Chemistry Reference
In-Depth Information
example of a long collective off event from1.2 until 1.4 s is presented in Figure 12.16c.
This rare long off has also been observed for PDI and other chromophores, where
similarly a long off state can be observed. Depending on the environment, these longer
off events follow a power-law or exponential distribution [28,117-120].
12.10 EXCITATION ENERGY TRANSFER BETWEEN
STRUCTURALLY DIFFERENT CHROMOPHORES
The above-mentioned F
orster resonant energy transfer processes are also applicable
between structurally different chromophores. Here we will mainly focus on energy
transfer from chromophores with a high S
0
-S
1
gap (donor) to chromophores with a
lower S
0
-S
1
gap (acceptor). The term dyad/triad that will be used refers to a bi- or
multichromophoric system that bears two/three different types of chromophores,
respectively.
Polyphenylene dendrimers can again provide a rigid framework for the intro-
duction of chromophores into the center, the scaffold or the periphery, by using
similar design concepts as presented in the previous paragraph. Different rylene
dyes such as naphthalenemonoimides (NMI), PMI, and TDI chromophores can be
attached in this way and act as light-harvesting arrays for directional transfer of
excitation energy [87,121-125]. Nature has brought this principle to perfection, as
can be seen from the crystal structure of the light-harvesting antenna complex (LH
2
)
of the purple bacterium
Rhodopseudomonas acidophila
[126]. In biological systems
a combination of Forster type energy transfer and strongly coupled chromophores
arrays is used to transfer energy over large distances [127,128]. On top, the
chromophore systems are protected from photobleaching by the presence of other
molecules like carotenoids.
Figure 12.17a shows the chemical structure of compound
€
, a dendritic dyad with
a TDI chromophore in the center and PMI chromophores at the periphery, which has a
high photostability suitable for single-molecule experiments. An efficient, vectorial
energy transfer via an energy gradient from the periphery toward the center of the
12
FIGURE 12.17
(a) Chemical structure of compound
12
containing one TDI chromophore as
a core and 8 PMI chromophores at the rim. (b) Steady-state absorption and emission spectra
(excitation wavelength at 500 nm) of compound
12
in toluene.
