Cyanine Dyes, Squarylium Dyes

Cyanine dyes contain some N-heterocycles on both sides of the polymethine structure. The N-heterocycle on the one side is a cationic ammonium with an electron-withdrawing effect, and the other N-heterocycle is a tertiary amine with an electron-donating effect. Accordingly, cyanine dyes show the light absorption of a charge transfer through the central polymethine structure. An extension of the polymethine structure exhibits a red shift of absorption. The absorption of cyanine dyes shifts to longer wavelengths with every extension by the addition of two methine moieties (or by the addition of one n moiety).¹⁾ There are cyanine dyes exhibiting near infrared absorption. The N-heterocycles contained in the cyanine dyes consist of indole, benzoxazole, benzothiazole, naphthothiazole, quinoline and so on. Proper selection of these N-heterocycles controls absorption wavelength. Increasing the number of aromatic rings shifts absorption to longer wavelengths. Counter anions consist of halides, sulfonate, perchlorate, tetrafluoroborate, hexafluoroantimonate and so on. Proper selection of these anions enhances solubility and durability.^2,3) Cyanine dyes are useful for silver photography, and optical recording media by laser sensitivity.⁴⁾ Water-soluble cyanine dyes are usable as fluorescent probes in biochemical research.⁵⁾

Squarylium dye (SQ), which is an analogue of cyanine dye, has a squaraine moiety in the center of the π-conjugate. The SQ dye has a zwitterion structure in which the cation and anion coexist in the molecule, although the cyanine dye has a separated cation component. The SQ was conventionally used as a charge generation material for an organic photo conductors.^6,7) Recently, SQ provided a sensitizer for organic solar cells originating from a strong absorption in near infrared area.^8,9) In addition, the SQ dye was applied to an organic light-emitting diode because it shows efficient red luminescence.¹⁰⁾