Exceptional Solar‐Blind Birefringence Engineered by Complementary Hydrogen‐Bond Ordering of π‐Conjugated Units

Wide‐bandgap transparency and large optical anisotropy are mutually exclusive in most solar‐blind birefringent crystals, severely limiting the miniaturization of solar‐blind polarization optics. Here, we show that this long‐standing trade‐off can be broken by a complementary hydrogen‐bond co‐assembly strategy, in which interstitial water molecules act as directional locks to compress the dihedral angles between urea building blocks and align their dipole moments. This yields two crystals, C 2 H 5 N 3 O 2 and its hydrated derivative C 2 H 5 N 3 O 2 ·0.75 H 2 O, which deliver birefringence values of 0.224 and 0.283 at 546 nm while maintaining absorption edges of 210 and 202 nm in the solar‐blind ultraviolet region, respectively. The hydrated phase achieves a 2.5‐fold birefringence enhancement over pristine urea, overcoming the inherent anisotropy bottleneck of the classical hydrogen‐bonded network. Our findings establish a rational paradigm, from excellent structural units to the ordered arrangement of complementary weak bonds, and then to outstanding optical anisotropy, showing that weak interactions can actively regulate polar units to drive birefringence close to its theoretical limit, offering a widely applicable design platform for the next generation of solar‐blind photonic materials.