Solid‐State Chemistry of Cyanine‐Type Dyes and the Effect of Two Novel Counterions on Their Crystal Properties

Organic dyes undergo a variety of solid‐state chemical processes, including intra‐ and intermolecular reactions, gas‐solid reactions, and polymorphic transformations. The properties of dye solids are markedly affected by this chemistry. This paper reviews solid state dye chemistry from the literature and reports in detail the chemistry of two novel cyanine dye salts whose properties are controlled by the nature of their counterions. In cyanine‐oxonol salts, the oxonol counterion is a large planar dye which forms crystalline dye aggregates with cyanine ions. There is a multiplicity of polymorphic forms of these mixed dyes reflecting multiple favorable dye aggregate geometries. The cyanine‐borate salts undergo intermolecular solid‐state reactions. In either large single crystals or dispersions of the latter salts in polymer binders, alkyl transfer from the anion to the chromophore can be induced thermally or photochemically.