Effect of (super) alkali doping and boron substitution on the nonlinear optical property of biphenalenyl diradical π dimer: A theoretical study

A series of novel biphenalenyl diradical π dimer have been theoretically designed via linking the (super) alkali to the biphenalenyl diradical π dimer with boron substitution. Firstly, we designed three molecules, Li-BC25H18 (1), Na-BC25H18 (2), K-BC25H18 (3). It is revealed that energy gaps (Egap) between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all molecules are in a range of 1.321-1.350 ev. The results indicate that the first hyperpolarizabilities of M-π dimer (M = Li, Na, K) increase with alkali atom number. Potassium atom can be more powerful in increasing the first hyperpolarizabilities of M-π dimer. For M3O-π dimer, they have structural isomers, LiO3-BC25H18 (1A, 1B), NaO3-BC25H18 (2A, 3B), KO3-BC25H18 (3A, 3B). Compared with M-BC25H18 (M = Li, Na, K), M3O-BC25H18 (M = Li, Na, K) exhibit a comparable electronic stability except 2A and 3A. Furthermore, introducing superalkali unit M3O as the electron source can lead to larger nonlinear optical response. The results show that M3O-π dimer exhibit the larger first hyperpolarizability (1616-11,902 au). Therefore, M3O-π dimer are expected to be potential candidates for NLO materials. We hope that this study could provide a new idea for designing nonlinear optical materials using diradical π dimer.