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

The introduction of optimized [4-HP/4-H 2 P] + cationic dimer pairs, instead of single monovalent [4-AP] + cations, enhances birefringence by increasing the density of cationic layers and improving the alignment of anionic [3-pySO 3 ] − groups.