Deep-Ultraviolet Transparent Mixed Metal Sulfamates with Enhanced Nonlinear Optical Properties and Birefringence

Enhancing anisotropy through the controlled arrangement of anionic groups is essential for improving the nonlinear optical (NLO) performance of non-π-conjugated NLO materials. In this study, we present the successful synthesis of the first examples of mixed alkali metal-alkaline earth metal sulfamate materials, including noncentrosymmetric Cs₂ Mg(NH₂ SO₃ )₄  ⋅ 4H₂ O (1), as well as centrosymmetric K₂ Ca(NH₂ SO₃ )₄ (2) and Rb₂ Ca(NH₂ SO₃ )₄ (3). All three compounds feature promising deep ultraviolet cut-off edges, notably 1 with a cut-off edge below 180 nm. The synergy of Cs⁺ and Mg²⁺ cations in 1 facilitated the successful alignment of polar [NH₂ SO₃ ] tetrahedra in a uniform orientation. Remarkably, 1 stands as the sole instance among reported sulfamate compounds with a co-parallel anionic arrangement, yielding a very large dipole moment compared to other non-π-conjugated NLO materials. Moreover, the substantial dipole moment of 1 yields an enhanced second harmonic generation response, approximately 2.3 times that of KH₂ PO₄ , and a large birefringence of 0.054 at 546.1 nm. The approach of regulating the arrangement of anionic groups using aliovalent cations holds promise for advancing the exploration of non-π-conjugated NLO materials.