Chemistry Reference
In-Depth Information
Two processes were proposed to explain the mechanism of energy transfer. In the first one, energy
transfers result from the interactions of the dipole fields of the excited donors and ground state
acceptor molecules (long-range: Forster (dipole-dipole)) [
86
,
90
]. This is referred to as the
resonance
transfer mechanism
Such transfer is rapid when the extinction coefficients for absorption to the
donor and acceptor-excited states involved in the process are large (10
4
-10
5
at the maximum). When
the dipolar interactions are large, resonance transfers are possible over distances of 50-100
˚
. Close
proximities of donors and acceptors, however, are required for weakly absorbing molecules. In the
second mechanism [
90
] (short-range: Dexter (exchange)), the excited donor and acceptor are in very
close proximity to each other, (up to
.
15
˚
) such that their electronic clouds overlap slightly. In the
region of the overlap, the location of the excited electron is indistinguishable. It may be at any one
instant on either the donor or on the acceptor molecule. Should the pair separate when the excited
electron is on the acceptor molecule, energy transfer has been achieved by the mechanism of electron
transfer, discussed in the next section.
Both absorption and emission processes may be intramolecular, localized in a single molecule.
On the other hand, they can also involve whole crystals that may act as absorbers and emitters. Such
energy transfers can manifest themselves in different ways that include sensitized fluorescence or
phosphorescence, concentration depolarization of fluorescence, photo-conduction, and formation of
triplet acceptor molecules.
Intermolecular energy transfer can be electronic and vibrational and can take place in solid, liquid,
and gaseous phases. In addition, the sensitized excitation of Q by S* has to take place within the time
that the molecule S remains in the excited state. In summary, theoretical and empirical considerations
suggest two modes of transfer, described above:
1. Only when the two molecules are in very close proximity to each other and their centers are
separated by the sum of their molecular radii will transfer take place.
2. When the two molecules are at distances that exceed their collision diameters, resonance transfer
or long range electronic excitation takes place though Coulombic interactions.
The transfers that take place by mechanism 1 are limited by diffusion of molecules in solution and
should be affected by the viscosity of the medium. Transfers by mechanism 2, on the other hand,
should be much less sensitive to the viscosity of the medium. It was shown by Foster [
86
] that the rate
constant of resonance-energy transfer (mechanism 1), as a function of distance, is:
6
S
!
Q
Þ¼
ð
ðR
o
=RÞ
=tS
Rate constant
1
where
tS
is the actual mean lifetime of S*,
R
is the separation between the centers of S
*
and Q, and
R
is the critical separation of donor molecules and the acceptor molecule. The efficiency of energy
transfer was expressed by Turro et al. [
94
] as follows:
S
½
S
½
S
g
F
et
¼ k
et
½
Q
=fk
et
½
Q
þk
d
½
The transfer by long-range excitation or mechanism 2 can be in the form a
singlet
-
singlet
transfer,
a
triplet
triplet transfer
. Due to the fact that the lifetime of triplet state
of molecule is longer than the singlet one, it is more probable to be the one to participate in energy
transfer. Molecules that undergo intersystem crossing with high efficiency, like benzophenone, are
efficient triplet sensitizers. Such molecules must possess high energy in the triplet state and a lifetime
of at least 10
4
s.
The two types of intermolecular energy transfers can be expressed as follows:
-
singlet transfer
, and a
triplet
-
S
SQ
0
6
Forster(dipole-dipole) long - range:
k
SQ
ðRÞ¼k
ðR
RÞ
