16.9:
UV–Vis Spectroscopy of Conjugated Systems
The UV–visible region of the electromagnetic spectrum extends from 200 to 800 nm.
Organic compounds with conjugated π systems strongly absorb in this region, causing electronic transitions.
Recall that 1,3-butadiene has four π molecular orbitals. The HOMO is a π-bonding orbital occupied by two electrons, and the LUMO is an empty π* antibonding orbital.
Absorption of UV light at 217 nm promotes one electron from the HOMO to the LUMO, resulting in a π → π* transition.
A plot of the absorption intensity versus wavelength gives the UV–visible absorption spectrum of 1,3-butadiene, where the absorption maxima, λmax, corresponds to the HOMO–LUMO energy gap.
Interestingly, as the extent of conjugation increases, the HOMO–LUMO energy gap decreases, and λmax shifts to higher wavelengths.
In highly conjugated systems, λmax extends into the visible region, imparting color to the molecule.
For example, β-carotene, a compound found in carrots, has 11 double bonds. It absorbs strongly at 457 nm, which is the blue region of the visible spectrum, but reflects the complementary color and appears orange.
16.9:
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 λmax.
One of the factors influencing λmax is the extent of conjugation in the molecule. As shown below, an increase in conjugation shifts λmax 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.
