An unprecedented planar π-conjugated [B2P5]5- group with ultra-large birefringence and nonlinearity: an ab initio study.

Searching for new nonlinear optical active (NLO-active) units would greatly promote the development of NLO materials. Herein, we theoretically investigate a newly reported [B2P5]5- group which consisted of two corner-sharing [BP3]3- trigonal planar units. Attributed to the coplanar π-conjugated configuration and the unique connection mode, the [B2P5]5- group can achieve both the largest birefringence (Δn∼ 0.68) and nonlinearity (d15∼ 57.14 pm V-1) among all inorganic planar NLO-active groups, which makes it a good NLO-active unit. Our analysis also reveals that the modulation of the electronegativity of constituent elements upon replacing with the elements in the same period can effectively improve the NLO responses, which would inspire a new direction for designing high performing NLO materials.