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

Abstract 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, Cs 2 [B 3 O 3 F 2 (OH) 2 ](NO 3 )·[B 3 O 3 (OH) 3 ]. Its anionic framework is constructed via the unprecedented, H‐bond‐directed assembly of three distinct, isolated π‐conjugated building blocks: [B 3 O 3 (OH) 3 ], [B 3 O 3 F 2 (OH) 2 ] − , and [NO 3 ] − . Complementary H‐bonding interactions strategically overcome the inherent difficulty of aligning these disparate π‐systems. This enforced coplanar alignment yields a record‐high birefringence (Δ n = 0.149@546 nm) among alkali/alkaline‐earth metal borates, surpassing commercialized α‐BaB 2 O 4 , 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. Cs 2 [B 3 O 3 F 2 (OH) 2 ](NO 3 )·[B 3 O 3 (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.