Ultraviolet–visible absorption Chromophores
Molecules that absorb light at these wavelengths are called chromophores. Chromophores are functional groups of a molecule that absorb light in this UV-Visible region. They are most of the time characterized by delocalized pi electrons. Pi electrons refer to a type of bond that occurs between electron orbitals called pi orbitals. When many of these pi bonds exist in a molecule this allows electrons to be delocalized or spread out across a molecule. An example of this type of molecule is pictured below. These molecules can be used for pH indicators to determine if a solution is acidic or basic. The addition of acid or base disrupts the delocalized pi electrons. This disruption causes a color change.