In pursuit of the ideal chromoionophores (part I): pH-spectrophotometric characteristics of aza-12-crown-4 ethers substituted with an anthraquinone moiety

• A set of new dyes based on anthraquinone moiety was synthesized and characterized.
• Unique technique of simultaneous pH-spectrometric titration was developed.
• The influence of number and location of N-atoms on the pKa values was determined.
• Protonation mechanism was proposed based on spectral data.
• Studied compounds present themselves as potentially good chromoionophores.

Seven new dyes – aza-12-crown-4 ether derivatives containing either one or two anthraquinone substituents - were synthesized and their spectroscopic and acid-base properties are discussed in this work. The number, location, type and class of the nitrogen groups in macrocyclic ring differentiate the analyzed compounds. Spectroscopic properties of all investigated derivatives were determined by UV–Vis spectroscopy in acetonitrile:methanol (9:1, v/v) mixture over a wide pH range. The combined potentiometric and spectrophotometric titrations method was used to determine the pKa values of studied compounds. The number and location of nitrogen atoms in the macrocycle ring affects the number of acid-base equilibria and the overall basicity of the compound. Spectral changes provide the information about which of the nitrogen atoms participates in the protonation process. Each of studied compounds presents itself as a potentially good chromoionophore, with the derivative as (3) the most promising one.

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