Breaking the Natural Tendency of Deep‐UV Polyborate Anion Clusters to Inducing Strong Optical Anisotropy

Polyborate anion is a tunable and versatile framework that offers a long-term structural stability, providing the active sites to tolerate various coordination ions to induce functionalities. However, their random arrangement typiaclly weakens the optical anisotropy despite high bandgaps. Converting these flexible frameworks into optical-active states remains challenging. Herein, a full optical-active model is proposed where all coordination groups are optimally arranged and fully contribute to the total optical anisotropy in the lattice. A new polyborate is reported with a high optical anisotropy quality factor (F = 0.984), based on a terminal stretch strategy using nucleophilic groups. This renders its framework almost entirely optically active and thus leads to a strong deep-ultraviolet (deep-UV) optical anisotropy of Δn_exp = 0.148 in borate system. Theoretical and structural evidence supports the role of nucleophilic groups and orbital hybridization in inducing a preferred configuration of the optical-active module. These findings not only validate the viability of using the full optical-active model to break the intrinsic defect of small optical anisotropy of polyanionic materials, but also expand the alternative system of promising deep-UV optical crystals with new polyanion system that has been neglected for a long time.