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While this method of classification is undoubtedly of importance theoretically, it is arguably of lesser practical importance, since the vast majority of commercial organic dyes and Chapter 2 26 pigments for traditional applications belong to the donor—acceptor category. The mechanism of the electronic excitation process, an understanding of which is fundamental in establishing relationships between the colour and the molecular constitution of dyes, is discussed in the following sections of this chapter.

The third attribute, brightness, may be described in various other ways, for example as brilliance or vividness. This characteristic of the colour depends on the absence of wavelengths of transmitted light other than those of the hue concerned. Electronic absorption bands of molecular compounds are not infinitely narrow because they are broadened by the superimposition of numerous vibrational energy levels on both the ground and excited electronic states. Brightness of colour is characterised, in terms of the UV/visible spectrum, by the shape of the absorption band.

Structural factors, both electronic and steric, which either stabilise or destabilise the first excited state relative to the ground state, are analysed to provide a qualitative explanation of colour. The assumptions which the approach makes concerning the structures of the ground and the first excited states are clearly approximations and cannot be rigorously justified. Nevertheless, evidence that the approximations are reasonable is provided by the fact that the method works well, at least in qualitative terms, in such a large number of cases.