UV Spectroscopy: What is chromophore

 Chromophoric group or simply chromophore is the part of the molecule or compound which is responsible for the characteristic absorption of light at a particular wavelength or we can say it is the part of molecule which is responsible for the colour.

Ensure that you'd read the UV spectroscopy earlier. Click to read

Well! Retinal - the vitamer of Vitamin A is also a chromophore, but we won’t study it here.

Lets explain chromophore somewhat more clearly..

We studied earlier that upon irradiation, the electrons gets excited from ground state to the excited state. Now if we recall the structure of an atom, the electrons remain strongly bound by the nucleus. So, nucleus plays a very important role here.

The nuclei decides the strength with which the electrons will be held and hence the difference between the ground and excited states. Therefore, the particular wavelength which will be absorbed by the compound and the energy of transition depends upon the nuclei instead of electrons themselves.  

 

Auxochrome

Replacement of the hydrogen of the chromophoric group with any other substituent (e.g. OH, CH3, Cl etc) changes the intensity of chromophoric absorption. The substituent do not give any characteristic absorption of radiation, but their presence can modify the absorption intensity of chromophore.

If that replacement leads to an increase in the absorption intensity, the groups are termed as Auxochromes.

The presence of other substituents can lead to the following effects:

• ·        Hyperchromic effect: There will be increase in the intensity of absorption.
• ·        Hypochromic effect:  Decrease in intensity of absorption.
• ·        Hypsochromic Shift(Blue shift): Shift to the higher energy (Shorter wavelength)
• ·        Bathochromic shift (Red shift):  Shift towards lower energy (Higher wavelength).

 

Effect of conjugation in the molecule on its absorption spectrum pattern

Conjugation is simply the presence of alternate single and double bonds.  For example, 1,3- Butadiene is  more conjugated than But-1-ene.

Now how does conjugation in a system can affect the absorption?

Conjugation moves the absorption pattern to the right side i.e. Bathochromic or red shift. Also, the intensity of absorption increases.

Explaining it briefly- Conjugation decreases the energy required by the electrons to get excited. As it can be explained on the basis of Molecular Orbital Theory (MOT), the increase in conjugation decreases the gap between the energy levels. So, the transition step from the HOMO to the LUMO becomes easier. That’s why the electrons need lesser energy to get excited which can be said that higher wavelength radiation will be used for the purpose leading to Bathochromic shift.