Cation‐Regulated [CH 3 BO 2 F]‐Derived Borates: Synergistic Fluorination and Organic Functionalization for High‐Birefringence UV NLO Materials

ABSTRACT Large birefringence is a key parameter for nonlinear optical (NLO) and birefringent crystals, as it is critical for extending the phase‐matching wavelength of NLO crystals and optimizing the light‐splitting performance of birefringent crystals. By rational structural design, two unprecedented functional building blocks (FBBs), [C 4 H 5 BO 4 F] and [C 5 H 7 BO 4 F], were successfully constructed from the newly identified [CH 3 BO 2 F] fragment via synergistic fluorination and organic functionalization. A cation‐mediated structural engineering approach was employed to achieve four centrosymmetric (CS) compounds (α‐KC 4 H 5 BO 4 F, β‐KC 4 H 5 BO 4 F, KC 5 H 7 BO 4 F and Na 0.5 Cs 0.5 C 4 H 5 BO 4 F) and one noncentrosymmetric (NCS) structure (NaC 4 H 5 BO 4 F). Systematic investigations reveal the unique structure‐directing role of Na + , yielding NaC 4 H 5 BO 4 F with a large birefringence (Δn = 0.139 at 1064 nm), strong SHG response (∼1.4 ×KDP) and a short phase‐matching wavelength of 208 nm. Additionally, the four CS compounds also exhibit considerable birefringence values of 0.138, 0.121, 0.074 and 0.140 at 1064 nm, respectively. These findings validate the synergistic modification of borate units and cation regulation as a feasible and effective design strategy, providing valuable guidance for the targeted development of high‐performance UV NLO materials with large birefringence and shortened phase‐matching cutoff wavelengths.