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

Birefringent crystals are central to polarization optics, yet pushing birefringence (Δn) beyond 1.0 while retaining transparency (Eg > 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 Δn = 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(I2Cl)·Cl (X = Cl, I; Br, II, Cy = cytosine), (HClCy)2(ICl2)·Cl (III), and (HBrCy)(BrCy)·IBr2 (IV) and·Br3 (V). I and II reach calculated Δn = 1.336 and 1.324 at 546 nm, the highest among all π-conjugated and inorganic crystals reported, while V retains Δn = 1.311 with a widened Eg = 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.