Instrumental Methods of Chemical Analysis

A covalently unsaturated group responsible for absorption in the UV or visible region is known as a chromophore. For example, C = C, C≡C, C=O, C≡ N, N = N, NO2 etc. If a compound absorbs light in the visible region (400-800 nm), only then it appears coloured. Thus, a chromophore may or may not impart colour to a compound depending on whether the chromophore absorbs radiation in the visible or UV region. Chromophores like C =C or C≡C having π electrons undergo π→ π* transitions and those having both π and non-bonding electrons, e.g. C = O, C≡ N or N=N, undergo π→ π*, n→ π* and n→ σ* transitions. Since the wavelength and intensity of absorption depend on a number of factors, there are no set rules for the identification of a chromophore