What are red shift and blue shift in UV spectroscopy?

Answer 1

The solvent in which the absorbing species is dissolved also has an effect on the spectrum of the species. Peaks resulting from n ® p* transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity. This arises from increased solvation of the lone pair, which lowers the energy of the n orbital. Often (but not always), the reverse (i.e. red shift) is seen for p ® p* transitions. This is caused by attractive polarisation forces between the solvent and the absorber, which lower the energy levels of both the excited and unexcited states. This effect is greater for the excited state, and so the energy difference between the excited and unexcited states is slightly reduced - resulting in a small red shift. This effect also influences n ® p* transitions but is overshadowed by the blue shift resulting from solvation of lone pairs.