Modulated nonlinear optical responses and charge transfer transition in endohedral fullerene dimers Na@C₆₀C₆₀@F with n -fold covalent bond (n = 1, 2, 5, and 6) and long range ion bond

Isomer structures of four endohedral fullerene dimers Na@C ₆₀C₆₀@F with an n-fold bond (n = 1, 2, 5 and 6) are obtained for the first time by using density functional theory. The n-fold bond depends on the dimeric pattern between endohedral fullerenes Na@C₆₀ and F@C₆₀. The pattern includes point-point (n = 1), side-side (n = 2), and face-face (n = 5, 6) mode. The four structures [1 + 1], [2 + 2], [5 + 5] and [6 + 6] have larger n-fold bond energies, as compared to that of the neutral π-(C₆₀)₂ dimer. In addition, the electronic properties of the endohedral dimers are first investigated. The dimers exhibit a strong nonlinear optical (NLO) response: large static first hyperpolarizabilities. Moreover, the first hyperpolarizability depends on the dimeric pattern. The large first hyperpolarizabilitiy is up to 25 169 au for [5 + 5], which is almost 110 times larger than that of the NaF molecule (228 au). The great enhancement of the first hyperpolarizability relates to the geometric effect of expanding the Na⋯F distance from 1.980 for NaF to 6.355 ̊ for [5 + 5], which provides a novel strategy for enhancing the first hyperpolarizability by altering the molecular structure. It is also found that the crucial charge transfer transition depends on the dimeric pattern, and controlling the transition may be performed by modulating the dimeric pattern. This work may promote the study of new nanomaterials, including high-performance NLO materials, and enrich knowledge of chemical bonds (for example, multifold bond between cages) and long-range interaction between two trapped atoms in different cages.