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—(C₅H₆NO)⁺(NO₃)^-(4HPN), (C₅H₆NO)⁺(HC₂O₄)^-(4HPO), (C₄N₃H₆)⁺(NO₃)^-(2APMN), and (C₄N₃H₆)⁺(HC₂O₄)^-·H₂O (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.