Designing hybrid materials of superalkali-complexed cyclo[18]carbon precursor, M3O@C18Br6 (M = Li, Na, and K), with advanced optical properties

Using stable precursor C18Br6 of cyclo[18]carbon (C18) as an electron acceptor and superalkali clusters M3O (M = Li, Na, and K) as electron sources, we theoretically designed M3O@C18Br6 (M = Li, Na, and K) with potential applications in optical and electronic devices. The geometric, electronic, and optical properties of M3O@C18Br6 were explored in detail by using (time-dependent) density functional theory [(TD-)DFT] combined with wavefunction analysis. About one unit of charge is transferred from M3O to C18Br6 resulting in charge-separated [M3O]+@[C18Br6] electrides, thus stabilizing the complexes by virtue of remarkable electrostatic interaction within the system. The isotropic polarizability (α0) of M3O@C18Br6 gradually increases with the atomic number of alkali metal due to the different abilities of the metal nuclei to bind to surrounding electrons. Li3O@C18Br6 has the first hyperpolarizability (β0) opposite in direction to its Na3O and K3O analogues, with a magnitude more than twice that of them. The essential differences in the β0, including magnitude and orientation, were elucidated through analyses of hyperpolarizability tensor and hyperpolarizability density. Electronic excitation studies showed that the absorption spectra of M3O@C18Br6 exhibit an obvious red-shift compared to that of the pristine C18Br6 and have transparency in the light range with wavelengths greater than 500 nm. Charge transfer spectrum (CTS) and hole-electron analysis revealed the nature of electronic excitation of M3O@C18Br6. The two-level model was used to correlate the response properties with excitation properties, explaining the different first hyperpolarizabilities of the complexes from the perspective of electronic excitation. This research demonstrates that superalkali complexing is an effective strategy for developing new nonlinear optical (NLO) materials in visible and infrared regions based on cyclocarbon analogues.