A multi-scale birefringence modulation strategy in DUV/UV crystals induced by organic cations

Abstract

Birefringent materials are essential for polarized light modulation in optical communications and laser technologies, yet synthesizing high-performance UV/DUV variants remains challenging. This study proposes a multiscale microstructural modulation strategy. First, introducing substituents, such as [CH₃], [NH₂], and [CH₃CO], into guanidinium cations enhances their polarizability anisotropy. Second, by leveraging the differences in basicity among guanidine derivatives, their pH control enables the precise regulation of B–O polymerization, achieving interconversion among the [B(OH)₃], [B₃O₃(OH)₄]⁻, [B₄O₅(OH)₄]²⁻, and [B₅O₆(OH)₄]⁻ groups. Third, hydrogen bond-directed self-assembly facilitates the coplanar arrangements and ordered stacking of π-conjugated cations and borate anionic clusters, further enhancing the optical anisotropy. Among the synthesized compounds, four exhibit DUV cutoff edges <200 nm and large birefringence (0.077–0.143 at 546 nm). Notably, DGBH combines a wide bandgap (HSE06 = 6.18 eV) with cm-scale crystal growth, showing strong DUV birefringent potential. GBH-2 achieves a birefringence (Δn_exp. = 0.311 at 546 nm) that is 14.80 times that of its structural analog Ba₂B₁₀O₁₆(OH)₂·(H₃BO₃)·H₂O. Validated experimentally and theoretically, this exceptional performance originates from the synergy among cation modification, precise control of B–O polymerization, and optimized functional motif arrangement, breaking conventional paradigms. This pioneering strategy provides a valuable framework for accelerating the discovery of high-performance birefringent materials.