The influence of chlorine and sulphonate substituents on the visible absorption maxima of some azo dyes

Abstract

A total of 110 arylazo dyes have been prepared, based on five benzenoid and heterocyclic coupling components, and containing combinations of chlorine and sulphonate (sulpho) substituents in the diazo components. Their visible absorption maxima have been analysed in order to determine the degree to which chloro and sulpho substituent influences are constant and transferable, both within a series, and between series. Additivity is found to pertain, if allowances are made for steric effects. A 2-sulpho group alone can induce nonplanarity, depending on the nature of the coupling component. The 2-sulpho-3-chloro combination is particularly responsible for hypsochromic shifts unexpected on the basis of strict additivity. Computer-aided molecular modelling studies have clarified the steric interactions present. Di-ortho-substitution also induces nonplanarity and hypsochromic effects. There is some evidence for non-equivalence of a substituent in the two distinct meta-orientations, although MO calculations do not reproduce this behaviour.