Dyes & color By Mansoor Iqbal Senior Research Associate, Textile PCSIR Lab Complex Karachi e-mail : mansoorprocessing@hotmail.com

Dyes & Color Any coloured compound is not a Dye or Dyestuff. A dye is a coloured

organic compound that absorbs light strongly in the visible region and can firmly attach to the fiber by virtue of chemical and physical bonding between group of the dye and group on the fiber. To be of commercial importance a dye should be fast to light, rubbing and water.

Colour and dye have always played an important role in the life of man from time immemorial. Preparation of a colour and dyeing of cloth date back to antiquity. Fabrics dyed in indigo were found in the tombs of predyanstic Egypt. Let us now try to understand how we get sensation of colour. Modern theory of colour:

Colour is a physiological sensation associated with the wavelength of light striking the retina of the eye. The sensation of colour is produced when light having a wavelength within the visible region of electromagnetic spectrum strikes the retina of the eye. The visible region of the spectrum extends from 4000 to 7500 Å in wavelength.

4000 4500 5000 5500 6000 7000 Ultra Viole

Violet Blue Green Yellow Orange Red Infra red

High Increasing Energy Low When white light falls on a substance, the light may be completely reflected and in this case substance will appear white. If it is completely absorbed, the substance will appear black. If a substance absorbs all visible light except that corresponding to e.g. yellow, it will transmit or reflect only yellow colour and will be seen as yellow. However, it is generally seen that, light of only one colour is absorbed in which case the substance will appear to have the complementary colour. Thus, if the light is absorbed from the violet region of spectrum, the substance will be seen as yellow. If light is absorbed from the red region, the substance will appear green.

Wavelength absorbed (Å)

Colour absorbed Visible colour (complementary colour)

4000 – 4350 Violet Yellow Green 4350 – 4800 Blue Yellow 4800 – 4900 Green blue Orange 4900 – 5000 Blue green Red 5000 –5600 Green Purple 5600 – 5800 Yellow green Violet 5800 – 5950 Yellow Blue 5950 – 6050 Orange Green blue 6050 – 7500 Red Blue green

Otto Witt theory of colour (1876): An early theory of dyes first formulated by O. Witt provided a basis for understanding the reaction between colour and structure of the molecule. According to the O. Witt colour theory a dye is made up of two essential kinds of parts, Chromophores and Auxochromes. He designated a group that produces colour as a chromophore (Gr, Kuroma. colour + Phors carrier). Chromophores are unsaturated groups. Presence of at least one such group is essential to produce a colour in an organic compound and a molecule containing such a group is called as chromogen. Some most effective chromophores are

0 00

O

-N= N--N= 0

Azo

Nitroso

N+

0

0O

P-Quinad O-Quinad

Thus for example nitrobenzene is pale yellow, azobenzene is orange-red, p-quinones are yellow and o-quinones are orange or red. Certain other unsaturated groups produce colour only when several of them are present in a molecule and when they are conjugated. They are

Thus though acetone is colourless, biacetyl colour.

C = C

Ethylene

C = O

Carbonyl

C = N -

Azomethine

O || CH3 – C – CH3 Acetone Colourless O O || || CH3 – C – C – CH3 Biacetyl Yellow O O || || CH3 – C – CH3 – C – CH3 Acetonyl Colourless

Acetone O. Witt also observed that certain groups, while not producing colour themselves, are able to intensify the colour when present in a molecule together with a chromophore. These are called auxochromes (Gr, auxanein = to increase). The most effective auxochromes

H

| –OH –OR –NH2 –N–R –NR2 Hydroxyl Alkoxy Amino Alkylated Amines Thus nitrophenols and nitroanilines are more intensely coloured than nitrobenzene and aniline and are deep yellow to orange. Further auxchoromes are salt forming groups, i.e., they are basic or acidic and makes the coloured compound to attach itself to the fabric, so that it is fast to light, soap and water. Acidic auxochromes like – OH, --COOH and – SO2H give acidic dyes and basic auxochromes like – NH2 – NHR and – NR2 gives basic dyes. Auxochromes like – SO3H group has little value as auxochrome but it has a solublishing effect. The halogen atom also functions as auxochrome and the relative order of colour intensifying effect is I>Br>Cl. It can be observed that all the auxochromic groups contain atoms with unshared pair of electrons. According to Witt theory of colour and constitution chromogen is a compound which contains a chromophore –N=N. It is a bright red compound but not a dye.

C6H5 – N = N – C6H5 On the other hand p-hydroxy-azo benzene is acid dye because

H2O – C6H4 – N = N – C6H5 It contains – OH group, an acid, auxochrome, and p-amino azobenzene is a basic dye, as it has basic auxochrome – NH2.

Azobenzene, anthraquinone, dinitro benzene are chromogens O O || || and are coloured due to the presence of –N=N, --C--C, --NO2, groups respectively. The chloromogens, on reduction give the colourless compounds, for examples azobenzene, a bright red compound, on reduction forms the colourless hydrazobenzene.

H2 C6H5 – N = N – C6H5 C6H5 – NH – NH –C6H5 Azobenzene Hydrazobenzene Sometimes the conversion is reversible. In this case the reduction products are called “Lecuo compounds”. H2 Azobenzene Red Hydrazobenzene (colourless)

Oxidation

H2 Indigo Blue Indigo white (colourless)

Oxidation Sometimes reduction completely decomposes the coloured compound, such reduction products are called “Leuco compounds”. Valence bond approach to colour: Like many other theories, the Witt theory has also been replaced by modern electronic theory. According to this theory, it is the resonance stabilization of excited states that is responsible for the absorption in the visible region. When ultraviolet or visible light is absorbed by a molecule, an electron is excited, that is, it is promoted to an orbital of higher energy. The wavelength of light absorbed depends on the energy difference between the excited and ground states of the molecule. The smaller difference between the two states, the longer is the wavelength of the light absorbed. The energy required to promote an electron depends upon the environment of the electron. Sigma (σ) bond electrons are firmly held and very high energy (or short wavelength) is necessary to promote electrons and may at times break the molecule and form free radical. Pi (π) electrons are less firmly held and require less energy (or longer wavelength) to excite. Electrons belonging to conjugated systems required even less energy (still longer length). Conjugation and resonance stabilize the excited state by sharing and delocalizing higher energy of the excited electron.

As conjugation and resonance increases, the wavelength of light absorbed also increases and when the wavelength is long enough to be in the visible region, we observe colour. This can be explained with the help of following example. Ethylene absorbs light in the ultraviolet part of the spectrum 1800 Å. Butadiene, with two conjugated double bonds, absorbs at 8170 Å (a wavelength closer to visible region) and hexatriene, with three conjugated double bonds, absorbs at 2580 Å (a wavelength still closer to visible region). But all the three compounds are colourless. However, as the number of conjugated double bonds increases, the absorption falls in the visible region, for example in β-carotene there are eleven conjugated double bonds and absorbs at 4510 Å, that is, in the visible region. The light absorbed is blue and we see the complementary orange colour.

CH2=CH–CH = CH2 1: 3 Butadiene (Colourless)

CH2 = CH – CH = CH – CH = CH2 1: 3: 5 Hexantriene (Colourless)

H2C + CH2 Ethylene

(Colourless)

H

H

2

22

2

23

C

C

H

H

H

H3 3C

C

H

H H32

CCH

H3 3C

C

H3 3 3 3

CCH=CHC=CHCH=CHC=CHCH=CHCH=CCH=CHCH=CCH=CHC

H H H H

Β-CAROTENE Benzene absorbs light at 2550 Å and is colourless. Aniline, which absorbs light at about 3000 Å, is also colourless; nitrobenzene absorbing light slightly above 4000 Å is pale yellow and p-nitro aniline absorbing light at 4500 Å is a yellow compound. Bathochromic effect: In this case benzene ring may be considered to be chromophore, while amino group and nitro group auxochromes. When they are conjugated, the longer resonance system decreases the energy gap between the ground state and excited state transitions, thus producing visible colour. All these groups, which lengthen wavelength of absorption, are bathochrome groups. Thus displacements (or shift) to longer wavelength are known as bathochromic effects or bathochromic shift and displacements to shorter wavelength are hypsochromic. Hypsochromes are groups which decrease resonance. This is done by forcing the pi (π) orbitals out of planarity. For example when alkayl group on benzene ring is ortho to adjacent rings or chains, the molecule is distorted out of planarity and resonance is decreased. As the number of fused

rings increases, the absorption in the visible region also increases e.g. naphthacene absorbs in blue region and is yellow. Pentacene absorbs in orange region and is blue. Graphite, which is a sheet of benzene rings is black, it absorbs all colours almost completely.