Linear π-conjugated units of the D_∞h point group as superior ultraviolet birefringent units

At present, birefringent materials face a limited selection of large structural anisotropic functional modules (FMs). In this paper, we present a series of linear units which belong to the D_∞h point group represented by (BO₂)^- proposed as novel birefringent active FMs. By analyzing the molecular orbital of the (BO₂)^- unit, it is found that there are relatively fewer non-bonding orbitals in (BO₂)^- than in (BO₃)^3- and the delocalized π bonds in (BO₂)^- appear in shallow energy levels, which are easily excited. Through first-principles modeling and simulation, it is found that the delocalized π bonds in (BO₂)^- can still show obvious transition processes, which produce a significant gain to the birefringence. Besides, a series of compounds containing linear anionic frameworks which also belong to the D_∞h point group show excellent optical anisotropy in the same way. Therefore, the linear anionic basic units which belong to the D_∞h point group have the great potential to become new birefringent FMs.