Dependence of the material properties of diimmonium dyes for NIR absorbing films on the type of counter anions

Diimmonium dye is one of the representative near-infrared (NIR) absorbing dyes. In general, diimmonium dyes, constructed with a cationic backbone and two counter anions, show strong and broad absorption in the near-infrared region. In this study, a series of diimmonium dyes were synthesized with varying types of counter anions, and the dependence of material properties of diimmonium dyes on the type of counter anions was investigated. The diimmonium dyes showed different colors in the solid-state as the crystal structures of dyes were change by the type of counter anions. One of the diimmonium dyes, DB-TFSI, showed the highest thermal stability and molar absorptivity because of the formation of a strong complex structure between the cationic backbone and TFSI, whose negative charges were extensively distributed over the whole molecule. A monatomic anion such as chloride did not form a strong complex structure, so that it showed low thermal stability and NIR absorbing ability. As the solvent polarity increased, the molar absorptivity of diimmonium dyes decreased due to the dissociation of counter-anions. The characteristic optical properties and solvatochromic behavior were also observed in NIR absorbing films made of the diimmonium dyes and a transparent polymeric binder. Thus, it is demonstrated by this study that the materials properties of diimmonium dyes are affected by the degree of charge distribution of counter anions, and the weakly coordinating anions such as TFSI are effective to improve the materials properties.