6. ELECTRONIC STATES AND STRUCTURES
6.4 Similarity of UV–Vis–NIR absorption spectra
Absorption spectra of endohedral metallofullerenes in the ultraviolet–
visible–near IR (UV–Vis–NIR) region are unique as compared with those of empty fullerenes. Normally, the absorption spectra of metallo- fullerenes have long tails to the red down to 1,500 nm or more. The absorption spectra of the major isomers of mono-metallofullerenes M@C82 (M¼Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu) are similar to each other and well represented by a sharp peak around 1,000 nm and a broad peak around 1,400 nm. These absorption peaks may be related to the intrafullerene electron transfers from the encaged metal atom to the carbon cage.
Figure 20 shows UV–Vis–NIR absorption spectra for the isolated group 3 metallofullerenes: La@C82 (Kikuchi et al., 1993; Yamamoto et al., 1994a), Y@C82 (Kikuchi et al., 1994b) and Sc@C82(Inakuma et al., 1995) in carbon disulfide solution. The spectrum of Sc@C82is very differ- ent from those of Y@C82and La@C82, indicating that the electronic struc- ture of Sc@C82is different from those of Y@C82and La@C82. As described in Section 4.3.2, the charge state of Sc2þ@C822 (divalency) is different from those of La3þ@C823and Y3þ@C823(trivalency) (Hino et al., 1993;
Nagase and Kobayashi, 1993; Takata et al., 1995). The UV–Vis–NIR absorption spectra of many mono-metallofullerenes encapsulating lan- thanide elements, Ln@C82(Ln ¼Ce–Nd, Gd–Er, Lu) (Ding et al., 1996b;
Suzuki et al., 1996), Pr (Ding and Yang, 1996; Shinohara et al., 1994b), Nd (Ding et al., 1996a), Gd (Kikuchi et al., 1994c; Shinohara et al., 1994b), Tb (Shi et al., 2003), Dy (Kikuchi et al, 1998; Tagmatarchis and Shinohara, 2000), Ho (Knapp et al., 1998; W Wang et al., 1997), Er (Ito et al., 2007a,b;
Kikuchi et al, 1998), Lu (Knapp et al., 1998)), are similar to those of La@C82
and Y@C82. However, the absorption spectra of the C82based metalloful- lerenes containing the divalent lanthanide elements (i.e., Sm, Eu, Tm, and Yb) (Kikuchi et al., 1997; Kirbach and Dunsch, 1996; Okazaki et al., 2000) are different from those of La@C82, Y@C82and Ln@C82but are similar to
that of Sc@C82in that the sharp absorption peak at 1,000 nm are missing in these divalent metallofullerenes.
UV–Vis–NIR absorption spectra of structural isomers for a metalloful- lerene generally differ from each other. Such differences stem from the fullerene cage (isomer) structure as well as from the difference in charge state of the fullerene. For example, isomers I and II of La@C82(Yamamoto et al., 1994b), Y@C82 (Inakuma et al., 1995) and Sc@C82 (Inakuma and Shinohara, 2000) have different spectral features in their respective absorption spectra (cf. Figure 21).
The absorption spectra of group 2 metallofullerenes are quite different from those of group 3 metallofullerenes. Figure 22 shows the UV–Vis–
NIR absorption spectra of major isomers of Ca@C82 (Dennis and Shinohara, 1997a,b, 1998; Dennis et al., 1998; Nakane et al., 1998;
Xu et al., 1996). This is due to the fact that the charge state of the encaged metal atoms for these group 2 metallofullerenes is 2þrather than 3þ.
Similar to the group 3 case, the absorption spectra of structural isomers of group 2 metallofullerenes also differ from each other. Such an example has been found for the four structural isomers of Ca@C82(I–IV) (Xu et al., 1996) (Figure 22). A similar observation has been reported for the three structural isomers of Tm@C82(A, B, C) (Dunsch et al., 1997; Kirbach and Dunsch, 1996). The absorption spectra of the three isomers (A, B, C) of Tm@C82are almost exactly the same as three structural isomers (III, I, IV) of Ca@C82, respectively, suggesting that each isomer shares the same isomer structure. Furthermore, the absorption spectrum of Ca@C82 (IV)
400 560 720 880 1040 1200 Wavelength (nm)
La@C82(isomer I)
Y@C82(isomer I)
Sc@C82(isomer I)
Absorbance (arb. units)
1360 1520 1680 1840 2000
FIGURE 20 UV–Vis–NIR absorption spectra of La@C82(I), Y@C82(I), and Sc@C82(I).
The spectral feature of Sc@C82is different from those of Y@C82and La@C82.
The absorption spectra of M@C82(M¼Ce, Pr, Gd, Tb, Dy, Ho, Er, and Lu) are essentially the same as that of La@C82.
was found to be similar to that of Sc@C82(I), indicating that the Ca and Sc atoms are trapped within the same structural isomer having the same oxidation state of 2þ.
Judging from the similarity on the absorption spectra, Ln@C82(Ln¼ Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu) metallofullerenes have 3þcharge state similar to La@C82and Y@C82, whereas the charge state of Ln@C82(Ln¼ Sm, Eu, Tm, Yb) is 2þas that of Sc@C82. It is interesting to note that the HPLC retention times of Ln@C82(Ln¼Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu) are similar to each other but are different from those of Ln@C82
(Ln¼Sm, Eu, Tm, Yb) which are also almost the same as each other (Kikuchi et al., 1997; Sueki et al., 1997). Yang and co-workers also made a systematic study on lanthanide metallofullerenes from the standpoint of Vis–NIR absorption spectra (Ding and Yang, 1997) together with their HPLC elution behavior (Huang and Yang, 1988). They found that there is
Sc@C82(II) Sc@C82(I)
×5
×5
2000 1600
1200 Wavelength (nm)
Absorbance (arb. units)
800 400
FIGURE 21 UV–Vis–NIR absorption spectra of isolated Sc@C82(I, II) in CS2solution.
The energy of the onset Sc@C82(II) (<0.95 eV) is much larger than that of Sc@C82(I) (0.62 eV). The overall spectral features are quite different with each other.
a fairly good correlation between the relative HPLC retention time and the charge state (i.e., divalent or trivalent) of encapsulated metal atoms.
According to their results, the four lanthanide atoms Sm, Eu, Tm, and Yb have divalent state whereas the other lanthanide elements (La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu) form a trivalent state in the C82 cage which is consistent with the former results.
Here we are able to derive an empirical rule regarding the relationship between absorption features and the isomer structure of a metallofullerene: ‘‘a UV–Vis–NIR absorption spectrum of a metallofullerene
800 1200 Wavelength (nm)
1600
800 1200 Wavelength (nm)
1600
×3 A Ca@C82(I)
×3 B Ca@C84(I)
×3 Ca@C82(II)
×3 Ca@C84(II)
×3 Ca@C82(III)
×3 C Y@C82(I)
×3 Ca@C82(IV)
FIGURE 22 UV–Vis–NIR absorption spectra of (A) Ca@C82(I–IV), (B) Ca@C84(I, II), and (C) Y@C82(I) for comparison.
(M@C82: M¼metal atom) is very similar to that of another metallofullerene irrespective of the kind of encaged metal atom when the cage (isomer) structure and the charge state of the atoms are the same.’’ This empirical rule can be best understood by molecular excitations of the isomer cage which should not be changed as long as the cage and the filling of the molecular levels are the same.
Fluorescence experiments of endohedral metallofullerenes, on the other hand, have been limited because of their weak emission feature.
IR emissions from Er3þfor Er2@C82 around 1.5mm have been observed (Ding et al., 1997; Hoffman et al., 1997; Ito et al., 2007a,b; Macfarlane et al., 1997; Plant et al., 2009). The emission was ascribed by the characteristic intraconfigurational 4f11 fluorescence of the trivalent Er 4I13/2 ! 4I15/2 transition.