Unlocking Structural Anisotropy by Guest-Symmetry Modulation Strategy to Achieve Giant Birefringence in Molecular Crystals.

While birefringence is determined by both the structural units and spatial arrangement within a crystal, the design of the latter remains highly challenging, largely due to the difficulty in controlling microscopic packing. Molecular crystals feature host-guest architectures, which offer an effective means of structural design by enabling the spatial arrangement of the host framework through the guest molecules. Herein, the molecule S8, characterized by its homoatomic bonding and planar-like configuration, exhibits significant polarizability anisotropy; then, by employing a new guest-symmetry modulation strategy, a series of guest molecules, including quasi-spherical SnI4, triangular pyramidal AsI3 and SbI3, and mixed-anion CHI3 that were successfully incorporated into the S8 host lattice, serves as structural directors that switch the S8 arrangement from an offset to a parallel alignment. Such control yields tunable birefringence, from a suppressed 0.182 (SnI4·(S8)2) to enhanced large values of 0.333 (S8), 0.596 (AsI3·(S8)3), 0.641 (SbI3·(S8)3), and finally giant 0.711 (CHI3·(S8)3) @546 nm, the last of which is the largest among known inorganic molecular crystals. The specific directionality of host S8 molecules is achieved via self-assembly, which is driven by the electrostatic potential of the incorporated guest molecules. These results not only validate the effectiveness of the guest-symmetry modulation strategy but also underscore its potential for the design of birefringent materials.