(C3N6H7)BF4·H2O and (C3N6H7)SO3CH3·H2O with large birefringence induced by coplanar π-conjugated [C3N6H7]+ groups

Abstract

Birefringent crystals are vital to modern optical devices as they can effectively modulate the polarization of light. Currently, limited by comprehensive properties such as crystal size and birefringence, the design and synthesis of birefringent crystals are challenging, especially in the short-wavelength spectral region. Herein, (C₃N₆H₇)BF₄·H₂O and (C₃N₆H₇)SO₃CH₃·H₂O were successfully synthesized, and they exhibit large birefringence and short UV cutoff edges (0.37/0.31 at 546 nm and 244/233 nm). Notably, the crystal of (C₃N₆H₇)BF₄·H₂O synthesised using a facile solvent growth method is over 20 mm in length. Theoretical analysis suggests that the parallel arrangement of [C₃N₆H₇]⁺ groups modulated by tetrahedral units induces large optical anisotropy. This work provides promising candidates with exceptional properties for short-wavelength ultraviolet applications.