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

Abstract

Using cyclo[2n]carbon (C2n, n = 5-10) that have been experimentally characterized as electron acceptor and superalkali clusters M3O (M = Li, Na, and K) with excess electrons as electron source, we designed nonlinear optical (NLO) electrides M3O@C2n. The detailed comparative analysis on geometric, electronic, and optical properties of different M3O@C2n were conducted using (time-dependent) density functional theory [TD-(DFT)] combined with wavefunction analysis methods. Due to the charge transfer from M3O to C2n, the complexes studied all display a charge-separated state in the form of M3O+@C2n-, in which the superalkali interact with the cyclocarbon mainly through electrostatic interaction. The isotropic polarizability (α0) of the M3O@C2n increases with the atomic number of alkali metal and the size of cyclocarbon, and the Li3O@C20 was found to possess the 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 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 electronic excitation dynamics of Li3O@C20. Generally, superalkali doping can serve as a good strategy for constructing novel NLO molecules based on cyclocarbons, and Li3O@C20 can be considered as a potential candidate for deep-ultraviolet NLO materials.