Chemistry Reference
In-Depth Information
FIGURE 10.12
Schematic representation of dendrimers composed of rigid and flexible
backbones. The rigid dendrimer can retain its topological architecture that has a dense shell and
sparse interior. In contrast, the termini of the flexible dendrimer intrude in its inner space (back
folding) and are be more compact.
p
-conjugating backbone effectively reduces the back folding due to the rigidity. As a
result, stiff dendrimers can retain their topological character in the actual confor-
mation [131]. In addition, they should have a relatively larger space than the flexible
dendrimers. This would allow these dendrimers to encapsulate large guest mole-
cules [103,132] and metal clusters [38].
10.4.2 Light-Emitting Materials
Many conjugated molecules and metal complexes have been developed as light-
emitting materials in photochemical devices such as OLEDs. The most important
requirement for the material is a bright emission even in the solid state. Although
many luminescent compounds show a strong fluorescence in a dilute solution, the
fluorescence intensity drastically decreases in their solid state due to the excimer
formation or intermolecular energy migration that leads to fluorescence quenching.
Thus, low-molecular-weight luminescent materials should be dispersed in photo-
chemically inert host materials, which allows spacing between the chromophores for
their efficient emission in photochemical devices (Figure 10.13). Macromolecular
light-emitting materials, which includes a fluorescent (phosphorescent) core as the
main chain or graft groups, have been extensively studied by many researchers.
Because the polymer chain helps with the homogeneous dispersion of the chromo-
phores into the solid state by suppressing their local crystallization, the luminescence
properties improved in many cases. We can modulate the property by modifying the
composition of the luminescent part and other inert moieties [133-139]. As noted in
the previous section, the design significantly depends on a statistical factor based on
the undefined geometry. As solid-state emitters, dendrimers with a luminescent
molecular group are attracting much attention because their molecular architecture
allows the design of the exact geometry of its key components, that is, the “core
and terminus.”
