Crystal Structures and Optical Properties of Cyanine Dyes Depending on Various Counter Anions

In this study, cyanine cations with various counter anions were prepared as examples of ionic materials constructed using charged pi-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 CHCl3 (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.