(C 5 H 6.16 N 2 Cl 0.84 )(IO 2 Cl 2 ): a birefringent crystal featuring unprecedented (IO 2 Cl 2 ) - anions and π-conjugated organic cations.

Birefringent crystals can manipulate the polarization state of lasers and have vital application in polarizers, optical isolators, phase compensators, etc. The design and synthesis of crystals with large birefringence remains a challenging task. To design crystals with large birefringence, we combine an unprecedented chloroiodate(v) group (IO 2 Cl 2 ) - featuring large polarizability anisotropy and a strong stereochemically active lone pair (SCALP) with the π-conjugated 2-amino-5-chloropyridine group. The superior synergy effect of (IO 2 Cl 2 ) - and 2-amino-5-chloropyridine groups produces a new birefringent crystal, namely (C 5 H 6.16 N 2 Cl 0.84 )(IO 2 Cl 2 ). It exhibits remarkably large birefringence of 0.67 at 546 nm, far exceeding those of most visible birefringent materials reported. This work discovers the first chloroiodate(v) group and provides a new synthetic route for birefringent materials.