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N. Tohnai
10.1 Introduction
The construction of porous structures using small organic compounds is of significant
interest, and this technique has a wide range of applications related to gas and molecular storage [1] as well as the fabrication of template response materials, including
chemical sensors [2]. Recently, the strategic construction of porous structures, such
as metal–organic frameworks (MOFs) [3] and covalent-organic frameworks (COFs)
[4], has attracted attention because such techniques provide efficient synthetic strategies and molecular compatibility. These structures are generated via the formation of
either coordination or covalent bonds and thus are robust. However, organic porous
structures held together by weak non-covalent bonds are also an attractive alternative, due to their processing advantages and good workability [5]. As an example,
porous structures can be designed based on organic ammonium sulfonate salts [6].
Organic salt systems comprising two components allow systematic structural design
simply by varying the combination of materials. In addition, ammonium sulfonate
ion pairs generate strong intermolecular hydrogen bonding and electrostatic interactions. Over the past 20 years, Ward et al. have studied a variety of porous structures
based on guanidine and disulfonic acids [7]. In these structures, two-dimensional
hydrogen-bonded sheets containing guanidinium and sulfonate ions are connected by
disulfonate pillars to produce a grid-like structure with one-dimensional open channels. Our own group has previously reported the preparation of organic structures
consisting of sulfonic acids and aliphatic amines, termed porous organic salts (POSs)
[8]. As an example, biphenyl-4,4
-disulfonic acid and aliphatic primary amines can
be employed to build a POS with a layered network via charge-assisted hydrogen
bonding [8a]. The size and shape of the void spaces in such materials can be readily
adjusted simply by changing the amine. These POS systems are expected to provide
tunable substances with highly versatile functions due to the variety of possible
combinations of sulfonic acids and amines.
Herein, a new and efficient strategy for building porous structures with diamondoid networks is proposed, using the POS approach. The construction of such
structures is attractive not only as a means of obtaining highly symmetric, wellshaped networks, but also because these materials can provide high stiffness, good
stability, and large voids. Since the first report of a stable organic diamondoid
network of tetrahedral tetracarboxylic acid derivatives in 1988 by Ermer, [9] several
porous materials utilizing similar single tetrahedral molecules as building blocks
have been prepared [10]. However, it is still difficult to achieve the desired diversity in such structures using this conventional strategy. This is because the structural design of the diamondoid network often restricts the molecular configuration
to a tetrahedral shape that cannot be further modified. In addition, these diamondoid networks tend to form highly interpenetrating structures [11], in which void
spaces are smaller or absent due to the use of “non-bulky” building blocks. To
overcome these problems, a supramolecular-based hierarchical strategy is proposed,
in which tetrahedral supramolecules formed by simple molecules are employed as
N. Tohnai
10.1 Introduction
The construction of porous structures using small organic compounds is of significant
interest, and this technique has a wide range of applications related to gas and molecular storage [1] as well as the fabrication of template response materials, including
chemical sensors [2]. Recently, the strategic construction of porous structures, such
as metal–organic frameworks (MOFs) [3] and covalent-organic frameworks (COFs)
[4], has attracted attention because such techniques provide efficient synthetic strategies and molecular compatibility. These structures are generated via the formation of
either coordination or covalent bonds and thus are robust. However, organic porous
structures held together by weak non-covalent bonds are also an attractive alternative, due to their processing advantages and good workability [5]. As an example,
porous structures can be designed based on organic ammonium sulfonate salts [6].
Organic salt systems comprising two components allow systematic structural design
simply by varying the combination of materials. In addition, ammonium sulfonate
ion pairs generate strong intermolecular hydrogen bonding and electrostatic interactions. Over the past 20 years, Ward et al. have studied a variety of porous structures
based on guanidine and disulfonic acids [7]. In these structures, two-dimensional
hydrogen-bonded sheets containing guanidinium and sulfonate ions are connected by
disulfonate pillars to produce a grid-like structure with one-dimensional open channels. Our own group has previously reported the preparation of organic structures
consisting of sulfonic acids and aliphatic amines, termed porous organic salts (POSs)
[8]. As an example, biphenyl-4,4
-disulfonic acid and aliphatic primary amines can
be employed to build a POS with a layered network via charge-assisted hydrogen
bonding [8a]. The size and shape of the void spaces in such materials can be readily
adjusted simply by changing the amine. These POS systems are expected to provide
tunable substances with highly versatile functions due to the variety of possible
combinations of sulfonic acids and amines.
Herein, a new and efficient strategy for building porous structures with diamondoid networks is proposed, using the POS approach. The construction of such
structures is attractive not only as a means of obtaining highly symmetric, wellshaped networks, but also because these materials can provide high stiffness, good
stability, and large voids. Since the first report of a stable organic diamondoid
network of tetrahedral tetracarboxylic acid derivatives in 1988 by Ermer, [9] several
porous materials utilizing similar single tetrahedral molecules as building blocks
have been prepared [10]. However, it is still difficult to achieve the desired diversity in such structures using this conventional strategy. This is because the structural design of the diamondoid network often restricts the molecular configuration
to a tetrahedral shape that cannot be further modified. In addition, these diamondoid networks tend to form highly interpenetrating structures [11], in which void
spaces are smaller or absent due to the use of “non-bulky” building blocks. To
overcome these problems, a supramolecular-based hierarchical strategy is proposed,
in which tetrahedral supramolecules formed by simple molecules are employed as