Hydrogen-Bonded Organic Frameworks as a Tunable Platform for Functional Materials

Abstract

As a novel class of porous crystalline materials, hydrogen-bonded organic frameworks (HOFs), self-assembled from organic or metal−organic building blocks through intermolecular hydrogen-bonding interactions, have attracted more and more attention. Over the past decade, a number of porous HOFs have been constructed through judicious selection of H-bonding motifs, which are further enforced by other weak intermolecular interactions such as π−π stacking and van der Waals forces and framework interpenetration. Since the H-bonds are weaker than coordinate and covalent bonds used for the construction of metal−organic frameworks (MOFs) and covalent organic frameworks (COFs), HOFs have some unique features such as mild synthesis condition, solution processability, easy healing, and regeneration. These features enable HOFs to be a tunable platform for the construction of functional materials. Here, we review the H-bonding motifs used for constructing porous HOFs and highlight some of their applications, including gas separation and storage, chiral separation and structure determination, fluorescent sensing, heterogeneous catalysis, biological applications, proton conduction, photoluminescent materials, and membrane-based applications.

Gas Storage in Porous Molecular Materials

“…These are related to systems linked through more specific interactions, such as hydrogen‐bonded organic frameworks (HOFs), which resemble 3D networks in their directional connectivity, although they typically display the hallmark solubility of related porous molecules. HOFs have been reviewed extensively elsewhere [66–68] . As an example of a porous molecule governed by relatively weak intermolecular forces, tris‐ o ‐phenylenedioxycyclotriphosphazene creates a nanoporous structure through weak van der Waals interactions [69] .…”

Achieving Bright Mechanoluminescence in a Hydrogen-Bonded Organic Framework by Polar Molecular Rotor Incorporation

“…Hydrogen-bonded organic frameworks (HOFs), as an emerging class of multifunctional porous materials, are constructed by discrete organic units through hydrogen-bonding interactions, which can be further stabilized by π•••π interactions, van der Waals interactions, and other weak intermolecular interactions [1][2][3][4] . HOFs show promising applications in such as gas separation, catalysis, biological and optical but suffer from porous instability due to weak noncovalent interactions [5][6][7] .…”

A Rod‐Packing Hydrogen‐Bonded Organic Framework with Suitable Pore Confinement for Benchmark Ethane/Ethylene Separation

“…[6,7] Since ethane has ah igher polarizability than ethylene, dispersion and induction interactions would make major contributions in C 2 H 6 -selective adsorbents; [8] so an adsorbent material with ap ore structure enriched with nonpolar/inert surfaces (e.g.,f eaturing aromatic or aliphatic moieties) may favor the preferential adsorption of C 2 H 6 over C 2 H 4 .I nt his regard, the emerging hydrogen-bonded organic frameworks (HOFs) are of particular interest to be developed as C 2 H 6selective adsorbents. [9,10] HOFs are formed from purely organic building blocks through intermolecular hydrogen bonds (H-bonds). [11][12][13][14] This metal-free nature endows HOFs with native nonpolar/inert pore surfaces to directly produce the desired C 2 H 6 -selective materials.O wing to the soft and flexible H-bonds,HOFs hold some inherent advantages such as high solution processability,e asy purification, and facile regeneration and healing by simple recrystallization.…”

“…Moreover, HOF materials can be highly crystalline that benefits not only structural determination but also the investigation of structure-property relationship.These attractive merits make the exploitation of porous HOFs as C 2 H 6 -selective adsorbents become very attractive.H owever,t he major drawback of HOFs is their structural fragility because of the weak nature of H-bonds,m aking their frameworks oftentimes collapse upon removal of solvent molecules. [15] It is also very difficult to precise control over pore size and functional sites in HOFs to strongly interact with gas molecules, [9,10] which has always limited their separation performance.I ti st herefore not surprising that the research on robust HOFs for gas separation is still at the comparatively early stage. [16,17] To date,only few porous HOFs have been established for the important hydrocarbon separations with quite limited separation per-formance.…”