Strategically designed metal-free deep-ultraviolet birefringent crystals with superior optical properties

Finding new birefringent materials with deep-ultraviolet (DUV, λ < 200 nm) transparency is urgent, as current commercial materials cannot meet the rapidly growing demands in related application fields. Herein, three guanidinium-based compounds, C(NH₂)₃CH₃SO₃, β-C(NH₂)₃Cl, and γ-C(NH₂)₃Cl, all featuring [C(NH₂)₃·X]_∞ (X = CH₃SO₃ and Cl) pseudo layers, were designed through structural motif tailoring. Theoretical calculations indicate that these metal-free compounds all possess broad bandgaps (6.49-6.71 eV, HSE06) and remarkable birefringence (cal. 0.166-0.211 @ 1064 nm). Centimeter-sized C(NH₂)₃CH₃SO₃ crystals have been grown using a feasible aqua-solution method. Subsequently, to further optimize the properties, β/γ-C(NH₂)₃Cl was remolded by further tailoring the [C(NH₂)₃]⁺ cationic unit and the acceptor Cl^- anion, and then the fourth compound NH₂COF was theoretically constructed. Interestingly, NH₂COF exhibits the desired coexistence of a wider bandgap (7.87 eV, HSE06) and giant birefringence (cal. 0.241 @ 1064 nm) attributed to its higher density of well-aligned birefringence-active groups (BAGs). Furthermore, among these four designed compounds, C(NH₂)₃CH₃SO₃ has been experimentally synthesized and exhibits a short UV cutoff edge. Centimeter-sized crystals have been grown using a feasible aqueous solution method. This study provides an effective strategy to optimize the density of BAGs for large birefringence and offers valuable insights into the strategic design of metal-free DUV birefringent crystals.