Constructing (super)alkali–boron-heterofullerene dyads: an effective approach to achieve large first hyperpolarizabilities and high stabilities in M3O–BC59(M = Li, Na and K) and K@n-BC59(n = 5 and 6)

Using DFT methods, the electronic properties and the first hyperpolarizabilities of boron-heterofullerene-(super)alkali dyads: M3O-BC59 (M = Li, Na and K) and K@n-BC59 (n = 5 and 6) were systematically investigated. It is found that both M3O and K can effectively bind to BC59 with high binding energies (2.50-2.69 eV for K and 4.24-5.14 eV for M3O). The interaction between K and BC59 in K@n-BC59 is identified as primarily ionic in nature, whereas that between the superalkali M3O unit and BC59 becomes much stronger owing to the formation of a strong chemical bond (B-O bond). Moreover, compared with the sole parent cluster BC59 (619 au), both K@n-BC59 (n = 5 and 6) and M3O-BC59 (M = Li, Na and K), possess large first hyperpolarizabilities (β0), which are 3352, 2621 and 4921, 5440 and 7800 au, respectively, where the superalkali doped dyads (M3O-BC59) are much superior to the simple alkali exo-hedral species (K@n-BC59), and heavier superalkali can be more powerful in enhancing the β0 values of M3O-BC59. Clearly, these superalkali doped dyads M3O-BC59, formal donor-acceptor (DA) chromophores, exhibit not only excellent stability but also large first hyperpolarizability; therefore, they are expected to be potential candidates for excellent second-order NLO materials.