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16.9: UV–Vis Spectroscopy of Conjugated Systems

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Organic Chemistry

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UV–Vis Spectroscopy of Conjugated Systems

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.



UV-Vis Spectroscopy Conjugated Systems Organic Compounds Absorption Features UV-visible Region Electronic Excitations Absorbance Vs Wavelength Maximum Absorbance Wavelength Of Maximum Absorbance Extent Of Conjugation Shift In Wavelength Highly Conjugated Systems Visible Region Colored Compounds Complementary Colors

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