Triple Integration of π‐Conjugated Building Blocks: Unprecedented Assembly for Large Optical Anisotropy

Assembling structurally diverse π-conjugated units into a single, coherently aligned architecture presents a significant challenge for achieving enhanced optical anisotropy for optical crystals. Herein, the directional molecular alignment driven by hydrogen bonding (H-bonding) is confirmed to be the key to addressing this challenge by designing and synthesizing a novel birefringent crystal, Cs₂[B₃O₃F₂(OH)₂](NO₃)·[B₃O₃(OH)₃]. Its anionic framework is constructed via the unprecedented, H-bond-directed assembly of three distinct, isolated π-conjugated building blocks: [B₃O₃(OH)₃], [B₃O₃F₂(OH)₂]^-, and [NO₃]^-. Complementary H-bonding interactions strategically overcome the inherent difficulty of aligning these disparate π-systems. This enforced coplanar alignment yields a record-high birefringence (Δn = 0.149@546 nm) among alkali/alkaline-earth metal borates, surpassing commercialized α-BaB₂O₄, while achieving a large bandgap of 5.82 eV, ideal for UV applications. First-principles calculations confirm that the planar π-conjugated groups dominate the giant optical anisotropy, while the H-bond network is crucial for enforcing their coplanarity and overcoming assembly limitations. Cs₂[B₃O₃F₂(OH)₂](NO₃)·[B₃O₃(OH)₃] establishes a new paradigm for engineering high-performance birefringent crystals by utilizing complementary H-bonding interactions to directionally assemble diverse π-conjugated units.