Chemistry Reference
In-Depth Information
dendrimers. The other is a low-molecular-weight material, such as the metal-ligand
complex, charge-transfer complex and
-conjugating organicmolecules. The strategy
to fabricate these materials in an integrated system (device) depends on whether we
select the polymer materials or low-molecular-weight material.
The low-molecular-weight materials are generally fabricated by a dry process,
such as the chemical vapor deposition (CVD) method. One advantage of using this
kind of material (method) is its applicability for a multilayer device structure, which
consists of metal, organic, and metal-complex composites. The CVD method
requires a high-vacuum condition; therefore, low cost and mass production of
these devices is rather difficult. In contrast, the polymer materials could be
fabricated by wet processes, which do not require any special equipment and
conditions, except for the solvent. The fabrication of a homogeneous film composed
of polymer materials on a surface is much easier even if the surface area is very large.
Althoughmultilayer modification through thewet process is unfavorable, the recent
development of a molecular-level implementation of multiple functions in one layer
enables the achievement of various device functions with a much simpler device
architecture.
The strategy to design polymer materials for electric or photochemical devices has
been limited by the selection of the chemical composition. The electron transfer and
luminescent property of the material were added as the unit structure was imple-
mented on the graft part or main chain of the polymer. However, the solid-state
structure of these conventional polymers is complicated because the main chains
entwine with each other. The statistical character of the polymer has been prevented
by fine structuring with geometric or orientational control. Under this situation, a new
class of macromolecule “dendrimers” was proposed by Tomalia and Naylor [3] and
Newkome et al. [92] in 1985. These molecules can be defined as highly regulated
branched polymers possessing a focal point [2,69,93-95]. The “divergent” method
was employed for the synthesis of the early dendrimers. It was later improved by
Fr
p
echet et al. and called as the “convergent method” [1,96] that enables the
preparation of a completely monodispersed dendrimer of over 10 kDa molecular
weight without any disorder in the terminal number.
One of the characteristics of the dendrimers is the “encapsulating effect” [2,97,98]
Based on their topology, the density of the molecular units should be higher than that
of the inner shells. Utilization of the nanospace inside the dendrimer shell is wide, for
example, as a microenvironment for shape-selective catalysis [99-102], a template
for metal nanoparticle synthesis [38,63,66,68], a molecular storage area in supra-
molecular host-guest systems [103-106], and so on.
Standard dendrimers consist of three different moieties called the core, dendron,
and terminus (Figure 10.11). Each part is important for characterizing the properties
of the dendrimers. In general, the core is located at the center of the dendrimer where
the environment is much different from that of the exterior due to steric factors from
the peripheral parts. This position is suitable for applications that utilize the unique
microenvironment, for example, a shape-selective catalyst [99-102], a protected
photoexcitation center [107-111], or chemical storage [112,113]. In contrast, the
terminal possesses a character opposite to the core. It can be used as a reactive
