Bathochromic and Hypsochromic Shift

 Bathochromic shift - It is the shift of the absorption maxima (λ_max) of a substance towards longer wavelengths. It is also known as red shift. 

Since, Energy (E) is inversely proportional to Wavelength. So, in Bathochromic shift----energy decreases and wavelength increases.

In Bathochromic shift, energy decreases and wavelength increases.

Factors causing Bathochromic shift: The bathochromic shift is caused by following:

A. Extending Conjugation

i. either by increasing double bonds

ii. or by adding electron donating groups (like -OH, -NH₂) in conjugation to double bonds.

B. Solvent Effects

i. The less polar (non-polar) solvents cause bathochromic shift for n-π* transitions.

ii. The more polar (polar) solvents cause bathochromic shift for π-π* transitions.

Example:

Benzene shows  π-π* electronic transitions at 255 nm. Adding auxochrome NH₂ to the benzene ring makes it aniline and shifts the absorptions to 280 nm.

  

Hypsochromic shift: It is the shift of the absorption maxima (λ_max) of a substance towards shorter wavelengths. It is also known as blue shift. 

In Hypsochromic shift, energy increases and wavelength decreases.

Factors causing Hypsochromic shift: The Hypsochromic shift is caused by following:

A. Removing Conjugation

i. either by decreasing double bonds

ii. or by adding electron withdrawing groups (like -CHO, -NO₂) in conjugation to double bonds.

B. Solvent Effects

i. The less polar (non-polar) solvents cause Hypsochromic shift for π-π* transitions.

ii. The more polar (polar) solvents cause Hypsochromic shift for n-π* transitions.

Example:

Aniline shows  π-π* electronic transitions at 280 nm. Adding proton to NH₂ group of aniline makes it anilium ion and lowers conjugation thereby shifting the absorptions to 203 nm.

The hypsochromic shift (Blue shift) and bathochromic shift (red shift) are opposite to each other.