The excitation energies and oscillator strengths of the first 100 low-lying excited states of OPP, C 18 @OPP, and 2C 18 @OPP were calculated, and the simulated UV-Vis absorption spectra were plotted accordingly (Figure ). The results of vertical excitation show that the only absorption of molecule OPP centered at 317.6 nm is reasonably consistent with the experimental observation (342.0 nm). A slight red-shift in absorption bands is observed with a significant reduction in absorption intensity after the complexation of host and guest molecules. Importantly, the electronic absorption spectra of complexes C 18 @OPP and 2C 18 @OPP with different numbers of guest C 18 show distinctive characteristics, that is, the maximum absorption peak of 2C 18 @OPP with one more guest is red-shifted more and its oscillator strength is weaker than that of C 18 @OPP. By utilizing our proposed charge-transfer spectrum (CTS) analysis method (see Supplementary Material of Ref. [34] for details), the contributions of intrafragment electron redistributions within OPP and C 18 moieties as well as interfragment charge transfers between OPP and C 18 to the UV-Vis absorption of C 18 @OPP and 2C 18 @OPP were evaluated (Figure ). As can be seen, the absorption spectra of the two complexes are almost entirely derived from the electron redistribution within OPP and the charge transfer from OPP to C 18 . So, the C 18 plays a certain role in the electron excitation of the complexes, which is the fundamental reason why complexation makes the UV-Vis absorption spectrum of the system change notably.