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

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, (C3N2H5)Na3(S2O3)2·2H2O and (C3N2H5)ClO4·(C3N2H4), with imidazolium/imidazole groups. Both crystals exhibit excellent optical performances, with a large birefringence of 0.158 @ 546 nm in (C3N2H5)ClO4·(C3N2H4) and a strong NLO effect approximately 2.6 times that of KH2PO4 in (C3N2H5)Na3(S2O3)2·2H2O, 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.