Chemistry Reference
In-Depth Information
devices and molecular sensors [
2
-
5
]. One merit of network formation is that the
properties of each unit can be accumulated in a single molecular system. However,
the effects of the accumulation are not always additive because of intramolecular
interactions through bonds or space. In some cases, new properties appear in 3D
networks even though each unit exhibits little or no such properties. For example,
although large 3D networks tend to form inner cavities to accommodate guest
molecules, this property should not be expected for each building unit. These
phenomena, called
emergence
, are one of the significant outcomes of in-depth
studies of 3D aromatic networks.
2 General Remarks
2.1 Aromatic Units
Any aromatic units can be used as a building unit for the construction of aromatic
networks [
6
]. In the case of aromatic hydrocarbons, benzene is the most frequently
used, although other polycyclic arenes, including naphthalene, anthracene, and
pyrene, are attractive units for the construction of various 3D structures [
7
].
Heteroaromatic units, such as pyridine and thiophene, are used to generate novel
functions by means of their coordination ability and novel electronic properties.
The number of aromatic units (two, three, four units, and so on) is a basic factor that
characterizes aromatic networks. Some aromatic networks consist of one kind of
aromatic unit, whereas others consist of multiple kinds. In an aromatic network,
each unit is connected to other units via two or more linkers in most cases.
Therefore, the position of the linkers is another important factor that controls the
direction and distance of network extension. For example, benzene offers 60
,120
,
and 180
angles at
ortho
,
meta
, and
para
positions, respectively. Large aromatic
units should have more variations in connectivity. For example, naphthalene and
anthracene have 10 and 15 combinations, respectively, for the introduction of two
identical linkers.
2.2 Linkers
Linkers serve to connect one unit to another in aromatic networks via chemical
bonds or attractive interactions. Linkers are classified according to such factors as
geometry, length, mobility, kind of bond, and number of bonds. These factors
control the arrangement and flexibility of arene units and thus greatly influence
the structures and dynamic behavior of aromatic networks in 3D space.
Ditopic linkers are the most common, which connect two units across
n
atoms or
n
+ 1 bonds (denoted as
n
-atom linkers hereafter). Typical examples of covalent
one-atom linkers are methylene (
CH
2
-) and oxygen (
O-), although other
