π → π∗ transitions:
These transitions are observed in molecules containing π = electrons and occur at wavelengths approaching near ultraviolet regions The hydrocarbons containing double and triple bonds are classic examples. For example, ethylene absorbs at 170 nm. The conjugation of unsaturation addition increases λmax;
But-1, 3-diene absorbs at 217 nm. The effect of conjugation is quite significant in aromatic molecules. For instance single ring aromatics absorb in the vicinity of 250 nm, naphthalene in the vicinity of 300 nm and anthracene within the vicinity of 360 nm.