1,3-Bis­(dicyanomethylidene)­indan anion (CMI–) is a stable π-electronic anion prepared by self-deprotonation. A series of ion-pairing assemblies based on CMI– were investigated by various π-electronic cations. The packing structures were revealed by single-crystal X-ray analysis, and charge-by-charge and charge-segregated assemblies were obtained. Interestingly, cation–cation and/or anion–anion stacking formations were observed in all the crystal structures, although electrostatic repulsion effects exist between identically charged species. The stabilization of the identically charged stackings by the dipole–dipole interactions was explained from the DFT calculations. In addition, the delocalization of negative or positive charge through the face-to-face stacking and hydrogen bonding between the differently charged species also play an important role for the formation of various assemblies.

In this study, cyanine cations with various counter anions were prepared as examples of ionic materials constructed using charged π-conjugated systems. A series of ion pairs was obtained by anion exchange reactions using iodide salts of carbocyanine dyes. The optical properties were measured by UV/vis absorption and fluorescence spectroscopy; measurements performed in CHCl 3 (less-polar solvent) were altered by the influence of the counter anions. The packing structures of nine crystals were determined by single-crystal X-ray analysis. Moreover, the locations of the anions relative to the cations were stabilized by hydrogen bonding and categorized into two types. In addition, delocalization of the negative charge of the anions on cyanine cations was explained by density functional theory calculations. Furthermore, it was concluded that the stack formation of cyanine cations depends on the size and structure of the anions.

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