Chemistry Reference
In-Depth Information
beautifully combine the advantages of both small molecules and polymers. On one
hand, they can be solution-processed like polymers; on the other hand, their
polydispersity can be reduced to close to unity by using highly efficient transforma-
tions and careful purification protocols. Structure-property relationship may thus be
deduced, just as for small molecules. In addition, the unique structure of dendrimers
offers new possibilities for molecular designs and for device configurations. For
example, independently tuning the properties of the core, the branches, and the
surface groups allows one to create a library of structurally related materials for
screening. In addition, employing dendrimers of different generations can be used as a
new approach to control intermolecular interactions [5].
Conjugation plays a central role in semiconducting dendrimers [12]. For example,
in light-emitting and photovoltaic materials, the
-delocalized structural units absorb
or emit light of various wavelengths depending on their conjugation lengths. In
dendrimers, chromophores can be linked by either saturated aliphatic chains or stiff
conjugate units. The former choice bestows some flexibility in the dendrimer
structure, hence improving its solubility. In addition, ground-state electronic coupling
among subunits is minimized. However, the introduction of insulating (nonconju-
gated) moieties has been shown to be detrimental to device performance [13].
Moreover, uncertainty in the distance between chromophores sometimes makes the
interpretation of photophysical processes difficult. In some cases, chromophores may
come into contact via folding of the flexible connecting chains, resulting in unde-
sirable chromophore-chromophore interactions, such as aggregation, excimer for-
mation, and/or self-quenching [14]. The use of rigid, conjugated linkages can prevent
these problems. More importantly, the stiffness of all building blocks in such
dendrimers results in a unique property: shape-persistency.
The Moore group pioneered the study of shape-persistent dendrimers [15]. Since
their first publication in 1993, many rigid, conjugated dendrimers have been re-
ported [16]. The use of stiff units ensures that the dimension of the entire dendrimer is
almost invariant of environmental conditions [17]. As we will see later, the ability to
retain a stiff conformation in the solid state results in many favorable properties, such
as reduced interchromophore interactions. Additionally, different functional groups
can be positionedwith exact distances as designed. Note that the conjugation length of
a shape-persistent dendrimer does not necessarily extend through the entire molecule.
For example, the commonly used meta-phenylene linkage disrupts conjugation to
large extent. In addition, nonplanarity among different aromatic rings (due to
torsional angle) also reduces coupling of
p
-electrons. Also, certain chromophores
such as pyrene [18] and perylene diimide [19] are known to exhibit nearly the same
photophysical properties regardless of their being incorporated into a dendrimer or
not. Therefore, it is usually appropriate to treat a shape-persistent dendrimer as an
ensemble of many chromophores. This fact simplifies the interpretation of the
structure-property relationship, and allows independent tuning of different segments
of a dendrimer.
These unique properties of shape-persistent dendrimers have brought about their
increasing applications in organic electronics. In this chapter, we present an overview
of recent researches in this area, including our own contributions.
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