Functions and fundamentals of porous molecular crystals sustained by labile bonds

Organic molecules favour dense packing so that they can maximise the enthalpic gain upon solidification. Multidentate organic molecules that can form reticular bonding networks have been considered essential to overcome this tendency and assemble the molecules in a porous manner. Meanwhile, contrary to this understanding, a few organic molecules have been found to form porous molecular crystals by simply stacking with each other via van der Waals forces or analogous very weak noncovalent interactions. Although the porous molecular crystals were relatively rare in the 2000s due to the difficulty in the synthesis, their number has increased in the last decade, and their functional uniqueness has been unveiled eventually. This article reviews the recent advances in such functional porous molecular crystals. Particularly, thermal stability, processability, structural dynamicity, reactivity, and self-healing ability are highlighted. In addition, fundamental principles behind their functionalities, including the history, energetics, and the effect of crystallization solvent, are also reviewed.