GENERAL INTRODUCTION TO THE CHEMISTRY （一般介绍化学）.pdf

GENERAL INTRODUCTION TO THE CHEMISTRY OF DYES 1. Principles of Colour Chemistry 1.1 Basis for colour Unlike most organic compounds, dyes possess colour because they 1) absorb light in the visible spectrum (400–700 nm), 2) have at least one chromophore (colour-bearing group), 3) have a conjugated system, i.e. a structure with alternating double and single bonds, and 4) exhibit resonance of electrons, which is a stabilizing force in organic compounds (Abrahart, 1977). When any one of these features is lacking from the molecular structure the colour is lost. In addition to chromophores, most dyes also contain groups known as auxochromes (colour helpers), examples of which are carboxylic acid, sulfonic acid, amino, and hydroxyl groups. While these are not responsible for colour, their presence can shift the colour of a colourant and they are most often used to influence dye solubility. Figure 1 shows the relationships between wavelength of visible and colour absorbed/observed. Other factors contributing to colour are illustrated in Figures 2–4. Fig. 1. Wavelength of light absorption versus colour in organic dyes Wavelength Absorbed (nm) Colour Absorbed Colour Observed 400–435 Violet Yellow-Green 435–480 Blue Yellow 480–490 Green-Blue Orange 490–500 Blue-Green Red 500–560 Green Purple 560–580 Yellow-Green Violet 580–595 Yellow Blue 595–605 Orange