Chemistry Reference
In-Depth Information
10.3.4 Energy Transfer Process
Photosensitizers are used in many photonic applications. To understand how they function, it is
necessary to understand the energy transfer process. The term
energy transfer
[
92
,
93
] refers
specifically to
one-step radiationless transfer
of electronic excitation from a
donor
molecule to
another, qualified,
acceptor
molecule, from one chromophore to another one. This excludes what is
referred to as
trivial energy transfers
that result from the donor emitting light that is subsequently
absorbed by an acceptor. Based on the energy and spin conservation laws, there are two a priori
requirements for efficient energy transfer: (1) the process must be thermoneutral or exothermic to
occur with highest efficiency, because the activation energies have to be low due to short lifetimes of
electronically excited state, and (2) no net spin changes should occur. If a donor molecule was in the
triplet state at the time of the energy transfer process, the acceptor molecule is then also promoted to
the triplet state. Transfer of singlet to singlet energy should be possible, but it occurs less frequently,
because of the shorter lifetimes of the singlet states [
92
,
93
].
Energy transfer is thus a process by which excitation energy passes from one photo-excited
molecule, often referred to as a
sensitizer
and in this case designated as S*, to another adjacent
molecule in its ground state, often referred to as a
quencher
, in this case designated as Q. The
quencher must have a thermodynamically accessible excited state, one whose energy is lower than
that of S*. A donor molecule must possess sufficiently long lifetime to be an efficient sensitizer. The
reaction of energy transfer can be illustrated as follows:
S
þ
Q
Q
!
S
þ
S
!
S
0
þ hn
where * designates an excited state. In the process of energy transfer, S* returns (or relaxes) to the
ground state S. Energy transfer is further categorized as involving singlet (paired electron spins) or
triplet (unpaired electron spins) states. Symmetry rules, as explained above, require a singlet S* to
produce a singlet Q* and a triplet S
3+
to produce a triplet Q
3+
.
The quenching reaction of the excited state was expressed in a equation by Stern-Volmer. The
reaction shown below is based on a quenching reaction that is accompanied by a release of heat:
S
þ
Q
þ
Q
!
S
0
ðk
2
Þ
S
!
S
0
þ
heat
ðk
3
Þ
The equation is written as follows:
F
0
=F
Q
¼fk
1
þ k
2
½
Q
þk
3
g=ðk
1
þ k
3
Þ
In experimental studies of energy transfer, it is convenient to express the experimental results in
another form of the Stern-Volmer equation, as follows,
F
0
=F
Q
¼
1
þ k
q
t½
Q
where
F
0
is the quantum yield for a particular process in the absence of a quenching molecule
F
Q
is the quantum yield of the quenched process
k
q
is the bimolecular rate constant for the quenching process
t
is the lifetime of the state in the absence of a quenching molecules. It is equal to 1/(
k
1
+
k
3
), and
[Q] is the concentration of the quenching molecules
