Conjugated system
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A chemically conjugated system, is a system of atoms covalently bonded with alternating single and double bonds (e.g., C=C-C=C-C) in a molecule of an organic compound. This system results in a general delocalization of the electrons which increases stability and thereby lowers the overall energy of the molecule.
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The electron delocalisation creates a region where electrons are not belonging to a single bond or atom, but rather a group. An example would be phenol (C6H5OH, benzene with hydroxyl group) (diagramatically has alternating single and double bonds) which has a system of 6 electrons above and below the flat planar ring, as well as around the hydroxyl group.
Conjugated systems often have unique properties, such as beta carotene's long conjugated hydrocarbon chain resulting in its strong color. This is because photons of lower energies can excite the bonds with slightly lower activation enthalpy, allowing photons in the visible region of the electromagnetic spectrum to be absorbed. The color is determined by the length of the conjugated system, since the electron can travel the whole length of the chain. A simple way to visualize this is by thinking of it as a particle in a box where E=hf ~ 1/L2, so the frequency, and thus color, of emitted light is proportional to 1/L2. This absorption of light means that conjugated systems are usually analysed using UV/VIS spectroscopy.
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Conjugated systems are often present in chromophores, which are light-absorbing parts of a molecule causing a compound to be colored. Such chromophores are often present in various organic compounds and sometimes present in polymers which are colored or glow in the dark. They are usually caused by conjugated ring systems with bonds such as C=O and N=N in addition to conjugated C-C bonds.