In the previous lecture, we studied that UV/Visible radiation is absorbed by the molecules through transition of electrons in the chromophore from low energy molecular orbitals to higher energy molecular orbitals. We are interested in the transitions that lie in the far UV, near UV, and visible regions of the electromagnetic spectrum. The molecules that absorb in these regions invariably have unsaturated bonds. Plants are green due to unsaturated organic compounds, called chlorophylls. A highly unsaturated alkene, lycopene, imparts red color to the tomatoes (Figure 5.1).

Figure 5.1 Structure of lycopene, the pigment that imparts red color to the tomatoes

As can be seen from its structure, lycopene is a highly conjugated alkene. As compared to the simple non-conjugated alkenes that typically absorb in vacuum UV region, absorption spectrum of lycopene is hugely shifted towards higher wavelengths (or lower energy). There can be factors that could shift the absorption spectra to smaller wavelengths or can increase/decrease the absorption intensity. Before understanding how conjugation causes shift in the absorption spectra, let us look at some important terms that are used to refer to the shifts in absorption spectra (Figure 5.2):

Bathochromic shift : Shift of the absorption spectrum towards longer wavelength

Hypsochromic shift : Shift of the absorption spectrum towards smaller wavelength

Hyperchromic shift : An increase in the absorption intensity

Hypochromic shift : A decrease in the absorption intensity

 

Figure 5.2 Terminology for shifts in absorption spectra