Reversible Symmetry Switching, Giant Birefringence, and UV Second-Harmonic Generation in Hybrid Niobium Oxyfluorides

Birefringent crystals that simultaneously exhibit large optical anisotropy and wide band gaps are highly desirable for advanced photonic technologies, yet these properties are often mutually constrained because structural features that enhance birefringence often compromise optical transparency. Herein, we report four new hybrid niobium oxyfluoride phenanthroline compounds, [Hphen]NbOF₄∙H₂O (NP1), α-[Hphen]NbOF₄ (NP2), β-[Hphen]NbOF₄ (NP3), and [Hphen]₂NbOF₅ (NP4), obtained through systematic variation of synthetic conditions. Thermal dehydration of centrosymmetric hydrated phase NP1 yields noncentrosymmetric (NCS) dehydrated phase NP2 through reversible crystal-to-crystal transformation, with birefringence increasing from 0.354 to 0.419 at 546 nm. Further optimization affords NP3, which exhibits an enhanced second-harmonic generation (SHG) response of 1.2 × KDP, compared with 0.09 × KDP for NP2. Strikingly, transformation from a one-dimensional chain-type niobium oxyfluoride framework (NP1-NP3) to a zero-dimensional structure of isolated NbOF₅ units (NP4) removes geometric constraints on phenanthroline alignment, enabling near-coplanar arrangement of π-conjugated chromophores and a record-high birefringence of 0.618 at 546 nm for a Nb-based crystal. All four compounds retain wide optical band gaps of 3.2-3.3 eV, demonstrating that giant birefringence and UV transparency are simultaneously achievable within a single material family. These results establish framework dimensionality as a viable strategy for overcoming the trade-off between optical anisotropy and transparency.