Chemistry Reference
In-Depth Information
directly that molecules display delayed fluorescence [49] Molecules with significant
longer decay time than 6 ns can be considered as having backward electron transfer
(colored yellow/green).
12.6 DENDRIMERS AS SCAFFOLDS FOR STUDYING ENERGY
TRANSFER PROCESSES IN MULTICHROMOPHORIC SYSTEMS
12.6.1 Introduction
In this section, we will see how dendrimers can act as a synthetic multichromophoric
model system that can be used to study energy transfer processes and excited-state
interactions. The study of these excited-state dynamics and multichromophoric
interactions plays an important role in the understanding of biological systems (like
the Light Harvesting 2 complex, DsRed, allophycocyanines, etc.) [62-67] and
material science applications (like for example MEH-PPV for organic light emitting
diodes LEDs) [68-70]. Again the single-molecule detection and spectroscopy aspect
will be highlighted.
12.6.2 Forster Type Energy Transfer Processes
Before going into detail on the experimental data it is maybe first important to briefly
review the processes that will be mentioned here. Also the role that the dendrimer
architecture plays will be highlighted. Forster type energy transfer processes occur
when chromophores are in close proximity to each other (1-10 nm), and this transition
dipole-dipole interaction decreases with the distance to the sixth power [71]. The rate
constant for energy transfer
k
ET
can be expressed by Eq. 12.1 where
k
D
is the decay
rate constant of the excited state of the donor,
R
is the interchromophoric distance, and
R
0
is the F
orster radius is the distance at which the rate constants
for energy transfer and decay of the excited state of the donor are equal. This
R
0
value
can be calculated from experimental data by Eq. 12.2
orster radius. The F
€
€
6
k
D
R
0
R
k
ET
¼
ð
Þ
12
:
1
2
9
ð
ln 10
5
N
A
k
Þ
F
D
n
4
R
0
¼
J
ð
12
2
Þ
:
128
p
2
is the orientation
In Eq 12.2, besides the presence of a number of constants,
k
factor,
F
D
is the quantum yield of fluorescence of the donor in absence of the energy
transfer,
J
is the overlap integral between the donor emission spectrum (area under
the spectrum normalized to 1) and the absorption spectrum of the acceptor,
n
4
is the
refractive index of the medium to the fourth power. More details on the F
orster type
energy transfer equations and possible pitfalls can be found in a recent article [72].
The two important parameterswith respect to studying accurately energy transfer
processes are the distance
R
and the orientation factor
€
2
. In order to determine
k
