Chemistry Reference
In-Depth Information
bring otherwise distant chromophores close enough so that energy can hop from one to the other,
skipping intervening units and thereby considerably shortening the effective migration distance along
an individual polymer chain [
93
]. For flexible polymers in solvents that promote folding, this motion
can take place even faster than excited state decay [
99
].
Intramolecular singlet energy migration can also proceed via electronic coupling through the bonds
that form the polymer backbone. In a random walk, the excitation energy migrates without directional
control, moving back and forth along a chain or across space. Through-space interactions between
pendant chromophores are also common in polymers with large numbers of absorbing units [
18
]. One
should also include movement of excitation across folds or loops that can form in polymeric chains.
Such folds can be the result of packing into crystalline domains or simply from temporary collisions.
In principle, the excitation can be localized for some finite time (however small) on a particular
chromophore before it is transferred to another one in the chain. Guellet [
94
] defines
intramolecular
energy migration
as any process that involves more than one exchange of excitation energy between
spectroscopically identical chromophores attached by covalent bonds to a polymeric chain [
12
]. He
further terms “energy transfer” as a single step migration between two chromophores, while one that
involves several or more chromophores as “energy migration” [
93
].
The polymers with multiple sensitizers offer several routes for energy migration. This can be
illustrated as follows [
99
]:
Route A
Route B
A very common arrangement is for the photosensitive groups to be aligned outside of a spiral
arrangement of the polymeric chain in close enough proximity to each other for energy transfer. Also,
as mentioned earlier, folding of a polymer before excitation into such a conformation that the
sensitizers are held within a hydrophobic pocket improves the efficiency of energy migration when
a large number of intramolecular hops. Efficiency of energy migration is also helped through-bond
interactions that intervene between the sensitizer and the ultimate trap [
99
]. Also, as mentioned
before, flexible polymer frameworks can bend the polymeric chains in such a manner as to bring
otherwise distant chromophores close enough together so that after excitation the energy can hop
from one to another. In such a case, the energy migration can skip intervening units and thereby
considerably shorten the effective migration distance along a single polymer chain. As stated above,
for flexible polymers in solvents that promote folding, this motion can be even faster than excited-
state decay [
99
].
Intermolecular energy migration can also occur between two different polymeric molecules. Thus,
for instance, Turro et al. [
95
] investigated inter- and intramolecular energy transfer in poly(styrene
sulfonate). They found that excimer formation between adjacent phenyl groups is a dominant reaction
both along a single chain and between two different chains [
95
]. At low densities of excited states,
singlet energy transfer between a sensitizer and its nearest quencher (perhaps on another chain)
dominates, whereas at high excited state densities, energy migration takes place through the series of
donors [
99
].
Guellet quotes Webber, who reported that he used the following equation (that he called crude but
useful) to obtain rough estimates of the energymigration diffusion rate along the polymer backbone [
94
]:
k
q
¼
4
pN
0
ðD
Q
þ k
g
ÞPR=
1
;
000
