Significant Enhancement of Birefringence Driven by Carboxylate Site Regulation in Pyridinedicarboxylic Acids

The limited optical modulation efficiency of existing birefringent crystals falls short of the demands for large optical anisotropy in optical communication and precision optical applications. In this work, we propose a hydrogen-bond-site synergistic strategy to improve birefringence by employing two isomeric pyridinedicarboxylic acids, pyridine-2,5-dicarboxylic acid (25Py) and pyridine-3,5-dicarboxylic acid (35Py), together with the guanidinium (G) cation, which provides abundant hydrogen-bond donors and a wide optical band gap. Variations in carboxylate substitution positions modulate the hydrogen-bonding sites, thereby directing the formation of distinct hydrogen-bonding networks. Two guanidinium pyridinedicarboxylates, 25PyG and 35PyG, exhibiting markedly different birefringence values, were assembled via a hydrothermal approach. Notably, the regulation in hydrogen-bonding sites visibly improves optical anisotropy. 35PyG exhibits an exceptionally large birefringence of 0.411@546 nm, ranking among the highest reported for guanidinium-containing UV-transparent crystals, while retaining a short UV cutoff edge. Consequently, it represents one of the highest-performing guanidinium-based birefringent crystals reported to date.