Obtaining (BN)4C10 with excellent optoelectronic properties by screening boron-nitrogen analogues of cyclo[18]carbon, (BN)nC(18-2n) (n = 1–9)

The geometric structures as well as electronic, nonlinear optical, and photophysical properties of boron-nitrogen (B-N) analogues of cyclo[18]carbon (C18), (BN)nC(18-2n) (n = 1–9), were comprehensively studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) for screening the ring with excellent optoelectronic performances. As the C-C units in the molecules are continuously replaced by B-N units, almost all geometric parameters change monotonically, while almost all electronic properties undergo a turning point at (BN)4C10, suggesting that (BN)4C10 may have special optoelectronic properties. The isotropic polarizabilities of (BN)nC(18-2n) (n = 1–9) decrease sequentially with an increase in the number of B-N units in the molecule, while the total first hyperpolarizabilities as well as the values corresponding to second harmonic generation (SHG) and hyper-Rayleigh scattering (HRS) experiments first increase and then decrease, reaching their respective maximum at (BN)4C10. Taking (BN)4C10 as an example, the anisotropic feature, spatial contribution, and structural origin of (hyper)polarizability were revealed thoroughly by analyzing (hyper)polarizability tensor, (hyper)polarizability density, and (hyper)polarizability decomposition, respectively. The frequency dispersion effect has a positive impact on increasing the response properties of (BN)nC(18-2n) (n = 1–9), and the higher the incident frequency of the applied field, the greater its effect on improving the molecular (hyper)polarizability. The absorption bands of (BN)nC(18-2n) (n = 1–9) lie in the ultraviolet range of 125–400 nm, indicating the transparency of these molecules in the visible light region. The hole-electron analysis of (BN)4C10 graphically and numerically illustrated that the maximum absorption of the molecule comes from localized electron excitation on the C-C moiety, which should also be a common characteristic of all (BN)nC(18-2n) (n = 1–9). The two-level model was used to rationalize the observed variation of the total first hyperpolarizability with replacement of groups from the perspective of electron excitation.