Decoupling Contributions of Distinct Strategies for Enhancing Polarizability Anisotropy in Crystals Containing π-Conjugated Groups

Birefringent crystals play indispensable roles in laser and telecommunication technologies. For UV birefringent crystals with suitable cutoff edges (<300 nm), the key to developing novel crystals with large birefringence (Δn >0.15) lies in strategically introducing π-conjugated groups with high polarizability anisotropy and reducing the dihedral angles between π-conjugated groups to minimize the cancellation of optical anisotropy. However, when multiple strategies are simultaneously operative, the relative importance of distinct strategies needs to be systematically evaluated. In this work, five new oxalate UV birefringent crystals and the known (CN₄H₇)₂C₂O₄ crystal were synthesized through a facile evaporation method. By employing three distinct strategies, halogen-centered secondary building unit (SBU) modulation, diversifying intermolecular hydrogen bond types, and group modification, remarkable birefringence enhancement was achieved. Through correlation analyses between polarizability anisotropy of groups, group density, dihedral angle, and birefringence across the six crystals and over 20 diverse π-conjugated systems, we reveal the hierarchical contributions of distinct birefringence-enhancement strategies. This work establishes a systematic framework for evaluating the birefringence potential of π-conjugated structures and provides a rational design principle for the development of next-generation UV birefringent crystals.