Designing Novel Solar‐Blind Ultraviolet Nonlinear Optical Crystals Based on Local Proton Distribution Complementary Effect

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ABSTRACT Dipolar π‐conjugated groups with large polarizability anisotropy and hyperpolarizability are generally regarded as important nonlinear optical (NLO) active building units. However, their strong dipole‐dipole interactions remain a key challenge in designing non‐centrosymmetric structures. To overcome this, we proposed a novel non‐centrosymmetric structure design strategy based on the local proton distribution complementary effect. Importantly, it could induce the groups to form a zigzag ordered arrangement along the direction perpendicular to the π‐conjugated plane, thereby significantly increasing the probability of forming non‐centrosymmetric structures. Based on this strategy, we successfully designed and synthesized two novel solar‐blind ultraviolet non‐centrosymmetric crystals, (C 3 N 2 H 5 )Na 3 (S 2 O 3 ) 2 ·2H 2 O and (C 3 N 2 H 5 )ClO 4 ·(C 3 N 2 H 4 ), with imidazolium/imidazole groups. Both crystals exhibit excellent optical performances, with a large birefringence of 0.158 @ 546 nm in (C 3 N 2 H 5 )ClO 4 ·(C 3 N 2 H 4 ) and a strong NLO effect approximately 2.6 times that of KH 2 PO 4 in (C 3 N 2 H 5 )Na 3 (S 2 O 3 ) 2 ·2H 2 O, which mainly result from the intrinsic characteristics of imidazolium/imidazole groups and their ordered arrangement. Consequently, this work not only develops two promising solar‐blind UV NLO crystals but also offers a novel structure design strategy that is expected to promote the development of NLO materials.

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