Azo-hydrazone tautomerism in azo dyes. IV. Colour and tautomeric structure of adsorbed 1-phenylazo-2-naphthylamine and 1-phenylazo-2-naphthol dyes

Abstract

The tautomeric compositions of 1-phenylazo-2-naphthylamines (1), -naphthols (2) and their monosulphonates ( 3 and 4) have been investigated in the adsorbed state either on dyed polyester and wool fibres or in KBr pellets by means of scattered transmission, diffuse reflectance and Raman spectroscopy. The azo structure predominates in the naphthylamine series ( 1 and 3) in accord with previous results in solutions or with predictions by the perturbational molecular orbital theory. The major tautomer of the investigated naphthols ( 2 and 4) is mainly h, but both forms are present in 2(p-MeO) and 4(p-MeO) in comparable concentrations. Their h content depends on the nature of the substrate and increases in the order PES < WO < KBr. The colour is influenced by crystal field effects in the solid state as was shown for 1(p-NO 2). Utilizing a set of inductively active substituents for R, the frequency of the first absorption band of the dyes correlates well with Hammett σ, yielding a positive slope with the hydrazones and a negative one with the azo tautomers. This behaviour leads to an inversion of the colour sequence upon changing the tautomer composition both in solution and in the adsorbed state and can be generalized by perturbational MO arguments.