Porous molecular crystals constructed through hydrogen-bonding are particularly called as "hydrogen-bonded organic frameworks (HOFs). Features of HOFs are as follows:

1. High crystallinity: HOFs are frequently obtained as a single crystal via a simple solution process thanks to the reversible H-bond formation.
2. Environmental friendliness: No heavy or transition metal species are necessary for framework construction, which can also provide lightweight materials.
3. Reusability and restorability: The materials are easily re-generated via a solution process and crystallinity can be recovered by vapor reannealing.
4. Flexibility: Rearrangement of H-bonds can provide structural changes to generate functionality.

Such intrinsic properties of HOFs, however, cause structural fragility and lack of general design strategy. Even when molecules are carefully designed, the desired porous HOFs do not always produce, but other structures such as nonporous crystals (Figure 1).

Figure 1. Construction of porous HOFs.

To obtain HOFs with permanent porosity by design, we have applied building block molecules involving both highly directional hydrogen bonding groups such as carboxy groups and shape persistent rigid pi-conjugate moieties. By connecting the molecules two-dimensionally or three-dimensionally via hydrogen bonds, we have constructed HOFs with various pi-systems and developed multi-functional porous materials (Figure 2).

Figure 2. Schematic representation for construction of 2D and 3D HOFs based on various pi-conjugated molecules.