UV–Vis Spectroscopy of Conjugated Systems

Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum attributed to π → π* electronic excitations. Generally, a UV–vis absorption spectrum is recorded as a plot of absorbance vs wavelength. The wavelength of maximum absorbance, which manifests as a peak in the absorption spectrum, is denoted as λ_max.

One of the factors influencing λ_max is the extent of conjugation in the molecule. As shown below, an increase in conjugation shifts λ_max to longer wavelengths.

Highly conjugated π systems absorb in the visible region rather than the UV and appear colored. The color reflected tends to be roughly complementary to the color that is absorbed. For example, lycopene, a compound found in tomatoes, absorbs in the blue region of the visible spectrum at 505 nm and reflects red, which is the perceived color.