Mechanochemical Synthesis of an Ionic Cocrystal with Large Birefringence Resulting from Neutral Planar π-Conjugated Groups

Birefringent materials with strong optical anisotropy are indispensable for optoelectronic devices as polarizers, isolators, and beam splitters, etc. Here, a new ionic cocrystal KBr·H 3 C 3 N 3 O 3 ( I ) was synthesized by a facile and fast mechanochemical reaction. I can be described as stacking of [(H 3 C 3 N 3 O 3 )Br] − layers via hydrogen bonds separated by K + cations along the a -axis. The experimental and theoretical results reveal that I exhibits a short ultraviolet cutoff (216 nm) and large birefringence (Δ n = 0.273 at 800 nm) due to the well-aligned planar π-conjugated H 3 C 3 N 3 O 3 molecules. In addition, I is thermally stable up to 400 °C, far higher than those of reported hydroisocyanurates. This work opens an avenue for the exploration for ionic cocrystals with π-conjugated units as prominent ultraviolet birefringent materials with strong optical anisotropy, wide band gap, easy synthesis, and good physicochemistry stability.