Chemistry Reference
In-Depth Information
and 9.7 nm for
, respectively. These were almost identical to the radii obtained from
the molecular modeling. Therefore, it was suggested that even in solution the extended
dendrimer molecules retained a relatively flat shape. This contrasts with the globular
shape of “flexible” dendrimers and compact conjugated dendrimers, in which steric
congestion overcame the stiffness of the building blocks [17]. Again, the length
gradient shows its importance; steric congestion was significantly reduced, allowing
themolecule to stretch to a flat conformation. This work is a nice validation of theword
“shape-persistent.”
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9.3.3 Outlook
In this section, we reviewed howshape-persistent dendrimerswere used to achieve fast,
directional ET that mimics some aspects of photosynthesis. The importance is clearly
shown of an energy gradient that funnels the peripheral excitons to the central core.
However, we are still far away from the grand goal of artificial photosynthesis. To
achieve that, instead of releasing the exciton energy as fluorescence, the excited state at
the core has to enter a charge separated state. Then, the electrons and holes should favor
different direction of movement to bring about a long-lived charge-separated state. We
note that several LH dendrimers with flexible chains have shown some primitive but
promising sign of this complicated function [99,100]. As a step further, electroactive
units should be introduced to take up the electron/hole, converting the light energy into
chemical energy and finishing a photochemical cycle.
The complex photophysical process in these dendrimers still needs proper
theoretical models; the classical Dexter or F
orster mechanismalone seems inadequate
to describe the process. For example, the distance between the donor and the acceptor
should be represented by a weighted distribution instead of a single value [88].
Clarification of the complex photophysics in these LH dendrimers would also help us
to understand the natural photosynthetic process.
Besides mimicking the photosynthetic process, the unique properties of rigid LH
dendrimers sparkled many interesting applications, some of which are described in
the next section of this chapter. But it should be mentioned that realization of
functional devices using these dendrimers is still rare. One reason is the tremendous
synthetic efforts needed to produce these large-sized molecules. Closer collaboration
among synthetic chemists and material scientists would allow a full exploration of the
application potential of these beautiful structures.
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9.4 MISCELLANEOUS APPLICATIONS
9.4.1 Introduction
In this section, we will present the application of shape-persistent conjugated
dendrimers in other areas of organic electronics not covered so far. It is not aimed
to be comprehensive; rather, we wish to illustrate how these large structures could be
tailored to meet different requirements in some main areas in organic electronics.
Examples include solar cell and nonlinear optics.
