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

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.