In chemistry, a conjugated system is a system of connected p-orbitals with
delocalized electrons in molecules with alternating single and multiple bonds,
which in general may lower the overall energy of the molecule and increase
stability. Lone pairs, radicals or carbenium ions may be part of the
system. The compound may be cyclic, acyclic, linear or mixed.
Conjugation is the overlap of one p-orbital with another across an
intervening sigma bond. A conjugated system has a region of
overlapping p-orbitals, bridging the adjacent single bonds. They allow a
delocalization of pi electrons across all the adjacent aligned p-orbitals. The
pi electrons do not belong to a single bond or atom, but rather to a group of
atoms. The largest conjugated systems are found
in graphene, graphite, conductive polymers, and carbon nanotubes.
Mechanism Conjugation is possible by means of
alternating single and double bonds. As long as each contiguous atom in a chain
has an available p-orbital, the system can be considered conjugated. For
example, furan is a five-membered ring with two alternating double bonds and an
oxygen in position 1. Oxygen has two lone pairs, one of which occupies a
p-orbital on that position, thereby maintaining the conjugation of that
five-membered ring. The presence of a nitrogen in the ring or groups α to the
ring like a carbonyl group, an imine group, a vinyl group, or an anion will
also suffice as a source of pi orbitals to maintain conjugation.
There are also other types of conjugation. Homoconjugation is an
overlap of two π-systems separated by a non-conjugating group, such as CH2. For
example, the molecule CH2=CH–CH2–CH=CH2 is homoconjugated because the two C=C
double bonds are separated by one CH2 group.
Conjugated cyclic compounds Cyclic compounds can be partly or
completely conjugated. Annulenes, completely conjugated monocyclic
hydrocarbons, may be aromatic, non-aromatic or anti-aromatic.
= Aromatic compounds= Conjugated, planar, cyclic compounds
that follow Hückel's rule are aromatic and exhibit an unusual stability. The
classic example benzene has a system of 6 π-electrons, which forms the benzene
ring along the planar σ-ring with its 12 electrons. For benzene there are two
equivalent conjugated Lewis structures, so that the true molecular state is a
quantum-mechanical combination of the two structures with six equivalent CC
bonds which are intermediate between single and double bonds, and all of
equal length and strength. = Non-aromatic compounds=
Not all compounds with alternating double and single bonds are aromatic.
Cyclooctatetraene, for example, possesses alternating single and double
bonds. The molecule typically adopts a "tub" conformation. Because the
p-orbitals of the molecule do not align themselves well in this non-planar
molecule, the electrons are not as easily shared between the carbon atoms.
The molecule can be still considered conjugated, but is neither aromatic, nor
antiaromatic. Conjugated systems in pigments
Many pigments 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 pi 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 and the lowest unoccupied molecular orbital. For a chain of n C=C
bonds or 2n carbon atoms in the molecular ground state, there are 2n pi
electrons occupying n molecular orbitals, so that the energy gap is
Since the box length L is proportional to the number of C=C bonds n, this means
that the energy ΔE of a photon absorbed in the HOMO-LUMO transition is
approximately proportional to 1/n. The photon wavelength λ = hc/ΔE is then
approximately proportional to n. Although this model is very approximate,
λ does in general increase with n 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.
Many electronic transitions in conjugated π-systems are from a
predominantly bonding molecular orbital to a predominantly antibonding MO, but
electrons from non-bonding lone pairs can also be promoted to a π-system MO 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 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 Phthalo Blue and Phthalo Green, 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 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. Conjugated chromophores are found in
many organic compounds including azo dyes, compounds in fruits and
vegetables, photoreceptors of the eye, and some pharmaceutical compounds such
as the following: See also
Resonance Hyperconjugation
Cross-conjugation Polyene
Conjugated microporous polymer List of conjugated polymers
References and notes