Chemistry Reference
In-Depth Information
O
OH
O
OH
C
C
+
light
10.3.5 The Electron Transfer Process
This process is particularly important in such photonic applications as solar cells. Simple migration of
energy is a thermodynamically neutral process. It allows the excitation energy deposited at a site in a
solid or in a concentrated solution to move to another position by transferring the excitation energy in
the absence of an intermediate quencher. Electron transfer, however, is a process by which an electron
is passed from an electron-rich donor to an electron-deficient acceptor [
95
]. This reaction is substan-
tially accelerated when the donor or acceptor is excited. Electron transfer from an excited state donor
molecule D* to a ground state acceptor A generates a radical cation D
+•
and a radical anion A
•
.
The resulting radical ion pair exists as a charge-separated pair of ions:
D
þ
D
þ
þ
A
A
!
The oxidized and reduced species are usually highly energetic, storing a substantial fraction of the
energy absorbed from the photon. The charge separation that occurs in such a photo-induced electron
transfer provides a way to convert the excitation energy of the excited molecule to a chemical
potential in the form of a radical ion pair.
Electron migration can also be a movement of an electron either to a neutral electron donor from an
oxidized one
A
Þ
[
93
]. These thermoneutral processes that are called
hole
and
electron migrations
, respectively, permit
further spatial charge separation between an excited donor, D*, and a reduced acceptor, A
•
.
This separation is beyond one that is initially produced in an ion pair by photo-induced electron
transfer. After the absorption of light by a to a sensitizer, the energy migrations or the energy transfer
move the excited state site where the excitation energy is converted to a radical ion pair by photo-
induced electron transfer. A kinetic competition then takes place between the rates of several possible
next steps. These steps can be chemical reaction of the radical ions, or they can be further charge
migrations by
sequential electron or hole transfers
, or actually nonproductive charge recombination,
called
back-electron transfer
. The back-electron transfer regenerates the ground states of both the
donor and the acceptor.
D
!
D
þ
A
þ
ð
D
þ
D
Þ
or from a reduced acceptor to a neutral one
ð
A
!
A
þ
10.3.6 The Charge Transfer Processes in Polymers
Charge transfer in polymers is either electronic (transfer of electrons or of positive charges alone) or it
is ionic (transfer of protons or larger charged species). Electronic conduction can be also of two types.
One type is conduction due to diffusion of electrons that are not localized on any particular molecule
(this is usually found in liquids or in gases). The other type can be by conduction due to positive or
negative charges that are localized on any particular part of the molecules. Such charges can be
exchanges between like polymeric molecules (or between segments of single polymeric molecules).
