What is Chromophore and auxochrome.

Chromophore is the part of the molecule responsible for the colour in any compound.
This can be any functional group with large number of 𝝅 electrons like conjugated double bonds. Conjugated Double bonds means alternating double and single bonds
For eg beta carotene contains 11 conjugated double bonds and this conjugated system imparts orange colour to beta carotene.
Some other examples of chromophores are Nitro group and Azo groups which are very common in colouring dyes.
Also it can be stated that the more number of pi bonds reduces the energy gap between bonding and anti bonding states, thus reducing the energy of transition.
As energy is inversely proportional to wavelength, the absorption band shifts towards higher wavelength. This type of shift is known as Bathochromic shift.
On the contrary if the conjugation is reduced, Energy gap between bonding and anti bonding increases and thus wavelength of absorption decreases. This shift towards lower wavelength is called Hypsochromic shift.
Auxochrome is the part of the molecule responsible for the intensity of colour in any compound. These are functional groups with lone pairs of electrons or non bonding electrons. Please note that auxochrome by themselves can not impart colour to any compound but they can only increase the intensity of colour. E.g. - OH group in p-nitro phenol.
For example benzene is a colourless liquid, also phenol is colourless. So addition of auxochrome -OH group to benzene does not impart any colour to benzene. On the other hand if nitro group which is chromophoric group is added to benzene. Nitrobenzene becomes slightly yellow or pale yellow. And now if we add OH group to nitrobenzene, it increases the intensity of yellow colour as in case of p-nitro phenol which is dark yellow in colour. So chromophores give colour to compounds but auxochromes only increases the intensity of the colour.
When intensity of colour increases the height of absorption peak in UV spectra also increases. This shift in intensity of absorption band is called as Hyperchormic shift.
On contrary if the auxochromic groups are reduced, intensity of peak reduces. This is called Hypochromic shift.
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