moieties in our red blood cells, whose primary function is to bind iron atoms which capture oxygen, result in the
heme chromophores which give human blood its red color. Heme is degraded by the body into
biliverdin (which gives bruises their blue-green color), which in turn is degraded into
bilirubin (which gives patients with
jaundice a yellow skin tone). , the molecule
retinal is a
conjugated chromophore.
Retinal begins as 11-cis-retinal, which, upon capturing a
photon γ (light) of the correct wavelength, straightens out into
all-trans-retinal which pushes against an
opsin protein in the
retina, which triggers a
chemical signaling cascade which results in
perception of light or images by the brain. Just like how two adjacent p-orbitals in a molecule will form a
pi-bond, three or more adjacent p-orbitals in a molecule can form a
conjugated pi-system. In a conjugated pi-system, electrons are able to capture certain photons as the electrons resonate along a certain distance of p-orbitals - similar to how a
radio antenna detects photons along its length. Typically, the more conjugated (longer) the pi-system is, the longer the wavelength of photon can be captured. In other words, with every added adjacent double bond we see in a molecule diagram, we can predict the system will be progressively more likely to appear yellow to our eyes as it is less likely to absorb yellow light and more likely to absorb red light. ("Conjugated systems of fewer than eight conjugated double bonds absorb only in the ultraviolet region and are colorless to the human eye", "Compounds that are blue or green typically do not rely on conjugated double bonds alone.") In the
conjugated chromophores, the
electrons jump between energy levels that are extended
pi orbitals, created by electron clouds like those in
aromatic systems. Common examples include
retinal (used in the eye to detect light), various
food colorings, fabric
dyes (
azo compounds),
pH indicators,
lycopene,
β-carotene, and
anthocyanins. Various factors in a chromophore's structure go into determining at what wavelength region in a spectrum the chromophore will absorb. Lengthening or extending a
conjugated system with more unsaturated (multiple) bonds in a molecule will tend to shift absorption to longer wavelengths.
Woodward–Fieser rules can be used to approximate
ultraviolet-visible maximum absorption wavelength in organic compounds with conjugated pi-bond systems. Some of these are metal complex chromophores, which contain a metal in a
coordination complex with ligands. Examples are
chlorophyll, which is used by plants for photosynthesis and
hemoglobin, the oxygen transporter in the blood of vertebrate animals. In these two examples, a metal is complexed at the center of a
tetrapyrrole macrocycle ring: the metal being iron in the
heme group (iron in a
porphyrin ring) of hemoglobin, or magnesium complexed in a
chlorin-type ring in the case of chlorophyll. The highly conjugated pi-bonding system of the macrocycle ring absorbs visible light. The nature of the central metal can also influence the absorption spectrum of the metal-macrocycle complex or properties such as excited state lifetime. The tetrapyrrole moiety in organic compounds which is not macrocyclic but still has a conjugated pi-bond system still acts as a chromophore. Examples of such compounds include
bilirubin and
urobilin, which exhibit a yellow color. ==Auxochrome==