Chemistry Reference
In-Depth Information
the extended electron- and hole-channel around the dendrimer architecture, an
efficient and stable charge separation could be achieved on the basis of the
molecular-level design. Not only for a photovoltaic application but the dendrimer
architecture also contributed various photochemical processes. For example, the
photochemical hydrogen evolution reaction was achieved using a molecular
sensitizer based on the dendrimer architecture.
10.6 CONCLUSION
The chemistry of a well-defined organic nanostructure has been developed over the
past decade. The advent of the dendrimer architecture provided a new strategy to
construct a fine nanostructure of discrete macromolecules, and possibility of broad
applications. In particular, a new class of
-conjugating dendrimers, which can
coordinate to multiple metal ions showed a very unique property in its metal-
assembling process. Although the dendrimer has many (30 or 60) ligation sites, the
metal ions are preferentially assembled at the ligation sites closer to the core. As a
result, metal ions are orderly assembled in the dendrimer scaffold through a step-by-
step fashion. This phenomenon allows suppression of the statistical distribution in the
number of assembled metal ions.
The metal assembly in a discrete molecular capsule can be converted to a size-
regulated metal cluster with a size smaller than 1 nm. Due to the well-defined
number of metal clusters in the subnanometer size region, its property is much
different from that of bulk or general metal nanoparticles. For example, the platinum
subnanocluster was determined to be a more active catalyst for the ORR than the
larger cluster. The size-specific variation in the onset potential where the ORR
started indicates that the poor catalytic performance as a fuel cell catalyst mainly
originated from the electron transfer process to the oxygenmolecule. Recent studies
have demonstrated that the electron transferprocesscanbeimprovedbythe
synthesis of a bimetallic composition. A similar synthetic protocol can be applied
to the synthesis of metal-oxide subnanoparticles, such as TiO
2
. Estimation of the
optical bandgap elucidated that the quantum size effect also appeared in TiO
2
nanoparticles smaller than 1 nm. This is the first investigation of subnanoclusters
properties that have not yet been determined.
We can now utilize this architecture in photochemical and electric devices, such as
light-emitting diodes or solar cells. Photochemical events (excitation, luminescence,
quenching or charge separation) should be implemented in the photoelectrochemical
devices as molecular-level functions. Although these essential functions are obtained
with organic
p
-extendedmolecules or metal complexes, it is not enough to use most of
the energy because the annihilation process (charge recombination) occurs during the
electron-transporting process through the films. A mechanism suppressing this
process can be realized by building a higher order structure of which the scale is
greater than single nanometers. In other words, the hierarchical structure from the
molecular level (functional unit) to the molecular assembly should be constructed
p
