Designing hybrid materials with advanced optical properties using superalkali M3O (M = Li, Na, and K) and isoelectronic species of cyclo[18]carbon (B6C6N6 and B9N9)

Abstract

The geometric, electronic, and optical properties of hybrid materials formed by complexing B₆C₆N₆ and B₉N₉ with superalkali M₃O (M = Li, Na, and K), that is, M₃O@B₆C₆N₆ and M₃O@B₉N₉ (M = Li, Na, and K), were studied systematically using (time-dependent) density functional theory [(TD-) DFT] and wavefunction analysis. The structures of B₆C₆N₆ and B₉N₉ in the complexes exhibit varying degrees of deformation under the influence of M₃O (M = Li, Na, and K). Natural population analysis (NPA) confirms charge transfer from M₃O (M = Li, Na, and K) to B₆C₆N₆ and B₉N₉, and an independent gradient model based on Hirshfeld partition (IGMH) analysis reveals the binding nature of the units. The M₃O@B₆C₆N₆ (M = Li, Na, and K) series exhibit higher polarizabilities (α_iso) than the M₃O@B₉N₉ (M = Li, Na, and K) series, and K₃O@B₆C₆N₆ and K₃O@B₉N₉ have the highest first hyperpolarizabilities (β_tot) in their respective series, comparable to that of the corresponding complex of cyclo[18]carbon (C₁₈). The M₃O@B₆C₆N₆ (M = Li, Na, and K) system remains almost transparent in the visible (Vis) region, but the absorption of M₃O@B₉N₉ (M = Li, Na, and K) gradually covers the Vis region with increasing alkali metal index. These results collectively demonstrate that complexing superalkali with molecular rings represents an effective strategy for designing high-performance nonlinear optical (NLO) materials, and K₃O@B₆C₆N₆ is an excellent candidate for a Vis NLO molecule.