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H. Ohtsu et al.
12.1.2 Kinetically Controlled Network Formation
Kinetic control of chemical reactions is a critical tool to produce desired products in
chemical synthesis and material sciences. Diamond, one of the carbon allotropes, is
less stable than graphite, which means that it is a metastable state. However, because
of the large activation energy required to transform diamond to graphite, it remains
as a stable crystalline solid under ambient conditions. This is a typical example of
kinetically stabilized phase found in nature. Many researchers have used kinetic
control to direct reactions along alternative pathways to produce desired products.
In particular, in organic syntheses, kinetic control (or resolution) is often crucial to
selectively obtain only one specific molecule and limit side reactions [1]. Biological
systems use kinetically trapped states in living cells to sustain their function. For
example, the protein folding can be finely tuned through kinetically guided assembly
[2, 3]. Furthermore, the synthesis of inorganic materials, such as zeolites, is typically
carried out under kinetic control [4, 5].
In addition, kinetic assembly was used to change the morphology of materials
to create functional nanostructures via weak intermolecular interactions [6–8]. For
example, the complexation of perylene tetracarboxylate (PTC) with Ni
2+ displays
selective kinetic/thermodynamic assembly depending on reaction temperatures. The
coordinating amphiphile, PTC, has a planar conjugated skeleton. At temperatures
below 25 °C, the intermolecular coordination between PTC and Ni
2+ extends the
coordinating system along the long axis of PTC, resulting in a microbelt structure
several micrometers in length. However, PTC in this coordination mode is not planar,
therefore, the system cannot minimize its energy via π–π stacking, indicating that
it is a kinetic product. In contrast, at temperatures above 60 °C, PTC–Ni undergoes
intramolecular coordination where the ligand adopts a planar conformation, facilitating π–π stacking. Consequently, it can be concluded that PTC–Ni self-assembles
into much shorter nanorods under thermodynamic conditions. The two morphologies exhibit vastly differing electronic properties. The kinetically assembled microbelts have excellent electronic conductivity [6]. In another example, kinetic control
was used to obtain various metal phenolic network (MPN) films. For this purpose,
temporal and spatial control of MPN growth by promoting self-correction of the coordinating building blocks through oxidation-mediated network assembly was utilized
[7]. The formation and growth mechanisms were investigated and used to engineer films with microporous structures and continuous gradients. These results show
that morphological structure control by kinetic assembly is a promising strategy for
producing functional materials.
However, for coordination networks, also known as coordination polymers (CPs)
or metal–organic frameworks (MOFs), kinetic control has not been commonly
employed in their synthesis [9–23]. Normally, the networks are synthesized under
harsh conditions that lead to thermodynamically stable crystalline structures. For
example, solvothermal reactions, which are carried out in a solvent close to or above
its boiling point, and often under autogenous pressure, produce network single crystals. Some of the most thermally stable materials reported, such as MOF-5, HKUST-1,
H. Ohtsu et al.
12.1.2 Kinetically Controlled Network Formation
Kinetic control of chemical reactions is a critical tool to produce desired products in
chemical synthesis and material sciences. Diamond, one of the carbon allotropes, is
less stable than graphite, which means that it is a metastable state. However, because
of the large activation energy required to transform diamond to graphite, it remains
as a stable crystalline solid under ambient conditions. This is a typical example of
kinetically stabilized phase found in nature. Many researchers have used kinetic
control to direct reactions along alternative pathways to produce desired products.
In particular, in organic syntheses, kinetic control (or resolution) is often crucial to
selectively obtain only one specific molecule and limit side reactions [1]. Biological
systems use kinetically trapped states in living cells to sustain their function. For
example, the protein folding can be finely tuned through kinetically guided assembly
[2, 3]. Furthermore, the synthesis of inorganic materials, such as zeolites, is typically
carried out under kinetic control [4, 5].
In addition, kinetic assembly was used to change the morphology of materials
to create functional nanostructures via weak intermolecular interactions [6–8]. For
example, the complexation of perylene tetracarboxylate (PTC) with Ni
2+ displays
selective kinetic/thermodynamic assembly depending on reaction temperatures. The
coordinating amphiphile, PTC, has a planar conjugated skeleton. At temperatures
below 25 °C, the intermolecular coordination between PTC and Ni
2+ extends the
coordinating system along the long axis of PTC, resulting in a microbelt structure
several micrometers in length. However, PTC in this coordination mode is not planar,
therefore, the system cannot minimize its energy via π–π stacking, indicating that
it is a kinetic product. In contrast, at temperatures above 60 °C, PTC–Ni undergoes
intramolecular coordination where the ligand adopts a planar conformation, facilitating π–π stacking. Consequently, it can be concluded that PTC–Ni self-assembles
into much shorter nanorods under thermodynamic conditions. The two morphologies exhibit vastly differing electronic properties. The kinetically assembled microbelts have excellent electronic conductivity [6]. In another example, kinetic control
was used to obtain various metal phenolic network (MPN) films. For this purpose,
temporal and spatial control of MPN growth by promoting self-correction of the coordinating building blocks through oxidation-mediated network assembly was utilized
[7]. The formation and growth mechanisms were investigated and used to engineer films with microporous structures and continuous gradients. These results show
that morphological structure control by kinetic assembly is a promising strategy for
producing functional materials.
However, for coordination networks, also known as coordination polymers (CPs)
or metal–organic frameworks (MOFs), kinetic control has not been commonly
employed in their synthesis [9–23]. Normally, the networks are synthesized under
harsh conditions that lead to thermodynamically stable crystalline structures. For
example, solvothermal reactions, which are carried out in a solvent close to or above
its boiling point, and often under autogenous pressure, produce network single crystals. Some of the most thermally stable materials reported, such as MOF-5, HKUST-1,