Lock‐and‐Key Assembly Enables Record Birefringence in Monocyclic π‐Conjugated Crystals via Spatial Confinement of Linear Polyhalide

ABSTRACT Birefringent crystals are central to polarization optics, yet pushing birefringence (Δ n ) beyond 1.0 while retaining transparency ( E g > 2.0 eV) has been pursued almost exclusively by extending π‐conjugation from monocyclic to polycyclic aromatics. Monocyclic π‐systems have long been considered intrinsically capped below Δ n = 1.0. Here, we challenge this assumption by demonstrating that geometric precision, rather than π‐system enlargement, can unlock this performance ceiling. We introduce a halogenation‐induced dimensional reduction strategy in which halogen substituents redirect the hydrogen‐bonding topology of cytosine from non‐directional 2D networks into wave‐like 1D chains, whose complementary concave pockets form capsule‐shaped cavities. These cavities act as lock ‐and‐ key templates that confine linear polyhalides into strict collinear alignment while enforcing π‐plane coplanarity. This strategy affords five new birefringent hybrid crystals: (HXCy) 2 (I 2 Cl)·Cl (X = Cl, I ; Br, II , Cy = cytosine), (HClCy) 2 (ICl 2 )·Cl ( III ), and (HBrCy)(BrCy)·IBr 2 ( IV ) and·Br 3 ( V ). I and II reach calculated Δ n = 1.336 and 1.324 at 546 nm, the highest among all π‐conjugated and inorganic crystals reported, while V retains Δ n = 1.311 with a widened E g = 2.25 eV, demonstrating that birefringence and transparency can be independently tuned. These results establish geometric precision, rather than π‐system enlargement, as a powerful design route to high‐performance birefringent crystals.