Dyes: beautifying substance?

What's up Guys? We are back after long time with some basic chemistry of dyes we used everyday. Hope you will enjoy it.

Dyes:
The chemical substances which are used to impart colour to fabrics, foods and others objects for their beautification and distinction are called dyes. Dye is a coloured substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber. A dye should have an attractive colour and it must be able to attach itself to the substrate to impart a washfast and light fast colour to it. Some substances which are themselves not coloured but they brighten the colour imparted by another dye are called white dyes. White dyes are optical brighteners or whiteners.

Why a substance appears colored?
All things around us are made up of compounds, and all compounds are made up from atoms. The colour is a physical property of chemicals that in most cases comes from the excitation of electrons due to an absorption of energy performed by the chemical. The study of chemical structure by means of energy absorption and release is generally referred to as spectroscopy. When light strikes the substance specific frequencies of the light are absorbed when electrons in the compound are promoted to higher energy levels, called an excited state.

The colour which we see is what we get when the remaining light is reflected back off the surface for most materials the electrons drop back down to the ground state in a number of steps. The amount of energy absorbed or released is the difference between the energies of the two quantum states. There are various types of quantum state, including the rotational and vibrational states of a molecule. On the other hand, if the substance absorbs only one wave length and reflects all other, then it attains the complementary colour of the absorbed wave length. A substance will appear coloured only when it absorbed some wave length in the visible range of the spectrum means the molecule of the substance absorb photons of the
visible light. The energy of photon in the visible range corresponds to the energy separation among the bonding and antibonding orbitals of the molecule. When a molecule absorbs a photon of the visible frequency, one of it is bonding or non bonding electrons jump to the vacant antibonding orbitals. The orbitals present in organic molecules are σ, π and nb orbitals and corresponding antibonding σ' and π' orbitals. When a molecule absorbs photon of visible Light commonly causes π→π' and n → π' transition of electrons. Higher energy is needed for σ→σ' and π→π' transition of electrons which takes place in ultraviolet region of the spectrum. Organic molecule containing NO2, NO, N = N and quinonoid structure absorb photon of visible light to undergo π→π' or n→π'electronic transition. Hence, these compounds are coloured compounds. For example, nitrobenzene is pale yellow, azobenzene is yellow orange and nitrosobenzene is green in colour. Such unsaturated groups capable of imparting colour to organic molecules are called chromophores and coloured compounds are known as chromogens. Dependent chromophores do not impart colour alone but impart colour in combination with other chromophores for examples,＞C=O, x=c<, etc.