@C60, has been extracted with aniline from soot prepared by arc-heating of a graphiterEu2O3
composite rod and obtained at high concentration by combining sublimation and repeated high-performance liquid
chromatography. The laser desorption time-of-flight mass spectrum showed a pronounced peak of Eu@Cq60. The UV–VIS
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absorption spectrum of this sample has a red-shift in the onset )900 nm in comparison with those for C60 and C70, as
expected from electron transfer from the Eu atom to the C60 cage. The valence state of the Eu atom in Eu@C60 has been
determined to be q2 by Eu LIII-edge XANES. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction
cations as superconducting or magnetic materials,
because the C60 cage is expected to be charged as a
result of electron transfer from the metal atom. It is
in fact known that electron transfer plays an impor-
tant role in the superconductivity of metal-inter-
calated C60 materials, such as Rb3C60 and K3C60
Encapsulation of an atom into the C60 cage is an
important subject in studies of fullerenes. For exam-
ple, N@C60 and RG@C60 RG: rare gas have been
produced by ion implantation and high pressure
methods, respectively, and their physical properties
have been elucidated 1–5 . The atoms inside the C60
cage in these compounds are electrically neutral, and
the cages have no electronic charge. These com-
pounds have basic importance in the chemistry and
physics of fullerenes. However, metal endohedral
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6 . However, physical and chemical properties of
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M@C60 have rarely been studied in spite of this
potential as new materials, because pure samples
could not be obtained.
Recently, we reported the extraction of some
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M@C60 with aniline 7,8 and the enrichment of
Ca@C60 , Y@C60, Ce@C60, Pr@C60, Gd@C60,
Nd@C60 and Dy@C60 by high-performance liquid
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C60 M@C60, M: alkaline, alkaline-earth, lanthanide
.
metals, etc. seem to have greater potential for appli-
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chromatography HPLC 9–11 . Furthermore, we
reported that the Eu atom in Eu@C60 is inside the
C60 cage on the basis of the Eu LIII-edge XAFS at
)
Corresponding author. Fax: q81-86-251-7853; e-mail:
kubozono@cc.okayama-u.ac.jp
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 99 01309-3

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T. Inoue et al.rChemical Physics Letters 316 2000 381–386
.
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295 K of the soot which contained a large amount of
dual pump with a Buckyclutcher I column Regis:
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Eu@C60 12 . However, no sample of M@C60 com-
pletely separated from other metallofullerenes has
yet been obtained, and the purification of M@C60
has remained a target of our continued efforts. The
present Letter reports on our attempts aimed at the
purification of Eu@C60 by combining sublimation
and HPLC techniques, by which we have confirmed
the valence state of the Eu atom in Eu@C60 .
10 mmf=250 mm using aniline as the eluent. The
flow rate of the eluent was 0.75 ml miny1, as
detected by UV at 330 nm. The compositions of the
samples were checked by a laser desorption time-of-
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flight LD–TOF mass spectrometer at 337 nm Fin-
nigan: Vision 2000 . Eu LIII-edge XANES and XAFS
.
spectra were measured in the fluorescence mode at
BL01B1 of SPring-8 and in the transmission mode at
BL-7C of KEK-PF, respectively. The UV–VIS ab-
sorption spectrum was measured with a Hitachi 228
spectrophotometer.
2. Experimental
Soot-containing Eu@C60 was prepared by arc-
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heating a Eu2O3rgraphite rod Toyo Tanso; Eu con-
3. Results and discussion
.
centration: 0.8 mole% at 27 V and 80 A under a 80
Torr He atmosphere. The soot was sublimed at 4508C
for 2 h under dynamical pumping at 10y6 Torr. The
powder obtained by sublimation was dissolved in
distilled aniline by applying ultrasonic excitation at
0–58C for 2 h, and the aniline solution was filtered
3.1. Characterization of the Eu@C60 sample
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The LD–TOF mass spectrum of the soot Fig. 1a
shows a pronounced peak of Eu@Cq60 and weaker
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peaks attributable to other fullerenes C60 , C70,
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with a 0.2 mm membrane filter Toso: H-25-2 . The
Eu@C70 , Eu@C74 . . . . The fraction of Eu@C60
aniline solution was passed through a semi-prepara-
was higher than those in the soots prepared by
arc-heating of graphite rods containing other metal
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tive HPLC system Toso: UV-8020 detector; CCPS
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HPLC procedure F2 in Fig. 2d . See text for symbols I and II.
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Fig. 1. LD–TOF mass spectra in arbitrary units of: a soot; b sublimate; c aniline extract; and d the fraction collected after the second
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T. Inoue et al.rChemical Physics Letters 316 2000 381–386
383
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oxides 7,8 . The powder obtained by sublimation of
ing that aniline is an effective solvent for the extrac-
q
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.
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the soot at 4508C Fig. 1b shows three peaks of C60,
Cq70 and Eu@Cq60 with a very weak peak correspond-
ing to Eu@Cq74; no peak was observed below mrzs
600. The mass spectrum of the powder obtained by
sublimation below 4008C showed only peaks corre-
sponding to Cq60 and Cq70, while that observed after
sublimation at 500–6008C was identical with that
observed at 4508C. No metallofullerene other than
Eu@C60 was sublimed at temperatures up to 6008C.
These findings show that sublimation above 4508C is
useful to obtain Eu@C60 from the soot by elimina-
tion of the other metallofullerenes. On the other
tion of M@C60 7–11 .
The HPLC profile of the aniline extract Fig. 2a
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shows two peaks and a shoulder at 14–25 min. The
fraction with a very intense peak at 17.5–25 min was
assigned to C60 and C70, because the peaks corre-
sponding to pure C60 and C70 were observed in this
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region Fig. 2b and c . When the fraction from
14.1–15.8 min denoted as F1 in Fig. 2a, which
exhibited a shoulder, was collected and injected again
into the HPLC system, the peaks of C60 and C70
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disappeared Fig. 2d and two peaks were observed
at 12.5–18 min. The essential component of the first
peak around 15 min was assigned to Eu@C60 . The
mass spectrum of the second peak denoted as A
showed peaks corresponding to Cq60, Eu@Cq60 and
impurities originating from aniline. This mass spec-
trum suggests the existence of clusters such as
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hand, empty fullerenes C60 , C70 and higher
fullerenes and metallofullerenes M@C60 , M@C70,
M@C76 , M@C82 and M@C90 were obtained by
sublimation at 4508C from the soots containing Y,
.
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Ce, Nd, Tb and Dy endohedral fullerenes 13 .
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. Ž .
C60 Eu@C60 .
m
The fraction from 12.5–14 min F2 was then
The LD–TOF mass spectrum of the filtered ani-
n
q
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.
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line solution Fig. 1c exhibits three peaks of C60,
Cq70 and Eu@Cq60, and a weaker peak of Eu@Cq74.
The peak intensity of Eu@Cq60 relative to that of Cq70
is about six times higher than that in Fig. 1b, show-
collected to avoid contamination from compounds in
Ž .
the second HPLC peak A . A dominant peak of
Eu@Cq60 was observed with very weak peaks corre-
Ž .
Ž .
Ž .
Ž .
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Ž ..
Fig. 2. HPLC profiles of: a aniline extract; b C60 ; c C70; and d the collected fraction F1 in a . See text for symbols A and F2.

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T. Inoue et al.rChemical Physics Letters 316 2000 381–386
sponding to Cq60 and Cq70 in the mass spectrum of the
3.2. Electronic structure
q
q
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fraction F2 Fig. 1d . The peaks of C60 and C70 were
weaker than 8% of that of Eu@Cq60. Very weak
peaks denoted as I and II were assigned to impurities
produced by laser irradiation. Very recently, Ogawa
et al. reported the purification and isolation of
The Eu@C60 powder sample was obtained by
evaporating the anline from the fraction F2. This
sample, hereafter denoted as S1, was used for the
measurement of the Eu LIII-edge XANES spectrum
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Er@C60 by a similar technique 14,15 .
Fig. 3a . Since Eu LIII-edge XANES is unaffected
Ž .
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Fig. 3. Eu LIII-edge XANES spectra of: a Eu@C60 sample S1; b Eu2O3; and c EuS.

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T. Inoue et al.rChemical Physics Letters 316 2000 381–386
385
by a trace of C60 and C70 in the sample, it can
selectively give information on the electronic proper-
ties of Eu@C60 . The inflection point E0 of the Eu
LIII-edge, which is 6 eV lower than that for Eu2O3
fullerenes can be associated with that of the 4f7 state
as in the 4f0 and 4f14 states.
The UV–VIS absorption spectrum of the aniline
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extract Fig. 4a is nearly identical with that for C60
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. Ž
.
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6979 eV Fig. 3b , is close to that for EuS 6971
Fig. 4b , but it has no clear similarity to that of C70
. Ž
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.
eV Fig. 3c ; the valences of Eu atoms are q3 in
Eu2O3 and q2 in EuS. It is further shown from the
Eu LIII-edge XANES spectra that the E0 for the
Eu2q compound is 7 eV lower than that for the
Fig. 4c . This is consistent with the LD–TOF mass
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spectrum Fig. 1c which exhibits a pronounced peak
corresponding of Cq60. The spectrum of the aniline
solution of the Eu@C60 sample S1 showed a struc-
tureless pattern different from those for C60 and C70
Eu3q compound 16 . We conclude that the valence
of the Eu atom in Eu@C60 is q2. The XANES
peaks are assigned to the 2p3r2–5d transitions of
Eu2q and Eu3q. Recently, Kikuchi et al. concluded
that the valence of the Eu atom in Eu@C82 was q2
from the similarity of UV–VIS–near IR absorption
spectra for Eu@C82 to those for Yb@C82 and
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Fig. 4d but similar to that recently observed for the
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aniline solution of Er@C60 15 .
The onset of the absorption spectrum of the
Eu@C60 sample, S1, shows a clear red-shift. This
red-shift may be attributed to the t1u –t1g transition
due to the electron transfer from the Eu atom to the
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Tm@C82 17 ; Yb and Tm atoms have the valence
C60 cage. Though intense peaks due to the t1u–t1g
state of q2. The stability of Eu2q in Eu endohedral
transition were observed for Cy60–C
18 , no corre-
4y
w
x
60
Ž .
Ž .
Ž .
Ž .
Fig. 4. UV–VIS absorption spectra in arbitrary units of the aniline solutions of: a extract; b C60 ; c C70; and d Eu@C60 sample S1.

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T. Inoue et al.rChemical Physics Letters 316 2000 381–386
sponding peaks were observed for Eu@C60 as in the
istry of Education, Science, Culture and Sports,
Japan.
w
x
case of Er@C60 15 . Consequently, there is no
characteristic feature that makes this spectrum re-
2q
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M
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semble those of M@C82
, Ca@C72 and
2q. w
x
17,19,20 .
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adduct may also be produced by the reaction of
Eu@C60 with aniline in the aniline solution, as in the
case of the photochemical reaction of La@C82 with
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x
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w
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sity of Tokyo, Professor Tunehiro Tanaka of Kyoto
University and Professor Takeshi Akasaka of Niigata
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