200
I. Hisaki et al.
11.1 Introduction: Porous Molecular Crystals
Porous molecular crystals (PMCs) are crystalline materials with permanent
porosity constructed from discrete organic molecules through reversible noncovalent intermolecular interactions. A pioneering work, for example, is of triso-phenylenedioxycyclotriphosphazene (TPP) [1–3]. Although PMCs are closely
related to organic inclusion crystals [4], an important feature of PMCs is that they
have self-standing pores (i.e., permanent porosity) that can accommodate various
and/or specific guest molecules reversibly. PMCs have recently attracted renewed
attention from viewpoint of applications such as selective gas storage/separation,
catalysis, chemical sensing, drug delivery, and optoelectronics [5–8].
Specifically, PMCs that are formed via hydrogen bonds (H-bonds) are often called
as hydrogen-bonded organic frameworks (HOFs) [9–13], which was introduced by
Chen [14]. Although several names and acronyms to describe PMCs constructed
through H-bonding can be found in the literature, we call such PMCs as HOFs in
this chapter. Typical merits of HOFs are as follows:
• HOFs are frequently obtained as single crystals via a simple solution process due
to the reversible nature of H-bonds, which enables to determine the precise crystal
structures by single-crystal X-ray diffraction (SXRD).
• HOFs do not need metal species for framework construction, allowing lightweight
and environmentally friendly porous materials.
• HOFs have ability to restore its original crystallinity by reannealing.
However, such intrinsic properties of HOFs simultaneously cause the following
problems:
• HOFs tend to collapse when solvent molecules are removed from voids to activate
the porous structures.
• HOFs are difficult to predesign: Even if building block molecules are preorganized
thoughtfully, the porous HOFs are not always produced as designed.
• The H-bonding moieties of building blocks are often trapped by polar solvent
molecules used for recrystallization, preventing formation of porous networked
frameworks.
Therefore, we need solve these kinds of dilemmas.
An important point to construct stable HOFs with permanent porosity is to
combine non-covalent intermolecular interactions such as π/π interactions with Hbonds, because a H-bond alone is too weak to maintain low-density porous materials,
compared with a dative or covalent bond. Fortunately, significant progress in the field
has resulted in the production of excellent HOFs with permanent porosity, thermal
and chemical durability, and functionality [15–41]. Some typical molecular skeletons
and supramolecular synthons are listed in Fig. 11.1. For construction of predesigned
molecular architectures, the appropriate selection of both a supramolecular synthon
(directionality and multiplicity of H-bonding) and molecular skeletons (geometry,
size, rigidity, planarity, symmetry, and functionality) to satisfy both H-bonds, and
I. Hisaki et al.
11.1 Introduction: Porous Molecular Crystals
Porous molecular crystals (PMCs) are crystalline materials with permanent
porosity constructed from discrete organic molecules through reversible noncovalent intermolecular interactions. A pioneering work, for example, is of triso-phenylenedioxycyclotriphosphazene (TPP) [1–3]. Although PMCs are closely
related to organic inclusion crystals [4], an important feature of PMCs is that they
have self-standing pores (i.e., permanent porosity) that can accommodate various
and/or specific guest molecules reversibly. PMCs have recently attracted renewed
attention from viewpoint of applications such as selective gas storage/separation,
catalysis, chemical sensing, drug delivery, and optoelectronics [5–8].
Specifically, PMCs that are formed via hydrogen bonds (H-bonds) are often called
as hydrogen-bonded organic frameworks (HOFs) [9–13], which was introduced by
Chen [14]. Although several names and acronyms to describe PMCs constructed
through H-bonding can be found in the literature, we call such PMCs as HOFs in
this chapter. Typical merits of HOFs are as follows:
• HOFs are frequently obtained as single crystals via a simple solution process due
to the reversible nature of H-bonds, which enables to determine the precise crystal
structures by single-crystal X-ray diffraction (SXRD).
• HOFs do not need metal species for framework construction, allowing lightweight
and environmentally friendly porous materials.
• HOFs have ability to restore its original crystallinity by reannealing.
However, such intrinsic properties of HOFs simultaneously cause the following
problems:
• HOFs tend to collapse when solvent molecules are removed from voids to activate
the porous structures.
• HOFs are difficult to predesign: Even if building block molecules are preorganized
thoughtfully, the porous HOFs are not always produced as designed.
• The H-bonding moieties of building blocks are often trapped by polar solvent
molecules used for recrystallization, preventing formation of porous networked
frameworks.
Therefore, we need solve these kinds of dilemmas.
An important point to construct stable HOFs with permanent porosity is to
combine non-covalent intermolecular interactions such as π/π interactions with Hbonds, because a H-bond alone is too weak to maintain low-density porous materials,
compared with a dative or covalent bond. Fortunately, significant progress in the field
has resulted in the production of excellent HOFs with permanent porosity, thermal
and chemical durability, and functionality [15–41]. Some typical molecular skeletons
and supramolecular synthons are listed in Fig. 11.1. For construction of predesigned
molecular architectures, the appropriate selection of both a supramolecular synthon
(directionality and multiplicity of H-bonding) and molecular skeletons (geometry,
size, rigidity, planarity, symmetry, and functionality) to satisfy both H-bonds, and