The M + @C18 in exhibit characteristic absorption in the wavelength range of about 170 nm to 260 nm, consisting of a super strong peak at 219 nm followed by an extremely weak absorption at 191 nm. It is obvious that M + doped C18 not only have virtually identical electronic absorption spectra with each other, but their absorption characteristics are also not distinguishably different from that of pristine C18, that is to say, the electronic absorption spectra of the complexes M + @C18 in are almost independent of the species or even the existence of the M + . This observation can be rationalized by the results of NPA charge analysis described above that there is not much charge transfer between M + and C18 as well as charge transfer spectrum (CTS) depicted below that the electronic absorption spectra of M + @C18 in are almost entirely contributed by the C18 moiety. So obviously, the M + @C18 in should still look colorless, just like the pristine C18. The concept of charge-transfer spectrum (CTS) proposed by us is helpful to understand the nature of electron excitation from the perspective of the contribution of electronic transitions within and between fragments to entirety. 32 By using this approach, the contributions of electron rearrangement and charge transfer to the absorption spectra of M + @C18 in are evaluated and plotted in Figure (a)-(e). As can be seen, almost the entire optical absorption of the M + @C18 in is assigned to be an electron excitation within C18 moiety, as the red curves representing the electron redistribution in C18 ring is extremely similar to the absorption spectrum of the complexes shown in Figure . This is precisely why the electronic absorption spectra of M + @C18 in are independent of the metal ion they contain. Furthermore, the electron transfers of M + →C18 and M + ←C18 contribute to the same and negligible remaining portion of the electronic absorption spectrum of M + @C18 in . As shown as an example in Figure , the maximum absorptions of Li + @C18 in originated from the degenerate electron excitation of S0→S25 and S0→S26 involve many MOs, and there is no specific orbital pair plays a dominate role. Other alkali-metal ions endohedral complexes also exhibit the same excitation characteristics, making it difficult to describe the electron excitation by means of MO picture. In this case, the hole-electron analysis method developed by us can more conveniently provide the characteristics of electron transitions. The isosurface maps of hole and electron distributions for the excited states involved in maximum absorption of M + @C18 in are given in Figure Green and blue regions denote the hole and electron distributions, respectively.