Cationic Dyes as Chromophores

Cationic dyes carry a positive charge in their molecule. The salt-forming counterion is in most cases the colorless anion of a low molecular mass inorganic or organic acid. Many of these dyes can be converted into water-insoluble dye bases by addition of alkali. For this reason, they were formerly called basic dyes although still in use today, the term should be abandoned. 

The positive charge of cationic dyes may be either localized or delocalized. In 1, the positive charge is localized on an ammonium group  

In the diazahemicyanine dye 2, the positive charge is delocalized across the dye cation. 

As in these examples, the charge-canying atom is usually nitrogen, but in some dyes this function is adopted by an oxygen, sulfur, or phosphorus atom. 

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These dyes possess two independent chromophoric chains of even methine (neutro) and uneven methine (cyanine) fixed on a central ketometbylene nucleus. The methylene reactive group is first used for the neutrocyanine synthesis in position 5. the, quaternization of which can ensure a subsequent polymethine synthesis in position 2 of a cationic dye by ordinary means (Scheme 58). As indicated, this quaternized neutrocyanine (37) may as well give another neutrocyanine. 

Stewart et. al.(21] have shown the usefulness of FI on-line ion-pair extraction in the determination of carboxylic acid drugs, using salicylic acid, valproic acid, and ibuprofen as model drugs. After a comparison of different chromophoric and fluorophoric cationic dyes in chloroform extractant. Gentian Violet was recommended as counterion for spec-trophotometric determination and Acridine Orange was recommended for fluorimetric determination. The system was used for post column detection in HPLC. 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

Small chromophores, such as -alkylquinolinium, and isoquinolinium cations can also be intercalated into the galleries of a-ZrP. In most cases, this did not bring about any change in the fluorescence spectra from that observed in solution (acetonitrile). The fluorescence decays of the intercalated compounds were, however, nonexponential and contained a very short-lifetime component. High loadings resulted in self-quenching of the fluorescence and this contributed to the shorter-lived component for the intercalated dyes [58],... 

This chapter is devoted to the chemical chromophores of dyes, but the term chromophore is used here in a somewhat extended manner that also considers dye classes such those as based on cationic, di- and triarylcarbonium, and sulfur compounds, and metal complexes. 

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