Chemistry Reference
In-Depth Information
This can occur without any net energy loss (
resonant charge transfer
). It was shown experimentally
that the electrical conductivity in many polymeric materials, subjected to short irradiation pulses,
consists initially of a “prompt” component. That means that very rapid transfer of a considerable
amount of charge takes place over a comparatively short distance (
100
˚
). The movement of the
charge is then terminated as a result of trapping in “shallow” traps [
90
,
93
,
97
]. This is followed by a
“delayed” component that is very temperature-dependent and probably indicates a thermally
activated charge-hopping process between the shallow traps. This continues until terminated (after
1
m) by trapping in deep traps or by recombination [
90
,
93
,
97
].
There is a major difference between
m
eximers
of polymers and those of small molecules. The
difference is that at least in some polymers a large part of the excitation of the excimer site appears
to be a result of singlet energy migration [
93
]. Also, in polymeric materials with a number of identical
chromophores, either in the backbone or as pendant groups, when photons are absorbed, the excited
states cannot be considered as localized. In simple cases of rigid lattices, the excitations are distributed
over the entire volume of the material as a wave-like linear combination of local excitations [
87
,
90
,
91
,
93
]. They are referred to as
tight-binding excitations
[
90
,
91
,
93
]. As one might expect, excimer
formations in polymers depend upon the properties of the chromophores and upon their location on the
polymeric chain [
90
]. In addition, polymer tacticity, conformation, and distance between
chromophores can greatly affect the formation of eximers. Also, it is possible to distinguish between
two different types of energy transfers in polymeric materials. The transfer of excitation can take place
either from or to large molecules from small ones. Thus, for instance, a polymer transfer of the
excitation energy can be localized from a chromophore on one polymeric chain to another. An
example of a transfer to a small molecule is an energy transfer from a polymer, like polystyrene to a
scintillator molecule, like 1,4-bis[2-(5-phenyloxazolyl)]benzene shown below [
95
]:
O
O
More than that, transfer can also take place from one group of atoms, or from a chromophore,
located on a polymeric chain in one section of the molecule, intramolecularly, to another one located
at another section of the same polymer. Thus, in copolymers from monomers with two different
chromophores groups, the energy absorbed by one group of chromophores can be transferred to the
chromophores from the other group. This can take place by either Foster or exchange mechanism.
The possibility of energy transfer from one chromophore to an adjacent different chromophore in
polymeric chains depends to a large extent upon the lifetime of the excitation and its alternative
modes of deactivation. For this reason, the most readily observed form of energy migration is one that
occurs through the mechanism of the triplet [
88
,
90
,
93
,
97
].
Intermolecular energy transfer from one polymeric material to another while the molecules are in
solution or in the melt can also take place [
17
]. This was demonstrated on an
intramolecular excimer
and
exiplex
formation in solutions of polyesters containing naphthalene or carbazole moieties in their
chemical structures [
98
].
In general, the migrations of energy in polymers are somewhat more complex, because chain
folding and conformations are additional factors that enter into the picture. The separation between
interacting units can be affected by the composition of the polymer, the geometry of the polymeric
chains, and the flexibility of the backbones [
99
].
There are two limiting cases for the effects of polymer folding on energy transfer efficiency.
Folding of a polymer before excitation into a conformation in which the sensitizers are held within a
hydrophobic pocket improves the efficiency of energy migration. This takes place with a large
number of intramolecular hops or when bond interactions intervene between the sensitizer and the
ultimate trap [
93
]. If the polymers are flexible, however, they can also bend after photo-excitation to
