by Professor Seth C. Rasmussen
Most organic polymers (i.e. plastics) are insulators and are often used to
isolate metallic conductors from other conducting materials. However, studies
beginning in the 1960s revealed that the electrical conductivity of conjugated
organic polymers (Figure 1) can be controlled through oxidation or reduction1.
These initial discoveries have led to a modern class of organic materials with
the conductivity of classical inorganic systems, but with many of the desirable
properties of organic plastics, including mechanical flexibility and low production
Commonly studied conjugated polymers.
Such organic materials, often called synthetic metals7,8,
are semiconductors in their neutral state and exhibit increased conductivity
upon oxidation or reduction. As a result, they have received considerable fundamental
and technological interest, leading to their current use in such applications
(Figure 2) as sensors, organic field effect transistors (OFETs), organic photovoltaic
(OPV) devices, electrochromic devices, and organic light emitting-diodes (OLEDs)2-6.
In addition, the flexible, plastic nature of the organic materials used as the
active layers in such electronic devices has led to the realistic promise of
flexible electronics in the near future9-12.
Free standing conjugated polymer film (A) and examples of electrochromics
(B), OPV devices (C)13 and OLED devices
One advantage of utilizing conjugated polymers for technological applications
is the ability to tune the material properties at the molecular level4. This
is typically accomplished through synthetic modification of the monomeric units,
the combination of dissimilar units to make copolymeric systems, or in some
cases through post-polymerization processes. The most promising organic semiconducting
materials combine a number of critical properties, including processiblity,
stability, conjugation length, band gap energy, and charge mobility. The ability
to modify these properties is of the utmost importance for the efficient application
of these materials. For example, soluble, stable systems are required for their
physical processing and incorporation in technological devices, while the remaining
properties determine the effectiveness of the polymer as an electronic material.
The stability of conjugated polymers can be divided into electrochemical stability
and stability of the polymer to chemical or photochemical reactions15,16.
For example, neutral polyacetylene is considered to be electrochemically stable,
but is susceptible to triplet oxygen insertion, resulting in chemical decomposition.
Due to both environmental stability and ease of synthetic modification, polythiophenes
are thought to be one of the most versatile classes of conjugated polymers and
thus exhibit perhaps the greatest amount of synthetic diversity3-6.
Due to their rigid nature, conjugated materials exhibit limited solubility
in comparison to saturated organic polymers. As a result, the majority of unfunctionalized
systems are insoluble. Beginning in 1986, however, it was found that substitution
of alkyl chains (butyl or longer) in the β-position of thiophenes resulted
in soluble and fusible polymers5,6.
The application of such flexible side chains to conjugated polymers in general
has since resulted in large numbers of soluble materials that can be easily
solution processed. It should be noted, though, that while such methods can
be used to enhance solubility, there are a number of unintended consequences,
the most significant of which is the addition of steric effects that can twist
the polymer backbone out of planarity4,6.
Optical and Electronic Properties
The optical and electronic properties of these materials are determined by
the extent of conjugation along the material backbone and thus a high degree
of conjugation is essential. Conjugation length can be determined by several
factors, of which the simplest are the overall length of the conjugation path
and the molecule's planarity. As the conjugation length is dependent on the
overlap of orbitals between neighboring π-bonds, it can be influenced by
torsional strain along the backbone and significant deviations from planarity
(i.e. > 40° between units)17 result in decreased
conjugation4,6. In addition, as
the potential conjugation path increases with molecular length, small molecules
generally exhibit shorter conjugation lengths than polymeric materials. Extension
of the conjugation length with molecular length is not infinite, however, as
delocalization limits result in a maximum effective conjugation length (~20-30
rings in polythiophene)18,19.
Further extending chain length can affect physical properties, but has little
effect on electronic and optical properties.
Directly related to conjugation is the material's band gap (Eg),
which is the energy between the filled valence and empty conduction bands and
thus corresponds to the HOMO-LUMO gap of the solid state material4,20,21.
As a result, it determines the lowest energy absorbance and the energy of any
emission. Bond length alternation is considered to have one of the largest effects
on the Eg of organic materials (greater alternation gives larger
based on aromatic rings, however, differ in that they have non-degenerate resonance
structures (Figure 3). Here, the aromatic form has a larger Eg, but is more
stable and thus represents the ground-state structure. For these systems, it
is thought that increasing quinoidal character in the ground state has a greater
effect than limiting bond length alternation22,23.
A) Aromatic and quinoid resonance structures of polythiophene; B) Low
band gap polymers due to enhanced quinoidal character4,24-26.
Additional factors include backbone planarity, the heteroatom in heterocyclic
systems, and monomer aromaticity (Figure 4)4,15,27-30.
The effect of the heteroatom is thought to strongly correlate with its electron
affinity, with higher affinities resulting in lower Eg values29.
It has also been proposed that monomer aromaticity determines the confinement
potential of the π-electrons. As the confinement within the ring becomes
stronger, the delocalization length along the backbone decreases, resulting
in larger Eg values30. The last factor
to consider is interchain coupling that can occur via stacking in the solid
state, resulting in increased electron delocalization and a reduction in Eg
. Reduced molecular ordering can increase the spatial distance between polymer
chains, reducing interchain coupling and increasing Eg. Such molecular
ordering can also directly affect the materials charge mobility.
Illustrative examples of molecular effects on band gap.
In conclusion, the combination of all the above factors determines the resultant
material properties and the key to designing next generation organic materials
is the enhancement of one or more of these factors. However, as many of these
factors are interrelated, it can be challenging to selectively modify one factor
without affecting others.
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ACS Symposium Series, American Chemical Society: Washington, DC, 2011.
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Copyright AZoM.com,Professor Seth C. Rasmussen (North Dakota State