π‐Conjugated Cations in Phosphates: A Pathway to Solar‐Blind UV Nonlinear Optical Crystals with Phase‐Matching

While birefringence phase-matching (PM) remains the most practical approach for nonlinear optical (NLO) frequency conversion, conventional phosphate crystals suffer from intrinsically low birefringence (Δn < 0.05) that hinders PM behavior in the solar-blind ultraviolet (UV) region (λ < 280 nm). Herein, we demonstrate a π-conjugated cation engineering strategy to break this limitation, reporting two phosphite-based NLO crystals: (C₂N₄OH₇)H₂PO₃ (GUPO) and C(NH₂)₃H₂PO₃ (GPO). The synergistic alignment of π-conjugated guanidinium cations and [H₂PO₃]⁻ anions enables record-breaking optical anisotropy (Δn = 0.19 @ 589.3 nm for GUPO), surpassing all known inorganic phosphates. Crucially, GUPO is the first phosphate to realize full-wavelength PM, in which the PM wavelength fully covers its optical transparency range down to 215 nm. Concurrently, GUPO exhibits exceptional second-harmonic generation (SHG) response (2.2 × KDP at 1064 nm and 1.0 × β-BBO at 532 nm), which may enable direct 266 nm laser generation. Mechanistic studies reveal that the giant birefringence originates from oriented π-π interactions between cations, while the SHG response stems from the cooperative polarization of cations. This work establishes π-conjugated cation engineering as a paradigm for designing UV NLO materials, with GUPO crystal emerging as a cheap, efficient alternative to conventional UV NLO crystals.