Dyes & Color - Fibre2fashion

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.
Ultra
Viole
4000 4500 5000 5500 6000 7000
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 Colour absorbed Visible colour
absorbed (Å)
(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
-N= 0
+
N
0
0
Nitroso
-N= N-
Azo
0 0
O O
P-Q uina d O-Quinad
Thus for example nitrobenzene is pale yellow, azobenzene is orange-red, pquinones
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
0
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.
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
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 βcarotene
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.
H2C + CH2
Ethylene
(Colourless)
H
3CH3 H
2C
H
2C
C
CH2 = CH – CH = CH – CH = CH2
1: 3: 5 Hexantriene
(Colourless)
H2
H
3
H3 H3 H3 H3
CCH=CHC=CHCH=CHC=CHCH=CHCH=CCH=CHCH=CCH=CHC
CH2=CH–CH = CH2
1: 3 Butadiene
(Colourless)
H
3CC
H
3CH3 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
C
H2
H2
H2

ings 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.