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@C60C60@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...