(C5H6.16N2Cl0.84)(IO2Cl2): a birefringent crystal featuring unprecedented (IO2Cl2)− anions and π-conjugated organic cations

Abstract

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 (IO₂Cl₂)⁻ 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 (IO₂Cl₂)⁻ and 2-amino-5-chloropyridine groups produces a new birefringent crystal, namely (C₅H_6.16N₂Cl_0.84)(IO₂Cl₂). 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.