The electronic absorption spectra of 2-substituted-4-NN-diethylamino-4′-nitroazobenzene dyes and their monoacid cations: the applicability of Dewar's rules to these and related dyestuffs

Abstract

A number of 2-substituted-4-NN-diethylamino-4′-nitroazobenzene dyestuffs have been prepared and their visible electronic spectra determined in neutral and acidic media. In ethanol, electron-donating substituents caused a bathochromic shift of λ_max. the extent of which was proportional to their electron-donating ability. Electron-withdrawing substituents caused a correspondingly smaller hypsochromic shift. When the 2-substituent was either carboxy, hydroxy, or acylamino, the results could be explained in terms of hydrogen-bonding to the azo-link. The tinctorial strength of the dyes, as expressed by the area under the absorption curve, remained essentially constant over a 100 nm range.

In ethanolic hydrogen chloride, electron-donating substituents stabilised the azonium tautomer and generally produced a hypsochromic shift of λ_max.; electron-withdrawing substituents generally had the opposite effect. With the more powerful electron-donating groups, the rare phenomenon of negative halochromism was observed.

The general applicability of Dewar's Rules to both neutral and protonated 4-aminoazobenzene dyes is questioned.