To understand how complexation influences molecular orbitals, which are key characters of molecular electronic structure, we plotted the total density-of-states (TDOS) maps and the partial DOS (PDOS) maps contributed by C 18 and OPP moieties for OPP, C 18 @OPP, and 2C 18 @OPP (Figure ). The PDOS of OPP in the two complexes are almost the same as TDOS of OPP in its free state, and the PDOS of C 18 ring in the two complexes are also very consistent, indicating that there is no obvious orbital mix between host and guest in the process of forming C 18 @OPP and 2C 18 @OPP. It is observed that the highest occupied molecular orbitals (HOMOs) of complexes C 18 @OPP and 2C 18 @OPP are distributed on the entire host molecule OPP, while their lowest unoccupied molecular orbitals (LUMOs) are only localized on the guest C 18 ring(s) (Figure ). It is noteworthy that the frontier molecular orbital (FMO) characteristics of these two complexes are very similar to those reported for C 60 @OPP and C 70 @OPP. The HOMO-LUMO energy gap of C 18 @OPP and 2C 18 @OPP is significantly smaller than that of the free OPP. Further observations revealed that the reduction of the HOMO-LUMO gap almost entirely comes from the decrease of the molecular LUMO after complexation, mainly because C 18 has a much lower LUMO level than OPP, while their HOMO level differs only slightly.