HOMO and LUMO determines the position of the absorption maximum
(l max ). Since wavelength is inversely proportional to ΔE, l max increases
with the length of the chain of the conjugated carbon atoms.
In the extreme case of a conjugated polymer, the π bonding orbitals and
the π* antibonding orbitals form broad bands separated by a small ΔE. A
long conjugated polymer is able to conduct electricity because electrons
in the valence band (π) can be excited into the conduction band (π*).
There, they are free to move and carry charge. Because electrons in
conjugated polymers must be excited in order to conduct electricity,
conjugated polymers make excellent semiconductors.
Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa shared the Noble
Prize in Chemistry in 2000 for their discovery and development of conductive (conjugated) polymers in the late 1960s and early 1970s. They did
much of their work on polyacetylene, the simplest conjugated polymer,
with alternating single and double bonds in a linear carbon chain.
Polyacetylene, however, is very susceptible to photo-oxidation, and more
recent studies have focused on other, more stable conjugated polymers.
Derivatives of polythiophene, polyaniline, polyfluorene, and poly(phenylene vinylene) or PPV have similar conductivities to polyacetylene but
are much more stable against oxidative degradation. Polymers and other
macromolecules will be discussed in Chapter 10.
5.4.3 Solvatochromism
The energy of electronic transitions in molecules and nanosystems are
affected by the environment that surrounds those molecules. When a
molecule undergoes an electronic excitation and electrons move into a
previously unoccupied orbital, the charge distribution in the molecule
changes. Such movement of charge may make a molecule or nanosystem
more or less polar. Recall that more polar molecules have a more
favorable potential energy of interaction with polar solvents due to
energetically favorable dipole–dipole and dipole-induced dipole interactions. If an electronic transition causes a molecule to become more polar,
its excited state will have a lower energy in a polar solvent than it would in
a nonpolar solvent, and the wavelength at which it is excited will become
longer (red-shifted) compared to the same molecule in a nonpolar
solvent. This is called a bathochromic shift. Conversely, if an electronic
transition causes a molecule to become less polar, the excited state will be
less energetically favorable in a polar solvent than in a nonpolar solvent
SIMPLE MODELS DESCRIBING ELECTRONIC STRUCTURE 173
(l max ). Since wavelength is inversely proportional to ΔE, l max increases
with the length of the chain of the conjugated carbon atoms.
In the extreme case of a conjugated polymer, the π bonding orbitals and
the π* antibonding orbitals form broad bands separated by a small ΔE. A
long conjugated polymer is able to conduct electricity because electrons
in the valence band (π) can be excited into the conduction band (π*).
There, they are free to move and carry charge. Because electrons in
conjugated polymers must be excited in order to conduct electricity,
conjugated polymers make excellent semiconductors.
Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa shared the Noble
Prize in Chemistry in 2000 for their discovery and development of conductive (conjugated) polymers in the late 1960s and early 1970s. They did
much of their work on polyacetylene, the simplest conjugated polymer,
with alternating single and double bonds in a linear carbon chain.
Polyacetylene, however, is very susceptible to photo-oxidation, and more
recent studies have focused on other, more stable conjugated polymers.
Derivatives of polythiophene, polyaniline, polyfluorene, and poly(phenylene vinylene) or PPV have similar conductivities to polyacetylene but
are much more stable against oxidative degradation. Polymers and other
macromolecules will be discussed in Chapter 10.
5.4.3 Solvatochromism
The energy of electronic transitions in molecules and nanosystems are
affected by the environment that surrounds those molecules. When a
molecule undergoes an electronic excitation and electrons move into a
previously unoccupied orbital, the charge distribution in the molecule
changes. Such movement of charge may make a molecule or nanosystem
more or less polar. Recall that more polar molecules have a more
favorable potential energy of interaction with polar solvents due to
energetically favorable dipole–dipole and dipole-induced dipole interactions. If an electronic transition causes a molecule to become more polar,
its excited state will have a lower energy in a polar solvent than it would in
a nonpolar solvent, and the wavelength at which it is excited will become
longer (red-shifted) compared to the same molecule in a nonpolar
solvent. This is called a bathochromic shift. Conversely, if an electronic
transition causes a molecule to become less polar, the excited state will be
less energetically favorable in a polar solvent than in a nonpolar solvent
SIMPLE MODELS DESCRIBING ELECTRONIC STRUCTURE 173