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

초록

Finding new birefringent materials with deep-ultraviolet (DUV, lambda < 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)(3)CH3SO3, beta-C(NH2)(3)Cl, and gamma-C(NH2)(3)Cl, all featuring [C(NH2)(3)<middle dot>X](infinity) (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)(3)CH3SO3 crystals have been grown using a feasible aqua-solution method. Subsequently, to further optimize the properties, beta/gamma-C(NH2)(3)Cl 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)(3)CH3SO3 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.

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