Local Conformational Confinement and Dipole Engineering for Phase-Matchable Solar-Blind Ultraviolet Nonlinear Optical Crystals

Solar-blind ultraviolet nonlinear optical crystals are difficult to design because efficient frequency conversion requires the simultaneous realization of noncentrosymmetry, phase-matchable birefringence, and a sufficiently wide band gap. Here, we introduce a local conformational-confinement strategy that transforms flexible malonate into chelated [C₃H₂O₄BFR]⁻ (R = F, Me, CF₃) chromophores, in which a six-membered ring fixes the relative orientation of two carboxylate π-conjugated units. This geometric locking enhances polarizability anisotropy and microscopic hyperpolarizability without sacrificing the band gap. In parallel, substitution at the boron-bound R site tunes the ground-state dipole moment of the anionic functional unit, suppressing antiparallel dipole packing and promoting noncentrosymmetric crystallization. Guided by this dual design principle, we obtained NaMaBFMe, which combines a UV cutoff edge near 220 nm, birefringence of 0.131 at 1064 nm, a phase-matchable SHG response of 2.4 × KDP, and a laser-induced damage threshold of 1.258 GW·cm⁻². These results establish conformational locking coupled with dipole engineering as a general route to high-performance solar-blind UV NLO crystals.