Chemistry Reference
In-Depth Information
where
D
Q
is the normal diffusion constant of the quencher and
k
g
is the energy migration diffusion
rate along the polymer.
In some aromatic vinyl polymers, excimer emission can occur after an initial excitation of an
aromatic chromophore. This is followed by intramolecular singlet energy migration, either along the
polymer chain, or intermolecularly along the chromophores. Here too, it can be through different
chains in a polymer that is in bulk form and the chains are in close proximity to each other. The
process generally continues until the excitation is trapped at some chain conformation that is suitable
for excimer formation. Such a chain conformation is referred to as
eximer-forming site
If the
polymer is in solution and viscosity is low, interconversion of chain conformations proceeds fairly
rapidly. In such cases, the lifetimes of any particular conformation are limited by the collision
processes as well as by the magnitude of the rotational barriers with respect to thermal energy [
93
].
In the solid state, however, the rotational freedom of the polymeric chain is considerably reduced.
Large-scale conformational changes are unlikely. There still is the possibility, however, that adjacent
chromophores will be in a marginal eximer-forming site [
94
].
.
10.3.7 The Antenna Effect in Polymers
It was originally observed by Schneider and Springer [
101
] that efficient fluorescence occurs from
small amounts of acenaphthalene that is copolymerized with styrene. Fox et al. [
100
] observed the
same effect in a copolymer of styrene with small amount of vinyl naphthalene. The emission of
naphthalene fluorescence is much higher than from solution of a mixture of the two homopolymers.
It was suggested by both groups that this phenomenon is due to energy migration between styrene
sequences to the naphthalene moieties. Guellet and coworkers carried out quantitative studies of this
phenomenon with various polymers that contained naphthalene or phenanthrene as the donors and
anthracene as the trap [
94
]. This effect is similar to one observed in ordered chlorophyll regions of
green plant chloroplasts (antenna chlorophyll pigments). It was, therefore, named the
antenna effect
.
Guellet [
94
] demonstrated that the effect is not entirely due to energy migration among the
chromophores that form the antenna, but rather a combination of migration and direct Forster energy
transfer to the trap [
94
]. It was concluded that energy migration and transfer in such systems are
primarily due to long-range Forster transfer by dipole-dipole mechanism (discussed earlier). In the
absence of any trap in the polymer, the energy will migrate along the backbone of the polymer chain
until it is deactivated by some other processes. In the presence of a singlet energy trap, the lifetime of the
excitation will be reduced and the length of energy migration will be reduced. The difference between
this form of energy transfer and one observed in solid aromatic polymers is that the photon energy is
collectedwithin a single polymer molecule and all energy transfer is intramolecular. The antenna effect
permits collection of the photon energy from the entire region of space (the hydrodynamic volume of
the polymer) and transmitting it to the traps located on the polymer chain. The efficiency is relatively
independent of concentration and can be very efficient even in dilute solutions [
93
].
10.4 Photosensitizers
As explained in Sect.
10.3
, photosensitizers are molecules that absorb the energy of light and act as
donors by transferring this energy to acceptor molecules. The molecules that receive the energy may in
turn undergo various reactions, such as polymerizations, isomerizations, couplings, and others. Many
different molecules can act as photosensitizers, but the most useful ones are various aromatic
compounds. In Table
10.1
are listed some common photosensitizes that appeared in various publications
