Cation Engineering of Nonlinear Optical Crystals by Introducing Fluorine Substituent for Terahertz Wave Generation

Organic nonlinear optical crystals are widely investigated for highly efficient terahertz wave generation. In particular, cation engineering plays a direct and crucial role in determining the frequency conversion characteristics of these ionic crystal systems. The methylbenzothiazol-3-ium based cations were introduced into benchmark organic nonlinear optical crystals. In this work, a fluorine substituent is introduced into the methylbenzothiazol-3-ium based cation’s acceptor group. Characterized by high electronegativity, it exhibits a strong inductive effect. Thus, the acceptor acts as a strong electron-withdrawing group. In addition, fluorine acts as a hydrogen bond or halogen bond acceptor sites. Through multiple non-covalent interactions, a tight molecular packing is achieved throughout the crystal structure. Unlike other halogen substituents, the small size of fluorine can introduce minimal steric hindrance, thereby promoting an even denser crystal packing.