Chemistry Reference
In-Depth Information
Pancake
Higher flexibility
Rigid
Globular
4,8,16 PMIs (G1, G2, G3)
Stepwise increase of flexibility and chromophore interactions
Stepwise Increase of Flexibility and Chromophore Interactions
y and Chromophore Interactions
O
O
para
-Substitution
meta
-Substitution
PMI
PMI
FIGURE 12.10
Overview over some of the building blocks to achieve dendritic multi-
chromophoric systems with varying numbers of PMI chromophores, predefined geometries,
different chromophore orientations and varying chromophore distances. In the top row the core
elements are depicted for creating the first-generation (G1) globular and pancake-like dendritic
structures containing four PMI chromophores. Second (G2) and third (G3) generations of this
core elements allow the attachment of 8 and 16 PMI chromophores, respectively. The bottom
row displays the building blocks used for attaching the PMI chromophores to the core elements.
attaching the PMI chromophores in the
meta
or
para
position of the outer phenyl
groups of the dendritic scaffold.
A further advantage of polyphenylene dendrimers is the absence of functional
groups within the dendritic scaffold. This makes themhighly chemically and thermally
stable. Also, the opportunity to modify the substitution pattern of the chromophores as
well as the scaffold of polyphenylene dendrimers allows for the precise adjustment of
chromophore distances and orientations. Polyphenylene dendrimers are optically inert
in the visible part of the spectrum, due to the twisting between the phenyl rings in the
dendritic building blocks. Polyphenylene dendrimers absorb light up to 350 nm,
depending on the generation considered and do not emit fluorescence above
450 nm [92]. As a result, they are an ideal scaffold since they do not interfere with
the energy transfer processes themselves, provided that the chromophores involved are
chosen to absorb and emit in the visible part of the spectrum. In this way, complex and
highly ordered multichromophore architectures were designed in which the energy is
collected and can be transferred in a directed fashion [77,93].
12.7 EXCITATION ENERGY TRANSFER BETWEEN
STRUCTURALLY IDENTICAL CHROMOPHORES
We will demonstrate in this section that excitation energy transfer processes
can be studied at the single-molecule level by making use of a first-generation
polyphenylene dendrimer system containing a sp
3
carbon atom as a core and 1-4
