Page 185 - Basic Principles of Textile Coloration

174 AN INTRODUCTION TO DYES AND DYEING

CHAPTER 10

An introduction to dyes and dyeing

10.1 DYES

10.1.1 The molecular structures of dyes

The structures of dye molecules are complex in comparison with those of most
common organic compounds. Compare the molecular formula of acetic acid,
C2H4O2, with that of even a simple dye such as Orange II, C16H11N2SO4Na
(Figure 1.2). Despite their complexity, dye structures have a number of common
features. Most dye molecules contain a number of aromatic rings, such as those of
benzene or naphthalene, linked in a fully conjugated system. This means that
there is a long sequence of alternating single and double bonds between the
carbon and other atoms throughout most of the formal written structure. This type
of arrangement is often called the chromophore or colour-donating unit. The
conjugated system allows extensive delocalisation of the p electrons from the
double bonds and results in smaller differences in energy between the occupied
and unoccupied molecular orbitals for these electrons. At least five or six
conjugated double bonds are required in the molecular structure for a compound
to be coloured. The wavelength of the absorbed light able to excite an electron
from an occupied orbital to the first unoccupied molecular orbital then
corresponds to visible light. The smaller the energy difference, the longer the
wavelength of the light absorbed, according to the Planck equation:

DE = hn = hc (1)

l

In this equation, DE is the energy difference between the implicated molecular
orbitals, h is Planck’s constant, n is the frequency and c is the speed of the
absorbed light, and l is its wavelength. Conjugation results in a framework with all
the atoms lying in the same plane. Figure 10.1 shows some typical examples of dye
structures with different chromophores.

Dyes with good water solubility usually have an ionic centre in their molecules.
This may be relatively localised, as in a sulphonate group attached to a benzene

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