what are dyes? why do dyes articles appear to have a characteristic colour
Answers
Answer:
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 inRegarding the requirement of a chromophore generating colour in organic compounds, it is important to note that the chromophore must be part of a conjugated system. This is illustrated through the examples in Figure 5 where it can be seen that placement of an azo group between methyl groups produces a colourless compound, while a yellow-orange colour is obtained when the azo group is placed between aromatic rings. Similarly, the structures in Figure 3 demonstrate the importance of having an extended conjugated system. It is evident that doubling the length of the conjugated system in Vitamin A to give β -carotene causes a significant bathochromic shift, i.e. to a darker colour.In addition to influencing solubility, auxochromes are essential ring substituents in providing target colours. This is illustrated in Figure 6, where the following effects of substituents are shownAdding groups of increasing electron-donating ability to the azobenzene structure has a bathochromic effect (cf. OH vs NH2).
Electron-donating (NH2) and electron-accepting (NO2) groups placed in conjugation provide a bathochromic effect. In this regard, nitro groups are especially beneficial, contributing to their prevalence in disperse dye structures.
Increasing the number of electron-attracting groups conjugated with the electron-donor has a bathochromic effect.
The electron-donating effects of an amino group are enhanced by adding alkyl groups to the N-atom.
Explanation:
Dyes posses colour because they absorp light in the visible spectrum they have at least one chromophore