Obtaining excellent optical molecules by screening superalkali-doped cyclo[2n]carbons, M3O@C2n (M = Li, Na, and K, n = 5–10)

Abstract

Using cyclo[2n]carbons (C_2n, n = 5–10) that have been experimentally characterized as electron acceptors and superalkali clusters M₃O (M = Li, Na, and K) with excess electrons as the electron source, we designed nonlinear optical (NLO) electrides, M₃O@C_2n. A detailed comparative analysis of the geometric, electronic, and optical properties of different M₃O@C_2n was conducted using the time-dependent density functional theory [TD-(DFT)] combined with wavefunction analysis methods. Due to the charge transfer from M₃O to C_2n, all the complexes studied demonstrated a charge-separated state in the form of M₃O⁺@C_2n⁻, in which the superalkali interacts with cyclocarbon mainly through electrostatic interactions. The isotropic polarizability (α₀) of M₃O@C_2n increased with the atomic number of the alkali metal and size of the cyclocarbon, and Li₃O@C₂₀ was found to possess an exceptionally large first hyperpolarizability (β₀) due to its perfectly planar wrapped configuration. The first hyperpolarizability anisotropy of Li₃O@C₂₀ was examined through the analysis of the hyperpolarizability tensor, offering insights into the intrinsic nature of hyperpolarizability. Electronic excitation studies showed that the absorption spectrum of Li₃O@C₂₀ exhibits a significant red shift relative to that of the pristine C₂₀, with its absorption band covering the entire visible region and being transparent in the deep-ultraviolet region below 200 nm. The hole–electron analysis of crucial excited states deepened the understanding of the electronic excitation dynamics of Li₃O@C₂₀. Conclusively, superalkali doping can serve as a good strategy for constructing novel NLO molecules based on cyclocarbons, and Li₃O@C₂₀ can be considered a potential candidate for deep-ultraviolet NLO materials.