Chemical structure of beta-carotene. The ten conjugated double bonds that form the chromophore of the molecule are highlighted in red.
UV spectroscopy is most useful when conjugated systems are present. The more extended the conjugation, the smaller the HOMO-LUMO energy gap (HOMO = Highest Occupied Molecular Orbital. LUMO = Lowest Unoccupied Molecular Orbital). The most extended conjugated systems are pigments that have absorption maxima in the visible range, not the UV range, although lmax may be in the UV range, the compound can still be colored with a broad peak that extends into the visible range.
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