Rapidly Diversifying Hydrogen-Bonded Organic Frameworks With Permanent Porosity

Throughout the long history of porous molecular crystals (PMCs), hydrogen-bonded organic frameworks (HOFs) have experienced a renaissance and have advanced significantly over the past two decades. HOFs can yield large single crystals with fewer defects due to their reversible bonding and dissociation processes, enabling precise analysis of the structure-property relationship. They are also attracting attention as candidates for sustainable materials. This article reviews the evolution of HOFs (primarily those with permanent porosity) over the past 15 years, based on the molecular structures of their constituent molecules (tectons). HOFs have been synthesized from a wide variety of tectons, ranging from simple substituted benzene derivatives to those with various π-conjugated polycyclic aromatic cores, as well as with rigid or flexible macrocyclic cores. Subsequently, we would like to focus on several distinctive aspects and concepts of HOFs. These include permanent porosity of rigid HOFs and flexible HOFs, which is crucial for porous materials; isostructural (isoreticular) HOFs, which can provide a series of continuous HOF libraries; and non-stoichiometrically mixed multicomponent HOFs, which aim to tune functions and create new ones by using multiple types of constituent molecules.