Amino‐Site Directed Molecular Engineering Enables Novel Wide‐Bandgap Organic Crystal With Record‐Breaking High Birefringence

Birefringent crystals are vital functional materials for light‐polarization modulation, widely utilized in optical communications, laser technology, displays, imaging, and photodetection. High‐performance birefringent crystals require broad optical band gaps to realize wide visible‐to‐ultraviolet transparency. However, large birefringence and wide band gap are difficult to achieve simultaneously, since highly polarizable structural units easily induce low‐energy electronic transitions. Herein, an amino‐position regulation strategy is proposed via proton‐transfer‐driven self‐assembly of 2‐, 3‐, and 4‐aminopyridines with 4, 5‐dicyanoimidazole, integrating prominent polarizability anisotropy and large molecular HOMO–LUMO gaps. Three organic crystals, [(C 5 N 2 H 7 ) + (C 5 N 4 H) − ]·H 2 O (2AP‐DCI‐W), [(C 5 N 2 H 7 ) + (C 5 N 4 H 2 )(C 5 N 4 H) − ] [3AP‐(DCI) 2 ], and [(C 5 N 2 H 7 ) + (C 5 N 4 H) − ] (4AP‐DCI), were successfully synthesized. Amino‐substitution position modulates the electrostatic distribution, intermolecular interactions, and packing modes of π‐conjugated units, progressively improving both birefringence and optical band gap. 4AP‐DCI exhibits the optimal overall performance with a calculated birefringence of 0.514 at 546 nm and a broad band gap of 4.06 eV. To our knowledge, this value represents the highest birefringence among all organic birefringent crystals with band gaps over 4.0 eV. This work provides an effective molecular strategy to balance large birefringence and broad transparency by tuning the position and spatial arrangement of organic π‐conjugated functional units.