Alkali-metal ions (M+ = Li+, Na+, K+, Rb+, and Cs+) endohedral cyclo[18]carbon (C18): Exploring binding interactions and predicting optical properties

With growing interest in carbon-based materials for energy storage and active research in the field of advanced optoelectronic devices, we theoretically designed ten complexes by cyclo[18]carbon (C18) inside and outside complexing alkali-metal ions (M+ = Li+, Na+, K+, Rb+, and Cs+), respectively referred to as M+@C18in and M+@C18out, and performed careful analyses of binding interaction between M+ and C18 as well as optical properties of stable endohedral complexes M+@C18in. The effects of atomic number of alkali-metals on electronic structures, binding interactions, electronic absorption spectra, and molecular (hyper)polarizabilities of the M+@C18in were studied using accurate density functional theory (DFT) calculations. The research results indicated that the differences in radius and properties of M+ lead to different binding modes and strengths with C18, but there is no difference in electronic absorption spectra between the complexes; the polarizability and second hyperpolarizability of M+@C18in containing different alkali-metal ions are similar due to their analogous electronic structures, but their first hyperpolarizability differ greatly due to discrepancies in molecular symmetry. The similarities and differences in intramolecular interactions, electronic absorption spectra, and (hyper)polarizability of M+@C18in were explored using advanced wavefunction analysis methods.