Birefringent crystals underpin polarization control and angular phase matching in nonlinear optics and therefore, attract sustained interest. However, achieving large birefringence (Δn) has largely depended on inorganic frameworks composed of metals with limited natural abundance or sustainability concerns. By contrast, organic cocrystals offer simple preparation and readily tunable packing. Here we design a series of diazapyrene derivatives and obtain 13 crystals, including three single-component crystals and ten two-component cocrystals formed separately with three distinct benzene derivatives, whose crystal-packing anisotropy can be programmed to yield Δn = 0.152-1.269. Two compositions, D27N and D13N, combine suitable optical band gaps with exceptional birefringence (Δn = 1.223 and 1.269 at 546 nm, respectively), ranking among the highest reported for purely organic crystals, and under identical conditions, surpassing all reported inorganic birefringent crystals. Across the series, we uncover a Boltzmann-type relationship between Δn and a geometric descriptor ΔS (minimal/maximal projected area of the conjugated core on crystallographic planes), thereby quantitatively linking molecular-level packing anisotropy to macroscopic Δn. Cocrystal engineering also modulates second-order nonlinear optical responses, including symmetry control and second-harmonic generation tuning. This work establishes a metal-free, designable route to high-Δn optical crystals and provides a predictive metric for anisotropy-driven materials discovery.
Sun et al. (2026) studied this question.