The New Paradigm\nof Ligand Substitution-Driven Enhancement\nof Anisotropy from SO<sub>4</sub> Units in Short-Wavelength Region

For non-π-conjugated [SO<sub>4</sub>] units, it\nis challenging\nto generate sufficient birefringence, owing to the high symmetry of\nthe regular tetrahedron. Unlike the traditional trial-and-error approach,\nwe propose a new paradigm for birefringence engineering to tune the\noptical properties based on [SO<sub>4</sub>] units. Through the strategy\nof ligand substitution, we can predict its effect on the band gap\nand anisotropy. Theoretical evaluations reveal generalized results\nthat the anisotropic electron distribution of new functional groups\ninduced by the suitable ligand substitution contributes to the band\ngap and birefringence. To further validate the correctness of the\nparadigm, we experimentally synthesized and characterized nine novel\ncompounds with selected functional modules. By the new paradigm of\nligand substitution, they can reach up to 4–6 times the birefringence\nof the corresponding sulfate and maintain the wide bandgap. Through\nrational design, (CN<sub>4</sub>H<sub>7</sub>)SO<sub>3</sub>NH<sub>2</sub> exhibits about 35 times the birefringence of Li<sub>2</sub>SO<sub>4</sub>, which is a significant order of magnitude improvement\nand verifies the superiority of our proposed paradigm. This work provides\na new paradigm for the modification to the non-π-conjugated\ngroup and will guide and accelerate the exploration of novel birefringent\ncrystals in the short-wavelength region.