Basics of UV-visible absorption spectroscopy

Absorption bands position

The position of an absorption band can be influenced by several factors such as solvent, steric, inductive or mesomeric effects …..

Absorption spectrum with a peak at 500nm. Before the peak: hypsochromic effect, after the peak: bathochromic effect.

Hypsochromic shift

A hypsochromic shift is a shift towards lower wavelengths (UV)

Bathochromic shift

A bathochromic shift is a shift towards higher wavelengths (IR)

Example

In chloroform, the absorbance of caffeine is upshifted (bathochromic shift) compared to its absorption in water. The energy difference between fundamental and excited states is smaller in chloroform.

Woodward-Fieser rules:

The Woodward and Fieser rules allow to predict the wavelength for the maximum of absorption of conjugated systems.

When the number of delocalized electrons increases in a molecule, its chromophores face a bathochromic effect.

Schematic representation

Two absorption spectra are present. The two red and green curves are identical, but shifted in abscissa of 35 nm.

Molecule A possesses one C=C bond responsible for its absorption at 185 nm. Molecule B possesses the same chromophore but with delocalized electrons. Its absorption band is then shifted to higher wavelengths: bathochromic effect.

Molecule C : lambda max = 255 nm, molecule B lambda max = 220 nm, molecule A lambda max = 185 nm.

Molecules A, B and C possess the same chromophore, C=C, but with an increasing number of delocalized electrons. Their absorption is therefore upshifted (bathochromic effect).