Designed Metal‐Free Quasi‐1D Crystals with Giant Birefringence

Abstract Birefringence, a vital property of anisotropic crystals, is indispensable for advanced optical technologies. Quasi‐1D structures, with their inherent structural anisotropy, offer fertile ground for discovering materials with exceptional optical anisotropy. In this work, we report four metal‐free quasi‐1D crystals—(C 5 H 6 NO) + (NO 3 ) ‐ ( 4HPN ), (C 5 H 6 NO) + (HC 2 O 4 ) ‐ ( 4HPO ), (C 4 N 3 H 6 ) + (NO 3 ) ‐ ( 2APMN ), and (C 4 N 3 H 6 ) + (HC 2 O 4 ) ‐ ·H 2 O ( 2APMO )—designed to feature pseudo‐chain structures and grown as centimeter‐scale crystals via a facile aqueous‐solution method. These crystals display wide bandgaps (3.40–4.48 eV) and exceptional birefringence, with calculated values up to 0.555@546 nm and an experimental maximum of 0.597@546 nm for 4HPN . The giant birefringence stems from their quasi‐1D architectures, high‐spatial density, and optimal arrangement of the birefringence‐active groups (BAGs). This study highlights the promise of metal‐free quasi‐1D materials for next‐generation miniaturized polarization optics and laser‐driven applications.