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 radio signals along its length. Typically, the more conjugated (longer) the pi-system is, the longer the wavelength of photon can be captured. Compounds whose molecules contain a sufficient number of conjugated bonds can absorb light in the visible region, and therefore appear colorful to the eye, usually appearing yellow or red. Many
dyes make use of conjugated electron systems to absorb
visible light, giving rise to strong colors. For example, the long conjugated
hydrocarbon chain in
beta-carotene leads to its strong orange color. When an electron in the system absorbs a
photon of light of the right
wavelength, it can be promoted to a higher energy level. A simple model of the energy levels is provided by the
quantum-mechanical problem of a one-dimensional
particle in a box of length L, representing the movement of a π electron along a long conjugated chain of carbon atoms. In this model the lowest possible absorption energy corresponds to the energy difference between the highest occupied molecular orbital (
HOMO) and the lowest unoccupied molecular orbital (LUMO). For a chain of
n C=C bonds or 2
n carbon atoms in the molecular
ground state, there are 2
n π electrons occupying
n molecular orbitals, so that the energy gap is :E_{n+1} - E_n = \frac{(2n+1)\hbar^2 \pi ^2}{2mL^2} Since the box length
L increases approximately linearly with the number of C=C bonds
n, this means that the energy Δ
E of a photon absorbed in the HOMO–LUMO transition is approximately proportional to 1/
n. The photon
wavelength λ =
hc/Δ
E is then approximately proportional to
n. Although this model is very approximate, λ does in general increase with
n (or
L) for similar molecules. For example, the HOMO–LUMO absorption wavelengths for conjugated
butadiene, hexatriene and octatetraene are 217 nm, 252 nm and 304 nm respectively. However, for good numerical agreement of the particle in a box model with experiment, the single-bond/double-bond bond length alternations of the polyenes must be taken into account. Alternatively, one can use the
Hückel method which is also designed to model the electronic structure of conjugated systems. Many electronic transitions in conjugated π-systems are from a predominantly
bonding molecular orbital (MO) to a predominantly
antibonding MO (π to π*), but electrons from non-bonding
lone pairs can also be promoted to a π-system MO (n to π*) as often happens in
charge-transfer complexes. A HOMO to LUMO transition is made by an electron if it is allowed by the
selection rules for
electromagnetic transitions. Conjugated systems of fewer than eight conjugated double bonds absorb only in the ultraviolet region and are colorless to the human eye. With every double bond added, the system absorbs
photons of longer wavelength (and lower energy), and the compound ranges from yellow to red in color. Compounds that are blue or green typically do not rely on conjugated double bonds alone. This absorption of light in the ultraviolet to visible spectrum can be quantified using
ultraviolet–visible spectroscopy, and forms the basis for the entire field of
photochemistry. Conjugated systems that are widely used for synthetic
pigments and
dyes are
diazo and
azo compounds and phthalocyanine compounds.
Phthalocyanine compounds Conjugated systems not only have low energy excitations in the visible spectral region but they also accept or donate electrons easily.
Phthalocyanines, which, like
Phthalocyanine Blue BN and
Phthalocyanine Green G, often contain a transition metal ion, exchange an electron with the complexed
transition metal ion that easily changes its
oxidation state. Pigments and dyes like these are
charge-transfer complexes.
Porphyrins and similar compounds Porphyrins have conjugated molecular ring systems (
macrocycles) that appear in many
enzymes of biological systems. As a
ligand, porphyrin forms numerous
complexes with metallic ions like
iron in
hemoglobin that colors blood red. Hemoglobin transports oxygen to the cells of our bodies. Porphyrin–metal complexes often have strong colors. A similar molecular structural ring unit called
chlorin is similarly complexed with
magnesium instead of iron when forming part of the most common forms of
chlorophyll molecules, giving them a green color. Another similar macrocycle unit is
corrin, which complexes with
cobalt when forming part of
cobalamin molecules, constituting
Vitamin B12, which is intensely red. The corrin unit has six conjugated double bonds but is not conjugated all the way around its macrocycle ring.
Chromophores Conjugated systems form the basis of
chromophores, which are light-absorbing parts of a molecule that can cause a compound to be colored. Such chromophores are often present in various organic compounds and sometimes present in
polymers that are colored or glow in the dark. Chromophores often consist of a series of conjugated bonds and/or ring systems, commonly aromatic, which can include C–C, C=C, C=O, or N=N bonds. . The eleven conjugated double bonds that form the
chromophore of the molecule are highlighted in red. Conjugated chromophores are found in many
organic compounds including
azo dyes (also
artificial food additives), compounds in fruits and vegetables (
lycopene and
anthocyanidins),
photoreceptors of the eye, and some pharmaceutical compounds such as the following: called
Amphotericin B has a conjugated system with seven double bonds acting as a chromophore that absorbs strongly in the
ultraviolet–visible spectrum, giving it a yellow color Conjugated polymer
nanoparticles (PDots) are assembled from hydrophobic fluorescent conjugated polymers, along with
amphiphilic polymers to provide water solubility. Pdots are important labels for
single-molecule fluorescence microscopy, based on high brightness, lack of
blinking or
dark fraction, and slow
photobleaching. ==See also==