Untersuchungen zum UV/Vis‐Spektralverhalten von Azofarbstoffen. XII. Eine Analyse der Korrelationen zwischen Absorptionsmaxima und den Hammett‐σ‐Konstanten der Substituenten

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Abstract
Studies on UV/Vis Absorption Spectra of Azo Dyes. XII. An Analysis of the Correlations between Absorption Maxima and Hammett‐σ‐Constants for the SubstituentsPreviously claimed relationships between wavelength shifts and Hammett‐σ‐constants for the substituents are mostly unfounded. In certain cases, good linear correlations are found, but in other cases the plots deviate appreciably from linearity.We have examined such correlations on the basis of the classification scheme of organic dyes suggested by Fabian (charge transfer‐(CT)‐chromophores, locally excited‐(LE)‐chromophores) respectively Griffiths (donor‐acceptor chromogens, cyanine‐type chromogens).It was shown that good linear correlations should be obtained by LE‐chromophores (cyanine‐type chromogens). In CT‐chromophores (donor‐acceptor chromogens) however, an electron withdrawing substituent in the acceptor fragment causes a bathochromic displacement of the absorption maxima, and in the donor fragment it has little influence. A reversed behaviour was assumed for electron releasing substituents. Thus in CT‐chromophores V‐type correlations should be obtained.These rules are equally applieable to interpret the linear relationships between σ‐constants and the difference between the absorption maxima of two different, equilibrating species.

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