The Valence-bond Approach to Colour and Constitution

The valence-bond approach may be used to provide a qualitative account of the /lmax values, and hence the hues, of many dyes, particularly those of the donor acceptor chromogen type. The use of this approach to rationalise differences in colour is illustrated in this section with reference to a series of dyes which may be envisaged as being derived from azobenzene, although in principle the method may be used to account for the colours of a much wider range of chemical classes of dye, including anthraquinones (see Chapter 4), polymethines and nitro dyes. 

Compound 20 is noteworthy in that it was the first azo dye reported whose absorption band is shifted beyond the visible region into the 

It can be seen that the valence-bond (resonance) approach may be used to provide a reasonable qualitative explanation of the dmax values of a 

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