Isomorphic organic crystal families: Analogous crystal structure with largely different physical and terahertz properties

Organic terahertz (THz) crystals exhibit complex and often unpredictable relationships among their chemical structures, crystal structures, and physical properties. This complexity makes it highly challenging to optimize crystals for efficient THz generation. Here, we introduce a design strategy based on isomorphic ionic organic crystal families to develop new organic nonlinear optical crystals that efficiently generate ultra-broadband THz waves extending up to 15 THz. Using three nonlinear optical cationic chromophores with different interionic assembly types, each combined with three molecular anions featuring different substituents (non-polar methyl or polar chloro and bromo groups), we investigated nine ionic crystals, including four reported here for the first time. The crystal members within each isomorphic crystal family show nearly identical crystal structures (i.e., isomorphic crystal structures), all of them achieving top-level macroscopic second-order optical nonlinearity and efficient ultrabroad THz wave generation capabilities. Remarkably, their physical (and terahertz) properties within each isomorphic crystal family differ dramatically according to a certain order of the introduced anions, and this ordering trend persists across all three isomorphic crystal families. These results provide a crucial advancement in the design and prediction of macroscopic properties in ionic organic THz and nonlinear optical crystals, marking an important step toward rational crystal design exhibiting enhanced physical properties.