Hydrogen-bonded organic frameworks (HOFs) are a class of porous polymers formed by hydrogen bonds among molecular monomer units to afford porosity and structural flexibility. There are diverse hydrogen bonding pair choices that could be used in HOFs construction, including identical or nonidentical hydrogen bonding donors and acceptors. For organic groups acting as hydrogen bonding units, species like carboxylic acid, amide, 2,4-diaminotriazine, and imidazole, etc., are commonly used for the formation of hydrogen bonding interaction. Compared with other organic frameworks, like COF and MOF, the binding force of HOFs is relatively weaker, and the activation of HOFs is more difficult than other frameworks, while the reversibility of hydrogen bonds guarantees a high crystallinity of the materials. Though the stability and pore size expansion of HOFs has potential problems, HOFs still show strong potential for applications in different areas. An important consequence of the natural porous architecture of hydrogen-bonded organic frameworks is to realize the adsorption of guest molecules. This character accelerates the emergence of various applications of different HOFs structures, including gas removal/storage/separation, molecule recognition, proton conduction, and biomedical applications, etc.
History Reports of extended 2D hydrogen-bonding-based porous frameworks can be traced back to the 1960s. In 1969, Duchamp and Marsh reported a 2D interpenetrated nonporous crystal structure with a honeycomb network constructed by benzene-1,3,5-tricarboxylic acid (trimesic acid or TMA). Then Ermer reported an adamantane-1,3,5,7-tetracarboxylic acid (ADTA) based hydrogen-bonded network with interpenetrated diamond topology. Meanwhile diverse works of guest-induced hydrogen-bonded frameworks were reported successively, which gradually developed the concept of hydrogen-bonded organic frameworks. Another milestone in the evolution of hydrogen-bonded organic frameworks was set by Chen. In 2011, Chen reported a porous organic framework with hydrogen bonding as binding force and demonstrated its porosity by gas adsorption for the first time. Since then, numerous HOF structures have been designed and constructed, meanwhile various applications related to porous frameworks have been attempted and applied to HOFs, whose effectiveness has been proved.
Hydrogen bonding pairs in HOFs Hydrogen bonds formed among various monomers guarantee the construction of hydrogen-bonded organic frameworks with different assembly architectures. The constitution of the hydrogen pairs is based on the structural and functional design of the HOFs, therefore different hydrogen bonding pairs should be selected following systematic requirements. The hydrogen bonding pairs generally include 2,4-diaminotriazine, carboxylic acid, amide, imide, imidazole, imidazolone and resorcinol, etc. Assorting with appropriate backbones, in every crystallization condition, the hydrogen-bonding pairs will exhibit specific assembly states, which means the morphologies with favored energy for this crystallization condition could be assembled by the monomers. In order to realize 2D or 3D HOFs, monomers with more than one hydrogen bonding pair are generally considered: the rigidity and directionality are also in favor of HOF construction.
Backbones of HOF monomer Rigidity and directionality of the constructional units offer HOFs various pore structures, topologies, and further applications. Therefore, a proper choice of monomer backbones plays an important role in the construction of HOFs. These backbones not only can combine with different hydrogen bonding pairs mentioned above to realize stable HOF structural design and expand pore size, but also give opportunities to offer more topologies of HOFs. Also, by using backbones with similar geometry and same connection pattern to generate the monomers and HOFs, the isoreticular expansion of the frameworks becomes a reliable method to expand the pore size effectively. As mentioned, for the sake of constructing porous and stable HOFs, multiple aspects should be considered simultaneously, such as the rigidity of the backbones, the orientation and binding strength of the hydrogen pairs, and other intermolecular interactions for orderly stacking. Therefore, the design of HOF monomers should focus on their H-bonds orientations and structural rigidity, and consequent framework stability and porosity.
Synthetic methods In principle, HOFs could be crystallized from solvents. However, the factors of solvent types, precursor concentration, crystallization time and temperature, etc., can have significant influence on HOFs crystallization process. Generally, the crystal products can correspond to kinetics through high concentration and short crystallization time, while slowing down the crystallization rate might yield thermodynamic crystals. One common method to produce HOF crystal is to slowly evaporate the solvent of the solution, which benefits the stacking of the monomers. Another widely used method is to diffuse low boiling point poor solvents into monomer solution with higher boiling point good solvents, in order to induce the assembly of the monomers. Depending on different crystallization systems, other methods have also been applied to HOF construction.
Characterization methods There are various methods to characterize HOF materials and their monomers. Nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HR-MS) are generally used for characterizing the synthesis of monomers. Single crystal X-ray diffraction (SCXRD) is the powerful tool for determining the structure of the HOF crystal packing. Powder X-ray diffraction (PXRD) is also a supported technique to demonstrate the pure phase formation of HOFs. The gas adsorption and desorption study through Brunauer-Emmett-Teller (BET) method could reasonably demonstrate some key parameters of HOFs, like pore size, specific gas adsorption amount and surface area from the adsorption isotherms. Depending on application directions and study fields, diverse techniques have been applied to the characterization of HOFs.
Applications The porous structures and unique properties guarantee HOFs good application performance in practical fields. The applications include but are not limited to gas adsorption, hydrocarbon separation, proton conductivity, and molecular recognition, etc.
… excerpt ends here. Continue reading the full article.
