Synergistic engineering of ultraviolet metal-free crystals with exceptional birefringence via pyridine-derived dimers

Crystals with giant birefringence are essential for practical applications in lasers, optical communication, and polarimetry, where precise control of polarized light is critical. Coplanar six-membered ring (6-MR) primitives with large polarizability anisotropy are particularly effective in enhancing birefringence. This study successfully combines different pyridine derivatives into two novel metal-free crystals: supramolecular one-dimensional [(4-HP)(4-H2P)][3-pySO3] (1; HP = hydroxypyridine; py = pyridine) and two-dimensional [4-AP][3-pySO3] (2; AP = aminopyridne), synthesized via a facile aqueous solution method. Compound 1 features unique [4-HP/4-H2P]+ cationic dimer pairs linked by hydrogen bonds, in contrast to the single monovalent [4-AP]+ cations in compound 2. This structural distinction leads to optimized anionic [3-pySO3]- arrangements, a reduced dimensionality of linkage, and denser spatial distribution of cationic pseudo-layers, significantly enhancing birefringence. Compound 1 exhibits a superior birefringence value of 0.443 at 546 nm, compared to 0.296 for compound 2, representing the highest birefringence among sulfate derivatives incorporating an additional birefringence-active group in the ultraviolet (UV) region. It also surpasses all reported metal-free compounds with single 6-MRs in the short-wave UV range. Theoretical calculations confirm the synergistic effects between cationic and anionic pyridine derivatives, further elucidating their contributions to enhanced birefringence. In addition, compound 1 demonstrates a short UV cut-off edge at 279 nm and favorable growth characteristics, making it a promising candidate for UV birefringent applications. This research offers new insights into designing and optimizing birefringence by exploring the relationship between the composition and arrangement of organic cations and their optical properties in UV metal-free systems through synergistic effects.