Research progress in optical materials with cationic organic planar π-conjugated groups containing CN bonds

Recently, organic planar π-conjugated groups have received increasing attention from researchers for the construction of linear/nonlinear optical crystals with excellent performance in the UV/DUV region. Compared with inorganic optically active units ([BO₃]³⁻, [B₃O₆]³⁻, [CO₃]²⁻, and [NO₃]⁻), organic planar π-conjugated groups have larger microscopic hyperpolarizability and polarizability anisotropy, which enable the synthesis of birefringent crystals with large birefringence or NLO crystals with a high SHG response and large birefringence. Among the various types of organic planar π-conjugated groups, cationic organic planar π-conjugated groups containing CN bonds show several great advantages, including high nonlinear polarizability, high flexibility in molecular design and assembly and easy large-size crystal growth. These merits have inspired researchers to make great efforts to develop high-performance optical crystals for application in the UV/DUV region. In this review, the compounds containing cationic organic planar π-conjugated groups (such as [C₃N₆H₇]⁺, [C(NH₂)₃]⁺, [C₅NH₆O]⁺, [C₄N₃H₆]⁺, etc.) are outlined. The relationship between the optical properties and the structure is discussed in accordance with the available computational and experimental data. The above types of compounds are summarised and evaluated. Finally, the main challenges and future opportunities for the construction of optical crystals using cationic organic planar π-conjugated groups are presented, and the prospects for development are outlooked.